Engineering of Self-Aggregation-Resistant MnO₂ Heterostructure with A Built-in Field for Enhanced High-Mass-Loading Energy Storage

Although MnO₂ has been intensively investigated for energy storage, further applications are limited by van der Waals force-triggered self-aggregation that always leads to poorly exposed active sites and compromised reaction dynamics, especially under high-mass-loading conditions. Herein, by synergistically coupling interfacial modulation with the Kirkendall effect, this work achieves in situ topological structure reorganization of MnOOH toward the high-aspect-ratio MnO₂ heterostructure (Heter-MnO₂) with fully exposed active sites, which is ready to assemble into self-supporting high-mass-loading film (30 mg cm⁻²) with restrained self-aggregation. Theoretical calculation and dynamics analysis results demonstrate the generation of the built-in field within the heterostructure, thus enhancing the electronic-transfer and ionic-adsorption/transport rates. As such, the 30 mg cm⁻² Heter-MnO₂ electrode achieves a superior areal capacitance of 4762 mF cm⁻² at 1 mA cm⁻² and a sound rate performance (79% at 100 mA cm⁻²) comparable to those of low-mass-loading/thin-film electrodes. As a proof of concept, the fabricated planar interdigital quasi-solid-state symmetric micro-supercapacitor (MSC) based on the Heter-MnO₂ electrode can deliver a remarkable areal capacitance of 181 mF cm⁻² and a considerable volumetric energy density of 10.3 mWh cm⁻³. This methodology highlights the promise of surface/interface chemistry modulation for the configuration of easy-to-integrate hierarchical nanostructures to better meet practical energy applications.