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,...

2,2'-Dithiodianiline (DTDA), a conducting polymer having a disulfide bond in it, has been proposed as a new class of high energy storage material. DTDA has one S-S bond interconnected between two moieties of anilines. DTDA was electrochemically polymerized to form an electroactive thin film. The structure of the poly(DTDA) was similar to that of polyaniline, in addition the S-S bond is preserved after electropolymerization of DTDA. The electropolymerized poly(DTDA) had enhanced redox processes due to an intramolecular electrocatalytic effect of aniline/anilinium and thiol/thiolate redox couples. The poly(DTDA) has some advantages because of its high theoretical energy density, faster kinetics, and higher electrical conductivity than an organosulfur cathode when used in lithium batteries. The discharge test of Li/poly(DTDA) cell showed a flat curve with a charge capacity of 270 Ah/kg-cathode and an energy density of 675 Wh/kg-cathode.

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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.

The quartz crystal microbalance (QCM) was used to investigate the formation of surface films on a lithium anode. Whereas few studies have dealt directly with electrode mass changes, QCM was employed here to investigate in situ the deposition and dissolution of lithium on a Ni substrate, with respect to mass changes and changes in the surface-film morphology. The formation and destruction of the deposited surface film was followed by a real-time measurement of the resonance frequency (AD and the resistance parameter (AR). AR expresses the energy loss to the oscillation of the crystal at resonance state, which gave us useful information on the variation of surface roughness during charge-discharge cycles. The Af and AR variations are discussed for three different electrolyte systems, viz, LiClO4, LiCF,SO,, and LiPF, in propylene carbonate or diethyl carbonate solutions. Upon cycling, Af showed an irreversible decrease indicating continuous formation and destruction of the surface film. The extent of Af shift depended strongly on the electrolyte being larger for LiC1O4/PC and LiCF,SO,/PC systems than that for LiPF,/PC. The AR measurement indicated a smoother and more uniform surface of the deposited lithium for LiPF6/PC compared to those for LiC1O4/PC and LiCF,SO,/PC. In the LiPF6/PC system, the mass was found to increase even during the initial dissolution process. Such behavior can be explained by a healing process involving the subsequent chemical formation of a surface film right after dissolution of lithium. This process is faster than the lithium dissolution process itself in this case.