Titanium dioxide nanotrees for high-capacity lithium-ion microbatteries

Li-ion microbatteries find wide applications in microdevices. It is important to develop electrode materials with high areal capacity, excellent rate capability and superior safety for microbatteries. TiO2 nanowire arrays satisfy the requirements of high areal capacity, short transport lengths and superior safety for microbatteries, but their areal/volumetric capacity is hindered by the insufficient surface area. TiO2 nanotrees can effectively eliminate these disadvantages of TiO2 nanowires; however, the electrochemical performances of TiO2 nanotrees for Li-ion microbatteries remain unexplored. It is highly desired yet challenging to construct two-dimensional TiO2 nanobranches with an ultrathin thickness and large length on nanoarrays, to maximize the surface area and structural hierarchy of electrodes. Herein, we developed a novel synthetic strategy to fabricate TiO2 nanotrees by depositing anatase/TiO2(B) mixed phase ultrathin nanobelts onto single-crystalline anatase nanowire arrays. The nanobelt branches were several nanometers in thickness and 200–260 nm in length. The growth process and the electrochemical mechanism were investigated. The unique nanoarchitecture and optimal phase structure endow the electrode with high areal capacity and rate capability, which is the best performance for TiO2 nanowire arrays ever documented. The 2nd discharge capacity of the TiO2 nanotrees at 0.1 mA cm−2 is ca. 267 μA h cm−2, corresponding to a volumetric capacity of 330 mA h cm−3. The TiO2 nanotrees have an areal capacity of 205, 141, 97 and 69 μA h cm−2 at a current density of 0.5, 2.0, 5.0 and 10.0 mA cm−2, respectively. The capacity can remain stable for 400 charge–discharge cycles at 1.0 mA cm−2. The present strategy may give hints to elegant electrode designs for energy applications.