A Co ₃ O ₄ Cage/MnO ₂ Nanosheet Composite: An Enhanced Electrode Material for Flexible All‐Solid‐State Supercapacitors

The development of flexible energy‐storage devices is limited by the poor conductivity, limited cycling stability, and slow electron/ion transport of metal oxides. Optimizing their structure and performance is crucial for advancing these technologies. Herein, a hollow cubic core–shell Co 3 O 4 @MnO 2 is synthesized via annealing and hydrothermal approaches. This composite enjoys advantages of the high electrical conductivity of Co 3 O 4 and the excellent pseudocapacitive properties of MnO 2 ; in addition, the hollow structure of the composite effectively mitigates the mechanical stress caused by volume changes during charge–discharge cycles. Owing to its large specific surface area and stable framework, Co 3 O 4 @MnO 2 achieves a high specific capacitance of 670.2 F g −1 and retains 83.18% of its initial capacitance after 10,000 cycles, demonstrating exceptional electrochemical stability and durability. Subsequently, an all‐solid‐state flexible supercapacitor was assembled using Co 3 O 4 @MnO 2 positive electrode and a negative electrode made of reduced graphene oxide hydrogel. The device achieves a high energy density of 78.2 W h kg −1 and a power density of 11,500 W kg −1 . Further, even after 10,000 charge–discharge cycles, it retained 73.1% of its initial capacitance, demonstrating potential for flexible energy‐storage applications. This article highlights the advantages of Co 3 O 4 @MnO 2 in enhancing supercapacitor performance and offers insights for cost‐effective cobalt‐based energy‐storage systems.