Synthesis of a full range Fe-doped ZnFeCo_2-O₄ and its application as anode material for lithium-ion battery

Fe-Doped ZnFeCo_2-O₄ (x = 0.00, 0.17, 0.33, 0.47, 0.67, 0.87, 1.17, 1.37, 1.67, 1.87, 2.00) compounds were prepared by a sol-gel method. X-ray diffraction measurements show that Fe-doping does not change the crystal structure of ZnCo₂O₄ and dopant Fe successfully occupies the 16c Co site. Because of the bigger radius of the doping ion, the cell parameters and cell volumes of ZnFeCo_2-O₄ compounds present an obvious linear increase with increasing Fe content. In addition, attributed to the similar crystal structures for ZnFe₂O₄ and ZnCo₂O₄, a full range (0 ≤ x ≤ 2) of ZnFeCo_2-O₄ solid solution phases was obtained. V/I measurement results show that a small Fe doping content obviously improved the electronic conductivity of the sample. In addition, due to the smaller particles size and uniform particle distribution caused by Fe doping, the lithium ion diffusion coefficient of the sample was increased by 2 orders of magnitude. Based on the improved electronic conductivity combined with the significantly increased lithium-ion diffusion coefficient, a sample with Fe doping content of x = 0.33, ZnFe_0.33Co_1.67O₄, presents a high reversible specific capacity and excellent rate cycle stability. At a rate of 100 mA g^-1, a relatively high discharge capacity of 850 mA h g^-1 can still be obtained after 100 cycles, which is obviously higher than that of pure ZnCo₂O₄ (only 295 mA h g^-1). Even at a higher discharge rate of 500 mA g^-1, a discharge capacity of 450 mA h g^-1 with a capacity retention of nearly 100% was obtained. Based on its excellent electrochemical properties, ZnFe_0.33Co_1.67O₄ will be a promising anode material for rechargeable lithium-ion batteries.