| Iron phosphates are very appealing lithium battery positive materials due to their low cost and low toxicity. The lithiated iron phosphates were attract the attention of the battery community, specially on LiFePO4, the well-known phospho-olivine. More recently, some attention has been devoted to the nonlithiated iron phosphates. This class of compounds includes very common materials, which occur in nature exist in the form of minerals, differing in stoichiometry, structure, and the content of crystallization of water. Clearly, not all these materials are suitable for battery application because, in this case, parameters such as the specific capacity and the cycle life must be taken into account.In this paper, different crystal systems of FePO4/FePO4·nH2O were synthesized. The mutual transformation relationships between them and the olive-style LiFePO4were systematic studied, using electrochemical intercalated-Li and solid phase intercalated-Li under high temperature methods. The research of structure and electrochemical properties which different crystal systems of FePO4/FePO4·nH2O and corresponding intercalated-Li products were carried out by XRD、SEM、IR、Raman、 TG-DSC、charge and discharge performance testing of the battery techniques. The major research contents and conclusions are as follows:The preparation of FePO4/FePO4·nH2O:The trigonal system of FePO4, P3132space group, which using Fe and H3PO4as raw materials was synthesized. The reactants were fully mixed by ultrasonic. The precursor under the air atmosphere firing at high temperature, which result to the particle size of the products is larger and highly crystallized; The monoclinic structure of FePO4·2H2O, P21/n space group, which using Na3PO4·12H2O and FeCl3·6H2O as raw materials was prepared. The pH of reaction system was controlled by HC1. FePO4·2H2O was hydrothermally synthesized at165℃for2days; Amorphous FePO4·nH2O was synthesized by mixing and FeCl3·6H2O in a stoichiometric ratio. The HC1was added to adjust the pH. Amorphous FePO4·nH2O was prepared using co-precipitation method, XRD showed that an irregular particle.1. Iron phosphate, FePO4which belong to trigonal system, electrochemical intercalated-Li process without altering the host structures. During the first discharge process, lithium can be intercalated into FePO4result to generate LixFePO4phase. The charge-discharge curve shows the average of FePO4is much smaller than that of LiFePO4olivine structure which display a plateau and do not have the typical characteristics of orthorhombic LiFePO4. Due to highly crystallized and larger particle size, the trigonal FePO4in electrochemical inert state. This paper uses a new processing method-wet ball milling, in other words, this material and acetylene black were mixed with acetone by ball milling, so that the electrochemical performance of it improved effectively. The crystal system, morphology and electrochemical performance of FePO4were influenced by the calcining temperature. The results show that the sample was calcined24h under400-700℃, the calcining temperature is400℃, the product exists in the form of amorphous; The heat temperature is500℃, the material start crystallizing, but not form crystalline phase and exit the amorphous in the system; while the temperature over500℃, the sample completely turn into hexagonal FePO4. The surface of FePO4has crystalline phase which strongly affect the activity of sample, the capacity of this system is small. The charge-discharge capacity of FePO4is greatly increased with good cycling performance after wet milling of crystalline FePO4with acetylene black in acetone. This shows the possibility of increasing its capacity by controlling the particle properties of FePO4, such as particle size and ratio area. However, the calcining time overlong, small size particles were reunited result to the sample become bigger and obtain the new area disappear so that contact portion of electrolyte/conductive agent was decreased. These factors for electrochemical performance are harmful, so controlling the ball-milling time is important.2. Electrochemical intercalated-Li process for the monoclinic FePO4·2H2O, the end of first discharge the material incorporates0.7Li per FePO4result to generate Li0.7FePO4·2H2O phase. From the battery voltage curve of this system was observed discharge platform at2.8-2.7V, which is much smaller than that of LiFePO4olivine structure which display the plateau. To illustrate that the crystal systems of product is not belong to orthorhombic. The reaction shows the characteristics of single-phase mechanism. Compared with the trigonal FePO4, the pure phase without any treatment, the first discharge capacity is106mAh/g. SEM pictures of FePO4·2H2O powders can be seen that the approximate rhombus particles in the sample. The size of particle is appropriate. Lattice dynamics studied by Raman and FTIR spectroscopy show the strong inductive effect in the [FePO4] framework, these factors can affect the electrochemical performance of it. Due to partial Li+-ions were difficult extracted after first discharge process. With the fraction of Li+-ion insertion decreasing, the special capacity becomes smaller and smaller. In addition, the decay capacity of material is related to the conductivity of sample itself.3. Electrochemical intercalated-Li course for the amorphous FePO4·nH2O, during the first discharge process of the material, lithium can be intercalated into the amorphous FePO4·nH2O host. Due to active substance of this material has not been activated, cause first discharge capacity is low. After recharge cycle process, sample fully displays its intercalated-Li ability. The second discharge capacity of this material was significantly increased. The bigger reversible scores of insertion and de-insertion of Li ions, the better electrochemical performance of this material. The discharge capacity affected by the conductivity of material which show light fade in later cycle. Electrochemical insertion-Li process for the amorphous FePO4·nH2O show the characteristics of single phase reaction which product the LixFePO4phase. The charge and discharge curves don’t have the typical characteristics of orthorhombic LiFePO4.This course different with LiFePO4-FePO4system which described as a two-phase reaction. Compared with trigonal FePO4and monoclinic FePO4·nH2O, electrochemical performance of amorphous FePO4·nH2O is better than the former. The reason is a natural form of this sample. The reversible scores of insertion and de-insertion of Li-ion is increased. The particle size is small result to shorten the distance of Li ions diffusion and speed up the Li-ion rate of expansion.4. Solid phase intercalated-Li at high temperature for trigonal FePO4, monoclinic and amorphous FePO4-nH2O. The results show that no matter what crystal system the samples are, the material was treated by high-temperature reduction process, the form of finial products were transform into another type. XRD pattern show the products are olive-type LiFePO4. The charge and discharge curve have the typical characteristics of orthorhombic LiFePO4. The reaction shows the distinguished feature of two-phase mechanism. Using different levels of carbon influencing the electrochemical performance of powder were studied. With the content of the carbon increasing, the first charge-discharge capacity of sample is increased. At small discharge rate, the decay of material capacity is slight; while at large rate condition, the polarization of electrode is relatively serious. The large current lead to structure of LiFePO4was light distort during charging and discharging process. Lithium-ion reversibly extracted and inserted in LiFePO4host frame work is restricted, so the capacity of material decay severely. Compared with the discharge capacity of three kinds of LiFePO4/C which have the same content of carbon, the sample was synthesized using amorphous FePO4·nH2O as precursor which the discharge capacity is best. The monoclinic system take the second place. The trigonal FePO4is the worst. The reason is the properties of the final product are affected by the particle size and morphology of the precursor. Because of high temperature reduction is the traditional synthesis method, and the characteristics of the materials were impacted by synthetic temperature, calcining time, the content of carbon additives, impurities. |
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