P2-Na_<i>x</i>Mn_1/2Fe_1/2O₂ Phase Used as Positive Electrode in Na Batteries: Structural Changes Induced by the Electrochemical (De)intercalation Process

The electrochemical properties of the P2-type Na_<i>x</i>Mn_1/2Fe_1/2O₂ (<i>x</i> = 0.62) phase used as a positive electrode in Na batteries were tested in various voltage ranges at <i>C</i>/20. We show that, even if the highest capacity is obtained for the first cycles between 1.5 and 4.3 V, the best capacity after 50 cycles is obtained while cycling between 1.5 and 4.0 V (120 mAh g^–1). The structural changes occurring in the material during the (de)­intercalation were studied by operando in situ X-ray powder diffraction (XRPD) and ex situ synchrotron XRPD. We show that a phase with an orthorhombic P′2-type structure is formed for <i>x</i> ≈ 1, due to the cooperative Jahn–Teller effect of the Mn³⁺ ions. P2 structure type stacking is observed for 0.35 < <i>x</i> < 0.82, while above 4.0 V, a new phase appears. A full indexation of the XRPD pattern of this latter phase was not possible because of the broadening of the diffraction peaks. However, a much shorter interslab distance was found that may imply a gliding of the MO₂ slab occurring at high voltage. Raman spectroscopy was used as a local probe and showed that in this new phase the MO₂ layers are maintained, but the phase exhibits a strong degree of disorder