Transport Number Determination and Relevance for Lithium Metal Batteries Using Localized Highly Concentrated...

Area of Science:

• Electrochemistry
• Materials Science
• Chemical Engineering

Background:

• Localized highly concentrated electrolytes (LHCEs) are promising for lithium metal batteries (LMBs).
• Fluorinated ethers like TFEE offer potential solvent options for LHCEs.
• Understanding electrolyte properties and their correlation with battery performance is crucial.

Purpose of the Study:

• To determine the lithium transport number of TFEE-based LHCEs with DOL and DME solvents.
• To evaluate the performance and stability of these electrolytes in LMBs.
• To elucidate the factors governing electrolyte performance in LMBs.

Main Methods:

• Molecular dynamics simulations
• Nuclear magnetic resonance spectroscopy
• Bruce-Vincent's method
• Electrochemical impedance spectroscopy (EIS)
• Scanning electron microscopy (SEM)
• X-ray photoelectron spectroscopy (XPS)

Main Results:

• TFEE-DOL LHCE showed a high transport number (0.65) but poor stability and low Coulombic efficiency (<90%) vs. Li metal.
• TFEE-DME LHCE exhibited high Coulombic efficiency (98.9%) and stability, despite a lower transport number (0.25).
• EIS revealed significant migration resistance through the solid electrolyte interphase (SEI) in the TFEE-DOL system.

Conclusions:

• Interfacial properties at the Li metal anode are more critical for LMB performance than bulk electrolyte transport.
• Focusing on full (operando) impedance spectra of Li metal anodes is recommended for characterizing interphase layers.
• Electrolyte design for LMBs should prioritize anode interfacial stability over high bulk transport numbers.