For the improvement of Lithium-ion Batteries (LiBs) performance, the optimum structure design of porous electrode is needed. In particular, the relationship between porous structure and the internal phenomena has to be understood well from the viewpoint of improvement of capacity, output power density and durability [1]. In LiBs electrode layer, the ratio of the electrode thickness and the diameter of active material is approximately 20, and this value is less than usual porous media electrode and devices, such as polymer electrolyte fuel cell [2,3], separation membrane and some packed bed system. So, it is difficult to treat LiBs electrode as homogeneous porous media. It is needed to evaluate the mass transport performance and the formation of electrochemical reaction field with the information of local structure that are the particle size and the position of each particle. Numerical simulation approach is effective to understand the internal phenomena and to design the optimum electrode structure. However, the microscopic structure, which is affected by particle size distribution, agglomeration and migration, cannot be targeted by usual homogeneous structure simulation [4]. Recently, though some microscopic simulation have already been tried to evaluate the effect of heterogeneous structure on cell performance, the calculation time and the calculation size are remarkable subjects.

In our group, we have already developed Multi-Element Network Model for LiBs simulation [5]. In porous electrode layer, imaginary pore spheres with greatest diameter were located between active material particles. These imaginary pore spheres represent electrolyte over the space by defining electrolyte information such as ion electric potential and Li ion concentration. After constructing electrode structures, the active material particle network and pore network were built. By building the Multi-Element Network, the electronic conduction can be calculated in particle network, as the ionic conduction and diffusion were calculated with pore network, while the electrode reaction occurring at interface between active material particles and electrolyte can be calculated. Based on the porous electrode theory, the reaction and mass transport phenomena at discharge condition was simulated.

As a result, in the case of small particle for enlargement of reaction interface, the continuity and the tortuosity of electrolyte phase was changed even if the volume ratio is constant. Especially, the bimodal particle size distribution strongly affect output density more than the monomodal distribution. By this simulation, the relationship between microscopic structure and the cell performance can be discussed. In addition, in our presentation, we will discuss the effect of the micro-scale structure on cell performance with actual porous electrode layer.

References

[1] G. Inoue et al., J. Power Sources, 342, 476-488 (2017).

[2] G. Inoue et al., J. Power Sources, 327, 610-621 (2016)

[3] G. Inoue et al., J. Hydrogen Energy, 41 (46), 21352-21365 (2016)

[4] G. Inoue et al., ECS Trans. 80(10), 275-282 (2017)

[5] K. Lin et al., ECS Trans. 80(10), 251-258 (2017)