A novel rechargeable Li/02 battery is reported. It comprises a Li conductive organic polymer electrolyte membrane sandwiched by a thin Li metal foil anode, and a thin carbon composite electrode on which oxygen, the electroactive cathode material, accessed from the environment, is reduced during discharge to generate electric power. It features an all solid state design in which electrode and electrolyte layers are laminated to form a 200 to 300 p.m thick battery cell. The overall cell reaction during discharge appears to be 2Li + °2 Li,0,. It has an open-circuit voltage of about 3 V, and a load voltage that spans between 2 and 2.8 V depending upon the load resistance. The cell can be recharged with good coulombic efficiency using a cobalt phthalocyanine catalyzed carbon electrode. btroduction Electrochemical power sources based on metal/oxygen chemical couples are unique because oxygen, the cathode active material, does not have to be stored in the battery, but rather it can be accessed from the environment. Past efforts to develop such batteries emphasized aqueous systems, utilizing either a KOH/H,0 alkaline electrolyte or a quasi-neutral electrolyte consisting of aqueous solutions of NaCl, NH4C1, (NH4),S04, or KNO,, and conventional design,1 for example as in the alkaline Zn/MnO, battery.Examples of aqueous metal/U, batteries include the Zn/U,, Al/U,, Ca/U,, and Li/0, systems,' although only the Zn/0, battery has become a commercial product; it is used for powering hearing aids.In this paper we report on a novel Li/0, battery that is unlike any metal/oxygen power sources developed to date. It is a nonaqueous thin film battery and consists of a thin L metal foil anode, a thin solid polymer electrolyte membrane that conducts Li ions, and a thin carbon composite electrode sheet made up of high surface area carbon on which oxygen, the electroactive cathode material, accessed from the environment, is reduced during battery discharge to generate electric power. The organic polymer electrolyte membrane serves both as the separator that electronically insulates the cathode from the anode and the medium through which Li ions are transported from the Li anode to the oxygen cathode during discharge.' The present Li/oxygen cell appears to be rechargeable due to the use of nonaqueous electrolyte. The design of this novel battery is a radical departure from that of traditional polymer electrolyte-based Li batteries in which the cathode comprises Li intercalating solid-state materials such as TIS,, V0,,, LiMn,04, and LiCoO,. 2 ExperimentdThe general experimental procedures, materials treatment, and cell construction were as follows. All experiments were carried out either in a Vacuum Atmospheres Corporation argon-filled dry box or in a dry room maintained with less than 1% humidity. Chevron carbon containing cobalt catalyst was prepared as follows: 0.5 g of cobalt phthalocyanine was dissolved in about 30 ml of concentrated [30 weight percent (w/o)] sulfuric acid. The resulting viscous liquid was poured onto 9.5 g of ...

A fundamental study of the influence of solvents on the oxygen reduction reaction (ORR) in nonaqueous electrolytes has been carried out for elucidating the mechanism of the oxygen electrode processes in the rechargeable Li−air battery. Using either tetrabutylammonium hexafluorophosphate (TBAPF6) or lithium hexafluorophosphate (LiPF6) electrolyte solutions in four different solvents, namely, dimethyl sulfoxide (DMSO), acetonitrile (MeCN), dimethoxyethane (DME), and tetraethylene glycol dimethyl ether (TEGDME), possessing a range of donor numbers (DN), we have determined that the solvent and the supporting electrolyte cations in the solution act in concert to influence the nature of reduction products and their rechargeability. In solutions containing TBA+, O2 reduction is a highly reversible one-electron process involving the O2/O2 − couple. On the other hand, in Li+-containing electrolytes relevant to the Li−air battery, O2 reduction proceeds in a stepwise fashion to form O2 −, O2 2−, and O2− as products. These reactions in the presence of Li+ are irreversible or quasi-reversible electrochemical processes, and the solvents have significant influence on the kinetics, and reversibility or lack thereof, of the different reduction products. The stabilization of the one-electron reduction product, superoxide (O2 −) in TBA+ solutions in all of the solvents examined can be explained using Pearson’s hard soft acid base (HSAB) theory involving the formation of the TBA+---O2 − complex. The HSAB theory coupled with the relative stabilities of the Li+−(solvent) n complexes existing in the different solvents also provide an explanation for the different O2 reduction products formed in Li+-conducting electrolyte solutions. Reversible reduction of O2 to long-lived superoxide in a Li+-conducting electrolyte in DMSO has been shown for the first time here. Our results provide a rational approach to the selection of organic electrolyte solutions for use in the rechargeable Li−air battery.

Unlocking the true energy capabilities of the lithium metal negative electrode in a lithium battery has until now been limited by the low capacity intercalation and conversion reactions at the positive electrodes. Abraham et al. (Abraham, K. M.; Jiang, Z. J. Electrochem. Soc. 1996, 143, 1-5) overcame this limitation by removing these electrodes and allowing lithium to react directly with oxygen in the atmosphere, forming the Li-air battery. The Li/O 2 battery redox couple has a theoretical specific energy of 5200 W h/kg and represents the ultimate, environmentally friendly electrochemical power source. In this work, we report for the first time the intimate role of electrolyte, in particular the role of ion conducting salts, in determining the reversibility and kinetics of oxygen reduction in nonaqueous electrolytes designed for such applications. Such fundamental understanding of this high energy density battery is crucial to harnessing its full energy potential. The kinetics and mechanisms of O 2 reduction in solutions of hexafluorophosphate of the general formula A + PF 6 -, where A ) tetrabutylammonium (TBA), K, Na, and Li, in acetonitrile are reported on glassy carbon electrodes using cyclic voltammetry (CV) and rotating disk electrode (RDE) techniques. The results show that the cations in the electrolyte strongly influence the reduction mechanism of O 2 . Larger cations represented by TBA salts displayed reversible O 2 /O 2 -redox couple, in contrast to those containing the smaller Li (and other alkali metal) cations, where an irreversible one-electron reduction of O 2 to LiO 2 , and other alkali metal superoxides, is shown to occur as the first process. It was also found the LiO 2 formed initially decomposes to Li 2 O 2 . Electrochemical data support the view that alkali metal oxides formed via electrochemical and chemical reactions passivate the electrode surface, making the processes irreversible. The O 2 reduction mechanisms in the presence of the different cations have been supplemented by kinetic parameters determined from detailed analyses of the CV and RDE data. The Lewis acid characteristics of the cation appear to be crucial in determining the reversibility of the system. The results of this study are expected to contribute to the rapid development of the Li-air battery.

A rechargeable Li-air cell utilizing an electrolyte composed of a solution of LiPF 6 in tetraethylene glycol dimethyl ether, CH 3 O͑CH 2 CH 2 O͒ 4 CH 3 ͑TEGDME͒, and an uncatalyzed porous carbon electrode, investigated to elucidate the baseline Li-air battery chemistry, is reported. From the x-ray diffraction patterns of the discharged carbon electrodes, the discharge product of the cell was identified to be Li 2 O 2 during normal discharge to 1.5 V. Discharging the cell to 1.0 V or below produces Li 2 O as well. The cell can be recharged without a catalyst in the carbon cathode, albeit at low depths of discharge. The high resistance of the discharged carbon cathode is a major impediment to recharging cells displaying a high specific capacity. The cell capacity decreases with continued cycling, which was found to be due to the poor cycling efficiency of the Li anode and the high resistance of the discharge products, which slowly accumulate in the porous electrode.

Prototype cells of the configuration Li/,~SM S as Li2S,, THF, 1M LiAsF6/C have been characterized with regard to capacity, rate, and rechargeability. Virtually 100% of the theoretical capacity could be realized at 50~ at rates below 1.0 mA/cm 2. In high rate cell configurations, 75% cathode utilization is possible at ,-.4 mA/cm 2 (C/3-C/4rate). The capacities at high rate are enhanced by Lewis acids, although the ultimate cause of rate limitation is passivation of the current collector by discharge products. The self-discharge rates of Li in contact with 4-5M S (as Li2Sn) solutions reveal capacity losses of 0.5%/day at 25~ to 4.4%/day at 71~ Based on the experimental results, a practical energy density of ~300 W-hr kg-' is possible using a standard cell design. Results on the battery's rechargeability are briefly reviewed.An ambient temperature Li/S battery utilizing an organic electrolyte has many attractive possibilities, among them high energy density, and the use of relatively low cost, relatively safe materials. Previous attempts to develop such a cell have involved the incorporation of insoluble Ss directly into the current collector (1-3). In general, very poor discharge efficiencies were obtained. One explanation put forth for these results is that some of the Ss was reduced to soluble polysulfide, Sn -2, which "escaped" from the cathode current collector and self-discharged on the Li anode (4). One recent moderately successful approach to increasing the S utilization efficiency was to add a Lewis acid, BF3, supposedly to suppress polysulfide formation (4, 5).The highest rate Li batteries, such as Li/SOC12 (6-8) and Li/SO2 (9, 10), have employed liquid cathodes. Here, very good electrical contact is maintained between the cathode material and the current collector through all phases of discharge. Both Ss and its ultimate discharge product, Li2S, are electrical insulators. Thus, it is likely that insulation of the cathode material, rather than S,~ -~ formation, led to poor results for Li/S cells. In our experience, the reaction between Li and dissolved Sn -2 is very slow, and the formation of dissolved Sn -2 enhances cathode utilization. In this regard, the explanation of the effect of BF8 on sulfur utilization is not very convincing.In contrast to most previous work on Li/S batteries, we have chosen to investigate a system where the S is completely dissolved in the electrolyte, as Li2S~ (11,12). Hence, the theoretical cell reaction becomes