In order to enhance the performance of Li or Li-ion batteries many kinds of electrolytes have been studied and developed over the years. 1,2 These electrolytes are needed to give optimum interfacial properties between the electrolyte and the various anodes or cathodes. So far, the salts such as LiClO 4 , LiPF 6 , LiBF 4 , LiAsF 6 , and LiOSO 2 CF 3 . LiOSO 2 CF 3 have been used in these batteries. However, there are advantages and disadvantages with each salt. These salts typically do not provide a balance of desired properties such as high ionic conductivity, high chemical/thermal stability, low toxicity, and a wide potential window. For example, LiClO 4 can induce explosions under some conditions. 3,4 LiPF 6 and LiBF 4 have poor chemical/thermal stability. 5-7 LiAsF 6 is regarded as one of the best salts because it gives the highest coulombic efficiency, but it degrades into toxic products. 8-12 LiOSO 2 CF 3 , which is more stable and safer than other salts, has the disadvantage of low ionic conductivity and like LiTFSI is corrosive toward Al current collectors at high potentials. 1 Due to its good ionic conductivity, electrochemical stability and low corrosion of Al, LiPF 6 is often used as the preferred salt in Li-ion rechargeable batteries.Recently, lithium bis(trifluoromethylsulfonylimide) [LiN(CF 3 SO 2 ) 2 , (HQ-115, available from 3M Company) LiTFSI] was introduced as a salt which acts as a good plasticizer for polyethylene oxide (PEO) electrolytes. [13][14][15] For application in polymer electrolyte systems, LiTFSI-PEO complexes exhibit rather low crystallinity and a low glass transition temperature (T g ), resulting in ionic conductivity above 10 Ϫ6 S cm Ϫ1 at 25ЊC. 14 The ionic conductivity of LiTFSI in propylene carbonate (PC) solvent is also comparable to that of LiPF 6 . Furthermore, this kind of imide salt has good chemical stability and safety characteristics. 13-15 Because of the above advantages of LiTFSI, several studies have been made of LiTFSI-based electrolytes. [16][17][18][19][20][21][22] However, the practical use of this salt in liquid electrolytes has not materialized due to its severe corrosion of Al which is commonly used as a cathode current collector above 3.6 V vs. Li/Li ϩ . 23 Very recently, a new imide salt, lithium bis(perfluoroethylsulfonylimide) [LiN(SO 2 C 2 F 5 ) 2 , Li-BETI] was introduced, in which the perfluoroalkyl groups are extended from -CF 3 (TFSI anion) to -C 2 F 5 (BETI anion). 23 Li-BETI has a relatively high ionic conductivity (ca. 10 mS cm Ϫ1 ) at a concentration of 1 mol dm Ϫ3 (ϭM) in PC:1,2-dimethoxyethane (DME) (1:1), and has excellent thermal stability up to 340ЊC compared to that of LiPF 6 , which undergoes thermal decomposition between 60 and 100ЊC. The relatively high ionic conductivity of Li-BETI/PC ϩ DME electrolyte can be attributed to the remarkable delocalization of charge in this molecule (see Fig. 1). 13 Compared to LiTFSI, Li-BETI offers a further advantage of improved corrosion performance with a repassivation potential of 4.2 V vs. Li/Li ϩ . 2,...

In order to stabilize and obtain highly ion-conductive surface films, lithium metal was modified using two kinds of nonionic polyether-type surfactants, viz., poly(ethylene glycol) dimethyl ether and dimethyl silicone/propylene oxide copolymer in propylene carbonate (PC) and ethylene carbonate (EC) + dimethyl carbonate (DMC) solutions. Impedance spectroscopy indicated that the surface films formed in the presence of these surfactants were very stable, whereas that in the absence of surfactant was not. Scanning electron microscopy images showed that localized deposition is suppressed for the modified systems compared with the unmodified system. The mass change associated with the deposition-dissolution of lithium was investigated with cyclic voltammetry (CV) and quartz crystal microbalance (QCM) methods. The CV/QCM analyses indicated that the extent of inactive lithium is diminished markedly (by ca. 50% for PC and ca. 30% for EC + DMC solutions) and that the surface film was very stable and its thickness was apparently invariant due to the presence of the surfactants on the electrode surface. The QCM analysis also suggested that the surface film may partly consist of the surfactant molecule constituents, including ethylene oxide (EO) groups reacted with lithium metal. It appears that a modified surface film including EQ chains enhances the ionic conductivity, surface uniformity, and cyclability. * Electrochemical Society Student Member. * Electrochemical Society Active Member.

In light of the possible utilization as a positive electrode material in lithium batteries, an attempt was made to improve the redox characteristics of the organodisulfide cathode (2,5-dimercapto-1,3,4-thiadiazole, DMcT) by trapping it within a basic polymer such as polyvinylpyridine (PVP). An investigation with cyclic voltammetry indicated that the redox process of DMcT on PVP film is enhanced. That is, the redox current increased in both the oxidation and reduction processes. Such an enhanced redox behavior of DMcT may be caused by the basic environment of the PVP matrix in which DMcT exists as a thiolate anion that can be oxidized at lower or favored, overpotentials. In fact, we confirmed by a separate experiment that in a pyridine environment, the standard rate constant which corresponded to the oxidation of DMcT increased by three orders of magnitude compared with that of the DMcT on a bare basal plane pyrolytic graphite (BPG) electrode. Due to the advantage of electrochemical binding of the thiolate anions to the PVP matrix, it is suggested that DMcT/PVP composite electrode may behave as improved cathode materials in lithium batteries.