Molecular fluorination towards deep eutectic amide-based electrolyte for stable high voltage lithium-metal batteries

Area of Science:

• Materials Science
• Electrochemistry
• Battery Technology

Background:

• Non-flammable deep eutectic electrolytes (DEEs) based on N-methylacetamide (NMAC) are used in lithium-metal batteries (LMBs).
• Challenges with NMAC-based DEEs include unstable interphase chemistry due to amide group reactivity and strong Li+ coordination.
• These issues limit the innovation and performance of DEEs in LMBs.

Purpose of the Study:

• To design a novel DEE with improved stability and electrochemical performance for LMBs.
• To address the limitations of NMAC-based DEEs by introducing a fluorinated amide.
• To investigate the correlation between Li+ coordination environment and interphase chemistry.

Main Methods:

• Synthesis of a fluorinated DEE (F-DEE) using N-Methyl-2,2,2-trifluoroacetamide (FNMAC) and LiTFSI salt.
• Electrochemical performance evaluation of F-DEE in NCM622||Li cells.
• Analysis of interphase chemistry and Li+ coordination environment.

Main Results:

• The electron-withdrawing -CF3 group in FNMAC enhances oxidation resistance and stabilizes interphase chemistry.
• F-DEE exhibits impaired Li+-amide coordination, leading to an anion-rich solvation sheath and a robust solid electrolyte interface (SEI) with high inorganic content.
• FNMAC-based DEE (F-DEE-1:4) enabled NCM622||Li cells to achieve excellent rate capability, long lifespan, and high capacity retention (~91.3% after 420 cycles).

Conclusions:

• FNMAC-based DEEs offer superior performance compared to NMAC-based DEEs in LMBs.
• The study highlights the importance of Li+ coordination environment for stable interphase chemistry in DEEs.
• This work provides insights for developing high-quality DEEs for advanced lithium-metal batteries.