Li-S batteries hold significant promise due to their high theoretical specific energy of 2,567 Wh kg⁻¹, assuming the complete electrochemical reduction of α-sulfur to S²⁻ (i.e., without the formation of lithium polysulfide). Altogether, these properties suggest promising implementations of the Li-S electrochemistry in transportation applications. However, the Li-S system is known for its complex and highly-interconnected chemistry, which gives rise to various problems throughout the battery.

… The solid sulfur cathode also has specific issues of its own, including low electronic and ionic conductivities and cumulative mechanical damage during cycling that results from volume changes associated with precipitation. Structural changes that occur during lithium sulfide precipitation and PS dissolution lead to swelling and pulverization. Although interesting solutions have been proposed and successfully implemented, such as the use of nanostructured composite sulfur-carbon compounds with reversible capacities up to 1,320 mAh g⁻¹, the development of a Li-S electrode couple into a commercially-viable battery has been hampered by poor reversibility during discharge and subsequent recharge cycles. This is due to the combination of the previously described issues, and the interrelated chemistries that connect the cathode, anode, and electrolyte reactions.

… This work reports the synthesis, characterization, electrochemical performance, and modeling of a nanosulfur-loaded CC composite as a novel cathode material for Li-S batteries.

—Dysart et al.

CCs were synthesized via pyrolytic heat-treatment of starch precursors within an inert atmosphere. The team then used ultrasonic irradiation to facilitate the reaction between dilute hydrochloric acid (HCl) solution and an aqueous sulfur precursor of sodium thiosulfate (Na₂S₂O₃) in the presence of CCs to yield a composite of CC particles with pure nano-size sulfur (CCs/S composite) and a water soluble by-product of sodium chloride (NaCl).

Schematic of the ultrasonic synthesis of the CCs/S composite. (Top) Graphical representation of the CCs synthesis process and the sonochemical sulfur loading technique. (Bottom) (a) SEM micrograph of a porous CC particle without sulfur. (b) SEM micrograph of the carbon-sulfur composite produced via the sonochemical sulfur deposition process. (c) Elemental mapping of the carbon-sulfur composite by EDS color pixel-mapping. Sulfur is highlighted in green, while carbon is highlighted in red. Dysart et al. Click to enlarge.

This approach avoids the common, less energy efficient “melt-down” method, which requires heating solid sulfur to form a liquid that permeates non-homogeneously into an available porous substrate.

The resulting CCs/S cathode architecture yields stable cycling performance and high cycling efficiency with a fluorine-containing electrolyte. Coulombic efficiency is greater than 96% for 100 cycles.

Resources

• Arthur D. Dysart, Juan C. Burgos, Aashutosh Mistry, Chien-Fan Chen, Zhixiao Liu, Chulgi Nathan Hong, Perla B. Balbuena, Partha P. Mukherjee, and Vilas G. Pol (2016) “Towards Next Generation Lithium-Sulfur Batteries: Non-Conventional Carbon Compartments/Sulfur Electrodes and Multi-Scale Analysis” J. Electrochem. Soc.163(5): A730-A741; doi: 10.1149/2.0481605jes