Lithium Intercalation into the Jarosite-type Hydroxysulfate: A Topotactic Reversible Reaction from a Crystalline Phase to an Inorganic Polymer-like Structure

Insertion of lithium into the jarosite hydroxysulfate NaFe3(SO4)2(OH)6 leads to an amorphous phase Li2+xNaFe3(SO4)2(OH)6 (with 0 < x<0.5), through a biphasic mechanism. The latter phase exhibits reversible electrochemical lithium intercalation/deintercalation through a solid-solution-like process, leading back to the oxidized jarosite LixNaFe3(SO4)2(OH)6 (with 0.3 < x < 0.6), with redox cycling at 2.82 V and a capacity of 110 mAh/g at C/20. This unique topotactic reaction between an amorphous and a well-crystallized phase is interpreted on the basis of the formation of an inorganic polymer.