Boosting Sodium Storage in TiO2 Nanotube Arrays through Surface Phosphorylation

: Sodium-ion batteries (SIBs) offer a promise of a scalable, low-cost, and environmentally benign means of renewable energy storage. However, the low capacity and poor rate capability of anode materials present an unavoidable challenge. In this work, it is demonstrated that surface phosphorylated TiO2 nanotube arrays grown on Ti substrate can be efficient anode materials for SIBs. Fabrication of the phosphorylated nanoarray film is based on the electrochemical anodization of Ti metal in NH4 F solution and subsequent phosphorylation using sodium hypophosphite. The phosphorylated TiO2 nanotube arrays afford a reversible capacity of 334 mA h g-1 at 67 mA g-1 , a superior rate capability of 147 mA h g-1 at 3350 mA g-1 , and a stable cycle performance up to 1000 cycles. In situ X-ray diffraction and transmission electron microscopy reveal the near-zero strain response and robust mechanical behavior of the TiO2 host upon (de)sodiation, suggesting its excellent structural stability in the Na+ storage application.