Researchers from the US’ Penn State University have proposed a new method of manufacturing solid-state lithium-ion batteries that eliminates material degradation issues.
The team cold sintered ceramic solid electrolytes at relatively low temperatures in a bid to avoid the issues of conventional sintering, which causes them to degrade because it is too hot for carbon and active materials.
Solving this issue could unlock solid-state battery technology, which is seen as the next big technical development for applications from electric cars to laptop computers.
The team reduced the sintering temperature of the ceramic solid electrolytes from the usual 1,200oC down to under 400oC, which enables the integration of solid electrolytes with everything else in the battery, including active material and the electrodes.
It was developed at Penn State by the research team led by Clive Randall, director of the Materials Research Institute, distinguished professor of materials science and engineering, and co-author of the study.
The study was published in the peer-reviewed journal ACS Applied Materials & Interfaces.
Zane Grady, doctoral student in materials science and lead author of the study, said it was not an overstatement to say solving the issue was one of the hottest topics in science at the moment.
He said: “What cold sintering does is really serve as an indication that it’s possible to make solid-state batteries out of ceramics.
“At low temperatures, you don’t need to compromise on density or conductivity in a way that I believe people assumed you had to with ceramics prior to low-temperature sintering.”
Ceramic electrolytes
Solid-state battery electrolytes are made out of ceramics, polymers, polymer composites or soft non-crystalline materials— but it’s ceramics that are considered among the best material types as far as ionic conductors and solid-state electrolytes.
In a prior study, the research team demonstrated how cold sintering could be used at below 300oF (150oC) to fabricate multilayered, solid-state lithium-ion batteries.
They relied on conductive salts to obtain suitable electrochemical properties, which undercut some of the conductive and safety advantages.
The team demonstrated that a solid electrolyte comprised of sodium zirconium silicate phosphate (referred to as the NASICON solid electrolyte) could be cold sintered at a slightly higher temperature, 707oF (375oC), by replacing the liquid transient solvent with a more reactive, solid sodium hydroxide transient solvent.
This resulted in a conductive ceramic solid electrolyte without the use of any additional conductive salts.
For the latest study, the team took a NASICON (sodium super ionic conductor) cathode ceramic powder, that was densified into a ceramic composite pellet with a transient solvent to help it densify, and used a carver press to apply necessary pressure to the powder.
Grady said: “We think that it’s possible to really explore the composition of the cold-sintered electrolytes, and research this relationship between ceramic mixed conduction and composition in a way that you can optimise for the most amount of active material, while also having the conductivity that you need to run the battery at a decent temperature.
“And then on the other side of things, we’re exploring layered structures as well, so that we can mix everything including the solid electrolyte in the cathode.”
The researchers will now look at how to place the cathode and the electrolyte on top of each other in a way that doesn’t cause a bottleneck of ions at that interface; and how thin they can make the electrolyte.
Along with Grady and Randall, additional authors of the study included: Zhongming Fan, post-doctoral researcher in materials science, and Arnaud Ndayishimiye, post-doctoral researcher in materials science.
