A New Class of Oxyhalide Solid Electrolytes NaNbClO for Solid-state Sodium Batteries.

Sodium-based batteries are gaining momentum due to the abundance and lower cost of sodium compared to lithium. Solid-state sodium batteries can also provide further safety advantages. However, sodium-based solid-state electrolytes (SSEs) that meet all the rigorous requirements, such as high ionic conductivity, oxidative stability with the cathode, and ease of processability, are lacking. We present here a new class of sodium-based oxyhalide electrolytes NaNbClO with a facile mechanochemical synthesis. The oxyhalide NaNbClO exhibits close to two orders of magnitude higher ambient-temperature sodium-ion conductivity (1.03×10 S cm) compared to the halide counterpart NaNbCl (3×10 S cm). Structural motifs unique to the oxygen content in NaNbClO are identified with Na and Nb magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and x-ray diffraction (XRD). Solid-state sodium batteries assembled with NaNbClO electrolyte and the cobalt- and nickel-free layered NaFeMnAlO cathode exhibit a maximum discharge capacity of 155 mAh g with good cycle life at ambient temperature.