In February 2024, the research group led by Professor Yu Yan from the University of Science and Technology of China published a paper online in the journal Angewandte Chemie International Edition (Impact Factor: 16.1) titled ‘Tailoring Na⁺ Solvation Environment and Electrode–Electrolyte Interphases with Sn(OTf)₂ Additive in Non-flammable Phosphate Electrolytes towards Safe and Efficient Na–S Batteries.’ This study ‘effectively regulated and improved the Na⁺ solvation environment and the chemical properties of the electrode-electrolyte interface by introducing a tin triflate (Sn(OTf)₂) additive into a phosphate-based electrolyte (1M NaTFSI-TEP-FEC), thereby enhancing the cycle stability of RT Na-S batteries.
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Research Background
Room-temperature sodium-sulfur (RT Na-S) batteries, known for their high energy density, low cost, and environmental friendliness, are one of the ideal choices for large-scale energy storage applications. However, the traditional electrolytes used in sodium-sulfur batteries are typically highly flammable, leading to safety concerns that limit their practical application. Phosphate-based flame-retardant solvents have shown advantages such as a wide operating temperature range, high dielectric constant, broad electrochemical window, and low viscosity. However, they exhibit poor interfacial compatibility with both the sodium anode and sulfur cathode. While these solvents enhance the safety of the battery, they also lead to a decline in electrochemical performance. Therefore, designing phosphate-based flame-retardant electrolytes that have good compatibility with both the sodium anode and sulfur cathode remains a significant challenge.
Research Question
Key Point 1: Solvation Structure Analysis
Theoretical calculations indicate that after introducing the Sn(OTf)₂ additive, the primary solvation structure of the electrolyte changes from Na⁺-1FEC-3TEP-1TFSI to Na⁺-1FEC-2TEP-2TFSI⁻, which helps to reduce the desolvation energy barrier of Na⁺. Additionally, Na⁺-1FEC-2TEP-2TFSI⁻ possesses a higher LUMO energy level (0.73 eV), which promotes the preferential decomposition of free fluoroethylene carbonate (FEC) to form a stable solid electrolyte interphase (SEI). This, in turn, inhibits the decomposition of triethyl phosphate (TEP), enhancing the stability of the electrolyte and its compatibility with sodium.
Figure 1. Solvation Structure Analysis of the Electrolyte.
Key Point 2: Sn(OTf)₂ Additive Suppresses Polysulfide Dissolution Shuttling
Molecular dynamics results demonstrate that the S₄²⁻ anions can be uniformly distributed in the base electrolyte (1M NaTFSI-TEP-FEC). However, in the electrolyte with Sn(OTf)₂ added, the Sn²⁺ cations interact with the S₄²⁻ anions, leading to the presence of S₄²⁻ in quasi-solid clusters rather than as solvent-separated ion pairs. Compared to the base electrolyte, the electrolyte with Sn(OTf)₂ additive significantly reduces the solubility and diffusion rate of sodium polysulfides (NaPSs), thereby suppressing the shuttling effect of NaPSs.
Figure 2. Study of Sodium Polysulfide Diffusion Behavior in the Electrolyte.
Key Point 3: Sn(OTf)₂ Additive Induces Formation of Na₁₅Sn₄-Rich SEI on Sodium Anode, Accelerates Na⁺ Diffusion, and Promotes Uniform Sodium Deposition
Ignition tests confirmed that the designed phosphate-based electrolyte containing the Sn(OTf)₂ additive has excellent flame-retardant properties. XPS and HRTEM further demonstrated that the introduction of Sn(OTf)₂ additive induces the formation of an SEI rich in Na₁₅Sn₄ on the sodium anode surface. Finite element simulations revealed that the Na₁₅Sn₄-rich SEI exhibits faster Na⁺ diffusion kinetics and higher Na⁺ diffusion flux, which can effectively alleviate Na⁺ depletion on the sodium anode surface, suppress dendrite growth, and promote uniform and dense sodium deposition.
Figure 3. Flame Retardancy Testing of the Electrolyte and Characterization of the Sodium Anode Interface.
Key Point 4: Sn(OTf)₂ Additive Induces Formation of Sn and NaSn Alloy-Rich CEI Film on Sulfur Cathode, Suppresses Polysulfide Shuttling
Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and HRTEM have confirmed that the CEI (cathode electrolyte interphase) on the sulfur cathode is rich in Sn and NaSn alloy. Theoretical calculations indicate that Sn and NaSn alloys exhibit significant adsorption energy towards NaPSs, effectively inhibiting their dissolution and shuttling behavior. With the modification using Sn(OTf)₂ additive, the resulting RT Na-S batteries show a substantial increase in reversible capacity from 322 mAh g⁻¹ to 906 mAh g⁻¹ after 100 cycles at a current density of 0.5 A g⁻¹. This study provides guidance for the design and development of new high-safety, high-performance RT Na-S batteries.
Figure 4. Electrochemical Performance and Interface Composition Analysis of RT Na-S Batteries.
Research Summary
In summary, guided by theoretical calculations, we developed a novel Sn(OTf)₂ additive based on TEP-based flame-retardant electrolytes to enhance the cycling stability of RT Na-S batteries by modulating the Na⁺ solvation environment and interface chemistry. On one hand, the Sn(OTf)₂ additive reduces the desolvation energy barrier of sodium ions and improves the stability of the electrolyte. On the other hand, it promotes the formation of Na-Sn alloy-based SEI on the anode and the construction of the cathode electrolyte interface, inhibiting dendrite growth and the dissolution and shuttling of NaPSs, thereby improving the cycling stability of RT Na-S batteries. Consequently, with the introduction of Sn(OTf)₂ additive, the specific capacity of RT Na-S batteries increased from 322 mAh g⁻¹ to 906 mAh g⁻¹ after 100 cycles at 0.5 A g⁻¹.
Original link: https://onlinelibrary.wiley.com/doi/10.1002/anie.202320060
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