According to Hongtao Sun, assistant professor of industrial and manufacturing engineering, solid-state batteries - which use SSEs instead of liquid electrolytes - are a leading alternative to traditional lithium-ion batteries. He explained that although there are key differences, the batteries operate similarly at a fundamental level.
"Rechargeable batteries contain two internal electrodes: an anode on one side and a cathode on the other," Sun said. "Electrolytes serve as a bridge between these two electrodes, providing fast transport for conductivity. Lithium-ion batteries use liquid electrolytes, while solid-state batteries use SSEs."
Solid-state batteries offer improved stability and safety when compared to traditional lithium-ion batteries but face several manufacturing and conductivity challenges, Sun explained. For example, the high temperatures introduced in the fabrication process, especially with ceramic-based SSEs, can hinder their production and practical implementation.
To overcome this challenge, Sun and his team used a technique known as cold sintering - a process where powdered materials are heated, treated with a liquid solvent, and compressed into a denser form - to incorporate a highly conductive ceramic-polymer composite SSE known as LATP-PILG. The method is referred to as "cold" because it operates at significantly lower processing temperatures than traditional sintering, instead relying on applied pressure and a small amount of liquid solvent to complete the process. They published their approach in Materials Today Energy.
Traditional ceramic-based SSEs are typically composed of polycrystalline grains - materials made up of hundreds of tiny crystals - separated by grain boundaries. According to Sun, these grain boundaries are considered defects that hinder the transport of conductive ions. To reduce conduction loss in ceramic-based SSEs, Sun's team co-sintered a poly-ionic liquid gel (PILG) with LATP ceramics to form a polymer-in-ceramic composite SSE, an ideal material for use due to its stability and high conductivity.
The PILG acts as a highly conductive "grain boundary" in the SSE, facilitating ion transport across engineered boundaries rather than through defect-prone natural interfaces. Sun said the team initially attempted to use traditional high temperature sintering to develop their new SSEs, but they immediately ran into problems.
"One of the fabrication challenges of LATP-based composite SSEs is that the sintering temperature for ceramic is very high, to the point that traditional sintering would actually burn up any additives such as the polymer compound before the ceramic could be properly densified," Sun said. "This is why we had to implement cold sintering, to keep temperatures much lower."
Cold sintering technology was originally developed in 2016 through a research project led by Clive Randall, director of Penn State's Materials Research Institute and distinguished professor of materials science and engineering. Its application to developing solid-state batteries came in 2018, when a postdoctoral scholar in the laboratory of Enrique Gomez, professor of chemical engineering and interim associate dean for equity and inclusion for the College of Engineering, cold sintered ceramic composite electrolytes.
According to Sun, traditional sintering requires temperatures around 80% of the melting point of the material, which for ceramic compounds like LATP can easily reach 900 to 1,000 degrees Celsius.
"For this application, we were able to keep our sintering temperatures very low, around 150 degrees Celsius," Sun said. "This allows us to integrate different kinds of materials into a highly dense form using the cold sintering process, regardless of their distinct processing temperatures."
By sintering the LATP ceramics with PILG gel, Sun's team developed composite SSEs with high ionic conductivity at room temperature, among other strengths.
"In addition to improved conductivity, our polymer-in-ceramic composite SSE showcased a very wide voltage window, between 0 to 5.5 volts," Sun said, explaining that traditional liquid electrolytes have a window of 0 to 4 volts. "The large voltage window of our ceramic SSEs supports the use of high-voltage cathodes, allowing the battery to generate more energy overall."
For Sun, the applications of this cold sintering technology can someday go beyond improving batteries. He said he believes that cold sintering has big implications for how companies approach using ceramic composite materials in general manufacturing, as well as in more specific industries like semiconductor manufacturing.
"Our next goal is to develop a sustainable manufacturing system that supports large-scale production and recyclability, as that will be the key towards industrial applications for this technology," Sun said. "That is the big vision we hope to work towards over the coming years."
Research Report:Probing cold sintering-regulated interfaces and integration of polymer-in-ceramic solid-state electrolytes
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