Yukio OUCHI1, Luoxiao SANG1, Chengzi QI1, Masahiro YOSHIZAWA-FUJITA2, Takashi IWAHASHI1
1Tokyo Institute of Technology, Department of Materials Science and Engineering, Tokyo, Japan
2Sophia University, Department of Materials and Life Sciences, Tokyo, Japan
Solvate ionic liquids (SILs) have been developed as potential electrolytes in lithium-based batteries (LBs). SILs are composed of salts and solvents, where solvent molecules strongly coordinate to the cations or anions of the salts to form stable complex ions. Most popular SILs are the mixtures of glyme and lithium bis(trifluoro methylsulfonyl)amide Li[TFSA]([Li(glyme)][TFSA]), where stable [Li(glyme)]+ complex cations are formed and behave as cationic species. In conventional LBs, solid electrolyte interface (SEI) and cathode electrolyte interface (CEI) are formed on the electrode by reduction and oxidation of the electrolytes. It is widely known that they play an important role in cycling performance of LBs. However, it is still unclear how SEI and CEI is formed from the electrolyte, and in particular, how SEI (and CEI) affects adsorption/desorption behavior of [Li(glyme)]+ complex cation and [TFSA]- anion at the electrode/SIL interfaces. In this work, infrared-visible sum frequency generation (IV-SFG) vibrational spectroscopy is applied to the in-situ studies of the potential dependence of Pt/[Li(G4)][TFSA] SILs (G4: tetraglyme) interfacial adsorption and desorption directly, where the SEI and CEI is prepared on the Pt electrode by the application of potentials around the reduction and oxidation limits of electrochemical window.
As for the case of reduction side where SEI is formed, we found that the SF signal of [TFSA]- anion did not respond to the change of the applied potential from -2.0 V to 0.5 V. Blue shifted peak position of the SO2ss of [TFSA]- anion at negative potentials suggests the strong interaction of [TFSA]- anion with Li+ cation on SEI rather than those with [Li(G4)]+ complex cation or Pt surface. In the case of oxidation side (CEI), the SF signal of [Li(G4)]+ complex cation increased by changing the applied potential from -2.0 V to 0.5 V. It reflects that the [Li(G4)]+ complex cation configuration at the surface dramatically changes from a flat-on to a vertical-on orientation configuration of the G4 ring, due to the specific interaction with CEI. Based on the full analyses of orientation configuration of ionic species, our IV-SFG results have successfully concluded that the SEI strongly influences the adsorption of [TFSA]- anion while the CEI mainly affects the configuration of [Li(G4)]+ complex cation.