Comparative Performance Evaluation of Flame Retardant Additives for Lithium Ion Batteries - II. Full Cell Cycling and Postmortem Analyses

Within this 2(nd) part of a comparative study five flame retardant additives (FRs) as candidates for lithium ion battery (LIB) electrolytes are evaluated in terms of their electrochemical performance in order to investigate performance differences and their long-term stability. FRs from four different phosphorus-containing molecule classes, (namely tris(2,2,2-trifluoroethyl)phosphate (TFP), tris(2,2,2-trifluoroethyl)phosphite (TTFPi), bis(2,2,2-trifluoroethyl)methylphosphonate (TFMP), (ethoxy)pentafluorocyclotriphosphazene (PFPN), (phenoxy)pentafluorocyclotriphosphazene (FPPN)) are investigated using MCMB graphite anode/NMC111 cathode full cells and cycled up to 501 times. A major part of the investigations focuses on the effect of different FRs on the first cycle performance, the raising additional resistance, the rate capability and the self-discharge behavior of the cells. It is shown that the addition of fluorinated cyclophosphazenes (PFPN and FPPN) provides the best electrochemical performance among the evaluated additives. Postmortem investigations by gas chromatography-mass spectrometry and scanning electron microscopy further validate the decomposition of TFP and TTFPi during prolonged cycling, thus explaining the detrimental impact on electrochemical performance. Hence, these additives are not suitable for application in LIB in terms of safety enhancement. In contrast, TFMP, PFPN and FPPN improve the electrolyte stability. The formation of typical decomposition products (e. g. dimethyl-2,5-dioxahexanedicarboxylate) that indicate severe electrolyte degradation, is avoided by using these additives.