Helmholtz research centre analyses solid-state cell degradation
The German Helmholtz Centre asserts that solid-state batteries can store more energy and are safer than batteries with liquid electrolytes. However, they do not last as long and their capacity decreases with each charging cycle. This is what the Helmholtz Centre aims to change and its researchers are already working on uncovering the causes.
Currently, solid-state batteries use a solid ion conductor between the battery electrodes instead of a liquid electrolyte, which allows lithium to be transported during charging and discharging. This makes for a higher degree of safety during operation, as well as a generally higher storage capacity. However, after each charge cycle, decomposition products and interphases form at the interfaces between the electrolyte and the electrode, which hinders the transport of the lithium ions and leads to the consumption of active lithium. This unfortunately ensures that the capacity of the batteries decreases with each charge cycle.
HZB researcher Dr Elmar Kataev poses the question: “Under what conditions and at what voltage do such reactions occur, and how does the chemical composition of these intermediate phases evolve during cell operation?” The researchers then analysed samples of the solid electrolyte Li6PS5Cl, a material which is considered the best candidate for solid-state cells due to its high ionic conductivity. “An extremely thin layer of nickel (30 atomic layers or 6 nanometres) served as the working electrode. A film of lithium was pressed onto the other side of the Li6PS5Cl pellet to act as a counter electrode.”
Using X-ray technology, the sample was analysed by exciting the atoms, causing the reaction products to be identifiable. “The results showed that the decomposition reactions were only partially reversible.”
A team from the Helmholtz Centre Berlin for Materials and Energy and Justus Liebig University Giessen presented the new method for tracking electrochemical reactions during the operation of a solid-state battery using the photoelectron spectroscopy ‘BESSY II’. The Helmholtz Centre explains that “BESSY II is a third-generation synchrotron radiation source that produces extremely bright X-ray light.” They say the results will help to improve battery materials and design.
“We demonstrate that it is possible to use an ultra-thin current collector to study the electrochemical reactions at the buried interfaces using surface characterisation methods,” said Kataev.
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