The high voltage cathode material, LiMn1.6Ni0.4O4 was prepared by a polymer-assisted method. The novelty of this work is the substitution of Ni with Mn, which already exists in the crystal structure instead of other isovalent metal ion dopants which would result in capacity loss. The electrochemical performance testing including stability and rate capability were evaluated. The temperature was found to impose a change on the valence and structure of the cathode materials. Specifically, manganese tends to be reduced at a high temperature of 800 oC, and leads to structural changes. The substitution of nickel (Ni) with manganese has proved to be simultaneously beneficial for the improvement of rate capability and capacity retention ability. The cathode materials processed at 550 oC showed a stable performance with negligible capacity loss for 400 cycles.