New microscopy techniques are providing an unprecedented view of the atomic-scale processes that cause rechargeable batteries to degrade over time. By directly observing the behavior of individual crystals and atoms within battery electrodes, scientists are uncovering the fundamental mechanisms of wear and tear. These new insights are expected to accelerate the development of more durable and higher-capacity batteries for everything from portable electronics to electric vehicles.
The longevity and performance of lithium-ion batteries are limited by the gradual breakdown of their electrode materials. During charging and discharging, lithium ions shuttle back and forth, causing the crystalline structures of the electrodes to expand and contract. Over many cycles, this repeated stress introduces microscopic damage, such as cracks and defects, that diminishes the battery’s ability to hold a charge. Until recently, researchers lacked the tools to directly witness these changes as they happened, relying instead on indirect measurements and simulations. Now, a suite of advanced imaging technologies is making it possible to see these degradation processes in real-time and at the atomic level.
Visualizing Atomic-Scale Processes
Understanding battery degradation requires tools that can see what is happening at the nanoscale, a scale thousands of times smaller than the width of a human hair. Traditional optical microscopes, which use light to create images, lack the magnification and resolution needed to see individual atoms and the tiny crystal structures that make up battery electrodes. To overcome this limitation, scientists have turned to more powerful methods that use electrons and X-rays instead of light. These advanced microscopy techniques can achieve atomic-level resolution, allowing researchers to map the precise location of atoms, identify different chemical phases, and observe the formation of microscopic defects.
A key challenge in studying battery materials is that they are often sensitive and can be damaged by the very tools used to observe them. For example, the high-energy electron beams used in some microscopes can alter the delicate crystal structures of the materials being studied. Researchers have therefore developed specialized techniques and “cryo-microscopy” methods to protect the samples and ensure that what they are seeing is an accurate representation of the processes happening inside a working battery. By combining multiple complementary techniques, scientists can build a comprehensive picture of the structural and chemical changes that occur during a battery’s life.
Advanced Imaging Technologies
Transmission and Scanning Electron Microscopy
Two of the most fundamental tools in this field are scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM provides detailed three-dimensional images of the surface of electrode particles, revealing their shape and texture. TEM, which passes a beam of electrons through a very thin slice of material, offers even higher resolution, allowing scientists to see the arrangement of atoms within a crystal. Advanced TEM techniques can also provide information about a material’s crystal structure, chemical composition, and the presence of disorder or defects. Some of the most powerful electron microscopes can achieve resolutions of less than 0.2 nanometers, making it possible to distinguish individual columns of atoms.
Atom Probe Tomography
While TEM provides a two-dimensional projection of a material’s atomic structure, atom probe tomography (APT) offers a three-dimensional view. This technique essentially reconstructs a material atom by atom, providing precise information about the location and identity of each element. APT is particularly useful for studying the distribution of lithium within an electrode, which is difficult to do with other methods. By combining APT with TEM, researchers can correlate the loss of lithium with specific structural changes in the cathode, providing a more complete understanding of the degradation process. This combination of techniques has been crucial in revealing how atomic-scale structural changes inhibit the ability of lithium to move back into the cathode as the battery discharges.
X-ray Microscopy
X-ray microscopy offers another powerful way to study battery materials without damaging them. At facilities like the Advanced Light Source at Lawrence Berkeley National Laboratory, scientists can use focused beams of soft X-rays to map the chemical and structural changes in electrode crystals as they charge and discharge. This technique, known as ptychographic microscopy, allows researchers to see where different chemical phases, such as lithium iron phosphate and its delithiated form, are located within a crystal. It can also pinpoint the location of undesirable cracks and defects, which often form at the interface between these two phases where the crystal structure is under the most strain.
Mechanisms of Battery Degradation
Using these advanced imaging tools, scientists have identified several key mechanisms that contribute to battery degradation at the nanoscale. One of the most important is the formation of irreversible structural changes in the cathode material. As lithium is removed from the cathode during charging, the crystal lattice can become unstable and transform into different structures, such as rock salt or spinel-like phases. These new phases can block the pathways for lithium ions to return during discharge, leading to a permanent loss of capacity.
Another critical factor is the non-uniform movement of lithium ions into and out of the electrode particles. Ideally, this process, known as lithiation and delithiation, should occur evenly across the entire electrode. However, research has shown that in reality, some areas of the electrode take on more ions than others, creating localized “hot spots” of high stress. This mechanical strain can cause the crystal lattice to fracture, creating tiny cracks that grow over time and degrade the battery’s performance. Interestingly, researchers have found that this non-uniformity can be influenced by the charging rate, with faster charging leading to more uniform delithiation.
Toward More Durable Batteries
The insights gained from advanced microscopy are not just of academic interest; they are providing a roadmap for designing better, longer-lasting batteries. By understanding the atomic-scale mechanisms of degradation, scientists can develop strategies to mitigate them. For example, they can design electrode materials with more stable crystal structures that are less prone to phase transformations. They can also explore new electrode architectures and coatings that promote more uniform ion flow, reducing mechanical stress and cracking.
The ability to observe battery processes in real-time also allows for the rapid testing of new materials and designs. Researchers can now directly see how a new material responds to charging and discharging, rather than relying on trial and error. This deeper understanding of the structure-property relationships in battery materials will be essential for developing the next generation of energy storage technologies, from smaller, more powerful batteries for our phones and laptops to the large-scale batteries needed to support the electric grid and a renewable energy future.