Zr-doped Li3V2–4x/3Zrx(PO4)3/C (x = 0, 0.05, 0.1, 0.15 or 0.2) composite cathode materials for lithium ion batteries had been prepared by a sol-gel method and characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, four-point probe measurements and electrochemical analyses. The cell volumes of the Zr-doped samples were larger than that of Li3V2(PO4)3, and Zr had an oxidation number of +4 in Zr-doped Li3V2–4x/3Zrx(PO4)3. The charge-transfer resistance of Li3V2(PO4)3/C was reduced and the reversibility was enhanced after Zr doping. All Zr-doped samples exhibited better electrochemical performance than pristine sample, and Li3V1.87Zr0.1(PO4)3/C composite displayed the highest capacity and the best cycle performance. The improved electrochemical performance mechanism for Zr-doped samples was discussed with respect to electronic structures using first-principle calculations. Zr doping did not significantly change the crystal structure of Li3V2(PO4)3, however, slight alterations in the lattice parameters and unit cell volume were observed. Li3V2(PO4)3 was affected by Zr doping at the V site with a narrower band gap as a result of the holes generated by activation of electrons to empty Zr states, leading to enhanced electronic conductivity of doped samples.
