Because of their very high theoretical energy density—more than five times the energy stored in a smaller capacity than state-of-the-art lithium-ion batteries—they are strong contenders for both small and large applications.
However, several performance issues—namely, poor conductivity and insufficient energy efficiency—must be addressed before practical applications can be realized. These shortcomings stem from the chemistry and reactions within the battery, as charge is transferred between lithium atoms between the two battery electrodes and through the electrolyte that separates them. These problems can be addressed by incorporating conductive metal sulfides such as copper sulfide (CuS), iron disulfide (FeS2), titanium disulfide (TiS2), and sulfur electrodes in other batteries. However, unique and distinct behaviors have been observed for each type of metal sulfide in Li-S batteries. To understand the underlying mechanisms of these differing behaviors, scientists need to be able to closely study these complex reactions in real time during battery discharge and charging, which is a challenge.
At the U.S. Department of Energy (DOE) Science User Facility Office at Brookhaven National Laboratory, the National Synchrotron Radiation Light Source II (NSLS-II), a team of researchers conducted a multi-technique X-ray study to learn more about the structural and chemical evolution of a metal sulfide additive—copper sulfide (CuS)—as lithium ions move between battery electrodes in this context. Their work is an example of operational studies that allow researchers to gather structural and chemical information while simultaneously measuring electrochemical activity. The team used a “multi-mode” approach involving a suite of X-ray techniques: X-ray powder diffraction to gather structural information, X-ray fluorescence imaging to visualize changes in elemental distribution, and tracking chemical reactions using X-ray absorption spectroscopy.
The results reported in the online scientific report on October 11, 2017, provide new insights into the structure and chemical evolution luminescence of this system.
Explore better performance additives
CuS is advantageous among metal sulfide additives for several reasons, including its high conductivity and energy density. In previous research, the team found that adding CuS to sulfur-only electrodes improved battery discharge capacity because sulfur is a poor conductor and CuS exhibits better conductivity and electrochemical activity. However, when using a mixed sulfur/CuS cathode (positive electrode), Cu ions dissolve in the electrolyte and eventually deposit on the lithium anode (negative electrode), disrupting the interfacial layer between the anode and electrolyte. This can lead to battery failure after only a few charge-discharge cycles.
“This observation represents a design challenge for multifunctional electrodes: when introducing new components with desired performance, parasitic reactions can occur and hinder the original design intent,” said Gan Hong, a scientist in the Sustainable Energy Technologies Department at Brookhaven.
He continued, "To address the specific challenges of Li-S batteries with CuS additives, and to guide future electrode design, we need a better understanding of the development of these systems in various ways: structurally, chemically, and morphologically."
Perform multiple modes and operations
“We believe it is necessary to develop a multimodal approach that not only studies one aspect of the system’s evolution but also uses a variety of complementary synchrotron technologies to provide a more comprehensive view of many aspects of the system,” said Karen Chen-Wiegart, another corresponding author of the paper and assistant professor in the Department of Materials Science and Chemical Engineering at Stony Brook University, who is also affiliated with NSLS-II.
To achieve this goal, the team first designed a battery that is fully compatible with all three X-ray techniques and can be used for studies on three different X-ray beamlines of the NSLS-II. Their design not only allows measurements to be taken at both electrodes of the battery but also possesses optical transparency, enabling researchers to perform in-line optical microscopy and alignment along the beamline.
Chen-Wiegart said, "These properties are critical because they allow us to address responses from different parts and multiple locations within the cell in space, which is one of our main research goals."
Furthermore, team members Sun Ke (Survival Energy Technologies Department, Brookhaven), Chonghang Zhao, and Chenghong Lin (both from Stony Brook University) noted that their versatile and simple design, using economical components, allows for the construction of numerous cells for each synchrotron experiment, greatly facilitating their research. Sun, Zhao, and Lin together developed a multi-mode field battery pack. In addition, the team designed a multi-cell holder that allows for the simultaneous cycling of several cells and continuous measurement of them.
This comprehensive approach requires a research team comprised of experts from diverse backgrounds. Scientists from the Brookhaven Sustainable Energy Technologies Department and Stony Brook University collaborated closely with scientists at NSLS-II. They used operando X-ray powder diffraction (XPD) with scientists Jianming Bai and Eric Dooryhee to study the structural evolution of the hybrid electrode during discharge. The XPD beamlines at NSLS-II are an effective tool for studying battery reactions, including Li-S batteries, and were used to capture the reaction time between lithium and CuS relative to its reaction with sulfur. The XPD data also indicated that the reaction products are not crystalline, as shown by the lack of diffraction peaks.
The team turned to operado X-ray absorption spectroscopy (XAS) using the inner-shell spectroscopy (ISS) beamline, collaborating with NSLS-II scientists Eli Stavitski and Klaus Attenkofer. The XAS data indicated that after the cell was fully discharged, the CuS had been converted to a Cu species ratio and somewhere between CuS and Cu ⇌ 2S. To further pinpoint the precise phase composition, the group will perform additional XAS studies in the future.
To observe the dissolution of CuS and its subsequent redeposition on the lithium anode, scientists, with the assistance of Garth Williams and Juergen, performed operational X-ray fluorescence (XRF) microscopy on the submicron resolution X-ray spectroscopy (SRX) beamline at THIEME. XRF imaging identifies elements in a sample by measuring the X-ray fluorescence emitted when the sample is excited by the main X-ray source. In this case, it allowed the team to image the distribution of elements in the cell, as well as how and over time this distribution evolved. This information can be correlated with chemical and structural evolution data obtained from XPD and XAS studies.
Put it together
When evaluating the findings of each X-ray technique in general, the image formation—though complex—is based on the evolution of the crystalline phases of the sulfur-CuS mixed electrode and how CuS dissolves during battery discharge. During the first portion of the discharge, the sulfur in the cathode is completely consumed, appearing to be converted into soluble lithium polysulfides, such as LiS3, LiS4, etc., up to LiS8. Next, the polysulfides then transform into amorphous Li2S2, which is then further converted into crystalline Li2S. This lithiation of sulfur stops at the end of the complete discharge phase. At this point, CuS begins to lithilate, forming amorphous Cu/S species.
CuS interacts strongly with some polysulfide substances. Cu ions dissolve into the electrolyte, where they migrate from the cathode to the anode. Various copper substances are deposited on the anode surface, and soon after, the battery fails.
The above work provides a clear mechanism explaining how sulfur and copper sulfide interact within Li-S batteries during discharge/charge cycles. The research team will use the multi-mode synchrotron method developed in this paper to investigate the cycling mechanisms of other battery systems. Research on multifunctional conductive additives for lithium-sulfur batteries has primarily focused on other more stable transition metal sulfides, such as titanium disulfide (TiS2), which do not release Ti ions from the electrolyte during battery discharge/charge.
