Researchers at Oak Ridge National Laboratory have developed a thin, flexible, solid-state electrolyte (SSE) that may double energy storage for next-gen electric vehicles, cell phones, laptops and other devices. The sheets may allow scalable production of future solid-state batteries with higher energy density electrodes.

The work, published in ACS Energy Letters, improved on a prior ORNL invention by optimizing the polymer binder for use with sulfide solid-state electrolytes. It is part of ongoing efforts that establish protocols for selecting and processing materials.

Our achievement could at least double energy storage to 500 watt-hours per kilogram. The major motivation to develop solid-state electrolyte membranes that are 30 micrometers or thinner was to pack more energy into lithium-ion batteries so your electric vehicles, laptops and cell phones can run much longer before needing to recharge.

—Guang Yang, co-corresponding author

The goal of this study was to find the “Goldilocks” spot—a film thickness just right for supporting both ion 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 incorporate liquid electrolytes to improve performance.

Sulfide solid-state electrolyte has ionic conductivity comparable to that of the liquid electrolyte currently used in lithium-ion batteries.

It’s very appealing,. The sulfide compounds create a conducting path that allows lithium to move back and forth during the charge/discharge process.

—Guang Yang

The researchers discovered that the polymer binder’s molecular weight is crucial for creating durable solid-state-electrolyte films. Films made with lightweight binders, which have shorter polymer chains, lack the strength to stay in contact with the electrolytic material. By contrast, films made with heavier binders, which have longer polymer chains, have greater structural integrity. Additionally, it takes less long-chain binder to make a good ion-conducting film.

We want to minimize the polymer binder because it does not conduct ions. The binder’s only function is to lock the electrolyte particles into the film. Using more binder improves the film’s quality but reduces ion conduction. Conversely, using less binder enhances ion conduction but compromises film quality.

—Guang Yang

Yang designed the study’s experiments and oversaw the project, collaborating with Jagjit Nanda, the 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 succeed in converting 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 made critical current density measurements. Daniel Hallinan from Florida State provided advice on polymer physics. Ella Williams, a summer intern from Freed-Hardeman University, helped with electrochemical cell fabrication and evaluations.

At the Center for Nanophase Materials Sciences, a DOE Office of Science user facility at ORNL, Yi-Feng Su and Wan-Yu Tsai conducted scanning electron microscopy and energy-dispersive X-ray spectroscopy to characterize the elemental composition and microscopic structure of the thin film. Sergiy Kalnaus, also from ORNL, used nanoindentation to measure local stress and strain on its 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 cathode particles.

These advanced characterization techniques were crucial for examining the intricate details of the sulfide solid-state electrolyte sheet.

By understanding these details, we were able to enhance the electrolyte’s ability to conduct ions effectively and maintain its stability. This detailed analysis is vital for developing more reliable and efficient solid-state batteries.

—Guang Yang

The scientists are expanding the capabilities of their 7,000 square feet of ORNL lab space by establishing low-humidity areas dedicated for research with sulfides, which tend to contaminate other materials.

The team will build a device that can integrate a thin film into next-generation negative and positive electrodes to test it under practical battery conditions. Then they will partner with researchers in industry, academia and government to develop and test the film in other devices.

This research was sponsored by the DOE Office of Energy Efficiency and Renewable Energy’s Vehicle Technologies Office.

Resources

• Elucidating Polymer Binder Entanglement in Freestanding Sulfide Solid-State Electrolyte Membranes; Anna Mills, Sergiy Kalnaus, Wan-Yu Tsai, Yi-Feng Su, Ella Williams, Xueli Zheng, Swetha Vaidyanathan, Daniel T. Hallinan Jr., Jagjit Nanda, and Guang Yang; ACS Energy Letters 2024 9 (6), 2677-2684 doi: 10.1021/acsenergylett.3c02813