Researcher:
Subaşı, Yaprak

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Yaprak

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Subaşı

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Subaşı, Yaprak

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2020 - 20234

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    The first alkaline-earth azidoaurate(III), Ba[Au(N-3)(4)](2) center dot 4 H2O
    (Wiley, 2023) Prots, Yurii; Jach, Franziska; Afyon, Semih; Höhn, Peter; Department of Chemistry; Department of Chemistry; Department of Chemistry; Department of Chemistry; Subaşı, Yaprak; Tekin, Elif Sena; Somer, Mehmet Suat; Researcher; Undergraduate Student; Faculty Member; College of Sciences; College of Sciences; College of Sciences; N/A; N/A; 178882
    Transparent, dark orange Ba[Au(N-3)(4)](2) center dot 4 H2O was synthesized by reaction of Ba(N-3)(2) and AuCl3 or HAuCl4 in aqueous solution. The novel barium tetraazidoaurate(III) tetrahydrate crystallizes in the monoclinic space group Cc (no. 9) with a=1813.68(17) pm, b=1737.95(11) pm, c=682.04(8) pm and beta=108.849(4)degrees. The predominant structural features of Ba[Au(N-3)(4)](2) center dot 4 H2O are two crystallographically independent discrete anions [Au(N-3)(4)](-) with gold in a tetragonal planar coordination by nitrogen. Vibrational spectra show good agreement with those of other azidoaurates(III). Upon drying, this salt was shown to be a highly explosive material.
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    High voltage LiCoO2 cathodes with high purity lithium Bis(oxalate) Borate (LiBOB) for lithium-ion batteries
    (American Chemical Society (ACS), 2022) Afyon, Semih; Department of Chemistry; Department of Chemistry; Subaşı, Yaprak; Researcher; Koç University Boron and Advanced Materials Application and Research Center (KUBAM) / Koç Üniversitesi Bor ve İleri Malzemeler Uygulama ve Araştırma Merkezi (KUBAM); College of Sciences; N/A
    Lithium bis(oxalate) borate, LiB(C2O4)(2) (LiBOB) can be used as an electrolyte additive for lithium-ion batteries (LIBs) to prevent structural change and electrolyte decomposition by developing a protective solid electrolyte interphase (SEI) on the cathode surface. However, impurities present in LiBOB result in significant electrochemical performance decays related to higher full cell impedance. Here, a practical purification technique is performed to remove these impurities within the as-synthesized anhydrous LiBOB in which we further add 1 wt % in 1 M LiPF6 in EC:DMC (1:1) electrolytes to achieve a more stable cycling performance for high voltage applications of LiCoO2 (LCO) cathodes. The phase and purity of as-synthesized LiBOB and recrystallized LiBOB is determined by a combination of X-ray powder diffraction (XRPD), Fourier-transform infrared (FTIR) spectra, and scanning electron microscopy (SEM) measurements. The LIB performance with the addition of high purity LiBOB as an electrolyte additive is investigated via galvanostatic charge-discharge cycling, rate capability, and cyclic voltammetry (CV) measurements within a voltage range of 3.0-4.4 V. The cell containing 1 wt % recrystallized LiBOB shows superior cycling performance, rate capability with higher energy density, and Coulombic efficiency in comparison with the reference cell through the formation of a passivation layer on the LCO surface. Thus, for the LiBOB added cell, the crystal structure of LiCoO2 is well-maintained even at higher potentials after 100 cycles according to the ex situ XRPD and SEM analyses. Therefore, high-purity LiBOB improves the interfacial stability of the LCO cathode by inhibiting oxidative decomposition of electrolytes, undesirable structural changes, and cobalt dissolution bringing about safer cycling even at high operation voltages.
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    Surface modified TiO2/reduced graphite oxide nanocomposite anodes for lithium ion batteries
    (Springer, 2020) Slabon, Adam; Afyon, Semih; Department of Chemistry; Department of Chemistry; N/A; Department of Chemistry; Subaşı, Yaprak; Somer, Mehmet Suat; Yağcı, Mustafa Barış; Researcher; Faculty Member; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Sciences; College of Sciences; N/A; N/A; 178882; N/A
    Anatase TiO2 nanoparticles with an average crystallite size of ~ 20 nm are synthesized through a sol-gel method. A composite anode for Li-ion batteries is prepared with the synthesized TiO2 nanoparticles and reduced graphite oxide (RGO) as the conductive carbon source. After the preparation of TiO2/RGO nanocomposite, a novel surface modification is carried out by the employment of H2O2 to enhance the overall electrochemical performance of nanocomposite anode (TiO2/RGO-P composite). The physical and chemical characterizations of the surface modified TiO2/RGO-P composites are performed with X-ray powder diffraction (XRPD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM) analyses. The electrochemical performance of TiO2/RGO-P composite electrodes is investigated via galvanostatic charge-discharge cycling tests in a potential window of 1.0-3.0 V. Compared to the plain TiO2/RGO composite anode, the TiO2/RGO-P composite anode has higher reversible capacities and better cycling performance due to the enhanced and stable formation of 3D channels of TiO2 nanoparticles with RGO stemming from the surface modification with H2O2. The TiO2/RGO-P composite anode delivers reversible discharge capacities around 291 mA h g(-1) at a rate of 100 mA g(-1), whereas the value stays at 214 and 143 mA h g(-1) for the plain TiO2/RGO composite and TiO2 nanoparticle without any RGO, respectively.
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    PublicationOpen Access
    Guide to water free lithium bis(oxalate) borate (LiBOB)
    (American Chemical Society (ACS), 2021) Zor, Ceren; Afyon, Semih; N/A; Department of Chemistry; Department of Chemistry; Haciu, Durata; Subaşı, Yaprak; Somer, Mehmet Suat; Teaching Faculty; Researcher; Koç University AKKİM Boron-Based Materials _ High-technology Chemicals Research _ Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); Graduate School of Sciences and Engineering; College of Sciences
    Lithium bis(oxalate) borate, LiB(C2O4)(2) (LiBOB), is one of the most important electrolyte additives for Li-ion batteries (LIBs) due to its numerous advantages such as thermal stability, good solubility in organic solvents, high conductivity, and low cost as well as providing safer operations with superior electrochemical performance compared to conventional electrolyte combinations. However, the use of LiBOB is limited due to slight instability issues under ambient conditions that might require extra purification steps and result in poorer performances in real systems. Here, we address some of these issues and report a high purity water free LiBOB synthesized with fewer processing steps, employing lithium carbonate, oxalic acid, and boric acid as low-cost starting materials, and via ceramic processing methods under protective atmosphere. The physical and chemical characterizations of both anhydrous and monohydrate phases are performed with X-ray powder diffraction (XRPD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and scanning electron microscopy (SEM) analyses to determine the degree of the purity and the formation of impurities, such as LiBOB center dot H2O, HBO2, and Li2C2O4, as a result of the aging investigations where the as-synthesized salt was exposed to ambient conditions for different durations. Differential thermal analysis (DTA) is applied to determine the optimum synthesis conditions for anhydrous LiBOB and to analyze the water loss and the decomposition of LiBOB center dot H2O. Aging experiments with the water free LiBOB are carried out to evaluate the effect of humidity on the phase changes and resulting impurities under various conditions. The detrimental effect of even slightest humidity conditions is shown, and protective measures during and after the synthesis of LiBOB are discussed. Anhydrous LiBOB could be widely used as an electrolyte additive to improve the overall electrochemical performances for LIBs through development of a protective solid electrolyte interface (SEI) on the surface of high voltage cathodes and by bringing about superior electrochemical properties with increased cycling stability, rate capability, and Coulombic efficiency, if synthesized, purified, and handled properly before use in real electrochemical systems.