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OCR for page 7
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.
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OCR for page 8
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.
OCR for page 9
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.
OCR for page 10
Representative terms from entire chapter:
ann marie