Storage-Environment-Dependent Surface Evolution and Electrochemical Degradation of High-Nickel NCM Single-Crystal Cathodes.

High-nickel ternary single-crystal cathodes are promising for high-energy-density batteries in low-altitude-economy applications owing to their high specific capacity. However, surface residual lithium formation during storage substantially impairs their electrochemical performance. Herein, we investigate the formation mechanisms of surface residual lithium and the associated storage-induced degradation of three single-crystal compositions-NCM613, NCM8866, and NCM9442-under vacuum, humid air, argon, pure CO2, and ultrapure water. Potentiometric titration, x-ray diffraction, x-ray photoelectron spectroscopy, electron microscopy, and electrochemical measurements were combined to elucidate the effects of storage environment and nickel content. The storage atmosphere plays a decisive role in residual-lithium formation. CO2 and humid air markedly promote Li2CO3 formation, while increasing nickel content leads to higher surface residual lithium levels and more pronounced structural degradation. The accumulation of residual lithium is accompanied by near-surface disorder and local layered-to-rock-salt-like reconstruction, together with changes in the surface oxygen and nickel chemical states. These alterations increase Li+ transport impedance and electrode polarization, causing substantial losses in specific capacity, rate capability, and cycling stability. In contrast, vacuum and inert-gas atmospheres effectively suppress surface degradation and preserve electrochemical performance. These findings provide useful guidance for optimizing the storage and handling protocols of high-nickel single-crystal cathodes.