Browsing by Author "Kim, SJ"
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- ItemLiTa2PO8: a fast lithium-ion conductor with new framework structure(Royal Society of Chemistry, 2018-10-29) Kim, JG; Kim, JH; Avdeev, M; Yun, H; Kim, SJA new lithium-ion conducting oxide, LiTa2PO8, with a three-dimensional (3D) framework structure was synthesized. The complementary use of synchrotron XRD and neutron diffraction revealed that LiTa2PO8 crystallized into a monoclinic system (space group C2/c) with lattice parameters a ≈ 9.716 Å, b ≈ 11.536 Å, c ≈ 10.697 Å, and β = 90.04°. In this structure, the TaO6 octahedra and PO4 tetrahedra are connected via corner-sharing to form an unprecedented anionic framework, [(TaO6/2)2(PO4/2)]−, providing a 3D pathway for lithium-ion conduction. The sintered LiTa2PO8 pellets exhibited excellent lithium-ion conductivities: bulk conductivity of 1.6 × 10−3 S cm−1 and total conductivity of 2.5 × 10−4 S cm−1 at 25 °C. © Royal Society of Chemistry
- ItemMagnetic transitions in the chiral armchair-kagome system Mn2Sb2O7(American Physical Society, 2017-01-20) Peets, DC; Sim, H; Choi, S; Avdeev, M; Lee, S; Kim, SJ; Kang, H; Ahn, D; Park, JGThe competition between interactions in frustrated magnets allows a wide variety of new ground states, often exhibiting emergent physics and unique excitations. Expanding the suite of lattices available for study enhances our chances of finding exotic physics. Mn2Sb2O7 forms in a chiral, kagome-based structure in which a fourth member is added to the kagome-plane triangles to form an armchair unit and link adjacent kagome planes. This structural motif may be viewed as intermediate between the triangles of the kagome network and the tetrahedra in the pyrochlore lattice. Mn2Sb2O7 exhibits two distinct magnetic phase transitions, at 11.1 and 14.2 K, at least one of which has a weak ferromagnetic component. The magnetic propagation vector does not change through the lower transition, suggesting a metamagnetic transition or a transition involving a multicomponent order parameter. Although previously reported in the P3121 space group, Mn2Sb2O7 actually crystallizes in P2, which allows ferroelectricity, and we show clear evidence of magnetoelectric coupling indicative of multiferroic order. The quasi-two-dimensional “armchair-kagome” lattice presents a promising platform for probing chiral magnetism and the effect of dimensionality in highly frustrated systems. ©2017 American Physical Society
- ItemOn disrupting the Na+-ion/vacancy ordering in P2-type sodium–manganese–nickel oxide cathodes for Na+-ion batteries(American Chemical Society, 2018-09-06) Gutierrez, A; Dose, WM; Borkiewicz, O; Guo, F; Avdeev, M; Kim, SJ; Fister, TT; Ren, Y; Bareño, J; Johnson, CSAn investigation of the electrochemical and structural properties of layered P2–Na0.62Mn0.75Ni0.25O2 is presented. The effect of changing the Mn/Ni ratio (3:1) from what is found in Na0.67Mn0.67Ni0.33O2 (2:1) and consequently the introduction of a third metal center (Mn3+) was investigated. X-ray powder diffraction (in situ and ex situ) revealed the lack of Na+-ion/vacancy ordering at the relevant sodium contents (x = 0.33, 0.5, and 0.67). Mn3+ in Na0.62Mn0.75Ni0.25O2 introduces defects into the Ni–Mn interplane charge order that in turn disrupts the ordering within the Na-plane. The material underwent P2–O2 and P2–P2′ phase transitions at high (4.2 V) and low (∼1.85 V) voltages, respectively. The material was tested at several different voltage ranges to understand the effect of the phase transitions on the capacity retention. Interestingly, the inclusion of both phase transitions demonstrated comparable cycling performance to when both phase transitions were excluded. Last, excellent rate performance was demonstrated between 4.3 and 1.5 V with a specific capacity of 120 mA h/g delivered at 500 mA/g current density. © 2018 American Chemical Society
- ItemSeasonality of Radon-222 near the surface at King Sejong Station (62°S), Antarctic Peninsula, and the role of atmospheric circulation based on observations and CAM-Chem model(Elsevier, 2022-11) Jun, SY; Choi, J; Chambers, SD; Oh, M; Park, SJ; Choi, T; Kim, SJ; Williams, AG; Hong, SBWe examined the seasonal cycle of radon concentration observed at King Sejong Station (KSG, 62°S), Antarctic Peninsula, during the period 2013–2016. The distribution of monthly radon concentration was found to be highly positively skewed from March through October (austral autumn to spring) due to large numbers of short-lived periods of high radon concentration. The global atmospheric chemistry model (CAM-Chem), which includes all global terrestrial sources of radon except for those in Antarctica, well reproduces the observed seasonal cycle of monthly-mean radon concentration at KSG. Further offline experiments suggest that uncertainties in radon emissions over South America and the Southern Ocean should be improved for the simulations of radon in Antarctica. The results demonstrate that seasonally varying transport of radon in the boundary layer from South America substantially affects the seasonality of monthly mean radon concentration at KSG. The composite analyses further reveal that high radon events at KSG are the result of a distinct east-west dipole-like structure associated with surface cyclonic circulation over the Bellingshausen Sea and anticyclonic circulation in the Weddell Sea. This atmospheric pattern provides favorable conditions for radon transport into KSG from the northwest. The relationship between radon concentration at KSG and climate variability is also discussed in this study. © 2022 Elsevier Inc.
- ItemTemperature dependence of structure and ionic conductivity of LiTa2PO8 ceramics(American Chemical Society, 2022-11-30) Dai, R; Avdeev, M; Kim, SJ; Rao, RP; Adams, StLiTa2PO8 has recently been reported as a new fast Li-ion conducting structure type within the series of Lix(MO6/2)m(TO4/2)n polyanion oxides. Here, we demonstrate the preparation of LiTa2PO8 by solid-state syntheses, clarify the temperature dependence of lithium distribution and ionic conductivity, and study the structural stability, densification, and achievable total conductivity as a function of sintering conditions synergizing experimental neutron and X-ray powder diffraction and electrochemical studies with computational energy landscape analyses and molecular dynamics simulations. A total room temperature conductivity of 0.7 mS cm-1 with an activation energy of 0.27 eV is achieved after sintering at 1323 K for 10 h. Spark plasma sintering yields high densification >98%, highly reproducible bulk conductivities of 2.8 mS cm-1, in agreement with our bond valence site energy-based pathway predictions, and total conductivities of 0.6 mS cm-1 within minutes. Powder diffraction studies from 3 to 1273 K reveal a reversible flipping of the monoclinic angle from above to below 90° close to room temperature as a consequence of rearrangements of the mobile ions that change the detailed pathway topology. A consistent model of the temperature-dependent Li redistribution, conductivity anisotropy, and transport mechanism is derived from a synopsis of diffraction experiments, experimental conductivity studies, and simulations. Due to the limited electrochemical window of Lix(TaO6/2)2(PO4/2)1 (LTPO), a direct contact with Li metal or high voltage cathode materials leads to degradation, but as demonstrated in this work, semi-solid-state batteries, where LTPO is protected from direct contact with lithium by organic buffer layers, achieve stable cycling. © 2022 American Chemical Society