Single phase, well-crystallized O3-type NaFeO 2 (alpha NaFeO 2 ) is prepared by a solid-state method. Electrode performance of O3-type NaFeO 2 is examined as positive electrode materials for rechargeable sodium batteries. O3-type NaFeO 2 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−x FeO 2 . 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−x FeO 2 . Acceptable cyclability is, therefore, achieved for O3-type NaFeO 2 as the positive electrode materials in the limited composition of x = 0-0.45 in Na 1−x FeO 2 .

New electrode materials of layered oxides, Na2/3Ni1/3Mn2/3-xTixO2 (0 ≤ x ≤ 2/3), are successfully synthesized, and their electrochemical performance is examined in aprotic Na cells. A Na//Na2/3Ni1/3Mn1/2Ti1/6O2 cell delivers 127 mA h g(-1) of reversible capacity and the average voltage reaches 3.7 V at first discharge with good capacity retention.

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Abstract

NaFex(Ni1/2Mn1/2)1-xO2 layered oxides are synthesized by a solid-state method, and their electrode properties as positive electrodes for rechargeable sodium batteries are examined. Crystallographic analysis on a series of samples reveals that NaFex(Ni1/2Mn1/2)1-xO2 samples crystallize into a solid solution between two end-members of O3-type Na(Ni1/2Mn1/2)O2 and NaFeO2. A Na/NaFe0.4(Ni1/2Mn1/2)0.6O2 cell delivers 130 mAh g−1 of reversible capacity in a voltage range of 2.0 – 3.8 V. Energy density available based on metallic sodium is estimated to be approximately 400 mWh g−1, which is considerably larger than that of NaFeO2 (∼300 mWh g−1). Moreover, the Na/NaFe0.4(Ni1/2Mn1/2)0.6O2 cell shows relatively good cyclability and rate-capability. From these results, a potential application of the NaFex(Ni1/2Mn1/2)1-xO2 solid solution for rechargeable Na-ion batteries is discussed.

Na‐ion batteries have become promising candidates for large‐scale energy‐storage systems because of the abundant Na resources and they have attracted considerable academic interest because of their unique behavior, such as their electrochemical activity for the Fe3+/Fe4+ redox couple. The high‐rate performance derived from the low Lewis‐acidity of the Na+ ions is another advantage of Na‐ion batteries and has been demonstrated in NaFe1/2Co1/2O2 solutions. Here, a solid solution of NaFeO2‐NaCoO2 is synthesized and the mechanisms behind their excellent electrochemical performance are studied in comparison to those of their respective end‐members. The combined analysis of operando X‐ray diffraction, ex situ X‐ray absorption spectroscopy, and density functional theory (DFT) calculations for Na1– x Fe1/2Co1/2O2 reveals that the O3‐type phase transforms into a P3‐type phase coupled with Na+/vacancy ordering, which has not been observed in O3‐type NaFeO2. The substitution of Co for Fe stabilizes the P3‐type phase formed by sodium extraction and could suppress the irreversible structural change that is usually observed in O3‐type NaFeO2, resulting in a better cycle retention and higher rate performance. Although no ordering of the transition metal ions is seen in the neutron diffraction experiments, as supported by Monte‐Carlo simulations, the formation of a superlattice originating from the Na+/vacancy ordering is found by synchrotron X‐ray diffraction for Na0.5Fe1/2Co1/2O2, which may involve a potential step in the charge/discharge profiles.

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