STAR Lab tests whether battery advances can scale

A battery material can meet key performance measures in a small laboratory cell yet fall short when researchers build it into a complete battery cell. At the Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL), researchers in the Sulfide/Sulfur Translational Advanced Research Lab, known as STAR Lab, are tackling that challenge by studying whether new battery technologies can withstand the demands of manufacturing and use.

"Our mission is to close the gap between promising battery materials and scalable, manufacturable cell technologies," said Guang Yang, an ORNL electrochemical scientist who leads STAR Lab.

Battery performance depends on more than the properties of a single material. Cells can degrade at interfaces where materials meet or when mechanical changes interrupt pathways for ions and electrons.

STAR Lab operates between materials discovery and industrial deployment. Researchers study how electrode formulations, binders, interfaces, cell designs, pressure and testing conditions affect performance after promising materials are incorporated into complete cells. The goal is to determine whether those materials can be processed reproducibly and continue to perform under realistic conditions.

The lab focuses on sulfide-based solid-state batteries and sulfur-based electrodes, two technologies with potential for higher energy density but significant manufacturing challenges. The research supports applications ranging from high-energy uses such as drones, robotics and transportation to stationary energy storage for the electric grid.

Why solid-state batteries require new approaches

Most commercial lithium-ion batteries use a liquid electrolyte to carry lithium ions between electrodes. Because liquids flow, they can fill pores and maintain contact as battery materials expand and contract during charging.

Solid-state batteries replace the liquid with a solid electrolyte, which can reduce leakage and flammability risks and allow different cell designs. But solid electrolytes cannot flow into gaps or restore contact when materials separate.

Researchers must therefore engineer the cell to maintain contact during operation.

Why researchers are studying sulfide electrolytes

Sulfide electrolytes can conduct lithium ions rapidly at room temperature. This property, known as ionic conductivity, affects how quickly a battery can charge and discharge and how much of its capacity can be used.

Under pressure, sulfide particles can deform and pack together, reducing gaps and improving pathways for lithium ions.

Compared with some oxide electrolytes, sulfides can require lower processing temperatures and can form close particle contact under pressure. Some polymer electrolytes can be easier to process but conduct ions more slowly at room temperature.

Why interfaces matter in solid-state batteries

High ionic conductivity alone does not guarantee a durable battery. Performance often depends on interfaces, the microscopic boundaries where solid materials meet.

Gaps can interrupt ion movement, while chemical reactions may form resistive layers called interphases. Cracking, warping or pressure loss can further reduce contact and performance.

STAR Lab researchers study these changes to identify why performance declines and how cell designs can be improved.

Sulfur electrodes offer high capacity but pose manufacturing challenges

STAR Lab also studies sulfur-based electrodes, which have high theoretical capacity and could increase the amount of energy stored per unit of mass. Sulfur is relatively inexpensive and widely available, with large quantities recovered as a byproduct of petroleum refining and natural gas processing.

Sulfur conducts electrons poorly, so researchers typically combine it with a conductive material, often carbon. The electrode must maintain connected pathways for both electrons and ions; otherwise, some sulfur cannot participate in the electrochemical reaction.

Manufacturing determines whether those pathways remain connected. Researchers must distribute sulfur evenly, preserve a continuous conductive network and balance electrode density with enough pore space for ion transport. Those requirements become more difficult in thicker electrodes with more sulfur per unit area.

High-energy sulfur cells designed for practical applications typically require about 4 to 5 milligrams of sulfur per square centimeter of electrode or more. As electrodes become thicker, ions and electrons travel farther, transport becomes more difficult and mechanical stresses increase. Changes in sulfur distribution and volume during cycling can also disrupt contact.

Designing experiments for manufacturing

STAR Lab researchers study advanced batteries as complete systems. Even a promising cathode can fail if particles lose contact, ion pathways break down or resistive layers form at interfaces.

The team designs experiments around manufacturability, reproducibility and quality control so results remain robust under practical processing and operating conditions.

Testing under practical conditions

Battery results can vary substantially with electrode thickness, charging conditions, temperature and pressure. Unless those variables are defined and controlled, results can be difficult to compare across laboratories.

STAR Lab is developing testing and evaluation approaches for sulfide solid-state and sulfur-based batteries with support from DOE's Office of Critical Minerals and Energy Innovation through the Advanced Battery Materials Research Program. Drawing on performance metrics from the United States Advanced Battery Consortium and DOE program objectives, researchers evaluate factors that can determine whether battery performance will hold up under more practical conditions, including how much active material it contains, the conditions under which it operates and how well it holds up through repeated charging and discharging. Researchers then analyze cells after cycling to determine why performance declines.

Identifying why cells fail

Measurements can show rising resistance or declining capacity, but identifying the cause is more difficult. Possible causes include limited ion transport, chemical reactions at interfaces, sulfur redistribution, loss of contact, material damage and electrolyte decomposition.

Researchers combine neutron and X-ray methods with spectroscopy, microscopy and electrochemical diagnostics to track structural, chemical and transport changes within cells and across critical interfaces. Researchers draw on capabilities at ORNL's Spallation Neutron Source (SNS), including the VENUS neutron imaging instrument. At ORNL's High Flux Isotope Reactor (HFIR), researchers use the CG-1D beamline, known as the Multimodal Advanced Radiography Station (MARS), for high-resolution neutron imaging and 3D computed tomography. SNS and HFIR are DOE Office of Science user facilities operated by ORNL.

ORNL researchers also collaborate with scientists at SLAC National Accelerator Laboratory and Stanford University to use synchrotron X-rays at the Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC. The measurements can reveal chemical and structural changes in battery materials and interfaces. SSRL is a DOE Office of Science user facility.

Using simulation to guide experiments

Computational models help researchers interpret experimental results and predict how materials, interfaces and manufacturing choices may affect battery performance. The models can screen possible designs and processing conditions, helping the team prioritize experiments.

Results from testing, characterization and modeling help researchers identify causes of performance loss. Those findings guide changes in materials, interfaces and cell designs for the next round of fabrication and testing.

Evaluating technologies for scale-up

STAR Lab provides a translational research environment at a scale larger than conventional laboratory experiments but smaller than industrial production, allowing researchers and partners to identify manufacturing challenges and process risks before technologies advance to larger-scale production.

STAR Lab's specialized capabilities complement ORNL's broader battery research and manufacturing portfolio, which includes DOE's Battery Manufacturing Facility, the nation's largest open-access battery manufacturing research and development center. There, researchers and industry partners can study the manufacturing process from raw materials through complete cells and performance testing.

Manufacturing complete cells introduces challenges that do not arise when materials are evaluated in isolation. That is particularly important for many sulfide solid electrolytes, which are sensitive to moisture and can react with some materials used during processing. STAR Lab researchers therefore study both dry and liquid-based manufacturing approaches, including how processing choices affect the materials and the resulting battery. Dry processing can limit exposure to liquids that may damage sensitive materials, while liquid-based methods may be more compatible with established lithium-ion battery manufacturing equipment.

STAR Lab accepts materials from external partners and can evaluate their processing compatibility, incorporate them into electrodes and full cells and investigate factors that limit performance.

"A material that works well in a small laboratory cell may fail when it is processed into a manufacturable electrode, separator or pouch cell," Yang said. STAR Lab researchers work to determine how those materials can be formulated and processed, how much pressure cells require and how to ensure consistent quality.

STAR Lab's goal is to determine whether promising materials can be manufactured consistently and continue to perform through repeated charging and discharging.

UT-Battelle manages ORNL for the Department of Energy's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science. - Scott Gibson