Unraveling the Structure of Iron(III) Oxalate Tetrahydrate and Its Reversible Li Insertion Capability

In the search for better batteries, the quest for new positive electrode materials, which are derived from earth-abundant elements and synthesized through low energy demanding processes, has become an area of interest to the scientific community. Many Li–iron based polyanionic compounds have already been considered for Li ion batteries, but there still remain a large variety of Fe-based polyanionic carboxylate compounds (carbonates, oxalates, malonates, etc.) to be explored, among which Fe2(C2O4)3·nH2O is commercially available, but for which structure and electrochemical reactivity toward Li are lacking. By means of combined X-ray and neutron powder diffraction, we unravel for the first time the structure of Fe2(C2O4)3·4H2O. It adopts a triclinic unit cell with iron atoms being octahedrally coordinated by one water molecule and three oxalate groups, two of them being tetradentate. This arrangement translates into zigzag chains linked one to another through the third oxalate molecule to lead to an opened layered structure into which two additional water molecules are located. Moreover, we demonstrate that it reacts with lithium at an average potential of 3.35 V vs. Li leading to a sustainable capacity of 98 mAh/g. The insertion–extraction of the lithium process follows a biphasic process and involves the Fe3+/Fe2+ redox couple as deduced from in situ X-ray powder diffraction and operando Mössbauer spectroscopy measurements.