Low-Crystalline Akhtenskite MnO₂-Based Aqueous Magnesium-Ion Hybrid Supercapacitors with a Superior Energy Density Boosted by Redox Bromide-Ion Additive Electrolytes

Recently, as a new type of hybrid supercapacitors, aqueous magnesium-ion hybrid supercapacitors (MHSs) have triggered continuous attention. Benefiting from the insertion/extraction processes of bivalent magnesium ions in the battery-type electrode, MHS offers the advantage of charging two electrons per ion into the battery-type material. However, the low energy density of reported MHSs is still unsatisfying for their practical applications. Herein, a novel redox bromide-ion additive aqueous MHS (B-MHS) has been designed via introducing the Br3-/Br- redox additive in 1 M MgSO4 electrolyte to promote their energy density. The optimally designed B-MHS exhibits the highest specific capacity of 268.1 mA h g-1 at a current density of 2 A g-1 in a wide voltage range of 2.6 V (0-2.6 V). Also, the maximum energy density of 262.3 W h kg-1 can be achieved at a power density of 1956.8 W kg-1, which is better than that of MHS. Most importantly, the energy storage mechanism and interactive correlation between magnesium ion insertion/extraction and redox reaction (Br3-/Br-) have been detailedly investigated. The proposed strategy provides a new route in promoting the energy density of MHS, which should be helpful in designing and constructing high-energy-density storage devices.