Non-stoichiometry and electrochemical properties of lithiated iron hydroxysulfides

LiFeOHS is a material with Li₂(OH)₂ layers intercalated between Fe₂S₂ planes. Its hydrothermal synthesis in various concentrations of LiOH yields materials with a high non-stoichiometry of the Li/Fe ratio which can be explained by partial substitution of Li⁺ for Fe²⁺ in the Li₂(OH)₂ layers. Thermogravimetry, X-ray diffraction and Mössbauer studies indicate that the charge balance is obtained by substitution of hydroxyl ions OH^- by oxide ions O^2-. This material has been tested as an electrode for lithium-ion batteries against lithium metal. Specific capacities above 200 mA h g^-1 at C/10 are achieved, involving 1 lithium per chemical formula when cycled between 1 V and 3 V vs. lithium. The first irreversible discharge leads to the insertion of one lithium atom and the evolution of hydrogen gas while iron remains in its +2-oxidation state. An original Li₂OFeS oxysulfide is formed. The following reversible oxidation/reduction cycles involve the Fe³⁺/Fe²⁺ redox couple between the two limiting compositions: Li₂OFe^IIS and LiOFe^IIIS.