Development of High-Performance, High-Loading Sulfur Cathodes
Authors: Chung Sheng-Heng , Chang Chi-Hao , Manthiram Arumugam 
Published: February 2020

The widespread application of lithium-ion batteries in portable electronics represents how electrical energy storage (EES) technologies can revolutionize human society. The transformation of the EES technologies to large-scale applications, such as electric vehicles and grid storage, depends heavily on the cost-competitiveness, cycle life, safety, and environmental compatibility. Lithium-sulfur (Li-S) batteries are one of the most promising future EES systems due to their high theoretical energy density of 2600 Wh kg-1. The high-capacity sulfur cathode (theoretical capacity: 1675 mAh g-1) has low production cost and high abundance. However, the scientific challenges of Li-S batteries mainly result from the sulfur core. First, the insulating nature of sulfur results in its low electrochemical utilization. Second, the generation of polysulfide intermediates (Li2S
x
, x = 4 – 8) induces the irreversible polysulfide diffusion from the cathode to the anode. The polysulfide migration leads to active-material loss, lithium-anode degradation, and low charge-discharge efficiency. Third, the conventional cathode configuration may not be able to make the best use of sulfur because of the differences in the battery chemistries between the lithium-insertion-compound oxide cathodes and the conversion-reaction sulfur cathodes. Fourth, the cyclability of Li-S cells faces a significant decline on going from low sulfur loadings (< 2 mg cm-2) to high sulfur loadings.

To overcome the challenges of the Li-S technology, this presentation will focus on the development of high-loading structural sulfur cathodes and the activation processes of such structuiral cathodes. A core-shell structural sulfur cathode is designed to investigate the feasibility of holding a high-loading sulfur core within a carbon-shell electrode configuration. This concept aims at utilizing the unique materials chemistry of sulfur rather than restricting it as usual. The formation and diffusion of polysulfides are now in-charge of activating the high-loading sulfur core and bettering the electrochemical utilization of sulfur. The carbon-shell electrode, on the other hand, offers the high-loading sulfur core with fast ion and electron transport and stabilizes the active material within the structural cathode configuration. As a result, the sulfur-carbon core-shell cathode effectively utilizes the stabilized sulfur core within the carbon-shell electrode, demonstrating an overall boost in the electrochemical utilization and polysulfide retention. The core-shell cathodes with high sulfur loadings of 4.0 to 30.0 mg cm-2 exhibit outstanding cycle stability at various cycling rates (0.05C to 0.5C rates). For example, the core-shell cathode with a 4 mg cm-2 sulfur loading exhibits the high electrochemical utilization of sulfur of above 96 % with the stable electrochemical cyclability for over 100 cycles at 0.2C rate. The high-loading core-shell cathodes with 20 and 30 mg cm-2 sulfur loadings attain peak discharge capacities of, respectively, 870 and 780 mAh g-1 at 0.2C rate. Such high electrochemical utilization facilitates a high areal capacity of 17 – 23 mAh cm-2.

A comparative analysis of the structural cathodes with increasing sulfur loading provides insights into the development of advanced sulfur cathodes with high electrochemical performance and attracting active-material loading. Our findings indicate that the insulating sulfur core may form within the active-material fillings and thereby reduce the initial active-material utilization. The possible solution presented in this work is to channel the dissolved polysulfides to activate the insulating sulfur clusters. Despite the vast number of publications on Li-S batteries employing regular-loading sulfur cathodes (sulfur loading < 2 mg cm-2), the new challenges including increasing polarization and cell resistance that arise with high-loading sulfur cathodes need to be tackled. In this regard, the findings and the cathode engineering of the core-shell structural cathodes presented here with high sulfur content (50 – 60 wt. %), sulfur loading (4.0 – 30.0 mg cm-2), and sulfur mass (4.0 – 30.0 mg cathode-1) might pave the way for the development of structural cathodes with high sulfur loading.