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 (LiNi0.83Co0.12Mn0.05O2) through a washing process. On-line electrochemical mass spectrometry (OEMS) revealed that the protons can be deintercalated at high voltages exceeding 4.5 VLi, 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.