Charge-Discharge Mechanisms in O3-Na_xFe_0.5Mn_0.5O₂ Na-Ion Battery Electrodes─Unraveling the Structure of the X-Phase

TY - JOUR

T1 - Charge-Discharge Mechanisms in O3-NaxFe0.5Mn0.5O2 Na-Ion Battery Electrodes─Unraveling the Structure of the X-Phase

AU - Drejer, Andreas Østergaard

AU - Schou Hansen, Maria

AU - Johansen, Morten

AU - Dunker, Josephine

AU - Poppe, Romy

AU - Hadermann, Joke

AU - Ravnsbæk, Dorthe Bomholdt

PY - 2025

Y1 - 2025

N2 - Layered sodium transition metal oxides, NaxTMO2, 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 Na0.95Fe0.5Mn0.5O2, 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.

AB - Layered sodium transition metal oxides, NaxTMO2, 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 Na0.95Fe0.5Mn0.5O2, 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.

UR - https://www.scopus.com/pages/publications/105009621376

U2 - 10.1021/acs.chemmater.5c00961

DO - 10.1021/acs.chemmater.5c00961

M3 - Journal article

AN - SCOPUS:105009621376

SN - 0897-4756

VL - 37

SP - 5234

EP - 5248

JO - Chemistry of Materials

JF - Chemistry of Materials

IS - 14

ER -