Thermal expansion/shrinkage measurement of battery separators using a dynamic mechanical analyzer

Polymer separators for lithium ion batteries are thin, porous membranes of 20-30 μm thickness. At elevated temperatures, some separators can shrink considerably. To predict the deformation and stresses in the separator in battery cells, it is necessary to measure the expansion/shrinkage property of a separator as expressed by the coefficient of thermal expansion (CTE). The reported CTE measurement techniques for thin films have their limitations. This paper examines the use of a dynamic mechanical analyzer (DMA) for the measurement of the thermal expansion/shrinkage property of polymer separators. The effect of the dimensional change of the testing fixture was considered. The influences of the preloading levels and heating rates were investigated. The measurements were carried out for three common types of polymer based separator. The CTE as a function of temperature was determined from the DMA data. DMA offers continuous measurements in an automatic fashion, which is an efficient and convenient method to characterize the thermal expansion/shrinkage behavior of thin polymer films.