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.