Article:
Better Lithium-Ion Batteries Are On The Way From Berkeley
Lab
From: NBC news
Editor: Paul Preuss
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, now available
online.
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.
At top, spectra of a series of polymers obtained with soft x-ray
absorption spectroscopy at ALS beamline 8.0.1 show a lower “lowest
unoccupied molecular orbital” for the new Berkeley Lab polymer,
PFFOMB (red), than other polymers (purple), indicating better potential
conductivity. Here the peak on the absorption curve reveals the lower key
electronic state. At bottom, simulations disclose the virtually complete,
two-stage electron charge transfer when lithium ions bind to the new
polymer.
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.
Cycling for success
Transmission electron microscopy reveals the new conducting
polymer’s improved binding properties. At left, silicon particles
embedded in the binder are shown before cycling through charges and
discharges (closer view at bottom). At right, after 32 charge-discharge cycles,
the polymer is still tightly bound to the silicon particles, showing why the
energy capacity of the new anodes remains much higher than graphite anodes
after more than 650 charge-discharge cycles during testing.
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.