Have you ever tried to look inside a running battery? I hope not! They contain toxic chemicals, die after exposure to water and oxygen, and cease to function when disassembled. So how do scientists explore inside batteries? Although visible light can’t penetrate into batteries, X-ray light pierces through packaging and interacts with the materials inside.
Advanced Photon Source synchrotron. The accelerator (blue) and storage ring (red) are highlighted. The APS storage ring is 380 yards across or about 0.7 miles around. Scientists perform experiments just outside the storage ring. This synchrotron has 66 stations at which experiments are performed simultaneously. Image courtesy of APS Image Bank.
In the past century, chemists and physicists have extended the use of X-rays far beyond medical imaging. In fact, the X-rays that doctors use are insufficient for the difficult research questions that scientists want to solve. First, doctors use a broad distribution of X-ray energies during medical imaging because they want X-rays to just pass through the body. In contrast, scientists use X-rays not only for their penetrating power but also for their ability to interact with bonds between atoms, which occurs because X-rays and bonds are about the same length. By carefully choosing an X-ray energy, scientists can characterize their material’s structure and drive X-rays to interact with an atom’s electrons, which make up the bonds between different atoms or lie closer to an atom’s center.
Customizable, or tunable, X-rays allow scientists to observe chemical changes at discrete energies, characterize bonds, and sample different depths into a material. Scientists also want to observe changes in real time. To achieve this, they bombard their samples with lots of X-rays at the same time, which provides bright, crisp images. Scientists can create these bright and highly tunable X-rays at synchrotrons.
Synchrotrons, like the Department of Energy (DOE) operated Advanced Photon Source, are composed of accelerators and storage rings (blue and red in the figure). Electrons are accelerated to almost (>99.999 percent) the speed of light and then injected into the storage ring. Strategically placed magnets in the storage ring bend the electrons’ trajectory into a segmented, but nearly circular path. Every time a magnet changes an electron’s path, very bright X-rays are emitted.
The emitted X-rays have a wide spread of different energies, from low to very high and anything in-between. To study chemical bonds and gather chemical information, scientists tune into a single, preferred X-ray energy. The energy chosen depends on the type of experiment and the type of information needed. Scientists tune their X-rays with crystals that allow only one X-ray energy to reach the sample. Researchers then focus the beam down onto a small area on their sample.
Not only do researchers choose their desired X-ray energy at the synchrotron, but they also perform different types of experiments that probe electron exchanges between atoms, electron energy changes within atoms, or the patterns in which atoms order. Scientists can perform all of these experiments—and more—simultaneously at different experiment stations located around a storage ring.
Researchers in the DOE’s Energy Frontier Research Centers (EFRCs), among others, exploit the synchrotron’s unique capabilities to peer inside batteries. Batteries are complex electrical and material systems. Scientists want to monitor the batteries during charge, such as when a phone is plugged into a socket, and discharge, such as using a phone on battery power. Scientists must study the processes occurring within batteries to understand how they work and how they might be improved. These processes include electrons passing through the battery, structural changes, and interfaces between materials reacting uniquely.
Scientists in the Center for Electrochemical Energy Science (CEES) EFRC are interested in an unorthodox type of lithium-ion battery reaction, called a conversion reaction, which has the potential to run electronics longer than ever before. They used a technique called X-ray reflectivity during which X-rays penetrate through the material’s layers and bounce off each smooth interface. Depending on their trajectory, the X-rays can add up or cancel each other out as they exit the sample. Using the resulting patterns, scientists interpret how the layers change thickness, composition, and regularity as the battery is charged or discharged.
Battery conversion reactions occur chaotically by destroying any semblance of uniformity in the film, a challenge scientists must overcome. How do they force order in a disordered system?
At CEES, scientists forced order into the chaos by creating layered structures (see figure) in which some of the layers (chromium metal) would change predictably during the reaction, preserve smooth interfaces, and allow the material of interest (chromium oxide) to react non-uniformly. As the battery discharges, lithium moves into the chromium oxide and steals the oxygen away to form lithium oxide and chromium metal. The newly formed chromium metal segregates towards the neighboring metal layers, preserving the overall structure of the layered film. These types of layered structures allow the battery to react reversibly, which is a challenge in these conversion reactions.
Researchers at the Center of Mesoscale Transport Properties (m2M) EFRC are interested in improving electrical conductivity in batteries. Ideally, battery electrodes should conduct electrons with ease, but electrical conductivity may be low in some types of materials. Because the passage of electrons fuels electronics, this is a problem. Researchers at m2M created a silver-containing material to combat this problem. During battery operation, silver leaves the material and disperses conductive silver throughout the electrode (see figure). Uniform distribution of silver will create a highway throughout the electrode, allowing electrons to travel efficiently. But if the silver coalesces in one area, this conductive highway across the battery fails to form.
At m2M, scientists monitored the electrode and the distribution of silver using energy dispersive X-ray diffraction. With this method, they used X-rays to “look inside” a working battery still in its own steel housing. Their technique bounces light off of the ordered layers of atoms inside the battery material. Different materials arrange uniquely, allowing researchers to differentiate between an electrode material and silver particles, even when they’re mixed together. During battery operation, researchers also moved their electrode up and down in the X-ray beam, just as you would move a flashlight along a wall to “see” the entire wall in the dark.
By combining the time- and space-dependent data, they created a clear account of silver particle dissemination and learned that silver particles distribute more evenly throughout the electrode if the battery operates at a slow rate. Researchers at m2M successfully identified a tactic to overcome a key obstacle—the lack of electron conductivity—in battery electrode performance.
Researchers at the NorthEast Center for Chemical Energy Storage (NECCES) EFRC are interested in finding the failure points of a battery material. They wanted to know what was stopping a new electrode material from reaching optimum performance. They hypothesized that previously observed composition changes at different depths were to blame, although they did not know which of the two reactions that the material experienced was causing the variations. They segregated the two separate processes (first and second lithium reactions) by synthesizing the material with the same composition as a material that had already undergone the first reaction. Additionally each of the lithium reactions occur at a specific energy, so they separately investigated the second reaction by applying only enough energy to activate the second reaction but not the first.
At NECCES, scientists took advantage of various X-ray spectroscopy techniques with different penetration depths into the material. Spectroscopy uses light to excite a material and observes how the electrons react. Depending on how much light is absorbed or how many electrons are ejected out of the material, researchers can deduce an atom’s chemical composition and nearest atom neighbors. As a battery charges or discharges, atoms can lose or gain electrons thereby modifying their surroundings.
The researchers observed the same spectroscopic response at different depths in the material as the second lithium was added or removed. In other words, atoms at the surface of the electrode experienced the same changes as those in the middle of the electrode. Therefore, the second lithium reaction did not start the detrimental composition variation with depth. The researchers effectively pinpointed the first lithium reaction as the obstacle for this material.
Scientists in EFRCs don’t use synchrotron X-rays to examine just batteries. In 2016 alone, EFRC scientists have also observed light absorption by bacteria, catalysts converting biomass, and high-pressure syntheses of incompressible materials at synchrotrons. State-of-the-art government-funded research facilities like the Advanced Photon Source might seem obscure. However, researchers at these facilities are hard at work studying new systems with outside researchers, expanding techniques, and pushing the boundaries of science to discover and characterize materials that could redefine how we live our lives.
