Observing Nucleation and Crystallization of Rocksalt LiF from Molten State through Molecular Dynamics Simulations with Refined Machine-Learned Force Field

Abstract

Lithium fluoride (LiF) is a critical component for stabilizing lithium metal anode and high-voltage cathodes towards the next-generation high-energy-density lithium batteries. Recent modeling study reported the formation of wurtzite LiF below about 550 K (J. Am. Chem. Soc. 2023, 145, 1327-1333), in contrast to experimental observation of rocksalt LiF under ambient conditions. To address this discrepancy, we employ molecular dynamics (MD) simulations with a refined machine-learned force field (MLFF), and demonstrate the nucleation and crystallization of rocksalt LiF from the molten phase at temperatures below about 800 K. The rocksalt phase remains stable in LiF nanoparticles. Complementary density functional theory (DFT) calculations show that dispersion interactions are essential for correctly predicting the thermodynamic stability of rocksalt LiF over the wurtzite phase on top of the commonly used PBE functional. Furthermore, we show that inclusion of virial stresses--alongside energies and forces--in the training of MLFFs is crucial for capturing phase nucleation and crystallization of rocksalt LiF under the isothermal-isobaric ensemble. These findings underscore the critical role of dispersion interactions in atomistic simulations of battery materials, where such effects are often non-negligible, and highlight the necessity of incorporating virial stresses during the training of MLFF to enable accurate modeling of solid-state systems.