Green quasi-solid-state planar asymmetric supercapacitors with high working voltage and extraordinary volumetric energy density

Abstract

Planar asymmetric supercapacitors (ASCs) hold great promise as micropower units for wearable and flexible electronics, but their inadequate energy density arising from low capacitance and narrow operating voltage remain serious challenges. Two-dimensional (2D) cation-intercalated manganese oxides possess distinctive electronic properties and outstanding capacitive performance as well as exhibit great potential in planar SCs. Herein, Na⁺ intercalated manganese oxide (Na_0.55Mn₂O₄·1.5H₂O) nanosheets are first reported as a cathode in planar ASCs, and exhibit an expanded working voltage of 0 to 1.2 V (vs. Ag/AgCl). Profiting from the complementary voltage windows and matchable specific capacities of the Na_0.55Mn₂O₄·1.5H₂O cathode and porous vanadium nitride/reduced graphene oxide (VN/rGO) anode, the as-assembled planar ASCs operate stably at 2.2 V and display a competitive volumetric energy density of 27.3 mW h cm⁻³, which are superior to state-of-the-art reported planar SCs and commercially available energy storage devices. Furthermore, they reveal long-term cycling stability (90.5% retention after 10 000 cycles) and outstanding mechanical flexibility. Notably, the planar ASCs demonstrate remarkably modular integration and can be easily integrated with commercial solar cells for efficient photorechargeable systems. Collectively, the production of planar ASCs with high-voltage and high-energy opens a new route for tremendous potential applications in domestic appliances and smart wearable microelectronics.