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

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 CuP bulks have higher initial reversible capacity (416 mAh g at 0.1 A g) and CE (74%) compared to the FeP, CoP, and NiP bulks, which is related to the unique crystal structure of CuP. The CuP electrode is optimized by the rational design of encapsulating CuP nanoparticles into three-dimensional graphene networks (CuP@GNs), leading to excellent electrochemical performance. In the carbonate electrolyte, the CuP@GNs electrode can deliver the reversible capacities of up to 804, 736, 685, 621, and 508 mAh g at 0.1, 0.5, 1, 2, and 5 A g, respectively, along with a first CE of 66%. The reversible capacity can be up to 737 mAh g at 0.1 A g with a first CE of 83% in the ether electrolyte. These excellent performance demonstrates that CuP@GNs could be a promising anode material for sodium-ion batteries.