Synergetic effect of Na-doping and carbon coating on the electrochemical performances of Li(3−x)Na(x)V(2)(PO(4))(3)/C as cathode for lithium-ion batteries

Carbon coated Li(3−x)Na(x)V(2)(PO(4))(3)/C (x = 0.04, 0.06, 0.10, 0.12, 0.18) cathode materials for lithium-ion batteries were synthesized via a simple carbothermal reduction reaction route using methyl orange as the reducing agent, which also acted as the Na and carbon sources. The influence of various Na-doping levels on the structure and electrochemical performance of the Li(3−x)Na(x)V(2)(PO(4))(3)/C composites was investigated. The valence state of vanadium, the form of residual carbon and the overall morphology of the Li(2.90)Na(0.10)V(2)(PO(4))(3)/C, which showed the highest initial specific discharge capacity of 128 mA h g(−1) at the current density of 0.1C (1C = 132 mA g(−1)) among this series of composites, were further examined by X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy and high-resolution transmission electron microscopy, respectively. The results indicated that a well crystallized structure of Na-doped Li(2.90)Na(0.10)V(2)(PO(4))(3) coated by a carbon matrix is obtained. In the further electrochemical measurements, the Li(2.90)Na(0.10)V(2)(PO(4))(3)/C cathode material shows superior discharge capacities of 124, 118, 113, 106 and 98 mA h g(−1) at 0.3, 0.5, 1, 2 and 5C, respectively. High capacity retention of 97% was obtained after 1100 cycles in long-term cyclic performance tests at 5C. The reason for such a promising electrochemical performance of the as-prepared Li(2.90)Na(0.10)V(2)(PO(4))(3)/C has also been explored, which revealed that the synergetic effect of the Na-doping and carbon coating provide enlarged Li(+) diffusion channels and the increased electronic conductivity.