Electrostatic energy (Madelung energy) is a major constituent of the cohesive energy of ionic crystals. Several physicochemical properties of these materials depend on the response of their electrostatic energies to a variety of applied thermal, electrical, and mechanical stresses. In the present study, a method has been developed based on Ewald's technique to compute the electrostatic energy arising from ion-ion interactions in ionic crystals such as LixMn2O4 with variable stoichiometries and mixed valencies. An interesting application of this method in computing the voltages of lithium ion batteries employing spinel cathodes is presented for the first time. The effect of including additional interaction terms such as the short-range and the multipolar was estimated and was found to necessitate only a fairly small change in the battery voltage. The advantages of the present method of computation over existing methods are also discussed.