Cross-Linking Hollow Carbon Sheet Encapsulated CuP2 Nanocomposites for High Energy Density Sodium-Ion Batteries

Sodium-ion batteries (SIB) are regarded as the most promising competitors to lithium-ion batteries in spite of expected electrochemical disadvantages. Here a "cross-linking" strategy is proposed to mitigate the typical SIB problems. We present a SIB full battery that exhibits a working potential of 3.3 V and an energy density of 180 Wh kg^-1 with good cycle life. The anode is composed of cross-linking hollow carbon sheet encapsulated CuP₂ nanoparticles (CHCS-CuP₂) and a cathode of carbon coated Na₃V₂(PO₄)₂F₃ (C-NVPF). For the preparation of the CHCS-CuP₂ nanocomposites, we develop an in situ phosphorization approach, which is superior to mechanical mixing. Such CHCS-CuP₂ nanocomposites deliver a high reversible capacity of 451 mAh g^-1 at 80 mA g^-1, showing an excellent capacity retention ratio of 91% in 200 cycles together with good rate capability and stable cycling performance. Post mortem analysis reveals that the cross-linking hollow carbon sheet structure as well as the initially formed SEI layers are well preserved. Moreover, the inner electrochemical resistances do not significantly change. We believe that the presented battery system provides significant progress regarding practical application of SIB.