Vanadium trioxide@carbon nanosheet array-based ultrathin flexible symmetric hydrogel supercapacitors with 2 V voltage and high volumetric energy density

Direct synthesis of large-scale electrode nanostructures on flexible current collectors is crucial to realize high-performance emerging flexible supercapacitors (SCs). However, three-dimensional (3D) macroporous and thick current collectors such as carbon cloth/foam were mostly utilized in previous studies. These 3D current collectors unfortunately wasted much internal space (tens of micrometer interspacing), resulting in quite low volumetric energy density of the flexible SC devices. To address this issue, herein, we choose highly conductive pseudocapacitive vanadium trioxide (V2O3) as an example and report for the first time the growth of a V2O3@carbon nanosheet array directly on a 10 μm ultrathin Ti current collector on a 600 cm2 scale through a hydrothermal & post-annealing strategy. The array is further utilized to assemble a symmetric ultrathin (40 μm) flexible quasi-solid-state SC with the integration of polyvinyl alcohol (PVA)–LiCl gel electrolyte. With different redox reactions of vanadium ions in the anode and cathode, our symmetric hydrogel SC achieved an exceptional cell voltage of 2.0 V, outstanding rate performance (∼47.2% capacitance retention with the rate increasing 80 times) and remarkable volumetric energy and power densities of 15.9 mW h cm−3 and 6800 mW cm−3, respectively (without considering the encapsulation film). It also exhibits excellent cycling stability (>6000 times) and ∼100% capacitance retention at various bending states.