Charge–Discharge Mechanisms in O3-Na$_{x}$Fe$_{0.5}$Mn$_{0.5}$O$_2$ Na-Ion Battery Electrodes-Unraveling the Structure of the X-Phase

Layered sodium transition metal oxides, Na$_{x}$TMO$_2$, based on abundant transition metals such as Fe and Mn, are promising low-cost and sustainable cathode materials for Na-ion batteries. However, their route to application is hampered by a limited understanding of the complex structural transformations entailing severe disordering during electrochemical cycling. In particular, lack of insight into the structure and formation mechanisms of the disordered high-potential phases poses a challenge, as these have been associated with rapid deterioration of electrochemical performance. In this work, we elucidate, for the first time, the structures of the high-voltage OP2- and X-phases in O3-type Na_0.95Fe_0.5Mn_0.5O₂, which are archetypical high-potential phases in layered NaTMO₂ materials. The unprecedented structural insight allows us to unravel the charge–discharge mechanism, hereunder showing that the first-to-second cycle asymmetry, common to layered NaTMO₂ materials, is caused by changes to the overlap between Fe³⁺/Fe⁴⁺ oxidation and oxygen redox processes. This promotes the migration of Fe-ions to tetrahedral sites in the NaO₂ slabs, which effectively “pins” the O-type layers, thus obstructing the O3 → P3 transition while it is ongoing and leading to formation of the highly disordered X-phase consisting primarily of O3-type stacking disordered by large domains of P3- and OP2-type stacking.