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　　X-ray study provides new findings for lithium-sulfur batteries

　　Lithium-sulfur (Li-S) batteries are a relatively new type of battery being studied and developed by researchers around the world. Because they have very high theoretical energy density - storing more than five times the energy in a smaller volume than state-of-the-art lithium-ion batteries - they are strong contenders for both small and large applications.

　　However, some performance issues - namely poor electrical conductivity and insufficient energy efficiency - must be addressed before practical applications can be achieved. These failures arise from the chemicals and reactions within the battery as charge is transferred through the lithium atoms between the two battery electrodes and through the electrolyte that separates them. These problems can be solved by adding conductive metal sulfides such as copper sulfide (CuS), iron disulfide (FeS2), titanium disulfide (TiS2) and other batteries to the sulfur electrode. 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 different behaviors, scientists need to be able to closely study these complex reactions in real time as the battery discharges and charges, which is a challenge.

　　At DOE's Brookhaven National Laboratory's U.S. Department of Energy (DOE) Office of Science User Facility, the National Synchrotron Radiation Light Source II (NSLS-II), a team of researchers conducted a multi-technology X-ray study to learn more about In this case, the structural and chemical evolution of a metal sulfide additive - copper sulfide (CuS) - moves as lithium ions move between the battery electrodes. Their work is an example of operational research, a method that allows researchers to gather structural and chemical information while measuring electrochemical activity. The team used a "multimodal" 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 X-ray absorption spectroscopy to track chemical reactions.

　　Discover better performance additives

　　Among metal sulfide additives of choice, CuS is advantageous for several reasons, including its high conductivity and energy density. In previous research, the team found that adding CuS to an electrode containing only sulfur increased the battery's discharge capacity because sulfur is a poor conductor and CuS has better electrical conductivity and electrochemical activity. However, when a mixed sulfur/CuS cathode (positive electrode) is used, Cu ions are dissolved in the electrolyte and eventually deposited on the lithium anode (negative electrode), destroying the interface layer between the anode and electrolyte. This can cause the battery to fail after a few charge and discharge cycles.

　　"This observation represents a design challenge for multifunctional electrodes: When introducing new components with desirable properties, parasitic reactions can occur and thwart the original design intent," said Gan Hong, a scientist in Brookhaven's Sustainable Energy Technologies Division.

　　He continued: "To address the specific issues of Li-S batteries with CuS additives, and to guide future designs of electrodes, we need to better understand 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 system evolution but also uses multiple complementary synchrotron techniques to provide a more comprehensive view of many aspects of the system," said another of the paper's authors Corresponding author Karen Chen-Wiegart, assistant professor in the Department of Materials Science and Chemical Engineering at Stony Brook University, who also serves on NSLS-II.

　　To achieve this goal, the team first designed a cell that is fully compatible with all three X-ray technologies and can be studied at NSLS-II's three different X-ray beamlines. Their design not only allows measurements to be made on both electrodes of the cell, but is also optically transparent, allowing researchers to perform in-line optical microscopy and alignment at the beamline.

　　"These properties are critical because they allow us to spatially resolve responses from different parts and multiple locations within the cell, which is one of our main research goals," Chen-Wiegart said.

　　Additionally, team members Ke Sun (Brookhaven's Sustainable Energy Technologies Division), Chonghang Zhao, and Chenghong Lin (both from Stony Brook University) noted that their versatile and simple design, using economical parts, can build many cells for each A synchrotron experiment greatly facilitated their research. Sun, Zhao and Lin worked together to develop a multi-mode field battery pack cell. In addition, the team of scientists designed a multi-battery holder that allows several batteries to be cycled simultaneously and measured continuously.

　　This comprehensive approach requires a research team of experts from diverse backgrounds. Scientists from Brookhaven's Sustainable Energy Technologies Division and Stony Brook University worked closely with NSLS-II scientists. Together with scientists Jianming Bai and Eric Dooryhee, they used operando X-ray powder diffraction (XPD) to study the structural evolution of the hybrid electrode as it discharges. NSLS-II's XPD beamline is an effective tool for studying battery reactions, including Li-S cells, where it is used to capture the reaction time between lithium and CuS relative to its reaction with sulfur. The XPD data also indicate that the reaction product is not crystalline, as shown by the lack of diffraction peaks.

　　The team turned to operando X-ray absorption spectroscopy (XAS) using the Inner Shell Spectroscopy (ISS) beamline, working with NSLS-II scientists Eli Stavitski and Klaus Attenkofer. The XAS data indicate that after the cell is fully discharged, the CuS has been converted to a species with a Cu 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, the scientists, assisted by scientists Garth Williams and Juergen, performed operational X-ray fluorescence at the Submicron Resolution X-ray Spectroscopy (SRX) beamline ( XRF) microscopy of THIEME. XRF imaging identifies elements in a sample by measuring the X-ray fluorescence emitted when the sample is excited by a primary X-ray source. In this case, it allowed the team to image the distribution of elements in the cell and how and when the distribution evolved. This information can be correlated with chemical and structural evolution data obtained from XPD and XAS studies.

　　put it together

　　When the findings from each X-ray technique are evaluated overall, the picture forms - albeit a complex one - formed by the evolution of the crystalline phase of the sulfur-CuS hybrid electrode and how CuS dissolves during battery discharge. During the first part of the discharge, the sulfur in the cathode is completely consumed and appears to be converted into soluble lithium polysulfides, such as LiS3, LiS4, etc., up to LiS8. Next, the polysulfide is then converted into amorphous Li2S2, which is then further converted into crystalline Li2S. This lithiation of sulfur stops at the end of the complete expulsion mark. At this time, CuS begins to lithiate, forming amorphous Cu/S species.

　　CuS interacts strongly with some polysulfide species. Cu ions dissolve into the electrolyte where they migrate from the cathode to the anode. On the surface of the anode, various copper species are deposited, and after a short time, the battery fails.

　　The above work provides a clear mechanism of how sulfur and copper sulfide interact inside Li-S batteries during discharge/charge cycles. The research team will use the multi-mode synchrotron method developed in this paper to study the cycling mechanisms of other battery systems. Research on multifunctional conductive additives for lithium-sulfur batteries mainly focuses on other more stable transition metal sulfides, such as titanium disulfide (TiS2), which does not release Ti ions in the electrolyte during the battery discharge/charge process.

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