Safer Lithium Battery Electrolyte Systems Based on Organic Solvent-Ionic Liquid Mixtures

The low safety of the actually commercially available lithium-ion batteries represents their main drawback because of the presence of flammable and volatile organic solvents in the electrolyte, resulting in possible fire and/or explosion of the electrochemical device. A very promising approach to solve this problem is the partial or the total replacement of the organic solvents with a new class of fluid materials called ionic liquids (ILs), e.g., molten salts at room temperature or below.

Despite their undoubtedly favorable characteristics (non- flammability, negligible vapor pressure, high chemical/ electrochemical/thermal stability), the conductivity of IL-based electrolytes is, however, still lower than that of the organic solutions, with a negative effect on the high rate performance of the electrochemical device. In this scenario, mixtures of ILs in combination with organic compounds have been proposed as electrolyte components for lithium batteries. Such an approach represents a very good compromise between high ionic conductivity (low viscosity) and high safety (low flammability) of the resulting electrolyte [1]. IL materials based on the pyrrolidinium (PYR_1A) cations and bis(trifluoromethanesulfonyl)imide (TFSI) anion have been found to be suitable as components for lithium battery electrolyte systems [1].

Here is reported a physicochemical investigation on mixtures composed by a lithium salt (LiTFSI), two of the most common organic solvents employed in lithium-ion battery systems (namely EC and EC) and a pyrrolidinium TFSI ionic liquid (namely PYR₁₃TFSI). These quaternary mixed electrolytes, investigated in terms of thermal, transport, rheological and flammability properties, were optimized modulating the ratio of the different components in way to achieve the best physicochemical characteristics.

The DSC heating trace of the (0.1)LiTFSI-(0.9-x)PYR₁₃TFSI-(x)EC/DEC mixtures are depicted in Figure 1. A progressive shift of the endothermic feature, due to the melting of the sample, towards lower temperatures is observed with increasing the EC/DEC mole fraction. This behavior may be addressed to a loss in ionicity of the electrolytic mixture. Furthermore, the presence of organic solvents interferes with the Li⁺ --- TFSI^-interactions ions, e.g., by solvation of the lithium cations.

The transport properties of the (0.1)LiTFSI-(0.9-x)PYR₁₃TFSI-(x)EC/DEC mixtures are reported in Figure 2 as conductivity vs. temperature dependence. The progressive replacement of PYR₁₃TFSI ionic liquid with EC/DEC results in increasing conductivity raise and in melting point decrease, this latter issue revealed by the shifting of the change slope of the conductivity plot to lower temperature values (in good agreement with the DSC data). At the same time, the increasing fraction of organic solvents progressively reduces the viscosity of the mixtures, resulting in enhanced ion mobility and, therefore, faster transport properties. The gain in ion conduction is much evident at low temperatures because of the melting point raise of the electrolytic mixtures with the EC/DEC content. Conductivity values interest for practical applications are matched at -20°C and -30°C for EC/DEC fractions equal to 0.3 and 0.5, respectively.

Finally, ignition tests have revealed the full non-flammability of the electrolyte mixtures for EC/DEC mole fraction up to 0.3. Therefore, this represents the optimal compromise among fast ion transport properties and non-flammable characteristics.

Figure 1. DSC trace of quaternary (0.1)LiTFSI-(0.9-x)PYR₁₃TFSI-(x)EC/DEC electrolyte mixtures. The EC:DEC weight ratio was kept equal to 1:1.

Figure 2. Ionic conductivity vs. temperature dependence of quaternary (0.1)LiTFSI-(0.9-x)PYR₁₃TFSI-(x)EC/DEC electrolyte mixtures. The EC:DEC weight ratio was kept equal to 1:1.

Acknowledgements

The financial support of the Italian Ministry of Economic Development (MSE), within the Program Agreement ENEA-MSE on Electric System Research, is kindly acknowledged.

References

[1] G.B. Appetecchi, M. Montanino, S. Passerini, Ionic liquid-based electrolytes for high-energy lithium batteries, in Ionic Liquids: Science and Applications, ACS Symposium Series 1117, A.E. Vissser, N.J. Bridges, R.D. Rogers eds., Washington, USA, 2013.