Selenium vacancy-rich and carbon-free VSe nanosheets for high-performance lithium storage.

VSe2 is a typical transition metal dichalcogenide with metallic conductivity, which makes it a potentially promising electrode material for lithium-ion batteries (LIBs). However, further research into the VSe2 nanomaterial for electrochemical applications has been seriously impeded by the practical difficulty of synthesizing phase-pure VSe2. In this work, Se vacancy-rich VSe2 nanosheets were synthesized by a one-step solvothermal method with suitable reactants. Benefiting from the strong reduction ability of hydrazine hydrate, V4+ was partly reduced into V3+, resulting in abundant Se vacancies being generated in situ in the as-obtained VSe2 nanosheets. Positron annihilation lifetime spectroscopy, X-ray absorption spectroscopy and photoluminescence spectroscopy all confirmed the existence of Se vacancies. When applied as the anode material for LIBs, the VSe2 nanosheets can deliver a remarkable reversible capacity of 1020 mA h g-1 at 0.1 A g-1 after 100 cycles, and even at 2 A g-1 a high specific capacity of 430 mA h g-1 is reached. Electrochemical characterizations further reveal that the Se vacancies in the VSe2 nanosheets can significantly enhance lithium-ion diffusion kinetics and increase the number of electrochemical active sites, which are responsible for the good lithium-storage performance. This work may provide an alternative approach for rational design of other high-performance electrode materials for LIBs to satisfy demand for future sustainable development.