In order to improve the performance of Li ion rechargeable batteries, solid-liquid interfacial phenomena taking place at the electrode during the operation should be well understood. For example, detailed analysis of solid electrolyte interphase (SEI) thin films formed at the electrode surface, which plays the key role to determine the battery’s properties, is a crucial issue. We have developed precise spectroscopy technique using surface enhanced Raman scattering (SERS), to realize highly selective interfacial analyses [1,2]. Here, transparent micro lens, partially covered with nano particles of plasmonic metals, such as Ag, Au, etc., is an essential component, which is called as “plasmonic sensor”. The sensor situated onto a particular sample enhances Raman signals from its surface, allowing us to get information only about the chemical state of the surface.

To employ this technique for in-situ analysis of SEI film formation on the electrode surface in Li ion rechargeable battery, this study optimizes the equipment and employs it for the analyses of solvent species dependence of SEI properties. The organic solvents employed here were the mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with 1:1 (volume), and that of propylene carbonate (PC) and EMC with 1:1. The graphite and LiCoO₂ films were used for anode and cathode, respectively. 1.0 mol/L of LiPF₆ was contained in each solvent. The design of plasmonic sensor implemented in the measurement cell was optimized so as to be movable by magnetic manipulation, which can avoid obstruction by the sensor to the diffusion of species during the reaction, realizing highly selective interfacial analyses. This setup was shown to serve also in effective Raman signal enhancement; the greater magnetic force to properly press down the sensor provides more significant signal enhancement. The SERS measurement results suggested that SEI formation depended on the solvent species employed: Li₂O₃ product was detected only in the case of PC-EMC. The time-dependent behavior of SEI formation was also obtained, indicating the mechanism of SEI formation.

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 2)” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

References:

[1] M. Yanagisawa, M. Saito, M. Kunimoto, T. Homma, Appl. Phys. Express, 9, 122002 (2016) .

[2] M. Kunimoto, Y. Sun, M. Yanagisawa, T. Homma, ECS Meeting Abstracts, MA2017-02, 131 (2017).