Fluorinated surface-to-bulk engineering enables high-rate and long-life sodium trititanate for sodium-ion batteries

Abstract

Layered sodium trititanate (Na2Ti3O7, NTO) is one of the most promising anodes for sodiumion battery (NIBs) owning to its merits of low charge potential and cost-effectiveness. However, it suffers from undesired interfacial reaction, sluggish Na+ transport kinetics and structural instability against cycling. Herein, a fluorinated surface-to-bulk modification strategy is applied to NTO, enabling surface NaF-coating and bulk F-doping, to enhance its rate capability and cycling stability. Experimental and computational calculation results demonstrate that F-doping lowers the bandgap energy of NTO and introducing the strengthen Ti-F bonds, facilitating electron transfer and enhancing the structural stability against cycling. Meanwhile, the NaF-coating minimizes electrolyte decomposition and facilitates the construction of NaF-enriched SEI, which is beneficial for Na+ diffusion. As a result, the fluorinated electrode (F-NTO) exhibits a reversible Na+ storage specific capacity of 172 mAh g-1 at 0.1C and a high-rate capability of 104 mAh g-1 at 5C. In addition, the F-NTO electrode delivers a long cycling life with 78.8% capacity retention after 2000 cycles at 1C, far outperforming the unmodified NTO electrode. This surface-to-bulk synergistic modification provides a robust approach to constructing high-performance titanate anode materials for developing super-charging and long-life NIBs.