Double-chain conjugated carbonyl polymer cathode for rechargeable magnesium batteries: Constructing active sites for reversible storage of bivalent magnesium cations

Rechargeable Mg batteries are advantageous substitutes for lithium-ion batteries in large-scale energy storage with low cost and high safety, but the lack of cathode materials is blocking the development. Organic polymers are promising Mg-storage materials, and the construction of suitable Mg-storage active sites is of great significance for the performance. In the present study, a double-chain conjugated carbonyl polymer is designed and investigated as cathode material of rechargeable Mg batteries. The double-chain structure builds favorable active sites for reversible storage of bivalent Mg2+ cation by means of immobilization of the two adjacent carbonyls. This double-chain polymer shows a higher Mg-storage capacity (194 mAg–1) than the corresponding single-chain polymer (116 mAg–1). The Mg-storage capacity of this double-chain polymer also exceeds the Li-storage capacity (106 mAg–1) significantly. A mechanism study demonstrates the reversible carbonyl enolization in Mg-storage reactions. A theoretical computation demonstrates the magnesiation structure has a higher HOMO and a lower LUMO energy level, as well as a smaller HOMO-LUMO gap. This is favorable for the Mg-storage reaction reversibility and stability of the magnesiation structure, thus improving the capacities and cycling stabilities. The principle revealed in the present study provides an important insight to design organic conjugated polymers with favorable Mg-storage active sites, which would assist the development of high-performance Mg-storage materials.