Effect of Ni/Fe/Mn ratio on electrochemical properties of the O3–NaNi1-x-yFexMnyO2 (0.25 ≤ x,y ≤ 0.75) cathode materials for Na-ion batteries

Layered O3-type NaMO2 oxides (M = transition metals) are the main candidates for cathode materials in the actively developing technology of sodium-ion batteries. Nevertheless, despite many years of research, the optimal composition of such materials is still a matter of debate. In this work, we studied the effect of the Fe, Mn and Ni content on the phase composition, structure, thermal stability and electrochemical properties of the cathode materials NaNi1/3Fe1/3Mn1/3O2 (NFM111), NaNi0.5Fe0.25Mn0.25O2 (NFM211), NaNi0.25Fe0.5Mn0.25O2 (NFM121) and NaNi0.25Fe0.25Mn0.5O2 (NFM112). All materials demonstrate similar discharge capacity (125–131 mAh·g−1) when cycling in the 1.9–4.0 V vs Na/Na+ potential range. However, only the NFM111 and NFM112 samples show a low increase in charge transfer resistance during cycling and acceptable thermal stability. Low-temperature experiments revealed good capacity retention for the NFM111 electrode material with loss of only ∼20 % of capacity when the temperature drops from +25 to −30°С. Thus, the NFM111 oxide exhibits a balanced combination of the electrochemical properties and, along with manganese-enriched composition, can be considered as a good choice for further development of cathode materials for sodium-ion batteries.