Reinforced birnessite derived from spinel Mn₃O₄ for sustainable energy storage

The layered birnessite derivatives as intercalation cathode materials for rechargeable batteries and supercapacitors receive great attention due to the abundant, low-cost, highly safe, and environmentally friendly manganese element. However, the practical application of chemically synthesized birnessite in energy storage has been restricted by low specific capacity and poor cyclability because of the limited interlayer metal ions intercalation and inferior structural stability during cycling. In this focused review, we discuss the origin of unsatisfying charge storage performance of the chemically synthesized birnessite and disclose the reinforced birnessite structures derived from spinel Mn₃O₄ by in-situ electrochemical conversion. With enhanced structural stability and large interlayer distance, the electrochemically converted birnessite shows promising electrochemical performance in various batteries and supercapacitors. Finally, critical perspectives on the future development of layered birnessite from spinel Mn₃O₄ are provided, which may guide advanced electrode design for high-performance sustainable batteries.