Researchers at Drexel University have stabilized a rare monoclinic γ-sulfur phase within carbon nanofibers that enables successful operation of Lithium-Sulfur (Li-S) batteries in carbonate electrolyte for 4000 cycles. AN open-access paper on their work is published in Communications Chemistry.

Carbonates are known to adversely react with the intermediate polysulfides and shut down Li-S batteries in first discharge. Through electrochemical characterization and post-mortem spectroscopy/ microscopy studies on cycled cells, we demonstrate an altered redox mechanism in our cells that reversibly converts monoclinic sulfur to Li2S without the formation of intermediate polysulfides for the entire range of 4000 cycles. To the best of our knowledge, this is the first study to report the synthesis of stable γ-sulfur and its application in Li-S batteries. We hope that this striking discovery of solid-to-solid reaction will trigger new fundamental and applied research in carbonate electrolyte Li-S batteries.

—Pai et al.

Li-sulfur batteries are attractive for use in EVs due to theoretically high energy density; in addition, sulfur is environmentally friendly and naturally abundant. However, current Li-S systems are challenged by a number of issues. The insulating nature of sulfur and the final discharge product—Li₂S—results in low material utilization during the redox processes. More challenging is the dissolution of the intermediate lithium polysulfides into the electrolyte. The polysulfide shuttle results in uncontrollable deposition of sulfide species on the lithium metal anode, reducing columbic efficiency and increasing capacity fade.

A much less discussed, but debilitating drawback for the commercial viability of Li–S batteries, is the use of the ether electrolyte itself. Ether-based solvents are highly volatile and have low flash points posing a significant risk of operating such batteries beyond room temperatures. For example, dimethoxyethane (DME), an important ingredient used in present-day Li–S batteries has a boiling point of only 42 °C. Therefore, despite tremendous research in overcoming Li–S battery challenges, the practicality of such battery chemistries is severely hindered due to severe safety concerns and transport issues.

—Pai et al.

The challenge of introducing sulfur into a lithium battery with commercially friendly carbonate electrolyte has been an irreversible chemical reaction between the intermediate polysulfides and the carbonate electrolyte. Because of this adverse reaction, previous attempts to use a sulfur cathode in a battery with a carbonate electrolyte solution resulted in nearly immediate shut down and a complete failure of the battery after just one cycle.

In the past decade, the majority of Li-S field adopted ether electrolytes to avoid the adverse reactions with carbonate. Then over the years, the researchers deep-dived into enhancing performances in ether-based sulfur batteries by mitigating what is known as polysulfide shuttle/diffusion—but the field completely overlooked the fact that the ether electrolyte itself is a problem. In our work, the primary objective was to replace ether with carbonate, but in doing so we also eliminated polysulfides, which also meant no shuttling, so the battery could perform exceptionally well through thousands of cycles.

—Vibha Kalra, George B. Francis Chair professor in the Department of Chemical and Biological Engineering, who led the research

Previous research by Kalra’s team also approached the problem by producing a carbon nanofiber cathode that slowed the shuttle effect in ether-based Li-S batteries by curtailing the movement of intermediate polysulfides. But to improve the commercial path of the cathodes, the group realized it needed to make them function with a commercially viable electrolyte.

Having a cathode that works with the carbonate electrolyte that they’re already using is the path of least resistance for commercial manufacturers. So rather than pushing for the industry adoption of a new electrolyte, our goal was to make a cathode that could work in the pre-existing Li-ion electrolyte system.

—Vibha Kalra

In hopes of eliminating polysulfide formation to avoid the adverse reactions, the team attempted to confine sulfur in the carbon nanofiber cathode substrate using a vapor deposition technique. While this process did not succeed in embedding the sulfur within the nanofiber mesh, it did something extraordinary, which revealed itself when the team began to test the cathode.

As we began the test, it started running beautifully—something we did not expect. In fact, we tested it over and over again—more than 100 times—to ensure we were really seeing what we thought we were seeing. The sulfur cathode, which we suspected would cause the reaction to grind to a halt, actually performed amazingly well and it did so again and again without causing shuttling.

—Vibha Kalra

Upon further investigation, the team found that during the process of depositing sulfur on the carbon nanofiber surface , it crystallized in an unexpected way, forming a slight variation of the element, called monoclinic gamma-phase sulfur. This chemical phase of sulfur, which is not reactive with the carbonate electrolyte, had previously only been created at high temperatures in labs and has only been observed in nature in the extreme environment of oil wells.

At first, it was hard to believe that this is what we were detecting, because in all previous research monoclinic sulfur has been unstable under 95 degrees Celsius. In the last century there have only been a handful of studies that produced monoclinic gamma sulfur and it has only been stable for 20-30 minutes at most. But we had created it in a cathode that was undergoing thousands of charge-discharge cycles without diminished performance — and a year later, our examination of it shows that the chemical phase has remained the same.

—Rahul Pai, a doctoral student in the Department of Chemical and Biological Engineering and coauthor

After more than a year of testing, the sulfur cathode remains stable and, as the team reported, its performance has not degraded in 4,000 charge-discharge cycles, which is equivalent to 10 years of regular use. And, as predicted, the battery’s capacity is more than three-fold that of a Li-ion battery.

While we are still working to understand the exact mechanism behind the creation of this stable monoclinic sulfur at room temperature, this remains an exciting discovery and one that could open a number of doors for developing more sustainable and affordable battery technology.

—Vibha Kalra

Kalra suggests that having a stable sulfur cathode, that functions in carbonate electrolyte, will also allow researchers to move forward in examining replacements for the lithium anode – which could include more earth-abundant options, such as sodium.

Resources

• Pai, R., Singh, A., Tang, M.H. et al. (2022) “Stabilization of gamma sulfur at room temperature to enable the use of carbonate electrolyte in Li-S batteries.” Commun Chem 5, 17 doi: 10.1038/s42004-022-00626-2