Conductivity and Viscosity of Solutions of LiCF3 SO 3, Li (  CF 3 SO 2 ) 2 N  , and Their Mixtures

Abstract

The high level of charge delocalization of the Li(CF3SO2)2N anion reduces ion pairing in nonaqueous electrolytes. This overrides the higher viscosity of the imide-contalning solutions making the electrolytes much more conductive than those containing LiCF3SO3, especially ir~ low dielectric ether-based solvent systems commonly used in secondary lithium batteries. In more viscous solvent systems (1:1 propylene carbonate: 1,2-dimethoxyethane) or at low temperatures, the imide affords less of an advantage in conductivity due to the more important role of viscosity. The cyclic imide LirSO2-(CF2)4-SO2-N 1 gives more viscous and less conductive solutions than does Li(CF3SO2)2N. The degree of dissociation in 1:1 PC:DME appears to beThe number of lithium salts suitable for use in lithium high energy density batteries is rather limited. Generally most salts have been found to be either insoluble (most dianions, for example) or unstable to the lithium anode (1). Of those reported, many have poor stability and/or undesirable safety characteristics, particularly those often used in secondary lithium systems. LiC104 can lead to explosions under certain conditions (2, 3) and while LiAsFs is not in itself particularly toxic (4), there is considerable concern regarding both the effect of disposal of such cells on the environment and the toxicity of salt degradation products (5-8) which may be present in the cells. LiBF4 and LiPF6 have been used but are themselves not as stable as one would wish. LiCF3SO3 (lithium "triflate") is a more stable, safer salt which is commonly used in primary lithium cells. Its main disadvantage is the low conductivity of lithium triflate electrolytes.Recently, the availability of the lithium imide salt Li(CF3SO2)2N (lithium bistrifluoromethylsulfonyl imide) has sparked considerable interest in the lithium battery field. This salt is reported to offer much greater conductivity than can be attained with the triflate salt ( 9) and yet the imide appears to have good stability (9) an d safety characteristics. Thus, it would seem possible to use the imide salt to produce highly conductive electrolytes with stability and safety characteristics similar to those of triflate solutions. Such electrolyte s could improve the characteristics of both primary and secondary lithium batteries.Apart from some conductivity studies (9) reported by Koch et aI., very little conductivity and no viscosity data

Safer Electrolytes for Lithium‐Ion Batteries: State of the Art and Perspectives

“…[8,97] The TFSI À anion provides high chemical inertness towards all cell components because it is characterized by ar ather low Lewis acidity due to the delocalized negative charge, [8,90] whichr esults in superiore lectrochemical stability towardso xidation. [33] In addition, LiTFSI exhibits good ionic conductivity, [63] outstanding solvation ability, [37] very low sensitivity towards hydrolysis, [77] and advantageous thermals tability. [42,43] These characteristicsr ender it ah ighly promising alternative for LiPF 6 .…”

Trifluoromethylsulfonyl-Based Salts of BEDT-TTF: Crystal and Electronic Structures and Physical Properties11

“…These compounds have excellent electrochemical and thermal stability in part because their negative charge is delocalized into the electron-withdrawing groups CF 3 and SO 2 (14). More recently, similar physical properties were observed for the more easily prepared lithium bis(trifluoromethanesulfonyl)methide, LiCH(SO 2 CF 3 ) 2 (15 3 .…”

“…Lithium bis(trifluoromethanesulfonyl)imide, LiN (SO 2 CF 3 ) 2 , and lithium tris(trifluoromethanesulfonyl)-methide, LiC(SO 2 CF 3 ) 3 , were developed as components for organic electrolyte-based lithium batteries (14). These compounds have excellent electrochemical and thermal stability in part because their negative charge is delocalized into the electron-withdrawing groups CF 3 and SO 2 (14).…”

Studies on the effect of anions of various lithium salts in PEMA gel polymer electrolytes

“…Sample A1 showed maximum ionic conductivity value, which is due to greater dissociative power and larger size of anion of LiClO 4 than the other salts overrides its high viscosity drawback and plays a crucial role in contributing towards the conductivity and towards the electrochemical oxidation stability of the electrolyte. 16 Tobishima and Yamaji 17 obtained this difference in incremental conductivity for LiClO 4 and LiAsF 6 and correlated them in a similar way. Hence, the ionic conductivity is affected by the diffusion rate of ions, which depends on the size of the ion.…”