Multi-scale model-based optimal design of electrode microstructures for lithium ion batteries

Lithium-ion batteries (LIBs) are known for their high specific capacity and good cycle stability. However, their further development is limited due to the low energy density of LiFePO₄ (LFP) electrodes. Optimization design for porous electrode structure is expected to break through this bottleneck. Furthermore, the electrochemical inhomogeneity of LIBs is closely related to the structure of the electrode. Currently there are mainly four methods to describe electrode structure: Models composed of homogeneous spherical particles, models composed of multi-size spherical particles, models simulated by complex mathematical methods and models reconstructed by focused ion beam-scanning electron microscopy (FIB-SEM) and X-ray computed tomography (XCT). Triply periodic minimal surfaces (TPMS) are described as surfaces with zero mean curvature. Compared with other porous structures, TPMS have the potential advantages of high specific surface area and enhanced pore connectivity. As interconnecting structures, TPMS are expected to be used as electrode structures to avoid battery degradation, because they are continuous and interconnected throughout three dimensions. In this paper, two methods were used to describe electrode structure: A filling model based on spherical particles and a model based on TPMS. Specific surface area and porosity were chosen as characteristic parameters to determine the appropriate size of the representative volume element (RVE). Furthermore, a multi-scale mathematical model of the LIBs discharge process was established and used to simulate the galvanostatic discharge process. The effects of TPMS structures and porosities on rate performance and solid diffusion performance were tested at varying discharge rates. The results show that the capacity loss of particles filling electrodes with particle sizes of 3200 nm is 47% when the discharge rate is 5C, while those of the four TPMS-based electrodes are all within 1%. The concentration difference on the electrode surface is 66% when the discharge rate is 1C, while that of the four TPMS-based electrodes are all within 1%. Therefore, TPMS-based electrodes perform much better than particles filling electrodes with particle sizes of 3200 nm, and the Gyroid structure has the best performance. Furthermore, low porosity electrode has higher discharge capacity but will restrict the transport of lithium ions which results in the loss of voltage at high discharge rates. This paper has guiding significance for the optimization design of LIBs and provides the theoretical basis and data support.