| As a kind of promising lithium ion battery, olivine structure of lithium iron phosphate has attracted much attention. Compared with Co, Ni and Mn based compounds; the overwhelming advantages of LiFePO4 are lower toxicity and higher stability and security than them. Nowadays, it has been used in commercial applications. However, several problems are still needed to be solved, such as reducing the cost of preparation of LiFePO4, improving the consistency and the pass rate of product, etc. Therefore, it is an important subject to search new methods of synthesis LiFePO4 at present. In this paper, LiFePO4/C composite and LiFePO4/C composite materials doped with Mn cathode materials have been synthesized by solid state-carbothermal reduction method using cheap Fe2O3 as iron precursor and sucrose as the reducing agent and carbon source through low cost and simple synthesis process with the orthogonal experiment design, then optimized the synthesis conditions. The crystal structure, electrochemical properties and the state of Fe2+/Fe3+ in the synthetic material were investigated by XRD, Laser particle size analyzer, SEM, Mossbauer spectrometer and electrochemical properties test systerm etc. The main experimental results were listed as following:(1) LiFePO4/C composite cathode materials were prepared by carbothermal reduction method, and according to the R value of orthogonal experiment, the factors of influencing the size of specific surface area are heating time (referred as "C"), heating temperature (referred as "B") and carbon content (referred as "A") in turn and the optimal combination was A1B1C1. XRD results showed that it was an olivine-type structure.(2) In analyzing the factors of affecting the initial discharge specific capacity of the materials, it has been found that the carbon content plays an important role on the electrochemical properties. The electrochemical properties of the A1B1C3 combination is better than that of the A1B1C1 combination, though the A1B1C1 combination with larger surface. That is, the optimal combination is A1B1C3. Mossbauer test results also showed that there is still partly Fe3+ in the samples not being reducted. So, anode materials with better electrochemical properties can be obtained by extending the heating time, improving material crystallinity and uniform particle size. The optimal combination of A1B1C3 shows the initial discharge capacities of 122.2 mAh·g-1,116.4 mAh·g-1 and 81.7 mAh· g-1 at 0.5C,1C and 5C, respectively. And after 50 times charge/discharge cycles, the discharge capacities of combination of A1B1C3 are 119.0mAhg-1, 115.mAhg-1 and 80.3mAhg-1 at 0.5C,1C and 5C, respectively. This means that the combination of A1B1C3 has an excellent cyclical stability.(3) XRD characteristic peak of the LiFePO4/C composite materials doped with Mn is found to be smooth, sharp, and there is no obvious diffraction peak of the Mn2+ in the XRD patterns, this means that the addition of small amount of the Mn2+ ion content into LiFePO4/C composite will not change the olivine-type crystal structure of LiFePO4/C, however, it will give rise to the homogenization of grain size and the inhibition of reunion.(4) Mossbauer experimental results show that the Fe3+ characteristic peaks of LiFePO4/C composite materials doped with Mn are higher than that of undoped Mn2+. In addition, the addition of Mn2+ will result in the change of the electron distribution outside of the iron nucleus, the decrease of its symmetry, the increase of the quadrupole splitting and octahedral distortion of LiFePO4. This obvious change has been found in the LiFePO4/C composite materials doped with 4% Mn2+.(5) The LiFePO4/C composite materials doped with 3% Mn2+ has a relatively good electrochemical property, which shows the initial discharge capacities of 121.9 mAh · g-1,108.5 mAh · g-1 and 90.7 mAh · g-1 at 0.5C,1C and 5C, respectively. This means that the LiFePO4/C composite materials doped with 3% Mn2+ has an excellent retention capacity.(6) In order to further improving the electrochemical properties of the materials, two stage heat treatments with different temperatures were used in synthesising the LiMno.o3Feo.97P04 composite, and the results show that the electrochemical properties of the materials prepared by two stage heat treatments, that is, synthesized at 700℃ for 2h first, and then at 660℃ for 14h, are obviously improved. The grain size distribution of this sample is uniform, and its electrochemical property is the best, which shows discharge capacity of 136.8 mAh · g-1 after 50 times charge/discharge cycle at 0.5C,121.8 mAh · g-1 after 100 times charge/discharge cycle at 1C and 92.8 mAh · g-1 after 150 times charge/discharge cycle at 5C. It indicates that this sample has an excellent large current discharge property. |
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