Low-cost, high-efficiency electrochemical separation of stable sulfur isotopes

"Our work has suggested that Li-S batteries built from the heavier S isotope could benefit from enhanced conversion reaction kinetics and suppressed active sulfur loss at the cathode, and inhibited parasitic reactions and joule heat generation at the anode." said Prof. Xin, "These results allow us to design a stable and safe high-energy rechargeable battery."

Yet there are more benefits beyond building a better Li-S battery.

Electrochemical Isotope Effects Unlock Low-Cost Isotope Mass Production

Sulfur occurs naturally in four stable isotopes: ³²S, ³³S, ³⁴S, and ³⁶S. Among these, ³²S (94.99 atom%) and ³⁴S (4.25 atom%) are the most abundant isotopes. Isotopically enriched S and its derivatives are essential for research in pharmacology, ecology, geology and other fields.

Conventional isotope separation methods, such as chemical exchange and distillation, typically show low separation factors (a typical value for single-stage separation is between 20‰ to 40‰), so that it would require hundreds of cascade separation steps to prepare high-purity S isotope products.  The complex production process for ³⁴S makes it incredibly costly due to high labor, material, and energy demands. As a result, it remains unaffordable priced at over $1,500 per gram, despite having an abundance close to that of copper and zinc.

By taking the advantage of isotope-variable polysulfides diffusion, the team's ongoing research into the electrochemical isotope effect yielded an unprecedentedly high ³²S/³⁴S separation factor of >1000‰, outperforming the conventional separation methods by two orders of magnitude.

"The development of cost-effective strategies for stable isotope separation is imperative," Prof. Xin emphasized. "Isotope separation by electrochemical methods such as electrolysis and discharging a battery seems promising, however, further studies are required to achieve scalable and cascadable isotope separation and tell a whole story."

Key Parameters Influencing the Isotope Separation Outcomes

The researchers noted that during the charge-discharge process of Li-S batteries, the transformation from solid sulfur (or Li₂S) to soluble lithium polysulfides, followed by their diffusion into the electrolyte, offers a feasible pathway for isotope separation in an electrolysis cell.

They tested this hypothesis using a Swagelok-type prototype electrolysis device, adjusting electrode distances by inserting varying numbers of glass fiber films. To evaluate separation performance, inductively coupled plasma mass spectrometry (ICP-MS) was employed to yield two critical parameters: the separation factor (α) and the yield (Y).

"Our results showed that α increases with electrode distance, whereas Y decreases," said Xi-Xi Feng, the first-author of the latest Energy Material Advances article. "Thus, a balance needs to be struck between α and Y to achieve optimal separation efficiency."

Based on these insights, the researchers developed a three-step electrochemical separation procedure and designed a scalable, cascaded device for stable sulfur isotope separation.

Toward Practical Applications

Under optimized conditions, the model predicts that high-purity ³⁴S can be obtained in just 14 cascade steps, while high-purity ³²S requires only 7 steps, dramatically fewer than the hundreds of steps required by conventional separation technologies.

These findings also have direct implications for Li-S battery technology, a next-generation energy storage system aiming at delivering two to three times higher specific energy than the conventional Li-ion batteries. Polysulfides migration from cathode to anode, which is considered the key factor that accounts for battery performance fade, could enable direct isotope recovery from an end-of-life Li-S battery.

"This means we could eventually repurpose spent batteries as 'urban mines'," said Prof. Xin. "Although our research is still preliminary, it provides more opportunities for isotope science and sustainable energy storage. We will continue to unlock new possibilities."

Acknowledgement

Sen Xin is also affiliated with School of Chemical Sciences, University of Chinese Academy of Sciences. Other contributors include Xi-Xi Feng, Yu-Hui Zhu, Prof. Yu-Guo Guo and Prof. Chunli Bai from Institute of Chemistry, Chinese Academy of Sciences (CAS) and School of Chemical Sciences, University of Chinese Academy of Sciences; Prof. Wen-Peng Wang and Prof. Yao Zhao from Institute of Chemistry, Chinese Academy of Sciences.

The National Natural Science Foundation of China (Grant Nos. 92472120, 52172252, 22209188 and 22421001), Beijing Natural Science Foundation (Grant Nos. JQ22005 and Z220021), Energy Revolution S&T Program of Yulin Innovation Institute of Clean Energy (Grant No. E412080705), and Agilent Technologies supported this work.

Sen Xin received his doctoral degree in Physical Chemistry in 2013. He worked as a postdoctoral researcher from 2015 to 2019, in the group of Prof. John Bannister Goodenough at the University of Texas at Austin, where he focused on the electrode materials and solid electrolytes for rechargeable alkali-metal batteries. In 2019, he started his independent research as a full professor at the Institute of Chemistry, Chinese Academy of Sciences (CAS). He showed a broad interest on the high-energy rechargeable lithium/sodium batteries, solid-state Li-metal batteries, battery materials for working at extreme conditions, energetic materials and battery safety, and stable isotope effects on battery chemistry.