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.