| The development of electric vehicle is recognized as one of the most effective way to alleviate the oil crisis and the urban air pollution originating from the internal-combustion engine. Because of its higher energy density, lithium ion battery is the preferred battery for the electric vehicle or hybrid electric vehicle, but the safety and cost are two main obstacles for its practical use. Because of the high abundance of raw materials and the low cost, non-toxicity, excellent thermal stability and safety, good electrochemical performance and high specific capacity, the olivine LiFePO4 is accepted as the most promising cathode material for the EV/HEV usage, but its shortcomings of low electric conductivity and slow lithium diffusion need to be overcome before its larger scale application. In view of its potential use in the near future, LiFePO4 material was chosen as our investigation object. The main results are summarized as follows:1. Synthesis of olivine LiMPO4 by high temperature solid-state reaction and the investigation on its electrochemical propertiesThe phase-pure LiFePO4 was prepared by high temperature solid-state method. Presintering at 350℃for 5hrs followed by a final calcination at 650℃for 8hrs was revealed as the optimized preparation condition and thus-obtained sample can deliver 136.4mAh/g at the room temperature and 160.8mAh/g at 60℃in its first cycle. However, its capacity retentivity is poor; and the results from CV and EIS tests indicate that along with the repeated charge/discharge, the reactivity of the electrode decreases accompanied by the increased polarization. Substituting 1% Mg2+ for Fe2+ can effectively enhance the cycleability of LiFePO4 material because that doping improves the electrochemical reactivity of the material. Coating LiFePO4 with conductive carbon is another effective way to improve the electrochemical properties of LiFePO4, but the electrochemical property of LiFePO4/C is also greatly influenced by the content of carbon. Careful comparison indicates that the LiFePO4/C material with 4.39wt% carbon has the best performance. As LiMnPO4 has the same olivine structure as LiFePO4, it can form a homogeneous solid solution with LiFePO4 in a wide content range and the obtained sample shows increased cell parameters with increasing x value in the solid solution of LiMnxFe1-xP04. Electrochemical measurements show that the higher Mn content in the LiMnxFe1-xP04 phase results to a degraded electrochemical performance.2. Hydrothermal synthesis of LiFePO4 and its relative investigationThe LiFePO4 material can be successfully synthesized by hydrothermal method using LiOH-H2O、FeSO4·7H2O and H3PO4 as the raw materials and the ascorbic acid as the antioxidant addition. The phase-pure LiFePO4 can be prepared when the pH value is between 7.01 and 9.53, and the crystallinity of the final product increases as the hydrothermal temperature increases. Comparison of the LiFePO4 samples prepared under different conditions shows that when the mixture solution with the Fe2+ concentration of 0.5M was hydrothermally heated at 200℃for 6h, the best electrochemical performance can be achieved. The TG analysis tells that LiFePO4 prepared by the hydrothermal method contains 0.72wt% H2O. A calcination of the hydrothermal sample can effectively remove the water, it also brings an increased crystallinity and improved electrochemical reactivity of LiFePO4. Further investigation shows that if the hydrothermal sample is calcined with glucose, the electrochemical performance of the final product can be greatly enhanced. Addition of the surfactants in the hydrothermal solution has some effects on the morphology and the electrochemical performance of the LiFePO4 sample. It is revealed that among different surfactants, such as CTAB, PEG400 or SDS, adding the cationic surfactant of CTAB in the hydrothermal solution leads to the most notable improvement in the particle morphology. It is found that the addition of CTAB helps to better disperse the particles, and with the increasing CTAB concentration, the discharge capacity of the final product gradually increases until the CTAB concentration reaches 40mmol/L. The influence of different starting material was also studied. It is shown that when choosing LiAc+NH4H2PO4+FeSO4 as the raw materials, impurities are generated in the final product; while using LiOH+H3PO4+FeSO4 or LiOH+NH4H2PO4+FeSO4 as the raw materials leads to the highest first discharge capacity or best cycleability, respectively.3. Microwave synthesis of LiFePO4/C materials and its relative investigationLiFePO4/C was synthesized by microwave heating for 2-7minutes, using the FePO4·4H2O and LiOH·H2O as the starting materials and the glucose as the reductant. A longer microwave heating time results to a larger particle size. SEM and particle size analysis tell that 4min-microwave heating leads to the formation of LiFePO4/C sample with narrowly distributed particle size and good electrochemical performance. Comparing with the LiFePO4/C prepared by solid-state reaction, the sample derived by the microwave method has a smaller particle size, smaller reactive resistance (Rct) and a larger discharge capacity. However, it is also found that the short microwave heating time also results to the relatively lower degree of graphitization and the obtaining of less conductive carbon. The LiFePO4/C synthesized by microwave method usually contains some ferric impurities, and the type of the impurity changes with the heating time. During a short microwave heating, the ferric intermediate is the major impurity phase; while the microwave heating longer than 5min favors the formation of Li3Fe2(PO4)3 impurity. These ferric impurities can be effectively removed by a further calcination at 650℃for 1h. Microwave heating the mixture of Fe2O3, Li2CO3, NH4H2PO4 and glucose also leads to the formation of LiFePOVC, but the product has large particle size and contains some impurities even starting from the nanosized precursor. Replacing nano-Fe2O3 by nano-FePO4 precursor results to the obtaining of nano-sized LiFePO4/C, but the electrochemical property does not greatly change.4. Investigation on the storage properties of LiFePO4/C and bare LiFePO4The storage properties of LiFePO4/C and bare LiFePO4 are investigated in detail. Storage tests under different conditions tell us that moisture and temperature in the storing environment has the most profound effect on the structure as well as the electrochemical property of LiFePO4/C. When storing LiFePO4/C in dry Ar or dry air, its structure or electrochemical almost remain unchanged; while exposing LiFePO4/C to saturated humidity air at 50℃for 12 weeks, its initial charging capacity decreases and the efficiency in the first cycle reaches 125%. This negative effect is found to be originated from the spontaneous delithiation and thus-resulted impurity formation. From FT-IR and XRD measurements, we directly observe the formation of olivine FePO4 and Li3PO4 during the storage in humid-hot air, and it is further confirmed that lithium extraction occurs in different degree depending on the storing time. Increasing temperature is found to favor the delithiation reaction. In the case that the LiFePO4/C sample is stored in saturated humidity air at 80℃, an 8-weeks storing will cause 18% delithiation, greatly shrunk initial charging capacity and degraded cycling stability. Storage tests of bare LiFePO4 indicate that unlike LiFePO4/C, exposing LiFePO4 to humid-hot air will not lead to the formation of olivine FePO4 and the aging mechanism for LiFePO4 differs from that of LiFePO4/C. XRD and FTIR measurements reveal no detectable structural changes in bare LiFePO4 after exposure to saturated humidity air at 80℃for 8 weeks; however, cycling tests show that the storage results in a notable degradation in the initial charging/discharging capacity and capacity retentivity. Although lithium extraction is unavoidable and significant once LiFePO4/C is stored in a humid-hot environment, thus-induced structure heterogeneity can be repaired by a re-calcination treatment. Re-calcining the stored LiFePO4/C sample at 650℃in H2/Ar for lhour, its structure and charging/discharging behavior both recover to the primitive state. Besides the storage behavior, we also investigated the thermal stability of LiFePO4 and LiFePO4/C. It is found that, comparing with bare LiFePO4 LiFePO4/C shows more serious structure damage and more notable deterioration in the electrochemical property after being heated at 300℃in air. These differences should be explained by different specific surface area. |
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