Spray-flame synthesis of Nb-doped TiO2 nanoparticles and their electrochemical performance in sodium-ion batteries

TiO2 nanomaterials are attractive anode materials in sodium-ion batteries (SIBs) and doping with niobium is known to improve the electronic conductivity as well as the kinetics of the de-/intercalation of Na+ ions. These effects are related to the formation of defects and lattice widening, leading to improved rate capability and stability. We report single-step synthesis of niobium-doped TiO2 nanoparticles by scalable spray-flame synthesis from titanium(IV) isopropoxide (TTIP) dissolved in mixtures of ethanol and ethyl hexanoic acid. Niobium(V) ethoxide was identified as the preferred precursor and materials with varying Nb/Ti ratios were synthesized. SEM/EDX analysis of the as-synthesized powders confirmed Nb/Ti ratios near to the target ratios determined by the precursor mixture. XRD with Rietveld-refinement revealed the dependence of the lattice parameters on doping. The materials synthesized mainly consist of anatase with a minor contribution of rutile and Nb2O5 at higher doping concentration. The increase of lattice parameters with higher Nb/Ti ratios, the formation of Nb-O-Ti bonds, and the detected main oxidation state of +5 of Nb indicate successful incorporation of Nb5+ into the TiO2 lattice and impedance measurements indicate higher electronic conductivity for the Nb-doped TiO2 in comparison to the undoped TiO2. The best electrochemical performance was observed for a material with 2 at.% Nb, which delivered high reversible capacity of 194 mAh/g at 0.1 C and 83.7 ± 1.5 % capacity retention at 0.5 C after 100 cycles. Moreover, rate capability tests indicate higher Na+ diffusion kinetics in the Nb-doped samples. This one-step synthesis route of these high-performing doped TiO2 nanomaterials provides promising anode material for sodium-ion batteries.