Mechanical behavior of Silicon-Graphite pouch cells under external compressive load: Implications and opportunities for battery pack design

The mechanical behavior of high capacity (1.4Ah) multilayer Si alloy-Graphite/NMC622 pouch cells under an external compressive load is presented in this research. The results show that their mechanical behavior is more complex compared to traditional cells with graphite anodes. Three distinct mechanisms are identified in the pressure evolution: i) a reversible pressure variation related to the lithiation of the anode, ii) an irreversible relaxation that occurs during early cycles and iii) an irreversible pressure growth related to capacity degradation. All mechanisms are investigated separately and a root cause is proposed for each one. Cells are cycled at different conditions to study the effect of initial compressive load, ambient temperature, current rate and depth of discharge on the mechanical behavior. Finally, a modeling methodology is proposed to estimate cell capacity fade based on cell pressure measurements and to model the expected pressure evolution during cycling. Presumably, this pressure behavior will become a key factor in designing future battery modules and packs containing energy-dense, volume changing electrode materials such as Silicon.