Cathode performance and voltage estimation of metal trihalides

Compounds with highly ionic metal-ligand bonds are attractive for use as high voltage cathodes. Therefore, we investigated metal trihalide cathodes, in particular trifluorides (MF(3)). FeF(3) shows a mean discharge voltage and quasi open-circuit voltage (QOCV) of 3.0 and 3.4 V respectively, X-ray analysis shows that the lithiation reaction proceeds in a topotactic manner. TiF(3) and VF(3) have mean discharge voltages of 2.5 and 2.2 V, respectively. These three compounds have the same layer structures and almost the same rechargeable capacity of 80 mAh g(-1). MnF(3), which has a monoclinic lattice, exhibits an initial voltage of 4.2 V, however, lithiation does not proceed due to a high overvoltage, These high voltages indicate the highly ionic nature of MF(3) cathodes, and they correspond to values estimated using the standard electrode potentials of the 'naked' ions. (C) 1997 Elsevier Science S.A.