In situ TEM observation of the electrochemical lithiation of N-doped anatase TiO₂ nanotubes as anodes for lithium-ion batteries

Due to their high specific capacity and negligible volume expansion during cycling, anatase titanium dioxide (a-TiO₂) nanotubes have been considered as a prime candidate for anodes in lithium-ion batteries. However, their rate capability for electrochemical cycling is limited by the low electronic conductivity of a-TiO₂ nanotubes. Here, we show that a desirable amount of nitrogen doping can significantly enhance the electronic conductivity in a-TiO₂ nanotubes, resulting in improvements in both the capacity stability and the rate capability at fast charge-discharge rates. Electron energy loss spectroscopy revealed a high doping concentration of nitrogen (∼5%) by substituting for oxygen ions in a-TiO₂ nanotubes. The lithiation mechanism of N-doped a-TiO₂ nanotubes was further investigated using in situ transmission electron microscopy, where a three-step lithiation mechanism was revealed. Lithium ions initially intercalate into the a-TiO₂ lattice structure. Further insertion of lithium ions triggers a phase transformation from a-TiO₂ to orthorhombic Li_0.5TiO₂ and finally to polycrystalline tetragonal LiTiO₂. Our results reveal that nitrogen doping significantly facilitates lithiation in TiO₂ through enhanced electronic conductivity, while the structural and chemical evolutions during the lithiation process remain similar to those of undoped TiO₂.