Improving the rate and low-temperature performance of LiFePO4 by tailoring the form of carbon coating from amorphous to graphene-like

A solid-state reaction process with poly(vinyl alcohol) as the carbon source is developed to synthesize LiFePO 4 -based active powders with or without modification assistance of a small amount of Li 3 V 2 (PO 4 ) 3 . The samples are analyzed by X-ray diffraction, scanning/transmission electron microscopy, and Raman spectroscopy. It is found that, in addition to the minor effect of a lattice doping in LiFePO 4 by substituting a tiny fraction of Fe 2+ ions with V 3+ ions, the change in the form of carbon coating on the surface of LiFePO 4 plays a more important role to improve the electrochemical properties. The carbon changes partially from sp 3 to sp 2 hybridization and thus causes the significant rise in electronic conductivity in the Li 3 V 2 (PO 4 ) 3 -modified LiFePO 4 samples. Compared with the carbon-coated baseline LiFePO 4 , the composite material 0.9LiFePO 4 ·0.1Li 3 V 2 (PO 4 ) 3 shows totally different carbon morphology and much better electrochemical properties. It delivers specific capacities of 143.6 mAh g −1 at 10 C rate and 119.2 mAh g −1 at 20 C rate, respectively. Even at the low temperature of −20 °C, it delivers a specific capacity of 118.4 mAh g −1 at 0.2 C.