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) . 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.
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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