HOUGHTON – Most
people see defects as flaws. A few Michigan Technological University
researchers, however, see them as opportunities. Twin boundaries, which are
small, symmetrical defects in materials, may present an opportunity to improve
lithium-ion batteries. The twin boundary defects act as energy highways and
could help get better performance out of the batteries.
This
finding, published in
Nano Letters earlier
this year, turns a previously held notion of material defects on its head. Reza
Shahbazian-Yassar helped lead the study and holds a joint appointment at
Michigan Tech as the Richard & Elizabeth Henes associate professor in
nanotechnology and an adjunct associate professor in materials science and
engineering. Anmin Nie, a senior postdoctoral researcher in his group,
conducted the study.
Nie says
that material defects, including twin boundaries, are naturally occurring and
majority of the past research has focused on removing them from materials.
“We look at
the nanostructure of the battery materials that are out there,” he explains. “We have noticed some defects, such as twin boundaries, that exist in these
materials can be good channels that will help us to transport lithium ions.”
That
movement of ions is key to making better, stronger batteries.
How
Lithium-Ion Batteries Work
Batteries
power most of our gadgets. Shahbazian-Yassar says, “The focus over the past few
years has been on rechargeable batteries, most specifically the lithium-ion
battery.”
That’s
because lithium-ion batteries are lightweight, pack a whopping punch of energy
density, and their efficiency continues to climb. Like all basic batteries,
ones run on lithium ions rely on shuttling ions from one place to another.
Technically speaking, that’s between the anode and cathode, and an electric
current coaxes ions to shuffle between them. A low battery means there is less
exchange happening between the anode and cathode. Twin boundaries could help hustle
that exchange along or perhaps extend it, hopefully without losing battery
life.
Twin
Boundaries in Tin Oxides
Twin
boundaries basically are mirror images, places in a material where one side of
atomic arrangements reflects another. They often result while making a
material, which shifts the atoms out of place a smidge.
“Without a
detailed view of the atomic arrangements, one might think the structure of
electrode material is perfect, but then when you pay attention at the atomic
level, you’ll notice that these atoms are all symmetric with one plane,” Nie
says, explaining that the symmetry causes problems because it creates weak
spots.
At the same
time, that symmetry is what provides a route for ions to travel along.
Shahbazian-Yassar and his team received a grant from the
Division of Materials Research at the National Science Foundation last fall to explore this and have now shown
that a twin boundary acts as a highway for lithium ion transport.
“Usually the
available free space within the crystal is what ions use to move in or out of
the electrode,” Shahbazian-Yassar says, explaining that the space is like a
crowded city with narrow streets and the ions resemble the moving cars. “If
there is an accident, road construction, or simply traffic, cars can not easily
pass through the streets, similar phenomenon happens in batteries.
Lithium ions
need wide and open roads in order to shuttle in and out of the battery
electrodes. Any obstruction to the moving ions will reduce the amount of energy
or power extracted from a battery.
Next Steps
in Energy Storage
The research
team examined twin boundaries in tin oxides, but Shahbazian-Yassar says it’s
applicable in many battery materials. The next step is finding out how to
optimize these defects to balance the mechanical integrity with the amount of
twin structures. Finding that balance will be the focus of the researchers’ next
steps, and this new finding about twin boundaries lays the groundwork for
improving lithium-ion batteries.
Allison
Mills is a Science and Tech Writer for Michigan Technological University
