With some custom-built hardware and an electron microscope, researchers at Oak Ridge National Laboratory recently invented a way to watch how lithium-ion batteries work in real time at nanometer scales.
A nanometer is one-billionth of a meter, a measure that scientists use to quantify unseeably tiny things, such as the width of a molecule. The technique helps scientists understand how batteries age and eventually fail, paving the way toward new cell designs that last longer and hold more energy. The researchers outlined their methods last month in the journal Chemical Communications.
Raymond Unocic, a co-author and a research and development staff scientist at Oak Ridge, explained that the team was investigating a structure known as a solid electrolyte interphase (SEI). It forms on a battery's electrodes as the electrolyte breaks down during the charge and discharge cycle.
As an SEI forms, it sops up some of the lithium ions in the cell, leading to a permanent capacity loss. It also hinders the remaining lithium ions as they shuttle back and forth between the electrodes, degrading the performance of the cell. "This is the first time people were able to understand how this interphase forms," Unocic said.
Researchers have linked SEI to dendrite formation, as well. Dendrites are tiny, growing branches of lithium metal that build up on electrodes as a cell ages. If the branches get long enough, they can pierce the membrane separating the anode and the cathode, shorting the cell and causing catastrophic failure.
These problems are becoming more pressing as lithium-ion cells move beyond the small packs powering laptops and phones -- a market they have mastered -- toward the hefty and expensive batteries that will drive electric cars and trucks for up to a decade.
Previous studies had a hard time replicating a real lithium cell because the liquid electrolyte would evaporate in the vacuum inside the transmission electron microscope specimen chamber. In this study, researchers designed a cell with gold electrodes that used a liquid electrolyte built into a bespoke microscope specimen holder.
"The most significant thing we have is the ability to image electrodes in a liquid cell, but at the same time, we're able to couple that with quantitative electrochemical measurements," Unocic said.
Moment-by-moment view of aging process
The researchers charged and discharged the cell and carefully watched what happened, sweeping across a range of voltages. They found that an SEI started forming early in the cell's operating life as polymer layers. "We see it right away," said Robert Sacci, a postdoctoral associate in the Materials Science and Technology Division at Oak Ridge and a co-author. "It happens really close to where we start depositing the lithium."
The researchers also saw what they interpreted as the seeds of lithium dendrite formation within the SEI. "If we push this hard enough and we did not care about breaking the microscope, we could probably grow lithium dendrites," Sacci said.
After substantial testing, automakers are confident lithium-ion batteries will last a long time. Researchers found that the aging process in the cells starts almost right away, with measurable changes in performance over time. With the new microscopy system, scientists can watch the breakdown carefully as it happens rather than simply conducting autopsies on decrepit batteries, using the observations to build better battery chemistries for the future.
"SEI is a critical component in all the advanced batteries," said Kang Xu, a chemist at the U.S. Army Research Laboratory who was not involved in this study. "With this technique, we can directly observe the growth of SEI and we can study its growth mechanistically."
However, visualizing how a battery works in real time is only one of the tools needed to understand a vexing problem. "Solid electrolyte interphases are very complex, and one needs a combination of different techniques to get a comprehensive picture of what is actually going on at the electrolyte/electrode surfaces," Xu said.
The Oak Ridge researchers said they want to start investigating other battery materials, like carbon nanofibers and silicon. They are especially interested in lithium metal, which has a very high energy density compared to conventional graphite electrodes but has a troubling tendency to grow dendrites. By figuring out how these structures form, scientists can come up with ways to slow them down or stop them altogether.
The ultimate goal, however, is to make battery energy storage more practical, whether in cars or on the grid, and figuring out how an SEI forms is a crucial step in that direction.
'Center stage' of research
"It's at center stage for a good reason," said Kevin Gering, technical lead for battery energy storage at the Idaho National Laboratory, who was not involved in this study. "The SEI is really the gatekeeper for battery performance and lithium-ion chemistries."
Automakers including General Motors Co. and Tesla Motors Inc. can't just offer batteries -- often the single most expensive component in a hybrid or an electric car -- with good sticker performance out of the gate; they have to guarantee that same performance years from now, whether in Phoenix or Minneapolis.
And how long a battery lasts is often different from how well it holds a charge. Nissan's all-electric Leaf, for instance, has an eight-year warranty against defects on its battery but only a five-year warranty against capacity loss below nine bars as shown on the car's capacity gauge.
"If you're talking about [battery] longevity in vehicles, the biggest issue right now is thermal management," Gering said. "In general, higher temperatures accelerate chemical kinetic reactions." This includes SEI formation, though the cold introduces its own challenges.
"Dendrites tend to be more of an issue at lower temperatures when you charge too aggressively," he added.
Oak Ridge's Sacci said the team is working on making a more realistic cell that they can observe at the nanometer level. "We're getting closer and closer to simulating an entire cell on a much smaller length scale," he explained.
Eventually, researchers expect their new visualization system will lead to payoffs like greater energy densities and lower costs. "We can look at site-specific phenomenon in these small cells, but we're trying to chip away at these real-world problems," Unocic concluded.