This image may not seem like much, but it’s actually the world’s smallest battery, consisting of a single tin oxide (SnO2) nanowire and that has been developed and visualized under a transmission electron microscope (TEM) by Sandia National Laboratories’ Jianyu Huang.
The battery prototype will help other scientists see what’s really going on in lithium ion storage devices and what happens when they charge/discharge, helping them design better units.
The microscopic battery has an anode made from a single nanowire 100 nanometers wide and 10 micrometers long, a “huge” lithium cobalt oxide cathode measuring three millimeters in length, and a low-vapor-pressure ionic liquid (basically a molten salt).
Nanowire batteries could not be studied so far, because the vacuum in the environment required by the TEM made it difficult to use a liquid electrolyte. Still, Huang demonstrated for the first time that his type of ionic liquid could be used in vacuum.
“What motivated our work,” says Huang, “is that lithium ion batteries [LIB] have very important applications, but the low energy and power densities of current LIBs cannot meet the demand. To improve performance, we wanted to understand LIBs from the bottom up, and we thought in-situ TEM could bring new insights to the problem.”
The most interesting part of his research, published today in the journal Science, is the discovery of how the nanowire anode increases its length during charging, as opposed to the popular belief that the battery actually increased its diameter during that time. This information could teach battery designers to avoid short-circuits in their newest models.
Huang’s group found this result by following the progression of the lithium ions as they travel along the nanowire and create what researchers christened the “Medusa front” – 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 also prove that nanowires can sustain large stress (>10 GPa) induced by lithiation without breaking, indicating that nanowires are very good candidates for battery electrodes,” said Huang.
Still, the researchers were surprised to see the length-wise elongations and dislocations. “No one had ever seen either before,” Huang said. “But our observations tell battery researchers how they are generated, how they evolve during charging, and offer guidance in how to mitigate them. This is the closest view to what’s happening during charging of a battery that’s been achieved so far.”
Huang also said that the volume expansion caused by lithiation, plasticity and pulverization of electrode materials are the major flaws that affect battery designs and reduce their performance. “So our observations of structural kinetics and amorphization [the change from normal crystalline structure] have important implications for high-energy battery design and in mitigating battery failure.”
The battery Huang developed had a current of a picoampere (a millionth of a millionth of an ampere) and 3.5 volts, but even that power could still move nanodevices in their medical quests through the human body to perform repairs. And the information obtained from such an in-depth analysis may be useful for electric cars and even larger applications. It all depends on the atoms, after all.
Below you can see how the lithiation process occurs in the nanowire. This clip was shot by Huang and his team and is the first of its kind. Observe the lithium entering the crystalline tin oxide lattice.