NaAlCl<sub>4</sub>: New Halide Solid Electrolyte for\n3 V Stable Cost-Effective All-Solid-State Na-Ion Batteries

Although high-voltage-stable halide solid electrolytes\n(SEs) have\nemerged, only a few Na⁺ halide SEs have been developed\nthus far. Moreover, the use of expensive elements reduces the suitability\nof all-solid-state Na-ion batteries (ASNBs). Herein, the new mechanochemically\nprepared orthorhombic NaAlCl₄ is demonstrated to exhibit\na 10-fold enhancement in Na⁺ conductivity (3.9 × 10^–6 S cm^–1 at 30 °C) compared\nto annealed samples. The feasibility of NaAlCl₄ for ASNBs\nis also validated for the first time. X-ray Rietveld refinement with\nbond valence energy landscape calculations reveals 1D-preferable 2D\nNa⁺ conduction pathways. High-voltage stability up to ∼4.0\nV (vs Na/Na⁺) is confirmed by electrochemical measurements\nand theoretical calculations. Furthermore, the outstanding electrochemical\nperformance of NaCrO₂/Na₃Sn ASNBs at 30 and\n60 °C is demonstrated (e.g., 82.9% capacity retention at the\n500th cycle at 60 °C and 1C), shedding light on the potential\nof the cost-effective and safe energy storage systems.