Better batteries from the ground up
ORNL microscopy explores the "room at the bottom" in lithium-ion batteries
Today, people talk about their electronic devices almost as if they were living, breathing beings. We wake our computers up, our cellphones die and we have longer conversations with our GPS devices than with many of our friends.
As new wireless technologies appear in devices from tablet computers to electric cars, efforts to improve these life forms focus on a common organ: the heart-like battery. Yet despite an accelerating demand for battery-powered devices, the pulse of the electronics world is not as well understood as you might think.
Kalinin is among a team of scientists at ORNL's Center for Nanophase Materials Sciences that is developing new microscopic methods to analyze and understand nanoscale complexities inside electrochemical systems such as lithium-ion batteries. "Richard Feynman famously noted that there is plenty of room at the bottom," Kalinin says, referring to the physicist's 1959 talk on the potential of nanoscience. "This room does not do us much good if we cannot explore it."
At first glance, batteries seem simplistic because they consist of only three major components— negative anode, positive cathode and electrolyte. But each part is itself a complex matrix of different materials that interact as ions and electrons flow through the battery in charging and discharging cycles. To top it off, batteries come in all shapes, sizes, materials and configurations, leaving scientists with myriad factors to consider when studying battery performance.
"The bottom line is that these systems are very complicated," Kalinin says. "Batteries are made of pieces that are tens of nanometers to microns in size. Unless you study them on these length scales, you cannot learn much about their structure and how they operate. They are defined at this scale. Beyond that, you're looking at the assemblies."
"It's like we're trying to understand the individual properties of a brick in a house while we're flying by in an airplane. We only see the house; we don't see the bricks," he says.
Fellow CNMS scientist Nina Balke explains that the operation of lithium-ion batteries at the nanoscale has been largely unexplored because few techniques operate at the appropriate resolution. "Batteries are well characterized on the device level, but most characterization techniques don't tell you what's going on at the nanoscale," Balke says. "Very few techniques allow you to look at ionic transport on a scale that shows grains, grain boundaries and defects—which, in sum, make up the battery."
Studying the nanoscale puzzle pieces that make up a battery is not an easy task for microscopists because of the dynamic nature of batteries. Electron microscopes, for instance, can produce images of individual atoms in a material, but only if the atoms remain motionless for several seconds. The flow of electrons and ions through batteries can easily disturb this atomically perfect picture and muddy the view that electron microscopes provide.
To tackle the challenge of analyzing battery dynamics at the proper resolution, Balke and Kalinin turned to scanning probe microscopy, or SPM. This well-known method uses a small probe that scans over a surface to measure different properties of the sample material. The ideal application would combine classical electrochemical measurement methods and SPM to have the ultimate characterization tool on the nanoscale. However, scanning probe microscopes typically depend on the measurement of electron flow, which has prohibited their use at the nanoscale for probing electrochemical reactions.
"The problem with most techniques is that they rely on Faradaic currents related to the number of electrons transferred through the system," Kalinin says. "The number of electrons required for measurements is actually quite large; they can measure billions of electrons, but probably not millions or thousands. When we study batteries at the nanoscale, we transfer hundreds to thousands of electrons. Therefore, these methods cannot be easily extended to the nanoscale."
To address this problem, Kalinin and Balke, along with ORNL researcher Stephen Jesse, have developed a new SPM technique called electrochemical strain microscopy, or ESM, that relies on the measurement of strain, or volume change, instead of electrons. The technique, based on an R&D 100 award-winning band excitation technology developed by Jesse and Kalinin, is now commercially available through Asylum Research.
"If we want to understand ionic materials like lithium-ion batteries, we need a technique that allows us to measure ionic flows and not electronic flows," Kalinin says. "We want to have a technique that is highly and exclusively sensitive to ions and not electrons." The ORNL team has used ESM to characterize individual parts of lithium-ion batteries and other electrochemical systems such as fuel cells at unprecedented resolutions—up to a million times greater than was previously possible. But Balke says their application of the ESM technique has only just begun.
"Right now, we are at the very beginning," Balke says. "We have developed the technique and tried it out on a few model systems. We can take a single battery component and look at it, but we aren't looking at a full battery yet. We have established knowledge that can move us to the next step."
The team's short-term goals include setting up an in-situ SPM to probe a realistic battery surface, which involves a transition to different conditions, given that most electrolytes are liquid. They also plan to start testing new and advanced battery materials instead of analyzing well-known model systems. The microscopy team is already collaborating with ORNL battery researchers, who study the mechanical properties and performance of batteries, with the aim of correlating nanoscale phenomena to overall battery functionality. Scientists from other institutions are also starting to come to the CNMS user facility to make use of the ESM capabilities. "Battery researchers provide us with samples, and we work with them to improve the samples. They look at the mechanical properties, so if they do something to the sample and see that it results in a better battery, they give it to us to find out if we see an improvement on the nanoscale. We're building up this relationship between the performance and what we see on the nanoscale. We want to understand, from a nanoscale perspective, what makes one battery work and another battery fail," Balke says.
While the microscopy team is dedicated to bringing the ESM technique to its full potential, they note that additional methods are needed to form a complete picture of battery dynamics. "ESM can only tell you so much," Balke says. "It can tell you that in some areas of the electrode you have much higher ionic transport than in others, but it doesn't tell you why. So you have to combine ESM with other techniques, such as microwave microscopy or theoretical analysis, to figure out what's so special about it and then draw conclusions or make suggestions about how to improve the samples. It's very much a work in progress."— Morgan McCorkle
Sidebar: Piezoresponse Force Microscopy workshops build microscopy community
When Intel invited ORNL's Sergei Kalinin in 2006 to speak to its staff about piezoresponse force microscopy (PFM), he asked how much time he was allowed. "As much time as you'd like," the Intel rep said. After a marathon presentation and discussion that lasted 10 hours, Kalinin recognized there was a glaring need to share knowledge around PFM techniques.
Kalinin addresses a recent PFM workshop in Beijing. Photo: University of Science and Technology Beijing
"I started to realize that it's a good idea to have all the community members who are engaged in this technique in one room—at least to talk to each other," Kalinin says. "It wasn't limited to just Intel. PFM was rapidly becoming the mainstream method for exploring ferroelectric memories, hard-drive-like storage and tunneling barrier–based devices in multiple research groups."
The result of this realization was the first meeting of the PFM workshop series, held at ORNL in September 2007, which attracted some 40 attendees from around the globe. PFM, a variant of scanning probe microscopy, was once used primarily to study ferroelectric materials, but its use has now been expanded to include other electrochemical systems, in large part thanks to the PFM workshop series.
"Before we started studying batteries, we spent a lot of time working on ferroelectric materials in terms of technique development and theoretical analysis," Kalinin said. "Many of these techniques can be applied to batteries or fuel cells with insignificant modifications."
Since its humble beginning, the PFM workshop series has grown to include international meetings in Europe and Asia that have attracted more than 100 attendees. The first dedicated PFM conference was held in Aveiro, Portugal, in 2009 and hosted 110 attendees. The 2011 meeting was held jointly with the International Symposium on Applications of Ferroelectrics in Vancouver, Canada, and was attended by 400 participants. Industry participation has led to the commercial availability of PFM techniques that were previously custom-built by individuals. The collaborative spirit of the workshops, however, remains the same.
"It's good to get people together who have a common language—people who understand our problems," ORNL's Nina Balke says. "Since there aren't many people who do these kinds of measurements, it's beneficial to get together to see where we stand and help each other out."— Morgan McCorkle