Structural, magnetic and redox properties of a new cathode material for Li-ion batteries: the iron-based metal organic framework

The iron-based metal organic framework (MOF) presently studied is the first example of MOF showing a reversible electrochemical Li insertion with a very good cycling life. Its potential application as a cathode material in Li-ion battery is nevertheless curbed by a rather poor capacity of 70 mAh/g. To figure out the origin of this limited insertion, first-principles density functional theory (DFT)+U calculations were performed. The results show that Fe-III {OH(BDC)} is a weak anti-ferromagnetic charge transfer insulator at T = 0 K with iron in the high-spin S=5/2 state. In agreement with the absence of electronic de-localisation along the inorganic chains, lithium insertion leads to the stabilisation of a Fe-II/Fe-III mixed-valence state of class I or II in the Robin-Day classification, whatever the Li sites considered in the calculations. Among these Li sites, the most probable site I (OH-Li) and site II (O=CO-Li) are shown to induce incompatible structural changes on the reduced Li0.5Fe{OH(BDC)} form that could be at the origin of the small capacity measured for this compound.