Rechargeable solid-state sodium-ion batteries (SSSBs) hold great promise for safer and more energy-dense energy storage. However, the poor electrochemical stability between current sulfide-based solid electrolytes and high-voltage oxide cathodes has limited their long-term cycling performance and practicality. Here, we report the discovery of the ion conductor Na_3-xY_1-xZr_xCl₆ (NYZC) that is both electrochemically stable (up to 3.8 V vs. Na/Na⁺) and chemically compatible with oxide cathodes. Its high ionic conductivity of 6.6 × 10^-5 S cm^-1 at ambient temperature, several orders of magnitude higher than oxide coatings, is attributed to abundant Na vacancies and cooperative MCl₆ rotation, resulting in an extremely low interfacial impedance. A SSSB comprising a NaCrO₂ + NYZC composite cathode, Na₃PS₄ electrolyte, and Na-Sn anode exhibits an exceptional first-cycle Coulombic efficiency of 97.1% at room temperature and can cycle over 1000 cycles with 89.3% capacity retention at 40 °C. These findings highlight the immense potential of halides for SSSB applications.

Erik A. Wu, Swastika Banerjee, Hanmei Tang, Peter M. Richardson, Jean-Marie Doux, Ji Qi, Zhuoying Zhu, Antonin Grenier, Yixuan Li, Enyue Zhao, Grayson Deysher, Elias Sebti, Han Nguyen, Ryan Stephens, Guy Verbist, Karena W. Chapman, Raphaële J. Clément, Abhik Banerjee, Ying Shirley Meng, Shyue Ping Ong