Kröhnkite-Type Na₂Fe(SO₄)₂·2H₂O as a Novel 3.25 V Insertion Compound for Na-Ion Batteries

Solid-state sulfate chemistry continues to yield promising high-voltage polyanionic cathode materials for rechargeable batter-ies (cf. fluorosulfates and bisulfates). As part of our ongoing exploration of new sulfate chemistry, here we report a new can-didate cathode material for sodium-ion batteries, Na2Fe(SO4)2.2H2O. Prepared by conventional low temperature (ca. 80 °C) dissolution and precipitation route, it stabilizes in a monoclinic unit cell with P21/c (#14) symmetry, isostructural to the mineral kröhnkite. Its crystal structure has refined against synchrotron X-ray diffraction. This bihydrated compound demon-strates reversible Na (de)insertion with a capacity exceeding 70 mAh/g, involving Fe3+/Fe2+ redox activity centered at 3.25 V (vs. Na/Na+). The synthetic, structural and electrochemical details of this new low-cost insertion compound are presented.