Percolation Behavior of a Sulfide Electrolyte-Carbon Additive Matrix for Composite Cathodes in All-Solid-State Batteries

To achieve high energy densities with sufficient cycling performance in all-solid-state batteries, the fraction of active material has to be maximized while maintaining ionic and electronic conduction throughout the composite cathode. It is well known that low-surface-area carbon additives added to the composite cathode enhance the rate capability; however, at the same time, they can lead to rapid decomposition of the solid electrolyte in thiophosphate-based cells. Thus, the fraction of such conductive additives has to be well balanced. Within this study we determined the electronic percolation threshold of a conducting matrix consisting of Li6PS5Cl and C65. Furthermore, we systematically investigated the microstructure and effective conductivity (sigma(eff)) of the conducting matrix. The percolation threshold p(c) was determined as similar to 4 wt.-% C65, and it is suggested that below p(c), the ionic contribution is dominant, which can be seen in temperature-dependent sigma(eff) and blocked charge transport at low frequencies. Above p(c), the impedance of the conducting matrix becomes frequency-independent, and the ohmic law applies. Thus, the conducting matrix in ASSB can be regarded as an electronic and ionic conducting phase between active material particles. Additionally, guidelines are provided to enable electronic conduction in the conducting matrix with minimal C65 content.