Investigation of Polymers Used in Lithium Oxygen Batteries as Electrolyte and Cathode Materials A thesis presented for the degree of Master by Research By Jinqiang Zhang, B. Sc. University of Technology, Sydney 2013
Certificate of original authorship I certify that the work in this thesis has not previously been submitted for a degree nor has it been submitted as part of requirements for a degree except as fully acknowledged within the text. I also certify that the thesis has been written by me. Any help that I have received in my research work and the preparation of the thesis itself has been acknowledged. In addition, I certify that all information sources and literature used are indicated in the thesis. Jinqiang Zhang May 2013 i
Acknowledgements Although it s only been one and half year since I started my Master research, I have received many help from many people who supported me finishing this Master degree project. First of all, I would like to express my sincere gratitude to my supervisor, Prof Guoxiu Wang, for the guidance, support, and encouragement. I cannot thank you enough for all the advice that leads to the improvement of me, all the patience when I made a mistake, and all the concern about my research and life. I m really grateful to have you as my teacher and supervisor. I wish to thank all my colleagues in the research team of Centre for Clean Energy Technology, Dr Hao Liu, Dr Xiaodan Huang, Dr Bei Wang, Dr Bing Sun, Dr Ying Wang, Mr Dawei Su, Mr Kefei Li, Mr Anjon Kumar Mondal, Mr Shuangqiang Chen, Mr Yiying Wei, and Mr Xiuqiang Xie for the help both in my research and life during my Master period. Special thanks would address to Dr Yueping (Jane) Yao, for the administrative assistance and lab management as well as the great support for our life. It is a great pleasure to work with all of you and I wish you all the best luck. I wish to thank Rochelle Seneviratne for the assistance and patience during my study. I am also grateful for the training and support in Faculty of Science from Dr Ronald Shimmon, Dr Linda Xiao, and all the MAU staff. The help from all the teachers and professors from school of chemistry and forensic science are much appreciated. Financial support provided by the Australian Research Council (ARC) through the ARC Linkage project (LP0989134), and ARC Discovery Project (DP1093855) is gratefully acknowledged. ii
Finally I would like to thank my family, my parents and my brother, for your help and support for me. You always have faith in me even when I was confused about my life and future in the toughest time. It is your loves that make me the person I am and allow me to chase my dreams. Thank you and I love you all. iii
Table of Contents Certificate of original authorship... i Acknowledgements... ii Table of Contents... iv List of Figures... viii List of Tables... xiii Abstract... xiv Introduction... 1 Chapter 1 Literature Review... 4 1.1 Li-O 2 batteries... 4 1.1.1 Anode... 7 1.1.2 Electrolyte... 8 1.1.3 Cathode... 16 1.1.4 Catalyst... 20 1.2 Polymer electrolyte... 25 1.2.1 Solid polymer electrolyte... 26 1.2.2 Gel polymer electrolyte... 29 1.3 Conducting polymer... 33 1.3.1 Synthesis method... 36 1.3.2 Application... 40 1.4 Summary... 46 iv
Chapter 2 Experimental Methods... 47 2.1 Overview... 47 2.2 Materials and chemicals... 48 2.3 Material preparation... 49 2.3.1 In situ oxidation... 50 2.3.2 Solution casting method... 50 2.4 Material characterization... 51 2.4.1 X-ray Diffraction (XRD)... 51 2.4.2 Scanning electron microscope (SEM)... 53 2.4.3 Fourier transform infrared spectroscopy (FT-IR)... 53 2.4.4 Thermogravimetric Analysis (TGA)... 54 2.5 Electrode preparation and cell assembly... 55 2.5.1 Electrode preparation... 55 2.5.2 Cell assembly... 55 2.6 Electrochemical characterization... 56 2.6.1 Cyclic Voltammetry (CV)... 56 2.6.2 Electrochemical Impedance Spectroscopy (EIS)... 57 2.6.3 Linear Sweep Voltammetry (LSV)... 58 2.6.4 Galvanostatic Charge and Discharge... 59 Chapter 3 Low Molecular Weight Polyethylene Glycol Based Gel Polymer Electrolyte Used in Li-O 2 Batteries... 61 3.1 Introduction... 61 v
3.2 Experiment... 62 3.2.1 Preparation of PEG based GPEs... 62 3.2.2 Material characterization... 63 3.2.3 Electrochemical testing... 63 3.3 Results and discussion... 64 3.4 Summary... 78 Chapter 4 Investigation of PVDF-HFP Based Gel Polymer Electrolyte Used in Li-O 2 Batteries... 80 4.1 Introduction... 80 4.2 Experiment... 81 4.2.1 Preparation of PVDF-HFP based GPEs... 81 4.2.2 Material characterization... 82 4.2.3 Electrochemical testing... 82 4.3 Results and discussion... 83 4.4 Summary... 93 Chapter 5 Conducting Polymer-Doped Polypyrrole as An Effective Cathode Catalyst for Li-O 2 Batteries... 94 5.1 Introduction... 94 5.2 Experiment... 95 5.2.1 Synthesis of materials... 95 5.2.2 Characterization of samples... 96 5.2.3 Electrochemical measurements... 96 vi
5.3 Results and discussion... 97 5.4 Summary... 106 Chapter 6 Conducting Polymer Coated CNT Used in Li-O 2 Batteries with Enhanced Electrochemical Performance... 107 6.1 Introduction... 107 6.2 Experiment... 109 6.2.1 Synthesis of materials... 109 6.2.2 Characterization of samples... 110 6.2.3 Electrochemical measurements... 110 6.3 Results and discussion... 111 6.4 Summary... 116 Chapter 7 Conclusions... 118 7.1 General conclusion... 118 7.2 Outlook and future work... 120 References... 122 vii
List of Figures Figure 1-1 The gravimetric energy density of commonly used rechargeable batteries... 5 Figure 1-2 Schematic mechanism of Li-O 2 batteries during discharge and charge process.... 6 Figure 1-3 Different types of Li-O 2 batteries based on different architectures... 9 Figure 1-4 Two models of reaction mechanisms of Li-O 2 batteries, (A) aqueous system and (B) non-aqueous system... 10 Figure 1-5 Three different types of electrolyte filling on cathodes, (A) flooding, (B) dry and (C) wetting... 11 Figure 1-6 Schematic mechanism of decomposition of PC electrolyte in Li-O 2 batteries... 12 Figure 1-7 Cycle performance of Li-O 2 batteries with TEGDME as electrolytes... 13 Figure 1-8 Schematic mechanism of discharge process on porous carbon cathodes... 17 Figure 1-9 The morphology study, discharge performance and discharge mechanism of a hierarchical graphene... 18 Figure 1-10 Schematic mechanism of (a) side reactions of carbon cathode and discharge products and (b) side reactions between electrolyte and carbon cathode... 19 Figure 1-11 Discharge/charge profiles (left) and cycle performance (right) of nano gold cathode in DMSO based electrolyte... 20 Figure 1-12 Discharge/charge profile (left) and cycle performance (right) of graphene cathode and carbon black cathode... 21 Figure 1-13 First galvanostatic charge of Li 2 O 2 oxidation for various Li O 2 cells... 23 Figure 1-14 Schematic mechanism of Li 2 O 2 and Li 2 O forming on MnO 2 catalyst... 24 Figure 1-15 Schematic mechanism of Li + movement through PEO based polymer electrolyte... 27 viii
Figure 1-16 Schematic mechanism of the addition of ceramic fillers and the effect of different particle sizes, (a) macro-size and (b) nano-size... 29 Figure 1-17 Schematic presentation for functional role of PDMITFSI ionic liquid on lithium deposition, (a) without and (b) with ionic liquid... 32 Figure 1-18 The structures of the most commonly used conducting polymers... 33 Figure 1-19 Conjugated orbitals formed in polyacetylene... 34 Figure 1-20 Schematic illustration of synthesis mechanism of PPy... 37 Figure 1-21 Schematic illustration of synthesis mechanism of (A) PPy nanotube and (B) PANI nanowire... 40 Figure 1-22 Cycling performance of PPy/FC at (a) constant current density of 50 mag -1 and (b) different current densities... 42 Figure 1-23 Discharge/charge profiles (left) and resistance (right) of the LiFePO 4 cathode (a) coated with PEDOT, (b) coated with PPy, (c) coated with C, and (d) pristine particles... 43 Figure 1-24 a) Electron-transfer pathway for LiFePO 4 particles partially coated with carbon. b) Designed ideal structure for LiFePO 4 particles with typical nano-size and a complete carbon coating. c) Preparation process for the LiFePO 4 /carbon composite including an in situ polymerization reaction and two typical restriction processes... 44 Figure 1-25 (A) Morphology and cycle performance of PPy cathode [70], (B) nitrogendoped graphene derived from PANI and (C) Performance of PEDOT catalyst... 45 Figure 2-1 Schematic illustration of the whole experiment process... 47 Figure 2-2 The preparation process of PPy when (NH 4 ) 2 S 2 O 8 was used as oxidant... 50 Figure 2-3 Schematic drawing of Bragg s law... 52 Figure 2-4 An example TGA result of polypyrrole coated silicon... 54 Figure 2-5 The structure of a Li-O 2 battery... 56 ix
Figure 2-6 A typical ESI Nyquist curve of a battery system... 58 Figure 2-7 A typical result of LSV measurement... 59 Figure 2-8 An example charge and discharge curve of a Li-O 2 battery... 60 Figure 3-1 The typical molecular structure of PEG or PEO... 61 Figure 3-2 Cyclic voltammetry results of Li/GPE/Li type cells with (a) PEG and (b) PEG with SiO 2 addition... 65 Figure 3-3 Linear sweep voltammetry results of (a) Li/GPE/SS and (b) Li/GPE/CB-air type cells... 67 Figure 3-4 The impedance spectra of PEG at different temperatures. (b) The calculated ionic conductivity of PEG at different temperatures... 69 Figure 3-5 (a) First discharge and charge profiles of Li-O 2 batteries with PEG, PEG- SiO 2, TEGDME as electrolytes. (b) Partial enlarged view of first discharge and charge profiles from 0-1500 mahg -1... 71 Figure 3-6 Discharge and charge profiles of Li-O 2 batteries with (a) PEG, (b) PEG- SiO 2, and (c) TEGDME as electrolytes at fixed capacity to 500 mahg -1... 72 Figure 3-7 Cycle profiles of Li-O 2 batteries with PEG, PEG-SiO 2, and TEGDME as electrolytes... 74 Figure 3-8 Structures of (a) PEG-based electrolyte and (b) PEG-SiO 2 -based electrolyte... 76 Figure 3-9 XRD pattern of PEG before and after made into polymer electrolyte... 77 Figure 3-10 XRD pattern of cathode after discharge in PEG polymer electrolyte... 78 Figure 4-1 The typical structure of PVDF-HFP... 80 Figure 4-2 The cyclic voltammetry curve of Li/GPE/Li typed cell with TEGDME based GPE as electrolyte... 84 x
Figure 4-3 Linear sweep voltammetry results of (a) Li/GPE/SS and (b) Li/GPE/CB-air type cells... 85 Figure 4-4 The calculated ionic conductivity of PEG at different temperatures... 87 Figure 4-5 The discharge and charge profiles in the first cycle of Li-O 2 batteries using different electrolyte... 88 Figure 4-6 Discharge and charge profiles of Li-O 2 batteries with TEGDME based GPE as electrolytes at fixed capacity to 500 mahg -1... 89 Figure 4-7 Cycle profiles of Li-O 2 batteries with PVDF-HFP based GPE and TEGDME as electrolytes... 90 Figure 4-8 Proposed structure of PVDF-HFP based GPE... 91 Figure 4-9 The discharge and charge profiles in the first cycle of PC (a) and DMSO (c) based GPEs and the cycling performance of PC (b) and DMSO (d) based GPEs... 92 Figure 5-1 The typical structure of PPy... 94 Figure 5-2 SEM images of the as-prepared (a) PPy-Cl and (b) PPy-ClO 4, and (c) FT-IR spectra of both PPy polymers... 97 Figure 5-3 The discharge-charge profiles and (b) cycling performance of PPy-Cl, PPy- ClO 4 and carbon black electrodes... 99 Figure 5-4 The mechanism of (a) oxygen activation of PPy and (b) doping-undoping process of PPy-Cl and PPy-ClO 4... 101 Figure 5-5 FT-IR spectra of (a) PPy-Cl, (b) PPy-ClO 4, and (c) carbon black electrodes before discharge, after discharge and after charge process... 101 Figure 5-6 (a) The charge-discharge profiles and (b) the cycling performance of carbon black electrodes with and without LiCl additive... 104 Figure 5-7 Schematic mechanism of discharge process on cathode with LiCl addition... 105 xi
Figure 6-1 The typical structure of PEDOT... 107 Figure 6-2 The SEM images of (a) the bare CNT, the as-prepared (b) PPy/CNT 1:2, (c) PPy/CNT 1:1, (d) PPy/CNT 2:1, (e) PEDOT/CNT 1:1, and (f) FT-IR spectra... 112 Figure 6-3 The TGA spectra of as-prepared (a) PPy/CNT 1:2, (b)ppy/cnt 1:1, (c) PPy/CNT 2:1, and (d) PEDOT/CNT 1:1... 113 Figure 6-4 (a) The discharge and charge profiles of as-prepared PPy/CNT 1:2, PPy/CNT 1:1, PPy/CNT 2:1, PEDOT/CNT 1:1, and CNT electrodes. (b) Partially enlarged profiles of as-prepared electrodes with capacity of 500 mah g -1... 114 Figure 6-5 The cycling performance of as-prepared prepared PPy/CNT 1:2, PPy/CNT 1:1, PPy/CNT 2:1, PEDOT/CNT 1:1, and CNT electrodes... 115 Figure 6-6 6 The schematic mechanism of (a) PPy/CNT during cycling, and (b) the block of O 2 from PEDOT structure... 116 xii
List of Tables Table 2-1 Materials and chemicals used in the research project... 49 Table 5-1 EDS results of PPy-Cl and PPy-ClO 4 electrodes after cycling... 103 xiii
Abstract It has been well established that the electrolytes and cathodes have a significant effect on the electrochemical performance of lithium oxygen batteries. In this Master project, polymers were employed as electrolyte and cathode materials due to their unique superior properties. Using different methods, we synthesized suitable gel polymer electrolytes and conducting polymer catalysts for lithium oxygen batteries. Techniques such as field emission gun scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy were used to characterize the physical properties. Electrochemical analyses including the galvanostatic discharge and charge method, the cyclic voltammetry, the linear sweep voltammetry and the impedance spectra were conducted to determine the electrochemical performance for the as-prepared materials. Gel polymer electrolytes based on low molecular weight polyethylene glycol were prepared and used as electrolyte in lithium oxygen batteries. The as-prepared polymer electrolytes showed improved stability compared with liquid electrolytes and exhibited good performance in lithium oxygen batteries. Additionally, the addition of ceramic filler SiO 2 was found to reduce the stability of polymer electrolyte towards oxygen reduction reaction although higher ionic conductivity was obtained. Polyethylene glycol based gel polymer electrolyte without SiO 2 addition exhibited excellent cycling performance and it could be used for achieving long-life lithium oxygen batteries. Poly(vinylidene fluoride-co-hexafluoropropylene) based gel polymer electrolytes were prepared by solvent casting and employed as electrolytes in lithium oxygen batteries. The stability of the gelled electrolyte with tetraethylene glycol dimethyl ether has been greatly increased than the liquid one. The as-prepared polymer electrolyte was demonstrated excellent cycling performances. This thesis also investigated the effect of xiv
different plasticizers on the performance of lithium oxygen batteries. The reason could lie on the interactions among the components when the gelled structure was set. The tetraethylene glycol dimethyl ether based gel polymer electrolyte showed the best electrochemical performance and can be used for long-life lithium oxygen batteries. Polypyrrole conducting polymers with different dopants have been synthesized and applied as the cathode catalysts in lithium oxygen batteries. Polypyrrole polymers exhibited an effective catalytic activity for oxygen reduction in lithium oxygen batteries. It was discovered that dopant significantly influenced the electrochemical performance of polypyrrole. The polypyrrole doped with Cl - demonstrated higher capactity and more stable cyclability than that doped with ClO - 4. Polypyrrole conducting polymers also exhibited higher capacity and better cycling performance than that of carbon catalyst. Conducting polymer coated carbon nanotubes were synthesized and used as catalysts in lithium oxygen batteries. It was found that both polypyrrole and poly(3,4- ethylenedioxythiophene) coated carbon nanotubes could provide high cycling performance while polypyrrole based one exhibited higher capacities. The ratio of conducting polymer coating also affected the electrochemical performance of lithium oxygen batteries. The conducting polymer coated carbon nanotubes also showed better performance than the bare carbon nanotubes. xv