| 1. |
- House, Robert A., et al.
(author)
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What Triggers Oxygen Loss in Oxygen Redox Cathode Materials?
- 2019
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In: Chemistry of Materials. - : AMER CHEMICAL SOC. - 0897-4756 .- 1520-5002. ; 31:9, s. 3293-3300
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Journal article (peer-reviewed)abstract
- It is possible to increase the charge capacity of transition metal (TM) oxide cathodes in alkali-ion batteries by invoking redox reactions on the oxygen. However, oxygen loss often occurs. To explore what affects oxygen loss in oxygen redox materials, we have compared two analogous Na-ion cathodes, P2-Na0.67Mg0.28Mn0.72O2 and P2-Na0.78Li0.25Mn0.75O2. On charging to 4.5 V, >0.4e(-) are removed from the oxide ions of these materials, but neither compound exhibits oxygen loss. Li is retained in P2-Na0.78Li0.25Mn0.25O2 but displaced from the TM to the alkali metal layers, showing that vacancies in the TM layers, which also occur in other oxygen redox compounds that exhibit oxygen loss such as Li[Li0.2Ni0.2Mn0.6]O-2, are not a trigger for oxygen loss. On charging at 5 V, P2-Na0.78Li0.25Mn0.75O2 exhibits oxygen loss, whereas P2-Na0.67Mg0.28Mn0.72O2 does not. Under these conditions, both Na+ and Li+ are removed from P2-Na0.78Li0.25Mn0.75O2, resulting in underbonded oxygen (fewer than 3 cations coordinating oxygen) and surface-localized O loss. In contrast, for P2-Na0.67Mg0.28Mn0.72O2, oxygen remains coordinated by at least 2 Mn4+ and 1 Mg2+ ions, stabilizing the oxygen and avoiding oxygen loss.
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| 2. |
- Kim, Eun Jeong, et al.
(author)
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Oxygen Redox Activity through a Reductive Coupling Mechanism in the P3-Type Nickel-Doped Sodium Manganese Oxide
- 2020
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In: ACS Applied Energy Materials. - : AMER CHEMICAL SOC. - 2574-0962. ; 3:1, s. 184-191
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Journal article (peer-reviewed)abstract
- Increasing dependence on rechargeable batteries for energy storage calls for the improvement of energy density of batteries. Toward this goal, introduction of positive electrode materials with high voltage and/or high capacity is in high demand. The use of oxygen chemistry in lithium and sodium layered oxides has been of interest to achieve high capacity. Nevertheless, a complete understanding of oxygen-based redox processes remains elusive especially in sodium ion batteries. Herein, a novel P3-type Na0.67Ni0.2Mn0.8O2, synthesized at low temperature, exhibits oxygen redox activity in high potentials. Characterization using a range of spectroscopic techniques reveals the anionic redox activity is stabilized by the reduction of Ni, because of the strong Ni 3d-O 2p hybridization states created during charge. This observation suggests that different route of oxygen redox processes occur in P3 structure materials, which can lead to the exploration of oxygen redox chemistry for further development in rechargeable batteries.
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| 3. |
- Kim, Eun Jeong, et al.
(author)
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Vacancy-Enhanced Oxygen Redox Reversibility in P3-Type Magnesium-Doped Sodium Manganese Oxide Na0.67Mg0.2Mn0.8O2
- 2020
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In: ACS Applied Energy Materials. - : AMER CHEMICAL SOC. - 2574-0962. ; 3:11, s. 10423-10434
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Journal article (peer-reviewed)abstract
- Lithium-rich layered oxides and sodium layered oxides represent attractive positive electrode materials exhibiting excess capacity delivered by additional oxygen redox activity. However, structural degradation in the bulk and detrimental reactions with the electrolyte on the surface often occur, leading to limited reversibility of oxygen redox processes. Here, we present the properties of P3-type Na0.67Mg0.2Mn0.8O2 synthesized under both air and oxygen. Both materials exhibit stable cycling performance in the voltage range of 1.8-3.8 V, where the Mn3+/Mn4+ redox couple entirely dominates the electrochemical reaction. Oxygen redox activity is triggered for both compounds in the wider voltage window 1.8-4.3 V with typical large voltage hysteresis from nonbonding O 2p states generated by substituted Mg. Interestingly, for the compound prepared under oxygen, an additional novel reversible oxygen redox activity is shown with an exceptionally small voltage hysteresis (20 mV). The presence of vacancies in the transition-metal layers is shown to play a critical role not only in forming unpaired O 2p states independent of substituted elements but also in stabilizing the P3 structure during charge with reduced structural transformation to the O'3 phase at the end of discharge. This study reveals the important role of vacancies in P3-type sodium layered oxides to increase energy density using both cationic and anionic redox processes.
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| 4. |
- Linnell, Stephanie F., et al.
(author)
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Enhanced Cycling Stability in the Anion Redox Material P3-Type Zn-Substituted Sodium Manganese Oxide
- 2022
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In: ChemElectroChem. - : John Wiley & Sons. - 2196-0216. ; 9:11
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Journal article (peer-reviewed)abstract
- Sodium layered oxides showing oxygen redox activity are promising positive electrodes for sodium-ion batteries (SIBs). However, structural degradation typically results in limited reversibility of the oxygen redox activity. Herein, the effect of Zn-doping on the electrochemical properties of P3-type sodium manganese oxide, synthesised under air and oxygen is investigated for the first time. Air-Na0.67Mn0.9Zn0.1O2 and Oxy-Na0.67Mn0.9Zn0.1O2 exhibit stable cycling performance between 1.8 and 3.8 V, each maintaining 96 % of their initial capacity after 30 cycles, where Mn3+/Mn4+ redox dominates. Increasing the voltage range to 1.8-4.3 V activates oxygen redox. For the material synthesised under air, oxygen redox activity is based on Zn, with limited reversibility. The additional transition metal vacancies in the material synthesised under oxygen result in enhanced oxygen redox reversibility with small voltage hysteresis. These results may assist the development of high-capacity and structurally stable oxygen redox-based materials for SIBs.
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| 5. |
- Luo, Kun, et al.
(author)
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Anion Redox Chemistry in the Cobalt Free 3d Transition Metal Oxide Intercalation Electrode Li[Li0.2Ni0.2Mn0.6]O-2
- 2016
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 138:35, s. 11211-11218
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Journal article (peer-reviewed)abstract
- Conventional intercalation cathodes for lithium batteries store charge in redox reactions associated with the transition metal cations, e.g., Mn3+/4+ in LiMn2O4, and this limits the energy storage of Li-ion batteries. Compounds such as Li[Li0.2Ni0.2Mn0.6]O-2 exhibit a capacity to store charge in excess of the transition metal redox reactions. The additional capacity occurs at and above 4.5 V versus Li+/Li. The capacity at 4.5 V is dominated by oxidation of the O-2(-) anions accounting for similar to 0.43 e(-)/formula unit, with an additional 0.06 e(-)/formula unit being associated with O loss from the lattice. In contrast, the capacity above 4.5 V is mainly O loss, similar to 0.08 e(-)/formula. The O redox reaction involves the formation of localized hole states on O during charge, which are located on O coordinated by (Mn4+/Li+). The results have been obtained by combining operando electrochemical mass spec on 180 labeled Li[Li0.2Ni0.2Mn0.6]O-2 with XANES, soft X-ray spectroscopy, resonant inelastic X-ray spectroscopy, and Raman spectroscopy. Finally the general features of O redox are described with discussion about the role of comparatively ionic (less covalent) 3d metal oxygen interaction on anion redox in lithium rich cathode materials.
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| 6. |
- Luo, Kun, et al.
(author)
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Charge-compensation in 3d-transition-metal-oxide intercalation cathodes through the generation of localized electron holes on oxygen
- 2016
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In: Nature Chemistry. - 1755-4330 .- 1755-4349. ; 8:7, s. 684-691
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Journal article (peer-reviewed)abstract
- During the charging and discharging of lithium-ion-battery cathodes through the de-and reintercalation of lithium ions, electroneutrality is maintained by transition-metal redox chemistry, which limits the charge that can be stored. However, for some transition-metal oxides this limit can be broken and oxygen loss and/or oxygen redox reactions have been proposed to explain the phenomenon. We present operando mass spectrometry of O-18-labelled Li-1.2[Ni0.132+Co0.133+Mn0.544+]O-2, which demonstrates that oxygen is extracted from the lattice on charging a Li-1.2[Ni0.132+Co0.133+Mn0.544+]O-2 cathode, although we detected no O-2 evolution. Combined soft X-ray absorption spectroscopy, resonant inelastic X-ray scattering spectroscopy, X-ray absorption near edge structure spectroscopy and Raman spectroscopy demonstrates that, in addition to oxygen loss, Li+ removal is charge compensated by the formation of localized electron holes on O atoms coordinated by Mn4+ and Li+ ions, which serve to promote the localization, and not the formation, of true O-2(2-)( peroxide, O-O similar to 1.45 angstrom) species. The quantity of charge compensated by oxygen removal and by the formation of electron holes on the O atoms is estimated, and for the case described here the latter dominates.
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| 7. |
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