Improved Na storage and Coulombic efficiency in TiP₂O₇@C microflowers for sodium ion batteries

Ti-based anode materials in sodium ion batteries have attracted extensive interests due to its abundant resources, low toxicity, easy synthesis and long cycle life. However, low coulombic efficiency and limited specific capacity affect their applications. Here, cubic-phase TiP₂O₇ is examined as anode materials, using in-situ/ex-situ characterization techniques. It is concluded that the redox reactions of Ti⁴⁺/Ti³⁺ and Ti³⁺/Ti⁰ consecutively occur during the discharge/charge processes, both of which are highly reversible. These reactions make the specific capacity of TiP₂O₇ even higher than the case of TiO₂ that only contains a simple anion, O^2-. Interestingly, Ti species participate only one of the redox reactions, due to the remarkable difference in local structures related to the sodiation process. The stable discharge/charge products in TiP₂O₇ reduce the side reactions and improve the coulombic efficiency as compared to TiO₂. These features make it a promising Ti-based anode for sodium ion batteries. Therefore, TiP₂O₇@C microflowers exhibit excellent electrochemical performances, ~ 109 mAh·g^-1 after 10,000 cycles at 2 A·g^-1, or 95.2 mAh·g^-1 at 10 A·g^-1. The results demonstrate new opportunities for advanced Ti-based anodes in sodium ion batteries.