In an attempt to achieve a higher energy density of secondary lithium batteries, cathode materials with higher potentials are investigated. LiMn_2?xNi_xO₄ samples are synthesized by a solid?state reaction in a controlled atmosphere. While the energy density and cycle life have improved with higher Ni contents in the phase, the electrical conductivity has decreased. In order to improve the rate capability of cells, surface modifications of the cathode material are carried out by a wet chemical method. Surface modified samples show improved rate capability and cyclability compared to the unmodified samples.