Comparative Performance Evaluation of Flame Retardant Additives for Lithium Ion Batteries – I. Safety, Chemical and Electrochemical Stabilities

Abstract

Within this 1st part of a comparative study, flame retardant electrolyte additives (FRs), as candidates for lithium ion battery electrolytes, from four different phosphorous-containing molecule classes, are investigated. The five FRs (tris(2,2,2trifluoroethyl)phosphate (TFP), tris(2,2,2-trifluoroethyl) phosphite (TTFPi), bis(2,2,2 trifluoroethyl)methylphosphonate (TFMP), (ethoxy)pentafluorocyclotriphosphazene (PFPN) and (phenoxy)pentafluoro-cyclotriphosphazene (FPPN)) are investigated in a comparative manner to conclude structure-property relationships according to their self-extinguishing time (SET), onset temperature of the thermal runaway, chemical and electrochemical stability. SET experiments using standard electrolyte (1 M LiPF 6 in ethylene carbonate : dimethyl carbonate 1 : 1 wt%) confirm high reproducibility of the used SET device. The results reveal a strong dependency of the ignition time on the SET. Therefore it is suggested, to investigate the SET for various ignition times. All FR additives remain chemically stable for weeks after added to the standard electrolyte during storage (approved by nuclear magnetic resonance). Cyclophosphazenes show superior results concerning their first time of inflammation and thermal electrolyte stability (approved by adiabatic reaction calorimetry). All additives show partial electrolyte decomposition only during the first charge in voltammetric experiments. . Molecule structures of a)phosphates, b) fluorinated phosphates, c) phosphites, d) phosphonates and e) cyclophosphazenes used as flame retardants in LIBs. The highlighted molecules are investigated in this study. Chemicals: The standard electrolyte (1 M LiPF 6 in ethylene carbonate (EC) : dimethyl carbonate (DMC) 1 : 1 wt%) (battery grade, BASF) was mixed with 5 wt% of the additives TFP (99 %, ABCR), TTFPi (> 98.5 %, Sigma Aldrich), TFMP (> 97.5 % Sigma Aldrich), PFPN (> 98 %, TCI) and FPPN (> 98 % TCI) under argon atmosphere (Westfalengas, purity 99.998 %) in a MBraun glove box with a water and oxygen content below 0.2 ppm. Electrolyte and additives were used as received without further purification. Electrochemical experiments: Lithium metal cells built in a glove box under argon atmosphere, with a three-electrode set-up were used to perform linear sweep voltammetry (LSV) (Figure 7a) and cyclic voltammetry (CV) experiments (Figure 7b) on a VSP potentiostat/galvanostat from BioLogic. . Scheme of a a) T-cell with three-electrode set-up for LSV experiments, b) for CV experiments and c) schematic plot of the NMR set-up for chemical stability investigation of additives in standard electrolyte.Figure 8. Schematic illustration of the heat-wait-search experiments.

In situpolymerization process: an essential design tool for lithium polymer batteries

“…Hetero functionalities often facilitate the fine-tuning of the chemical resistance, 430 interfacial and transport properties (e.g., ionic conductivity, T Li + ), and thermal properties (e.g., nonflammability, high-temperature stability) of PEs. [431][432][433][434][435][436] For this purpose, the in situ processable organic-inorganic hybrid monomers/oligomers are explored. In line with this, functional moieties such as siloxane, boranes, and phosphate groups are introduced into the monomer/oligomer structure.…”

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].…”

Advances in Prevention of Thermal Runaway in Lithium‐Ion Batteries

“…[84] Organic fluorocompounds are [121] currently under development to allow formation of a more stable SEI that is resistant to high voltages. [84] The electrolyte can also be manufactured with flame-retardant properties [85][86][87][88] to stop mixtures of gases from igniting, at the cost of increasing electrolyte volume with complex additives such as cyclophosphazenes. [85] However, more fundamentally, the electrolyte can be designed to be nonliquid, to prevent the release of flammable gases, which progresses TR.…”

“…[84] The electrolyte can also be manufactured with flame-retardant properties [85][86][87][88] to stop mixtures of gases from igniting, at the cost of increasing electrolyte volume with complex additives such as cyclophosphazenes. [85] However, more fundamentally, the electrolyte can be designed to be nonliquid, to prevent the release of flammable gases, which progresses TR. Certain polymer electrolytes, either gel or solid, are used in this respect but suffer lower ionic conductivities.…”