Carbon-encapsulated LiMn2O4 (LMO@C) spheres were prepared using polymer microgel reactor, in which Mn2+ ions are hydrolyzed in situ to form Mn(OH)2, and followed by annealing at a high temperature. The LMO@C spheres are constructed with the spinel LiMn2O4 nanospheres that embedded in a porous carbon matrix uniformly. Owing to possessing three-dimensional (3D) electron-conductive and 3D ion-conductive networks, the LMO@C spheres exhibit high rate capability. They can deliver the specific capacities of 142, 137, 126, 107, and 91 mAh g−1 at the rates of 0.1C, 1C, 5C, 10C, and 20C (1C = 148 mA g−1), respectively. Owing to carbon encapsulation, the LMO@C spheres can retain 80% of the initial capacity at 1C rate after 1000 cycles at 25 °C, displaying stable cycling performance. The results suggest that the LMO@C spheres are promising cathode materials for high-power lithium-ion batteries.