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Synthesis And Properties Of Cationic Doped LiMnPO 4 + / Cathode Cathode Materials

Posted on:2017-01-23Degree:DoctorType:Dissertation
Country:ChinaCandidate:E R DaiFull Text:PDF
GTID:1101330488464650Subject:Non-ferrous metallurgy
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Lithium ion batteries have been widely used due to their favorable advantages of high voltage, high energy density, long cycle life and environmental friendliness. The characteristics of lightweight, portable and high-energy have kept expanding its application range. High performance lithium ion batteries are essential to meet the requirements, and the performance of lithium ion batteries is to a great extent determined by the cathode materials. Based on reviewing the development of lithium ion batteries and cathode materials in detail, olivine structured LiMnPO4 was selected as the prototype of cathode materials. The modifications including carbon coating and Mn-site Fe2+, Mg2+, V3+ ion doping were adopted to overcome the problem of low electronic conductivity and slow Li ionic diffusion for LiMnPO4. The crystal structure, surface morphology and electrochemical properties of the materials were studied by XRD, XPS, SEM, TEM, EDS, Raman and electrochemical tests.In order to investigate the influence of Li source on the properties, LiMn0.8Fe0.2PO4/C composite materials were synthesized by solid-state method. The results show that the material synthesized with LiH2PO4 has better performance. According to the ionic conductivity of material can be improved by creating a fast ion-conducting surface phase through controlled off-stoichimetry, off-stoichiometric Li(Mn0.8Fe0.2)0.9P0.95O4/C and related materials with different off-stoichiometric ratio were synthesized. It found that off-stoichiometric method didn’t improve the performance of the material, indicating electron conduction was probably more important than ionic conduction in the LiMn0.8Fe0.2PO4/C. After using Fe2+ substituted partially Mn2+ in LiMnPO4, LiMn0.8Fe0.2PO4/C material has the best electrochemical performance at 700℃ by changing the source of Li and stoichiometric ratio. Then further mixed with Mg2+, it was found that Mg2+ doping not only can improve the electrochemical performance of the material, also can change the optimum calcination temperature of the material (800℃).During the synthesis process of carbon coated LiMn0.9Fe0.09Mg0.01PO4/C composite material, and its feed methods of carbon source will affect the effect of carbon coating and electrochemical performance of material. Combined with the feed methods of carbon source by twice, the sample will enjoy better surface carbon coating effect when added carbon source for one time, which is more favorable for charge transfer and Li+ diffusion. In this regard, the content of Fe in LiMn0.99-xFexMg0.01P04/C was optimized, and the best comprehensive electrochemical performance was obtained when x=0.19, its discharge capacity are 128.4 mAh/g and 115.1 mAh/g at 5 C and 10 C respectively.For the purpose of obtaining material with low carbon content and high rate performance, Fe&Mg co-doped LiMn0.8Fe0.19Mg0.01PO4/C composite material was prepared by spray drying method. The particles were spherical in 10~30μm for the sample got by the first time. And it had poor electrochemical performance, the discharge capacity only 31.3 mAh/g at 0.2 C. The particle size of the precursor was refined after ball milling three hours and the electrochemical performance of the material was significantly improved. We found that the synthesis conditions affected the performance of materials, as to change the iron source (ferrous oxalate and ferric citrate), ball milling time (0 h,3 h,6 h) and methods (dry-milling and wet-milling), solution concentration (the amount of deionized water is 400 ml and 800 ml), feed rate (35 rpm and 70 rpm), calcination temperature (700℃ and 800℃), feed order of raw materials (Mn-Mg-Fe-Li and Mn-Mg-Li-Fe) and drying method of precursor (spray drying and water bath method) and so on. LiMn0.8Fe0.19Mg0.01PO4/C material which synthesized by spray drying method and ball milling three hours, exhibits the best electrochemical performance when using ferric citrate as iron source, feed order is Mn-Mg-Fe-Li, the amount of deionized water is 800 ml, feed rate is equal to 35 rpm and calcination temperature is 800℃. At this time, the carbon content in material is only 3.64 wt.%, and the discharge capacity of LiMn0.8Fe0.19Mg0.01PO4/C are 143.6 mAh/g,141.7 mAh/g,121.9 mAh/g and 96.2 mAh/g at 0.2 C,1 C,5 C and 10 C respectively.The synthesis, structure and properties of V doped LiMnPO4 materials were studied. It turned out that V can be successfully doped into LiMnPO4 crystal lattice, and exist in +3 valence. What’s more, V doping can facilitate charge transport and the diffusion of Li ions, so as to improving the electrochemical properties of LiMnPO4 material. When Mn2+ was substituted partially by the supervalent V3+, charge-compensating defect is inevitable in LiMnP04. On the basis of electronic defect, Mn vacancy and Li vacancy, LiMn1-xVxPO4/C, LiMn1-3x/2VxPO4/C and Li1-xMn1-xVxPOC (0≤x≤0.15) materials were synthesized respectively. However, the solid solubility limit and the best doping amount of V are different in the three series of V-doped LiMnPO4/C samples, the Li3V2(PO4)3 and V2O3 will be formed beyond the solid solubility limit. According to minimum energy principle, the defect formation mechanism was analyzed and discussed from the perspective of synthesis. For the first time, we found the type of charge-compensating mechanism was Li vacancy in V3+ doped LiMnPO4 systems.
Keywords/Search Tags:Lithium ion batteries, cathode materials, LiMnPO4, carbon coating, cation substitution
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