Real-Time Observation of Nanowire Anode to Improve Lithium Batteries - World's Smallest Battery

A benchtop version of the world's smallest battery has been created by a team led by Sandia National Laboratories researcher. The anode of this battery is a single nanowire which is claimed to be one seven-thousandth the thickness of a human hair.

To better study the anode's characteristics, the tiny rechargeable lithium-based battery was formed inside a transmission electron microscope (TEM) at the Center for Integrated Nanotechnologies (CINT), a Department of Energy research facility jointly operated by Sandia and Los Alamos national laboratories.

This experiment facilitates the researchers to study the charging and discharging of a battery in real time and at atomic scale resolution, so that they can understand the fundamental mechanism how batteries work.

The motivation behind this work lies in the fact that current lithium ion batteries have very important application but cannot meet the demand due their low power and energy density. To improve performance they need to be investigated from the bottom up; and TEM could bring new insights to the problem.

As nanowire-based materials in lithium ion batteries significantly improved in power and energy density over bulk electrodes, more stringent investigations of their operating properties should improve new generations of plug-in hybrid electric vehicles, laptops and cell phones.

Battery research groups do use nanomaterials as anodes, but in bulk rather than individually -- a process, Scientist Huang says, that resembles "looking at a forest and trying to understand the behavior of an individual tree."

The tiny battery consists of a single tin oxide nanowire anode 100 nanometers in diameter and 10 micrometers long, a bulk lithium cobalt oxide cathode three millimeters long, and an ionic liquid electrolyte. The device offers the ability to directly observe change in atomic structure during charging and discharging of the individual wires.

An unexpected find of the researchers was that the tin oxide nanowire rod nearly doubles in length during charging which is far more than its diameter increases -- a fact that could help avoid short circuits that may shorten battery life. In future manufacturers should take account of this elongation in their battery design.

Huang's group found this flaw by following the progression of the lithium ions as they travel along the nanowire and create what researchers described as "Medusa front" defined by an area where high density of mobile dislocations cause the nanowire to bend and wiggle as the front progresses. The web of dislocations is caused by lithium penetration of the crystalline lattice. These observations prove that nanowires can sustain large stress (>10 GPa) induced by lithiation without breaking; a clear indicating that these nanowires are very good candidates for battery electrodes.

Lithiation-induced volume expansion, plasticity and pulverization of electrode materials are the major mechanical defects that plague the performance and lifetime of high-capacity anodes in lithium-ion batteries. So these observations of structural kinetics and amorphization have important implications for high-energy battery design and in mitigating battery failure.

Researchers estimated a current level of a picoampere flowing in the nanowire during charging and discharging.

Although the work was carried out using tin oxide (SnO2) nanowires, the experiments can be extended to other materials systems, either for cathode or anode studies.

The methodology that was 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.