Properties of fused Polysulphides—VIII. Sulphur activity and polysulphide chain-length distribution in molten lithium and sodium polysulphides and their mixtures

The sulphur activity has been measured at 400°C as a function of sulphur mole fraction in a series of molten polysulphides containing lithium and sodium in defferent proportions. The measurements provide confirmation of the positions of the phase boundaries in the (Li/NaS/S) system, and also allow calculation of the emf of a hypothetical (Li,Na)/S cell as a function of the charged passed. From this the theoretical cell energy and specific energy and specific energy can be found as a function of the Li/Na ratio. Since the measurement were difficult to make and do not cover a wide range of conditions, the results were supplemented by calculations of sulphur activity on the basis of a previously developed model for the chain-length distribution in molten polysulphides. As a by-product, the polysulphide chain-length distribution has been derived for different values of the lithium/sodium ratio. From the calculated discharge curves of hypothetical cells containing both sodium and lithium, it is concluded that the stored energy per mole of sulphur decreases slightly as sodium is replaced by lithium, but the specific energy(per kg of electroactive material) increases by nearly 30%