Most studies on sodium chloride SEs to date focus on rare earth metals, especially Ta and Y, or expensive In, limiting their economic potential. NaAlCl4 is known for its past use (in the molten state) as a catholyte in Zebra batteries. This material may offer a cost-effective alternative due to its raw materials' abundance and low cost. However, its conductivity as a solid is still low, which did not prevent anyhow its recent implementation in SSBs as a catholyte (but not as a separator). In this work, we aim to increase the conductivity of NaAlCl4 without the use of rare earth elements. We synthesized and studied the influence of Zn 2+ aliovalent substitution in NaAlCl4 (according to the nominal Na1+xZnxAl1-xCl4 formula unit). So far, very few investigations on the cationic doping of NaAlCl4 have been reported. Since the second end member Na2ZnCl4 belongs to a different structural type than NaAlCl4, a solid solution is also not guaranteed. Hence, we also investigate the introduction of Al 3+ (and Na vacancies) in the Na2ZnCl4 end member. The whole series of samples has been synthesized by mechanochemical methods. Their structure and properties will be discussed based on X-ray diffraction (XRD), synchrotron X-ray diffraction (sXRD), Electrochemical Impedance Spectroscopy (EIS), and solid-state Nuclear Magnetic Resonance (ss-NMR) results. ss-NMR was conducted to investigate the local environment of Na and explore the origin of the conductivity increase, in combination with computational methods such as Density Functional Theory (DFT) and Molecular Dynamics (MD).