Researchers from Sandia National Laboratories have identified a big hurdle to progressing solid-state lithium-ion battery performance in small electronics: the movement of lithium ions across battery interfaces.
Sandia’s three-year Laboratory Directed Research and Development project examined the nanoscale chemistry of solid-state batteries, concentrating on the region where electrolytes and electrodes make contact. A majority of commercial lithium-ion batteries contain a liquid electrolyte and two solid electrodes, but solid-state batteries instead have a solid electrolyte layer, allowing them to last longer and run more safely.
The underlying goal of the work is to make solid-state batteries more efficient and to improve the interfaces between different materials. In this project, all of the materials are solid; we don’t have a liquid-solid interface like in traditional lithium-ion batteries.
Farid El Gabaly, Physicist, Sandia
The research paper titled, “Non-Faradaic Li+ Migration and Chemical Coordination across Solid-State Battery Interfaces” was published in Nano Letters. Authors include Sandia postdoctoral scientist Forrest Gittleson and El Gabaly. The study was supported by the Laboratory Directed Research and Development program, with additional funding by the Department of Energy’s Office of Science.
El Gabaly clarified that in any lithium battery, the lithium has to travel to and fro from one electrode to the other when it is charged and discharged. However, the movement of lithium ions is not the same in all materials, and interfaces between materials are a key obstacle.
Speeding up the intersection
El Gabaly compares the research to finding out how to make traffic flow rapidly through a busy intersection.
For us, we are trying to reduce the traffic jam at the junction between two materials.
Farid El Gabaly
El Gabaly compared the electrode-electrolyte interface to merge on a freeway or a tollbooth.
“We are essentially taking away the cash tolls and saying everybody needs to go through the fast track, so you’re smoothing out or eliminating the slowdowns,” he said. “When you improve the process at the interface you have the right infrastructure for vehicles to pass easily. You still have to pay, but it is faster and more controlled than people searching for coins in the glove box.”
There are two crucial interfaces in solid state batteries, he explained, at the electrolyte-anode junction and cathode-electrolyte junction. Either could be ordering the performance limits of a full battery.
When we identify one of these bottlenecks, we ask, ‘Can we modify it?’ And then we try to change the interface and make the chemical processes more stable over time.
Forrest Gittleson, Postdoctoral Scientist, Sandia
Sandia’s interest in solid-state batteries
El Gabaly said Sandia is keen on the research largely because solid-state batteries are reliable, low maintenance and safe. Liquid electrolytes are usually reactive, volatile, and extremely flammable and are a leading reason for commercial battery failure. Removing the liquid component can make these devices work better.
“Our focus wasn’t on large batteries, like in electric vehicles,” El Gabaly said. “It was more for small or integrated electronics.”
Since Sandia’s California laboratory did not do solid-state battery research, the project initially built the base to prototype batteries and examined interfaces.
“This sort of characterization is not trivial because the interfaces that we are interested in are only a few atomic layers thick,” Gittleson said. “We use X-rays to probe the chemistry of those buried interfaces, seeing through only a few nanometers of material. Though challenging to design experiments, we have been successful in probing those regions and relating the chemistry to full battery performance.”
Processing the research
The research was done using materials that have been used in earlier proof-of-concept solid-state batteries.
“Since these materials are not produced on a massive commercial scale, we needed to be able to fabricate full devices on-site,” El Gabaly said. “We sought methods to improve the batteries by either inserting or changing the interfaces in various ways or exchanging materials.”
The research employed pulsed laser deposition and X-ray photoelectron spectroscopy integrated with electrochemical methods. This allowed very small-scale deposition since the batteries are thin and combined on a silicon wafer.
“Using this method, we can engineer the interface down to the nanometer or even subnanometer level,” Gittleson said, adding that a number of samples were made.
Making batteries in this way allowed the team to obtain a precise view of what that interface looks like because the materials can be put together so controllably.
The subsequent phase of the research is to enhance the performance of the batteries and to assemble them together with other Sandia technologies.
“We can now start combining our batteries with LEDs, sensors, small antennas or any number of integrated devices,” El Gabaly said. “Even though we are happy with our battery performance, we can always try to improve it more.”