Amorphous Cobalt Polyselenides with Hyperbranched Polymer Additive as High-Capacity Magnesium Storage Cathode...

Rechargeable magnesium batteries (RMBs) are a promising energy-storage technology with low cost and high reliability, while the lack of high-performance cathodes is impeding the development. Herein, a series of amorphous cobalt polyselenides (CoSe_x, x>2) is synthesized with the assistance of organic amino-terminal hyperbranched polymer (AHP) additive and investigated as cathodes for RMBs. The coordination of cobalt cations with the amino groups of AHP leads to the formation of amorphous CoSe_x rather than crystalline CoSe₂. The amorphous structure is favorable for magnesium-storage reaction kinetics, and the polyselenide anions provide extra capacities besides the redox of cobalt cations. Moreover, the organic AHP molecules retained in CoSe_x-AHP provide an elastic matrix to accommodate the volume change of conversion reaction. With a moderate x value (2.73) and appropriate AHP content (11.58%), CoSe_2.7-AHP achieves a balance between capacity and cycling stability. Amorphous CoSe_2.7-AHP provides high capacities of 246.6 and 94 mAh g^‒1, respectively, at 50 and 2000 A g^‒1, as well as a capacity retention rate of 68.5% after 300 cycles. The mechanism study demonstrates CoSe_x-AHP undergoes reversible redox of Co^2+/3+↔Co⁰ and Se_n ^2‒↔Se^2‒. The present study demonstrates amorphous polyselenides with cationic-anionic redox activities is as a feasible strategy to construct high-capacity cathode materials for RMBs.