Insight into structural degradation of NCMs under extreme fast charging process

Lithium-ion batteries (LIBs) with extreme fast charging (XFC) capability are considered an effective way to alleviate range anxiety for electric vehicle (EV) buyers. Owing to the high ionic and electronic conductivity of LiNixCoyMnzO2 (x + y + z = 1, NCM) cathodes, the inevitable Li plating of graphite in NCM | graphite cell is usually identified as a key bottleneck for XFC LIBs. However, the capacity decay mechanism of cathode materials under XFC has not been fully investigated. In this work, three typical NCM cathode materials with different Ni fractions were chosen and their electrochemical performances under XFC associated with structural evolution were investigated. A faster capacity decay of NCMs under XFC conditions is observed, especially for Ni-rich NCMs. In-situ X-ray diffraction (XRD) reveals that the multiple c-axis parameters appear at the high-voltage regions in Ni-rich NCMs, which is probably triggered by the larger obstruction of Li (de)intercalation. Particularly, NCMs with moderate Ni fraction also present a similar trend under XFC conditions. This phenomenon is more detrimental to the structural and morphological stability, resulting in a faster capacity decay than that under low current charging. This work provides new insight into the degradation mechanism of NCMs under XFC conditions, which can promote the development of NCM cathode materials with XFC capability.