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Reports: DNI1052834-DNI10: Identification of Lithium Ion Battery Electrode Structural Inhomogeneity and Its Effect on Battery Performance

Likun Zhu, PhD, Indiana University-Purdue University, Indianapolis

The main objective of this project is to understand the effect of electrode structural inhomogeneity on lithium ion battery (LIB) performance. During the last year, our research efforts have been focused on generating realistic LIB electrode microstructures, analyzing local microstructural properties, simulating full cell charging and discharging processes, and analyzing the local effects based on the simulation results. A brief description of the research work is provided below.

1.       Generation and analysis of realistic LIB electrode microstructures

The realistic 3D microstructures of LIB electrode play a key role in studying the effects of inhomogeneous microstructures on the performance of LIBs. In this study, we investigated geometric characteristics of inhomogeneous anode electrodes of a commercial LIB. An Xradia microXCT-400 system was employed to obtain tomographic data of the electrode. A 112 x 112 x 39.2 μm3 volume of the anode structure was reconstructed from the X-ray projected image data with a 0.56 μm voxel resolution. First, three domains (112 × 112 × 39.2 μm3) were selected to decide the minimum size of the representative volume element (RVE) from the CT data. As shown in Fig. 1a, the porosity profile converges to the domain’s porosity by increasing the volume. For all domains, the deviation of the porosity of sub-divisions remains within 2% of the domain porosity when the lengths of x and y directions are longer than 44.8 μm. Based on this result, the size of the RVE was chosen as 44.8 × 44.8 × 39.2 μm3. Adapting the size, we collected 16 sample RVE candidates from the domain 1, which have porosities within 2% deviation from the porosity of the domain 1 (0.27). Then the pore size distribution, specific surface area, and tortuosity of these 16 sample RVEs were analyzed. Figs. 1b – c show the deviations of structural properties of the 16 samples from the domain value.

Fig.1 (a) Porosity profiles of three domains by increasing volume. (b) Specific surface area, (c) tortuosity, and (d) pore size distribution of the domain 1 and its subsamples.
2.     Identification of local effects in LIB electrodes

In order to reveal the detailed local effects in LIB electrodes, the charging and discharging processes need to be simulated based on the 3D realistic microstructure. In this study, we used the microstructure of both anode and cathode electrodes to reconstruct a testing cell. The c++ software has been developed to rebuild and mesh the realistic microstructures of cathode and anode electrodes together based on both the nano-CT scanned images of cathode electrode and the micro-CT scanned images of anode electrode. The full cell model was composed of electrolyte, cathode and anode active materials, and current collectors, which was given below as well as the simulation boundary conditions.

Fig.2 Schematic of a testing cell including anode, cathode, separator, and current collectors.

In addition, c++ software based on 3D finite volume method (FVM) has been developed to simulate the discharging and charging processes of the LIB cell. The simulation software solved the coupled equations of charge conservation, mass conservation and electrochemical dynamics simultaneously. Fig. 3 shows the distribution of reaction current density at the interface between anode/cathode active material particles and electrolyte during a 1 C galvanostatic discharging process. Local effects can be clearly identified and they were studied further.

Fig.3 Reaction current density distribution at the interface between anode and cathode active material particles and electrolyte respectively (unit: A / mm2).

In order to explore the correlation of local effects and electrode structural inhomogeneity, both the cathode and anode electrode were divided equally into several sub-divisions. Then the consequent porosity, tortuosity, specific surface areas of each sub-division were calculated. Three groups of sub-processes were considered to cause polarization in our current research. They were activation of the electrochemical reactions, charge transport of species and contact resistance between active materials and current collectors respectively. Charge transport of species was further divided into electronic conduction and ionic conduction because both electrons and ions were charges in the LIB cell. The polarizations in each sub-division were calculated at different time and compared with sub-division porosities. Fig. 4 shows the polarization of each sub-division due to ionic conduction and intercalation reaction at 300 s during a 10 C discharge process. The results demonstrate the correlation between the polarizations and the porosities of sub-divisions.

Fig.4 Polarization of each sub-division due to ionic conduction and intercalation reaction at 300 s during 10 C discharge process at 25 ℃.

3.     Summary

In the last funding period, we have obtained the realistic microstructure of LIB electrodes using micro and nano CTs and analyzed the local microstructural properties. The results demonstrate the inhomogeneity nature of the LIB electrodes. We also established a modeling and simulation framework based on FVM and the 3D LIB microstructures. The correlations between structural inhomogeneity and polarization of sub-processes during discharging processes have been investigated. In the next funding period, we will continue to study the correlations between structural inhomogeneity and LIB performance. This analysis method can open up the possibility of correlating the structural inhomogeneity to the limiting features of a LIB cell in an effective way. And 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.