60th Annual Report on Research 2015

As shown in Fig. 1, a C++ code was developed to build a half cell LIB model with the reconstructed realistic microstructure of LiCoO₂ cathode electrode. Discharge processes of the half cell LIB was simulated at 1 C and 5 C rates using 3D finite volume method framework. In order to relate local effects to geometric properties, the cathode electrode is equally divided into 8 subdomains and the geometric properties and polarization were calculated in each subdomain. As shown in Fig. 2, the geometric properties from 3D electrode model are calculated, i.e. the porosity, tortuosity in x axial direction and specific interfacial area of each subdomain. The results exhibit that the cathode electrode has an inhomogeneous microstructure.

With the half cell model shown in Fig. 1, the galvanostatic discharge and charge processes were simulated. Due to microstructural inhomogeneity, the physical and electrochemical processes in the electrode are inhomogeneous. Fig. 3 shows the inhomogeneous distribution of Li ion concentration and reaction current density in the cathode electrode at 3200 sec at 1 C discharge rate. Fig. 3(c) shows that the electrochemical reaction is concentrated in the fore group with small porosity at the end of discharge.

In order to investigate the correlation between LIB performance and electrode microstructure, the polarization inside the electrode are calculated. The polarization due to ionic transport in electrolyte (PIT) and the polarization due to activation of electrochemical reaction (PAER) are shown in Fig. 4. The simulation results of the 3D model with realistic cathode electrode are different from those of pseudo 2D model. Fig. 4(d) shows that the PIT in pseudo 2D model is much smaller than that in 3D model. The PIT of the fore group is much smaller than that of the back group in the pseudo 2D model and the PIT does not change much during the discharge process, which are different from the 3D model results. It is because that the PIT depends on the porosity, tortuosity and position in 3D model but it only depends on position in pseudo 2D model. Fig. 4(e) exhibits that the PAER in pseudo 2D model is also smaller than that in 3D model due to the homogeneity assumption, especially at the end of discharge. Figs. 4(c and f) show the total polarization in the electrode due to PIT and PAER in 3D model and pseudo 2D model, respectively. The microstructural inhomogeneity in the 3D model leads to the larger polarization than pseudo 2D model, especially for PIT. The result demonstrates that the microstructural inhomogeneity has more impact on Li ion transport. Another interesting feature shown in Figs. 4(c and f) is the variation of PIT and PAER during the discharge process. The PIT in 3D model is not monotonic, but it is monotonically increasing in pseudo 2D model. We believe that the microstructural inhomogeneity causes inhomogeneous Li ion transport and electrochemical reaction, which could result in the nonmonotonicity of PIT during the discharge process.

Fig. 5 exhibit PIT and PAER in each subdomain inside cathode electrode for 5 C discharge process. In general, the phenomena and mechanisms that cause the phenomena are the same as we discussed for the 1 C discharge results. Here, we want to highlight several important differences between 1 C and 5 C processes. (1) The PIT and PAER in the subdomains of 3D model span in a much wider range for 5 C processes. (2) PIT counts more weight in the total polarization for both 3D model and pseudo 2D model for 5 C processes compared to 1 C processes. (3) At 5 C discharge and charge, PIT starts to count for a significant weight of total polarization in 3D model, but it still not significant in pseudo 2D model.

Overall, the porosity, specific interfacial area, tortuosity, position and SOD / SOC have significant influence on the average variables and polarization. Compared with the results from pseudo 2D model, the results from 3D model show that microstructural inhomogeneity has significant impact on the polarization and results in more inhomogeneous distributions of Li ion concentration, electrochemical reaction, and polarization in the electrode. The knowledge obtained in this study can offer the possibility to find a LIB’s optimal operational condition and design. This ACS PRF award has had a significant effect on my career by allowing me to establish a leading battery modeling and characterization program that is making fundamental discoveries. Two graduate students and one postdoc were supported by this award. Because the students and postdoc are in engineering major, this work has provided them a unique opportunity to study batteries and electrochemistry. They have to learn a lot on electrochemistry and they also need to use what they have learned in their engineering major for this research. This award helped them to develop their expertise in the battery research area.