Graphene-Encapsulated CuP₂: A Promising Anode Material with High Reversible Capacity and Superior Rate-Performance for Sodium-Ion Batteries

Abstract

Metal polyphosphides are regarded as the ideal anode candidates for sodium storage because of their high theoretical capacity, reasonable potential, and abundant resource alternative. However, most of them suffer from irreversibility problems, as reflected by their low reversible capacity, inferior Coulombic efficiency (CE), low rate capability, and poor cycling stability. In this work, we systematically compare the electrochemical behavior of a variety of polyphosphides bulks, discovering that the CuP₂ bulks have higher initial reversible capacity (416 mAh g^-1 at 0.1 A g^-1) and CE (74%) compared to the FeP₂, CoP₃, and NiP₂ bulks, which is related to the unique crystal structure of CuP₂. The CuP₂ electrode is optimized by the rational design of encapsulating CuP₂ nanoparticles into three-dimensional graphene networks (CuP₂@GNs), leading to excellent electrochemical performance. In the carbonate electrolyte, the CuP₂@GNs electrode can deliver the reversible capacities of up to 804, 736, 685, 621, and 508 mAh g^-1 at 0.1, 0.5, 1, 2, and 5 A g^-1, respectively, along with a first CE of 66%. The reversible capacity can be up to 737 mAh g^-1 at 0.1 A g^-1 with a first CE of 83% in the ether electrolyte. These excellent performance demonstrates that CuP₂@GNs could be a promising anode material for sodium-ion batteries.