POLYMER ELECTROLYTES FOR HIGH CURRENT DENSITY LITHIUM STRIPPING/PLATING TEST
A Thesis Presented to The Graduate Faculty of the University of Akron
In Partial Fulfillment Of the Requirements for the Degree Master of Science
Yuhan Zhang May, 2019 POLYMER ELECTROLYTES FOR HIGH CURRENT DENSITY LITHIUM STRIPPING/PLATING TEST
Yuhan Zhang
Thesis
Approved: Accepted:
Advisor Interim Dean of the College Dr. Yu Zhu Dr. Ali Dhinojwala
Faculty Reader Dean of the Graduate School Dr. Steven S.C. Chuang Dr. Chand Miaha
Department Chair Date Dr. Tianbo Liu
ii ABSTRACT
Metallic lithium, performing high theoretical capacity, have been regarded as one of the most promising anode materials for lithium batteries. However, dendrite formation on lithium surface results in safety risks and poor cyclability. To suppress the dendritic morphology formation, a freestanding ternary solid polymer electrolyte film which is based on polyethylene glycol diacrylate monomer and plasticized by a small molecule plasticizer (succinonitrile) has been studied. For an optimized thickness of thin film and a uniform morphology of Li deposits, Celgard® 3501 and fluoroethylene carbonate were incorporated into the solid polymer electrolyte system. The SPEs achieved high ionic conductivities in a range of 0.72 mS cm-1 to 1.79 mS cm-1 at room temperature and a wide electrochemical window of 0-5.0 V (vs. Li+/Li). The lithium stripping/plating experiments indicated that the polymer electrolyte can suppress the dendrite formation under current density from 0.1 mA cm-2 to 1.0 mA cm-2. The lifetime of the cell after added FEC easily exceeded 700 hours at 0.5 mA cm-2.
iii ACKNOWLEDGEMENTS
First and foremost, I would like to express my deepest gratitude to my supervisor, Dr.
Yu Zhu, a respectable and resourceful scholar, who has provided me with valuable guidance and constant encouragement on every stage of the experiments. Also, I shall extend my thanks to Wenfeng Liang and Yunfan Shao for all their important guidance and assistance in my experiments. Additionally, I want to express my appreciation to my friends and family, for their supporting and encouragement. Finally, I am grateful to Dr.
Steven Chuang for being so generous and spending his valuable time on my thesis.
iv TABLE OF CONTENTS
Page
ABSTRACT ...... iii
ACKNOWLEDGEMENTS ...... iv
LIST OF FIGURES ...... vii
CHAPTER
I. INTRODUCTION ...... 1
1.1 Lithium metal batteries ...... 1
1.2 Challenges of Li metal anodes...... 2
1.3 Polymer based solid-state electrolyte ...... 4
II. EXPERIMENTAL SECTION ...... 9
2.1 Materials preparation ...... 9
2.2 Fabrication of solid polymer electrolyte ...... 9
2.2.1 Precursor preparation ...... 9
2.2.2 Solid polymer electrolyte formation ...... 10
2.3 SPE electrochemical measurements ...... 11
2.4 Porous membrane characterization ...... 12
III. RESULTS AND DISCUSSION ...... 13
3.1 Freestanding solid polymer electrolyte lithium stripping/plating results ...... 13
3.2 Porous membrane SEM ...... 16
3.3 Separator supported polymer electrolyte lithium stripping/plating results ...... 17
3.4 5% FEC improved separator supported polymer electrolyte ...... 20
v 3.4.1 5% FEC improved polymer electrolyte electrochemical properties ...... 20
3.4.2 Lithium stripping/plating results ...... 22
IV. CONCLUSION AND PROSPECT ...... 24
BIBLIOGRAPHY ...... 25
vi LIST OF FIGURES
Figure 1. Bar chart showing the practical specific energy (pink) and energy densities (blue) of petrol (gasoline) and typical Li batteries including the state-of-the-art Li-ion battery, the Li metal/LMO cell, Li–S and Li–air cells...... 2
Figure 2. Correlations among the different challenges in the Li metal anode, originating from high reactivity and infinite relative volume change ...... 3
Figure 3. Chemical structure of PEGDA ...... 6
Figure 4. Lithium ion transportation a) amorphous region b) crystalline region ...... 7
Figure 5. Electrochemical performance of the freestanding SPE in the symmetric Li/SPE/Li cells.
Potential profiles of the lithium plating/stripping cycling using two different ratios a) 25/45/30 c) d) 20/50/30 of SPE with current densities of a) d)0.2 mA/cm2 and c) 0.1 mA/cm2 at 30 oC, b)
Enlarged view of a), showing voltage drop at short circuit point...... 15
Figure 6. a) Lifetime variations for individual samples with two different current densities. b) The voltage-current relationship for freestanding SPE film with a ratio of 25/40/35...... 16
Figure 7. SEM images of a) Sterlitech polycarbonate membrane, b) Celgard polypropylene separator 3501...... 18
Figure 8. Electrochemical performance of separator supported thin SPE in the symmetric
Li/SPE/Li cells. Potential profiles of the lithium plating/stripping cycling using two different ratios of a) b) 25/40/35, c) d) 20/50/30 with current densities of a) b) c) 0.2 mA/cm2 and d) 0.4 mA/cm2 at 30 oC...... 19
vii Figure 9. a) Lifetime variations for individual samples with four different current densities. b) The voltage linearity for freestanding SPE film with a ratio of 20/50/30 at different current densities
...... 21
Figure 10. Electrochemical stability characterization of SPE with 5% FEC. a) LSV of freestanding
SPE film with 5% FEC. b) CV of freestanding SPE film with 5% FEC at room temperature .... 23
Figure 11. Electrochemical performance of the FEC improved thin SPE in the symmetric
Li/SPE/Li cells. Potential profiles of the lithium plating/stripping cycling using a ratio of 20/50/30 with current densities of a) 0.5 mA/cm2 b) 0.6 mA/cm2 and c) 1.0 mA/cm2 at 30 oC...... 25
vii CHAPTER I
INTRODUCTION
1.1 Lithium metal batteries
Nowadays there are two urgent challenges in energy sciences: shifting electricity production from traditional energy resources like fossil fuel to sustainable energy sources, and storing electricity energy efficiently and safely. Lithium ion battery (LIB) is one of the most critical electrical energy storage technologies over last two decades.1 Since the introduction of commercial LIBs in 1991, we have witnessed extraordinary development in portable electronic devices, and the recent utilization of electric vehicles promises to reform personal transportation too.2 The intrinsic limitations of
Li-ion chemistry make this type of batteries unlikely to meet the growing desire for high-energy density, and it is now widely acknowledged that battery chemistries beyond Li-ion need to be developed.3
Lithium metal batteries (LMBs) appeared to be one of the optimum energy storage systems with high energy density because the Li metal anode has a superhigh specific capacity of 3860 mAh/g, and a very low redox potential (-3.040 V versus standard hydrogen electrode).4 The benefits of Li metal batteries are summarized in
Figure 13. State-of-the-art LIBs can achieve a specific energy of ~250 Wh/kg, which is one order of magnitude lower than the practical value of petrol (gasoline). Once the anode is taken place by Li metal, a Li–LMO cell (where LMO is a lithium transition-
1 metal oxide) can extend the specific energy to ~440 Wh/kg. The greatly reduced weight of Li carries out a higher energy density.
Figure 1. Bar chart showing the practical specific energy (pink) and energy densities
(blue) of petrol (gasoline) and typical Li batteries including the state-of-the-art Li-ion
battery, the Li metal/LMO cell, Li–S and Li–air cells.
1.2 Challenges of Li metal anodes
Although metallic lithium is a promising alternative for next generation high- energy density batteries, its applications are still bound due to the limited charging/ discharging rate and bad cycling performance.5 Alike to many other metals, Li trends to plate in dendritic form4, which is considered to be the primary cause of thermal runaway and explosion risks caused by internal short circuit of the cells.
2 Figure 2. Correlations among the different challenges in the Li metal anode, originating
from high reactivity and infinite relative volume change
Owing to the extremely negative electrochemical potential of Li+/Li and thermodynamically unstable nature of lithium, almost all available electrolytes can be reduced on the Li surface.6 A solid electrolyte interphase (SEI) passivated on the surface of Li metal. The initial SEI constitution is principally the product of Li alkyl carbonates
(ROCOOLi) result from one-electron reduction of alkyl carbonates, which will be further converted to Li2CO3 in the present of trace amounts of H2O. Hence the inner layer of the
SEI is dominated by more stable components such as Li2O, Li2CO3 and Li halides, while metastable components like ROCOOLi distribute on the outer layer. The SEI passivation layer can be further described as a “mosaic model” formed by the heterogeneous stacking
3 of tiny domains with different compositions.3 Overall, SEIs of this type lack flexibility, making them weak and brittle during interfacial fluctuation.
Dendritic deposition is a common occurrence at high rate electroplating of metals.
There is a concentration gradient of metal cation in electrolyte between two electrodes during the electroplating. Once a critical current density J* is reached, the current can only be maintained for a certain period called the Sand’s time, τ, after which cations be depleted in the electrolyte, interrupting the electrical neutrality at the deposited metal surface. This cations nucleation builds up a partial space charge, producing the formation of dendritic metal depositions. This theory helps predict the electroplating of Li dendrites for current densities higher than J*.7
Lithium dendrite formation is self-boosted, and various theories have been proposed to explain this phenomenon. One is that protrusions possess a fairly higher electric field at their tips with high curvature, which consequently attend to extract more
Li-ions, leading to further growth of the protrusions and eventually developing into dendrites.8 Meanwhile the hemispherical tips of protrusions provides three-dimensional
(3D) Li-ion diffusion, instead of the one-directional diffusion detected under flat surfaces, resulting in faster Li nucleation on the tips.9
1.3 Polymer based solid-state electrolyte
With recent incidences in batteries failure due to lithium dendrite growth leading to potential hazard of fire and explosion, the solid-state electrolyte (SSE) becomes one of the promising candidates that can provide both high ionic conductivity and improve the safety of batteries at the same time. In comparison to their liquid electrolyte counterparts,
4 where usually organic carbonate compounds predominate, hazard properties including high volatility, potential leakage and no mechanical strength could be eliminated.
Solid Polymer electrolytes (SPEs) are composed of a Li salt immobilized in a polymer matrix to form a macromolecular architecture to conduct Li-ion for use in batteries. Till now, the family of SPE can be categorized to 5 sorts: conventional polymer-salt complexes (dry SPE), plasticized polymer-salt complexes, polymer gel electrolytes, composite polymer electrolytes and rubbery electrolytes.10
The first solid polymer electrolyte (SPE) material is discovered by P.V. Wright in
1973. At that time poly(ethylene oxide) (PEO) was associated with alkali metal salt to provide ionic conductivity11, and it was applied on batteries in 1979 for the first time12.
PEO is the most widely used polymer in SPEs, for its reliable performance as a host polymeric material. Aside from PEO, many other polymers such as poly(acrylonitrile)
(PAN), poly(vinylidene fluoride) (PVDF), poly(methyl methacrylate) (PMMA) and poly(vinyl chloride) (PVC) were utilized in different SPE systems as host for the salt and other components.13 It is not satisfactory PEO (or POM, PEG) provides a ionic conductivity of 10-6 to 10-5 S/cm at room temperature with its semi-crystalline structure14, which hindered its application in practical batteries. Nonetheless, the PEO, with repeating unit of (-CH2-O-) in its backbone, can be good solvent of diverse salts due to the interaction of oxygen atom in ether groups with cations.15 Furthermore, the PEO is highly efficient in coordinating metal ions according to orientation and optimal distance of the ether oxygen atoms in polymer chains.16 Considering all those advantages of PEO based system, polyethylene glycol diacrylate (PEGDA) was used as polymer ingredients of SPE. The chemical structure of PEGDA is given by Figure 3. The similar backbone
5 structure comparing to PEO allows for a promising performance while 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 cross-linked network enhances the mechanical strength while decreasing crystallinity.
Figure 3. Chemical structure of PEGDA
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 417, there are two kinds of conduction mechanism in SPE. First in amorphous region, a Li salt interacts with coordination sites to form a complex in a polymer matrix. In this state, cations dissociate with anions, hop and coordinate repeatedly with new adjacent coordination sites as the amorphous polymer chain segments movement in free volume space. Directional movement of cations and anions is achieved under the influence of an external electric field.
In crystalline region, polymer chains fold to form interlocked spiral channels inside the ordered framework. The Li-ions travel in the channels applied external electric
6 field and it is crucial to obtain a desired crystallinity with a regular lattice to transport ions efficiently. The anions are located outside and are not coordinated to Li-ions.
Figure 4. Lithium ion transportation a) amorphous region b) crystalline region
Thus, to improve the performance of SPE, highly amorphous structure is needed.
The higher amorphous of the SPE achieves, the higher the ionic conductivity, which is widely recognized. Introducing plasticizer into the polymeric host can effectively increase the amorphousness together with ionic conductivity, by the small molecules of plasticizer hindering the crystallization. With plastic crystals such as succinonitrile, the
SPE could be fabricated as a free-standing solid film and performs an acceptable ionic conductivity.
7 In previous research of our group, a highly conductive and electrochemically stable dual-salt solid polymer electrolyte (SPE) for a high-capacity cathode was developed. A ternary phase solid polymer electrolyte system was advanced to achieve a high ionic conductivity (over 1.76 mS/cm at 30oC). The synergy of combining different salt rendered superior electrochemical stability to the SPE with an electrochemical window from 0 to 5.0 V (vs Li+ /Li) in linear sweep voltammetry and up to 4.15 V in
LMBs.18
Herein, the solid polymer electrolyte with PEGDA, succinonitrile and bis(trifluoromethane)sulfonamide lithium salt (LiTFSI) is reported. Two different ratio of ingredients with different mechanical strength and ionic conductivities were utilized in this work. And 5 wt.% FEC was added to the system which was reported that an addition of small amount fluorinated compounds contributes to form a robust LiF-rich SEI19. The
FEC-induced SEI layer is compact and stable, and thus beneficial to obtain a uniform morphology of Li deposits.
8 CHARPTER II
EXPERIMENTAL SECTION
2.1 Materials preparation
All materials were used as received without further purification. Poly(ethylene glycol) diacrylate (PEGDA, Sigma Aldrich, 99%, Mn=700), Succinonitrile (C2H4(CN)2,
TCI Corp., 99%), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, Matrix Scientific,
97%), lithium bis(oxalato)borate (LiBOB, Sigma Aldrich), monofluoroethylene carbonate (FEC, BASF), and bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide
(Irgacure® 819, Sigma Aldrich, 97%) were used as received without further purification.
Lithium foil (Li, MTI Corp., 99.9%) was purchased from Scientific Polymer Products,
Inc. and punched into round chips with diameter of 0.5 inch. Separate film 3501 was obtained from Celgard® and the thickness was 25 µm.
2.2 Fabrication of solid polymer electrolyte
The fabrication of solid polymer electrolyte was carried out inside an argon-filled glovebox (O2< 0.5ppm, H2O< 0.5ppm).
2.2.1 Precursor preparation
The entire preparation processes were blocked out of UV light in case of crosslinking of PEGDA. First, the ternary precursor was prepared into a homogenous mixture; for instance, 20/50/30 is corresponding to mixture of 20 wt.% PEGDA, 50 wt.%
9 succinonitrile and 30% LiTFSI. 2 wt.% LiBOB and 5 wt.% FEC were added as additives into the system. The 2 wt.% LiBOB was first mixed with PEGDA and succinonitrile. The mixture was vigorously shaken for 10 min by a vortex mixer (120V, VWR International) after heating at 60oC for 10 min on a hot plate. After three components completely dissolved, LiTFSI was added into the mixture by proportion. The mixture was also shaken by the vortex mixer and assisting dissolving by heating at 60 oC. After the mixture became a homogeneous clear liquid, 0.5 wt.% of Irgacure® 819 photoinitiator was introduced into the homogeneous mixture and stirred for 1 min to dissolve. The final mixture was put aside for at least 15 min until no bubble suspends in the liquid.
2.2.2 Solid polymer electrolyte formation
After the final precursor is completely homogenous, clear and no bubble, it is ready to crosslink into solid polymer electrolyte. To form a freestanding SPE film with thickness of 250 μm, the precursor was poured into a round stainless steel mold with 19 mm diameter and 0.25 mm height. The filled mold was covered by a microscope slide on the top from one side carefully avoid capturing small bubble into the viscous precursor to obtain a smooth surface and controlled thickness. A UV light source with wavelength of 365 nm (25 W Mineralight, UVP) was exposed to the sample for 1 min to obtain a transparent, solvent-free, freestanding SPE film. The
UV light intensity was measured as 0.02 mW/cm2. For the separator supported thin polymer electrolyte with thickness of 50 μm, it was formed using two microscope slides. The polypropylene separator (3501, Celgard) was punched into round pieces with 19 mm diameter. The thickness of the separator is 25 μm as indicated by the company. Then the cut separator was presoaked by the uninitiated precursor until it
10 was fully wetted into a darker color. The well wetted membrane was placed between two microscope slides with adding two drops of precursor on each side and pressed them tight. The entire process should be carefully operated making sure no bubble trapped in and the electrolyte was homogenous, the surface was smooth.
In cell assembly, in order to get a better contact of SPE with lithium metal piece, the pressed lithium disks were pasted onto the SPEs with precursor to reduce resistance at the contacting surface.
2.3 SPE electrochemical measurements
Ionic conductivity measurements were taken with the stainless steel (SS)/SPE/SS configuration coin cell. The thickness of the SPE was fixed to 1mm by using a validated using a thickness gauge (547S-401, Mitutoyo). And the diameter of the SPE film was controlled to 7/16 inch using a round punch. A.C. impedance was measured by an electrochemical workstation (CHI608E Electrochemical Analyzer, CH Instrument). The electrochemical impedance spectroscopy (EIS) test was scanned from 1 MHz to 1 Hz with a perturbation voltage of 10 mV. Ionic conductivity at various temperature with a range of 30 oC to 70 oC using the same cell configuration. All the data points were measured in a GC oven (HP 5890 series II). Cells were placed in the condition and rested for more than 30 min to isothermal state before test. Each data point was tested for 3 samples and 3 times each sample, the result is given by averaged value.
Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were tested using configuration of SS/SPE/Li block cell and same electrochemical workstation under room temperature. Thickness of the SPE films used in LSV and CV measurements is 1 mm
11 with diameter of 7/16 inch. In LSV tests, the range of potential is 0 to 6.0 V with a sweeping rate of 0.5 mV s-1. In CV tests, the range of potentials is -0.5 V to 5.0 V with a sweeping rate of 0.5 mV s-1 for 6 cycles.
The lithium metal electrode-SPE membrane interfacial was investigated by
“stripping/plating” experiments, using symmetric cell with configuration of Li/SPE/Li. In the “stripping /plating” test, the cells were charged and discharged with different constant current density by an eight channels battery analyzer (BST8-WA 1 mA, MTI Corp.). The lithium metal tapes were pressed into 150 μm thickness by an electric precision width rolling press with dual micrometer (MSK-HRP-MR100A, MTI Corp.) and smooth surface was obtained with metallic luster. Then lithium tapes were punch into round pieces with diameter of 1.27 cm. All the tests were carried out in an isothermal chamber
(30 oC) connected to a temperature controller (Omron E5AK). The period time was controlled as 1 hour per half cycle.
2.4 Porous membrane characterization
The morphology of two types of porous membrane were observed with scanning electron microscopy (SEM) (JSM-7401F, JOEL) technique. The pore structure was observed.
12 CHAPTER III
RESULTS AND DISCUSSION
3.1 Freestanding solid polymer electrolyte lithium stripping/plating results
Two different ratios of precursor have different ionic conductivity and different mechanical strengths but with relatively good electrochemical stability.18
First, freestanding solid electrolyte films were obtained with the ratio of 25/40/35.
For this ratio, it processes an ionic conductivity of 0.72 mS/cm at room temperature.18
The thickness was controlled by the depth of stainless steel mold as 250 μm. The films, clear and transparent with smooth surface, were assembled in Li/Li symmetric cells and characterized by stripping/plating test at a current density of 0.2 mA/cm2. It is to apply positive and negative constant current alternatively to dissolve and deposit lithium on each side. Figure 5a shows the galvanostatic cycling profiles of it. At beginning, an overpotential of 0.06 V was observed. Ideally, there should be no potential difference of two lithium electrodes, however, this overpotential reflects the energy that a cell needs to drive Li-ions through electrolytes. The potential increased slightly with increasing time which indicates a higher impedance. To the best of our knowledge, a passivation layer forms on the Li surface once the Li metal and electrolyte contact each other due to the unstable nature of Li. In step 1, Li plating causes volume expansion, which cracks the
SEI film. And further plating causes Li dendrites to shoot out through the cracks. During
Li stripping procedure, Li dendrites produce isolated Li which becomes part of the “dead”
Li. With volume contraction during stripping process, those SEI further fractures and accumulates. After continuous cycling these steps occur repeatedly, and this finally
13 results in accumulated dead Li, thick SEI and porous Li electrode. These demonstrates the major failure mechanism of lithium metal anodes and how the internal resistance built up.
A sudden voltage drop was observed at 627 h and was enlarged into Figure 5b.
The cycles were continued with a much smaller voltage of 0.02 V in square shape which refers to short circuit of the cell induced by the lithium dendrites growth that penetrate through the SPE. Once two lithium electrodes contact each other by tips of dendrites, the resistance reduces in large scale and there will be no polarization in when applied constant current.
14 Figure 5. Electrochemical performance of the freestanding SPE in the symmetric
Li/SPE/Li cells. Potential profiles of the lithium plating/stripping cycling using two
different ratios a) 25/40/35 c) d) 20/50/30 of SPE with current densities of a) d) 0.2 mA/cm2 and c) 0.1 mA/cm2at 30 oC. b) Enlarged view of a), showing the voltage change
at short circuit point.
15 Figure 5d shows the result of the same stripping/plating experiment with a different ratio of electrolyte (20/50/30). This ratio has a higher ionic conductivity of 1.76 mS/cm. The longest lifetime could reach 1200 hours. At first 400 hours the potential profile stayed pretty stable and smooth, however some sudden voltage changes happened from time to time. But it did not drop into a tiny value compared to the voltage shown in
Figure 5b, and self-healed to the same level as the beginning. These might be resulted from the contact issue of two solid surface with continuous morphology change caused by dendrites growth. When the cell was conducted at 0.1 mA/cm2, it showed an excellent performance for more than 2400 hours in Figure 5c.
Figure 6. a) Lifetime variations for individual samples with two different current
densities. b) The voltage-current relationship for freestanding SPE film with a ratio of
25/40/35.
In stripping/plating experiments, lifetime refers to the period during which the
Li/Li symmetric cells give steady voltage response with applying positive and negative current alternatively. There should not be large voltage change during lifetime. When the
16 cell fails with large voltage drop, the lifetime finishes. The cell failure could be determined by the sharp rise in waveform of the voltage profile and there will be a sudden voltage drop into a very small value after lifetime which indicates short circuit induced by the dendrites growth that penetrate through the SPE. Figure 6a shows lifetime variation of individual samples at same current density which caused by the fabrication and cannot be eliminate. But they are quite close to each other and have good reliability.
Although lifetimes at 0.2 mA/cm2 were around 800 hours, it behaved very poor at a higher current density of 0.4 mA/cm2. The relationship between voltage and current is not that linear as shown in Figure 6b. The voltage for 0.2 mA/cm2 was around 0.1 V but became 4 times larger while current doubled. It might because of thickness of the electrolyte, Li-ions need to travel for a long distance which generated a non-uniform Li- ion flux and facilitated Li dendrite growth. Therefore, the thickness of SPEs need be decreased to reduce the impedance thus lower the potential.
3.2 Porous membrane SEM
Previous SPE films were freestanding with a thickness of 250 μm, but the mechanical strength of this system is not sufficient enough to fabricate a thinner freestanding film. In order to be supported to obtain a thinner solid polymer electrolyte film, porous thin membranes were incorporated into the system.
17
Figure 7. SEM images of a) Sterlitech polycarbonate membrane, b) Celgard
polypropylene separator 3501.
There were two types of porous thin film observed under scanning electron microscope. The pores of polycarbonate film shown in Figure 7a were directly punched through, while the Celgard polypropylene separator (Figure 7b) has a torturous structure.
Higher tortuosity will contribute to more homogenous Li-ion flow, leading to a more uniform current. Consistently, the right one showed a better result in preliminary test. So
Celgard separator was chosen as the porous thin film to support our SPE system to fabricate thinner polymer electrolyte.
3.3 Separator supported polymer electrolyte lithium stripping/plating results
First, a separator supported polymer electrolyte (25/40/35) was fabricated with thickness of 200 μm. The stripping/plating result conducted at 0.2 mA/cm2 only survived
100 hours and was followed by a sharp rise in voltage hysteresis20 with the cycle time and the sudden drop after 120 hours as shown in Figure 8a. In Figure 8b, the thickness was decreased to 67 μm measured by gauge (547S-401, Mitutoyo), as a result the lifetime prolonged to 500 hours. An asymmetric potential curve was observed at the beginning 30
18 hours in Figure 8b. The positive and negative woltage denotes lithium stripping and lithium plating, respectively. The cell exhibited a high voltage (~1.5 V) for lithium stripping and a low woltage (~0.3 V) for lithium plating, which is likely caused by contact area differences21 between lithium metal and polymer electrolyte due to the difference of the surface morphology for each Li metal piece. The initial SEI layers formed on the interface between lithium metal and polymer electrolytes were distinct.
Thus the overpotentials for Li-ions to go through it were different. But the unique situation became close to each other after cycles.
Figure 8. Electrochemical performance of the separator supported thin SPE in the
symmetric Li/SPE/Li cells. Potential profiles of the lithium plating/stripping cycling using two different ratios of a) b) 25/40/35, c) d) 20/50/30 with current densities of a) b)
c) 0.2 mA/cm2 and d) 0.4 mA/cm2 at 30 oC.
19 For the higher conductivity ratio (20/50/30), the lifetime can easily exceed 800 hours like Figure 8c at current density of 0.2 mA/cm2. A decrease of overpotential was observed. Overpotentials are generated by kinetic hindrances in the system. In lithium plating/stripping processes, these may include the Li-ion transport in solid polymer electrolyte and in the electrode/electrolyte interphase, such as the SEI, and always the kinetic hindrance of the Li-ion reduction and oxidation processes at the electrode itself, influencing the charge transfer resistance.22 The contributions of these processes to the overall cell resistance can be identified by electrochemical impedance spectroscopy (EIS).
The beginning potential was a little bit higher than the steady status voltage. Only after very little current flow, the overpotential is cut down immediately to around continues to decrease rapidly at first and then decreases slower and slower. This decrease of the overpotential during lithium deposition can be explained by deposition of new lithium on previously deposited lithium, which means lithium was driven through the SEI layer formed during the time of rest.
As shown in Figure 8d, the separator supported thin polymer electrolyte can be charged and discharged at 0.4 mA/cm2 for more than 450 hours with electrolyte ratio of
20/50/30 and polarization happened causing sharp rise of voltage.
20
Figure 9. a) Lifetime variations for individual samples with four different current
densities. b) The voltage linearity for freestanding SPE film with a ratio of 20/50/30 at
different current densities.
Figure 9a shows the lifetime for each individual samples conducted at different current densities. The average lifetime for 0.4 mA/cm2 batch is around 400 hours, which was enhanced pretty much than that of the freestanding thick SPE. However, this thin supported polymer electrolyte still cannot run for long time at higher current densities like 0.6 mA/cm2 and 1.0 mA/cm2.
3.4 5% FEC improved Separator supported polymer electrolyte
Lithium deposition behavior has been understood well in Li/Li symmetric cell.
When charging and discharging at high rate, large dendrites and dead lithium formed on lithium metal, leading to a large polarization. The highly resistive porous dead lithium layer dramatically increased the cell impedance and resulted in cell degradation and final failure. To suppress lithium dendrite growth, Zhang et al. investigate the cycling performance in liquid electrolyte with additives.19
21 Fluoroethylene carbonate (FEC) is beneficial for robust SEI formation on lithium metal anode. This FEC-induced SEI is more stable and conductive, promoting the uniform lithium stripping/plating and superior long-term cycling performance. FEC additive was reported to lead to LiF-rich components on the surface of lithium metal which prevents the side reactions between electrolyte and lithium metal.
To form a robust SEI layer, suppress the severe lithium dendrite growth and improve the cycling performance at higher current densities, FEC was incorporated as additive of 5 wt.%.
3.4.1 5% FEC improved polymer electrolyte electrochemical properties
Thin separator supported polymer electrolytes with 5 wt.% FEC were formed with thickness in the range from 50-70 μm and no liquid left throughout under observation by naked eye. The films are uniform, smooth, soft and can be bend, twist readily. The improved mechanical strength of the films promises an easier handling to fabricate a coin cell. The ionic conductivity of this system with 5 wt.% FEC is
1.796 mS/cm. It is almost the same as the one without FEC which is 1.76 mS/cm at
30 oC18. It denoted that the addition of FEC did not change the ionic conductivity, but promoted the electrochemical stability of the electrolyte.
22
Figure 10. Electrochemical stability characterization of SPE with 5% FEC. a) LSV of
freestanding SPE film with 5% FEC. b) CV of freestanding SPE film with 5% FEC at
room temperature.
A linear sweeping voltammetry (LSV) test was carried out to determine the electrochemical working potential range (Figure 10a). The FEC promoted SPE exhibits a stable potential window up to 5.0 V vs Li +/Li. The major oxidation degradation of SPE was initiated at voltage above 5.0 V. Further CV evaluation
(Figure 10b) on the oxidation and reduction was carried out from -0.5 to 5.0 V with the same scanning rate as the LSV (0.5 mV/s). The peaks between -0.5 and 0.4 V (vs
Li+/Li) correspond to the lithium plating and stripping processes.
From the CV results, the SPE was stable up to 5.0 V (vs. Li +/Li) without showing any obvious oxidation peaks and remains unchanged after six cycles. As comparison, the current response of FEC-improved SPE was three times smaller than that without FEC in CV experiments, which indicated that reactions became milder here. The reduction and oxidation peaks remained at the same position in each cycle, indicating the lithium stripping/plating process was a reversible process. The
23 aforementioned results confirmed that FEC could help stabilize the interphase between electrolyte and lithium metal, accommodate the serious deterioration at a higher potential range, and hence, improve the electrochemical compatibility with high-potential.
3.4.2 Lithium stripping/plating results
When the lithium cycles at higher current density, there will be more serious decomposition of electrolyte and severer lithium dendrites growth, which resulting in higher overpotential and leading to faster cell failure.23
24 Figure 11. Electrochemical performance of the FEC improved thin SPE in the
symmetric Li/SPE/Li cells. Potential profiles of the lithium plating/stripping cycling
using a ratio of 20/50/30 with current densities of a) 0.5 mA/cm2 b) 0.6 mA/cm2 and c)
1.0 mA/cm2 at 30 oC.
By addition of 5 wt.% FEC, the lifetime reached 730 hours with current density of 0.5 mA/cm2 in Figure 11a, demonstrating the durability and electrochemical
25 stability of FEC-promoted SPE. For current density of 0.6 mA/cm2, the lifetime exceeded
490 hours. However, potential spikes were observed after 150 hours stable cycling, which might be ascribed to the nucleation reaction of the lithium plating reaction at the electrolyte/lithium interface.24 The voltage shape change was observed just after the potential spikes, later, it was self-healed to steady status. The cell charged and discharged at current density of 1.0 mA/cm2 exhibited a stable performance for 220 hours. These results demonstrated the capability of FEC to mitigate the shoring of cell by the lithium dendrites.
26 CHAPTER IV
CONCLUSION
Lithium metal as electrodes were stripped and plated at different current densities in this work, and the lithium dendrites growth were mitigated by solid polymer electrolyte due to the strong shear modulus of it. The solid polymer electrolyte is based on polyethylene glycol diacrylate monomer, plasticized by a small molecule plasticizer (succinonitrile) and incorprated with lithium bis(trifluoromethanesulfonyl)imide as lithium salt. The SPEs achieved high ionic conductivities in a range of 0.72 mS cm-1 to 1.79 mS cm-1 at room temperature and a wide electrochemical window of 0-5.0 V (vs. Li+/Li). The lithium stripping/plating experiments indicated that the polymer electrolyte can suppress the dendrite formation under current density from 0.1 mA cm-2 to 1.0 mA cm-2. Freestanding SPE films with thickness of 250 μm showed duration at 0.2 mA/cm2 for more than 600 hours. The performance of this SPE system was enhanced to 450 hours at 0.4 mA/cm2 by thinning it supported by porous separator. After adding 5 wt.% FEC, the lifetimes of the cells fabricated with thin separator supported polymer electrolyte exceeded 730, 490 and 220 hours at different current densities of 0.5, 0.6 and 1.0 mA/cm2, respectively.
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