How Dipole Moment Affects Lithium Metal Anodes

How Dipole Moment Affects Lithium Metal Anodes
First Author: Li Pengcheng
Corresponding Author: Li Ge
Affiliation: University of Alberta, Canada
[Research Background]
Current lithium-ion batteries rely on intercalation chemistry, achieving an energy density of about 250 Wh kg-1. However, there is a continuous demand for higher energy density (500 Wh kg-1), especially for electric vehicles and even electric aircraft. One potential method to achieve this goal is to replace graphite with metallic lithium as the anode, which increases the theoretical capacity from 372 mAh g-1 to 3860 mAh g-1. Notably, metallic lithium has the lowest working voltage (compared to the standard hydrogen electrode at -3.04 V), maximizing the energy density of the anode. Despite these advantages, the practical application of lithium metal as an anode faces numerous challenges due to safety issues (fire, explosion, etc.) and poor reversibility.
[Work Introduction]
Recently, Professor Li Ge’s team at the University of Alberta proposed guidelines for selecting solvents for lithium metal anodes aimed at achieving high coulombic efficiency while minimizing corrosion. This work reveals for the first time the significant impact of solvent molecular dipole moments and orientations on the reversibility and corrosion of lithium metal. Solvents with large dipole moments tend to adsorb more readily on the surface of lithium metal, affecting the solid electrolyte interface and the corrosion behavior of lithium metal. Utilizing this principle, the researchers demonstrated that using LiNO3 as the sole salt in NCM811 (20mg cm-2) / Li (50 μm) full cells can achieve excellent cycling stability. This work connects the molecular structure of solvents with the reversibility and corrosion of lithium metal, concepts that can be extended to other batteries using metals as anodes. The results were published in “Dipole Moment Influences the Reversibility and Corrosion of Lithium Metal Anodes” in Advanced Materials. PhD student Li Pengcheng is the first author.
[Content Description]
The design and development of electrolytes is a key aspect in the development of high energy density lithium metal batteries, and is currently a hot research topic. However, most research in this field still relies on a “trial and error” approach to determine new electrolytes, highlighting an urgent need for a universal strategy to guide electrolyte design principles. In recent years, “solvated structures” have gained widespread attention in liquid electrolytes. However, for highly reactive alkali metal (Li, Na, K) anodes, the double-layer structure at the solid-liquid interface and the orientation of molecules on the metal surface may be more critical than the bulk solvated structure. While the double layer in aqueous solutions has been well studied, there is still a lack of comprehensive research on the electric double layer in organic solutions. Therefore, deepening the understanding of interface adsorption and the influence of different particle orientations near the metal surface is crucial.
Extending the cycle life of lithium metal batteries has been a major focus of many studies; however, the corrosion of lithium metal in electrolytes has received little attention. Reports indicate that lithium metal continues to corrode during contact with the electrolyte; unfortunately, no effective methods have yet been found to suppress this corrosion. Recent studies suggest that controlling stacking pressure can inhibit the chemical corrosion of metallic lithium; however, research to understand the corrosion behavior of lithium metal at the atomic and molecular levels is still lacking. Therefore, there is an urgent need to establish guidelines to understand and reduce the corrosion of metallic lithium from a microscopic perspective. Such principles would greatly contribute to the development of lithium metal batteries with extended calendar life.
In this study, the solvent’s dipole moment is highly correlated with the reversibility and corrosion of lithium metal. The research found that high dipole moment solvents preferentially adsorb on the surface of lithium metal. This preferential adsorption and the corresponding orientation of the adsorbed molecules significantly affect the solid electrolyte interface (SEI) and the corrosion behavior of lithium metal. Based on this principle, an electrolyte with LiNO3 as the sole lithium salt was designed. Using this electrolyte, under a N/P ratio of 2.5, LiFePO4/Li maintained a capacity retention of 93.4% after 150 cycles, while the NCM811 (20mg cm-2) / Li (50 μm) battery showed an 80.0% capacity retention after 170 cycles. More importantly, through the rational design of solvent molecules, particularly by introducing fluorine atoms on the positively charged side of the molecule, the corrosion of metallic lithium can be effectively mitigated. These findings are significant for the rational design of lithium metal electrolytes. Furthermore, these results provide new insights for future research aimed at improving the cycling stability and corrosion resistance of lithium metal batteries.
How Dipole Moment Affects Lithium Metal Anodes
Figure 1: Theoretical calculations and electrochemical performance of lithium metal anodes.
How Dipole Moment Affects Lithium Metal Anodes
Figure 2:Solvation structure of lithium ions and the experimental and theoretical study of structure-performance correlation.
How Dipole Moment Affects Lithium Metal Anodes
Figure 3: Study of lithium metal deposition behavior and solid electrolyte interface (SEI).
How Dipole Moment Affects Lithium Metal Anodes
Figure 4: Electrochemical performance and interface characterization of lithium metal full cells.
How Dipole Moment Affects Lithium Metal Anodes
Figure 5: Study of the corrosion behavior of lithium metal and the molecular orientation at the interface.
[Conclusion]
In this study, the researchers investigated the impact of solvent dipole moments on the reversibility and corrosion of metallic lithium. The results indicate that solvents with high dipole moments tend to preferentially adsorb on the surface of lithium metal. Additionally, the researchers observed that the dipole moment and orientation of solvents significantly influence the corrosion behavior of metallic lithium. Interestingly, the researchers noted that fluorine atoms located on the positively charged side of the solvent can mitigate corrosion. Based on these fundamental concepts, the researchers demonstrated that using LiNO3 as the sole salt can achieve good cycling performance for cathodes such as LiFePO4 or NCM811 under low N/P ratios. The researchers believe that the results of this work will provide valuable insights for future solvent designs aimed at achieving long cycle life and calendar life in lithium metal batteries.
P.-C. Li, Z.-Q. Zhang, Z.-W. Zhao, J.-Q. Li, Z.-X. Xu, H. Zhang, G. Li, Dipole Moment Influences the Reversibility and Corrosion of Lithium Metal Anodes. Adv. Mater. 2024.
https://doi.org/10.1002/adma.202406359
Author Biography
Professor Li Ge is an independent PI in the Department of Mechanical Engineering at the University of Alberta, Canada. His research focuses on lithium metal batteries, lithium-sulfur batteries, zinc-ion batteries, next-generation battery technologies, and electrocatalysis. He has led or participated in multiple R&D projects on energy storage battery electrode materials and their critical technologies, designing and synthesizing catalytic materials, and has published over 50 papers, including those as corresponding/first author in journals such as Nat. Commun, Angew. Chem, Adv. Mater, Energy Environ. Sci, Adv. Energy Mater, Adv. Funct. Mater, Nano Energy, etc.

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