Monitoring the Sodiation Mechanism of Anatase TiO₂ Nanoparticle-Based Electrodes for Sodium-Ion Batteries by Operando XANES Measurements

Anatase TiO2 represents an attractive electrode material for application in sodium-ion batteries due to its relatively low cost, high environmental compatibility, high intrinsic safety, conferred by the relatively high operating voltage, and satisfactory theoretical capacity. Nonetheless, a comprehensive understanding of the Na uptake and release mechanism is still missing, which is crucial for further insight-driven optimization of the electrode material. This work presents for the first time an extensive operando X-ray absorption near-edge structure spectroscopy (XANES) study at the Ti K-edge of a TiO2 anatase nanoparticle-based electrode, aiming at unraveling the structural evolution and consequent Ti oxidation state and coordination changes upon sodiation and following desodiation. Using two approaches, i.e., an analytical fit and principal component analysis (PCA) with a linear combination analysis (LCA) for the evaluation of operando data, this study reveals the amount of irreversible and reversible Na+ inserted upon cycling. In addition, a change of Ti coordination during the first cycle is monitored, observing a decrease of the original six-coordinated symmetry. Simultaneously, the irreversible loss of the nanoparticle structural ordering due to the effect of initial Na insertion in the anatase lattice is detected. These results support some of the (ex situ) findings reported previously and give a more comprehensive picture of the highly discussed sodiation mechanism of TiO2-based anodes under more realistic operating conditions.