A stable cathode-solid electrolyte composite for high-voltage, long-cycle-life solid-state sodium-ion batteries

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.