Nanoscale Thermal and Mechanical Responses of Lithium Fluoride in the Solid Electrolyte Interphase under Coupled Temperature and Pressure Conditions

Lithium fluoride (LiF)-rich solid electrolyte interphases (SEIs) are promising for improving the safety and durability of lithium-based batteries due to their mechanical strength, thermal stability, and chemical inertness. However, a fundamental understanding of the behaviors of LiF under coupled thermal and mechanical stress conditions, commonly encountered during battery cycling and abuse, remains unclear. In this study, we investigate the thermal-mechanical responses of LiF at the nanoscale using molecular dynamics simulations and phonon analysis. LiF exhibits excellent mechanical stability under uniaxial and torsional loading with its fracture threshold surpassing the typical stress levels encountered in operating batteries. In contrast, the thermal conductivity (k) of LiF is highly sensitive to the strain and temperature: tensile strain leads to a ∼50% reduction in k at 300 K, which is primarily attributed to phonon softening, increased anharmonicity, and suppressed group velocities; compressive strain enhances k by up to ∼300%, due to phonon hardening and improved phonon velocities; elevated temperatures also degrade k by increasing phonon scattering. These results reveal the strain-temperature coupling effects on the behaviors of LiF, where its ability to dissipate heat can be severely compromised under inhomogeneous strains or elevated temperatures. Such thermomechanical coupling effects may cause localized heat accumulation in LiF-rich SEIs, accelerating degradation under harsh conditions. Our findings provide atomic-level insights into the coupled and coevolving effects of thermal and mechanical stresses in SEI performance and emphasize the importance of optimizing both mechanical and thermal properties for safer battery interfaces.