Decomposition reaction of LiPF 6-based electrolytes for lithium ion cells

Abstract

The decomposition of LiPF 6 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[H 2O]/d t = k[H 2O] 2[LiPF 6]. 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 LiPF 6 dissociates to PF 5 and LiF in organic solvents, and PF 5 reacts with water. A solvent with a high dielectric constant increases the ionization of LiPF 6, thereby suppressing the reaction with water.