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

Abstract

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.