A new family of salts for lithium Secondary batteries
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ABSTRACT A novel family of salts suitable for lithium battery application was synthesized and characterized. These salts have a large delocalized anion whose charge is spread over a single SO 2 and a phenyl ring. Remarkable properties were obtained for the lithium N-(3-trifluoromethyl phenyl) trifluoromethanesulfonamide salt or LiTFPTS. The electrochemical stability window is around 4.0 V and its conductivity in solid poly(ethylene oxide) or PEO is close to the one of the lithium perchlorate salt. Calorimetric analysis also showed that LiTFPTS behaves as a plasticizer since it hinders, to a certain extent, the PEO crystallization when it is used in a solid polymer matrix. Above all, its synthesis is quite straightforward and leads to potentially inexpensive salts as the starting amines are made commercially on a large scale.
INTRODUCTION For secondary lithium battery application, solid polymer electrolyte technology has its advantages over liquid or gel electrolytes. Safety was greatly improved since no flammable liquids are present. A solid polymer film can also withstand abusive treatments (overcharge, short circuit, excess heating) without venting dramatically or explode. However, the safety of this technology has been achieved so far at the expense of operating at high temperatures (60'C < T). During the past twenty years, much work has been done to modify the polymer backbone or salt structure in order to increase the electrolyte ionic conductivity. So far, the lithium bis-trifluoromethyl sulfonyl imide salt or LiTFSI [1-31 can be considered as one of the main breakthrough of this technology. Since then, an increasing number of lithium salts have been developed [4-9]. Unfortunately, few of them have shown suitable properties for solid polymer electrolyte (SPE) applications. Commercially available lithium salts are generally unfit because of the relatively small anion size that are not able to hinder the polymer crystallization; the latter effect is responsible for poor conductivity. Salts with low lattice energy do not guarantee good performances in a solid polymer matrix. The anion must be large and capable of spreading its charge over a wide structure. Furthermore, it must have a plasticizing effect. To be suitable for SPE battery application, a salt must have a good ionic conductivity in the polymer and a high thermal, chemical and electrochemical stability. This work deals with the characterization of a new lithium salt family containing a sulfonamide function combined with a phenyl ring in which, different functional groups can be attached. A typical anion structure was given by the following chemical formula:
f'
CF3-SO2%
CF3 e1 %
0
Present address: Rh6ne-Poulenc, 85 ave des Fr6res Perret. BP. 62, 69192 Saint-Fons CEDEX France. 123 Mat. Res. Soc. Symp. Proc. Vol. 575 02000 Materials Research Society
Similarly to the TFSI, the [(SO 2 CF 3 )N(C6H 4 )CF 3 ]- anion is large, flexible and capable of charge delocalization over the whole structure. By adding a second electron-withdrawing group to the aroma
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