Unravelling the convoluted and dynamic interphasial mechanisms on Li metal anodes

Area of Science:

• Battery technology
• Materials science
• Electrochemistry

Background:

• Solid-electrolyte interphase (SEI) formation is critical for high-energy lithium metal batteries.
• Current understanding suggests SEI primarily forms from reactions between lithium metal and electrolytes.

Purpose of the Study:

• To elucidate the complex formation mechanism of the SEI.
• To identify all contributing factors and dynamic behaviors during SEI evolution.
• To explore pathways for forming desirable SEI compositions in low-cost electrolytes.

Main Methods:

• Synchrotron-based X-ray diffraction (XRD) and pair distribution function (PDF) analysis.
• Isotope labeling techniques to trace the origin of SEI components.
• Electrochemical cycling of lithium metal cells with varying lithium accessibility and electrolyte concentrations.

Main Results:

• SEI formation is a convoluted process influenced by electrolyte, cathode, moisture, and native lithium metal surface species.
• Lithium hydride (LiH) originates from electrolyte solvent, moisture, and native lithium hydroxide (LiOH).
• LiOH transforms into LiH and Li2O with abundant lithium, or forms large crystals when lithium is limited.
• Anion-derived LiF-rich SEI can form in low-concentration electrolytes through crosstalk effects.

Conclusions:

• The SEI formation mechanism is more complex than previously thought, with significant contributions from various sources.
• Understanding these contributions is vital for designing advanced electrolytes and optimizing battery performance.
• The formation cycle protocol significantly influences SEI composition, offering opportunities for cost-effective electrolyte development.