Investigation of structural, optical, and electrochemical properties of niobium-doped Li4Ti5O12 for high-performance aqueous capacitor electrode

In this study, nanocrystalline niobium-doped lithium titanate Li4Ti5O12 (LTO) was successfully synthesized with various stoichiometric ratios (x = 0, 0.025, 0.05, 0.075, 0.1) using a niobium ethoxide precursor via the solid-state synthesis method. X-ray diffraction spectroscopy results revealed that the synthesized samples possess a spinel structure without any impurities, and lattice constants expanded from 8.357 Å to 8.363 Å as the concentration of niobium ions increased. Scanning electron microscopy (SEM) analysis demonstrated that particle sizes ranged from 1.13 to 1.56 μm. The smallest particle size was observed in the LTO-Nb-0.075 sample. The maximum absorptions were detected in the ultraviolet region, and the band gaps of Nb-doped Li4Ti5-xNbxO12 (x = 0, 0.025, 0.05, 0.075, 0.1) were determined using Tauc plots. These results showed that the band gaps of Nb-doped LTO were lowered compared to pure LTO, suggesting enhanced electron conductivity. X-ray photoelectron spectroscopy (XPS) measurements confirmed that the band gaps of Nb-doped samples decreased as the formation of oxygen vacancies and Ti3+ ions increased due to Nb5+ substitution. All synthesized samples displayed well-defined redox peaks in pairs during the cyclic voltammetry (CV) measurement. The optimal stoichiometric ratio for doping lithium titanate was found to be x = 0.075, as it yielded the highest specific capacitance of 3.59 F/g and the lowered band gap value of 3.13 eV. Therefore, this study provides further encouragement for applying LTO electrodes in aqueous capacitors and electrical uses.