Preparation and characterization of chlorine doped Li₃V₂(PO₄)₃as high rate cathode active material for lithium secondary batteries

Monoclinic Li₃V₂(PO₄)_2.99Cl_0.01was synthesized using the conventional solid state method and the X-ray diffraction pattern was indexed based on P 2₁/n space group. The sharp cyclic voltammetric curves clearly revealed three lithium extraction/insertion processes at approximately 3.64, 3.72, 4.13, and 4.58 V during the anodic scan and 3.96, 3.58, and 3.48 V during the cathodic scan. Charge/discharge studies showed reduced electrolyte decomposition contribution in the case of the chlorine doped Li₃V₂(PO₄)_2.99Cl_0.01sample with an initial capacity of 176 mA h g^-1at a 0.1 C current rate. The chlorine doped Li₃V₂(PO₄)₃sample showed an increased capacity retention with an increase in current rate, even at a very high C-rate (20 C), than the pristine and carbon coated samples. The pristine and carbon coated Li₃V₂(PO₄)₃samples showed a lower capacity retention of 71% and 84%, respectively, at a current rate of 0.1 C. In contrast, the chlorine doped Li₃V₂(PO₄)₃sample retained 87% of the initial capacity (176 mA h g^-1) at the same current rate but with a higher coulombic efficiency of 91%. The enhanced capacity retention for the chlorine doped Li₃V₂(PO₄)₃was attributed to the reduction in polarization and decreased charge transfer resistance of the electrode.