Browsing by Author "Chang, H"

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    Capacity enhancement of the quenched Li-Ni-Mn-Co oxide high-voltage Li-ion battery positive electrode
    (Elsevier, 2017-03-23) Jena, A; Lee, CH; Pang, WK; Peterson, VK; Sharma, N; Wang, CC; Song, YF; Lin, CC; Chang, H; Liu, RS
    Li-rich metal oxides, regarded as a high-voltage composite cathode, is currently one of the hottest positive electrode material for lithium-ion batteries, due to its high-capacity and high-energy performance. The crystallography, phase composition and morphology can be altered by synthesis parameters, which can influence drastically the capacity and cycling performance. In this work, we demonstrate Li1.207Ni0.127Mn0.54Co0.127O2, obtained by a co-precipitation method, exhibits super-high specific capacity up to 298 mAh g−1 and excellent capacity retention of ∼100% up to 50 cycles. Using neutron powder diffraction and transmission X-ray microscopy, we have found that the cooling-treatments applied after sintering during synthesis are crucially important in controlling the phase composition and morphology of the cathodes, thereby influencing the electrochemical performance. Unique spherical microstructure, larger lattice, and higher content of Li-rich monoclinic component can be achieved in the rapid quenching process, whereas severe particle cracking along with the smaller lattice and lower monoclinic component content is obtained when natural cooling of the furnace is applied. Combined with electrochemical impedance spectra, a plausible mechanism is described for the poorer specific capacity and cycling stability of the composite cathodes. © 2017 Elsevier Ltd.
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    Chromium ion pair luminescence: a strategy in broadband near- infrared light-emitting diode design
    (American Chemical Society, 2021-11-04) Rajendran, V; Fang, MH; Huang, WT; Majewska, N; Leśniewski, T; Mahlik, S; Leniec, G; Kazmarek, SM; Pang, WK; Peterson, VK; Lu, KM; Chang, H; Liu, RS
    Portable near-infrared (NIR) light sources are in high demand for applications in spectroscopy, night vision, bioimaging, and many others. Typical phosphor designs feature isolated Cr3+ ion centers, and it is challenging to design broadband NIR phosphors based on Cr3+–Cr3+ pairs. Here, we explore the solid-solution series SrAl11.88–xGaxO19:0.12Cr3+ (x = 0, 2, 4, 6, 8, 10, and 12) as phosphors featuring Cr3+–Cr3+ pairs and evaluate structure–property relations within the series. We establish the incorporation of Ga within the magentoplumbite-type structure at five distinct crystallographic sites and evaluate the effect of this incorporation on the Cr3+–Cr3+ ion pair proximity. Electron paramagnetic measurements reveal the presence of both isolated Cr3+ and Cr3+–Cr3+ pairs, resulting in NIR luminescence at approximately 650–1050 nm. Unexpectedly, the origin of broadband NIR luminescence with a peak within the range 740–820 nm is related to the Cr3+–Cr3+ ion pair. We demonstrate the application of the SrAl5.88Ga6O19:0.12Cr3+ phosphor, which possesses an internal quantum efficiency of ∼85%, a radiant flux of ∼95 mW, and zero thermal quenching up to 500 K. This work provides a further understanding of spectral shifts in phosphor solid solutions and in particular the application of the magentoplumbites as promising next-generation NIR phosphor host systems. © 2021 American Chemical Society
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    Monitoring the phase evolution in LiCoO2 electrodes during battery cycles using in-situ neutron diffraction technique
    (John Wiley & Sons, Inc, 2019-12-03) Jena, A; Lee, PH; Pang, WK; Hsiao, KC; Peterson, VK; Darwish, TA; Yepuri, NR; Wu, SH; Chang, H; Liu, RS
    LiCoO2 (LCO) with average particle distribution of 8 μm (LCO-A) and 11 μm (LCO-B) exhibit substantial differences in cycle performance. The half-cells have similar first-cycle discharge capacities of 173 and 175 mAh/g at 0.25 C, but after 100 cycles, the discharge capacities are substantially different, that is, 114 and 141 mAh/g for LCO-A and LCO-B, respectively. Operando neutron powder diffraction of full LCO||Li4Ti5O12 batteries show differences in the LCO reaction mechanism underpinning the electrochemical behavior. LCO-A follows a purely solid solution reaction during cycling compared to the solid solution and two-phase reaction mechanism in LCO-B. The absence of the two-phase reaction in LCO-A is consistent with a homogeneous distribution of Li throughout the particle. The two-phase reaction in LCO-B reflects two distinguishable distributions of Li within the particles. The faster capacity decay in LCO-A is correlated to an increase in electrode cracking during battery cycles. © 2019 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co.
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    Pentavalent manganese luminescence: designing narrow-band near-infrared light-emitting diodes as next-generation compact light sources
    (American Chemical Society, 2023-11-30) Rajendran, V; Chen, KC; Huang, WT; Majewska, N; Leśniewski, T; Grzegorczyk, M; Mahlik, S; Leniec, G; Kaczmarek, SM; Pang, WK; Peterson, VK; Lu, KM; Chang, H; Liu, RS
    Manganese in the pentavalent state (Mn5+) is both rare and central in materials exhibiting narrow-band near-infrared (NIR) emission and is highly sought after for phosphor-converted light-emitting diodes as promising candidates for future miniature solid-state NIR light source. We develop the Ca14Zn6Ga10-xMnxO35 (x = 0.3, 0.5, 1.0, 1.25, 1.5, and 3.0) series that exhibit simultaneous Mn4+ (650-750 nm) and Mn5+ (1100-1250 nm) luminescence. We reveal a preferential occupancy of Mn in regular octahedral and tetrahedral environments, with the short bond length between these responsible for luminescence. We present a theoretical spin-orbital interaction model in which breaking the spin selection rule permits the luminescence of Mn4+ and Mn5+. A total photon flux of 87.5 mW under a 7 mA driving current demonstrates its potential for real-time application. This work pushes our understanding of achieving Mn5+ luminescence and opens the way for the design of Mn5+-based narrow-band NIR phosphors. © 2022 American Chemical Society.