Battery separators shrinkage

One way to achieve some of these goals will be to develop mathematical models that reflect the effects of separator resistance, thickness, pore size, shrinkage, tortuosity, and mechanical strength on the final performance and safety of batteries. The battery separators for tomorrow will demand more than just good insulation and mechanical filtration they will require unique electrochemical properties. 

Kim et al. [15] showed that dne to cross-linking, gamma irradiation of polyethylene improved the thermal shrinkage resistance of polyethylene battery separation materials compared to the thermal shrinkage of unexposed separations. 

Compared to standard polyolefin separators, Freudenberg sets itself apart with its nonwovens in a few instances. Citing the fact that internal short circuits are a major cause of failed lithium-ion batteries and that temperatures can rise very quickly (fractions of a second) in dangerous battery cells, Freudenberg s nonwovens aim to improve on three areas compared to these polyolefin separators separator shrinkage, penetration resistance, and separator meltdown. AU three of these areas greatly influence cell reliability and safety (eg, particle penetration via electrodes leads to premature cell failure). 

Battery separators are characterized by numerous properties, including material nature, membrane stractural and functional properties. Material nature includes chemical stability, crystalline structure, hydrophilicity, thermal shrinkage, melting point, M and Mv,/M of polyolefin materials. Structural properties include thickness, porosity, pore size, pore shape, pore tortuosity, and pore distribution. Functional properties include mechanical strength, electrical resistivity, air permeability, thermal shutdown, electrolyte wettability and retention. Many of the above properties are affected with each other and may be in a trade-off relationship. For example, the mechanical strength is affected in opposite manner by the thickness, porosity and permeability, as required by the battery performance. 

Thermal Stability. Lithium-ion batteries can be poisoned by water, and so materials going into the cell are typically dried at 80 °C under vacuum. Under these conditions, the separator must not shrink significantly and definitely must not wrinkle. Each battery manufacturer has specific drying procedures. The requirement of less than 5% shrinkage after 60 min at 90 °C (in a vacuum) in both MD and TD direction is a reasonable generalization. 

Jeong HS, Hong SC, Lee SY (2010) Effect of microporous structure on thermal shrinkage and electrochemical performance of A1203/poly(vinylidene lluoride-hexalluoro-propylene) composite separators for lithium-ion batteries. J Membr Sci 364 177-182. doi 10.1016/j.memsci.2010.08.012... 

DuPont polyimide separators are thin, offer lower ionic resistance when filled with electrolyte, are made with higher-temperature stable materials that offer very low shrinkage at high temperatures, and offer very good wettability with typical organic electrolytes used in batteries and capacitor applications. 

The most common separator for these batteries is the PP separator. The separator s properties for these batteries are not very stringent because there are no undesirable electrochemical deposits (e.g., dendrites), and electrodes are very smooth. Thermal runaway is not a major concern because it does not occur unless the temperature reaches 180°C. Thus, a simple separator with low electric resistance, high strength, and low shrinkage is adequate. Of course, the separators need to be thin and defect free, and should have desirable pore size distribution. 

The desirable properties of separators for lithium-ion batteries include low resistance, low shrinkage, and uniform pore structure. 

---