Neutrons unlock the secrets of solid-state batteries

Researchers at the U.S. Department of Energy’s Oak Ridge National Laboratory have used neutron reflectometry to peer inside a working solid-state battery and monitor its electrochemistry. In doing so, it was discovered that its excellent performance is due to an extremely thin layer, across which charged lithium atoms quickly flow as they move from anode to cathode and blend into a solid electrolyte. ORNL’s Andrew Westover, who co-led the study published in ACS Energy Letters with James Browning, said the need for better batteries means more energy density, lower cost, faster and safer battery charging and longer life.

Rechargeable batteries rely on lithium, a small metal atom that packs tightly into the negatively charged anode to maximise energy density. However, lithium is unstable with most electrolytes — a factor in the flammability of smartphone, laptop and electric vehicle batteries that use liquid electrolytes. “To fix the flammability issue, we want to switch to solid electrolytes,” Westover said.

Lithium phosphorous oxynitride, or LiPON, is a solid electrolyte developed at ORNL nearly 30 years ago. Westover said that while it has never been understood why it works so well, the researchers want to make what works with LiPON work on a larger scale. Prior work showed the solid electrolyte interphase, or SEI — a layer that forms to protect and stabilise the solid-state battery — is key to its ability to charge and discharge repeatedly. In this case, the interphase is a chemical gradient consisting of a lithium-rich layer whose lithium content decreases as it blends into pure LiPON. In a normal battery, an interphase forms between the electrolyte and the working electrode. Over time, as the battery is charged and discharged, that material can change in composition and thickness.

“If you have a good SEI, your battery works. If you have a bad SEI, it doesn’t. The reason that the capacity of your cell phone battery slowly decreases year after year is because your SEI is expanding and consuming your electrolyte in the liquid-based battery,” Westover said.

In a LiPON-based solid-state battery, however, a thin SEI layer forms to passivate lithium, making it unreactive, and does not grow like the SEI in a traditional battery. Scientists coupled neutron reflectometry with electrochemistry to measure this stable interphase between LiPON and lithium and found that it was as thin as seven nanometres. “We discovered with this study that the layer formed is about 70 atoms thick. This work shows it is possible to make interfaces in solid-state batteries that are thin and provide excellent performance,” Westover said.

That small scale and the solid state of the materials prompted the researchers to use neutrons to look inside the battery. “Prior to the discovery of X-rays, you couldn’t look under skin to see bones inside a body. You had to cut the skin open. Until now, that’s basically been the approach that most people have used to look at interphases in batteries. In this case the scale is too small to cut anything open. We needed a tool that would allow us to go through the material, to probe it non-destructively at that scale and understand what’s happening at the interphase. That’s where neutron reflectometry came in,” Westover said.

Coupling neutron reflectometry with electrochemistry enhanced understanding of the interphase between lithium metal and solid electrolytes in solid-state batteries, and opened the door for researchers to look at the entire spectrum of solid-state electrolyte materials to determine which ones will enable fast-charging, high-energy batteries.

“We’ve already started version 2.0, where we’re looking at a different type of solid electrolytes and starting to understand what they look like. New materials need to be invented that have this stability,” Westover said.

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