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

O3-type NaFeO2 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 Na1-xFeO2) 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 NaFeO2 demonstrates excellent electrode performance in a non-aqueous Na cell at room temperature. NaFe0.5Co0.5O2 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 NaFe0.5Co0.5O2. The partial substitution of metal element for Fe in O3-type NaFeO2-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. (C) 2013 The Authors. Published by Elsevier B.V. All rights reserved.