Graphene-Encapsulated CuP2: 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 behaviour 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 3D graphene networks (CuP2@GNs), leading to excellent electrochemical performance. In the carbonate electrolyte, the CuP2@GNs electrode can deliver the reversible capacities of 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 746 mAh g-1 at 0.1 A g-1 with a first CE of 83% in the ether electrolyte. These outstanding properties demonstrate that CuP2@GNs could be a promising anode material for sodium-ion batteries.