Solid-state synthesized LiNi0.5−y M y Mn1.5O4 spinels, where M=Fe, Mg, Al, or Cu, and y=0.0–0.4, have been studied as high-voltage cathode materials. Powder X-ray diffraction studies showed that all the substituents displayed a propensity for the 8a tetrahedral site at high concentrations. Cyclic voltammetric studies showed electrochemical activity around 4V as well as above 4.4V. While the 4-V activity was related solely to the Mn4+/Mn3+ couple, the 5-V activity was due to the redox reactions of Ni and the other transition metal ions. The co-substituents reduced the 5-V capacity and shifted the redox potentials in the 5-V region to higher values. At high concentrations, the co-substituents tended to occupy the 8a sites, which may lead to a blockage of lithium transport during the charge–discharge processes. LiNi0.4Fe0.1Mn1.5O4 registered the best performance with a first-cycle capacity of 117mAh/g and 78% capacity retention over 60 cycles. Electrochemical impedance spectroscopic studies showed a decrease in the charge transfer resistance at high deintercalation levels.