Reversible transformation from amorphouse Na3Fe3(SO4)2(OH)6 to crystallized NaFe3(SO4)2(OH)6 Jarosite-type hydroxysulfate

•Unique reversible topotactic reaction from a complete amorphous polymer like phase back to a well crystallized phase.•Reversible insertion of sodium into NaFe3(SO4)2(OH)6 to NaFe3(SO4)2(OH)6 at 2.72 V for 120 mAh/g

An amorphous iron hydroxysulfate Na3Fe3(SO4)2(OH)6 has been prepared for the first time from sodium insertion into the crystalline phase NaFe3(SO4)2(OH)6. We show that this transformation is completely reversible, leading back to the well crystallized hexagonal phase on oxidation. This material could then be proposed as positive electrode material for Na ion batteries with a reversible capacity of 120 mAh/g at 2.72 V vs Na+/Na.