### Research Team at Tsinghua University's Department of Chemical Engineering Achieves Significant Progress in Lithium Battery Electrolyte Design Lithium-ion batteries have played a pivotal role in advancing the智能化, 便携化, and 多元化 of modern society, significantly enhancing the quality of human life. The 2019 Nobel Prize in Chemistry was awarded to three scientists for their contributions to lithium-ion battery technology. However, the theoretical energy density limitations of conventional lithium-ion batteries pose a significant challenge in meeting the future demands of society. Developing next-generation lithium batteries with metal lithium anodes is considered the ultimate solution for high-energy-density battery systems. #### Challenges and Solutions in Metal Lithium Anode Utilization The use of metal lithium anodes introduces new challenges to the overall battery design, with the most critical being the design of electrolytes and their interfaces. Metal lithium has an extremely low electrode potential and strong reducing properties, leading to intense reactions at the anode interface. These reactions can cause electrolyte dry-out, leading to battery failure, and more critically, the decomposition of electrolytes can produce flammable gases, posing safety risks. Understanding the solvation structure of electrolytes and the relationship between their structure and function is essential for designing more stable and efficient electrolytes, which can suppress the anode interface reactions and stabilize the metal lithium anode, thereby enabling the practical application of lithium metal batteries. #### Theoretical Insights and Innovations Professor Zhang Qiang's team at Tsinghua University's Department of Chemical Engineering has made significant strides in this area by leveraging lithium bond chemistry theory. They have gained a deep understanding of the mutual interactions among electrolyte components and their impact on electrolyte properties, leading to the rational design of highly efficient electrolyte systems. The team's research has resulted in several original contributions. In electrolytes, the microscopic interactions can be categorized into those between lithium ions, solvent molecules, and salt anions. These interactions not only determine the structure of the electrolyte and influence its physical and chemical properties but are also regulated by solvation effects. Specifically, solvation through the dielectric constant affects the interaction forces between ions and dipoles, thereby modulating the strength of interactions between lithium ions and solvent molecules or anions. The varying influence of the dielectric constant on different interaction forces provides a means to control the microscopic interactions within the electrolyte. #### The Concept of "Lithium Bonds" Similar to hydrogen bonds in aqueous solutions, the team introduced the concept of "lithium bonds" to understand the microscopic interactions between lithium ions and electrolyte components and electrode materials. Lithium bonds, which lack saturation and directionality, can form various cluster structures, offering more possibilities for electrolyte design. These bonds can regulate the interactions between lithium ions and solvent molecules, influencing the solvation and desolvation processes of lithium ions and altering their transport and transformation properties during charging and discharging. Additionally, lithium bonds can modulate the interactions between lithium ions and anions, affecting the solubility of different lithium salts in the same solvent and the stability of the solid electrolyte interphase (SEI) and cathode-electrolyte interphase (CEI) layers. #### Innovative Electrolyte and Interface Design Based on these insights, the team has proposed several electrolyte and interface design strategies to stabilize the metal lithium anode. For example, introducing fluoroethylene carbonate (FEC) molecules, which have a higher affinity for lithium ions, into the solvation shell can reduce the desolvation energy barrier, lowering the polarization during lithium ion deposition and extraction. The FEC molecules preferentially decompose on the metal lithium surface, forming a SEI rich in lithium fluoride (LiF), which reduces the diffusion energy barrier of lithium ions through the SEI and promotes uniform lithium deposition. Another strategy involves incorporating nitrate ions into the lithium ion solvation shell, which can form larger solvation clusters and promote the decomposition of FSI-anions, creating a LiF-rich interface layer that broadens the stability window of the electrolyte. The team has also explored the synergistic effects of FEC and lithium nitrate, forming a fluorine-nitrogen SEI that reduces interfacial resistance and adapts to the interface evolution during the cycling of metal lithium, maintaining the SEI's structure and properties. This approach has been successfully applied in pouch cell batteries. #### General Applicability and Future Directions The principles governing lithium battery electrolytes are applicable to other secondary battery systems, such as sodium metal batteries. Given the higher electrode potential of sodium metal compared to lithium metal, adding lithium ion additives to the electrolyte can stabilize the interface between the electrolyte and the metal sodium. The lithium ions accumulate at the surface of sodium metal, forming an electrostatic shielding layer that inhibits the growth of sodium dendrites. #### Comprehensive Review and Machine Learning Integration The team's deep understanding of the solvation chemistry in lithium battery electrolytes is encapsulated in a comprehensive review published in *Accounts of Chemical Research* titled "Atomic Insights into the Fundamental Interactions in Lithium Battery Electrolytes." This review summarizes the interactions among lithium ions, solvent molecules, and anions, providing atomic-level insights into the general principles of electrolyte design. The team also proposes combining lithium bond chemistry theory with machine learning methods to accelerate the development and design of electrolytes. #### Research Impact and Recognition The research papers were co-authored by Professor Zhang Qiang and led by Tsinghua University Ph.D. student Chen Xiang. Professor Zhang Qiang's research group focuses on energy materials chemistry and engineering. Their work aims to develop high-capacity-density electrode materials and energy chemistry principles to create high-energy-density storage systems. The team has extensively studied lithium-sulfur batteries, which rely on multi-electron chemistry to generate energy, and has proposed lithium bond chemistry and ion-solvent complex concepts. They have developed various high-performance energy materials, including solid electrolyte interphase-protected lithium anodes and carbon-sulfur composite cathodes, and have constructed lithium-sulfur pouch cell devices. The group has also explored "lithium-friendly" chemistry to control the deposition behavior of metal lithium through advanced techniques, such as introducing nano-skeletons and surface modification layers. These efforts have been published in prestigious journals such as *Advanced Materials*, *Journal of the American Chemical Society*, *Angewandte Chemie International Edition*, *Energy Storage Materials*, *Chem*, *Joule*, *Nature Communications*, and *Proceedings of the National Academy of Sciences*. Furthermore, the team has filed a series of Chinese and PCT patents related to lithium-sulfur batteries and metal lithium anodes. #### Conclusion The research conducted by Professor Zhang Qiang's team at Tsinghua University represents a significant advancement in the field of lithium battery electrolytes. By understanding and leveraging the principles of lithium bond chemistry, the team has developed innovative electrolyte and interface designs that enhance the stability and efficiency of metal lithium anodes. These findings not only contribute to the practical application of lithium metal batteries but also offer valuable insights for the development of other secondary battery systems. The integration of machine learning with lithium bond chemistry theory promises to further accelerate the design and optimization of electrolytes, paving the way for safer and more efficient energy storage solutions.