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.
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