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3D porous nano-LiMn2O4 (nano-LMO) is successfully prepared at 250 °C by a low temperature pyrolysis, using porous MnO2 and lithium acetate as reactants. The as-formed porous samples are demonstrated to be constructed by well-defined and uniform cubic nanocrystals with particle sizes regulated from tens to hundreds of nanometers by adjusting the annealing temperature. Due to its proper particle size with perfect crystallization (∼50 nm) and rich pores, sample LMO-265, prepared at 265 °C, exhibits high performance as an aqueous lithium ion cathode. It can present a high specific capacity of 102 mAh g−1 at a current density of 5C with a greatly improved rate capability (57.3%, 68C) and capacity retention (70.7%, after 5000 cycles at 13.5C) in 1 M Li2SO4. When assembled with commercial used active carbon (AC) to form an aqueous hybrid capacitor, it can deliver a high energy density of 15.8 Wh kg−1 at a large power density of 9 kW kg−1 with a good cycling life of 71.6% after 5000 at 2 A g−1. Considering of its simple and economical preparation, as-prepared porous LMO can be regarded as a promising high-performance cathode material in aqueous electrolyte.