Carbon-coated LiMn1-xFexPO4 (0≤x≤0.5) nanocomposites as high-performance cathode materials for Li-ion battery

Abstract To improve the rate capability and cycle performance of the LiMnPO 4 cathode, carbon-coated LiMn 1- x Fe x PO 4 (0 ≤  x  ≤ 0.5) nanocomposites have been successfully prepared by hydrothermal process. The carbon-coating does not affect the morphology of LiMn 1- x Fe x PO 4 (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 LiMn 1- x Fe x PO 4 . Fe doping has an important influence on the morphology of LiMnPO 4 , and carbon-coated LiMn 0.5 Fe 0.5 PO 4 obviously shows a nanorod morphology with a length of 100–200 nm. Carbon-coated LiMn 0.5 Fe 0.5 PO 4 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 LiMnPO 4 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 LiMn 0.5 Fe 0.5 PO 4 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 LiMn 0.5 Fe 0.5 PO 4 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 LiFe 0.5 Mn 0.5 O 4 will be improved obviously in comparison with LiMnPO 4 , which well explains the better cycling stability of LiFe 0.5 Mn 0.5 O 4 than LiMnPO 4 observed experimentally.