Controllable Carbon Shell Encapsulation via Rapid Joule Heating Calcination for High‐Performance Asymmetric Supercapacitor With Suppressed Self‐Discharge and Robust Cycling Stability

Asymmetric supercapacitors (ASCs) comprising two different pseudocapacitive electrodes offer a promising route toward higher energy density, yet the serious self‐discharge behavior and poor cycle life hinder their wider applications. This work proposes a controllable carbon shell encapsulation strategy based on rapid Joule heating calcination to construct high‐performance ASC with suppressed self‐discharge and robust cycling stability. As a result, the as‐assembled ASC (H‐Fe 3 O 4 @C‐15//H‐NiCo 2 S 4 @C‐40) exhibits a maximum energy density of 105.6 W h kg −1 at a power density of 749 W kg −1 , and long cycling lifespan with 93.7% capacitance retention after 25 000 cycles. Furthermore, current ASC also demonstrates moderated self‐discharge, with its open‐circuit voltage decaying from 1.48 to 0.75 V over 31102 s. Theoretically, the density functional theory (DFT) results adequately uncover that the significantly improved self‐discharge performance should originate from the increased adsorption energy between the electrode and the electrolyte ions. Meaningfully, this controllable carbon shell encapsulation strategy represents a universal and feasible approach to effectively suppress self‐discharge and extend the cycle life of ASC.