Atsushi Funabiki

Impedance Study on the Electrochemical Lithium Intercalation into Natural Graphite Powder

Electrochemical lithium intercalation into natural graphite powder of different sizes was studied by alternating current impedance spectroscopy. Impedance spectra at various potentials were fitted with a modified Randles equivalent circuit including a pseudocapacitance to express the observed finite diffusional behavior. The variations of electrochemical parameters with electrode potential, such as the charge-transfer resistance, the pseudocapacitance, the Warburg prefactor, and, finally, the chemical diffusion coefficient of lithium ion within graphite, were evaluated and discussed. It was shown that the charge-transfer reaction takes place on the whole surface of graphite particles, whereas lithium ion is intercalated from the edge plane and diffuses to the interior. The kinetics of the charge-transfer reaction was independent of the structure of the host. In contrast, the diffusivity of lithium ion within graphite was strongly dependent on the host structure, and the dependence was explained in terms of differences in in-plane and stacking order of lithium-graphite intercalation compounds formed by the intercalation.

A.c. impedance analysis of electrochemical lithium intercalation into highly oriented pyrolytic graphite

Abstract Electrochemical lithium intercalation into graphite was studied by cyclic voltammetry and a.c. impedance spectroscopy. Highly oriented pyrolytic graphite was used as a model graphite material to distinguish the difference in electrochemical behavior between the basal and the edge planes at graphite. A comparison between cyclic voltammograms of the basal plane and the whole surface of highly oriented pyrolytic graphite revealed that electrochemical lithium intercalation proceeds predominantly at the edge plane/electrolyte interface. The charge-transfer resistance changed continuously with electrode potential, and no significant change was observed at stage transition potentials (210, 120, and 90 mV versus Li/Li + ). From the variations of the Warburg impedance of samples of different sizes, it was concluded that lithium diffuses from the edge plane to the interior in the direction parralel to the basal plane and that its diffusivity changes with the stage structure of the bulk lithium—graphite intercalation compound.

STM study on graphite/electrolyte interface in lithium-ion batteries: solid electrolyte interface formation in trifluoropropylene carbonate solution

Abstract Lithium intercalation within graphite was studied in an electrolyte system, 1 M LiClO 4 dissolved in trifluoropropylene carbonate (TFPC). Lithium was intercalated within graphite in TFPC. The reversible capacity obtained (275 mAh g −1 ) was smaller than that in ethylene carbonate-based solutions while the irreversible capacity was larger (335 mAh g −1 ). The morphology change of the basal plane of highly oriented pyrolytic graphite (HOPG) was observed by electrochemical scanning tunneling microscopy (STM) to obtain information about passivating film (solid electrolyte interface, SEI) formation in this solvent system. The exfoliation of graphite layers was observed at 1.1 and 1.0 V vs. Li + /Li, and then swelling of graphite layers appeared along step edges at 0.5 V. The feature observed at 0.5 V was considered as SEI itself in this solvent system.

Stage Transformation of Lithium‐Graphite Intercalation Compounds Caused by Electrochemical Lithium Intercalation

The kinetics of the stage transformation of lithium-graphite intercalation compounds from dilute stage 1 to stage 4 were studied using potential-step chronoamperometry and alternating current impedance spectroscopy. Highly oriented pyrolytic graphite was used as a host material. The current-transient curve showed a current hump, suggesting that the stage transformation was initiated by the nucleation and growth of stage 4. The phase-boundary movement was discussed quantitatively using a simple geometric model. The phase boundary progressed in proportion to time during the initial stage. The rate constant was inversely proportional to the product of the interfacial resistance and the geometric edge-plane area, indicating that the phase-boundary movement was determined by the rate of the reaction at the graphite/electrolyte interface. In the following stage, the phase boundary advanced in proportion to the square root of time. The parabolic rate constant obtained experimentally was in satisfactory agreement with that calculated using Wagner`s classical model which describes the diffusion within two phases separated by a phase boundary. These results indicated that the phase-boundary movement was initially determined by the rate of the interfacial electrochemical reaction and was controlled thereafter by a diffusion process.

Electrochemical scanning tunneling microscopy analysis of the surface reactions on graphite basal plane in ethylene carbonate-based solvents and propylene carbonate

Abstract In order to elucidate the mechanism of surface film formation on graphite negative electrodes of rechargeable lithium-ion batteries, topographical changes of the basal plane of a highly oriented pyrolytic graphite were observed in a few electrolyte solutions under polarization by electrochemical scanning tunneling microscopy. In 1 M LiClO4/ethylene carbonate (EC) + diethyl carbonate, a hill-like structure of ∼ 1 nm height appeared on the surface of highly oriented pyrolytic graphite at 0.95 V versus Li/Li+, and then changed at 0.75 V to irregular shaped blister-like features with a maximum height of ∼ 20 nm. In 1 M LiClO4/EC + dimethoxyethane, hemispherical blisters of ∼ 20 nm height appeared at 0.90 V. These morphology changes, hill and blister formation, were attributed to the intercalation of solvated Li− ions into graphite interlayers and to the accumulation of its decomposed products, respectively. On the other hand, only rapid exfoliation and rupturing of graphite layers were observed in 1 M LiClO4/propylene carbonate (PC), which was considered to be responsible for ceaseless solvent decomposition when graphite electrodes are charged in PC-based solutions. From the observed topographical changes, it was concluded that the intercalation of solvated Li+ ions is a necessary step for stable surface film formation on graphite.

Nucleation and phase-boundary movement upon stage transformation in lithium–graphite intercalation compounds

Abstract Nucleation and phase-boundary movement upon stage transformation in lithium–graphite intercalation compounds were investigated by potential-step chronoamperometry and alternating current impedance spectroscopy. Highly oriented pyrolytic graphite was used as a host graphite material. The current response obtained by potential step was strongly dependent on the kind and direction of stage transformation. A current hump due to the nucleation and nuclear growth of a new stage was observed during the stage transformations from dilute stage 1 to stage 4 and from stage 2 to stage 1 upon lithium intercalation, and from stage 4 to dilute stage 1 upon deintercalation. The rate-determining steps for nucleation and for phase-boundary movement were discussed in the stage transformation between dilute stage 1 and stage 4. It was shown that the nucleation rate was controlled by the reaction at the graphite/electrolyte interface in both directions. The rate of the phase-boundary movement was affected greatly by the diffusivity of lithium ions in a growing phase; that is, it was controlled mainly by the diffusion in stage 4 in the direction from dilute stage 1 to stage 4, whereas by the interfacial reaction in the reverse direction.

Influence of defects on the phase-boundary movement in a stage transformation of lithium-graphite intercalation compounds

Abstract The influence of defects on the phase-boundary movement in the stage transformation from random stage (dilute stage 1) to stage 4 of lithium-graphite intercalation compounds was investigated using highly oriented pyrolytic graphite (HOPG). Potential-step chronoamperometry and in situ optical microscopy coupled with micro Raman spectroscopy were used to clarify macroscopic and microscopic influence of defects. Potential-step chronoamperometry confirmed that the diffusion-limited rate of the phase-boundary movement in the bulk of HOPG was independent of the crystallinity of the samples. This result indicates that defects have macroscopically no appreciable influence on the phase-boundary movement. In situ optical microscopy on the basal plane of HOPG revealed that the nucleation and growth of the stage-4 phase took place quite inhomogeneously in the initial stage. The rate of the phase-boundary movement in the surface region of HOPG was roughly evaluated to be in the range of 5 to 10 μm min −1 . The phase-boundary movement was seriously retarded by cracks formed on the basal plane during the stage transformation.

Lithium ion diffusion through glassy carbon plate

The electrochemical permeation method was applied to the determination of the diffusion coefficient of Li{sup +} ion (D{sub Li{sup +}}) in a glassy carbon (GC) plate. The cell was composed of two compartments, which were separated by the GC plate. Li{sup +} ions were inserted electrochemically from one face, and extracted from the other. The flux of the permeated Li{sup +} ions was monitored as an oxidation current at the latter face. The diffusion coefficient was determined by fitting the transient current curve with a theoretical one derived from Ficks law. When the potential was stepped between two potentials in the range of 0 to 0.5 V, transient curves were well fitted with the theoretical one, which gave D{sub Li{sup +}} values on the order of 10{sup {minus}8} cm{sup {minus}2} s{sup {minus}1}. In contrast, when the potential was stepped between two potentials across 0.5 V, significant deviation was observed. The deviation indicated the presence of trap sites as well as diffusion sites for Li{sup +} ions, the former of which is the origin of the irreversible capacity of GC.