VO2@Carbon foam as a freestanding anode material for potassium-ion batteries: First principles and experimental study

Abstract

Potassium-ion batteries (PIBs) are advanced and promising energy storage systems because of their abundant potassium resources and low cost. However, the large radius of K+ (1.38 Å) makes it diffuse slowly in the solid electrode and the electrode material volume changes greatly, resulting in the lack of satisfactory materials with high cycle performance and rate capability. Herein, first-principles calculations indicate that VO2 is a semiconductor and has two-dimensional diffusion path and a low diffusion energy barrier for K+, making it a potential anode material for PIBs. Considering the semiconductor characteristics of VO2, the VO2/carbon foam (VOCF) composites with carbon foam (CF) as a conductive substrate are prepared and studied as a freestanding electrode. The interconnected CF frames with high conductivity provide a three-dimensional conductive network to ensure efficient electron transport, and also prevent VO2 from losing electrical contact due to the volume changes caused by K+ intercalation/de-intercalation. In addition, the porous structure of the composites electrode can facilitate electrolyte penetration and improve the K+ transport. As expected, VOCF exhibits high specific capacity (443.7 mAh g−1 at 0.1 A g−1), cycle performance (236.4 mAh g−1 after 400 cycles at 1 A g−1) and rate performance (366.1, 319.3 and 251.7 mAh g−1 at 0.5, 1 and 2 A g−1). Further, the research on potassium storage mechanism of VOCF reveals that the reversible intercalation/de-intercalation reaction contributes to the potassium storage capacity.