Size-Dependent Electrochemical Performance of Monolithic Anatase TiO2 Nanotube Anodes for Sodium-Ion Batteries

Well-defined, monolithic TiO2 nanotube thin films have been used as model anode electrodes to study Na-ion storage in anatase TiO2. It is shown that anatase TiO2 nanotubes with wall thicknesses up to 50 nm can be transformed into amorphous sodium titanate (e.g. Na0.2TiO2) nanotubes via an electrochemical activation process at about 0.2 V vs. Na+/Na. Due to the Na+ insertion and extraction reactions at about 0.55 and 0.75 V vs. Na+/Na, respectively, the activated TiO2 nanotubes exhibit reversible capacities of 170 mA hg(-1). For the first time, it is shown that the nanotube length and wall thickness play critical roles in determining the electrochemical performances of this type of electrodes in Na-ion cells. An excellent rate performance, yielding capacities of about 33 mA hg(-1) at 20 C and 161 mA hg(-1) at C/5 rates, as well as a capacity retention of more than 97% after more than 350 cycles, could be achieved with nanotubes with a wall thickness of up to 20 nm. The cycling rate for the nanotubes with a tube length of 4.5 mu m should, however, be limited to 1 C to guarantee a cycle life of about 200 cycles.