Flammability of Electrolytes – a Criticial Discussion of Flash Point and Self-Extinguishing Time Measurements

The electrolyte is a critical component for the safety of Li-ion batteries (LIBs). Under critical conditions it may take part in exothermal reactions with the electrodes which cause a rise of the cell temperature up to critical levels. Once the cell is ignited, the electrolyte may act as the fuel which keeps the flame alive. In LIBs with “safe” electrode materials such as Li₄Ti₅O₁₂ anodes and LiFePO₄ cathodes, the electrolyte constitutes the weakest (unsafest) cell component. Consequently, the thermal safety of a LIB can be improved by carefully adjusting the electrolyte composition. Reliable data on the flammability behaviour of the electrolyte components are helpful in this respect.

Two commonly used values to describe the flammability of a liquid are the flash point and the self-extinguishing time (SET) [1].

The flash point is the lowest temperature at which a liquid forms an ignitable vapour mixture with air near the surface of the liquid. It can be measured in different ways, e.g. with open or “closed” sample cups, or with ignition with open flames or glowing filaments. Ignition by open flame is the method which is usually covered in national and international standard specifications. In practice also ignition by glowing solids is frequently found. A common case is, for instance, that during overheating the cell opens and first electrolyte evaporates out from the cell forming a solvent / air gas mixture, which is then ignited by glowing particles (sparks) which are emitted from the cell.

The SET describes how long an ignited sample keeps burning. A glance at the LIB literature shows that a multitude of methods are used to perform SET measurements. These include both the ignition of the electrolyte soaked into different kinds of substrates (e.g. [2,3]) or direct ignition of the liquid in an open container (e.g. [4]). The amount of electrolyte and ignition time as well as the geometry of the container will influence the observed behaviour.

Table 1 lists the flash points and SETs of several solvents and electrolytes. The flash points were measured in “closed” cups and with ignition by a glowing filament (modified Pensky-Martens method), and the SETs have been measured for ~0.5 g liquid and 3 s exposure to an open flame. Solvents with a flash point higher than approx. 74 °C cannot be ignited any longer.

Strategies to reduce the flammability of electrolytes are the use of flame-retardant electrolyte additives [1-3,5] or of ionic liquids [6]. As shown for selected substances a significant increase of the flash point and decrease of the SET is only obtained by adding major amounts of the flame-retardant. In these amounts it will influence the bulk properties of the electrolyte (such as conductivity, salt solubility, etc.) and can no longer be regarded as an “additive”.

Finally, the measured flash point values are compared with estimated values derived from empirical methods, where the flash point is calculated from thermodynamic parameters, such as boiling point or vapour pressure, and/or group contributions for each functional building unit in the molecule. Whereas such methods are well known in the chemical engineering sector, they are rarely used in the battery community. Within a certain error margin these methods may be used to predict the flash points of new substances or of substances which are not available in amounts large enough to perform the flame point measurements.

Funding of this work by the German Federal Ministry for Economic Affairs and Energy (BMWi) within the project “EiSiBatt” (contract No. 03ET2015C) is gratefully acknowledged.

[1]     K. Xu; Chem. Rev. 104 (2004), 4303.

[2]     K. Xu, M.S. Ding, S.S. Zhang, J.L. Allen, T.R. Jow; J. Electrochem. Soc. 149 (2002), A622.

[3]     G. Nagasubramanian, K. Fenton; Electrochim. Acta 101 (2013), 3.

[4]     L. Lombardo, S. Brutti, M.A. Navarra, S. Panero, P. Reale; J. Power Sources 227 (2013), 8.

[5]     C.W. Lee, R. Venkatachalapathy, J. Prakash; Electrochem. Solid-State Lett. 3 (2000), 63.

[6]     C. Siret, L. Caratero, P. Biensan; WO 2009/0076540

Table 1: Flash points (FPs) and SETs of several solvents and electrolytes. (NF = non-flammable during 3 s exposure to open flame). (More examples are shown on the poster).