The State of Understanding of the Lithium-Ion-battery Graphite Solid Electrolyte Interphase (SEI) and Its Relationship to Formation Cycling

An in-depth historical and current review is presented on the science of lithium-ion battery (LIB) solid electrolyte interphase (SEI) formation on the graphite anode, including structure, morphology, composition, electrochemistry, and formation mechanism. During initial LIB operation, the SEI layer forms on the graphite surfaces, the most common anode material. The SEI is essential to the long-term performance of LIBs, and it also has an impact on its initial capacity loss, self-discharge characteristics, rate capability, and safety. While the presence of the anode SEI is vital, it is difficult to control its formation and growth, as they depend on several factors. These factors include the type of graphite, electrolyte composition, electrochemical conditions, and temperature. Thus, SEI formation and electrochemical stability over long-term operation should be a primary topic of future investigation in the LIB development. This article covers the progression of knowledge regarding the SEI, from its discovery in 1979 to the current state of understanding, and covers differences in the chemical and structural makeup when cell materials and components are varied. It also discusses the relationship of the SEI layer to the LIB formation step, involving both electrolyte wetting and subsequent slow charge–discharge cycles to grow the SEI.