Cold sintering may pave the way for improved production of solid-state batteries
According to Grady, solid-state battery electrolytes are made from ceramics, polymers, polymer composites, or soft non-crystalline materials. Ceramic is considered one of the best types of materials when it comes to ionic conductors and solid state electrolytes.
“So there’s this disconnect in the research world between knowing what material would be perfect for solid electrolytes and what materials you can work with, and because of the limitations of the sintering process for ceramics, no one has been able to really solve that,” Gradi said. “So what cold sintering does is really an indication that it’s possible to make ceramic semiconductor batteries. At low temperatures you don’t have to compromise on density or conductivity in a way that I believe people assumed you had to do with ceramics before low temperature sintering.
In an earlier study, the research team demonstrated how cold sintering can be used at temperatures below 300 degrees Fahrenheit (150 degrees Celsius) to make multilayer solid-state lithium-ion batteries. They relied on conductive salts to achieve proper electrochemical properties, which undermine some of the conduction and safety benefits of solid-state batteries. Next, the team demonstrated that a solid electrolyte composed of sodium zirconium silicate phosphate, often referred to as NASICON solid electrolyte, could be cold sintered at a slightly higher temperature, 707 degrees Fahrenheit (375 degrees Celsius), replacing the transitional liquid solvent with a more reactive solid sodium hydroxide transitional solvent. This resulted in a highly conductive ceramic solid electrolyte without the use of additional conductive salts.
For this ongoing study, the team demonstrated a new route to fabricating mixed conductive 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 die press to apply the necessary pressure to the powder. Pressure is applied and heated for three hours at 707 degrees Farenheit (375 degrees Celsius).
Next steps for the research team include developing the process for cold sintering solid-state batteries.
“We think it’s possible to really explore the composition of cold-sintered electrolytes and look for this relationship between ceramic mixed conduction and composition in a way that maximizes the greatest amount of active material, while 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 also exploring layered structures, so we can mix everything together, including the solid electrolyte in the cathode.”
Researchers will then explore a few additional issues and work to resolve them.
“We will then ask questions such as how do you slap the cathode and electrolyte on top of each other so that you don’t get an ion bottleneck at that interface?” said Grady. “How thin can you make the electrolyte? These are important steps in the transition to a real and practical solid-state battery. »
Besides Grady and Randall, other study authors include Zhongming Fan, post-doctoral researcher in materials science, and Arnaud Ndayishimiye, post-doctoral researcher in materials science.
The US Department of Energy and the US Department of Defense supported this research.