Better batteries that charge quickly
and last a long time are a brass ring for engineers. But despite decades of
research and innovation, a fundamental understanding of exactly how batteries
work at the smallest of scales has remained elusive.
In a paper published this week in the
journal Science, a team led by William Chueh, an assistant professor of
materials science and engineering at Stanford and a faculty scientist at the
Department of Energy's SLAC National Accelerator Laboratory, has devised a way
to peer as never before into the electrochemical
reaction that fuels the most common rechargeable cell in use today:
the lithium-ion battery.
By visualizing the fundamental building
blocks of batteries - small particles typically measuring less than 1/100th of
a human hair in size - the team members have illuminated a process that is far
more complex than once thought. Both the method they developed to observe the
battery in real time and their improved understanding of the electrochemistry
could have far-reaching implications for battery
design, management and beyond.
"It gives us fundamental insights
into how batteries work," said Jongwoo Lim, a co-lead author of the paper
and post-doctoral researcher at the Stanford Institute for Materials &
Energy Sciences at SLAC. "Previously, most studies investigated the
average behavior of the whole battery. Now, we can see and understand how
individual battery particles charge and discharge."
The heart of a
battery
At the heart of every lithium-ion
battery is a simple chemical reaction in which positively charged lithium ions
nestle in the lattice-like structure of a crystal electrode as the battery is
discharging, receiving negatively charged electrons in the process. In
reversing the reaction by removing electrons, the ions are freed and the
battery is charged.
These basic processes - known as
lithiation (discharge) and delithiation (charge) - are hampered by an
electrochemical Achilles heel. Rarely do the ions insert uniformly across the
surface of the particles. Instead, certain areas take on more ions, and others
fewer. These inconsistencies eventually lead to mechanical stress as areas of
the crystal lattice become overburdened with ions and develop tiny fractures,
sapping battery
"Lithiation and delithiation should be homogenous and uniform," said Yiyang Li, a doctoral candidate in Chueh's lab and co-lead author of the paper. "In reality, however, they're very non-uniform. In our better understanding of the process, this paper lays out a path toward suppressing the phenomenon."
For researchers hoping to improve batteries, like Chueh and his team, counteracting these detrimental forces could lead to batteries that charge faster and more fully, lasting much longer than today's models.
This study visualizes the
charge/discharge reaction in real-time - something scientists refer to as
operando - at fine detail and scale. The team utilized brilliant X-rays and
cutting-edge microscopes at Lawrence Berkeley National Laboratory's Advanced
Light Source.
"The phenomenon revealed by this
technique, I thought would never be visualized in my lifetime. It's quite
game-changing in the battery field," said Martin Bazant, a professor of
chemical engineering and of mathematics at MIT who led the theoretical aspect
of the study.
Chueh and his team fashioned a
transparent battery using the same active materials as ones found in
smartphones and electric vehicles. It was designed and fabricated in
collaboration with Hummingbird Scientific. It consists of two very thin,
transparent silicon nitride "windows." The battery electrode, made of
a single layer of lithium iron phosphate nanoparticles, sits on the membrane
inside the gap between the two windows. A salty fluid, known as an electrolyte,
flows in the gap to deliver the lithium ions to the nanoparticles.
"This was a very, very small
battery, holding ten billion times less charge than a smartphone battery,"
Chueh said. "But it allows us a clear view of what's happening at the
nanoscale."
Significant advances
In their study, the researchers
discovered that the charging process (delithiation) is significantly less
uniform than discharge (lithiation). Intriguingly, the researchers also found
that faster charging improves uniformity, which could lead to new and better
battery designs and power management strategies.
"The improved uniformity lowers
the damaging mechanical stress on the electrodes and improves battery
cyclability," Chueh said. "Beyond batteries, this work could have
far-reaching impact on many other electrochemical materials." He pointed
to catalysts, memory devices, and so-called smart glass, which transitions from
translucent to transparent when electrically charged.
In addition to the scientific knowledge
gained, the other significant advancement from the study is the X-ray
microscopy technique itself, which was developed in collaboration with Berkeley
Lab Advanced Light Source scientists Young-sang Yu, David Shapiro, and Tolek
Tyliszczak. The microscope, which is housed at the Advanced Light Source, could
affect energy research across the board by revealing never-before-seen dynamics
at the nanoscale.
"What we've learned here is not
just how to make a better battery, but offers us a profound new window on
the science of electrochemical reactions at the nanoscale," Bazant said.
EmoticonEmoticon