An experimental sodium-sulfur cell using a beta-alumina electrolyte is described. The cell uses carbon felt as a cathode current collector with molybdenum wires as terminals and with a beta-alumina tube. At temperatures between 300 ~ and 350~ and at a current density of 0.2 A/cm 2 lifetime is about 200 cycles with a 30-35% depth of discharge and a power density of 0.3 W/cm 2 at the working potential. Experiments at greater depth of discharge are described.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.218.248.209 Downloaded on 2015-04-05 to IP Vol. I20, No. 10
The influence of some impurities on the aging and the lifetime of sodiumsulfur and sodium-sodium experimental cells is studied. The aging mechanism involves an ion exchange between impurities of the reactants and the sodium ions of the E-alumina lattice. In particular, introduction of potassium changes lattice parameters of ;~-alumina and Na* mobility. An important conclusion of this work is that pure E-alumina does allow the transfer of large amounts of total charge (i.e., of sodium ions) before failure. The present figures are in excess of 1700 A-hr/cm 2. These results have enabled the authors to construct Na-S cells which have given in excess of 4,000 cycles or 16,000 hr of continuous operation.The sodium-sulfur battery, based on the sodium ;~alumina electrolyte, was first described by Weber and Kummer (1) and is to date the most promising hightemperature, high-energy density battery. The reasons for this developmental success depend on the use of the solid sodium-conducting E-alumina between the molten sodium anode and molten sulfide-polysulfide electrolyte in contact with the sulfur cathode. This solves immediately the self-discharge and constructional problems involving the use of matrix or pastes to retain molten components (either liquid alkali, metal, or molten salt electrolyte) in all-liquid hightemperature systems (2). Development of the system so that a wide range of practical applications will result requires the solution of two major difficulties, both economic. The first is clearly one of initial investment cost per kW-hr of capacity or kW of power. This depends on many factors, the most important being:(i) The cost of the E-alumina used, which depends not only on precise conditions of manufacture and sintering, but also on the purity of the a-alumina used.(ii) The cost of the current collector material at the sulfur electrode (graphite or carbon felt).(iii) The total cost of manufacture, which depends on the development of efficient sealing techniques.The second factor is battery lifetime. Economic considerations dictate that a minimum lifetime of 2-3 years should be aimed at for urban electric vehicles, and of about 5 years for urban mass transit (buses). On ~he off-peak power storage application, for which the sodium-sulfur system seems eminently suitable provided that its cost-lifetime ratio is sufficiently low, a lifetime at the order of 10 years should be aimed at.In the present paper, some factors influencing cell lifetime will be discussed.
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