The synthesis and characterization of sodium polysulfides for Na-S battery application

The limited understanding of the electrochemical mechanism of Na-S battery systems is a barrier to further improve the performance of the Na-S energy storage. The characterization of sodium polysulfides in the Na-S battery systems can offer insightful information to understand the electrochemical reaction mechanism of the Na-S batteries and overcome the "inert" nature of short-chain polysulfides (Na2Sn, 1<n<4) during charge and discharge of the batteries. Up to now, there are limited studies on the sodium polysulfides compound in the Na-S batteries. Meanwhile, although many synthesis methods for sodium polysulfides have been reported, many related studies offer unclear and misleading parameters.

This work examines several reported synthesis methods for sodium polysulfide. The results show that the sodium polysulfides cannot be obtained by the reaction of Na2S and S using anhydrous ethanol as the reaction media. In "dry" synthesis method, the pressure will influence the components in the product, and only Na2S4 can be synthesized at atmospheric pressure. This study also revises the synthesis method for Na2S2 and Na2S5, and detailed synthesis parameters are presented.

The properties of sodium polysulfides in tetraethylene glycol dimethyl ether TEGDME were studied in this thesis. The solubility of Na2Sn in TEGDME increases with increasing sulfur in sodium polysulfide. The corresponding solution gets to longer-wavelength color and contains more long-chain polysulfides anions.