Fast sodium storage kinetics of titanate-based anode materials via Bi3+/S co-doping engineering for advanced sodium-ion capacitors

Abstract

Layered titanates (LTs) are typical intercalated anode materials for sodium-ion capacitors (SICs). However, the sluggish diffusion kinetics of LTs fail to match the rapid insertion/extraction of capacitive cathodes. It is important to enhance the diffusion kinetics of LTs and achieve better compatibility with the cathode material. In this study, the sodium storage kinetics of titanate (H₂Ti₂O₅, HTO) nanoarrays are optimized by protonation and co-doping engineering of Bi³⁺ ion intercalation and S-doped lamella (S-HBTO). On one hand, the interlayer Bi³⁺ can supply additional capacity while reducing volume expansion. On the other hand, Bi³⁺/S co-doping effectively narrows the band gap in HTO, enhances its electronic conductivity, and constructs a sodium-ion diffusion channel with a low diffusion barrier, which is also supported by the outcomes of density functional theory (DFT) computations. The energy densities of SICs assembled with the S-HBTO anode and an activated carbon cathode are 147.5 and 60.9 Wh kg⁻¹ at power densities of 150 and 3000 W kg⁻¹, respectively. Furthermore, they exhibit an exceptional cycle life, maintaining a capacity of 81.5% after 5000 cycles. This work opens up a new path for LT anode materials of SICs with fast sodium storage kinetics and excellent cycling performance.