Improving the electrochemical performance of Na3V2O2(PO4)2F cathode by using a defect-containing TiO2-x coating for sodium ion batteries

Abstract

Na3V2O2(PO4)2F (denoted as NVOPF) is one of the most promising cathode materials for sodium-ion batteries because of its three-dimensional NASICON structure and high reversible capacity. However, the sluggish sodium-ion diffusion and poor electronic conductivity of NVOPF still hinder their practical application. The oxygen vacancies on the surface of anode materials could promote their charge-transfer kinetics. So far, there are many reports about metal oxide coating to improve the performance of the cathode material. However, it has rarely been reported that the performance of the cathode material is improved by coating with an oxygen-deficient metal oxide. Herein, in this paper, we have successfully synthesized the oxygen-deficient titanium dioxide coated Na3V2O2(PO4)2F (denoted as NVOPF@TiO2-x) nanosheets by a simple hydrothermal method combined with a high temperature calcination in Ar/H2. As a cathode for a sodium-ion half-cell, the NVOPF@TiO2-x delivers excellent cycling stability and rate capability. A reversible capacity of 105 mAh g−1 at 0.2 C after 100 charge-discharge cycles and 54 mAh g−1 at 1 C can be achieved after 500 cycles. The prominent electrochemical properties should be attributed to the effective diffusion of ion and improved transportation of electron in the TiO2-x layer, which is due to the enhanced donor density and number of active sites resulting from the formation of oxygen vacancies.