The story of liquid sulfur goes back to our work which was published in the Proceedings of the National Academies of Sciences (116 (3) 765-770, 2019) where we reported that super-cooled liquid sulfur can be generated electrochemically at room temperature (well below its melting temperature 115°C). During this discovery, as Prof. Steven Chu named it a "surprise by accident," we believed that significant scientific advances would follow. 

For years, sulfur (as an electrode material) had always been assumed to be solid during the battery operation. With our discovery of electrochemically generated super-cooled liquid sulfur, we immediately asked ourselves the question: how would liquid sulfur influence lithium sulfur batteries? To answer this question, we needed to controllably generate both liquid and solid sulfur. Lucky for this project, Prof. Yi Cui's research group is incredibly interdisciplinary – students and scholars have a diverse range of backgrounds and interests.

While contributing to the supercooled sulfur project (led by Nian and Guangmin), I was simultaneously working alongside Jinsong on a two-dimensional (2D) materials project. It was during this time that my PI (Prof. Yi Cui) and I decided to use 2D materials as a substrate to control the phase of sulfur. The top surface of 2D materials have completely passivated bonds while the edges have an abundance of dangling bonds – we hypothesized that this difference in bonding structure would promote the generation of sulfur differently at the top surface versus at the edges.

Figure 1. Liquid sulfur droplets on the basal plane and solid sulfur crystal from the edges of molybdenum disulfide (MoS₂).

As we hypothesized, but still very exciting, we observed an interesting phenomenon: sulfur grows as liquid droplets on the molybdenum disulfide (MoS₂) basal plane and grows as solid crystal from the edges of MoS₂ grains. We spent some time characterizing the compositions of liquid and solid sulfur. These include an overnight beam-line experiment at the Advanced Light Source with Xueli and several late-night sessions on Stanford's aberration-corrected electron microscope with Rafael (both of which were a lot of fun).

Understanding the growth mechanism was not straightforward. Initially, we thought the different phases result from interaction of generated sulfur with different atomic sites on MoS₂. Soon two pieces of evidence changed our minds. First, DFT calculations performed with help from Dr. Cong Su and Prof. Ju Li from MIT showed that the binding of sulfur at the basal plane and edges of MoS₂ grains are energetically very similar. Second, we tested additional 2D materials including other transition metal dichalcogenides and graphitic materials which showed similar growth behaviors of sulfur (liquid droplets on basal planes and solid crystal at grain edges). These two pieces of evidence demonstrated that the phenomenon does not depend on atomic composition. We further showed that the concentrated electric field from the edges of 2D materials due to their high aspect ratio plays an important role, as supported by the finite-element simulations from Allen, molecular dynamics simulations from Xian, and various experimental observations.

Figure 2. Liquid and solid sulfur provide very different areal capacities. (Cartoon courtesy of Xiaoyun)

We then proceeded to characterize the electrochemical performance of liquid and solid sulfur. One way to manage the sulfur phase is to take advantage of the edge-induced crystallization phenomenon. By suppressing sulfur crystallization at the edges we can force the phase of sulfur to remain in the super-cooled liquid phase. Using our in situ optical set-up, we could characterize the material morphology while simultaneously recording the charge and discharge capacities in order to understand the relationship between sulfur phase and electrochemical performance. To our surprise we found that liquid sulfur enables much higher areal capacities because these liquid droplets only require minimal point-contact with the electrode which maximizes the electrolyte-electrode contact area during cycling. In contrast, solid sulfur results in much lower areal capacities because crystallized sulfur occupies the entire electrode surface which slows down the electrochemical reaction. These experiments directly show that super-cooled liquid sulfur enables faster reaction kinetics in comparison to the solid sulfur.

This study demonstrates the great potential of exploiting the faster reaction kinetics of liquid sulfur in batteries, particularly when fast charging is required. However, there are still many challenges to address before we can take full advantage of this newly discovered phase, for example, how to host the liquid sulfur and minimize its dissolution into the electrolyte.

If you're interested in more details, please give our paper a read:

A. Yang, G. Zhou, X. Kong, R.A. Vilá, A. Pei, Y. Wu, X. Yu, X. Zheng, C.-L. Wu, B. Liu, H. Chen, Y. Xu, D. Chen, Y. Li, S. Fakra, H.Y. Hwang, J. Qin, S. Chu, Y. Cui, “Electrochemical generation of liquid and solid sulfur on two-dimensional layered materials with distinct areal capacities”, Nature Nanotechnology, 2020.

Written by Ankun Yang and edited by Rafael A. Vilá.