Synthesis and Electrochemistry of O3-type NaFeO2–NaCo0.5Ni0.5O2 Solid Solutions for Na-Ion Positive Electrodes
2018
Thorne, J. S. | Zheng, Lituo | Lee, Christopher L. D. | Dunlap, R. A. | Obrovac, M. N.
The synthesis, structure, and electrochemistry in Na cells of NaFeₓM₁–ₓO₂ 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 NaFeₓ(Co₀.₅Ni₀.₅)₁–ₓO₂ 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, NaFeₓM₁–ₓO₂ 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 NaFeₓ(Co₀.₅Ni₀.₅)₁–ₓO₂ and NaFeₓCo₁–ₓO₂ (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.
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