Sodium preintercalation-induced oxygen-deficient hydrated potassium manganese oxide for high-energy flexible Mg-ion supercapacitors

Abstract Layered potassium manganese oxides are promising candidates for use in aqueous supercapacitors owing to their wide potential windows, layered feature, and Faradaic redox reactions that occur on surfaces and in bulk regions. However, the practical application is hindered by rapid performance degradation due to their inherently low electrical conductivities and inferior structural stabilities. Here, we develop ultralong nanobelts comprising hydrated Na-intercalated oxygen-deficient potassium manganese oxide (H-Na-D-KMO), in which the Na⁺ ions are preintercalated and synchronously induce the generation of oxygen vacancies, as high-energy-density and durable electrodes for Mg-ion supercapacitors. The experimental results indicated that preintercalation of Na⁺ ions and formation of oxygen vacancies improved the electrical properties and ion diffusion, which accounted for the fast reaction kinetics and good cycling performance of H-Na-D-KMO. The optimized H-Na-D-KMO delivered a significantly enhanced specific capacitance and cycling performance compared to those of pure H-KMO. Asymmetric supercapacitors with H-Na-D-KMO as the cathode and as-prepared MoO₂@carbon as the anode exhibited an ultrahigh energy density of 108.4 Wh kg^–1 at 11,000 Wh kg⁻¹, which is superior to most supercapacitors reported in the literature. Moreover, the assembled device exhibited good cycling stability for over 5000 cycles with a fading rate of 0.002% per cycle and good mechanical flexibility, which opens an avenue for further advancements in high-energy supercapacitors.