Application Research Of The Electrolyte Used In Lithium Iron Phosphate Batteries At Low Temperature

Lithium iron phosphate batteries are the most promising power in the area of mobile phone and pure electric vehicles, power hybrid vehicles, because of low cost, high stability. However, the low intrinsic conductivity and low lithium ion diffusion coefficient in the solid phase of lithium iron phosphate, limit the olivine batteries operating in low temperature. Methods to improve the low temperature performance of lithium iron phosphate batteries, other than cathode and graphite, there is an important direction-electrolyte.In this paper, we focus on the solvents and additives of electrolyte. The special capacity of LiFePO4/Li and MCMB/Li half cells delivered in room and low temperature, cycle time and rate performance were invested. The solvents are fluorinated ethylene carbonate and ethyl acetate, the additive are gama-butyrolactone. The reasons for the low temperature performance deterioration of LiFePO4/Li and MCMB/Li half cells were studied by electrochemical test. The surface films on the negative were analyzed by SEM and Fourier transform infrared spectroscopy. Conclusions as followed:(1) Adding the new solvent FEC can decrease the viscosity of electrolyte, so that the conductivity of the electrolyte in low temperature can be improved. The batteries performance related to the volume fraction of solvent FEC. When the volume fraction of the FEC was25%, the electrolyte had the maximum conductivity at-40to25centigrade. The LiFePO4/Li half cells had a71.7mAh/g discharge capacity when discharge at0.1C under-40centigrade. MCMB/Li half cells delivered discharge capacity of250.6mAh/g at0.1C under0centigrade.When EA as the new solvent added to electrolyte, the amount of EA was60%to achieve best battery performance at-40to25centigrade. The LiFePO4/Li half cells had86mAh/g discharge capacity when discharge at0.1C under-40centigrade. MCMB/Li half cells delivered discharge capacity of177.6mAh/g at0.1C under0centigrade.(2) The performance of LiFePO4/Li and MCMB/Li half cells in low temperature were affected by charge transfer resistance. At low temperature the polarization of the battery was severe, and discharge plateau was lower than that of25centigrade. Before discharging the battery reached cut-off voltage, so that they wouldn’t deliver capacity normally. Because of new co-solvent FEC and EA, the polarization of the battery was decreased, also the electrolyte interface resistance decreased1time, the value of charge transfer resistance decreased3to4times. By adding new solvent, the MCMB/Li half cells charge voltage was increased at low temperature.(3) After adding the new co-solvent FEC and EA, reduced the proportion of EC and PC in the electrolyte. As a result, the MCMB electrode surface formed a dense, stable and low-impedance SEI film. This film coved on the MCMB electrode surface uniformly, reduced the impedance of the batteries.(4) On the basis of the optimization of the solvent,3.0%GBL was added as an additive, the conductivity of the electrolyte was2.875mS/cm at-40centigrade. Positive half-cells discharge capacities at room temperature were improved, and the capacity retention rate was86.2%,60%at-20centigrade and-40centigrade relatively. Discharge capacities of negative half-cells were significantly improved at0centigrade, but the capacity retention rate at-40centigrade was still low. The main reason was the polarization and impedance limitations when the battery subjected to low temperature. GBL was added to promote the formation of the negative electrode surface of the low resistance film, reducing the polarization of the battery, the conductivity of the film was increased.