How Surface Reconstruction Drives Heterogeneous Bulk Delithiation in Single-crystalline Ni-rich Layered Oxide Cathodes

Abstract

The electrochemical performance of single-crystalline Ni-rich layered oxide cathodes is fundamentally limited by bulk Li⁺ diffusion within micrometre-sized particles. During high-voltage cycling—necessary for high-energy applications—intraparticle Li⁺ diffusion is further impeded by oxygen-loss-induced surface reconstruction from the layered phase to spinel/rock-salt structures. Therefore, to fully understand how bulk Li⁺ transport kinetics influences electrochemical degradation, it is necessary to establish the correlation between surface reconstruction and bulk delithiation during the anisotropic structural evolution (i.e., expansion of the layers followed by their contraction) of the cathode particles during long-term cycling. In this work, we accomplish this using multi-rate operando X-ray diffraction studies of single-crystalline Ni-rich layered oxide cathodes aged under different voltage windows in single-layer pouch full cells. We quantify how increased surface reconstruction leads to greater heterogeneity in bulk delithiation, thereby promoting phase separation and exacerbating electrochemical capacity fade. These results provide a direct mechanistic link between surface degradation and bulk delithiation in such cathodes and offer a framework for non-destructively probing kinetics-dependent degradation under practically relevant conditions to guide strategies for improved cycling stability.