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 Na₄Co_0.5Mn_0.5Fe₂(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.