Identifying the Key Ageing Mechanism and its Effects on NMC622 Li-Ion Battery

The exponential increase in demand for energy requires indispensable energy sources such as lithium-ion batteries. While batteries play a crucial role in meeting the global energy demand, during long operation lithium-ion batteries go through undesirable effects causing the battery to age. Battery aging is caused by side reactions and degradation processes at various places in the battery leading to capacity loss. The rate of degradation depends on cycling conditions, potential, local concentrations, and temperature. Moreover, different materials used within the cell degrade in unique ways, and interactions between these materials—commonly referred to as “cross-talk” between electrodes—can intensify the overall aging of the battery.
Through pseudo-two-dimensional model (P2D), this study investigates one such degradation known as the solid-electrolyte-interphase (SEI) formation; that mainly occurs in the anode side of the lithium-ion battery causing loss in the cyclable lithium. It also accounts for the increasing potential drop caused by the increasing resistance of the thickening SEI layer on electrode particles. Additionally, the model considers how the reduction in electrolyte volume fraction impacts charge transport within the electrolyte. The model uses 21700 NMC622/Gr cylindrical prototype cells from commercial manufacturer and parameters of the cell were obtained from the manufacturer, project partners, literature and experimental data.
The study emphasizes how the formation and thickening of the parasitic SEI layer contributes significantly to the degradation of the lithium-ion cell. Simulations were conducted over 300 charge-discharge cycles and validated using experimental cell test data. The results indicated an approximate SEI layer thickness increase of 70 nm, which correlated with the observed rise in potential drop. For accuracy assessment, the root-mean-square error (RMSE) between the simulated and experimental results was calculated to be less than 0.05. This degradation model provides valuable insights into the performance and aging behavior of lithium-ion batteries under varying operational conditions.