Thermal Runaway Mechanism in Ni-Rich Cathode Full Cells of Lithium-Ion Batteries— The Role of Multidirectional Crosstalk

This study investigates the thermal runaway (TR) mechanism in Ni-rich cathode lithium-ion batteries (LIBs), focusing on the role of multidirectional crosstalk between electrodes. Ni-rich layered oxide cathodes and graphite anodes are widely used in electric vehicles (EVs) due to their high energy density, but safety concerns related to TR remain significant. The severity of TR, characterized by rapid temperature escalation and catastrophic failure, makes it essential to understand and mitigate the underlying causes.

Our research addresses the complex multidirectional crosstalk between the cathode and anode, often overlooked in previous studies focused on single-directional oxygen (O₂) migration from the cathode. Using synchrotron-based high-temperature X-ray diffraction (HT-XRD), mass spectrometry (MS), and differential scanning calorimetry (DSC), we captured the evolving chemical species and phase transformations during TR. The results reveal that ethylene (C₂H₄) gas, generated from the anode through the decomposition of the solid electrolyte interphase (SEI), promotes O₂ evolution at the cathode. This O₂ migrates back to the anode, forming a self-amplifying loop that exacerbates TR. Additionally, carbon dioxide (CO₂), traditionally considered an inert gas, reacts with lithium at the anode to form lithium carbonate (Li₂CO₃), contributing to further exothermic reactions.

DSC analysis showed unique exothermic peaks in the full cell, not present in cathode-only or anode-only configurations, highlighting the synergistic effects of crosstalk. These reactions were significantly more intense in the full-cell setup, suggesting an unrecognized mechanism that intensifies heat generation. We also observed distinct phase transitions in the cathode material, from a layered to a spinel structure and subsequently to an oxygen-deficient rock salt phase, which further promoted lattice oxygen release and intensified TR.

To mitigate the adverse effects of multidirectional crosstalk, we developed an anode coating strategy using a thermally stable Al₂O₃ coating. This coating reduced the formation of C₂H₄ and O₂, mitigating the self-amplifying loop and improving the thermal stability of the full cell. DSC results of the coated anode showed a significant reduction in heat generation above 230°C, indicating improved TR resistance. Postmortem analyses using scanning electron microscopy (SEM) and X-ray diffraction (XRD) confirmed the absence of Li₂O and Li₂CO₃ deposits on the coated anode surface.

This study provides new insights into TR mechanisms in Ni-rich LIBs, emphasizing the critical role of multidirectional crosstalk between electrodes. The application of an anode coating presents a practical solution for enhancing battery safety, paving the way for safer, high-energy LIBs suitable for EV applications.
Keywords: thermal runaway, lithium-ion batteries, Ni-rich cathode, multidirectional crosstalk, anode coating, battery safety, electric vehicles

Reference
- Advanced Materials, 2024, 36, 2402024