Professor Zhang Qiang of Tsinghua University in Chem: A New Breakthrough in Electrolytes for Sodium-Metal Anodes!
Release Date:
2020-08-05 14:31
Source:
[Research Background]
Lithium (Li) and sodium (Na) metal batteries have attracted considerable attention due to their high energy density. Compared with lithium metal batteries, sodium is more abundant and cost-effective, making sodium metal batteries promising for large-scale energy storage applications. However, unlike conventional carbon anodes, metallic sodium’s high reactivity and propensity for dendrite formation impose stricter requirements on the electrolyte used.
Currently, the role of cationic additives in electrolyte formulations remains incompletely understood. Therefore, investigating the functions of cationic additives in electrolytes—such as their electrostatic shielding effects, their ability to modulate the solvation structure of the electrolyte, and their impact on electrolyte stability—is of great significance. Based on these insights, fundamental principles for the rational design of cationic additives can be established, thereby enabling the development of batteries that are both stable and safe.
[Article Summary]
Recently, Professor Zhang Qiang’s team at Tsinghua University published their latest research in Chem, titled “Ion-Solvent Chemistry-Inspired Cation-Additive Strategy to Stabilize Electrolytes for Sodium-Metal Batteries,” proposing a rational and feasible strategy for stabilizing sodium-metal-battery electrolytes through the use of cation additives. Through first-principles calculations and molecular dynamics simulations, they demonstrated three key principles: the electrode potential upon introduction of a cation, the lowering of the lowest unoccupied molecular orbital energy level of the solvent after coordination with the cation, and the enhanced strength of cation–solvent interactions. This well-established cation-additive strategy opens up new opportunities for the rational design of stable and safe electrolytes for sodium-metal batteries.
[Article Interpretation]
1. Thermodynamic Analysis
Given that the electrode potential of cationic additives should be lower than that of the sodium metal anode, the authors recommend using Li+, K+, and Ca2+. Taking into account both the lowering of the LUMO energy level and the binding energy, Li+ emerges as the optimal candidate. The reduction potential of Li+ is −0.08 V vs Na/Na+, and this potential will remain virtually unchanged if the concentration of the additive is much lower than that of the working ions in a practical battery. Furthermore, Li+ exhibits a stronger binding affinity for DME molecules than Na+, making it less prone to desolvation. Consequently, Li+ cannot be reduced at the sodium metal anode. More importantly, the extent to which Li+ lowers the LUMO level of DME is smaller than that of Na+, thereby helping to stabilize the electrolyte solution.
Figure 1. Thermodynamic analysis of cationic additives.
2. Molecular Dynamics Simulation
Molecular dynamics simulations were further employed to investigate the influence of lithium ions on electrolyte properties. To quantitatively analyze the solvation structure, radial distribution function (g(r)) analysis was conducted, and the coordination numbers of Na+ with O and F in different electrolytes were summarized.
In addition to the solvation structure, the transport properties of cations in the electrolyte were further analyzed by calculating the mean squared displacement. Introducing cation additives at a given concentration proves more effective than adding an equivalent amount of Na+ in maintaining the diffusion coefficient of Na+ in the electrolyte; moreover, by precisely tuning the concentration of cation additives, the negative electrode–electrolyte interface can be optimized while keeping any adverse effects on the bulk electrolyte within a controllable range.
Figure 2. Molecular dynamics simulation analysis of cationic additives.
3. Finite Element Simulation and Experimental Validation
To demonstrate the electrostatic shielding effect of cationic additives on sodium deposition, finite-element simulations and in-situ optical microscopy were conducted. The cationic additives exert an electrostatic shielding effect on sodium deposition, thereby suppressing the growth of sodium dendrites.
Figure 3: Finite element simulation and in-situ optical microscopy observation.
4. Electrochemical Performance
Further electrochemical tests were conducted to demonstrate the practical performance of lithium-ion additives in sodium-metal batteries. The results show that the use of Li+ additives can significantly enhance the Coulombic efficiency after 200 cycles. More importantly, Li+ additives markedly reduce voltage hysteresis. In addition, Li+ additives have a pronounced effect on Na stripping behavior.
Figure 4. Electrochemical testing of the cationic additive strategy.
Figure 5 Schematic illustration of the effect of cationic additives. (A) Electrolyte without cationic additives: vigorous gas evolution occurs, and the anode exhibits a dendritic morphology. (B) Electrolyte with cationic additives: due to the electrostatic shielding effect, gas evolution is partially suppressed, resulting in a smooth anode surface.
【Conclusion】
In summary, based on ion–solvent chemistry and employing first-principles calculations, molecular dynamics simulations, finite-element modeling, in situ optical microscopy, and electrochemical testing, we have developed a cationic additive strategy for stabilizing the electrolyte–anode interface in sodium-metal batteries. We have proposed three design principles for cationic additives: electrode potential, reduction of the solvent’s lowest unoccupied molecular orbital (LUMO) energy level, and binding energy with the solvent. Consequently, this cationic additive strategy offers a new opportunity for the rational design of electrolytes, thereby enabling the construction of safe and stable batteries.
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