2.5 V “water in salt” aqueous micro-supercapacitors based on polypyrrole-coated NiCo layered double hydroxides

Layered double hydroxides (LDHs) have been considered promising electrode materials in micro-supercapacitors (MSCs) for their high theoretical capacity, adjustable composition, designable function, and facile synthesis. However, the low energy density and disappointing cycle stability are still the main problems. Herein, water-in-salt (WIS) aqueous electrolyte and PPy coating of metal hydroxides are applied to broaden the working potential and stabilize the structure during the charge/discharge process. The asymmetric MSCs with PPy@NiCo-LDH@ITO NWs as the cathode, PPy@FeOOH@ITO NWs as the anode, and WIS electrolyte as the electrolyte have been assembled. The hybrid MSCs perform a high voltage window of 2.5 V, a high areal capacitance of 63.04 mF cm−2 at 0.4 mA cm−2, a maximum volumetric energy density of 32.2 mWh cm−3 (54.7 μWh cm−2) at a power density of 294.1 mW cm−3 (0.5 mW cm−2), and remarkable cyclic stability with 85 % capacitance retention after 8000 cycles at 5 mA cm−2. Remarkably, the series–parallel lighting tests demonstrate good device conformance. These results suggest a new perspective for applying WIS electrolytes for high-performance LDH-based MSCs.