Fast and Reversible Four‐Electron Storage Enabled by Ethyl Viologen for Rechargeable Magnesium Batteries

Abstract Magnesium (Mg) batteries are the most promising “post‐lithium‐ion” energy storage technologies owing to their high theoretical energy density, low cost, and intrinsic safety with air and moisture. However, the development of Mg batteries has been limited to cathode materials leading to low power, low reversible energy density, and poor cycle life. Here, a new Mg cathode is reported based on ethyl viologen (EV), which not only has a fast redox couple EV²⁺/EV⁰ but also is capable of coupling with redox‐active anions, such as iodide (I⁻), achieving a total four‐electron storage. The EV²⁺/EV⁰ redox couple demonstrates a superior rate performance (10 C) and stable cycle life (500 cycles) owing to intrinsic fast electrode kinetics. A high material utilization (>80%) can be achieved at 1.0 C under a high areal loading of 5 mg cm⁻². When coupling with iodide I⁻, a reversible four‐electron storage is achieved with a high energy density (304.2 Wh kg⁻¹) and a stable cycle life (>100 cycles). This study provides effective strategies for designing reversible multielectron storage for high‐rate and high‐energy rechargeable Mg batteries.