In-Situ AFM Observation of Highly Oriented Pyrolytic Graphite in PC-Based Electrolyte Solution Containing Magnesium Ion

Abstract

Introduction Graphite is used as a negative active material in lithium-ion batteries. The surface film, called solid electrolyte interphase (SEI) formed in ethylene carbonate (EC)-based electrolyte solution, plays an important role in reversible lithium-ion intercalation and de-intercalation reaction. When propylene carbonate (PC) is used as organic solvent, electrolyte decomposition and exfoliation of graphite layers occurred. We reported that lithium-ion intercalation and de-intercalation in PC-based electrolytes was succeeded using high concentrated lithium salt electrolytes or containing multivalent cations[1-3]. However, the SEI formation mechanism in PC-based electrolyte solution containing multivalent cation was not clarified. In this study, we investigate the surface morphology changes of highly oriented pyrolytic graphite (HOPG) electrode in PC-based electrolytes containing Mg2+ by in-situ Atomic Force Microscopy (AFM) to clarify the SEI formation mechanism. Experimental A three-electrode cell was used for electrochemical measurements. A HOPG was used as a working electrode and lithium foils were used as a counter electrode and a reference electrode. Electrolytes were prepared from lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2, LiTFSA), magnesium bis(trifluoromethanesulfonyl)imide (Mg(N(SO2CF3)2)2, Mg(TFSA)2), and PC. The molar ratio of two cation was fixed to be Mg2+/Li = 1.2. AFM observation was conducted during the CV measurements. All potentials were referred to Li/Li+. Results Figure 1 shows the cyclic voltammograms and in-situ AFM image at 1st cycle of HOPG. Large reduction peak at around 0.7 V appeared in the first cycle but disappeared in the second cycle. This result indicated that irreversible electrolyte decomposition, which was related to SEI formation, occurred at the initial cycle. In the AFM image obtained between 0.508 V and 1.204 V, precipitates appeared on the HOPG surface at 0.66 V. In EC-based electrolyte solution, hill-like structures and blister structures by the co-intercalation and decompostion of solvated-lithium ion were observed around 0.9 V. However, these structures were not observed in this system. Therefore, it is proposed that the addition of Mg2+ suppressed the co-intercalation of solvated lithium ion and SEI was formed by the surface reaction. Reference [1] S.-K. Jeong et al., J. Power Sources, 175 (2008) 540. [2] S. Takeuchi et al., Electrochim. Acta, 56 (2011) 10450. [3] S. Takeuchi et al., submitted. Acknowledgment This work was partially supported by CREST, JST and JSPS KAKENHI Grant Number 16H04216. Figure 1