Persistent Inter-particle State-of-Charge Heterogeneity in Fast-Charged High-Loading NMC811 Electrodes Revealed by Ex Situ Raman Spectroscopy

Designing high-energy lithium-ion battery (LIB) cells based on LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes that combine high areal capacity, fast-charge capability, and long-term cycling stability remains challenging. Particle de-/lithiation and degradation do not proceed uniformly across composite electrodes or within NMC secondary particles, resulting in inter- and intra-particle state-of-charge (SOC) heterogeneity that can accelerate electrode degradation and limit rate capability. This work introduces an ex situ Raman-based approach to estimate the local SOC in NMC811 particles in ion-milled electrode cross-sections with sub-micron lateral resolution. The method is based on an empirical correlation between SOC and the wavenumber of the minimum/inflection point near 500 cm–1 in the Raman spectra. We investigated high-loading NMC811 electrodes at charge rates from 0.5 to 3C (with 0.1C used for calibration) and after 100 aging cycles at 0.5C. Our key finding is the charge-rate-dependent occurrence of a particle subgroup with Raman-derived SOC values near 0% after open-circuit relaxation. At charge rates ≤1C, the measured particles exhibit a continuous, unimodal SOC distribution across particles, whereas at ≥1.5C, a distinct near-0% SOC particle subgroup is still detectable after >1 h under open-circuit conditions, with Raman signatures consistent with a highly lithiated state. The absence of this near-0% SOC particle subgroup in aged electrodes cycled at 0.5C argues against degradation as the primary origin and indicates that the phenomenon is primarily rate-driven. Overall, this work reveals persistent heterogeneous active-material utilization in high-loading NMC811 electrodes and suggests that fast charging can kinetically isolate a subset of particles or particle regions even after open-circuit relaxation, which may contribute to electrode degradation and capacity fade.