Effect of Hf-doping on electrochemical performance of anatase TiO(2) as an anode material for lithium storage

Hafnium-doped titania (Hf/Ti = 0.01; 0.03; 0.05) had been facilely synthesized via a template sol–gel method on carbon fibre. Physico-chemical properties of the as-synthesized materials were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray analysis, scanning transmission electron microscopy, X-ray photoelectron spectroscopy, thermogravimetry analysis and Brunauer–Emmett–Teller measurements. It was confirmed that Hf(4+) substitute in the Ti(4+) sites, forming Ti(1–x)Hf(x)O(2) (x = 0.01; 0.03; 0.05) solid solutions with an anatase crystal structure. The Ti(1–x)Hf(x)O(2) materials are hollow microtubes (length of 10–100 µm, outer diameter of 1–5 µm) composed of nanoparticles (average size of 15–20 nm) with a surface area of 80–90 m(2) g(–1) and pore volume of 0.294–0.372 cm(3) g(–1). The effect of Hf ion incorporation on the electrochemical behaviour of anatase TiO(2) in the Li-ion battery anode was investigated by galvanostatic charge/discharge and electrochemical impedance spectroscopy. It was established that Ti(0.95)Hf(0.05)O(2) shows significantly higher reversibility (154.2 mAh g(–1)) after 35-fold cycling at a C/10 rate in comparison with undoped titania (55.9 mAh g(–1)). The better performance offered by Hf(4+) substitution of the Ti(4+) into anatase TiO(2) mainly results from a more open crystal structure, which has been achieved via the difference in ionic radius values of Ti(4+) (0.604 Å) and Hf(4+) (0.71 Å). The obtained results are in good accord with those for anatase TiO(2) doped with Zr(4+) (0.72 Å), published earlier. Furthermore, improved electrical conductivity of Hf-doped anatase TiO(2) materials owing to charge redistribution in the lattice and enhanced interfacial lithium storage owing to increased surface area directly depending on the Hf/Ti atomic ratio have a beneficial effect on electrochemical properties.