In situ electrochemical atomic force microscopy (AFM) observation of the basal plane of highly oriented
pyrolytic graphite (HOPG) was performed during cyclic voltammetry in 1 M LiClO4/propylene carbonate
(PC) containing 3 wt % vinylene carbonate (VC), fluoroethylene carbonate (FEC), and ethylene sulfite (ES)
in order to clarify the roles of these additives in the formation of a protective surface film on a graphite
negative electrode in lithium-ion batteries. Particle-like precipitates appeared on the HOPG surface at
the potentials 1.35, 1.15, and 1.05 V versus Li+/Li in PC + VC, PC + FEC, and PC + ES, respectively,
and covered the whole surface at lower potentials. No evidence for cointercalation of solvent molecules was
observed in the presence of each additive. It was concluded that the layer of the precipitates functions as
a protective surface film, which suppresses cointercalation of PC molecules as well as direct solvent
decomposition on the surface of the graphite negative electrode.
Electrochemical lithium intercalation within graphite was investigated in propylene carbonate ͑PC͒ containing different concentrations, 0.82 and 2.72 mol dm Ϫ3 , of bis͑perfluoroethylsulfonyl͒imide, LiN(SO 2 C 2 F 5 ) 2 . Lithium ion was reversibly intercalated into and deintercalated from graphite in the latter concentrated solution in spite of the use of pure PC as a solvent, whereas ceaseless solvent decomposition and intensive exfoliation of graphene layers occurred in the former solution. X-ray diffraction analysis revealed that a stage I graphite intercalation compound was formed after being fully charged in the 2.72 mol dm Ϫ3 solution. The results of Raman analysis indicated that no free PC molecules are present in the concentrated solution, which suggested that the ion/solvent interactions would be an important factor that determines the ability of stable surface film formation in PC-based solutions.Lithium ion is intercalated within graphite to form lithiumgraphite intercalation compounds ͑Li-GICs͒. Li-GICs have been prepared by reduction of graphite in nonaqueous media, typically ethylene carbonate ͑EC͒-based solutions, containing lithium salts such as LiClO 4 , LiPF 6 , LiBF 4 etc. 1 The invention of the EC-based electrolyte systems in 1981 was the most important breakthrough for realizing the electrochemical preparation of Li-GICs, 2 which enabled us to use graphite as a negative electrode in commercially available lithium-ion cells. Before this finding, all attempts to electrochemical lithium intercalation within graphite had been unsuccessful because they had been conducted in propylene carbonate ͑PC͒-based solutions, 3-5 in which only ceaseless solvent decomposition and intensive exfoliation of graphene layers, instead of lithium intercalation, take place when a graphite electrode is polarized to negative potentials.It is widely recognized that a surface film, which is often called the solid electrolyte interface ͑SEI͒, is formed on graphite by decomposition of EC upon reduction, and it effectively passivates the graphite surface and prevents further cointercalation and decomposition of solvent molecules, allowing only lithium ion to migrate and to be intercalated. Such an effective surface film is not formed in PC-based solutions; 1 however, they are still attractive as electrolyte solutions for lithium-ion cells because of their superior ionic conductivity at low temperatures. 6 For this reason, many researchers have made many efforts to explore effective additives or cosolvents that give a stable surface film on graphite in PC-based solutions.We recently found that Li-GICs can be prepared in a solution of concentrated lithium bis͑perfluoroethylsulfonyl͒imide ͓LiN(SO 2 C 2 F 5 ) 2 , LiBETI͔ dissolved in pure PC, which does not contain any film-forming reagents ͑additives or cosolvents͒. The results of charge/discharge tests, X-ray diffraction ͑XRD͒ analysis, and laser Raman spectroscopic analysis of the solution are presented in this report.
ExperimentalNatural graphite powder ͑The Kansai Coke an...
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