Potential-Dependent Structure of Ionic Liquids and Solvated Lithium Ions on a Gold Electrode: A Surface-Enhanced Infrared Study

Room temperature ionic liquids (RTILs), especially those containing TFSA and FSA anions, are promising electrolyte materials for Li ion batteries and other electrochemical devices due to high thermal and electrochemical stabilities. It has been reported that the cathodic decomposition potential of the FSA-based RTIL is less negative than that of TFSA-based RTIL when Li salt is added, despite the similar chemical structures of FSA and TFSA anions (Figure).¹ A mechanism for stabilization of the FSA-based RTIL on the electrode involving an interfacial structure of the RTIL has been proposed;¹ however, the structure and dynamics of RTILs and dissolved ions at the vicinity of electrodes are still under debate. Direct observation using spectroscopic technique is required to understand the environment in the vicinity of the electrode: interfacial structures of RTILs, solvation structure around Li⁺, and their potential-dependent behavior. Surface-enhanced infrared absorption spectroscopy (SEIRAS) that can selectively probe solid/liquid interfaces is suited for this purpose as represented in our studies on the potential-dependent behavior of a pure TFSA-based RTIL on an Au electrode using this method.² In the present work, we investigated the effect of the concentration of Li ion on the potential-dependent structure of [BMIM][FSA] and [BMIM][TFSA] (Figure) on an Au electrode by using SEIRAS.

SEIRAS experiments have been performed on an Au thin film electrode chemically deposited on a Si prism in a vacuum-compatible glass cell. The electrode potential was controlled by a potentiostat against a Pt quasi-reference electrode and corrected by the redox potential of Fc/Fc⁺ couple. LiFSA and LiTFSA are added to [BMIM][FSA] and [BMIM][TFSA], respectively, to control the concentrations of Li⁺. SEIRAS measurements have been performed during potential scans where under potential deposition of Li is not observed.

Potential-dependent SEIRA spectra of [BMIM][TFSA]/Au electrode showed the threshold potentials for steep increasing/decreasing in the intensities of the vibrational bands, which indicated the existence of the energy barrier for anion-cation exchange at the first ionic layer on the electrode.² Addition of the Li salt resulted in more positive threshold potential for the exchange of cations to anions at the first layer. We concluded that the anionic complex (e.g. [Li(TFSA)₂]^-) have higher potential barrier to replace cations at the first layer than TFSA^- owing to its bulky structure. The formation of anionic Li complex is supported by the decrease of the band intensity of the solvated TFSA^- at potentials negative of pztc (potential of zero total charge). Anionic Li complex moves further away from the electrode at the negative potential. The difference between [BMIM][FSA] and [BMIM][TFSA] will be reported in the presentation.

 ¹M. Yamagata et al., Electrochim. Acta 110, 181 (2013). ²K. Motobayashi et al., J. Phys. Chem. Lett. 4, 3110 (2013).