Electrochemical lithium-ion intercalation/de-intercalation at a graphite electrode were studied in propylene carbonate-based electrolyte solutions containing both lithium bis(trifluoromethanesulfonyl)amide and magnesium bis(trifluoromethanesulfonyl)amide salts by using lithium/graphite cell, and the influence of the salt concentration was investigated. In the higher concentration electrolyte solutions, reversible lithium-ion intercalation/de-intercalation at the graphite electrodes occurred. In contrast, only the exfoliation of graphite occurred in the lower concentration electrolyte solutions. The threshold ratio of the magnesium salt (Mg 2+ /Li + = 0.9) was lower than that of the calcium salt (Ca 2+ /Li + = 1.1 Electrochemical intercalation/de-intercalation of lithium ions at graphite electrodes is a negative electrode reaction that occurs in lithium-ion batteries (LIBs). Ethylene carbonate (EC) is an essential solvent for commercialized LIBs principally due to its high dielectric constant and its ability to form a stable solid electrolyte interphase (SEI). Owing to the relatively high melting point of EC (34• C), the low-temperature performance of LIBs requires improvement if they are to be used as power sources for electric vehicles in cold areas. One way to improve the low-temperature performance of LIBs is to replace EC with another solvent such as propylene carbonate (PC), which has a low melting point of −49• C. However, it is well known that intercalation/de-intercalation of lithium ions into/from graphite electrodes in common PC-based electrolyte solutions does not occur. 1According to the Besenhard model, 2 this intercalation failure mechanism can be explained briefly as follows; PC-solvated lithium ions co-intercalate into graphite 2,3 and PC-solvated lithium ions are reduced by one electron reaction and gas evolution occurs.4 During this process, exfoliation of graphite continues to occur. As a result, lithium ions do not intercalate into the graphite electrodes. The stable intercalation/de-intercalation of lithium ions into/from graphite electrodes in EC-based electrolyte solutions occurs due to the SEI 5,6 formed from by the products of reductive decomposition of EC on graphite electrodes. However, in common PC-based electrolyte solutions without additives, the effective SEI was not formed on graphite. Thus, one possible ways to achieve lithium-ion intercalation in PCbased electrolyte solutions is the addition of substances whose reductive decomposition products form an SEI. [7][8][9] In contrast to reports that focus on the SEI, we have focused on the impact of the structure of solvated lithium ions on electrochemical lithium-ion intercalation into graphite electrodes in PC-based electrolyte solutions.10-16 Our previous study showed that the reaction that occurred at a graphite electrode in PC-based electrolyte solutions, which was either lithium-ion intercalation or exfoliation, depended on the concentration of the lithium salt 10,11 and that a high concentration of lithium salt enabled successf...
Propylene carbonate (PC) is one of the promising solvents of lithium-ion batteries for use in cold area. Lithium salt concentrated PC and both lithium salt and bivalent salt dissolved PC enabled lithium ion to intercalate at graphite negative electrodes. The key to use PC is a solid electrolyte interphase (SEI). In this study, the SEI formation process on a highly oriented pyrolytic graphite was investigated by in-situ atomic force microscopy (AFM) in 4 mol dm−3 lithium bis(trifluoromethanesulfonyl)amide (LiTFSA)/PC and 1 mol dm−3 LiTFSA + 1.2 mol dm−3 M(TFSA)2 (M = Ca or Mg)/PC. Based on the in-situ AFM observation, the SEI formation processes in 4 mol dm−3 LiTFSA/PC and 1 mol dm−3 LiTFSA + 1.2 mol dm−3 Ca(TFSA)2/PC were the co-intercalation type and that in 1 mol dm−3 LiTFSA + 1.2 mol dm−3 Mg(TFSA)2/PC was the surface decomposition type. Relation between the SEI formation process and the charge–discharge properties was also discussed.
It is well-known that charge-discharge reactions of graphite electrode cannot take place in propylene carbonate (PC) based electrolyte solutions. Since the melting point of PC is much lower than that of ethylene carbonate (EC), PC-based electrolyte solutions are preferable when lithium-ion batteries are operated in the lower temperature regions. Vast work has been done on graphite electrodes in PC-based electrolyte solutions. One of our colleagues, Jeong found that concentrated salt PC solutions enabled charge-discharge reactions at graphite electrodes [1]. This was the first approach for use of concentrated salt electrolyte solutions. The concept of the concentrated electrolyte solutions was the change of the solvation states of lithium-ion. We also found that the addition of divalent cations such as calcium ion and magnesium ion in the PC-based electrolyte solutions also made graphite electrode active for the intercalation/de-intercalation of lithium-ion [2,3]. Since the Lewis acidities of Mg2+ and Ca2+ are higher than that of Li+, PC should preferentially solvate with the divalent cations. This will result in the change of solvation states of lithium-ion. Then, charge-discharge reactions could take place at graphite electrode. Further, we used “in situ” atomic force microscopy (AFM) for the understanding of the formation of solid electrolyte interface (SEI) on graphite electrode in PC-based electrolyte solutions containing Mg2+. As a result, we found the co-intercalation of lithium-ion and solvent was suppressed by the addition of Mg2+. The detail will be reported in the conference. References [1] S. K. Jeong, Electrochem. Solid State Lett. 6, A13 (2003) [2] S. Takeuchi et al., Electrochim. Acta, 56 (2011) 10450. [3] S. Takeuchi et al., submitted. Acknowledgement This work was partially supported by CREST, JST and JSPS KAKENHI Grant Number 16H04216.
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
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