Composite Cathodes for Solid-State Lithium Batteries: "Catholytes" the Underrated Giants

To expedite the large-scale adoption of electric vehicles (EVs), increasing the gravimetric energy density of batteries to at least 250 Wh kg(-1) while sustaining a maximum cost of $120 kWh(-1) is of utmost importance. Solid-state lithium batteries are broadly accepted as promising candidates for application in the next generation of EVs as they promise safer and higher-energy-density batteries. Nonetheless, their development is impeded by many challenges, including the resistive electrode-electrolyte interface originating from the removal of the liquid electrolyte that normally permeates through the porous cathode and insures efficient ionic conductivity through the cell. One way to tackle this challenge is by formulating composite cathodes (CCs) that employ solid ionic conductors as "catholytes" in their structure. Herein, it is attempted to shed light on this less studied and poorly understood approach. The different classes of catholytes that have been reported in literature alongside the most common fabrication techniques used to prepare CCs are presented. Next, the interplay between the microstructure and design parameters of CCs with the electrochemical performance of solid-state batteries (SSBs) and the techniques used to measure their transport properties is well documented. Finally, general guidelines surrounding CC research are outlined.