LiFe_0.4Mn_0.6−xCr_xPO₄/C (x ≤ 0.01) cathode materials with different Cr-doping were synthesized by a nano-milling assisted solid-state method. The experimental results demonstrated that Cr-doping can significantly improve the electrochemical performance of the target material. Among the samples synthesized, the LiFe_0.4Mn_0.595Cr_0.005PO₄/C (with 0.5 atm% of Cr) sample exhibited the highest specific capacity and the best rate performance. It delivered initial discharge capacities of 164.0, 156.2, 147.5 and 139.3 mA h g⁻¹ at 0.1C, 0.5C, 2C and 5C, respectively. Moreover, it showed the best cycle stability with capacity retention of 99.2% after 50 cycles at 0.1C. Such enhancement can be ascribed to the improvements in not only the electronic conductivity, but also the Li ion diffusion coefficient. Powder conductivity tests revealed that the conductivity of the powder sample with 0.5% Cr doping presents the highest conductivity of 5.91 × 10⁻⁵ S cm⁻¹, which is almost 5.3 times that of the pristine sample. Calculations of diffusion coefficients using the EIS data also suggested a highest Li ion diffusion coefficient of 4.36 × 10⁻¹⁰ cm² s⁻¹ for the sample with 0.5% Cr doping, which is almost 4.6 times that of the pristine sample. The synthesized LiFe_0.4Mn_0.595Cr_0.005PO₄/C with such excellent electrochemical performance showed great potential for application in high-power devices.