We synthesized LiNi0.5Mn1.5O4 cathode materials by using ammonia-mediated carbonate precipitation, and we studied how the ammonia concentration during synthesis (0.1–0.5mol/L) affected the structural, physicochemical, and electrochemical characteristics of the prepared materials. X-ray diffraction and Rietveld refinement reveal that the amount of Li/Ni antisite defects in the LiNi0.5Mn1.5O4 depend much on the ammonia concentration. The LiNi0.5Mn1.5O4 prepared at 0.3mol/L has the fewest Li/Ni antisite defects. This sample also delivers the best electrochemical performance, retaining a capacity of 133.5 mAh g−1 after 20 cycles at 0.1C. Electrochemical impedance spectroscopy confirms that the LiNi0.5Mn1.5O4 prepared at 0.3mol/L has lower charge transfer resistance, correlating with its better ion diffusion kinetics than the other materials.