Direct Assessment of Separator Strain in Li-Ion Batteries at the Onset of Mechanically Induced Short Circuit

In the literature, mechanical deformation of Li-ion batteries (LIB) is characterized in terms of global or volumetric strain of the entire cell to develop load vs. strain plots. In characterizing the mechano-electrical-thermal-chemical interaction of the battery in relation to internal short circuit (ISC) due to mechanical load, these estimated strains are indirect strains at best. Direct evaluation of internal local strains between the layers, particularly, in the first separator layer should be a critical material parameter as it relates to separator rupture and should be the key link in ISC in LIBs. We make an effort to assess internal local strains which is not reported elsewhere, first by using the Oak Ridge National Laboratory (ORNL) approach to use plastic deformation of aluminum casing to freeze deformation states of the LIBs followed by microscopy to image undeformed and deformed cells. An image analysis procedure is developed to estimate transverse compression strains in the cells, e.g., in Cu anode, Al cathode, and the polymeric separator. The local strain experienced by the polymeric separator nearest to ball indentation is found to be close to 65-70% and this strain level is much higher than 40-50% maximum average strains estimated for the same sample.