Understanding calendar aging degradation in cylindrical lithium-ion cell: A Novel P4D electrochemical-thermal model

Lithium-ion batteries are widely used in modern applications. Despite their improvements, understanding how they degrade over time is still a challenge. Battery degradation involves complex chemical and mechanical processes that occur simultaneously and interact with each other, making it challenging to isolate individual degradation phenomena. Calendar aging denotes the decline in a battery's performance when no current is delivered or absorbed, resulting in capacity deterioration and increased resistance. The principal cause of battery degradation during calendar aging is the growth of the solid electrolyte interphase (SEI) at the surface of the anode. Additionally, high state of charge (SoC) can induce cathode electrolyte interface (CEI) formation and the dissolution of transition metals, particularly in cathodes containing manganese. Battery models help in the understanding of these mechanisms. Based on the pseudo-two-dimensional (P2D) model various mathematical models have been developed to investigate degradation. However, these models cannot simulate the local heterogeneity of real electrochemical-thermal processes, crucial for understanding degradation in commercial cells with numerous electrode layers. In this work, commercial cylindrical cells with LiNi₀.₈Mn₀.₁Co₀.₁O₂ (NMC811) cathode and a silicon-doped graphite anode, underwent three months of accelerated calendar aging at 60°C and different SoCs. We introduced a pseudo-four-dimensional (P4D) battery model, which extends the original 1D geometry of the P2D model into a 3D geometry including both electrochemical and thermal physics. This novel approach accounts for non-uniformities within the battery and the influence of current flow within the electrode plane, providing detailed insights into the cell behaviour and degradation processes. The model contains various degradation mechanisms, including SEI growth at the anode, CEI formation, and transition metal dissolution at the cathode. Validation of the model was achieved through experimental data obtained from charge/discharge tests, pulse tests at 25°C, and calendar aging tests at 60°C. Model results exhibit strong agreement with experimental temperature and voltage profiles. The P4D model introduced in this study offers a valuable tool for investigating battery degradation mechanisms by capturing local electrochemical and thermal variations within a cylindrical cell.