Influence of Mn/Fe Ratio on Electrochemical and Structural Properties of P2-Na_xMn_1–yFe_yO₂ Phases as Positive Electrode Material for Na-Ion Batteries

The comparative structural, Mössbauer, and electrochemical studies of the NaxMn2/3Fe1/3O2 and NaxMn1/2Fe1/2O2 systems show that the change in the Mn/Fe ratio has a significant influence on the overlap between the Mn3+/4+ and Fe3+/4+ redox couples. The P2-type structure is maintained in the 0.3 < x < 0.8 domain. For the highest intercalation amount, structural distortions occur due to the Jahn–Teller effect of the Mn3+ ions. The macroscopic distortion results from a competition between the opposite effects of Mn3+ and Fe3+: the isotropic character of Fe3+ tends to prevent the macroscopic distortion. For the lower sodium amounts, the instability of the interstitial trigonal prismatic space leads to the formation, by slab gliding, of a very disordered structure. Even if this structural transition is reversible, a strong capacity fading is observed if the cell is charged above 4 V verus Na/Na+.