Four-fold coordinated Co 3+ and Co 2+ in LLZO is in contrasts with observations made previously, where predominantly octahedral-coordinated Co 3+ has been identified via XPS for LLZO-LCO blends annealed at temperatures between 400 and 900 °C22 . To clarify oxidation state and local coordination of Co in LLZO Co Emission Mössbauer spectroscopy has been performed on samples herein, whereby only for LLZO-b a sufficient Co signal (that is not related to Co-adsorption on LLZO) has been observed (see Figure ). The signal characteristic for Co 3+ in octahedral coordination originate, however, from extra phases, rather than from Co incorporated in LLZO (cf Supplementary Note 3). Hence, spatially resolved studies on phasepure regions of the single crystals have been performed. Although the signal to noise ratio of the Co 2p XPS spectra is low due to the small sample size and the low Co content (cf Supplementary Notes 4), the chemical state of Co 2+ was determined for LLZO-b (see Figure ). The asymmetric line shape of the Co 2p3/2 requires a main component (A) and satellite (B), whose binding energy shift is characteristic for Co , which further supports the findings made by SCXRD and UV-Vis spectroscopy.