Swiss researchers have lifted the lid on electromechanical processes that take place in solid-state batteries in a move they say could make the technology safer and more efficient for use in electric vehicles.
Researchers at the Paul Scherrer Institute (PSI) used X-ray tomography to observe the batteries “more precisely than ever before” and discovered how fissures in the battery material propagate.
Xiaohan Wu, who carried out the study as part of his doctoral work in PSI’s battery materials and diagnostics research group, said researchers had now advanced “a critical step” towards understanding electromechanical processes that occur in order to further develop the technology.
The researchers used X-ray tomographic microscopy at PSI’s Swiss Light Source (SLS) to investigate a battery that comprised mainly lithium and phosphorus sulfide.
Small tin pellets with a diameter of about 30 micrometres, half as thick as a human hair, were embedded in the battery. When the battery was charged, lithium ions were deposited in the tin pellets. The lithium was forced into the lattice structure of the tin. The volume of the pellets grew and they expanded. This caused the surrounding electrolyte material to fracture. The resulting cracks then hindered the movement of lithium ions through the electrolyte, which significantly reduced the performance of the solid-state battery.
The researchers continuously monitored the battery during charging and discharging and saw that the tin pellets expanded up to around 300%.
Researchers were also able to see how cracks in the electrolyte spread. Wu said: “We didn’t expect the cracks to spread in such a way that they would exactly traverse the lithium ions’ path through the battery cell.” As a result, the ions have to make extreme detours, which greatly inhibits the charging and discharging process.
It was also observed that the battery “virtually repairs itself when discharged”. When the lithium ions migrate out of the tin pellets, the cracks in the surrounding electrolyte close again. Wu said: “The solid electrolyte is elastic, so it can self-heal.”
The next step in this research is to find other electrolyte materials that react to the expansion of the tin spheres to a smaller extent, Wu said.
Details of the research are online here.
