A Na-rich fluorinated sulfate anti-perovskite with dual doping as solid electrolyte for Na metal solid state batteries

High-conductivity solid electrolytes are crucial components for the development of Na-based solid state batteries . However the electrolyte structure prototype and corresponding synthesis method are still lacking. The conventional oxide and sulfide electrolytes are precursor-expensive (e.g. Na 2 S) or energy-intensive in sintering synthesis (e.g. at 1000 ​°C). Here, we propose a novel anti-perovskite solid electrolyte of Na-rich fluorinated sulfate (Na 3 SO 4 F) benefiting from Mg and Cl co-doping by solid state reaction from low-cost precursors at moderate temperature (500 ​°C). The tailored dual doping enables an improvement of ionic conductivity by three orders of magnitude close to 10 −4  ​S ​cm −1 at 60 ​°C. The creation of Na vacancies (by aliovalent Mg doping) and lattice expansion (by substituting F sites with larger-sized Cl ions) are responsible for the conductivity upgrade. A Na–Sn/Fe [Fe(CN) 6 ] 3 solid state battery based on Na 2.98 Mg 0.01 SO 4 F 0.95 Cl 0.05 electrolyte can reversibly run with a first discharge capacity as high as 91.0 mAh g −1 and a reversible capacity preserved at 77.0 mAh g −1 . This result paves a way to novel anti-perovskite family of fluorinated sulfates as potential alkali metal ion solid electrolytes beyond already reported A 3 OX (A ​= ​Li or Na, X ​= ​heavy halogen or hydrogenide anions) anti-perovskites.