Insights into the storage mechanism of 3D nanoflower-like V3S4 anode in sodium-ion batteries

Over the past years, transition metal sulfides, with remarkable redox reversibility, favorable electrical conductivity and superb theoretical capacity, have received considerable attention on the development of electrode materials that are capable of efficient and reversible storage of sodium ions. In this work, three-dimensional (3D) nanoflower-like V3S4 was applied into sodium-ion batteries (SIBs) as anode, which was successfully synthesized via a facile solvothermal process with post-calcination treatment. The electrochemical measurements demonstrate that the as-prepared V3S4 delivers a high reversible capacity of 499 mAh g−1 after 100 cycles at 0.1 A g−1. Furthermore, even at a high current density of 5 A g−1, the V3S4 also exhibits an outstanding specific capacity of 299 mAh g−1 and a capacity retention rate of 58% compared with the capacity at 0.1 A g−1, which are superior to those of most vanadium sulfides for SIBs reported previously. Remarkably, the ultrahigh sodium-ion storage performance of V3S4 should be ascribed to the unique nanoflower-like structure assembled by ultrathin nanosheets, which not only provides a large specific surface area, but also offers an adequate cushion to suppress the volume expansion during the sodiation-desodiation processes. More importantly, operando Raman spectra, ex-situ X-ray diffraction and ex-situ X-ray photoelectron spectroscopy were employed to demonstrate the evolution of the structure and chemical valence of V3S4, revealing that the high capacity of V3S4 results from the cooperation of Na+ intercalation and conversion reaction. The findings of this study provide a better understanding on the sodium storage mechanism of V3S4.