Elucidating the detrimental effect of intercalated protons in Ni-rich NCMs on structural stability and cycle life

Abstract

Ni-rich NCMs are widely used as commercial cathode active materials for high-energy lithium-ion batteries due to their high specific capacity. However, upon exposure to liquid or gaseous water or protons from other sources, these materials are prone to cation-exchange reactions, in which lithium ions in the near-surface region of the NCM particles are replaced by protons. To investigate the impact of the protonation on electrochemical performance, controlled amounts of protons were introduced deliberately into NCM831205 (LiNi_0.83Co_0.12Mn_0.05O₂) through a washing process. On-line electrochemical mass spectrometry (OEMS) revealed that the protons can be deintercalated at high voltages exceeding 4.5 V_Li, while X-ray photoelectron spectroscopy (XPS) showed their gradual removal during extended cycling even at low upper cutoff potentials. As protons are removed –at least in part– as water, an oxygen-depleted surface layer forms. As confirmed by electrochemical impedance spectroscopy (EIS), this degraded surface gives rise to a detrimental charge-transfer resistance, thereby compromising both rate capability and long-term capacity retention. This newly identified, proton-induced degradation pathway underscores the importance of optimized dry processing and storage conditions to mitigate performance losses and extend the cycle life of layered-oxide-based lithium-ion batteries.