HIGHLY ELECTROCHEMICAL STABLE QUATERNARY SOLID POLYMER
ELECTROLYTE FOR ALL-SOLID-STATE LITHIUM METAL BATTERIES
A Thesis
Presented to
The Graduate Faculty of the University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Yunfan Shao
March, 2018 HIGHLY ELECTROCHEMICAL STABLE QUATERNARY SOLID POLYMER
ELECTROLYTE FOR ALL-SOLID-STATE LITHIUM METAL BATTERIES
Yunfan Shao
Thesis
Approved: Accepted:
Advisor Dean of the College Dr. Yu Zhu Dr. Eric J. Amis
Faculty Reader Dean of the Graduate School Dr. Steven S.C. Chuang Dr. Chand K. Midha
Department Chair Date Dr. Coleen Pugh
i
ABSTRACT
Lithium metal batteries are regarded as promising electrochemical energy storage solutions due to their high energy densities. However, the safety issues including lithium dendrite formation impede their practical uses. In this thesis, solid polymer electrolytes based on polyethylene glycol diacrylate and additional acrylate comonomer plasticized by an organic plasticizer (succinonitrile) were studied. The lithium stripping/plating experiments indicated that the polymer electrolyte can suppress the dendrite formation under current density from 0.05 mA/cm2 to 0.5 mA/cm2. With a high room temperature conductivity of 7.8x10-4 S/cm and an wide electrochemical window of 0-5.0 V (vs. Li+/Li), the solid polymer electrolyte was used to fabricate all- solid-state lithium batteries with NCA cathode and operated at cut-off voltage of 4.3 V and current density of 0.5 C (1 C= 180 mA/g). The solid-state battery exhibited capacity retention of 47.6 % after 450 cycles with an average coulombic efficiency of
99.80 %, indicating the long-term stability of the polymer electrolyte.
ii
ACKNOWLEDGEMENT
Firstly, I would like to express my deep sense of gratitude to Dr. Yu Zhu to be my advisor and helping me with extraordinary guidance and constant encouragement on my study. My heartfelt thanks also go to Dr. Feng Zou, Kewei Liu, Wenfeng Liang and Si Li for their timely suggestions and assistance with kindness which helped me throughout my research. Additionally, I want to express my appreciation to my friends and family, for their supporting and encouragement. Finally, I am genuinely grateful to Dr. Steven Chuang for being so generous and spending his valuable time to be reader of this thesis.
iii
TABLE OF CONTENTS Page
ABSTRACT………………………………………………………………………….ii
CHAPTER
I INTRODUCTION ...... 1
1.1 Lithium Ion Battery ...... 1
1.2 Polymer based solid-state electrolyte ...... 2
1.3 Mechanism of ion transportation in SPEs ...... 4
II EXPERIMENTAL SECTION ...... 8
2.1 Materials preparation ...... 8
2.2 Fabrication of solid polymer electrolyte ...... 8
2.3 SPE thermal measurements ...... 9
2.4 SPE electrochemical measurements ...... 10
2.5 Lithium metal battery fabrication ...... 11
2.6 Electrochemical measurements for lithium metal battery ...... 12
III RESULTS AND DISSCUSSION ...... 13
3.1 Optimization of quaternary SPE recipe ...... 13
3.2 Thermal analysis of SPE ...... 15
3.3 Electrochemical measurements of SPE ...... 16
3.4 Lithium stripping/plating experiments ...... 21
iv 3.5 lithium metal batteries ...... 23
IV CONCLUSION...... 29
BIBLIOGRAPHY ...... 30
v LIST OF FIGURES
1. Chemical Structure of PEGDA...... 4
2. Lithium ion transportation a) amorphous region b) crystalline region.19 Copy right
2017, University of Akron ...... 6
3. Ionic conductivity at various momoer2 percentage in polymer part...... 14
4. LSV of the SPEs with various momoer2 content of polymer part...... 15
5 TGA thermograms of SPE film...... 16
6. Ionic conductivity at various temperature of 23.3/11.7/35/30 SPE...... 18
7. Variation of current versus polarization time with 50 mV bias. Inset is the
electrochemical impedance spectra before and after polarization...... 19
8 CV of 23.3/11.7/35/30 SPE, from -0.5 V to 5.0V (vs. Li/Li+)...... 21
9. Lithium stripping/plating experiment with current density of 0.05
2 mA/cm ...... 22
2 10. Lithium stripping/plating experiment with current density of 0.5mA/cm ...... 23
11. SEM image of the cross section of the NCA|SPE composite electrode ...... 24
12. Cycle performance of NCA|SPE|Li lithium metal batteries at 30 °C...... 26
13. Cycle performance of NCA half-cell with EC/DEC 1M LiPF6 electrolyte at 30 °C ...... 27
14. Rate performance of NCA|SPE|Li lithium metal battery at 30 °C ...... 28
vi CHAPTER I
INTRODUCTION
1.1 Lithium Ion Battery
Today the world faces energy challenges in two distinct aspects: shifting electricity production from fossil fuel to sustainable energy sources, and how to efficiently and safely store electricity energy. Lithium ion battery (LIB) is one of the most important electrical energy storage technology in recent years which has been widely used in portable electronic devices such as smartphones and laptops.1 In various kinds of electrochemical energy storage methods, Li-ion battery has both a high specific energy and a high specific power comparing to competing technologies such as nickel (Ni)-
2 metal hydride, Ni-cadmium(Cd), and lead (Pb)-acid batteries.
To fulfill the requirements lithium of the electric vehicles (EVs) and battery energy storage stations, advanced energy storage system with higher energy density of >250Wh/kg and higher power density of >2000W/kg is needed. For lithium ion batteries, utilizing lithium metal anode and advanced high energy density cathodes has been researched for decades. The metallic lithium is regarded as the ultimate anode material for its high specific capacity (~3680 mAh/g) low density (0.534g/cm3) and the
1 lowest redox potential (-3.040V vs. SHE)3. However, technical challenges such as
4 lithium dendrite formation and low coulombic efficiency limited its application.
Advanced cathode materials such as Li-rich and Ni-rich layered materials (LiNi1-
5,6 7 xMxO2, M=Metal) and high voltage spinal materials (LiNi0.5Mn1.5O4) can provide higher energy density and power density than commercial materials such as lithium iron phosphate (LFP, LiFePO4) and lithium cobalt oxide (LCO, LiCoO2). However, the redox potential of these advanced cathode materials usually exceeds 4.3V which is the upper limit of the EC-Based electrolyte8.
Thus, a new electrolyte system with wide electrochemical window, high electrochemical stability and capability to suppress lithium dendrite formation is required for applying advanced cathode materials and lithium metal anode in lithium ion battery systems.
1.2 Polymer based solid-state electrolyte
With recent incidences in batteries failure due to lithium dendrite formation leading to overheating and a potential fire source, the solid-state electrolyte (SSE) is one of the promising candidates that can provide high ionic conductivity and enhance the safety of batteries at same time.9,10 In comparison to their liquid electrolyte counterparts, where
2 usually organic carbonate compound predominates, potential hazard-prone properties including high volatility and no mechanical strength could be ruled out.
Up to now, the family of SPE can be classify to 5 kinds: conventional polymer-salt complex or dry SPE, plasticized polymer-salt complexes, polymer gel electrolytes,
11 rubbery electrolytes and composite polymer electrolytes.
The first solid polymer electrolyte material is discovered by P. V. Wright in 1973 where poly(ethylene oxide) (PEO) was combined with alkali metal salt to provide ionic conductivity12, and first applied on batteries in 197913. PEO is the most common polymer used in SPEs which has been the most researched, for its reliable performance as a host polymeric material. Other than PEO, many other polymers including poly(acrylonitrile)
(PAN)14-15, poly(vinylidene fluoride) (PVDF)16,17, poly(methyl methacrylate)
(PMMA)18 and poly(vinyl chloride) (PVC)19,20 were used in different SPE systems to host the salt component. PEO (or PEG) with semi-crystalline structure can only provide poor ionic conductivity of 10-6 to 10-5 S/cm at room temperature21,22, which hindered its application in LIBs. However, the PEO, with repeating unit of (-CH2-O-) in its backbone, can be good solvent of various of salts due to the interaction of oxygen atom in ether groups with cations.23 Additionally, the PEO is highly efficient in coordinating metal ions due to orientation and optimal distance of the ether oxygen atoms in polymer chains.24 Considering all those advantages of PEO based system, we use polyethylene glycol diacrylate (PEGDA) as one of the ingredients of quaternary SPE. The chemical
3 structure of PEGDA is given by Figure 1. The similar backbone structure comparing to
PEO allows for a promising performance and the two end groups with carbon-carbon double bond can be polymerized which enable it to crosslink and form a polymer network.
The backbone is flexible which is favorable to efficiently transportation of ions, and crosslinked network enhances the mechanical strength while decreasing crystallinity.
Figure 1. Chemical Structure of PEGDA.
In previous research of our group, the electrochemical window of PEG based SPE is still limited in LIB applications.25 In order to enlarge the electrochemical window and increase the stability, we use a halogenated monomer2 copolymerized with PEGDA.
Monomer2 is a halogenated monomer containing one vinyl group, and it can be initiate by the same method with PEG ternary system SPE.
1.3 Mechanism of ion transportation in SPEs
For conventional binary SPEs or known as dry SPEs, the ionic conductivity of PEO- salt system could vary with the type of salt solvated. The formation of the polymer-salt is determined by the competition between lattice energy and solvation energy between
4 23 polymer and salt. PEO is able to form complex with various salt including LiClO4,
-5 LiBF4, LiCF3SO3. However, the ionic conductivity has a maximum of the order of 10
26 S/cm at room temperature.
PEO is a semi-crystalline polymer which have both amorphous and crystalline regions. The lithium ions follow different transportation mechanism in these two regions.
As it is shown in Figure 2, in amorphous region, the ions transport with the polymer chain segment movement, which gives high ionic conductivity. And in crystalline region, the ion transportation through a continuous interlocked channel between the folded chains and it is necessary to obtain a high crystallinity with a regular lattice to transport ions efficiently.
5
Figure 2. Lithium ion transportation a) amorphous region b) crystalline region.27 Copy right 2017, University of Akron
Thus, to improve the performance of SPE, highly amorphous structure is needed.
The higher amorphousness of the SPE increases, the higher the ionic conductivity, which is widely recognized. Adding plasticizer into the polymeric host can effectively increasing the amorphousness together with ionic conductivity, by the small molecules of plasticizer hindering the crystallization. Adding liquid plasticizers including ethylene carbonate and dimethyl carbonate can obtain gel-like SPE. And with plastic crystals such
6 as succinonitrile28, the SPE could be a free-standing solid and perform a good ionic conductivity.
Herein, a novel solid polymer electrolyte was reported. The electrolyte is prepared by PEGDA, monomer2, succinonitrile, bis(trifluoromethane)sulfonamide lithium salt
(LiTFSI) and lithium bis(oxalate)borate (LiBOB) is reported. The crosslinked network of PEGDA and monomer provides an enlarged electrochemical window of 0-5 V (vs.
Li+/Li) and high electrochemical stability. The superionic conductivity of ~1.0 mS/cm at
30 °C was obtained by ternary phase diagram analysis. Lithium stripping/palting experiments of more than 3000 hours indicating the SPE can suppress the lithium dendrite formation. Electrochemical performance of lithium metal battery of
Li/SPE/NCA suggesting excellent electrochemical stability and durability.
7
CHAPTER II
EXPERIMENTAL SECTION
2.1 Materials preparation
Poly(ethylene glycol) diacrylate (PEGDA, Sigma Aldrich, 99%, Mn=700g/mol),
succinonitrile (C2H4(CN)2, TCI, >99%), photoinitiator bis(2,4,6-trimethylbenzoyl)-
phenylphosphine oxide (Irgacure® 819, Sigma Aldrich, 97%), Lithium
bis(trifluoromethanesulphonyl)imide (LiTFSI, Matrix Scientific) and lithium
bis(oxalate)borate (LiBOB, Sigma Aldrich) were purchased without further purification.
Poly(vinylidene fluoride) (HSV900 PVDF, Arkema, >99.5%), carbon black (Timcal
Super P, MTI corp.) and cathode powder Lithium Nickel Cobalt Aluminium Oxide
(LiNiCoAlO2, NCA, Targary) were placed in vacuum oven at 80°C overnight to remove
residual moisture before use. Anhydrous 1-methyl-2-pyrrolidinone (NMP, Sigma Aldrich,
99%), round punched lithium metal pieces (Li, MTI Corp.) were used as purchased.
2.2 Fabrication of solid polymer electrolyte
In a typical fabrication process in making the SPEs, the prepolymer host PEGDA,
8 comonomer , the lithium slats and the plasticizer succinonitrile were kept and stored in
an argon filled glovebox(O2 < 0.5 ppm, H2O <0.5 ppm). The ratio of the quaternary
mixture PEGDA + monomer2:succinonitrile:Lithium salt was 35:35:30. Samples with
different ratio between PEGDA and monomer2 for optimization was prepared. The
optimized ratio of the PEGDA and monomer2 was 2:1, which was used for all phase
diagram, thermal, electrochemical measurement and batteries fabrication. For
electrochemical and thermal measurements, PEGDA, LITFSI, succinonitrile and 2 wt%
of LiBOB salt in respect to the whole solution was was added and heated at 60°C until
the salt completely dissolved. After that, monomer2 was added and stirred to obtain a
transparent homogeneous solution. 1 wt% of 1 wt% of Irgacure® 819 photoinitiator in
respect to PEGDA and monomer2 was introduced into the quaternary mixture and further
stirred for 5 minutes. Subsequently, the mixture was poured in to a mold with desired
depths of thickness with a glass cover on top to obtain a smooth surface. An UV light
with 350nm wavelength (25W Minerallight®, UVP LLC) was used to cure the precursor
for 3 minutes to obtain a solvent-free, free-standing SPE film. The light intensity was
measured as 0.02mW/cm2.
2.3 SPE thermal measurements
Thermal gravimetric analysis (TGA, TA Q50, TA Instruments Inc.) was used to
determine the thermal stability of the SPE system and the weight percentage component
of the quaternary system. Glass transition temperature and crystallinity of the SPE was 9 determined by differential scanning calorimetry (DSC, TA Q200, TA Instruments Inc.).
The temperature range was scanned from -60 °C to 60 °C with the ramp rate of 10°C/min.
All the sample was tested in fresh.
2.4 SPE electrochemical measurements
Ionic conductivity measurements were taken by utilizing the Stainless Steel
(SS)|SPE|SS configuration. The thickness of SPE films was ~1mm and further measured
by a thickness gauge (547S-401, Mituoyo). AC impedance was carried out by using the
electrochemical workstation (CHI608E Electrochemical Analyzer, CH Instrument),
where the electrochemical impedance spectroscopy (EIS) test was scanned from the
range of 100 KHz to 0.1 Hz with a perturbation voltage of 5mV. Ionic conductivity at
different temperature was measured with same cell configuration yet placed in a GC oven
(5890 series II, HP) to control the temperature. Serveral data points were collected within
the temperature range of 20 °C to 100 °C. For instance, at 30 °C, the cell was placed in
the oven and rested for 30 minutes before conduct AC impedance. Linear sweep
voltammetry (LSV) was tested under the SS|SPE|Li block cell configuration using the
same electrochemical workstation at 30 °C. The thickness of the free-standing SPE film
10 is ~1 mm with a diameter of 7/16’’. In the LSV test, the potential ranges from 0 V to 5.5
V (vs. Li+/Li) with a sweeping rate of 0.5 mV/s. The same block cell configuration was
also for the cyclic voltammetry (CV) measurement, where the potential range is -0.5 V
to 5.0V (vs. Li+/Li) with a sweeping rate of 0.5 mV/s. In the lithium stripping/plating
experiments, symmetric cell (Li|SPE|Li) configuration was used. The thickness of the
free-standing SPE film was 250μm. The cells were cycle under different current density
for a fixed period of time at 30 °C. The potentiostatic polarization experiments were
carried out with the same symmetric cell configuration. A constant voltage of 50 mV was
applied between the cell for 10000 s at 30 °C using an 8 Channel Battery Analyzer BTS-
8A (5V, 1mA, MTI Corp.). The interfacial impedances were measured in 1Hz – 100 KHz.
Electrochemical impedance spectroscopy (EIS) with different time experiments were
using the same symmetric cell configuration and placed in an isothermal chamber at
30 °C. The impedances were measured in 1 Hz – 100 KHz.
2.5 Lithium metal battery fabrication
Lithium Nickel Cobalt Aluminium Oxide, carbon black and PVDF was first
measured at weight ratios of 80:10:10, respectively. The powders were placed in a Nylon
container with ceramic balls and NMP solvent. The container is then placed on a ball
milling machine (MTI Corp.) to allow vigorous mixing and from a homogeneous slurry.
11 The slurry is then casted on a carbon coated Al foil using a doctor blade and dried at
80 °C overnight in a vacuum oven. The electrode was punched into 5/16’’ circular discs
with active material mass loading of approximately 1.5 - 2.0 mg/cm2. The electrode was
then moved into a glovebox and soaked in liquid precursor of the SPE without
photoinitiator for overnight. After that, the electrode was placed at the bottom of a mode.
The mode was then filled with SPE precursor with photoinitiator and cured with UV light.
A Li foil was then placed on the top of the SPE to complete the configuration of
Li|SPE|NCA.
2.6 Electrochemical measurements for lithium metal battery
The galvanostatic charge/discharge cycle test for the Li|SPE|NCA all solid-state
lithium metal battery was carried out on an 8 Channel Battery Analyzer BTS-8A (5V,
1mA, MTI Corp.). The battery was placed in an isothermal chamber at 30 °C. The initial
3 cycles have a potential range of 2.5 V to 4.1 V (vs. Li+/Li) at a current density of 0.2 C
(1 C =180 mA/g). The following cycles have the potential range of 2.5 V to 4.3 V (vs.
Li+/Li) at a current density of 0.5 C.
12
CHAPTER III
RESULTS AND DISSCUSSION
3.1 Optimization of quaternary SPE recipe
The PEGDA/Succinonitrile/LiTFSI ternary system SPE and ternary phase diagram has been reported25,29 and the working range of the ternary system has been measured.
So for this quaternary system SPE, the ratio between monomers and succinonitrile and
LiTFSI is fixed as 35:35:30 for optimizing the monomer2 content. Different ratio of the two monomers (PEGDA, monomer2) in SPE is tested with their ionic conductivity and electrochemical stability through LSV test to optimize the recipe.
The ionic conductivity of the SPEs are calculated by the EIS method followed by the
Eq. 1.
= (Eq. 1) 𝑙𝑙
σ 𝑅𝑅𝑏𝑏∙𝐴𝐴 In Eq. 1 the σ is the ionic conductivity, l is the thickness of the film, Rb is the bulk resistance and A is the area of SPE. All the results are given in Figure 4. The room temperature ionic conductivity of SPEs is between 0.6x10-3 to 2.2x10-3 which has order of magnitude similar to some liquid electrolyte system which is due to the low crystallinity of the polymer network. And there is a trend of SPE with higher fraction of monomer2 gives a higher ionic conductivity.
13
Figure 3. Ionic conductivity at various momoer2 percentage in polymer part.
Result of LSV tests is given by Figure 5. The stable potential (vs. Li+/Li) of SPE is determined by the starting point of oxidation. Comparing to
PEGDA/Succinonitrile/LiTFSI ternary system (black line in Figure 5), the electrochemical stability has an obvious enhancement after introducing monomer2. And there is a trend that SPE with higher fraction of monomer2 gives a sharper oxidation peak at 5.1-5.3V however it gives a worse stability.
14
Figure 4. LSV of the SPEs with various momoer2 content of polymer part.
Although 25% of monomer2 sample has a higher oxidation potential, a small peak occurs at around ~4.9V indicating unstable behavior at this potential range. Considering both ionic conductivity and electrochemical stability, 33% monomer sample was chosen to use in following work.
3.2 Thermal analysis of SPE
Thermal stability of the SPE was tested by thermogravimetric analysis (TGA).
It is crucial for a safe battery to remain stable when heated, because heat is induced upon cycling. The result of TGA is given by Figure 4. It could be seen the SPE remain stable 15 until approximately 100 ˚C without significant deterioration, which suggesting that the
SPE film could be utilized as high as 100 ˚C. The initial weight loss is corresponding to the decomposition of the plasticizer succinonitrile with ~35% weight loss. The polymer matrix and LiTFSI decompositions start at 350 ˚C and ~390 ˚C with the weight loss of
35% and 30% respectively which is coherent with the initial mixing ratio.
Figure 5 TGA thermograms of SPE film.
3.3 Electrochemical measurements of SPE
16 The ionic conductivity of 33% monomer2 SPE at various temperature is given by
Figure 7. The ionic conductivity is 7.6x10-4 S/cm at 30 °C and increases as the increasing temperature. The increase in ionic conductivity is ascribed to the higher chain mobility of polymer and can be plotted following the Vogel-Tamman-Fulcher (VTF) empirical formula (Eq. 2)30,31 which is suitable in describing polymer electrolytes.
1 ) 2 ( 0) 0 𝐸𝐸𝑎𝑎 (Eq. 2) (− − 𝑘𝑘𝐵𝐵 𝑇𝑇−𝑇𝑇 σ = 𝜎𝜎 ∙ 𝑇𝑇 ∙ 𝑒𝑒 where σ0 is the pre-exponential factor related to number of charge carriers, kB is the
Boltzmann constant, Ea is the pseudo-activation barrier related to critical free volume for ion transport, T0 is a reference temperature 10-50K lower than Tg (glass transition temperature) of polymer.
The ionic conductivity is fitted with VTF formula with Least squares method. The
-0.5 -1 -1 fitted values of constant σ0, T0 and Ea were 0.733 S·K ·cm , 2.58kJ·mol and 228K, respectively.
17 T (°C) 100 80 60 40 20 0.01
VTF Fitting Ionic Conductivity
(S/cm)
1E-3
Conductivity Ionic 1E-4 2.8 3.0 3.2 3.4 1000/T (K-1)
Figure 6. Ionic conductivity at various temperature of 23.3/11.7/35/30 SPE.
The lithium ion transference number (tLi+) is the ratio of lithium ion in respect to the total transferred ion including cations and anions. The lithium ion transfer number implies the efficiency of lithium ion transfer, which is widely used to characterize electrolyte of lithium ion battery. The lithium transference number of SPE is obtained by
AC/DC method32 which is DC polarization combined with electrochemical impedance
+ spectroscopy. The value of tLi can be calculated by Eq. 3.
( 0 0) + = (Eq. 3) ( ) 0𝐼𝐼𝑆𝑆𝑆𝑆 𝑉𝑉−𝐼𝐼 𝑅𝑅 𝑡𝑡 𝐼𝐼 𝑉𝑉−𝐼𝐼𝑆𝑆𝑆𝑆𝑅𝑅𝑆𝑆𝑆𝑆
18 In Eq. 3 V is the polarization potential that is applied onto the cell, I0 and Iss are initial and steady-state currents, R0 and Rss are the initial and steady-state interfacial resistances before and after the polarization. The result of AC/DC method is given by
Figure 8. The tLi+ calculated is 0.327 for 33% monomer2, which is compatible with the conventional PEO-based polymer electrolyte whose lithium transference number
33,34,35 typically is around 0.2 - 0.5.
Figure 7. Variation of current versus polarization time with 50 mV bias. Inset is the electrochemical impedance spectra before and after polarization.
Other than LSV test, cyclic voltammetry (CV) test is used to characterize the electrochemical stability of SPE. CV test gives a stability information on cyclic voltage
19 scanning. In CV curves, peaks indicate redox reaction at certain potential range, and for reversible reaction there are usually a pair of peak on opposite direction which has similar area. For SPE, the peaks are not favored because the peaks mean the ingredients has been unstable when redox reaction is occurring, especially irreversible reaction. To avoid irreversible reaction the scan range is set to -0.5 to 5.0V.
The result of CV test is given by Figure 9. The peaks at -0.5V to 0.5V are ascribed to the lithium plating/striping on stainless steel electrode.36 The oxidation peak at high voltage range is not obvious in all three cycles and the curve of the all three cycles entirely overlaps indicating no irreversible reaction was happen in the test confirmed the electrochemical window of the electrolyte is -0.5 V to 5.0V (vs. Li/Li+). Comparing to the EC based electrolyte which has electrochemical window of 1.3 V to 4.3V 8, the SPE provides an expanded electrochemical window with high stability which is suit for applying high energy density cathode materials such as Ni-rich layered materials and high voltage spinel oxides.
20
0.10 1st Cycle 2nd Cycle 3rd Cycle
0.05
0.00
Current (mA) -0.05
-0.10
-1 0 1 2 3 4 5 Potential (V vs. Li+/Li)
Figure 8 CV of 23.3/11.7/35/30 SPE, from -0.5 V to 5.0V (vs. Li/Li+).
3.4 Lithium stripping/plating experiments
In order to evaluate the durability of the SPE film against the lithium dendrite, lithium stripping/plating experiments were conducted with both low and high current density. Samples with configuration of Li|SPE|Li is placed in an isothermal chamber at
30 °C. The results were shown in Figure 10 and Figure 11. In Figure 10, the current density is 0.05 mA/cm2 and the time for each cycle was fixed to 90 min. The overpotential of the lithium anode at beginning is ~0.03 V (vs. Li/Li+) and then gradually increased to
~0.06V after 2000 hours. The experiment lasts for over 2000 hours, indicating good
21 cycling performance and capability of suppressing lithium dendrite formation at low current density condition. In Figure 11, the current density is 0.5 mA/cm2 and the time of each cycle was fixed to 40 min. Because of the hash condition, the cell only can cycling for ~500 hours and then become unstable with asymmetric overpotential. The obvious peak of the overpotential at 0- 50 h can be ascribed to the stabilization process of the interface between SPE and lithium anode.
The lithium stripping/plating experiments showing the SPE has a good durability against the lithium dendrite low current density of 0.05mA/cm2 to high current density of 0.5 mA/cm2, suggesting the electrolyte can be applied in lithium metal batteries with different operating parameters and remain stable for hundreds of cycles.
Figure 9. Lithium stripping/plating experiment with current density of 0.05 mA/cm2.
22
Figure 10. Lithium stripping/plating experiment with current density of 0.5mA/cm2.
3.5 lithium metal batteries
To improve the contact between solid polymer electrolyte and cathode, a composite electrode with mixed active materials, conducting filler and solid polymer electrolyte is needed. In this work, liquid precursor of SPE containing monomer is infiltrated into the casted NCA cathode and then UV-crosslinked with a layer of SPE film.
The cross section of the composite electrolyte is shown in Figure 12. The layered
structures from top were solid polymer electrolyte, cathode materials and aluminum
current collector, respectively. The thickness of the SPE film is ~ 130μm.
23
Figure 11. SEM image of the cross section of the NCA|SPE composite electrode.
Lithium nickel cobalt aluminum oxide (NCA) cathode was chosen for lithium metal battery fabrication. The casted electrode with active material mass loading of 1.5-
2 mg/cm2 was soaked with non-crosslinked SPE precursor for overnight. Figure 13 shows the galvanostatic charge/discharge cycles of the all-solid-state NCA lithium metal battery with the SPE. The first 3 cycles (not shown in Figure 13) were operated with voltage range of 2.5 V - 3.9 V and current density of 0.2 C (1 C =180 mA/g) to initiate the cell with passivation layers formation on both anode and cathode sides. The following cycles was operated with voltage range of 2.5 V - 4.3 V and current density of 0.5 C. Specific capacity of ~144 mAh/g was achieved at the beginning and remain 43.2% capacity retention after 450 cycles with an averaged coulombic efficiency of 99.80%. The capacity fading was continuing decrease during the cycles and stabilized after 100 cycles. The
24 specific capacity of the 200th cycle is ~93 mAh/g with 64.6% retention which is higher than battery with liquid electrolytes showing in Figure 17 (200th cycle retention: 56.5%), indicating good stability and cycling performance of the SPE.
The rate performance test is shown in Figure 15, where the cell exhibited specific capacities of ~157 mAh/g, ~129 mAh/g, ~108 mAh/g, ~92 mAh/g, ~63 mAh/g, ~39 mAh/g and 145 mAh/g with current density of 0.2 C, 0.5 C, 0.8 C, 1 C, 1.5 C, 2 C and
0.2 C, respectively. The specific capacity difference between the ratability test and long- cycle test due to the quick capacity fading of the first few cycles.
25
200 100
) 150 80
60 100
40 CE(%)
50 Charge 20 Discharge
SpecificCapacity (mAh/g CE(%) 0 0 0 50 100 150 200 250 300 350 400 450 Cycle
Figure 12. Cycle performance of NCA|SPE|Li lithium metal batteries at 30 °C.
26
300 100 250
80 200
60 150
CE(%) 40 100
Charge 50 20 Discharge
SpecificCapacity (mAh/g) CE(%) 0 0 0 50 100 150 200 Cycle
Figure 13. Cycle performance of NCA half-cell with EC/DEC 1M LiPF6 electrolyte at 30 °C.
27
250 100
200 80 0.2C 150 0.2C
0.5C 60 0.8C 100 1C 40 CE(%)
1.5C
2C 50 Charge 20 Discharge
SpecificCapacity (mAh/g) CE(%) 0 0 0 10 20 30 40 50 60 70 Cycle
Figure 14. Rate performance of NCA|SPE|Li lithium metal battery at 30 °C.
28
CHAPTER IV
CONCLUSION
Addition halogenated monomer2 as a comonomer in polymeric matrix for SPE can increase the ionic conductivity and electrochemical stability at same time comparing to ternary SPE. The quaternary SPE system of PEGDA/monoer2/succinonitrile/LiTFSI with a ratio of 23.3/11.7/35/30 could provide an ionic conductivity of 7.6x10-4 S/cm at room temperature and remain stable over 5.3V (vs. Li+/Li), which is ideal in lithium ion battery applications.
By the thermal and electrochemical characterizations, this light-weight, solvent-free, self-standing and transparent quaternary SPE film, which is comparable to or better than those of conventional electrolytes that required organic solvents.
Lithium stripping/plating experiment exceeding over 2000h, showing the good cycling stability and capability of suppression against lithium dendrite. Lithium metal battery with NCA cathode showing a good cycling performance and ratability of the optimized SPE.
29
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