We have studied in detail the electrochemical reaction of nickel in several kinds of molar ratio-controlled molten salts consisting of AlCl 3 and 1-butylpyridinium chloride (BPC) at 40 • C. We observed NiCl 2 as an oxidation product from nickel on the surface of the electrode in slightly acidic AlCl 3 /BPC salts with molar ratios of 1.05/1.0 and 1.1/1.0. However, in strongly acidic salt with the ratio of 1.5/1.0, NiCl 2 deposits on the electrode less than when in the above salts, and no NiCl 2 is observed in basic and neutral salts with the ratio of 1.0/1.0 or less AlCl 3 content. This suggests that [NiCl 4 Lithium-ion batteries are the most appealing power sources that operate at a higher voltage and achieve a higher energy density, compared with nickel metal-hydride batteries, and are now widely used in commercial hybrid electric vehicles.1,2 Lithium-ion batteries, however, are not competitive with gasoline engines to date because of their limited energy density. Therefore, intensive efforts have been done to develop new rechargeable batteries with far higher energy density than the present lithium-ion batteries.3-8 One effective approach to developing batteries with high energy density is to find a new type of cell systems that is charged and discharged accompanied by multiple-ions transport.Nickel compounds have been expected to be one of the cathode active materials for high energy batteries. Nickel oxyhydroxide, for example, has been used as a cathode active material for commercial nickel-cadmium and nickel-metal hydride batteries 9 and also investigated as cathode active materials for other high energy batteries such as zinc-nickel batteries.10,11 Nickel compounds are also expected to be cathode active materials for high energy batteries having nonaqueous electrolytes. Lithium nickel oxides and their derivatives have also been expected to be cathode active materials for high energy density lithium-ion batteries.

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