Modeling Reversible Expansion of Porous Electrodes in Si/NMC Cells within the Framework of Multi-Species, Multi-Reaction Theory

Abstract

We formulated a model that describes the diffusion, volume change and mechanical compression, coupled with multi-site-multi-reaction theory of the porous electrodes, and we apply the treatment to battery cells with silicon as anode active material. Irreversible thermodynamics and conservation laws have been used to tie all the equations together. For cell lithiation (charge), changes in the porosity, cell thickness and cell electrochemical resistance due to increase in active material volume and mechanical compression are calculated. Experimental data on cell expansion is collected on pouch cells with silicon anode and NMC622 the cathode; the model compares favorably with the data. Model simulations show that during the C/5 charge cycle, particle expands by 10% and porosity of the electrode decreases by approximately 8%. The model can be exercised to evaluate the cell operating regime for meeting targets and design considerations. Simulation studies revealed the importance of compression pressure and the spring constant on cell expansion.