As an electrolyte of lithium ion battery (LIB), a lithium salt as ion source, such as lithium hexafluorophosphate, is usually dissolved into an organic solvent, such as mixture of ethylene carbonate and dimethyl carbonate because the lithium salt is solid phase at room temperature and its melting point is above 200 ºC. We have focused on the electrolyte lithium salts themselves and attempt to use a “lithium molten salt” as electrolyte with no solvent. Firstly, molten salt has low-vapor pressure and non-flammability. A low-melting lithium melt enables a molten salt battery, which has been practically used as rugged storage,[1] to be used for broad application similar to LIB. On the other hand, the lithium melt contain no organic solvents, organic cations, which are composed in room temperature ionic liquid (RTIL). Therefore, discussion about alkali metal melt is free from complicated effects of the solvents and organic cations. The lithium melt is expected to be a base on all of the electrolytes containing the lithium salt.

We found that lithium (fluorosulfonyl)(trifluoromethylsulfonyl)amide (Li[fTfN]) possesses a significantly low melting point (100 ºC) among the lithium salts due to asymmetric structure of fTfN anion.[2] This melt allows stable charge-discharge of composite LiCoO₂ or LiFePO₄ positive electrode[3] and graphite negative electrode,[4] in half-cell. The Li[fTfN] can be used as a single lithium molten salt electrolyte without any organic solvents or RTILs. A cation mixture of the Li[fTfN] and its corresponding cesium salt (Li_0.4Cs_0.6[fTfN]) shows another interesting phenomena.[5] The Li_0.4Cs_0.6[fTfN] has 10²~10³ times higher viscosity, and 10^–2~10^–3 times lower specific conductivity than the organic electrolyte. However, its rate performance in LiFePO₄composite electrode was equal to those of the organic electrolyte.

In this study, we investigated charge-transfer resistance with these composite electrode and lithium metal in the Li[fTfN] and Li_0.4Cs_0.6[fTfN] by electrochemical methods, such as ac impedance, in order to discuss about factors of the stable charge-discharge without solvents and specific rate performance against its electrolyte property. Based on the single Li[fTfN] melt, we prepared the cation mixture added the other alkali metal fTfN salts, the RTIL electrolyte added the organic cation and the organic electrolyte solvated in the organic solvent. They will be compared for the physicochemical properties of electrolyte itself, the charge-transfer resistance with each electrode, and charge-discharge property.

References

[1] P. Masset and R. A. Guidotti, J. Power Sources, 164, 397 (2007).

[2] H. Matsumoto, N. Terasawa, T. Umecky, S. Tsuzuki, H. Sakaebe, H. K. Asaka, K. Tatsumi, Chem. Lett., 37, 1020 (2008).

[3] K. Kubota and H. Matsumoto, J. Phys. Chem. C, 117, 18829 (2013).

[4] K. Kubota and H. Matsumoto, ECS Trans., 61, 231 (2014).

[5] K. Kubota and H. Matsumoto, J. Electrochem. Soc., 161, A902 (2014).

 

Acknowledgements

This work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA-SPRING) of Japan Science and Technology Agency (JST) and Grant in Aid for scientific Research from Japan Society for the Promotion of Science (JSPS).