Hierarchical flower-like VS2 nanosheets - A high rate-capacity and stable anode material for sodium-ion battery

Hierarchical flower-like VS2 nanosheets assemblies are successfully synthesized via a facile solvothermal method, and their Na+ storage behavior is systematically studied with respect to the galvanostatic charge-discharge profiles, cyclic voltammograms, rate capability and long-term cycle stability. With a well-controlled cut-off voltage (3.0-0.3 V) and suitable electrolyte (1.0 M NaCF3SO3 in diglyme), flower-like VS2 delivers a high reversible capacity of around 600 mAh g(-1) at 0.1 A g(-1) and excellent cycle stability with 83% and 87% of its initial capacities retained after 700 cycles at 2 and 5 A g(-1), respectively. Remarkably, the VS2 anode shows a high initial Coulombic efficiency of 94% and nearly 100% in the subsequent cycles, which points to the promising application of the present material in the commercial sodium-ion batteries. Moreover, VS2 nanostructures also exhibit superior rate performance with a discharge capacity of 277 mAh g(-1) at a current density of as high as 20 A g(-1). Quantitative kinetic analysis indicates that the sodium storage is governed by a pseudocapacitance mechanism, particularly at high current rates. Combined with ex-situ Raman, HRTEM and SAED characterizations further reveal that the Na+ storage is based on electrochemical intercalation-de-intercalation reactions.