Decomposition of LiPF6 and stability of PF5 in Li-ion battery electrolytes -: Density functional theory and molecular dynamics studies

The decomposition of LiPF6 and the stability of PF5 in organic solvents, diethyl carbonate (DEC), dimethyl carbonate (DMC), gamma-butyrolactone (GBL), and ethylene carbonate (EC), have been investigated through density functional theory (DFT) calculations, in which solvent was modeled as a dielectric continuum, and also by molecular dynamics (MD) simulations which treated solvents explicitly. Both calculations showed a similar trend in which the decomposition was further promoted in more polar solvents, yet the DFT calculations predicted an endothermic decomposition, while the MD simulations indicated exothermic. This sharp contrast in the results suggests strong solute-solvent interactions, especially for PF5, which were not accounted for in the DFT calculations. The specific interaction between PF5 and solvent was further investigated by DFT calculations for adduct models and also by the MD simulations for solutions. Both calculations suggest a stable formation of a PF5-solvent adduct in solution and its stability depends on the solvent. It was found that PF5 is more stabilized in polar and sterically compact solvents such as EC and GBL than in less polar and bulky, linear carbonates such as DMC and DEC. The reactivity of PF5 with organic solvents and the difference in the stability of LiPF6 between organic and aqueous solution are also discussed. (C) 2003 The Electrochemical Society.