Organic Compounds as Electrodes for Rechargeable Mg Batteries
Redox active organic materials can be considered as a suitable host matrix for multivalent cations due to their open structure caused by relatively weak intermolecular forces. Their application in realistic two electrode rechargeable magnesium batteries has been realized after the discovery of non-nucleophilic electrolytes. Different groups have tested and proposed several families of redox active organic materials. Among them, materials based on conjugated carbonyls can be seen as the most promising direction, since practical batteries with an average discharge voltage of 2 V could be obtained and with proper design of redox active organic polymers, a battery with a theoretical energy density close to 700 W h kg−1 could be realized. The mechanism of the redox process of conjugated carbonyl compounds occurs through the reversible reduction of carbonyl bonds. It is proposed that the low volumetric energy density of a cathode based on redox active organic materials is compensated by the high volumetric energy density of metallic magnesium. The remaining problems of redox active organic materials are mainly due to their solubility in electrolytes based on organic solvents, which affects the cycling stability. In addition, reports on water-based electrolytes show much better electrochemical stability. Stability issues, together with the optimization of several battery engineering parameters, such as the content of electron conductive additives, the amount of required electrolyte and areal loadings of both electrodes will in future determine the practical viability of Mg–organic batteries.