Sodium in situ Intercalated Ultrathin δ-MnO₂ Flakes Electrode with Enhanced Intercalation Capacitive Performance for Asymmetric Supercapacitors

The sodium ion intercalated ultrathin δ-MnO₂ flakes (abbreviated as Na_xKMO) are synthesized by one-step hydrothermal synthesis. Benefiting from the synergistic effect of sodium ion intercalating process and the homogeneous few-layered microstructure, the Na_xKMO electrode shows increased electron conduction as well as largely enhanced intercalation capacitive performance of 324.2 F g⁻¹ at 0.5 A g⁻¹ and 40.91 % capacitance retention at 8 A g⁻¹ is obtained. Meanwhile, the 91.9 % diffusion contribution for Na_xKMO electrode is clarified to be higher than that of KMO electrode, which indicates the preintercalated sodium ions caused a promoted diffusion-controlled kinetic in charge-discharge processes for the Na_xKMO electrode. An asymmetric supercapacitor device (ASC) is assembled with the Na_xKMO cathode and activated carbon anode. With a voltage window of 1.6 V, the fabricated ASC device delivers a high energy and power densities of 31.2 Wh kg⁻¹ and 1571 W kg⁻¹, respectively. With the continuous charge-discharge tests, the device demonstrates outstanding capacitance retention of 98.2 % after 5000 cycles at 1 A g⁻¹, suggesting a great potential of sodium ion intercalated MnO₂ film electrodes in practical applications.