Cryo-Sampling Enables Precise Evaluation of Thermal Stability of a Ni-Rich Layered Cathode

Thermal runaway of a Ni-rich LiNixCoyMn1–x–yO2 (NCM, x ≥ 0.8) cathode-based lithium ion battery (LIB) remains as a key safety concern for its utilization in power batteries, necessitating the precise evaluation protocol for the thermal stability of an NCM cathode. Herein, we reveal that conventional-sampling methods encounter a significant self-discharge issue during cell disassembly, artificially lowering the cathode’s state of charge (SOC) and leading to inaccuracy of thermal stability evaluation. To address this, we developed a cryo-sampling protocol to disassemble the coin cells by flash-freezing in liquid nitrogen. This technique immobilizes the electrolyte, effectively blocking ionic transport pathways and preserving the cathode’s intrinsic SOC by suppressing parasitic lithium redistribution. By coupling this method with multiscale characterization (in situ XRD, DSC-TG, and cross-sectional electron microscopy), we systematically elucidated the high-temperature failure mechanisms of single-crystal LiNi0.88Co0.05Mn0.07O2 (Ni88). Results show cryogenically disassembled cathodes exhibit ∼20% higher residual capacity retention and 15 °C earlier onset of oxygen release compared with conventionally processed samples, confirming that SOC preservation critically reshapes thermal degradation pathways. The cryo-disassembled electrodes further revealed accelerated phase transitions (layered-to-spinel/rock-salt) and anisotropic microcrack propagation at 190 °C, phenomena masked in conventional sampling. This work establishes a methodological framework for the reliable thermal stability assessment of Ni-rich cathodes while revealing SOC-dependent lattice oxygen release behavior and its correlation with microcrack formation and propagation, offering actionable strategies for battery safety enhancement.