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Microstructure Effects on Conductive Pathways in Polymer and Hybrid All-Solid-State Electrodes

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Summary:

All-Solid-State Electrodes as part of the next generation of batteries promise increased energy densities, fast charging properties and higher safety compared to traditional liquid-based systems. In order to promote ionic and electric conductivity 3D-structured and graded composite cathodes from composite particles are intended. These particles consist of active material, electrolyte and conductive additive and have a great impact on the resulting cathode properties.
Here we present a dry and sustainable manufacturing process for polymer-electrolyte based Solid-State Batteries with LFP and NCM as active material. The LFP composite particles are produced by a melt granulation process where the components are uniformly distributed and the carbon black is dispersed. The composite particles are extruded or directly calendered to form a film that is laminated onto a current collector at target thickness. To increase energy and power density specific adjustment of composite particles and their characteristics is necessary for graded cathodes with higher area capacities.
To understand how the charge transport is affected by the inner structure and the process parameters, voxel-based FEM-simulations are executed. Advantageous transportation routes require low electrolyte tortuosity and a percolated conductive network as well as a high contact between active material and electrolyte. The simulations are validated by measurements of ionic and electrical conductivity and used to identify favorable parameters in regard to particle size, and material distribution gradients.
To enable NCM as cathode active material instead of LFP a two-step process is used to firstly coat NCM and secondly form composite particles. The composite particles are then pressed, calendered to target thickness and laminated onto a current collector. The coating enhances contact between NCM and the solid polymer electrolyte and carbon black, therefore reducing interface resistance and buffer volume changes during cycling.
To manufacture composite particles for oxide-electrolyte cathodes intensive mixing of components is used to coat NCM with binder and LATP as electrolyte. The binder is used as a conductive additive after pyrolysis and improves the compaction properties. During sintering mass loss of binders increase the porosity of the LATP pellets. The electrolyte particle size is reduced to nanoscale with a wet ball milling process resulting in oxide-electrolyte coated composite particles.
Reducing transport limitations within the cathode by utilizing a multilayer graded cathode with composite particles designed to meet the demanded characteristics in each layer can enhance the energy and power density of polymer-electrolyte solid-state-batteries. Enabling NCM as active material as well as hybrid oxide-polymer-electrolytes increasing ionic conductivity at lower temperatures could improve the cell performance.

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