Modeling the Impact of Active Material Volume Change on Accessible Capacity in Automotive Pouch Cells

Abstract

Lithium-silicon chemistries show promise to increase battery capacity; however, silicon expands significantly during lithium intercalation, leading to inaccessible capacities as well as cell or pack failure due to pressure generation. Silicon-graphite (Si/C) composite anodes are used to increase the anode capacity while maintaining a tolerable degree of active material volume expansion. Recently, Dash and Pannala modeled anodes with increasing Si/C ratios and explored the tradeoff between increasing gravimetric capacity and the increasing active material volume expansion upon Li intercalation [1]. Their conclusion highlights that while the gravimetric capacity increases, there is ultimately a “threshold value” where the volumetric capacity will begin to decrease to account for the significant silicon volume expansion.This concept was built upon to account for electrochemical and mechanical design limitations applicable to battery packs for electric vehicles (EVs). We propose that increasing the Si/C ratio does not directly lead to an increase in the accessible capacity, because excessive volume expansion can lead to unacceptable cell pressure or electrode porosity. In order to predict the accessible capacity as a function of Si/C ratio, we integrated mechanical behavior for each individual cell component (e.g., composite anode, cathode, and foam packing) into our previous battery model [2-4]. This model can predict the split between changes in electrode porosity and dimensions by coupling component mechanical behavior to the volume change governed by electrochemical phenomena.In this presentation, we will focus on how the Si/C ratio will impact both mechanical and electrochemical behavior using our mechano-electrochemical model. Figure 1 shows an example of the pressure generation seen when considering (a) foam packaging and (b) a rigid volume for cells with varying percentages of Si in the anode. Accessible capacities of these materials can then be determined for a specific design’s pressure cutoff. Preliminary modeling of the links between active material Eeq, anode/cathode capacity balance, and volume change will also be discussed.