Abstract LiFe0.8Mn0.2PO4/C composites are synthesized via solid state reaction promoted by wet ball milling. Synthesis conditions are optimized to obtain desirable crystallization, morphology, particle size and specific surface area. Our experiments show that the LiFe0.8Mn0.2PO4/C sample sintered at 600°C exhibits the best performance with initial discharge capacities of 160mAhg−1 at 0.1C, 133mAhg−1 at 5C and 122mAhg−1 at 10C. At 5C, the cathode exhibits an excellent cyclability over 100 cycles, and the average specific energy density is as high as 414Whkg−1. We attribute the superior performance to well crystallization, uniform morphology, small particle size and high specific surface area. Our results indicate that partial substitution of Mn ions for Fe ions can significantly improve rate capability and energy density in LiFePO4. LiFe0.8Mn0.2PO4 should be identified as highly promising cathode material with high energy density.