1P049
IV-SFG studies on lithium salt concentration dependent interface structureof Pt/glyme-lithium solvate ionic liquids○Chengzi Qi1, Takashi Iwahashi1, Seitaro Yamaguchi2, Masahiro Fujita3, Yukio Ouchi1
1

Department of Materials Science and Engineering, Tokyo Institute of Technology, Japan2
New Materials Research Dept. Lintec Corporation, Japan3
Department of Materials and Life Sciences, Sophia University. Japan[Abstract] IV-SFG spectroscopy has been used to explore Li[TFSA] concentration
dependence of microscopic structure at Pt/[Li(glyme)x][TFSA] (x: mole ratio of glyme and
Li[TFSA]) SILs interface. LSV results show that the anodic stability of [Li(G4)x][TFSA] SIL
is enhanced with an increase in Li[TFSA] concentration. SFG results confirmed that theenhanced anodic stability can be caused by the decreased adsorption in the number of free G4adsorbed on the electrode surface.
[Introduction] Recently, solvate ionic liquids (SILs) have been developed and extensively
studied as electrolytes in lithium-based batteries. SILs are composed of a salt and a solventthat strongly coordinates to the cation or anion of salt to form stable complex ions. One of themost popular SILs is glyme and lithium bis(trifluoromethylsulfonyl)amide Li[TFSA] mixture[Li(glyme)][TFSA], where stable [Li(glyme)]+ complex cations can be formed. It has beenreported that the electrochemical anodic stability of [Li(glyme)x][TFSA] SILs depends on theconcentration of Li[TFSA][1]. However, it is still unclear how Li[TFSA] concentration affects
the oxidation stability of [Li(glyme)x][TFSA] SILs and how [TFSA]- anion and [Li(glyme)]+complexes behave on electrical interface. Elucidating Li[TFSA] concentration dependentinterface structure of [Li(glyme)x][TFSA] SILs on the electrode surface is very helpful toexplore these problems. In this work, infrared-visible sum frequency generation (IV-SFG)
vibrational spectroscopy is applied to study Li[TFSA] concentration and potential dependenceof platinum (Pt)/[Li(G4)x][TFSA] (x = 8, 4, 2, 1) SILs (G4: tetraglyme) interfacial structure.
The chemical structures of G4 and Li[TFSA] are shown in Fig.1.
[Methods] Li[TFSA] salt was added into G4 solvent toform different Li concentration [Li(G4)x][TFSA] SILs ,where the mole ratio of G4 and Li[TFSA] was fixed at
x= 8, 4, 2 and 1. The electrochemical properties of[Li(G4)x][TFSA] SILs as well as potential dependent
SFG measurements are carried out with aFig.1 Structure of G4 and Li[TFSA]
three-electrode cell. The working electrode is apolycrystalline Pt disk. The reference and counter electrodes are Ag/Ag+ and Pt wire,
respectively. HZ-5000 electrochemical workstation is employed to control the potential of Pt
disk in the SFG measurement and also to perform linear sweep voltammetry (LSV). Amode-locked picosecond Nd:YAG laser (pulse duration of 20 ps, repetition rate of 10 Hz)
system is used to carry out SFG measurement. The polarization combination used in this work
is ssp (denoting in order s-polarized SF, s-polarized visible and p-polarized field).
[Results and Discussion] Fig.2 shows LSV results of [Li(G4)8][TFSA] (black line) and[Li(G4)1][TFSA] (red line) at Pt electrode with a scan rate 50 mV s-1. The oxidation limit in[Li(G4)1][TFSA] is located at around 1.7 V, which is much higher than that in[Li(G4)8][TFSA] (around 1 V). The results show thatthe anodic stability of [Li(G4)x][TFSA] SIL is enhancedwith an increase in Li[TFSA] concentration. In fact, theanodic limit of [Li(G4)8][TFSA] is mainly determinedby the anodic stability of free G4, which is confirmed tobe easily oxidized at positive potential. While in[Li(G4)1][TFSA] system, the amount of free G4 in SIL
is considered to be negligible, and electrochemicallystable[Li(G4)]+complexes,
are formed in the bulk, possibly resulting in highanodic stability.
Potential dependent SFG spectra of Pt/[Li(G4)8][TFSA]
and Pt/[Li(G4)1][TFSA] SILs are shown in Fig.3 andFig.4, respectively. In [Li(G4)8][TFSA] system, thepeaks at around 1040 cm-1 and 1078 cm-1 are due to freeG4 and Li+-G4 complex, respectively. The peak
appeared at 1143 cm-1 is originated from SO2 symmetricstretching vibration mode (νss-SO2) of [TFSA]- anion,
which is very small even at positive potential. The SFG
spectra of [Li(G4)8][TFSA] in Fig.3 indicates that Pt
electrode surface is mainly occupied by Li+-G4complex and free G4 both at -2.4 V and at 0.5 V. The
increased peak intensity of Li+-G4 complex is due to its
orientation change as potential shifts positively.

Fig.2 LSV results of [Li(G4)8][TFSA]
and [Li(G4)1][TFSA] SILs at Pt

Fig.3 SFG spectra in [Li(G4)8][TFSA]

In [Li(G4)1][TFSA] system, two main peaks are
attributable to νss-SO2 and CF3 anti-symmetric stretchingvibration mode (νas-CF3) of [TFSA]- anion respectively,
and there is no peak from Li+-G4 complex or free G4 at-2.5 V. It means that [TFSA]- anion may adsorb on thenegatively charged Pt surface by interacting with Li+
cation. At 1.3 V, the new peak at 1083 cm-1 may becontributed by both Li+-G4 complex and SNS
anti-symmetric stretching vibration mode (νas-SNS) of Fig.4 SFG spectra in [Li(G4)1][TFSA][TFSA]- anion. The very small peak appeared at 1035
cm-1 is due to free G4. Moreover, the increase in νss-SO2 peak intensity and decrease on νas-CF3
peak intensity may be caused by the orientation change of [TFSA]- anion. Comparing SFG
results at positive potential of two systems, the relative intensity ratio of Li+-G4 complex toνss-SO2 is decreased as Li[TFSA] concentration increase in SILs. It demonstrates that theamount of [TFSA]- anion absorbed on Pt surface is increased. In addition, it should be notedthat the amount of free G4 in [Li(G4)1][TFSA] bulk is just tiny, so the available free G4adsorbing on the Pt surface is much less than that in [Li(G4)8][TFSA] system. Finally, we canconclude that positively charged Pt surface in high Li concentration SIL is largely covered by[TFSA]- anion, resulting in the decrease of free G4 adsorbed on the positively charged Pt
surface and therefore the enhancement of anodic stability. In the poster presentation, the othertwo different Li concentration SILs will be discussed.
[Reference][1] K. Yoshida et al. J. Am. Chem. Soc., 133, 13121(2011)