LiNi0.5Mn1.5O4, a lithium-ion battery cathode material, is prepared using co-precipitation via a two-step drying method with Ni–Mn mixed hydroxide as the precursor. This study examines the effects of precursor pretreatment with hydrazine (a reductant) or with H2O2 (an oxidant) in solutions of NiSO4 and MnSO4. The results indicate substantial differences in the structure and electrochemical properties of LiNi0.5Mn1.5O4 depending on whether the precursor is pretreated with reductant or oxidant. For the hydrazine-treated precursor, the synthesized LiNi0.5Mn1.5O4 has a very pure spinel phase and an ordered, octahedral crystal morphology (ca. 100–300nm). In contrast, the material synthesized using the H2O2-treated precursor shows numerous impurity phases (Na0.7MnO2.05) with a layer-by-layer crystal structure. The control sample (prepared without precursor pretreatment) maintains an octahedral structure but still retains a few impurity phases of Na0.7MnO2.05. The electrochemical results show that LiNi0.5Mn1.5O4 synthesized using a hydrazine-treated precursor has a higher specific capacity (especially under high discharge current) and a higher cyclic life than the control sample, whereas the sample using H2O2-treated precursor shows almost no special capacity due to changes in crystal structure.