Decomposition reaction of LiPF6-based electrolytes for lithium ion cells

The decomposition of LiPF6 electrolytes with water was studied by measuring the water content after storage. The solvents used were ethylene carbonate (EC) + diethyl carbonate (DEC), EC + dimethyl carbonate (DMC), propylene carbonate (PC) + DEC, and PC + DMC mixed by the volume ratio 1 to 1, respectively. The experimental results were in good agreement with −d[H2O]/dt = k[H2O]2[LiPF6]. We obtained rate constants (k) from this empirical equation, with k increasing in the order of EC + DMC < EC + DEC < PC + DMC < PC + DEC. This order is in inverse proportion to the order of the dielectric constants of these solvents. Nonionized LiPF6 dissociates to PF5 and LiF in organic solvents, and PF5 reacts with water. A solvent with a high dielectric constant increases the ionization of LiPF6, thereby suppressing the reaction with water.