K⁺ intercalated MnO₂ with ultra-long cycling life for high-performance aqueous magnesium-ion hybrid supercapacitors

Aqueous magnesium-ion hybrid supercapacitors (MHSs) are very safe, low-cost, and green energy storage systems but confronted with several weaknesses such as low-capacity cathode materials and limited cycling life. In this work, K⁺ intercalated MnO₂ is developed as a cathode material for MHSs to maintain reversible phase transition during cycling. The interplanar spacing is expanded by introducing K⁺ ions between the MnO₂ layers, which shortens the ion diffusion path and exposes more ion reaction/storage sites. Therefore, the K-MnO₂-2 electrode shows a high specific capacitance of 333.3 F g⁻¹ and good rate capability. Furthermore, the aqueous MHS based on the K-MnO₂-2 cathode and AC anode exhibits a superior energy density of 85.2 W h kg⁻¹ at a power density of 360 W kg⁻¹ and an amazing cycling life with a capacity retention of 96.7% after 20 000 cycles, surpassing most of the Mn-based electrode materials for MHSs reported in the literature. This strategy to design high-performance cathodes for aqueous MHSs will provide new opportunities to develop high-efficiency energy storage devices for practical applications.