Spinel LiMn2O4 has been prepared by a facile hydrothermal procedure and the prepared LiMn2O4 exhibits superior electrochemical performance, such as high specific capacitance (409Fg−1 at 2mVs−1) and excellent rate capability (85% retention at 100mVs−1). In order to achieve high energy and power densities, a high-voltage asymmetric supercapacitor (ASC) is successfully assembled using the as-prepared LiMn2O4 as the positive electrode and nitrogen-doped graphene/porous carbon composite as the negative electrode in a Li2SO4 aqueous electrolyte. The optimized ASC delivers a high specific energy of up to 44.3Whkg−1 at a specific power of 595Wkg−1 with an output voltage of 1.8V based on the total weight of the active electrode materials. This novel strategy provides a promising route to design high-performance supercapacitors with high energy density for next-generation storage systems.