For future BEV applications, the use of Si-containing negative electrode has shown to be beneficial for meeting aggressive cell energy density targets of 750 Wh/L which are driven by performance objectives. However, Si-containing negative electrodes come with a drawback: the cell volume increases and decreases significantly during Li⁺ intercalation and deintercalation, respectively. Cell expansion and contraction are major concerns for battery pack and module developers. This talk will focus on cell expansion due to formation and cycling. Specifically, measurements of cell expansion were made with a system that uses a Linear Variable Differential Transformer (LVDT) sensor. Data showed that reversible expansion during charge/discharge in a cycle is a function of cell capacity and it can diminish with increasing the initial cell compression. Irreversible expansion during cycling grows linearly with the number of cycles and is reduced with initial compression of the cell. With initial compression of 45 psi, measurements showed 3% cell expansion during formation, 4% reversible expansion in a charge/discharge cycle with C/5 rate, and 12% irreversible expansion over 220 cycles.

We also formulated a mathematical model that describes the diffusion, volume change and mechanical compression coupled with multi-site-multi-reaction theory of porous electrodes and we apply this to battery cells with Si as the anode active material. Changes in the porosity, cell thickness, and cell electrochemical resistance were calculated due to an increase in active material volume and mechanical compression. Model simulations (Figure 1) show that during the C/5 charge cycle, silicon particle radius expands by 10%, and the porosity of the electrode decreases by ~8%. The model can be exercised to evaluate the operating regime for meeting targets and design considerations. Simulation studies revealed the importance of compression pressure and the spring constant on cell expansion.