Generation and evolution of materials in the anode Solid Electrolyte Interphase (SEI) of lithium ion batteries

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 40 years, the SEI remains poorly understood.  Over the last decade additional investigations of the structure of the initial SEI formed on graphite, silicon and lithium metal 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 SEI formation reactions have been complemented by investigations utilizing the one electron reducing agent, lithium napthalenide, to independently prepare and investigated 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 and related lithium alkyl carbonates 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, and GC-MS.  Mechanisms for the generation of the complicated mixture of products will be discussed along with the differences in the SEI structure and function for different anodes including graphite, silicon and lithium metal.