Safety Testing with Nonwoven Nanofiber Separators: Comparing Shutdown Separators to Thermally Stable Separators

摘要

Pouch cells of LiFePo4 and graphite at 2700 mAh capacity were made from commercial electrodes with four different separators: 25 micron polypropylene (PP), 25 micron ceramic coated polyethylene (PE/C), 25 micron nanofiber nonwoven (Dreamweaver Silver - DWI/S), and 25 micron nanofiber nonwoven containing para-aramid fibers (Dreamweaver Gold - DWI/G). The cells had nearly identical electrical performance. The cells underwent hard short, overcharge, hot box and nail penetration tests. All cells passed hard short and overcharge tests. In hot box tests, the PE/C and PP cells experienced internal shorts when the cell temperature reached 145 C. The DWI/S and DWI/G cells showed a steady voltage for 1 hour, and continued to function after the test was completed. In the nail penetration test, the PP and PE/C cells experienced an immediate drop to zero voltage, and the temperature rose quickly to 115 C (PE/C) and 85 C (PP). The DWI/S and DWI/G cells rose to 60 C and 40C respectively, but exhibited only a ～100 mV drop in voltage and continued to function with the nail in the cell. Autopsies of the cells revealed significant shrinkage and cracking of the polyolefin separators, but primarily mechanical damage to the DWI separators.