Mechanical Investigations of Composite Cathode Degradation in All-Solid-State Batteries

Despite ongoing efforts aimed at increasing energy density in all-solid-state batteries (ASSBs), the optimal composite cathode morphology, which requires minimal volume change, small void development, and good interfacial contact, remains a significant concern within the community. In this work, we focus on the theoretical investigation of the aforementioned mechanical defects in the composite cathode during electrochemical cycling. It is demonstrated that these mechanical defects are highly dependent on the solid electrolyte (SE) material properties, the external stack pressure, and the cathode active material (CAM) loading. The following conclusions are highlighted in this study: (1) Higher CAM loading (>50 vol %) causes an increase in mechanical defects, including large cathode volume change (>5%), contact loss (50%), and porosity (>1%). (2) High external stack pressure up to 7 MPa reduces mechanical defects while preventing internal fracture in the cathode. (3) Soft SE materials with small Young's modulus (<10 GPa) and low hardness (<2 GPa) can significantly minimize these mechanical defects during cycling. (4) A design strategy is proposed for high CAM loading with minimal mechanical defects when different SE materials are utilized in the composite cathode, including an oxide-type SE, a sulfide-type SE, and a halide-type SE. The research provides specific guidelines to optimize the composite cathode in terms of its mechanical properties. These guidelines broaden the design approach toward improving the performance of ASSBs, by highlighting the importance of considering the mechanical properties of battery materials.