An Air‐Stable Sodium‐Containing Metal‐Sulfide Cathode for the Development of Sodium‐Metal‐Free Room‐Temperature Sodium‐Sulfur Batteries

The cathode materials comprising an alkali metal ion are considered promising for high‐energy rechargeable batteries. In room‐temperature sodium‐sulfur batteries (RT‐Na/S), the sodium metal anode is used in excess, posing safety challenges; the elemental sulfur cathode is susceptible to polysulfide dissolution, leading to cell failure. These issues can simultaneously be addressed by incorporating sodium ions into the cathode matrix. Unfortunately, most sodium‐containing sulfur cathodes are either chemically unstable or prone to disintegration, limiting their use under ambient conditions. In this contribution, we report the development of an air‐stable sodium‐containing metal sulfide (NaCu ₃ S ₂ (NCS)) and demonstrate it as a potential cathode for sodium‐metal‐free RT‐Na/S batteries. In a half‐cell configuration, the cathode demonstrates a remarkably high cycling stability of over 1300 cycles with a capacity fade of around 0.31 mAh g ⁻¹ per cycle. Further, a full RT‐Na/S cell is fabricated with the NCS cathode and a hard‐carbon anode. The full cell delivers a high discharge capacity of about 460 mAh g ⁻¹ and functions reversibly for over 500 cycles. The postmortem analysis of the cathode unravels that the as‐developed cathode undergoes conversion reactions. We believe these findings are critical and may pave the way for the development of a safe, high‐energy RT‐Na/S battery.