Tech Briefs Magazine - October 2021 - 37

module performed far better under the
LED than sunlight. Both the GaInP and
GaAs modules significantly outpaced silicon
indoors, converting 23.1% and 14.1%
of the LED light into electrical power,
respectively, compared with silicon's 9.3%
power conversion efficiency.
The rankings were the same for a charging
test in which they timed how long it
took the modules to fill a half-charged 4.18volt
battery, with silicon coming in last by a
margin of more than a day and a half. The
team was interested in learning if the silicon
module, despite its poor perfor mance
relative to its top-shelf competitors, could
generate enough power to run a lowdemand
Internet of Things (IoT) device.
The IoT device chosen for the experiment
was a temperature sensor that was
hooked up to the silicon PV module,
placed once more under an LED. Upon
turning the sensor on, the researchers
found that it was able to feed temperature
readings wirelessly to a computer nearby
that was powered by the silicon module
alone. After two hours, they switched off
Long-Life Lithium-Metal Battery
The technology could be key to lighter, less expensive, and long-lasting batteries for future
electric vehicles.
Pacific Northwest National Laboratory, Richland, Washington
ithium-metal batteries hold almost
twice the energy of their widely used
lithium-ion counterparts and they're
lighter. That combination offers the
prospect of an electric vehicle that would
be lighter and go much farther on a single
charge. But lithium-metal batteries in the
laboratory have been plagued by premaL
ture
death, lasting only a fraction of the
time of today's lithium-ion batteries.
Researchers have created a lithiummetal
battery that lasts for 600 cycles,
meaning it can be fully charged and discharged
600 times before it dies. While
the lithium-ion batteries used in electric
vehicles today hold less energy, they last
longer - typically at least 1,000 cycles.
But vehicles won't go as far on one
charge as they would with an effective
lithium-metal battery.
Instead of using anodes with more
lithium, the team found a way to in -
crease the battery's lifetime by using
incredibly thin strips of lithium just 20
microns wide, far thinner than the
width of a human hair. The lithiummetal
battery has an energy density of
350 watt-hours per kilogram (Wh/kg)
and after 600 cycles, the battery re -
tained 76 percent of its initial capacity.
The battery is a pouch cell, which more
closely mirrors real-world conditions
than does a coin cell - a less realistic
type of device used in many battery
research projects.
The scientists found that thicker strips
contribute directly to battery failure.
That's due to complex reactions around a
film on the anode known as the solid electrolyte
interphase (SEI). The SEI is the
byproduct of side reactions between lithium
and the electrolyte. It acts as an important
gatekeeper that allows certain molecules
to go from the anode to the electrolyte
and back again while keeping
other molecules at bay.
The team worked on pouch cells (top), which are much more realistic compared to coin cells (bottom).
(Photos courtesy of Jie Xiao/Pacific Northwest National Laboratory)
Tech Briefs, October 2021
Cov
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ToC
An SEI working effectively allows certain
lithium ions to pass through but
limits unwanted chemical reactions that
reduce battery performance and accelerate
cell failure. A primary goal for re -
searchers has been to reduce unwanted
side reactions between the electrolyte
and the lithium metal - to encourage
vital chemical reactions while restraining
unwanted ones.
37
the light in the black box and the sensor
continued to run, its battery depleting at
half the rate it took to charge.
The researchers' findings suggest that
an already ubiquitous material in outdoor
PV modules could be repurposed
for indoor devices with low-capacity batteries.
The results are particularly applicable
to commercial buildings where
lights are on around the clock.
For more information, contact Jonathan
Griffin at jonathan.griffin@nist.gov; 301975-4117.

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Tech Briefs Magazine - October 2021

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