Correlating charge heterogeneity and first-cycle irreversible capacity loss in lithium-ion batteries

Abstract

Establishing a direct correlation between nickel charge heterogeneity and first-cycle irreversible capacity (IRC) loss is essential for optimizing high-energy lithium-ion battery (LIB) cathodes. However, the interplay between charge distribution and IRC loss remains poorly understood. In this study, we reveal that spatial variations in nickel oxidation states are a key contributor of IRC loss by systematically investigating their evolution in NCM 333 and NCM 811 cathodes using diffraction, spectroscopy, and nanoscale microscopy techniques. Using LiNiO₂ (Ni³⁺) as a reference, first-derivative XANES analysis determined the pristine-state Ni oxidation states of NCM 333 and NCM 811 to be approximately +2.2 and +2.6, respectively. Nanoscale transmission X-ray microscopy (TXM-XANES) enabled direct quantitative visualization of Ni chemistry and charge distribution, revealing pronounced spatial Ni charge heterogeneity at different cycling states. Single-pixel XANES analysis further uncovered localized Ni chemistry variations, highlighting distinct redox behaviors between core and near-surface regions that strongly correlate with IRC loss. Notably, in low-Ni-content NCM 333, the Ni valence state remains uniformly distributed across all states of charge during cycling. In contrast, high-Ni-content NCM 811 exhibits significant Ni valence state inhomogeneity, disrupting charge balance and hindering complete capacity utilization, ultimately leading to IRC loss. By quantitatively linking Ni charge heterogeneity to irreversible lithium loss, this study provides a mechanistic foundation for mitigating IRC loss and optimizing Ni-rich cathode performance. These insights pave the way for designing next-generation cathodes with suppressed IRC loss, prolonged cycle life, and enhanced electrochemical stability, addressing critical challenges in high-performance LIB development.