(350r) Numerical Simulation of Packing Structure and Compression Process of Cohesive Bimodal Particles.

All-solid-state lithium ion batteries (ASSLIBs) are composed of electrode active materials and nonflammable inorganic solid electrolytes instead of flammable liquid electrolytes. ASSLIBs are expected as next-generation batteries for electric vehicles due to their high energy densities and safety. Bulk-type solid state batteries with advantages of high capacity are prepared by compressing electrode and solid electrolyte powders. Hence, voids between the particles reduce the energy density. To solve this problem, the densification of ASSLIBs is necessary. The straightforward approach to the further densification includes an addition of bimodal particles with coarse and fine particles. An increase in packed active materials by adding fine particles will improve the energy density. The densification also leads to a reduction of internal resistance due to an increase in conductive paths. Here, it is crucial to predict the particle size ratio and the mixing ratio of fine particles for optimal packing structure by the aspects of a void fraction and a coordination number. Several studies have revealed that adhesion forces dominated in the case of fine particles, such as battery materials, strongly affects the packing structure of cohesive particles. However, the optimal packing structure of adhesive bimodal particles has not still been analyzed theoretically.

In this study, the effects of the particle size ratio, the volume fraction of fine particles, and the cohesion force on the packing and compression process were investigated by using the discrete element method (DEM). EDEM2019.1 (DEM Solutions Co. Ltd.) was used for DEM calculation. The Hertz-Mindlin with JKR model (hereinafter called JKR model) was applied as a contact model to describe the adhesive forces between particles. Adhesive forces were as a function of surface energy in the JKR model.The simulation parameters were as follows: particle size ratio α = 1-4, volume fraction of fine particles S_vf = 0-0.5, and surface energy γ = 0-0.2 J/m². Particle properties were assumed to be LiNi_1/3Co_1/3Mn_1/3O₂ (NCM) of positive active materials; however, only young’s modulus was halved to reduce calculation costs. The particles were randomly generated above the cylindrical container to form a stable packing under gravity. To simulate the compression and decompression processes, the upper punch was vertically moved. A pressure at 400 MPa was applied to the powder bed in the compression process. The lower punch remained stationary throughout simulation. All compacts were conducted at a constant loading and unloading rate of 25 mm/s. The total simulation time was 0.09 s to obtain the stable packing structure after compression.

In the non-cohesive condition, void fractions of particle size ratio 1 and 4 were 0.410 and 0.313, respectively, demonstrating that the densification was achieved by the addition of fine particles. In addition, under the all volume fractions of fine particles, void fractions in cohesive conditions were higher than those in non-cohesive conditions. Moreover, in the cohesive condition, the void fraction and the contacts between fine particles increased with the cohesive forces. On the other hand, the densification effect due to the addition of fine particles decreased as the cohesive force increased. After the compression at 400 MPa, the cohesive forces had little effect on the void fraction at either particle size ratio.