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 (σ_eff) of the conducting matrix. The percolation threshold p_c was determined as ~4 wt.-% C65, and it is suggested that below p_c, the ionic contribution is dominant, which can be seen in temperature-dependent σ_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.

Transition metal oxides exhibit a high potential for application in the field of electronic devices, energy storage, and energy conversion. The ability of building these types of materials by atomic layer-by-layer techniques provides a possibility to design novel systems with favored functionalities. In this study, by means of the atomic layer-by-layer oxide molecular beam epitaxy technique, we designed oxide heterostructures consisting of tetragonal K₂NiF₄-type insulating La₂CuO₄ (LCO) and perovskite-type conductive metallic LaNiO₃ (LNO) layers with different thicknesses to assess the heterostructure—thermoelectric property—relationship at high temperatures. We observed that the transport properties depend on the constituent layer thickness, interface intermixing, and oxygen-exchange dynamics in the LCO layers, which occurs at high temperatures. As the thickness of the individual layers was reduced, the electrical conductivity decreased and the sign of the Seebeck coefficient changed, revealing the contribution of the individual layers where possible interfacial contributions cannot be ruled out. High-resolution scanning transmission electron microscopy investigations showed that a substitutional solid solution of La₂(CuNi)O₄ was formed when the thickness of the constituent layers was decreased.