Résumé
A solvothermal method was used to prepare Na1.25Ni1.25Fe1.75(PO4)3 nanoparticles, a new promising electrode material for lithium-ion batteries. The composition and the crystal structure were determined by 57Fe Mössbauer spectroscopy and powder X-ray diffraction Rietveld refinements and confirmed by magnetic measurements. The structural formula □0.75Na1.25Ni1.25Fe1.75(PO4)3 was obtained showing a significant amount of Na vacancies, which enhances Li diffusion. Na1.25Ni1.25Fe1.75(PO4)3 was used as negative and positive electrode material and shows excellent electrochemical performances. As negative electrode in the voltage range 0.03-3.5 V vs. Li+/Li, the first discharge at current density of 40 mA g−1 delivers a specific capacity of 1186 mAh g−1, which is almost three times its theoretical capacity (428 mAh g−1). Then, reversible capacity of 550 mAh g−1 was obtained at 50 mA g−1 with high rate capability (150 mAh g−1 at 500 mA g−1) and capacity retention of 350 cycles. As positive electrode material, specific capacities of about 145 and 99 mAh g−1 were delivered at current densities of 5 and 50 mA g−1, respectively, in the voltage range of 1.5–4.5 V vs. Li+/Li. In addition, we show that the use of solvothermal synthesis contributes to the synthesis of small sized particles leading to good electrochemical performances.