Higher Performing and Longer Lasting Rechargeable Batteries

June 2, 2014

Improvements in discharge capacity and cyclability of Li-S batteries have been achieved using sulfur-functionalized mesoporous carbons as the sulfur host material.

Figure 1. Preparation of cubic ordered mesoporous carbon (CMC) materials with and without sulfide functionality.

Li-S Battery: A Source of Renewable Energy
The Li-S system offers a promising battery for electric vehicles and renewable energy storage due to its high theoretical capacity and its use of inexpensive and earth abundant materials. The Li-S battery differs from the traditional lithium ion intercalation battery in that it relies on a series of conversion reactions between lithium and sulfur. The capacity of intercalation systems is limited to about 300 mAh/g, making them a poor candidate for lightweight batteries. The Li-S conversion reaction battery has one of the highest theoretical capacities of 1675 mAh/g, more than five times higher than intercalation systems, making this system an attractive choice for lightweight batteries. However, due to the low electrical and ionic conductivities of sulfur, it is necessary to incorporate conductive additives with the sulfur cathode to facilitate electron transfer, usually carbons, and use electrolytes that slightly solubilize the sulfur active material to promote solution-based interactions. Major challenges in the Li-S system are the underutilization of the sulfur active material, leading to lower than theoretical capacities, and the loss of sulfur to the electrolyte as soluble intermediate polysulfides are formed during discharge, leading to poor cyclability and stability.

Figure 2. (a) The discharge capacities as a function of cycle number show enhancement of sulfur utilization (capacity) provided by the mesoporous materials as a result of their high surface area. The S-functionalized CMC materials show enhanced capacity retention. The weight % sulfur functionality is denoted by S#-CMC. (b) The effect of sulfur functionality is more clearly demonstrated when the discharge capacities are normalized to the unfunctionalized CMC. The S5.5-CMC shows a 55% increase in capacity after 100 cycles.


UC Santa Barbara Research on Enhancing the Li-S System
Researchers at UC Santa Barbara have recently made progress in addressing these issues and achieved higher capacities and enhanced cyclability in the Li-S system using sulfur-functionalized mesoporous carbons as the sulfur host material. The porous structure of a mesoporous carbon host, shown in Figure 1, combines electrical conductivity with high surface area, facilitating electron and mass transfer to the sulfur while allowing for increased utilization of the sulfur active material. Additionally, functionalizing the surface of the carbon host with sulfur increases the affinity of the intermediate polysulfides, allowing for better retention of the polysulfides within the porous network of the carbon host and preventing loss of the sulfur active material to the electrolyte during cycling. As the sulfur functionality increases, a higher discharge capacity is retained during cycling, an enhancement of up to a 55% after 100 cycles for the most functionalized material, shown in Figure 2b. This research has shown that interactions between the intermediate polysulfides and the carbon host have a significant impact on the performance of the battery. These interactions can be tuned by slight modifications of the carbon hosts’ surface chemistry, resulting in enhanced cyclability and leading the way toward higher performing and longer lasting rechargeable batteries.

Author: Kristin Denault, May 2014
Materials Department, UC Santa Barbara

To read the full research paper, click here.

Author and publication details:
Kimberly A. See, Young-Si Jun. Jeffrey A. Gerbec, Johannes K. Sprafke, Fred Wudl, Galen D. Stucky, and Ram Seshadri, “Sulfur-Functionalized Mesoporous Carbons as Sulfur Hosts in Li-S Batteries: Increasing the Affinity of Polysulfide Intermediates to Enhance Performance.” ACS Appl. Mater. Interfaces, 6 xxx (2014).

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