Effect of Hf-doping on electrochemical performance of anatase TiO₂ 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 Hf⁴⁺ substitute in the Ti⁴⁺ sites, forming Ti_1- Hf O₂ (x = 0.01; 0.03; 0.05) solid solutions with an anatase crystal structure. The Ti_1- Hf O₂ 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 m² g^-1 and pore volume of 0.294-0.372 cm³ g^-1. The effect of Hf ion incorporation on the electrochemical behaviour of anatase TiO₂ in the Li-ion battery anode was investigated by galvanostatic charge/discharge and electrochemical impedance spectroscopy. It was established that Ti_0.95Hf_0.05O₂ 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 Hf⁴⁺ substitution of the Ti⁴⁺ into anatase TiO₂ mainly results from a more open crystal structure, which has been achieved via the difference in ionic radius values of Ti⁴⁺ (0.604 Å) and Hf⁴⁺ (0.71 Å). The obtained results are in good accord with those for anatase TiO₂ doped with Zr⁴⁺ (0.72 Å), published earlier. Furthermore, improved electrical conductivity of Hf-doped anatase TiO₂ 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.