BYLINE: Daniela Levy
Led by the Department of Energy’s Oak Ridge National Laboratory, scientists are developing a polymer into a strong yet elastic thin film, accelerating the development of next-generation solid-state batteries. The effort is advancing the development of electric vehicle propulsion systems enabled by flexible, durable layers of solid-state electrolytes.
The films could enable scalable production of future solid-state batteries with higher energy density electrodes. By separating negative and positive electrodes, they would prevent dangerous short circuits while providing highly conductive paths for ion movement. These achievements suggest increased safety, performance and energy density compared to current batteries that use liquid electrolytes that are flammable, chemically reactive, thermally unstable and prone to leakage.
“Our achievement could double energy storage to at least 500 watt-hours per kilogram,” said ORNL’s Guang Yang. “The main motivation for developing solid-state electrolyte membranes that are 30 microns or thinner was to pack more energy into lithium-ion batteries, so your electric vehicles, laptops and cell phones can run much longer before they need to be recharged.”
The work, published in ACS Energy Letters, improved on an earlier ORNL invention by optimizing the polymer binder for use with solid sulfide electrolytes. It is part of an ongoing effort to establish protocols for materials selection and processing.
The goal of this study was to find the “Goldilocks point” – a film thickness that is just right to support both ionic conduction and structural strength.
Current solid-state electrolytes use a plastic polymer that conducts ions, but their conductivity is much lower than that of liquid electrolytes. Sometimes polymer electrolytes contain liquid electrolytes to improve performance.
Sulfide solid electrolyte has an ionic conductivity comparable to that of the liquid electrolyte currently used in lithium-ion batteries. “This is very appealing,” Yang said. “The sulfide compounds create a conductive path that allows the lithium to move back and forth during the charge/discharge process.”
The researchers discovered that the molecular weight of the polymer binder is critical for producing durable solid-state electrolyte films. Films made from lightweight binders with shorter polymer chains do not have the strength needed to stay in contact with the electrolyte material. Films made from heavier binders with longer polymer chains, on the other hand, have greater structural integrity. In addition, less long-chain binder is needed to produce a good ion-conducting film.
“We want to minimize the polymer binder because it does not conduct ions,” Yang said. “The only function of the binder is to fix the electrolyte particles in the film. Using more binder improves the quality of the film but reduces ion conduction. Conversely, using less binder improves ion conduction but compromises film quality.”
Yang designed the study’s experiments and oversaw the project, working with Jagjit Nanda, executive director of the SLAC Stanford Battery Center and a Battelle Distinguished Inventor. Yang was recently recognized by DOE’s Advanced Research Projects Agency-Energy as a scientist likely to translate innovative ideas into impactful technologies.
Anna Mills, a former graduate student at Florida A&M University-Florida State University College of Engineering, focused on nanomaterial synthesis. She tested thin films using electrochemical impedance spectroscopy and performed critical current density measurements. Daniel Hallinan of Florida State offered advice on polymer physics. Ella Williams, a summer intern at Freed-Hardeman University, helped fabricate and evaluate electrochemical cells.
At the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL, Yi-Feng Su and Wan-Yu Tsai performed scanning electron microscopy and energy-dispersive X-ray spectroscopy to characterize the elemental composition and microscopic structure of the thin film. Sergiy Kalnaus, also of ORNL, used nanoindentation to measure local stresses and strains on the surface and applied theory to understand the results.
Xueli Zheng and Swetha Vaidyanathan, both of SLAC National Acceleratory Laboratory, performed measurements at the Stanford Synchrotron Radiation Lightsource to reveal the morphology of the cathode particles.
These advanced characterization techniques were critical to studying the intricate details of the sulfide solid-state electrolyte layer. “By understanding these details, we were able to improve the electrolyte’s ability to effectively conduct ions and maintain its stability,” Yang said. “This detailed analysis is critical for developing more reliable and efficient solid-state batteries.”
The scientists are expanding the capabilities of their 650-square-meter lab space at ORNL by creating low-humidity areas for research involving sulfides, which tend to contaminate other materials. “To accomplish this, we need special glove boxes in our chemistry lab,” Yang said. “In many settings, it can be challenging to dedicate resources to such specialized equipment. At ORNL, we have eight glove boxes specifically for this work.”
The team will build a device that integrates a thin film into next-generation negative and positive electrodes to test it under practical battery conditions. They will then collaborate with researchers from industry, academia and government to develop and test the film in other devices.
“This work is ideally suited to the capabilities of a national laboratory,” said Yang, praising the teams of diverse experts with access to valuable materials, characterization tools and special facilities.
This research was funded by the Vehicle Technologies Office of the DOE Office of Energy Efficiency and Renewable Energy.
UT-Battelle manages ORNL for DOE’s Office of Science. As the largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to solve some of the most pressing challenges of our time. For more information, visit energy.gov/science. — Dawn Levy
