Synthesis and Electrochemistry of O3-type NaFeO2–NaCo0.5Ni0.5O2 Solid Solutions for Na-Ion Positive Electrodes

• Published in: ACS applied materials & interfaces Vol. 10; no. 26; pp. 22013 - 22022
• Main Authors: Thorne, J. S, Zheng, Lituo, Lee, Christopher L. D, Dunlap, R. A, Obrovac, M. N
• Format: Journal Article
• Language: English
• Published: American Chemical Society 05.07.2018
• Subjects: NaFe0.5Co0.5O2; batteries; solid solution; O3-type; NaFeO2; Na-ion; sodium-ion cathodes; NaCo0.5Ni0.5O2
• ISSN: 1944-8244; 1944-8252
• DOI: 10.1021/acsami.8b03336

The synthesis, structure, and electrochemistry in Na cells of NaFe x M1–x O2 positive electrode materials with M = Ni, Co0.5Ni0.5, and Co are reported. In particular, the properties of O3-NaFeO2–NaCo0.5Ni0.5O2 solid solutions having compositions NaFe x (Co0.5Ni0.5)1–x O2 with 0 ≤ x ≤ 0.5 are explored. It is found that the substitution of Fe in NaNi0.5Co0.5O2 causes an increase in first cycle energy density from 320 to 440 mWh/g in a 1.5–4.0 V test. However, capacity retention is generally reduced when x is increased for all M = Ni, Co0.5Ni0.5, and Co. In general, NaFe x M1–x O2 samples with M = Co had the highest capacity retention for all values of x. Ex situ X-ray diffraction and Mössbauer results of as-prepared and charged materials are directly compared for NaFe x (Co0.5Ni0.5)1–x O2 and NaFe x Co1–x O2 (x = 0.4, 0.5). Iron was found to be in the +3 oxidation state in the as-prepared materials. A significant fraction of Fe3+ is oxidized to Fe4+ in these samples when they are charged to 4.0 V.