Surface enhanced Raman scattering (SERS) effect to magnificently intensify Raman signals, provided by nano-structured metal surface or plasmonic nano particles, is a powerful tool for analyzing interfacial phenomena on the electrodes [1,2]. To produce this SERS effect handily in practical analyses of solid-liquid interfacial reaction at the electrode, we have developed optical components made of transparent materials with nano particles of plasmon-active metals (Ag, Au, etc.), which we call as “SERS sensor” [3]. Just locating this sensor onto the electrode surface magnifies the Raman signals about chemicals at the electrode, providing the information to investigate the interfacial reaction phenomena. We employs this SERS measurement technique to reaction mechanism analyses for electrode surfaces.

One of our present targets is the reaction behavior at electrodes in Li ion secondary battery (LIB), focusing on formation mechanism of solid-electrolyte interface (SEI) [4]. SEI formation is one of the critical factors to determine the performance of LIB [5,6], which could avoid farther decomposition of electrolyte, while permitting Li⁺ to penetrate. Such properties of SEI films strongly depend on the composition of the film itself, which is determined by unclear mechanism of SEI formation. To understand the SEI film formation mechanism in the LIB in detail, Raman spectroscopy including highly precise SERS measurements was performed. Here, Li₂CO₃ generation was particularly focused on, whose formation mechanism is not clear in spite of its critical roles in the SEI films. Two kinds of solvent were used as electrolytes: mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with 1:1 in volume, called as EC-EMC, and mixture of propylene carbonate (PC) and EMC with 1:1 (v/v), PC-EMC. This is because these two organic solvents, EC and PC, show clearly different performances on SEI formation. The graphite anode and the LiCoO₂thin film cathode composed LIB model cell systems.

SERS measurements using the SERS sensor revealed clear difference between EC-based and PC-based electrolytes for the SEI formation behavior. Formation of Li₂CO₃ in the SEI film was not detected in EC-EMC, while that appeared in the case of PC-EMC, which should be due to the difference in the electrolyte distribution; PC-based electrolytes locate relatively lower amount of PC at the graphite electrodes than the amount of EC at the electrode in the case of EC-EMC, which leads to 2-electron reduction mechanism of SEI formation, resulting in Li₂CO₃formation.

Acknowledgements

This research was partially supported by the “Development of Systems and Technology for Advanced Measurement and Analysis”, and the “Research & Development Initiative for Scientific Innovation of New Generation Batteries (RISING II)” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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