Unveiling the reversible sodium-ion storage mechanism in rutile TiO₂ nanorods

Rutile titanium dioxide (TiO₂) is an abundant, cost-effective material with a one-dimensional ion diffusion pathway along the c-axis. However, its potential as an anode material for Na-ion batteries has been long underestimated due to its low electronic conductivity and restricted ion diffusion across the ab-plane. Despite its promising electrochemical properties, the origin of the electrochemical performances in TiO2 rutile is still largely unclear. In this work, the Na+storage mechanisms of TiO2(R) nanorods, 50 nm in length and 5 nm in width, are systematically investigated. The overall charge storage in TiO2 nanorutile is dominated by a mix of surface pseudo-capacitive and diffusion-control Na+ intercalation, whose contributions strongly depend on the C-rate employed. Using operando X-ray diffraction, we demonstrate for the first time reversible Na+ intercalation in the rutile tunnels at low C-rates, favored by the specific nanoarchitecture of TiO2. In line with this, sodium diffusion coefficient is several orders of magnitude higher compared with previous reports (10−16 - 10−17 cm2 s−1 vs. 10−20 cm2 s−1), ensuring a high reversible capacity of ∼ 210 mAh g−1 at low 17 mA g−1 (C/20), and 140 mAh g−1 at 67 mA g−1 (C/5) with little capacity fade and improved cycling stability. ... mehrWhereas high-rate capability of 71 mAh g−1 at 1.7 A g−1 is justified by the dominant pseudo-capacitive contribution to the total capacity at high cycling rates. Carbon-enriched TiO2 (6.5:2.5:1) electrodes exhibit enhanced electrochemical sodium charge storage and kinetics, delivering a higher reversible capacity of 280 mAh g−1 at 17 mA g−1 and an improved high-rate capability of 128 mAh g−1 at 1.7 A g−1. These results demonstrate the significant potential of TiO2 nanorutile for high-rate sodium-ion battery applications.