Modeling tailor-made 3D-structures of solid-state polymer cathodes

Summary:

Solid electrolytes in lithium-ion batteries are promising regarding increased energy densities, fast charging properties and offer advantages in terms of safety due to the lower fire risk.
However, besides the electrochemical properties of the materials used, the chemical processes are limited by the transport mechanism and transportation routes of charged species, which effect the cell performance significantly. The transportation routes are affected by the electrode’s inner structure and play an important role especially for solid electrolytes, which usually exhibit lower conductivities compared to solvent electrolytes. Thus, processes, as well as manufacturing techniques are developed, to improve the structural design of those electrodes. Parallel to these developments, simulations can provide a more detailed inside view on the structure in regard to advantageous transportation routes and material distribution.
To utilize this simulation-based approach, it is essential to create a digital twin of the investigated electrode. This naturally requires knowledge about material as well as structural properties, which in some cases are directly measured, or, calibrated from the comparison of experimental and simulation data. These properties include, among others, intrinsic charge transport properties, transport via interfaces, particles sizes and structural features of the transport network.
The poster presents the generation of a digital twin of a polymer based solid state cathode with LFP active material within the voxel-based GeoDict FEM-framework (math2market). The intrinsic electrical conductivity of the cathode active material (CAM) and carbon black (CB) is determined from a calibration simulation of powder conductivity, taking the porosity and the particle size into account. Based on these values, the structuring algorithm is adjusted, to match the physical structural properties. The LFP is placed without percolating in a stable network at low conductivity, while the subsequent insertion of CB provides for the network formation. To validate the results, the effect of CB agglomerate size and CB content is investigated. The conductivity measurement of pure PEO/LiTFSi solid electrolyte provides for the required input parameters of diffusivity and equilibrium lithium concentration. To simulate the ionic conductivity, the models of Bruggeman, and Nernst-Einstein, as well as different structuring algorithms to generate the solid electrolyte phase, are compared. To account for the charge transport via the CAM/solid electrolyte interface, the poster presents methods to access the Butler-Volmer rate constant via EIS measurements and calibration simulations.
The aim is to investigate tailor-made cathode structures with graded material contents that account for changes of the local current density under charging/discharging, which may enhance the transport of charged species and thus, improve the cell performance.