N-doped TiO2 with a disordered surface layer fabricated via plasma treatment as an anode with clearly enhanced performance for rechargeable sodium ion batteries

Sodium ion batteries (SIBs), which share a similar electrochemical reaction mechanism to lithium ion batteries (LIBs), have already attracted much attention because of the rich reserves and low cost of sodium. TiO2 is considered as one of the promising anodes for sodium ion batteries due to its large sodium storage capacity and potentially low cost. However, low electrical conductivity limits the wide application of TiO2 for sodium ion batteries. Here nitrogen doped TiO2 (N-TiO2) nanoparticles are prepared via nitrogen plasma treatment and investigated as anode materials for sodium ion batteries. The N-TiO2 nanoparticles demonstrate a much better rate performance, yielding discharge capacities of about 621 mA h g(-1) at 0.1C and 75 mA h g(-1) at 5C (1C = 335 mA h g(-1)), and a clearly enhanced capacity retention (more than 98% after more than 400 cycles) compared with those of pristine TiO2. What is different from the other nitrogen doped TiO2 reported in the literature is that a disordered surface layer with a thickness of around 2.5 nm is formed on the N-TiO2 nanoparticles after N-2 plasma treatment, which is barely found in normal nitrogen doping processes. Both the doped nitrogen and the disordered surface layer play significant roles in enhancing the sodium storage performance.