Solid electrolytes in lithium-ion batteries promise increased energy densities, fast charging properties and advantages in terms of safety due to the lower fire risk. Sufficient safety properties are required especially in applications such as electrical vehicles .
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 size distribution of the particles, and the composition which needs to be balanced in terms of transport characteristics and capacity, affect the electrode’s inner structure and thus the charge transport. Even at a good structural design, solid state battery cells often exhibit interfacial limitations induced by the poor connection of electrolyte and electrode or electrode and current collector which raises difficulties to distinguish between transportation limits contributed to the structural design and interfacial resistances. Simulations can provide an inside view on the structure in regard to advantageous transportation routes obtained by low tortuosity of the electrolyte phase and a well-connected or percolated conductive network as well as a high contact area between active material and electrolyte. A tailor-made cathode structure with graded material contents that account for changes of the local current density under charging/discharging may enhance the transport of charged species and thus, improve the cell performance.
The presented study investigates graded polymer solid state cathodes by voxel-based FEM simulations. The electrode’s material composition shows a gradient with varied fractions of carbon black, PEO solid electrolyte and LFP active material. In order to create a representative digital twin, different structuring algorithms are available, that control the network connectivity as well as the tortuosity and remaining pores, and thus, a suitable algorithm is validated via ionic and electronic transport properties.
To account for the electronic conductivity network, the LFP is placed without percolating in a stable network at low conductivity, while the subsequent insertion of carbon black provides for the network formation. The comparison with experimental results allows to choose the size and shape of CB agglomerates and is in very good agreement for the variation of CB contents up to 11 %. The calculated ionic conductivity values show the same dependencies as function of the active material content as the measured conductivity  obtained from pressed PEO/LiTFSI-cathodes. Based on the validated parameters, cathodes consisting of three different layers are created and investigated regarding their charge/discharge behavior in cells with lithium anodes. By characterizing the cell potential, the lithium concentration and the capacity, the optimization potential of graded solid-state cathodes is evaluated. All structural and battery simulations were executed with GeoDict 2020 (Math2Market).
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