Advanced Cathodes for Practical Lithium-Sulfur Batteries

Abstract

Sulfur, being lightweight, cost-effective, and offering a remarkably high lithium-ion storage capacity, has positioned lithium-sulfur (Li-S) batteries as promising candidates for applications that demand high energy density. These range from electric vehicles (EVs) to urban air mobility (UAM) systems. Despite this potential, Li-S batteries still face significant performance challenges, limiting their practical application. Chief among these challenges are the limited lifespan and low charge-discharge efficiency, predominantly caused by the dissolution of lithium polysulfide intermediate products formed during battery cycling in ether-based electrolytes. Moreover, sulfur and lithium sulfide, which constitute the active material in the cathode, are intrinsically insulating, complicating efforts to increase the active material content in the cathode and fabricate thick cathodes with high conductivity. These issues have long stood in the way of Li-S batteries achieving commercial viability. Overcoming these obstacles requires a multifaceted approach that focuses on modifications at the level of the cathode materials such as the active material, conductive agents, binders, and additives. This Account delves into these key challenges and presents a comprehensive overview of research strategies aimed at enhancing the performance of Li-S batteries with a particular focus on the sulfur cathode. First, the Account addresses practical challenges in Li-S batteries, such as the complex composition of the cathode, the low sulfur utilization efficiency, suboptimal electrolyte-to-sulfur ratios, and nonuniform sulfur conversion reactions. Strategies to overcome these barriers include the design of advanced cathode architectures that promote high sulfur utilization and an improved energy density. Modifications to the components of the cathode and the adjoining materials, such as the incorporation of conductive additives, help mitigate the insulating nature of sulfur.Additionally, the Account places particular emphasis on the innovative use of pelletizing techniques in sulfur cathode fabrication, which has demonstrated notable improvements in the cathode performance. One of the Account's highlights is the discussion of low-temperature operation strategies for Li-S batteries, which is a critical area for real-world application, especially in aerospace and cold-environment operations. There are significant performance differences when transitioning from lab-scale coin cells to larger pouch cells, underscoring the importance of considering cell geometries and their impact on the scalability and performance. Finally, the Account explores the development of all-solid-state Li-S batteries, a promising approach that could fundamentally address the issue of lithium polysulfide dissolution by eliminating the use of liquid electrolytes altogether. The inherent drawbacks of Li-S batteries, such as the insulating nature of sulfur and the challenges of high sulfur loading, can be strategically addressed to pave the way for their commercialization. In doing so, Li-S batteries offer a clear pathway beyond the limitations of conventional lithium-ion batteries, making them a highly attractive option for applications requiring high gravimetric and volumetric energy densities.