Effect of Anion Size on Conductivity and Transference Number of Perfluoroether Electrolytes with Lithium Salts

Deep B Shah,Nitash P Balsara,Sue J Mecham,Kevin R Olson,Adar Karny,Joseph M Desimone

doi:10.1149/2.0301714jes

Abstract

Mixtures of perfluoropolyethers (PFPE) and lithium salts with fluorinated anions are a new class of electrolytes for lithium batteries. Unlike conventional electrolytes wherein electron-donating oxygen groups interact primarily with the lithium cations, the properties of PFPE-based electrolytes appear to be dependent on interactions between the fluorinated anions and the fluorinated backbones. We study these interactions by examining a family of lithium salts wherein the size of the fluorinated anion is systematically increased: lithium bis(fluorosulfonyl)imide (LiFSI), bis(trifluoromethanesulfonyl)imide (LiTFSI) salts and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI). Two short chain perfluoroethers (PFE), one with three repeat units, C6-DMC, and another with four repeat units, C8-DMC were studied; both systems have dimethyl carbonate end groups. We find that LiFSI provides the highest conductivity in both C6-DMC and C8-DMC. These systems also present the lowest interfacial resistance against lithium metal electrodes. The steady-state transference number (t+ss) was above 0.6 for all of the electrolytes and was an increasing function of anion size. The product of conductivity and the steady-state transference number, a convenient measure of the efficacy of the electrolytes for lithium battery applications, exhibited a maximum at about 20 wt% salt in all electrolytes. Amongst the systems studied, LiFSI/PFE mixtures were the most efficacious electrolytes.

Highlights

Title Effect of anion size on conductivity and transference number of perfluoroether electrolytes with lithium salts
This paper is part of a series wherein we examine the possibility of using fluorinated oligomers to dissolve lithium salts with fluorinated anions.[7,8,9,10,11]
A large majority of studies on PEO-based electrolytes use lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the salt.[17,18,19]. While these electrolytes exhibit reasonable conductivity at room temperature and lithium salts can be dissolved at high concentrations, the transference number based on the steady-state current method can be as low as 0.05.20 This is due to specific interactions between the solvent and the cation

Summary

Mixtures of oligomeric and long chain polyethylene oxide and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) have been studied for lithium battery applications These electrolytes contain electronegative oxygen atoms and lithium salts with fluorinated anions. A large majority of studies on PEO-based electrolytes use LiTFSI as the salt.[17,18,19] While these electrolytes exhibit reasonable conductivity at room temperature and lithium salts can be dissolved at high concentrations, the transference number based on the steady-state current method can be as low as 0.05.20 This is due to specific interactions between the solvent and the cation. The constant c was determined by measuring the conductivity of four electrolytes in a liquid cell described in Reference 22: C6-DMC with 3 wt% LiTFSI and C8-DMC with 3 wt% LiTFSI, C6-DMC with 20 wt% LiFSI, and C6-DMC with 15 wt% LiBETI at 30◦C. Equation 3 was used to calculate the steady-state transference number: ts+s

Iss I

Results and Discussion

Conclusions