Symposium Organizers
Enrico Traversa University of Rome Tor Vergata
Timothy Armstrong Carpenter Technology Corporation
Koichi Eguchi Kyoto University
M. Rosa Palacin Institut de Ciencia de Materials de Barcelona (CSIC)
S1: Micro SOFCs: From Materials to Devices
Session Chairs
Tim Armstrong
Enrico Traversa
Monday PM, December 01, 2008
Back Bay A (Sheraton)
9:30 AM - **S1.1
Investigation of Cathode Behavior of Model Thin Film SrTi1-xFexO3-δ Mixed Ionic-Electronic Conducting Electrodes.
Harry Tuller 1 , WooChul Jung 1
1 Materials Science & Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractThe defect structure, transport properties and band structure of the perovskite solid solution SrTi1-xFexO3-δ(STF) have recently been refined and reported1. Of particular interest is the ability to systematically increase both the electronic and ionic conductivity, over orders of magnitude, by increase of the Fe fraction on the Ti site. This suggests that STF may serve as a model cathode material, given the ability to systematically vary levels of mixed ionic-electronic conductivity in the system by control of x. STF cathodes, with compositions x = 0.05 to 1, prepared as dense films by pulsed layer deposition (PLD) were therefore investigated by electrochemical impedance spectroscopy (EIS) as a function of electrode geometry, temperature and oxygen partial pressure. The STF cathode was observed to exhibit typical mixed ionic-electronic behavior with the electrode reaction occurring over the full electrode surface area rather than being limited to the triple phase boundary. The electrode impedance was observed to be independent of electrode thickness and inversely proportional to the square of the electrode diameter pointing to surface exchange limited kinetics. Furthermore, a GDC interlayer was found to have no effect on the electrode impedance. Values for the surface exchange coefficient, κ, were calculated and found to be comparable in magnitude to those exhibited by other popular mixed ionic-electronic conductors such as (La,Sr)(Co,Fe)O3, thereby, confirming the suitability of STF as a model mixed conducting cathode material. The observed trends are discussed in relation to the known defect and transport properties of STF._____1. A. Rothschild, W. Menesklou, H. L. Tuller, and E. Ivers-Tiffee, Chem. Mater., 18, 3651 (2006).
10:00 AM - S1.2
Local Electronic Structure and Surface Chemistry of Solid Oxide Fuel Cell Cathodes.
Bilge Yildiz 1 , Burc Misirlioglu 1 , Richard Schalek 2 , Balasubramaniam Kavaipatti 3 , Paul Salvador 3
1 Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Center for Nanoscale Systems, Harvard University, Cambridge, Massachusetts, United States, 3 Materials Science and Engineering, Carnegie Mellon University, Pittsburgh,, Pennsylvania, United States
Show AbstractSurface structure has a major role on the electrocatalytic activity of the solid oxide fuel cell (SOFC) cathodes. We are investigating the correlations of the crystallographic structure, cation composition, and lattice strain to the electronic structure, defect chemistry, and electronic and ionic transport characteristics of cathode surfaces. The understanding and tailoring of the activity at the “inhomogeneities” on the surfaces, namely grain boundaries and surface segregates, is important for our research.We are employing variable tempearture Scanning Tunneling Microscopy and Spectroscopy (STM/STS) to probe the localized topological and electronic properties at the nanoscale confined features and inhomogeneities on the cathode surfaces. We correlate the surface structure and electronic properties found by STM/STS to electrocatalytic activity using electrochemical characterization and Auger electron nano-spectroscopy. Our prototype cathode structures include the epitaxial single crystal and textured polycrystalline dense thin-films in the presence of nano-scale grain boundaries. The dense thin film cathode samples consisted of 10-200 nm thick La0.7Sr0.3MnO3 (LSM) and La0.7Sr0.3CoO3 (LSC) on single crystal yttria stabilized zirconia and 0.7% Nb-SrTiO3 substrates.We found that an in-plane retexturing of the dense thin film cathodes in the form of small rectangular islands has occurred upon electrochemical treatment at 300 mA/cm2 and 800C. The restructuring can be related to the preferential stabilization of an in-plane crystallographic epitaxy domain that LSM and LSC can form on the YSZ upon high-temperature treatment. Furthermore, we observed that fine segregate particles of few-nm size exist on the surfaces of the 100nm LSM film. These fine segregates can be related to the cation enrichment that we identified upon depth profiling with Auger Electron Spectroscopy.We found that the Fermi-level tunneling conductance of the surfaces exhibited thickness dependence. The thin 10-50 nm LSM films have zero near-Fermi level tunneling conductance with an electronic band-gap of 1.5-2.2 V, while the thick 100 nm LSM displays a large spread in the number of surface sites with dI/dV greater than zero. We expect that the 100 nm thick LSM have higher activity due to more available electronic states to exchange electrons with oxygen on its surface. Non-zero tunneling conductance on the 100nm-LSM film can be a consequence of more favorable relaxation of the local and overall strain states. Electrochemical activity of these electrodes ranked consistently with the findings from the STM/STS analysis. Increasing the temperature upto 680K have shown a spread of band-gap values on the surface, with a fixed conduction band lower level and varying valance band and Fermi level, with a more semiconductor-like behavior at 680K and 10-8 bar. We will continue to probe the evolution of spatially localized surface structure and electronic behavior on SOFC cathodes upto 1100K.
10:15 AM - S1.3
In situ Characterization of Equilibrium Strontium Surface Segregation in Epitaxial La0.7Sr0.3MnO3 at Solid Oxide Fuel Cell Conditions.
Tim Fister 1 , Matthew Highland 1 , Marie-Ingrid Richard 1 , Dillon Fong 1 , Jeffrey Eastman 1 , Peter Baldo 1 , Paul Fuoss 1 , Kavaipatti Balasubramaniam 2 , Joanna Meador 2 , Paul Salvador 2
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 2 Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractLa1-xSrxMnO3 (LSMO) is a widely used cathode material in solid oxide fuel cells (SOFCs) because of its excellent catalytic properties, good electronic conductivity, and chemical stability at high temperatures. In each case, the relevant properties depend on the interplay between the strontium concentration, the manganese charge state, and oxygen nonstoichiometry. Surface composition is particularly important in controlling both the catalytic and transport properties of LSMO thin films and heterostructures, but previous studies have only probed nonequilibrium states far from typical SOFC operating conditions. We employ in situ synchrotron measurements of total reflection x-ray fluorescence to monitor the behavior of (001)-oriented La0.7Sr0.3MnO3 thin film surfaces under a wide range of temperatures (25-900°C) and oxygen partial pressures (pO2 = 0.15-150 Torr). The strontium surface concentration is observed to increase with decreasing pO2 at all measured temperatures, suggesting that the surface oxygen vacancy concentration plays a significant role in controlling the degree of segregation; in contrast, La0.7Sr0.3MnO3 film thickness and epitaxial strain state display little impact on segregation behavior. Interestingly, the enthalpy of segregation becomes less exothermic with increasing pO2, varying from -9.5 to -2.0 kJ/mol over the range of measured conditions. We also observe that the surface strontium enrichment persists in LaMnO3-capped LSMO thin films, suggesting that the segregation occurs during the growth process, possibly driven by the requirement of providing charge compensation of the polar LaMnO3 surface. Using these results, we discuss the impact of surface polarity and point defect chemistry on the performance of solid oxide fuel cells.This work is supported by the U.S. Department of Energy (DOE) Strategic Energy Conversion Alliance (SECA) program, and at Argonne by DOE under contract DE-AC02-06CH11357.
10:30 AM - S1.4
Application of Nanoscaled LSC Thin Film Cathodes in SOFCs: FEM Modelling and Experimental Verification.
Jan Hayd 1 , Ellen Ivers-Tiffee 1 , Bernd Rueger 1 , Uwe Guntow 2
1 Institute of Materials for Electrical Engineering (IWE) , Universität Karlsruhe (TH), Karlsruhe Germany, 2 , Fraunhofer-Institut für Silicatforschung, Würzburg Germany
Show Abstract10:45 AM - S1.5
Oxygen Exchange Kinetics of Epitaxial PrBaCo2O5+x and LaBaCo2O5+x Thin Films.
Xuening Jiang 1 , Shuangyan Wang 1 , Guntae Kim 1 , Jian Liu 2 , M. Liu 2 , Wenquan Gong 1 , Chonglin Chen 2 , Allan Jacobson 1
1 Chemistry, University of Houston, Houston, Texas, United States, 2 Physics, University of Texas at San Antonio, San Antonio, Texas, United States
Show AbstractReducing the operating temperature of solid oxide fuel cells from 1000 oC to intermediate temperatures (IT, 500-700 oC) has been a research focus in recent years. At ITs, cathode polarization of conventional (La,Sr)MnO3 becomes prohibitively large, therefore, cathode materials with faster oxygen transport and exchange kinetics are desired. Oxygen-deficient double perovskite with mixed oxygen ionic-electronic conductivities are promising cathode materials for use with ceria based electrolytes. In our work, we prepared dense epitaxial PrBaCo2O5+x (PBCO) and LaBaCo2O5+x (LBCO) thin films on SrTiO3 (STO) and LaAlO3 (LAO) single crystalline substrates by Pulsed Laser Deposition (PLD). The rocking curve measurements indicated an excellent epitaxial nature for these films. The oxygen exchange kinetics of PBCO and LBCO thin films have been determined by electrical conductivity relaxation (ECR). Surface oxygen exchange coefficients and/or bulk oxygen diffusion coefficients were obtained by fitting the ECR profiles. The results for PBCO and LBCO thin films will be compared.
11:00 AM - S1:mSOFCs
BREAK
11:30 AM - **S1.6
Micro-Solid Oxide Fuel Cells: From Thin Films to Power Delivering Membranes.
Jennifer L.M. Rupp 1 , Anja Bieberle-Huetter 1 , Anna Evans 1 , Henning Galinski 1 , Ashley Harvey 1 , Thomas Ryll 1 , Barbara Scherrer 1 , Rene Tolke 1 , Ludwig Gauckler 1
1 Nonmetallic Inorganic Materials, ETH Zurich, Zurich Switzerland
Show Abstract12:00 PM - S1.7
Deposition of a Proton Conductive Oxide Thin Film on a Porous Stainless Steel Substrate using Liquid Delivery MOCVD.
Kiyoshi Uchiyama 1 , Hiroyuki Sakairi 1 , Hiroshi Ichise 1 , Gakuko Lin 1 , Tadashi Shiosaki 1 , Hiroki Ikeda 2 , Katsu Yanagimoto 2 , Akira Nakayama 3 4 , Yoji Yamada 3
1 School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara, Japan, 2 , Sanyo Special Steel, Himeji Japan, 3 , Ion Engineering Research Institute, Hirakata Japan, 4 , Osaka Scienece & Technology Center, Osaka Japan
Show AbstractSolid oxide fuel cells (SOFCs) are considered to be one of the solutions for future alternative energy sources of automobiles and home electricity. However, one of the issues for enlarging their applications is reducing their operating temperatures. Generally, they need about 800°C for and it is too high to use general metallic materials, i.e. stainless steel, as their components. As a result, device costs of SOFCs become too expensive to use for consumer applications. In 2004, one solution for reducing operating temperatures was proposed by Toyota Motor Corp. In their new SOFC, they adopted a proton conductive oxide, i.e. doped-BaCeO3, as an electrolyte which operates as low as 500°C. In addition, this kind of SOFCs brings other benefit. In this configuration, the water vapor is generated in the air side and no mixing of the water and hydrogen occurred in a fuel gas. This makes an SOFC device structure much simpler and reducing a device cost. However, this proton conductive type SOFC still has some issues for realizing actual SOFC applications. One of the issues is reducing the electrolyte resistibility. The proton conductive material has about 10-2-10-4S/cm and it is said at least 10-1S/cm is required for the SOFCs. The second issue is what kind of materials are suitable as an electrode. In the Toyota’s SOFC, they used 100μm-thick Pd as a bottom electrode. They needed 100μm thickness for Pd because it also acts as a supporting material of the MEA (Membrane Electrode Assembly) itself. However, the use of 100μm-thick Pd results in higher SOFC costs. In order to solve these issues, we proposed a new MEA structure with proton conductive thin films on a porous stainless steel substrate. A proton conductive films as thin as 200nm brings a lower resistibility of the electrolyte and the use of porous stainless steel as a substrate also solves cost issues. However, it was quite difficult to deposit such kind of thinner oxide films on the porous stainless steels. Sol-gel and sputtering could not fabricate fine films and only MOCVD could fabricate dense and defect-less oxide thin films. The films do not show leaky properties for gases at all. In this paper, we will describe the deposition conditions and film properties in detail. Acknowledgements: This work was supported by the Japan Science and Technology Agency (JST) and the Foundation for NAIST. We also thank to ADEKA Corp. for providing MOCVD precursors.
12:15 PM - S1.8
Microstructure and Electrochemical Properties of Epitaxial Samarium Doped Ceria (SDC)-SrTiO3 (STO) Buffered-MgO Films by Pulsed Laser Deposition.
Simone Sanna 1 2 , Vincenzo Esposito 1 , Silvia Licoccia 1 , Giuseppe Balestrino 2 , Enrico Traversa 1
1 NAST Center & Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma Tor Vergata, Rome Italy, 2 INFM – CNR Coherentia and Dipartimento di Ingegneria Meccanica, Università di Roma Tor Vergata, Rome Italy
Show AbstractSolid oxide fuel cells (SOFCs) can be miniaturized for portable applications (micro-SOFCs) leading to energy power density 4-times larger then the state-of-the-art batteries. This needs the reduction of the operative temperature and therefore the development of materials in thin-film form. Samarium oxide doped ceria (SDC) has much interest as electrolyte for IT-SOFCs due to its high-ionic conductivity compared to other conventional materials such as yttria stabilized zirconia (YSZ). In the relevant literature, many works are reported about gadolinium doped ceria (GDC) and SDC grown by pulsed laser deposition (PLD) on several perovskite substrates such as SrTiO3 (STO) providing a good lattice match for the fluorite structure of ceria. However, STO substrates are not stable at high temperatures and might show electronic conduction at temperatures above 300 °C.In this work, samarium oxide 20%-doped ceria, and STO thin films were grown by pulsed-laser deposition (PLD) on single crystal (001) MgO substrates. The STO thin film was used as buffer layer to provide the proper crystalline match to grow epitaxial SDC fluorite on the (001) MgO. The prepared thin films were analyzed by X-ray diffraction (XRD). Microstructures and geometrical features of the SDC/STO on (001) MgO layered system were observed by Field Emission Scanning Electron Microscopy (FE-SEM). SDC film thickness was around 250 nm and it resulted (001) mono-oriented and grain-boundary-free. Finger-type Au electrodes were deposited on the SDC surface at a distance of about 2 mm. Electrochemical impedance spectroscopy (EIS) measurements showed large ionic conductivity and no interfacial blocking effects. Ionic and electronic conductivity were separated by varying the oxygen partial pressure (pO2) in the 10-25 to 1 atm range at several temperatures (400-700 °C). SDC/STO films on (100) MgO showed dominant ionic conductivity up to pO2 ≈10-15 atm with ionic conductivity around 5 10-2 S cm-1 at 600 °C. The very low electronic conductivity demonstrated the lack of electronic contribution given by the STO buffer layer.
12:30 PM - S1.9
Sol-gel and PLD Preparation of Lanthanum Gallate-based Mixed Ionic-electronic Conducting Oxides.
Natalia Golubko 2 , Galina Kaleva 2 , Yuliana Roginskaya 2 , Sergey Kabanov 2 , Alexander Avetisov 2 , Gunnar Suchaneck 1 , Ekaterina Politova 2
2 , L.Y.Karpov Institute of Physical Chemistry, Moscow Russian Federation, 1 Solid State Electronics Lab, TU Dresden, Dresden Germany
Show Abstract12:45 PM - S1.10
Enhanced Oxide Ion Conductivity at (Y2O3)x(ZrO2)1-x /SrTiO3 Ultra Thin-Film Epitaxial Heterostructures.
Javier Garcia-Barriocanal 1 , Alberto Rivera-Calzada 1 , Maria Varela 3 , Zouhair Sefrioui 1 , Enrique Iborra 2 , Carlos Leon 1 , Steve Pennycook 3 , Jacobo Santamaria 1
1 Dpto. Fisica Aplicada III, Universidad Complutense Madrid, Madrid Spain, 3 Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Escuela Técnica Superior de Ingenieros de Telecomunicaciones, Universidad Politécnica de Madrid, Madrid Spain
Show AbstractIt has been recently reported a systematic increase of the oxide ion conductivity in thin films of yttria stabilized zirconia (Y2O3)x(ZrO2)1-x (YSZ) when decreasing their thickness below 60 nm down to 15nm [1]. These results could be explained by assuming that the oxide ion conductivity is several orders of magnitude higher at the interface between the substrate and the YSZ thin film than for bulk YSZ. In order to test this attractive scenario of a highly oxygen conducting interfacial plane, we have grown heterostructures where ultra thin YSZ layers (with 8 %mol nominal yttria content) of thickness down to 1 nm, were sandwiched between two layers of insulating SrTiO3 (STO) 10 nm thick. Also, superlattices were grown alternating the same thickness of YSZ and STO as in the trilayers. YSZ and STO layers were grown on STO (100) substrates in a high pressure (3 mbar) pure oxygen RF sputtering system. High pressure and high substrate temperature (900 °C) ensures a slow (1 nm/min) and highly thermalized growth of complex oxides providing excellent epitaxial properties [2, 3]. The structural characterization by XRD and STEM has evidenced the high quality of our epitaxial heterostructures, and that the interfaces between YSZ and STO layers are continuous and atomically flat over long lateral distances (a few microns). Lateral electrical conductivity measurements show an enhanced oxygen conductivity in ultra thin film heterostructures, which for 1 nm thick YSZ layers is found to be as high as 0.014 S/cm at 357 K (about eight orders of magnitude higher than that of bulk YSZ), with a substantial decrease of the activation energy for the dc ionic conductivity from 1.1 eV down to 0.64 eV. EELS analysis is consistent with a partial occupancy and high disorder in the interface oxygen plane between YSZ and STO layers, which would yield a large number of interfacial oxygen vacancies and simultaneously give rise to a decrease in the activation energy for oxygen migration. Thus, our results demonstrate that the observed high oxygen conductivity in ultra thin layers is a genuine interface process, and that the design of suitable heterogeneous interfaces in epitaxial heterostructures might have important implications in the search of artificial nanostructures with high ionic conductivity for application in solid oxide fuel cells or other technological devices.[1] I. Kosacki, C. M. Rouleau, P. F. Becher, J. Bentley, and D. H. Lowndes, Solid State Ionics 176, 1319 (2005).[2] M. Varela, W. Grogger, D. Arias, Z. Sefrioui, C. León, C. Ballesteros, K. M. Krishnan, and J. Santamaría, Phys. Rev. Lett. 86, 5156 (2001).[3] V. Peña, Z. Sefrioui, D. Arias, C. Leon, J. Santamaria, J. L. Martinez, S. G. E. te Velthuis, and A. Hoffmann, Phys. Rev. Lett. 94, 57002 (2005).
S2: Modelling and Fundamental Studies
Session Chairs
Monday PM, December 01, 2008
Back Bay A (Sheraton)
2:30 PM - **S2.1
Solid State NMR Studies of Lanthanum Gallate Solid Electrolytes: Ion Mobility and Trapped Defects.
Frederic Blanc 1 , John Palumbo 1 , Clare Grey 1
1 Chemistry, Stony Brook University, Stony Brook , New York, United States
Show AbstractNew materials with improved ionic conductivity at moderate temperatures, and hence materials with higher numbers of mobile oxygen-ion vacancies O2- (the charge carriers), are required in order to lower the operating temperature of a solid oxide fuel cell (SOFC). Often dopant ions Mn+ are substituted for higher valent ions, M(n+1)+ or M(n+2)+ in order to create these vacancies. Typically, the dopant sits on a normal crystallographic site, so it is not straighforward to distinguish between the mobility near the dopant and near the normal site using diffraction techniques. To date, the electrolyte of choice for SOFC is yttrium stabilized zirconia (YSZ). However, lanthanum gallate compounds are under consideration for their lower operating temperature and better ionic conductivities. Lanthanum gallate, LaGaO3, adopts the perovskite structure ABO3 and can accommodate a wide range of dopants on both the A and B sites to induce vacancies. In particular, the combination of Sr2+ doping on the La3+ (A) site and Mg2+ doping on the Ga3+ (B) site (thereafter named LSGM) has been shown to result in the highest increase in ionic conductivity in the series, the conductivity for La0.9Sr0.1Ga0.9Mg0.1O2.9 (0.1 S.cm-1 at 800 °C) being approximately an order of magnitude higher at similar temperatures than that achieved with only either Mg2+ or Sr2+ doping (T. Ishihara et al. J. Am. Chem. Soc. 1994, 116, 3801-3803 and Solid State Ionics 1995, 79, 147-151). Clearly the vacancies cannot be observed directly by Nuclear Magnetic Resoance (NMR) spectroscopy. However, it is possible to identify the cation sites with lower coordination environments than those of the bulk. We, therefore, use a combination of different NMR experiments involving various nuclei such as 139La, 69/71Ga, 25Mg and 17O NMR to examine the local structures in these materials. 69/71Ga is a relatively straightforward (I = 3/2) nucleus to study by NMR and like 27Al, its chemical shift correlates with coordination number (T. J. Bastow et al. Solid State Ionics 2004, 175, 129-133 and D. Massiot et al. Solid State NMR 1995, 4, 241-248) and lineshapes analysis which will give insight into the presence of vacancies. These experiments are performed in parallel with 25Mg NMR. This (I = 5/2) nucleus has a quadrupole moment that is similar to that of 27Al, its disadvantages being its low natural abundance (10 %) and its low gyromagnetic ratio. Nonetheless, a series of recent experiments have identified the chemical shift ranges for this nucleus and have shown that typical quadrupole coupling constants are not prohibitive. Multiple Quantum Magic Angle Spinning (MQMAS, L. Frydman and J.S. Harwood J. Am. Chem. Soc. 1995, 117, 5367-5368 and A. Medek et al. J. Am. Chem. Soc. 1995, 117, 12779-12787) and Satellite Transition Magic Angle Spinning (STMAS, S.E. Ashbrook and S. Wimperis P. Nucl. Mag. Res. 2004, 45, 53-108) NMR will be used, where necessary, to help separate the signals from the different coordinate environments. 17O Magic Angle Spinning (MAS) NMR (including MQMAS and others techniques) on 17O-enriched samples are used to identify the Ga-O-Ga and Ga-O-Mg environments and probe the mobility.
3:00 PM - S2.2
Influence of the Chemical Strain Effect on the Lattice Parameter of Ce0.8Gd0.2O1.9.
Anna Kossoy 1 , Isai Feldman 2 , Roman Korobko 1 , Ellen Wachtel 2 , Joachim Maier 3 , Igor Lubomirsky 1
1 Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot Israel, 2 Department of Chemical Research Support, Weizmann Institute of Science, Rehovot Israel, 3 , Max Planck Institute for Solid State Research, Stuttgart Germany
Show AbstractStrong deviation from linear elasticity observed in thin films of Ce0.8Gd0.2O1.9 is attributed to the association/dissociation of point defects, presumably aliovalent ions and oxygen vacancies (so-called “chemical stain effect”). We investigated the kinetics and the magnitude of the chemical strain effect in substrate-supported and self-supported films of Ce0.8Gd0.2O1.9. The direct measurements of stress and strain in the substrate-supported films indicate that the “apparent” elastic modulus of these films is more than ten times lower than that measured in bulk ceramics, which matches our previous data obtained with self-supported films. By monitoring the rate of the chemical strain effect in substrate-supported and in self-supported films, it was found that the activation energy of the defect reaction responsible for the chemical strain effect is about 1 eV. In addition, on the basis of temperature-dependent XRD measurements, it was concluded that the complexes of the aliovalent ions and oxygen vacancies completely dissociate above 300 °C. From these data and from the strain-temperature dependence, it was determined that the formation of the aliovalent ion - oxygen vacancy complex is accompanied by volume increase (i.e. the volume effect of the complex formation is positive).Our findings may explain large discrepancies in the literature data on the lattice parameter of Ce0.8Gd0.2O1.9. Since, the time necessary to establish the defect equilibrium at room temperature is of the order of a few months, the “observable” lattice parameter may differ by a few tenths of a percent depending on the sample prehistory and preparation route.
3:15 PM - S2.3
First-Principles Study of ZrO2 – In2O3 Heterointerface Structure and Properties.
Hakim Iddir 1 , Peter Zapol 1 , Fong Dillon 1 , Paul Fuoss 1 , Jeffrey Eastman 1
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractOxide heterostructures of ZrO2 and In2O3 have potential for producing novel mixed conduction properties, since they exhibit ionic and electronic types of conductivity, respectively. Since these oxides exhibit poor solid miscibility and good epitaxial match, they can form nanostructured materials, which would be of interest for a wide range of applications, including solid oxide fuel cells. A first-principles study of ZrO2 – In2O3(001) interfaces is performed to identify the stable configurations under different oxygen and electron chemical potentials. Two different interfaces, each sharing a common oxygen plane, are considered for the (001) orientation: (1) a flat interface with two types of In atoms (8b and 24d) present at the interface, and (2) a buckled interface with one type of In atom (24d). We present results on the atomic and electronic structure of different interfaces as a function of oxygen partial pressure. Theoretical studies of oxygen vacancy diffusion at the interface have also been carried out, providing insight into the effects of interfacial proximity on optical and conduction properties. The results are compared with those from synchrotron x-ray scattering and transport measurements on sputter-deposited ZrO2 – In2O3 heterostructures. Finally, we discuss the impact of Y and Sn dopants in ZrO2 and In2O3, respectively, on interfacial segregation behavior and electrical transport. This work is supported by the U.S. Department of Energy under Contract No. DE-AC02-06CH11357.
3:30 PM - S2.4
Examination of the LaFeO3 (010) Surface using Density Functional Theory and Thermodynamics.
Chan-Woo Lee 1 , Eric Wachsman 1 , Simon Phillpot 1 , Ram Devanathan 2 , Susan Sinnott 1
1 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractElectronic structure and thermodynamic calculations are combined to understand the stability of the LaFeO3 (010) surface, which is the base material of La1-xSrx CoyFe1-yO3 (LSCF), a potential candidate for solid oxide fuel cell (SOFC) cathodes. In particular, classical thermodynamics are combined with density functional theory calculations to predict a surface phase diagram of the LaFeO3 (010) plane of varying terminations. Specifically, the LaO3 -, LaO2 -, LaO -, La -, FeO3 -, FeO2 -, FeO - and Fe - terminations are considered. Preliminary calculations indicate that the FeO2 - and LaO2 - terminated surfaces are stable under typical operating conditions for SOFCs: PO2 ≈ 0.21 atm and T ≈ 900 – 1300 K. In addition, charge compensation mechanisms of the (010) plane with varying PO2 and T are investigated. The mechanisms of molecular oxygen adsorption and dissociation on the various terminations with and without intrinsic and extrinsic defects, are also examined.
3:45 PM - S2.5
Ab initio Thermokinetic Modeling of Defect Chemistry, Oxygen Transport, and Surface Oxygen Reduction Reactions for (La,Sr)MnO3 for Solid Oxide Fuel Cells.
Yueh-Lin Lee 1 , Dane Morgan 1
1 Materials Science Program, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractPerovskite (La,Sr)MnO3 (LSM) is the primary cathode catalyst used in commercial solid oxide fuel cells (SOFC). Essential to its use in SOFCs is the oxygen reduction reaction (ORR) on the surface and oxygen ion transport during operation. Ab initio based thermokinetic models are being developed to investigate bulk LSM defect chemistry, bulk vs. surface diffusivity, and surface ORR to understand the reaction mechanisms and rate-limiting steps. The bulk defect reaction energies and interactions have been calculated using density functional theory (DFT) for a wide range of defect species (e.g., antisites and vacancies). Our defect reaction results show that Sr and La antisite defects are unstable but that Mn antisites are quite low energy, and vacancy formation on the Mn sublattice is much easier than for the La sublattice. Defect interaction results reveal that the defects in this system are highly non-ideal, with nearest-neighbor repulsion and attraction energies as large as 1-2 eV. This suggests that non-ideal defect interactions will play a significant role in LSM defect behavior both thermodynamically (e.g. nonstoichiometry vs P (O2)) and kinetically (e.g. ionic transport), especially at low temperatures. We have explore the most stable {100} LSM surfaces, where vacancies are found to be stabilized by well over 1 eV compared to bulk, implying an enormous increase in vacancy content in near surface layers compared to bulk. Preliminary results using the an ideal solution defect model have demonstrated that near-surface transport can be ~106 times faster than the bulk, mediated by the higher vacancy content. The key energy terms that must be determined to model the surface ORR are the oxygen adsorption, O2 dissociation, surface vacancy, and oxygen hopping energies. These are difficult to determine experimentally and have been calculated directly by ab initio methods. Ab initio ORR energetics are being incorporated into the surface model to determine rates of LSM surface ORR processes vs. oxygen partial pressure and temperature.
4:30 PM - S2.6
First-principles and CALPHAD Thermodynamic Modeling of Defects in La1-xSrxCoO3-δ.
James Saal 1 , Mei Yang 1 , Venkateswara Rao Manga 1 , Zi-Kui Liu 1
1 Materials Science and Engineering, Penn State University, University Park, Pennsylvania, United States
Show AbstractThe perovskite La1-xSrxCoO3-δ (LSCO) has a unique range of properties that are strongly dependent on the temperature, oxygen partial pressure, and strontium content, which makes LSCO an important material in a range of applications, including fuel cells and gas sensors. To fully exploit the properties of LSCO, a comprehensive and quantitative description of its defects is necessary. Typically, thermodynamic models for the defects in ionic systems evaluate the parameters of the Gibbs free energy by fitting to experimental measurements of the defect concentrations. Such an approach reproduces the defect quantities but is not always capable of uniquely describing the thermodynamic properties of the material. We overcome this difficulty by incorporating first-principles calculations of LSCO. This allows the model to predict not only measurable quantities, such as the oxygen vacancy concentration δ, but also properties that are difficult to determine experimentally, such as the concentration of the valences of cobalt. The resulting model is able to describe the material as a function of temperature, oxygen partial pressure, and strontium content.This technology development has been supported in part by the U.S. Department of Energy under Contract No. DE-FC26-98FT40343. The Government reserves for itself and others acting on its behalf a royalty-free, nonexclusive, irrevocable, worldwide license for Governmental purposes to publish, distribute, translate, duplicate, exhibit and perform this copyrighted paper.
4:45 PM - S2.7
Computing Electrochemical Impedance of Solid Electrolyte from Fluctuations.
Eunseok Lee 1 , Wei Cai 1 , Fritz Prinz 1 2
1 Mechanical Engineering, Stanford University, Stanford, California, United States, 2 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractWe present a new method for computing the electrochemical impedance of solid electrolyte from kinetic Monte Carlo (kMC) simulations of the ionic diffusion. Using the fluctuation-dissipation theorem, the impedance at overall frequency can be obtained from the correlation function of microscopic current fluctuations through a single equilibrium simulation. In the conventional approach, the impedance at a given frequency has been obtained by a non-equilibrium kMC simulation subjected to an AC voltage at this frequency. Our fast method enables us to obtain the overall impedance and infer the governing mechanisms of ionic diffusion by decomposition of the impedances of materials into the impedances of basic chemical reactions. The effects of doping concentration and the doping distribution to the ionic diffusion in yttria-stabilized zirconia (YSZ) are discussed.
5:00 PM - S2.8
Thermodynamics and Kinetics of Conversion Reaction Cathodes from First Principles.
Robert Doe 1 , Kristin Persson 1 , Gerbrand Ceder 1
1 Dept. of Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show Abstract5:15 PM - S2.9
DFT-based Design of Orthosilicate-based Cathode Materials for Li Ion Batteries.
Anti Liivat 1 , Kinson Kam 1 , John Thomas 1
1 Materials Chemistry, Uppala University, Uppsala Sweden
Show AbstractSeveral lithium-rich materials with tetrahedrally or tetragonally coordinated ions are known to be good Li-ion conductors and have been studied as cathode materials for lithium ion batteries for almost three decades [1]. They typically contain the polyanion AO4n-, with covalent A-O bonds (for A=Al, Si, Ge and P), or involve redox-active Transition Metal (TM) atoms (for A=V, Ti, Cr and Mn). The structural stability of the polyanion matrix allows for a wide range of different cation compositions, with polymorphs having vacancy- or interstitialcy-driven Li-ion conductivity. Furthermore, these structures could give rise to dramatically improved energy density, if multi-electron redox-active TMs (such as Mn, V, Cr) could be embedded in the materials, and if the otherwise rather poor electron conductivity could be improved.It is known that the TM-ion redox potential varies with crystal structure; e.g., it is ca. 2.9 V for Fe in LiFeP2O7, but 3.5 V for Fe in LiFePO4. The redox potential also varies with the induction effect, originating from the covalency of the A-O bonds in the AO4n- polyanion [2]. Here, we used Density Functional Theory (DFT) to study the substitution of SiO44- polyanions in Li2MSiO4 (M=Fe, Mn) [3-6] with AO4n-, A=V,Ti, for the purpose of:i) lowering of the redox-potential for M=Mn systems; ii) enhancing electron transfer between the TM-ion and the polyanion;iii) introducing redox-active polyanions; especially in vanadate.Our calculations show that such substitutions lead to significant changes in the electronic structure and intercalation potentials of the host system. For example, in Li2FeVO4, besides a ca. 10% increase in unit-cell size, no major structural distortions could be observed. The delithiation starts with the oxidization of V4+ in VO44- to V5+ at a potential of 2.2 V, followed by the complete delithiation of the Li2FeVO4 host structure to FeVO4, with the Fe2+/Fe3+ oxidation occurring in the voltage range 3.1-3.5 V. Models involving the co-doping of Mn into Fe-sites are also investigated for systems containing VO44- and/or other polyanions. Furthermore, the insights from these calculations have been used in synthesis of such electrode materials. References1. A. D. Robertson, A. R. West and A. G. Ritchie, Solid State Ionics, 1, 104 (1997).2. A. K. Padhi, K. S. Nanjundaswamy, C. Masquelier, S. Okada, and J. B. Goodenough, J. Electrochem. Soc., 144, 1609 (1997). 3. A. Nytén, A. Abouimrane, M. Armand, T. Gustafsson, and J. O. Thomas. Electrochem. Commun., 7, 156 (2005).4. P. Larsson, R. Ahuja, A. Nytén, and J. O. Thomas, Electrochem. Commun., 8, 797 (2006).5. A. Nytén, S. Kamali, L. Häggström, T. Gustafsson, and J. O. Thomas. J. Mater. Chem., 16, 2266 (2006).6. A. Kokalj, R. Dominko, G. Mali, A. Meden, M. Gaberscek, and J. Jamnik. Chem. Mater., 19, 3633 (2007).
5:30 PM - S2.10
Polyatomic Anions as Mobile Species in Solid Electrolytes.
Stefan Adams 1 , Yongkai Zhou 1 , R. Prasada Rao 1 , Arkady Neiman 2 , Doreen Edwards 3
1 Mater. Science and Eng. , Nat. University of Singapore, Singapore Singapore, 2 Chemical Department, Ural State University, Ekaterinburg Russian Federation, 3 Inamori School of Engineering , Alfred University, Alfred, New York, United States
Show AbstractIon transport in solids has been observed for a limited number of structure types typically containing low-valent mobile ions. In contrast scandium tungstate has been reported as the prototype of a class of trivalent cation conductors. Here we reinvestigate ion transport in scandium (and related trivalent cation) tungstates to clarify the mobile species and the ion transport mechanism. Structural and electrochemical properties are monitored over an extended temperature range by Rietveld analysis of non-ambient XRD, impedance spectroscopy and electrolysis experiments. Results are interpreted with the help of molecular dynamics (MD) simulations and bond valence (BV) ion transport pathway modeling. XRD data for 11 – 1300 K were used to fine-tune a force-field. MD simulations this force-field qualitatively reproduce the structure variation, especially the negative thermal expansion and the pressure-dependant phase transition to a monoclinic phase. Isothermal-isobaric MD simulations as well as constant volume MD simulations for fully ordered, initially defect-free Sc2(WO4)3 structure models containing 1224 atoms show that ion transport starts with a high energy Frenkel defect creation, where an entire WO42- group hops into a neighboring interstitial site leaving a vacancy behind. This triggers a chain of WO42- hops, which due to the limited size of the structure models ends in translation copies of the initially created vacancy. Thereby the activation energy for diffusion in the simulation significantly exceeds the experimentally observed value of 45kJ/mol for T > 1100K. In reality defects are probably created during sample preparation, i.e. the defect concentration is fixed extrinsically throughout the temperature range studied. The increase of activation energy to 65kJ/mol for T<800K may then be tentatively ascribed to defect association. This is consistent with further MD simulations, where a WO42- defect is built into the initial model, leading to a close match of the simulated and experimental activation energies as well as absolute conductivities. No matter whether a Sc3+ and WO42- is introduced, no Sc3+ migration is observed in the simulations. Further evidence that anions rather than the previously assumed trivalent cations cause the ion transport stems from Tubandt-type electrolysis experiments, which show mass transfer from the anode pellet to the cathode pellet. Our earlier BV analyses of pathways for Sc3+, O2- or WO42- ions in time averaged structure models suggested that low energy diffusion pathways exist only for WO42-. Using series of instantaneous structure models from the MD simulations, a BV analysis of the transport pathway dynamics was achieved. WO42- hops between equilibrium sites closely follow the instantaneous diffusion pathways, except for the high energy defect creation. The individual tungstate transport step combines features of an intersticialcy mechanism with a unique rotation of the polyatomic anion.
5:45 PM - S2.11
Point Defect Mediated Cation Transport in MgAl2O4.
Samuel Murphy 1 , Blas Uberuaga 2 , Kurt Sickafus 2 , Roger Smith 3 , Robin Grimes 1
1 Materials, Imperial College London, London United Kingdom, 2 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 Mathematical Sciences, Loughborough University, Loughborough United Kingdom
Show AbstractThe mechanisms by which Mg2+ and Al3+ ions are transported through the MgAl2O4 spinel lattice are investigated using atomic scale computer simulation. Both vacancy and interstitial cation processes are considered. Stable vacancies can be generated on either the magnesium or aluminium sublattices but the Mg2+ and Al3+ cation interstitials are most stable when incorporated into a split interstitial configuration along with another Mg2+ ion, centred about a vacant Mg2+ site. There are three principle factors which can affect transport of cation species in spinel, these are: (i) the concentration and hence availability of the mediating point defect, (ii) the activation energy for the isolated jump process and (iii) the diffusion prefactor, D0. The prefactor contains information such as the number of equivalent pathways, jump distances and the attempt frequency. Combining all of this information allows us to determinate the preferred mechanisms for cation transport in spinel. Our calculations have demonstrated the importance of considering the affects that antisite defects may have on both the availability of transport mediating point defects as well as the migration pathways themselves. We have also demonstrated that, in the case of spinel, the concentration and activation energies are the dominant factors in determining how cations diffuse.
S3: Poster Session: Batteries
Session Chairs
Tim Armstrong
Toshiaki Matsui
Rosa Palacin
Enrico Traversa
Tuesday AM, December 02, 2008
Exhibition Hall D (Hynes)
9:00 PM - S3.1
Silicon Cobalt Graphite Composite as the Anode Materials of Lithium Ion Secondary Batteries.
Jian Hong 1 , Stanley Whittingham 1
1 , State University of New York at Binghamton, Binghamton, New York, United States
Show Abstract Silicon is an attractive candidate for high-energy anode material for Li-ion batteries. Although the introduction of transition metal as buffer media could release some of the stain during the lithium ion intercalation and extraction into the silicon host, the extreme large volume change between the de-lithiated and fully lithiated states of silicon makes this kind of material unsuitable for industrial application at the current stage. The strain may be released by creating nano-composite materials such as silicon particles embedded completely and uniformly in the carbonaceous matrix. In this work, high-energy ball milling and carbon coating route to such structure are investigated.Si-Co-graphite composites were synthesized from nano-silicon, cobalt and graphite powders in a high-energy milling machine followed by heat treatment in N2 atmosphere. The products were characterized by x-ray diffraction (XRD) and scanning electron microscopy. The electrochemical properties were tested in 1M LiPF6 in EC/EMC (1:1) liquid electrolyte. The XRD data indicates diamond structure of the nano-silicon powder and poor crystallinity of the composite material. The initial cycle of the composite has large irreversible capacity, which may come from the formation of solid electrolyte interface (SEI). In the following cycles, the reversible capacity increased initially and then continuously decreased. After 50 cycles, the composite electrode has a reversible capacity of 350 mAh/g, which is half of initial one (~700 mAh/g). Possible reason for the degradation of cycle life is a poor attachment between the graphite matrix and silicon active material. The SEM image of the electrode after 50 cycles revealed cracks on the surface of about 0.5 mm width. Future effort will be focused on the improvement of the bonding strength between the nano-silicon and carbon matrix and on elimination of first-cycle irreversible capacity. This work is supported by the US Department of Energy, Office of FreedomCAR and Fuel Partnership through the BATT program at Lawrence Berkeley National Laboratory.
9:00 PM - S3.10
New Polymeric Electrolytes with Titanium Oxide as Inert Fillers Prepared by Sol-gel Technique.
Liliana Hechavarria 1 , Narcizo Mendoza 1 , Hailin Hu 1 , Jose Campos 1
1 Centro de Investigacion en Energia, Universidad Nacional Autonoma de Mexico, Temixco, Morelos, Mexico
Show AbstractTransparent and conductive polymeric gels have been prepared from polyethylen glycol (PEG), polyethylene oxide (PEO) or Pluronic F127 by incorporating titanium isopropoxide into the polymer matrixes with sol-gel method. The presence of titanium oxide compound and its interaction with the polymers were confirmed by FT-IR. Optical transmittance spectra of the polymeric gels were studied as a function of titanium isopropoxide percentage, as well as the molecular weight of each polymer. For the dye-sensitized solar cell (DSSC) or photoelectrochromic cell (PEC) application purpose, LiI was added into the polymeric gels to form polymeric gel electrolytes. The electrical conductivity of each electrolyte was measured by electrochemical impedance spectroscopy at a temperature interval from 0 to 80 C. To keep the transparency of the electrolytes, it is important to avoid the reduction of iodine by controlling the electrolyte preparation conditions. The increase of the electrical conductivity in the resulting polymeric gel electrolytes can be explained by a higher microscopic disorder or larger space between polymeric chains created by the titanium oxide compound in the original polymers, that leads to a faster ion transport.
9:00 PM - S3.11
Component Diffusion in the Miscible Polymer Blend of PEGDME and PMMA Doped with LiTFSI Measured by Multinuclear PFG NMR.
Yan Meng 1 , Luis Smith 1
1 Carlson School of Chemistry and Biochemistry, Clark University, Worcester, Massachusetts, United States
Show AbstractThe blending of miscible or compatible polymers based on available polymers is an attractive and economic alternative in the development of new materials rather than designing and synthesizing completely new polymers because of the energy, materials and time savings. When a miscible polymer blend in the solid state is formed by a rigid solid polymer and a liquid polymer, it can have relatively good mechanical strength, and at the same time the liquid component in it still has high mobility. Such a property can be utilized to develop solid polymer electrolytes with good performance for secondary lithium batteries. It has been reported that poly (vinyl methyl ether)/polystyrene (PVME/PS, 50/50 wt.) with PVME being a liquid at the temperature of measurement exhibits almost the same Li+ conductivity with that in pure liquid PVME [1]. To further explore the possibility of this kind of polymer blends being a good lithium electrolyte, some important questions urgently need to be answered. Specifically, while the liquid polymer is the carrier of Li+, what is the effect of the addition of lithium salts on the self-diffusion of the liquid polymer? How do the apparent self-diffusion coefficients (ADC) of Li+, TFSI- and PEGDME in the polymer blend change as a function of salt concentration and temperature? How does the ADC value of the liquid polymer change after it is blended with the other rigid polymer? In this work, these studies were done on the liquid polymer of poly (ethylene oxide), which has been the most representative and widely studied polymer for lithium polymer electrolytes. A novel solid state polymer electrolyte is prepared by doping poly (ethylene glycol) dimethyl ether / poly(methyl methacrylate) (PEGDME/PMMA, 40/60 wt.) with LiN(CF3SO2)2 (LiTFSI) at different concentrations. The effect of the addition of LiN(CF3SO2)2 (LiTFSI) at different concentrations on the self-diffusion of PEGDME is observed using Li7, F19 and H1 pulsed field gradient (PFG) NMR experiments at different temperature. The apparent self-diffusion coefficients (ADC) of Li+, TFSI- and PEGDME were measured, and were found to be in the range of 10e-9 to 10e-8 cm2/s at 60 degree C. For PEGDME in the same polymer blend without the lithium salt, ADC values of an order of magnitude greater were observed and are comparable with pure liquid PEGDME. The results show that the addition of lithium salts slows down the self-diffusion of PEGDME, but does not substantially limit diffusion, making this blend an interesting electrolyte candidate. [1] D. Cangialosi, A. Alegria, J. Colmenero. Macromolecules, 41, 1565-1569 (2008).
9:00 PM - S3.12
Atomistic Studies of Ionic Diffusion in Lithium Aluminosilicate.
Alexander Chroneos 1 2 , Hartmut Bracht 2 , Robin Grimes 1 , Andre Schirmeisen 3 , Bernhard Roling 4
1 Department of Materials, Imperial College London, London United Kingdom, 2 Institut für Materialphysik and Collaborative Research Centre (SFB 458), Westfälische Wilhelms-Universität Münster, Münster Germany, 3 Centre for Nanotechnology (CeNTech) and Collaborative Research Centre (SFB 458), Westfälische Wilhelms-Universität Münster, Münster Germany, 4 Fachbereich Chemie, Philipps-Universität Marburg, Marburg Germany
Show AbstractLithium aluminosilicate (LiAlSiO4) is a highly conductive solid electrolyte material and is technologically important due to its potential application in microlithium batteries. Importantly, crystalline LiAlSiO4 exhibits one-dimensional superionic conductivity as the lithium ions are transported in structural channels. In a recent study [Schirmeisen et al. Phys. Rev. Lett. 98, 225901 (2007)] it was determined that in a nanostructured LiAlSiO4 glass ceramic, the interfacial regions between the glass matrix and the embedded crystallites exhibit ionic conductivity that is orders of magnitude higher than that of the glass or the crystallites. In the present study, we use density functional theory calculations to predict the migration energy barriers for Li+ transport in LiAlSiO4. Atomistic simulation based in pair potentials is used to identify the mechanisms for the increased ionic conductivity observed at the interfaces between the glass and crystalline LiAlSiO4.
9:00 PM - S3.14
Synthesis of Mesoporous Manganese Oxide with a High Surface Area.
Akira Endo 1 , Bao Wang Lu 1 , Takao Ohmori 1 , Keigo Matsuda 2
1 Research Institute for Innovation in Sustainable Chemistry, National Institute of Advance Industrial Science and Techology (AIST), Tsukuba, Ibaraki, Japan, 2 Department of Chemistry and Chemical Engineering, Yamagata University, Yamagata, Yamagata, Japan
Show Abstract9:00 PM - S3.15
Transmission Electron Microscopy Observation of Li Extraction/insertion Behaviors in Li1.2Mn0.4Fe0.4O2 Positive Electrode Material for Li-ion Battery.
Jun Kikkawa 1 , Tomoki Akita 1 , Mitsuharu Tabuchi 1 , Masahiro Shikano 1 , Kuniaki Tatsumi 1 , Masanori Kohyama 1
1 Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology, Ikeda, Osaka, Japan
Show AbstractThe electrochemical performance of Li-ion batteries depends on the extraction/insertion behaviors of Li ions in positive electrode materials during charge/discharge. Clarifying the distribution of Li ions in positive electrode materials at the nanometer or atomic scales is a crucial issue. Especially, real-space observations of the behaviors of Li ions are strongly demanded. In this study, we have achieved the ‘Li mapping’ of lithiated transition-metal oxides with spatial resolutions of about 1–2 nm, by using a spectrum imaging scheme based on scanning transmission electron microscopy-electron energy-loss spectroscopy (STEM-EELS). The ‘Li mapping’ scheme was applied to Li1.2Mn0.4Fe0.4O2 (Li2MnO3–LiFeO2 system) as a high-capacity 3V-class positive electrode material at each stage of the first charge-discharge cycle. Each Li1.2Mn0.4Fe0.4O2 (synthesized using MnCl2 and Fe(NO3)3 as manganese and iron sources) particle was found to be comprised of both Li2MnO3-like and α-LiFeO2-like nanodomains on a common oxygen sub-lattice of the cubic close-packed structure. The Li ions are firstly extracted from α-LiFeO2-like nanodomains and subsequently extracted from the whole region during charging to 4.5 V at 60 degree C. Both nanodomains are really activated, which is different from inactive bulks of α-LiFeO2 and Li2MnO3. We have also observed that Li ions are inserted into both nanodomains after the discharge to 2.0 V. The results have provided a design concept of novel positive electrode materials forming nanodomains even though each pure phase is inactive. The present ‘Li mapping’ technique will bring great advances of the science and technology of Li-ion batteries. [This work was financially supported by R&D project for Li batteries (“Li-EAD” project) by METI and NEDO, Japan]
9:00 PM - S3.16
Structural and Electrochemical Studies of Chemical Solution Derived xLiMn2O3-(1-x)Li[Mn0.5Ni0.5]O2 Cathodes.
Naba Karan 1 , Dillip Pradhan 1 , Jose Saavedra-Arias 1 , Reji Thomas 1 , Ram Katiyar 1
1 Physics, University of Puerto Rico, San Juan, Puerto Rico, United States
Show AbstractRecently, layered LiMn0.5Ni0.5O2 has drawn attention as an alternate cathode material for secondary lithium-ion batteries due to its lower cost, better stability at high voltages and improved thermal safety characteristics compared with LiCoO2. It has been reported recently that the electrochemical properties of layered lithium–manganese–nickel electrodes are enhanced by the addition of excess lithium, typically 10–15%. The effect of lithium content in LiMn0.5Ni0.5O2,or more precisely, in Li1+y[Mn0.5Ni0.5]1−yO2, is of particular interest because, unlike LiCoO2 or LiNiO2, LiNi0.5Mn0.5O2 can accommodate considerable amount of lithium in the transition metal sites without disturbing the overall rhombohedral structure (R-3m) of the material. Since the lithium layer is fully occupied by 1Li in the layered structure of Li1+y[Mn0.5Ni0.5]1−yO2, the excess lithium yLi resides in the transition metal layer to form cation ordering with Mn ions, resulting in local Li2MnO3-like domains. Such compounds can also be reformulated in the two-component notation as xLi2MnO3-(1-x)LiMn0.5Ni0.5O2. The addition of Li2MnO3 to this system is thought to structurally stabilize the overall electrode while providing diffusion pathways for lithium cations. The structures of mixed xLi2MnO3-(1-x)LiMn0.5Ni0.5O2 are extremely complex and the cation distribution and ordering is highly dependent on the synthesis methods, which has profound influence on the electrochemical behavior. In the present work, xLiMn2O3-(1-x)Li[Mn0.5Ni0.5]O2 bulk cathodes (x=0.0–0.4) were synthesized using a cost effective solution technique. The structural properties and oxidation state of Mn and Ni were investigated using X-ray diffraction, Raman Spectroscopy and X-ray photoelectron spectroscopy. The electrochemical measurements of the synthesized cathodes were performed in a two-electrode coin-cell configuration (CR2032), using liquid electrolyte [1M LiPF6 in (1EC:1DMC by volume)] and Li-metal as anode. The major peaks in the XRD patterns of the synthesized xLiMn2O3-(1-x)Li[Mn0.5Ni0.5]O2 cathode materials could be indexed to R-3m symmetry, which characterizes layered LiCoO2. Weak peaks that are characteristic of cation ordering in the transition metal layer, as in the case of LiMn2O3, were present between 20-25o. They could be indexed to the [020] and [110] lattice planes of a LiMn2O3 like unit cell with C2/m symmetry.The first charge and discharge capacity for the composition 0.4LiMn2O3-0.6Li[Mn0.5Ni0.5]O2 were 399 and 225 mAh/g, respectively correspond to a coulombic efficiency of 56%. However, the coulombic efficiency considerably increased to 78% in the 2nd cycle. The effect of LiMn2O3 fraction on the charge and discharge capacity along with the effect of annealing conditions upon electrochemical properties will be presented and discussed in conjunction with local structure of the as prepared and electrochemically cycled electrode materials using ex-situ Raman spectroscopy.
9:00 PM - S3.17
Size Control and Mechanistic Study of LiFePO4 in the Polyol Process.
Wang Hay Kan 1 , Christian Maunders 2 , Shri prakash Badi 1 , Brian L. Ellis 1 , Gianluigi Botton 2 , Linda Nazar 1
1 Chemistry, University of Waterloo, Waterloo, Ontario, Canada, 2 Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada
Show AbstractOlivine lithium metal phosphates, including LiFePO4, are amongst the most promising cathode materials for lithium ion rechargeable batteries. We have investigated the polyol method as a route to prepare nanocrystallites of LiMPO4 (M = Fe, Mn), in order to understand size dependent solid solution behavior. Nanocrystallites of varying dimensions were synthesized through the manipulation of experimental parameters such as reaction time, choice of solvent, and reagent concentration. We found that phase pure samples could be isolated in remarkably short reaction times. Characterization by XRD revealed that the coherence length in the long [200] direction (SG:Pnma) was as low as 12 nm under optimized reaction conditions, which was consistent with our TEM measurements. Longer reaction times or increased concentrations resulted in the formation of substantially larger, and more anisotropic nanocrystallites. High resolution electron loss spectroscopy studies (HREELS) conducted in the TEM revealed the existence of Fe3+ defects on the surface, and also within the bulk of nanocrystallites which can be explained by nucleation and growth mechanisms that will be discussed. The solid solution behavior of the nanocrystallites were examined by a combination of Mössbauer, electrical conductivity and variable temperature XRD studies, which showed that the transition temperature decreases with decreasing particle size, for both the Fe and Mn olivines, although the activation energy for small polaron hopping is relatively size independent.
9:00 PM - S3.18
Crystal Structure and Lithium Diffusion in Copper Doped LiFePO4.
Shailesh Upreti 1 , Natalya Chernova 1 , M. Whittingham 1 , Olga Yakubovich 2 , J. Cabana 3 , Clare Grey 3 , Janice Musfeldt 4
1 Chemistry and Materials, Binghamton University, Binghamton, New York, United States, 2 Geology, Moscow State University, Moscow, Moscow, Russian Federation, 3 Chemistry , SUNY Stony Brook, Stony Brook, New York, United States, 4 Chemistry , University of Tennessee, Knoxville, Tennessee, United States
Show AbstractModern LiFePO4 technology for lithium ion batteries is already in commercial growth owing to the excellent electrochemical performance, high-rate capability and structural stability of this cathode material. However, the two main functional characteristics of LiFePO4 – electronic and ionic conductivity – still remain a subject of controversy. The material behaves like an electronic insulator, but its band structure is not clear. The delithiation of LiFePO4 proceeds as a two-phase reaction, making the determination of the Li diffusion coefficient impossible by means of standard impedance techniques. In nature, lithium-deficient olivine phosphates are stabilized by divalent ions, such as Mg2+ or Ca2+, on the Fe sites of the structure. This inspires us to produce a series of doped olivine phosphates and study their structure and electrochemical behavior.In this work, Cu-substituted LiFePO4 obtained under high-pressure, high-temperature hydrothermal conditions (4000C, 1000 atm) is reported. Dark brown single crystals of composition Li0.95[(Fe2+)0.61(Fe3+)0.10Cu0.24Li0.05]PO4 were successfully grown. A remarkably high level of Cu-substitution is achieved compared to those reported earlier. A systematic single crystal x-ray examination revealed Pnma space group with unit cell dimensions a = 10.226(2) Å, b = 6.012(1) Å, c = 4.682(1) Å and V = 287.8(1) Å3. In order to ascertain the presence of Li ions and/or holes at the transition metal sites, a detailed solid-state Li NMR study is in progress. Temperature dependent magnetic studies confirm the oxidation states expected from the composition and show an antiferromagnetic behavior below 49.5 K with magnetic moments aligned along [010]. Optical properties are being examined with the goal of confirming that Fe2+/Fe3+ charge transfer is responsible for the dark color, and to determine the electronic band structure and vibrational properties.The compound delivers a discharge capacity of 100 mAhg-1 when cycled between 2 and 4.5 V at a current density of 0.1 mAcm-2; this capacity is sustained at higher current rates. A 3.4 V plateau is observed at the charge curve, indicating that Li removal occurs mostly as a two-phase reaction. Upon chemical delithiation in Br2, the single crystal structure is preserved, which allows us to conduct comprehensive structural characterization. In situ and ex situ chemical delithiation studies using single-crystal x-ray diffraction are attempted with the goal of estimating Li diffusion rate. The results will be discussed in comparison with LiFePO4. This work is supported by the US Department of Energy, Office of FreedomCAR and Vehicle Technologies through the BATT program
9:00 PM - S3.19
Morphology and Electrochemistry of Hydrothermally Synthesized Na2FePO4F.
Linda Nazar 1 , Brian Ellis 1 , Emilyne Nicolas 1
1 , University of Waterloo, Waterloo, Ontario, Canada
Show Abstract Among the many positive electrode materials being studied for rechargeable lithium batteries, iron phosphates in particular, are promising electrode materials due to their minimal environmental impact and low cost of the raw materials. We have recently reported on a new iron fluorophosphate, Na2FePO4F,(1) which like other iron phosphates, has a low inherent electrical conductivity.(2) To produce Na2FePO4F that exhibits close to theoretical capacity at high rates, the crystallites must be coated with a conductive material, and most importantly, the particle size must be minimized to reduce the transport path length. We report here a novel synthetic method based on hydrothermal synthesis in the presence of carbon-containing additives, which results in nanocrystallite, single-phase Na2FePO4F. The morphology of the end product can be controlled by the manipulation of the concentration of precursors, hydrothermal pressure and the nature of the additive. Control of these factors results in crystallites with an average size of less than 300nm and a narrow particle size distribution. The effects of morphology and particle size on electrochemical performance will be described. Temperature-dependent Mössbauer spectroscopy measurements show that the compound, when partially oxidized, undergoes a transition into a regime of rapid electron hopping. The onset of this phenomenon occurs at a lower temperature than that for LiFePO4,(3) suggesting a lower activation energy for electron hopping in the bulk phase of the material. References1. Ellis, B.; Makahnouk, W. R. M.; Makimura, Y.; Toghill, K.; Nazar, L.F. Nat. Mat. 2007, 6, 749.2. Chung, S-. Y.; Bloking, J. T.; Chiang, Y-. M. Nat. Mat. 2002, 1, 123.3. Ellis, B.; Perry, L. K.; Ryan, D. H.; Nazar, L. F. J. Am. Chem. Soc. 2006, 128, 11416.
9:00 PM - S3.2
Growth, Characterization of Silicon Nanostructures and their Application as an Anode Material for Rechargeable Lithium Batteries.
Sri Lakshmi Katar 1 2 , Deepak Varshney 1 3 , Gerardo Morell 1 3
1 Institute of Functional nanomaterials, University of Puerto Rico, San Juan, Puerto Rico, United States, 2 Chemistry, University of Puerto Rico, San Juan, Puerto Rico, United States, 3 Physics, University of Puerto Rico, San Juan, Puerto Rico, United States
Show AbstractSilicon nanowires were selectively grown by vapor liquid solid method (VLS) by simply placing a mixture of silica and graphite powder in a flowing nitrogen gas environment on substrates(Cu,Si,Quartz,Mo). The obtained silcon nanostructures is characterized by X ray Diffraction , Scanning electron microscopy (SEM), Energy dispersive x ray análisis (EDAX) Raman spectroscopy, and Transmission electron microscopy(TEM). The electrochemcial properties are studied by cyclic voltammetry, and charge discharge measurements.
9:00 PM - S3.21
Surface Modification of LiCoO2 for Improvement of Cycle Life Performance at High Voltage.
Jae Won Lee 1 , Sun Min Park 1
1 , Korea Inst. Ceramic Eng. & Tech., Seoul Korea (the Republic of)
Show AbstractA transition metal oxide was coated on the surface of LiCoO2 to improve the cycle life performance and increase the energy capacity. The metal oxide was coated via sol-gel process. SEM, TEM analysis were applied to observe the surface of the orginal and the metal-oxide coated LiCoO2. The change in crystal structure after the coating was estimated through XRD analysis. The charge and discharge characteristics and cycle life performance were measured and compared before and after the coating and the difference was discussed.
9:00 PM - S3.22
Orientation Dependence of Microstructure and Electrochemical Properties of LiCoO2 Cathode Films Deposited on Single-Crystalline La2/3-xLi3xTiO3.
Kengo Goto 1 , Kyosuke Kishida 1 , Yuji Yamaguchi 1 , Norihiko Okamoto 1 , Katsushi Tanaka 1 , Haruyuki Inui 1 , Yasutoshi Iriyama 2 , Zempachi Ogumi 3
1 Department of Materials Science and Engineering, Kyoto University, Kyoto Japan, 2 Department of Materials Science & Chemical Engineering, Shizuoka University, Hamamatsu Japan, 3 Department of Energy and Hydrocarbon Chemistry, Kyoto University, Kyoto Japan
Show AbstractSolid state lithium-ion conductors have received a considerable amount of attentions as solid electrolytes for all solid-state lithium rechargeable batteries. In the case of the all solid-state batteries especially with a crystalline solid electrolyte, the solid / solid interface between the electrode and electrolyte are considered to have significant influences on the battery performance such as resistivity and mechanical stability upon charging and discharging. Recently, we have studied influences of microstructures of the solid electrolyte / solid cathode interfaces on the electrochemical properties using model assemblies with a perovskite-based lithium lanthanum titanate (LLT: La2/3-xLi3xTiO3) solid electrolyte and a LiCoO2 cathode with a layered rock-salt type rhombohedral structure. We have revealed the orientation relationship between the LLT and LiCoO2 and effects of the lattice defects on the battery performance. These preliminary results suggest the resistivity as well as the mechanical stability of the interface can be controlled partly by the geometrical configuration of the Li layers in the LiCoO2 crystal against the interface plane. In order to elucidate the influences of the geometrical configuration of the layered structure of the LiCoO2 on the battery performance, we have prepared single-crystalline LLT by a unidirectional solidification process and investigated the variation of the microstructure and electrochemical properties of the LiCoO2 thin films deposited by PLD as a function of the surface orientation of the LLT solid electrolyte. The LiCoO2 thin-film cathode consisting of differently oriented fine domains is epitaxially grown on the LLT with the orientation relationships: {111}LLT//(0001) LiCoO2 and <110>LLT