Controlled synthesis of 3D porous VO2(B) hierarchical spheres with different interiors for energy storage

Abstract

A strategy for the synthesis of three-dimensional (3D) porous VO₂(B) hierarchical spheres with different interiors (solid spheres and hollow spheres) was developed by a template-free method. 3D VO₂(B) solid spheres were synthesized by the precipitation of precursor spheres at room temperature and subsequent calcination, while 3D VO₂(B) hollow spheres were prepared by the hydrothermal treatment of precursor spheres and subsequent calcination. BET specific surface areas of VO₂(B) solid spheres and hollow spheres were measured to be 18 and 29 m² g⁻¹, respectively, and their porosity varied from mesoporous to macroporous with hierarchical structures. Electrochemical properties of 3D porous VO₂(B) hierarchical spheres were studied by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS). As electrode materials, the specific capacitances of 3D VO₂(B) hollow spheres and solid spheres reach as high as 1175 mF cm⁻² (336 F g⁻¹) and 951 mF cm⁻² (272 F g⁻¹) at 2 mA cm⁻², respectively. After 10 000 cycles, the capacitance retention of about 68% and 49% was observed for VO₂(B) hollow spheres and solid spheres, respectively. Furthermore, symmetric supercapacitor (SSC) devices using VO₂(B) hollow spheres and VO₂(B) solid spheres were assembled, and they exhibited good pseudocapacitive properties. VO₂(B) hollow sphere-SSC device delivered an areal capacitance of 246 mF cm⁻² at 1 mA cm⁻², while VO₂(B) solid sphere-SSC device delivered an areal capacitance of 122 mF cm⁻² at 1 mA cm⁻². VO₂(B) hollow spheres exhibit better electrochemical properties, including specific capacitance, areal energy density and cycling stability, compared with those of VO₂(B) solid spheres. The findings in this work demonstrate that 3D porous VO₂(B) hierarchical spheres with different interiors can improve the electrochemical properties of VO₂(B) and can be considered promising candidates for high-performance energy storage materials.