Isovalent doping of tin in sodium trititanate for enhanced sodium-ion battery performance

Layered sodium trititanate (Na 2 Ti 3 O 7 , NTO) is a promising anode material for sodium-ion batteries (NIBs) for large-scale energy storage applications because of its relatively low charge potential and low cost. However, NTO suffers from unsatisfactory structural stability against cycling and poor electron conductivity. Herein, an isovalent doping strategy using Sn 4+ to partially replace Ti 4+ is demonstrated for improving the cycling stability and rate capability of NTO. The isovalent doping of Sn 4+ does not alter the valence state of Ti 4+ , thus maintaining the lattice integrality and structural stability. Moreover, the Sn 4+ dopant creates more Na + -preferable travel channels and expands the interlayer spacing, thus increasing Na + diffusivity. As a result, a Sn 4+ -doped Na 2 Ti 3 O 7 (NSTO) electrode exhibits a reversible Na + storage specific capacity of 176 mA h g −1 at 0.1C and an ultra-long cycling life with 80.2% capacity retention after 5000 cycles at 1C, far outperforming the undoped and aliovalent-doping NTO electrodes reported in the literature. In addition, the NSTO electrode delivers a rate capability of 102 mA h g −1 at 5C, higher than that of the NTO electrode (62 mA h g −1 ). In situ X-ray diffraction characterization results reveal that Na + storage in NSTO undergoes a partial solid-solution reaction mechanism, which is completely different from the two-phase transition mechanism of NTO. Density functional theory calculation results demonstrate that Sn 4+ doping strengthens the Ti–O bond, contributing to structural stability. This work provides a robust approach to significantly improving the electrochemical performance of NTO-based anode materials for developing long-life NIBs.