Multiscale Interface Engineering of Sulfur-Doped TiO₂ Anode for Ultrafast and Robust Sodium Storage

Sodium-ion batteries (SIBs) are a promising electrochemical energy storage system; however, their practical application is hindered by the sluggish kinetics and interfacial instability of anode-active materials. Here, to circumvent these issues, we proposed the multiscale interface engineering of S-doped TiO₂ electrodes with minor sulfur/carbon inlaying (S/C@sTiO₂), where the electrode-electrolyte interface (SEI) and electrode-current collector interface (ECI) are tuned to improve the Na-storage performance. It is found that the S dopant greatly promotes the Na⁺ diffusion kinetics. Moreover, the ether electrolyte generates much less NaF in the cycled electrode, but relatively richer NaF in the SEI in comparison to fluoroethylene carbonate-contained ester electrolyte, leading to a thin (9 nm), stable, and kinetically favorable SEI film. More importantly, the minor sodium polysulfide intermediates chemically interact with the Cu current collector to form a Cu₂S interface between the electrode and the Cu foil. The conductive tree root-like Cu₂S ECI serves not only as active sites to boost the specific capacity but also as a 3D "second current collector" to reinforce the electrode and improve the Na⁺ reaction kinetics. The synergy of S-doping and optimized SEI and ECI realizes large specific capacity (464.4 mAh g^-1 at 0.1 A g^-1), ultrahigh rate capability (305.8 mAh g^-1 at 50 A g^-1), and ultrastable cycling performance (91.5% capacity retention after 3000 cycles at 5 A g^-1). To the best of our knowledge, the overall SIB performances of S/C@sTiO₂ are the best among all of the TiO₂-based electrodes.