Sandwiched MnO₂-Fe(CN)₆^4--Doped PPy-MnO₂ Cathodes with Tunable Interlayer Spaces and Dual Redox Active Sites for Enhanced Na Storage

The limited interlayer spacing and irreversible structure transition often lock the practical potential of 2D manganese oxide nanosheets as cathodes for Na secondary batteries. Herein, an effective nondestructively interfacial engineering approach at the atomic-level was proposed to transform the monolayer building blocks into efficient layered Na intercalation materials with tunable physiochemical properties. The 3D layered sandwiched MnO₂-Fe(CN)₆^4--doped PPy-MnO₂ superlattice was self-assembled via an electrostatic coprecipitation and layer-by-layer orderly rearrangement of MnO₂ nanosheets with guest unilamellar polymer nanoflakes. As for the redox active Fe(CN)₆^4--doped PPy interpenetrated MnO₂ architecture, the polymer species served as structural stabilizers to prevent the restacking/aggregating of metal oxide nanosheets and expand the interlayer channels to accommodate fast Na-ion diffusion, effectively enabling barely inactive MnO₂ to reach the highest Na storage capabilities among Na-intercalation cathodes. Accordingly, the remarkable specific capacity of 180 mAh g^-1, prominent rate performance, and excellent long-term cycling stability of up to 1000 cycles are obtained for the sandwiched cathode with dual active centers for Na-ion storage. Meanwhile, the proof-of-concept Na-ion pouch cell delivers a high energy of 405 Wh kg^-1 and validates the promising practical potential of the designed 3D layered intercalative cathodes.