Unveiling LiTFSI Precipitation as a Key Factor in Solid Electrolyte Interphase Formation in Li-Based Water-in-Salt...

Area of Science:

• Electrochemistry
• Materials Science

Background:

• Water-in-salt electrolytes (WiSE) offer wider electrochemical stability windows for lithium-ion batteries.
• High salt concentrations in WiSE reduce water activity, suppressing decomposition.
• Challenges remain in utilizing WiSE for enhanced energy density and cycling stability due to parasitic reactions like hydrogen evolution.

Purpose of the Study:

• To investigate the composition and behavior of the solid electrolyte interphase (SEI) layer in WiSE.
• To understand the role of LiTFSI precipitation in SEI formation.
• To analyze the dynamic nature of the SEI layer during battery operation.

Main Methods:

• Operando high-resolution optical microscopy to observe SEI formation.
• Electrochemical analysis to study hydrogen evolution reactions.
• Chemical analysis to identify SEI components.

Main Results:

• A micrometric white layer, identified as a component of the SEI, was observed.
• LiTFSI precipitation was confirmed as the primary species within the SEI layer.
• The SEI layer demonstrated dynamic behavior, dissolving back into the electrolyte during open circuit voltage.

Conclusions:

• The SEI layer in WiSE is not solely nanoscale and contains LiTFSI precipitates.
• The dynamic nature of the SEI layer, including its dissolution, is a critical factor in battery performance.
• Understanding SEI dynamics is crucial for optimizing WiSE in lithium-ion batteries.