Kinetics and Heterogeneity Changes in Single-Crystal Ultrahigh-Ni Cathodes Accompanying Phase Transition Progression

Abstract

With the increasing demand for high-energy-density lithium-ion batteries (LIBs), ultrahigh-nickel (Ni > 90%) layered oxide cathodes have attracted significant research interest. The transition from polycrystalline to single-crystalline particle architectures has been widely adopted to suppress intergranular cracking and enhance structural integrity. However, this structural evolution also introduces new challenges, including kinetically limited lithium transport arising from extended diffusion lengths, which can promote non-uniform electrochemical reactions. Despite growing recognition of reaction heterogeneity as an important factor influencing electrochemical behavior, a comprehensive understanding of how heterogeneity emerges and evolves in relation to the sequential phase transitions (H1 → M1 → M2 → H2 → H3) during charge remains elusive. In this study, the origin and evolution of reaction heterogeneity in single-crystal ultrahigh-Ni layered cathodes are systematically investigated from a phase-transition-resolved perspective. By integrating galvanostatic intermittent titration technique (GITT), operando two-dimensional X-ray absorption near- edge structure transmission X-ray microscopy (2D XANES TXM), and Transmission electron microscopy (TEM), heterogeneity is tracked across multiple length scales throughout the charging process. The results reveal that heterogeneity evolves in a strongly phase-dependent manner rather than increasing monotonically with state of charge. Distinct forms of spatial non-uniformity emerge depending on the phase transition region, reflecting changes in the balance between surface reaction kinetics and internal lithium diffusion. Atomic-scale structural analysis further demonstrates that these chemical heterogeneities are closely associated with phase-dependent lattice distortion, symmetry changes, and the formation of internal phase boundaries, particularly in kinetically constrained transition regions. Taken together, this study redefines reaction heterogeneity in ultrahigh-Ni layered cathodes not as a passive consequence of degradation but as a dynamic phenomenon driven by phase-specific kinetic limitations. By establishing a phase-informed analytical framework that links crystallographic phase evolution, lithium transport kinetics, and heterogeneity development across multiple length scales, this work provides a mechanistic basis for understanding reaction non-uniformity and offers new insights toward phase-informed strategies for controlling electrochemical behavior in next-generation high- energy-density LIBs.