Impact of cobalt content in Na0.67MnxFeyCozO2 (x + y + z = 1), a cathode material for sodium ion batteries

Abstract

Due to the toxicity and price of cobalt (Co), Na_0.67Mn_xFe_yCo_zO₂ (NaMFC) was tested as a cathode material for Na-ion batteries with lower cobalt contents. The objective was to set the Co to a maximum of 0.15 and compensate its loss by changing the ratio of Mn or Fe. The synthesized compounds were compared to two reference materials Na_0.67Mn_0.5Fe_0.25Co_0.25O₂ and Na_0.67Mn_0.5Fe_0.5O₂. Microscopy (SEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and electrochemical tests were performed to characterize the pristine samples and to understand the impact of the cobalt on the electrochemical mechanism. A link was established between the amount of Co in the NaMFC material and the oxidation state of the Mn in the discharged sample. We confirmed that the role of the cobalt is to prevent a high Mn(III) ratio in the compound and thus help to limit the fading of the electrochemical performance. The compound Na_0.67Mn_0.5Fe_0.35Co_0.15O₂ offers a good compromise between electrochemical stability and lower Co content.