AIM

The research-objective of this work is the deeper understanding of the development reactions of the solid electrolyte interphase (SEI) on lithium metal anodes. As the SEI develops initially during the first contact of this type of anode with the electrolyte and is therefore present in all lithium metal batteries being assembled. The goal is realized by a newly developed sample-preparation method which will be the main focus of this presentation together with the applied analytical methods.

METHODOLOGY

As the SEI is a very thin layer on the anode, one must deal with low amounts of the compounds of interest, especially when it comes to the initially developed layers. Therefore, a new method for accumulating these compounds in the sample container was developed to assure deeper insights into the ongoing reactions and the resulting compounds using analytical methods like liquid NMR, GC-TCD (1) , GC-BID (1), SPME-GC-MS (2) and HPLC-IT-TOF. The electrolytes for the experiments were LiPF6 (1.2 M) in EC:EMC (3:7 b.w.) with and without 5 % VC.

RESULTS

The results of the accumulation method of SEI-derived compounds in general demonstrate the proof of principle of this kind of method. The different kinds of samples assure the correctness of the method together with the expected signals in the GC-BID measurements, as well as the structures being identified from the SPME-GC-MS and HPLC-IT-TOF data. In total the applied methods and results help to create a negative picture of the SEI by focusing on the liquid and gaseous phase. To determine the species of the solid SEI directly it would be necessary to use other analytical methods such as FTIR or XPS.

DISCUSSION

The results of the GC-BID and GC-TCD together with GC-MS reference data allow the identification and semi-quantification of gases that form as side products of the SEI-development.
An experiment to check for the changes using different treatment-durations demonstrate the formation of different gases and their total percentage of the overall formed gases over time. In addition, even after 24 h of treatment no plateau-level of the gasvolume is achieved. Such a plateau would be equivalent to a total turn-over of all available lithium metal and would allow full quantification (see attached figure).
Via the SPME-GC-MS and HPLC-IT-TOF results it was also possible to characterize further reaction products based on the used carbonate solvents and their reaction with freshly exposed lithium metal forming new SEI-layers.