| Because of the safety and low cost of the Lithium iron phosphate battery, it is the most popular kind of many commercial batteries, and owns broad prospects for application. However, the low intrinsic conductivity and low lithium ion diffusion coefficient of the lithium iron phosphate lead to the bad performance in the low temperature. We usually compare the electrolyte to the blood of battery, so it is a good idea to improve the low temperature performance by adjusting the formula of electrolyte.In this paper, by changing the solvents and additives of electrolyte, we invest the special capacity, cycle time and rate performance of LiFePO4/Li and MCMB/Li half cells in both room temperature and low temperature. The solvents are ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate and propylene carbonate. The additive is propylene sulfite. Firstly we invest the lithium iron phosphate produced by commercial method in all aspects. Then we test the electrochemical performance of the battery assembly. Conclusion as followed:(1) We use the method of carbon thermal reduction to produce the lithium iron phosphate, and the particles are similar to the shape of sphere, the diameter of which is about 1μm. According to the energy spectrum analysis, its surface covers a layer of carbon film. Because of the low heating temperature and the short time of the progress, this craft is suitable for mass production.(2) To improve the low temperature performance of battery, we feel about the ratio of solvents. As the high dielectric constant of EC, the high content of EC degrades the low temperature performance, then we ultimately determine the solvents ratio of EC, DMC, EMC, which is 1:1:3. In the case, the conductivity of electrolyte is 2.206mS/cm under the temperature of-40℃. At the same time, the special discharge capacity of half cell is 40mAh/g. Both the low temperature performance and room temperature performance are very good.(3) Due to the low melting point of PC, we optimize the formula of the electrolyte through replacing EC by PC according to certain proportion. Through exploration, we find that when the ratio of EC and PC is 2:1, Both the low temperature performance and room temperature performance are improved dramatically. Under low temperature, not only the special discharge capacity is 50.1mAh/g, but also the discharge voltage is as high as 2.9V. And both the cycle performance and rate performance are outstanding.(4) Through the EIS test at both room and low temperature, we find the charge transfer impedance of LiFePO4/Li half cell increases rapidly under low temperature, which is 1000 times impedance under room temperature. But this kind of half battery, whose ratio of EC and PC is 2:1, not only owns a lower impedance of 160Ω, but also owns the lowest impedance among these five kinds.(5) Due to PC destroying the MCMB seriously, we need to add a little amount of addictive PS. The aim of adding PS is to form a stable film on the anode material. We eventually determine to add PS of 3%, which is volume fraction. This method improves not only low temperature performance of LiFePO4/Li half cell to 62.1 mAh/g, but also stability of MCMB/Li half battery. The special capacity of MCMB/Li half battery is very high, and through 80 charge and discharge cycles, the capacity keep rate is as high as 98.5%. |
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