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Topic Name: Future Batteries
Category: Chemical
Research persons: Venkat Srinivasan& his team at Lawrence Berkeley National Laboratory (LBNL)
Location: Berkeley, CA-94720, United States
Details
The BATT Program
addresses the fundamental problems of chemical and mechanical instabilities that
have impeded the development of EV, HEV, and FCEV batteries with acceptable
costs, lifetimes, and safety. The aim is to identify and better understand cell
performance and lifetime limitations before initiating battery scale-up and
development activities. Emphasis is placed on the synthesis of components into
battery cells with determination of failure modes, while maintaining strengths
in materials synthesis and evaluation, advanced diagnostics, and improved
electrochemical model development. The selected battery chemistries are
monitored continuously with timely substitution of more-promising components or
modifications thereof, as appropriate. This is done with advice from within the
BATT Program and from outside experts, including consultation with automotive
companies and DOE. Also factored into the BATT Program decision-making process
is the continuous monitoring of world-wide battery R&D activities, including
assessments carried out by others. This strategy constitutes a systematic
screening of battery chemistries/designs that not only has a built-in
methodology for reselection but also provides a clear focus for the development
of new materials
The BATT
Program (Batteries for Advanced Transportation Technologies) is a $6 million DOE
program that aims to develop the next-generation batteries for use in electric,
hybrid-electric, and plug-in hybrid-electric vehicles. Berkeley Lab's
Environmental Energy Technologies Division (EETD) assists the U.S. Department of
Energy in managing research conducted under this program, which takes place not
only at Berkeley Lab, but other national labs, universities, and private
companies.
The next
generation of batteries in your car is coming from laboratories and from
computer models. Advanced battery development is no longer just a question of
trial and error engineering; scientists increasingly use computer models to
design the best possible battery.
Batteries based
on lithium are considered by many experts to be the most promising, in part
because of their high cell voltage as much as 3.7 volts, as compared to 2.0
volts for a lead-acid battery or 1.2 volts for a nickel metal hydride cell. This
high voltage translates directly into higher energy, which has been key to
commercializing lithium ion (Li-ion) batteries for cellphone and laptop
applications.
And lithium
batteries, says Venkat Srinivasan, a staff scientist in Berkeley Lab's
Environmental Energy Technologies Division (EETD), "will also allow for
significant improvements in the presently available hybrid-electric vehicles,
HEVs. In addition, it is hoped that lithium batteries will pave the way for the
development of plug-in HEVs and the electric vehicles of the future."
For lithium
batteries to become widespread in vehicular applications, however, their
performance and life need to improve, their safety must be enhanced, and their
costs need to decline. "While the HEV market will be the low-hanging fruit, with
plug-in HEVs expected within the next decade, pure electric vehicles will be a
major challenge," Srinivasan says. Even fuel-cell-powered vehicles will need
high-performance batteries, because only batteries can provide the necessary
acceleration. Fuel cells can't ramp power up and down fast enough for rapid
acceleration.
"The mechanism of
charge/discharge in lithium cells involves shuttling the lithium between an
anode and a cathode," explains Srinivasan. "The choice of materials for the
anode, cathode, and electrolyte has a major impact on the various problems
facing lithium batteries today. Even after a decade of research, no magic
combination of material has been found that has all the good attributes. So,
research continues on three classes of cathode materials, four classes of
anodes, and three classes of electrolytes, all in the hope of finding the right
combination that will allow for commercialization."
Srinivasan and
other researchers in EETD are studying batteries in many different ways,
including synthesizing new anodes, cathodes, and electrolytes; fabricating test
batteries with advanced materials and measuring their performance in the lab;
understanding their behavior using advanced diagnostics, including microprobe
techniques; and by creating computer models of battery behavior.
This last is the
approach taken by Srinivasan, who works in EETD's Electrochemical Technologies
Group. Typically the attempt to produce improved batteries involves trial and
error, but Srinivasan is using a more systematic approach to help both the
materials scientists who develop new materials and the engineers who are trying
to optimize whole battery systems.
Srinivasan uses
mathematical models of battery chemistry to evaluate the performance limitations
of particular Li-ion chemistries. He simulates the performance of a particular
chemistry and compares it to experiments performed in the lab to see how well
his model results hold up. From the results he extracts information about what
factors in a particular material are limiting the performance of the battery.
Material developers and battery engineers can use the information to design a
better battery that comes closer to meeting the needs of real applications.
"We get the
physics from simple lab-scale experiments," Srinivasan says, "and then we use
equations to describe this physics. If the model shows that the material looks
promising for, say, a plug-in HEV, then we can spend the time and effort to make
large amounts of this material, to make prototype batteries with it, and to see
how they will perform when used in the real world." What particularly interests
Srinivasan about the work "is that I can connect the materials development
scientists with those who are optimizing the batteries, and I can make this
connection quickly."
Acceleration and
range
In Srinivasan's
presentations he uses a key image, which has become widely popular because of
how clearly it summarizes where the field lies right now. It's a map depicting
the current performance of batteries and other technologies, and where they have
to go to be useful for electric vehicles.
The map's
horizontal axis is power, and represents acceleration; for acceleration
comparable to internal combustion engines, electric cars need to be able to ramp
up power quickly. The map's vertical axis is energy, representing the amount of
energy a battery can store. It's a measure of range the more energy the
battery stores, the farther the car can travel.
Different types
of batteries are represented on the map by curved lines, which show the decrease
in stored energy as power increases. All batteries show a big decline in energy
that is, range as they achieve more and more power, or acceleration.
A star on the
lower right of the map represents the U.S. Department of Energy's goal for
hybrid electric vehicles. Some lithium-ion batteries on the market today already
meet the goal established for hybrid vehicles; these batteries provide
sufficient acceleration but not much range. Nickel metal hydride batteries fall
just short, and lead acid batteries, the oldest of all technologies, trail the
pack.
The upper star on
the map represents DOE's range and acceleration goal for future electric
vehicles. Internal combustion engines sit high on the performance curve, but no
battery technology currently meets the goal, although lithium-ion batteries come
closest. According to some claims, fuel cells could theoretically come close to
the range and acceleration needs of electric vehicles, but this technology is
still unproven.
From real
batteries to models and back again
Srinivasan models
lithium-ion materials sent to Berkeley Lab from many groups throughout the world
who are developing these materials. A model's output for a specific material
might be a plot of how its voltage and capacity changes with increasing power,
for example.
Srinivasan and
other Berkeley Lab researchers perform lab tests on the materials, and similar
battery chemistries from different sources are compared. Srinivasan's model can
tell whether differences in performance are caused by a battery's design or by
something intrinsic to the material itself. Anything from electrode thickness,
to porosity, to particle size, to the parameters of the battery's chemical
reactions can affect the results.
The basic model
that Srinivasan starts with was developed by John Newman, head of the
Electrochemical Technologies Group at Berkeley Lab and a professor of chemical
engineering at UC Berkeley. Newman's group has been modeling batteries since the
1970s, and their approach is widely used throughout the field. Fitting the model
to the specific chemistry he's working with allows Srinivasan to get close to a
battery's actual performance.
"This is what I
love about batteries," he says. "Each one has its own idiosyncrasies; there's
something a little different about each battery chemistry. To get the right
physics, you have to keep adding more details."
Srinivasan has
graphically summarized some of the materials he has modeled recently, again
plotting their energy against their power. Materials come from all over the
world from Berkeley Lab's own groups, from MIT, from a researcher in Slovenia,
and from the Canadian power company Hydro-Quebec, which sent a commercial
prototype. So far no material has come close to the theoretical maximum
performance, which Srinivasan represents by a curve labeled "ideal." The ideal
battery material would have the particle size of the MIT sample and the
conductivity of the Hydro-Quebec sample, so there is still a lot of room for
improvement in this particular set of chemical combinations. Particularly
promising are compounds of lithium iron phosphate with graphite, an electrically
conductive form of carbon.
One important
conclusion Srinivasan drew from this study was that research groups who provide
the materials could identify the maximum energy density of a battery cell by
varying the porosity and thickness of the electrodes.
"My hope is that
five years from now, we will have a plug-and-play model for these battery
materials," says Srinivasan. "Lithium ion batteries are much more complex than
lead acid cells, partly because of the wide variety of materials under
consideration."
Although he
concedes that "We are not at that stage right now," he notes that computer
models have gotten better over the years. "This is because our understanding of
the physics is getting better. As better diagnostics tools are developed,
researchers are beginning to understand the numerous complexities that
characterize batteries."
This has happened
because interest in batteries has led to increased funding and more people
studying the problems. "You need a critical mass of researchers thinking about
batteries every day to make progress," he says.
"And there are
still other battery-related problems to solve," he adds. "For example, we don't
really understand why batteries fail."
Future
Batteries 2:
Developing the
science and technology for next-generation battery systems has long been a focus
of research at Lawrence Berkeley National Laboratory, dating back to the early
1980s. Lithium-ion batteries (sometimes abbreviated Li-ion) are the primary
focus of current research, because their light weight and high energy-density
make them ideal candidates for transportation use.
The Department of
Energy's Office of FreedomCar and Vehicle Technologies is supporting researchers
in the Lab's Environmental Energy Technologies Division (EETD) who are
developing high-performance rechargeable batteries for use in a veritable
alphabet soup of transportation: electric vehicles (EVs), hybrid-electric
vehicles (HEVs), plug-in hybrid-electric vehicles (PHEVs), and fuel-cell
electric vehicles (FCEVs).
Batteries and
other energy storage technologies are critical to advanced, fuel-efficient
transportation so much so that they are one of DOE's Energy Strategic Goals.
The automotive industry is working together with the FreedomCAR and Vehicle
Technologies Program to identify technical barriers to improving energy storage
technologies; DOE-funded research is aimed at toppling these barriers.
The BATT Program,
Batteries for Advanced Transportation Technologies, is a $6 million program
being carried out at Berkeley Lab and other institutions to research fundamental
problems, those chemical and mechanical instabilities which have impeded the
development of EV, HEV, PHEV, and FCEV batteries with acceptable costs,
performance, lifetimes, and safety. The aim is to better understand battery cell
performance and the factors that limit battery lifetime.
Microscale and
nanoscale probes
The third
lightest element on the periodic table, following hydrogen and helium, is
lithium a rising star in battery chemistry. Lithium-ion batteries are
considered the state of the art, the future of battery technology. Energy is
stored in these batteries through the movement of lithium ions between the
cathode, or positive terminal, and the anode, or negative terminal, electrodes
which effectively "house" the ions. (Ions are charged particles, in this case
atoms with net positive charge.)
For
transportation purposes, lithium's very light weight can provide a substantial
savings compared to batteries made of heavier metals. Another big advantage of
Li-ion chemistry is that compared to aqueous batteries such as lead acid, nickel
metal hydride, or nickel cadmium, it yields high open-circuit voltage the
higher the voltage, the higher the power and the better the acceleration.
But these methods
usually cannot sense local phenomena in the electrodes, which take place at the
microscale (measured in millionths of a meter) or even the nanoscale (billionths
of a meter). Kostecki and his Berkeley Lab colleague Frank McLarnon were among
the first to apply instrumental methods allowing them to monitor the composition
and structural changes of battery electrodes at nanoscale or microscale
resolution.
Using
current-sensing atomic force microscopy (CSAFM), Kostecki and McLarnon studied
the surface and electric conductance of composite electrodes used in various
lithium-ion batteries. A single scan of the conductive AFM tip over the cathode
surface produces two images simultaneously, a topographic image and a
conductance image.
The tip of the
current-sensing atomic force microscope is in physical contact with the oxide.
The magnitude of the current is determined by the local electronic properties of
the electrode and the tip, the voltage difference between tip and sample, and
the geometries of the CSAFM tip and the local electrode surface.
The researchers
also used Raman microscopy to carry out a microanalysis of the electrode
surface. Raman microscopy, a spectroscopic technique used in physics and
chemistry, measures laser light scattered from the sample to provide information
about its chemical composition and structure. By collecting thousands of Raman
spectra from small sections of electrode surfaces, they were able to produce and
compare unique color-coded surface composition maps of electrodes from Li-ion
cells.
Our diagnostic
evaluations of composite electrodes revealed changes in electrode surface
composition, structure, electronic conductivity, and local state of charge,
which accompany cell cycling and aging," says Kostecki. "Our hypothesis is that
the phenomena that cause degradation in batteries occur at micro- and nanometer
scale and can only be detected with appropriate microscopic techniques. To
detect them, we have developed and applied techniques and methodologies never
used before in this field. We were the first to use high-resolution Raman
microscopy mapping, which revealed the nonuniform distribution of the electrode
state of charge at a micrometer scale. The data allowed us to identify the local
processes that contribute to significant loss of electronic conductivity within
the electrode and consequently to the capacity loss."
A dopant effect
is not what it seems
Kostecki applied
these local-probe techniques to investigate lithium iron phosphate, LiFePO4,
considered one of the most promising cathode materials for the next generation
of Li-ion batteries.
However, says
Kostecki, "The poor electronic conductivity of LiFePO4 compared to
transition-metal oxide cathodes is a serious limitation on its use in high-power
Li-ion systems. Controversial reports in the literature suggested different
pathways to improving its electrochemical performance. The lack of understanding
of the LiFePO4 operating mechanism has delayed introducing this material into a
new generation of Li-ion batteries."
Kostecki and his
colleagues performed CSAFM and Raman microscopy on two samples of lithium iron
phosphate powder, one pure and one that had been one-percent doped with niobium
by a team of MIT scientists, replacing some lithium atoms with niobium atoms in
an attempt to improve the compound's electronic conductivity.
In the CSAFM
images, the researchers detected no electronic conductance in pristine lithium
iron phosphate at any location; the conductance image was pure white. The
niobium-doped sample, on the other hand, had better electronic conductivity, at
first suggesting that the niobium was indeed responsible for adding
conductivity.
A closer look at
the niobium-doped sample raised questions, however. In the conductance image,
black splotches revealed numerous small sites with good conductance, scattered
across the surface but curiously, on the corresponding morphology image, the
researchers identified grains of active electrode material that had become
completely insulating. Indeed, conductivity was very nonuniform, localized
mainly in deep crevices and pockets between agglomerates.
Typical Raman
spectra of lithium iron phosphate powder showed that the material consisted not
just of lithium iron phosphate but of iron oxides, phosphides, and elemental
carbon impurities as well. The Raman microscopy images of the same powders,
niobium-doped and pure, revealed that the carbon content in the niobium-doped
sample was much higher than in the pristine lithium iron phosphate.
An organic,
niobium-containing precursor used in the doping process was the source of the
extra carbon. The researchers observed that the carbon distribution in the
niobium-doped sample corresponded exactly to the pattern of conductivity
observed in the CSAFM images. They concluded that it was actually the carbon
additive, not the niobium, which was responsible for the doped material's
conductivity increase and better electrochemical performance.
"We would not
have been able to reach this conclusion without the unique combination of
nanoprobe techniques and innovative methodologies that we applied to study this
system," says Kostecki. "We determined that the key to increasing the
conductivity of the material and making it a more effective cathode material was
to incorporate more conductive carbon and to improve the distribution of
carbon deposits. This shifted the focus away from materials science toward
better engineering."
Improving
battery longevity
Kostecki applied
the same set of instrumental methods to study the mechanism of Li-battery
degradation over time. First, using Raman and CSAFM imaging, he and his
colleagues characterized the surface chemical composition, structure,
morphology, and electronic conductivity of a fresh Li-ion electrode. The imaging
allowed them to identify the original distribution of the electrode components
the active material and carbon additives at the surface of the electrode. They
reexamined the electrode with the same tools after prolonged charge/discharge
cycling and storage, looking for changes that might be linked to detrimental
surface phenomena responsible for the loss of electrochemical performance.
The Raman images
showed a marked change in the material's structure as well as its surface
composition and distribution. While some areas of the sample remained relatively
unchanged, elsewhere there were large changes in both surface structure and
composition the more active material was exposed and less carbon additive was
present.
"Loss of surface
electronic conductivity accompanied the observed changes in the surface
chemistry," Kostecki explained. As a result, some particles of the electrode
active material became partially or fully electronically disconnected from the
rest of the electrode and become inactive. "These highly localized phenomena had
severe impacts on the overall electrochemical performance of the electrode and
the whole Li-ion battery. Its charge capacity was diminished and impedance
significantly increased."
Kostecki says,
"The nano- and microprobe analytical tools allowed us to demonstrate that the
localized deactivation processes that occur on a microscopic scale can be
directly linked with the macroscopic behavior. It was the first time these
techniques were applied in such an efficient and concerted way to study battery
surface phenomena. The results of these studies have given us a better
understanding of both the nature of the process, and some ideas about how to
prevent them or slow them down."
It has also
motivated materials scientists and battery engineers to work together more
closely, so that the dream of cheaper, longer lasting, and safer lithium
batteries for advanced electric vehicles becomes a reality sooner.
About
researcher:
Venkat Srinivasan's research
interest is in understanding electrochemical systems. He works with a
team of researchers at the
Lawrence Berkeley National Lab
as part of the
Batteries for Advanced
Transportation Technologies
(BATT) program, involved in solving the multitude of problems that
prevent lithium-ion batteries from being used in electric- and
hybrid-electric-vehicles. BATT is funded
by the US
DOE,
as part of the
FreedomCAR
program.
Venkat Srinivasan
Scientist/Engineer
Lawrence Berkeley National Lab
MS 70 R-108B
Berkeley, CA-94720
Ph: (510) 495 2679
Fax: (510) 486 4260
e-mail:
vsrinivasan@lbl.gov
Financing:
The Batteries for
Advanced Transportation Technologies (BATT) Program is supported by the U.S.
Department of Energy FreedomCAR andVehicle Technologies Program (FCVT)
to help develop high-performance rechargeable batteries for use in electric
vehicles (EVs) and hybrid-electric vehicles (HEVs). The work is carried out by
the Lawrence Berkeley National Laboratory (LBNL)
and several other organizations, and is organized into six separate research
tasks.
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