Ionic liquids for electrochemical applications: Correlation between molecular structure and electrochemical stability window

Abstract

Ionic gating has emerged as an effective and versatile tool to tune the charge-carrier density of a material and control its electronic ground state, as well as to develop low-temperature devices such as electrochemical transistors. Ionic liquids are a promising gating agent due to their high thermal- and electrochemical stability for both fundamental and applied research. However, the understanding of the correlation between the molecular structure of ionic liquids and their electrochemical stability is quite limited. For this reason, this study aims at determining the guidelines for synthesizing ionic liquids suitable for their use as electrolytes at low temperatures. A series of twenty-three ionic liquids having various ammonium cations, composed of three ''short chains'' and one ''long chain'', and Tf2N as the anion, were synthesized. Afterwards, their thermal behavior was determined to identify those ionic liquids exhibiting Tg < −50 °C. The anodic and cathodic limits of the selected ionic liquids were measured via linear-sweep voltammetry using an electrochemical transistor configuration, working at −33 °C. Electrochemical windows having absolute values from 2.9 to 5.7 V were measured. Overall, five guidelines were determined from the experimental results: first, the cations influence both cathodic and anodic limits; second, the asymmetric ammonium cations show larger electrochemical stability than symmetric ones; third, the electrochemical stability decreases at the increase of the length of the ''long chain''; fourth, alkyl long chains show a larger anodic limit, but smaller cathodic limit than ether long chains having the same length; fifth, the ether chain with largest electrochemical stability comprises three carbon atoms and one oxygen.