A major hurdle still hindering the practical development of the highly desirable lithium-sulfur batteries is the solubility in organic, carbonate-based electrolytes of polysulfide compounds formed as intermediates during charge and discharge processes. This high solubility results in a loss of active mass, which is reflected in a low utilization of the sulfur cathode and in severe capacity decay upon cycling. The dissolved polysulfide anions, by migration through the electrolyte, may react with the lithium metal anode to form insoluble products on its surface; this process also negatively impacts the battery operation. Various strategies to address the solubility issue have been explored, including the design of modified organic liquid electrolytes, the use of ionic liquid solutions and of polymer electrolytes [1, 2].

In this work we present a combined strategy, where optimized liquid electrolytes, composed by ether-based solvents and an ionic liquid additive, were used to swell a selected polymer matrix. This allows the formation of composite, highly conductive, gelled polymer electrolytes able to control the dissolution of the sulfide anions. In our approach, a dry poly(ethylene oxide) (PEO) membrane was first prepared, through a solvent-free route [3, 4], and then activated by the liquid electrolyte, according to a well addressed swelling procedure.

Thermal, spectroscopical and electrochemical characterizations were performed on both liquid and gelled polymer electrolytes and will be discussed. The successful practical application in Li-S cells was demonstrated by prolonged galvanostatic cycles where very high and stable capacity values (i.e., 900 mAh/g) were achieved.

Acknowledgments:

The results of this work have been obtained by the financial support of the European Community within the Seventh Framework Program LISSEN (Lithium Sulfur Superbattery Exploiting Nanotechnology) Project (project number 314282).

[1] Junghoon Kim , Dong-Ju Lee , Hun-Gi Jung , Yang-Kook Sun , Jusef Hassoun , Bruno Scrosati, Adv. Funct. Mater. 2013, 23, 1076–1080

[2] Rezan Demir-Cakan, Mathieu Morcrette, Gangulibabu, Aur´elie Gu´eguen, R´emi Dedryv`ere, Jean-Marie Tarascon, Energy Environ. Sci., 2013, 6, 176–182

[3] P.P Prosini, S Passerini, R Vellone, W.H Smyrl - J. Power Sources, 75 (1998), p. 73

[4] G.B Appetecchi, S Scaccia, S Passerini - J. Electrochem. Soc., 147 (2000), p. 4448