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ORNL's STAR Lab hunts the battery failures that only show up in a finished cell
Oak Ridge's STAR Lab tests sulfide solid-state and sulfur chemistries as complete cells, because the particle gaps, interface reactions and swelling that kill them do not appear in a materials test. Its results are scored against USABC metrics.
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What happened
- Oak Ridge National Laboratory's STAR Lab is set up to find the failures in sulfide solid-state and sulfur battery technologies before those technologies reach large-scale manufacturing.
- ORNL names three ways a good material still fails in a cell: gaps between particles, chemical reactions at interfaces, and mechanical changes during charging that block the movement of ions and electrons.
- Practical high-energy sulfur cells typically need around 4 to 5 milligrams of sulfur per square centimeter of electrode or more, and raising that loading makes ion transport harder and adds mechanical stress.
- The lab tests complete cells with electrode thickness, temperature, charging conditions and pressure controlled, and measures the results against USABC metrics and Department of Energy program objectives.
- Diagnosis runs on the VENUS neutron imaging facility at the Spallation Neutron Source and the MARS beamline at the High Flux Isotope Reactor, which supports 3D computed tomography.
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Why it matters
- constraint A solid electrolyte cannot flow into the space that charging opens, so whoever ships a sulfide cell owns the job of holding particle contact for the life of the pack.
- decision Anyone qualifying a battery supplier now has a defensible ask: results on a complete cell at a stated loading and pressure. A vendor who can only answer with materials data has told you what stage the technology is at.
- exposure Programs that picked a sulfur chemistry from thin-electrode data are exposed at the loading at which the energy density is claimed, because the transport and stress problems arrive with the loading.
- capability Reconstructing a cell's interior in 3D lets a team attribute capacity loss to a place in the electrode instead of arguing about it from the discharge curve.
The two properties that make sulfide electrolytes attractive are measured on the material by itself. The particles conduct lithium ions rapidly at room temperature, and they deform under pressure, so they pack together with fewer gaps [7]. Put that material into a working cell and the cell expands and contracts, and a solid electrolyte cannot flow into the space that opens. Maintaining contact is a major engineering challenge, in ORNL's account [6].
Sulfur hits the limit from the other direction. It is inexpensive and widely available, including as a byproduct of petroleum refining and natural gas processing [9], and it conducts electrons poorly, so manufacturers combine it with conductive material such as carbon [10]. The finished electrode has to hold connected pathways for electrons and for lithium ions at the same time, and that gets harder as the electrode grows thicker [10][21]. At 4 to 5 milligrams per square centimeter, a square meter of coated electrode carries 40 to 50 grams of sulfur [20].
A reaction at an interface is chemistry and swelling during charge is mechanics [2]. What the lab holds and varies are the process choices around them: electrode formulations, binders, interfaces, pressure, cell designs and testing conditions [8]. The object it measures is the assembled cell [13].
"Our mission is to close the gap between promising battery materials and scalable, manufacturable cell technologies," Guang Yang, an electrochemical scientist at ORNL, said in a press release quoted by Interesting Engineering [4]. The same account describes outside access the lab draws on: synchrotron X-rays at the Stanford Synchrotron Radiation Lightsource, through collaborations with SLAC National Accelerator Laboratory and Stanford University [18]. Computational models predict how manufacturing choices and material properties affect performance, and the results guide the next round of cell fabrication and testing [19].
The teams this is for are the ones specifying higher-energy packs for drones, robotics, transportation and grid storage [5]. When a supplier's one-page result lands, ask at what loading, and at what pressure. An ionic conductivity figure with neither attached is a material property, useful for deciding what to fund and thin for deciding what to design a pack around. A capacity and cycling figure at a stated loading, pressure and temperature is a claim about a cell, and those are the terms ORNL's own evaluation uses when it checks against USABC metrics and Department of Energy program objectives [14].
What to watch
- Whether STAR Lab publishes full-cell results at stated sulfur loadings and pressures, which would let buyers compare two suppliers on the same basis.
- Whether sulfide cell vendors start quoting the pressure their cells needed alongside room-temperature conductivity.
- Whether the USABC and DOE targets the lab scores against are published per chemistry, since those numbers decide what counts as a pass.