リチウム電池用LiPF$_6$電解液の分解反応

The decomposition of LiPF$_6$ electrolytes with water was studied by measuring the water content after storage. The solvents used are ethylene carbonate (EC) + diethyl carbonate (DEC), EC + dimethyl carbonate (DMC), propylene carbonate (PC) + DEC and PC + DMC. The experimental results were in good agreement with $-d[{\rm H}_2{\rm O}]/dt = k [{\rm H}_2{\rm O}]^2 [{\rm LiPF}_6]$. We obtained rate constant (k) from this empirical equation and rate constant (k) increased in the order of EC+DMC < EC+DEC < PC+DMC < PC+DEC. This order is inverse proportion to order of dielectric constant of these solvents. Non-ionized LiPF$_6$ dissociates to PF$_5$ and LiF in organic solvents and PF$_5$ reacts with water. High dielectric constant solvent increases the ionization of LiPF$_6$. Therefore, the reaction with water is suppressed in high dielectric constant solvent.