Distribution of anion chain lengths in molten sodium polysulphides

Cleaver, B.; Sime, S. J.

doi:10.1016/0013-4686(83)85068-3

Cited by 11 publications

(9 citation statements)

References 6 publications

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“…We have summarized in Figure the Raman spectra of the liquids obtained during the cooling step at temperatures between 225 and 300 °C for n = 2−6 and at 400 °C for n = 1. The strong changes observed between the compositions with n = 1−3 are in agreement with the presence of the S n +1 2- anion as the predominant species, in each of these compositions, as indicated from the various thermodynamic models used to interpret the open circuit voltage measurements , and vapor pressure measurement in the sodium−sulfur battery.…”

Section: Discussionsupporting

confidence: 75%

Raman Spectroscopy Study of the Reaction between Sodium Sulfide or Disulfide and Sulfur: Identity of the Species Formed in Solid and Liquid Phases

et al. 1999

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The reactions of sodium sulfide or disulfide with sulfur, when heated, are examined through Raman spectroscopy. It is shown that whatever the composition of the mixtures, the solid sodium sulfide or disulfide transforms into the crystalline alpha-Na(2)S(4) phase in a first step, with alpha- or beta-Na(2)S(2) as an intermediate. The reaction, which proceeds when the sulfur melts, is assumed to be related to the polymerization-depolymerization mechanism responsible for the formation of smaller rings and sulfur chains in molten S(8). This confirms the strong reactivity of the radical sulfur chain molecules. This solid alpha-Na(2)S(4) formed may further react around 200 degrees C with Na(2)S in excess. This solid-state reaction leads to the formation of beta-Na(2)S(2). It is shown that, after the liquid of composition Na(2)S(4) is heated above 400 degrees C, a glass is formed upon cooling. Annealing this glass around 124 degrees C yields a new gamma-Na(2)S(4) crystalline phase where the S(4)(2-) anions have a smaller torsion angle. This new phase is metastable and transforms into the alpha phase upon prolonged heating at 200 degrees C. The solids, formed from heating the mixtures Na(2)S + (n/8)S(8) or Na(2)S(2) + (n'/8)S(8) with n' = n - 1, for n 4, higher polysulfides decompose under crystallization into Na(2)S(5) and sulfur. The liquids formed from these mixtures show the formation of all the S(n)()(+1)(2-) anions although Na(2)S(3) and Na(2)S(6) do not crystallize from these liquids.

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Section: Discussionsupporting

confidence: 75%

Raman Spectroscopy Study of the Reaction between Sodium Sulfide or Disulfide and Sulfur: Identity of the Species Formed in Solid and Liquid Phases

et al. 1999

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“…When K–S and Na–S batteries were under heavy investigation, attempts were made to use thermodynamic knowledge of the systems to estimate S n 2– chain lengths in the melt and thereby explain the electromotive force across Na–S cells. − Cleaver and Sime’s investigation of Na 2 S n proposed a broad distribution of S n 2– chain lengths, smoothly varying for 2 < n < 6 at 300 °C . In this case, where a wide distribution of S n 2– species would be present in the melt, the Raman spectra should be smoothly varying from molten K 2 S 3 to K 2 S 5 , with all modes present and in gradually shifting intensity.…”

Section: Resultsmentioning

confidence: 99%

“…The temperature range over which this relationship holds should be investigated. At 400 °C we do not find a broad distribution of molten polysulfide chain lengths where S n 2– are in equilibrium with S n –1 2– , S n +1 2– , etc., as was originally proposed to explain the electromotive force of Na 2 S n batteries …”

Section: Discussionmentioning

confidence: 99%

“…Large distributions of S n 2− chain lengths were predicted in molten Na 2 S n to explain the electromotive force in Na−S batteries, which is nearly linear versus n, but these were not substantiated by any structural or spectroscopic techniques. 36,37 In fact, neither conductivity nor density of Na 2 S n varies smoothly over the same range. 38,39 We utilized in situ high-temperature Raman spectroscopy to probe S n 2− chain formation in molten K 2 S n .…”

Section: ■ Introductionmentioning

confidence: 95%

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Understanding Fluxes as Media for Directed Synthesis: In Situ Local Structure of Molten Potassium Polysulfides

et al. 2012

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Rational exploratory synthesis of new materials requires routes to discover novel phases and systematic methods to tailor their structures and properties. Synthetic reactions in molten fluxes have proven to be an excellent route to new inorganic materials because they promote diffusion and can serve as an additional reactant, but little is known about the mechanisms of compound formation, crystal precipitation, or behavior of fluxes themselves at conditions relevant to synthesis. In this study we examine the properties of a salt flux system that has proven extremely fertile for growth of new materials: the potassium polysulfides spanning K(2)S(3) and K(2)S(5), which melt between 302 and 206 °C. We present in situ Raman spectroscopy of melts between K(2)S(3) and K(2)S(5) and find strong coupling between n in K(2)S(n) and the molten local structure, implying that the S(n)(2-) chains in the crystalline state are mirrored in the melt. In any reactive flux system, K(2)S(n) included, a signature of changing species in the melt implies that their evolution during a reaction can be characterized and eventually controlled for selective formation of compounds. We use in situ X-ray total scattering to obtain the pair distribution function of molten K(2)S(5) and model the length of S(n)(2-) chains in the melt using reverse Monte Carlo simulations. Combining in situ Raman and total scattering provides a path to understanding the behavior of reactive media and should be broadly applied for more informed, targeted synthesis of compounds in a wide variety of inorganic fluxes.

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“…An upper limit is not imposed on the chain length. In order to reduce the number of parameters required to fit the model to the data for sulfur activity, they assumed that the equilibrium constant for the reaction na2Sj-, + S --* na2Sj kj - [1] XNazs(~-Das had some functional dependence on j, where S is dissolved sulfur, and as its activity in the polysulfide melt. The relationship between kj and j, that provided the best fit between the predictions of the model, and the experimental activity data is as follows…”

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confidence: 99%

The Electromotive Force of the Na/S Cell

McKubre

Tanzella

Smedley

1989

J. Electrochem. Soc.

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We have adapted the anion chain length model of Cleaver and Sime to predict the distribution of anions in a polysulfide melt, Na2Sx (1 -< X -6), as a function of composition the mole fraction of total sulfur. This was then used to calculate the EMF of a Na/S celt from Xs = 0.75 to 0.56, where Xs is the mole fraction of sulfur. This range corresponds to the entire two-phase region. The predicted anion distribution is similar to that of Cleaver and Sime except for the prediction of the appearance of Na2S to high states of discharge; the calculated EMF provides a good fit to the literature values. Figure 1 illustrates how the EMF of the Na/S cell undergoes a transition from 2.076V when the sulfur electrode is in the two-phase, sulfur plus sodium polysulfide state, to 1.74V in the two-phase sodium polysulfide plus Na2S2(s) state. The former two-phase zone terminates when the mole fraction of total sulfur Xs = 0.729, the latter occurs at Xs = 0.571. Between these two zones, when 0.571 < Xs < 0.729, only one phase is present, a sodium polysulfide liquid, and the EMF decreases almost linearly with decreasing Xs.If at Xs = 0.714, a ratio corresponding to the stoichiometric formula Na2Ss, the sulfur electrode half-reaction is 4Na2S5 + 2Na § + 2e ~ 5Na2S4 then, as the cell discharges, and Xs decreases, the EMF would exhibit the behavior of a typical EMF titration curve. In fact, as the composition of the sulfur electrode passes through values of Xs corresponding to the stoichiometric compositions of Na2Ss, Na2S4, Na2S3, the EMF would be expected to pass through successive titration curves.The fact that the EMF follows a smooth transition from one two-phase region to the other suggests that the polysulfide melt may be comprised of several polysulfide species in equilibrium and that each of these make a contribution to the EMF. Thus, as the cell discharges one species may be in the majority and make the major contribution to the EMF, but this prodominance diminishes on further discharge, and another species of lower chain length becomes the majority species. If a sufficiently large number of species are present, then the EMF would be expected to behave in the experimentally observed manner.Cleaver and Sime (1) have developed a model that predicts the distribution of anion chain lengths in the polysulfide melt, and they indicate how it can be used to calculate the EMF of the Na/S cell as a function of the composition of the sulfur electrode. They assumed that the melt contains a distribution of polysulfides from Na~S2 upwards. An upper limit is not imposed on the chain length. In order to reduce the number of parameters required to fit the model to the data for sulfur activity, they assumed that the equilibrium constant for the reaction na2Sj-, + S --* na2Sj kj -XNazs(~-Das had some functional dependence on j, where S is dissolved sulfur, and as its activity in the polysulfide melt. The relationship between kj and j, that provided the best fit between the predictions of the model, and the experimental activity data i...

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Distribution of anion chain lengths in molten sodium polysulphides

Cited by 11 publications

References 6 publications

Raman Spectroscopy Study of the Reaction between Sodium Sulfide or Disulfide and Sulfur: Identity of the Species Formed in Solid and Liquid Phases

Raman Spectroscopy Study of the Reaction between Sodium Sulfide or Disulfide and Sulfur: Identity of the Species Formed in Solid and Liquid Phases

Understanding Fluxes as Media for Directed Synthesis: In Situ Local Structure of Molten Potassium Polysulfides

The Electromotive Force of the Na/S Cell

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