Polyimides as Promising Cathodes for Metal-Organic Batteries: A Comparison between Divalent (Ca²⁺, Mg²⁺) and Monovalent (Li⁺, Na⁺) Cations

Ca- and Mg-based batteries represent a more sustainablealternativeto Li-ion batteries. However, multivalent cation technologies sufferfrom poor cation mass transport. In addition, the development of positiveelectrodes enabling reversible charge storage currently representsone of the major challenges. Organic positive electrodes, in additionto being the most sustainable and potentially low-cost candidates,compared with their inorganic counterparts, currently present thebest electrochemical performances in Ca and Mg cells. Unfortunately,organic positive electrodes suffer from relatively low capacity retentionupon cycling, the origin of which is not yet fully understood. Here,1,4,5,8-naphthalenetetracarboxylic dianhydride-derived polyimide wastested in Li, Na, Mg, and Ca cells for the sake of comparison in termsof redox potential, gravimetric capacities, capacity retention, andrate capability. The redox mechanisms were also investigated by meansof operando IR experiments, and a parameter affecting most figuresof merit has been identified: the presence of contact ion-pairs inthe electrolyte.