VSC-doping and VSU-doping of Na3V2-xTix(PO4)2F3 compounds for sodium ion battery cathodes: Analysis of electrochemical performance and kinetic properties

Ion doping strategy is employed to improve the rate performance of insulated Na3V2(PO4)2F3 (NVPF) as cathode of sodium ion batteries. The concepts of VSU-doping (the valence state of the doping ion keeps unchanged during the synthesis) and VSC-doping (the valence state changes during the synthesis) have been introduced in this research. Titanium with different valence states (Ti2+, Ti3+, Ti4+) is selected as the doping element to investigate the VSU-doping and VSC-doping effects on NVPF for the first time. Based on a series of characterizations, it is verified that a moderate Ti doping is able to effectively improve the rate performance of NVPF through the enhancing of electron conduction and sodium ion diffusion, and the advantage is more obvious for the VSC-doping samples. Among all the NVPF-Ti0.1x+ (x = 2, 3, 4) samples, the NVPF-Ti0.12+ treated with a VSC-doping (Ti source: TiO) possesses the minimum particle size distribution (~40 nm), exhibits the highest initial specific capacity of 125 mA h g-1 at 0.2 C, and a very high rate capability (104 mA h g-1 at 40 C, 81 mA h g-1 at 80 C, 41 mA h g-1 at 200 C) as well, which has surpassed most of the reported results. When coupling with Na3V2(PO4)3 (NVP) as the anode, the NVPF-Ti0.12+-NVP pseudo-symmetric full cell shows a high initial specific capacity of 99.6 mA h g-1 at 0.2 C and an outstanding rate capability (60% of initial specific capacity at 40 C). Subsequently, a detailed investigation on the effects of the VSU-doping and VSC-doping processes on the battery performance, in combination with DFT calculations and analysis of kinetic properties, has been conducted, especially from the perspective of electron conduction and sodium ion diffusion.