Rechargeable lithium-ion batteries can only run so long, and extended use shortens this life. But what causes this lifetime to decline? Scientists at the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center, are working to answer this question by taking a look inside a battery while it is operating. These experiments focus on commonly used positive electrodes to understand just how these materials work.
In a battery, the positive and negative electrodes are essential to storing and delivering energy. When storing energy in a battery (charging), lithium ions are transported from the positive to the negative electrode. By reversing this process (discharging), the stored energy can be used to run everyday devices. The positive electrode, usually referred to as the cathode, plays a key role in these processes.
One type of commonly tested battery design is the coin cell, in which the battery components are stacked inside a button-like casing. The cathode is just one of the many parts in a typical battery. NECCES scientists use X-rays to selectively look at the cathode. X-rays provide sensitivity to different types of materials. In medical X-rays, for example, denser bones absorb more X-rays, distinguishing them from skin and tissues. By choosing the desired X-ray wavelength and experimental conditions, NECCES scientists can probe a range of information, from individual atoms to groups of particles within the cathode.
Probing the battery during operation requires modification to the coin cell design. The button-like casing is too thick for X-rays to penetrate. Adding windows to the top and bottom of the casing allows X-rays to transmit through. However, the windows can alter how the battery performs. To overcome this issue, a team of NECCES researchers, led by Karena Chapman, helped develop a specialized cell, known as the AMPIX cell, in 2012. They continue to use it today. This modified cell performs like a typical coin cell so reliable measurements can be made.
“Drilling a hole in a coin cell to let X-rays through will alter the pressure and create inhomogeneities in the cathode. The AMPIX was especially designed to avoid this issue,” says Antonin Grenier, a researcher in Chapman’s group.
Taking advantage of the AMPIX cell, these scientists investigate the nature of lithium storage in the cathode material. As lithium moves out of the cathode, the crystal structure of the material, related to the local arrangement of atoms, will change to accommodate this loss. X-ray diffraction is used to correlate changes in lithium content to changes in the crystal structure.
Looking at the cathode during the first cycle, these researchers identified non-uniform structural changes when lithium is removed. This non-uniformity occurred for particles stored in air, before they were used in the battery. During storage in air, these particles can form a surface layer like the process of rust forming on a car. When these particles were used as the cathode, lithium was not evenly removed from all particles. Instead, some particles lagged behind, which led to non-uniform structural changes. This non-uniformity was removed when the particles were kept in inert storage conditions such as argon.
While the first cycle is often looked at for the basics of how the cathode material works in the battery, long-term cycling provides insight into battery failure. Crystal structure changes of the cathode material were compared over long-term cycling using the AMPIX cell. Over this long-term cycling, 20 percent of the cathode’s storage capacity was lost.
During the first cycle, the cathode structure changed uniformly when removing and inserting lithium. This is similar to when food coloring is added to water: All the water changes to the new color. But on the 93rd cycle, a fraction of the cathode material did not fully participate in the cycling process. Microscopy images revealed cracks forming between groups of particles. Cracking leads to the isolation of individual particles, and they become less active during battery operation.
Hao Liu, the lead author for this study, explains, “With the optimized AMPIX cell, we looked at how capacity fading related to crack development over the period of a few months.” Looking at how the battery operated during the 93rd cycle revealed the culprit for capacity fade.
Diffraction experiments provide just one type of video of lithium motion into the cathode. Another NECCES group, led by Jordi Cabana, combines spectroscopy with microscopy to visualize particles’ shapes and chemical states. Spectroscopy uses X-rays to excite atoms that can be used to distinguish the chemical state of the atom. In the cathode, an atom’s chemical state changes when lithium ions move into the system or can change from undesirable reactions. By combining spectroscopy with microscopy (referred to as spectromicroscopy), these chemical states can be mapped across a range of particles.
Using spectromicroscopy, with a similarly modified coin cell, these researchers obtain a video of the particle shape and local chemical environments. Yet, for current specialized coin cells, these videos are only 2-D projections of the whole particle.
Mark Wolf, a graduate researcher in Cabana’s group, said, “Since they’re transmission techniques, you can’t tell where in the depth of the particle a certain signal originates.” As part of NECCES, he is developing a specialized cell to overcome some of the limitations of coin cells and these 2-D projections.
These various X-ray measurements during battery operation shine a new light on a range of long-standing problems. Rather than snapshots of the endpoints, these operation experiments provide a time-lapse video of battery operation. As batteries are vital for everyday devices, these time-lapse measurements provide insight into making the best and most reliable battery.
