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Iron and vanadium oxides have a rich structural chemistry when combined with phosphate groups; the transition metal most commonly in an octahedral coordination. The inductive effect increases the potential difference between Fe3+/Fe2+ and Li/Li+ couples in phosphate lattices relative to the pure iron oxides; a similar behavior is found for the corresponding vanadium compounds. Of the iron phosphates, the olivine phase LiFePO4 has high thermal and chemical stability, even when lithium-free; the challenges of low electronic conductivity are being overcome, but data is lacking on the true lithium diffusion behavior. The all-ferric lipscombite-type phase, Fe1.33PO4OH, shows the highest capacity of the iron phosphates for lithium intercalation. The ε-VOPO4 material, formed by the oxidative de-intercalation of protons from H2VOPO4, can reversibly react with two lithium atoms in two steps. The face- and edge-sharing transition metal octahedra lead to a range of interesting and structurally revealing magnetic interactions. A number of vanadium oxide phases are known, with those containing VO6 octahedra showing the greatest stability when undergoing redox reactions. Such structures have been synthesized using xerogel, hydrothermal and electrochemical methods. The double-sheet delta structures show reversible lithium intercalation of up to one lithium ion per vanadium, leading to the highest storage capacities. However, the large potential width of discharge and the apparent low reaction rates will minimize their use unless improved.
Whittingham et al. (Sat,) studied this question.