Formerly unobserved increase in length and twist of the
anode in a nanobattery (Image: DOE Center for Integrated
Nanotechnologies)
Because battery technology hasn’t developed as quickly as the
electronic devices they power, a greater and greater percentage
of the volume of these devices is taken up by the batteries
needed to keep them running. Now a team of researchers working
at the Center for Integrated Nanotechnologies (CINT) has created
the world’s smallest battery, and although the tiny battery
won’t be powering next year’s mobile phones, it has already
provided insights into how batteries work and should enable the
development of smaller and more efficient batteries in the
future.
The tiny rechargeable, lithium-based battery was created by a
team led by
Sandia National Laboratories researcher Jianyu Huang. It
consists of a bulk lithium cobalt cathode three millimeters
long, an ionic liquid electrolyte, and has as its anode a single
tin oxide (Sn02) nanowire 10 nanometers long and 100 nanometers
in diameter – that’s one seven-thousandth the thickness of a
human hair.
Because nanowire-based materials in lithium-ion batteries
offer the potential for significant improvements in power and
energy density over bulk electrodes the researchers wanted to
gain an understanding of the fundamental mechanisms by which
batteries work. They therefore formed the battery inside a
transmission electron microscope (TEM) so they could study the
charging and discharging of the battery in real time and at
atomic scale resolution.
By following the progression of the lithium ions as they
travel along the nanowire, the researchers found that during
charging the tin oxide nanowire rod nearly doubles in length.
This is far more than its diameter increases and could help
avoid short circuits that may shorten battery life. This
unexpected finding goes against the common belief of workers in
the field that batteries swell across their diameter, not
longitudinally.
“Manufacturers should take account of this elongation in
their battery design,” Huang said. “These observations prove
that nanowires can sustain large stress (>10 GPa) induced by
lithiation without breaking, indicating that nanowires are very
good candidates for battery electrodes,” he added.
Atomic-scale examination of the charging and discharging
process of a single nanowire had not been possible before
because the high vacuum in a TEM made it difficult to use a
liquid electrolyte. Huang’s group overcame this problem by
demonstrating that a low-vapor-pressure ionic liquid –
essentially molten salt – could function in the vacuum
environment.
This means that although the work was carried out using tin
oxide nanowires, Huang says the experiments could be extended to
other materials systems, either for cathode or anode studies.
“The methodology that we developed should stimulate extensive
real-time studies of the microscopic processes in batteries and
lead to a more complete understanding of the mechanisms
governing battery performance and reliability,” he said. “Our
experiments also lay a foundation for in-situ studies
of electrochemical reactions, and will have broad impact in
energy storage, corrosion, electrodeposition and general
chemical synthesis research field.”
The research team’s work is reported in the December 10 issue
of the journal
Science.
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