| Summary: | High-voltage generation (over 4 V versus Li<sup>+</sup>/Li) of polyanion-positive electrode materials is usually achieved by Ni<sup>3+</sup>/Ni<sup>2+</sup>, Co<sup>3+</sup>/Co<sup>2+</sup>, or V<sup>4+</sup>/V<sup>3+</sup> 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<sup>3+</sup>/Fe<sup>2+</sup> and Cr<sup>4+</sup>/Cr<sup>3+</sup> redox couples are summarized. Most successful examples are alluaudite Na<sub>2</sub>Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> (3.8 V versus sodium and hence 4.1 V versus lithium) and <i>β<sub>1</sub></i>-Na<sub>3</sub>Al<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>F<sub>3</sub>-type Na<sub>3</sub>Cr<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>F<sub>3</sub> (4.7 V versus sodium and hence 5.0 V versus lithium), where maximizing Δ<i>G</i> by edge-sharing Fe<sup>3+</sup>-Fe<sup>3+</sup> Coulombic repulsion and the use of the 3d<sup>2</sup>/3d<sup>3</sup> configuration of Cr<sup>4+</sup>/Cr<sup>3+</sup> are essential for each case. Possible exploration of new high-voltage cathode materials is also discussed.
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