Simulation of Lithium-Ion Conduction and Deposition with Heterogeneous Porous Structure of Electrode Layers and Separators

Abstract

Recently, innovative design of electrode structure on the basis of kinetics and transport phenomena of ion is needed in order to increase the high-rate performance. Especially, its local ion flux distribution relates ununiformed reaction distribution and local Li metal deposition. So, the relationship between heterogeneous porous structure and its mass transport and reaction phenomena has to be known well to increase cell performance and durability. In our previous study, we focused on the actual heterogeneous structure and the Li ion and electron conductivity was evaluated by the effective conductive path with the actual reconstruction electrode structure of LiCoO2 cathode and graphite anode [1], [2]. Furthermore, the effect of volume expansion of charge-discharge state was evaluated by using simulation with structure change based on the actual porous electrode structure, and the relationship between structural change and battery performance was considered. In this study, Li deposition phenomena was modeled and combined to our previous simulation. And, the effect of porous separator structure on its phenomena was examined. Firstly, actual separator structure was reconstructed by focused-ion beam scanning electron microscopy (FIB-SEM: Helios NanoLab 600, FEI). In this study, Cryo-FIB-SEM with the function of frozen-cutting was used to prevent polymer material from melting by beam damage. The slice pitch was 20 nm in 200 slices. The validity of this structure was checked by comparing pore size distribution. Furthermore, in order to compare between the property of actual structure and theoretical physical limiting, various simulated structure were made by simulation, such as biaxial stretching structure, fibrous structure and foam structure. In simulation structure, the resolution is 0.1 mm/mesh, the porosity is 0.5, the thickness is 20 mm, and the size is (50 mm)2. Figure 1 shows reconstructed and simulated structure, Li+ flux distribution, standardized statistics of local Li+ flux and relative conductivity. In this figure, it was found that Li flux distribution of actual separator was not uniform. Comparing actual structure and foam structure, the relative conductivity of actual separator which was defined by the ratio between porosity and tortuosity can be increased 1.7 times, and the local flux can be uniformed with optimal structure. With these structures, the reaction and mass transfer in these reconstructed electrode structure and simulated separator structure were simulated by multi-block method [3] which is the method of coarse graining to keep the information of heterogeneous structure. Li concentration and potential distribution in active material (AM) and electrolyte (EL) were calculated with Butler-Volmer equation as the boundary condition of the interface between AM and EL. In addition, Li metal deposition reaction model and structural change effect were included our previous simulation. It was assumed that temperature is constant and that the formation of SEI is ignored. Figure 2 shows the example of Li metal deposition distribution in separator. Moreover, the relationship between porous structure of separator and deposition structure was examined.