Template-free synthesis of VO2 hollow microspheres with various interiors and their conversion into V2O5 for lithium-ion batteries

Hollow microand nano-structures have attracted great interest because of their promising applications in a wide range of areas. Substantial efforts have been dedicated to the facile synthesis of hollow structures with different complex interiors, such as core-shelled, yolk-shelled, and multi-shelled hollow structures, in view of their fantastic architectures and tunable physical and chemical properties, which are attractive for many applications, such as drug delivery, 8] photocatalysis, dye-sensitized solar cells, gas sensors, and lithium-ion batteries (LIBs). A common synthetic strategy for the fabrication of hollow structures with complex interiors employs sacrificial templates, such as uniform polymer, silica, carbon, and metal oxide nanoparticles. For example, multi-shelled metal oxide hollow spheres, including TiO2, iron oxides, and SnO2, have been synthesized by employing sacrificial templates. Multi-shelled Cu2O hollow spheres have also been prepared through vesicle templating and an intermediate-templating phase-transformation process by Xu et al. However, templating methods for constructing complex hollow structures are usually time consuming and costly because of the need for the synthesis of the templates and the multi-step templating process. Therefore, it is highly desirable to develop facile, scalable template-free approaches for the rational synthesis of hollow structures with designed multilevel interior structures. Vanadium oxides, especially vanadium pentoxide (V2O5), have been extensively studied as high capacity cathode materials for LIBs in the past decades. In particular, porous or hollow-structured V2O5 materials have drawn special interest because of their advantageous features for facile Li ion insertion and good cycling stability. However, the controllable fabrication of V2O5 hollow structures with high uniformity and complex interiors through simple synthesis procedures still remains very challenging. Herein, we report the controllable synthesis of uniform VO2 microspheres with various hollow structures, including yolkshelled, multi-shelled, and single-shelled structures, through a one-step template-free solvothermal method (for detailed experimental procedures, see the Supporting Information). The complex interior structures can be simply tailored by adjusting the solvothermal reaction duration and the concentration of the VOC2O4 precursor solution. The resulting VO2 hollow complex structures are robust and could be readily transformed into V2O5 hollow microspheres, which manifest high capacity and good cycling stability when evaluated as cathode materials for LIBs. The powder X-ray diffraction (XRD) pattern of a representative sample prepared by solvothermal synthesis is shown in Figure 1a. The pattern can be assigned to the recently