Synthesis and Electrochemistry of O3-type NaFeO 2 -NaCo 0.5 Ni 0.5 O 2 Solid Solutions for Na-Ion Positive Electrodes.

The synthesis, structure, and electrochemistry in Na cells of NaFe x M1- x O2 positive electrode materials with M = Ni, Co0.5 Ni0.5 , and Co are reported. In particular, the properties of O3-NaFeO2 -NaCo0.5 Ni0.5 O2 solid solutions having compositions NaFe x (Co0.5 Ni0.5 )1- x O2 with 0 ≤ x ≤ 0.5 are explored. It is found that the substitution of Fe in NaNi0.5 Co0.5 O2 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, Co0.5 Ni0.5 , 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 (Co0.5 Ni0.5 )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.