Phase Field Modeling of SEI layer morphology in Li-Ion Batteries
Authors: Lin Xianke , Liu Lin , Guan Pengjian 
Published: February 2020

Lithium-ion batteries (LIBs) suffer severe performance degradation during their lifetime application because of the undesired chemical reactions, ageing, corrosion, structural integrity compromise, and the threat of thermal runaway. Among those degradation mechanisms, much attention has been focused on LIBs materials decomposition, new components formation and growth due to undesired side reactions. The electrode is covered by a passivation layer named as the solid electrolyte interface (SEI), which is one of dominant LIBs degradation mechanisms. The SEI layer affect the LIBs cyclability, life time, power and rate capability, and even their safety. Growth of the SEI reduces the lifetime of LIBs and increases cell internal resistance.

Many researchers have investigated the SEI layer in term of structure, formation and compositions, thickness growth prediction and measurement. The SEI is believed to have multilayered structure, a thin layer of inorganic components close to the anode, and followed by a thick porous organic or polymeric layer close to the electrolyte phase. The composition of SEI layer can be distinguished to include Li2CO3, LiOH, LiF, Li2O, ROCO2Li and RCOLi (1). The SEI on the cross-section is dominated by Li and F and on the basal plane is dominated by organic materials. Heavy organic fragments were found mainly at the surface of the basal SEI. The thickness of the SEI on the basal plane was found to be three to five times smaller than that on the cross-section (2).

Compare to the experiment, the simulation work will be more flexible. Yan et al phenomenologically modeled the formation and evolution of the SEI on the graphite electrode. Within the framework of classical nucleation theory (CNT), they qualitatively explained the origin of the two-layer structure of SEI films (3). Broussely et al. proposed their diffusion-limited SEI growth model, then tried to explain the mechanism of lithium loss for LIBs during storage and gave us the rate of lithium loss is proportional to the SEI electronic conductance(4). Kim et al. simulated the effect of electrolytes on the structure and evolution of the SEI (5).

Although much work has already been done with the SEI study, the morphology of SEI layer is still not fully understood, especially in term of SEI structure prediction. In this work, we will predict the morphologies of the SEI using the phase field (6-8). The phase field model has been applied in many electrochemical studies to simulation the electrochemical reactions and to model the phenomena as roughness formation and electronically mediated reactions between interfaces in electronic contact. Also the phase field model is more accurate in regions with sharp composition gradients and can treat phase boundaries without the need to explicitly track interfaces. Phase field model can be used as a potentially powerful tool for modeling the structure of the SEI in LIBs.

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