Interestingly, for LLZO-b, an oxidative reaction could be observed at about 4.0 V. To check if this process is reversible, we cycled the cell in a range of -1.0 V to 4.5 V. However, no indication of a corresponding reduction peak occurred. In subsequent cycles, neither oxidation nor reduction peaks could be observed, indicating a kinetically induced stability. We hypothesize, that the oxidation of Co 2+ to Co 3+ leads to a destabilization of the LLZO structure (due to high Co concentration), hence, to a decomposition of LLZO, probably forming a stable interphase. Alternatively, this could be also related to the decomposition of the found extra phase LaLi0.95Co0.26O2.0. Further studies are, however, needed to evaluate our assumptions.