Additionally, ²³Na solid-state nuclear magnetic resonance (ss-NMR) spectroscopy was performed to determine the distribution of Na local environments in Na SSEs. The AlCl₄⁻ tetrahedra are isolated from each other but are connected to Na⁺ capped-trigonal prisms through both edge-sharing and corner-sharing (Figure ). The isolated nature of the AlCl₄⁻ tetrahedra allows them to deviate from their equilibrium positions at room temperature, reflected in the higher Biso value of Cl atoms in the refinement results. All Na⁺ cations occupy the Wyckoff 4a site, and the seven Cl⁻ anions surrounding one Na⁺ form a capped-trigonal prism, which generates a framework of capped prisms through corner-sharing. The facile Na⁺ ionic conduction pathway in NAC is illustrated in electrochemical impedance spectroscopy (EIS) data. Figure shows the Nyquist plot and Z-fitting of the equivalent circuit (details provided in Figure ). The semicircle on the left side at high frequencies represents the Na⁺ transport processes in both the bulk and grain boundaries, which are difficult to deconvolute due to their similar capacitance values (around 10⁻⁹ F). The smaller semicircle on the right side at low frequencies with capacitance around 10⁻⁶ F, can be attributed to the transport mechanism at the SSE/hard metal interface. From the EIS-fitting, we obtained a conductivity of 3.28 x 10 -6 S/cm, which is in good agreement with literature values. There, it is suggested that the ionic conduction in NAC originates from two factors: (i) the movement of AlCl₄⁻ tetrahedra, facilitated by their isolated nature, and (ii) the empty "Na2" sites in the unit cell, which serve as intermediate sites for Na⁺ hopping from one site to another. To be noted that the unoccupied free volume in the unit cell is a prerequisite for efficient ionic conduction.