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

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Smaller, more charge-intensive electrodes are needed for "safe" stimulation of the nervous system. In this paper we review critical concepts and the state of the art in electrodes. Control of charge density and charge balance are essential to avoid tissue electrolysis. Chemical criteria for "safe" stimulation are reviewed ("safe" is equated with "chemically reversible"). An example of a safe, but generally impractical, charge-injection process is double-layer charging. The limit here is the onset of irreversible faradaic processes. More charge can be safely injected with so-called "capacitor" electrodes, such as porous intermixtures of Ta/Ta2O5. BaTiO3 has excellent dielectric properties and may provide a new generation of capacitor electrodes. Faradaic charge injection is usually partially irreversible since some of the products escape into the solution. With Pt, up to 400 muc/cm2 real area can be absorbed by faradaic reactions of surface-adsorbed species, but a small part is lost due to metal dissolution. The surface of "activated" Ir is covered with a multilayer hydrated oxide. Charge injection occurs via rapid valence change within this oxide. Little or no metal dissolution is observed, and gassing limits are not exceeded even under stringent conditions.

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