[Image above] Shuttling of polysulfide compounds (shown as yellow and blue chains) impairs the performance of lithium-sulfur batteries. Polar host materials for the cathode’s sulfur can mitigate this effect, but does that make up for the materials’ low conductivity? Credit: Egibe, Wikimedia (CC BY-SA 4.0)
At the virtual industry exposition Ceramics Expo Connect last week, manufacturers and researchers alike spoke at length about the future of ceramics in a wide variety of fields, from sustainable energy technologies and aerospace applications to the state of additive manufacturing techniques and material testing methods.
The move to renewable energy sources and electrification in particular was the focus of Monday’s sessions, with panelists discussing some of the technologies that are expected to challenge lithium-ion batteries in the coming years.
One energy technology that researchers hope to see achieve its potential soon is lithium-sulfur batteries. Li-S batteries have a high theoretical specific energy of more than 2,500 Wh/kg, which is much higher than the average specific energy of 100–265 Wh/kg for current Li-ion batteries. However, to date the experimental values of Li-S battery specific energy have been far below theoretical values.
The main mechanisms hindering Li-S battery performance are irreversible loss of sulfur from the cathode (the polysulfide “shuttle” effect) and unstable lithium deposition on the anode. A CTT post from May explains these mechanisms in detail and also discusses one approach to solve the latter problem.
These mechanisms are not the only challenges, however. Sulfur also has low electrical conductivity (5 × 10-30 S/cm at room temperature), which hinders the cycling efficiency of Li-S batteries.
To improve conductivity, researchers have experimented extensively with placing the cathode’s sulfur within highly conductive carbon host materials, such as hollow porous carbon, graphene, mesoporous carbon, and microporous carbon. Unfortunately, long-term cycling stability continues to be a problem because of the nonpolar covalent bonds that carbon forms with itself, which prevent polysulfides on the carbon surface from attaching strongly, and instead they diffuse away—leading to the notorious polysulfide “shuttle” effect.
Researchers have investigated employing oxide additives, polymers, or other inorganic materials on the carbon framework to enhance polysulfide confinement and mitigate the polysulfide shuttle effect. But these methods often require complicated and expensive synthesis processes, plus they limit the accommodation of sulfur by reducing available surface area.
Based on these challenges, the question of the best host material for sulfur in Li-S batteries remains wide open. But a new study by an international research team may help guide future explorations on this question.
Polarity or conductivity: Which property helps ensure cycling stability?
Jong-Su Yu from Daegu Gyeongbuk Institute of Science & Technology (Korea) and Khalil Amine from Argonne National Laboratory led the recent study, which looked to determine the respective importance of two key properties—polarity and conductivity—in improving the cycling stability of Li-S batteries.
As mentioned above, the polysulfide “shuttle” effect continues to be a problem for Li-S batteries with carbon host materials because of the carbon surface’s nonpolar nature, despite having high conductivity. In contrast, oxide materials usually are polar and thus provide secure interaction points for the polysulfides, but these materials tend to be less conductive or are even insulating.
Because of the high cost and complicated processing associated with synthesizing carbon host materials that contain oxide additives, commercializing a battery that exhibits both good conductivity and polarity does not appear feasible at this point. So, the researchers of the recent study wanted to know if only one of these two properties can be maximized, which plays the bigger role in achieving cycling stability?
To answer this question, they designed two cathodes, one made from platelet ordered mesoporous silica (pOMS) and one made from platelet ordered mesoporous carbon (pOMC).
“The two cathodes were designed to be exact replicas of one another apart from the use of either silica or carbon,” Amine says in an Argonne press release. “This way, we could determine whether a more polar cathode or a more conductive cathode improved the longevity of the battery.”
Upon testing, the researchers found that while the conductive carbon host with a higher specific surface area of 1,597 m2 g−1 showed better initial capacity, “the polar [silica host] with a lower surface area of 844 m2 g−1 reveals much more stable performance for long cycles and eventually outperforms the conductive counterpart after 500 cycles.”
In addition, the silica host also demonstrated outstanding low fading rates, even at high current density, and comparable and improved areal and volumetric capacities, respectively, compared to carbon hosts.
“These outstanding areal and volumetric capacities, as well as cycle stability, which have not been achieved by even state-of-the-art carbon hosts, clearly indicate that the polar [silica] host, despite nonconductivity, has high promising potential for energy storage in [Li-S batteries],” the researchers write in the paper.
Of course, electrical conductivity is still necessary to achieve good electrochemical performance. “However, the conductivity is not a big issue in the host itself since the poor conductivity of the host can be compensated by the conducting agent involved as a required electrode material during electrode preparation,” the researchers add.
In the conclusion, the researchers note they are currently investigating ways to improve electron pathways in the silica host while maintaining the high surface polar properties, such as by adding a thin conductive carbon coating to the silica to enhance conductivity. Overall, they hope this study “will broaden the selection of host options and open up new innovative paradigms and cell designs for next-generation metal–sulfur batteries.”
The paper, published in Advanced Energy Materials, is “Revisiting the role of conductivity and polarity of host materials for long‐life lithium–sulfur battery” (DOI: 10.1002/aenm.201903934).