Computational determination of the solvation structure of LiBF4 and LiPF6 salts in battery electrolytes

Abstract

Li-ion battery technology has come a long way since its initial development a few decades ago. However, expectations of energy storage continue to grow, making the optimization of battery technology vital. Along with improving the electrochemical performance of electrodes, investigation of the properties of the electrolyte has thus become necessary. In this work, we use first principles computational modeling to investigate the structural and physical interactions of a common solvent solution, ethylene carbonate:dimethyl carbonate, with two salts, LiPF6 and LiBF4. We look at how the solvation shells form and evolve using ab initio molecular dynamics (AIMD) simulations. The solvation structures extracted from the calculated trajectories are used to investigate the energetics of removal of individual solvent molecules using density functional theory (DFT). We also investigate the order in which the solvent molecules form a shell around the salts. LiBF4 and LiPF6 have similar solvation structures and interaction energies with the solvent, with a preference for the carbonyl oxygens and the ether oxygens to interact with the Li+ cations, and the methyl or methylene groups to interact with the BF4− or PF6− anions. Our calculations also find that the solvation shell formed around LiPF6 can desolvate more easily compared to LiBF4, suggesting easier kinetics/diffusion.