Lithium Ion Conduction in Single Lithium Perfluorosulfonylamides
Authors: Kubota Keigo , Matsumoto Hajime 
Published: February 2020

A liquid electrolyte of lithium ion battery (LIB) is usually composed of a lithium salt and a solvent, such as an organic solvent or a room temperature ionic liquid (RTIL), which is usually necessary to dissolve the lithium salts. Therefore, the electrolytes must contain characteristics of both of the lithium salt and solvent. For example, an organic electrolyte usually has high vapor pressure and flammability derived by the organic solvent. The solvent or the other additive materials are absolutely necessary for practical use but also complicate properties of the electrolytes.

We reported that a lithium (fluorosulfonyl)(trifluoromethylsulfonyl)amide (Li[(FSO2)(CF3SO2)N]), which is denoted as Li[fTfN], formed a molten salt above 100 ºC.[1] The Li[fTfN] melt is extremely simple construction as the electrolytes for the lithium secondary battery, only Li+ and fTfN–. Therefore, it is free from the effect of the solvents and its concentration of Li+ is extremely high (about 7.0 mol·dm–3) among liquid electrolytes. Charge-discharge in the single Li[fTfN] was successful with conventional electrodes for LIB, such as LiCoO2, LiFePO4[1] and graphite.[2]

The Li[fTfN] has many interesting characteristics compared with the conventional electrolytes of LIB, such as much high viscosity and superionic conductivity.[1] In this study, we focused on no concentration gradient in the Li[fTfN] when applying current. In the conventional liquid electrolytes, the concentration gradient of lithium salt is generated in the electrolytes when current is applied in order to maintain electroneutrality in the electrolytes. However, the concentration gradient of lithium salt is not generated in the single Li[fTfN] melt because this melt is a lithium salt itself. In chronoamperometry of the Li[fTfN] using the two-electrode lithium metal symmetrical sell, there is little difference between an initial current, which caused by Li+ and fTfN−, and a steady current, which caused by only Li+.[1] This indicates that the only Li+ migrate by difference of voltages and fTfN– does not diffuse by concentration gradient. Its conduction behavior is similar to a solid electrolyte. Therefore, it is expect that Li+ is not drained near electrode if large current is applied in high rate charge-discharge.

To the best of our knowledge, there is a few kind of single lithium salt which melts below melting point of lithium metal (180 ºC), such as the Li[fTfN] and bis(fluorosulfonyl)amide salt (Li[f2N]). These melts have possibility of electrolyte possessing high rate performance due to the no concentration gradient. In this presentation, the conduction behavior of Li+ in these low-melting single lithium melts.

Acknowledgement

This work was supported by the Advanced Low Carbon Technology Research and Development Program Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING) of Japan Science and Technology Agency (JST).

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

[2] K. Kubota, H. Matsumoto, ECS Trans., 62, 231 (2014).