The Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) is a member of the Helmholtz Association, Germany’s biggest research organisation. It is funded by the Federal Republic of Germany and the State of Berlin. The HZB cooperates with more than 400 German and international universities, research institutions and companies. The institutes and departments in the HZB research areas are headed by professors holding joint appointments. More than 100 doctoral students from academic institutions in the region conduct research at HZB for their degrees. One of the newer departments is the Institute for Soft Matter and Functional Materials, which has also a research focus on electrochemical energy storage.
In theory, silicon anodes could store ten times more lithium ions than graphite anodes, which have been used in commercial lithium batteries for many years. However, the amp-hour capacity of silicon anodes has so far decreased sharply with each further charge-discharge cycle. Our HZB team has now succeeded in demonstrating what happens on the surface of the monocrystalline silicon anode during charging and which processes reduce the capacity. With the help of neutron experiments at the neutron source BER II (Berlin, Germany) and ILL (Grenoble, France), the research team was able to observe how a blocking layer formed and dissolved on the silicon surface during lithiation. This layer hindered the penetration of lithium ions.
This surface layer consists of organic molecules from the liquid electrolyte and inorganic components. During lithiation, this 30-60 nanometer thin layer partially dissolves so that the lithium ions can migrate into the silicon anode. However, energy is needed to form and dissolve the layer, which is then no longer available for storage. The physicists used the same carbonate based electrolyte fluid with lithium hexafluorophosphate salt that is also used in commercial lithium batteries.
Several cycles observed
After preliminary investigations of the neutron source BER II of the HZB, the experiments at the Institut Laue-Langevin (ILL) in Grenoble provided a precise insight into the processes. Cold neutrons with a very high flux are available at the reactor of the ILL, with which we were able to investigate the silicon anode non-destructively during several charge cycles. Using a measuring cell developed at the HZB, our physicists were able to study the silicon anodes during the charge-discharge cycles (operando) with neutrons and also measure a number of other values such as electrical resistance using impedance spectroscopy.
As soon as this blockage layer is dissolved, the efficiency of the charge-discharge cycles increases to 94 percent (94 % of the stored charge can be delivered again). This value is higher than that of lead-acid batteries (90%), but slightly lower than that of technically very mature lithium-ion batteries, which reach up to 99.9%.
The next step in our work will now be to investigate whether the formation of the blocking layer can be prevented by applying a very thin protective layer of metal oxide, so that the capacity of silicon anodes decreases less in the course of many charge-discharge cycles.
Dr. rer. nat. Sebastian Risse,
Physicist, Institute Soft Matter and Functional Materials,
Helmholtz Zentrum Berlin for Materials and Energy
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