Lithium insertion into Fe 2(SO 4) 3 frameworks

The two polymorphs of Fe ₂(SO ₄) ₃ consist of framework structures built up of tetrahedra sharing corners with octahedra and vice versa. One is rhombohedral, the other is monoclinic. Two moles of lithium insert rapidly into both structures at room temperature. However, lithium insertion into the rhombohedral phase is topotactic without any change of symmetry of the framework, whereas the monoclinic modification is converted to an orthorombic Li ₂Fe ₂(SO ₄) ₃ phase via a displacement transition; the existence of a two-phase region between Fe ₂(SO ₄) ₃ and Li ₂Fe ₂(SO ₄) ₃ results in a flat OCV of 3.6 V versus lithium, which is 600 mV higher than is found for Li _xFFe ₂(WO ₄) ₃ or Li _xFe ₂(MoO ₄) ₃. This difference is discussed in terms of the influence of the counter cation on the solid-state Fe ^{3+}/{2+} redox couple.