For the optical absorption measurements, the absorption A of differently LCO treated LLZO samples was investigated. The measurements were conducted using a UV-Vis spectrometer equipped with optical fibres and a 1024-pixel diode-array detector (J&M Analytik AG, Essingen, Germany) in a range of 200 nm to 1000 nm while the sampling time was set to 2 minutes. The samples were abraded until a thickness of h ~ 0.8 mm and pre-polished with a ZrO2 sandpaper (4000 grit).