Large Conjugated Polyimide Cathodes of Rechargeable Magnesium Batteries: Electron Delocalization Induced by Bivalent Magnesium Cations

Rechargeable magnesium batteries are promising for grid energy-storage applications, and organic conjugated polymers are highly advantageous magnesium battery cathodes with open and amorphous frameworks. Herein, a series of polyimides with different conjugated structures are fabricated and studied as cathodes for rechargeable magnesium batteries. The capacity increases with extension of the conjugated structure, and the large conjugated poly(benzene perylene imide) shows a high magnesium storage capacity as well as outstanding long-term cycling stability and rate capability. Mechanism investigation indicates the large conjugated polyimide shows enhanced carbonyl enolization, prominent electron delocalization of the conjugated structure, and highly reversible electron density change during magnesium association. Theoretical calculation reveals the magnesiation structure shows a uniform distribution of HOMO and LUMO on the conjugated framework, leading to a moderate change of the HOMO and LUMO energies during magnesium storage and thus higher tolerance against oxidation and higher reduction tendency than the lithiation structure.