Dual anion-doped porous carbon embraced TiO2 nanocrystals with long-term cycling performance for sodium ion batteries

• N, S dual-anion doped porous carbon embraced ultrafine TiO 2 nanocrystals (NCs) (TiO 2 @NSPC) is synthesized. • TiO 2 @NSPC was synthesized by using NH 2 -MIL-125 (Ti) and sulfur powder as precursor and sulfur source, respectively. • TiO 2 @NSPC delivers a high reversible capacity and unprecedented cycling stability. • Double heteroatom doping has higher pesudocapacitance than single heteroatom doping. TiO 2 as one of critical anode materials for sodium ion batteries (SIBs) has excellent characteristics such as low cost, high safety, small volume expansion and high packing density. However, low conductivity and poor sodium ion diffusion ability prevent its further applications in SIBs. Thus, achieving functionalized carbon embraced TiO 2 nanocrystals (NCs) becomes an alternative to boost the TiO 2 performance in SIBs. Herein, using NH 2 -MIL-125 (Ti) and sulfur powder as template and sulfur source, N, S dual-anion doped porous carbon embraced ultrafine TiO 2 NCs (TiO 2 @NSPC) are successfully constructed. Due to a large surface-to-volume ratio of TiO 2 NCs, the transport pathway of sodium ions is greatly shortened. Meanwhile, N, S dual-anion doped porous carbon can ameliorate the electrical conductivity and transport efficiency of ions, effectively inhibiting the agglomeration of TiO 2 NCs. As a result, when used as the anode of SIBs, TiO 2 @NSPC shows a reversible capacity of 230.2 mAh g -1 after 300 cycles at a current density of 500 mA g -1 , with high capacity retention of 88.9%. Moreover, it exhibits extremely high cycling stability with a capacity of 63 mAh g -1 even at 10 A g -1 after 20000 cycles, and higher pseudocapacitive sodium storage than single heteroatom doping, causing its superior sodium ion storage capability. This strategy opens up a new situation to design new electrode materials with enhanced pseudocapacitance and superior sodium storage.