The ultrafast charge-discharge rate and robust cycling performance associated with capacitive electrochemical energy storage devices, makes them more appealing than batteries in some specific applications, such as electric vehicles. However, despite their high power performance, capacitive electrochemical energy storage devices suffer from relatively low energy density. Enhancing the capacitance density of these devices is one effective way to improve their overall energy density. Usually there are three types of capacitive energy storage mechanisms in electrochemical supercapacitors: double-layer capacitance, surface pseudocapacitance, and intercalation capacitance. Much has been done to improve the double-layer and pseudocapacitance of electrode materials. However, intercalation capacitance has not been sufficiently exploited. The Li+ intercalation properties of layered valence-sensitive VO2 (B), which the layers are linked by strong covalent bond, has been heavily investigated due to its stable open-framework and high theatrical capacity (323 mAh g-1 or 1163 C g-1). However, the expected high-capacity and cycling stability have not been experimentally achieved despite many attempts with various VO2 (B) nanostructures, probably due to the slow reaction kinetics.
To this end, we propose a monomer-assisted strategy to fabricate atomic thin two-dimensional (2D) VO2 (B) nanoribbons to improve its capacitive energy storage performance. We demonstrate that these 2D-VO2 (B) nanoribbons deliver unexpectedly high energy density, rate capability (> 140 mAh g-1 at 100C) and ultralong lifespan (> 9000 cycles at 20C). Furthermore, we show that the 2D-VO2 (B) offers much faster charge storage kinetics and enables fully reversible Li ions uptake and removal in its lattice using cyclic voltammetry, in-situ Raman, ex-situ XRD and ex-situ XPS studies. The higher specific capacity (> 600 mAh g-1) of VO2 (B) is attributed to its atomic-level thickness (theoretical capacity: 884 mAh g-1 for monolayer VO2 (B)). What’s more, we prove that the atomic-thin 2D feature can strongly decrease the intercalation energy of Li+ into VO2 (B) crystal and further lead to lower diffusion barriers using detailed theoretical and experimental methods. We believe that transformation of the atypical layered (or non-layered) materials into ultrathin 2D geometry could lead to significantly enhanced pseudocapacitive performance.