Synthesis and Electrochemistry of O3-type NaFeO₂-NaCo_0.5Ni_0.5O₂ Solid Solutions for Na-Ion Positive Electrodes

The synthesis, structure, and electrochemistry in Na cells of NaFe _xM_{1- x}O₂ positive electrode materials with M = Ni, Co_0.5Ni_0.5, and Co are reported. In particular, the properties of O3-NaFeO₂-NaCo_0.5Ni_0.5O₂ solid solutions having compositions NaFe _x(Co_0.5Ni_0.5)_{1- x}O₂ with 0 ≤ x ≤ 0.5 are explored. It is found that the substitution of Fe in NaNi_0.5Co_0.5O₂ 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, Co_0.5Ni_0.5, and Co. In general, NaFe _xM_{1- 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(Co_0.5Ni_0.5)_{1- x}O₂ and NaFe _xCo_{1- 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.