Attempts to gain a better understanding of the reactive species in the sodium-sulfur battery system developed at Ford Motor Co. have led us to reexamine the sodium-sulfur phase diagram published by Pearson and Robinson in 1930. A modified phase diagram was constructed based on dta studies. In addition, dta revealed that when a mixture of Ña2S and NajSj or Ña2S4 and S8 is heated, a reaction takes place around the melting point of sulfur, with the formation of the pentasulfide, Na2Ss, as the initial step. Unless the sulfur:sodium ratio in the mixture is 5:2 or higher, further interaction between the sulfides can be observed by dta, until at equilibrium only those species are observed corresponding to the given stoichiometric Na:S ratio. The highest sulfide is Na2Ss, whereas the trisulfide, Na2S3, does not exist at the melting point, and the mixture with the same stoichiometry was found to be a 1:1 Na2S2-Na2S4 eutectic.
The polysulfides of potassium-K2S2, K2S3, K2S4, K2S5, and K2S6-are all known and well-characterized compounds. In the case of the analogous sodium compounds, only the disulfide, tetrasulfide, and pentasulfide are reported in the literature, whereas reports pertaining to the existence and isolation of the trisulfide, Na2S3, have been disputed. Past efforts to synthesize Na2S3 have always yielded the 1:1 eutectic Na2S2-Na2S4. A novel method to synthesize the sodium polysulfides in liquid ammonia, based on the reaction scheme 2NaCl + K2SX -+ 2KC4 + Na2Sx {x = 3, 5, 6), is described, yielding Na2S3 and Na3S5, but not Na2S6. The results of ESCA (electron spectroscopy for chemical analysis) applied to the polysulfides are discussed. The density and surface tension of molten Na2S4 and Na2S5 were determined at various temperatures.
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