Lithium-ion (Li-ion) batteries have become the cornerstone of modern technology, powering everything from smartphones to laptops and electric vehicles. Over the years, they have proven to be reliable, efficient, and versatile, making them the go-to solution for a wide variety of energy storage needs. However, as the demand for higher energy densities, lower costs, and more sustainable solutions intensifies, scientists are increasingly looking beyond Li-ion batteries. The search for “beyond Li-ion” batteries that can improve on their limitations and offer better performance is underway, and lithium-sulfur (Li-S) batteries are emerging as one of the most promising candidates.
While Li-ion batteries offer a relatively high energy density, they rely on expensive materials like cobalt and nickel, which are not only scarce but are also subject to price fluctuations and ethical concerns associated with mining practices. Furthermore, as we move toward an electrified future, these batteries are becoming less suitable, both in terms of energy storage capacity and sustainability. In contrast, lithium-sulfur batteries use much more abundant materials—lithium and sulfur—which makes them an attractive alternative. In theory, Li-S batteries can achieve energy densities that are two to three times higher than those of commercial Li-ion batteries, potentially offering longer-lasting and less expensive power solutions.
Despite their advantages, lithium-sulfur batteries face significant challenges that have hindered their widespread adoption. These include the issue of short battery life and performance degradation due to unwanted chemical reactions. When lithium and sulfur react within a Li-S battery, elemental sulfur from the cathode forms polysulfide compounds. Some of these polysulfides dissolve into the electrolyte, leading to a phenomenon known as “shuttling,” where the polysulfides move back and forth between the cathode and the anode. This shuttling of polysulfides causes several problems, including a loss of sulfur at the cathode, which reduces the battery’s energy capacity and cycle life.
Furthermore, the chemical interactions within the battery can be uneven, which exacerbates the already complex and fragile chemistry. These challenges have led researchers to explore different strategies to improve the efficiency and longevity of Li-S batteries, with a particular focus on the electrolyte and its role in controlling polysulfide movement.
Researchers at the U.S. Department of Energy’s Argonne National Laboratory have made significant strides in addressing some of the core challenges facing lithium-sulfur batteries. They have developed a new electrolyte design that introduces a novel additive to mitigate the issue of polysulfide shuttling and improve the overall performance of Li-S batteries. The findings, published in the journal Joule, provide an innovative solution that could bring Li-S batteries one step closer to practical commercial use.
The Role of the Electrolyte in Lithium-Sulfur Batteries
In any battery, the electrolyte is a critical component that facilitates the movement of ions between the anode and cathode during charging and discharging. In Li-ion batteries, the electrolyte is typically a lithium salt dissolved in an organic solvent, and the lithium ions travel between the electrodes via electrochemical reactions. However, in Li-S batteries, the chemistry is more complicated.
In a Li-S cell, lithium ions move between the anode and cathode, where sulfur is converted into polysulfide compounds. These polysulfides, which consist of chains of sulfur atoms, can dissolve in the electrolyte, leading to the aforementioned polysulfide shuttling effect. The mobility of these compounds results in a loss of material from the cathode and deposits of sulfur at the anode, thereby reducing the overall efficiency and lifespan of the battery. Additionally, the irregular distribution of chemical reactions within the battery leads to imbalances that affect its performance.
While researchers have proposed various strategies to mitigate these problems, including designing new electrode materials and improving electrolyte compositions, one potential solution has been to add special chemical additives to the electrolyte that can better manage the polysulfides.
Breaking the Barriers: The Lewis Acid Additive
Up until now, the use of additives in Li-S battery electrolytes has been seen as a double-edged sword. Adding certain chemicals could improve the battery’s efficiency, but there were concerns that they might also react with the sulfur cathode or other battery components, leading to undesirable side reactions. However, the team at Argonne National Laboratory, led by chemist Guiliang Xu, has developed an innovative class of additives—called Lewis acid additives—that can help stabilize the battery’s chemistry while simultaneously improving performance.
A Lewis acid is a substance that can accept electrons from other molecules. In this case, the additive reacts with the polysulfide compounds in the battery’s electrolyte to form a protective film over both the anode and the cathode. This film is crucial because it helps suppress the polysulfide shuttling effect, stabilizing the sulfur in the cathode and preventing material loss. Additionally, this protective layer provides an “ion transport highway,” which enables easier movement of lithium ions between the electrodes.
The key to this solution lies in the control of the chemical reaction between the Lewis acid additive and the polysulfides. Rather than causing a continuous, high-rate reaction that could consume the additive and reduce the energy density of the battery, the additive forms a mild and self-limiting reaction. This ensures that the additive does not deplete the sulfur and the overall energy storage capacity of the battery is maintained.
By using this additive, the battery chemistry is altered in a way that suppresses the dissolution of sulfur into the electrolyte while encouraging more uniform and stable chemical reactions. In turn, this enhances the overall performance and stability of the battery, offering a potential route to extend the cycle life of Li-S cells.
Validating the Breakthrough
To ensure that this new electrolyte design would function as expected, the Argonne team compared it with the conventional electrolyte typically used in Li-S batteries. Using X-ray imaging and other techniques, they observed a significant reduction in the formation of polysulfides in the new electrolyte. Importantly, the new electrolyte also showed a much lower degree of polysulfide dissolution, as confirmed using advanced X-ray techniques at Argonne’s Advanced Photon Source (APS) and Brookhaven National Laboratory’s National Synchrotron Light Source II, both facilities operated by the U.S. Department of Energy’s Office of Science.
These cutting-edge synchrotron techniques allowed the researchers to map out the dissolution process and observe how the polysulfide compounds interacted with the electrolyte during charging and discharging cycles. X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and X-ray fluorescence mapping (XRF) all played a vital role in understanding how the electrolyte and the additive influenced the behavior of the sulfur cathode. The results confirmed that the new electrolyte design significantly reduced polysulfide shuttling and minimized the formation and dissolution of polysulfides, both of which are key challenges for lithium-sulfur batteries.
Additionally, these experiments demonstrated that the new electrolyte enabled smoother ion transfer, meaning that reactions within the cell were more homogeneous, further improving performance consistency.
Solving Safety Concerns
Along with enhancing performance, another major concern for Li-S batteries is the stability and safety of the lithium metal anode. Lithium metal can be highly reactive and flammable, posing safety risks. Researchers like Xu and his team are focused on improving the stability of lithium metal by developing safer electrolyte compositions that prevent the buildup of dendrites (tiny lithium crystals that can grow into the battery and cause short circuits or fires). The development of safer, more stable electrolyte formulations will be a crucial step toward making Li-S batteries commercially viable.
Conclusion
Lithium-sulfur (Li-S) batteries hold significant promise as a viable alternative to traditional lithium-ion batteries, offering the potential for higher energy densities, reduced costs, and reliance on more abundant materials. However, challenges such as polysulfide shuttling and the stability of the lithium metal anode have hindered their widespread adoption. The recent advancements made by researchers at Argonne National Laboratory, particularly their innovative use of Lewis acid additives in the electrolyte, represent a major step forward. By mitigating polysulfide shuttling and enhancing ion transport, these improvements boost the overall performance and stability of Li-S batteries. While further research is required to optimize these solutions and ensure the long-term safety of Li-S batteries, these developments suggest a promising future for their commercial use. Ultimately, lithium-sulfur batteries could play a vital role in the global transition toward more sustainable and efficient energy storage technologies.
Reference: Chen Zhao et al, Polysulfide-incompatible additive suppresses spatial reaction heterogeneity of Li-S batteries, Joule (2024). DOI: 10.1016/j.joule.2024.09.004