A New Class of Oxyhalide Solid Electrolytes NaNbCl 6-2x O x 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 NaNbCl O with a facile mechanochemical synthesis. The oxyhalide NaNbCl O 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 NaNbCl O 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 NaNbCl O electrolyte and the cobalt- and nickel-free layered Na Fe Mn Al O cathode exhibit a maximum discharge capacity of 155 mAh g with good cycle life at ambient temperature.