Relationship between Multivalent Cation Charge Carriers and Organic Solvents on Nanoporous Carbons in 4 V‐Window Magnesium Ion Supercapacitors

Abstract Multivalent charge carriers with a smaller ionic radius exhibit strong ionic interactions with solvent molecules. This can lead to unusual characteristics that typically are not observed in conventional monovalent charge carriers. Herein, the capacitive Mg ion storage behavior on nanoporous carbons is investigated within different solvent systems. A larger multivalent charge carrier, Ca ions, and alkali cations with different ionic radii are used as a comparison. In addition, the effects of nanopores on solvated Mg ion physisorption are observed in two types of nanoporous carbon, namely typical microporous carbon and ultramesoporous carbon (MEC). The oxidation stability of dimethoxyethane (DME) solvent is significantly improved by forming a solvation complex with Mg ions, while the destabilization effect of DME induced by anions is suppressed by the Mg ion charge carriers. The use of MEC as an active electrode material in a Mg ion‐DME electrolyte system leads to high electrochemical performance of the Mg ion supercapacitor over a wide range of operating voltages. A high‐performance 4 V Mg ion supercapacitor with charge‐injected symmetric MEC‐based electrodes is evaluated, where excellent specific energy and power densities of ≈ 106 W h kg −1 and ≈ 11870 W kg −1 , respectively, are achieved.