Advancing Electrochemical Performance of Li-S Batteries: Strategies for Polysulfide Immobilization and Catalytic Enhancement

Abstract

Lithium-sulfur batteries are regarded as a promising next-generation energy storage system, featuring a theoretical energy density that is five to ten times higher than that of conventional Li-ion battery (LIB) cathode material. Furthermore, natural abundance, low cost, and environmental benignity make sulfur an appealing candidate for sustainable energy solutions. Despite the potential of Lithium-sulfur batteries, their commercialization is hindered by several challenges. One major issue is the shuttle effect of soluble long-chain lithium polysulfides (LiPSs), which migrate uncontrollably through the electrolyte, leading to rapid capacity fade, poor sulfur utilization, and severe corrosion of the lithium anode. Furthermore, the sluggish redox kinetics of LiPSs exacerbate these issues, reducing the overall electrochemical efficiency. Additionally, sulfur's intrinsically poor electrical conductivity and the significant volume expansion during charging-discharging create mechanical instability in the cathode. These combined factors significantly impair the long-term cycling stability, Coulombic efficiency, and overall performance, limiting their feasibility for practical application.

This thesis explores strategies for immobilizing lithium polysulfides and enhancing electrocatalytic activity in Lithium-sulfur batteries to improve overall performance. The first project investigated the energy losses induced by modified membranes in Lithium-sulfur batteries. By utilizing a commercial PP separator, energy density decreases by 20% when overpotentials exceed 250 mV in a Lithium-sulfur battery. Symmetric cell with lithium metal as electrode measurements demonstrates that certain types of membranes (GO, MoS2, and rGO) display overpotentials surpassing 250 mV at a 1C rate. This issue becomes more pronounced when these membranes are tested within H-cells with Li2S6 as catholyte. Fabrication and testing of the porous membrane with an induced level of mesoporosity and modeling confirm that the overpotentials stem from physical obstruction of lithium-ion transport, revealing the need to compromise between soluble polysulfide blocking and energy loss.

The second study was designed to redefine the approach to enhance Lithium-sulfur battery performance by emphasizing the significant role lithium-ion plays in sulfur redox reactions (SRR). Traditional membrane design strategies focus on managing the migration of lithium polysulfides (LiPSs) and catalyzing the sulfur redox reactions (SRR). However, the critical influence of lithium-ion transport on SRR kinetics is often neglected. A novel holey graphene membrane, embedded with Co9S8 (Co9S8/HG) was designed to facilitate rapid lithium-ion diffusion and efficient polysulfide conversion. This approach tackles the notorious shuttle effect of LiPSs and significantly improves SRR kinetics, overcoming the limitations of conventional membrane designs that impede lithium-ion movement. The cell incorporated with Co9S8/HG/PP achieved a notable discharge capacity of 671 mAh/g over 900 cycles at 0.2C and exhibited robust rate performance, maintaining 784 mAh/g capacity when switching from 2C to 0.2C.

In the third project, we incorporated holey Co3S4 nanosheets into the sulfur composite cathodes to enhance the conversion kinetics of LiPSs by facilitating rapid lithium-ion diffusion. Through overpotential measurements, Tafel plots, and lithium-ion diffusion coefficient analysis, we compared the electrocatalytic efficiency of the synthesized materials which exhibit varying particle sizes and porous structures due to their formation at various temperatures.

These studies have uncovered several previously unexplored challenges, which will inspire research in the sustainable energy storage field.