Abstract:A solid electrolyte
interphase (SEI) is generated on the anode of lithium ion batteries during the
first few charging cycles. The presence and stability of the SEI is critical to
the performance of the battery. 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 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
vinylene carbonate (VC) and fluoroethylene carbonate (FEC). While transition
metal catalyzed degradation of the anode SEI has been widely proposed as a
primary source of capacity loss for high voltage lithium ion batteries, we
propose a related acid induced degradation of the anode SEI. The role of
potential on the generation of soluble acidic fluorophosphates crossover
species and the impact of these species on the structure and stability of the
SEI will be presented.
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