Lithium storage in microstructures of amorphous mixed-valence vanadium oxide as anode materials

Constructing three-dimensional (3D) nanostructures with excellent structural stability is an important approach for realizing high-rate capability and a high capacity of the electrode materials in lithium-ion batteries (LIBs). Herein, we report the synthesis of hydrangea-like amorphous mixed-valence VO_x microspheres (a-VO_x MSs) through a facile solvothermal method followed by controlled calcination. The resultant hydrangea-like a-VO_x MSs are composed of intercrossed nanosheets and, thus, construct a 3D network structure. Upon evaluation as an anode material for LIBs, the a-VO_x MSs show excellent lithium-storage performance in terms of high capacity, good rate capability, and long-term stability upon extended cycling. Specifically, they exhibit very stable cycling behavior with a highly reversible capacity of 1050 mA hg^-1 at a rate of 0.1 Ag^-1 after 140 cycles. They also show excellent rate capability, with a capacity of 390 mAhg^-1 at a rate as high as 10 Ag^-1. Detailed investigations on the morphological and structural changes of the a-VO_x MSs upon cycling demonstrated that the a-VO_x MSs went through modification of the local V-O coordinations accompanied with the formation of a higher oxidation state of V, but still with an amorphous state throughout the whole discharge/charge process. Moreover, the a-VO_x MSs can buffer huge volumetric changes during the insertion/extraction process, and at the same time they remain intact even after 200 cycles of the charge/discharge process. Thus, these microspheres may be a promising anode material for LIBs.