Research on all-solid-state batteries has accelerated in the last decade, mainly because of their potential safety at high current densities compared with lithium-ion batteries (LIBs). As an energy storage device with high energy and power density, they promise to further increase electric vehicle range and operation time for portable electronics. Their key component is a solid electrolyte, which replaces the flammable organic liquid electrolyte used in LIBs. Among the plethora of available materials, thiophosphate-based solid electrolytes such as Li₆PS₅Cl are very promising because of their high lithium-ion mobility. Furthermore, their malleable nature makes them easily processable at room temperature.

However, the use of solid electrolytes in composite cathodes faces different challenges such as insufficient charge transfer between cathode active material and solid electrolyte due to low contact areas. In contrast to LIBs, where infiltration of porous electrodes by liquid electrolyte ensures contact by wetting of the active material particles, all-solid-state batteries rely solely on solid-to-solid contacts, which are more difficult to create and maintain. Sufficient ionic and electronic transport pathways in composite cathode structures are essential because cathode active material particles that are either electronically or ionically isolated cannot contribute to the charging or discharging process. The issue of ionic isolation is amplified for composite cathodes containing high amount of cathode active material, which is necessary for achieving high energy density. Thus, optimizing cathode microstructures becomes increasingly important for the improvement of all-solid-state batteries.

In this study, we investigated both ionic and electronic percolation in composite cathodes made of LiNi_0.6Co_0.2Mn_0.2O₂ and Li₆PS₅Cl. Different cathode architectures were investigated by varying the volumetric content of cathode active material and solid electrolyte between 20% and 70%. Partial conductivities are used to evaluate the effectivity of the electronic and ionic conduction pathways. To quantify the respective partial conductivities, electronically as well as ionically blocking electrodes were used for the electrochemical impedance spectroscopy measurements.

From the obtained partial conductivities, tortuosity factors were calculated and correlated to cell performance with complimentary cycling data of all-solid-state batteries in order to determine charge transport bottlenecks.^[1] Application of FIB-SEM tomography to a composite cathode allowed a detailed investigation of the cathode microstructure and the particle size of the solid electrolyte was found to influence the electrochemical performance of composite cathodes containing high amount of cathode active material.

[1] P. Minnmann et al.: Editors Choice - Quantifying the Impact of Charge Transport Bottlenecks in Composite Cathodes of All-Solid-State Batteries - 2021 J. Electrochem. Soc. 168