The small signal a-c impedance of the cell Li]LiAsF6 (0.75M) in propylene carbonatel Lip V~O3 thin film on tin oxide covered glass substrate has been measured at room temperature as a function of frequency from * Electrochemical Society Active Member.

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Measurements of currents of the oxygen reduction reaction (ORR) at a Pt RDE coated with a recast Nafion film yield the permeability of O2 through such films. Combined with results from linear potential scanning experiments on similar electrodes, these measurements also allow the separate evaluation of the concentration of O2 and its diffusion coefficient in such films. Similar recast Nafion films have been used successfully to promote protonic access to catalyst sites in fuel cells. Accordingly, the maximum film thickness which would allow adequate oxygen flux through such a layer is evaluated. Measurements in 85% H3PO4 at room temperature show that a very significant enrichment of O2 (e.g., by a factor of 20) in a region close to the electrode can be achieved by coating the electrode with a recast Nafion film. This result is expected from the high solubility of O2 in Nafion. However, the measured resulting enhancement of the kinetic ORR currents was found to be only a factor of two. The interpretation suggested for the lack of direct proportionality between average O2 concentration near the electrode and ORR kinetic current density is that enhanced local ORR rates are possible only at a limited number of free Pt sites that are immediately adjacent to a surface region covered by the hydrophobic O2‐normalrich phase. Contamination by organics of lower molecular weight originating from the Nafion film may contribute to site‐blocking. With increase in temperature the O2 enrichment effect becomes less pronounced, because the solubility of O2 in 85% H3PO4 increases with temperature while in Nafion it decreases.

q A B S T R A C TIn an attempt to overcome problems associated with the reversible lithium electrode in organic solvent-based electrolytes, the electrochemical characteristics of a number of binary and ternary lithium alloys in propylene carbonate have been studied. Thermodynamic and kinetic results for the Li-Sn, Li-Sb, and Li-Bi systems are reported here. In each of these systems, one or more two-phase regions with apparently rapid transformation kinetics at ambient temperature have been identified. Lithium chemical potentials and partial molar entropies for the alloys have been determined. Constant current charge-discharge characteristics and diffusion coefficient data have also been obtained.Although a number of attractive positive electrode materials have been developed for use in secondary lithium ambient temperature organic solvent-based electrolyte cells, significant problems are associated with the use of pure lithium negative electrodes in such cells (1-5). The primary problem is the interaction between pure lithium and species within the solvent which causes the formation of filamentary growth or dendrites as well as macroseopic shape changes during the recharge process. This can result in disconnection and electrical isolation of active material, and sometimes electrical shorting between the electrodes. Many different possible approaches to solve this problem have been explored (6). One of these is to use a lithium alloy instead of pure lithium. In this case, one can avoid plating lithium at unit activity. If the diffusion of the lithium into an alloy is fast enough, so that the lithium activity at the surface never reaches unity, no filaments or dendrites will be formed. Disadvantages in the use of such alloys are lowered cell voltage and increased weight, and thus, decreases in the associated specific energy and energy density. Therefore, such an approach can be viewed as a trade-off in which one sacrifices specific energy for an improvement in cycle life.Lithium aluminum alloys (Li-Al) have been widely investigated for use in organic solvent-based electrolytes at room temperature (7-9). Considering the potential of the relevant two-phase plateau and its specific capacity, such lithium aluminum alloys are among the better candidates for use in lithium batteries (0.385V against lithium, 0.79 Ah/g at 25~ However, due to the large volume change accompanying the phase transformation that occurs during the removal or addition of lithium, such electrodes show unsatisfactory mechanical stability during chargedischarge cycles.A series of investigations of the thermodynamic and kinetic properties of this alloy, as well as other alloy systems, such as Li-Sn, Li-Sb, Li-Bi. Li-Pb, and Li-Cd, has been carried out at elevated temperatures, using LiCI-KC1 molten salts at approximately 400~ (10-13) and LiNOz-KNO3 molten salts at approximately 150~ (14). It was found that some phases in these systems show very high rates of lithium diffusion: up to 10 -4 cm~/s in the ease of Lie.6Sn at 400~ (14). Some of these mater...