An Aqueous Mg2+‐Based Dual‐Ion Battery with High Power Density

Abstract Rechargeable Mg batteries promise low‐cost, safe, and high‐energy alternatives to Li‐ion batteries. However, the high polarization strength of Mg 2+ leads to its strong interaction with electrode materials and electrolyte molecules, resulting in sluggish Mg 2+ dissociation and diffusion as well as insufficient power density and cycling stability. Here an aqueous Mg 2+ ‐based dual‐ion battery is reported to bypass the penalties of slow dissociation and solid‐state diffusion. This battery chemistry utilizes fast redox reactions on the polymer electrodes, i.e., anion (de)doping on the polyaniline (PANI) cathode and (de)enolization upon incorporating Mg 2+ on the polyimide anode. The kinetically favored and stable electrodes depend on designing a saturated aqueous electrolyte of 4.5 m Mg(NO 3 ) 2 . The concentrated electrolyte suppresses the irreversible deprotonation reaction of the PANI cathode to enable excellent stability (a lifespan of over 10 000 cycles) and rate performance (33% capacity retention at 500 C) and avoids the anodic parasitic reaction of nitrate reduction to deliver the stable polyimide anode (86.2% capacity retention after 6000 cycles). The resultant full Mg 2+ ‐based dual‐ion battery shows a high specific power of 10 826 W kg −1 , competitive with electrochemical supercapacitors. The electrolyte and electrode chemistries elucidated in this study provide an alternative approach to developing better‐performing Mg‐based batteries.