Lithium-ion batteries now in widespread use for everything from mobile electronics to electric vehicles rely on a liquid electrolyte to carry ions back and forth between electrodes within the battery during charge and discharge cycles. The liquid uniformly coats the electrodes, allowing free movement of the ions.
Rapidly evolving solid-state battery technology instead uses a solid electrolyte, which should help boost energy density and improve the safety of future batteries. But removal of lithium from electrodes can create voids at interfaces that cause reliability issues, limiting how long the batteries can operate.
Using X-ray tomography, researchers observed the internal evolution of the materials inside solid-state lithium batteries as they were charged and discharged. Detailed three-dimensional information from the research could help improve the reliability and performance of the batteries, which use solid materials to replace the flammable liquid electrolytes in existing lithium-ion batteries.
Operando synchrotron X-ray computed microtomography imaging revealed how the dynamic changes of electrode materials at lithium/solid-electrolyte interfaces determine the behavior of solid-state batteries. The researchers found that battery operation caused voids to form at the interface, which created a loss of contact that was the primary cause of failure in the cells.
The team built special test cells about two millimeters wide and studied the changes in battery structure during a five-day period. The test instrument took images from different directions; the images were reconstructed using computer algorithms to provide 3D images of the batteries over time.
Because lithium is so light, imaging it with X-rays can be challenging and required a special design of the test battery cells. The technology used is similar to what is used for medical computed tomography (CT) scans. Because of limitations in the testing, the researchers were only able to observe the structure of the batteries through a single cycle. In future work, they would like to see what happens over additional cycle and whether the structure somehow adapts to the creation and filling of voids. The results would likely apply to other electrolyte formulations and the characterization technique could be used to obtain information about other battery processes.
Battery packs for electric vehicles must withstand at least 1,000 cycles during a projected 150,000-mile lifetime. While solid-state batteries with lithium metal electrodes can offer more energy for a given size battery, that advantage won’t overcome existing technology unless they can provide comparable lifetimes.