X-rays offer clear view of how lithium-ion batteries work

(Credit: iStockphoto)

Despite decades of research, scientists haven’t been able to fully understand how batteries work at the smallest of scales.

In a paper published in the journal Science, researchers describe a way to peer as never before into the electrochemical reaction that fuels the most common rechargeable cell in use today: the lithium-ion battery.

“The phenomenon revealed by this technique, I thought would never be visualized in my lifetime.”

By visualizing the fundamental building blocks of batteries—small particles typically measuring less than 1/100th of a human hair in size—the team from Stanford University has illuminated a process that is far more complex than once thought. Both the method they developed to observe the battery in real time and their improved understanding of the electrochemistry could have far-reaching implications for battery design, management, and beyond.

“It gives us fundamental insights into how batteries work,” says Jongwoo Lim, a co-lead author of the paper and postdoctoral researcher at the Department of Energy’s SLAC National Accelerator Laboratory. “Previously, most studies investigated the average behavior of the whole battery. Now, we can see and understand how individual battery particles charge and discharge.”

Make batteries last longer

At the heart of every lithium-ion battery is a simple chemical reaction in which positively charged lithium ions nestle in the lattice-like structure of a crystal electrode as the battery is discharging, receiving negatively charged electrons in the process. In reversing the reaction by removing electrons, the ions are freed and the battery is charged.

nanoparticles charging and discharging
Greatly magnified nanoscale particles are shown here charging (red to green) and discharging (green to red). The animation shows regions of faster and slower charge. (Credit: SLAC National Accelerator Laboratory)

These basic processes—known as lithiation (discharge) and delithiation (charge)—are hampered by an electrochemical Achilles heel. Rarely do the ions insert uniformly across the surface of the particles.

Instead, certain areas take on more ions, and others fewer. These inconsistencies eventually lead to mechanical stress as areas of the crystal lattice become overburdened with ions and develop tiny fractures, sapping battery performance and shortening battery life.

“Lithiation and delithiation should be homogenous and uniform,” says Yiyang Li, a doctoral candidate and co-lead author of the paper. “In reality, however, they’re very non-uniform. In our better understanding of the process, this paper lays out a path toward suppressing the phenomenon.”

This safe battery won’t burst into flames

For researchers hoping to improve batteries, counteracting these detrimental forces could lead to batteries that charge faster and more fully, lasting much longer than today’s models.

This study visualizes the charge/discharge reaction in real-time—something scientists refer to as operando—at fine detail and scale. The team utilized brilliant X-rays and cutting-edge microscopes at Lawrence Berkeley National Laboratory’s Advanced Light Source.

“The phenomenon revealed by this technique, I thought would never be visualized in my lifetime. It’s quite game-changing in the battery field,” says Martin Bazant, a professor of chemical engineering and of mathematics at MIT who led the theoretical aspect of the study.

A transparent battery

The researchers fashioned a transparent battery using the same active materials as ones found in smartphones and electric vehicles. It was designed and fabricated in collaboration with Hummingbird Scientific. It consists of two very thin, transparent silicon nitride “windows.”

The battery electrode, made of a single layer of lithium iron phosphate nanoparticles, sits on the membrane inside the gap between the two windows. A salty fluid, known as an electrolyte, flows in the gap to deliver the lithium ions to the nanoparticles.

illustration of battery particles
Artist’s rendition shows lithium-ion battery particles under the illumination of a finely focused X-ray beam. (Credit: Courtesy Chueh Lab)

“This was a very, very small battery, holding ten billion times less charge than a smartphone battery,” says William Chueh, an assistant professor of materials science and engineering at Stanford and a faculty scientist at SLAC, who led the team. “But it allows us a clear view of what’s happening at the nanoscale.”

“What we’ve learned here is not just how to make a better battery, but offers us a profound new window on the science of electrochemical reactions at the nanoscale.”

The researchers discovered that the charging process (delithiation) is significantly less uniform than discharge (lithiation). Intriguingly, the researchers also found that faster charging improves uniformity, which could lead to new and better battery designs and power management strategies.

“The improved uniformity lowers the damaging mechanical stress on the electrodes and improves battery cyclability,” Chueh says. “Beyond batteries, this work could have far-reaching impact on many other electrochemical materials.”

He points to catalysts, memory devices, and so-called smart glass, which transitions from translucent to transparent when electrically charged.

“What we’ve learned here is not just how to make a better battery, but offers us a profound new window on the science of electrochemical reactions at the nanoscale,” Bazant says.

The US Department of Energy, Office of Basic Energy Sciences, and the Ford-Stanford Alliance funded the work. Bazant was a visiting professor at Stanford and was supported by the Global Climate and Energy Project.

Source: Stanford University