Preparation And Characterization Of Composite Separators With High Thermal Stability For Lithium-ion Batteries

With the widely use in portable electronic devices and electric vehicles, the safetyof lithium-ion battery has raised serious concerns, in which the thermal stability ofseparator plays an essential role in preventing thermal runaway reactions. Two sortsof composite separators are developed in this paper.Firstly, we coated commercialized polyethylene (PE) separator and trilayerpolypropylene/polyethylene/polypropylene (PP/PE/PP) separator with ethylcellulose(EC), a thermally stable and renewable biomass. The formation of the EC layer withhigh porosity is through a simple dipping and extracting process. The effects of theEC layer on thermal shrinkage, electrolyte wettability, tensile strength and cellperformance are investigated. After coating, the thermal shrinkage of PE separator atshutdown and meltdown point is reduced from20%to9%and42%to23%respectively, while the drop of OCV under increasing temperature is also postponedfrom130℃to160℃. The electrolyte wettability of pristine trilayer PP/PE/PPseparator is greatly improved, leading to increased capacity retention from28%to99%of the cell.Secondly，PVDF-HFP/ePTFE composite separator with high thermal stability andlow thermal shrinkage characteristic was developed. The PVDF-HFP acts to absorbthe electrolyte and shutdown at elevated temperature. The thermally stable ePTFEmatrix is adopted to improve the mechanical strength and sustain the insulation afterthe shutdown. This novel separator presents good ion conductivity (up to1.29mS·cm-1) and has a low thermal shrinkage of8.8%at162℃. The composite separatorshutdown at162℃and keep its integrity before329℃. Cells based on the compositeseparator show excellent capacities at high rate discharge and stable cyclingperformance.