SEI development study – Accumulation and identification of SEI-derived species from lithium metal anodes

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Up to now there have been many studies on the morphology and the components of the SEI, always trying to add the next piece in solving the puzzle of this interphase. But the main issues with the analysis of this interphase remain. The fact that the SEI changes over the quantity and parameters of performed cycles by the cell and its low thickness, which is in the lower nanometre scale results in low abundances of compounds. In total this makes it even harder to perform studies on elucidating the componential chemical structures.
To allow state-of-the-art studies performing structure elucidation on SEI-derived components, it is necessary to have a method to accumulate those compounds in the first place.
Herein we want to introduce a method for the accumulation of SEI-derived species in a lithium metal system using standard carbonate-based electrolytes with and without 5% VC and their analytical results.
Our method is based on the fact, that the primary SEI is spontaneously developed, whenever bare lithium is in contact with the electrolyte. We use this characteristic to our benefit and physically destroy this layer repeatedly to accumulate the formed compounds in the liquid and gaseous phase.
In practice we follow a workflow, in which we prepare the sample, treat it for a certain amount of time, followed by a specific resting phase and the subsequent quantitative gas analysis via two gas-chromatographic methods and the qualitative analysis of the electrolyte via HPLC-IT-TOF, SPME-GC-MS and liquid NMR.
The qualitative results reveal several differences between the two electrolytes concerning their absolute and relative gas volumes that are formed due to primary SEI growth.
The quantitative results demonstrate the wide variety of compounds that are formed during the interphase development but also show how much of an impact the additive VC has on the process.
In summary the novel sample preparation in combination with the applied analytical methods allows to sharply raise the detectability of several compounds, as well as gather repeatable and reliable results for SEI-growth studies. In addition, this method is applicable for all liquid electrolytes in combination with lithium metal. Following this course, it is possible to elucidate and work further on the network of chemical reactions building the primary SEI.

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