Mapping of Redox Energies

Electrochemical discharge/charge curves vs Lithium of coin cells using as cathodes various transition-metal oxide hosts for lithium insertion/extraction reactions provide information on the operative redox energies of the transition-metal atoms. The relative positions of the redox energies were found to vary little, but their absolute positions by as much as 1 eV, with changes in structure or, for an isostructural series, with changes in counter cation. Use of polyanions to obtain a more open framework with a larger free volume for Li+-ion motion was found to be more important for high-power applications than the loss in electronic mobility, but a larger free volume for ionic motion reduces the capacity per unit volume. It was shown that introduction of a second phase with a higher redox energy provides a buffer against over discharge. A reversible decrease in capacity with increasing current density was identified and its origin discussed. Substitution of the polyanions (PO4)3− or (SO4)2− for oxide ions brings the V4+/V3+ and Fe3+/Fe2+ redox energies to levels of interest for cathodes in a lithium-ion battery.