Low Temperature Performance Of Lithium-ion Batteries That Used LiFePO₄ As Cathode Material

This paper demonstrates the low temperature performance of lithium-ion batteries that used LiFePO4 as cathode material. The reason that limits lithium-ion batteries' low temperature performance is researched. The way to improve lithium-ion batteries' low temperature performance is provided. Lithium-ion batteries'low temperature performance is improved by carbon doping, using new electrolyte and the optimization of the preparation about button type battery.LiFePO4/C composite cathode material was synthesized by liquid phase method. The battery has 65.6mAh/g capacity at -20℃of 0.1C rate, which is only 41.67 percent of it at 25℃. At low temperature, the polarization phenomenon of battery is much more serious. EIS test tells us that the apparent chemical diffusion coefficient of lithium ions at -20℃is 0.15×10-11m2/s, which has 10 percent of it at 25℃. The poor performance of lithium ion battery at low temperature is mainly due to the sharp reduction of lithium ion diffusion rate. The battery has lower lithium intercalation potential at low temperature. Li+ will precipitate at the surface of anode. The precipitation of lithium makes SEI impedance much larger. This can limit the performance of lithium-ion batteries' charge capacity.LiFePO4/C composite cathode material was synthesized using single molecular weight PEG as carbon source. This low temperature performance of lithium-ion batteries is studied by changing the quantity of carbon doping. When the quantity of carbon doping is 0%,9%,12%,17%, the batteries' discharge capacity are 58.6,76.7,81.2,95.1 and 111.7mAh/g. It has higher discharge capacity, when he quantity of carbon doping is more. TEM test is also used to characterize the sample's structure. There is uniform carbon film at the surface of sample grain, about lOnm thick. The lithium ion diffusion rate is improved by carbon doping.ES and LiBOB are used as additive to improve the low temperature performance of electrolyte. The doping quantity of ES and LiBOB is researched. When the doping quantity of ES is more or less, it will limit batteries' low temperature performance. The best doping quantity is 3%. But the doping quantity of LiBOB, the more the better during 2%～5%. It suggests that ES and LiBOB used together can improve batteries' low temperature performance largely.The preparation process of button type battery is researched. The conductive agent content is studied. Much more conductive agent content is good for battery's electrochemical performance. The battery's discharge capacity is 96.5mAh/g when the conductive agent content is 20%. The best press is 14MP during the preparation of positive pole piece. It's much better to control the film thickness at 300μm. The better activation temperature is 50℃.