Upton, Stephen Mark (1981) Properties of sodium and lithium polysulphides. University of Southampton, Doctoral Thesis.
Abstract
The sodium-sulphur battery offers great promise of satisfying the criteria for a high energy density battery system suitable for use in automotive,rail traction and load levelling applications. A potential improvement to the sodium-sulphur system is the use of a mixed sodium/lithium electrode. Various methods of synthesis of pure samples of sodium and lithium polysulphides have been developed, including two novel preparations of lithium polysulphides. One of these uses an organic solvent reaction medium, the other makes use of an aluminium separator in an electrochemical cell. Several high temperature techniques involving the use of sodium, silver and sodium/lithium beta-aluminas were investigated and this required the preparation of these materials. The phase diagrams of both the sodium-sulphur and lithium-sulphur systems exhibit a two liquid region. The behaviour of the edge of this region in the ternary system lithium monosulphide-sodium monosulphide sulphur has been investigated with a view to improving the performance of a sodium-sulphur battery by using a mixture of sodium and lithium. The procedure consisted of quenching equilibrated samples followed by mechanical separation of the different phases present and identification by chemical analysis. A similar procedure has been applied to the one liquid one solid phase boundary of the ternary system for the same purpose. In this case it was necessary to design and build a high temperature centrifuge to achieve satisfactory phase separation. A solid state electrochemical cell has been used to measure the activity of sulphur vapour above sodium/lithium polysulphide melts. Using a mathematical model for the melt, the activity data were used to calculate free energy values for the stepwise formation of polysulphide anions it in the liquid state. These values have been used to calculate the distribution of anions of different chain lengths in a melt and to calculate a theoretical ternary phase diagram. The density and conductivity of a lithium polysulphide melt containing 66 atom percent sulphur has been determined as a function of temperature.
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