Investigation On The PTEE/PP Composite Separators For Lithiumion Battery

Nowadays, secondary lithium-ion battery is one of high-profile clean energy which is widely used in electronics, transportation, military, medical and other fields. These fields have higher request for the safety performance of the battery pack, not only overcharge- discharge but also thermal effect have a negative impact on it. Battery is mainly composed by the anode, cathode, separator and electrolyte. Separator is a microporous membrane which provides ion channel and prevents the cathode and anode contact each other in order to prevent a short circuit. All above means that the safety performance and efficiency of battery is related to the performance of the separators. Uniaxial tensile polypropylene（PP） membrane, one of microporous membrane, which has good permeability and chemical stability is commonly used in lithium ion batteries. However, since the thermal stability of PP membrane, the heat that generated during battery run makes it shrink so that the safety performance of the battery assembled with PP membrane can not be guaranteed. “Teflon”, chemical called polytetrafluoroethylene（PTFE） has excellent thermal stability, chemical stability and non-tackiness, etc.In this article, via coating the uniaxial tensile PP membrane with PTFE dispersion,we improve its thermal stability, wetting and permeability. And then comparison with the battery assembled with PP membrane, the performance of the battery assembled with the new separator has been improved. By scanning electron microscope we observe the surface and cross section morphology of composite separators. We then observe whether the coating is effective or not by permeability test, thermal stability test and linear sweep voltammetry curve test. Moreover, we use half-cell performance test to compare the performance of half-cells assembled with these separators.It was found in above-experiments that PTFE has a good non-stick properties. The performance leads to excessive use of binders. It means that such composite separator does not get the desired effect. Fumed silica（A-SiO₂）, whose thermal stability is good, is one of conventional binder additives. So we put fumed silica particles into PTFE dispersion to further improve the performance of the separators. Owing to the stability of both dispersion is worse, along with long-time stirred prone to flocculation, we first prepare A-SiO₂ dispersion. Then we mix up these two dispersion at different ratio, and coat PP membrane with this mixed dispersion to obtain PTFE―A-SiO₂/PP composite separator.Finally, we characterize performance of this composite separators.