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 (MF₃). FeF₃ 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. TiF₃ and VF₃ 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. MnF₃, 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 MF₃ cathodes, and they correspond to values estimated using the standard electrode potentials of the 'naked' ions.