A solid - rather than liquid - electrolyte between the opposite electrodes of a battery should, in theory, enable a rechargeable lithium metal battery that is safer, packs much more energy, and charges considerably faster than the lithium-ion batteries commercially available today. For decades, scientists and engineers have explored several paths to realize the great promise of lithium metal batteries. A major problem with the solid, crystalline electrolytes under study has been the formation of microscopic cracks that grow during use until the battery fails.
Stanford researchers, building on findings they published three years ago that identified how these tiny imperfections form and expand, have discovered that annealing an extremely thin silver coating on the solid electrolyte's surface seems to largely solve the problem. As reported in Nature Materials today, this coating toughens the surface of the electrolyte fivefold against fracturing from mechanical pressure. It also makes existing imperfections much less vulnerable to lithium burrowing inside, especially during fast recharging, which turns nano fissures into nano crevices and eventually renders the battery useless.
"The solid electrolyte that we and others are working on improving is a kind of ceramic that allows the lithium-ions to shuttle back and forth easily, but it's brittle," said Wendy Gu, associate professor of mechanical engineering and a senior author of the study. "On an incredibly small scale, it's not unlike ceramic plates or bowls you have at home that have tiny cracks on their surfaces."
"A real-world solid-state battery is made of layers of stacked cathode-electrolyte-anode sheets. Manufacturing these without even the tiniest imperfections would be nearly impossible and very expensive," said Gu. "We decided a protective surface may be more realistic, and just a little bit of silver seems to do a pretty good job."
Silver-lithium switch
Previous research by other scientists investigated the use of metallic Ag coatings on the same solid electrolyte material - known as "LLZO" for its mix of lithium, lanthanum, and zirconium atoms, as well as oxygen - with which the current study worked. While the earlier studies used metallic silver to improve battery performance, the new study used a dissolved form of silver that has lost an electron (Ag+). This dissolved, charged silver - unlike metallic, solid silver - is directly responsible for hardening the ceramics against crack formation.
The researchers deposited a 3-nanometer-thick layer of silver onto LLZO surfaces, then heated the samples up to 300 degrees Celsius (572° Fahrenheit). During heating, the silver atoms diffused into the surface of the electrolyte, exchanging places with much smaller lithium atoms to a depth of 20 to 50 nanometers. The silver remained as positively charged ions rather than metallic silver, which the scientists think is key to preventing cracks from forming. Where imperfections exist, the presence of some positive silver ions also prevents lithium from intruding and growing destructive branches inside the electrolyte.
"Our study shows that nanoscale silver doping can fundamentally alter how cracks initiate and propagate at the electrolyte surface, producing durable, failure-resistant solid electrolytes for next-generation energy storage technologies," said Xin Xu, who led the research as a postdoctoral scholar at Stanford and is now an assistant professor of engineering at Arizona State University.
"This method may be extended to a broad class of ceramics. It demonstrates ultrathin surface coatings can make the electrolyte less brittle and more stable under extreme electrochemical and mechanical conditions, like fast charging and pressure," said Xu, who at Stanford worked in the laboratory of Professor William Chueh, a senior author of the study and director of the Precourt Institute for Energy, which is part of the Stanford Doerr School of Sustainability.
Using a specialized probe inside a scanning electron microscope, the researchers measured the force required to fracture the surface. The silver-treated, solid electrolyte required almost five times more pressure to crack, compared with the untreated material.
Looking ahead
So far, the experiments worked with small test samples rather than complete batteries. The researchers are now applying the silver-based surface treatment to full lithium metal batteries to see how well the coating performs under real-world conditions, such as repeated fast charging and long-term use.
In addition, the team is researching various strategies for using mechanical pressure at different angles that may extend battery life. They are also studying methods to prevent failure in additional types of solid electrolytes, like those based on sulfur, which may have additional benefits such as improved chemical stability with lithium. The application of these findings to emerging, sodium-based batteries is an intriguing possibility, which may help alleviate supply-chain constraints on lithium-based batteries.
Silver is not the only option, the researchers said. Early tests with other, less expensive metals - copper, for example - have shown encouraging results. Together, these findings suggest a new and flexible approach to toughening the fragile materials that may be crucial for next-generation batteries.
