2D Na- Na exchange (EXSY) MAS NMR spectra (Figure -5e) were conducted using a mixing time of 3 ms for NZAC compositions (x = 0.045, 0.25, 0.5, 0.75) to probe which Na sites exchange on the millisecond timescale. The spectra show no cross-correlation (off-diagonal) signals for the NAC phase (-14.2 ppm) for all compositions, but strong exchange (off-diagonal intensity) is observed for the broad signal(s) between 0 and -13 ppm for compositions x = 0.25 and 0.5. This indicates that on the mstimescale Na + exchange between different environments takes place in the NZC phase. Furthermore, the 2D 23 Na- Na exchange (EXSY) NMR spectrum of x = 0.75 compositions (Figure ) shows also some cross-correlation intensity for the broad signals between 0 and -8 ppm. However, the intensity is significantly weaker compared to x = 0.25 and 0.5 compositions. The observed exchange intensity in the 2D EXSY NMR spectra suggests that Na in different environments is only present in the NZC phase, as no signals exchange with the NAC phase. Therefore, we conclude that predominantly Zn 2+ is substituted by Al 3+ in the NZC phase likely due to the smaller ionic radius of Al³⁺ (53.5 pm) compared to Zn²⁺ (74 pm) and the relative ease of removing Na⁺ from the NZC structure. Furthermore, the 2D EXSY NMR spectra indicates that the more mobile Na ions are mostly associated with the Na environments which were introduced through the substitution of Zn 2+ with Al 3+ . Finally, DC polarization measurements (Figure ) reveal that the electronic conductivity of all selected NZAC compositions is below 10⁻⁹ S/cm. For the composition Na1.5Zn0.5Al0.5Cl₄, an electrochemical stability window ranging from 0.9 to 3.6 V vs. Na3Sn was determined using the cell architecture Na3Sn/NPS/NZAC (Figure ). Regarding the oxidation stability, two broad oxidation peaks at 3.6 V and 3.9 V vs. Na3Sn are distinguishable during the first cycle. These peaks are attributed to the oxidation of NAC-type and NZC-type components, respectively. In the following cycles, no peaks are observed, possibly indicating the formation of a stable passivation layer. Only the NZAC (x = 0.5) composition was tested for the reduction stability due to its high ionic conductivity. The first cycle of linear sweep voltammetry (LSV) indicates a slight reduction when the potential decreases below 0.9 V vs. Na₃Sn, which can be attributed to the reduction of Zn 2+ and Al 3+ . Furthermore, during the second and third cycles, the reduction stability window expanded to approximately 0.6 V vs. Na₃Sn, but a fully stable passivation layer does not seem to form.