N-doped TiO2 with Disordered Surface Layer Fabricated via Plasma Treatment as Anode with Clearly Enhanced Performance for Rechargeable Sodium Ion Battery

Sodium ion batteries (SIB), which shares a similar electrochemical reaction mechanism with lithium ion batteries (LIB), have already attracted more attentions 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 the large sodium storage capacities with potentially low cost. However, the 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 material of sodium ion batteries. The N-TiO2 nanoparticles demonstrate a much better rate performance, yielding discharge capacities of about 621 mAh·g-1 at 0.1 C and 75 mAh·g-1 at 5 C (1 C =335 mAh·g-1 ), as well as a clearly enhanced capacity retention (more than 98% after more than 400 cycles) compared with the pristine TiO2. What is different from the other nitrogen doped TiO2 reported in the literatures is that a disordered surface layer with the thickness of around 2.5 nm is formed in the N-TiO2 nanoparticles after the N2 plasma treatment, which is barely found with normal nitrogen doping processes. Both the doped nitrogen and the disordered surface layer play significant roles on enhancing the sodium storage performance.