(Invited) Generation and Evolution of Materials in the Anode Solid Electrolyte Interphase (SEI) of Lithium Ion Batteries

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A solid electrolyte interphase (SEI) is generated on the anode of lithium ion batteries during the first few charging cycles. The SEI provides a passivation layer on the anode surface which inhibits further electrolyte decomposition and affords the long calendar life required for many applications. However, despite thorough investigation of the SEI for the last few decades, the SEI remains poorly understood. Over the last several years additional investigations of the structure of the initial SEI formed on graphite electrodes along with changes which occur to the SEI upon additional cycling have been conducted. The investigations provide significant new insight into the structure and evolution of the anode SEI. The characterization of SEI components via ex-situ surface analysis of cycled electrodes has been complemented by investigations utilizing the one-electron reducing agent, lithium naphthalanide, to independently prepare and investigate the reduction products which constitute the SEI. The initial reduction products of ethylene carbonate (EC) are lithium ethylene dicarbonate (LEDC) and ethylene. However, the instability of LEDC generates an intricate mixture of compounds which greatly complicates the composition of the SEI. The reduction products and their subsequent decomposition products have been thoroughly investigated via a combination of NMR, XPS, IR-ATR, TGA, GCMS, and OEMS. Mechanisms for the generation of the complicated mixture of products are presented along with the differences in the SEI structure and function in the presence of electrolyte additives and the effect of crossover reactions related to electrolyte oxidation on the cathode will be discussed.