With lithium-sulfur batteries (LiSBs)  becoming the focus of increasing research, a research team at Southern  University of Science and Technology (SUSTech) has found a new direction  in the electrolytes that could lead to their improved performance and  stability.

    Associate Professor Yonghong Deng (Materials Science and Engineering) has led her research team to publish their latest research progress in new lithium salts in the high-impact academic journal, Advanced Energy Materials (IF = 24.88). Their paper was titled, “New Lithium Salt Forms Interphases Suppressing Both Li Dendrite and Polysulfide Shuttling.” Nano Letters (IF = 12.7) published a second paper, under the title, “Lithophilic Zn Sites in Porous CuZn Alloy Induced Uniform Li Nucleation and Dendrite-free Metal Deposition.”

    The first paper was published in Advanced Energy Materials, under the title, “New Lithium Salt Forms Interphases Suppressing Both Li Dendrite and Polysulfide Shuttling.”

    LiSBs offer enormous promise as high-specific energy batteries,  primarily due to their high energy density. However, LiSBs experience  significant problems around the anode and cathode that hinder their  potential for commercial applications. Scholars have studied new  electrolyte additives and solvents, as well as using unsaturated or Li  salts containing fluorine (Fl) to solve the problems. The larger  challenge has been around the design and synthesis of Li-salt anions,  which has lacked significant scientific research. There are many  physical and chemical properties associated with Li-salt anions that  mean that they have not been effective thus far, so finding a Li-salt  that can form a stable electrolyte around both electrodes is vital.

    The research team developed a new unsaturated Li-salt (LiHFDF) that  would decompose on the surface of positive and negative electrodes to  form a LiF-rich electrolyte environment during charge-discharge cycles.  It suppressed the growth of Li-dendrites and the shuttle effect of Li₂Sn.

    Their findings showed a better LiF-rich electrolyte environment  around the electrodes, far more than found under solvents and additives.  Their testing under a transmission electron microscope (TEM) indicated  that their LiHFDF Li-salt would form a CEI film on the surface of both  electrodes during different stages of the charge-discharge cycle. They  were also able to show that their scanning electron microscope (SEM)  testing that the lithium plating that would occur was dense and plat,  preventing significant lithium dendrite growth.

    Further testing also showed improved charge-discharge cycle stability  and reduced battery polarization, which means increased stability from  the LiHFDF and reduced resistance. The LiSBs with LiHFDF and these new  batteries were shown to have significantly improved cycle stability and  coulomb efficiency (proportion of electrons supplied which are recovered  from storage), even as an additive.

    This research has developed a new Li-salt that can provide a more  stable and efficient lithium-sulfur battery. Such a development can be  applied for many lithium-ion battery systems around the world.

    The co-first authors were SUSTech-Peking University join-trained doctoral candidates Yingling Xiao and Bing Han. SUSTech Associate Professor Yonghong Deng and US Army Research Laboratory Dr. Kang Xu are the co-correspondent authors, with SUSTech as the first communication unit. Additional contributing units were the School of Advanced Materials at Peking University (PKU) and the Institute of Textile and Clothing (IKTC) at the Hong Kong Polytechnic University (PolyU). The research received support from the R&D Projects in Key Areas of Guangdong Province.

Paper link: https://onlinelibrary.wiley.com/doi/10.1002/aenm.201903937

    The paper, “Lithophilic Zn Sites in Porous CuZn Alloy Induced Uniform  Li Nucleation and Dendrite-free Metal Deposition,” published in Nano Letters  examined new techniques for minimizing lithium (Li) dendrites. It also  sought to regulate the infinite changes in volume lithium undergoes  during long-term repetitive stripping and plating cycles. These problems  occur during charge and discharge cycles, so being able to find a  solution to these problems would improve the effective use of Li-salts  in high-energy batteries.

    A lot of research has been done on the use of three-dimensional (3D)  lithiophilic hosts, as they are the most effective ways to regulate Li  dendrites and volume change in the working Li metal anodes. In  high-temperature environments, these 3D lithiophilic hosts melt or lose  their adhesive bonds under the thermal infusion method. Thus, finding a  3D lithiophilic host that can handle a high-temperature environment  while exploring an internal mechanism for homogenous Li deposition is  highly desirable to creating Li metal anodes.

    The research team used a copper-zinc (CuZn) alloy in a 3D porous host  to contain anchored lithiophilic Zn sites. This new approach ensures  that the host can handle a thermal melting high-temperature environment  under the thermal infusion strategy. It dispersed the Li nucleation  sites, which encouraged the uniform deposition of Li through the  stripping/plating cycle.

    On testing the new host, the researchers found that the Li host sites  absorb more than previous examples, based on the analysis of nucleation  overpotential, binding energy calculation and optical observation of Li  plating/stripping cycles. It shows that Li nucleation and dendrite-free  Li deposition can occur within the host while significantly improving  its electrochemical performance.

    The co-first authors were SUSTech postdoctoral researcher Shang-Sen Chi and research assistant Qingrong Wang. The co-correspondent authors were Hefei National Laboratory for Physical Sciences at the Microscale (University of Science and Technology of China) Professor Yan Yu and SUSTech Associate Professor Yonghong Deng. Additional contributors came from the Academy for Advanced Disciplinary Studies (AAIS) at SUSTech,  the Shenzhen Key Laboratory of Solid-State Batteries, the Guangdong  Provincial Key Laboratory of Energy Materials for Electric Power, the Research Institute of Materials Science at South China University of Technology (SCUT), Dalian National Laboratory for Clean Energy (DNL) at the Chinese Academy of Sciences (CAS), and the State Key Laboratory of Fire Science (SKLFS) at USTC.

    This research received support from the National Key R&D Program  of China, R&D Project in Key Areas of Guangdong Province, the  National Natural Science Foundation of China, the Fundamental Research  Funds for the Central Universities, Dalian National Laboratory For Clean  Energy (DNL) Cooperation Fund, the CAS, Shenzhen Key Laboratory of  Solid-State Batteries, Guangdong Provincial Key Laboratory of Energy  Materials for Electric Power, and the China Postdoctoral Science  Foundation.

    Article link: https://pubs.acs.org/doi/10.1021/acs.nanolett.0c00352