Solvation-Mediated Shift From Solvent- to Anion-Derived Solid Electrolyte Interphases for Stable Calcium Metal Anodes

Area of Science:

• Energy Storage
• Materials Science
• Electrochemistry

Background:

• Calcium metal batteries offer high capacity and abundance but suffer from poor reversibility in ether-based electrolytes.
• Detrimental passivation layers form due to unstable Ca2+-DME coordination and reactive solvent-separated ion pairs (SSIPs).

Purpose of the Study:

• To elucidate Ca2+ solvation structures and understand electrolyte decomposition mechanisms.
• To design a co-solvent strategy using ionic liquids to stabilize calcium metal batteries.

Main Methods:

• Theoretical screening identified 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI) as an effective additive.
• Investigated the impact of EMIMTFSI on Ca2+ solvation shells and electrolyte decomposition pathways.
• Tested Ca||Ca symmetric cells with optimized electrolytes.

Main Results:

• The optimized electrolyte, featuring EMIMTFSI, reshaped the Ca2+ coordination environment, reducing SSIPs and promoting contact-ion-pair (CIP)-rich configurations.
• Electrolyte decomposition shifted from solvent-derived organics to inorganic-rich SEI, forming a denser, ion-permeable solid-electrolyte interphase.
• Ca||Ca symmetric cells achieved stable cycling for 160 hours with low overpotentials, significantly outperforming the base electrolyte.

Conclusions:

• A co-solvent design strategy using ionic liquids effectively stabilizes calcium metal batteries by controlling solvation structures.
• The developed electrolyte enables stable cycling and improved performance, addressing key limitations of calcium-based energy storage.
• Further optimization with OTf- anions extended stable cycling to 180 hours.