Electron passivation in CaF₂ on calcium metal anodes.

Area of Science:

• Materials Science
• Electrochemistry
• Computational Physics

Background:

• Calcium-ion battery electrolytes face stability issues and degradation from anode reduction.
• The solid-electrolyte interphase (SEI) on anodes, often containing calcium fluoride (CaF2), limits electron flow.
• Different phases of CaF2 (crystalline, polycrystalline, amorphous) exhibit varying electronic conductivities.

Purpose of the Study:

• To investigate the electronic transport properties of different CaF2 phases within the SEI.
• To compare the electron tunneling enhancement in amorphous CaF2 films versus crystalline and polycrystalline forms.
• To determine which CaF2 phase offers superior electrolyte protection in calcium-ion batteries.

Main Methods:

• Utilized the non-equilibrium Green's function technique combined with density functional theory (NEGF-DFT).
• Simulated electron transport through various amorphous, crystalline, and polycrystalline CaF2 structures.
• Analyzed transport properties using the decay constant (β) and low-bias conductance.

Main Results:

• Amorphous CaF2 systems demonstrated a 1-2 order of magnitude enhancement in electron tunneling compared to crystalline and polycrystalline phases.
• Despite structural randomness, amorphous CaF2 exhibited similar conductance properties across different simulated structures.
• Crystalline and polycrystalline CaF2 phases showed significantly lower electron transport, indicating better insulating properties.

Conclusions:

• Amorphous CaF2 in the SEI facilitates greater electron tunneling, potentially impacting battery performance.
• Crystalline and polycrystalline CaF2 phases provide more effective electrolyte protection due to their lower conductivity.
• Phase-dependent properties of CaF2 are critical for designing stable and efficient calcium-ion battery electrolytes.