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, Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (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 Na 4 Co 0.5 Mn 0.5 Fe 2 (PO 4 ) 2 P 2 O 7 (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 Mn 3+ /Mn 4+ 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.