Investigation on the Organic/Polypropylene Composite Separators for Lithium-ion Battery

Organic/PP composite separators were prepared by coating polymers (PVDF, PTFE, PEK-C, PES and PPSU), which are of high temperature resistance, onto uniaxially stretched PP separators. The porous coating layers of PVDF, PEK-C, PES and PPSU were obtained by phase inversion method. And PTFE coating layer were formed by coating a suspension of PTFE nanoparticles. It could be found by SEM that pores were formed in the inter-particle area when the PTFE deposited on surface of the bare PP separator. The permeability and thermal stability of the composite separators were studied. It could be found that although the permeability of the organic composite separators decreased slightly, the thermal shrinkage properties could improve obviously, which meant that organic coating layers were helpful for improving the thermal stability of the composite separators. Linear sweep voltammetry was measured and showed that the composite separators were stable in the electrochemical window for lithium-ion batteries. The organic/PP composite separators were also used to assemble cells and evaluated by the charge and discharge performance. Cycle performance showed that the cells with composite separators had higher discharge capacity and better rate discharge performance than those cells with bare PP separator. We attributed the improvement of discharge capacity and rate discharge performance to the better wetting ability of the electrolyte to the composite separators.