Structural Directed Growth of Ultrathin Parallel Birnessite on β-MnO₂ for High-Performance Asymmetric Supercapacitors

Two-dimensional birnessite has attracted attention for electrochemical energy storage because of the presence of redox active Mn⁴⁺/Mn³⁺ ions and spacious interlayer channels available for ions diffusion. However, current strategies are largely limited to enhancing the electrical conductivity of birnessite. One key limitation affecting the electrochemical properties of birnessite is the poor utilization of the MnO₆ unit. Here, we assemble β-MnO₂/birnessite core-shell structure that exploits the exposed crystal face of β-MnO₂ as the core and ultrathin birnessite sheets that have the structure advantage to enhance the utilization efficiency of the Mn from the bulk. Our birnessite that has sheets parallel to each other is found to have unusual crystal structure with interlayer spacing, Mn(III)/Mn(IV) ratio and the content of the balancing cations differing from that of the common birnessite. The substrate directed growth mechanism is carefully investigated. The as-prepared core-shell nanostructures enhance the exposed surface area of birnessite and achieve high electrochemical performances (for example, 657 F g^-1 in 1 M Na₂SO₄ electrolyte based on the weight of parallel birnessite) and excellent rate capability over a potential window of up to 1.2 V. This strategy opens avenues for fundamental studies of birnessite and its properties and suggests the possibility of its use in energy storage and other applications. The potential window of an asymmetric supercapacitor that was assembled with this material can be enlarged to 2.2 V (in aqueous electrolyte) with a good cycling ability.