NaAlCl₄: New Halide Solid Electrolyte for 3 V Stable Cost-Effective All-Solid-State Na-Ion Batteries

Although high-voltage-stable halide solid electrolytes (SEs) have emerged, only a few Na+ halide SEs have been developed thus far. Moreover, the use of expensive elements reduces the suitability of all-solid-state Na-ion batteries (ASNBs). Herein, the new mechanochemically prepared orthorhombic NaAlCl4 is demonstrated to exhibit a 10-fold enhancement in Na+ conductivity (3.9 × 10-6 S cm-1 at 30 °C) compared to annealed samples. The feasibility of NaAlCl4 for ASNBs is also validated for the first time. X-ray Rietveld refinement with bond valence energy landscape calculations reveals 1D-preferable 2D Na+ conduction pathways. High-voltage stability up to ∼4.0 V (vs Na/Na+) is confirmed by electrochemical measurements and theoretical calculations. Furthermore, the outstanding electrochemical performance of NaCrO2/Na3Sn ASNBs at 30 and 60 °C is demonstrated (e.g., 82.9% capacity retention at the 500th cycle at 60 °C and 1C), shedding light on the potential of the cost-effective and safe energy storage systems.