The effect of O3–P3–P′3 phases coexistence in NaxFe0.3Co0.7O2 cathode material on its electronic and electrochemical properties. Experimental and theoretical studies

Within this study, we present the comprehensive study on structural, electrochemical and electronic properties of the NaxFe0.3Co0.7O2 compound as a cathode material for Na-ion type batteries. The XRD in-situ, as well as ex-situ results, revealed that the structural evolution upon Na deintercalation proceeds via intermittent regions composed of O3, P3, and P′3 type structures. Along with the deintercalation reaction, the O3 phase disappears accompanied with profound ~0.2 V potential jump on the EMF profile and P3 phase begins deformation to P′3 form. The analysis of the unit cell parameters of the O3, P3 and P′3 type structures confirmed the electrochemical activity of all phases, while the Mössbauer spectroscopy of iron valence states showed that mainly Co3+ ions are involved in electrochemical oxidation processes. The calculated difference in energy between the ΔEF(x) of both O3 and P3 phases was estimated as ~0.18 eV for the Na concentration x~0.75 and collaborated well with ~0.2 V potential jump on the EMF profile, indicating that observed variation is linked to the abrupt change of the Fermi level when passing from O3 to P3 phase.