Metal cation and crystal lattice water molecule stabilized highly mesoporous manganese oxide network for excellent durable electrode in sodium-ion storage

Potassium birnessite is a remarkable material with a wider inter-planar spacing, which enables to accommodate more electrolytic ions to improve overall electrochemical performances. In this work, controlled synthesis of K_0.46Mn₂O4(H₂O)_1.4 (HKMO) nanosheets were interconnected mesoporous networks uniformly grown on carbon cloth (CC) via a one-step hydrothermal process. Specifically, the HKMO sample synthesized at 100 °C for 12 h (100@HKMO-12 h) exhibited a mesoporous morphology with a large specific surface area. The binder-free 100@HKMO-12 h electrode exhibits a maximum specific capacitance of 255F g⁻¹ (323F cm⁻³) in 1 M NaClO₄/acetonitrile electrolyte over a broad potential range of 3 V. DFT studies demonstrated the interlayer distance increased by the insertion of K⁺ ions into the MnO₂ matrix. Bader charge analysis showed a 12.09 |e| charge difference for K-birnessite in the inter-layer region compared to the normal birnessite, supported the increase of inter-layer region in the MnO₂ matrix. Significantly, the increased interlayer the distance, promoted rapid intercalation/deintercalation of Na⁺ ions and allowed the reversible faradic pseudocapacitance reaction to occur at a wider potential window. Moreover, the symmetric full-cell fabricated utilizing the 100@HKMO-12 h electrodes have a wide voltage of 2 V and the device delivered a maximum specific energy of 43 Wh kg⁻¹ (28 Wh cm⁻³) at a minimum specific power of 556 W Kg⁻¹ (349 W cm⁻³). Besides, the device showed an excellent capacitance retention of ∼94 % even after 10,000 continuous charge–discharge cycles at a current of 5 A/g, indicating it is a potential candidate for next-generation sodium energy storage devices.