High-temperature batteries based on sodium as a negative electrode, sulphur as a positive, and b00-Al2O3 as a Naþ-conducting solid electrolyte are now used for energy storage applications. The energy density of the Na/S system is very high and by far exceeds that of aqueous systems (Pb-acid and Ni–Cd) thus far used for energy storage.
Other distinct advantages of the Na/S battery are: good power density, high cycle life, independent of external temperature and moderate cost. The operating temperature of this system is between 300 and 350C. In this temperature range, both Na and S are liquid, while the solid electrolyte has a high Naþ conductivity, thus ensuring good kinetics.
During discharge, Naþ migrates from Na to S and forms polysulphides; formation of Na2S3, at 1.78 V, is taken as the discharge limit. At the C/3 rate, the average voltage is 1.9 V.
During charge, the reactions leading to Na2S3 are reversed and, in the final stages, there is a marked resistance increase due to the insulating character of sulphur. Therefore, the charge has to be stopped before complete Na recovery, and subsequent discharges provide 85–90% of the theoretical capacity. As already mentioned, this system has a high cyclability (up to 5000–6000 cycles). This is mainly due to the liquid state of reactants and products: the aging mechanism based on morphological changes of the electrodes does not operate here. An essential prerequisite for the Na electrode is high purity, that is other metals and Na compounds are not allowed. Contaminants tend to concentrate at the interface with the electrolyte, reducing the electrode-active area or even causing its failure.
The sulphur electrode is impregnated into a layer of carbon or graphite felt. The carbon fibres ensure a good electronic conductivity, as sulphur is an insulator for both electrons and ions. Fortunately, Na polysulphides are good ionic conductors.
The electrolyte, b00-Al2O3, has a negligible electronic conductivity and is impermeable to molten Na and S. The idealized composition of b00-Al2O3 is Na2O5.33Al2O3. Pure b00-Al2O3 is not easy to prepare, so it has to be stabilized with Mg or Li-ions that substitute for Al ions.
The ionic conductivity of this electrolyte is 0.5 –1 cm–1 at 350C for the polycrystalline form. However, b00-Al2O3 is rather sensitive to moisture, this favouring deterioration of its mechanical properties. Therefore, some b-Al2O3 (idealized formula, Na2O11Al2O3) is enclosed in the mixture, in spite of its lower conductivity, as it is less hygroscopic. A conductivity of 0.2 –1 cm–1 is regarded as acceptable for practical electrolytes . Production of Na/S batteries for stationary applications is particularly active in Japan. Very large energy-storage systems can be built with Na/S modules: their storage capacity may be as high as 57MWh. They can live in this application for up to 15 years providing thousands of cycles.
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