Preparation And Properties Of High-performance Lithium Iron Manganese Phosphate Cathode Materials

Lithium-Ion Batteries (LIBs) are suitable for high and safe energy applications such as portable power, electric vehicles and stored energy because of their high energy, power density and long cycle life. Building a batter performance and cheaper LIB depends to a high degree on cathode material. Olivine structure lithium iron phosphate cathode material is the preferred cathode material for electric vehicles and stored energy applications, because of its excellent cycle stability and thermal stability. However, lithium iron phosphate faces of two major drawbacks, such as relatively low working potential (～3.5V vs Li+/Li), low intrinsic conductivity. In this work, the study focus on the phosphate composite LiFe1-xMnxPO4/C (0≤x≤1), including using Mn replaced Fe to increase the working potential from 3.5V to 4.1V, and designing a variety of carbon-coated techniques to improve the conductivity of the material. The main studies are generalized as follows:Effects of preparation process on the sample structure and morphology of LiFePO4/C has been studied. Stoichiometric amounts of Li2CO3, FeC2O4·2H2O and NH4H2PO4 were ball milled in a planetary mill (Fritsch-P5) with a zirconia container, and then annealed at 600℃ for 10 hours under N2 gas flow to obtain the single-phase olivine LiFePO4 sample. Compared with the heat treatment time, the formation of olivine structure was more dependent on the temperature changed. Thus, the nucleation and growth process of LiFePO4 can be described as homogeneous nucleation process.The relationship between Mn contents and the electrochemical properties of LiFe1-xMnxPO4/C has been studied. The lattice parameters increase with the Mn content raised which followed by the Vegard’s law. It indicates that the series samples of LiFe1-xMnxPO4 were continuous solid solution between LiFePO4 and LiMnPO4. In the charge-discharge process, two oxidation reduction reactions were occurred around 3.5V and 4.1V, corresponding to Fe2+/3+ and Mn2+/3+ redox couples, respectively. With the increasing of Mn content, discharge capacity decreased, rate performance deteriorated, but the cycling stability remained. In the series of composites, the energy density was expected to increase due to the more Mn content. Unfortunately, the electrochemical performance would be deteriorated, especially when the Mn content exceeded half.Comparatively study on the effects of different electrochemical performance products, which were obtained from three carbon technologies. Added carbon source into precursor (in-situ carbon-coated), after formed olivine structure (ex-situ carbon-coated), and added twice before/after formed olivine structure (double carbon-coated). Electrochemical performance results suggested that the double carbon-coated sample had the best electrochemical properties, followed by the in-situ carbon-coated sample, and the ex-situ carbon-coated sample were the worst.After the process optimization, traditional processing techniques (i.e. ball milling, spraying and thermal annealing widely used in industry for LiFePO4 production) were used to produce an excellent cathode material (LiFe0.4Mn0.6PO4/C microspheres) with excellent electrochemical performance. A remarkable cathode material of LiFe0.4Mn0.6PO4/C microspheres had been produced via a double carbon coating process, which established a 3D carbon network between LiFe0.4Mn0.6PO4 nanoparticles, ensuring most particles contribute to electron charge/discharge. The microspheres exhibited excellent electrochemical performance with high capacity, impressive rate capability and good cycling life. The specific capacity of the microsphere was 165.8mAhg-1 at a rate of 0.1C, and reached 142, 132.3,103.3 and 72.4mAhg-1 at 3C,5C, 10C and 20C, respectively. A high reversible capacity of 152 mAhg-1 at 1C had been obtained after 500 cycles. The results demonstrated outstanding cathode materials could be produced using industrial techniques by fine designing/tuning operation conditions. The use of glucose as the carbon source further reduced the production cost. The double coating process discovered in this study could be directly applied to current industrial cathode material production process for mass production of high-energy density cathode materials for LIBs.