Because of the limitation of lithium resources, sodium-based rechargeable batteries started to emerge as potential candidates for medium and large-scale stationary energy storage system (ESS) again. Na-S batteries have been used for large-scale stationary ESS but intrinsically have a risk for explosion. Sodium metal chloride batteries based on Na-NiCl₂ chemistry, however, are consisted of safer electrode configuration with nickel, NaCl and NaAlCl₄ as cathode ingredients than the Na-S batteries. The Na-NiCl₂ batteries offer many attractive properties such as high cell voltage, high theoretical specific energy, and broad range in operating temperature. The nickel chloride electrode plays an important role in the cell performance. It will be discussed why a metal additive must be considered in Na-(Ni,M)Cl₂ batteries [M: a metal additive] in this paper. It is because the metal additive protects the chains of the conductive metal from disconnection in the cathode layer of the Na-(Ni,M)Cl₂ cells. This concept has been confirmed through investigation of electrochemical behaviors in the Na-(Ni,Fe)Cl₂ cells. Especially, Fe shows excellent cycle performance in the Na-(Ni,M)Cl₂ cells. Other metal additives can be considered to protect the conductive chains in the Na-(Ni,M)Cl₂ cells also.