Implications of Material Reponse for Battery Safety

Abstract

There has been a significant amount of effort to make lithium ion batteries safer, resulting in improvements across the board from safer cathode chemistries, alternate candidates for anode materials, electrolytes with wider operating windows, separators with better mechanical and thermal properties, to binders that are less reactive under abuse conditions. At the cell level, safety improvements include tabs that limit electrical conductivity at high temperatures, alternate pathways to dissipate heat and/or abuse reaction currents and preferential channeling of vent gases. Test methodologies have improved a great deal over the last few years: we are able to selectively investigate the effect of SEI additives on abuse response of the cell in situ and obtain tomographic images during thermal runaway. All these capabilities have enabled our understanding of the interactions among the different cell components during cell abuse scenario considerably. As reported previously, improvements to individual components are not always adequate to address cell safety. Given the extent of advancements in characterization techniques and material properties, the next logical step is to understand the influence of material-scale-improvements on the cell level response during short-circuit. However, this step is not straightforward. There are several chemical/ electrochemical reactions involving multiple components that take place simultaneously. Material properties such as thermal diffusivity and electrical conductivity change during both nominal operation as well as during the abuse reactions. In this study, we present a rigorous characterization of the reactivity, as well as material properties with aging of the cell. We then proceed to use these properties to better explain the temperature distribution within the cell during short-circuit. Implications of changes in material properties for propagation of thermal runaway at the cell level are described based on experimental observations. The results are used in mathematical models to assess effectiveness of different design options in improving cell-level safety.