Alkoxy-functionalized ionic liquid electrolytes: understanding ionic coordination of calcium ion speciation for the rational design of calcium electrolytes

• Published in: Energy & environmental science Vol. 13; no. 8; pp. 2559 - 2569
• Main Authors: Gao, Xinpei, Liu, Xu, Mariani, Alessandro, Elia, Giuseppe Antonio, Lechner, Manuel, Streb, Carsten, Passerini, Stefano
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 13.08.2020
• Subjects: Ammonium; Batteries; Calcium; Calcium ions; Cations; Coordination; Crystallography; Density functional theory; Design; Diffusion layers; Electrolytes; Ion diffusion; Ionic liquids; Ions; Oxygen atoms; Solid electrolytes; Speciation; Surface chemistry; Vanadium pentoxide
• ISSN: 1754-5692; 1754-5706
• DOI: 10.1039/d0ee00831a

There is growing interest in the rational design of electrolytes for multivalent-ion batteries by tuning the molecular-level interactions of solvate species present in the electrolytes. Herein, we report our effort to control Ca-ion speciation in ionic liquid (IL) based electrolytes through the design of alkoxy-functionalized cations. Quantitative analysis reveals that the alkoxy-functionalized ammonium cation (N 07 + ), bearing seven ether oxygen atoms, can effectively displace the bis(trifluoromethanesulfonyl)imide anion (TFSI − ) from the Ca 2+ ion coordination sphere, facilitating the reversible Ca deposition/stripping process. More importantly, post-analysis of Ca deposits surface chemistry and density functional theory calculations of Ca-ion speciation indicate the formation of an organic-rich, but inorganic-poor solid electrolyte interphase layer, which enables Ca 2+ ion diffusion rather than passivating the Ca metal electrode. Finally, as a proof-of-concept, a prototype Ca/V 2 O 5 cell using the optimized IL-based electrolyte ([Ca(BH 4 ) 2 ] 0.05 [N 07 TFSI] 0.95 ) is demonstrated for the first time, exhibiting a remarkable initial discharge capacity of 332 mA h g −1 and reversible capacity of 244 mA h g −1 . The proof-of-concept demonstration of a rechargeable Ca/V 2 O 5 cell is given, taking advantage of an alkoxy-functionalized ionic liquid-based electrolyte enabling the displacement of the anions from the coordination of the Ca 2+ cations.