The structure of electrolyte solutions plays an important role in the lithium-ion intercalation reaction at graphite negative electrodes. The solvation structure of an electrolyte solution in bulk has been investigated previously. However, the structure of an electrolyte solution at the graphite negative electrode/electrolyte solution interface, where the lithium-ion intercalation reaction occurs is more important. In this study, the structure of electrolyte solutions in the vicinity of a graphite negative electrode was investigated using in situ Raman spectroscopy during the 1st reduction process in 1 mol dm LiClO/ethylene carbonate (EC) + diethyl carbonate (DEC) (1 : 1 volume ratio), 1 mol dm LiCFSO/propylene carbonate (PC), and 1 mol dm LiCFSO/PC + tetraethylene glycol dimethyl ether (tetraglyme) (20 : 1 volume ratio). As a result, in the electrolyte solutions in which the lithium-ion intercalation reaction can occur (LiClO/EC + DEC and LiCFSO/PC + tetraglyme), the Raman spectra of free solvent molecules (EC or PC) and anions showed a positive vibrational frequency shift during the co-intercalation reaction, and these shifts returned to their original positions during the lithium-ion intercalation reaction. On the other hand, there is no vibrational frequency shift in LiCFSO/PC, an electrolyte in which the lithium-ion intercalation reaction cannot occur. Based on our results, the relationship between the Raman shift and the solid electrolyte interphase (SEI) formation process was discussed.

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Suppression of a co-intercalation reaction into a graphite negative electrode is an important issue to use propylene carbonate (PC) as a base electrolyte solvent in lithium-ion batteries. In order to suppress the co-intercalation reaction, diethylene glycol dimethyl ether (diglyme) was added to the PC-based electrolyte solution. Interfacial reactions at the graphite negative electrodes were investigated by an in-situ scanning probe microscope (SPM) observation and an in-situ Raman spectroscopy to elucidate effects of diglyme in the PC-based electrolyte solution. Reversible intercalation and de-intercalation reactions of lithium ion took place in the PC-based electrolyte solution by adding small amount of diglyme (PC:diglyme = 20:1, vol. ratio). Based on the results of in-situ SPM and in-situ Raman spectroscopy, it is found that diglyme-solvated lithium ion preferentially intercalated and decomposed within the graphite and form a surface film. This surface film lowered the co-intercalation reaction potential of PC-solvated lithium ion, and co-intercalated PC-solvated lithium ion was easily decomposed within the graphite and then formed the effective surface film on the graphite surface.

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.