High voltage spinel oxides with composition LiMn2− x M x O4 (M, a transition metal element) have remarkable properties such as high potential, high energy density and high rate capability. We believe that these positive electrode materials could replace the widespread commercial layered nickel cobalt oxides in some applications. The present assessment highlights electrochemical performance of optimized LiNi0.5Mn1.5O4 and substituted counterparts, all having a spinel structure (cubic close-packed oxygen array) similar to the relative LiMn2O4. To fully emphasize the benefit from high potential spinel oxides, tests have been performed versus lithium metal, Li4Ti5O12 and graphite, using various electrode loadings (0.3–4.5mAhcm−2) and cycling rates (from C/20 to 60C rate). Steady capacity retention (130–140mAhg−1 for nearly 500 cycles) and flat voltage (4.7V vs. Li+/Li) have been obtained at C/5 rate at room temperature. Effect of cycling at high temperature has been shown to be less critical than for LiMn2O4. High voltage spinel oxides still sustain 100mAhg−1 and over after 400 cycles at 55°C at 1C rate. Rate capability is also excellent, with only 4% loss of capacity when comparing C/8 and 8C rates (thin electrodes).