P2-Na$_{2}$/$_{3}$[Fe$_{1}$/$_{2}$Mn$_{1}$/$_{2}$]O$_{2}$ layered oxide is a promising high energy density cathode material for sodium-ion batteries. However, one of its drawbacks is the poor long-term stability in the operating voltage window of 1.5–4.25 V vs Na$^{+}$/Na that prevents its commercialization. In this work, additional light is shed on the origin of capacity fading, which has been analyzed using a combination of experimental techniques and theoretical methods. Electrochemical impedance spectroscopy has been performed on P2-Na$_{2}$/$_{3}$[Fe$_{1}$/$_{2}$Mn$_{1}$/$_{2}$]O$_{2}$ half-cells operating in two different working voltage windows, one allowing and one preventing the high voltage phase transition occurring in P2-Na$_{2}$/$_{3}$[Fe$_{1}$/$_{2}$Mn$_{1}$/$_{2}$]O$_{2}$ above 4.0 V vs Na+/Na; so as to unveil the transport properties at different states of charge and correlate them with the existing phases in P2-Na$_{2}$/$_{3}$[Fe$_{1}$/$_{2}$Mn$_{1}$/$_{2}$]O$_{2}$. Supporting X-ray photoelectron spectroscopy experiments to elucidate the surface properties along with theoretical calculations have concluded that the formed electrode-electrolyte interphase is very thin and stable, mainly composed by inorganic species, and reveal that the structural phase transition at high voltage from P2- to “Z”/OP4-oxygen stacking is associated with a drastic increased in the bulk electronic resistance of P2-Na$_{2}$/$_{3}$[Fe$_{1}$/$_{2}$Mn$_{1}$/$_{2}$]O$_{2}$ electrodes which is one of the causes of the observed capacity fading.