Lean-Electrolyte Lithium-Sulfur Cells with a Core-Shell Cathode Configuration with High Active-Material Loading and Energy Density
Authors: Chung Sheng-Heng , Wu Yun-Chen 
Published: February 2024

Introduction

The growing demand for rechargeable batteries with increasingly high energy density and low-cost active material has inspired the research of low-cost cathodes with higher specific capacities than those based on intercalation lithium-ion cathodes. Among these novel energy storage technologies, the lithium-sulfur electrochemical cell is the most promising because of the high energy density, low cost, wide availability, and low environmental impact of the sulfur cathode. However, the high-capacity sulfur cathode forms insulating solid sulfur and lithium sulfide in the fully charged and discharged states. The resulting high cathode resistance limits electrochemical utilization and reversibility, and the resulting low redox kinetics leads to the poor rate performance of sulfur cathodes. Moreover, these solid-state active materials are converted to liquid-state polysulfides during intermediate charging and discharging. The liquid-state polysulfides readily dissolve in the lithium-sulfur battery electrolyte, resulting in irreversible polysulfide loss and poor discharge-charge efficiency of the cell. The inefficient and unstable cell electrochemistry of sulfur can be mitigated by designing appropriate configurations of cathode materials. However, the modification of cathode materials often encounters the inability to accommodate a sufficient amount of sulfur in the cathode under lean electrolyte conditions while maintaining electrochemical stability.

Results and Discussion

In this presentation, we present the design of a core-shell architecture for the entire cathode, which features a conductive carbon shell having a fabric-like tubular structure to accommodate an active material core and provide fast charge transfer to the active material. In addition to the novel cathode configuration, we present a summary of a number of active material cores, including a pure sulfur core and a pure lithium sulfide core as solid-state active material cores, and a polysulfide catholyte core as a liquid-state active material (Figure 1) [1-3]. With these three different core-shell cathodes, we will comprehensively report the material design with high electrochemical utilization and efficiency, and the designed cell configuration stabilizes the reversible lithium-sulfur electrochemistry. Most importantly, we propose the sulfur core to demonstrate the high-loading sulfur cathode with the highest sulfur loading and content, and the sulfide and polysulfide cores to demonstrate the lean-electrolyte cells with low electrolyte to active material ratios. Specifically, we design a series of lithium-sulfur battery cathodes with a unique cathode material and configuration. In terms of the cathode material, a high-loading sulfur cathode prepared with pure sulfur powder, a high-concentration polysulfide prepared as the catholyte, and a sulfide cathode prepared in an ether-based solvent are reported. The sulfur, polysulfide, and sulfide serve as a high-loading active materials. The conductive carbon shell encapsulates the active material during cycling and resting. The integrated design of the cathode material and configuration enables the core-shell cathodes to achieve high sulfur loadings ranging from of 4 to 30 mg cm−2 and high sulfur contents in the range of 60-70 wt%. The high-loading core-shell cathodes excellent electrochemical utilization of the large amount of active material, showing high peak charge-storage capacity of 600-800 mA∙h g−1 at C/10 rate, long cycle life of 100-200 cycles, and high rate performance of C/10-C/2 rates with long-term cyclability and high capacity retention of 70% after 200 cycles. Moreover, by using polysulfide and sulfide cores, we demonstrate a lean electrolyte cell with low electrolyte/sulfur ratios of 6-3 μL mg-1. Accordingly, the designed core-shell cathodes achieve a high areal capacity of about 10 mA∙h cm-2, high gravimetric capacities of ~500 mA∙h g-1, and an energy density of about 20 mW∙h cm-2.

Conclusion

In summary, we report in the presentation a summary of our high-loading core-shell sulfur cathodes and the design of their application in the development of the lean-electrolyte lithium-sulfur cells. Our core-shell cathodes exhibit excellent electrochemical utilization and retention of active material in both solid and liquid state, which shows the long cycle life and long shelf life of the high energy density lithium-sulfur cell.

Figure 1. Illustration of a lithium-sulfur cell with the core-shell cathode equipped with sulfur, polysulfide, and sulfide cores.

References

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-H. Chung, A. Manthiram, Adv. Energy Mater.
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-H. Chung, A. Manthiram, Energy Environ. Sci.
2016, 9, 3188.

Figure 1