A comprehensive understanding of the structural evolution and capacity contribution of fast-charging NCM cathodes

Achieving extreme fast charging (XFC, ∼6 C) capability remains a challenge for Li ion batteries in electric vehicle applications. This work employs time-resolved X-ray diffraction (XRD) to investigate the structural evolution and capacity contributions of a series of LiNixCoyMnzO2 (x + y + z = 1, NCM) cathodes under XFC conditions. All NCM cathodes (NCM-92, NCM-83, and NCM-622) deliver ∼60 % of their capacities with less than 2 % unit cell volume expansion during the H1-H2 phase transition, but the subsequent H2-H3 phase transition exhibits significant compositional and rate dependence. The NCM-92 cathode shows a maximum d-spacing shrinkage of −5.3 % at 6 C, which is larger than that of NCM-83 (−4.1 %) and NCM-622 (−0.05 %). Furthermore, NCM-92 follows a “phase heterogeneity” pathway for its structural evolution above 4.2 V, distinct from the “solid-solution” pathway observed in NCM-83 and NCM-622. This phase heterogeneity is evidenced by the splitting of the (0 0 3) diffraction peak and a decrease in intensity during the H2-H3 phase transition, indicating the formation of lithium-rich/depleted domains. These findings establish a direct correlation between cathode composition, structural dynamics, and XFC performance, highlighting a critical trade-off between structural stability and fast-charging capability in nickel-rich layered oxides.