Investigations on the Fundamental Process of Cathode Electrolyte Interphase Formation and Evolution of High-Voltage Cathodes

Cathode electrolyte interphase (CEI) layer plays an essential role in determining electrochemical performances of Li-ion batteries (LIBs), but the detailed mechanisms of CEI formation and evolution are far from fully understood. With the pursuit of LIBs with increased energy density, the fundamental investigations on CEI are becoming even more important. Herein X-ray photoelectron spectroscopy (XPS) is employed to fingerprint CEI formation and evolution on three most prevailing high voltage cathodes, including layered Li1.144Ni0.136Co0.136Mn0.544O2 (LR-NCM), Li2Ru0.5Mn0.5O3 (LRMO), and spinel LiNi0.5Mn1.5O4 (LNMO). The impact of crystal structure, chemical constitution and cut-off voltage on CEI compositions is well clarified. Among these cathodes, spinel cathode demonstrates most stable CEI layer throughout battery cycling. While for layered oxide cathode, anionic redox reaction (ARR) at high charging voltage largely dominates CEI evolution, and 4d Ru further favors CEI formation upon contacting electrolyte. The distinct CEI evolution in diverse cathodes can be attributed to a series of entangled processes, including decomposition of electrolyte/Li salt, dissolution and re-deposition of CEI components. Based on these findings, rational guidelines are provided for interface design for high voltage LIBs.