Low-valence titanium oxides synthesized by electric field control as novel conversion anodes for high performance sodium-ion batteries

Abstract

A series of low-valence titanium oxides (TiO and Ti₂O) were fabricated by the electric field control approach and their stoichiometric ratios were precisely regulated by adjusting the electrolytic voltage and time. The Na-storage behaviours of the low-valence titanium oxides were investigated for the first time. Compared to conventional TiO₂, low-valence titanium oxides undergo conversion reactions with excellent electrochemical performances. The Ti₂O electrode delivers a high reversible capacity of 515 mA h g⁻¹, outstanding rate capability of 173 mA h g⁻¹ at 20 A g⁻¹ and superior long-term cycling stability over 1800 cycles. Ex situ characterization reveals the reversible transformation of Ti₂O to Ti and Na₂O during repeated cycling. Density functional theory (DFT) calculations confirm that the low Ti–O bond energy is in favor of the fracture and recombination of Ti–O bonds, resulting in high capacity through multi-electron conversion reactions. The high conductivity of low-valence titanium oxides is conducive to the rapid transfer of electrons, achieving high electrochemical utilization. In addition, the porous nanostructure generated in situ by chemical conversion reactions could improve Na⁺ diffusion kinetics and buffer volume changes during sodiation/desodiation processes, thus realizing superior cycling stability and ultrafast charge/discharge ability. Moreover, a full cell coupled with a carbon-coated Na₃V₂(PO₄)₃ (Na₃V₂(PO₄)₃/C) cathode was assembled, which exhibited a high reversible capacity of 258.4 mA h g⁻¹ and extraordinary cycling performance of 86.5% capacity retention over 100 cycles, demonstrating the potential of low-valence titanium oxides in sodium ion batteries.