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1 TOPIC 2: NANOSCALE PROPERTIES OF ENERGY STORAGE MATERIALS Three presentations were mace on this topic, by Dane Morgan of the Massachusetts Institute of Technology, Dan Scherson of Case Western Reserve University, and Ann Marie Sastry of the University of Michigan. Their papers are summarized below. AB INITIO METHODS IN POWER TECHNOLOGIES: BATTERIES AND NANOSCALE MATERIALS Dane Morgan made the case for rational materials design based on ab initio calculations involving well-characterized systems having a small number of atoms (calculations with 100-150 atoms are average; 1,000 atoms would be a maximum). When such calculations are combined with empirical potentials, it is possible to treat a billion atoms. The calculations can be uses! to predict energy-relatec} properties such as structural stability, lattice parameters, voltages, and reaction rates, as well as charge- density-relatecl properties such as band structure, optical properties, and electrical conductivity. Compared with experimentation, calculations offer a fast and inexpensive way to explore many possibilities. However, Morgan stressed that it is essential to have experimental knowledge of structures in order to use calculations effectively. He presented results for Li intercalation oxides such as LiXCoO2 in batteries as a substitute for Co in order to decrease price and increase energy density. Calculations were also performed to understand the strains induced in LiXCoO2 during battery cycling as a result of Li transport. These strains can cause the particles to fracture and reduce battery performance. Such calculations are expecter! to be more accurate and useful at the nanoscale, where fewer atoms are involved. For example, one collie compare the properties of carbon nanotube bundles with those of graphite or catalytic oxidation processes on 10-atom gold nanoparticles. Morgan noted that ah initio calculations have become a standard too} of materials science and should be a part of nanoscience research, complementary to experimentation. 1 7 . a
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1 8 Summary of the Power Systems Workshop ELECTROCHEMICAL CHARGE STORAGE DYNAMICS Dan Scherson presented research aimed at characterizing the in situ dynamics of battery electrode changes cluring charge/discharge cycles. The objective is to monitor the extent of Li+ intercalation within various Li+ battery constituents so as to provide a rational basis for improving battery performance. The experimental approach is to use Raman spectroscopy, which is a vibrational probe that reflects local forces and small volumes in a time-resolved fashion (but provides only indirect structural information), and correlate Raman spectral changes observed on charging and discharging with data obtained using x- ray diffraction (XRD), a structural probe that provides information on long-range order in bulk materials. If one can establish a direct ant} unique link between the vibrational and the structural information, one can use Raman microscopy as a quantitative time- and space-resolved probe of dynamic events in the material. Scherson described Raman spectra of mode! materials: single, m~crometer-sized particles of LiMn2O4 (Li+ intercalation cathode for Li+ batteries) ant! graphite (Li+ intercalation anode for Li+ batteries) while recording cyclic voltammetry. He showed that specific Raman peak heights varied according to the extent of Li+ intercalated, ant! that these variations could be quantitatively related to phase changes observed in XRD spectra on the same material. Scherson then performed the experiment on an operating graphite-LixCoO2 Li-ion battery and obtained encouraging results on the anode, allowing time-resolved line maps of lithiatec] graphite as a function of the state of charge. Further experiments across the entire cross-sectional edge of the battery will enable assessment of models describing the Li+ dynamics in real devices. NANOSTRUCTURED MATERIALS FOR POWER SUPPLIES: DESIGN OF MATERIALS Ann Marie Sastry began by noting that designing smaller power supplies means making better use of volume. How can we model the more stochastic nature of performance properties as we move to smaller length scales? Small is good, but we have to strategize about the architectural arrangement of materials. In particular, both the electron and ion pathways must be percolated (connected in a continuous path through the space of interest) to facilitate the electrochemical reaction. Active particles usually suffer phase and volume changes during electrochemical processes, so conduction and mechanics are linker! at all scales. Conduction physics is critical, and this points to the need for more work on synthesis/property modeling. Is two-dimensional ideal, or three-dimensional, or something intermediate? The creation of designed properties in surface layers/films depends on the dispersion of particles on the surface. Sastry discussed models of the percolation probability as a function of the volume fraction and aspect ratio of particles. For the constant area/volume fraction, there is a greater probability of percolation in three dimensions than in two dimensions. Particles with higher aspect ratios are superior for achieving percolation at any given volume fraction, but processing is critical. Utilization of nanostructured materials as coatings can inhibit unwanted chemical reactions without sacrificing the electron conductivity. In the design of materials, there needs to be a greater emphasis on the internal mechanics of heterogeneous systems, continued focus on percolative phenomena, and development of optimal multiphase blencis.
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Topic 1: Overview of Power Technologies 9 1 TOPIC 2 DISCUSSION The pane} discussion focused on the extent to which major advances in power crevice properties are still possible with known chemistries as opposed to new chemistries. By using higher aspect ratio particles to reduce electrode mass, it appears possible to get 20 to 30 percent higher specific properties. By investing a lot of energy to achieve optimal self organization) of components, it was felt that perhaps a twofold} improvement was possible. However, the primary interest of the committee was in opportunities to improve properties by more than an order of magnitude. It was pointed out that in capacitors, one couicl. improve capacity by an order of magnitude from the increased packing fraction enabled by nanoparticles. Furthermore, asymmetric EC capacitors hac! increased energy capacity an order of magnitude by better design rather than by different materials. A further twofold increase can be obtained by cycling deeper (e.g., 20 percent discharge as opposer} to the current typical discharge of 10 percent). Regarding the development of new materials for power devices, it was pointed out that a small number of researchers for example, John Goodenough at the University of Texas- had identified the bulk of the new cathode materials that have been tested. It was suggested that expert system software incorporating the analysis used by Goodenough would be useful for developing a list of promising new materials to explore. ~ In this report, the term "self-organization" refers to construction of a regular array such as a CVD diamond film or Langmuir-Blodgett film, with relatively high levels of associated defects. The term "self-assembly" refers to construction of complex three-dimensional structures via selective pattern matching and sticking of complementary surfaces. An example would be T4 phage parts self-assembling in solution. This process is associated with a relatively low level of defects by comparison.
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