Studies On Iron-based Phosphate Cathode Material

Lithium iron phosphate is focused in the world wild as a promising cathode material for the power batteries in terms of its excellent structural stability, low cost, and environmental benignity and safety. Compared with LiCoO2 and LiMn2O4, LiFePO₄ is low conductivity and low tap density, which are fundamental limitations for this material to be used. Researchers in the world-wide have made efforts on this issue. To improve its electronic conductivity, two main means were explored: one is the synthesis of LiFePO₄ with carbon coating; the other is the modification by doping cations into LiFePO₄. The presence of carbon in LiFePO₄ decreases the energy density, but the modification by doping cations doesn't induce this problem. However, it is still an open question as to where the doped cations in LiFePO₄ go and no clear evidence exists on Li and Fe sites.To address this issue in this work, several works have been done, including the synthesis of cation-doped LiFePO₄ by solid-state reaction or hydrothermal method, the investigation of doping cations occupancies and charge/discharge mechanism of LiFePO₄ and the exploration of iron hydroxyl-phosphates, a new cathode material for lithium ion batteries. The results we got list in the following.Spherical-like LiFePO₄ was synthesized firstly by hydrothermal method using Phenanthroline as complexing-agent to avoid the Fe(II) ions from oxidation and control the growth of the crystal. The sample possesses uniform spherical-like particles with average size of 0.5μm. Test showed that the reversible capacity of spherical-like LiFePO₄ was about 140mAh/g at 0.1C and the capacity fading was neglectable after 20 cycles.The doping cations occupancies of the Mo-doped LiFePO₄ system were investigated by XAS. Data support a concurrent replacement of Mo ions at Fe and Li sites. The electronic structures of Mo-doped LiFePO₄ were studied by ab initio calculations using VASP. With respect to the density of states (DOS) of the LiFePO₄, in which no electronic states were at the Fermi level, the Mo doping is predicted largely impacting the conductivity. The test showed the electronic conductivity of Mo-doped LiFePO₄ increased by 8 times relative to the pure LiFePO₄. The Mo-doped LiFePO₄ sample exhibits a better cycling performance with an initial specific capacity of 161 mAh/g at a 0.1 C rate, near to the theoretical limit of 170 mAh/g, with negligible fading. In particular, at the higher rate (1C), the Mo-doped LiFePO₄ shows remarkable power capability ¹40 mAh/g, which is 50 mAh/g more than that of pure LiFePO₄ powders.LixFePO₄ (x=0.89) solid solution was synthesized firstly by a hydrothermal method combined with a solid-state reaction. On another hand, we give the experimental evidence that the solid solution LixFePO₄ (x=0.89) exists at room temperature. The two phase and one single phase mechanism were investigated during charge/discharge process of LiFePO₄. The solid solution charge/discharge of LiFePO₄ is relative to the particle size, ion replacements and ion vacancies.Another cathode material, Fe_2-yâ–¡y(PO₄)(OH)_3-3y(H₂O)_3y-2 was very easy synthesized by hydrothermal methods. It exhibits solid solution mechanism during the charge/discharge process which is different with that of LiFePO₄. In Fe_2-yâ–¡y(PO₄)(OH)_3-3y(H₂O)_3y-2 framework, a face is shared by two neighboring FeO6 octahedra. According to the results of the GGA+U calculation, Fe5(PO₄)₄(OH)₃·H₂O is a higher electronic conductivity than LiFePO₄. The diffusion coefficient of lithium ion in this material is 10-12cm²/s derived from PITT, which is higher than 10-15cm²/s in LiFePO₄ and comparable with 10-11cm2/s in LiMn2O4 and 0-10～10-12cm²/s in LiCoO₂. The compound exhibits good electrochemical performance, with reversible capacities of around 150 mAh/g and 120 mAh/g at current densities of 170 mA/g and 680 mA/g, respectively, and sloping voltage charge/discharge curves characteristic of single-phase behavior.