Solvent Free Lithium Molten Salt as Electrolyte of Lithium Secondary Battery

As an electrolyte of lithium ion battery (LIB), a lithium salt as conductive ion source, such as lithium hexafluorophosphate (LiPF₆), is dissolved into an organic solvent, such as mixture of ethylene carbonate and dimethyl carbonate. The organic solvents do not only dissolve the lithium salts but form solid electrolyte interface to enable stable charge-discharge. The organic electrolytes have high specific conductivity and wide electrochemical window, which allows charge-discharge of 4V-class electrode materials. However, they are weak to heat in unexpected condition due to high vapor pressure and flammability of organic solvent.

We have focused on the electrolyte lithium salts themselves and studied about a low-melting lithium molten salt electrolyte, which is composed of lithium cation and its counter anion for two aims. Firstly, the lithium molten salt has low-vapor pressure and non-flammability as same as the room temperature ionic liquids (RTILs). A battery using high-melting lithium molten salt, such as LiCl-KCl eutectic, is rugged and high storage property. It has been practically used at above 350 ºC.[1] The low-melting lithium molten salt enables the molten salt battery to be used for broad application similar to LIB. On the other hand, the lithium molten salt contains only lithium cation and the counter anion, that is, there is no organic solvents, organic cations of RTILs, and the other additive materials. Its properties are affected by only its counter anions. Thus, properties of this simple electrolyte are expected to be a base on the solvent-mixed electrolytes.

Various lithium salts were synthesized and their melting point and thermal stability were investigated. Almost lithium perfluoro salts, such as LiPF₆, which are used as the electrolyte lithium salts of LIB due to its high oxidative resistance, are not used as molten salts because they thermally decomposed before melting. Meanwhile, lithium perfluorosulfonylamides, such as lithium bis(trifluoromethylsulfonyl)amide, are respectively low-melting (below 250 ºC) and stable liquid temperature range (to above 300 ºC). In particular, 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] Its single melt is also wide electrochemical window of 5.0 V from lithium metal deposition to oxidative decomposition of fTfN anion. Composite LiCoO₂ or LiFePO₄positive electrode and graphite negative electrode, which were prepared for the solvent-mixed electrolytes at room temperature, were successful in charge and discharge with the Li[fTfN] single molten salt at above 100 ºC.[3] Therefore, the Li[fTfN] can be used as a single lithium molten salt electrolyte without any organic solvents or RTILs. Reduced melting point, lower viscosity and higher specific conductivity have been attempted by addition to the other salts. These electrolyte properties and results of charge-discharge test will be presented.

[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).

Acknowledgements

This work was supported by the ALCA program of Japan Science and Technology Agency (JST) and Grant in Aid for scientific Research from Japan Society for the Promotion of Science (JSPS).