New understanding of reaction mechanisms leads to design of Li-S cells with unprecedented performance
Daniel Robertson

Everyone wants their phone, laptop computer, or even electric car to run longer without needing a recharge. Currently, these devices are all powered by Li-ion batteries, which have seen tremendous improvements over the past few decades. In the next decade, however, this rate of growth looks to be slowing down, and our battery-powered society will need the next generation of energy storage technology to keep on pace with the energy demands of our portable electronics. 

There are a variety of emerging battery chemistries in development that could allow for extended runtimes. Some options are similar to traditional Li-ion batteries, but use alternative ions like Na+, Mg2+, or Zn2+. Other candidates, like lithium air or lithium sulfur batteries, are considerably different from the current Li-ion technology. When considering the next generation of batteries, the amount of energy stored, the number of stable charge-discharge cycles, and the cost of the materials used are all essential characteristics. At this point, only one thing is clear: all of these potential types of new batteries need huge improvements to compete with, let alone surpass, the Li-ion batteries we currently use.

Lithium-sulfur (Li-S) batteries are one of the most promising options being considered, since sulfur is abundant and inexpensive. Plus, with the right engineering strategy, Li-S batteries could store up to several times the energy of our current Li-ion batteries.

However, slow charging rates, and rapid capacity fading, where the battery stores less and less with each use, have limited the viability of Li-S since its beginning. Recently, a team of researchers at the Synthetic Control Across Length-scales for Advancing Rechargeables (SCALAR) Energy Frontier Research Center (EFRC) developed a unique approach to address the most critical issues with Li-S chemistry, and in doing so, demonstrated one of the best performing Li-S batteries thus far.

Li-S batteries rely on conversion of S8 to Li2S and back

Figure 1. The conversion from S8 to Li2S and back again in a Li-S battery involves multiple lithium polysulfide intermediates, which can "shuttle" away and decrease the battery's capacity. Modified from Peng et al.

In any rechargeable battery, the goal is to store electricity by driving a chemical reaction, and later extract the stored electricity by driving that reaction in reverse. For Li-S batteries, the chemical reaction involves taking elemental sulfur (S8) in its solid form and reacting it with lithium ions to produce lithium sulfide (Li2S). The full net chemical reaction is depicted in Figure 1. Importantly, the 2 electrons per sulfur that are used in this reaction lead to Li-S batteries’ high theoretical charge storage capacity.

However, this reaction scheme only gives the start and the end of the reaction in a Li-S battery. On the way to forming Li2S, sulfur must combine with the lithium ions one at a time, forming chain-like molecules called lithium polysulfides (Li2S8, Li2S6, L2S4, Li2S2). Figure 1 shows a schematic of one S8 molecule progressing through these intermediate polysulfides to eventually form Li2S. As one might expect, this journey to get from S8 all the way to Li2S is complex and doesn’t necessarily finish. Often, short-chain polysulfides like Li2S4 and Li2S2 dissolve and float away from the electrode, never to return. This process is called the shuttle effect, and it causes loss of sulfur, and loss of capacity, during Li-S battery operation.

Because of this shuttle effect, Li-S batteries currently exhibit dramatic capacity loss with each charge-discharge cycle. Imagine if the battery on your phone or laptop lost 10% or more of its capacity every day! Unless this problem is addressed, Li-S batteries can’t be a viable option for rechargeable energy storage.

Solving the polysulfide shuttle problem with a new catalyst

Figure 2. The graphene-based catalyst designed for the SCALAR team, shown in a photograph (left) and a scanning electron micrograph (right) at different length scales. Modified by Peng et al.

The polysulfide shuttle effect has been a notoriously tricky problem to address. One approach that researchers have used to prevent the loss of sulfur from this process in Li-S batteries is by catching the polysulfide molecules with tiny nanoscale webs or nets. This way, the molecules can’t float away and they stay near the electrode surface. While this strategy has seen some success in designing better-performing batteries, it requires detailed engineering that likely won’t be usable in a real battery due to cost and size concerns.

More recently, some scientists have begun to develop catalytic approaches to control the reactions inside Li-S batteries. A catalyst is any material or molecule that increases the rate of a chemical reaction by changing the way that reaction happens without itself being permanently altered by the interaction. The idea is that, if the lithium polysulfide molecules react fast enough, they won’t have time to float away. The only problem is that the development of catalysts is difficult and mostly relies on trial and error to see what works.

Recently, researchers working in the SCALAR EFRC took a new approach to designing a catalyst for Li-S batteries. First, the team studied the mechanism of the Li-S battery to figure out which step of the reaction was slowest. Typically, the slowest, or rate-limiting, step of a reaction has a disproportionate effect on the overall reaction speed, so figuring out this part was a key first step. Then, by utilizing computer simulations of the molecules in this reaction step, the researchers were able to design and synthesize a graphene-based catalyst that was highly effective at facilitating that specific part of the reaction. A photograph and a scanning electron micrograph of the graphene catalyst material that the researchers made are shown in Figure 2. After this detailed study on the mechanism in a Li-S battery, the researchers put their knowledge to work and made a working Li-S cell using the catalyst they designed. The resulting performance was among the best, if not the best, thus far: most Li-S batteries lose their capacity after fewer than 100 charge-discharge cycles, but including the graphene catalyst enabled stable charging and discharging for more than 500 cycles!

Fundamental studies inform applied solutions

One takeaway from this work was the importance of fundamental understanding in guiding energy-related research. Although many energy-related technological challenges require engineering and design, basic information about the mechanism of a chemical reaction or other processes can lead to breakthroughs in the long term. Here, the SCALAR team demonstrated that gaining insight from fundamental chemical processes was able to inform and guide their design strategy in making a high-performance Li-S battery.

More Information

Robertson, D. (2020). Powering up Li-ion batteries with anion redox chemistry

Stewart, D. M. (2019). What Stress Means for Batteries

Sahadeo, E. (2019). Meet a Better Battery: All Solid Materials Facilitate Safer Energy Storage.

Bruck, A. (2018). Batteries Through the Looking Glass.

Lebens-Higgins, Z. (2018). Catching a Battery in the Act.

Peng, L., Wei, Z., Wan, C., Li, J., Chen, Z., Zhu, D., Baumann, D., Liu, H., Allen, C.S., Xu, X., Kirkland, A.I., Shakir, I., Almutairi, Z., Tolbert, S.H., Dunn, B., Huang, Yu., Sautet, P., Duan, X. A fundamental look at electrocatalytic sulfur reduction reaction. Nature Catal., 2020, 3(9), 762-770.


This work is supported by the Center for Synthetic Control Across Length-Scales for Advancing Rechargeables, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science Basic Energy Sciences programme under award DE-SC0019381. Y.H. acknowledges the support by Office of Naval Research through grant no. N00010141712608 (initial effort on catalyst preparation and rotating disc electrode electrochemical characterizations). I.M. and Z.A. acknowledge the support by the International Scientific Partnership Program (ISPP-147) at King Saud University. We acknowledge the Electron Imaging Center at UCLA for SEM technical support and the Nanoelectronics Research Facility at UCLA for device fabrication technical support. We thank Diamond Light Source for access and support in use of the electron Physical Science Imaging Centre (MG23956). The calculations were performed on the Hoffman2 cluster at UCLA Institute for Digital Research and Education (IDRE), The National Energy Research Scientific Computing Center (NERSC), and the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1548562, through allocation TG-CHE170060.

About the author(s):

Danny Robertson is a graduate student in chemistry in Sarah Tolbert’s group at UCLA. He studies nanostructured materials for energy storage applications. Through his collaboration with other researchers in the SCALAR EFRC, he is working to develop nanoscale architectures of battery materials to access fast-charging capability.