A previously unknown path for high-energy, low-cost, long-life batteries

The road from advances in the laboratory to practical technology can be long and bumpy. An example is the lithium-sulfur battery. It has notable advantages over the current lithium-ion batteries that power vehicles. But it has not affected the market yet despite the intensive development over many years.

This situation could change in the future, thanks to the efforts of scientists at the US Department of Energy’s Argonne National Laboratory. Over the past decade, they have made several pivotal discoveries related to lithium-sulfur batteries. Their latest discovery, published in natureIt unlocks a previously unknown reaction mechanism that addresses a major drawback, which is the very short life of the batteries.

“Our team’s efforts can bring the United States a huge step closer to a greener, more sustainable transportation landscape,” said Gui-Liang Xu, a chemist in Argonne’s Department of Chemical Science and Engineering.

Lithium-sulfur batteries offer three significant advantages over current lithium-ion batteries. First, it can store two to three times more energy in a given volume, resulting in a longer vehicle range. Second, its low cost, facilitated by the abundance and affordability of sulfur, makes it economically viable. Finally, these batteries do not depend on vital resources such as cobalt and nickel, which may face shortages in the future.

Despite these benefits, the transition from laboratory success to commercial viability has proven elusive. Laboratory cells have shown promising results, but when scaled up to commercial size, their performance declines rapidly with repeated charging and discharging.

The underlying reason for this decrease in performance lies in the solubility of sulfur from the cathode during discharge, which leads to the formation of soluble lithium polysulfide (Li₂s₆). These compounds flow to the negative electrode of lithium metal (the anode) during charging, exacerbating the problem. Thus, the loss of sulfur from the cathode and changes in the anode composition significantly impede battery performance during cycling.

In a recent previous study, Argonne scientists developed a catalyst that essentially eliminated the problem of sulfur loss when added in a small amount to a sulfur cathode. While this catalyst has shown promising results in both commercial and laboratory-sized cells, its mechanism of action at the atomic level has so far remained a mystery.

The team’s most recent research has shed light on this mechanism. In the absence of the catalyst, lithium polysulphide forms on the surface of the cathode and undergoes a series of reactions, eventually converting the cathode into lithium sulfide (Li₂s).

“But having a small amount of catalyst in the cathode makes a big difference,” Xu said. “This is followed by a completely different reaction pathway, one devoid of intermediate reaction steps.”

The key is the formation of dense nanobubbles of lithium polysulfide on the surface of the cathode, which would not appear without the catalyst. Lithium polysulfides rapidly diffuse throughout the cathode structure during discharge and transform into lithium sulfide which is composed of nano-sized crystals. This process prevents sulfur loss and reduced performance in commercial size cells.

To unlock this black box around the reaction mechanism, the scientists used advanced characterization techniques. Analyzes of the catalyst’s structure using intense synchrotron X-ray beams at the 20-BM beamline of the Advanced Photon Source, a user facility of the DOE’s Office of Science, revealed that it plays a critical role in the reaction pathway. The structure of the catalyst affects the shape and composition of the final product at discharge, as well as intermediate products. With the catalyst, nanocrystalline lithium sulfide is formed at full discharge. Without the catalyst, microscopic rod-shaped structures are formed instead.

Another biotechnology, developed at Xiamen University, allowed the team to visualize the interface between the electrode and the electrolyte at the nanoscale while the test cell was in operation. This newly invented technique helped link changes at the nanoscale to the behavior of a functioning cell.

“Based on our exciting discovery, we will conduct more research to design better sulfur cathodes,” Xu noted. “It would also be useful to explore whether this mechanism applies to other next-generation batteries, such as sodium sulfur batteries.”

And with this latest achievement by the team, the future of lithium-sulfur batteries looks even brighter, providing a more sustainable and environmentally friendly solution to the transportation industry.

In addition to Xu, authors include Xiyuan Zhu, Ji Shi, Sangui Liu, Jin Li, Fei Bai, Yuehu Chen, Junxian Deng, Qizheng Zheng, Jiayi Li, Chen Zhao, Anhui Huang, Cheng Junsun, Yuzi Liu, Yu Ding Wuling Huang, Yu Qiao, Jianfeng Chen, Khalil Amin, Shi Gangsun, and Hong Gang Liao.

Other participating institutions include Xiamen University, Beijing University of Chemical Technology, and Nanjing University.