Spinel LiNi0.5Mn1.5O4 is rapidly synthesized by using microwave heating at 700°C for 7min (MW700C7m). The crystal structure, morphology and elemental deficiency of MW700C7m are carefully studied by Rietveld refinement of XRD patterns, scanning and transmission electron microcopy, and redox titration, in comparison with the sample synthesized by conventional heating at 700°C for 48h (CP700C48h). Both MW700C7m and CP700C48h are well refined using the Fd-3m space group. Microwave irradiation not only accelerates the crystal growth of spinel LiNi0.5Mn1.5O4, but also results in preferential growth behavior. Cyclic voltammetry (CV) of MW700C7m presents low polarization and better structural stability. Galvanostatic charge–discharge cycling of MW700C7m and CP700C48h have minor Mn3+/4+ redox peaks beside Ni2+/4+ redox peaks. At normal current rate, both MW700C7m and CP700C48h present good capacities, close to the theoretical value. At high current rate, MW700C7m delivers much better capacity than CP700C48h. MW700C7m delivers capacity of 108.7mAhg−1 at 10C-rates and fades less than 1% after 150 cycles. The severe capacity loss of LiNi0.5Mn1.5O4 cathodes is still a problem when cycling is carried out at high and low temperatures. Under various temperatures, the contribution and effect of Mn3+/4+ and Ni2+/4+ redox for the capacity are studied in detail.