Heterostructure of SnO2/TiO2 Nanotubes with Precise Wall Thickness Control As Anodes for Lithium Ion Battery

Devices of energy storage and conversion draw much attention recently, among the devices, secondary lithium-ion batteries for energy storage will play an increasingly important role. SnO₂ as anode materials have attracted a lot of attention and are regarded as one of the most promising candidates for lithium-ion battery, with the theoretical reversible capacity of 782 mAhg^-1 and low enough discharge potential of 0.01V. Like to other alloy-type anode materials, its practical application has been impeded by the poor cycling performance owing to the pulverization and subsequent electrical disconnection of the electrode caused by extremely large volume change (about 300%) during the insertion and extraction processes of lithium ions. Li₂O phases, produced by the reaction of SnO₂ and lithium ions, are electrochemically inactive, which is the main reason for the large initial irreversible capacity. To overcome this problem, feasible strategy is to design the nanostructure. 1-dimensional nanotubular structures exhibit enhanced cyclability due to the short diffusion path of lithium ions within the wall of nanotubes and the local empty space in the core of tubular structures can accommodate its volume expansion. SnO₂ and TiO₂ nanotubes as well as their heterostructures were synthesized by anodic aluminum oxide (AAO) template-directed ALD (Atomic Layer Deposition) process. Nanotubes and their heterostructures were characterized by thermogravimetric/differential thermal analysis (TG/DTA), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and X-ray diffraction (XRD).  

The wall thickness of nonotubes can be easily controlled by the number of cycles during ALD processes. The porous nanotubes and their heterostructures exhibit high lithium ion storage capacity, fairly high rate performance, and high cyclability when evaluated as anode materials. As expected, the nanotubes of SnO₂ with the wall thickness of about 10 nm show the initial capacity of 900 mAhg^-1 and reduces down to 300 mAhg^-1 after 50 cycle. The conversion reaction of SnO₂ to Sn and further Li and Sn alloys were clealy observed by ex sity TEM. Composite heterostructures of TiO₂/SnO₂  show much improved cycle performance without any degradation during Li insertion and di-insertion. Advantages of using heterostructure of TiO₂/SnO₂  nanotubes as anode materials will be further discussed.