Recent advances and challenges in MnO₂-based composite cathodes for high-performance flexible zinc-ion batteries: A critical review

Flexible zinc-ion batteries (FZIBs) have attracted significant attention due to their potential for use in wearable electronics and portable energy storage devices. However, challenges such as low conductivity, structural instability, and limited cycling performance in traditional MnO₂-based cathodes limit their practical application. This review explores recent progress in the development of MnO₂-based composite cathodes for FZIBs, focusing on their enhanced electrochemical performance, mechanical flexibility, and structural stability. We examine various synthesis techniques, including hydrothermal, electrodeposition, sol-gel, and chemical deposition methods, which play a crucial role in tailoring the morphology and enhancing the electrochemical properties of MnO₂ composites. Emphasis is placed on the incorporation of conductive materials, such as carbon nanofibers (CNFs), graphene, MXenes, and conductive polymers like polypyrrole (PPy) and polyaniline (PANI), to improve ion transport, reduce internal resistance, and provide structural flexibility. The role of metal oxides and alkylphosphonic acids in further enhancing cycling stability and rate performance is also discussed. Key findings highlight the significant improvements in discharge capacity, rate capability, and mechanical durability achieved through these composite structures. The review also addresses the challenges associated with scalability, material cost, and potential solutions for optimizing MnO₂-based composites in flexible and wearable energy storage systems. This review paves the way for the next generation of high-performance, flexible zinc-ion batteries that meet the demands of modern energy storage applications.