LiMn2O4 spinel was prepared by a hydrothermal method using α-MnO2 nanotubes as precursor at 180 °C, a temperature much lower than that in previously reported methods. It is nanoporous with a pore size of about 40–50 nm and a BET surface area of 9.76 m2 g−1. It exhibits a high specific capacitance of 189 F g−1 at 0.3 A g−1 as a cathode for an aqueous supercapacitor. Even at 12 A g−1, it still has a capacitance of 166 F g−1. After 1500 cycles, there is no evident capacity fading. The LiMn2O4 cathode can deliver an energy density of 31.9 Wh kg−1 at 3480 W kg−1 and even maintain 19.4 Wh kg−1 at about 5100 W kg−1 based on the mass of LiMn2O4.