The Role of Thiourea As an Electrolyte Additive in Lithium-Sulfur Batteries

Abstract

Lithium-sulfur batteries offer five times more energy density compared to commercialized Lithium-ion batteries. Theoretical capacity in Li-S batteries reaches to ~1675 mAh/g making them a great candidate for next-generation batteries. Despite the advantages of Li-S batteries, they suffer from poor cycle life. The main challenge in achieving long-term cycling in Li-S batteries, known as polysulfide shuttling, is the dissolution and diffusion of highly soluble intermediate species, lithium polysulfides. Polysulfide shuttling results in a loss of active material and low efficiency of the Li-S battery. Moreover, Li2S, as the final discharge product, is an electronic and ionic insulator; consequently, activation of such material in the charge process of a Li-S battery becomes a challenge.In the present work, thiourea is used as an additive in the ether-based electrolyte, commonly used in Li-S batteries. We integrated electrochemical characterization and testing with in-situ spectroscopy to understand the potential role of thiourea in enhancing battery cycle life. Specifically, we hypothesized that thiourea facilitates stable cycling via favorable interactions between its C=S and C-NH2 bonds and intermediate lithium polysulfides. To understand the reaction mechanism and to test our hypothesis, we carried out in situ FTIR experiments. Using in situ FTIR results, lithium polysulfide interaction can be studied through monitoring the changes in the vibrational mode of existing bonds. Our electrochemical results show that the Li-S battery with a very low thiourea concentration of 0.02 M, enabled an initial capacity of 800 mAh/g at C/2 rate. The discharge capacity stabilized at 700 mAh/g after 20 cycles and remained stable for up to 200 cycles (figure 1a), unlike the reference battery with no-additive electrolyte that remained stable for only 80 cycles (figure 1a inset). As a result of reduced polysulfide shuttling, the coulombic efficiency of the battery remained >98% throughout the cycling. The electrochemical results show that this additive plays a significant role in enhancing the cycling stability of Li-S batteries. Moreover, we found out that thiourea is electrochemically active in this voltage range. Based on the redox activity of this additive, we believe that thiourea can potentially be used as a redox mediator to activate Li2S in the charging process. To understand the potential effect of thiourea as a redox mediator, we synthesized Li2S/CNF nanofibers. The electrochemical testing of Li2S/CNF nanofibers shows a significant difference when thiourea is added to the conventional ether electrolyte. Based on the electrochemical results obtained, we believe that thiourea has two simultaneous effects on the performance of Li-S batteries. It can perform as an additive to bind Lithium polysulfides and reduce shuttling of these species. Moreover, as a redox mediator, it can activate Li2S in the charging process. Figure 1