Low‐Cost Aqueous Magnesium‐Ion Battery Capacitor with Commercial Mn3O4 and Activated Carbon

Abstract In this work, an aqueous rechargeable magnesium‐ion battery capacitor with low cost and great application potential is constructed for the first time by using commercial spinel Mn 3 O 4 as the cathode, coupling with activated carbon as the anode. Spinel Mn 3 O 4 experiences a phase transformation to layered Birnessite nanoflakes during the electrochemical process in aqueous MgSO 4 electrolyte, which acts as the actual cathode material. By decreasing the particle size of commercial Mn 3 O 4 from several micrometers to about 50–100 nm through a facile milling method, the nanoparticles can more easily experience the phase transformation, which results in significantly enhanced electrochemical properties, with a high reversible capacity of about 81 mAh⋅g ‐1 at a current density of 0.5 A g −1 after 300 cycles in a three‐electrode system. For comparison, the Mn 3 O 4 without milling exhibits only 33 mAh g −1 . Coupling with activated carbon, the full cell also exhibits excellent rate and cycle performance; for example, even after 6000 cycles at the rate of 0.5 A g −1 , it still delivers a high capacity of about 81 mAh g −1 . Such outstanding electrochemical properties are obtained without adding high cost components such as graphene or forming complex nanostructures. Its low cost and scalable preparation procedure make this approach very promising for safe energy storage.