Bulk O2 formation and Mg displacement explain O-redox in Na0.67Mn0.72Mg0.28O2

Abstract

•Out-of-plane Mg and in-plane Mn migration enable formation of trapped molecular O2•Quantification of molecular O2 by magnetometry agrees well with charge passed•The molecular O2 is almost fully reduced to O2− upon discharge•Local reformation of honeycomb ordering induces the persistence of the charging plateau Layered intercalation compounds based on transition-metal oxides with alkali ions on the transition-metal (TM) and alkali-metal layers, such as Li[Li0.2Ni0.13Mn0.54Co0.13]O2 and Na0.75[Li0.25Mn0.75]O2, can store charge by oxidation of the O2− ions forming molecular O2 trapped in the particles. This mechanism involves TM rearrangement as the alkali ions migrate out of the TM layers on charge, resulting in the loss of the honeycomb ordering and a large voltage hysteresis during the first charge-discharge cycle. Here, we demonstrate that the same mechanism of O2 formation occurs in compounds with divalent ions on the TM layers: Na2/3Mn0.72Mg0.28O2. The mechanism is shown by RIXS and SQUID magnetometry. However, in contrast to materials with alkali ions on the TM layers, the honeycomb ordering in Na0.67Mn0.72Mg0.28O2 is partially reformed at the local scale upon discharge, leading to the persistence of the high-voltage charging plateau on subsequent cycles, although it fades on cycling. O-redox in compounds with Li on the transition-metal layers (TML) has recently been attributed to the formation of molecular O2 on charge, trapped in the lattice. Here, we show that a similar process occurs for P2-Na0.67[Mn0.72Mg0.28]O2, which contains Mg2+ on the TML. The molecular O2 is identified by high-resolution RIXS and quantified by magnetometry, showing that it equates to the charge passed. This O2 is trapped in voids that are formed by Mg2+ out-of-plane displacement and Mn4+ in-plane disordering and is then reduced on discharge associated with a large voltage hysteresis. In contrast to compounds containing Li+ in the TML, in which the honeycomb ordering and the high-voltage plateau are irreversibly lost after the first cycle, in P2-Na0.67[Mn0.72Mg0.28]O2, the plateau reappears partially on the second charge due to the partial reversibility of Mn in-plane and Mg out-of-plane migration and the local reformation of the honeycomb ordering. O-redox in compounds with Li on the transition-metal layers (TML) has recently been attributed to the formation of molecular O2 on charge, trapped in the lattice. Here, we show that a similar process occurs for P2-Na0.67[Mn0.72Mg0.28]O2, which contains Mg2+ on the TML. The molecular O2 is identified by high-resolution RIXS and quantified by magnetometry, showing that it equates to the charge passed. This O2 is trapped in voids that are formed by Mg2+ out-of-plane displacement and Mn4+ in-plane disordering and is then reduced on discharge associated with a large voltage hysteresis. In contrast to compounds containing Li+ in the TML, in which the honeycomb ordering and the high-voltage plateau are irreversibly lost after the first cycle, in P2-Na0.67[Mn0.72Mg0.28]O2, the plateau reappears partially on the second charge due to the partial reversibility of Mn in-plane and Mg out-of-plane migration and the local reformation of the honeycomb ordering. Alkali-metal (AM) intercalation compounds based on layered transition-metal (TM) oxides and containing Li in the TM layers, such as Li1.2Ni0.13Mn0.54Co0.13O2, exhibit capacities in excess of that associated with TM redox by invoking redox reactions on the O2− ions.1Luo K. Roberts M.R. Hao R. Guerrini N. Pickup D.M. Liu Y.S. Edström K. Guo J. Chadwick A.V. Duda L.C. Bruce P.G. Charge-compensation in 3d-transition-metal-oxide intercalation cathodes through the generation of localized electron holes on oxygen.Nat. Chem. 2016; 8: 684-691Crossref PubMed Scopus (664) Google Scholar, 2Bhardwaj A. Kaur J. Wuest M. Wuest F. In situ click chemistry generation of cyclooxygenase-2 inhibitors.Nat. Commun. 2017; 8: 1Crossref PubMed Scopus (385) Google Scholar, 3Koga H. Croguennec L. Ménétrier M. Mannessiez P. Weill F. Delmas C. Different oxygen redox participation for bulk and surface: a possible global explanation for the cycling mechanism of Li1.20Mn0.54Co0.13Ni0.13O2.J. Power Sources. 2013; 236: 250-258Crossref Scopus (255) Google Scholar The coordination of the O2− ions by alkali ions in the transition-metal as well as the AM layers raises the energy of the O2p states, rendering them accessible to oxidation within the voltage stability window of common organic electrolytes.4Seo D.H. Lee J. Urban A. Malik R. Kang S. Ceder G. The structural and chemical origin of the oxygen redox activity in layered and cation-disordered Li-excess cathode materials.Nat. Chem. 2016; 8: 692-697Crossref PubMed Scopus (723) Google Scholar The nature of the oxidized oxygen has been widely debated with localized holes (O−), peroxo (O22−), peroxo-like (O2n−), and superoxo (O2−) species being proposed.1Luo K. Roberts M.R. Hao R. Guerrini N. Pickup D.M. Liu Y.S. Edström K. Guo J. Chadwick A.V. Duda L.C. Bruce P.G. Charge-compensation in 3d-transition-metal-oxide intercalation cathodes through the generation of localized electron holes on oxygen.Nat. Chem. 2016; 8: 684-691Crossref PubMed Scopus (664) Google Scholar,5Hong J. Gent W.E. Xiao P. Lim K. Seo D.H. Wu J. Csernica P.M. Takacs C.J. Nordlund D. Sun C.-J. et al.Metal–oxygen decoordination stabilizes anion redox in Li-rich oxides.Nat. Mater. 2019; 18: 256-265Crossref PubMed Scopus (186) Google Scholar, 6McCalla E. Abakumov A.M. Saubanère M. Foix D. Berg E.J. Rousse G. Doublet M.L. Gonbeau D. Novák P. Van Tendeloo G. et al.Visualization of O-O peroxo-like dimers in high-capacity layered oxides for Li-ion batteries.Science. 2015; 350: 1516-1521Crossref PubMed Scopus (505) Google Scholar, 7Chen Z. Li J. Zeng X.C. Unraveling oxygen evolution in Li-rich oxides: a unified modeling of the intermediate peroxo/superoxo-like dimers.J. Am. Chem. Soc. 2019; 141: 10751-10759Crossref PubMed Scopus (55) Google Scholar It has been shown recently that in such compounds, specifically O3-Li1.2Ni0.13Mn0.54Co0.13O2 and P2-Na0.75Li0.25Mn0.75O2, the oxidation of O2− results in O− disproportionation, forming O2 molecules which are physically trapped in voids within the bulk of the particles formed by in-plane TM migration.8House R.A. Maitra U. Pérez-Osorio M.A. Lozano J.G. Jin L. Somerville J.W. Duda L.C. Nag A. Walters A. Zhou K.-J. et al.Superstructure control of first-cycle voltage hysteresis in oxygen-redox cathodes.Nature. 2020; 577: 502-508Crossref PubMed Scopus (225) Google Scholar,9House R.A. Rees G.J. Pérez-Osorio M.A. Marie J. Boivin E. Robertson A.W. Nag A. Garcia-Fernandez M. Zhou K. Bruce P.G. First-cycle voltage hysteresis in Li-rich 3d cathodes associated with molecular O2 trapped in the bulk.Nat. Energy. 2020; 5: 777-785Crossref Scopus (123) Google Scholar This O2 is then reduced on discharge, explaining the first cycle voltage hysteresis systematically observed. Investigation of P2-Na0.67Mn0.72Mg0.28O2 and more recently P2-Na0.67Mn0.78Zn0.22O2 have shown that O-redox is not restricted to alkali-rich compounds but can be activated in compounds with divalent ions, such as Mg2+ or Zn2+.10Maitra U. House R.A. Somerville J.W. Tapia-Ruiz N. Lozano J.G. Guerrini N. Hao R. Luo K. Jin L. Pérez-Osorio M.A. et al.Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2.Nat. Chem. 2018; 10: 288-295Crossref PubMed Scopus (284) Google Scholar,11Bai X. Sathiya M. Mendoza-Sánchez B. Iadecola A. Vergnet J. Dedryvère R. Saubanère M. Abakumov A.M. Rozier P. Tarascon J. Anionic redox activity in a newly Zn-doped sodium layered oxide P2-Na2/3 Mn1−yZnyO2 (0 < y < 0.23).Adv. Energy Mater. 2018; 8: 1802379Crossref Scopus (102) Google Scholar However, the nature of oxidized oxygen in the bulk and the persistence of charging plateaus as well as the voltage hysteresis in these materials remains to be understood. P2-Na0.67Mn0.72Mg0.28O2 is a useful material to study this question as it shows almost pure O-redox, no O2 loss from the lattice, and Mn is the only TM. Here, we show that molecular O2 physically trapped in the bulk of particles is the primary form of oxidized oxygen species formed on charging P2-Na0.67Mn0.72Mg0.28O2. The O2 was identified by using high-resolution O K-edge resonant inelastic X-ray scattering (RIXS) and quantified by magnetometry, which showed close agreement with the charge passed across the plateau corresponding to the O-redox process. Mg2+ is displaced from the TM layers into the alkali-metal layers, as also observed for Zn2+ upon charging P2-Na0.67Mn0.78Zn0.22O2.11Bai X. Sathiya M. Mendoza-Sánchez B. Iadecola A. Vergnet J. Dedryvère R. Saubanère M. Abakumov A.M. Rozier P. Tarascon J. Anionic redox activity in a newly Zn-doped sodium layered oxide P2-Na2/3 Mn1−yZnyO2 (0 < y < 0.23).Adv. Energy Mater. 2018; 8: 1802379Crossref Scopus (102) Google Scholar The Mg2+ migrates to tetrahedral and octahedral sites in the AM layer, with half the Mg2+ in each site. The vacancies generated in the TM layer allow in-plane disorder of Mn4+. The in-plane Mn/□ redistribution, confirmed by neutron pair distribution functions (n-PDF), converges into stable vacancy-cluster voids that can accommodate O–O dimers with a bond length of 1.23 Å, characteristic of molecular oxygen. In contrast to compounds with Li in the TM layers where the honeycomb ordering and the high-voltage plateau are irreversibly lost on the first cycle,2Bhardwaj A. Kaur J. Wuest M. Wuest F. In situ click chemistry generation of cyclooxygenase-2 inhibitors.Nat. Commun. 2017; 8: 1Crossref PubMed Scopus (385) Google Scholar,5Hong J. Gent W.E. Xiao P. Lim K. Seo D.H. Wu J. Csernica P.M. Takacs C.J. Nordlund D. Sun C.-J. et al.Metal–oxygen decoordination stabilizes anion redox in Li-rich oxides.Nat. Mater. 2019; 18: 256-265Crossref PubMed Scopus (186) Google Scholar,12Sathiya M. Abakumov A.M. Foix D. Rousse G. Ramesha K. Saubanère M. Doublet M.L. Vezin H. Laisa C.P. Prakash A.S. et al.Origin of voltage decay in high-capacity layered oxide electrodes.Nat. Mater. 2015; 14: 230-238Crossref PubMed Scopus (610) Google Scholar in P2-Na0.67Mn0.72Mg0.28O2, the initial honeycomb ordering, observed at long range in the pristine material, is partially reformed at the local scale on discharge. After several cycles, the high-voltage plateau is no longer observed, indicating complete loss of honeycomb ordering. Time-of-flight neutron powder diffraction (TOF-NPD) data were used to refine the structure of P2-Na0.67Mn0.72Mg0.28O2 using the P63/mcm space group, which captures the honeycomb ordering of the Mg/Mn within the TM layer. Na0.67Mn0.72Mg0.28O2 was the closest composition to the ideal honeycomb (i.e., Na2/3Mn2/3Mg1/3O2) that we could prepare as a pure phase. The difference is accounted for in the structure by some occupancy of Mn in the Mg site (Figure S1; Table S1). The galvanostatic cycling of Na0.67Mn0.72Mg0.28O2 is shown in Figure 1. The initial sloped region corresponds to the small amount of Mn3+/Mn4+ redox activity and accounts for the extraction of ~0.14 Na+, whereas the high-voltage plateau observed upon further Na+ extraction is charge compensated through O-redox rather than O-loss from the lattice. Indeed, the lack of O2 gas loss from the particle surface for this material has been shown previously.10Maitra U. House R.A. Somerville J.W. Tapia-Ruiz N. Lozano J.G. Guerrini N. Hao R. Luo K. Jin L. Pérez-Osorio M.A. et al.Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2.Nat. Chem. 2018; 10: 288-295Crossref PubMed Scopus (284) Google Scholar The plateau is not preserved upon discharge and instead a sloping profile is seen with a large voltage hysteresis. This behaviour is common to most O-redox cathode materials and has been assigned previously to the formation of molecular O2 trapped in the bulk of particles.8House R.A. Maitra U. Pérez-Osorio M.A. Lozano J.G. Jin L. Somerville J.W. Duda L.C. Nag A. Walters A. Zhou K.-J. et al.Superstructure control of first-cycle voltage hysteresis in oxygen-redox cathodes.Nature. 2020; 577: 502-508Crossref PubMed Scopus (225) Google Scholar,9House R.A. Rees G.J. Pérez-Osorio M.A. Marie J. Boivin E. Robertson A.W. Nag A. Garcia-Fernandez M. Zhou K. Bruce P.G. First-cycle voltage hysteresis in Li-rich 3d cathodes associated with molecular O2 trapped in the bulk.Nat. Energy. 2020; 5: 777-785Crossref Scopus (123) Google Scholar However, the presence of a shortened plateau during subsequent charge of such a honeycomb-ordered compound is a feature observed only upon cycling of P2-Na0.67Mn0.72Mg0.28O2, P3-Na0.67Mn0.67Mg0.33O2, and P2-Na0.67Mn0.78Zn0.22O2.13Song B. Hu E. Liu J. Zhang Y. Yang X. Nanda J. Huq A. Page K. A novel P3-type Na2/3Mg1/3Mn2/3O2 as high capacity sodium-ion cathode using reversible oxygen redox.J. Mater. Chem. A. 2019; 7: 1491-1498Crossref Google Scholar,14Wang Y. Wang L. Zhu H. Chu J. Fang Y. Wu L. Huang L. Ren Y. Sun C. Liu Q. et al.Ultralow-strain Zn-substituted layered oxide cathode with suppressed P2–O2 transition for stable sodium ion storage.Adv. Funct. Mater. 2020; 30: 1910327Crossref Scopus (56) Google Scholar In order to identify the nature of oxidized oxygen formed in Na0.67Mg0.28Mn0.72O2, and in particular to see whether O2 is formed and trapped in the bulk, O K-edge X-ray absorption spectroscopy (XAS) and RIXS measurements were carried out (Figure 2). O K-edge XAS and high-resolution RIXS spectra for pristine and cycled Na0.67Mn0.72Mg0.28O2 are shown in Figures 2A and 2B, respectively. Charging to the beginning of the plateau (i.e., 4.2 V versus Na+/Na) generates electron-hole states in hybridized Mn–O orbitals of primarily Mn character and associated with Mn redox, observed between 528 and 529 eV in XAS, whereas oxide oxidation across the high-voltage plateau creates electron-hole states at 531 eV.10Maitra U. House R.A. Somerville J.W. Tapia-Ruiz N. Lozano J.G. Guerrini N. Hao R. Luo K. Jin L. Pérez-Osorio M.A. et al.Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2.Nat. Chem. 2018; 10: 288-295Crossref PubMed Scopus (284) Google Scholar The RIXS spectra collected at this excitation energy reveal new oxygen valence states (centered at 523 eV) in addition to a strong increase of the elastic peak, in good agreement with that reported previously for this system.10Maitra U. House R.A. Somerville J.W. Tapia-Ruiz N. Lozano J.G. Guerrini N. Hao R. Luo K. Jin L. Pérez-Osorio M.A. et al.Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2.Nat. Chem. 2018; 10: 288-295Crossref PubMed Scopus (284) Google Scholar The higher resolution RIXS that we employed here compared with past studies reveals the underlying fine structure of the “elastic” contribution (Figure 2C), which is composed of a succession of energy loss peaks similar to that observed for charged O3-Li1.2Ni0.13Mn0.54Co0.13O2 and P2-Na0.75Li0.25Mn0.75O2, as well as O2 gas.8House R.A. Maitra U. Pérez-Osorio M.A. Lozano J.G. Jin L. Somerville J.W. Duda L.C. Nag A. Walters A. Zhou K.-J. et al.Superstructure control of first-cycle voltage hysteresis in oxygen-redox cathodes.Nature. 2020; 577: 502-508Crossref PubMed Scopus (225) Google Scholar,9House R.A. Rees G.J. Pérez-Osorio M.A. Marie J. Boivin E. Robertson A.W. Nag A. Garcia-Fernandez M. Zhou K. Bruce P.G. First-cycle voltage hysteresis in Li-rich 3d cathodes associated with molecular O2 trapped in the bulk.Nat. Energy. 2020; 5: 777-785Crossref Scopus (123) Google Scholar,15Århammar C. Pietzsch A. Bock N. Holmstrom E. Araujo C.M. Grasjo J. Zhao S. Green S. Peery T. Hennies F. et al.Unveiling the complex electronic structure of amorphous metal oxides.Proc. Natl. Acad. Sci. USA. 2011; 108: 6355-6360Crossref Scopus (82) Google Scholar They are associated with transitions to different vibrational energy levels of an O–O dimer and the peak spacing indicates the nature of this O–O dimer.16Rubensson J.E. Pietzsch A. Hennies F. Vibrationally resolved resonant inelastic soft X-ray scattering spectra of free molecules.J. Electron Spectrosc. Relat. Phenom. 2012; 185: 294-300Crossref Scopus (13) Google Scholar The extrapolation of the Birge-Sponer plot, shown in Figure 2D at n = 0, gives a fundamental vibrational energy of 0.194(1) eV, which corresponds to 1,565(8) cm−1, consistent with that of molecular O2 (~1,556 cm−1).17Radjenovic P.M. Hardwick L.J. Evaluating chemical bonding in dioxides for the development of metal-oxygen batteries: vibrational spectroscopic trends of dioxygenyls, dioxygen, superoxides and peroxides.Phys. Chem. Chem. Phys. 2019; 21: 1552-1563Crossref PubMed Google Scholar This wavenumber value is very different from the vibrational frequency of any negatively charged oxygen dimer (either O22− at ~750 cm−1, O2− at ~1,100 cm−1 or in between for O2n−).17Radjenovic P.M. Hardwick L.J. Evaluating chemical bonding in dioxides for the development of metal-oxygen batteries: vibrational spectroscopic trends of dioxygenyls, dioxygen, superoxides and peroxides.Phys. Chem. Chem. Phys. 2019; 21: 1552-1563Crossref PubMed Google Scholar This in accord with the absence of a signal in the 700–1,200 cm−1 range in the charged state of Na0.67Mn0.72Mg0.28O2 measured by bulk sensitive Raman spectroscopy.10Maitra U. House R.A. Somerville J.W. Tapia-Ruiz N. Lozano J.G. Guerrini N. Hao R. Luo K. Jin L. Pérez-Osorio M.A. et al.Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2.Nat. Chem. 2018; 10: 288-295Crossref PubMed Scopus (284) Google Scholar Elsewhere, indications of peroxo-like species seen by using surface enhanced Raman (SERS) are most likely to arise from surface-localized phenomena (i.e., CEI formation or surface decomposition products)18Li X. Qiao Y. Guo S. Xu Z. Zhu H. Zhang X. Yuan Y. He P. Ishida M. Zhou H. Direct visualization of the reversible O2−/O− redox process in Li-rich cathode materials.Adv. Mater. 2018; 30: 2-7Google Scholar,19Qiao Y. Guo S. Zhu K. Liu P. Li X. Jiang K. Sun C. Chen M. Zhou H. Reversible anionic redox activity in Na3RuO4 cathodes: a prototype Na-rich layered oxide.Energy Environ. Sci. 2018; 11: 299-305Crossref Google Scholar that might surface bias bulk sensitive Raman.20Cao X. Li H. Qiao Y. Li X. Jia M. Cabana J. Zhou H. Stabilizing reversible oxygen redox chemistry in layered oxides for sodium-ion batteries.Adv. Energy Mater. 2020; 10: 1-7Crossref Scopus (36) Google Scholar The RIXS data in the discharged state (i.e., 2.1 V, the composition of the pristine material) reveal the persistence of a much weaker progression of energy loss peaks associated with the elastic peak, indicating that the O2 is significantly reduced but not entirely on discharge and thus that some Mn4+ starts to be reduced before all the O2 is reduced completely to O2− consistent with previous X-ray absorption near edge spectroscopy (XANES) studies.10Maitra U. House R.A. Somerville J.W. Tapia-Ruiz N. Lozano J.G. Guerrini N. Hao R. Luo K. Jin L. Pérez-Osorio M.A. et al.Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2.Nat. Chem. 2018; 10: 288-295Crossref PubMed Scopus (284) Google Scholar Although RIXS presents evidence for O2 molecules trapped in the bulk of the particles, it is difficult to quantify the amount of O2 and hence to show that it accords with the charge passed. Quantification of the O2 can be achieved by exploiting its paramagnetism with magnetometry measurements, which also possess the advantage of being free from any possibility of beam damage. The formation of molecular oxygen O2, localized holes O−, peroxo O22−, peroxo-like O2n−, or superoxo O2− species would generate unpaired electrons to different extents for the same degree of oxidation, i.e., Na removed (as represented by the dash lines at Figure 3). As an example, the exclusive formation of peroxo species across the high-voltage plateau would not lead to an increase of the Curie constant, O22− being diamagnetic. Also, O− and O2, which nominally have the same number of unpaired electrons per O, can be distinguished given that there would need to be twice as many of the former than the latter to account for the same degree of oxidation. The experimental data shown in Figure 3 and summarized in the Table S2 demonstrate the gradual rise of the Curie constant along with the activation of O-redox until 1.458(9) cm3 K mol −1 at the charged state. This value is in good agreement with charge compensation occurring predominantly via the O2−/O2 redox couple (Cth = 1.453 cm3 K mol−1). The quantity of charge passed based on the amount of O2 estimated from magnetometry is 0.432 ± 0.036 electrons, which is consistent with 0.41 electrons exchanged across the high-voltage plateau and corresponds to approximately 10% of the oxide ions being in the form of O2. Note that Na0.67Mn0.72Mg0.78O2 provides a good system with which to quantify O2 with magnetometry given that spin counting is simple with Na+, Mg2+, and Mn4+ as the only cations. In materials with Li in the TM layers, Li extraction and in-plane TM disordering on charging form voids that accommodate O2, and the TM disorder is associated with the irreversible loss of the honeycomb superstructure.8House R.A. Maitra U. Pérez-Osorio M.A. Lozano J.G. Jin L. Somerville J.W. Duda L.C. Nag A. Walters A. Zhou K.-J. et al.Superstructure control of first-cycle voltage hysteresis in oxygen-redox cathodes.Nature. 2020; 577: 502-508Crossref PubMed Scopus (225) Google Scholar,9House R.A. Rees G.J. Pérez-Osorio M.A. Marie J. Boivin E. Robertson A.W. Nag A. Garcia-Fernandez M. Zhou K. Bruce P.G. First-cycle voltage hysteresis in Li-rich 3d cathodes associated with molecular O2 trapped in the bulk.Nat. Energy. 2020; 5: 777-785Crossref Scopus (123) Google Scholar In order to probe the evolution of the long-range ordering within the TM layer in P2-Na0.67Mg0.28Mn0.72O2, operando PXRD was employed (Figure 4). On charging, the continuous shift in the PXRD peak positions in the initial sloped region of the voltage curve is in accord with this being a single-phase reaction. The reaction associated with the high-voltage plateau corresponds to the two-phase structural transition from a honeycomb-ordered P2 phase to an O2 phase, which is in good agreement with results reported previously.10Maitra U. House R.A. Somerville J.W. Tapia-Ruiz N. Lozano J.G. Guerrini N. Hao R. Luo K. Jin L. Pérez-Osorio M.A. et al.Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2.Nat. Chem. 2018; 10: 288-295Crossref PubMed Scopus (284) Google Scholar,21Yabuuchi N. Hara R. Kubota K. Paulsen J. Kumakura S. Komaba S. A new electrode material for rechargeable sodium batteries: P2-type Na 2/3 [Mg 0.28 Mn 0.72 ]O 2 with anomalously high reversible capacity.J. Mater. Chem. A. 2014; 2: 16851-16855Crossref Google Scholar Not all of the Na is removed at the end of charge, commensurate with the persistence of peaks associated with some P2 phase. It is noteworthy that at the end of subsequent discharge the honeycomb superstructure peak is again present, although reduced in magnitude compared with that observed in the pristine material. In order to form O2 trapped within the particles, structural changes have to occur. To investigate the structural changes on charging, Mg and Mn K-edge extended X-ray absorption fine structure (EXAFS) were employed. Except for a tiny shortening of the Mn–O distance upon charge, the Mn K-edge EXAFS does not show any significant change in the first coordination shell on cycling. In contrast, in the Mg K-edge EXAFS, a shoulder arises at low radial distance (Figure 5A, red arrow) indicating that Mg occupies a new site in the charged state. The short Mg-O bond (ca. 1.8 Å) is in accord with that of Mg in a tetrahedral site (as seen for instance in the spinel MgM2O4 with M = Al, Cr, Mn, or Fe).22Hellenbrandt M. The inorganic crystal structure database (ICSD) - present and future.Crystallogr. Rev. 2004; 10: 17-22Crossref Scopus (324) Google Scholar In the O2 structure of the charged material, two distinct tetrahedral sites are accessible, either in the TM layer or in the AM layer. However, the migration of Mg from its original octahedral site toward the tetrahedral site of the TM layer would imply face sharing with 2 MnO6 octahedra with a Mg-Mn distance of approximately 2.5 Å, which is unlikely and clearly not seen either in the Mg K-edge or the Mn K-edge EXAFS. Therefore, in the charged state, Mg migrates into tetrahedral sites in the AM layer as also observed in the charged P3-Na2/3Mn2/3Mg1/3O213Song B. Hu E. Liu J. Zhang Y. Yang X. Nanda J. Huq A. Page K. A novel P3-type Na2/3Mg1/3Mn2/3O2 as high capacity sodium-ion cathode using reversible oxygen redox.J. Mater. Chem. A. 2019; 7: 1491-1498Crossref Google Scholar and for Zn2+ in charged P2-Na0.67Mn0.78Zn0.22O2.11Bai X. Sathiya M. Mendoza-Sánchez B. Iadecola A. Vergnet J. Dedryvère R. Saubanère M. Abakumov A.M. Rozier P. Tarascon J. Anionic redox activity in a newly Zn-doped sodium layered oxide P2-Na2/3 Mn1−yZnyO2 (0 < y < 0.23).Adv. Energy Mater. 2018; 8: 1802379Crossref Scopus (102) Google Scholar Octahedral vacancies now surround Mn in the TM layer enabling Mn in-plane disordering. Mg is also likely to disorder within the AM layer to avoid face sharing along the c-axis with the Mn ions that migrate in-plane into the sites vacated by Mg2+. The persistence of a peak in the Mg EXAFS corresponding to a Mg–O distance of 2.05 Å, indicates that some of the Mg also occupies octahedral sites in the AM layers, as discussed in the following section. At the end of the discharge, when the composition has returned to the original value, the EXAFS spectra of the Mn and Mg K-edges are very similar to those observed for the pristine material, indicating that Mg returns to octahedral sites in the TM layer. To explore the Mn/□ (□ = cation vacancy) disorder formed on charging, density functional theory (DFT) and n-PDF were employed and are summarized in Figures 6, 7, and S2. The fully charged structure composed of [□2/3Mg1/3]O2 (AM) and [Mn2/3□1/3]O2 (TM) layers stacked in a O2 sequence was employed as the DFT model to explore the Mn/□ and Mg/□ distribution within the TM and AM layers, respectively. The ideal honeycomb structure of Mg1/3Mn2/3O2 was used for DFT to ensure a tractable cell size for the calculations, as employed previously.10Maitra U. House R.A. Somerville J.W. Tapia-Ruiz N. Lozano J.G. Guerrini N. Hao R. Luo K. Jin L. Pérez-Osorio M.A. et al.Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2.Nat. Chem. 2018; 10: 288-295Crossref PubMed Scopus (284) Google Scholar Although most of the Mn/□ distributions explored gave rise to energies comparable with kBT, one Mn/□ configuration leads to a large stabilization (−410 meV/f.u.) (Figure S3). This structure involves clusters of four vacancies, as shown in Figure 6E, and the formation of O–O dimers with a bond length of 1.23 Å, as observed for molecular oxygen (1.21 Å for O2 versus 1.28 Å for O2−, 1.48 Å for O22−, or in the 1.4 to 2.5 Å range for O2n−).17Radjenovic P.M. Hardwick L.J. Evaluating chemical bonding in dioxides for the development of metal-oxygen batteries: vibrational spectroscopic trends of dioxygenyls, dioxygen, superoxides and peroxides.Phys. Chem. Chem. Phys. 2019; 21: 1552-1563Crossref PubMed Google Scholar This four-vacancy cluster with two O2 molecules trapped inside is similar to the moiety recently identified in charged Li1.2Ni0.13Mn0.54Co0.13O2.9House R.A. Rees G.J. Pérez-Osorio M.A. Marie J. Boivin E. Robertson A.W. Nag A. Garcia-Fernandez M. Zhou K. Bruce P.G. First-cycle voltage hysteresis in Li-rich 3d cathodes associated with molecular O2 trapped in the bulk.Nat. Energy. 2020; 5: 777-785Crossref Scopus (123) Google Scholar Moreover, within the AM layer of the O2 structure, the DFT studies indicate that Mg is distributed between tetrahedral sites (sharing a face with a vacant octahedral site in the TM layer) and the octahedral sites of the O2 structure (sharing three edges with Mn/□ octahedra of the TM layer above and a face with a vacancy of the TM layer below). The fitting of the Mg K-edge EXAFS shows that approximately 50% of the Mg in the O2 phase is found in the tetrahedral sites and 50% in the octahedral sites (Figure S2; Table S3), in line with the lowest energy configuration found by DFT.Figure 7Local structure evolution upon cyclingShow full caption(A–C) Fits of the n-PDF data for (A) pristine, (B) charged, and (C) discharged materials. Inset, an enlargement of the short-range region (1–4 Å) highlighted in gray. The relaxed DFT structures (illustrated in inset, Mg is in orange, Mn in purple, and O in red) have been used as starting models for the fits of n-PDF data. Honeycomb ordering and cluster disordering are systematically compared at each state of charge. Based on the agreement factors, it is shown that the honeycomb ordering is lost on charge with vacancy clusters showing a better fit and reformed at the local scale on discharge.View Large Image Figure ViewerDownload Hi-res image Download (PPT) (A–C) Fits of the n-PDF data for (A) pristine, (B) charged, and (C) discharged materials. Inset, an enlargement of the short-range region (1–4 Å) highlighted in gray. The relaxed DFT structures (illustrated in inset, Mg is in orange, Mn in purple, and O in red) have been used as starting models for the fits of n-PDF data. Honeycomb ordering and cluster disordering are systematically compared at each state of charge. Based on the agreement factors, it is shown that the honeycomb ordering is lost on charge with vacancy clusters showing a better fit and reformed at the local scale on discharge. On discharge, when Mg2+ returns to the TM layers, two options for Mn/Mg distribution are possible: either Mg2+ occupies the octahedral sites in the TM layer within the vacancy cluster (similar to that observed for Li+ returning to the TM layer in Na0.75Mn0.75Li0.25O2),8House R.A. Maitra U. Pérez-Osorio M.A. Lozano J.G. Jin L. Somerville J.W. Duda L.C. Nag A. Walters A. Zhou K.-J. et al.Superstructure control of first-cycle voltage hysteresis in oxygen-redox cathodes.Nature. 2020; 577: 502-508Crossref PubMed Scopus (225) Google Scholar Figure 6F or the Mn/Mg honeycomb ordering is recovered, implying the reversibility of the in-plane Mn migration (Figure 6B). Both possibilities have been explored with DFT. As expected, when Mg and Mn occupy the TM layer, the honeycomb ordering is the most stable configuration in the discharge state (i.e., the composition of the pristine material). In comparison, the Mg in the cluster configuration is metastable by 180 meV/f.u. (Figure S3). To probe the formation of the vacancy-cluster voids experimentally and thereby corroborate the DFT model, n-PDF was employed. The models obtained from DFT were used as a starting point for the analysis (see experimental procedures and Figure 7). For the pristine material, the fit of the n-PDF data using the DFT model gives similar agreement factors to those using the long-range structure determined by diffraction (Rwp = 0.24 versus Rwp = 0.22). This indicates that, despite a composition slightly different to the one determined by NPD (i.e., Na0.67Mn0.67Mg0.33O2 vs. Na0.67Mn0.72Mg0.28O2), the relaxed DFT models can be used to fit the n-PDF patterns. Given that at the end of charge the conversion from P2-[Na0.56][Mn0.72Mg0.28]O2 to O2-[□0.67Mg0.28][Mn0.72□0.28]O2 is incomplete, a two-phase refinement was employed, fixing the P2/O2 ratio to that obtained experimentally based on the amount of the Na+ electrochemically extracted (see experimental procedures). For the O2-[□0.67Mg0.33][Mn0.67□0.33]O2 phase, two models were explored: Mn/□ clusters or a Mn/□ honeycomb ordering. The ordering of the Mn ions into four-vacancy-clusters provides a significantly better agreement factor than Mn remaining in the sites occupied in the pristine material (Rwp = 0.29 versus Rwp = 0.37, see Figure 7B). The n-PDF data confirm that the honeycomb ordering is lost even at short range on charging and, hence, that the decrease of the superstructure peak intensity seen on charge by operando PXRD (see Figure 4) is due to the loss of honeycomb ordering rather than to the disruption of the coherence arising from stacking faults. On discharge, the honeycomb ordering of Mg/Mn provides a better agreement with the n-PDF data than Mg/Mn clusters (Rwp = 0.25 for honeycomb versus Rwp = 0.34 for the four-Mg-cluster) (Figure 7C). Therefore, the n-PDF data show that the honeycomb ordering present in the pristine material is lost on charge and then reformed at the local scale on the first discharge in accord with the formation and annihilation of the four-vacancy-clusters on charge and discharge. Previous studies of P2-Na0.67Mn0.72Mg0.28O2 have employed only standard RIXS spectroscopy.10Maitra U. House R.A. Somerville J.W. Tapia-Ruiz N. Lozano J.G. Guerrini N. Hao R. Luo K. Jin L. Pérez-Osorio M.A. et al.Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2.Nat. Chem. 2018; 10: 288-295Crossref PubMed Scopus (284) Google Scholar,23Dai K. Wu J. Zhuo Z. Li Q. Sallis S. Mao J. Ai G. Sun C. Li Z. Gent W.E. et al.High reversibility of lattice oxygen redox quantified by direct bulk probes of both anionic and cationic redox reactions.Joule. 2019; 3: 518-541Abstract Full Text Full Text PDF Scopus (149) Google Scholar Although such spectroscopy can identify that oxidation is taking place on the O2−, it cannot identify the species formed, i.e., molecular O2, something that is possible with the high-resolution RIXS used here. Furthermore, P2-Na0.67Mn0.72Mg0.28O2 permits investigation of O-redox by magnetometry as there is only one other redox-active species. Magnetometry has enabled the quantification of O2 trapped in the particles and shows that this is in close agreement with the charge passed, providing valuable confirmation of the assignment of O-redox to O2 formation. The O-redox behaviour of P2-Na0.67Mn0.72Mg0.28O2 not only exhibits similarities but also important differences when compared with compounds possessing Li in the TM layer, such as O3-Li1.2Ni0.13Mn0.54Co0.13O2 and P2-Na0.75Li0.25Mn0.75O2. In terms of similarities, alkali extraction is charge compensated for by oxidation of O2− to form O2 trapped in cation vacancy-cluster voids in the TM layer generated by in-plane TM disordering, the O2 being reduced back to O2− on discharge. The similarity in bond length between trapped and gaseous O2 indicates the O2 molecules are physically confined with relatively little chemical interaction with their surroundings. Previous 17O MAS NMR measurements have suggested that the trapped O2 exists in a solid-like environment with a lack of rotational motion. However, in contrast to O3-Li1.2Ni0.13Mn0.54Co0.13O2 and P2-Na0.75Li0.25Mn0.75O2, in which Li is removed from the TM layers on charge (either occupying the octahedral sites in the AM layer in the case of P2-Na0.75Li0.25Mn0.75O2 or extracted from O3-Li1.2Ni0.13Mn0.54Co0.13O2),8House R.A. Maitra U. Pérez-Osorio M.A. Lozano J.G. Jin L. Somerville J.W. Duda L.C. Nag A. Walters A. Zhou K.-J. et al.Superstructure control of first-cycle voltage hysteresis in oxygen-redox cathodes.Nature. 2020; 577: 502-508Crossref PubMed Scopus (225) Google Scholar,9House R.A. Rees G.J. Pérez-Osorio M.A. Marie J. Boivin E. Robertson A.W. Nag A. Garcia-Fernandez M. Zhou K. Bruce P.G. First-cycle voltage hysteresis in Li-rich 3d cathodes associated with molecular O2 trapped in the bulk.Nat. Energy. 2020; 5: 777-785Crossref Scopus (123) Google Scholar for P2-Na0.67Mn0.72Mg0.28O2 the Mg2+ ions are displaced from the TM layers into tetrahedral and octahedral sites of the AM layers then return to octahedral sites in TM layers on discharge. In a previous paper on P2-Na0.67Mn0.72Mg0.28O2, we did not detect Mg displacement. Here we have used EXAFS and PDF data that provide a bulk average understanding of local structure, whereas the previous study used STEM, which might not have shown obvious Mg displacement due to a lack of alignment in the Mg columns.10Maitra U. House R.A. Somerville J.W. Tapia-Ruiz N. Lozano J.G. Guerrini N. Hao R. Luo K. Jin L. Pérez-Osorio M.A. et al.Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2.Nat. Chem. 2018; 10: 288-295Crossref PubMed Scopus (284) Google Scholar The low and sloping voltage curve on the discharge following the first charge is similar to that seen for O3-Li1.2Ni0.13Mn0.54Co0.13O2 and P2-Na0.75Li0.25Mn0.75O2, consistent with Mg2+ repopulating the octahedral sites in the vacancy clusters as discharge proceeds. However, in contrast to O3-Li1.2Ni0.13Mn0.54Co0.13O2 and P2-Na0.75Li0.25Mn0.75O2 cases, the n-PDF data at the end of discharge for P2-Na0.67Mn0.72Mg0.28O2 and the presence of a plateau on the subsequent charge are all consistent with the reformation of honeycomb ordering. It is important to note that only ~70% of the first charge plateau is observed on the second charge, whereas the rest is accompanied by a sloping curve, indicating that the honeycomb ordering is not completely reformed on the first discharge. The second charge of P2-Na0.67Mn0.72Mg0.28O2 can be viewed as a hybrid of the voltage curves seen for P2-Na0.75Li0.25Mn0.75O2 on the first charge (plateau) and second charge (sloping) in which the plateau is associated with loss of honeycomb ordering and the sloped region to the depopulation of the vacancy clusters by Li+. We, therefore, suggest that for P2-Na0.67Mn0.72Mg0.28O2 during the first discharge, as Na re-enters the structure, Mg is displaced back into the TM layers partly into vacancy-cluster sites but also into honeycomb sites which are reformed by reversing the Mn migration. The second charge involves depopulation of Mg from the clusters and O2 formation, giving rise to the sloping region of the charging curve—which also accounts for a minor contribution of the Mn3+/Mn4+ redox as shown previously by quantitative XAS23Dai K. Wu J. Zhuo Z. Li Q. Sallis S. Mao J. Ai G. Sun C. Li Z. Gent W.E. et al.High reversibility of lattice oxygen redox quantified by direct bulk probes of both anionic and cationic redox reactions.Joule. 2019; 3: 518-541Abstract Full Text Full Text PDF Scopus (149) Google Scholar,24Wu J. Zhuo Z. Rong X. Dai K. Lebens-Higgins Z. Sallis S. Pan F. Piper L.F.J. Liu G. Chuang Y.-D. et al.Dissociate lattice oxygen redox reactions from capacity and voltage drops of battery electrodes.Sci. Adv. 2020; 6: eaaw3871Crossref PubMed Scopus (59) Google Scholar - and loss of honeycomb, resulting in the second charge plateau. This second charge plateau is also observed in Na0.67Mn0.78Zn0.22O2, where tetrahedral Zn2+ has also been identified at a charged state. The reduced mobility of Mg2+ and Zn2+ ions that remain predominantly in the sites adjacent to their original octahedral site in the TM layer in the charged structure might offer an explanation as to why the honeycomb ordering is partially reformed on discharge. On the other hand, Li is much more mobile and can diffuse quickly to find an available vacancy clusters elsewhere in the TM layer. As shown recently, the O K-edge RIXS signature remains mostly unchanged at the hundredth charge compared with the first charge,23Dai K. Wu J. Zhuo Z. Li Q. Sallis S. Mao J. Ai G. Sun C. Li Z. Gent W.E. et al.High reversibility of lattice oxygen redox quantified by direct bulk probes of both anionic and cationic redox reactions.Joule. 2019; 3: 518-541Abstract Full Text Full Text PDF Scopus (149) Google Scholar and hence, although the structural transition from vacancy cluster to honeycomb ordering is partly irreversible, the formation of molecular O2 is almost fully reversible. Eventually, the honeycomb ordering of Mg is completely lost leading to the loss of the reordering phenomenon and the charging plateau over cycling. Further studies are required to understand the behaviour of O2 trapped in voids over extensive cycling and especially its influence on the complex issues of voltage fade and capacity decay. High-resolution RIXS spectroscopy demonstrates that O-redox in P2-Na0.67Mn0.72Mg0.28O2 involves formation of molecular O2 on charge, which is reduced back to O2− upon subsequent discharge. The quantity of O2 trapped in the structure on charging has been determined by magnetometry and is in good agreement with the quantity of charge passed associated with the O-redox plateau, reinforcing the assignment of O-redox to O2 formation. Mg2+ is displaced into tetrahedral and octahedral sites in the AM layers, with equal amounts of Mg2+ in each, and enables the Mn in-plane disordering in the TM layers generating vacancy-cluster voids that accommodate O2 molecules. On discharge, partial reformation of the honeycomb ordering occurs as seen in n-PDF and consistent with a partial plateau on the second charge. The occupancy of tetrahedral sites on charge and the partial reformation of honeycomb ordering on discharge make the divalent cation O-redox distinct from their monovalent counterparts.