Researchers at Argonne and partner institutions point to the significant implications of "aggregates" for future battery performance.
Keeping track of the latest scientific literature is an essential part of a scientist's job because it could yield insights that turn into tomorrow's breakthroughs.
In 2018, Lei Cheng, a battery chemist at the U.S. Department of Energy's (DOE) Argonne National Laboratory, was doing just that when she came across a few studies on battery electrolytes that described the presence of structures called nanometric aggregates. These are clusters of tens to hundreds of charged particles called ions with a total diameter greater than one nanometer. Up until that point, most battery electrolyte research had focused on much smaller structures.
"One important goal of the research going forward is to find out when aggregates are beneficial and when they are not. When they have adverse impacts, you would want to eliminate them from the electrolyte." -; Larry Curtiss, Argonne senior chemist and Distinguished Fellow
Electrolytes are chemical solutions that serve an essential role in the operation of a battery. They contain positively charged ions that move back and forth between a battery's positive and negative sides.
Cheng is a technical lead at the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub started by the DOE and led by Argonne. JCESR brings together more than 150 researchers from 20 institutions -; including national laboratories, universities and industry -; to design and build materials to enable next-generation batteries. Such batteries can help usher in major energy transitions in vehicles, the grid and even electric flight.
Cheng and several other JCESR researchers agreed that aggregates were worth a deeper look. After all, the team was well aware that the structure of electrolytes can significantly impact their properties and ultimately play a big role in the performance of batteries. For example, working to develop better lithium-ion batteries, researchers have found that adding small amounts of certain salt molecules to the electrolytes can make them more stable.
"Aggregates weren't a big deal at the time," said Cheng. "Scientists weren't talking much about how they impacted electrolyte properties. That's why we decided to launch research projects to investigate further."
Fast forward from 2018 to 2021: The JCESR researchers accumulated a large enough body of research to conclude that aggregates are an important emerging topic with potentially significant implications for the performance of next-generation batteries. To alert the battery science community, they published a survey and analysis of aggregate research in the American Chemical Society's Energy Letters. The Perspective article incorporates the results of 60 studies by JCESR researchers and other scientists.
Impacts on Electrolyte Properties
The Perspective article explores how aggregates can have unique effects on electrolyte properties, including stability and ion transport.
Stability can impact many critical aspects of battery performance. These include lifetime (the number of charge and discharge cycles), safety, energy density and charge and discharge rates. For example, an unstable electrolyte tends to decompose. This may shorten how long a battery lasts and lead to safety concerns.
Ion transport refers to how fast ions move through an electrolyte. This property can affect a battery's charge and discharge rate. Rapid ion transport can enable faster electric vehicle charging as well as faster discharge by grid-scale batteries. Another potential benefit could be improved performance of electrolytes made of very large molecules known as polymers. Such electrolytes are safer than liquid electrolytes.
Aggregates could have either beneficial or adverse impacts on battery performance. For instance, they could slow down or speed up ion transport.
"One important goal of the research going forward is to find out when aggregates are beneficial and when they are not," said Larry Curtiss, a veteran Argonne chemist and one of the authors of the Perspective article. "When they have adverse impacts, you would want to eliminate them from the electrolyte."
An example of a known beneficial impact of aggregates occurs in lithium-oxygen batteries. These next-generation batteries work by transporting oxygen through an electrolyte to the cathode. There, it reacts with lithium to form lithium peroxide. Relative to lithium-ion batteries, lithium-oxygen devices have a much higher energy density and could potentially be used to electrify long-haul trucking and aviation. Simulations and calculations by Curtiss and other researchers suggest that aggregates may improve oxygen transport as well as the reactions at the cathode-electrolyte interface. However, it's not understood why these effects occur.
"That's an area for future study," said Curtiss.
How Aggregates form
The formation of aggregates is not fully understood yet. Researchers believe it depends on the strength of the various interactions among ions and solvent molecules in an electrolyte. Solvents are materials capable of dissolving other materials.
"If ions react weakly with solvent molecules, you may get much smaller structures like ion pairs," said Curtiss. "If the ion-ion interactions are very strong, then you may get aggregates."
"There's not a complete and uniform theory behind how aggregates form," said Cheng. "We still need to know what parameters you can tune to manipulate their formation and structure."
Many knowledge Gaps and Research Needs
Most aggregate research to date has focused on lithium-ion batteries. However, the electrolytes used in lithium-ion batteries -; such as ethylene carbonate and propylene carbonate -; are not compatible with the electrode materials found in many next-generation batteries under development. Such batteries include lithium-oxygen and lithium-sulfur batteries. As researchers develop alternative electrolytes for these advanced batteries, they will need to investigate the effects of aggregates.
In addition, most existing research on aggregates has examined their effects only on electrolytes. "There have been very few studies on how they impact the electrode-electrolyte interface, which is critical to battery performance," said Curtiss. "We don't know much about how they affect ion transfer across the interface. And we don't know whether they could cause electrons to leak out of the cathode and destroy the electrolyte."
"A big knowledge gap is how aggregates organize themselves at the interfaces and how that affects charge transfer," said Cheng.
Cheng added that there is a need to develop new experimental characterization tools able to target these interfaces. These may include spectroscopic tools, which use light to characterize the composition and structure of materials. Enhanced X-ray techniques, such as those under development at Argonne's Advanced Photon Source, can help detect the presence of aggregates and characterize their composition and evolution over time.
An active area of research is improving computational and simulation methods to accurately describe the complex interactions among aggregates, ions and molecules. Machine learning can potentially be used to draw insights from the large amounts of data collected on these interactions.
Cheng, Curtiss and other JCESR researchers plan to continue several lines of aggregate research. One ongoing area involves varying ions and other parameters to better understand how aggregates form. Argonne researchers also plan to continue research with the University of Illinois Urbana-Champaign on the effects of aggregates at electrode interfaces.
Interestingly, aggregate formation is not unique to battery electrolytes. Aggregates may play a role in material production processes used in other industries such as pharmaceuticals. Insights from aggregate research on battery electrolytes could benefit these other processes.
The other authors of the Perspective paper are Zhou Yu, Argonne National Laboratory; Nitash Balsara, University of California, Berkeley/Lawrence Berkeley National Laboratory; Oleg Borodin, U.S. Army Research Laboratory; Andrew Gewirth, University of Illinois at Urbana−Champaign; Nathan Hahn, Sandia National Laboratories; Edward Maginn, University of Notre Dame; Kristin Persson, Lawrence Berkeley National Laboratory/University of California, Berkeley; Venkat Srinivasan, Argonne Collaborative Center for Energy Storage Science; Michael Toney, University of Colorado, Boulder; Kang Xu, U.S. Army Research Laboratory; and Kevin Zavadil, Sandia National Laboratories.