At the LCO|LLZO interface region of LLZO-b(s) a thin decomposition zone is observed. Due to prolonged exposure and high temperature, this zone is well developed and consists mainly of Li2CO3, blue-black CoO (see black spots in Figure ) and small amounts of a dark brownish phase. No indications of La2Zr2O7 or LaCoO3 as reported by Varda et al (2018) were found by PXRD (see Figure ). A single-crystal of the brown phase, around 60 µm in size, was extracted and examined by single-crystal X-ray diffraction. This phase is cubic, space group 𝐼𝑚3 & 𝑚, with refined composition LaLi0.95Co0.26O2.0, like an iron containing compound reported by Mazza, et al., containing octahedral coordinated Co 3+ . This further supports investigations made by Vardar et al. that La-Co-O decomposition phases with varying stoichiometry are formed at the LLZO|LCO interface. No crystalline Zr-containing decomposition phase is observed, supporting the assumption that Zr is rigidly bonded in LLZO and is not taking majorly part in inter-diffusion processes.