Lithium Ion Battery Autopsies after Safety Testing Reveal Safety Benefits of Thermally Stable Separators
Authors: Morin Brian , Kaschmitter James , Khokhlov Pavel , Hu Carl , Cho Sung-Jin 
Published: February 2020

Pouch cells of LiFePo4 and graphite were constructed with 10 layers and a total capacity of 3 Ah, with four different separators: 25 micron polypropylene dry process, 25 micron ceramic coated polyethylene wet process, Dreamweaver (DWI) Silver 25 micron and Dreamweaver Gold 25 micron. DWI Silver is a wet laid nonwoven composed of nanofibers of cellulose and polyacrylonitrile and microfibers of polyethlyene terephthalate. DWI Gold is a wet laid nonwoven composed of nanofibers of cellulose, fibrids of para-aramid, and microfibers of polyethylene terephthalate. The cells were tested for hard short, overcharge, hot box at 170 C, and nail penetration.

Hard Short: The cells were shorted with 16 gauge wire and a switch rated to 100A. The cells all performed the same, with the anode tab (4 mm tab) getting red hot, and the cells slowly heating and the electrolyte expanding until the seal broke.

Overcharge: The cells were charged at 2C from 30% SOC until failure. All cells failed at 5.5 V by outgassing. There were no flames, and the temperature rise was uniform between cells made with different separators.

Hot Box: Cells made with PP and PE/ceramic separators both had faster temperature rise than either the DWI Silver or Gold cells. They also both experienced a catastrophic short after around 20 minutes, compared to the DWI Silver and Gold cells, which maintained voltage for 1 hour and never experienced a short. The short for the PE/ceramic cell occurred with onset at around 140 C, and for the PP cell at around 145 C. Both of the DWI cells rose to nearly the oven temperature without experiencing a short.

Nail Penetration: The nail penetration cells experienced likewise a significantly improved behavior in the Dreamweaver cells. The PP and PE/ceramic cells voltage both went immediately to zero, and the temperature rise was fast, to 90 C for PP and to 120 C for PE/ceramic. For the DWI Silver and Gold cells, the voltage dropped only by ~200 mV, and the temperature rise was slower and to a lower temperature, to 80 C for Silver and to 40 C for Gold. Autopsies of the cells performed after testing revealed even more of what went on inside. The PP cells showed a separator with splitting and shrinking around the hole made by the nail, and significant shrinkage at one end of the cell. The PE/ceramic cells showed significant splitting, cracking and shrinking around the nail hole, but no other shrinkage. The cells made with DWI Silver showed no shrinkage, cracking or splitting caused by heat, but only a little browning of the fibers and the physical damage caused by the nail. The cells made with DWI Gold showed only the physical damage of the nail, with no other effects observable.
The significant difference in cell performance is exemplified in that after both the hot box and nail penetration tests, the DWI Silver and DWI Gold cells were still functioning cells, while the cells made with PP and with PE/ceramic separators all experienced internal shorts and catastrophic failures, and the cells were not functioning after the tests.

Figure 1