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    Boron activity in the inactive Li2MnO3 cathode material
    (Elsevier, 2019) Ozkendir, O. Murat; Harfouche, Messaoud; Ulfat, Intikhab; Kaya, Cigdem; Celik, Gultekin; Ates, Sule; Aktas, Sevda
    Boron substituted Li2MnO3 cathode materials have frequently been studied and found to be inactive. In this reseach, crystal and electronic properties of the Li-ion cathode material with boron substitution having general formula; Li(2)Mn(1-x)BxO(3) (where x has values of 0.00, 0.05, 0.10, 0.15 and 0.20, respectively) were studied. The study revealed that, with the substitution of boron atoms in the manganese coordinates in the Li2MnO3 cathode materials, boron atoms acted as a landmark for the parent oxides by forming LiBO2 crystal domains. In the studied samples, boron atoms were determined as the key to construct new crystals and to restrict the excess of oxygen in the materials. Beyond the boron coordinations, the manganese atoms formed a cubic LiMn2O4 crystal structure which was reported to have good electrochemical properties. With the formation of the LiMn2O4 crystal, the substituted samples became a polycrystalline material with Li2MnO3- LiBO2- LiMn2O4 crystals. The obtained structure in the cathode materials have similarity with the general formula xLi(2)MnO(3)center dot(1-x)LiMn2O4 which had been reported to have good intercalation and electrochemical properties.
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    Öğe
    Crystal and electronic structure study of the Li2Mn1-xNdxO3 battery cathode
    (Elsevier Science Inc, 2019) Celik, Gultekin; Aktas, Sevda; Ates, Sule; Ozkendir, O. Murat; Klysubun, Wantana
    Neodymium substituted Li2MnO3 samples were prepared by the solid-state reaction method. The crystal and electronic structure properties of the prepared samples were investigated by x-ray based techniques. According to the analysis, it was determined that Nd atoms did not sit in to the Cr coordinations in Nd substituted samples and built up a different crystal structure. To support the analysis of the crystal structure study of the samples, electronic structure properties of the Nd substituted samples were studied via the collected data by x-ray absorption fine structure (XAFS) spectroscopy from the Mn K-edge. The results of the data analysis revealed that due to the misfit with the ionic radii and also the oxidation states, substituted neodymium atoms built up isolated crystal domains in Li8Mn5Nd18O39 cubic crystal structure. The symmetry in Mn K-edge absorption data of the sample both in parent Li2MnO3 and Nd substituted samples confirmed that Mn coordinations has not been influenced by the Nd substitution.
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    Öğe
    Electronic structure and electrochemical analysis of the Li2Mn1-xSexO3 materials
    (Elsevier, 2020) Ozkendir, O. Murat; Celik, Gultekin; Ates, Sule; Aktas, Sevda; Gunaydin, Selen; Harfouche, Messaoud; Bondino, Federica
    With the aim of probing the influence of the highly oxidizable selenium in the electrochemically inactive cathode Li2MnO3 material, samples were prepared with selenium substitution in the manganese coordination according to the general formula of Li2Mn1-xSexO3. In Li-ion batteries, oxygen instabilities are one of the major problems confronted that effect the performances of the cathode materials. The crystal and electronic structure properties of the materials were studied with x-ray absorption techniques. Selenium atoms were determined to build Li2SeO4 crystal and due to the oxygen removal during sample preparation mechanisms were determined to cause lower ionic conductivity than the parent Li2MnO3 oxide. The atomic distances in the materials were determined by the fits performed by the commercial code FEFF 8.2. Li2SeO4 crystal was determined as stacked between manganese and lithium atoms and isolated with each other.
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    Study of the local structure and electrical properties of gallium substituted LLZO electrolyte materials
    (Elsevier Science Sa, 2019) Aktas, Sevda; Ozkendir, Osman Murat; Eker, Yasin Ramazan; Ates, Sule; Atav, Ulfet; Celik, Gultekin; Klysubun, Wantana
    The solid-state lithium batteries are more stable than the batteries with liquid electrolyte, however their performance are also worse especially due to the low ionic conduction of their electrolyte. Garnet-type Li7La3Zr2O12 is a promising solid-state electrolyte candidate for lithium batteries. In this work the influence of gallium substitution on the electrical, crystal and electronic structure properties in the Li7La3Zr2O12 material were studied. Li7-3xGaxLa3Zr2O12 solid electrolytes were synthesized by solid state reaction method and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), AC impedance spectroscopy and X-ray absorption spectroscopy (XAS) techniques. XRD and XAS analyses showed that the x = 0.05 sample is formed in the tetragonal phase with a space group of I4(1)/ acd:2 and the rest are formed in the cubic phase with a space group of I-43d due to strong coupling between outer shell electrons of the oxygen (O) and gallium (Ga) atoms. Electrochemical impedance spectroscopy (EIS) studies indicated that the tetragonal phase has the highest ionic conductivity, 3.04 x 10(-6) S cm(-1), among all other cubic phases. (C) 2019 Published by Elsevier B.V.