Robust graphene layer modified Na2MnP2O7 as a durable high-rate and high energy cathode for Na-ion batteries

Na2MnP2O7 has been considered as a promising cathode candidate for advanced sodium-ion batteries due to its high potential, low cost and non-toxicity. However, the low initial Coulombic efficiency, poor high-rate and unsatisfactory cycling ability originated from the intrinsic inferior electronic conductivity and manganese dissolution severely hinder its practical application. Herein, we report an approach based on a feasible high energy vibrating activation process to fabricate a robust graphene layers (GL) modified Na2MnP2O7 material (noted as NMP@GL) for the first time. The as-prepared NMP@GL could exhibit an ultrahigh initial Coulombic efficiency of 90%, and a high energy density over 300 Wh kg-1. In addition, rate performance and cycling stability were also improved, with high capacity retention of 83% after 600 cycles at 2 C. These impressive progresses should be ascribed to the enhanced electron transportation with distinctive framework through graphene layer modifying, and structural stability of triclinic Na2MnP2O7 with spacious 3D ion migration channels. Ex-situ XRD and GITT demonstrate a consecutive multi-phase reaction mechanism with facile sodium diffusion. Our design makes Na2MnP2O7@GL to achieve its potential for practical application.