Experimental and theoretical investigation of cobalt and manganese substitution in Na4Fe3(PO4)2P2O7 as a high energy density cathode material for sodium-ion batteries

As an inexpensive and environmentally-friendly cathode for sodium-ion batteries, Na4Fe3(PO4)2P2O7 (NFPP) is considered to be a promising candidate for practical applications. However, the theoretical capacity of NFPP is limited to 129 mAh g-1, due to the fact that only three electrons can be participated in the reaction during the charge/discharge process. Herein, the critical role of cobalt and manganese in elevating the energy density of the pristine NFPP structure was investigated, and Na4Co0.5Mn0.5Fe2(PO4)2P2O7 (NCMFPP) is prepared in order to trigger a redox reaction with 3.5 electrons during the charge/discharge process. It is found that the NCMFPP can provide a high initial discharge capacity of 139 mAh g-1 at 0.1C (1C = 129 mA g-1), with an excellent rate capacity (75 mAh g-1 at 10C) and a remarkable long cycling stability with a capacity retention of 65.2 % after 2000 cycles at 10C. The reaction of the Mn3+/Mn4+ redox couple was characterized by X-ray photoelectron spectroscopy analysis during the charge/discharge process. These results verified the effectiveness of cobalt and manganese in elevating the energy density of NFPP as a cathode as a cathode material for low-cost sodium-ion batteries.