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Obrovac, Mark N.

Permanent URI for this collectionhttps://hdl.handle.net/10222/27728

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  • ItemOpen Access
    Quantitative Measurement of Compositional Inhomogeneity in NMC Cathodes by X-ray Diffraction Postprint
    (IOPscience, 2023-08-14) Obrovac, Mark N.; Tahmasebi, Mohammad
    A novel X-ray diffraction (XRD) analysis technique is described for quantitatively measuring compositional inhomogeneity in Li[NixMnyCoz]O2 (NMC) cathode materials and NMC precursors. Single-phase rock salt precursors with varying degrees of compositional inhomogeneity were prepared by grinding mixtures of Ni, Mn, and Co oxides for different times and then heating. These precursors were then heated with lithium to form cathode materials. A modified Williamson-Hall analysis was used to measure the degree of compositional inhomogeneity in the precursors and the final NMC materials. This analysis showed that precursors made with low grinding times had higher compositional inhomogeneity and that this compositional inhomogeneity was amplified in the final NMC, leading to interlayer mixing and poor electrochemical performance. Higher precursor grinding times lead to more compositionally homogeneous NMC, while even higher compositional homogeneity was achieved by NMC made from conventional hydroxide precursors, with correspondingly improved electrochemical performance. The ability described here to measure the degree of compositional homogeneity in NMC precursors and NMC cathode materials by simple XRD measurements presents a powerful tool for the research and development of NMC and other cathode materials.
  • ItemOpen Access
    Hydrothermally Deposited Carbon Coatings for Li-Ion Battery Active Materials Postprint
    (IOPscience, 2023-08-14) Obrovac, Mark N.; Wang, Jun; Cao, Yidan
    Hydrothermally deposited carbon coatings were evaluated for Li-ion battery materials. The applied coatings were found to be highly uniform and porous, with a density of 1.89 g/ml and a surface area of 370 m2/g. In lithium cells, the hydrothermally deposited carbon was found to have a reversible capacity of 230 mAh/g when cycled between 7 mV and 0.9 V. Carbon coatings applied hydrothermally onto graphite particles were found to be effective at reducing surface reactions and increasing coulombic efficiency during cycling in lithium cells. However, because of the high surface area and high porosity of the hydrothermal coating it likely has limited applicability in commercial Li-ion cells.
  • ItemOpen Access
    Combinatorial Investigations of Ni-Si Negative Electrode Materials for Li-Ion Batteries
    (IOPscience, 2015-06-30) Obrovac, MN; Hatchard, T. D.; Du, Zhijia; Dunlap, RA
    Sputtered thin films in the Ni-Si system (0 ≤ x ≤ 0.65 in NixSi1-x) were studied for use as anode materials in Li-ion cells. All compositions were found to be amorphous. The Ni in Ni-Si films was found to suppress the lithiation voltage, resulting in a reduction in capacity. The delithiation voltage was not affected. No capacity was observed when Ni content was more than 50 at% because at this composition the lithiation voltage was suppressed to 0 V. In contrast to previous models of capacity in transition metal-Si films, all Si atoms were found to be active in Ni-Si films at all compositions. Capacity reduction is only caused by a suppression of the Si lithiation voltage. We attribute this voltage suppression to internal stress in the thin film during lithiation from the presence of Ni.
  • ItemOpen Access
    Lab-scale chemical vapor deposition onto powders
    (AIP Publishing, 2022-07-06) Wang, Jun; Obrovac, Mark N.
    In this paper, a laboratory chemical vapor deposition (CVD) reactor is described, which features an opposing screw rotating fluidized bed. The reactor efficiently concentrates powdered reactants in the reaction zone while maintaining fluidization independent of powder properties. This allows for lab-scale CVD processing of many small powder samples at high yields and without the need for re-adjustment of fluidization parameters for different samples. Alumina and graphite were carbon-coated and characterized in lithium batteries. The deposited carbon layer had a density of 1.84 g/ml and a capacity of 225 mA h/g when cycled between 7 mV and 0.9 V in lithium cells. Carbon coatings applied by this CVD reactor onto graphite particles were found to be effective at reducing surface reactions during cycling in lithium cells. We suggest that utilization of this opposing screw rotating fluidized bed reactor can effectively apply CVD coatings to small laboratory powder samples, with particular utility for Li-ion battery materials.
  • ItemOpen Access
    Li-ion Battery Negative Electrodes Based on the FexZn1-x Alloy System (Preprint)
    (Elsevier B.V., 2014-12-10) Obrovac, Mark N.; Dunlap, RA; MacEachern, L
    Thin-film Fe-Zn libraries were investigated as negative electrode materials for Li-ion batteries using combinatorial and high-throughput techniques. X-ray diffraction, Mössbauer effect spectroscopy and electron microprobe were used to characterize the library structure. A new Fe-Zn phase with an unknown structure was observed in the ζ (FeZn13) region on the phase diagram. The electrochemistry of FexZn1-x (0.01< x <0.45) was studied at 30ºC. The cycle life and coulombic efficiency improved as Fe concentration in electrodes increased to x = 0.12. However, the capacity decreased as the iron content increased and the FexZn1-x alloys became completely inactive when the Fe content was above 12 atom %. Ex-situ X-ray diffraction and Mössbauer measurements were used to explain the structural changes that occur during cycling. This is the first report of sputtered amorphous Fe-Zn alloys in the literature.
  • ItemOpen Access
    Investigation of the NaNixMn1-xO2 (0 ≤ x ≤ 1) System for Na-ion Battery Cathode Materials (Preprint)
    (IOP Publishing, 2015-01-06) Obrovac, Mark N.; Fielden, Ryan
    Layered NaNixMn1-xO2 (0 ≤ x ≤ 1) oxides were prepared via solid state reactions. Different reaction conditions were required to obtain phase pure samples, depending on the value of x. The 0 ≤ x ≤ 0.1 compositions were prepared in an inert argon atmosphere at 700°C and had a monoclinically distorted O'3 type structure. The 0.25 ≤ x ≤ 0.33 compositions were prepared in air at 850°C and had a P2-type structure. Compositions in the range of 0.5 ≤ x ≤ 0.66 were synthesized in air at 850°C and had an O3-type structure. Lastly, compositions with 0.9 ≤ x ≤ 1 were prepared in an oxygen atmosphere at 700°C and had a monoclinically distorted O'3 type structure. Electrochemical experiments were performed on pure phase samples. All showed reversibility of sodium ions and high capacities. The highest reversible capacity was achieved for x = 0.66, with a capacity of ∼190 mAh/g and an average discharge voltage of 3.07 V, corresponding to a high energy density of 2705 Wh/L. This is among the highest reported volumetric energy densities for Na-ion battery electrodes.
  • ItemOpen Access
    Mössbauer and Electrochemical Investigations of Carbon-Rich Fe1-xCx Films (Preprint)
    (2015 Elsevier Ltd., 2015-07-10) Obrovac, Mark N.; Xiuyun, Zhao; R.J., Sanderson; L, MacEachern; Dunlap, R. A.
    A thin film binary library of carbon-rich Fe1-xCx (0.47 ≤ x ≤ 0.97) alloys was prepared by combinatorial sputtering of carbon and iron. The sputtered library was characterized by X-ray diffraction and room temperature 57Fe Mössbauer effect spectroscopy to determine its microstructure. X-ray diffraction results show that the Fe1-xCx film is amorphous in the whole composition range of the library. For 0.52 ≤ x ≤ 0.59, a hyperfine field distribution and a quadrupole splitting distribution as obtained from Mössbauer spectra indicate the presence of a ferromagnetic phase and a paramagnetic phase in this regime. With increasing of carbon content, for 0.61 ≤ x ≤ 0.97, the sextet disappears and two paramagnetic doublets splitting appear suggesting two different Fe sites. The electrochemical performance of the Fe1-xCx film was investigated in lithium cells and the presence of Fe was found to increase the reversible capacity per mass of carbon over that of a pure carbon electrode.
  • ItemOpen Access
    The Reversible Magnesiation of Pb (Preprint)
    (Elsevier Ltd, 2015-03-04) Obrovac, Mark N.; Periyapperuma Achchige, Mary Kalani Erangi; Tran, Tuan T.; M.I, Purcell
    Sputtered Pb films have been found to reversibly alloy electrochemically with magnesium in Grignard based electrolytes. The voltage curve shows a single plateau at about 125 mV vs. Mg, corresponding to the formation of Mg2Pb, as confirmed by ex-situ X-ray diffraction. Pb was found to be the highest energy density Mg alloy yet reported, with the lowest voltage and highest volumetric capacity of any Mg alloy.
  • ItemOpen Access
    Mixed Transition Metal Titanate and Vanadate Negative Electrode Materials for Na-Ion Batteries (Preprint)
    (IOP Publishing, 2014-11-07) Obrovac, Mark N.; Brown, Z. L.; Smith, S
    Sodium-ion batteries have the potential to be a low cost, sustainable replacement for lithium-ion batteries in large scale energy storage. The lack of practical negative electrode materials limit the development of Na-ion batteries. In this study, mixed transition metal titanates and vanadates were synthesized and electrochemically characterized in Na cells as well as Li cells for comparison. Some of these materials were found to have volumetric capacities that far exceeded that of Li in graphite. CoV3O8, CoTiO3 and Ca5Co4(VO4)6 were found to have sodiation mechanisms that were not simple conversion reactions. CoTiO3 in particular had low hysteresis and good reversibility, which is suggestive of an intercalation mechanism.
  • ItemOpen Access
    A Combinatorial Investigation of Fe-Si-Zn Thin Film Negative Electrodes for Li-ion 2 Batteries (Preprint)
    (IOP Publishing, 2014-11-07) Obrovac, MN; Brown, Z. L.; Smith, S
    Sodium-ion batteries have the potential to be a low cost, sustainable replacement for lithium-ion batteries in large scale energy storage. The lack of practical negative electrode materials limit the development of Na-ion batteries. In this study, mixed transition metal titanates and vanadates were synthesized and electrochemically characterized in Na cells as well as Li cells for comparison. Some of these materials were found to have volumetric capacities that far exceeded that of Li in graphite. CoV3O8, CoTiO3 and Ca5Co4(VO4)6 were found to have sodiation mechanisms that were not simple conversion reactions. CoTiO3 in particular had low hysteresis and good reversibility, which is suggestive of an intercalation mechanism.
  • ItemOpen Access
    A Combinatorial Investigation of Fe-Si-Zn Thin Film Negative Electrodes for Li-ion 1 Batteries (Preprint)
    (IOP Publishing, 2014-12-04) Obrovac, Mark N.
    Thin-film Fe-Si-Zn libraries were investigated as negative electrode materials for Li-ion batteries using combinatorial and high-throughput techniques. X-ray diffraction and electron microprobe measurements were used to characterize the library structure. A large amorphous region exists in Si-rich compositions of this system. The electrochemistry of numerous compositions of Fe-Si-Zn materials was determined near room temperature. Both Zn and Fe additions to Si were found to suppress the formation of Li15Si4 during cycling.
  • ItemOpen Access
    Synthesis, Lithium Insertion and Thermal Stability of Si–Mo Alloys
    (The Electrochemical Society, 2020-10-05) Obrovac, MN; Cao, Simeng; Gracious, Shayne; Craig Bennett, J.
    Li insertion was investigated in SixMo100−x (90 ≥ x ≥ 70, Δx = 10) alloys prepared by mechanical ball milling. X-ray diffraction (XRD) and quantitative phase analysis were used to analyze phase compositions of these Si–Mo alloys, and how these phase compositions changed with milling times. The results of quantitative phase analysis showed that cr-Si converted into a-Si within 1 h during milling, and the Si–Mo reactions were nearly complete after only 4-h milling. During electrochemical cycling, the Si–Mo samples with high initial Mo contents and long milling times displayed good crystalline Li15Si4 suppression and stable cycling performance. In addition, thermal stability of some selected Si–Mo alloys was studied. The Si80Mo20 16 h alloy combines good thermal stability and a high volumetric capacity of about 1300–1400 Ah L−1 after heat treatment at 600 °C or 800 °C, which may allow the alloy to be further improved by carbon coating at high temperature.
  • ItemOpen Access
    Electrochemistry and Thermal Behavior of SiOx Made by Reactive Gas Milling
    (The Electrochemical Society, 2020-06-30) Obrovac, MN; Cao, Yidan; Dunlap, R. A.
    SiOx with various oxygen contents were synthesized from Si powder by a simple room-temperature ball milling method by controlling the air exposure time during milling. The resulting SiOx consists of nano and amorphous Si dispersed in an amorphous silicon oxide matrix. The oxygen saturated composition of SiO0.37 is thermally stable up to 800 °C and has improved cycling performance after annealing. The 1st irreversible capacity is reduced by high temperature annealing due to defect healing, while the high reversible capacity (1500–2000 mAh g−1 or 1600–1800 Ah l−1) is well maintained. This work demonstrates the thermal properties of SiOx made by reactive gas milling and how internal defects directly influence its electrochemistry.
  • ItemOpen Access
    Engineered Particle Synthesis by Dry Particle Microgranulation
    (Elsevier Inc, 2020-04-14) Obrovac, MN; Zheng, Lituo; Garayt, M.D.L.
    Highlights * A method of making monodisperse engineered ∼10-μm particles is introduced * Core-shell, spherical, and tetrahedral particles demonstrated at ∼100% yield * Spherical layered oxides for Li-ion batteries made from metal oxide powders * Spherical onion-type graphite synthesized in bulk from fine natural graphite flakes
  • ItemOpen Access
    Polyaniline Electrode Activation in Li Cells
    (The Electrochemical Society, 2020-04-17) Obrovac, MN; Charlton, Michael; Hatchard, T. D.
    Polyaniline (PANi) can be used as an electroactive organic cathode material in Li-cells with multiple redox states. The theoretical specific capacity of the emeraldine base (PEB) is 150 mAh g−1. In this study, we show how the specific capacity of PEB90−xCBxPVDF10 cathodes is dependent upon the mass percentage of PEB and carbon black (CB) in the potential range of 2.0–4.0 V. The capacity of PEB90−xCBxPVDF10 cathodes is initially very low, steadily increasing during initial cycling. A constant capacity is eventually reached with continued cycling. Microscopic imaging and elemental analysis of cycled electrodes reveals how the electrode composition and morphology of PEB90−xCBxPVDF10 cathodes plays an integral role in the magnitude of the specific capacity during the initial and steady state cycles. We propose a diffusion-limited model as an attempt to elucidate the differences in the total specific capacity among PEB90−xCBxPVDF10 cathodes. From our findings, we propose that increasing CB content provides more ion-diffusion channels throughout the PEB90−xCBxPVDF10 cathodes. This model is consistent with our findings in that more CB content reduces the amount of cycles required to reach steady state cycling and increases the magnitude of the specific capacity at steady state.
  • ItemOpen Access
    Spherically Smooth Cathode Particles by Mechanofusion Processing
    (The Electrochemical Society, 2019-08-27) Obrovac, MN; Zheng, Lituo; Wei, Congxiao; Garayt, M. D. L.; MacInnis, Judy
    Surface modification has been shown to be useful for improving the cycling performance of cathode materials. Typically hetero-compositional coatings are applied on cathode particle surfaces using methods, such as aqueous deposition and atomic layer deposition (ALD), that can be expensive and inefficient. In this report, a dry mechanofusion method was used to treat cathode particles with no auxiliary coating applied. This resulted in a drastic reduction in surface area and the elimination of surface features on the particles. Furthermore, the processing results in an iso-compositional amorphous shell on the surface of each particle. The resulting particles have improved cycling. We believe the mechanofusion process is an important step toward the goal of improving the cycling performance of cathode materials.
  • ItemOpen Access
    Quantitative Determination of Carbon Dioxide Content in Organic Electrolytes by Infrared Spectroscopy
    (The Electrochemical Society, 2019-07-09) Obrovac, MN; YU, HAONAN
    CO2 has been shown to be an effective additive to improve the cycling characteristics of silicon negative electrodes for Li-ion batteries. However, a quantitative technique for measuring the CO2 content in electrolyte is not readily available. Here, FTIR was used to accurately determine the CO2 content of various carbonate-based Li-ion battery electrolytes. The accuracy of this method was validated with the weight variation method. Using this method, it was found that in EC/DEC electrolytes with 1M LiPF6 and LiTFSI salts, CO2 was found to have a maximum solubility of 0.37 wt% when the LiPF6:LiTFSI molar ratio is 3:1. In 1M LiPF6 solutions, CO2 solubility is in the order of PC ≈ EMC > DEC for the pure solvents. Mixed non-polar solvents are also favorable for CO2 dissolution. When used in silicon alloy cells, electrolytes saturated with CO2 (0.33% CO2 in EC/DEC) resulted in the best cycling stability.
  • ItemOpen Access
    Stabilizing NaCrO2 by Sodium Site Doping with Calcium
    (The Electrochemical Society, 2019-06-14) Obrovac, MN; Zheng, Lituo; Bennett, J. C.
    Layered cathode materials based on abundant, low cost raw materials have garnered interest in recent years. O3-type NaCrO2 is a promising cathode material as it offers decent energy density and is easy to synthesize. In this study, calcium doped NaCrO2 was synthesized using a solid-state method and the resulting [Na1-2xCax]CrO2 materials were studied in sodium cells. Compared to calcium-free NaCrO2, [Na0.9Ca0.05]CrO2 has improved capacity retention without sacrificing reversible capacity. The enhanced performance was ascribed to structural stabilization of NaCrO2 by Ca-doping, as observed by ex-situ X-ray diffraction. Finally, calcium doped NaCrO2 was also found to have improved air-stability than the pristine material.
  • ItemOpen Access
    An Investigation of the Fe-Mn-Si System for Li-Ion Battery Negative Electrodes
    (The Electrochemical Society, 2019-01-03) Obrovac, MN; Cao, Yidan; Scott, Benjamin; Dunlap, R. A.; Wang, Jun
    Fe-Mn-Si alloys prepared by ball milling were investigated as negative electrodes for Li-ion batteries. X-ray diffraction and room temperature 57Fe Mössbauer measurements were used to characterize alloy structure, which revealed the formation of ternary (Fe,Mn,)11Si19 solid solutions. During cycling in Li cells, the formation of crystalline Li15Si4 was effectively suppressed by inactive transition metal silicide phases, resulting in good cycling performance. In addition, Fe was found to be superior to Mn in avoiding excessive side reactions with electrolyte during cycling.
  • ItemOpen Access
    Li Insertion in Ball Milled Si-Mn Alloys
    (The Electrochemical Society, 2018-06-08) Obrovac, MN; Cao, Yidan; Craig Bennett, J; Dunlap, R. A.
    Si1-xMnx (0 ≤ x ≤0.5, x=0.05)alloyswereprepared by ball milling and studied as negative electrode materials in Li cells. These alloys were found to be unique amongtransition metals. When x ≤ 0.35, the alloys are essentially homogeneous with nanocrystallites of Si19Mn11 uniformly dispersed in a matrix of amorphous silicon. Nanocrystallites of SiMn and Si19Mn11 coexist in samples with x > 0.35. The formation of Li15Si4 during lithiation was totally suppressed for alloys with x ≥ 0.3. Cycling performance was significantly improved by Mn addition, especially for the samples in which the formation of Li15Si4 was suppressed. The addition of Mn increases the particle size, which results in reduced surface area. Capacity trends suggest that inactive Si19Mn11 and SiMn phases were formed in these alloys, consistent with XRD results. The x = 0.25 alloy achieved a volumetric capacity of 1526 Ah/L with 95% capacity retention after 50 cycles.