Vacancy engineering in VS2 nanosheets for ultrafast pseudocapacitive sodium ion storage

Abstract Slow diffusion kinetics of Na+ in electrode is a major obstacle for the rate capability of Na-ion batteries. In this work, super-fast Na+ diffusion in a VS2 anode was achieved by creating ordered (OSV-VS2-x) or disordered S vacancies (DSV-VS2-x) in VS2 nanosheets under a H2/Ar reduction atmosphere. Both the disordered and ordered S vacancies can improve the Na+ diffusion kinetics in VS2. Especially, disordered S vacancies provided an extra ion diffusion pathway perpendicular to the VS2 plane, which significantly reduced the Na+ diffusion barrier of DSV-VS2-x to 0.229 eV, much smaller than the 0.473 eV for VS2 and 0.401 eV for OSV-VS2-x. Taking this advantage, the DSV-VS2-x nanosheets exhibited impressive rate capability (117 mA·h·g−1 at 100 C) and ultra-long cycle life (~100% capacity retention after 5000 cycles at 50 C) in the 1.0–3.0 V voltage window. About 95% of the capacity was attributed to a pseudocapacitive process. The vacancy engineering strategy proposed in this work could significantly improve the rate capability of VS2 and other transition metal dichalcogenides. This technology that combines the high rate performance of supercapacitors with the high energy density of rechargeable batteries would promote the development of high power energy storage devices.