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

Abstract

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 ...

Recent progress and strategic perspectives of inorganic solid electrolytes: fundamentals, modifications, and applications in sodium metal batteries

“…, C 2/ m -Na 3 XI 6 with room-temperature ionic conductivity over 10 −1 mS cm −1 , the experimentally obtained samples usually result in significantly lower ionic conductivity at the level of 10 −3 to 10 −7 mS cm −1 , as well as large deviation from the theoretical crystal structures calculated based on their Li-ion counterparts. 627,649,652 For instance, the water-mediated synthesized Na 3 InCl 6 had an ionic conductivity of merely 2.23 × 10 −8 S cm −1 at 80 °C, 627 in line with the experimentally reported ionic conductivity from the previous literature. 653,657 As illustrated, the major determinant of the ion conduction within halide-based SEs is their structure and geometry.…”

“…Meanwhile, in an attempt to achieve the practical costeffectiveness in terms of raw materials, the expensive rareearth metals, e.g., Y, Sc, La, Er, etc., in halide-based SEs were replaced by the cheap and easily accessible Al, and the NaAlCl 4 halide-based SE was successfully developed by facile mechanochemical ball-milling and was found to exhibit an ionic conductivity of 3.9 Â 10 À6 S cm À1 at 30 1C. 649 Although this ball-milled and cold-pressed sample provided improved ionic conductivity compared to the annealed ones by an order of magnitude, owing to the cationic disorders 531,654,655 and stacking faults, 656 such a value is still undesirable for practical application.…”

“…658 With such a concern, almost all the halide-based SEs are still using the Na alloy metal anode instead of metallic Na. 642,647,649 6.4. Guidelines for future interfacial modification Considering that halide-based SEs are intrinsically compatible with high-voltage cathodes, it would be a pity if the Na metal anode cannot be added into the combination.…”

“…, Na 2 ZrCl 6 , 642 Na 3 YX 6 (X = Cl and Br), 643 Na 3 XI 6 (X = Sc, Y, La, and In), 644,645 Na 3 MCl 6 (M = In and Sc), 646 Na 3– x M 1– x Zr x Cl 6 (M = Y, Er), 647,648 and NaAlCl 4 . 649…”

Boosting the interfacial superionic conduction of halide solid electrolytes for all-solid-state batteries

“…These results boosted research to develop halide SEs, such as Li 3 InCl 6 (1.5 mS cm −1 at 25 °C), Li 3 ScCl 6 (3 mS cm −1 at 25 °C), Li 2 Sc 2/3 Cl 4 (1.5 mS cm −1 at 30 °C), Li 2 ZrCl 6 (0.40 mS cm −1 at 30 °C), and Li 3 YbCl 6 (0.19 mS cm −1 at 30 °C) 22 – 26 . Similar to other types of SE materials 2 , 4 , 6 , 13 , 27 – 29 , the conventional strategy of compositional tuning, e.g., aliovalent substitution, to control the charge carrier concentration and/or structural framework was applied to enhance the ionic conductivity of halide SEs: 22 , 26 , 30 , 31 Li 3-x M 1-x Zr x Cl 6 (M = Y, Er, max. 1.4 mS cm −1 at 25 °C), Li 3-x M 1-x Zr x Cl 6 (M = In, Sc, max.…”