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Lithium-ion batteries are everywhere, in smart
phones, laptops, an array of other consumer electronics, and the newest
electric cars. Good as they are, they could be much better, especially when it
comes to lowering the cost and extending the range of electric cars. To do
that, batteries need to store a lot more energy.
The anode is a critical component for storing
energy in lithium-ion batteries. A team of scientists at the U.S. Department of
Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has designed a
new kind of anode that can absorb eight times the lithium of current designs,
and has maintained its greatly increased energy capacity after over a year of
testing and many hundreds of charge-discharge cycles.
The secret is a tailored polymer that conducts
electricity and binds closely to lithium-storing silicon particles, even as
they expand to more than three times their volume during charging and then
shrink again during discharge. The new anodes are made from low-cost materials,
compatible with standard lithium-battery manufacturing technologies. The
research team reports its findings in Advanced Materials.
High-capacity
expansion
“High-capacity lithium-ion anode materials have always confronted the
challenge of volume change—swelling—when electrodes absorb lithium,” says Gao
Liu of Berkeley Lab’s Environmental Energy Technologies Division (EETD), a
member of the BATT program (Batteries for Advanced Transportation Technologies)
managed by the Lab and supported by DOE’s Office of Vehicle Technologies.
Says Liu, “Most of today’s lithium-ion batteries have anodes
made of graphite, which is electrically conducting and expands only modestly
when housing the ions between its graphene layers. Silicon can store 10 times
more—it has by far the highest capacity among lithium-ion storage materials—but
it swells to more than three times its volume when fully charged.”
This kind of swelling quickly breaks the electrical contacts
in the anode, so researchers have concentrated on finding other ways to use
silicon while maintaining anode conductivity. Many approaches have been
proposed; some are prohibitively costly.
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One less-expensive approach has been to mix silicon
particles in a flexible polymer binder, with carbon black added to the mix to
conduct electricity. Unfortunately, the repeated swelling and shrinking of the
silicon particles as they acquire and release lithium ions eventually push away
the added carbon particles. What’s needed is a flexible binder that can conduct
electricity by itself, without the added carbon.
“Conducting polymers aren’t a new idea,” says Liu, “but
previous efforts haven’t worked well, because they haven’t taken into account
the severe reducing environment on the anode side of a lithium-ion battery,
which renders most conducting polymers insulators.”
One such experimental polymer, called PAN (polyaniline), has
positive charges; it starts out as a conductor but quickly loses conductivity.
An ideal conducting polymer should readily acquire electrons, rendering it
conducting in the anode’s reducing environment.
The signature of a promising polymer would be one with a low
value of the state called the “lowest unoccupied molecular orbital,” where
electrons can easily reside and move freely. Ideally, electrons would be
acquired from the lithium atoms during the initial charging process. Liu and
his postdoctoral fellow Shidi Xun in EETD designed a series of such polyfluorene-based
conducting polymers—PFs for short.
When Liu discussed the excellent performance of the PFs with
Wanli Yang of Berkeley Lab’s Advanced Light Source (ALS), a scientific
collaboration emerged to understand the new materials. Yang suggested
conducting soft x-ray absorption spectroscopy on Liu and Xun’s candidate
polymers using ALS beamline 8.0.1 to determine their key electronic properties.
Says Yang, “Gao wanted to know where the ions and electrons
are and where they move. Soft x-ray spectroscopy has the power to deliver
exactly this kind of crucial information.”
Compared with the electronic structure of PAN, the
absorption spectra Yang obtained for the PFs stood out immediately. The
differences were greatest in PFs incorporating a carbon-oxygen functional group
(carbonyl).
“We had the experimental evidence,” says Yang, “but to
understand what we were seeing, and its relevance to the conductivity of the
polymer, we needed a theoretical explanation, starting from first principles.”
He asked Lin-Wang Wang of Berkeley Lab’s Materials Sciences Division (MSD) to
join the research collaboration.
Wang and his postdoctoral fellow, Nenad Vukmirovic,
conducted ab initio calculations of the promising polymers at the
Lab’s National Energy Research Scientific Computing Center (NERSC). Wang says, “The calculation tells you what’s really going on—including precisely how the
lithium ions attach to the polymer, and why the added carbonyl functional group
improves the process. It was quite impressive that the calculations matched the
experiments so beautifully.”
The simulation did indeed reveal “what’s really going on”
with the type of PF that includes the carbonyl functional group, and showed why
the system works so well. The lithium ions interact with the polymer first, and
afterward bind to the silicon particles. When a lithium atom binds to the
polymer through the carbonyl group, it gives its electron to the polymer—a
doping process that significantly improves the polymer’s electrical
conductivity, facilitating electron and ion transport to the silicon particles.
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Cycling for success
Having gone through one cycle of material synthesis at EETD, experimental
analysis at the ALS, and theoretical simulation at MSD, the positive results
triggered a new cycle of improvements. Almost as important as its electrical
properties are the polymer’s physical properties, to which Liu now added
another functional group, producing a polymer that can adhere tightly to the
silicon particles as they acquire or lose lithium ions and undergo repeated
changes in volume.
Scanning electron microscopy and transmission electron
microscopy at the National
Center for Electron
Microscopy (NCEM), showing the anodes after 32 charge-discharge cycles,
confirmed that the modified polymer adhered strongly throughout the battery
operation even as the silicon particles repeatedly expanded and contracted.
Tests at the ALS and simulations confirmed that the added mechanical properties
did not affect the polymer’s superior electrical properties.
“Without the input from our partners at the ALS and in MSD,
what can be modified and what should not be modified in the next generation of
polymers would not have been obvious,” says Vince Battaglia, Program Manager of
EETD’s Advanced Energy Technologies Department.
“This achievement provides a rare scientific showcase,
combining advanced tools of synthesis, characterization, and simulation in a
novel approach to materials development,” says Zahid Hussain, the ALS Division
Deputy for Scientific Support and Scientific Support Group Leader. “The cyclic
approach can lead to the discovery of new practical materials with a
fundamental understanding of their properties.”
The icing on the anode cake is that the new PF-based anode
is not only superior but economical. “Using commercial silicon particles and
without any conductive additive, our composite anode exhibits the best
performance so far,” says Gao Liu. “The whole manufacturing process is low cost
and compatible with established manufacturing technologies. The commercial
value of the polymer has already been recognized by major companies, and its
possible applications extend beyond silicon anodes.”
Anodes are a key component of lithium-ion battery
technology, but far from the only challenge. Already the research collaboration
is pushing to the next step, studying other battery components including
cathodes.
