A thin, glass-like layer could be the key to longer-lasting, cost-effective solid-state batteries.
In everyday life, we use many protective barriers: Sunscreen shields us from the sun, umbrellas keep us dry in the rain and oven mitts protect our hands from hot pans. Similarly, batteries need protection to stop their internal components from breaking down due to environmental exposure.
Inside of a battery, the electrolyte is the chemical medium that allows the electrical charge to flow between its components. Solid-state batteries (SSBs) use solid electrolytes instead of the liquid ones found in regular lithium-ion batteries. By using solid electrolytes, SSBs could revolutionize the energy storage industry by offering better energy density, safety and lifespan than lithium-ion ones.
However, a big challenge for SSBs is that solid electrolytes can break down when exposed to atmospheric conditions like humidity and oxygen. This challenge is particularly severe for high-performance, sulfide-based solid electrolytes such as lithium phosphorus sulfur chloride (LPSCl). Making SSBs with these materials requires maintaining a dry room below -40 C (-40 F), which makes production costly.
To improve the chemical stability and make manufacturing more affordable, researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory have developed a method to coat sulfide-based solid electrolytes. They use a process called atomic layer deposition (ALD) to apply a protective layer. This coating improves the chemical stability of the electrolyte not only by acting as a physical shield, but also by modifying the surface’s electronic structure, resulting in materials that are more stable to moisture and oxygen.
“Our research shows that even a very thin coating — just a few nanometers thick, or about 100,000 times thinner than a human hair — can act as a strong barrier, keeping the electrolyte intact and boosting its performance,” said Argonne materials scientist Justin Connell. ?“This breakthrough not only can extend the battery’s life but also can lower manufacturing costs by allowing production in less controlled environments.”
The ALD process, commonly used in making computer chips, deposits a layer of aluminum oxide onto the electrolyte particles. Aluminum oxide is similar to glass, with many of the same properties.
“We’ve coated the solid electrolyte powder with an ultrathin, glass-like layer that stops it from reacting with the atmosphere,” said Jeffrey Elam, a senior chemist and Argonne Distinguished Fellow. ?“This material can be so thin that it’s less than one atomic layer, meaning it is thinner than the diameter of a single atom. At first, this result puzzled us, but computational modeling helped uncover an explanation.”
Peter Zapol, a computational scientist, explained, ?“We initially thought that the coating was just a physical barrier, but we discovered a lot more about the electronic properties of the electrolyte. The ALD coating alters the electronic structure of the electrolyte surface, which helps suppress degradation and maintain lithium-ion conductivity. This dual role — acting as both a physical shield and an electronic structure modifier — makes the coating particularly effective.”
The protective layer not only keeps the electrolyte stable but also ensures efficient lithium-ion movement, which is essential for the battery’s operation.
In tests with high humidity and oxygen, comparable to ambient air, the coated electrolytes performed much better than uncoated ones. The coated materials remained stable with little degradation, while the uncoated ones showed significant breakdown and atmospheric reactivity.
The ability to work with these materials in less controlled environments is a key advantage of this coating. Materials scientist Zachary Hood noted that handling these materials under harsher conditions would simplify the manufacturing process.
“It would allow manufacturers to use existing infrastructure, similar to what is used for lithium-ion batteries,” he said. ?“This would result in significant savings in the upfront cost of factories needed to make batteries out of these materials, while also improving reliability since there is less concern of materials degradation during assembly.”
The team is also working to scale up this method. They are currently collaborating with a commercial partner to produce larger quantities of the coated electrolyte for demonstration in larger format batteries.
While the team has achieved success with the current aluminum oxide coating, they acknowledge that it is just one of many possible coating chemistries. There are many others to explore, and future research will focus on these alternatives.
Other contributors to this work include Taewoo Kim, Aditya Sundar, Anil Mane, Francisco Lagunas, Jordi Cabana and Sanja Tepavcevic from Argonne; Khagesh Kumar, who is affiliated with both Argonne and the University of Illinois at Chicago; and Neelam Sunariwal from the University of Illinois at Chicago.
Results of this research were published in ACS Materials Letters. This study was funded by the DOE Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office.