Browsing by Author "Andersen, HL"
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- ItemConsequences of long-term water exposure for bulk crystal structure and surface composition/chemistry of nickel-rich layered oxide materials for Li-ion batteries(Elsevier, 2020-06-10) Andersen, HL; Cheung, EA; Avdeev, M; Maynard-Casely, HE; Abraham, DP; Sharma, NWater exposure of layered nickel-rich transition metal oxide electrodes, widely used in high-energy lithium-ion batteries, has detrimental effects on the electrochemical performance, which complicates electrode handling and prevents implementation of environmentally benign aqueous processing procedures. Elucidating the degradation mechanisms in play may help rationally mitigate/circumvent key challenges. Here, the bulk structural consequences of long-term (>2.5 years) deuterated water (D2O) exposure of intercalation materials with compositions LixNi0.5Co0.2Mn0.3O2 (NCM523) and LixNi0.8Co0.1Mn0.1O2 (NCM811) are studied by neutron powder diffraction (NPD). Detailed inspection of the NPD data reveals gradual formation of a secondary crystalline phase in all exposed samples, not previously reported for this system. This unknown phase forms faster in liquid- compared to vapor-exposed compounds. Structural modelling of the NPD data shows a stable level of Li/Ni anti-site defects and does not indicate any significant changes in lattice parameters or hydrogen-lithium (D+/Li+) exchange in the structure. Consequently, the secondary phase formation must take place via transformation rather than modification of the parent material. X-ray photoelectron spectroscopy data indicate formation of LiHCO3/Li2CO3 at the surface and a Li-deficient oxide in the sub-surface region of the pristine compounds, and the presence of adsorbed water and transition metal hydroxides at the exposed sample surfaces. © 2020 Elsevier B.V.
- 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.
- ItemElucidating the relationship between nanoparticle morphology, nuclear/magnetic texture and magnetic performance of sintered SrFe12O19 magnets(Royal Society of Chemistry, 2020-04-22) Saura-Múzquiz, M; Eikeland, AZ; Stingaciu, M; Andersen, HL; Granados-Miralles, C; Avdeev, M; Luzin, V; Christensen, MSeveral M-type SrFe12O19 nanoparticle samples with different morphologies have been synthesized by different hydrothermal and sol–gel synthesis methods. Combined Rietveld refinements of neutron and X-ray powder diffraction data with a constrained structural model reveal a clear correlation between crystallite size and long-range magnetic order, which influences the macroscopic magnetic properties of the sample. The tailor-made powder samples were compacted into dense bulk magnets (>90% of the theoretical density) by spark plasma sintering (SPS). Powder diffraction as well as X-ray and neutron pole figure measurements and analyses have been carried out on the compacted specimens in order to characterize the nuclear (structural) and magnetic alignment of the crystallites within the dense magnets. The obtained results, combined with macroscopic magnetic measurements, reveal a direct influence of the nanoparticle morphology on the self-induced texture, crystallite growth during compaction and macroscopic magnetic performance. An increasing diameter-to-thickness aspect ratio of the platelet-like nanoparticles leads to increasing degree of crystallite alignment achieved by SPS. Consequently, magnetically aligned, highly dense magnets with excellent magnetic performance (30(3) kJ m−3) are obtained solely by nanostructuring means, without application of an external magnetic field before or during compaction. The demonstrated control over nanoparticle morphology and, in turn, crystal and magnetic texture is a key step on the way to designing nanostructured hexaferrite magnets with optimized performance. © Royal Society of Chemistry 2020
- ItemNanoengineered high-performance hexaferrite magnets by morphology-induced alignment of tailored nanoplatelets(American Chemical Society, 2018-11-15) Saura-Múzquiz, M; Granados-Miralles, C; Andersen, HL; Stingaciu, M; Avdeev, M; Christensen, MMagnetic materials are ubiquitous in electric devices and motors making them indispensable for modern-day society. The hexaferrites currently constitute the most widely used permanent magnets (PMs), accounting for 85% (by weight) of the global sales of PMs. This work presents a complete bottom-up nanostructuring protocol for preparation of magnetically aligned, high-performance hexaferrite PMs with a record-high (BH)max for dry-processed ferrites. The procedure includes the supercritical hydrothermal flow synthesis of anisotropic magnetic-single-domain strontium hexaferrite (SrFe12O19) nanocrystallites of various sizes, and their subsequent compaction into bulk magnets by spark plasma sintering (SPS). Interestingly, Rietveld modeling of neutron powder diffraction data reveals a significant difference between the magnetic structure of the thinnest nanoplatelets and the bulk compound, indicating the Sr-containing atomic layer to be the termination layer. Subsequently, high-density SrFe12O19 magnets (>95% of the theoretical density) are produced by SPS of the flow-synthesized nanoplatelets. Texture analysis by X-ray pole figure measurements demonstrates how the anisotropic shape of the nanoplatelets causes a self-induced alignment during SPS, without application of an external magnetic field. The self-induced texture is accompanied by crystallite growth along the magnetic easy-axis, i.e., the thickness of the platelets, resulting in high-performance PMs with square hysteresis curves and (BH)max of 30 kJ/m3. The (BH)max is further enhanced by annealing, reaching 36 kJ/m3 after 4 h at 850 °C, which exceeds the (BH)max of the highest grade of dry-processed commercial ferrites worldwide. © 2018 American Chemical Society
- ItemNeutron diffraction studies of nanostructured SrFe12O19 magnets(International Conference on Neutron Scattering, 2017-07-12) Saura-Múzquiz, M; Stingaciu, M; Eikeland, AZ; Andersen, HL; Granados-Miralles, C; Lucin, V; Avdeev, M; Christensen, MPhase pure, highly crystalline SrFe12 19 nanoparticles have been synthesized by hydrothermal and sol-gel synthesis methods. By varying synthesis parameters and method, SrFe12 19 nanoplatelets of various sizes and morphologies can be obtained. The nuclear and magnetic structure of the samples have been studied by neutron and X-ray diffraction, revealing a clear size dependency on the long range magnetic order. Subsequent compaction of the tailor-made powder samples into bulk magnets is carried out by Spark Plasma Sintering. Powder diffraction as well as X-ray and neutron pole figure analyses were performed on the compacted magnets. The obtained results, together with macroscopic magnetic measurements, reveal a direct influence between nanoparticle morphology, texture and magnetic performance. The platelet-like morphology of the nanoparticles leads to highly aligned magnets without the need of an externally applied magnetic field. Therefore, by varying the morphology of the platelets prior to compaction, the final magnetic properties of the sample can be tuned. Meticulous characterization based on neutron and X-ray diffraction techniques reveals the relationship between synthesis conditions, crystal-, nano- and magnetic structure, and macroscopic magnetic performance. Extensive control over each step of the nanostructuring process is essential in the design of materials with tailored physical properties.
- ItemThe Sc2WxMo3−xO12 series as electrodes in alkali-ion batteries(Royal Society of Chemistry, 2021-04-29) Liu, JN; Johannessen, B; Brand, HEA; Andersen, HL; Sharma, NHerein, the series Sc2WxMo3−xO12 (0 ≤ x ≤ 3) is synthesised and the structure and electrochemical performance in alkali-ion batteries is characterised. The structures remain in the orthorhombic Pnca space group for the whole series with the lattice parameters increasing approximately linearly from {a = 9.6336(2) Å, b = 13.2406(3) Å, c = 9.5413(2) Å} in Sc2Mo3O12 to {a = 9.6735(2) Å, b = 13.3218(3) Å, c = 9.5811(2) Å} in Sc2W3O12. Discharge against Li delivers a high initial discharge capacity of 1200 mA h g−1 for Sc2Mo3O12 with a reversible capacity of about 150 mA h g−1 after 100 cycles. Meanwhile the increase of W content reduces both the initial and overall capacities for all lithium, sodium and potassium half cells. The initial discharge capacity for Sc2W3O12 against lithium is only about 700 mA h g−1 with a reversible capacity of about 100 mA h g−1 after 100 cycles. For all sodium and potassium half cells across the series, the capacities drop dramatically after a few cycles and the reversible capacities are low, below 50 mA h g−1. Structurally, the fully potassium discharged Sc2Mo3O12 partially transforms into a new P[4 with combining macron] space group KMo4O6 phase, while the crystallinity decreases in both fully lithium and sodium discharged Sc2Mo3O12. For Sc2W3O12, only fully potassium discharged Sc2W3O12 shows a decrease in crystallinity, while the fully lithium and sodium discharged Sc2W3O12 appears to become amorphous (or particles are too small to be examined with X-ray diffraction). X-ray absorption spectroscopy demonstrates that the Mo oxidation state changes with different type and amount of alkali-ion discharge. This work illustrates the influence of composition on the electrochemical performance in this family of compounds. © 2021 The Royal Society of Chemistry
- ItemStructural evolution and stability of Sc2(WO4)3 after discharge in a sodium-based electrochemical cell(Royal Society of Chemistry, 2017-12-13) Andersen, HL; Al Bahri, OK; Tsarev, S; Johannessen, B; Schulz, B; Liu, JN; Brand, HEA; Christensen, M; Sharma, NSc2(WO4)3, prepared by solid state synthesis and constructed as an electrode, is discharged to different states in half-cell batteries, versus a Na negative electrode. The structural evolution of the Na-containing electrodes is studied with synchrotron powder X-ray diffraction (PXRD) revealing an increase in microstrain and a gradual amorphization taking place with increasing Na content in the electrode. This indicates that a conversion reaction takes place in the electrochemical cell. X-ray absorption spectroscopy (XAS) at the tungsten L3 absorption edge shows a reduction in the tungsten oxidation state. Variable temperature (VT) PXRD shows that the Sc2(WO4)3 electrode remains relatively stable at higher temperatures, while the Na-containing samples undergo a number of phase transitions and/or turn amorphous above ∼400 °C. Although, Sc2(WO4)3 is a negative thermal expansion (NTE) material only a subtle change of the thermal expansion is found below 400 °C for the Na-containing electrodes. This work shows the complexity in employing an electrochemical cell to produce Na-containing Sc2(WO4)3 and the subsequent phase transitions. © 2018 The Royal Society of Chemistry.