Effect of Process Duration on Electrochemical Performance in Composite Cathodes for All-Solid-State Li-Ion Batteries Processed via Warm Isostatic Pressing

Isostatic pressing is frequently used to densify various components of all-solid-state batteries (ASSBs). Among these, warm isostatic pressing (WIP) has proven to be a useful technique for ASSB production. We conducted a parametric study of the performance of composite cathodes treated with WIP to investigate the effect of the process duration on the performance of ASSBs. Electrochemical impedance spectroscopy (EIS) and charge-discharge performances of the test cells were examined to determine the interfacial properties of the composite cathodes. X-ray computed tomography (CT) using synchrotron radiation was performed before and after WIP to correlate the interfacial properties with the microstructures of the composite cathodes. Our findings showed that the test cells had specific capacities of around 125 mAh g^-1 when the composite cathodes were treated with WIP for 60-1800 s. However, for shorter WIP treatments (1 s), it remained around 80 mAh g^-1, which is two-thirds of those obtained from longer WIP. According to the X-ray CT analysis, the cathodes treated with one s WIP contained a large number of small voids in the solid electrolytes (SE). Such voids were eliminated during longer WIP treatments. The change in the void ratio correlated well with the EIS results, suggesting that the resistance related to charge transfer was the dominant factor determining the performance of the ASSBs. The lower-temperature WIP resulted in insufficient densification. Additional degradation modes were enhanced in the low-temperature regime, probably due to the formation of gaps and cracks. In conclusion, process duration is a crucial factor in determining the performance of composite cathodes and hence should be carefully controlled to obtain suitable interfacial properties between the active materials and SEs. The presented results give insights into the comprehension of the interface issues in the ASSBs from both scientific and industrial aspects. They contribute to further improvement of the electrochemical performance of ASSBs.