Cathode Electrochemistry in Nonaqueous Lithium Air Batteries
This chapter summarizes the authors’ results and opinions of the electrochemistry occurring at a principally C cathode during Li–O2 discharge and charge. Ideally this reaction is only 2(Li+ + e −) + O2 ↔ Li2O2 that involves 2e −/O2 consumed during discharge and 2e −/O2 liberated during charge. Using quantitative DEMS and other spectroscopies, however, we find significant other chemistry/electrochemistry occurring as parasitic processes in Li–O2 discharge/charge. Much of this is related to electrolyte stability issues (and is electrolyte specific), while some is related to C stability as a cathode material. Much of the work presented in this chapter is an attempt to isolate and study the ideal Li–O2 electrochemistry in order to answer a fundamental question. Even if there are no parasitic chemical processes or practical cell-dependent limitations, is the Li–O2 electrochemistry sufficient to build a high-energy and high-power battery? In this regard, we report a wide variety of experiments and theory on the mechanism, kinetic overpotentials, and charge transport through Li2O2. We then combine our understanding of these fundamental aspects of the electrochemistry with what we know about limitations (parasitic chemistry and cell limiting properties) to understand observed galvanostatic discharges and charges. We describe origins of the current dependent loss of potential in discharge, the cell sudden death or capacity limitations, and the potential rise during charging. At present, the fundamental Li–O2 electrochemistry appears very promising for ultimate use in a high-energy battery. However, both electrolyte stability and the poor electrical conductivity through the Li2O2 remain as challenges to developing a practical lithium air battery.
KeywordsOxygen Reduction Reaction Charge Transport Oxygen Evolution Reaction Charge Cycle Electrolyte Decomposition
The authors wish to thank their many collaborators, both at IBM and SLAC/Stanford, for their contributions to the work presented here. These include at IBM, Girish Gopalakrishnan, Angela Speidel (VW), Rouven Scheffler (VW), and Greg Wallraff and at SLAC/Stanford, Jens Hummelshøj and Jens Nørskov. In addition, we especially thank Winfried Wilcke for initiating the program in lithium air at IBM and for stimulating interest in this field throughout the world. V. V. also acknowledges support from the US Department of Energy, Basic Energy Sciences, through the SUNCAT Center for Interface Science and Catalysis and for a UTRC fellowship.
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