| 1. |
- Linnell, Stephanie F., et al.
(författare)
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Effect of Cu substitution on anion redox behaviour in P3-type sodium manganese oxides
- 2022
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Ingår i: Journal of Physics. - : Institute of Physics (IOP). - 2515-7655. ; 4:4
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Tidskriftsartikel (refereegranskat)abstract
- Sodium layered oxides which display oxygen anion redox behaviour are considered promising positive electrodes for sodium-ion batteries because they offer increased specific capacities. However, they suffer from irreversible structural changes resulting in significant capacity loss and limited oxygen redox reversibility. Here the effect of Cu substitution on the electrochemical performance of P3-type sodium manganese oxide is examined by evaluating the structural and electronic structural evolution upon cycling, supported by density functional theory (DFT) calculations. Over the voltage range 1.8-3.8 V vs. Na/Na+, where the redox reactions of the transition metal ions contribute entirely towards the charge compensation mechanism, stable cycling performance is maintained, showing a capacity retention of 90% of the initial discharge capacity of 166 mA h g(-1) after 40 cycles at 10 mA g(-1). Over an extended voltage range of 1.8-4.3 V vs. Na/Na+, oxygen anion redox is invoked, with a voltage hysteresis of 110 mV and a greater initial discharge capacity of 195 mA h g(-1) at 10 mA g(-1) is reached. Ex-situ powder x-ray diffraction patterns reveal distortion of the P3 structure to P ' 3 after charge to 4.3 V, and then transformation to O ' 3 upon discharge to 1.8 V, which contributes towards the capacity fade observed between the voltage range 1.8-4.3 V. DFT with projected density of states calculations reveal a strong covalency between the copper and oxygen atoms which facilitate both the cationic and anionic redox reactions in P3-type Na0.67Mn0.9Cu0.1O2.
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| 2. |
- Linnell, Stephanie F., et al.
(författare)
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Effect of Ti-Substitution on the Properties of P3 Structure Na2/3Mn0.8Li0.2O2 Showing a Ribbon Superlattice
- 2022
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Ingår i: ChemElectroChem. - : Wiley-Blackwell. - 2196-0216. ; 9:19
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Tidskriftsartikel (refereegranskat)abstract
- Oxygen anion redox offers an effective strategy to enhance the energy density of layered oxide positive electrodes for sodium- and lithium-ion batteries. However, lattice oxygen loss and irreversible structural transformations over the first cycle may result in large voltage hysteresis, thereby impeding practical application. Herein, ribbon superstructure ordering of Li/transition-metal-ions was applied to suppress the voltage hysteresis combined with Ti-substitution to improve the cycling stability for P3-Na0.67Li0.2Ti0.15Mn0.65O2. When both cation and anion redox reactions are utilized, Na0.67Li0.2Ti0.15Mn0.65O2 delivers a reversible capacity of 172 mA h g(-1) after 25 cycles at 10 mA g(-1) between 1.6-4.4 V vs. Na+/Na. Ex-situ X-ray diffraction data reveal that the ribbon superstructure is retained with negligible unit cell volume expansion/contraction upon sodiation/desodiation. The performance as a positive electrode for Li-ion batteries was also evaluated and P3-Na0.67Li0.2Ti0.15Mn0.65O2 delivers a reversible capacity of 180 mA h g(-1) after 25 cycles at 10 mA g(-1) when cycled vs. Li+/Li between 2.0-4.8 V.
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| 3. |
- Linnell, Stephanie F., et al.
(författare)
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Enhanced Cycling Stability in the Anion Redox Material P3-Type Zn-Substituted Sodium Manganese Oxide
- 2022
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Ingår i: ChemElectroChem. - : John Wiley & Sons. - 2196-0216. ; 9:11
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Tidskriftsartikel (refereegranskat)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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| 4. |
- Linnell, Stephanie F., et al.
(författare)
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Enhanced oxygen redox reversibility and capacity retention of titanium-substituted Na-4/7[1/7Ti1/7Mn5/7]O-2 in sodium-ion batteries
- 2022
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry. - 2050-7488 .- 2050-7496. ; 10:18, s. 9941-9953
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Tidskriftsartikel (refereegranskat)abstract
- Anion redox reactions offer a means of enhancing the capacity of layered sodium transition metal oxide positive electrode materials. However, oxygen redox reactions typically show limited reversibility and irreversible structural changes upon cycling, resulting in rapid capacity loss. Here, the Ti-substituted Na-4/7[1/7Ti1/7Mn5/7]O-2 (where represents a transition metal vacancy) is presented as a positive electrode material for sodium-ion batteries. Na-4/7[1/7Ti1/7Mn5/7]O-2 delivers a reversible capacity of 167 mA h g(-1) after 25 cycles at 10 mA g(-1) within the voltage range of 1.6-4.4 V and presents enhanced stability compared with Na-4/7[Mn-1/7(6/7)]O-2 over the voltage range 3.0-4.4 V. The structural and electronic structural changes of this Ti4+ substituted phase are investigated by powder X-ray diffraction, X-ray absorption spectroscopy, electron paramagnetic resonance and Raman spectroscopy, supported by density functional theory calculations. These results show that the Na-4/7[Mn-1/7(6/7)]O-2 structure is maintained between 3.0 and 4.4 V, and the presence of TiO6 octahedra in Na-4/7[1/7Ti1/7Mn5/7]O-2 relieves structural distortions from Jahn-Teller distorted Mn3+O6 between 1.6 and 4.4 V. Furthermore, Ti4+ substitution stabilises the adjacent O 2p orbitals and raises the ionicity of the Mn-O bonds, increasing the operating potential of Na-4/7[1/7Ti1/7Mn5/7]O-2. Thereby providing evidence that the improved electrochemical performance of Na-4/7[1/7Ti1/7Mn5/7]O-2 can be attributed to Ti4+ substitution. This work provides insight and strategies for improving the structural stability and electrochemical performance of sodium layered oxides.
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