High-Performance Flexible Solid-State Supercapacitors Based on MnO₂-Decorated Carbon Nanotube Sponge with Ultra-high Capacitance, Long Cycling Life, and Enhanced Mechanical Strength

Manganese dioxide (MnO₂) has emerged as one of the most promising pseudo-capacitive materials with ultra-high theoretical specific capacitance in the energy storage field. Unfortunately, poor electrical conductivity and unfavorable cycling stability greatly impede its application as an excellent electrode material. In this paper, manganese dioxide (MnO₂) nanoparticles with well-controlled morphology and density are deposited on the surface of three-dimensional porous carbon nanotube sponge (CNTS) through a controllable electrochemical deposition method. The conductive and flexible CNTS skeleton greatly enhances the conductivity of a MnO₂-composed electrode and dramatically improves the cycling stability. The ratio of the pseudo-capacitive contribution to the electrochemical double-layer capacitive contribution can be tuned by MnO₂ deposition time. The CNTS@MnO₂ composite electrode and assembled symmetric flexible solid-state supercapacitor (FSSC) device using the CNTS@MnO₂ composite as electrodes achieved superior volumetric capacitances of up to 4.62 F·cm–³ (at 10 mA·cm–³) and 3.72 F·cm–³ (at 5 mA·cm–³), respectively. Meanwhile, the capacitance preservation remained around 100% after 10,000 rapid charge–discharge cycles. Moreover, the salt-induced Hofmeister effect further increased the tensile strength of the CNTS@MnO₂@PVA composite-based FSSC device to as high as 2.02 MPa (with an elongation at the break of 354%), which is almost two times tougher than the initial. Considering the excellent electrochemical and mechanical properties as well as environmental friendliness, our CNTS@MnO₂ hybrids have a feasible application prospect in flexible, wearable, and implantable electronic devices.