The formation process of the spinel phase by the melt-impregnation method was extensively investigated by the pore volume distribution measurement. Two kinds of the spinel structure compounds Li1+xMn2O4 optimized in this method were examined as a 4V cathode in a lithium nonaqueous cell. The first one has an initial charge capacity of 135-147 mAh/g and an unstable rechargeability with the characteristic two-step process, and another one delivers a slightly lower capacity of 105-120 mAh/g and ideal rechargeability with the quasi-one step process. These compounds preserve a capacity of more than 110 mAh/g for the first 100 cycles. The capacity fading on cycling of the former spinel only occurs at the second charge plateau in the rage of x<0.4 in LixMn2O4, and is due to the unstable two-phase structure for lithium intercalation coexisting in this region. The excellent rechargeability for the later spinel results from a homogeneous reaction occurring over the entire intercalated region.