Defect engineering assisting in high-level anion doping towards fast charge transfer kinetic

The research team of Prof. Xiaobo Ji and associate Prof. Guoqiang Zou has proposed an ingenious oxygen vacancy (OV)-engineering strategy to realize high content anionic doping in TiO2 and offered valuable insights into devise electrode materials with fast charge transfer kinetics in the bulk phase. The article titled "High content anion (S/Se/P) doping assisted by defect engineering with fast charge transfer kinetics for high-performance sodium ion capacitors" is published in Science Bulletin. Xinglan Deng is listed as first author and Prof. Guoqiang Zou as corresponding author.

The rate-determining process for sodium storage in TiO2 is greatly depending on charge transfer happening in the electrode materials owing to its inferior diffusion coefficient and electronic conductivity. Apart from reducing the diffusion distance of ion/electron, the increasement of ionic/electronic mobility in crystal lattice is very important for charge transport. Here, an OV engineering assisted in high content anion (S/Se/P) doping strategy to enhance its charge transfer kinetics for ultrafast sodium-storage performance is proposed. The theoretical calculations have predicted that OV-engineering evokes the spontaneous S doping into TiO2 phase and achieves high anionic dopant concentration to bring about impurity state electron donor and electronic delocalization over S occupied sites, which can largely reduce the migration barrier of Na+. Accordingly, experimental measurements validate the realization of high content anion (S/Se/P) doping and the significantly enhanced Na ion diffusivity and conductivity in prepared electrode materials.

The optimized A-TiO2-x-S/C anode (with S content of 9.82 at%) exhibits extraordinarily high-rate capability with 209.6 mAh g-1 at 5000 mA g-1. When applied as anode materials, the assembled SIC delivers an ultrahigh energy density of 150.1 Wh kg-1 at a power density of 150 W kg-1. This work provides a new strategy to realize the high content doping of anion, and enhance the charge transfer kinetics for TiO2, which sheds a light on the design of electrode materials with fast kinetic.