High-Voltage Polyanion Positive Electrode Materials.

High-voltage generation (over 4 V versus Li/Li) of polyanion-positive electrode materials is usually achieved by Ni/Ni, Co/Co, or V/V redox couples, all of which, however, encounter cost and toxicity issues. In this short review, our recent efforts to utilize alternative abundant and less toxic Fe/Fe and Cr/Cr redox couples are summarized. Most successful examples are alluaudite NaFe(SO) (3.8 V versus sodium and hence 4.1 V versus lithium) and -NaAl(PO)F-type NaCr(PO)F (4.7 V versus sodium and hence 5.0 V versus lithium), where maximizing Δ by edge-sharing Fe-Fe Coulombic repulsion and the use of the 3d/3d configuration of Cr/Cr are essential for each case. Possible exploration of new high-voltage cathode materials is also discussed.