Lithium-ion batteries store and release energy by embedding and removing lithium ions between transition metal oxides containing lithium and lithium-poor graphite materials. The application of graphite material in lithium-ion batteries depends on the solid electrolyte interface (SEI) film formed by the decomposition of electrolyte on the graphite surface. This protective film will reduce the stability of the electrolyte is much lower than the lithium embedding potential (0.01V) and the graphite electrode isolation, so as to ensure that under the lithium embedding potential electrolyte does not occur reduction decomposition, so that lithium ions in graphite materials reversible inlay.

How is the SEI membrane so important formed? Why is it that some electrolyte decomposition products can form stable SEI films, while some electrolytes continue to undergo reductive decomposition at a potential higher than that of lithium embedding, which eventually leads to the collapse of the graphite layer structure? Runze: The most typical solution to this interfacial behavior difference is the well-known "difference between Propylene carbonate (PC) and ethylene carbonate (EC)" in the history of lithium ion battery. PC has a continuous reduction decomposition at the lithium embedding potential (~0.7V), which eventually leads to the collapse of the graphite structure and the failure of normal lithium embedding. However, EC, whose molecular structure is only one methyl group less than PC, decomposes at a potential slightly higher than 0.7V to form a stable SEI film, which inhibits the decomposition of electrolyte at a lower potential and enables lithium ions to be normally embedded and released in graphite materials. Over the past two decades, a number of scientists have tried to explain the differences between PC and EC behavior, but no mechanistic model has been fully convincing. Zhuang et al. proposed, for example, that the difference between PC and EC is due to PC undergoing two-electron reduction on the electrode surface, directly generating Li2CO3 and propylene gases, the latter leading to structural destruction of the graphite layer. In contrast, EC undergoes one-electron reduction to form carbonate polymers. However, this mechanism could not explain the experimental results of Xu et al., which detected carbonate oligomers, a one-electron reduction product, in both PC and EC reduction reactions. Tasaki believed that this difference was mainly due to the fact that the volume of the co-embedded [Li(PC)n]+ structure formed by PC in the graphite layer was larger than the layer spacing of the graphite layer, thus splitting and destroying the graphite layer. However, the volume of co-inserts formed by EC system is smaller than the spacing between graphite layers, so it will not lead to the destruction of graphite layers. However, this mechanism cannot explain the experimental phenomenon that the interface behavior of solvent molecules with larger molecular volume than PC is similar to that of EC.