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Salt-in-presalt electrolyte solutions for high-potential non-aqueous sodium metal batteries

Nature Nanotechnology volume 20, pages 388–396 (2025)Cite this article

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Abstract

Room-temperature non-aqueous sodium metal batteries are viable candidates for cost-effective and safe electrochemical energy storage. However, they show low specific energy and poor cycle life as the use of conventional organic-based non-aqueous electrolyte solutions enables the formation of interphases that cannot prevent degradations at the positive and negative electrodes. Here, to promote the formation of inorganic NaF-rich interphases on both negative and positive electrodes, we propose the salt-in-presalt (SIPS) electrolyte formulation strategy. In SIPS, sodium bis(fluorosulfonyl)imide (NaFSI) salt is dissolved in the liquid precursor of the sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) salt, that is, N,N-dimethyltrifluoromethane-sulfonamide, called PreTFSI. The prepared 0.5 M NaFSI in PreTFSI (SIPS5) electrolyte solution shows an electrochemical stability up to 6.7 V versus Na|Na+ and enables a Na stripping/plating average Coulombic efficiency of 99.7% at 2.0 mA cm−2 and 4.0 mAh cm−2 in Na||Al cell configuration. By testing SIPS5 in Na metal and ‘anode-less’ coin and pouch cell configurations using NaNi0.6Mn0.2Co0.2O2 or sulfurized polyacrylonitrile as positive electrode active materials, we demonstrate the ability of the SIPS strategy to deliver improved specific discharge capacity and capacity retentions at high cell potentials and moderate applied specific currents for cell cycle life up to 1,000 cycles.

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Fig. 1: High-energy SMBs enabled by electrolyte design.
Fig. 2: The SIPS electrolyte design.
Fig. 3: Physicochemical characterizations of the SIPS5 and EE electrolyte solutions and SEI formed on negative electrodes.
Fig. 4: Na plating/stripping in Na metal asymmetric and symmetric cells with SIPS5 and EE electrolytes at 23 ± 3 °C.
Fig. 5: Na|SIPS5|NaNi0.6Mn0.2Co0.2O2 (NMC622) battery testing.
Fig. 6: Battery performance of Na metal cells with SPAN-based positive electrodes.

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Data availability

The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers CCDC 2298530 (neat NaFSI) and CCDC 2298531 (NaFSI/PreTFSI, 3:1). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. The authors declare that the data supporting the findings of this study are available within the article and its Supplementary Information files. Source data are provided with this paper.

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Acknowledgements

This work is supported by the US Department of Energy (DOE) Office of Electricity (OE) under contract DE-AC06-76LO1830 through the Pacific Northwest National Laboratory (70247) (C.W. and X.L.).

Author information

Authors and Affiliations

  1. Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, MD, USA

    Ai-Min Li & Chunsheng Wang

  2. Department of Chemistry and Biochemistry, University of Maryland, College Park, MD, USA

    Peter Y. Zavalij

  3. Energy & Environment Directorate, Pacific Northwest National Laboratory, Richland, WA, USA

    Fred Omenya & Xiaolin Li

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  1. Ai-Min Li
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  4. Xiaolin Li
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  5. Chunsheng Wang
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Contributions

A.-M.L. and C.W. conceived the idea for the project and wrote the paper. P.Y.Z. helped with the single-crystal data collection and analysis. F.O. and X.L. helped with the NMC622-based positive electrode preparation and casting.

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Correspondence to Chunsheng Wang.

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Nature Nanotechnology thanks Seung Woo Lee and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 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. (E–F) 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.

Extended Data Fig. 2 Linear sweep voltammetry (LSV) anodic scanning in Na||Al cells at a scan rate of 0.5 mV s-1 for SIPS5 (magenta) and EE (blue) electrolytes (23 ± 3 ˚C).

The LSV scan data was shown with different current density resolutions for comparison, note that the current density unit on the y-axis for (A) is mA cm-2, and for (B) is μA cm-2.

Extended Data Fig. 3 Morphology and composition of CEI formed in the NMC622-based positive electrode after 100 charge/discharge cycles in SIPS5 and EE electrolytes.

(A, B) Ex situ HRTEM images of NMC622-based positive electrode after 100 cycles at 23 ± 3 ˚C (50 mA g-1) in EE (2.0–4.2 V) (A) and SIPS5 (2.0–4.7 V) (B) electrolytes. Yellow dashed lines indicate the interfaces between the CEI and NMC622 bulk phase or the surface transition region of the NMC622 particle. (C–D) Ex situ C1s XPS spectra of the cycled NMC622-based positive electrodes cycled in (C) EE or (D) SIPS5 electrolyte solutions. The XPS raw data were fitted with CasaXPS software with binding energy calibrated at 284.8 eV of C1s. Shirley BG type was used for background subtraction and GL(30) line shape was used for peak fitting. (E–H) Ex situ SEM micrographs of NMC622-based positive electrodes cycled in EE (E) and SIPS5 (F) electrolyte solutions; Schematic representation of the CEI role on the stability of positive electrode active material particles (G, H). Thick, organic-rich EE-derived CEI cracks at the NMC622 particle surface during the sodiation/desodiation process (G), while thinner NaF-rich SIPS5-derived CEI is stable and effectively protects NMC622 particles from degradation (H).

Supplementary information

Supplementary Information

Supplementary Notes 1–7, Tables 1–7, Figs. 1–35 and references.

Supplementary Data 1

Crystal data for NaFSI.

Supplementary Data 2

Crystal data for NaFSI/PreTFSI (3:1).

Source data

Source Data Fig. 3

Statistical source data for Fig. 3a.

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Li, AM., Zavalij, P.Y., Omenya, F. et al. Salt-in-presalt electrolyte solutions for high-potential non-aqueous sodium metal batteries. Nat. Nanotechnol. 20, 388–396 (2025). https://doi.org/10.1038/s41565-024-01848-2

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