Lithium-sulfur (Li|S) batteries with metallic lithium as the negative electrodes and elemental sulfur as the positive electrodes’ active mass are electrochemical systems with the potential for extremely high gravimetric energy density. The theoretical specific capacity of elemental sulfur cathodes is 1675 mAh g-1S and the theoretical energy density of Li|S cells is 2600 W h kg−1, which is much higher than that of Li-ion batteries (about 415-670 W h kg−1 based on graphite anodes and Li[MnNiCo]O2 or Li[NiCoAl]O2 cathodes). Besides, sulfur is abundant, cost-effective, and batteries is constrained by several problems, including shuttle side reactions of lithium polysulfide intermediates between the working cathode and anode and Li metal surface side reactions that lead to inevitable loss of active materials.
We showed that the formation of effective solid electrolyte interphase (SEI) on the surface of sulfur/microporous carbon composite electrodes during the initial discharge in fluoroethylene carbonate-based electrolyte solutions makes it possible the operation of S/carbon composite electrodes via the quasi-solid state (QSS) mechanism. The surface films thus formed prevent the encapsulated sulfur from detrimental direct contact with the liquid electrolyte solution and facilitate the desolvation of the Li ions before they react with the sulfur. We developed in our lab a facile synthesis of sulfur/carbon composite cathode based on activated microporous carbon with narrow pores up to 1 nm derived from polyvinilydene chloride. Thousands of stable cycles were observed for these systems.
The traditional solid-liquid-solid operation mechanism of Li|S cells with ethereal electrolyte solutions is characterized by faster kinetics than QSS mechanism. For these applications we showed that the addition of lithium polysulfides have a positive effect on the capacity retention and cycling efficiency of Li|S cells with different types of composite sulfur cathodes operating with commercial parameters in terms of areal capacities and current densities.
The development of advanced Li|S batteries with practical parameters is in progress in our lab.
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