In 2018, Lei Cheng, a battery chemist at the U.S. Department of Energy's (DOE) Argonne National Laboratory, stumbled across studies on battery electrolytes that described the existence of nanoaggregate structures. These are clusters of tens to hundreds of charged particles, called ions, with an overall diameter greater than one nanometer. Until now, most battery electrolyte research has focused on smaller structures.
An electrolyte is a chemical solution that plays an important role in battery operation. The electrolyte contains positive 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. The Joint Energy Storage Research Center brings together more than 150 researchers from 20 institutions, including national laboratories, universities and companies, to design and manufacture the materials that will enable the next generation of batteries. Such batteries help enable significant energy conversions in cars, power grids and even electric aircraft.
Cheng and several other JCESR researchers agreed that aggregates deserve further study. After all, the research team is fully aware that the structure of an electrolyte can significantly affect its properties, and ultimately, a battery's performance. For example, in order to develop better lithium-ion batteries, researchers have found that adding small amounts of salt can make them more stable.
"Aggregates are not a big problem," Cheng said. "Scientists don't talk too much about how to affect the properties of the electrolyte. That's why we decided to start a research project to investigate further."
From 2018 to 2021, researchers at the research center have accumulated enough research results that aggregates are a very important emerging topic with a potentially significant impact on the performance of next-generation batteries. To alert the battery science community, the researchers published a survey and analysis of aggregate studies in American Chemical Society's Energy Letters. This article brings together the results of 60 studies by researchers at the research center and other scientists.
Effects on Electrolyte Properties
This article explores how aggregates have unique effects on electrolyte properties, including stability and ion transport.
Stability affects many key aspects of battery performance. These include lifetime (number of charge-discharge cycles), safety, energy density, and charge-discharge rate. For example, unstable electrolytes are easily decomposed. This may shorten battery life and lead to safety issues.
Ion transport refers to the speed at which ions move through an electrolyte. This characteristic can affect the charge-discharge rate of the battery. Fast ion transport could allow electric vehicles to charge more quickly, while also allowing grid-scale batteries to discharge more quickly. Another potential benefit is to improve the performance of electrolytes made from macromolecular polymers. This electrolyte is safer than liquid electrolytes.
Aggregates of electrolytes can have favorable or unfavorable effects on battery performance. Therefore, aggregates may slow or accelerate ion transport.
Larry Curtiss, an experienced Argonne chemist and one of the authors of the paper, said: "An important goal of research is to find out when aggregates are beneficial and when they are not. Detrimental effects should probably be removed from the electrolyte."
One known beneficial effect of aggregates occurs in lithium-oxygen batteries. The new generation of batteries works by delivering oxygen to the cathode through the electrolyte. The aggregates react with lithium to form lithium peroxide. Compared with lithium-ion batteries, lithium-oxygen batteries have a higher energy density and have the potential to be used for long-distance trucking and air transportation. Through simulations, Curtiss and other researchers concluded that aggregates could improve oxygen transport and reactions at the catholyte surface. However, it is unclear why these phenomena occur.
"This is an area of future research," Curtiss said.
Formation of aggregates
The formation of aggregates is not fully understood. The researchers believe that this depends on the strength of various interactions between ions and solvent molecules in the electrolyte. Solvents are substances capable of dissolving other materials.
"If the ion reacts weakly with the solvent molecules, you might get smaller structures, such as ion pairs. If the ion-ion interaction is strong, you might get aggregates," Curtiss said.
"There is no complete and unified theory behind aggregate formation," Cheng said. "We also need to know which parameters to tune to manipulate aggregate formation and structure."
There are many knowledge gaps and research needs
So far, most aggregate research has focused on Li-ion batteries. However, the electrolytes used in lithium-ion batteries, such as ethylene carbonate, propylene carbonate, etc., are not compatible with many electrode materials for next-generation batteries under development. Including lithium-oxygen batteries and lithium-sulfur batteries. As researchers develop alternative electrolytes for these advanced batteries, they need to conduct more investigations into the effects of aggregates.
Furthermore, most studies on aggregates have only examined their effects on electrolytes. "Research on how aggregates affect the electrode-electrolyte surface is very sparse, but it is critical for battery performance," Curtiss said. "We don't understand how aggregates affect ion transport at the interface. We It was also unclear whether the aggregates would cause electrons to leak out of the cathode and destroy the electrolyte."
"A huge knowledge gap is how aggregates assemble themselves on surfaces and how that affects charge transport," Cheng said.
Cheng also added that we need to develop new experimental characterization tools specifically for these interfaces. Spectroscopic tools may be required to document the composition and structure of materials. Enhanced X-ray technology, such as the one being developed at the Argonne Advanced Photon Source, can help detect the presence of aggregates and record how they are made up and how they change over time.
An active area of research is improving computational and simulation methods to accurately describe complex interactions between aggregates and ions and molecules. Machine learning can collect massive amounts of data from these interactions.
Cheng, Curtiss, and other researchers at the Joint Energy Storage Research Center plan to continue research on several aggregates. One area of ongoing research involves different ions and other elements to better understand aggregate formation. The Argonne researchers plan to continue their work with the University of Illinois Urbana-Champaign to study the effects of aggregates at the electrode interface.
Interestingly, the formation of aggregates is not unique to battery electrolytes. Aggregates may play an important role in the production of materials in other industries such as pharmaceuticals. Insights from the study of aggregates for battery electrolysis will also bring benefits to other processes.