context: The fast advancement in portable and wearable electronics has aroused growing demand for energy storage devices not only with high electrochemical performance but also having extra appealing characteristics, such as high safety, low cost, environmental friendliness, and so on. Supercapacitors are a kind of energy storage device demonstrating distinct advantages of simplicity in fabrication and high power density. However, the insufficient energy density has indeed hampered their application. In contrast, lithium-ion batteries possess a much higher energy density, but suffer considerably from cost and safety issues especially under some extreme conditions. Recently, rechargeable Zn–MnO2 batteries have attracted significant research interest due to their unique features, including high safety, light weight, nontoxicity and ease of fabrication. Solid-state rechargeable Zn–MnO2 batteries coupled with hydrogel electrolytes show excellent cycling stability, making them a promising alternative to lithium-ion batteries. Nevertheless, most electrodes for rechargeable Zn–MnO2 batteries are made from synthetic and expensive materials like graphene, carbon nanotubes, acetylene black, etc., which encounter obstacles, such as complex synthesis process, low yield, insufficient capacity, and environmental incompatibility. It is also important to note that most of the recent studies on rechargeable Zn–MnO2 batteries were focused on the optimization of electrochemically active substances, but ignored the structural design of the bulk electrode materials. As is well recognized, the hierarchical microstructure of electrodes may affect their electrochemical properties profoundly, in particular when the electrode is thick or the load of active substances is high.
answer: hydrogel