Crystal Structures and Electrode Performance of Alpha-NaFeO₂ for Rechargeable Sodium Batteries

Abstract

Single phase, well-crystallized O3-type NaFeO₂ (alpha NaFeO₂) is prepared by a solid-state method. Electrode performance of O3-type NaFeO₂ is examined as positive electrode materials for rechargeable sodium batteries. O3-type NaFeO₂ can deliver 80–100 mAh g⁻¹ of reversible capacity with a nearly flat voltage profile at approximately 3.3 V vs. Na metal. The electrode performance is significantly deteriorated by oxidation beyond x > 0.5 in Na_1−xFeO₂. X-ray diffraction study reveals that loss of electrode reversibility originates from irreversible structural change, possibly accompanied by iron ion migration in layered host structures. The sodium ion insertion into the host structures would be disturbed by the irreversible structural change when charged beyond x > 0.5 in Na_1−xFeO₂. Acceptable cyclability is, therefore, achieved for O3-type NaFeO₂ as the positive electrode materials in the limited composition of x = 0–0.45 in Na_1−xFeO₂.