Post-Mortem Investigations of Fluorinated Flame Retardants for Lithium Ion Battery Electrolytes by Gas Chromatography with Chemical Ionization

Abstract

Using flame retardants (FRs) in lithium ion battery (LIB) electrolytes is usually a tradeoff between electrochemical performance and electrolyte flammability. Fluorinated FRs are a promising class of FRs which are currently under investigation. During this work, three FRs originating from triethyl phosphate with varying degree of fluorination were investigated regarding their electrochemical stability on cathode (LiNi0.33Co0.33Mn0.33O2, NCM) and anode (graphite) in half cells. During long-term cycling, changes in performance were observed. Especially on the anode side the FR addition showed a decrease in performance in comparison to the standard electrolyte (DEC/EC 1:1, 1M LiPF6). The electrolytes containing the three FRs were extracted from the cells and analyzed regarding their changes in composition and structural degradation. The decomposition products were investigated by gas chromatography (GC) with electron impact (EI) ionization and mass selective (MS) detection. To obtain more information with regard to the identification of unknown decomposition products further GC‐MS experiments with positive chemical ionization (PCI) and negative chemical ionization (NCI) were performed. Twelve different volatile organic decomposition products were identified. These decomposition products can be subdivided regarding their basic structure. Ether based, carbonate based and phosphate based fluorinated and non-fluorinated decomposition products were identified. Furthermore, possible formation pathways for all groups of decomposition products were postulated taking existing literature into account.