Unlocking safer batteries: New study uncovers key insights into electrolyte materials for all-solid-state batteries

A joint computational and experimental study examined how adding certain dopants to a solid electrolyte could improve its interaction with a lithium metal electrode. The result could be safer, more energy-efficient batteries.

From cellphones to laptops to electric vehicles, lithium-ion batteries" target="_blank">batteries power many of the devices on which we rely. Given the important role this technology plays in the modern world, scientists are continually trying to develop safer and more energy-efficient battery technology.

In a recently published paper, a team led by researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory revealed key insights into solid electrolytes they are testing for use in all-solid-state batteries. Their findings could lead to safer, more energy-efficient batteries.

Electrolytes are like membranes that allow an electrical charge carried by lithium ions to flow between the positive and negative electrodes of a battery. All-solid-state batteries use solid instead of liquid electrolytes. They are emerging as a critical technology for the future development of lightweight, energy-dense, longer lasting and safer lithium-ion batteries. Solid electrolytes are neither volatile nor flammable, unlike the liquid electrolytes used in conventional lithium-ion batteries. 

“It’s important to know how a dopant will react with lithium. It’s another requirement for good electrolytes, not just high conductivity.” — Peter Zapol, Argonne physicist

They are also less reactive with lithium metal, making solid electrolytes more compatible with lithium metal electrodes than liquid electrolytes. Because all atoms in lithium metal can participate in the charge and discharge of a battery — enabling it to store more energy — lithium metal has a higher energy density than graphite, a conventional electrode material. 

Solid electrolytes made of lithium lanthanum zirconium garnet (LLZO) are a leading candidate for such a battery. This material stands out because of its strength and durability. It’s also notable for its conductivity, or the ease with which it moves lithium ion between electrodes during charge and discharge.

To make LLZO even better, researchers have been experimenting with adding small amounts of elements like aluminum or gallium to improve how well the LLZO conducts lithium ions. This process is known as doping. Doping means adding small amounts of another element to change and improve the properties of a material. It’s like adding a pinch of spice to a recipe to make the dish better. 

Doping with aluminum and gallium helps LLZO to retain the most symmetric structure and creates vacant spaces. These spaces allow lithium ions to escape more readily from electrodes and improve conductivity. However, doping can make the LLZO more reactive with lithium metal, shortening the cycle life of the battery.

In the study, researchers examined what happens when LLZO containing aluminum or gallium dopants contacts metallic lithium. Using computational and experimental techniques, the researchers found that gallium tends to move more easily out of the electrolyte and has a stronger tendency to react with the lithium to form an alloy. This causes the amount of gallium to decrease. The loss of gallium can cause the lithium garnet to change its structure and decrease ionic conductivity. Conversely, aluminum-doped LLZO remains intact.

Gallium-doped LLZO is attractive because it has a much higher ionic conductivity than aluminum-doped LLZO. However, the reactivity of these dopants when put in contact with lithium is what led researchers to determine that in order to use gallium, an interfacial layer is needed to protect and preserve its conductivity but prevent its reactivity. 

Understanding why the LLZO behaves differently, depending on which dopant has been added, will help scientists design better materials for stable and reliable solid-state batteries. 

“It’s important to know how a dopant will react with lithium,” said Peter Zapol, an Argonne physicist and lead researcher on the paper. ​“It’s another requirement for good electrolytes, not just high conductivity.” 

If dopants are unstable, having improved conductivity is not enough, explained Sanja Tepavcevic, an Argonne chemist and lead experimentalist on the study.

“If we can separate reactivity from conductivity, or if we can develop one material that has both high conductivity and stability, that’s basically what we are trying to show with this work,” she said. 

By combining computational and experimental techniques, the researchers were able to measure key properties of the doped materials. At the same time, they gained atomic-level insights into what’s happening at the interface between the lithium metal and solid electrolyte. 

Using a powerful computer-based method known as density functional theory to study how atoms and electrons behave in materials, the researchers were able to predict the stability of various dopants and how they would react with other substances. 

There are few experimental techniques that allow scientists to look at the solid electrolyte-electrode interface, especially while an electrochemical reaction is occurring during battery operation. That’s because these interfaces are ​“buried” and not visible with most experimental techniques, according to Tepavcevic. 

One technique researchers used was X-ray photoelectron spectroscopy to study changes in the surface chemistry of LLZO. Another was electrochemical impedance spectroscopy to analyze the movement of lithium ions in electrolytes and at the electrolyte-electrode interface. 

Another experimental technique the researchers used, neutron diffraction, helps determine how atoms are arranged in a material. In this case, it helped researchers confirm that gallium became less stable and more reactive once it interacted with lithium, whereas aluminum remained stable.

This research benefited from collaborations with several other institutions, including the University of California, Santa Barbara, which provided high-quality LLZO. Meanwhile, the neutron diffraction experiments were conducted at user facilities at the Heinz Maier-Leibnitz Zentrum in Germany and the Nuclear Physics Institute of the Czech Academy of Sciences in the Czech Republic. 

“The role of the U.S.-German collaboration was absolutely critical for this work,” Zapol said. ​“Looking ahead, these findings open new avenues in the international pursuit of safer, more efficient solid-state batteries.”

The U.S.-German Cooperation on Energy Storage, which funded the study, was established by DOE’s Office of Energy Efficiency and Renewable Energy for its Vehicle Technologies Office to collaborate on lithium battery research.

This research first appeared in ACS Materials Letters. In addition to Tepavcevic and Zapol, Argonne authors include Matthew Klenk, Michael Counihan, Zachary Hood, Yisi Zhu and Justin Connell. Also contributing were Neelima Paul and Ralph Gilles from the Heinz Maier-Leibnitz Zentrum; Charles Hervoches from the Nuclear Physics institute of the Czech Academy of Sciences; and Jeff Sakamoto from the University of California, Santa Barbara.

Argonne National Laboratory seeks solutions to pressing national problems in science and technology by conducting leading-edge basic and applied research in virtually every scientific discipline. Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy’s Office of Science.

The U.S. Department of Energy’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://​ener​gy​.gov/​s​c​ience.