Enhanced electrochemical performance of LiVPO4F/C composite with in-situ generated Li3V2(PO4)3 for lithium-ion batteries

The vanadium-containing phosphate polyanion LiVPO4F has been attractive as a novel 4 V cathode for lithium -ion batteries. Herein, a hybrid cathode containing two active components of LiVPO4F and Li3V2(PO4)3 was successfully obtained by single-step solid-state reaction. The crystalline grains are coated by an amorphous carbon layer of about 8 nm thick. The composite presents reversible capacities of 102.8 and 98.2 mAh g-1 at the rates of 10C and 20C, respectively. Even at an ultrahigh rate of 100C (14.5 A g-1), it remains a discharge capacity of 70.1 mAh g- 1 with a high average working potential of 3.5 V. After cycled 500 times at 10C, the obtained sample shows a capacity retention of 78.6%, implying outstanding cycling performance. The electrochemical impendence spectroscopy demonstrates that the hybrid cathode delivers higher lithium-ion diffusion coefficient. It is believed that the electrochemical performance is enhanced synergistically by the in-situ generated ionic conductor of Li3V2(PO4)3. Finally, when charged and discharged at around 4.1 V, the intermediate phase of LixVPO4F is clearly revealed by CV tests, which likely derives from a symmetry phase transform mechanism.