Detailed analysis of Li₆PS₅Cl/LiNi_0.5Mn_0.3Co_0.2O₂ composite electrode before electrochemical cycling

Solid-state batteries show promise as future energy storage systems. However, reactions between oxide and thiophosphate materials at their interface often cause decomposition, which shortens cycle life. Until now, these interfaces have mainly been studied electrochemically after cell cycling. Nonetheless, ageing may occur while cycling, or even when composite electrodes, blends of electroactive materials and solid electrolytes, are ground together and stored in a glovebox for a week. This paper examines how different storage conditions affect composite electrodes made from LiNi_0.5Mn_0.3Co_0.2O₂ (NMC532) paired with Li₆PS₅Cl solid electrolyte. Variables like powder versus pellet form, and storage temperatures at 25 °C or 60 °C, were evaluated. After just one week, results showed that storing the material as pellets accelerated decomposition due to the close contact between the active material and the electrolyte. Ageing for only one week significantly reduced the specific charge and increased the polarisation during cycling compared to fresh samples. These changes stem from bulk modifications, including the formation of side phases, like transition metal sulfides and LiCl, and surface reactions producing Li₂S. Detailed electrochemical impedance studies revealed that ionic conduction dropped sharply while electronic conductivity rose because of transition metal sulfide formation. The findings highlight how managing electrode ageing is crucial when developing solid-state batteries featuring composite electrodes.