The tube was then sealed under vacuum before it was heat treated at different temperatures and dwelling times under Ar to avoid Cu oxidation. After the heat treatment, the embedded single crystals were covered in a solid coating being surrounded by a metallic-appearing surface, which was removed by sanding. After removing the LCO coating, we obtained single crystals with colors ranging from almost transparent to blue depending on the treatment history. For simulating degradational effects on the LCO-LLZO interface, two experiments were conducted: