New internal stress mechanisms of phase transformations caused by lattice distortion in single-crystal Ni-rich cathodes

Abstract Operating LiNixCoyMn1-x-yO2 (NCM, x ≥ 0.92) cathodes at high voltages (>4.3 V) to achieve high capacity inevitably leads to accelerated capacity fade. Despite extensive research into cycling behaviour under various voltage cut-off conditions and the resulting multimodal phase degradations, the fundamental mechanisms governing internal phase transformations, lattice deformations, and internal stress generation remain poorly understood. We hypothesize that crystal degradation manifesting as intracrystalline cracking is directly linked to severe lattice bending, accompanied by Li+ loss and transition metal dissolution, along with the formation of an SEI at different voltage cut-off levels. Severe lattice bending directly causes concentrated chemical–mechanical stress, where O3–O1, O1–LiNi2O4, and LiNi2O4–rock salt (RS) phase transformations form in bending structures. The O1 phase transition to LiNi2O4 spinel in curved bands induces rapid crystal structure changes due to stress concentration from lattice bending. Using HAADF–STEM with density functional theory (DFT) calculations and MD simulations, we found a new chemo-mechanical degradation rule: phase transformation via lattice bending produces both ordered and discontinuous disordered RS phases. The relationship between phase transformation in cracked areas and stress has been confirmed. Unlike previous findings, both RS and Ni3O4/LiNi2O4 phases were detected in various crack regions. Stress concentration from bending-induced O1–LiNi2O4 and LiNi2O4–RS phase transformations leads to intracrystalline cracking, impairing capacity retention. This research provides new insights into lithium-ion batteries, paving the way for achieving higher energy density and enhanced safety.