Carbon-coated LiMn1-Fe PO4 (0≤x≤0.5) nanocomposites as high-performance cathode materials for Li-ion battery

To improve the rate capability and cycle performance of the LiMnPO4 cathode, carbon-coated LiMn1-xFexPO4 (0 ≤ x ≤ 0.5) nanocomposites have been successfully prepared by hydrothermal process. The carbon-coating does not affect the morphology of LiMn1-xFexPO4 (0 ≤ x ≤ 0.5), but restrains the aggregation of particles, and an obvious carbon film with a thickness of about 2.5 nm can be observed on the surface of LiMn1-xFexPO4. Fe doping has an important influence on the morphology of LiMnPO4, and carbon-coated LiMn0·5Fe0·5PO4 obviously shows a nanorod morphology with a length of 100–200 nm. Carbon-coated LiMn0·5Fe0·5PO4 shows excellent rate capability, and delivers specific capacities of about 156.4, 151, 147.6, 145.7 and 137.3 mAh g−1 at 0.05, 0.1, 0.2, 0.5 and 1 C, respectively. However, carbon-coated LiMnPO4 only delivers specific capacities of about 109.5, 100, 82.4, 79 and 65.8 mAh g−1 at corresponding current densities. The carbon-coated LiMn0·5Fe0·5PO4 also shows a large initial specific capacity of 134.5 mAh g−1 at 5 C rate with outstanding capacity retention of 84.6% even after 100 cycles. The enhanced rate capability and cycling stability of carbon-coated LiMn0·5Fe0·5PO4 at high rate are attributed to the decreased charge transfer resistance, decreased electrode polarization, enhanced reversibility of extraction and insertion of Li-ions, and increased Li-ion diffusion coefficient. DFT calculation shows that Fe–O bond is stronger than Mn–O bond, and it can be expected that the thermodynamic stability of LiFe0·5Mn0·5O4 will be improved obviously in comparison with LiMnPO4, which well explains the better cycling stability of LiFe0·5Mn0·5O4 than LiMnPO4 observed experimentally.