X-ray research provides new discoveries for lithium-sulfur batteries

　　Lithium-sulfur (Li-S) battery is a relatively new battery that is being researched and developed by researchers all over the world. Because of their very high theoretical energy density-more than five times the energy stored in a smaller capacity than the most advanced lithium-ion batteries-they are a strong contender for small and large applications.

　　Lithium-sulfur (Li-S) battery is a relatively new battery that is being researched and developed by researchers all over the world. Because of their very high theoretical energy density-more than five times the energy stored in a smaller capacity than the most advanced lithium-ion batteries-they are strong competitors for small and large applications.

　　However, before realizing practical applications, some performance problems must be solved-namely, poor conductivity and insufficient energy efficiency. These failures originate from the chemical substances and reactions within the battery, because the 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 battery sulfur electrodes. However, unique and unique behaviors have been observed for each type of metal sulfide in Li-S batteries. In order to understand the basic mechanisms of these different behaviors, scientists need to be able to closely study these complex reactions in real time while the battery is being discharged and charged. This is a challenge.

　　At DOE’s Brookhaven National Laboratory’s Office of Scientific User Facilities, National Synchrotron Radiation 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 the metal sulfide additive—copper sulfide (CuS)—as the lithium ions move between the battery electrodes. Their work is an example of operational research. This method allows researchers to collect structural and chemical information while measuring electrochemical activity. The team used a "multi-modal" approach involving a set of X-ray techniques: X-ray powder diffraction to collect structural information, X-ray fluorescence imaging to visualize changes in element distribution, and to track chemical reactions in X-ray absorption spectroscopy.

　　Explore better performance additives

　　"In the choice of metal sulfide additives, CuS is advantageous for several reasons, including its high conductivity and energy density. In previous research, the team found that adding CuS to electrodes containing only sulfur can increase the discharge capacity of the battery, because sulfur is a poor conductor, and CuS has better 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 the 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 new components with ideal performance are introduced, parasitic reactions may occur and hinder the original design intent," said Hong Gan, a scientist at Brookhaven's Sustainable Energy Technology Department.

　　He continued: "In order to solve the specific problems of Li-S batteries with CuS additives and to guide the future design of electrodes, we need to better understand the development of these systems in various ways: structurally, chemically and morphologically. "

　　Various modes and operations are performed

　　"We believe that it is necessary to develop a multi-modal approach that not only studies one aspect of system evolution, but also uses a variety of complementary synchrotron technologies to provide a more comprehensive view of many aspects of the system," another author of the paper Corresponding author Karen Chen-Wiegart, assistant professor in the Department of Materials Science and Chemical Engineering, Stony Brook University, who also holds a position at NSLS-II.

　　In order to achieve this goal, the team first designed a battery that is fully compatible with all three X-ray technologies and can be studied on three different X-ray beam lines of NSLS-II. Their design not only allows measurements on the two electrodes of the battery, but also has optical transparency, enabling researchers to perform online optical microscopy and alignment on the beamline.

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

　　In addition, team members Ke Sun (Brookhaven’s Sustainable Energy Technology Department), Zhao Chonghang and Lin Chenghong (both from Stony Brook University) pointed out that their multi-functional and simple design, using economical parts, can build many cells for each A synchrotron experiment greatly facilitated their research. Sun, Zhao, and Lin jointly developed a multi-mode field battery pack battery. In addition, the team of scientists designed a multi-battery holder that allows several batteries to be recycled at the same time and to continuously measure them.

　　This comprehensive approach requires a research team composed of experts from different backgrounds. Scientists from Brookhaven's Sustainable Energy Technology Department and Stony Brook University work closely with NSLS-II scientists. Together with scientists Jianming Bai and EricDooryhee, they used operando X-ray powder diffraction (XPD) to study the structural evolution of the hybrid electrode during discharge. The XPD beam line of NSLS-II is an effective tool for studying battery reactions, including Li-S batteries, which are used to capture the reaction time between lithium and CuS, relative to its reaction with sulfur. The XPD data also indicated that the reaction product was not crystalline, as indicated by the lack of diffraction peaks.

　　The team turned to operando X-ray absorption spectroscopy (XAS) using Inner Shell Spectroscopy (ISS) beamline, and collaborated with NSLS-II scientists Eli Stavitski and Klaus Attenkofer. The XAS data shows that after the battery is fully discharged, the CuS has been converted to a species’ Cu ratio and somewhere between CuS and Cu 2S. In order to further pinpoint the precise phase composition, the group will Perform additional XAS research in the future.

　　In order to observe the dissolution of CuS and its subsequent redeposition on the lithium anode, scientists, with the help of scientists GarthWilliams and Juergen, performed operational X-ray fluorescence ( XRF) Microscopic examination of THIEME. XRF imaging identifies the elements in the sample by measuring the X-ray fluorescence emitted when the sample is excited by the primary X-ray source. In this case, it allows the team to image the distribution of elements in the battery and how and when the distribution evolves. This information can be correlated with the chemical and structural evolution data obtained from XPD and XAS research.