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

Abstract

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.