Innovative Ionic Liquid Electrolytes, Based on the FSI Anion, for Highly Safe Lithium Batteries

There is a strong request to improve the safety of lithium battery systems, and researches towards novel generation solvents have been aimed. Among these, ionic liquids (ILs) are excellent candidates as electrolytes and/or electrolyte components in the place of volatile and hazardous alkyl carbonates, resulting in improved safety in case of overheating and thermal runaway of the lithium battery [1].

ILs based on N-alkyl-N-ethylpyrrolidinium cations (PYR_2A)⁺(the subscripts indicate the number of carbon atoms in the alkyl side chains) have been favorably used as electrolyte components for lithium batteries because of their sub-ambient melting point, high room temperature conductivity, and wide electrochemical stability [1].

The feasibility to reversibly cycling lithium into graphite anodes [1] and LiCoO₂cathodes [1] using IL electrolytes, based on the bis(fluorosulfonyl)imide (FSI) anion, was previously reported. The FSI anion was found able to form stable passive layer, preventing even the reduction of the unstable EMI cation [1] and allowing excellent lithium stripping/plating cycling behavior [1]. In addition, it is well known that FSI-based ILs display fast transport properties (due to their lower viscosity) [1] and low melting point.

Therefore, we combined the PYR₂₄ cation with the FSI anion to synthesize a novel IL (PYR₂₄FSI), which was used for preparing PYR₂₄FSI-LiFSI innovative electrolyte. The physicochemical properties of these mixtures as well as the electrode interfacial compatibility were investigated.

The PYR₂₄FSI-LiFSI electrolyte mixtures showed melting point close to -40 °C (Figure 1), resulting appealing for low temperature applications. It is worthy to note here that the crystallized PYR₂₄FSI and PYR₂₄FSI-LiFSI mixture (solid traces) do not show any shift of the endothermic melting peak with respect to not-crystallized corresponding samples. This behavior, previously observed in other pyrrolidinium FSI materials [1], clearly indicated that crystallization took place very quickly on cooling. Conductivity approaching 4´10^-3 S cm^-1 was achieved at ambient temperature (Figure 2A) whereas 1´10^-2 S cm^-1was matched at room-medium values. The conductivity trend shows a VTF behavior (Figure 2B), typical ion conduction in amorphous matrices and previously observed in different IL family electrolytes [1].

Figure 1. DSC traces, obtained after quenching (dotted traces) and slow cooling/annealing (solid traces) from room temperature, of the PYR₂₄FSI ionic liquid (panel A) and the (0.9)PYR₂₄FSI-(0.1)LiFSI mixture (panel B).

Figure 2. Ionic conductivity vs. temperature dependence (panel A) and VTF plot (panel B) of the PYR₂₄FSI ionic liquid and the (0.9)PYR₂₄FSI-(0.1)LiFSI mixture.

Acknowledgements

J.S.M. and G.B.A. thank the Italian Institute of Technology (IIT) for the financial support within the SEED Project “REALIST”. Y.D. and H.O. acknowledge the financial support from the Japan Society for the Promotion of Science (Grant No. 21225007).

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.