Characterization of Chemical Degradation in Lithium-Ion Batteries Using Secondary Ion Mass Spectrometry (SIMS) and Hard X‑ray Photoelectron Spectroscopy (HAXPES).

LiNi x Mn y Co z O2 layered oxide cathodes are widely used in lithium-ion batteries, with LiNi1/3Mn1/3Co1/3O2 being the most commercially popular. Increasing the nickel ratio enhances the battery capacity and energy density but also quickens electrolyte degradation and capacity fade. This work presents a comparative analysis of the noncharged and single-charged states of NMC111, NMC532, and NMC811 cathodes using SIMS and HAXPES. The results reveal that electrolyte degradation initiates before applying a voltage bias and increases during the first charge, correlating with both increasing nickel content and charging process. Further comparison of NMC811 cathodes cycled in two different electrolytes (E1 and E2) across different electrochemical states reveals distinct cathode electrolyte interphase (CEI) evolution. In E1, the CEI layer reaches maximum thickness after a single charge and subsequently decreases upon discharge and at the end of life. In contrast, in the E2 electrolyte, the CEI layer continues to grow progressively throughout cycling. SIMS depth profiling at the discharged and end-of-life states shows a dual-layer CEI structure in both electrolytes with deeper penetration and denser accumulation observed in the E2 electrolyte system. Consistent with previous hypotheses, this study demonstrates that the CEI thickness is affected by both the nickel content and electrolyte reactivity.