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. Na2S) or energy-intensive in sintering synthesis (e.g. at 1000 degrees C). Here, we propose a novel anti-perovskite solid electrolyte of Na-rich fluorinated sulfate (Na3SO4F) benefiting from Mg and Cl co-doping by solid state reaction from low-cost precursors at moderate temperature (500 degrees 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 degrees 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 Na2.98Mg0.01SO4F0.95Cl0.05 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.