Polyanionic Halide Glasses: A New Family of Sodium Superionic Conductors

Abstract The development of safe, low-cost, and high-performance solid-state electrolytes (SSEs) is critical for the next-generation all-solid-state sodium-ion batteries. However, state-of-the-art mixed-anion SSEs often rely on complex synthesis routes or reactive and expensive precursors, hindering their large-scale applicability. Here, we introduce a general and highly cost-effective mechanochemical strategy that leverages common, commercially available sodium oxo-anion salts to react with various transition metal halides, thereby creating a new and broad family of sodium superionic conductors. Notably, the champion 1/2Na₂CO₃-TaCl₅ composition exhibits a record-breaking ionic conductivity of 5.641 mS cm⁻ ¹ at 30°C with a low activation energy of 0.216 eV, a value that surpasses all previously reported sodium halide and oxyhalide-based SSEs. Crucially, in-depth structural characterization reveals that this exceptional performance originates from a novel "anion-cluster-retention" mechanism, where the carbonate anion (CO₃²⁻) is not decomposed but is fully retained as a structural building block. This process forms a previously unexplored amorphous carbonato-chloride network of [TaClx(CO₃)y]n−-polyhedra, which facilitates rapid Na⁺ migration. This work establishes a new design paradigm based on an anion-cluster-retention mechanism, unlocking a vast compositional space for advanced SSEs using cost-effective commodity chemicals.