Dissolution of Mn ions and capacity loss of spinel-type Li-Mn oxide cathodes in LiPF6-containing electrolytic solutions at 60 degrees C were investigated. The amounts of dissolved Mn decreased with the increase in the Li/Mn molar ratios in Li-Mn oxides. The dissolution reaction of Mn ions was enhanced on charged cathodes, resulting in a cathode capacity loss and a contraction of the lattice parameter. An increase of the polarization voltage was considered to cause a capacity loss on the charged cathodes during the course of immersion at 60 degrees C, as well as the dissolution of Mn ions.