Direct Optical Visualization of Kinetic Homogenization in Al‑Doped LiCoO ₂ Cathodes

ABSTRACT

Phase‐transformation dynamics govern the performance and degradation of layered oxide cathodes but remain difficult to observe at the single‐particle level under operating conditions. Here, we introduce optical scattering microscopy for real‐time visualization of phase‐transition pathways in individual LiCoO ₂ (LCO) particles. By decoding optical signatures of the H1, H2, and H1–3 phases, we track phase‐boundary evolution during battery operation. Undoped LCO shows progressive kinetic destabilization under high‐voltage cycling: the H1→H2 transition evolves from symmetric shrinking‐core propagation to asymmetric motion, and interlayer kinetic asynchrony intensifies during the H3→H1–3 transition. In contrast, Al‐doped LCO exhibits highly homogeneous and reversible lithium (de)intercalation, suppressing phase separation and interlayer heterogeneity. Combined with in situ XRD, thermodynamic analysis, and phase‐field simulations, we demonstrate that Al doping smooths the free‐energy landscape and reduces the driving force for phase separation, shifting the reaction from abrupt two‐phase to solid‐solution‐like behavior. This kinetic homogenization promotes uniform structural evolution, alleviates stress accumulation, and enhances cycling stability. Our work resolves a long‐standing mechanistic question in high‐voltage LCO chemistry and establishes a generalizable optical methodology for probing phase transformations in energy materials.