In-Situ Raman Spectroscopy Study on the Preferential Adsorption of Electrolyte Species on Electrode Surface of Lithium Ion Battery
Authors: Sun Yingying , Yanagisawa Masahiro , Homma Takayuki 
Published: February 2020

The Solid-electrode interphase (SEI) plays an important role in the performance of Li-ion battery, while the mechanism of SEI was still unclear, such as the accurate interior structure, the formation process, the Li+ ion diffusion through SEI, and so on. All these aspects have a closed relationship with the reaction of electrolyte at the electrode surface during the first charging cycle. In this study, we employed in-situ Raman spectroscopy and an effective depth-profiling measurement to investigate the electrode/electrolyte interfacial process. The results suggested that there might be an unequal opportunity for each electrolyte species to react at the electrode surface, due to their adsorption properties. Motivation, experimental methods and results will be explained as below.
To make clear the role of each electrolyte component in interphase reaction, we focused on the responsibility of two conventional electrolyte solvents, cyclic carbonate (polarity) and acyclic carbonate (high viscosity). During the SEI formation process, one of them would be preferentially reduced on electrode surface. The interfacial reaction behavior concerns to the origins of reduction products, the distribution of products on electrode surface and the local arrangement at interface, which are significant to dictate SEI and batteries properties. Some researches were conducted to analyze the preferential reduction at interphase, yet the conclusion is much limited and the controversies still exist. [1].
Thus, to explore more detailed mechanism for better understanding for the SEI formation, we pursued the preferential interfacial adsorption behavior of three electrolyte species, Ethylene Carbonate (EC), Propylene (PC) and Ethylmethyl Carbonate (EMC), on electrode surface. A Li-ion battery model cell was fabricated with Graphite nanoparticles (GrNPs) and LiCoO2as anode and cathode. Two electrolyte solvent mixed solution PC/EMC (cyclic/acyclic) and EC/EMC (cyclic/acyclic) were flowed into the Li-ion battery model cell, to observe and compare their interfacial phenomena with no charging cycle. Raman spectroscopy was employed with a laser of wavelength 532 nm and power 4.3 mW. The depth profiling measurements were performed with step of 0.5 μm and 0.1 nm respectively. XZ mapping was operated to record cross section information with depth step 0.5 μm and horizontal step 1μm.
Figure 1 shows the XZ mapping result of PC/EMC and EC/EMC on GrNPs surface. The intensity of the peak at 1753 cm-1 (EMC) over that at 1807 cm-1 (EC), and the intensity of the peak at 1753 cm-1 (EMC) over that at 1798 cm-1 (PC) were calculated to compare their abundance ratio in Figure 1 (a1) (b1). From the mapping results, we can find that ratio of EMC/PC was increased toward GrNPs surface, but the ratio of EMC/EC show less obvious trend than EMC/PC. It demonstrated that EMC has more preferential adsorption ability around GrNPs surface while in the solution of PC/EMC, but shows no competition with EC in the solution of EC/EMC. A more accurate depth profile was done with step 0.1 nm and indicated the same result. We suspect that, when the PC-based electrolyte was used, the adsorbed EMC molecule would be preferentially reduced at the GrNPs surface before Li-ion intercalated into GrNPs. Considering that the PC-based electrolyte was already demonstrated to be a bad solvent to form a SEI film with well property, the interfacial adsorption competition from acyclic electrolyte solvent may be one of the reasons. What kind of disadvantageous product would be reduced from acyclic was not well understood so far. We will keep focusing on the demonstration of products from acyclic and cyclic solvent in future research.
Acknowledgements
This research was partially supported by "Development of Systems and Technology for Advanced Measurement and Analysis", and Grant-in-Aid for Scientific Research of the MEXT Japan.
References:
[1] K. Xu, Electrolytes and interphases in Li-ion batteries and beyond, Chemical Reviews, 114 (2014) 11503-11618.

Figure 1