Elucidating the Detrimental Effect of Intercalated Protons in Ni-rich NCMs on Structural Stability and Cycle Life

Abstract

Li-ion batteries are a crucial technology to decarbonize transportation and contribute to a greener future. To achieve, e.g., the necessary driving ranges of electric vehicles, high-energy materials such as layered lithium transition-metal oxides (NCMs) are commercially established cathode active materials. However, the processing of Ni-rich NCMs on an industrial scale is challenging since the washing after synthesis, improper storage, and/or aqueous slurry mixing introduces protons into the NCM structure, which partially replace the lithium ions. Within this study, we discover a novel, proton-driven degradation mechanism by investigating a state-of-the-art NCM material with controlled amounts of deliberately introduced protons. We find that intercalated protons decrease rate capability while, upon extended charge/discharge cycling, the protons deintercalate, form an oxygen-depleted, highly resistive layer at the surface of the NCM, and decompose the LiPF₆ conductive salt in the electrolyte. Overall, protons present in the NCM destabilize its structure, which then degrades during battery operation, limiting the lifetime of NCM-based battery cells drastically. Therefore, proper storage and handling of NCMs is crucial.