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

Abstract

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. Whereas 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.