By Muhammad OsamaReviewed by Lexie CornerMay 15 2025A recent study published in Small examines how dual-salt electrolytes with reduced fluorine content can enhance the performance of sodium-ion batteries (SIBs), particularly those using layered transition metal oxide cathodes.
The researchers developed and tested alternative formulations to improve electrochemical stability and address the limitations of conventional sodium and lithium-ion batteries (LIBs). Their goal: to support more efficient, affordable, and environmentally safer energy storage systems.
Image Credit: IM Imagery/Shutterstock.com
Progress in Sodium-Ion Battery Technology
SIBs have become a practical alternative to LIBs, mainly due to sodium’s abundance and lower cost.
While their theoretical energy density is lower, SIBs can deliver strong performance when paired with suitable cathode materials such as layered transition metal oxides or Prussian blue analogs. These characteristics make them well-suited for large-scale and cost-sensitive applications like grid energy storage.
One of the major challenges lies in the electrolyte. Common options like sodium hexafluorophosphate (NaPF₆) raise safety and environmental concerns due to their high fluorine content. In response, researchers are investigating alternatives that reduce fluorine while maintaining performance.
Among them are dual-salt systems that combine sodium bis(fluorosulfonyl)imide (NaFSI) and sodium difluoro(oxalato)borate (NaDFOB) in carbonate-based solvents.
Study Overview: Testing Dual-Salt Electrolyte Formulations
The study evaluated three electrolyte formulations:
- 1 m NaFSI
- 0.8 m NaFSI + 0.2 m NaDFOB
- 0.5 m NaFSI + 0.5 m NaDFOB
These were dissolved in propylene carbonate (PC) and tested with a layered oxide cathode, P2-Na₂/₃Al₁/₉Fe₁/₉Mn₂/₃Ni₁/₉O₂ (P2-AFMNO).
Researchers used a range of electrochemical techniques—including linear sweep voltammetry (LSV), galvanostatic cycling, and electrochemical impedance spectroscopy (EIS)—to assess capacity retention, Coulombic efficiency, and cycling stability.
They also evaluated thermal behavior, ionic conductivity, and anodic aluminum dissolution across temperatures from −30 to 80 °C. To study the cathode–electrolyte interface (CEI) and gas evolution, they used X-ray photoelectron spectroscopy (XPS) and differential electrochemical mass spectrometry (DEMS).
Key Findings: Impacts of Using Dual-Salt Electrolytes
The study showed that dual-salt electrolytes combining NaFSI and NaDFOB provide a wide electrochemical stability window of about 5 V vs Na⁺/Na and exhibit strong ion transport properties. Adding NaDFOB improved anodic stability, making these formulations suitable for high-voltage applications.
Among the tested options, the 0.5 m NaFSI + 0.5 m NaDFOB in propylene carbonate delivered the most consistent performance. It achieved an initial capacity of 95 mAh g⁻¹ at 0.5 C and retained over 90 % of this capacity after 100 cycles, placing it among the highest-performing systems for P2-AFMNO cathodes cycled at 4.3 V.
NaDFOB also helped reduce anodic aluminum dissolution, a common degradation issue in SIBs. The reduction in dissolution was directly related to the concentration of NaDFOB. While the dual-salt systems showed a slight increase in viscosity and a minor drop in conductivity, they maintained stable ionic conductivity across a broad temperature range (−30 to 80 °C), supporting effective ion transport under varying conditions.
XPS confirmed that the improved cycling behavior was linked to the formation of a thin, uniform, and inorganic-rich CEI. This CEI helped limit electrolyte breakdown and reduce side reactions. DEMS results indicated that the mixed-salt system produced fewer gases than the pure NaFSI electrolyte, which contributes to safer cell operation.
Download your PDF copy now!
Relevance and Applications
The study highlights how low-fluorine dual-salt electrolytes can improve key performance factors in SIBs, including stability, safety, and cycle life. These findings are especially relevant for applications such as renewable energy storage, electric vehicles, and stationary grid systems, where cost and environmental impact are critical.
Because the composition of dual-salt systems can be adjusted, this approach may also prove useful across a broader range of battery chemistries and operating conditions.
Journal Reference
Lu, Y., et al. The Impact of Dual-Salt Electrolyte with Low Fluorine Content on the Performance of Layered Transition Metal Oxides for Sodium-Ion Batteries. Small, 2410704 (2025). DOI: 10.1002/smll.202410704, https://onlinelibrary.wiley.com/doi/10.1002/smll.202410704
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.