Nb-Doped TiO₂ as a Promising Anode Material for Sodium-Ion Batteries

Due to their low costs and high sustainability, sodium-ion batteries (SIBs) represent an alternative to lithium-ion batteries (LIBs) especially in the area of stationary battery storage and for small BEVs. Besides hard carbon, nanoscale TiO₂ is an attractive anode material in SIBs and doping with niobium is known to improve its 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 electrochemical properties in comparison to undoped TiO₂, i.e., capacity and rate capability.<br/>We report a single-step synthesis of niobium-doped TiO₂ nanoparticles by scalable spray-flame synthesis based on a solution of titanium (IV) isopropoxide (TTIP) dissolved in mixtures of ethanol and ethyl hexanoic acid [1]. Niobium(V) ethoxide was identified as a preferred Nb precursor and added as dopant to the solution and materials with varying dopant concentrations were synthesized. The as-synthesized powders were analyzed with SEM/EDX confirming Nb concentrations matching the precursor mixture. XRD with Rietveld refinement revealed that the synthesized materials mainly consist of anatase with a minor contribution of rutile and Nb₂O₅ at higher doping concentration. Doping with Nb was also confirmed due to an increase of lattice parameters with higher Nb concentration. XPS analysis reveals the formation of Nb–O–Ti bonds, and the detected main oxidation state of +5 of Nb indicates successful incorporation of Nb⁵⁺ into the TiO₂ lattice.<br/>Electrochemical analyses show an improved electronic conductivity and a higher Na⁺ diffusion kinetics for the Nb-doped TiO₂ in comparison to undoped TiO₂. The highest reversible capacity of 194 mAh/g at 0.1 C and 83.7 ± 1.5 % capacity retention at 0.5 C after 100 cycles was observed for a material with 2 at.% Nb. This single-step synthesis route of these high-performing doped TiO₂ nanomaterials provides promising anode material for sodium-ion batteries.<br/><br/>References:<br/>[1] Eitner et al., Spray-flame synthesis of Nb-doped TiO₂nanoparticles and their electrochemical performance in sodium-ion batteries, <i>Applications in Energy and Combustion Science</i> 2024, 17, 100252