Evaluating the Pressure Dependence of Charge Transport in Composite Cathodes for Solid-State Batteries

Solid-state batteries (SSBs) are seen as a promising advancement of conventional lithium-ion batteries that use liquid electrolytes. Yet, their current reliance on operation under high stack pressures is a key challenge for wide-scale application. In this context, the composite cathode microstructure is of crucial importance as interface contact between the active material and solid electrolyte as well as charge transport pathways must remain intact under low-pressure operation. In this work, we evaluate the electrochemical charge transport properties of composite cathodes consisting of LiNi$_{0.82}$Mn$_{0.07}$Co$_{0.11}$O$_2$ and Li$_6$PS$_5$Cl at different stack pressures. By using solid electrolytes with different particle size distributions, we investigate how a change in the composite microstructure affects the pressure dependence of effective electronic and ionic conductivities. The effective electronic conductivity is found to be highly sensitive to pressure. Even though the effective ionic conductivity is less sensitive to pressure, it also diminishes when the stack pressure is decreased and approaches practically relevant values. A reduced solid electrolyte particle size leads to higher effective ionic conductivity, but it has no influence on the pressure dependence as such. This demonstrates the need for solid electrolytes that not only feature an optimized particle size distribution but also tailored mechanical properties. Considering the strong pressure dependence of electronic charge transport, SSB cells with and without conductive carbon were tested at different stack pressures. The results highlight the benefits of an electronically conductive additive during low stack pressure operation.