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 NaAlCl 4 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 NaAlCl 4 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 NaCrO 2 /Na 3 Sn 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.L i-ion batteries (LIBs) have expanded into application areas such as large-scale energy storage systems (ESSs) that could stabilize the power grid. 1,2 However, the price of Li rose more than 10-fold over the past decade (from 6.06 USD kg −1 in 2012 to 75 USD kg −1 in 2022 for Li 2 CO 3 ). 3 In addition, the uneven distribution of Li mines has caused political and economic issues. 4−7 Moreover, there are safety concerns about LIBs, as seen from frequent fire accidents associated with the use of flammable organic liquid electrolytes. 8−10 These factors have impeded the widespread use of LIBs for ESSs. 8,9,11 Solidifying electrolytes with nonflammable inorganic Na + superionic conductors could improve safety and reduce cost, making all-solid-state Na-ion or Na batteries (ASNBs) promising for use as ESSs. 4,12−26 Sulfide SEs have been extensively investigated due to their high ionic conductivity and mechanical deformability which allows for the cold-pressing-based fabrication of all-solid-state batteries. 12−19,24,27−32 The first sulfide Na + superionic conductor, cubic Na 3 PS 4 , had a Na + conductivity of 0.2 mS cm −1 . 13 Since then, various aliovalent and/or isovalent substitutions for Na 3 PS 4 have been investigated, which produced compounds with improved Na + conductivity, [15][16][17][18][19]24,30,33 including Na 3 SbS 4 (1 mS cm −1 ), Na 1−2x Ca x PS 4 (1 mS cm −1 ), Na 3−x PS 4−x Cl x (1 mS cm −1 ), and Na 3-x Sb 1−x W x S 4 (maximum ∼40 mS cm −1 ). However, the electrochemical oxidation stability of sulfide SEs is as low as ∼2 V (vs Na/Na + ). 25,28 Recently, halide SEs have emerged as a game changer because they are mechanically sinterable, similarly to sulfide SEs, and exhibit excellent electrochemical oxidation stability and interfacial compatibility with LiMO x (M = Ni, Co, Mn, and Al mixture) cathode active materials (CAMs). 31,34,35 Since the seminal report on Li 3 YCl 6 (0.5 mS cm −1 ) that demonstrated stable cycling of LiCoO 2 with a high initial Coulombic efficiency (ICE) of ...

T-Na2Fe2F7 based on three-dimensionally connected FeF6 octahedra exhibits large specific capacity and ultra-high-stable cycling performance as a promising cathode for NIBs.

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