We studied the contribution of internal energy and entropy change to cathode voltage assuming that the cathode material LixMn2O4 was completely ionic and only the Coulomb potential was effective. We calculated the cathode voltage of an ideal spinel structure LixMn2O4 (Fd3m) (Mn-O: 2 Angstrom, Li-O: 1.732 Angstrom, O-O: 2.828 Angstrom). From this calculation, we found that LixMn2O4 has a multiphase co-existence for 0 < x < 1 with a very high voltage (8.7 V versus Li), and has a negative voltage when 1 < x. If we suppose that inserted Lif and electron have 40% of the formal charge, LixMn2O4 has a voltage from 4.55 to 4.79 V for 0 < x < 1, and has a voltage of 1.75 V when 1 < x.