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Polymer Electrolytes for High Current Density Lithium Stripping/Plating Test

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Polymer Electrolytes for High Current Density Lithium Stripping/Plating Test 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)
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