Spinel LiNi0.5Mn1.5O4 (Fd-3m) powders are synthesized by a facile one-step sol-gel approach with a resorcinol formaldehyde (RF) resin as a chelating agent. The cross-linked metal-containing RF xerogel particles are sintered at different high temperatures from 750 to 950°C to produce several micron-sized LiNi0.5Mn1.5O4 powders. Electrochemical measurements suggest that the 850°C-sintered (in air) sample (Fd-3m phase) performs the best with a discharge capacity of 141 mAh g−1 at 0.1C and 110 mAh g−1 at 10C, and capacity-retention of 96.3% after 60 cycles at 0.25C and 89% after 200 cycles at 1C. For comparison, the LiNi0.5Mn1.5O4 sample sintered at 850°C in O2 (P4332 phase) presents limited rate performance (45 mAh g−1 at 10C) and higher values in both AC impedance and DC-method derived resistance. A characteristic double “w”-shape curve of DC resistance against cell potential can be possibly considered as an indicator to probe the material structure transition during the charge/discharge process of the cell.