Transfer-Lithiation Causes Material Stress in Silicon/Graphite Composite Anodes

Silicon content in graphite electrodes can substantially reduce the anode volume of lithium ion batteries and relevantly increase the energy density. However, they lead to a loss of capacity and thus to a reduced lifetime of the cells. In a joint study, MEET Battery Research Center and the Institute of Physical Chemistry at the University of Münster as well as Helmholtz Institute Münster of Forschungszentrum Jülich have examined this type of composite anode in detail. The team discovered that the material stress of silicon cannot yet be minimized in long term – even if the material is only added to the graphite anode in small amounts.

Silicon Fully Lithiates After a Certain Period

Among other things, the researchers analyzed battery cells with composite anodes that had been stored in a charged state for a certain period. “These cells suffered from material stress caused by a fully lithiated silicon, which was actually aimed to be avoided via minimization of Silicon content in the anode. This in turn could be the cause of the reduced lifetime of the batteries,” says MEET scientist Dr Lars Frankenstein. The research team identified transfer lithiation as the reason of the silicon becoming fully lithiated both during and especially after charging. While the lithium initially goes to the graphite during charging, the ‘harmful’ transfer to the silicon begins over time until it is fully lithiated and overdosed.

Dr Johannes Kasnatscheew, Head of the Research Division Materials at MEET Battery Research Center, adds: “To minimize the negative effects in application, the material is currently prominent in research and development. In our study, we identified the duration in a charged state as a relevant factor to further investigate, evaluate and develop composite anodes based on silicon and graphite.”

Entire Study Online Available

The detailed results have been published by the authors Dr Lars Frankenstein, Pascal Jan Glomb, Leon Focks, Dr Aurora Gomez-Martin, Dr Tobias Placke and Dr Johannes Kasnatscheew, MEET Battery Research Center, Prof. Dr Michael Ryan Hansen and Steffen Böckmann, Institute of Physical Chemistry at the University of Münster, Marvin Mohrhardt, Helmholtz Institute Münster of Forschungszentrum Jülich as well as Prof. Dr Martin Winter, MEET Battery Research Center and Helmholtz Institute Münster, in the journal “ChemSusChem”.