Browsing by Author "Cheong, S"

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    Alkali 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, N
    Sodium-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
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    Characterisation and controls on mineral-sorbed organic matter from a variety of groundwater environments
    (EarthArXiv, 2019-12-13) Oudone, PP; Rutlidge, H; Andersen, MS; O'Carroll, DM; Cheong, S; Meredith, KT; McDonough, LK; Marjo, CE; Baker, AA
    Detailed investigations into natural groundwater organic matter (OM) as carbon sources or sinks in the natural carbon cycle are generally limited. Groundwater OM concentration and composition is altered by biodegradation and sorption to minerals. In the saturated zone of an aquifer, dissolved organic matter (DOM) may represent a significant fraction of the natural groundwater dissolved organic carbon (DOC) pool, therefore understanding how mineral sorption influences OM will contribute to our understanding of how DOC is processed in groundwater. In this study we investigate the dominant fractions of natural DOC in groundwater and the extent of sorption on three common minerals found in the environment: iron-oxide coated sand, calcite and quartz sand. DOM sorption on these minerals was studied using groundwaters from three different geological environments in New South Wales, Australia: Anna Bay (quartz-sand coastal aquifer); Maules Creek (alluvial gravel and clay aquifer); and Wellington (alluvial karst limestone aquifer). Each groundwater and surface sample were characterised before and after sorption using size exclusion liquid chromatography with organic carbon detection (LC-OCD). Analysis revealed that humic substances (HS) are the dominant (13 – 70%) fraction of natural groundwater DOC. HS sorption on iron-oxide coated sand was higher than that on calcite and quartz sand, respectively while sorption on the calcite was also higher than on quartz sand. In shallow-sandy aquifer groundwater, due to less DOC sorption in sandy environment (Anna Bay), DOC concentration was found to be the highest compared to that from karst and other alluvial boreholes from Maules Creek and Wellington. HS sorption increases with the mineral mass and DOC concentration indicating that DOC sorption to the mineral surface did not reach saturation under the study conditions. Only the high-DOC alluvial groundwater produced significant sorption to each mineral phase and of the chemical fractions present (85% of 72 batch systems that HS sorption was found). Multiple linear regression showed that mineral mass, mineral type, depth of groundwater sample, DOC concentration, aqueous Fe2+ concentration and DOM aromaticity are the controlling factors of DOC sorption in the various groundwater environments. The regression analysis showed sorption decreases with depth, which could be because of DOC sorption along the groundwater flow path, resulting in less DOC at depth. The multiple linear regression predicts less DOC (HS) sorption in quartz sand system, agreeing with laboratory sorption results. HS sorption also correlated with aromaticity suggesting the chemical character of HS will control the degree of mineral sorption. The model also indicated that DOC sorption is negatively correlated with dissolved Fe2+ concentration in water samples presumably due to redox condition which is under anoxic environment iron oxide became electron acceptors under the process of DOC biodegradation leaving higher Fe2+ concentration and less available DOC for sorption.
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    Investigation 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, N
    Sodium-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