Abstract:

Sodium-ion batteries (SIBs) are promising energy storage devices because of their low cost and high safety compared with traditional lithium-ion batteries (LIBs). The electrochemical performance of the electrode material determines the whole battery's function. As sodium ion's radius is larger than lithium ion's, the ion embedding/removal is relatively slow, and the electrode material is prone to structural damage after multiple cycles, resulting in capacity decay. Therefore, high energy density and long-life electrode materials are the breakthroughs to achieve high-performance SIBs. Meanwhile, the battery energy storage mechanism and electrode reaction dynamics still need to be further explored. Given the above problems, designing advanced cathode materials to achieve a good match with the existing positive electrodes is one of the urgent problems to be solved in developing high-performance SIBs to improve the energy density and cyclic life of the battery. The TiO₆ octahedrons were connected to each other by corners or edges to form tunnel- and layer-structured sodium titanates (NTO). This open structure made NTO promising anode materials for SIBs. In this work, we compared the electrochemical behavior difference of P- and S-doped NTO as anode materials for SIBs. We found that phosphorus-doped NTO (P-NTO) had excellent electrochemical performance compared with sulfur-doped NTO (S-NTO); when it was used as an electrode for SIBs, it exhibited outstanding long-term cycling stability and rate performance. When the current density was high, up to 2000 mA/g, the P-NTO delivered a reversible capacity of 111 mAh/g. Even after 1300 cycles (500 mA/g), the electrode retained a capacity of 150 mAh/g. These excellent performances are mainly attributed to the open structure of NTO, and doping P drastically boosted the electron movement within the nanosheets.

Key words: heteroatoms, doping, Na₂Ti₃O₇ nanosheets, sodium ion storage