Self‐Formulated Na‐Based Dual‐Ion Battery Using Nonflammable SO₂‐Based Inorganic Liquid Electrolyte

Abstract Sodium secondary batteries have gained much attention as alternative power sources to replace lithium secondary batteries. However, some technical issues must be solved to ensure their success. Here, a highly safe and cost‐effective Na‐based dual‐ion battery system employing self‐formulated CuCl cathode material starting from a mixture of Cu and NaCl in conjunction with a nonflammable NaAlCl₄·2SO₂ inorganic liquid electrolyte is demonstrated. It is found that CuCl is spontaneously formed by redox coupling of Cu/Cu(I) and SO₂/SO₂⁻ anion radical. In the proposed battery, Na⁺ and Cl⁻ are employed as energy carriers for the anode and cathode, respectively, and it is further demonstrated that the Na‐metal‐free battery configuration is possible using a hard carbon anode. Owing to the use of cheap electrode materials and a highly conductive and safe electrolyte, the proposed batteries deserve to be regarded as a promising approach for next‐generation Na rechargeable batteries.