Lithium-sulfur batteries can potentially store five to 10 times more energy than current state-of-the-art lithium-ion batteries at much lower cost. Current lithium-ion batteries use cobalt oxide because the cathode, an expensive mineral mined in ways in which harm people and the environment. Lithium-sulfur batteries replace cobalt oxide with sulfur, which is abundant and low cost, costing lower than one-hundredth the worth of cobalt.
But there is a catch: Chemical reactions, particularly the sulfur reduction response, are very complex and never well understood, and undesired side reactions could end the batteries’ lives well before those of traditional batteries.
Now, researchers led by UCLA chemists Xiangfeng Duan and Philippe Sautet have deciphered the important thing pathways of this response. These findings, outlined in a paper published within the journal Nature, will help fine-tune the response to enhance battery capability and lifelong.
The sulfur reduction response in a lithium-sulfur battery involves 16 electrons to convert an eight-atom sulfur ring molecule into lithium sulfide in a catalytic response network with quite a few interwoven branches and different intermediate products called lithium polysulfides and lots of other byproducts. Since it is such a fancy response, with many paths branching off from one another and lots of intermediate products which can be necessary for continuing the response, it has been hard to review and even harder to determine which parts of the response to focus on for improved battery performance.
“Despite extensive efforts dedicated to improving the apparent performance of lithium-sulfur batteries, the basic response mechanism stays unsettled,” said Duan, corresponding writer and UCLA professor of chemistry and biochemistry. “The primary branch on this response network for the sulfur reduction response stays a subject of considerable debate.”
An issue of particular interest is a side response by which the polysulfide intermediates migrate, called shuttling, to the lithium metal anode and react with it, consuming each sulfur and lithium and resulting in energy loss and rapidly reduced storage capability. A transparent identification of the important thing intermediates and higher understanding of how these intermediates are produced or consumed would help scientists control this migration between electrodes and minimize the waste of sulfur and lithium.
The brand new study deciphers the entire response network for the primary time, determines the dominant molecular pathway and unveils the critical role of electrocatalysis in modifying the response’s kinetics.
The team first used theory calculations to map out all possible response pathways and the associated intermediates, after which electrochemical and spectroscopic evaluation to validate the computational findings.
Battery performance was dominated by Li2S4 because the primary intermediate and catalysis turned out to be crucial for fully converting the Li2S4 to the ultimate discharge product (Li2S). Carbon-based electrodes doped with sulfur and nitrogen can effectively facilitate this conversion. Their study also found that the intermediate Li2S6 does circuitously take part in the electrochemical process but is present as a serious product from side chemical reactions and contributes significantly to the unwanted polysulfide shuttling effect.
“Our study provides a fundamental understanding of the sulfur reduction response in lithium-sulfur batteries and demonstrates that a properly designed catalytic electrode material can speed up the charging and discharging reactions, mitigate the side reactions and improve the cycle life,” said Duan, who in December was chosen as a 2023 fellow by the National Academy of Inventors.
“The mixture of battery technology and catalysis science opens recent avenues for fast and high-capacity energy conversion devices,” said Sautet, who’s the Levi James Knight, Jr. Term Chair for Excellence.
This work was supported by the Center for Synthetic Control Across Length-scales for Advancing Rechargeables, an Energy Frontier Research Center funded by the U.S. Department of Energy Office of Science Basic Energy Sciences program.
