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This past decade has seen extensive research in lithium-sulfur batteries with exemplary works mitigating the notorious polysulfide shuttling. However, these works utilize ether electrolytes that are highly volatile severely hindering their practicality. Here, we stabilize a rare monoclinic γ-sulfur phase within carbon nanofibers that enables successful operation of Lithium-Sulfur (Li-S) batteries in carbonate electrolyte for 4000 cycles. Carbonates are known to adversely react with the intermediate polysulfides and shut down Li-S batteries in first discharge. Through electrochemical characterization andpost-mortemspectroscopy/ microscopy studies on cycled cells, we demonstrate an altered redox mechanism in our cells that reversibly converts monoclinic sulfur to Li₂S without the formation of intermediate polysulfides for the entire range of 4000 cycles. To the best of our knowledge, this is the first study to report the synthesis of stable γ-sulfur and its application in Li-S batteries. We hope that this striking discovery of solid-to-solid reaction will trigger new fundamental and applied research in carbonate electrolyte Li-S batteries.

 

In Li–S batteries, the insulating nature of sulfur and Li 2 S causes enormous challenges, such as high polarization and low active material utilization. The nucleation of the solid discharge product, Li 2 S, during the discharge cycle, and the activation of Li 2 S in the subsequent charge cycle, cause a potential challenge that needs to be overcome. Moreover, the shuttling of soluble lithium polysulfide intermediate species results in active material loss and early capacity fade. In this study, we have used thiourea as an electrolyte additive and showed that it serves as both a redox mediator to overcome the Li 2 S activation energy barrier and a shuttle inhibitor to mitigate the notorious polysulfide shuttling via the investigation of thiourea redox activity, shuttle current measurements and study of Li 2 S activation. The steady-state shuttle current of the Li–S battery shows a 6-fold drop when 0.02 M thiourea is added to the standard electrolyte. Moreover, by adding thiourea, the charge plateau for the first cycle of the Li 2 S based cathodes shifts from 3.5 V (standard ether electrolyte) to 2.5 V (with 0.2 M thiourea). Using this additive, the capacity of the Li–S battery stabilizes at ∼839 mA h g −1 after 5 cycles and remains stable over 700 cycles with a low capacity decay rate of 0.025% per cycle, a tremendous improvement compared to the reference battery that retains only ∼350 mA h g −1 after 300 cycles. In the end, to demonstrate the practical and broad applicability of thiourea in overcoming sulfur-battery challenges and in eliminating the need for complex electrode design, we study two additional battery systems – lithium metal-free cells with a graphite anode and Li 2 S cathode, and Li–S cells with simple slurry-based cathodes fabricated via blending commercial carbon black/S and a binder. We believe that this study manifests the advantages of redox active electrolyte additives to overcome several bottlenecks in the Li–S battery field.