Designing flexible asymmetric supercapacitor with high energy density by electrode engineering and charge matching mechanism

•An accordion-like layered vanadium nitride electrode material was synthesized.•Asymmetric supercapacitor has 2.4 V high voltage window.•A high energy density of 8.5 mWh cm−3 at 34.3 mW cm−3 was realized. Extensive exploratory works are dedicating to increase the energy density of aqueous asymmetric supercapacitors (AASCs), from the perspective of augmenting the specific capacitance and widening the operating voltage window. Herein, Mxene (V2C) derived unique accordion-like layered vanadium nitride coated by ultrathin amorphous carbon layer (L-VN@AC) is synthesized via a facile nitride strategy as negative electrode materials for AASC. Profiting from the distinctive layered structure inherited from V2C, the resultant L-VN@AC electrode exhibits a more remarkable areal capacitance (631.4 mF cm−2) and outstanding rate capability compared to the V2C precursor and commercial vanadium nitride (C-VN). Furthermore, a brand-new charge matching mechanism is proposed for the assembly of AASCs with capacitive-type (pseudocapacitance/electric double layer capacitance) electrode materials to soften the impact of energy density ascribed to the capacitance gap between the asymmetrical electrodes. Based on this charge matching mechanism, an optimized L-VN@AC//α-MnO2 AASC with 2.4 V operating voltage is assembled to achieve the enhancements in energy densities (597.5 μWh cm−2 at 2.4 mW cm−2 and 8.5 mWh cm−3 at 34.3 mW cm−3). It is believed that the design of high-performance L-VN@AC electrode materials and the proposal of balanced capacitance mechanism will provide new insights for the development of supercapacitors from two aspects of material synthesis and device assembly mechanism.