With electron microscopes and tiny wires far thinner than a human hair, U.S. Department of Energy researchers have pinpointed key events in the life of a consumer electronics staple — the lithium ion battery.
Their findings could lead to smaller, longer-lasting, more powerful batteries ready to rev up next-gen electric vehicles, laptops, cellphones and tablets.
“We think this work will stimulate new thinking for energy storage,” said Chongmin Wang, a materials scientist at the Department of Energy’s Pacific Northwest National Laboratory (PNNL). “We hope that with continued work, it will show us how to design a better battery.”
World’s Smallest Battery
A desire to understand batteries “from the bottom up” motivated Wang, fellow PNNL researcher Wu Xu, DOE Sandia National Laboratories nanotechnology scientist Jianyu Huang, and researchers from the University of Pittsburgh and University of Pennsylvania to create the world’s smallest lithium ion battery, a feat they reported in the Dec. 10 issue of the journal Science.
One seven-thousandth the thickness of a human hair, the battery’s 100-nanometer-wide anode, through which electric charge flows in, is a single nanowire made of tin oxide, Xu explained. From the single-wire anode, the nanobattery’s electric current flows through a liquid electrolyte to a lithium-cobalt oxide cathode.
It’s a design that mimics the ubiquitous consumer electronics battery, albeit on a far smaller scale.
In a rechargeable lithium ion battery (LIB), positively charged lithium ions move from a negative electrode (the anode) to a positive electrode (the cathode) during electric discharge, and back again during recharge.
Lithium ions make great battery chargers because they strongly gravitate toward electrons, initially clustering around the cathode. As charging pumps free electrons into the anode, lithium ions make haste across an electrolyte fluid, flowing from the cathode to the anode.
Playing tunes on an iPod or downloading email on a notebook depletes the newly charged battery, causing electrons to flee the anode while leaving lithium ions behind. In time, those ions return to the cathode, back across the electrolyte fluid.
Atomic-scale examination of battery life was a scientific pipe dream until the DOE team invented a new type of electrolyte, a molten salt that functions under the high-vacuum conditions of transmission electron microscopy.
Use of single nanowires rather than bunched wires or bulk materials was another novel approach, like assessing the strength of a rope by studying its individual threads. Previous battery researchers have studied bulk materials, a process Huang likened to “looking at a forest and trying to understand the behavior of an individual tree.”
These dual innovations provided what he termed “the closest view to what’s happening during charging of a battery that researchers have achieved so far,” including how so-called “lithiation stresses” — physical nanowire distortions — take a toll on battery life.
“Lithiation means squeezing lithium into a material, which happens during battery charging,” Huang told TechNewsWorld. “Our observations — which initially surprised us — tell battery researchers how lithiation distortions are generated, how they evolve during charging, and offer guidance on how to mitigate them.”
Medusa’s Hair – and Glare
The distortions and contortions the nanowires sustain during lithiation create a many-headed area of atomic dislocations the researchers christened the “Medusa front.”
Medusa was a Greek Gorgon, a mythological female monster with snakes for hair whose countenance could turn a person to stone.
“The dislocations emanating from the Medusa front are just like Medusa’s hair snaking out of her head,” said Huang.
A high-resolution video of the tin oxide wires shows them behaving like snakes during a meal, writhing and fattening by as much as 250 percent as lithium ions feed them with electricity.
The nanowire’s lively behavior is important for several reasons, PNNL’s Xu explained. Repeated distortions can introduce tiny defects that accumulate, damaging electrode materials. Indeed, over time lithiation changes the tin oxide from a neatly arranged crystal to an amorphous glass — not unlike the Medusa’s flesh-to-stone changing glare.
“The insertion of lithium ions into tin oxide crystals leads to a phase transformation, from crystalline to amorphous,” PNNL’s Wang told TechNewsWorld. “Accompanying this phase transformation is the volume expansion.”
Along with the volume expansion, the researchers observed that upon recharging the battery, the tin oxide nanowires nearly double in length, a finding that conflicts with the conventional wisdom — that batteries swell across their diameter.
To help avoid short circuits that shorten battery life, “manufacturers should take account of this elongation in their battery designs,” said Sandia’s Huang. “The gap between the cathode and anode needs to be more than double the length of the nanowire, so that no short circuit will occur during charging.”
Lab to Market
Observing that nanowires “were able to withstand the deformations associated with electrical flow better than bulk tin oxide, which is a brittle ceramic,” PNNL’s Wang envisions a rudimentary design for a nanoscale battery that works as well in the marketplace as it does in the laboratory.
“It reminds me of making a rope from steel — you wind together thinner wires rather than making one thick rope,” he said.
Presently studying silicon, which works extremely well with lithium ions, the DOE research team’s nano-sized rechargeable battery “might look like a human hair,” Huang said.
Until they finish such a design, however, “the methodology we developed should stimulate extensive real-time studies of the microscopic processes in batteries,” he said, “and lead to a more complete understanding of the mechanisms governing battery performance and reliability.”