Dendrite-Free Polygonal Sodium Deposition with Excellent Interfacial Stability in a NaAlCl₄–2SO₂ Inorganic Electrolyte

Room-temperature Na-metal-based rechargeable batteries, including Na–O₂ and Na–S systems, have attracted attention due to their high energy density and the abundance of sodium resources. Although these systems show considerable promise, concerns regarding the use of Na metal should be addressed for their success. Here, we report dendrite-free Na-metal electrode for a Na rechargeable battery, engineered by employing nonflammable and highly Na⁺-conductive NaAlCl₄·2SO₂ inorganic electrolyte, as a result, showing superior electrochemical performances to those in conventional organic electrolytes. We have achieved a hard-to-acquire combination of nondendritic Na electrodeposition and highly stable solid electrolyte interphase at the Na-metal electrode, enabled by inducing polygonal growth of Na deposit using a highly concentrated Na⁺-conducting inorganic electrolyte and also creating highly dense passivation film mainly composed of NaCl on the surface of Na-metal electrode. These results are highly encouraging in the development of room-temperature Na rechargeable battery and provide another strategy for highly reliable Na-metal-based rechargeable batteries