A transient component of the solid electrolyte interphase in the Li-ion battery

The solid electrolyte interphase (SEI) is one of the critical components of Li-ion batteries, which largely determines their performance and safety. Despite being studied for almost three decades, it remains one of the least understood battery components, especially at the early stage of formation. In this work we provided microscopic evidence of formation of a transient layer inside graphite caused by cointercalation and reduction of solvated Li. Using Scotch-tape cleavage we directly accessed the intercalated species and traced their redox transformations at different stages of the formation cycling. The results show that solvated Li intercalates inside graphite in the surface vicinity at a potential above 1 V vs. Li + /Li and well before SEI formation. The reduction of the intercalated components proceeds simultaneously with the on-surface SEI formation and leads to graphite exfoliation and blistering. The internal reduction products are mostly inorganic and presumably disappear after oxidation, which constitute its transient nature. Schematic graphical representation of the SEI formation process accompanied by cointercalation of electrolyte components into graphite.