To estimate the band gap we plotted the absorption spectra (see Figure ) in a Tauc plot fashion ((Ahν)² versus hν), with A being the absorption coefficient and hν describing the energy of light at a certain measured wavelength λ, allows the direct read out of the optical band gap at the intercept of the first slope of the linear part of the Tauc plot with the x-axis (see Figure ). The band gap for LLZO-p is 4.78 eV and in good agreement with values reported previously. The band gap significantly narrows in dependence of the amount of Co incorporated. For LLZO-o and LLZO-b, the optical band gap is as low as 2.6 eV and 1.6 eV, respectively.