Fluorinated organic solvents in electrolytes for lithium ion cells
Introduction
Currently, commercial graphite-based lithium ion batteries use mixed solvent electrolytes containing highly viscous ethylene carbonate (EC) and low viscosity dilutants such as dimethyl carbonate or diethyl carbonate (DMC, DEC) as main solvents. EC is indispensable, because of its excellent filming properties, DMC and/or DEC are required to get the low temperature performance of the electrolyte at least reasonable [1], [2]. Nevertheless, the low temperature performance of these EC based electrolytes still needs some improvement. In addition, DMC and DEC are highly volatile and their flash points are quite low (DMC 18°C, DEC 31°C), which may have a considerable impact on the battery safety.
In previous work we employed partly fluorinated solvents like glycol ethers [3] of the type HC2F4O(C2H4O)nC2F4H, n=1, 2, 3, (4, 6), urethanes [4] such as (CH3)2NCO2CH2CF3 or glycol esters [5], [6] as dilutants for more viscous solvents such as propylene carbonate (PC), the latter being known for its better low temperature behaviour compared to EC. Exchange of some hydrogen for fluorine causes significant differences in the physical and chemical properties of polar solvents. In particular, viscosities, but also melting and boiling points of fluorinated solvents are in most cases significantly lower, compared to their hydrogenated counterparts. Moreover, fluorinated solvents are in general much less flammable as less hydrogen is available. As a rule, oxidation stability of partly fluorinated solvents is quite good, due to the stability of the carbon–fluorine bond, and thus promising for lithium battery applications. On the other hand, costs are obvious disadvantages of fluorinated solvents.
It turned out that several of the investigated fluorinated solvents showed very good anode filming properties, as was observed also in other works using fluoroethylene carbonate [7]. Here, we present results where small amounts of 10 vol% of the fluorinated solvent allow to operate graphite anodes in PC based electrolytes; PC being known to lead to strong solvent co-intercalation into graphite, of refs. [6], [8].
Section snippets
Experimental
Commercially available solvents were used without further purification. N,N-dimethyl trifluoracetamide (DTA) was synthesised and purified according as described [8]. Lithium bis-(trifluormethansulfonyl) imide (LiTFSI) was dried at 140°C in vacuum with a turbo molecular pump for 5 days. The water content of the prepared electrolytes was determined by Karl Fischer titration to be less than 15 ppm. Graphite based anodes were made from TIMREX® synthetic graphites SFG 44 or KS 6 (Timcal Group Ltd.)
Results and discussion
The physical properties of DTA compared to the unfluorinated homologue N,N-dimethyl acetamide (DMAc) and some commercially used solvents are given in Table 1. Compared to low viscosity solvents as DMC or DEC the boiling point and in particular the flash point of DTA is higher, so that a better cell safety can be anticipated. As expected, the melting point and boiling point of DTA are significantly lower than those of DMAc. The melting point of −42°C is comparable to propylene carbonate (PC) and
Conclusions
N,N-dimethyl trifluoraceatamide (DTA) is a promising solvent for use as electrolyte component in lithium ion batteries. The advantageous properties of DTA presented in this study are: (i) beneficial filming behaviour on graphite, even in combination with propylene carbonate resulting in excellent anode cycling stability; (ii) satisfactory oxidation stability at a typical lithium ion battery cathode; and (iii) moderate viscosity, low melting point and comparatively high boiling point and flash
Acknowledgements
Financial support by the Austrian Science Fund (FWF) in the “Electroactive Materials” special research programme is gratefully acknowledged. We would like to thank Merck KGaA, Germany, and the Timcal Group, Switzerland, for supplying samples.
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