Effects of Electrolyte Salts on Ionic Transport in Solid Polymer Electrolytes Investigated by Operando Raman Spectroscopy
Authors: Ito Ayana , Hiraoka Koji , Seki Shiro 
Published: December 2024

Introduction

To achieve carbon-neutral society, battery technologies have been attracting attention in recent years owing to its potential for storing renewable energy and serving as a load leveling power sources. In particular, all-solid-state batteries (ASSBs) utilizing non-volatile and flame-resistant solid electrolytes are actively investigated worldwide for practical implementation, as they are expected to enhance safety. Among solid electrolytes, solid polymer electrolytes (SPEs), which exhibit high flexibility and formability, are expected for their application in electrochemical devices such as ASSBs. In SPEs, cations are solvated by the host polymer and occurs ionic conducte through cooperative transport associated with segmental motion. The ionic transport properties of SPE, affected by molecular structures of electrolyte salt, and are further influenced by processes solvation/desolvation of cation and decomposition reactions under electrochemical conditions, is considered different from static states. Therefore, we focused on applying [Li | SPE | Li] symmetric cells and evaluated the ionic transport processes within SPE under reaction conditions using concentration changes through operando Raman spectroscopy measurements. In this study, we applied the measurement approach for use on SPEs incorporating LiN(SO2F)2 (LiFSA), LiN(SO2CF3)2 (LiTFSA), and LiN(SO2C2F5)2 (LiBETI) as Li salts to investigate the impact of anion structure/molecular weight on ionic transport mechanisms under actual reaction conditions.

Experiments

Li-based solid polymer electrolytes were prepared by mixing cross-linked polyether-based macromonomer P(EO/PO) (EO: PO = 8:2), Li salts (LiFSA, LiTFSA, LiBETI), and photopolymerization initiator DMPA in acetonitrile under an Ar atmosphere within a glovebox. The resulting solution was vacuum-dried and filled between glass plates using a spacer (thickness: 0.5 mm). Each SPE film was then fabricated by UV irradiation. To investigate the solvation structure and salt dissociation characteristics of SPE (salt concentration: [Li]/[O] = 0.04-0.16) under static conditions, Raman spectroscopy measurements were performed (NRS-4500: Jasco). A 9 mm × 9 mm rectangular-shaped [Li | SPE([Li]/[O]=0.04, 0.16) | Li] symmetric cell was prepared for in situ monitoring of concentration variations into the SPE during electrochemical reactions. The symmetric cell was introduced into a sealed cell with a quartz glass having monitoring window and connected to a potentiostat (HZ-7000, Hokuto Denko). In o
perando Raman spectroscopy, Raman spectra were obtained from the vicinity of the working electrode (W.E.) and the counter electrode (C.E.) within the SPE during voltage application. The W.E. and C.E. regions were measured at points approximately 10-30 µm from the electrode/electrolyte interface.

Results & Discussion

Raman spectrum of the P(EO/PO)-LiTFSA(1) and LiBETI(2) system exhibited the peak at approximately 740 cm-1 derived from SNS vibration of TFSA and BETI anion. This peak, which increases with salt concentration, can be utilized as an indicator to evaluate concentration changes within the SPE induced by electrode reactions. To evaluate the ionic transport properties, operand
o Raman spectroscopy measurements were performed using [Li | SPE([Li]/[O]=0.04, 0.16)|Li] symmetry cells. The peak area change (A/A
0) from the initial state was calculated by using the obtained Raman spectra. Fig. 1 and 2 exhibited the relationship between A/A
0 and applied voltage time calculated from measured points near W.E. and C.E. in symmetrical cells using the LiTFSA and LiBETI systems. In both Fig. 1 and 2, the peak area changes (A/A
0) were observed that concentrations near W.E. and C.E. increased or decreased with voltage application. In particular, the peak area changes (A/A
0) of the LiTFSA system exhibited a rapid increase in W.E. and decrease in C.E. within the initial 1 h of measurement, followed by no significant changes over 5 h (Fig. 1). This result suggests that the SPE has reached a steady state within at the end of the experiment. And in Fig. 1, the area changes (A/A
0) of [Li]/[O]=0.04 was significantly higher than that of [Li]/[O]=0.16. This results suggests that the faster ionic transport characteristics of [Li]/[O]=0.04 compared to [Li]/[O]=0.16 can be attributed to the lower concentration of the former. No significant changes were observed both [Li]/[O]=0.04 and 0.16 owing to changes in A/A
0, similar to LiTFSA system (Fig. 2). The lower ionic transport properties of LiBETI can be attributed to the larger molecular weight of BETI anion. These results indicated that the ionic transport mechanisms under reaction conditions differ depending on the anion structure (molecular weight).
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Figure 1