UNIVERSITY PARK, Pa. — Compared to their traditional battery counterparts, solid-state batteries have higher energy potential and are safer, making them key to advancing electric vehicle development and use. Penn State researchers have proposed an improved method of solid-state battery production that enables multi-material integration for better batteries — cold sintering.  

Traditional batteries have a liquid electrolyte, which enables the ions to move between the cathode and the anode, the battery’s two electrodes. Solid-state batteries have a thin electrolyte made of a solid material.  

"Solid-state batteries have a lot of advantages from a safety perspective in that they don't catch on fire, because they're a lot more stable owing to their stronger bonding,” said Zane Grady, doctoral student in materials science and lead author of the study that was published in ACS Applied Materials & Interfaces. "Because of that stronger bonding, they're also more mechanically robust. This prevents fire-causing short circuits, but also in theory it enables solid-state batteries to have higher energy density. They have an order of magnitude increase in performance relative to the batteries that we have now, which are reaching their limit. But there's also a lot of problems in making solid-state batteries.” 

One of the larger issues for solid-state batteries making the leap from laboratory to the market is the great challenges inherent in their production. Current battery electrodes are a mixture of active material, carbon, and the liquid electrolyte. Without a liquid electrolyte, there is no longer any direct path for the ions to move around in the electrode. The best way to give ions a path is by introducing a solid electrolyte, which requires sintering, and conventional sintering is too hot for carbon and active material, causing them to degrade. Cold sintering enables introducing the sintered solid electrolyte at very low temperatures.   

“In liquid batteries, you can take your two electrodes, and then add the electrolyte and as long as there's something separating the two, usually a polymer, you have a battery,” Grady said. “But making a solid-state battery involves producing a material such as a very thin layer of dense, conductive ceramic glass for a solid electrolyte, which is very difficult to do at scale.” 

Solving this issue is a current subject of deep interest for science and industry. This is because of the future potential for electric cars and their ability to lower emissions and combat climate change. Solid-state batteries have other potential benefits, such as longer-lasting laptop batteries.  

“There are a lot of people and companies right now who are researching different ways to produce solid-state batteries at scale,” Grady said. “It's not an overstatement to say it is one of the hottest topics in science at the moment, solving this problem.” 

According to the researchers, cold sintering may offer a solution. Cold sintering is a revolutionary process that enables sintering of ceramics at a much lower temperature than traditional methods, therefore using much less energy and enabling potential new material combinations. 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.  

“What we do in cold sintering is we reduce the sintering temperature of the ceramic solid electrolytes from the usual 1,200 degrees Celsius down to under 400 degrees Celsius,” Grady said. “When you do that, now you can integrate your solid electrolytes with everything else in the battery, like your active material and the electrodes, and cold sinter the interfaces together. It addresses all the different manufacturing challenges that are vexing everyone else who wants to make solid-state batteries by lowering the temperature. It opens up a whole window of co-processability between the solid-state battery materials that you can't get out for any other ceramic processing method.” 

According to Grady, solid-state battery electrolytes are made out of ceramics, polymers, polymer composites or soft non-crystalline materials. Ceramics are considered among the best material types as far as ionic conductors and solid-state electrolytes. 

“So, there's this disconnect in the research world between knowing what material would be perfect to have for solid electrolytes and what materials you can work with, and because of limitations of the sintering process for ceramics, no one's been able to really solve that,” Grady said. “So, 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.” 

In a prior study, the research team demonstrated how cold sintering can be employed at temperatures below 300 degrees Fahrenheit (150 degrees Celsius) 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 of solid-state batteries. Then, the team demonstrated that a solid electrolyte comprised of sodium zirconium silicate phosphate, often colloquially referred to as the NASICON solid electrolyte, could be cold-sintered at a slightly higher temperature, 707 degrees Fahrenheit (375 degrees Celsius), by replacing the liquid transient solvent with a more reactive, solid sodium hydroxide transient solvent. This resulted in a highly conductive ceramic solid electrolyte without the use of any additional conductive salts.  

For this current study, the team demonstrated a novel route toward the fabrication of mixed conducting electrodes for solid-state batteries. The team took a NASICON cathode ceramic powder that is 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. The pressure is applied and heated for three hours at 707 degrees Farenheit (375 degrees Celsius).  

Next steps for the research team include fine-tuning the cold sintering process of the solid-state batteries.  

“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 optimize for the most amount of active material, while also having the conductivity that you need to run the battery at a decent temperature,” Grady said. "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 then explore some additional issues and work to solve them.  
 
“We will then ask questions such as how do you slap the cathode and the electrolyte on top of each other in a way that you don't get a bottleneck of ions at that interface?” Grady said. “How thin can you make the electrolyte? These are important steps in moving towards an actual, practical solid-state battery.” 

Along with Grady and Randall, other authors of the study include Zhongming Fan, post-doctoral researcher in materials science, and Arnaud Ndayishimiye, post-doctoral researcher in materials science. 

The U.S. Department of Energy and the U.S. Department of Defense supported this research.