Lotus leaf-derived capacitive carbon for zinc-ion hybrid supercapacitors prepared by one-step molten salt carbonization

High-performance carbon-based cathode materials were prepared by means of a facile eco-friendly and cost-effective molten salt carbonization of lotus leaves in eutectic (Na/K)₂CO₃ melt at 850 °C for aqueous zinc-ion hybrid supercapacitors (ZHSCs). Coin-type ZHSCs assembled as Carbon//Zn@Zn₃(PO₄)₂ delivered 164.3 F g^− 1 at 0.2 A g^− 1 and 95.2 F g^− 1 at 20 A g^− 1 with capacitance retention of 57.9% using 2 M ZnSO₄ solution as electrolyte. Meanwhile, it delivered the maximum energy density of 65.2 Wh kg^− 1 at 169.0 W kg^− 1 and the maximum power density of 13.3 kW kg^− 1 at 23.3 Wh kg^− 1. Benefitting from the multifunctionally interface-modified Zn₃(PO₄)₂ layer acting as physical barrier and Zn²⁺-transfer ionic conductor, it revealed outstanding recyclability with capacitance retention of 96.6% and coulombic efficiency of 99.6% after 10,000 charge-discharge cycles at 1 A g^-1. The synergistic effect on energy storage performance was discussed between porous structure, specific surface area, heteroatom doping and electrical conductivity.