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33 items found (0 items not approved), displaying 31 to 33.[First/Prev] 1, 2, 3, 4 [Next/Last]

Instruments / Technical Area	Publication Details	Published
B18-Core EXAFS	Vacancy enhanced oxygen redox reversibility in P3-type magnesium doped sodium manganese oxide Na0.67Mg0.2Mn0.8O2 Eunjeong Kim , Le Anh Ma , David M. Pickup , Alan V. Chadwick , Reza Younesi , Philip Maughan , John T. S. Irvine , Robert Armstrong Acs Applied Energy Materials DOI: 10.1021/acsaem.0c01352 Diamond Proposal Number(s): [14239] Show Abstract ▼ 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 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 non-bonding O 2p states generated by substituted Mg. Interestingly, for the compound prepared under oxygen, an additional reversible oxygen redox activity is shown with 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 stabilising 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.	Sep 2020
B18-Core EXAFS	Exploring the effects of synthetic and postsynthetic grinding on the properties of the spin crossover material [Fe(atrz)3](BF4)2 (atrz = 4-amino-4h-1,2,4-triazole) Jed H. Askew , David M. Pickup , Gareth O. Lloyd , Alan V. Chadwick , Helena J. Shepherd Magnetochemistry 6 DOI: 10.3390/magnetochemistry6030044 Diamond Proposal Number(s): [14239] Open Access Show Abstract ▼ Abstract: The effects of mechanochemical synthesis and postsynthetic grinding on the spin crossover material [Fe(atrz)3](BF4)2 was examined in detail using a combination of X-ray diffraction, magnetometry, EXAFS and TEM. Mechanochemical synthesis yielded a different polymorph (β-phase) to the solution synthesised sample (α-phase), with a lower temperature spin crossover. Milling duration did not significantly affect this temperature but did result in the production of smaller nanoparticles with a narrower size distribution. It is also possible to convert from α- to the β-phase via postsynthetic grinding.	Sep 2020
B18-Core EXAFS	Activation of anion redox in P3 structure cobalt-doped sodium manganese oxide via introduction of transition metal vacancies Eun Jeong Kim , Kenza Mofredj , David M. Pickup , Alan V. Chadwick , John T. S. Irvine , A. Robert Armstrong Journal Of Power Sources 481 DOI: 10.1016/j.jpowsour.2020.229010 Diamond Proposal Number(s): [14239] Show Abstract ▼ Abstract: Additional capacity delivered by oxygen redox activity may in principle represent a means of enhancing the electrochemical performance of layered sodium transition metal oxides. However, irreversible structural changes occurring during cycling typically cause significant capacity fade with limited reversibility of oxygen redox processes. Here, P3-structure Na0.67Co0.2Mn0.8O2 was synthesised under two different reaction conditions. Both materials exhibit very stable cycling performance in the voltage range 1.8–3.8 V where the redox couples of transition metals entirely dominate the electrochemical reaction. For the compound prepared under more oxidising conditions, anion redox activity is triggered in the wider voltage window 1.8–4.4 V in a reversible manner with 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 generating unpaired O 2p states but also in stabilising the crystal structure in the high voltage region.	Nov 2020

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