Which Exchange Current Densities Can Be Achieved in Composite Cathodes of Bulk-Type All-Solid-State Batteries? A Comparative Case Study

The performance of bulk-type all-solid-state Li batteries (ASSBs) depends critically on the contacts between cathode active material (CAM) particles and solid electrolyte (SE) particles inside the composite cathodes. These contacts determine the Li⁺ exchange current density at the CAM | SE interfaces. Nevertheless, there is a lack of experimental studies on Li⁺ exchange current densities, which may be caused by the poor understanding of the impedance spectra of ASSBs. We have carried out a comparative case study using two different active materials, namely, single-crystalline LiCoO₂ particles and single-crystalline LiNi_0.83Mn_0.06Co_0.11O₂ particles. Amorphous 0.67 Li₃PS₄ + 0.33 LiI particles act as a solid electrolyte within the cathode and separator, and lithiated indium acts as the anode. The determination of the cathode exchange current density is based on (i) impedance measurements on In–Li | SE | In–Li symmetric cells in order to determine the anode impedance together with the anode | separator interfacial impedance and (ii) variation in the composite cathode thickness in order to differentiate between the ion transport resistance and the charge transfer resistance of the composite cathode. We show that under the application of stack pressures in the range of 400 MPa, the Li⁺ exchange current densities can compete with or even exceed those obtained for CAM | liquid electrolyte interfaces.