Doping of potassium by partial substitution of sodium in sodium trititanate for improved sodium-ion storage properties

Layered sodium trititanate (Na 2 Ti 3 O 7 , NTO) is a safe and cost-effective anode material for sodium-ion batteries. However, its sluggish Na + diffusion kinetics, poor electron conductivity, and unstable layered structure against cycling pose major barriers for practical applications. Here, we show a novel doping strategy for improving Na +  storage properties of NTO. Instead of doping transition metals in the Ti 4+ site, doping of alkaline metal K + by partial substitution of Na + in NTO is demonstrated. The K + -doped NTO displays enhanced Na + -storage capacity and transport kinetics, as well as improved cycling stability. It delivers a reversible Na + storage specific capacity of 162 mAh g −1 at 0.1C, a capacity retention of 75 % after 8200 cycles at 1C or after 4000 cycles at 10C. In situ X-ray diffraction characterization and density functional theory calculation results reveal that the improved performance is mainly attributed to increased exposure of the Na + storage (100) plane, widened interlayer spacing and stabilized layered structure. A full cell assembled with the K + -doped NTO as anode and Na 3 V 2 (PO 4 ) 3 @C as cathode delivers a specific energy density of 165 Wh kg −1 . This work provides an alternative approach to modifying NTO for improving its Na  +  storage properties.