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 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.