High-Voltage Polyanion Positive Electrode Materials

High-voltage generation (over 4 V versus Li+/Li) of polyanion-positive electrode materials is usually achieved by Ni3+/Ni2+, Co3+/Co2+, or V4+/V3+ 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 Fe3+/Fe2+ and Cr4+/Cr3+ redox couples are summarized. Most successful examples are alluaudite Na2Fe2(SO4)3 (3.8 V versus sodium and hence 4.1 V versus lithium) and β1-Na3Al2(PO4)2F3-type Na3Cr2(PO4)2F3 (4.7 V versus sodium and hence 5.0 V versus lithium), where maximizing ΔG by edge-sharing Fe3+-Fe3+ Coulombic repulsion and the use of the 3d2/3d3 configuration of Cr4+/Cr3+ are essential for each case. Possible exploration of new high-voltage cathode materials is also discussed.