Salt-In-Presalt Electrolyte Design for High-energy Sodium Batteries

Our present work introduces a new electrolyte design named “Salt-In-Presalt” (SIPS). This strategy maximizes the fluorinated anion functions at a low salt concentration and promotes inorganic electrolyte-electrode interphases to realize long-cycle performance for high-energy sodium batteries.
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The Origin of the Idea

The SIPS electrolyte design began with a simple question: could the solvent in the electrolyte not only function as a medium for ion solvation but also play a key role in stabilizing the electrode-electrolyte interfaces? Anions from the conducting salts have traditionally been viewed as the major passivation enablers in the electrolytes, the contribution from the organic solvent is very limited and highly overlooked.1 Fluorinated anions, such as sodium bis(fluorosulfonyl)imide (NaFSI) and sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), have been recognized for their ability to form stable, inorganic-rich interphases like sodium fluoride (NaF).2 These interphases are critical for suppressing dendrite formation on Na metal anodes and stabilizing high-potential cathodes. However, achieving these benefits typically requires high-concentration electrolytes in ether-based solvents due to the incompatibility between carbonate-based solvents and the Na metal anodes, which come with major drawbacks such as high viscosity, low conductivity, and high price. We sought to overcome these limitations by rethinking the role of solvents in the electrolyte.

Building the SIPS Concept

Our breakthrough came when we considered using the molecular precursors of NaFSI and NaTFSI salts, specifically N,N-dimethylfluoro-sulfonamide (PreFSI) and N,N-dimethyltrifluoromethane-sulfonamide (PreTFSI), as the solvent themselves. This approach allowed us to develop an electrolyte system entirely consisting of anion and anion-functionalized components, creating what we proposed as “Salt-In-Presalt” (SIPS) electrolytes (Figure 1).

Figure 1. The SIPS electrolyte design. Schematic representation of PreFSI and PreTFSI molecular construction, which can be viewed as the liquefaction of solid NaFSI and NaTFSI salts via chemical methylation. The use of salt precursor as the solvating medium will maximize the fluorinated anion functions at a low salt concentration, forming the “Salt-In-Presalt” electrolytes, which will be beneficial for NaF-rich interphase formation on various electrode surfaces.

The key innovation of SIPS lies in its ability to maximize the presence of fluorinated anion functions in the electrolyte at a low salt concentration. By dissolving 0.5 M NaFSI salt in PreTFSI solvent, we formulated a low-viscosity, high-conductivity electrolyte (SIPS5) that promotes the formation of NaF-rich interphases on both Na, hard carbon anodes and Ni-rich O3-NaNi0.6Mn0.2Co0.2O2 (NMC622), sulfur-based cathodes.3 The results were striking: In the Na||NMC622 cells (2.0 mAh cm-2, N/P = 1.7), SIPS5 enabled over 1000 cycles with a capacity retention of 89% at a cut-off voltage of 4.2 V. Even in “anode-less” configurations of Al||NMC622 cells, SIPS5 demonstrated a 200-cycle performance at a high cut-off voltage of 4.7 V, achieving a specific energy of 231 Wh kg-1. For sulfurized polyacrylonitrile (SPAN) cathodes, which are known for their high theoretical capacity but challenging cycle stability, SIPS5 formed robust NaF-rich interphases in the Na||SPAN cells (3.3 mAh cm-2, N/P = 1.7) that accommodated the material's volume changes during repeated charge/discharge process, resulting in 93% capacity retention for 1000 cycles, showing great promise for low-cost, sulfur-based sodium batteries.

Challenges and Insights

The development of SIPS was not without challenges and difficulties. Early tests showed that while PreTFSI is highly effective in stabilizing the sodium metal anodes compared to PreFSI, the solubility of NaFSI is very limited in PreTFSI solvent, which crystallizes easily in the 1.0 M electrolyte solution, isolating two solid crystals simultaneously (Figure 2A-2D). 

Figure 2. The solid crystal structure of NaFSI salt and NaFSI/PreTFSI (3:1 by mole) solid and liquid solvation of 0.5 M NaFSI in PreTFSI liquid electrolyte solution. (A) The asymmetry unit of NaFSI crystal shows the mono-capped octahedron, where six oxygen atoms facilitate the octahedral geometry with one nitrogen atom capped to the top. The O-η2- solvation mode of the FSI anion is highlighted in a red circle. (B) The close-packing structure of NaFSI crystal, in which the cationic layer with fluorine atoms is pushed aside. (C) The asymmetry unit of NaFSI/PreTFSI (3:1) solid structure with two octahedral solvated Na+ centers and one distorted trigonal bipyramidal site. All the FSI anion oxygen atoms adapt the O-η1- solvation mode as highlighted in the red circle. (D) The scattered packing layer in NaFSI/PreTFSI (3:1) crystal structure. (EF) Solvation of SIPS5 electrolyte. (E) Proposed solvation cluster {Na3(FSI)4(PreTFSI)4} in SIPS5 electrolyte, where four NaFSI is solvated by four PreTFSI molecules to have the dispersed cluster rather than the infinite sheets in the isolated crystals showing in B and D. (F) 2D {7Li–1H} (orange-cyan, below) HOESY contour plot of SIPS5 electrolyte, the inset shows the chemical shifts of 19F-NMR of SIPS5 electrolyte and PreTFSI molecule.

By fine-tuning the salt-to-solvent ratio, we minimized these sediment reactions and achieved optimal electrolyte performance at a NaFSI salt concentration of 0.5 M (SIPS5). Another rewarding aspect of this work was uncovering the unique solvation characteristics of the SIPS electrolyte from the two in-situ formed solid crystals (Figure 2A-2D). Unlike traditional electrolytes, where cations and anions are loosely coordinated and separated by organic solvents, the SIPS system forms a highly structured solvation shell through anion-functionalized solvents (Figure 2E-2F). This unique feature not only contributes to the formation of uniform, stable inorganic-rich interphases but also enhances the ionic conductivity of the electrolyte.4

Toward a Sustainable Future

Our work on SIPS5 represents a significant step forward in the development of high-energy sodium batteries. By leveraging the dual functionalities of anion functions as both solvents and interphase formers, we have created a versatile electrolyte platform that addresses the critical challenges for sodium batteries, where high potential, long cycle life, and improved operation safety can all be achieved at the same time.

We believe that our work will provide a deep understanding for electrolyte innovation, and bring inspiration to the electrochemical field for effective interphase construction on various high-energy electrodes in the near future, which will bring the low-cost sodium batteries one step further towards practical application.5

More details of this study can be found in our recent article "Salt-In-Presalt Electrolyte Solutions for High-Potential Non-aqueous Sodium Metal Batteries" published in Nature Nanotechnology.

Ai-Min Li, …et al. Chunsheng Wang* Salt-in-presalt electrolyte solutions for high-potential non-aqueous sodium metal batteries. Nat. Nano. (2025).

DOI: https://doi.org/10.1038/s41565-024-01848-2

Contributors: Ai-Min Li & Chunsheng Wang

Reference

1. Li, A. M. et al. High voltage electrolytes for lithium-ion batteries with micro-sized silicon anodes. Nat. Commun. 15, 1206 (2024).
2. Choudhury, S. et al. Designing solid-liquid interphases for sodium batteries. Nat. Commun. 8, 1–10 (2017).
3. Song, J. et al.  Interphases in sodium‐ion batteries. Adv. Energy Mater. 8, 1703082 (2018).
4. Wang, L. et al. Identifying the components of the solid–electrolyte interphase in Li-ion batteries. Nat. Chem. 11, 789–796 (2019).
5. Yao, A. et al. Critically assessing sodium-ion technology roadmaps and scenarios for techno-economic competitiveness against lithium-ion batteries. Nat. Energy. 1-13 (2025).

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Go to the profile of Thiyagaraj S
13 days ago

Fabulous work! I am very much interested in electrolyte-based work; recently I have also been working on the carbon-based electrolyte. Definitely, your work has really encouraged me more. With your permission, I would like to cite your paper for my future research work.

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