Browsing by Author "Rawal, A"
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- ItemAlkali metal-modified P2 NaxMnO2: crystal structure and application in sodium-ion batteries(American Chemical Society, 2020-08-18) Sehrawat, D; Rawal, A; Cheong, S; Avdeev, M; Ling, CD; Kimpton, JA; Sharma, NSodium-ion batteries (NIBs) are an emerging alternative to lithium-ion batteries because of the abundance of sodium resources and their potentially lower cost. Here we report the Na0.7MnO2 solid state synthesized at 1000 °C that shows two distinct phases; one adopts hexagonal P2-type P63/mmc space group symmetry, and the other adopts orthorhombic Pbma space group symmetry. The phase ratio of P2 to the orthorhombic phase is 55.0(5):45.0(4). A single-phase P2 structure is found to form at 1000 °C after modification with alkali metals Rb and Cs, while the K-modified form produces an additional minor impurity. The modification is the addition of the alkali elements during synthesis that do not appear to be doped into the crystal structure. As a cathode for NIBs, parent Na0.7MnO2 shows a second charge/discharge capacity of 143/134 mAh g–1, K-modified Na0.7MnO2 a capacity of 184/178 mAh g–1, Rb-modified Na0.9MnO2 a capacity of 159/150 mAh g–1, and Cs-modified Na0.7MnO2 a capacity of 171/163 mAh g–1 between 1.5 and 4.2 V at a current density of 15 mA g–1. The parent Na0.7MnO2 is compared with alkali metal (K, Rb, and Cs)-modified NaxMnO2 in terms of surface morphology using scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, scanning electron microscopy, 23Na solid-state nuclear magnetic resonance, and X-ray photoelectron spectroscopy and in terms of electrochemical performance and structural electrochemical evolution using in situ or operando synchrotron X-ray diffraction. © 2020 American Chemical Society
- ItemDefect structure and property consequence when small Li+ ions meet BaTiO3(American Physical Society, 2020-08-31) Narayanan, N; Lou, Q; Rawal, A; Lu, T; Liu, Z; Chen, J; Langley, J; Chen, H; Hester, JR; Cox, N; Fuess, H; McIntyre, GJ; Li, G; Yu, DH; Liu, YIn the present work the longstanding issue of the structure and dynamics of smaller ions in oxides and its impact on the properties was investigated on 7% Li-doped BaTiO3. The investigation combined several techniques, notably neutron powder diffraction (NPD), nuclear magnetic resonance (7Li-NMR), electron paramagnetic resonance (EPR), electron microprobe, electric polarization (EP) measurement, and electronic structure calculations based on density-functional theory (DFT). Electron microprobe confirmed multiple phases, one containing incorporated Li in the BaTiO3 host lattice and another glassy phase which breaks the host lattice due to excessive Li accumulation. While the average structure of Li in BaTiO3 could not be determined by NPD, 7Li-NMR revealed one broad “disordered” and multiple “ordered” peaks. Local structure models with different defect types involving Li+ were modeled and the corresponding chemical shifts (δ) were compared with experimental values. It is found that the closest defect model describing the ordered peaks, is with Ti4+ being replaced by four Li+ ions. The biexponential behavior of the spin-lattice relaxation of the ordered peaks each with a short and a long relaxation discloses the existence of paramagnetic ions. Finally, EPR revealed the existence of the paramagnetic ion Ti3+ as a charge-transfer defect. DFT calculations disclosed local antipolar displacements of Ti ions around both types of defect sites upon insertion of Li+. This is in accordance with the experimental observation of pinching effects of the EP in Li-doped BaTiO3. These studies demonstrate the huge impact of the local structure of the doped smaller/lighter ions on the functional properties of oxides. ©2020 American Physical Society
- ItemDefect structure-property correlations in Li doped BaTiO3(Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Narayanan, N; Lou, Q; Rawal, A; Lu, T; Liu, Z; Chen, J; Langley, J; Chen, H; Hester, JR; Cox, N; Fuess, H; McIntyre, GJ; Li, G; Yu, DH; Liu, Y; Li, GIn the present work we investigate the important issue of the structure and dynamics of smaller ions in oxides and the resulting impact on its functional properties. For this purpose, we selected a 7% Li-doped BaTiO3. Li is a vital ingredient in novel energy storage technologies such as Li-ion batteries. The smaller Li-ion can influence the structural stability, homogeneity, local environment, and dynamic behavior of the host lattice, affecting and optimizing the dielectric and multiferroic properties of novel polar functional materials [1-2]. However, the Li-ion positions and dynamics in functional materials are not completely understood, controversially discussed and are the subject of extensive ongoing research . Furthermore, sample inhomogeneity due to Li migration to the grain boundary and/or development of multiple phases complicates the elucidation of the structure-property correlations that may lead to incorrect interpretations . The selection of BaTiO3 as the host lattice is due to materials based on this being considered as the alternative to the piezoelectric lead zirconate titanate, citing environmental issues . BaTiO3 also crystallizes in a simple perovskite structure and Li ions can be effectively doped into it at lower doping levels. Very recently, field-dependent electric polarization measurements on BaTiO3 exhibited a polarization–electric field double hysteresis loop upon Li doping . These drastic changes to the electric polarization, related to the doping poses a good test case for the investigation of the Li induced defect structure model and its influence on the functional properties. To elucidate the above structure-property correlations, we combined several techniques, such as neutron powder diffraction electron microprobe associated with the wavelength-dispersive spectroscopy, 7Li nuclear magnetic resonance spectroscopy (NMR), electron paramagnetic resonance (EPR), electric polarization measurement, and theoretical calculations based on density functional theory .
- ItemElectrochemically activated solid synthesis: an alternative solid-state synthetic method(Royal Society of Chemistry, 2018-09-29) Liu, JN; Andersen, HL; Al Bahri, OK; Bhattacharyya, S; Rawal, A; Brand, HEA; Sharma, NSolid-state synthesis is one of the most common synthetic methods in chemistry and is extensively used in lab-scale syntheses of advanced functional materials to ton-scale production of chemical compounds. It generally requires at least one or several high temperature and/or high-pressure steps, which makes production of compounds via solid-state methods very energy and time intensive. Consequently, there is a persistent economic and environmental incentive to identify less energy and time consuming synthetic pathways. Here, we present an alternative solid-state synthetic method, which utilizes structural changes, induced by an electrochemical "activation" step followed by a thermal treatment step. The method has been used to synthesize a Sc0.67WO4-type phase where Sc0.67WO4 has previously only been obtained at 1400 °C and 4 GPa for 1 h. Through our method the Sc0.67WO4-type phase has been prepared at only 600 °C and ambient pressure. Experimental factors that influence phase formation from the electrochemical perspective are detailed. Overall, the method presented in this work appears to be able to generate the conditions for unusual and new phases to form and thus becomes another tool for synthetic solid-state chemists. This in turn permits the exploration of a larger synthetic parameter space. © 2018 The Royal Society of Chemistry.
- ItemInvestigation of K modified P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 as a cathode material for sodium-ion batteries(Royal Society of Chemistry, 2018-11-19) Sehrawat, D; Cheong, S; Rawal, A; Glushenkov, AM; Brand, HEA; Cowie, BCC; Gonzalo, E; Rojo, T; Naeyaert, PJP; Ling, CD; Avdeev, M; Sharma, NSodium-ion batteries (NIBs) are emerging as a potentially cheaper alternative to lithium-ion batteries (LIBs) due to the larger abundance of sodium and in some cases the similar intercalation chemistry to LIBs. Here we report the solid state synthesized K-modified P2 Na0.7Mn0.8Mg0.2O2 which adopts hexagonal P63/mmc symmetry. The second charge/discharge capacity for the as-prepared material is 115/111 mA h g−1 between 1.5–4.2 V at a current density of 15 mA g−1, which reduces to 61/60 mA h g−1 after 100 cycles. Scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM-EDS) analysis shows a heterogeneous distribution of K and solid state 23Na NMR illustrates that the presence of K perturbs the local environment of Na within the P2 Na0.7Mn0.8Mg0.2O2 crystal structure. Larger scale X-ray absorption near-edge structure (XANES) data on the K L-edge also illustrate that K is present on the surface of electrodes in preference to the bulk. In situ synchrotron X-ray diffraction (XRD) data illustrates that the P2 structural motif is preserved, featuring a solid solution reaction for most of charge–discharge except at the charged and discharged states where multiple phases are present. The K-modified sample of P2 Na0.7Mn0.8Mg0.2O2 is compared with the K-free samples in terms of both structural evolution and electrochemical performance. © The Royal Society of Chemistry 2019
- ItemNanoporous zirconium phosphonate materials with enhanced chemical and thermal stability for sorbent applications(American Chemical Society, 2020-04-01) Veliscek-Carolan, J; Rawal, A; Oldfield, DT; Thorogood, GJ; Bedford, NMNanoporous zirconium phosphonate (ZrP) materials are considered to be promising candidates for practical applications such as catalysis and separation, in particular because of their excellent stability, resulting from the strength of the P–O–Zr bond. However, the functionality of ZrP materials is dependent on the availability of free phosphonate groups uncoordinated by zirconium, the presence of which can decrease the stability. The mechanisms by which nanoporous ZrP materials degrade and lose functionality during thermal and chemical treatment are not well understood. Herein, we address this knowledge gap using nanoporous zirconium aminotris(methylenephosphonic acid) (Zr-ATMP) sorbent materials. Thermal treatment up to 150 °C caused collapse of the nanoporous structure of some Zr-ATMP materials without a significant effect on the chemical structure. On the other hand, contact with 5 M nitric acid changed the chemical structure of the Zr-ATMP materials by catalyzing the formation of P–O–Zr bonds and elemental leaching. Enhancement of the thermal and chemical stability of the Zr-ATMP materials was achieved by decreasing the pH of the synthesis and, interestingly, changing the counterion of the hydroxide used to control the pH also impacted the structure and stability of the resulting materials. The most stable Zr-ATMP material was produced at pH 3 using LiOH, but this material demonstrated lower selectivity than other Zr-ATMP materials, which decreases its practicality for separation applications. The Zr-ATMP material synthesized at pH 3 with NaOH showed an optimal balance between the stability and sorption performance. The enhanced chemical and thermal stability of this material drastically improves its applicability for use in harsh environments, such as in the treatment of radioactive wastes. © 2020 American Chemical Society
- ItemSynthesis of per-deuterated alkyl amines for the preparation of deuterated organic pyromellitamide gelators(Pergamon-Elsevier Science Ltd, 2013-05-15) Yepuri, NR; Jamieson, SA; Darwish, TA; Rawal, A; Hook, JM; Thordarson, P; Holden, PJ; James, MA general, direct and scalable synthesis of per-deuterated alkyl amines is reported, together with their incorporation into pyromellitamides, which form self-assembled gels in cyclohexane. The deuterium labelling of these gelators allows the study of the dynamic intermolecular interactions in these materials using solid-state 2H NMR spectroscopy.© 2013, Elsevier Ltd.