High-voltage LiNi_0.5Mn_1.5O₄ cathode materials were synthesized using urea-based solution combustion synthesis combined with a calcination treatment. The morphology and particle size distribution of the products were considerably dependent on the amount of urea fuel. The electrochemical characterization illustrated that the sample that was produced with a fuel ratio of ϕ = 0.5 had a homogenous particle size distribution of approximately 8 μm, and showed the best cycling and rate performance. LiNi_0.5Mn_1.5O₄ with two different structures of disordered Fdm and ordered P4₃32 were obtained by controlling the calcination process. The samples, which were calcined at 800 °C with fast cooling, presented a disordered structure of Fdm, and the samples, which were calcined at 800 °C with slow cooling and reannealing at 600 °C, demonstrated an ordered structure of P4₃32. The sample with a disordered structure exhibited a better electrochemical performance than the sample with an ordered structure. The disordered sample produced at ϕ = 0.5 presented a discharge capacity of 130.73 mA h g⁻¹ and a capacity retention of 98.43% after 100 cycles at 1 C. Even at a higher current rate of 3 C, the sample still showed a high discharge capacity of 117.79 mA h g⁻¹ and a capacity retention efficiency of 97.63% after 300 cycles.