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This image shows an aggregate of battery electrolyte. (Image courtesy of Argonne National Laboratory)
In 2018, Lei Cheng, a battery chemist at Argonne National Laboratory of the U.S. Department of Energy (DOE), stumbled upon research on battery electrolytes that described the existence of nano-aggregate structures. These are clusters of tens to hundreds of charged particles, called ions, with a total diameter greater than one nanometer. Prior to this, most battery electrolyte research had focused on even smaller structures.
"One of the key goals of this research is to determine when aggregates are beneficial and when they are detrimental," said Larry Curtiss, a senior chemist and distinguished scholar at Argonne University. "When aggregates have adverse effects, they should be removed from the electrolyte."
An electrolyte is a chemical solution that plays a crucial role in battery operation. It contains charged ions that can move back and forth between the positive and negative electrodes of the battery.
Cheng is the technical lead at the Joint Energy Storage Research Center (JCESR), an Energy Innovation Hub initiated by the Department of Energy and led by Argonne. JCESR brings together over 150 researchers from 20 institutions, including national laboratories, universities, and companies, to design and manufacture materials that can become the next generation of batteries. These batteries could enable significant energy conversions in automobiles, power grids, and even electric aircraft.
Cheng and several other JCESR researchers agree that aggregates deserve further study. After all, the research team is well aware that the structure of the electrolyte can significantly affect its properties and ultimately play a major role in battery performance. For example, to develop better lithium batteries, researchers have found that adding a small amount of salt can make them more stable.
Cheng said, "Agglomerates aren't a major problem. Researchers don't discuss in much detail how they affect the properties of electrolytes. That's why we decided to launch a research project to investigate further."
From 2018 to 2021, researchers at the research center accumulated a wealth of research findings. Aggregates are a very important emerging topic with potentially significant implications for the performance of next-generation batteries. To inform the battery science community, researchers published a survey and analysis of aggregate research in *American Chemical Society's Energy Letters*. This article compiles the results of 60 studies by researchers at the research center and other scientists.
Effects on electrolyte properties
This article explores how aggregates influence electrolyte properties in unique ways, including stability and ion transport.
Stability affects many key aspects of battery performance, including lifespan (number of charge-discharge cycles), safety, energy density, and charge/discharge rate. For example, an unstable electrolyte is prone to decomposition. This can shorten battery life and lead to safety issues.
Ion transport refers to the speed at which ions pass through an electrolyte. This property can affect the charge and discharge rate of a battery. Faster ion transport can enable faster charging of electric vehicles and also allow grid-scale batteries to discharge more quickly. Another potential benefit is improved performance of electrolytes made from macromolecular polymers. These electrolytes are generally safer than liquid electrolytes.
Electrolyte aggregates can have both beneficial and detrimental effects on battery performance. Therefore, aggregates may slow down or accelerate ion transport.
"A key objective of the study was to determine when aggregates are beneficial and when they are detrimental," said Larry Curtiss, an experienced Argonne chemist and one of the authors of the paper. "When aggregates exhibit adverse effects, they should perhaps be removed from the electrolyte."
One known beneficial effect of aggregates appears in lithium-oxygen batteries. The next-generation battery works by transporting oxygen to the cathode via an electrolyte. The aggregates react with lithium to form lithium peroxide. Compared to lithium-ion batteries, lithium-oxygen batteries have a higher energy density, potentially making them suitable for long-haul trucking and transportation. Curtiss and other researchers, through simulations, suggest that aggregates can improve oxygen transport and reactions on the cathode electrolyte surface. However, the reasons for these phenomena are currently unclear.
"This is an area for future research," Curtiss said.
formation of aggregates
The formation of aggregates is not yet fully understood. Researchers believe it depends on the strength of various interactions between ions and solvent molecules in the electrolyte. A solvent is a substance capable of dissolving other materials.
Curtiss says, "If the reaction between ions and solvent molecules is weak, smaller structures, such as ion pairs, may be obtained. If the interaction between ions is strong, aggregates may be obtained."
Cheng said, "There is no complete and unified theory behind aggregate formation. We still need to understand what parameters to adjust to manipulate the formation and structure of aggregates."
There are many knowledge gaps and research needs.
To date, most research on aggregates has focused on lithium-ion batteries. However, the electrolytes used in lithium-ion batteries, such as ethylene carbonate and propylene carbonate, are incompatible with the electrode materials of many next-generation batteries under development, including lithium-oxygen and lithium-sulfur batteries. As researchers develop alternative electrolytes for these advanced batteries, they need to conduct more research into the impact of aggregates.
Furthermore, most studies on aggregates have only examined their effects on the electrolyte. Curtiss said, "Studies on how aggregates affect the electrode-electrolyte surface are very scarce, but this is crucial for battery performance. We don't understand how aggregates affect ion transport at the interface. We also don't know whether aggregates cause electrons to leak from the cathode and damage the electrolyte."
Cheng said, "A huge knowledge gap is about how aggregates aggregate on a surface and how that affects charge transport."
Cheng added that we need to develop new experimental characterization tools specifically for these interfaces. These might include spectroscopic tools to record the composition and structure of materials, and enhanced X-ray techniques, such as those being developed at Argonne Advanced Photon Source, which help detect the presence of aggregates and record their composition and how it changes over time.
An active area of research is improving computational and simulation methods to accurately describe the complex interactions between aggregates and ions and molecules. Machine learning can collect vast amounts of data on these interactions.
Cheng, Curtiss, and other researchers at the Joint Energy Storage Research Center plan to continue several studies on aggregates. One ongoing research area involves different ions and other elements to better understand aggregate formation. Argonne researchers plan to continue their collaboration with the University of Illinois at Urbana-Champaign to study the effects of aggregates at the electrode interface.
Interestingly, aggregate formation is not unique to battery electrolytes. Aggregates may play an important role in material production processes in other industries, such as pharmaceuticals. Insights gained from studying aggregates in battery electrolysis can also benefit other processes.
