Self-Improving Na Ion Storage in Oxygen Deficient, Carbon Coated Self-Organized TiO₂ Nanotubes

The development of sodium ion batteries is largely motivated by the growing cost and limited resources of lithium. Titanium dioxide (TiO2), in the form of self-organized and well-oriented nanotube arrays, are considered as a highly attractive anode material for sodium ion batteries, due to their inherent safety, low cost, and structural stability. This work reports on the sodiation and desodiation characteristics of anodically grown, self-organized TiO2 nanotubes annealed in an Ar/C2H2 atmosphere (TiO2–x–C). It is found that anatase TiO2–x–C nanotubes demonstrate substantial self-improving charge storage capacities as cycling proceeds, leading to a specific capacity of 202.2 ± 50.6 mAh g–1 at a current rate of 30 mA g–1 (C/20). Subsequent kinetic analysis reveals a pseudocapacitive contribution which dominates the Na storage process in TiO2–x–C nanotubes at fast sodiation rates. This pseudocapacitance in TiO2–x–C nanotubes is found to enable exceptional high-rate capabilities with a specific capacity of 58.4 ± 14.6 mAh g–1 at a current rate of 12 A g–1 (20C).