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

Abstract

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 chromatographymass 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. . Chromatograms of acetonitrile (black), pure additive diluted in acetonitrile (grey) as well as fresh and cycled electrolytes diluted in acetonitrile (blue and green) are shown for each FR respectively. a) Standard electrolyte (black), standard electrolyte and 5 wt % of b) TFP, c) TTFPi, d) TFMP, e) PFPN and f) FPPN. Final concentrations are presented in Figure SI 5. . SEM images of anodes obtained from pouch bag full cells cycled for 501 times (or SOH80) containing different FRs. Black: standard, pink contains 5 wt % TFP, red: TTFPi, yellow: TFMP, green: PFPN and blue: FPPN. Cathodes are depicted in Figure SI 6. . Comprehensive overview of the six used electrolytes in terms of safety (onset temperature and nonflammability, see Part I), electrochemical performance (rate capability at 3 C, first cycle efficiency, self-discharge and additional resistance at cycle 460), long-term durability (additive ratio after cycling) and costs. Outer values of each axes are desired. Black: standard, pink contains 5 wt % TFP, red: TTFPi, yellow: TFMP, green: PFPN and blue: FPPN.

Chromatographic Techniques in the Research Area of Lithium Ion Batteries: Current State-of-the-Art

“…Furthermore, additives (up to 5%, either by v/v or w/w) [93] can be applied to further modify the different electrolyte properties. These modifications include flame retardancy [28][29][30][94][95][96] or the enhanced formation of SEI and CEI [97][98][99][100][101][102]. During the first charge/discharge cycles, also called formation cycles, the SEI is formed and, thus, further prevents decomposition of electrolyte at the Cathode materials can be divided into three structural groups: one-dimensional materials (e.g., olivine structures) with a highly anisotropic lithium diffusion in a single direction, two-dimensional materials (layered structures) allowing diffusion in two spatial dimensions, and three-dimensional materials (e.g., spinel structures) which allow diffusion in all three dimensions [72][73][74].…”

Non-flammable liquid electrolytes for safe batteries

“…52 Among these five flame retardants it was shown that the fluorinated cyclophosphazenes (PFPN and FPPN) outperform the other additives (phosphates, phosphites and phosphonates) both in terms of electrolyte safety and electrochemical performance. 53 Although the fluorinated cyclophosphazenes are most expensive, they are promising for future investigations. Future work on this electrolyte may include the application in larger cells and compatibility with other electrode configurations.…”

High‐Throughput Experimentation and Computational Freeway Lanes for Accelerated Battery Electrolyte and Interface Development Research

“…Discovery and development of novel functional materials with targeted properties traditionally involves a large number of trial tests following a series of procedures, see, e.g., on flame retardants for liquid electrolytes. [14][15][16][17] However, these efforts are inevitably far from time-efficient considering the near infinite chemical space. Current materials design approaches are still mostly based on human knowledge and intuition, as well as on low throughput experiments.…”