Operando Infrared Spectroscopy Study on Electrode-Electrolyte Interface in Lithium Secondary Batteries Using Polyethylene Oxide-Based Electrolyte
Authors: Yamada Koki , Sai Ryansu , Hitaka Tasuku , Katayama Yu , Tsutsumi Hiromori 
Published: December 2020

(Electro-)chemical reaction at the electrode-electrolyte interface, including the formation of solid electrolyte interphase (SEI), is a crucial element to improve the performance of lithium secondary batteries. Numerous operando measurements, for instance, XPS1, Raman2, and infrared spectroscopy (IR)3 technique have been performed to study the reaction proceeding on the electrode. Operando attenuated total reflection (ATR) infrared spectroscopy technique especially has received much attention due to its high sensitivity to organic molecules, and have successfully revealed the oxidation mechanisms of carbonate solvents on the common electrode (LiNixMnyCo1-x-yO2)4. On the contrary, the electrode-electrolyte interface of the polymer electrolytes, which is considered as a next-generation electrolyte owing to its flame retardancy and structural stability5, is not well understood.
In this study, operando ATR-IR was newly employed to understand (electro-)chemical reaction at the interface between electrode and polymer electrolyte, combined with various electrochemical measurements, including electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV). We selected highly ion-conductive polyethylene oxide (PEO)-based electrolyte as a target electrolyte. (Electro)chemically inert Cu nanofilm was used as a model electrode to achieve good S/N while avoiding the undesirable side reaction.
Linear sweep voltammetry (LSV), performed from 2.8 VLi to -0.25 VLi, confirmed (1) no side reaction related to Cu electrode and (2) successful Li deposition at -0.25 VLi. Furthermore, LSV showed a small reduction current at ca. 1.0 VLi and ca. 0.5 VLi, which may correspond to the growth of the SEI layer on the Cu electrode. Meanwhile, we observed two semicircles in the EIS spectra, which can be assigned to the bulk resistance of the electrolyte (high-frequency semicircle) and charge transfer and SEI resistance (low-frequency semicircle) due to the following reasons; No visible change was observed for high-frequency semicircle during LSV while the low-frequency semicircle changed significantly during LSV. The change in the low-frequency semicircle suggests the formation of the SEI layer, which is in agreement with the observation of the small reduction current during LSV. Operando ATR-IR spectra, obtained simultaneously with LSV, showed the voltage-dependent peaks at ca. 1000 cm-1 and ca. 1200 cm-1, further indicating the SEI layer formation.
We will discuss the formation mechanisms of the SEI during the LSV, based on the detailed analysis of operando ATR-IR spectra. It will also be demonstrated that the holistic information about the electrode-electrolyte interface can provide additional knobs to design the highly-stable and highly-ion conductive polymer electrolyte, leading the implementation of all-solid-state battery technologies.

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