Preparation and Electrochemical Properties of Li3V2(PO4)3-xBrx/Carbon Composites as Cathode Materials for Lithium-Ion Batteries

Li3V2(PO4)(3-x)Br-x/carbon (x = 0.08, 0.14, 0.20, and 0.26) composites as cathode materials for lithium-ion batteries were prepared through partially substituting PO43- with Br-, via a rheological phase reaction method. The crystal structure and morphology of the as-prepared composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and electrochemical properties were evaluated by charge/discharge cycling and electrochemical impedance spectroscopy (EIS). XRD results reveal that the Li3V2(PO4)(3-x)Br-x/carbon composites with solid solution phase are well crystallized and have the same monoclinic structure as the pristine Li3V2(PO4)(3)/carbon composite. It is indicated by SEM images that the Li3V2(PO4)(3-x)Br-x/carbon composites possess large and irregular particles, with an increasing Br content. Among the Li3V2(PO4)(3-x)Brx/carbon composites, the Li3V2(PO4)(2.86)Br-0.14/carbon composite shows the highest initial discharge capacity of 178.33 mAh.g(-1) at the current rate of 30 mA.g(-1) in the voltage range of 4.8-3.0 V, and the discharge capacity of 139.66 mAh.g(-1) remains after 100 charge/discharge cycles. Even if operated at the current rate of 90 mA.g(-1), Li3V2(PO4)(2.86)Br-0.14/carbon composite still releases the initial discharge capacity of 156.57 mAh.g(-1), and the discharge capacity of 123.3 mAh.g(-1) can be maintained after the same number of cycles, which is beyond the discharge capacity and cycleability of the pristine Li3V2(PO4)(3)/carbon composite. EIS results imply that the Li3V2(PO4)(2.86)Br-0.14/carbon composite demonstrates a decreased charge transfer resistance and preserves a good interfacial compatibility between solid electrode and electrolyte solution, compared with the pristine Li3V2(PO4)(3)/carbon composite upon cycling.