With neutron experiments at the BER II research reactor of the Helmholtz Centre in Berlin and at the Laue-Langevin Institute in Grenoble, the scientists were able to observe how a blocking layer forms on the silicon surface when the battery is charged, preventing lithium ions from penetrating. This 30-60 nanometer thin layer consists of organic molecules from the electrolyte liquid and inorganic components. When discharged, the layer partially dissolves again so that the lithium ions can penetrate the silicon anode. However, energy is required to dissolve the layer, which then is no longer available for storage. The physicists used the same electrolyte fluid that is also used in commercial lithium batteries.
After preliminary investigations of the neutron source BER II at the HZB, the experiments at the Institut Laue-Langevin (ILL) in Grenoble provided a precise insight into the processes. Using a measuring cell developed at the HZB, the physicists were able to study the anodes with neutrons during the charge-discharge cycles and also record a series of other measured values such as the electrical resistance using impedance spectroscopy.
As soon as this blockade layer is resolved, the efficiency of the charge-discharge cycles increases to 94 percent. This value is higher than that of lead-acid batteries (90%), but slightly lower than that of technically very mature lithium-ion batteries, which today reach up to 99.9%.
Now the scientists want to investigate whether the formation of the blockade layer can be prevented by applying a very thin protective layer of metal oxide. Then the capacity of silicon anodes would decrease less in the course of many charge-discharge cycles.