A physics-based side-reaction coupled electrochemical model for capacity fade of a graphite/LiMn₂O₄ cell is developed by including the key degradation mechanisms in both anode and cathode. The side reactions considered in this study include 1) solid electrolyte interphase (SEI) growth and manganese deposition on the anode and 2) manganese dissolution, electrolyte oxidation and salt decomposition on the cathode. Our study reveals three stages of capacity fade upon long term cycling: acceleration, stabilization, and saturation. In the acceleration stage, capacity fade is due mainly to the cyclable lithium loss induced by the anode SEI growth. In the stabilization stage, the anode SEI growth slows down as it gets thicker, the cathode Mn dissolution-induced capacity loss outpaces cyclable lithium loss, and the cathode becomes more intercalated at the end of discharge. In the saturation stage, cathode capacity degrades further and becomes the limiting factor, the cyclable lithium is shifted to the anode and the cathode reaches end-of-discharge saturation due to the severe cathode capacity fade. This study shows that the cyclable lithium loss and the cathode capacity loss are the two major contributors to the cell capacity fade, and the interaction between them determines the cell capacity.

In this study, a phase-field model is developed to simulate the microstructure morphology evolution that occurs during solid electrolyte interphase (SEI) growth. Compared with other simulation methodologies, the phase-field method has been widely applied in the solidification modeling that has great relevance to SEI formation. The developed model can simulate SEI structure and morphology evolution, and can predict SEI thickness growth rate. X-ray photoelectron spectroscopy (XPS) experiments are performed to confirm the major SEI species as LiF, Li₂O, ROLi, and ROCO₂Li. Transmission electron microscopy (TEM) experiment is performed to present the SEI layer structures. The experiments reduce the complexity of the model development and provide validation to some extent. Fick's law and mass balance are applied to investigate lithium-ion concentration distributions and diffusion coefficients in different types of SEI layers predicted by the phase-field simulations. Simulation results show that lithium-ion diffusion coefficients between 298 K and 318 K are 1.340–7.328(10⁻¹⁶) cm²/s, 1.734–3.405(10⁻¹²) cm²/s, and 2.611–2.389(10⁻¹⁵) cm²/s in the compact, porous, and multilayered structures of SEI layer, respectively. The resistances between 298 K and 318 K are 0.740–1.693 Ω⋅cm², 2.827–5.517 Ω⋅cm², and 3.726–5.839 Ω⋅cm² in the compact, porous, and multilayered structures of SEI layer, respectively.