The effect of cathode particle size on cell potential was investigated using a voltage prediction method from the Coulomb potential created by the atoms of a cathode active material. LiNiO2 was selected as a typical cathode of an ordered rock salt structure. From the calculation, it was found that the voltage changes when the size becomes less than about 10 nm. The voltage increased with decreasing in-plane size, and decreased with decreasing the number of layers. (c) 2005 Elsevier B.V. All rights reserved.