Possible carbon-carbon bond formation during decomposition? Characterization and identification of new decomposition products in lithium ion battery electrolytes by means of SPME-GC-MS

Commercially available lithium ion batteries (LIBs) of the 18650 cell format were aged with different cycling protocols. After cell opening and electrolyte extraction, the obtained electrolyte was characterized with respect to the occurring organic aging products. Therefore, gas chromatography - mass spectrometry (GC-MS) with standard liquid injection as well as solid phase microextraction (SPME) was applied. The composition of the pristine electrolyte and the main decomposition products have already been discussed in a preceding study. However, the SPME method provides access to study aging products, which are present in lower concentrations. Structural elucidation was done for several signals of interest. For ongoing validation of previously unknown compounds, selected standards were synthesized in order to compare fragment patterns and retention times. Three carbonates with butoxy-moieties were identified, although the longest carbon chain in the constituents of this specific electrolyte are the C2 chains of ethyl methyl carbonate (EMC) and ethylene carbonate (EC). Furthermore, the longest chain in standard carbonate based electrolytes in general is the C3-chain of propylene carbonate (PC). Thus, the formation of a carbon-carbon bond during cycling of LIB electrolytes has to be considered. In addition to the well-known transesterification reactions the occurrence of this type of reaction leads to a variety of new carbonate based decomposition products. Hence, their presence has to be taken into account with respect to possible influences on the interphases formed at the electrode/electrolyte interfaces