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Öğe Electrochemical effects and magnetic properties of B substituted LiCoO2: Improving Li-battery performance(Elsevier Science Sa, 2016) Oz, E.; Altin, S.; Demirel, S.; Bayri, A.; Altin, E.; Baglayan, O.; Avci, S.LiCo(1-x)BxO(2) (x = 0, 0.125, 0.25, 0.375, 0.5, 0.75 and 1) samples were synthesized via solid state reactions. They were characterized by SEM, X-ray powder diffraction, FTIR, Raman, electrical and magnetic measurements. XRD data show impurity phases for x >= 0.25 samples due to boron content. Based on FTIR studies, the peak position of CoO6 shifts with increasing B content. The Raman spectra of LiCoO2 shows the presence of three broad bands, indicating the possible formation of spinel structure. The effective magnetic moments of the samples were calculated using Curie-Weiss law with temperature independent term. The M-H curves of x = 0.375 and 0.5 samples show very weak hysteresis loops. Effects of boron substitution on battery performance are also investigated. x = 0.125 and x >= 0.25 samples give promising results due to their enhanced reversibility of cycling compared to pristine LiCoO2. Capacity retentions for 100 cycles for x = 0, 0.125 and 0.25 samples are 22%, 81% and 83%, respectively. (C) 2015 Elsevier B.V. All rights reserved.Öğe Elucidating the optimized P2 type Na0.67Mn1-xCuxO2 cathode active material via operando XAS(Elsevier Science Sa, 2023) Altin, S.; Altundag, S.; Altin, E.; Vanpoucke, D. E. P.; Avci, S.; Ates, M. N.Here we report on the structural and electrochemical properties of P2-type Na0.67Mn1-xCuxO2 (where x = 0.20-0.50) via various techniques. X-ray diffraction (XRD) reveals a reduction of the unit cell volume upon substitution of Cu elucidated through detailed Rietveld analysis. The cyclic voltammetry (CV) behavior is also affected by the Cu substitution indicating new redox reactions stemming from Cu substitution. Galvanostatic cycling measurements at room temperature show that when x = 0.35 in a P2-type Na0.67Mn1-xCuxO2 cathode active material, the best electrochemical performance is obtained. The Na-ion diffusion rate is found to be strongly dependent upon the environmental temperature. Changes in the valence state and the local structures of P2-type Na0.67Mn1-xCuxO2 during the charge/discharge are in-vestigated through the operando X-ray absorption spectroscopy (XAS) technique. (c) 2022 Elsevier B.V. All rights reserved.Öğe Enhanced thermoelectric properties induced by chemical pressure in Ca3Co4O9(Elsevier Sci Ltd, 2014) Demirel, S.; Avci, S.; Altin, E.; Altin, S.; Yakinci, M. E.We report the investigation of boron substitution on structural, electrical, thermal, and thermoelectric properties of Ca-3_xBxCo4O9 (x=0, 0.5, 0.75, and 1) in the temperature range between 300 K and 5 K. X-ray diffraction studies show that the Ca3Co4O9 phase is successfully preserved as the majority phase in the x=0.5 sample despite the small size of boron ions. Electrical transport measurements confirm that B3+ substitution for Ca2+ causes an increase in resistivity due to the decrease in carrier concentration. x=0.5 sample is found to have a Seebeck coefficient of 181 mu V/K at room temperature which is similar to 1.5 times higher than that of the pure Ca3Co4O9. Our results indicate that the chemical pressure due to the large ionic radii difference between B3+ (0.27 angstrom) and Ca2+ (1 angstrom) enhances the thermoelectric properties as long as the unique crystal structure of Ca3Co4O9 is preserved. (c) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.Öğe Enhancement of battery performance of LiMn2O4: correlations between electrochemical and magnetic properties(Royal Soc Chemistry, 2016) Demirel, S.; Oz, E.; Altin, S.; Bayri, A.; Altin, E.; Avci, S.We report the results of a systematic investigation of structural, electrical transport, magnetic, and electrochemical properties of LiBxMn2-xO4 (where x = 0.0-1), synthesized via a one-step solid state reaction technique. We find that the parent compound (x = 0) has the non-stoichiometric Li1.05Mn2O4 phase and the B ions successfully incorporate the structure for x <= 0.5. The resistivity anomaly of the parent compound below 100 K is attributed to the antiferromagnetic correlations. The linear part of the chi(-1)-T data are used to calculate the effective magnetic moments and to determine the total number of Mn3+ and Mn4+ ions. Magnetization measurements show that Jahn-Teller active Mn3+ ions have high spin configuration. Average valance of Mn ions increases with increasing boron content. Electrochemical studies show improved capacity retention for x = 0.125 for 100 charge-discharge cycles. This improvement is attributed to subtle modifications in the structural and magnetic properties upon substitution.Öğe Fabrication of Ca-Mn-Nb-O compounds and their structural, electrical, magnetic and thermoelectric properties(Iop Publishing Ltd, 2018) Oz, E.; Demirel, S.; Altin, S.; Altin, E.; Baglayan, O.; Bayri, A.; Avci, S.CaMn1-xNbxO3-delta (0 <= x <= 1) were synthesized by conventional solid state reaction method. The structural properties were determined by FTIR, Raman, XRD, XAS measurements. The FTIR and Raman modes change by increasing Nb content and the lattice volume increases by increasing Nb content. The solubility limit of Nb is determined as x <= 0.3 and impurity phases start to appear above this limit. The temperature dependence of the magnetization data shows an antiferromagnetic transition below 120 K for low Nb content. Increasing the Nb content causes a change in the magnetic phase from antiferromagnetic to paramagnetic. The oxygen deficiency in CaMnO3-delta may cause the formation of polaron effect which is destroyed by the Nb ions. The difference graph of XAS data for x = 0 and 0.5 show that the number of Mn3+ ions increases by increasing Nb content to maintain the charge neutrality.Öğe Na0.67Mn0.33Ni0.33Co0.33O2: Effect of synthesis technique on competing P3 and P2 phases br(Elsevier, 2023) Ecer, S.; Altundag, S.; Altin, S.; Avci, S.In layered NaxTMO2 (TM = transition metal) common understanding attributes the emergence of P2 and P3 phases to the synthesis temperature. In this work, we show that the synthesis technique is the main responsible for these phases. We synthesize Na0.67Mn0.33Ni0.33Co0.33O2 with two techniques, solid-state and electrospinning, using two temperatures, 700 degrees C and 900 degrees C. Both electrospun samples sintered at 900 degrees C and 700 degrees C show single phase P2 and P3, respectively. Both solid-state synthesized samples at 900 degrees C and 700 degrees C show mixture of P2/P3. Electrochemical properties of the sample with only P3 phase has the highest initial capacity of 130 mAhg(-1) at C/ 3 rate. The sample with only P2 phase has very stable cycle performance with 87 % capacity retention over 100 cycles consistent with its large interlayer separation. Our results show that the synthesis technique plays a crucial role in the crystal structure and the electrochemical performance of the layered cathode materials for Na-ion batteries.Öğe Structural, magnetic, electrical and electrochemical properties of SrCo02.5, Sr9Co2Mn5O21 and SrMnO3 compounds(Elsevier Sci Ltd, 2017) Demirel, S.; Oz, E.; Altin, S.; Bayri, A.; Baglayan, O.; Altin, E.; Avci, S.SrCo1-xMnxO3-d (0 <= x <= 1) were synthesized by conventional solid state reaction method. We found that Mn substitution hinders the decomposition of carbonate at 900 degrees C and increases the synthesis temperature of SrCo1-xMnxO3-d to 1200 degrees C. Three different main phases emerge as the Mn content is increased; SrCoO2.5, Sr9Co2Mn5O21 and SrMnO3. Structural properties of the samples were investigated by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, x-ray diffraction (XRD) and scanning electron microscopy (SEM). Valance states of the transition metal (TM) ions were estimated based on lattice parameters and ionic radii of TM ions. High temperature resistivity measurements show hysteresis upon heating and cooling and is attributed to oxygen release without any endothermic/exothermic activity. The spin configurations of Co and Mn ions in SrCoO2.5, Sr9Co2Mn5O21. and SrMnO3 were estimated by Curie-Weiss fitting of X-T curves. Electrochemical measurements show two anodic/cathodic peaks indicating two main types of oxygen sites in the unit cell.Öğe Synthesis of Na2Ti3O7 nanorods by a V-assisted route and investigation of their battery performance(Royal Soc Chemistry, 2020) Altin, S.; Demirel, S.; Oz, E.; Altin, E.; Hetherington, C.; Bayri, A.; Avci, S.We report the V-assisted synthesis of Na2Ti3O7 nanorods via a conventional solid state reaction technique. Energy dispersive X-ray spectroscopy (EDS) mapping showed that V-ions are not incorporated into the main structure of the nanorods but rather V behaves as a flux agent during the growth of the nanorods. The cyclic voltammetry (CV) analysis of the samples shows changes in the redox peaks as a function of V content. Our detailed ex situ X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) analyses after 1000 cycles show that the degradation mechanism is the formation of various titanium oxide impurity phases which inhibits the Na-ion diffusion.Öğe Synthesis of ultra-thin nanobristles of Na-Mn-O compounds and their magnetic and structural properties(Elsevier Sci Ltd, 2016) Oz, E.; Demirel, S.; Altin, S.; Bayri, A.; Avci, S.Boron substituted Na0.44MnO2 nanorods were synthesized via conventional solid state reaction technique. Optimum synthesis temperature of the nanorods is determined as 750 degrees C. As the temperature increases, the nanorods start merging each other. As the boron content increases, amount of nanorods decrease due to the emergence of impurity phases. 12.5% boron substitution (x=0.25 in NaMn2-xBxO4 nominal composition) into Mn site striggers the formation of ultra-thin nanobristles on the surface of the nanorods. These nanobristles disappear completely in x=0.75 sample. We attribute the formation of these nanobristles to the increase in the internal energy upon increase in the unit cell volume. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
