Multinuclear NMR Study of the Solid Electrolyte Interface Formed in Lithium Metal Batteries

Area of Science:

• Electrochemistry
• Materials Science
• Solid-state NMR Spectroscopy

Background:

• Solid electrolyte interphase (SEI) layers are crucial for lithium metal battery safety and performance.
• Formation of "dead" metallic lithium during cycling is a major safety concern.
• Electrolyte composition significantly influences SEI layer properties.

Purpose of the Study:

• To investigate the composition of SEI layers formed with LiFSI and LiTFSI in DME electrolytes.
• To understand the impact of electrolyte concentration on SEI layer characteristics.
• To correlate SEI composition with battery safety and electrochemical performance.

Main Methods:

• Multinuclear solid-state Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) spectroscopy at high magnetic field.
• Analysis of SEI layers formed in Cu|Li cells with varying concentrations of LiFSI and LiTFSI in DME.
• Quantitative 6Li MAS NMR for SEI density determination.

Main Results:

• Reduced "dead" metallic lithium in SEI layers formed with 4 M LiFSI-DME compared to 1 M LiFSI-DME.
• Significant LiF formation in SEI layers from concentrated 4 M LiFSI-DME and 3 M LiTFSI-DME electrolytes.
• Denser SEI layer formation with 4 M LiFSI-DME electrolyte compared to other systems.
• Absence of substantial LiF in SEI from diluted 1 M LiFSI-DME electrolyte.

Conclusions:

• Concentrated LiFSI-DME electrolytes, particularly at 4 M, promote the formation of SEI layers with reduced "dead" lithium and increased LiF content.
• The presence of LiF contributes to superior mechanical strength and ionic conductivity of the SEI.
• Higher electrolyte concentrations lead to denser SEI layers, correlating with improved electrochemical performance and safety in lithium metal batteries.