Nitrogen and fluorine co-doped TiO2/carbon microspheres for advanced anodes in sodium-ion batteries: High volumetric capacity, superior power density and large areal capacity

Fast charging and high volumetric capacity are two of the critical demands for sodium-ion batteries (SIBs). Although nanostructured materials achieve outstanding rate performance, they suffer from low tap density and small volumetric capacity. Therefore, how to realize large volumetric capacity and high tap density simultaneously is very challenging. Here, N/F co-doped TiO2/carbon microspheres (NF-TiO2/C) are synthesized to achieve both of them. Theoretical calculations reveal that N and F co-doping increases the contents of oxygen vacancies and narrows the bandgaps of TiO2 and C, improving the electronic conductivity of NF-TiO2/C. Furthermore, NF-TiO2/C exhibits the high binding energy and low diffusion energy barrier of Na+, significantly facilitating Na+ storage and Na+ diffusion. Therefore, NF-TiO2/C offers a high tap density (1.51 g cm−3), an outstanding rate performance (∼125.9 mAh g−1 at 100 C), a large volumetric capacity (∼190 mAh cm−3 at 100 C), a high areal capacity (∼4.8 mAh cm−2) and an ultra-long cycling performance (∼80.2% after 10,000 cycles at 10 C) simultaneously. In addition, NF-TiO2/C||Na3V2(PO4)3 full cells achieve an ultrahigh power density of 25.2 kW kg−1. These results indicate the great promise of NF-TiO2/C as a high-volumetric-capacity and high-power-density anode material of SIBs.