Flammability of Li-Ion Battery Electrolytes: Flash Point and Self-Extinguishing Time Measurements

Abstract

The flammability of electrolytes is an important aspect of the thermal safety behavior of Li-ion batteries. Flash points (FPs) and self-extinguishing times (SETs) of 25 solvents (including carbonates, ethers, esters, lactones, dinitriles, a sulfone, and others), 3 solvent mixtures, and 15 electrolytes are presented. The FPs have been measured according to the Abel and Pensky-Martens closed-cup methods using 12 mL of sample and electric ignition. The SETs have been determined with the pure liquids, without any carrier substrates. A correlation of the FPs with the SETs, as well as with vapor pressures and boiling points is attempted. Furthermore, the effect of the addition of two non-flammable solvents [1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide and methyl nonafluorobutyl ether] and a flame-retardant additive (trimethyl phosphate) on both the SET and the FP of a carbonate-based electrolyte is discussed. As an alternative to their experimental determination, the FPs of the pure solvents have been calculated from other physical and chemical properties of the substance, and the FPs of the solvent mixtures and electrolytes from the FPs of their flammable constituents. Some of the models predict the FPs with an accuracy of ± 10 • C, which may be sufficient to estimate the flammability hazards for many applications. Within less than 25 years Li-ion batteries (LIBs) have become the battery system of choice for consumer electronics, automotive, and stationary applications. With the increasing circulation of LIBs and with the growth of cell size, safety -especially the thermal safety behavior -has become a focus of public interest.The inherent (thermal) safety of a cell is determined by the safety of each component as well as by the interactions between the single components. To obtain a complete picture, safety should be assessed on multiple levels. On a component level this includes tests of electrolyte flammability and thermal analysis (such as differential scanning calorimetry) of electrolytes and of electrodes without and with electrolyte. On a cell level this includes accelerated rate calorimetry, to determine how thermal effects created at the various cell components affect the overall heating of the cell, and standardized safety tests (such as hot-box, fire, nail penetration, crush tests, etc.) of cells, battery modules, and battery packs, to obtain information about the thermal behavior under abuse conditions. [1][2][3][4][5] One of the most critical cell components is the electrolyte, and only with safe electrolytes will it be possible to build safer LIBs and batteries beyond LIBs.6-9 The most commonly used electrolytes for LIBs are based on LiPF 6 and mixtures of cyclic and linear carbonate solvents. The linear carbonates, such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), or diethyl carbonate (DEC), are required to keep the electrolyte viscosity low and the electrolyte conductivity high. Unfortunately, they are highly volatile and flammable and show flash points (FPs) around ro...

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