We studied the contribution of internal energy change to cathode voltage assuming that the cathode material was completely ionic and only the Coulomb potential was effective. From the internal energy change caused by one Lif and one electron insertion, we derived a cathode voltage equation related to the Coulomb potential, which we calculated using a personal computer. We calculated the cathode voltage of an ideal structure LixMO2 (R (3) over bar m) (Li-O: 2A, Ni-O: 2A), which is a model compound of LixNiO2. From this calculation, we found that LixMO2 has a two-phase co-existence for 0 < x < 0.25, and one phase for 0.25 < x < 1.