Cycling mechanism of Li2MnO3: Li–CO2 batteries and commonality on oxygen redox in cathode materials

Abstract

•Charge plateau of Li2MnO3 is from oxygen release and surface carbonate reactions•Mn(III/IV) redox is solely responsible for the reversible bulk Li2MnO3 cycling•Oxygen redox shares common nature in both Li-rich and conventional cathodes•Li2MnO3 and alkali-rich materials could be superior catalysts for Li–CO2/air batteries In the debates on how to achieve high energy density battery cathodes, Li-rich compounds are often considered superior over conventional materials due to their high capacity associated with the oxygen redox reactions. Here, we clarify both the bulk and surface reaction mechanisms of Li2MnO3 during the initial and later cycles. Our results reveal that the initial charge plateau is from two types of surface activities, followed by predominating Mn redox reactions with no sign of reversible lattice oxygen redox. The surface chemistry of Li2MnO3 indicates a highly reactive surface to facilitate carbonate formation and decomposition, inspiring a Li-CO2/air battery with Li2MnO3 as a superior electrocatalyst. The comparison between Li-rich, conventional, and Li2MnO3 suggests that the oxygen redox in Li-rich and conventional materials is of the same nature and origin. Li2MnO3 has been considered to be a representative Li-rich compound with active debates on oxygen activities. Here, by evaluating the Mn and O states in the bulk and on the surface of Li2MnO3, we clarify that Mn(III/IV) redox dominates the reversible bulk redox in Li2MnO3, while the initial charge plateau is from surface reactions with oxygen release and carbonate decomposition. No lattice oxygen redox is involved at any electrochemical stage. The carbonate formation and decomposition indicate the catalytic property of the Li2MnO3 surface, which inspires Li-CO2/air batteries with Li2MnO3 acting as a superior electrocatalyst. The absence of lattice oxygen redox in Li2MnO3 questions the origin of the oxygen redox in Li-rich compounds, which is found to be of the same nature as that in conventional materials based on spectroscopic comparisons. These findings provide guidelines on understanding and controlling oxygen activities toward high-energy cathodes and suggest opportunities on using alkali-rich materials for catalytic reactions. Li2MnO3 has been considered to be a representative Li-rich compound with active debates on oxygen activities. Here, by evaluating the Mn and O states in the bulk and on the surface of Li2MnO3, we clarify that Mn(III/IV) redox dominates the reversible bulk redox in Li2MnO3, while the initial charge plateau is from surface reactions with oxygen release and carbonate decomposition. No lattice oxygen redox is involved at any electrochemical stage. The carbonate formation and decomposition indicate the catalytic property of the Li2MnO3 surface, which inspires Li-CO2/air batteries with Li2MnO3 acting as a superior electrocatalyst. The absence of lattice oxygen redox in Li2MnO3 questions the origin of the oxygen redox in Li-rich compounds, which is found to be of the same nature as that in conventional materials based on spectroscopic comparisons. These findings provide guidelines on understanding and controlling oxygen activities toward high-energy cathodes and suggest opportunities on using alkali-rich materials for catalytic reactions. The pressing demand for high-energy batteries has triggered tremendous efforts in the development of transition-metal-oxide (TMO)-based Li-ion battery (LIB) cathode materials operated at high voltages.1Li W. Song B. Manthiram A. High-voltage positive electrode materials for lithium-ion batteries.Chem. Soc. Rev. 2017; 46: 3006-3059Crossref PubMed Google Scholar, 2Li M. Liu T. Bi X. Chen Z. Amine K. Zhong C. Lu J. Cationic and anionic redox in lithium-ion based batteries.Chem. Soc. Rev. 2020; 49: 1688-1705Crossref PubMed Google Scholar, 3Goodenough J.B. Kim Y. Challenges for rechargeable Li batteries †.Chem. Mater. 2010; 22: 587-603Crossref Scopus (7534) Google Scholar, 4Ellis B.L. Lee K.T. Nazar L.F. Positive electrode materials for Li-Ion and Li-batteries †.Chem. Mater. 2010; 22: 691-714Crossref Scopus (1455) Google Scholar The high-voltage operation triggers complex chemical reactions, with its concepts and mechanism under active debate. One of the key practical concerns of the TMO electrode into the high-capacity high-voltage region is the spontaneous involvement of oxygen activities. If the oxygen activities are reversible oxygen redox reactions, they could potentially contribute to charge compensation for the high capacity.5Grimaud A. Hong W.T. Shao-Horn Y. Tarascon J.M. Anionic redox processes for electrochemical devices.Nat. Mater. 2016; 15: 121-126Crossref PubMed Scopus (373) Google Scholar However, if the reactions are irreversible oxygen oxidation and electrolyte decomposition, they lead to detrimental effects with irreversible oxygen release and parasitic surface reactions.6Wu 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 At present, both the fundamental understanding and the practical control of the oxygen reactions in TMO remain formidable challenges.7Assat G. Tarascon J.-M. Fundamental understanding and practical challenges of anionic redox activity in Li-ion batteries.Nat. Energy. 2018; 3: 373-386Crossref Scopus (611) Google Scholar In LIB research, majority of the studies on oxygen redox activities have been focused on layered Li-rich material,2Li M. Liu T. Bi X. Chen Z. Amine K. Zhong C. Lu J. Cationic and anionic redox in lithium-ion based batteries.Chem. Soc. Rev. 2020; 49: 1688-1705Crossref PubMed Google Scholar,7Assat G. Tarascon J.-M. Fundamental understanding and practical challenges of anionic redox activity in Li-ion batteries.Nat. Energy. 2018; 3: 373-386Crossref Scopus (611) Google Scholar, 8Seo 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 (717) Google Scholar, 9Luo K. Roberts M.R. Hao R. Guerrini N. Pickup D.M. Liu Y.-S. Edström K. Guo J. Chadwick A.V. Duda L.C. et al.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 (663) Google Scholar, 10McCalla 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 (504) Google Scholar a compound with excessive Li occupying part of the TM sites in the TM–O layer, which is typically described as xLi2MnO3·(1−x)LiMO2 (M = Ni, Co, Mn, etc.), indicating the “Li-rich” parent compound of Li2MnO3 and the conventional part of LiMO2.11Thackeray M.M. Kang S.-H. Johnson C.S. Vaughey J.T. Benedek R. Hackney S.A. Li2MnO3-stabilized LiMO2 (M = Mn, Ni, Co) electrodes for lithium-ion batteries.J. Mater. Chem. 2007; 17: 3112-3125Crossref Scopus (1682) Google Scholar At present, it has been realized that the superior capacity of most Li-rich layered electrodes is enabled by the reversible oxygen redox reactions, confirmed by many experimental and theoretical studies.8Seo 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 (717) Google Scholar,9Luo K. Roberts M.R. Hao R. Guerrini N. Pickup D.M. Liu Y.-S. Edström K. Guo J. Chadwick A.V. Duda L.C. et al.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 (663) Google Scholar,12Gent W.E. Lim K. Liang Y. Li Q. Barnes T. Ahn S.-J. Stone K.H. McIntire M. Hong J. Song J.H. et al.Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides.Nat. Commun. 2017; 8: 2091Crossref PubMed Scopus (327) Google Scholar Quantitative values of 44% retention rate of the oxygen redox reaction after 500 cycles are identified in a typical Li-rich material, Li1.17Ni0.21Co0.08Mn0.54O2.13Dai 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. 2018; 3: 518-541Abstract Full Text Full Text PDF Scopus (149) Google Scholar This is impressive, especially considering oxygen activities are often associated with fast decays. As another Li-rich system, the rock-salt disordered Li-rich materials could display a majority of cationic redox through fluorination14Lee J. Kitchaev D.A. Kwon D.H. Lee C.W. Papp J.K. Liu Y.S. Lun Z.Y. Clément R.J. Shi T. McCloskey B.D. et al.Reversible Mn2+/Mn4+ double redox in lithium-excess cathode materials.Nature. 2018; 556: 185-190Crossref PubMed Scopus (379) Google Scholar; however, oxygen redox activities are triggered in most of these materials too at high voltages, and fluorination is hard to achieve in layered compounds.15Clément R.J. Lun Z. Ceder G. Cation-disordered rocksalt transition metal oxides and oxyfluorides for high energy lithium-ion cathodes.Energy Environ. Sci. 2020; 13: 345-373Crossref Google Scholar We, therefore, note that the discussions on the Li-rich material in this work refer to the Li-rich layered compounds without fluorination. Although oxygen redox has also been a topic of some non-Li-rich conventional LIB electrodes,16Lebens-Higgins Z.W. Faenza N.V. Radin M.D. Liu H. Sallis S. Rana J. Vinckeviciute J. Reeves P.J. Zuba M.J. Badway F. et al.Revisiting the charge compensation mechanisms in LiNi 0.8 Co 0.2−y Al y O 2 systems.Mater. Horiz. 2019; 6: 2112-2123Crossref Google Scholar, 17Li N. Sallis S. Papp J.K. Wei J. McCloskey B.D. Yang W. Tong W. Unraveling the cationic and anionic redox reactions in a conventional layered oxide cathode.ACS Energy Lett. 2019; 4: 2836-2842Crossref Scopus (61) Google Scholar, 18Zhang J.-N. Li Q. Ouyang C. Yu X. Ge M. Huang X. Hu E. Ma C. Li S. Xiao R. et al.Trace doping of multiple elements enables stable battery cycling of LiCoO2 at 4.6 V.Nat. Energy. 2019; 4: 594-603Crossref Scopus (312) Google Scholar, 19Lee G.-H. Wu J. Kim D. Cho K. Cho M. Yang W. Kang Y.-M. Reversible anionic redox activities in conventional LiNi1/3Co1/3Mn1/3O2 cathodes.Angew. Chem. Int. Ed. Engl. 2020; 132: 8759-8766Crossref Google Scholar as well as some Na-ion compounds,13Dai 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. 2018; 3: 518-541Abstract Full Text Full Text PDF Scopus (149) Google Scholar,20Dai K. Mao J. Zhuo Z. Feng Y. Mao W. Ai G. Pan F. Chuang Y.-d. Liu G. Yang W. Negligible voltage hysteresis with strong anionic redox in conventional battery electrode.Nano Energy. 2020; 74: 104831Crossref Scopus (40) Google Scholar, 21House 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 (223) Google Scholar, 22Maitra 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 (283) Google Scholar the oxygen redox almost always exhibits fast decay once triggered in conventional materials at high voltages, e.g., 63% of reversible oxygen redox is retained after only 10 cycles in the LiNi1/3Co1/3Mn1/3O2 (NCM111) electrode.19Lee G.-H. Wu J. Kim D. Cho K. Cho M. Yang W. Kang Y.-M. Reversible anionic redox activities in conventional LiNi1/3Co1/3Mn1/3O2 cathodes.Angew. Chem. Int. Ed. Engl. 2020; 132: 8759-8766Crossref Google Scholar Therefore, in LIB cathode research, Li-rich has been the role model of reversible oxygen redox and has dominated recent oxygen redox studies on both fundamental understandings and practical developments.2Li M. Liu T. Bi X. Chen Z. Amine K. Zhong C. Lu J. Cationic and anionic redox in lithium-ion based batteries.Chem. Soc. Rev. 2020; 49: 1688-1705Crossref PubMed Google Scholar,7Assat G. Tarascon J.-M. Fundamental understanding and practical challenges of anionic redox activity in Li-ion batteries.Nat. Energy. 2018; 3: 373-386Crossref Scopus (611) Google Scholar, 8Seo 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 (717) Google Scholar, 9Luo K. Roberts M.R. Hao R. Guerrini N. Pickup D.M. Liu Y.-S. Edström K. Guo J. Chadwick A.V. Duda L.C. et al.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 (663) Google Scholar, 10McCalla 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 (504) Google Scholar,12Gent W.E. Lim K. Liang Y. Li Q. Barnes T. Ahn S.-J. Stone K.H. McIntire M. Hong J. Song J.H. et al.Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides.Nat. Commun. 2017; 8: 2091Crossref PubMed Scopus (327) Google Scholar,23Chen H. Islam M.S. Lithium extraction mechanism in Li-Rich Li2MnO3 involving oxygen hole formation and dimerization.Chem. Mater. 2016; 28: 6656-6663Crossref Scopus (164) Google Scholar, 24Xie Y. Saubanère M. Doublet M.-L. Requirements for reversible extra-capacity in Li-rich layered oxides for Li-ion batteries.Energy Environ. Sci. 2017; 10: 266-274Crossref Google Scholar, 25Saubanère M. McCalla E. Tarascon J.-M. Doublet M.-L. The intriguing question of anionic redox in high-energy density cathodes for Li-ion batteries.Energy Environ. Sci. 2016; 9: 984-991Crossref Google Scholar, 26Oishi M. Yamanaka K. Watanabe I. Shimoda K. Matsunaga T. Arai H. Ukyo Y. Uchimoto Y. Ogumi Z. Ohta T. Direct observation of reversible oxygen anion redox reaction in Li-rich manganese oxide, Li2MnO3, studied by soft X-ray absorption spectroscopy.J. Mater. Chem. A. 2016; 4: 9293-9302Crossref Google Scholar, 27Han S.J. Xia Y.G. Wei Z. Qiu B. Pan L.C. Gu Q.W. Liu Z.P. Guo Z.Y. A comparative study on the oxidation state of lattice oxygen among Li1.14Ni0.136Co0.136Mn0.544O2, Li2MnO3, LiNi0.5Co0.2Mn0.3O2 and LiCoO2 for the initial charge–discharge.J. Mater. Chem. A. 2015; 3: 11930-11939Crossref Google Scholar, 28Massel F. Hikima K. Rensmo H. Suzuki K. Hirayama M. Xu C. Younesi R. Liu Y.-S. Guo J. Kanno R. et al.Excess lithium in transition metal layers of epitaxially grown thin film cathodes of Li2MnO3 leads to rapid loss of covalency during first battery cycle.J. Phys. Chem. C. 2019; 123: 28519-28526Crossref Scopus (12) Google Scholar, 29Xu J. Sun M. Qiao R. Renfrew S.E. Ma L. Wu T. Hwang S. Nordlund D. Su D. Amine K. et al.Elucidating anionic oxygen activity in lithium-rich layered oxides.Nat. Commun. 2018; 9: 947Crossref PubMed Scopus (190) Google Scholar, 30Ji H. Wu J. Cai Z. Liu J. Kwon D.-H. Kim H. Urban A. Papp J.K. Foley E. Tian Y. et al.Ultrahigh power and energy density in partially ordered lithium-ion cathode materials.Nat. Energy. 2020; 5: 213-221Crossref Scopus (91) Google Scholar To clarify the discussions on the lattice oxygen reactions in this work, we should first try to define the scope of the oxygen redox concept. Unfortunately, the definition of the oxygen redox reaction has yet to be clearly established, mainly due to the elusive mechanism of the oxidized oxygen state that remains a challenging topic and is still under active debates. However, we could at least remove part of the common confusions by labeling what should not be considered as the oxidized oxygen states in the scope of oxygen redox concept: (1) the loss of electron density around an O atom through covalency or hybridization should not be considered as the oxidized oxygen in the oxygen redox concept, e.g., TM–O hybridized states or highly covalent states in molecules like CO2.31Zhuo Z. Liu Y.S. Guo J. Chuang Y.D. Pan F. Yang W. Full energy Range resonant inelastic X-ray scattering of O2 and CO2: direct comparison with oxygen redox state in batteries.J. Phys. Chem. Lett. 2020; 11: 2618-2623Crossref PubMed Scopus (23) Google Scholar As a matter of fact, all TM-based battery cathodes display TM–O hybridization features in oxygen absorption spectroscopy, e.g., LiFePO4, and the hybridization gets enhanced upon electrochemical charging.32Roychoudhury S. Qiao R. Zhuo Z. Li Q. Lyu Y. Kim J.-H. Liu J. Lee E. Polzin B.J. Guo J. et al.Deciphering the oxygen absorption pre-edge: a caveat on its application for probing oxygen redox reactions in batteries.Energy Environ. Mater. 2020; https://doi.org/10.1002/eem2.12119Crossref Scopus (24) Google Scholar Therefore, depopulation of electrons around O through TM–O hybridization itself is not an oxidized oxygen state in oxygen redox concept, otherwise, all the TM-based cathode compounds, including LiFePO4, would be called oxygen redox systems. (2) A justified oxygen redox reaction should have at least one reversible cycle of oxidation (“ox”) and reduction (“red”) reactions. The obviously irreversible oxygen oxidation, e.g., released oxygen, should not be defined as oxygen “redox” reactions. For the same reason, the oxygen release signal itself does not signify lattice oxygen redox reactions. This is particularly important for understanding this work here. As we will see later, although Li2MnO3 is well known to display a significant amount of gas released during charging, it does not involve any oxygen “redox” reaction in the lattice even in the very first cycle. However, it is important to emphasize that the regulations here do not mean the TM–O hybridization and oxygen release are unrelated to the lattice oxygen redox reactions; instead, they both show strong correlations with the oxygen redox behaviors on various properties, especially reversibility and stability, but with an unclear mechanism by this time.33Yang W. Oxygen release and oxygen redox.Nat. Energy. 2018; 3: 619-620Crossref Scopus (31) Google Scholar Therefore, although the mechanism of the oxidized oxygen is still under debate, we could loosely define the lattice oxygen redox reaction in TMO-based battery cathodes: the oxygen oxidization in the oxygen redox concept should be a depopulation of oxygen electrons not through the TM–O hybridization itself but through a literal electron loss process such as a chemical peroxo-bond formation or a physical electron charge transfer from oxygen through a reshuffling of the electron states. Additionally, the reduction of the oxidized oxygen should take place for at least one cycle, otherwise, it is merely an irreversible oxygen oxidation process, which was found to take place mostly in the near-surface region.34House R.A. Maitra U. Jin L. Lozano J.G. Somerville J.W. Rees N.H. Naylor A.J. Duda L.C. Massel F. Chadwick A.V. et al.What triggers oxygen loss in oxygen redox cathode materials?.Chem. Mater. 2019; 31: 3293-3300Crossref Scopus (83) Google Scholar Electrochemically, Li2MnO3 has long been identified to be responsible for the characteristic high-voltage plateau of Li-rich electrodes.35Lu Z. Dahn J.R. Understanding the anomalous capacity of Li/Li[ NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 cells using in situ x-ray diffraction and electrochemical studies.J. Electrochem. Soc. 2002; 149: A815-A822Crossref Scopus (940) Google Scholar This plateau is now considered the region of oxygen oxidation reactions during the initial charge of Li-rich electrodes.2Li M. Liu T. Bi X. Chen Z. Amine K. Zhong C. Lu J. Cationic and anionic redox in lithium-ion based batteries.Chem. Soc. Rev. 2020; 49: 1688-1705Crossref PubMed Google Scholar,9Luo K. Roberts M.R. Hao R. Guerrini N. Pickup D.M. Liu Y.-S. Edström K. Guo J. Chadwick A.V. Duda L.C. et al.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 (663) Google Scholar,12Gent W.E. Lim K. Liang Y. Li Q. Barnes T. Ahn S.-J. Stone K.H. McIntire M. Hong J. Song J.H. et al.Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides.Nat. Commun. 2017; 8: 2091Crossref PubMed Scopus (327) Google Scholar,13Dai 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. 2018; 3: 518-541Abstract Full Text Full Text PDF Scopus (149) Google Scholar As a result, Li2MnO3 is often considered the key model system of a Li-rich configuration, which is defined in this work as a system with a considerable amount of lithium ions occupying the transition metal layer. Such a configuration has been widely believed to be responsible for the reversible oxygen redox reactions in Li-rich electrodes.23Chen H. Islam M.S. Lithium extraction mechanism in Li-Rich Li2MnO3 involving oxygen hole formation and dimerization.Chem. Mater. 2016; 28: 6656-6663Crossref Scopus (164) Google Scholar, 24Xie Y. Saubanère M. Doublet M.-L. Requirements for reversible extra-capacity in Li-rich layered oxides for Li-ion batteries.Energy Environ. Sci. 2017; 10: 266-274Crossref Google Scholar, 25Saubanère M. McCalla E. Tarascon J.-M. Doublet M.-L. The intriguing question of anionic redox in high-energy density cathodes for Li-ion batteries.Energy Environ. Sci. 2016; 9: 984-991Crossref Google Scholar, 26Oishi M. Yamanaka K. Watanabe I. Shimoda K. Matsunaga T. Arai H. Ukyo Y. Uchimoto Y. Ogumi Z. Ohta T. Direct observation of reversible oxygen anion redox reaction in Li-rich manganese oxide, Li2MnO3, studied by soft X-ray absorption spectroscopy.J. Mater. Chem. A. 2016; 4: 9293-9302Crossref Google Scholar, 27Han S.J. Xia Y.G. Wei Z. Qiu B. Pan L.C. Gu Q.W. Liu Z.P. Guo Z.Y. A comparative study on the oxidation state of lattice oxygen among Li1.14Ni0.136Co0.136Mn0.544O2, Li2MnO3, LiNi0.5Co0.2Mn0.3O2 and LiCoO2 for the initial charge–discharge.J. Mater. Chem. A. 2015; 3: 11930-11939Crossref Google Scholar,36Radin M.D. Vinckeviciute J. Seshadri R. Van der Ven A. Manganese oxidation as the origin of the anomalous capacity of Mn-containing Li-excess cathode materials.Nat. Energy. 2019; 4: 639-646Crossref Scopus (99) Google Scholar, 37Rana J. Papp J.K. Lebens-Higgins Z. Zuba M. Kaufman L.A. Goel A. Schmuch R. Winter M. Whittingham M.S. Yang W. et al.Quantifying the capacity contributions during activation of Li2MnO3.ACS Energy Lett. 2020; 5: 634-641Crossref Google Scholar, 38Yu D.Y.W. Yanagida K. Kato Y. Nakamura H. Electrochemical activities in Li2MnO3.J. Electrochem. Soc. 2009; 156: A417Crossref Scopus (333) Google Scholar However, it is important to note that oxygen redox itself is not bound to this charging plateau, as evidenced by the clear signature of oxygen redox reactions in the Li-rich electrode over hundreds of extended cycles with a complete disappearance of the high-voltage plateau.12Gent W.E. Lim K. Liang Y. Li Q. Barnes T. Ahn S.-J. Stone K.H. McIntire M. Hong J. Song J.H. et al.Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides.Nat. Commun. 2017; 8: 2091Crossref PubMed Scopus (327) Google Scholar Therefore, the high-voltage plateau itself is not a fingerprint of the reversible oxygen redox and one should not attribute this initial charging of Li2MnO3 to oxygen redox reactions merely because of its charging plateau.39Yang W. Devereaux T.P. Anionic and cationic redox and interfaces in batteries: advances from soft X-ray absorption spectroscopy to resonant inelastic scattering.J. Power Sources. 2018; 389: 188-197Crossref Scopus (132) Google Scholar The reaction mechanism of Li2MnO3 has seen a long history of contradictory debates between models of oxygen and Mn activities. Besides the early proposals of Mn4+/5+ redox,40Kalyani P. Chitra S. Mohan T. Gopukumar S. Lithium metal rechargeable cells using Li2MnO3 as the positive electrode.J. Power Sources. 1999; 80: 103-106Crossref Scopus (196) Google Scholar Li+/H+ exchange,41Robertson A.D. Bruce P.G. The origin of electrochemical activity in Li2MnO3.Chem. Commun. 2002; : 2790-2791Crossref PubMed Scopus (162) Google Scholar and Li2O extraction,35Lu Z. Dahn J.R. Understanding the anomalous capacity of Li/Li[ NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 cells using in situ x-ray diffraction and electrochemical studies.J. Electrochem. Soc. 2002; 149: A815-A822Crossref Scopus (940) Google Scholar etc., debates continue with revitalized concepts of reversible oxygen redox models,26Oishi M. Yamanaka K. Watanabe I. Shimoda K. Matsunaga T. Arai H. Ukyo Y. Uchimoto Y. Ogumi Z. Ohta T. Direct observation of reversible oxygen anion redox reaction in Li-rich manganese oxide, Li2MnO3, studied by soft X-ray absorption spectroscopy.J. Mater. Chem. A. 2016; 4: 9293-9302Crossref Google Scholar,27Han S.J. Xia Y.G. Wei Z. Qiu B. Pan L.C. Gu Q.W. Liu Z.P. Guo Z.Y. A comparative study on the oxidation state of lattice oxygen among Li1.14Ni0.136Co0.136Mn0.544O2, Li2MnO3, LiNi0.5Co0.2Mn0.3O2 and LiCoO2 for the initial charge–discharge.J. Mater. Chem. A. 2015; 3: 11930-11939Crossref Google Scholar irreversible oxygen release and surface reactions23Chen H. Islam M.S. Lithium extraction mechanism in Li-Rich Li2MnO3 involving oxygen hole formation and dimerization.Chem. Mater. 2016; 28: 6656-6663Crossref Scopus (164) Google Scholar, 24Xie Y. Saubanère M. Doublet M.-L. Requirements for reversible extra-capacity in Li-rich layered oxides for Li-ion batteries.Energy Environ. Sci. 2017; 10: 266-274Crossref Google Scholar, 25Saubanère M. McCalla E. Tarascon J.-M. Doublet M.-L. The intriguing question of anionic redox in high-energy density cathodes for Li-ion batteries.Energy Environ. Sci. 2016; 9: 984-991Crossref Google Scholar,37Rana J. Papp J.K. Lebens-Higgins Z. Zuba M. Kaufman L.A. Goel A. Schmuch R. Winter M. Whittingham M.S. Yang W. et al.Quantifying the capacity contributions during activation of Li2MnO3.ACS Energy Lett. 2020; 5: 634-641Crossref Google Scholar,38Yu D.Y.W. Yanagida K. Kato Y. Nakamura H. Electrochemical activities in Li2MnO3.J. Electrochem. Soc. 2009; 156: A417Crossref Scopus (333) Google Scholar,42Lebens-Higgins Z.W. Chung H. Zuba M.J. Rana J. Li Y. Faenza N.V. Pereira N. McCloskey B.D. Rodolakis F. Yang W. et al.How bulk sensitive is hard X-ray photoelectron spectroscopy: accounting for the cathode–electrolyte interface when addressing oxygen redox.J. Phys. Chem. Lett. 2020; 11: 2106-2112Crossref PubMed Scopus (23) Google Scholar,43Guerrini N. Jin L. Lozano J.G. Luo K. Sobkowiak A. Tsuruta K. Massel F. Duda L.-C. Roberts M.R. Bruce P.G. Charging mechanism of Li2MnO3.Chem. Mater. 2020; 32: 3733-3740Crossref Scopus (32) Google Scholar, and Mn4+/7+ reactions.36Radin M.D. Vinckeviciute J. Seshadri R. Van der Ven A. Manganese oxidation as the origin of the anomalous capacity of Mn-containing Li-excess cathode materials.Nat. Energy. 2019; 4: 639-646Crossref Scopus (99) Google Scholar Attributing the Li2MnO3 cycling mechanism to reversible oxygen redox is mostly based on O-K X-ray absorption spectroscopy (XAS) and photoelectron spectroscopy (XPS) results.26Oishi M. Yamanaka K. Watanabe I. Shimoda K. Matsunaga T. Arai H. Ukyo Y. Uchimoto Y. Ogumi Z. Ohta T. Direct observation of reversible oxygen anion redox reaction in Li-rich manganese oxide, Li2MnO3, studied by soft X-ray absorption spectroscopy.J. Mater. Chem. A. 2016; 4: 9293-9302Crossref Google Scholar,27Han S.J. Xia Y.G. Wei Z. Qiu B. Pan L.C. Gu Q.W. Liu Z.P. Guo Z.Y. A comparative study on the oxidation state of lattice oxygen among Li1.14Ni0.136Co0.136Mn0.544O2, Li2MnO3, LiNi0.5Co0.2Mn0.3O2 and LiCoO2 for the initial charge–discharge.J. Mater. Chem. A. 2015; 3: 11930-11939Crossref Google Scholar This model has been appreciated because it provides the natural rationality of the reversible oxygen redox found in Li-rich systems. However, recent clarifications have shown that XPS signals, even with hard X-rays, may not indicate the bulk oxygen states,42Lebens-Higgins Z.W. Chung H. Zuba M.J. Rana J. Li Y. Faenza N.V. Pereira N. McCloskey B.D. Rodolakis F. Yang W. et al.How bulk sensitive is hard X-ray photoelectron spectroscopy: accounting for the cathode–electrolyte interface when addressing oxygen redox.J. Phys. Chem. Lett. 2020; 11: 2106-2112Crossref PubMed Scopus (23) Google Scholar and variations of O-K XAS pre-edge is dominated by the changing TM characters and, thus, is not a reliable oxygen redox probe.32Roychoudhury S. Qiao R. Zhuo Z. Li Q. Lyu Y. Kim J.-H. Liu J. Lee E. Polzin B.J. Guo J. et al.Deciphering the oxygen absorption pre-edge: a caveat on its application for probing oxygen redox reactions in batteries.Energy Environ. Mater. 2020; https://doi.org/10.1002/eem2.12119Crossref Scopus (24) Google Scholar,33Yang W. Oxygen release and oxygen redox.Nat. Energy. 2018; 3: 619-620Crossref Scopus (31) Google Scholar Other than the reversible oxygen redox model, it has long been