The empirically modified spectral density that fits the observed relaxation rate well most likely leads to different activation energies than the BPP model due to the empirical parameter β, which needs to be further investigated, as it is not clear whether the values obtained from these models can be compared. Measuring at a larger Larmor frequency or temperature range may be beneficial, which is a subject of further study. From the data presented here, it can be concluded that the Zr-substituent also affects the high-frequency Li-ion motion, as indicated by the different shape of the relaxation curves.