A Thesis
Entitled
Novel Conductive Glass-Perovskites as Solid Electrolytes in Lithium – ion Batteries
By
Taiye J. Salami
Submitted to the graduate faculty as partial fulfillment of the requirements for a Master of
Science Degree in Chemical Engineering
Joseph Lawrence, Ph.D., Committee Chair
Sam Imanieh, Ph.D., Committee Member
Kim, Dong Shik, Ph.D., Committee Member
Amanda C. Bryant-Friedrich, Ph.D., Dean, College of Graduate Studies
The University of Toledo August 2018
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Copyright © 2018, Taiye Salami This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author.
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An Abstract of
Novel Conductive Glass-Perovskites as Solid Electrolytes in Lithium – ion Batteries
By Taiye J. Salami
Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Masters of Arts Degree in Economics
The University of Toledo August 2018
Despite commanding a huge market share of rechargeable batteries, current lithium ion batteries have safety concerns due to their use of flammable organic solvents as electrolytes. Successfully replacing the liquid electrolyte in a lithium ion battery with a solid electrolyte with comparable capability to the organic liquids would result in batteries that are safer to use, have a longer cycle life, and possess minimal self-discharge, wider operating potential and temperature window.
Solid electrolytes currently have a limitation in that they do not match the ability of the organic liquids in conducting lithium ions because they almost always have ionic resistive components called grain boundaries in their microstructure.
Appropriate combination of a glass with a perovskite-type ceramic that contains a lithium-ion conductive phase is shown to result in an amorphous composite having denser microstructure, better stability and no grain boundary effects. This is a pioneering breakthrough and a major upgrade to the ordinary crystalline ceramic, which, previously, had been shown to have one of the highest bulk ionic conductivity among solid electrolytes
iii but greatly limited in application because of the much higher ionic resistance of its grain boundaries.
In this research, different molar composition of glass and ceramics were melted and cooled using varying techniques, including a dual roller-quencher built from a rolling mill.
A phase diagram for the mixture at different compositions was proposed and the composition giving a nucleation & growth morphology, where the lithium – ion conductive phase was the amorphous matrix was found to be one order higher in ionic conductivity than the ordinary Li0.5La0.5TiO3 perovskite-type ceramic. The results from other cooling rates and doping of the glass – ceramics with foreign ions were also reported and explained in this report.
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This work is dedicated to my parents, rtd Engr. & Mrs. R.O. Salami, for nurturing me to strive for excellence in all my pursuits.
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Acknowledgement
I am greatly appreciative of my advisor, Dr Joseph Lawrence for always standing by me and advising me in his ever gentle manner even at times when I thought I was not seeing enough progress as I would have liked to. His professionalism was obvious right from the get go and I’m happy I learnt how to become a more active researcher from listening to him.
Words would fail me to express my gratitude and indebtedness to my co-advisor,
Dr. Sam Imanieh, assistant research professor at the Center for Materials and Sensor
Characterization, who taught me a lot of valuable academic and life lessons right from the start of my research till this moment. He was very open and ever ready to listen and advise me on what to do in all the various concerns I had. With his accessibility, I got more curious and ever willing to search deeper into the reasons why research is done and ultimately to be able to answer the question “Why?” Without a doubt, this whole research would never have been possible without his constant support.
To Dr Dong Shik Kim, who despite the short notice was willing to stand as a member in my defense committee where he offered constructive feedback on my work, I am most grateful. To everyone who taught me to be a better person, team player and scientist, I appreciate your support, patience and effort.
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Table of Contents
Acknowledgement ...... vi
Table of Contents ...... vii
List of Tables ...... xi
List of Figures ...... xii
List of Abbreviations ...... xvi
List of Symbols ...... xvii
Preface ...... xviii
1 Introduction ...... 1
1.1 Background ...... 1
1.2 Problem Statement ...... 6
1.3 Aim, Objectives and Scope ...... 7
2 Literature Review ...... 9
2.1 Battery Developments before Lithium-ion ...... 9
2.2 Lithium - ion Battery Electrodes ...... 10
2.2.1 Anode ...... 11
2.2.2 Cathode ...... 12
2.3 Lithium Ion Battery Electrolytes ...... 14
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2.4 Separator ...... 16
2.5 All Solid-state Batteries ...... 17
2.5.1 Lithium Phosphorus Oxynitride (LIPON) ...... 17
2.5.2 Lithium Superionic Conductor (LISICON) ...... 18
2.5.3 Sodium Superionic Conductor (NASICON) ...... 18
2.5.4 Garnet-type Oxides ...... 19
2.5.5 Perovskites ...... 19
2.5.6 Ionic Conductivity Measurement of Solid Materials ...... 23
2.6 Glass ...... 25
2.6.1 Glass Systems ...... 26
2.6.2 Properties of Glass ...... 29
2.6.3 Phase Separation in Glass ...... 30
2.7 Glass-Ceramics ...... 33
3 Methodology ...... 36
3.1 Equipment & Processing Materials ...... 36
3.1.1 Chemical Raw Materials Used in Glass-Perovskite Production ...... 36
3.1.2 Equipment for Processing & Finishing ...... 37
3.1.3 Characterization equipment ...... 38
3.2 Glass - Perovskite Production ...... 39
3.2.1 Mold - cooling and Water – cooling Processes...... 39
3.2.2 Roller – Quencher Fabrication and Use in Melt Cooling ...... 40
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3.3 Glass-Perovskite Characterization ...... 42
3.3.1 Differential Thermal Analysis ...... 42
3.3.2 Electron Microscopy Characterization ...... 42
3.3.3 X-Ray Diffraction Analysis ...... 43
3.3.4 Electrochemical Impedance Spectroscopy ...... 43
3.4 Crystalline LLTO Fabrication ...... 46
3.5 Glass-Perovskite Doping ...... 47
3.6 Heat treatment of Samples ...... 48
4 Results & Discussion ...... 49
4.1 Physical Examination of Samples ...... 49
4.2 Characterization Results ...... 53
4.2.1 DSC & TGA Response of Samples ...... 53
4.2.1 Glass – Perovskite EDX and Ordinary Perovskite SEM ...... 55
4.2.2 Phase Diagram for Glass-Lithium Lanthanum Titanate ...... 63
4.3 X-Ray Diffraction ...... 66
4.4 Impedance Analysis ...... 69
5 Effects of Varying Cooling Rates on Nucleation & Growth Morphology ...... 72
5.1 Water-cooling of Glass-perovskite melts ...... 73
5.2 Cooling Using a Roller-Quencher ...... 75
5.3 Impedance Response for Samples Obtained From Various Cooling Methods 77
5.4 Water-cooled Glass-Perovskite vs Sintered Perovskite ...... 80 ix
6 Glass – Perovskite Doping ...... 82
6.1 Physical Attributes of Doped Glass-perovskites ...... 82
6.2 SEM Morphology ...... 83
6.3 Impedance Analysis ...... 87
7 Conclusion, Recommendations & Future Work ...... 89
7.1 Conclusion ...... 89
7.2 Recommendations ...... 91
7.3 Future Work ...... 92
References ...... 94
Appendices ...... 100
A Other Information for Heat-treated Samples ...... 100
B Melting and Material Processing Pictures ...... 101
C Other Secondary Information ...... 105
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List of Tables
1.1 Technical performance of the current battery types ...... 4
3.1 Raw materials used in the production of glass - perovskite...... 37
3.2 Processing & finishing equipment for glass - perovskite manufacture ...... 38
3.3 Characterization equipment used ...... 39
3.4 Impedance analysis for glass perovskites ...... 45
3.5 Raw material composition for perovskites ...... 47
3.6 Concentration of the dopants used in 60%-40% perovskite glass ...... 48
4.1 Melting and boiling points of the raw materials used for glass-perovskite production
...... 52
4.2 Mass loss data in the melted glass-perovskite batch for 60% perovskite-40% glass . 52
5.1 Impedance data from various samples ...... 800
6.1 The impedance parameters obtained from the cerium – doped and fluoride – doped samples ...... 88
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List of Figures
Figure 1-1 Specific power versus specific energy plot for various secondary battery technologies ...... 3
Figure 1-2 The worldwide battery production & market worth ...... 5
Figure 2-1 The charge/discharge mechanism in a lithium-ion battery [21]...... 13
Figure 2-2 Structure of LiCoO2 (layered) and LiMn2O4 (spinel) ...... 14
Figure 2-3 Structure of the cubic crystalline lithium lanthanum titanate ...... 22
Figure 2-4 (a): Typical impedance cell arrangement for solid materials (b): bode plot and
(c): nyquist plots (via O’Hara corp.) ...... 24
Figure 2-5 Enthalpy - temperature curve for crystalline and glass materials [66] ...... 26
Figure 2-6 (a): Tetrahedral structure of silica (b): crystalline silica network and (c): amorphous silica networks in glass...... 28
Figure 2-7 Plot of Free energy vs composition for an ideal glass system [66] ...... 31
Figure 2-8 Immiscibility region in a binary system [66] ...... 32
Figure 2-9 (a): Nucleation & growth and (b): spinodal morphology of glass systems [66]
...... 33
Figure 3-2 Roller – quencher (right) made from rolling mill (left) used for high speed cooling of glass - perovskite ...... 41
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Figure 3-3 (a): The different electrode leads for the Gamry impedance analyzer, (b): a physical impedance cell setup ...... 45
Figure 4-1a Physical attributes of glass-perovskite cooled in mold ...... 49
Figure 4-1b Before and after sintering of perovskite sample ...... 50
Figure 4-2 Cross-sectional electron image for mold-cooled samples ...... 51
...... 51
Figure 4-3a DSC/TGA data for amorphous 60% perovskite - 40% glass ...... 53
Figure 4-3b TGA for ordinary perovskite 1st run vs 2nd run ...... 54
Figure 4-4 EDX showing the elements present in glass-perovskite ...... 55
Figure 4-5 SEM image for sintered perovskite ...... 56
Figure 4-6 SEM image of 50% perovskite – 50% perovskite showing spinodal decomposition ...... 56
Figure 4-7 SEM image of 55% perovskite – 45% glass showing a growing spinodal decomposition ...... 58
Figure 4-8 (a): SEM image showing nucleation & growth morphology of 60% perovskite and (b): Selected Area Diffraction Pattern using TEM ...... 59
Figure 4-9 TEM- EDS mapping of nucleation & growth morphology for 60% perovskite
- 40% glass ...... 60
Figure 4-10 SEM images of layered dendrites - in- glass morphology for 65% perovskite
- 35% glass at different magnifications (a. 250x, b. 500x, c. 2000x and d. 5000x) ...... 61
Figure 4-11 SEM image for (a and b) 70% perovskite - 30% glass showing the pattern for the formation of dendrites and (c) the dendrites growth along with their branches .... 62
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Figure 4-12 SEM image of 80% perovskite - 20% glass showing dendritic LLTO matrix and much reduced glass sphere areas at different magnifications (a. 5000x, b. 8000x and c. 20000x) ...... 63
Figure 4-13 Phase diagram for glass - perovskite for all compositions fabricated ...... 65
Figure 4-14 (a) XRD profiles for 60% LLTO and (b) other glass - perovskites ...... 67
Figure 4-15 Crystalline phases present in glass - perovskites ...... 67
Figure 4-16 XRD profile of perovskite showing cubic crystallinity ...... 68
Figure 4-17 (a & b) Impedance spectrum (Nyquist plot) for perovskite, (c) glass – perovskite, and (d) perovskite impedance from literature ...... 69
Figure 4-18 Temperature dependence of conductivity ...... 71
Figure 5-1 (a & b) Processing of water-cooled sample into pellets; (c) the SEM morphology after water-cooling, (d) SEM morphology after heating at 1100℃ ...... 75
Figure 5-2 Physical features of the roller – quenched sample ...... 76
Figure 5-3 SEM morphologies at 5000x for (a) mold - cooled, (b) water - cooled and (c) roller - quenched samples...... 77
Figure 5-4 Impedance response for (a) Roller – quenched sample and (b) mold – cooled vs roller – quenched sample impedance ...... 78
Figure 5-5 Impedance from all three cooling techniques used ...... 79
Figure 5-6 Impedance response of water –cooled sample heated at 1100°C vs perovskite
...... 81
Figure 6.1 Physical attributes of doped glass - perovskites ...... 83
Figure 6-2 SEM morphology with different magnifications (a. 250x, b. 1000x, c. 5000x and (d) EDS spectrum of cerium - doped glass - perovskite ...... 84
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Figure 6-3 SEM morphology of fluoride - doped glass – perovskite at (a) 1000x, (b)
2500x and (c) 5000x ...... 85
Figure 6-4 Strontium - doped glass - perovskite morphology at (a) 1000x, (b) 2500x and
(c) 5000x ...... 85
Figure 6-5 Proposed phase diagram for cation – site doped samples showing eutectic, dendritic and nucleation & growth regions ...... 86
Figure 6-6 Impedance response from cerium and fluoride - doped samples ...... 87
Figure A-1 XRD profiles for heat-treated samples ...... 100
Figure B (1-14) Pictures of processing materials...... 104
Figure C-1 Atomic arrangement in crystalline and amorphous SiO2 ...... 105
Figure C-2 impedance response from all three cooling processes, enlarged ...... 105
Figure C-3 (a-h): Nyquist curve fittings for doped samples, mold-cooled samples, roller- quenched samples, heat-treated sample and ordinary perovskite sample, (i): model used for fitting nyquist curve semi-circles ...... 106
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List of Abbreviations
ASSB……………………………………………...…………… All Solid-State Batteries
DSC ………………………………………………..Differential Scanning Calorimetry
EIS ……………………………………….…Electrochemical Impedance Spectroscopy
LIPON ……………………………………..………….Lithium Phosphorus Oxynitride LISICON ……………………………...……………Lithium SuperIonic CONductor LLTO ……………………………....Lithium Lanthanum Titanium Oxide (a ceramic)
NASICON ……………………………………….…….Sodium SuperIonic CONductor
ORNL ……………………………………………...….Oak Ridge National Laboratory
SADP ……………….………………………………..Selected Area Diffraction Pattern SAXS …………….….…………………………………….Small Angle X-Ray Scanning SEI …………………………………………………………Solid Electrolyte Interphase SEM …………………………………………………….Scanning Electron Microscopy
TEM …..……………………………………………Transmission Electron Microscopy TGA ………………………………………………………ThermoGravimetric Analysis
XRD ……………………………………………………………….…..X-Ray Diffraction
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ZRA …………………………………………………………..Zero Resistance Ammeter
List of Symbols
∆Gm ………………………………………………………….…...Free Energy of Mixing
∆Gs …………………………………………………………...Free Energy of Separation
∆Hm ……………………………………………………………….…Enthalpy of Mixing
∆Sm ……………………………………………………………………Entropy of Mixing °C ………………………………………………………………………….Degree Celsius Hz ……………………………………………………………………………..……..Hertz Rpm ……………………………………………………………..Revolutions per minute σ ……………………………………………………………………………..Conductivity
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Preface
Electrochemical energy storage is a subject that has the prospect of transforming technology as is known today. The importance of mobile digital electronic devices and equipment in the current age cannot be overemphasized and one subject that often comes to mind is how long these devices can be kept running continually. While the current electrical energy storage systems (or batteries) that power these devices vary with regard to end use, the lithium – ion battery, which operates via lithium ions migrating between its terminals is the most prevalent. This research seeks to address a very urgent question that arises with the use of lithium – ion batteries: the prospects of changing from its conventional unsafe liquid electrolyte to safer solid electrolytes.
Chapter one presents the current challenges with the most important safety risks in lithium-ion batteries: the organic liquid electrolyte, as well as the reasons why their solid state alternatives are not so attractive. The aim of the research – elimination of the challenges with a specific solid ceramic electrolyte by combination with glass was also outlined. In chapter two, a thorough literature review on lithium-ion battery components, solid electrolytes, glass and glass ceramics are given. Most importantly, the phase diagram in the glass-ceramics, which gives a representation of the thermodynamic and kinetic contributions during fabrication was proposed as this is vital for understanding microstructure variation with composition.
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In chapter three, the equipment used, fabrication and characterization methods used for the perovskite and glass – perovskites were discussed. This included a description of the impedance spectroscopy method used for understanding the behavior of the samples towards lithium migration and the parameters employed in the analysis. Chapter four considers the results from mold cooling of glass perovskites and also gives a proposed phase diagram for all compositions fabricated as well as important impedance response as different from the ordinary ceramic.
In chapter five, other cooling methods used, like water cooling and the use of high speed rolling equipment for the glass perovskites were considered to determine their effects on the microstructure and impedance response while chapter six considered the effects of doping glass – perovskites with various cations and anion. The motive for this was because of the high tolerability of perovskites in accepting foreign cations in their structure which further impacts their properties.
The appendix contains some DSC - informed results, a pictorial representation of the equipment used during this research and some secondary impedance fittings and results that could not be included in the chapters.
Taiye J. Salami June 2018
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Chapter 1
1 Introduction
1.1 Background
Batteries are important electrical energy storage devices required for the continued use of energy in the absence of the source. They are devices that convert stored chemical energy into electrical energy and vice-versa [1]. They are classified as primary or secondary, depending on the nature of the electrochemical reaction that brings about the generation of electrical energy within them. Primary batteries undergo non-reversible chemical reaction during the production of electricity and are used for powering devices ranging from television remote controller to intelligent drill bits; generally, in applications where charging is impractical. Because they need to be replaced after each complete use, they are considered costly, and they require a proper system in place for their disposal; hence their disadvantage.
Secondary batteries have a system of restoring the energy lost when an external potential is applied to them. Secondary batteries are otherwise called rechargeable batteries and find applications in electronic devices like phones, laptops, toys, power tools, etc.
Lithium-ion battery, the most common type of secondary batteries operates via lithium ions
1 migrating between electrodes through an electrolyte and electrons flowing simultaneously through a connected external circuit to provide the electrical energy that powers a load [2].
Lithium ions have smaller sizes compared to other ions in the periodic table, hence, they are known to transport charges fastest, and this gives them better power and energy densities [3, 4]. The energy density of a battery (measured in Watt-hours/gram or Watt- hours/liter) is what determines how much weight or volume is required by a battery to achieve a given electric range, obtained from the product of the capacity and the voltage; the capacity being a measure of how much current a cell provides [5]. Usually, the limiting electrode (in this case the cathode) determines the capacity of the battery. The power density is measured in Watts/gram or Watts/liter and is calculated as energy divided by time; this is what tells of how much load a battery can support. Lithium – ion batteries, in figure 1-1 is seen to have better specific power and energy combinations among various secondary battery systems, as adapted from visual capitalist.
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Figure 1-1 Specific power versus specific energy plot for various secondary battery technologies Source: visualcapitalist.com
Other important properties of batteries are the cycle number and the efficiency. The cycle number gives information about the battery’s expected life by showing how many times the battery can be charged or discharged before it begins to lose its capacity [6]. The efficiency defines how much of the total electrons are transferred from one electrode to the other in the process of charging or discharging. Lithium-ion batteries are again better when these factors are considered holistically, and table 1.1 below shows some of these comparisons.
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Table 1.1 Technical performance of the current battery types
Battery Lead-acid Ni-Cd Ni-MH Li-ion Type/Properties Energy density 35 40-60 60 120 (Wh/kg) Power density 180 150 250-1000 1800 (W/kg) Cycle life 4500 2000 2000 3500
Cost ($/kWh) 269 280 500 - 1000 Consumer electronics: 300-800 Vehicles: 1,000 - 2,000 Battery High reliability, Good memory Currently, best Small size, Characteristics low cost effect value and most lightweight popular battery for HEVs [6] Applications Starting & Replacement for Hybrid Electric Consumer ignition for cars, a flashlight Vehicles (HEVs), electronics forklift, golf cart battery a replacement for and backup flashlight power batteries Source: (Deutsche Bank, 2009; METI, 2009a; Nishino, 2010; the Institute of Applied
Energy, 2008)
Lithium-ion batteries because of these advantages have been the subject of scientific research since its commercialization by Sony in 1991 [7] due to the success of
Prof John Goodenough of the University of Texas at Austin in designing an efficient cathode material for it several years prior [8]. As expected, their production and market worth has also significantly increased, primarily because it has found continued applications in many recent electronic technologies including laptop computers, cell phones and more recently, hybrid cars. Figure 1-2 shows the steady increase in these values for different battery chemistries, as observed by AVICENNE energy, a consulting firm
4 specializing in battery market development for consumer electronics, automotive and industrial [9].
Lithium – ion battery enjoys high enough acceptability world over, however, there are some dangers with its use – the requirement of an organic liquid with a toxic lithium salt as an electrolyte. The combination is flammable, unstable and also limits the amount of potential difference allowed across electrodes [10]. When exceeded, this has the potential of causing fires and explosions. Also, the formation of dendrites at the anode causes short-circuit in the battery, resulting in a thermal runaway and ultimately battery fires. Altogether, these disadvantages have triggered research into the development of better battery electrolytes and hence the consideration of the All Solid-State Batteries
Figure 1-2 The worldwide battery production & market worth
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(ASSB) where the liquid electrolyte is replaced by a solid material which can conduct lithium ions efficiently [11].
Lithium lanthanum titanate, a ceramic material of the perovskite family is one of the most promising solids that can be used as electrolytes. This is because they have been shown in times past, to not only have higher ionic conductivities than most other solids but also to be more stable and non-reactive to air and moisture [12].
1.2 Problem Statement
Even though perovskite crystals have impressive ionic conductivity values termed as the bulk conductivity, their total ionic conductivity at room temperature is lower when compared with liquid electrolytes, and this significantly limits the overall power they can supply, thus discouraging further research and possible commercialization and adoption of their technology [13, 14]. This current challenge is the result of the grains existing as separate crystals in their morphology, hence creating spaces between crystals. These spaces are called grain boundaries. The grain boundary in effect offers additional ionic resistivity to the bulk resistivity, hence reducing the overall or total conductivity.
The formation of the separate crystals is due to the high-temperature sintering process [15] usually employed in perovskite fabrication. It is a relatively cost-effective production method where the processing conditions determine the size of the final grains but not the spacing between them [16]. Grain sizes are known to have a slight influence on conductivity values, with increased conductivities shown for the larger grains. However, this does not affect the overall conductivity significantly.
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1.3 Aim, Objectives and Scope
The present study aims to eliminate the grain boundary challenge present in lithium lanthanum titanate perovskite to make them less ionic resistive, and in effect increasing the ionic conductivity. Several methods have been used in the past, including cation doping and the use of pulse laser deposition but these processes have been shown to be not very productive and overly expensive respectively [17]. In this research, my goal is to incorporate a glass system having an open structure with the crystalline perovskite. The amorphous and crystalline morphology that results based on composition is characterized and the effect on lithium – ion conductivity analyzed.
The following are the objectives of my research: Fabrication of glass-perovskite with varying molar compositions: 50% perovskite – 50% glass, 55% perovskite – 45% glass, 60% perovskite – 40% glass, 65% perovskite – 35% glass, 70% perovskite – 30% glass and 80% perovskite – 20% glass by cooling from a constant melt temperature in a graphite mold; Characterization of these composites to determine the different morphologies and prediction of a two-component phase diagram for them; Also, fabrication of glass perovskite with specific molar composition (based on information from the phase diagram) using water as a cooling medium; Design and construction of high- speed roller-quenching equipment; Fabrication of glass perovskite of specific molar composition using the roller-quencher; Characterization of samples made from water cooling and roller-quenching; Fabrication and characterization of cerium, fluoride and strontium-doped glass-perovskite samples.
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Chapter 2 gives a thorough literature review on traditional lithium-ion battery electrolytes, materials used as solid electrolytes and their impedance measurement using blocking electrodes, glass processing, phase separation in glass systems, and glass- ceramics. In chapter 3, a description of the laboratory methods used for producing, preparing and characterizing the glass – perovskites were given. In chapter 4, the results obtained from traditional mold – cooling of glass – perovskite melt were presented.
In chapter 5, results obtained upon using other cooling methods for 60% glass – perovskites (water and roller- quencher) were presented and analyzed to determine the effects of the various cooling methods. Chapter 6 then considers the result of doping the glass ceramics by certain choice ions to know their implications on the glass perovskites produced. In chapter 7, the conclusions made in this research along with the recommendations for a better research were outlined, as well as the future studies on this research.
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Chapter 2
2 Literature Review
2.1 Battery Developments before Lithium-ion
We make use of stored energy in our daily life activities, from small flashlights and drills to cellphones and laptops. These days, we know about the manufacture of cars that are powered by rechargeable batteries as well, even though they are not yet commonplace like the conventional combustion engine vehicles. And it doesn’t end at that: batteries used for storing solar energies are being researched as potentially eliminating the use of heavy, high voltage capacity wires across streets when powering homes while others are considered for use in aircrafts as well. Batteries are cleaner and more flexible technology whose place in the future is undeniable as the world look towards not just minimizing careless energy production that use non – renewables in large quantities, but also careful and targeted storage of the energy produced to minimize wastage.
Electrochemical batteries convert electrical energy into chemical energy and comprise of three active components: the anode, cathode and electrolyte. The first battery invention was in 1799 when Italian physicist Alessandro Volta invented the voltaic pile.
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Issues with hydrogen gas evolution was a major limitation to the voltaic pile and was inevitably superseded by the Daniell cell introduced by British chemist John Daniell.
The accumulator was invented by French physicist Gaston Plante, having the advantage of being able to supply large inrush currents. Commonly called lead – acid batteries, they are used for starting and ignition in combustion engines in cars today. By
1899, the current design and appearance of batteries started to take effect with the advent of the Nickel – cadmium battery which had metallic casings containing a sheet roll of the active materials nickel and cadmium. Due to the toxicity of cadmium and the high rate of self – discharge however, the Nickel Metal Hydride or Ni – MH) was developed. Alkaline batteries were then invented by Canadian engineer Lewis Urry in 1950 and contained an alkaline potassium hydroxide as its electrolyte. It is used in domestic applications in flashlights, wall clocks and other simple devices with Duracell and Energizer being common brands of this battery type.
2.2 Lithium - ion Battery Electrodes
The most popular batteries used in the present age no doubt are the lithium-ion batteries, which have the advantage of using the lightest and most reductive ion (lithium) in its electrochemistry. They make use of components that make them suitable as the secondary batteries of choice for most devices.
The electrodes in a lithium-ion battery need have some very important attributes to be useful as one, chief among these being the ability to store/release lithium ions reversibly.
The ease or difficulty of this usually depends on the properties of the material used as the
10 electrodes and the chemistry of the interface between the electrodes and the electrolyte.
The lithium-ion battery uses intercalating graphite and lithium cobalt oxide electrodes when it was commercialized by Sony in 1991 [18] and even though new materials are being developed on a regular basis, they are still being used at present as they can deliver up to
3.6 Volts between them without giving off reaction products.
2.2.1 Anode
The anode is the negative electrode of a battery and where oxidation takes place during discharge. During charging, lithium ions are inserted into the anode by forming a lithium/carbon intercalation compound indicated as LixCn. The typical requirements for an anode material are: high capacity, long cycle life, high rate capability, electrochemical stability. The first categories of lithium – ion batteries had lithium anodes, because of the high specific capacity (3860mAh/g) it possesses but challenges with dendrites formation leading to short-circuiting meant these materials would need replacements [19, 20].
Usually, the more lithium ions a material can store, the better its use as an anode and for this reason, intercalation materials like graphite are the best candidates, giving the highest possible lithium content (372mAh/g,) at room temperature and standard pressure.
Intercalation materials are those that have structures capable of expanding and allowing intake as well as the release of ions.
There are other potential materials considered for use as the anode in a lithium – ion battery, the most important of which is the transition metal oxides and chalcogenides.
However, their inherently unstable nature and the offer of a higher potential versus Li/Li+
11 makes them undesirable for use. Typically, these materials have a potential of between 0.3
– 1.0Volts vs Li/Li+ while carbon-based anode materials offer 0.1Volts vs Li/Li+ , making for a higher potential between the electrodes [21].
2.2.2 Cathode
Cathodes are the positive electrodes of a lithium-ion battery during discharge. This means they accept electrons coming from the anode and are electrochemically reduced in the process. The cathode is usually the limiting electrode in a lithium – ion battery because its electrochemical properties determine the capacity of the battery [2]. The higher the amount of lithium it can store during charge, the greater the amount of power the battery can offer during discharge. They are broadly classified as intercalation and conversion cathodes with the former being the preferred choice, where the ions can be inserted into or removed from the host network repeatedly. The most popular intercalation cathode is the transition metal oxides family and rightly so because of the high operating voltage it can withstand and relatively higher energy density compared with other materials. Lithium cobalt oxide (LiCoO2), a layered oxide, is the first commercialized and most common transition metal oxide used as the cathode. It has a high theoretical volumetric energy density of 274mAh/L and a high theoretical specific energy capacity of 1363mAh/g. Fig
2.1 shows the charge/discharge mechanism that takes place in a lithium-ion battery [22]:
The cathodic half reaction is given by:
푒 퐿푖퐶표푂2 ↔ 퐿푖1−푥퐶표푂2 + 푥퐿푖 + 푥 …….. (Eq. 1)
Where the forward reaction is the charging process
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The anodic half reaction is provided by:
+ − 6퐶 + 푥퐿푖 + 푥푒 ↔ 퐶6퐿푖푥 ……….. (Eq. 2) Where the forward reaction again is the charging process.
The overall reaction in the lithium – ion battery is given by:
퐿푖퐶표푂2 + 퐶 ↔ 퐿푖1−푥퐶표푂2 + 퐶퐿푖푥 …… (Eq. 3)
Figure 2-1 The charge/discharge mechanism in a lithium-ion battery [21].
At over-discharge conditions, the cathode becomes supersaturated and produces irreversible products, and this marks the end of the life of the cathode. At overcharge conditions up to 5.2V, cobalt (IV) oxide is produced [23] with the battery being unstable.
These possibilities are best avoided and thus the reason why lithium-ion battery packs come with special protection and control measures to preempt the occurrence. Some drawbacks with the use of the LiCoO2 are firstly because of much lower discharge current (2.4A) due of safety limits and secondly because of increasing internal resistance during cycling / aging causing a large voltage - drop after few years of usage [24].
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Lithium manganese oxide cathodes were developed in 1996 and could allow for higher current flow (20-30A) within the battery, because of having a three – dimensional spinel structure but service life is not much different from that of lithium cobalt oxide cathodes [25]. They however have the advantage of requiring lesser safety circuitry than their correspondent because of their inherently higher thermal stability. Figure 2-2 shows the structures of the cobalt oxide and manganese oxide materials. The drawbacks in the usage of the manganese oxide battery is the much lower specific capacity, providing only about 1200mAh in a typical cylindrical battery [26].
Figure 2-2 Structure of LiCoO2 (layered) and LiMn2O4 (spinel) Source: batteryuniversity.com
2.3 Lithium Ion Battery Electrolytes
The electrolyte is the conductive medium for migration of lithium ions and requires careful selection so as not to limit the capacity of the electrodes in the battery or pose safety hazard that is unmanageable [27]. The electrolyte is the only component of the lithium-ion
14 battery that contacts all the other elements as a result, its selection needs a lot of considerations that cannot be made without first considering the components altogether.
Some of the properties desired of an electrolyte are: high ionic conductivity and electronic resistivity [28], be accommodative of the working potential of the electrodes in order not to limit them and to be inert to the other materials in the battery, like the anode, cathode and the separator. Electrolytes also need to be thermally stable, cost-effective and sustainable [29].
There has been many lithium – ion battery electrolytes developed over the years.
Some of these include ionic liquids, polymer electrolytes, organic solvents and hybrid electrolytes [30]. Of all these categories, the organic liquid electrolyte made by dissolving a lithium salt in an organic solvent is the most popular, having the best balance between ionic conductivity, stability and cost.
The electrolyte is usually made of Lithium hexafluorophosphate salt in a combination of ethyl and dimethyl carbonates [31, 32]. Ordinarily, the lithium hexafluorophosphate (LiPF6) salt is unstable and gives off hydrogen fluoride on reaction with water. It is thus mixed in appropriate proportions with the organic liquids to help enhance its properties while suppressing these disadvantages [33, 34]. Additives like dimethyl, diethyl and ethylmethyl carbonates make the electrolyte more stable at about 4V and as well make it more polar. Also, in the first few cycles of the lithium-ion battery, the ethylene carbonate additive in the electrolyte forms an ion conductive solid electrolyte interphase (SEI) with the electrode, and this helps to prohibit further depletion of the electrolyte as the SEI is electronically non-conductive [35-37].
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The combinations of solvents used for the lithium-ion battery electrolyte gives products that are irritants, toxic, flammable and harmful, thus requiring careful safety measures in place to make them less dangerous to use [32]. Potential hazards possible in a lithium – ion battery can be classified as chemical, electrical and thermal.
Chemical hazards result when there’s a spillage of the electrolyte or escape of the gas, electrical hazards occur when the state of charge is so high or so low that it makes the electrolyte unstable thus causing other unwanted reactions with other battery components while thermal run-away occurs when the heat buildup in a battery system is trapped, causing increase in internal temperature which produces other toxic gases [38] and ultimately to an explosion of the battery.
2.4 Separator
A separator is a permeable membrane that is usually placed between a battery’s anode and cathode to prevent short-circuiting by keeping the electrodes apart [39]. It, however, must also allow the transport of ions as this is necessary to complete the lithium- ion battery chemistry due to electrons migration through an external circuit. It generally must be chemically and electrochemically stable with the electrolyte and electrode materials and as well mechanically strong enough to withstand the high stress it is subjected. They are essential to batteries because their structure and properties considerably affect the battery performance, including the battery energy and power densities, cycle life, and safety [40].
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2.5 All Solid-state Batteries
Solid electrolytes are being developed because of their safety and stability advantages over their liquid counterparts [41]. They also would encourage a more significant manufacturing volume of much smaller electronic devices, also called micro devices because they do not require extra safety measures in place to ensure their safe operation. After many years into the research of the right solid electrolyte to replace the organic liquids, there haven’t been much success in this area because of the generally lower ionic conductivity values typical of solid electrolytes [42].
There are a good number of startup and auto-manufacturing companies that have built and continue to improve on an all-solid lithium ion battery technology based on different electrolyte chemistries, some of these include O’Hara Corporation, Sakti3,
Toyota, and Tesla Motors. Even though the development has been slow, there have been lots of promise and success in this field and as a result, it can be expected that solid lithium- ion batteries would be the batteries for the next generation of advanced electronics and cars. Some of the current solid electrolytes being researched and having the most significant potentials are enumerated below:
2.5.1 Lithium Phosphorus Oxynitride (LIPON)
This glass was initially prepared by researchers at the Oak Ridge National
Laboratory (ORNL) in 1991. LIPON is usually prepared by radio frequency sputtering of a mixture target of P2O5/Li2O in a nitrogen atmosphere and used in systems requiring ultra- low power supply systems and has been reported as showing negligible self – discharge
17 rate when used as the electrolyte in a lithium / lithium cobalt oxide battery, noticed at fully charged and fully discharged conditions when stored for a year or more [43]. It is known to be stable to Li/Li+ from 0 up to 5.5V [44] and due to the targeted end use [45], the low power density of LIPON is not much of a big concern, however, it is unideal for higher energy-requiring applications.
2.5.2 Lithium Superionic Conductor (LISICON)
LISICON are solid electrolytes known to have higher ionic conductivities than most of their solid electrolyte counterparts at room temperature (≈ 2.2 x 10-3S/cm). The thio-
LISICON LGPS system (Li10GeP2S12) was discovered in 2011 and found to satisfy most conditions required for advanced applications like electric vehicles, backup power systems, grid – level storage of renewably generated energy with their very high lithium ion conductivity of 1.2 x 10-2 S/cm [46, 47]. It would thus be expected that solid electrolytes based on this system would be the choice technology for the realization of an all solid-state battery, but the scarcity and cost of the germanium element and instability of the material with lithium metal has stifled its reality.
2.5.3 Sodium Superionic Conductor (NASICON)
This class of solid electrolytes have been found to exhibit high ionic conductivity of up to 10-4 S/cm at room temperature [48, 49]. It is currently used in high temperature applications and known to be stable to higher potentials and in particular, some carefully prepared NASICON have been shown to have a good stability with lithium anodes, giving it its most significant advantage over other solid-state electrolytes [50]. Also, is the
18 advantage that sodium systems do not have stringent rules like their lithium-ion counterpart, in that they have widespread availability [51] and are disposable without posing serious health risks. However, they are not known for higher specific energy and power densities, because of their higher mass than the lithium metal, hence a lot of work needs to be done to make them meet the requirements of more massive electronic systems.
2.5.4 Garnet-type Oxides
Garnet-type compounds have structures having a general chemical formula of
A3B2C3O12 ideally, where A, B and C are eight, six and four-oxygen coordinated cation sites respectively [52]. They are suitable materials for various cation sites doping and they vary extensively because of this. Various lithium-based garnet-type metal oxides exists in the literature, with Thangadurai, et al stating according to experimental and theoretical studies, that their lithium ion conductivity and conduction pathways depend on their lithium ions content and distribution in the various crystallographic sites.
Ionic conductivities of 10-6 S/cm have been reported for this category of electrolytes and some reports by Lu, et al have shown that appropriate doping of these materials give improved ionic conductivities of up to 2 x 10-4 S/cm [53]. The generally low conductivity of garnets at room temperature however prevents a wider application of the materials and some reports also indicate the instabilities with moisture and oxygen [54].
2.5.5 Perovskites
Calcium Titanate (CaTiO3) was discovered few centuries ago by Gustav Rose and named perovskite, after Russian mineralogist Alekseevich Perovski. It has since been
19 widely adopted as the general name for compounds having similar structure as CaTiO3
[55]. Perovskites are ceramics combining metallic elements with non-metals, usually oxygen. They show a good mix of electrical properties as they can be insulators, conductors or semiconductors; properties depending on the structures they possess [56, 57].
Ideally, they are described by the formula ABX3 where the A and B are metallic cations with A being the larger of the two while X is a non-metallic anion, mostly oxygen.
In the ideal cubic structure, the B cations lie at the center of the cube, position (½, ½, ½) and 6-fold coordinated with the oxygen ions. The A cations occupy the corners, at position
(0, 0, 0) and are 12-fold coordinated with the oxygen ions. The oxygen ions are face- centered at (½, ½, 0) [58].
There is usually some form of distortion in perovskite structures, making for the realization of other structures like rhombohedral, tetragonal, monoclinic and triclinic. This is usually caused by disparity in size or non-electro neutrality between A, B and oxygen ions, resulting in either a tilting effect, off-centering of the B cation or a combination of both. Tilting causes the symmetry of the perovskite to change and this instills new properties in the structure formed. It is known that the cubic structure is not prevalent at room temperature.
The tolerance factor is a parameter used to predict how much deviation from ideality would result from a combination of cations in the perovskite structure [59]. It is given by:
푟 + 푟 푡 = 퐴 푂 …… 4 √2 (푟퐵 + 푟푂)
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To maintain the perovskite structure, there is need for the presence of multivalent ions at the B – site to accommodate the effect of varying valency of the A cation. For this reason, the B site is mostly occupied by transition metal ions able to possess multi valency and thereby adjust to maintain the perovskite structure. When the centered ion in a perovskite structure is a transition metal ion, a variety of intriguing electronic and magnetic properties is achievable, partly because transition metals are known to possess various electronic properties because of having unfilled 3d shell structure and for the fact that they form complexes with halides and oxygen. Perovskites are ceramics known to accommodate a wide range of cations in their structure, from monovalent to multivalent, depending on the Goldschmidt tolerance factor of the resulting compound.
Some known families of Perovskites useful as electrolytes are Lithium Lanthanum
Titanium Oxide and Lithium-Strontium-Tantalum-Zirconium oxide. The typical fabrication process for perovskites includes preparation by the solid-state reaction from stoichiometric amounts of the raw materials, sintering, milling and pressing into pellets, furnace cooling to room temperature and characterization of perovskites prepared to determine its properties.
2.5.5.1 Lithium Lanthanum Titanate
Of the categories of perovskites available, the family of A – site deficient perovskite, called lithium lanthanum titanates Li3xLa(2/3)-xTiO3 (fig. 2-3) are the most promising, in terms of bulk ionic conductivity [60, 61]. Also, it is expected that due to their stability with moisture, oxygen, temperature and electric potential, they would allow for electrodes with higher voltage hence the possibility of powering more massive devices [62]
21 and reduction in the constant need to recharge current mobile electronic devices like phones and laptop computers.
Figure 2-3 Structure of the cubic crystalline lithium lanthanum titanate
Image generated by the VESTA (Visualization for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/
Li-ionic conductivity of the perovskite Li0.3La0.59TiO3 is very dependent on the processing conditions and the composition of the raw materials used in fabrication. This is because they determine the microstructure obtained which in turn gives information on the densification, grain sizes and grain boundary properties [63]. Lithium lanthanum titanate has in recent years been shown to exhibit enhanced properties upon appropriate doping of the parent material. This is made possible because of the ability of the foreign cations to reduce the activation energy of the perovskite by reducing both the defect – formation energy and the potential barrier energy.
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2.5.6 Ionic Conductivity Measurement of Solid Materials
Ionic conductivity for solid samples is usually measured by performing an impedance analysis using ion – blocking electrodes like platinum, gold and stainless steels
[64]. This system is applicable for use in thin – film batteries, super capacitors, fuel cells and electrochemical sensors, where the spectra is taken as a function of frequency. Two different plots are generated in a typical impedance analysis: A frequency dependent sinusoidal curve otherwise called the Bode plot and a complex impedance plot otherwise called the Nyquist plot.
In the Bode plot, the impedance magnitude and the phase angle are generated as a function of frequency. In the Nyquist plot [65], the different impedance regions in the solid samples are displayed as grain, grain boundary and electrode polarization regions at higher, lower and much lower frequencies respectively. Figure 2-4 (a-c) below shows the typical
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a
b
c
Figure 2-4 (a): Typical impedance cell arrangement for solid materials (b): bode plot and (c): nyquist plots (via O’Hara corp.) setup used for solid sample impedance characterization and the responses obtained in impedance analysis.
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A model for the impedance generated in the Nyquist plot is used to extract the approximate values of resistances for the material under analysis whereupon this is combined appropriately with the sample parameters to obtain the ionic conductivity values for the sample.
Generally:
푠 푡ℎ𝑖푐푘푛푒푠푠 휎 ( ) = ……. 5 푐푚 (퐴푟푒푎 푥 푟푒푠𝑖푠푡푎푛푐푒)
2.6 Glass
Glasses are essentially amorphous solids having no long range periodic arrangement and exhibiting a time-dependent glass transformation temperature, a property signifying the region at which a material begins to change from solid to slightly rubbery state, instead of straight transition into a liquid such as occurs when crystalline materials are melted. They are produced traditionally by heating the constituting materials to temperatures beyond their melting temperature and cooling the melt fast enough so that there is not enough time for crystallization to be reached. This has the effect of making for a much smaller dip in an enthalpy – temperature curve, more than would be observed if enough time is allowed for the material to cool to equilibrium conditions [66].
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On the enthalpy – temperature curve in the figure 2-5 below, the deviation from crystallinity is shown and the glass transformation region lying between the equilibrium liquid and the frozen solid regions.
Figure 2-5 Enthalpy - temperature curve for crystalline and glass materials [66]
2.6.1 Glass Systems
Typically, glass constituents are divided into network formers, network modifiers/fluxes and intermediates. Other glass constituents are fining agents used for removal of bubbles during glass production.
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Depending on the constituting elements in the networks of glasses, they can be either silicates, phosphates or borates. Glass made from a silicate network are the first and the most common glasses known and thus, many glass systems are explained based on this system.
2.6.1.1 Glass Formers
Glass network formers are the most important constituents of glass systems. There have been some theories over the years that helped to give some description of glass network formers which are applied in most glass making considerations. Because formation of a non-periodic, non-symmetrical and open network is pertinent to the formation of glass, Zachariasen proposed that it is necessary that glass network formers have structures where the oxygen atom is not linked to more than two network cations, so as not to prevent the variation in cation-oxygen-cation bond angles necessary for a non- periodic network.
Also, Zachariasen proposed that glass network formers needed to have structures such that a high proportion of the cations is surrounded by oxygen triangle or tetrahedra to form a continuous structure and that the triangle or tetrahedra needs to be connected by corners to other triangles and tetrahedra in the network. This was important for the open structure requirement of glass.
Other theories of glass formation were also considered by various early glass analysts. Smekal considered that glass melts needed to have properties intermediate between those of ionic and covalent bonds with oxygen, because purely ionic bonds would
27 result in a non-directional framework and purely covalent bonds would result in sharply- defined bond angles, hence preventing the formation of a non-periodic network.
Sun proposed that glass – forming ability of melts is dependent on the bond strength of the constituents, with higher values meaning more impediment to crystal formations while Rawson considered temperature along with bond strength in determining glass formers that are better. According to Rawson, a high, single bond strength and low melting temperature would make for a much better glass former.
These considerations suggest that silicates (SiO2), borates (B2O3) and phosphates
(P2O5) are the common glass formers of choice with silicates being the earliest studied.
Borates have triangular arrangements of the cation and oxygen while silicates and phosphates have a tetrahedral arrangement, figure 2-6 shows the structure (a), atomic arrangements in crystalline SiO2 (b) and SiO2 glass (c).
Figure 2-6 (a): Tetrahedral structure of silica (b): crystalline silica network and (c): amorphous silica networks in glass
Source: www.heraeus.com
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2.6.1.2 Modifiers & Intermediates
Stanworth classified oxides into three groups based on cation electronegativity. The first categories of cations are those that form bonds with oxygen with a fractional ionic character near 50%, termed network formers. Some cations however form very ionic bonds with oxygen and thus are generally called network modifiers. This category of cations cannot form glass networks based on the non-directional nature of ionic bonds but are added to help reduce the melting temperatures of the network formers by occupying the interstices in the network, thus reducing the unoccupied free volume of the structure and creating occasional breaks in the connectivity of the network and in effect producing a non- bridging oxygen.
The third category of oxides exist between these two extremes and are called intermediates. This category of oxides cannot form glasses out rightly but because they are tetrahedrally coordinated, can substitute directly into the network for silicon – oxygen tetrahedra. Alumina is an example of a glass intermediate. Because it offers 1.5 oxygen atom per alumina molecule, it requires oxygen supply from the modifier included in the composition to meet the requirement of 2 oxygen atoms per tetrahedron for fully linked tetrahedra. This has the effect of removal of a non-bridging oxygen from the structure.
2.6.2 Properties of Glass
Glasses exhibit some properties dependent on their brittle and transparent nature.
Mechanical properties, elastic modulus and hardness are factors dependent on the brittle nature of glasses, where properties like fracture behavior dependent on external/environmental factors, elastic modulus and hardness are considered. Elastic
29 modulus and hardness are both dependent on the type and strength of containing bonds respectively. Other factors like network structure and atomic density packing also affect these properties respectively.
Glasses are also known to transmit light in the visible region. Refractive index and optical dispersion properties, wavelength-dependent optical effects and modern glass technology are factors collectively determining the optical properties of glasses, accelerating their applications in optical instruments, telecommunication systems and their general esthetic appeal in homes.
2.6.3 Phase Separation in Glass
This phenomenon is so common in melts that it is thought that formation of homogeneous glasses from melts is unusual. The behavior of the free energy of the system is what determines if phase separation would occur, with the lower value between the free energy of mixing and the free energy of separation determining the path taken.
The behavior can be modeled by the mathematical expression for the free energy of mixing ∆Gm, given by:
∆퐺푚 = ∆퐻푚 − 푇∆푆푚
∆Hm is the enthalpy of mixing and expressed as
∆퐻푚 = 훼푥1푥2
Where α is a term related to the bond energies among constituent components.
∆Sm, the entropy of mixing is given by:
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−푅(푥1 ln 푥1 + 푥2 ln 푥2)
Where X1 and X2 are concentrations expressed as mole fractions and R is the universal gas constant.
Since the T∆Sm term on the right would always result in a negative value, the sign on ∆Gm would depend on the sign on α. If negative, the free energy of mixing term would be lower and the system would be homogeneous.
If α is positive, ∆Gm becomes a function of the absolute temperature, where at 0
Kelvin, it is positive and the system would phase separate. At sufficiently high temperature however, the T∆Sm controls and glass system stay mixed.
If temperature is between 0 K and the sufficiently high temperature, there is a competition between the enthalpy and the entropy contributions, resulting in a saddle in the curve of free energy vs composition (fig. 2-7). Above this solid curve, the melt is
Figure 2-7 Plot of Free energy vs composition for an ideal glass system [66]
31 homogeneous and miscible and below this curve, the melt separates into the respective phases if allowed by kinetics. This curve is called the binodal or immiscibility curve.
Phase separation occurs by two mechanisms: nucleation & growth and spinodal decomposition. Nucleation & growth starts with the precipitation of the minority phase from the melt which grows with time. This can be homogeneous or heterogeneous, depending on if the nucleus is formed within the melt or from external sources respectively.
Spinodal decomposition on the other hand, depends on the varying compositions of the system until it approaches.
Based on the classical Gibbs theory, the determining phase is based on the local curvature of the free energy of mixing at the bulk composition of the melt [67]. Depending
Figure 2-8 Immiscibility region in a binary system [66] on the position in the free energy vs composition curve of figure 2-6, a system would either have positive (between a and c or b and d) or negative ( between c and d) values of the
32 second derivative of the free energy of mixing, where a significant or slight change in composition respectively would be required for an overall decrease in the free energy to attain equilibrium [66]. These regions are the metastable nucleation & growth and the unstable spinodal decomposition regions respectively, depicted in the temperature – composition curve of figure 2-8 below. The morphology of nucleation & growth as well as spinodal decomposition for a glass system are shown in figure 2-9 a and b respectively.
a b
Figure 2-9 (a): Nucleation & growth and (b): spinodal morphology of glass systems [66]
2.7 Glass-Ceramics
Glass ceramics were discovered accidentally in 1953 by controlled crystallization of glass and found to possess enhanced properties like toughness, low coefficient of thermal expansion, low dielectric loss, high abrasion resistance to name a few. They may be crystalline or may contain a high amount of glass, with the crystals developing in the glass and thus changing the latter’s composition.
Microstructure and composition are the most important factors determining glass ceramics properties. The bulk chemical composition controls its ability to form a glass and
33 if homogeneous or heterogeneous nucleation is achieved. They can have varying morphological arrangements of microstructures, which is important to optical and mechanical properties and promote/diminish the characteristics of key crystals. The microstructure however depends on bulk composition and crystalline phase assemblage and can be modified by varying thermal treatment [68].
Glass - ceramics are applicable in electronics, optics, acoustics, mechanics, etc. with the most important system being the Li2O–Al2O3 –SiO2 (LAS) system. Based on field of applications, glass-ceramics can be high-strength, machinable, dental, bioactive, conducting & insulating glass-ceramics. Technology used in primary glass shaping, like rolling, pressing, casting and press-blowing can be applied to glass – ceramics fabrication.
Glass – ceramics are produced by a typical glass-manufacturing process of cooling of the melts and they have the fabrication advantage of glass and the special properties of ceramics [66]. They possess improved mechanical strength and have better thermal shock resistance due to having lower coefficient of thermal expansion [69]. Typically, they are produced in two steps: formation of glass by a glass – manufacturing process and reheating of the formed glass in a heat-treatment process.
The most important factor to consider during its fabrication is the cooling rate of the melt formed, which can be varied either by the amount of the undercooling or the cooling time allowed for the melt to solidify. Traditionally, this is done by using a high heat conductive material which draws heat continuously from the melt thus solidifying the melt in the process. This production method however has a limitation in that the cooling profile within the melt is uncontrollable and brings about largely differing microstructures.
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Water – cooling of glass – ceramic is another method used to obtain samples to have a more uniform microstructure within the sample, because of the complete immersion of the melt in water at a temperature lower than that of the traditional mold used. Here, the samples are not able to maintain a continuous solid shape but shatter under the inbuilt stress developed within them as they cool from very high to very low temperature. The sample recovered from this cooling method generally have lower crystallinity than that from the mold – cooled processes.
High speed roller-quenching equipment are also used for fabrication of glass and glass-ceramic [70]. With this system, the highest possible cooling rate is obtainable and this is useful particularly for melts that have little or no possibility of forming glasses using normal cooling methods. The samples from this method are usually drawn into thin sheets by the dual – rolling action of the roller-quencher.
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Chapter 3
3 Methodology
This section is broadly divided into two parts: part one lists the raw materials and the equipment used for glass-perovskite manufacture, processing, and characterization while part two describes the actual processing methods and the different ways the materials were characterized.
3.1 Equipment & Processing Materials
3.1.1 Chemical Raw Materials Used in Glass-Perovskite
Production
Table 3.1 contains a list of the raw materials that were used in making the glass – lithium lanthanum titanate composite. The molar ratio of the perovskite materials was chosen to obtain perovskite that would give the cubic phase if crystalline materials were to result. These perovskite-related chemical raw materials are listed as the first three materials in table 3.1. Items four to seven are the raw materials for the glass component, made of the glass network former, modifier and intermediate respectively. All the elements have no
36 less than 99% purity levels and were obtained from Alfa Aesar, Aldrich Chemicals and
Aero Organics, renowned chemical powder manufacturers in the United States.
Table 3.1 Raw materials used in the production of glass - perovskite
S/N Chemical Name Name Maker
1 Lithium carbonate Li2CO3 Alfa Aesar
2 Lanthanum oxide La2O3 Acros Organics
3 Titanium oxide TiO2 Alfa Aesar
4 Silicon oxide SiO2 Alfa Aesar
5 Aluminum oxide Al2O3 Alfa Aesar
6 Potassium oxide K2CO3 Aldrich Chemicals 7 Zinc oxide ZnO Alfa Aesar
3.1.2 Equipment for Processing & Finishing
In getting the raw materials into desired shapes and for subsequent analysis and spectroscopic probing, it is necessary that they are processed and finished with the right equipment to be able to make informed judgments on new observations. The equipment are classified into two categories: melting & forming equipment and sample processing equipment. Table 3.2 below shows this while a section in the appendix shows the pictures of some of the equipment and devices used.
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Table 3.2 Processing & finishing equipment for glass - perovskite manufacture
Melting & forming materials Sample processing materials
Zircar zirconia oven Fume hood
Lindberg/Blue M Mini-Mite Tube Furnace Emery cloth, different grits
Cold water in a trough Sonicator
Graphite molds Acetone
Hot tongs Methanol & Propanol
Face mask Carbon tapes
Gloves Stubs
Roller-quencher Silver Paste
Platinum crucible Connecting wires
Lubricating oil Sputter Coater
Ceramic crucible Stainless steel discs
Hydrofluoric acid, Nitric acid
3.1.3 Characterization equipment
Upon fabrication of the glass-perovskite and subsequent processing, the spectroscopic properties of the samples were analyzed and determined using robust characterization equipment like the Scanning Electron Microscope, X-Ray Diffractometer,
Scanning Transmission Electron Microscope, and Electrochemical Impedance
Spectroscopy. Table 3.3 below shows the various characterization equipment used alongside their make.
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Table 3.3 Characterization equipment used
Equipment Make
Electron Microscopes (1) FEI Quanta 3D FEG ESEM (2) Hitachi S – 4800 SEM
(3) Hitachi HD – 2300A TEM X-Ray Diffractometer Rigaku Ultima III with SAXS
Impedance Analyzer Gamry Potentiostat Reference 600
Differential Scanning SDT Q600
Calorimetry/TGA
Thermogravimetric Analyzer TGA Q50
3.2 Glass - Perovskite Production
The following section discusses the roles of the materials listed above in the whole glass - perovskite electrolyte fabrication & analysis process. The second part of this section explained the design process and fabrication of the roller-quencher, an alternative equipment used for the fast cooling of glass-perovskite melts.
3.2.1 Mold - cooling and Water – cooling Processes
Glass-perovskite composite was fabricated by the traditional melt cooling process used to produce glasses. A 10g equivalent of the combined materials was prepared in different mole ratio composition of glass and perovskite. Separate compositions containing
50%, 55%, 60%, 65%, 70% & 80% mole of perovskite were weighed, ground and mixed in a ceramic crucible to initiate closer contact of the raw materials. The process was
39 followed by transfer into a platinum crucible where the raw materials would be heated in the zircar zirconia oven at temperatures of 1450°C.
The graphite mold was heated separately in the Lindberg/Blue M Mini-Mite Tube
Furnace to 200ºC. The mold has different circular cavity sizes into which the melt was poured, and this was followed by annealing at 550ºC for 3 hours in an alumina crucible to remove the inbuilt stress. The annealed samples were recovered, weighed and polished using various sandpaper of different grits as applicable until a 1mm thick sample was obtained, to be used for characterization.
In a separate fabrication process, the melt from the oven was cooled using tap water in a stainless-steel trough at room temperature, and again, the crucible containing the bottom layer was transferred into a separate water trough.
3.2.2 Roller – Quencher Fabrication and Use in Melt Cooling
The use of twin rollers rotating in opposite directions as a method of production of glass had been touted as being able to offer the fastest cooling rates of glass melts possible.
To this end, a roller – quencher equipment was designed and fabricated from the combination of a rolling mill bought via amazon, electric motor, rubber & pulley, polycarbonate plates and mounted on a metallic industrial cart. Figure 3-1 shows the image of this roller-quencher fabricated at the University of Toledo.
In cooling the melt, the roller-quencher was switched to the “ON” position and made to attain its highest rotation speed of 3000rpm with time. When this was achieved, the melt at 1450°C in the platinum crucible was charged through the spacing between the
40 two rollers where a stream of a mixture of water and lubricating oil was running through.
This cooling method for the glass perovskites yielded materials with different physical attributes from the mold cooled and the water-cooled samples. Colorless, ultra-thin sheets of brittle materials were formed when the double rollers had spacing on a micro scale order while brittle materials quite like those obtained from the water-cooled process resulted when the spacing was a little bigger [71].
The samples were collected, rinsed clear of lubricating oil with acetone, dried at
80°C and sonicated with isopropanol to rid the samples of unwanted particles. Analysis of the samples with characterization equipment like the scanning electron microscope, X-Ray diffraction, and Gamry impedance analyzer was next performed, as described in the following sections.
Figure 3-2 Roller – quencher (right) made from rolling mill (left) used for high speed cooling of glass - perovskite
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3.3 Glass-Perovskite Characterization
The different materials obtained from the cooling methods used were analyzed to know more about the chemical identity of each of the samples fabricated.
3.3.1 Differential Thermal Analysis
The DTA analysis of some samples were done using the SDT Q600 from TA instruments which uses an empty alumina crucible as the reference material. A 10-degree step increase in temperature was used up to a temperature of 1200°C. Before this analysis, the sample was grounded into fine powder in a ceramic crucible. The equipment also measured the thermogravimetric response of the glass perovskite sample simultaneously.
The thermogravimetric analysis of the sintered perovskite was also performed using the TGA Q50 to determine its weight loss upon thermal treatment and a second TGA analysis on the same sample to confirm the mass loss. A 10-degree step increase from room temperature up to 950°C was used in this operation.
3.3.2 Electron Microscopy Characterization
The samples were collected, polished flat and placed on the carbon tape which is on a metallic sample stub. The sample was then coated with a Cressington 108 Auto sputter coater using a gold-palladium plate for thirty seconds, because of the need to make the semi conductive sample detectable by the microscope. A pathway for electron flow was created using a thin strip of carbon tape to avoid electron charge buildup. The FEI Quanta
3D FEG was used afterwards to determine the morphology of the different compositions of glass-perovskite fabricated using backscattered detector at 30kV and a working distance
42 of 10mm. The morphologies were taken at a lower and a higher magnification at different regions of each material. Energy Dispersive X-Ray Spectroscopy (EDX) was also performed with the Quanta 3D FEG to confirm the percentage of the different elements present in the samples.
In the case of the amorphous 60% perovskite, mapping analysis and Selected Area
Diffraction Pattern (SADP) was done using the Transmission Electron Microscope (TEM) to have a better understanding of the various zones in the sample morphology and to prove the amorphous nature of the sample. Cross section morphology of a sample was performed to have an idea of the cooling process prevalent in each layer of material.
3.3.3 X-Ray Diffraction Analysis
X-Ray diffraction analysis was performed with the Rigaku Ultima III X-Ray diffractometer using Cu Kα radiation (λ = 1.5406Å). The samples from each composition were prepared for diffraction by grinding into fine powder, placed on an appropriate glass holder and fitted into the diffractometer to obtain the diffraction peaks at scanning parameters: two seconds count time and 0.02 step width. The diffractometer was operated using a focusing beam method with monochromator installed and nickel β-filter absent.
3.3.4 Electrochemical Impedance Spectroscopy
The impedance analysis employed for solid samples is quite different from the methods used for corrosion studies using a liquid solution which is a 3 – electrode mode arrangement for probing the impedance characteristics in samples. Because the samples in this research contained lithium, round, stainless steel ion-blocking electrodes were used in
43 place of traditional graphite rod as the counter and silver/silver chloride reference electrodes which is applied for tests using liquid electrolytes, because it has the possibility of dissolving the lithium in the samples.
Impedance analysis of the annealed bulk samples cooled in the mold, the water – cooled sample heated at 1100℃ and the sheets from the roller-quencher was carried out.
The annealed sample was filed into shape to get rid of unwanted parts using sanding papers of various grits, typically from 80 to 600. This was followed by wet cloth surface finishing using a 0.05-micron-sized alumina powder solution. Afterward, sonication of the sample in isopropanol was performed for ten minutes and the samples were dried with acetone.
The surfaces were then coated with gold-palladium for thirty seconds.
The stainless steel plates (1cm diameter) were rid of rust using a 1000 grit sand paper after which they were cleaned with acetone and dried. These were used with double- sided carbon tape to initiate contact between metal and sample material. Ionic conduction property was then investigated using an AC impedance method on a Gamry Instrument
Reference 600 Potentiostat / Galvanostat / ZRA by connecting the working and working sense leads to a wire extending outward from one plate. The counter, reference and the counter – sense electrodes were also connected to the second blocking plates. Finally, the floating ground was connected to the faraday cage which housed the cell setup in order to prevent noises from external effects.
The impedance measurements were carried out in a frequency [72] range of 1mHz
- 106Hz with a stabilization time of 100 seconds. The temperature – dependent conductivity
44 was measured between 19º – 60ºC. Table 3.4 shows the parameters used in the impedance analysis.
Table 3.4 Impedance analysis for glass perovskites
Parameter Value
Initial frequency 1000000Hz
Final frequency 0.01Hz
A/C voltage 10rms
DC voltage 0Volts vs open circuit
Sample area 0.254cm2 (for the 60% composite)
Sample thickness 0.055cm (for the 60% composite)
Stabilization time 60 seconds
a b
Figure 3-3 (a): The different electrode leads for the Gamry impedance analyzer, (b): a physical impedance cell setup
45
3.4 Crystalline LLTO Fabrication
To compare the properties of the fabricated glass-perovskite, cubic crystalline lithium lanthanum titanate perovskite Li0.5La0.5TiO3 was synthesized by solid-state sintering. The starting materials were Li2CO3, (>99%, Alfa Aesar), La2O3 (99%, Aeros
Organics) and TiO2 (99.9%, Alfa Aesar) with the following reaction equation:
0.25 Li2CO3 + 0.25 La2O3 + TiO2 Li0.5La0.5TiO3 + 0.25CO2
Table 3.5 shows the masses and the molar percentage of starting materials used for the sintering procedure. A 10g equivalent of the combined raw materials was ground in a ceramic crucible to bring about closer contact of the starting materials after that 100ml of water was added for further mixing using zirconia balls in a ball mill. The resulting mixture was oven dried to drive out water and then pressed into pellets with a pressure of 7 tonnes with the S. Carver hydraulic press; this was done to ensure the grains are well-packed together with spaces as small as possible.
The pellet was subsequently transferred to a platinum foil and transferred into the zircar zirconia oven where a series of controlled heating took place. There was first a 10- degree step in temperature per minute up until a temperature of 500°C was achieved. This was done to drive out the carbonates from the total mass on the platinum foil. The temperature was then maintained for many hours at 500°C before again stepping it up to
750°C for a total of 3 hours. Finally, the pellet was heated to 1200°C to make the grains stick tightly together and form a hard, single piece. Furnace cooling of the pellet ensued after that it was retrieved for subsequent characterization. The total sintering time for the perovskite was twenty-two hours.
46
Table 3.5 Raw material composition for perovskites
Raw material Mole percent Per 10 grams Li2CO3 16.7 1.03 La2O3 16.7 4.53 TiO2 66.7 4.44 Total = 100 % Total = 10 grams
3.5 Glass-Perovskite Doping
Because of the high tolerance of perovskites to different cations and anions, and because of the number of property variation possible with this, several cations and anions of varying sizes were doped on the parent glass - lithium lanthanum titanate perovskite.
The A, B, and X sites doping of the LLTO was studied in this research using similar preparation techniques as my undoped samples. For the A – site doping, Strontium carbonate and cerium oxide were added as cation donors in 0.25 molar concentration replacement of the lanthanum oxide concentration. Potassium fluoride was also selected as the anion donor in a 0.05 molar addition. This was done in the hopes to obtain materials with improved desired properties.
With this arrangement, cations cerium and strontium, as well as the fluoride anion, were chosen as the dopants in this study, with the molar concentration of the dopants chosen from literature studies [73-75] on the doping of lithium lanthanum titanate perovskite. The molar concentration of strontium carbonate, cerium oxide and potassium fluoride used in this study are outlined in table 3.6 below:
47
Table 3.6 Concentration of the dopants used in 60%-40% perovskite glass
Dopant Mole percent (%) Proposed perovskite chemical Reference
formula
SrCO3 5.72 Li0.5La0.35Sr0.15TiO3 [73]
KF 0.05 Li0.5La0.5TiO2.95F0.05 [74]
CeO2 5.72 Li0.5La0.35Ce0.15TiO3 [75]
3.6 Heat treatment of Samples
Electrochemical impedance spectroscopy requires solid samples to be hard, incompressible and able to withstand the large stress offered by metallic blocking electrodes. Usually, a heat-treatment step is required in order to achieve this status for some samples. This procedure is typically performed using Lindberg/Blue M Mini-Mite Tube
Furnace at an appropriate temperature which is sample dependent.
48
Chapter 4
4 Results & Discussion
4.1 Physical Examination of Samples
In the series of glass – perovskite experiments undertaken, a constant melting temperature of 1450°C was used with all compositions showing increasing resistance to pour from 50 mole% perovskite – 50 mole% glass to 80 mole% perovskite – 20 mole% glass. The physical attributes of the formed glass-perovskite composite depended on many factors which takes effect right from when the melt is taken out of the furnace to the first few seconds afterward.
The annealed samples were observed to have some color difference, ranging from white to yellow, with the white section being the surface cooled on the graphite mold
(fig.4.1 a) , and having a thickness between 1.5mm to 3mm which was non-uniform across the whole sample. The upper section had little or no contact with the graphite mold, and thus heat transfer was slowest, making it yellowish. For the pelletized and sintered perovskite, the physical features of this sample before and after sintering is shown in figure
4-1b. a
Figure 4-1a Physical attributes of glass-perovskite cooled in mold
49
b
Figure 4-1b Before and after sintering of perovskite sample
Some of the samples retained their compact shape without breaking off, a close examination of these samples revealed the nature of cooling in the mold: the weight of the glob poured from the melt was well supported by the surface of the mold cavity. Thus, the spherical glob tend to flatten out at the top in other to accommodate the upward force offered by the graphite surface. It was observed that the thickness of the base section of the formed mass reduced with increasing perovskite composition, giving a firsthand guess that the top layer of the annealed sample is made of majorly the perovskite. The electron microscopy image of the cross – sectional area is shown (fig. 4-2) to reveal these regions.
The presence of dendrites is noticed at the left section of the electron image.
50
Top Bottom
Figure 4-2 Cross-sectional electron image for mold-cooled samples During melting there is a decrease in the total mass of the raw material going into the furnace as evident from the differences in the masses input and output for each composition fabricated. This can be attributed in part to the carbon dioxide escape from the whole batch of material and as well as the loss of lithium and potassium in the form of the oxides (Li2O and K2O). The table 4.1 below shows the melting and boiling points of the products of the melting reaction for glass-perovskite production where lithium and potassium oxides show the highest tendency of escape at higher temperatures.
51
Table 4.1 Melting and boiling points of the raw materials used for glass-perovskite production
Sample Melting point (°C) Boiling point (°C)
Li2CO3 723 1310
Li2O 1570 --
La2O3 2315 4200
TiO2 1843 2972
SiO2 1710 2230
Al2O3 2045 2980
K2CO3 891 --
K2O 350 --
ZnO 1975 --
The mass loss in the glass – perovskite after melting at higher temperature is given in table 4.2 below:
Table 4-2 Mass loss data in the melted batch for 60% perovskite-40% glass
Parameter Value Melting temperature 1450ºC Melting time 1 hour Anneal temperature 550°C Anneal time 3 hours Empty crucible weight 13.45 grams Crucible weight with sample 23.45 grams Crucible weight after melting 22.25 grams Mass loss 1.2 grams
52
4.2 Characterization Results 4.2.1 DSC & TGA Response of Samples
The Differential Scanning Calorimetry (DSC) of the 60% perovskite – 40% glass was done to understand its thermal response in terms of mass and enthalpy. This sample was chosen based on its amorphous response from the XRD. The sample was found to be very stable to heat up until a temperature of about 1200°C, the maximum DSC temperature used. Figure 4.3a shows the joint DSC and TGA responses of the glass-perovskite sample.
Between room temperature and this limit, only a 0.6% drop in its weight was observed.
The glass transition temperature (first point on the “blue” DSC curve of figure 4.3a) of the sample was found to be approximately at 707°C while there was an onset of phase separation via crystallization at about 788°C (second point), marked by the heat increase due to exothermic reaction. a c
b d g
Figure 4-3a DSC/TGA data for amorphous 60% perovskite - 40% glass 53
2nd
1st
Figure 4-3b TGA for ordinary perovskite 1st run vs 2nd run
This is the point where lithium lanthanum titanate crystals begin to grow. There was more growth of crystals at 844°C, where the heat released increases. The temperature at 1057°C (point d) marks the melting temperature for the sample, showing a dip along the curve which signifies the intake of heat to break the crystal bonds. On the TGA curve for the glass-perovskite, the most noticeable dip in the weight of the sample came at 1200°C
(final point), signifying a drop of 0.2% after crystal melting.
The TGA result for the ordinary perovskite (fig.4-3b) indicated the loss of some components beginning at about 250°C and a constant mass afterwards. This mass loss was from CO2 escape during the thermal treatment. On the second run of the same sample, there
54 is no mass loss whatsoever as the material was roughly at 100%. When the TGA in figure
4-3a is compared to that of the first run in figure 4-3b, it is seen that the mass loss is reduced in the glass perovskite sample by approximately 0.4%, proving that the glass incorporated into the perovskite brings about even more thermal stability.
4.2.1 Glass – Perovskite EDX and Ordinary Perovskite SEM
The different glass-perovskite samples were analyzed and found to have microstructures varying from spheres and matrix to dendrites and ultimately to a combination of both. The EDX spectrum in figure 4-4 confirms the presence of the raw
Figure 4-4 EDX showing the elements present in glass-perovskite material elements used in the fabrication of the glass – perovskite for all the compositions produced. All of the elements were verified, other than lithium, which was undetectable because of the detection limit of the equipment.
55
For the 100% perovskite prepared by solid – state sintering, the backscattered SEM image showed the grains and the grain boundaries of the lithium lanthanum titanate crystals. These had sizes ranging between 1 – 3 microns which were mostly cubic. The
SEM image is shown in figue 4-5.
Figure 4-5 SEM image for sintered perovskite
4.2.1.1 50% Perovskite – 50% Glass composition
The 50 – 50 LLTO samples showed a morphology that had broad demixing boundaries made up of a matrix of the white phase with the dispersed nuclei of the dark a b
Figure 4-6 SEM image of 50% perovskite – 50% perovskite showing spinodal decomposition
56 phase on one side and a matrix of the dark phase with the dispersed nuclei of the white phase on the other side of the curved boundary. Figure 4-6a and b shows the scanning electron image of the glass-perovskites having equal molar compositions of glass and perovskite. The images collected at low magnification (500x) can be seen to be a random collection of interconnecting matrices in which the continuous phases are formed from both the dark and the white regions [50].
Because these two distinct layers co-occur and are interconnected as described, this mechanism proceeds by way of spinodal decomposition which takes place in many glass systems. At higher magnification (figure 4-6b), the minor phase suspended in the matrices are droplets in both cases, showing that the two components are liquids at the melting temperature and the morphology remained fixed at those conditions upon cooling. The sizes of the dark “glass” spheres ranged from 500nm to 6μm in the LLTO matrix while the white “LLTO” spheres have a uniform size of about 200nm.
57
4.2.1.2 55% LLTO – 45% Glass Composition
The SEM morphology of this composition (fig. 4-7a and b) showed nucleus growth by spinodal decomposition but with shorter boundary than the 50% glass-perovskite composition analyzed above. The grey (LLTO) phase appeared as the matrix in certain areas and the dark phase as the matrix in other areas, but with slightly lesser total area. The nucleus however is seen to be slightly bigger than those of the 50% compositions in that their sizes ranged from 1 – 3 microns at a 5000x magnification.
a b
Figure 4-7 SEM image of 55% perovskite – 45% glass showing a growing spinodal decomposition
58
4.2.1.3 60% LLTO – 40% Glass Composition
This morphology showed a nucleation and growth mechanism for the glass phase in a matrix of the amorphous LLTO. As seen from the backscattered SEM images, the glass spheres are fairly uniform and well-dispersed across the white LLTO matrix (fig. 4-8a).
The size of the glass spheres in this morphology were observed to fall within the range of
0.325 to 0.401μm2 (based on a working distance of 9.6mm and a magnification of 5000).
a b
Figure 4-8 (a): SEM image showing nucleation & growth morphology of 60% perovskite and (b): Selected Area Diffraction Pattern using TEM
In most of the different regions examined, the matrix was seen to be amorphous and contained no visible dendrites and this was proved by the selected area diffraction pattern (SADP) on the sample using the transmission electron microscope, which gave a ring structure signifying the amorphous nature (figure 4-8b). Also, mapping analysis was done to have an idea of the position of the various elements present in the morphology (fig.
4-9). It was seen here that the glass – related elements were found in the spheres in the transmission electron image while the perovskite – related elements were found in the matrix.
59
TEM image O K
Si A Zn l
La Ti
Figure 4-9 TEM- EDS mapping of nucleation & growth morphology for 60% perovskite - 40% glass
60
4.2.1.4 65% LLTO – 35% Glass Composition
In this composition, the morphology (figure 4-10 a - d) was observed to contain a different makeup than the first three discussed above. Here, there was no presence of spherical droplets or nucleus of the glass phase but there was the formation of white, swordlike dendrites which were composed primarily of the LLTO and a dark matrix within which the dendrites were arranged in a regular manner. The length of these dendrites was between the 100μm and 130μm range and obviously had boundaries between them. The dendrites appeared to exist in layers and completely separated from the dark glass matrix existing between them, as is pronounced in the magnified images in c and d.
a b
c d
Figure 4-10 SEM images of layered dendrites - in- glass morphology for 65% perovskite - 35% glass at different magnifications (a. 250x, b. 500x, c. 2000x and d. 5000x)
61
4.2.1.5 70% LLTO – 30% Glass Composition
For the 70% LLTO morphology, the white matrix was seen to be non-amorphous with a growing dendrite phase that was non-uniform across the sample. In some regions, the dendrites were observed to be fully formed, having a long backbone with branches protruding in all directions (figure 4-11c). In other regions, the dendritic backbone pattern could be seen as growing out of the matrix-spheres morphology (fig. 4-11 b). The spheres in this morphology had sizes bigger than those from the 60% LLTO morphology. They were found to be between 0.238 to 0.273μm2 (based on working distance of 14.3mm and magnification of 5000).
a b
c
Figure 4-11 SEM image for (a and b) 70% perovskite - 30% glass showing the pattern for the formation of dendrites and (c) the dendrites growth along with their branches
62
4.2.1.6 80% LLTO – 20% Glass Composition
The morphology of this sample (fig. 4-12 a - c) showed a retarded growth of the dark spheres in the matrix of the white LLTO phase. The sizes of the spheres are pretty uniform and in the 500nm range (at a working distance of 12.8mm and a magnification of
5000), thus existing literally as tiny particles. The dendrites here are thinner and more closely packed together. The sample did not melt after heating at 1450℃, reason being primarily because of the higher amount of the LLTO perovskite present which supersaturated the glass melt. A portion of the sample was however retrieved from the crucible and was analyzed.
a b c
Figure 4-12 SEM image of 80% perovskite - 20% glass showing dendritic LLTO matrix and much reduced glass sphere areas at different magnifications (a. 5000x, b. 8000x and c. 20000x)
4.2.2 Phase Diagram for Glass-Lithium Lanthanum Titanate
The various samples produced showed very different morphologies which reflect the different regions in their phase diagram. As the melts are composed of silica glass former, alumina and rare-earth metals, it is expected that they have a liquidus temperature greater than their eutectic temperature [66]. Figure 4-13 shows a proposed phase diagram
63 based on a binary system for the compositions analyzed, where A represents the perovskite phase whose composition decreases from left to right and B the glass phase, whose composition increases.
For the 50% glass – 50% perovskite composition, the cooling line is well in the spinodal decomposition range (the curve under the binodal curve) and this explains why the morphology was made up of two different matrices separated by the boundary (fig.
4.6b). The variation in the sizes of the dark spheres is a result of diffusion and coalescence of the droplets within the white matrix shortly before the solidus line is reached. The growth of the nucleus is seen to proceed in no definitive order, giving a rather spatial ordering of the spheres. Here, both the predominantly perovskite phase and the predominantly glass phase constitute the matrix morphology.
As the LLTO composition increases to 55%, there is now an uneven proportion in the areas of both matrices (fig.4-7). The boundary between the matrices become shorter and even though it is still largely a case of spinodal decomposition, the interconnection between the phases is less obvious.
Composite containing 60% by mole of LLTO are able to show amorphous LLTO matrices (fig. 4-8) and finely dispersed glass spheres with no interconnection. This is typical of the metastable region (between the spinodal and the binodal regions) found in phase diagram of glass systems shown by Shelby (fig. 2-8). Being in this region, the glass spheres decomposed from the LLTO matrix when it hits the immiscibility curve and does not touch the unstable spinodal region before it gets to the solidus line. Giving an amorphous LLTO matrix (fig. 4-8) means the sample possessed no crystals and this was
64 proved from the X-Ray diffraction pattern obtained from it. As the melt cooled down further, the glass spheres became locked into the crystals and hardened.
The 65% LLTO composite had dendrite layers existing separately with the glass
(fig. 4-10). At this region of the phase diagram, the melt cooled from liquid conditions but
Figure 4-13 Phase diagram for glass - perovskite for all compositions fabricated
encountered no immiscibility curve. Thus, the crystals contained in the melt grew as the temperature dropped further. This invariant condition exists at the eutectic temperature of
1056°C and below this limit, the melt solidifies.
65
Composites 70% LLTO (fig. 4-11) contained more perovskite composition than the previous composites fabricated. A closer look at the morphology present in the compositions confirms that as the melt was cooled, it encountered a liquidus line beyond which the glass separated out of the melt as spheres. Shortly before getting to the solidus line, the perovskite separated out as dendrites. For the 80% LLTO melt however, it was impossible to melt the raw material batch at 1450℃ and thus there was no flow of the sample from the crucible. However, it was observed that the morphology had a dendritic matrix.
4.3 X-Ray Diffraction
Other than the diffraction pattern of the 60% perovskite – 40% glass sample, all other samples from 50%-50% glass – perovskite to 80%-20% glass perovskite samples were found to have diffraction peaks similar to those of crystalline lithium lanthanum
a
66
b
Figure 4-14 (a) XRD profiles for 60% LLTO and (b) other glass - perovskites titanate perovskite, in both cubic and tetragonal phases. There was also a secondary
LiTi2O4 phase crystallized from the melt. The 60% perovskite sample showed an amorphous spectrum typical of glass XRD and quantitatively about 98% of the whole material. (fig. 4-14 a). Figure 4-14b gives the XRD profile for the crystalline phases in all the compositions except 60%-40% while figure 4- 15 shows the position of the phases, using the 80% composite as basis.
Figure 4-15 Crystalline phases present in glass - perovskites
67
The XRD profile of the perovskite sample is shown in fig. 4-16 below, confirming the presence of the cubic phases.
Figure 4-16 XRD profile of perovskite showing cubic crystallinity
68
4.4 Impedance Analysis
The impedance analysis of the 60% LLTO – 40% Glass composite was considered
because of the amorphous nature of its LLTO matrix. Being amorphous, the lithium
lanthanum titanate showed a single impedance response in all the frequency range tested.
This result differed from the impedance response offered by crystalline perovskite which
had both the bulk and grain boundary responses, the grain boundary being the principal
limitation to its commercialization. Compared to literature values [76] and figure 4-17d,
the conductivity of the crystalline lithium lanthanum titanate sample fabricated for the
purpose of this research showed a valid correspondence. The conductivity results (fig.
a b
c d
Figure 4-17 (a & b) Impedance spectrum (Nyquist plot) for perovskite, (c) glass – perovskite, and (d) perovskite impedance from literature
69
4.17a) obtained showed an increase of one order than the crystalline perovskite. Because the active lithium component of the glass-perovskite composite available for ionic transport is only 60% of the total available in the crystalline perovskite, this result is a major upgrade and with a little tweak and optimization of the production process used in this research, an even better result is possible. The bulk conductivity of the ordinary perovskite sample was unnoticeable at all the frequency range tested, however, a magnification of the higher frequency region (4-17b) reveals this.
Compared to the impedance response for the perovskite, there was no presence of the polarization effects of ion blocking noticeable. This was due to the fact that the sample was not thick enough to reveal the blocking effect, rather there was a cluster of impedance points at the end of the curve. The equivalent circuit used to obtain the different values from the nyquist curve is shown in the appendix while figure 4-17d shows the impedance response of crystalline lithium lanthanum titanate obtained from the works of Inaguma, et al [76] where the bulk contribution is shown separately in the inset for better visibility.
Table 4.4 shows the parameters used in the impedance analysis.
70
Room temp. 40 degrees 60 degrees
Figure 4-18 Temperature dependence of conductivity
The conductivity results were also tested at higher temperatures (40°C and 60°C) and was found to vary linearly with temperature. Figure 4-18 shows the Nyquist plots at these temperatures.
71
Chapter 5
5 Effects of Varying Cooling Rates on Nucleation & Growth Morphology
The glass-perovskites fabricated by the use of graphite mold were observed to be non-uniform across the whole samples; the 60% mole perovskite – 40% mole glass composition with no grain-boundaries or dendrites in its morphology had different physical features all over. White coloration of the solid formed was most noticeable at the region where the melt glob contacts the cooling surface of the graphite. A much larger portion of the sample had either different morphologies than the amorphous nucleation & growth or a combination with dendrites of different lengths. These were found in parts farther away from the mold surface and were the results of much reduced cooling rates.
Because of the relationship between cooling rates, temperature difference and time of cooling, expressed as follows:
푡푒푚푝푒푟푎푡푢푟푒 푑𝑖푓푓푒푟푒푛푐푒 푅푎푡푒 표푓 푐표표푙푖푛푔 = ….5 푐표표푙𝑖푛푔 푡𝑖푚푒
72
Other processing methods that could give faster cooling rates for glass-perovskites were analyzed. Cooling with water at room temperature was considered because it helps to bring the temperature of the cooling sample to final values lower than that obtained from the mold cooling (200°C).
Also considered, was an oppositely rotating dual roller-quencher equipment, which was designed by me and constructed in the university of Toledo. The process was based on the report from Feller, et al where they obtained cooling rates of up to 106 °C/seconds. This was achievable because the process focused on reducing the cooling time to several orders lower than that for the mold-cooling process [70].
5.1 Water-cooling of Glass-perovskite melts
Rather than the usual bulk sample obtained from graphite mold processing of glass- perovskite melts, the samples gotten from water-cooling appeared as broken pieces of the composite. The reason for this was because of the residual stress in the melt as it cooled from very high temperature to very low temperature, making it difficult to maintain a continuous, cohesive bulk sample. As the melts release all thermal residual stress upon contact with water, broken pieces result.
The samples collect at the base of the trough after which the water is decanted and the samples dried. Upon physical analysis, the samples were observed to have more white coloration than yellow. The SEM morphology of the water-cooled sample was done with phase separation via nucleation & growth well pronounced. The glass spheres in this case, were seen to be smaller in size than those obtained in the mold-cooled sample. This is
73 because the rate of nucleation was higher than the nucleus growth rate as the mass is constant, caused by temperature dropping lower than the 200℃ for the mold used for the initial sample fabrication route, hence stiffling appreciable growth of the glass spheres
Because of the brittle and particulate nature of the samples, it was impossible to get them in the required shape and size for impedance analysis using blocking electrodes. The particles were thus collected, grounded, pelletized and heated at 1100°C (the temperature of melt from the DSC result) in the hopes of bringing about solidification of the pellet.
Figure 5-1 shows the process used for pelletizing the water – cooled sample, the final product obtained after heating the sample at 1100℃, the SEM morphologies of the samples after water-cooling and after heating.
74
5.2 Cooling Using a Roller-Quencher
The roller-quencher designed and constructed in the university of Toledo was oiled with multipurpose, high-performance EP gear lubricant oil SAE 80W – 90 before use.
Water-cooled pellet Pelletizer heated at 1100ºC
a b
c d
Initial SEM image of SEM image of water-cooled water-cooled pellet pellet heated at 1100ºC
Figure 5-1 (a & b) Processing of water-cooled sample into pellets; (c) the SEM morphology after water-cooling, (d) SEM morphology after heating at 1100℃
75
The 60% perovskite - 40% glass melt was then retrieved from the furnace and poured on the roller-quencher as it was turned on, with the spacing between both rollers adjusted to approximately 100 microns.
Ribbon-like pieces of the sample (fig. 5-2) was obtained from using high – speed roller- quencher for cooling of glass- perovskites. Because the melt poured had spherical shape like a glob, the first point of contact with the dual rollers is the external surface of the spherical drop. Immediately after this contact, a small percentage of the melt is able to find its way through the opening between the dual rollers, pressed tightly together into sheets because of the counter rotation of the rollers. Simultaneously, the remaining part of the glob is retained during this rolling operation and solidifies as a deformed spherical mass sitting atop the dual rollers. Generally, the ultra – thin ribbons are transparent, lacking any particular coloration.
Figure 5-2 Physical features of the roller – quenched sample For the SEM morphology, it was observed that the lithium active region (grey area in the morphology) was much bigger than that obtained for the mold-cooled and water-cooled samples, with the glass spheres showing a high level of connectivity due to interconnection.
76
Reason for this was because the growth of the spheres was hindered by the speed of the dual rollers and nucleation was only partial. Figure 5-3 shows the different SEM morphologies of the samples obtained with different cooling mechanisms.
a b c
Figure 5-3 SEM morphologies at 5000x for (a) mold - cooled, (b) water - cooled and (c) roller - quenched samples.
5.3 Impedance Response for Samples Obtained From Various Cooling Methods
The impedance analysis of the transparent, ultra-thin sample obtained from the roller – quencher was undertaken using stainless steel blocking electrodes as in the previous samples. Because the thickness of this sample was on the order of hundreds of microns, they were susceptible to tear if the applied force was too great for them to withstand. The approach in preparing this sample for impedance was quite different from that of the mold
– cooled sample. Because the thin, transparent sample would be irrecoverable after the test, they were carefully placed on the carbon tape – covered blocking electrodes so that there was no uncovered portion of the carbon tape left. This, in some cases, required the placement of one thin sheet over another. If this was not done and there was contact
77 between the carbon tapes from both blocking electrodes, a massive error in the impedance spectrum would result.
The impedance spectrum obtained from these ribbons also showed a single RC semicircle (fig. 5-4) having no grain boundary contribution, just like previously obtained for the mold – cooled sample having a nucleation and growth morphology. In this case however, there was a four times increase in the conductivity value than in the sample from the mold – cooled process.
The conductivity measured for the roller-quencher sample was gotten as 5.1 x 10-
5 S/cm.
a b
Figure 5-4 Impedance response for (a) Roller – quenched sample and (b) mold – cooled vs roller – quenched sample impedance
78
The impedance response comparism from all three cooling methods are shown in part in figure 5-5 below, the bigger of the two arcs being for the mold – cooled sample.
The curved line is a part of the semicircle from the water – cooled process that was pelletized and heated at 1100°C.
Figure 5-5 Impedance from all three cooling techniques used
79
Table 5.1 Impedance data from various samples
S/N sample Area Thickness R1 R2 Rtotal Conductivity
(cm2) (cm) (Ω) (Ω) (Ω) (S/cm)
1 Perovskite 0.577 0.088 4341 318300 3.2 x 105 4.73 x 10-6
2 Glass-perovskite 0.254 0.055 49.66 16630 1.7 x 104 1.3 x 10-5
(mold-cooled)
3 Roller-quenched 1.131 0.088 54.99 1300 1354.99 5.6 x 10-5
samples
4 Glass-perovskite 0.347 0.1 1.2 x 10-5 2.2 x 10-7 2 x 107 1.3 x 10-8
(water – cooled
and 1100°C
sintered)
5 Glass-perovskite 0.254 0.055 0.048 16910 16910 1.2 x 10-5
(mold – cooled)
@ 40°C
6 Glass-perovskite 0.254 0.055 0.052 11960 11960 1.8 x 10-5
(mold – cooled)
@ 60°C
5.4 Water-cooled Glass-Perovskite vs Sintered Perovskite
The impedance response of the ordinary perovskite was further compared with that
of the heat-treated sample as shown in figure 5-6. The bulk response of the water – cooled
80 sample heated at 1100C was seen to be higher in value upon fitting than the perovskite, suggesting that heat-treatment of glass – perovskite changes the structure of the perovskite in such a way that the conductivity of the composite drops considerably.
Water-cooled sample heat-treated at 1100⁰C Crystalline Perovskite
Figure 5-6 Impedance response of water –cooled sample heated at 1100°C vs perovskite
81
Chapter 6
6 Glass – Perovskite Doping
The physical attributes, SEM, XRD and EIS results obtained from doping glass- perovskites revealed interesting properties about the LLTO – glass systems quite different from those of the undoped amorphous 60% LLTO sample discussed in the preceeding chapters.
6.1 Physical Attributes of Doped Glass-perovskites
The doped glass – perovskite samples made by mold – cooling of the melt showed physical attributes very similar to the undoped samples, apart from the cerium-doped sample. The strontium and fluoride – doped samples had both white and yellow coloration, while the cerium – doped sample was brownish, beause of the color of the cerium ion. The thickness of the glassy surface did not proceed in any definite pattern, only being a function of the contact time the melt made with the mold surface. There was not much of a difference between the physical attributes of the undoped samples and that of the doped glass – perovskites in terms of the shape, size and texture. Figure 6-1 below shows the physical attributes of the doped and undoped samples
82
Fluoride - doped Cerium - doped Strontium - doped
Undoped sample
Figure 6.1 Physical attributes of doped glass - perovskites
6.2 SEM Morphology
The cerium – doped sample was composed principally of dendrites which had no noticeable branches. These were thin, swordlike components with the dark glass phase in between them. The dendrites – in – glass matrix morphology was locked in as the melt cooled down to the solidus line, marking the end of any significant morphological change.
When compared with the undoped glass – perovskite, the cerium – doped sample is seen to change from nucleation & growth to layered morphology as was with the undoped 65%
LLTO sample of figure 4-10.
The morphology and EDS mapping for the cerium – doped glass perovskite are shown in figure 6-2 (a-c) at different magnifications. Lanthanum, cerium and titaium were found on the dendritic phase and zinc, potassium, aluminum and silicaon on the dark (glass) zones.
83
a b c
d O Al
d
Electron image
Si K Ti
Zn Ce La
Figure 6-2 SEM morphology with different magnifications (a. 250x, b. 1000x, c. 5000x and (d) EDS spectrum of cerium - doped glass - perovskite
The fluoride doped samples showed mainly a nucleation & growth mechanism, but with spheres smaller than obtained from the undoped 60% LLTO morphology of figure 4-
84
7. These SEM morphology results are presented at different magnifications in figure 6-3
(a-c) below:
a b c
Figure 6-3 SEM morphology of fluoride - doped glass – perovskite at (a) 1000x, (b) 2500x and (c) 5000x
In the strontium – doped sample, there were dendrites growth in all regions of the sample
similar to those obtained for the 70% LLTO undoped sample of figure 4-11c. The presence
of glass was not uniform in this sample as they appeared to be clustered at some areas and
scarce at other areas, as revealed at different magnifications in figure 6-4 (a-c).
Each of the cation-doped glass – perovskite showed well pronounced dendrites
formation for 60:40 composition of perovskite and glass respectively. For this system, the
position of the desired nucleation & growth morphology is different, observed to shifted to
a b c
Figure 6-4 Strontium - doped glass - perovskite morphology at (a) 1000x, (b) 2500x and (c) 5000x
85 regions of lower composition than the 60% used (fig.6-5). For the fluoride - doped sample however, the nucleation & growth was unchanged for the 60:40 glass – perovskite and this was because the cation sizes were unchanged in the combination. The fluoride ion only replaces some oxygen ions thereby strengthening the bond in the TiO2.95F0.05 as compared to that of the TiO3.
Figure 6-5 Proposed phase diagram for cation – site doped samples showing eutectic, dendritic and nucleation & growth regions
86
6.3 Impedance Analysis
The impedance of the cerium and fluoride – doped samples showed a single impedance region for the fluoride – doped sample and double arcs for the cerium-doped sample, also, an inclined line signifying the blocking effects of the stainless steel electrodes used was observed. The impedance response (fig. 6-6) showed a bigger arc for the fluoride doped sample than the cerium doped sample, as a result of the anionic doping of the former, which only strengthens the Li-O and La-O bonds, but does not increase the bottleneck size which results from cerium doping. The impedance response of the strontium – doped sample could however not be obtained because of a high number of noises generated during the analysis. Table 6-1 gives the impedance parameters obtained from the cerium and strontium – doped samples.
Fluoride- doped Cerium- doped
Figure 6-6 Impedance response from cerium and fluoride - doped samples
87
Table 6.1 The impedance parameters obtained from the cerium – doped and fluoride – doped samples
S/N sample Area Thickness R1 R2 Rtotal Conductivity
(cm2) (cm) (Ω) (Ω) (Ω) (S/cm)
1 Cerium doped 0.175 0.101 58640 2833830 2.9 x 10-6 2 x 10-7
2 Fluoride doped 0.298 0.115 0 4.8 x 10-6 4.8 x 10-6 8.1 x 10-8
88
Chapter 7
7 Conclusion, Recommendations & Future Work
7.1 Conclusion
Lithium lanthanum titanate, a perovskite known to have the highest potential of all solid electrolytes in the realization of an all solid state battery, has a chance of achieving this feat in the nearest future. Appropriate formulation of a glass-coupled composite with a fast enough cooling rate can reveal morphologies comprising of an amorphous lithium lanthanum titanate matrix in which the glass phase is dispersed as droplets. This particular morphology, unlike the crystalline perovskite has no grain boundary contribution in its impedance spectrum and thus limiting the overall resistance which, in translation, means a higher overall conductivity. It’s intriguing to find that with only 60% of the total percentage of the conductive lithium phase present, the bulk conductivity of the glass – perovskite mix
(1.3 x 10-5 S/cm) was one order better than the overall conductivity of the ordinary perovskite (4.6 x 10-6 S/cm).
A simplified phase diagram for the glass-perovskite based on a two-component assumption reveals a region in the immiscibility gap where the nucleation & growth
89 mechanism is present, and in totality giving an idea of how the glass-perovskite properties would change upon variation of the molar composition of the melt. From the diagram, it is seen that matrix & spheres morphology also existed for the 50% and 55% lithium lanthanum titanate, but in this case, the composition favored a spinodal decomposition mechanism as opposed to the nucleation & growth.
An eutectic composition of the composite exists at 65% composition of the perovskite, where the dendrites appeared as layers with the glass phase in between them.
This particular composition is significant, happening at invariant conditions of composition and temperature in that at this point, the melt cooled has dendrites made up of the 2 components as well as the liquid phase all in equilibrium before the melt cooled further to become all-solid.
At compositions 70%, the microstructure consists of smaller number of glass spheres in a matrix of the dendrites of the perovskite. Here, since crystallization is the obvious result with a little area signifying the presence of glass, the melt formed at some region farther away from the eutectic because no crystals of the predominantly glass phase was noticeable.
Using cooling procedures that increases the cooling rate of glass – perovskites either by increasing the amount of undercooling (ΔT) in the case of the water – cooling process or by reducing the cooling time in the case of the high speed roller – quencher process was seen to change the nucleation & growth microstructure by reducing the area of the glass phase and by increasing the area of the lithium – active phase respectively. The
90 increased area of the lithium-active matrix was seen to give increase the conductivity value for the 60% LLTO glass – perovskite by four times (5.6 x 10-5S/cm).
Doping of 40% glass – 60% perovskite in the A and X sites showed microstructures quite different from the undoped samples, with the inability to obtain nucleation & growth morphology in the microstructure. For the A – site doping, layered eutectic and dendritic phases were obtained with the doped 60% LLTO sample, meaning that the nucleation & growth would only be obtained at compositions lower than 60% LLTO.
Overall, this research showed two important results: that the combination of 60% mole of lithium lanthanum titanate with a 40% mole of aluminosilicate glass yields a composite without the grain boundary contributions common with the crystalline perovskite, and also, the phase prediction possible with different combinations of the glass and the perovskite. The significant advantage of this research route is the very low cost of production of the sample compared to other much more expensive production routes like pulsed layer deposition, basically requiring just a means of melting of the batch and subsequent melt-cooling.
7.2 Recommendations
The series of experiments conducted in this research was done by making intelligent use of the materials available to me. Since the fabrication of perovskite having specific desired properties comes at a great cost, this research focused primarily at low- cost ways of making the material and thus helping to realize its potential use as a solid electrolyte for lithium ion batteries.
91
During the use of the roller – quencher for glass – perovskite fabrication, there was a large dissimilarity in the resulting formed samples, some appeared like ultra-thin ribbons while the others as small, hard pieces that shattered upon application of mild force, making it difficult to carry out the impedance analysis on these category of samples. It is advised that for persons using this equipment as a fabrication route for glass perovskite, the spacing between the dual rollers, the method of melt pour and the viscosity of the glass melt is carefully selected to give samples that are neither super thin nor overly thick for the purpose of impedance analysis.
7.3 Future Work
The incorporation of 40 mole % of aluminosilicate glass to 60 mole % of lithium lanthanum titanate resulted in a composite having a one order increase in ionic conductivity. While this result is definitely an improvement on the ordinary perovskite, it should not go without saying that there is a possibility for more improvement in this value.
For the undoped samples, the differential scanning calorimetry (DSC) of each of the samples needs to be done for the purpose of comparism in order to better understand the varying thermal responses, if any, between the samples. This could not be done because of equipment availability as at when needed and moreso, the focus in this research was on the
60mole % perovskite sample because of its amorphous microstructure.
Compositions other than 60% perovskite – 40% glass should be fabricated using water and the roller-quencher to determine if it’s possible to get a morphology that would impact ionic conductivity positively or otherwise, an understanding of their behavior towards changing cooling rates.
92
In all of the various experiments performed, a fixed glass component ratio of
5:1:1:1 between the network former, modifier, intermediate and zinc oxide was employed.
If this composition is varied and/or if other network formers are used in place of these, it is possible to obtain other interesting results for each compositions.
For the doped glass-perovskite system, the molar combination that would produce the nucleation & growth morphology in the microstructure should be optimized and analyzed because of the elimination of grain boundary contributions. Moreso, the
DSC/TGA analysis of other samples need to be done in order to have a knowledge of the thermal behaviour compared to the undoped sample.
93
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Appendices
Appendix A
A Other Information for Heat-treated Samples
The SEM images and XRD profiles for the glass perovskite at temperatures 708,
789, 1056 and 1294°C were analyzed to understand the microstructure at these temperatures and the results are shown in figure A-1 (a - c) below.
Figure A-1 XRD profiles for heat-treated samples
100
Appendix B
B Melting and Material Processing Pictures
The figure below shows the pictorial representation of the materials that were used in the processing of the perovskite and glass – perovskites with their appropriate labels.
101
Fig B-1 Fig B-2
Raw materials Material mixing
Fig B-3 Fig B-4
Oven Graphite mold
Fig B-5 Fig B-6
Sanding paper and drill Platinum crucible after for polishing melt pour
102
Fig B-7 Fig B-8
Water for melt Sample from cooling water cooling
Fig B-9 Fig B-10
Oven for annealing Epoxy cast for annealed sample to ensure controlled polishing mold – cooled melts
103
Blocking electrodes Fig B-11 Glass-perovskite Fig B-12 used with after polishing connecting wires.
Impedance cell Fig B-13 The twin rollers in Fig B-14 comprising two the roller-quencher blocking electrodes equipment and solid sample at center
Figure B (1-14) Pictures of processing materials
104
Appendix C
C Other Secondary Information
Figure C-1 Atomic arrangement in crystalline and amorphous SiO2
Figure A-4: impedance response from all three cooling processes, enlargedFigure A-3: Atomic arrangement in crystalline and amorphous SiO2
Figure C-2 impedance response from all three cooling processes, enlargedFigure C-1 Atomic arrangement in crystalline and amorphous SiO2
Figure A-4: impedance response from all three cooling processes, enlargedFigure A-3: Atomic arrangement in crystalline and amorphous SiO2 Figure C-2 impedance response from all three cooling processes, enlarged
Figure C-2 impedance response from all three cooling processes, enlarged 105
Fig C-3a 60% glass perovskite @ 40°C CCdegrees
Fig C-3a 60% LLTO glass perovskite @ 40°C CCdegrees
60% glass perovskite @ 40°C CCdegrees
60% LLTO glass perovskite @ 40°C CCdegrees
106
Fig C-3b 60% LLTO glass perovskite @ 60°C
Fig C-3b 60% LLTO glass perovskite @ 60°C
107
Fig C-3c Fluoride – doped bulk impedance fitting
Fig C-3c Fluoride – doped bulk impedance fitting
108
Fig C-3d Cerium – doped bulk impedance fitting
Fig C-3d
109
Fig C-3e Cerium – doped grain boundary impedance fitting
Fig C-3e
110
Fig C-3f 60% perovskite – 40% glass roller-quenched sample
Fig C-3f 60% perovskite – 40% glass roller-quenched sample
Fig C-3g Perovskite bulk impedance
Fig C-3g Perovskite bulk impedance
111
Fig C-3h Perovskite grain boundary impedance
Fig C-5h Perovskite grain boundary impedance
Fig C-3i
Figure Model used for fittings C-3 (a- h): Nyquist Model used for curve fittings Figurefittings C-3 (a -h): Nyquist curve fittings for doped samples, mold-cooled samples,for roller-quenched samples, heat-treated sample and ordinary perovskitedoped sample, (i): model used for fitting nyquist curve semi-circles samples 112 , mold- cooled samples , roller-