The effects of some film-forming organic additives, fluoroethylene carbonate (FEC), vinylene carbonate (VC), and ethylene sulfite (ES), on lithium deposition and dissolution were investigated in 1 M LiClO4 dissolved in propylene carbonate (PC) as a base solution. When 5 wt % FEC was added, the cycling efficiency was improved. On the contrary, addition of 5 wt % VC or ES significantly lowered the cycling efficiency. The surface morphology of lithium deposited in each electrolyte solution was observed by in situ atomic force microscopy (AFM). In PC+FEC, the surface was covered with a uniform and closely packed layer of particle-like deposits of about 100-150 nm diam. The surface film seemed to be more solid in PC+VC, and inhomogeneous in PC+ES. From ac impedance measurements, it was revealed that the surface film formed in PC+FEC has a lower resistance than that in the additive-free solution, whereas that formed in PC+VC or PC+ES has a higher resistance. Large volume changes during lithium deposition and dissolution require that the surface film should be elastic (or soft) and be self-repairable when being damaged. In addition, a nonuniform current distribution is liable to cause dendrite formation, which requires that the surface film should be uniform and its resistance should be as low as possible. PC+FEC gave a surface film that satisfies all these requirements, and therefore only FEC was effective as an additive for deposition and dissolution of lithium metal. © 2002 The Electrochemical Society. All rights reserved.

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...

In situ atomic force microscopic (AFM) observation of the basal plane of highly oriented pyrolytic graphite was performed during cyclic voltammetry at a slow scan rate of 0.5 mV s−1 in 1 mol dm−3 LiClO4 dissolved in a mixture of ethylene carbonate and diethyl carbonate. In the potential range 1.0-0.8 V, atomically flat areas of 1 or 2 nm height (hill-like structures) and large swellings of 15-20 nm height (blisters) appeared on the surface. These two features were formed by the intercalation of solvated lithium ions and their decomposition beneath the surface, respectively, and may have a role in suppressing further solvent cointercalation. At potentials more negative than 0.65 V, particle-like precipitates appeared on the basal plane surface. After the first cycle, the thickness of the precipitate layer was 40 nm, and increased to 70 nm after the second cycle. The precipitates were considered to be mainly organic compounds that are formed by the decomposition of solvent molecules, and they have an important role in suppressing further solvent decomposition on the basal plane. © 2001 The Electrochemical Society. All rights reserved.

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