Fluorinated Ether Based Electrolyte Enabling Sodium-Metal Batteries with Exceptional Cycling Stability

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Conventional sodium-metal batteries face challenges with dendrite formation and safety due to organic liquid electrolytes.
• Existing electrolytes exhibit poor cycling performance and low Coulombic efficiency, particularly with high-capacity cathodes like Na3V2(PO4)3 (NVP).
• Fluoroethylene carbonate (FEC) improves Coulombic efficiency but causes unstable solid electrolyte interphase (SEI) formation on sodium anodes.

Purpose of the Study:

• To develop a low-flammable electrolyte for sodium-metal batteries that enhances cycling stability and safety.
• To address the issues of dendrite deposition and unstable SEI formation encountered with conventional electrolytes.
• To improve the electrochemical performance of sodium-metal batteries, specifically focusing on NVP cathodes.

Main Methods:

• Formulation of a novel low-flammable electrolyte (NaPF6-FRE) comprising NaPF6 in 1,2-dimethoxyethane (DME), fluoroethylene carbonate (FEC), and 1,1,1,3,3,3-hexafluoroisopropylmethyl ether (HFPM) at a 2:1:2 volume ratio.
• Evaluation of symmetric Na and Na||Cu cells using the NaPF6-DME electrolyte and a porous separator.
• Testing of Na3V2(PO4)3 (NVP)||Na cells with the developed NaPF6-FRE electrolyte.
• Characterization of electrochemical window, cycling performance of Na symmetric cells, and capacity retention/Coulombic efficiency of NVP||Na cells.

Main Results:

• The NaPF6-FRE electrolyte exhibits a wide electrochemical window of 5.2 V and superior cycling performance in Na symmetric cells for over 800 hours.
• Addition of HFPM stabilizes the sodium electrode surface, forming a protective fluorine-containing organic layer and mitigating issues caused by FEC reactivity.
• NVP||Na cells demonstrate excellent capacity retention (94% after 2000 cycles) and high average Coulombic efficiency (99.9% at 5 C).

Conclusions:

• The developed NaPF6-FRE electrolyte offers a promising solution for safer and more stable sodium-metal batteries.
• The combination of DME, FEC, and HFPM effectively balances electrolyte stability, electrochemical performance, and safety.
• This electrolyte formulation significantly advances the potential for practical application of high-energy sodium-metal batteries.