A high voltage cathode material, LiNi0.5Mn1.5O4, is synthesized with a two-step approach, in which the nickel–manganese oxalate precipitate is firstly obtained by adding oxalic acid to the solution of nickel and manganese ions precursors, followed by calcining the oxalates to obtain spinel nickel–manganese oxide, incorporating lithium ions with ball milling and calcining at 900 °C for 15 h. The materials are characterized with TG, XRD, SEM, BET and FTIR; it is revealed that both nickel–manganese oxide and final LiNi0.5Mn1.5O4 have well defined spinel structure. The LiNi0.5Mn1.5O4 spinel materials exhibit high capacities and good cyclic stability, the capacity of the materials is in the range from 126 to 136 mAh −1, depending on the calcining temperatures. The sample calcined at an optimal temperature of 900 °C exhibits best performance, the capacity is high up to 136 mAh g−1 at tenth cycle and the capacity retention after 50 cycles is 93%. For the sample prepared by mixing and milling oxalate with lithium salt, the discharge capacity is only 115 mAh g−1. We suggest that the spinel oxide derived from oxalate may play an important role for the high performance and high stability of the final cathode materials.