|
SUMMARIES OF FY 1996
RESEARCH IN THE CHEMICAL
SCIENCES |
Advanced Battery Research:
National Laboratory Projects
Ames Laboratory
Iowa State University
Ames, IA 50011
Department of Materials Science and Engineering
Design and Processing of High Purity and Ultrafine Electrodes for High Performance
Nickel/Metal Hydride Batteries
All major battery manufacturers are aware that their rechargeable Ni/Cd products are currently
poised on the edge of an outright ban or a heavy tax due to the extreme toxicity of Cd. As a
consequence, an acceptable substitute must be found. The Ni/metal hydride (Ni/MH) rechargeable
battery has emerged as one of the most promising environmentally acceptable substitutions for
current Ni/Cd rechargeable batteries in nonautomotive applications for the consumer and
industrial market. A key to realization of this potential lies in coupling of the development of both
high performance positive and negative electrode materials with the development of efficient,
well-controlled electrode processing approaches for each new electrode material. Another key is
the close collaboration of a high level research team with a capable and aggressive manufacturer
of Ni/MH batteries. This coupled development and industrial collaboration can produce not only
improved battery performance and effective manufacturability but also will promote a rapid
transfer of materials and process research results from the laboratory to the manufacturing floor
and marketplace. This program's focus includes the fundamental, systematic development of a
significantly improved alloy composition (especially with respect to chemical and phase purity
levels) and electrode fabrication technique for the negative electrode centered on the
AB5 (based on LaNi5) intermetallic compounds. The objective of
this work is to greatly reduce self-discharge and to double cycling life characteristics of this
electrode material over the competing AB2 compounds. Also, focus is on
development of a unique processing approach for the NiOOH positive electrode that will permit
fabrication of new ultrafine pitch electrodes. Such an ultrafine pitch electrode with
homogeneously dispersed particulate has the potential to increase the energy storage density by
about 50%, thereby reducing size and weight of this new generation of Ni/MH batteries. This
program is being carried with intimate contact between the Ames Laboratory and Harding Energy
Systems Inc., an emerging domestic battery manufacturer.
Argonne National Laboratory
Argonne, IL 60439
Chemical Technology Division
A Fundamental Study of Lithium Polymer Electrolytes: Neutron and X-ray Scattering
Experiments and Electronic Structure Calculations
| Investigator(s) |
Curtiss, L.A.; Price, D.L.; Saboungi, M.L.
|
$110,000 |
| Phone | 630-252-7380 |
| E-mail |
curtiss@cmt.anl.gov |
This project is a complementary experimental and theoretical study of the structure, dynamics,
and transport properties of lithium polymer electrolytes. The goal is to obtain a fundamental
understanding of the ionic transport properties in these electrolytes in order to help optimize their
performance in lithium secondary batteries. The experimental part of the study involves neutron
scattering measurements at the Intense Pulsed Neutron Source (IPNS) at Argonne and X-ray
scattering measurements at the Advanced Photon Source (APS) at Argonne. Application of
scattering techniques to lithium polymer electrolytes will provide information on ion solvation and
the attendant effects of ion pairing, which affect the ionic transport in these systems. The
theoretical part of the project involves electronic structure calculations based on ab initio
molecular orbital theory. These calculations are being used to investigate energetic, structural, and
dynamical properties of ion-ion and ion-polymer interactions at the molecular level. The
computational studies are providing accurate potentials for molecular dynamics studies being
carried out at the University of Minnesota. The calculations will also be used to help interpret the
experimental data. At a later stage these methods will be used to study processes ocurring at
electrode/electrolyte interfaces in batteries.
Argonne National Laboratory
Argonne, IL 60439
Chemical Technology Division
Ion Transport Properties Determined by In-Situ NMR Spectroscopic Imaging
This program uses in situ magnetic resonance imaging (MRI) to better define electrode-electrolyte
interfaces and solid-state ion transport mechanisms. Recent results have demonstrated that toroid
cavity NMR imaging has sufficient sensitivity and distance resolution to accurately map out the
ionic concentration profiles that result during the electrochemical cycling of lithium
polymer-electrolyte battery materials. In addition, a new spin labeling method for the
determination of diffusion coefficients has been developed. Nuclear spin labeling has been
achieved at a distance resolution of 2 µm in low-viscosity organic solvents. A method has
been proposed for distinguishing between short-and long-range ion motion using diffusion
coefficient measurements and is being tested on polymer electrolyte materials. The in situ NMR
imaging techniques that are being developed should be generally useful for the analysis of the
chemical composition at electrode-electrolyte interfaces, growth factors responsible for passive
film formation, ion concentration gradients within solid-state batteries, conformational dynamics
of polymeric electrolytes, ion penetration depths within graphite insertion electrodes, and dendrite
formation on lithium anodes.
Argonne National Laboratory
Argonne, IL 60439
Chemical Technology Division
Development of New Electrode Materials for Battery Applications: Synthesis,
Characterization and Modeling
This is a new project which will focus on the search for, and characterization of novel or modified
metal oxide electrodes for non-aqueous rechargeable lithium batteries. The advent of lithium-ion
batteries in products such as cellular phones and laptop computers has led to an increasing
awareness of their possible application in heavy duty devices such as electric vehicles. However,
state-of-the-art metal oxide electrodes are limited by performance factors such as structural
instability to lithium insertion/extraction reactions, particularly at the end of discharge/charge, and
an instability to temperature rise effects. Furthermore, the capacity of the metal oxide electrodes
and cycle life of lithium batteries need to be improved. These limitations are slowing the progress
of lithium battery technology. There is therefore an urgent need to explore the possibility of
finding alternative materials for these batteries. This project, which is exploratory in nature, has
two complementary tasks. The first task will be to search for novel or improved electrode
materials (notably metal oxides), to optimize materials processing techniques and to characterize
the materials in terms of their structural and electrochemical properties. The second task will
focus on the modeling of both "known" and "hypothetical" materials using ab initio methods:
theoretical calculations and modeling can provide invaluable insight and guidance for
understanding and tailoring the structure and behavior of materials. The project will make use of
the extensive battery expertise that exists at Argonne National Laboratory, and its unique
facilities, for example, the Intense Pulsed Neutron Source (IPNS) for undertaking detailed
structure analyses of materials. Materials characterization will also include convergent-beam
electron diffraction (CBED) and high-resolution transmission electron microscopy (HRTEM) to
provide "real-life" pictures of the structures of materials which cannot be obtained from
"time-averaged" structural representations provided by X-ray or neutron diffraction techniques
alone.
Argonne National Laboratory
Argonne, IL 60439
Chemistry Division
Template Mediated Synthesis of New Carbon Negative Electrode
| Investigator(s) |
Sandi, G.; Winans, R.E.; Carrado, K.A.
|
$140,000 |
| Phone | 630-252-7479 |
| E-mail |
rewinans@anl.gov |
This program seeks to understand the chemistry and physics of carbon anodes in lithium
secondary batteries. The approach is to design and prepare carbons with specific molecular
properties. Inorganic templates such as pillared clays are being used to prepare molecularly
porous, disordered carbons. The carbons have been characterized by a number of methods,
including small angle neutron scattering (SANS) and synchrotron X-ray spectroscopy. SANS data
have shown that after removal of the pillared clays, the carbon sheets contain holes the size of the
original pillars. In electrochemical studies, these carbons exhibit high reversible capacities (up to
850 mAh/g) and coulombic efficiencies higher than 80%.
Argonne National Laboratory
Argonne, IL 60439
Material Science Division
The Study of Electrochemical Interfaces Important to Energy Storage and Conversion
Processes at the Advanced Photon Source
| Investigator(s) |
You, H.; Nagy, Z.
|
$150,000 |
| Phone | 630-252-3429 |
| E-mail |
you@anl.gov |
The primary objective of this interdisciplinary research is the fundamental understanding of the
solid/solution interfacial structure of materials important to energy storage and to energy
conversion. The problem areas include electrocatalysis, surface morphology of metal
deposition/dissolution, intercalation/deintercalation mechanism, and structure of the electric
double layer. The final aim is to contribute seminal guidance to the development of improved
energy storage/conversion materials with increased energy and power density and
charge/discharge rate for a variety of battery and fuel cell systems. While we expect that the
results of our investigation will provide impetus for technological developments, they will also be
of fundamental scientific importance in the field of interfacial electrochemistry. The research
program that we carry out couples in situ synchrotron-based x-ray measurements with
electrochemical transient techniques and theoretical modeling. An example of our investigations is
the study in real time of the actual and complete phenomena occurring in ultracapacitor storage
devices. The ultracapacitor stores energy using several different phenomena of interfacial changes
restricted within a 1-100Å layer such as the electrochemical double layer and the
adsorption pseudocapacitance involving redox. All these phenomena can be observed at a
third-generation-synchrotron facility such as the Advanced Photon Source.
Brookhaven National Laboratory
Upton, L.I., NY 11973
Department of Applied Science
Synthesis and Characterization of Metal Hydride Electrodes
The purpose of this work is to elucidate the structural, thermodynamic, and metallurgical
parameters that affect the stability, kinetics, and energy density of metal hydride electrodes. It is
focused on alloy development and the application of in situ methods such as X-ray absorption
(XAS), X-ray diffraction (XRD), X-ray tomography, magnetic susceptibility, and scanning
tunneling microscopy (STM) to determine the roles that various hydride phases and alloying
elements play in hydrogen storage and corrosion inhibition. In situ methods are complemented by
ex situ studies such as neutron diffraction, electrochemical, thermodynamic, and metallurgical
characterization of candidate alloys. The information thus gained will be used to develop metal
hydride electrodes with improved cycle life and energy density.
Brookhaven National Laboratory
Upton, NY 11973
Department of Applied Science
Chemistry and Electrochemistry of Hydrogen Insertion Electrodes
| Investigator(s) |
Reilly, J.J.; Feldberg, S.W.; Johnson, J.R.; McBreen, J.
|
$150,000 |
| Phone | 516-344-4502 |
| E-mail |
jreillys@bnl.gov |
The purpose of this effort is to elucidate the structural, thermodynamic, and metallurgical
parameters that affect the stability, kinetics and energy density of hydrogen insertion electrodes. It
is focused on multicomponent alloy hydrides and develops and applies in situ methods such as
x-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), x-ray tomography, magnetic
susceptibility and scanning tunneling microscopy (STM) to determine the roles of various hydride
phases and alloying elements in hydrogen storage and corrosion inhibition. In situ methods are
complemented with ex-situ studies employing x-ray and neutron diffraction as well as
electrochemical, thermodynamic and metallurgical characterization of candidate alloys. The
information thus gained will be used to develop metal hydride electrodes with improved cycle life,
energy density and lower costs.
Lawrence Berkeley National Laboratory
University of California
Berkeley, CA 94720
Chemical Sciences Division
Characterization of the Li-Electrolyte Interface
| Investigator(s) |
Ross, P.N., Jr.
|
$110,000 |
| Phone | 510-486-6226 |
| E-mail |
PNRoss@lbl.gov |
A detailed understanding of the reactions that occur between metallic Li and the individual
molecular constituents of electrolytes used in Li batteries will be developed. Ultrahigh vacuum
(UHV) deposition methods are used to prepare ultraclean Li surfaces of preferred orientation.
Molecular films of solvent and/or solute molecules are deposited onto the clean surfaces in UHV
at a very low temperature. The reaction between Li and the molecular films is followed using a
combination of UHV surface analytical techniques, including Auger electron spectroscopy (AES),
secondary ionization mass spectroscopy (SIMS), vacuum UV and X-ray photoelectron
spectroscopy (UPES and XPS), and the recently developed variant of XPS termed photoelectron
diffraction. The connection between films formed on Li in UHV and films formed at ambient
temperature and pressure on Li in liquid electrolyte is made by the use of a common
spectroscopy, ellipsometry. Using the fingerprint method, the ellipsometric signatures obtained in
UHV for different surface layers having various known structures and compositions are used to
identify the structure and composition of the film formed on the Li electrode in liquid electrolyte.
Lawrence Berkeley National Laboratory
University of California
Berkeley, CA 94720
Energy and Environment Division
Fundamental Studies of Materials and Processes in Rechargeable Lithium Batteries
| Investigator(s) |
Cairns, E.J.; McLarnon, F.R.; Ross, P.N.
|
$150,000 |
| Phone | 510-486-5028 |
| E-mail |
ejcairns@lbl.gov |
Rechargeable lithium batteries are attractive for vehicular energy storage applications because of
the low equivalent weight and low electronegativity of lithium. However, large-size (capacities
greater than a few Ah) rechargeable lithium batteries have not been developed because of
problems with rechargeability, safety and cost. Fundamental problems associated with interfacial
processes, sensitivity to overcharge and overdischarge, material stability, and electrolyte chemistry
have prevented successful battery scale-up and commercialization. We propose to address these
problems by carrying out fundamental studies of key interfacial processes, cell overcharge
chemistry, and cell materials. The theme of our research closely follows the approach
recommended on p. 8 of the document "Basic Research Needs for Vehicles of the Future" (P.
Eisenberger, ed., published by the Princeton Materials Institute, 1995).
Lawrence Berkeley National Laboratory
University of California
Berkeley, CA 94720
Energy and Environment Division
Application of Pulsed Laser Deposition to the Study of Rechargeable Battery Materials
| Investigator(s) |
Cairns, E.J.; Striebel, K.A.
|
$125,000 |
| Phone | 510-486-5028 |
| E-mail |
ejcairns@lbl.gov |
The aim of this project is to study the performance-limiting phenomena of complex transition
metal oxides of significance to high-performance rechargeable batteries and to suggest practical
means for improving their performance and lifetime. This is done by preparing thin dense oxide
films on electronically conductive and/or transparent substrates with the pulsed laser deposition
(PLD) technique. The films are characterized using such techniques as XRD, SEM, EDS, RBS,
and XPS. Transmission FTIR spectra can also be recorded for films deposited on transparent
substrates. Electrochemical and other properties of the thin-film oxides on stainless steel
substrates, such as active species diffusivity, electrocatalyst kinetics and film corrosion behavior,
are measured for geometries with well-defined electrode-electrolyte interfaces. Thin dense films of
many complex oxides have been prepared, including La0.6Ca0.4CoO3, Bi2Ru2O7 and
LixNi0.15Mn1.85O4. Films of LixMn2O4 have been cycled electrochemically over various
voltage ranges to study the mechanisms for the capacity fade which is often reported for
LixMn2O4 electrodes. The FTIR spectra of LixMn2O4 powders at various states-of-charge and
after cycling have been recorded for reference purposes. Preliminary FTIR characterization of
thin-film LixMn2O4 electrodes suggest that significantly higher resolution spectra are obtained by
using thin-film materials.
Lawrence Berkeley National Laboratory
University of California
Berkeley, CA 94720
Energy and Environment Division
EXAFS and NMR Studies of Electrode Materials for Lithium Batteries
| Investigator(s) |
Cramer, S.P.; Cairns, E.J.; Reimer, J.A.
|
$50,000 |
| Phone | 510-486-4720 |
| E-mail |
spcramer@lbl.gov |
This project seeks to correlate the electrochemical performance of lithium insertion materials with
the atomic and electronic structural changes that occur as a result of the insertion process. X-ray
Absorption Spectroscopy (XAS) is used to characterize the atomic and electronic structure of the
insertion electrode, whereas electrochemical performance is determined through cell cycling and
cyclic voltammetry. Performance yardsticks include reversible capacity, cycle life, and rate
capability. The XAS and electrochemical information are used to determine the solid-state
reaction mechanism of the lithium insertion process, identify performance hampering
characteristics of the electrode material, and point the way towards improved electrode
compositions. The LixMn2O4 spinel system is
the baseline system for this study. This electrode material possesses a 4 volt potential (versus
Li/Li+) for x <1.0 and a 3 volt potential (versus Li/Li+) for x
1.0. It is known
from the literature that LixMn2O4 remains a
single-phase, cubic spinel in the 4 volt region (x <1.0) and transforms to a tetragonal spinel in
the 3 volt region ( x
1.0). During the past year a set of lithium inserted
LixMn2O4 electrodes were studied with XAS at
the Mn K-edge and Mn L2,3-edge. The lithium
content varied from x = 0.1 to x = 2.0 which comprises the useful range for this material. The Mn
K-edge XAS techniques utilized were X-ray Absorption Near Edge Structure (XANES),
Extended X-ray Absorption Fine Structure (EXAFS), Site-Selective X-ray Absorption Near Edge
Structure (SSXANES), and K
Emission Spectroscopy. These measurements took place at the Stanford
Synchrotron Radiation Laboratory and the National Synchrotron Light Source. The XANES data
showed the features of cubic Mn-O octahedral symmetry throughout the 4 volt region (x <
1.0). For samples in the 3 volt region ( x 1.0), the XANES possessed a feature similar to square-planar
transition-metal compounds where a tetragonal-type of spectra was expected. The reason for the
difference between the expected and experimental spectra is attributed to the presence of the
inserted Li altering the Mn electronic structure. K Emission Spectroscopy showed that the Mn in
LixMn2O4 for all x is present in the high-spin
state which supports the hypothesized Jahn-Teller basis of the cubic to tetragonal phase
transformation occurring in the 3 volt region (x 1.0). The EXAFS and SSXANES data are presently undergoing
analysis. Mn L2,3-edge absorption measurements
took place at the Advanced Light Source. L-edge techniques are extremely sensitive to first row
transition-metal oxidation state since transitions to the metal 3d levels are involved. The
L2,3-edge absorption spectra of the
LixMn2O4 samples revealed the presence of
Mn(II) for x = 2.0. The ability to detect Mn(II) makes this technique useful for studies of capacity
fading via electrode dissolution since Mn(II) is soluble in the organic electrolytes employed in
these systems. The L2,3-edge absorption spectra
also showed that the LixMn2O4 system is a
Mn(III)/Mn(IV) mixed valent system which is consistent with literature reports that
LiMn2O4 is a small-polaron semiconductor. These results
illustrate the value of XAS in characterizing the insertion behavior of lithium battery electrodes
because of the sensitivity to both the atomic and electronic structure of the absorbing element. In
the coming year, these XAS techniques will be applied to spinels with Mn substituted by other
transition-metal cations. Evaluating the resulting spectra against those from
LixMn2O4 and comparing the electrochemical
data will lead to greater insight into how atomic and electronic structure correlates with
electrochemical performance. The NMR portion of this project will be initiated within the next
few weeks. It is intended that the same family of materials will be studied using both XAS and
NMR, allowing the study of both the Mn atoms (by XAS), and the Li atoms by NMR. This set of
techniques, combined with the electrochemical studies will provide excellent power in fully
understanding a range of important materials for rechargeable Li cells.
Lawrence Berkeley National Laboratory
University of California
Berkeley, CA 94720
Energy and Environment Division
Fundamental Characterization of Carbon-Based Materials for Electrochemical Systems
The objective of this research is to develop an understanding of the relationship between the
physicochemical properties of carbon-based materials and their electrochemical properties. The
role of surface modification on the electrochemical behavior of carbonaceous materials and
techniques to develop carbon electrodes with well-defined structures and with the appropriate
edge orientation to enhance the rate of electrode kinetics are being investigated. Experiments
were conducted to modify carbon electrodes by electrochemical and chemical techniques, to
utilize various analytical techniques to identify the surface functional groups, and to determine
their influence on double-layer charge storage. Dramatic changes in the electrochemical
double-layer capacitance were evident when a technique involving catalytic etching was used to
modify the surfaces of low-surface-area carbon fibers (rayon-based, PAN-based, and mesophase
carbon fibers). The capacitances of the untreated rayon and the PAN fibers were much higher than
that of the untreated mesophase carbon fiber. After oxidation in air, the capacitance of the rayon
and PAN fibers increased about 60% and the mesophase carbon fibers showed a capacitance
increase of about 100%. After the catalytic treatment the capacitance of the fibers increased
dramatically. The mesophase carbon fibers showed an increase of capacitance of about twenty
times while those for the other two fibers doubled.
Los Alamos National Laboratory
University of California
Los Alamos, NM 87545
Center for Materials Science
Development of Materials for Advanced Ni/Metal Hydride Cells
| Investigator(s) |
Schwarz, R.B.
|
$110,000 |
| Phone | 505-667-8454 |
| E-mail |
rxzs@lanl.gov |
We investigate materials for rechargeable nickel--metal hydride (Ni--MHx) cells with emphasis on
(1) a high-energy storage density; (2) high cyclic life; (3) low H2 overpressure
operation; (4) low cost; and (5) minimal impact of manufacturing, disposal, or recycling on the
ecology. This investigation concentrates on alloys of the type AB5 (e.g.,
LaNi5) and AB2 (e.g., TiV2). Alloy powders are
prepared by mechanical alloying, a high-energy ball milling technique. This technique enables the
preparation of homogeneous alloy powders and two-phase powders with layered morphologies.
The program investigates the relation of composition and microstructure to the kinetics of
hydrogen absorption--desorption, the hydrogen storage capacities, and the degradation of the
hydride material after a number of charging--discharging cycles in concentrated KOH containing
various additives. Under--deposition of zinc from the electrolyte is being investigated as a
dynamic means to decrease the corrosion of the hydride electrode.
National Renewable Energy Laboratory
Golden, CO 80401
Basic Sciences Division
Microstructure and Ion Transport in Vanadium Oxide Electrodes for Rechargeable
Batteries
| Investigator(s) |
Turner, J.A.; Zhang, J.G.
|
$90,000 |
| Phone | 303-384-6667 |
| E-mail |
jturner@nrel.gov |
Vanadium oxide is one of the most promising electrode materials used in secondary lithium
batteries and its development is actively pursued by U.S. industry. This task investigates the
fundamentals of ion transport, charging/discharging and the failure mechanisms in these vanadium
oxide electrodes, to gain a better understanding of the factors which affect the performance,
lifetime and safety of lithium rechargeable batteries. Pulsed Laser Deposition (PLD) has been used
in this work to prepare vanadium oxide films with improved charging capacity and long term
stability. Crystalline vanadium oxide films can be prepared at 200 °C. These films exhibit a
capacity of ~1.5 lithium per vanadium when cycled at a current density of 0.02
mA/cm2. The capacity dropped to 1.2 lithium per vanadium when cycled at a
current density of 0.1 mA/cm2. The capacity loss in these films is less than 2%
after 100 cycles. Although thermally evaporated films exhibited similar initial capacity under the
same charging conditions, they lose more than 17% of their charging capacity after 100 cycles.
The improved cycle stability can be partially attributed to the improved morphology of the PLD
films. The deterioration mechanisms of these electrodes in the cycling process will be the subject
of further examination. Based on both the kinetic data and a new theoretical model/calculation
algorithm developed recently in our laboratory, we will simulate the ion transport processes in a
solid state battery to maximize performance and minimize material requirements under typical
charge/discharge conditions. The potential failure mechanisms and inherent safety problems in the
batteries involving vanadium oxide electrodes will be investigated. Application of our expertise in
the charging/discharging processes of electrochromic materials is expected to provide valuable
cross-fertilization and new insights into the ion insertion kinetics of vanadium oxide and related
components for lithium rechargeable batteries.
Oak Ridge National Laboratory
Oak Ridge, TN 37831
Solid State Division
Rechargeable Thin-Film Lithium Batteries
The purpose of this program is to conduct the research needed for further advancement and
commercialization of thin film rechargeable lithium batteries developed at the Oak Ridge National
Laboratory. These batteries, based on V2O5,
LiMn2O4,
LiMnxM2-xO4, M = Ni, Ti, Al, and
LiCoO2 cathodes, have potentially many applications as small power supplies that
can be integrated into electronic circuits at the board, chip carrier, or the chip level. Several
important issues addressed in this research are crucial to the commercialization of this technology.
These include exploring the structure and transport properties of the cathode-electrolyte interface
which is the major source of cell impedance, conducting long-term cycle testing and investigating
the origin of capacity fading in crystalline and amorphous cathodes, determining cell performance
as a function of temperature and discharge current density, and devising methods for fabricating
multicell batteries. The program also includes investigating the possible use of the lithium
phosphorus oxynitride (Lipon) electrolyte discovered at ORNL as a protective coating that could
improve the performance of existing rechargeable lithium-solid polymer electrolyte and
lithium-liquid electrolyte batteries. Part of this research is carried out under a Cooperative
Research and Development Agreement with a commercial firm. Joint research is also conducted
with groups at the Argonne National Laboratory as well as groups at several universities.
Sandia National Laboratories, New Mexico
Albuquerque, NM 87185
Exploratory Batteries Department
Doping of Lithium Manganese Oxide for Improved Battery Performance
Lithium manganese spinel compounds demonstrate considerable promise as cathodes in the next
generation of rechargeable batteries due to their high specific capacity, long cycle life, and benign
effect upon the environment. In order to evaluate potentially new cathode materials based on the
LiMn2O4 spinel structure, we have used an atomistic theoretical
approach. The atomic simulations employ an energy optimization of the crystal structure based on
the summation of Coulombic, short-range repulsive, and van der Waals interactions. A minimum
energy structure is obtained under the constraint of P1 symmetry and constant pressure
conditions, thereby allowing all 56 atoms of the spinel unit cell and the cell parameters to relax
while maintaining an isometric crystal. A shell model that accounts for electronic polarization is
used to refine the model. Pure LiMn2O4 and various doped
spinels were examined in this study in order to determine the lattice energy, unit cell volume, and
the relative stability of the doped structures. We are studying the effect selected dopants on the
theoretical lattice parameter of spinels as a function of the ionic radii of the substituted metal ion
and dopant amount. Having validated the model, we are developing structure/property
relationships that impact cathode performance in rechargeable lithium battery systems. The doped
lithium manganese dioxide materials were prepared at Sandia using a proprietary non-aqueous
precipitation technique. Ti4+ and Nb5+ doped spinels exhibit
significant unit cell expansion (and energy destabilization), while Al3+ and
Co2+ doped compounds exhibit only slight unit cell contraction. Cathode
performance will be intimately linked to lattice expansion and contraction upon lithiation and
delithiation of the spinel. Molecular dynamics simulations are being used to compare the relative
diffusion rates of lithium ions in these various structures. We are also measuring kinetic
parameters in these materials by electrochemical impedance spectroscopy (EIS). A single EIS
experiment can provide information on both electron transfer and diffusion limited reactions.
Smaller diffusion constants are observed for doped materials with contracted unit cells.
Last updated by Harry J. Dewey, (hd@lanl.gov) on December 16,
1996.