Modulating Solvation Structure through Steric Hindrance: A Diluent Strategy for Low-Temperature Sodium Metal...

Area of Science:

• Electrochemistry
• Materials Science
• Energy Storage

Background:

• Sodium metal batteries (SMBs) face challenges in low-temperature performance due to poor cation-solvent interactions and slow desolvation kinetics.
• Electrolyte issues like salt precipitation and solidification hinder ion transport, leading to dendrite growth and capacity decay in SMBs.
• Localized high-concentration electrolytes (LHCEs) show promise but often use expensive and volatile fluorinated ethers.

Purpose of the Study:

• To introduce cyclopentyl methyl ether (CPME) as a cost-effective diluent for formulating a NaPF6-diethylene glycol dimethyl ether (G2)/CPME LHCE for low-temperature SMBs.
• To investigate the mechanism by which CPME influences Na+ solvation and SEI formation.
• To evaluate the electrochemical performance of SMBs using the developed LHCE at various temperatures.

Main Methods:

• Formulation of a NaPF6-G2/CPME LHCE.
• Theoretical and experimental analyses of cation-solvent interactions and solvation sheath structure.
• Electrochemical testing of Na‖Na symmetric cells and Na‖Na3V2(PO4)3 full cells at 25 °C and -30 °C.

Main Results:

• CPME acts as a sterically hindering diluent, kinetically excluding it from the Na+ solvation sheath and weakening Na+-G2 coordination.
• This facilitates PF6- anion entry, forming an anion-enriched structure and promoting a gradient NaF-rich solid electrolyte interphase (SEI).
• Na‖Na symmetric cells showed stable cycling over 3600 h at 25 °C. Full cells retained 90.1% capacity after 1000 cycles at 1 C (25 °C) and 90.9% after 500 cycles at 1 C (-30 °C), outperforming the standard NaPF6-G2 electrolyte (72.7% retention at -30 °C).

Conclusions:

• CPME is an effective and cost-efficient diluent for developing low-temperature-tolerant SMBs.
• The CPME-based LHCE promotes stable Na plating/stripping and dendrite suppression through controlled SEI formation.
• This research paves the way for practical, high-performance SMBs operating under demanding cold conditions.