Post Mortem Analysis of Fluorinated Flame Retardants for LIB Electrolytes by GC-MS
The high flammability of the organic solvents in LIB electrolytes leads to a safety concern regarding smartphones, tablets, laptops and electric vehicles. The addition of flame retardants (FRs) to the electrolyte decreases the flammability or in an ideal case results in a non-flammable electrolyte formulation.
Organophosphates like triethyl phosphate (TEP) were often used. Due to a low electrochemical stability of these organophosphates, the use as a flame retardant results in a trade-off between electrolyte flammability and electrochemical performance of the cell. Thus, two fluorinated FRs originating from TEP were investigated regarding their electrochemical stability in the following publication. Using Shimadzu’s GCMS QP-Series, post mortem analysis was performed to investigate changes in the electrolyte composition. Furthermore, chemical ionization in positive and negative mode was used to obtain additional structural information for the identification of the volatile electrolyte decomposition products.
SFE of Electrolyte from Spent LIBs and its Characterization by GC-MS
A long lifetime of LIB cells is one of the most important criteria when evaluating cells for a specific application e.g. in a defined operating temperature range. The temperature has a major influence on the electrochemical performance and lifetime of the cells. Operating a commercially available 18650 type cylindrical LIB cell within the specified operating temperature range, we observe a significant difference in performance. During long term stability investigations, the end-of-life criteria was reached after 300 cycles at 20 °C, whereas 1500 cycles were achieved at 45 °C. To better understanding this temperature depending aging mechanism, a post mortem investigation of the electrolyte is necessary.
The electrolyte was extracted from the spent cells using supercritical fluid extraction (SFE) and analysed by Shimadzu’s GCMS QP-series. The results were compared to electrolyte extracted from cells after formation (as supplied from the manufacturer). For the identification of unknown electrolyte decomposition products, additional structural information are needed. Thus, analyses with positive and negative chemical ionization were performed.