LiNixMn2-xO4 has been synthesized using sol-gel and solid-state methods for 0 < x < 0.5. The electrochemical behavior of the samples was studied in Li/LiNixMn2-xO4 coin-type cells. When x = 0, the capacity of Li/LiMn2O4 cells appears at 4.1 V. As x increases, the capacity of the 4.1 V plateau decreases as 1-2x Li per formula unit, and a new plateau at 4.7 V appears. The capacity of the 4.7 V plateau increases as 2x Li per formula unit, so that the total capacity of the samples (both the 4.1 and 4.7 V plateaus) is constant. This is taken as evidence that the oxidation state of Ni in these samples is +2, and therefore they can be written as Li+Nix+2Mn1-2x+3Mn1+x+4O4-2. The 4.1 V plateau is related to the oxidation of Mn3+ to Mn4+ and the 4.7 V plateau to the oxidation of Ni2+ to Ni4+. The effect of synthesis temperature, atmosphere, and cooling rate on the structure and electrochemical properties of LiNi0.5Mn1.5O4 is also studied on samples made by the sol-gel method. LiNi0.5Mn1.5O4 samples made by heating gels at temperatures below 600 degrees C in air are generally oxygen deficient, leading to Mn oxidation states significantly less than 4. LiNi0.5Mn1.5O4 samples heated above 650 degrees C suffer due to disproportionation into LiNixMn2-xO4 with z < 0.5 and Li2Ni1-zO with z approximate to 0.2, which occurs above about 650 degrees C. Pure LiNi0.5Mn1.5O4 materials can be made by extended heatings near 600 degrees C or by slowly cooling materials heated at higher temperatures. LiNi0.5Mn1.5O4 made at 600 degrees C has demonstrated good reversible capacity at 4.7 V in excess of 100 mAh/g for tens of cycles.