Vertical β-MnO₂@δ-MnO₂ Core–Shell Heterostructures with Superior Cycling Stability for All-in-One Flexible Supercapacitors

β-MnO2, as the most stable phase of polycrystalline MnO2, has a narrow tunnel structure that limits ion diffusion and electron transfer, restricting its application in supercapacitors. In this work, we present a novel all-in-one MnO2-based material, β-MnO2@δ-MnO2, featuring a unique three-dimensional architecture with ultrathin δ-MnO2 nanostructures vertically grown on the β-MnO2 network. This innovative design leverages the structural support of β-MnO2 to enhance the electrode material’s specific surface area and effectively mitigate volume changes during ion adsorption/desorption, a key factor for improving cycling stability. As a result, the β-MnO2@δ-MnO2 composite demonstrates exceptional electrochemical performance, with a remarkable cycling stability of 98% capacitance retention after 10,000 cycles. Additionally, it maintains 74.2% of its capacitance when the current density increases from 1.0 to 20 A g–1, showcasing an outstanding rate capability. When assembled into an asymmetric supercapacitor device, the β-MnO2@δ-MnO2 electrodes deliver a high surface capacitance of 287.3 mF cm–2 and a significant area energy density of 159.3 μWh cm–2. This work represents a significant advancement in the optimization of β-MnO2 for supercapacitor applications, demonstrating the practical benefits of rational nanostructure design. The β-MnO2@δ-MnO2 composite not only enhances performance in energy storage devices but also holds potential for other applications, offering new opportunities for the development of flexible, high-performance energy storage systems.