As the cathode materials in conversion-type lithium ion batteries, transition-metal chlorides are well-known to easily dissolve in many organic solvents. However, the LiPF 6 /methyl difluoroacetate (MFA; CHF 2 COOCH 3 ) electrolyte is effective in suppressing the dissolution of the CuCl 2 electrode, as reported for a Li/CuCl 2 battery. This effect did not arise in the MFA solvent without LiPF 6 , but did on slightly dissolving LiPF 6 in the MFA solvent. This was because a very small amount of film was formed on the surface of the CuCl 2 electrode due to the addition of the small amount of LiPF 6 , and the film was composed of copper fluoride hydroxide, partial release of Cl from CuCl 2 , and CuCl.