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(3+)/Ni(2+), Co(3+)/Co(2+), or V(4+)/V(3+) 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(3+)/Fe(2+) and Cr(4+)/Cr(3+) redox couples are summarized. Most successful examples are alluaudite Na(2)Fe(2)(SO(4))(3) (3.8 V versus sodium and hence 4.1 V versus lithium) and β(1)-Na(3)Al(2)(PO(4))(2)F(3)-type Na(3)Cr(2)(PO(4))(2)F(3) (4.7 V versus sodium and hence 5.0 V versus lithium), where maximizing ΔG by edge-sharing Fe(3+)-Fe(3+) Coulombic repulsion and the use of the 3d(2)/3d(3) configuration of Cr(4+)/Cr(3+) are essential for each case. Possible exploration of new high-voltage cathode materials is also discussed.