NaFe0.5Co0.5O2 as high energy and power positive electrode for Na-ion batteries☆

Abstract O3-type NaFeO 2 is a promising candidate as positive electrode materials for rechargeable Na batteries. However, its reversible range of sodium extraction is relatively narrow ( x  = ca. 0.4 in Na 1 −  x FeO 2 ) because of the irreversible structural change, presumably associated with the iron migration into the adjacent tetrahedral sites in Na layers as we recently reported. Herein, we report that cobalt-substituted NaFeO 2 demonstrates excellent electrode performance in a non-aqueous Na cell at room temperature. NaFe 0.5 Co 0.5 O 2 delivers approximately 160 mAh g − 1 of reversible capacity with relatively good capacity retention and excellent rate-capability in a voltage range of 2.5–4.0 V, resulting in reversible formation of Na 0.3 Fe 0.5 Co 0.5 O 2 . The partial substitution of metal element for Fe in O3-type NaFeO 2 -based materials is proved to be the important strategy to suppress the irreversible phase transition, and thus improving the reversibility of sodium removal/insertion as the electrode materials.