Molecular Electrolyte Design Enables Sodium Batteries with LIBs-parity Energy-Density and Improved Sustainability

The Eureka Moment: Methylation as an Alternative to Fluorination

The long-term paradox in the field is: ether-based electrolytes provide excellent reversibility in metal anode plating/stripping but fail to withstand high-voltage cathodes, whereas carbonate- and fluorinated ether-based electrolytes that tolerate high voltages compromise metal anode Coulombic efficiency (CE) and sustainability. To enable anode-free sodium metal batteries (SMBs) to compete with lithium-ion batteries (LIBs) in terms of energy density, electrolytes must simultaneously achieve >99.9% Na CE and >4.3 V anodic stability, which is challenging for existing electrolyte chemistries. Our initial investigation involved using fluorinated ether solvents to improve the anodic stability of the electrolytes. However, we quickly encountered an unsolvable limitation: fluorinated groups increased the solvents’ reduction potentials, resulting in organic-rich solid electrolyte interphase layers that significantly reduced Na CE and promoted Na dendrite formation.

Our previous report on lithium metal batteries utilized methylation instead of fluorination to modify ether solvents, where the electron-donating methyl groups with spatial hindrance could protect α-H atoms and fine-tune electrolyte solvation. [1] However, the solvation behavior and electrochemical reactivity between Na and Li batteries are quite different. On the one hand, the salt dissociation is totally different for Na and Li salts. For example, LiPF₆ is found not compatible in dimethoxyethane (DME), while NaPF₆/DME makes a clear electrolyte solution. On the other hand, the redox pair of Na/Na⁺ is 0.3V higher than that of Li/Li⁺, enabling NaPF₆/DME electrolyte for a Na CE of 99.9%. [2] However, the upshift of the Na/Na⁺ redox pair also brings additional challenges to the cathode stability of the electrolytes. [3]

Figure 1. Electrolyte design strategy and solvation. (a) The electrolyte design route and principle, with key properties of the solvents highlighted under the molecule. b-c, MD simulation snapshots highlighting NaPF₆ as cyan isosurfaces (b) and nano-domain formation for DEE/Na⁺ as blue isosurface and DBE as yellow isosurface for DEBE (c).

Building on these insights, we paired fully terminal-methylated DME, 1,2-di-tert-butoxyethane (DBE), with 1,2-diethoxyethane (DEE) in a 1:1 volume ratio, loaded with 1.0 M NaPF₆ to develop the DEBE electrolyte. This formulation balanced salt solubility, ionic conductivity, and interfacial stability without using fluorinated solvents (Figure 1a). The DEBE electrolyte showed a distinctive solvation structure: DEE formed Na⁺-rich nano-domains facilitating ion transport, while DBE served as a stable diluent (Figure 1b-1c). This architecture with anion aggregates promoted PF₆^- decomposition for NaF-rich interphase formation on both Na anode (Na CE of >99.9%) and Ni-rich cathodes (4.3V stability). In addition, we found that the addition of 1, 1, 2, 2-tetrafluoroethyl 2, 2, 3, 3-tetrafluoropropyl ether (TTE) to DEE/NaPF₆ cannot form the anion-aggregated solvation commonly observed in LiFSI/ether/diluent electrolytes [4] due to the strong interactions between PF₆^- anion and fluorine atoms on TTE molecules, resulting in a low Na CE.

Pushing Battery Cycle Limits: From Coin Cells to Anode-Free Pouch Cells

We evaluated the DEBE electrolyte in both traditional Na-metal coin cells and anode-free pouch cells. For coin cells (2.0 mAh cm⁻², N/P=1.7), sodium metal batteries using NASICON-type sodium vanadium phosphate cathode (Na₃V₂(PO₄)₃, NVP) exhibited 5,000 cycles while retaining >83% capacity under fast charging of 5C (12-minute charge). High-voltage batteries using Ni-rich cathode (NaNi_0.6Mn_0.2Co_0.2O₂, NMC622) endured 500 cycles with a capacity retention of >80%, far outperforming previous sodium batteries. Notably, for 50 mAh anode-free pouch cells (2.0 mAh cm⁻², N/P=0) with any pre-deposited sodium on the anode, the 4.0 Al||NVP pouch cells achieved 500 cycles with 75% capacity retention. Even more impressive, the 4.3 V Al||NMC622 anode-free pouch cells demonstrated for 300 cycles with 78% capacity retention and an energy density exceeding 360 Wh kg_electrode^-1, delivering LIB-parity energy density with low costs and high sustainability, paving the way for high-energy SMBs. [5]

Sustainability and Broader Impact

Beyond technical performance improvement, this research emphasizes the sustainability of energy storage. Sodium is 1000 times more abundant than lithium, and the DEBE electrolyte uses low-cost NaPF₆ salt and non-fluorinated solvents, avoiding toxic fluorine byproducts and reducing manufacturing carbon footprints. By extending battery cycle life, we also minimize raw material consumption and waste generation, addressing environmental, economic, and supply chain challenges all at once.

Looking Ahead

While the DEBE electrolyte shows promise in the present work, there are still hurdles to overcome. The next step will be optimizing the electrolyte’s performance under extreme conditions (wide temperature and high current), improving nonflammability and safety for both electrolytes and SMB cells. Addressing easy scalability and intrinsic safety for SMBs is critical before any commercialization. Additionally, we aim to explore the application of the molecular electrolyte design principles to other battery chemistries, such as potassium and magnesium systems. [6]

This journey has taught us that breakthroughs often come from challenging conventional thoughts, for example, abandoning fluorination for methylation, and prioritizing sustainability alongside performance. We hope this work inspires more researchers to explore eco-friendly electrolyte designs, accelerating the transition from LIBs to low-cost, high-energy sodium batteries and contributing to a more sustainable energy future.

More details of this study can be found in our recent article "Non-fluorinated Electrolyte for High-voltage Anode-free Sodium Metal Battery" published in Nature Sustainability. DOI: https://www.nature.com/articles/s41893-025-01710-w

Contributors: Prof. Ai-Min Li & Prof. Chunsheng Wang

Reference

1. Li, A.M., et al. Methylation enables the use of fluorine-free ether electrolytes in high-voltage lithium metal batteries. Nat. Chem. 16, 922–929 (2024).
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3. Cao, X. et al. Effects of fluorinated solvents on electrolyte solvation structures and electrode/electrolyte interphases for lithium metal batteries. Proc. Natl Acad. Sci. USA 118, 2020357118 (2021).
4. Li, A. M. et al. Salt-in-presalt electrolyte solutions for high-potential non-aqueous sodium metal batteries. Nat. Nanotechnol. 20, 388–396 (2025).
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