Phase Transformation Accompanied by Evolution of Internal Stress and the Coupling Mechanism of Chemical-Mechanical Degradation in Single-Crystal NiRich Cathodes.

Abstract

Operating LiNi_xCo_yMn_1-x-yO₂ (NCM, x ≥ 0.92) cathodes at high temperature/voltages (≥4.3 V or 45°C) to achieve high capacity inevitably leads to accelerated capacity fade. Despite extensive research into cycling behaviour under various cut-off voltage and phase degradations, the fundamental mechanisms governing internal phase transformations, lattice deformations, and internal stress generation remain poorly understood. By using HAADF-STEM characterization with DFT and MD simulations, we disclose a new chemo-mechanical degradation rule: lattice bending leads to the formation of O1/LiNi₂O₄ (Fd-3m) and unstable intermediate transition phase Ni₃O₄ (Cmmm), the bending and distortion of the lattice are the direct causes of internal stress. Unlike previous findings, both RS, Ni₃O₄/LiNi₂O₄ and O1 phases were detected in various crack regions. Stress concentration from bending-induced O1-LiNi₂O₄ and LiNi₂O₄-Ni₃O₄-RS (Fm-3m) phase transformations leads to intracrystalline cracking, impairing capacity retention. Lattice deformation can lead to the emergence of stress and the formation of micro-cracks, even during the O3-O1 phase transition. This work confirmed the relationship between phase transformation and stress in the in cracked areas and stress. Meanwhile, this research provides new insights into the degradation mechanism for lithium-ion batteries, specifically paving the way for the design and optimization of high-energy-density.