Yasutoshi Iriyama

Solvated Li-Ion Transfer at Interface Between Graphite and Electrolyte

Solvated lithium-ion transfer at the interface between the graphite and electrolyte was investigated by ac impedance spectroscopy. Electrolytes consisting of 1 mol dm -3 LiCF 3 SO 3 dissolved in 1,2-dimethoxyethane (DME) and dimethylsulfoxide (DMSO) were used. Cyclic voltammograms of highly oriented pyrolytic graphite in the above electrolytes showed that solvated lithium ion intercalation occurred below a potential of 1.5 V (vs. Li/Li + ). In I mol dm -3 LiCF 3 SO 3 /DME electrolyte, impedance spectra of graphite were measured at a potential of 1.1 V. One semicircle was found in the Nyquist plot with a characteristic frequency of 15.8 Hz. For only lithium (nonsolvated lithium) ion intercalation, charge (lithium ion) transfer resistance was observed at a characteristic frequency of less than 0.1 Hz. The temperature-dependence of the charge-transfer resistances for solvated lithium ion transfer and lithium ion-only transfer gave activation energies of around 25 and 53-59 kJ mol -1 , respectively. These results suggest that solvated lithium ion transfer at the interface between graphite and electrolyte should be very fast, and the desolvation process for Li + ion intercalation and deintercalation at the graphite electrode should play an important role in intercalation/deintercalation kinetics at the interface between electrode and electrolyte.

Effects of Some Organic Additives on Lithium Deposition in Propylene Carbonate

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 LiClO 4 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 paced layer of particle-like deposits of about 100-150 nm dram. 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.

Kinetics of Lithium Ion Transfer at the Interface between Graphite and Liquid Electrolytes: Effects of Solvent and Surface Film

The kinetics of lithium ion transfer at an interface between graphite and liquid electrolyte was studied by ac impedance spectroscopy. Using highly oriented pyrolytic graphite (HOPG) as a model electrode, we evaluated the activation energies of the interfacial lithium ion transfer from the temperature dependences of the interfacial conductivities. When a binary electrolyte consisting of LiClO(4) dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume) was used, the activation energy of the interfacial lithium ion transfer was 58 kJ mol(-1), while an electrolyte consisting of LiClO(4) dissolved in DMC gave an activation energy of 40 kJ mol(-1). A calculation with the density functional theory clarified that the solvation ability of EC is higher than that of DMC. Therefore, we concluded that the activation energies of the interfacial lithium ion transfer at graphite reflected the energies for the desolvation of lithium ion from the solvent molecule. Furthermore, the activation energies of the interfacial lithium ion transfer varied in the presence of different surface films (solid electrolyte interphase, SEI). These results suggest that the kinetics of the interfacial lithium ion transfer at graphite is influenced by the compositions of SEI films as well as the desolvation of lithium ion from solvent molecules.

Raman study of layered rock-salt LiCoO2 and its electrochemical lithium deintercalation

Unpolarized and polarized Raman spectra (200–800 cm-1) of LiCoO2 with a layered rock-salt structure were measured. The Raman-active lattice modes of LiCoO2 were assigned by polarized Raman measurements of a c-axis oriented thin film. The variation of the Raman spectra of Li1-xCoO2 powder prepared by electrochemical lithium deintercalation was investigated, and the spectral changes were well correlated with the structural changes determined by x-ray diffraction except that peak splitting by the distortion in the monoclinic phase was not observed. The observed line broadening of the second hexagonal phase and the monoclinic phase indicated that the lithium ions remaining in the lattice after deintercalation randomly occupy the available sites on the lithium planes in the lattice the layered rock-salt structure.

Lithium-Ion Transfer at the Interface Between Lithium-Ion Conductive Ceramic Electrolyte and Liquid Electrolyte-A Key to Enhancing the Rate Capability of Lithium-Ion Batteries

In this study, lithium-ion transfer through the electrode/electrolyte interface was examined using a model interface composed of a lithium-ion-conductive ceramic and liquid electrolytes to focus on lithium-ion transfer. Lithium-ion transfer resistances at the interface and their activation energies were evaluated by impedance spectroscopy. The activation energies were quite large and consistent with the interaction between lithium-ion and solvents in an electrolyte as determined by a theoretical calculation.

Surface film formation on a graphite negative electrode in lithium-ion batteries: AFM study on the effects of co-solvents in ethylene carbonate-based solutions

Abstract In situ AFM observation of the basal plane of highly oriented pyrolytic graphite (HOPG) was performed before and after cyclic voltammetry in 1 mol dm −3 LiClO 4 dissolved in ethylene carbonate (EC), EC+diethyl carbonate (DEC), and EC+dimethyl carbonate (DMC) to clarify the effects of co-solvents in EC-based solutions on surface film formation on graphite negative electrodes in lithium-ion cells. In each solution, surface film formation involved the following two different processes: (i) intercalation of solvated lithium ions and their decomposition beneath the surface; and (ii) direct decomposition of solvent molecules on the basal plane to form a precipitate layer. The most remarkable difference among these solvent systems was that solvent co-intercalation took place more extensively in EC+DEC than in EC+DMC or EC. Raman analysis of ion–solvent interactions revealed that a lithium ion is solvated by three EC molecules and one DEC molecule in EC+DEC, whereas it is solvated exclusively by EC in EC+DMC and in EC, which suggested that the presence of linear alkyl carbonates in the solvation shell of lithium ion enhance the degree of solvent co-intercalation that occurs in the initial stage of the surface film formation.

Electrochemical Intercalation of Lithium Ion within Graphite from Propylene Carbonate Solutions

Electrochemical lithium intercalation within graphite was investigated in propylene carbonate (PC) containing different concentrations, 0.82 and 2.72 mol dm - 3 , of bis(perfluoroethylsulfonyl)imide, 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 alter being fully charged in the 2.72 mol dm - 3 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 Transfer at the Interface between Lithium-Ion-Conductive Solid Crystalline Electrolyte and Polymer Electrolyte

Lithium-ion transfer at the interface between the lithium-ion-conductive ceramic of La 0.55 Li 0.35 TiO 3 (LLT) and the polymer electrolyte of polyethylene oxide (PEO) complexed with LiCF 3 SO 3 (PEO-LiCF 3 SO 3 ) was studied by ac impedance spectroscopy for the system of Li/PEO-LiCP 3 SO 3 /LLT/PEO-LiCF 3 SO 3 /Li. The impedance ascribed to ion transfer through the interface was observed. The resistance due to the lithium-ion transfer at the interface was larger than those through phases of LLT and PEO-LiCF 3 SO 3 , and the temperature dependence of the interfacial resistances showed Arrhenius-type behavior. The activation energy of the interfacial resistances was larger than those for lithium-ion conduction in LLT and PEO-LiCF 3 SO 3 . The design of the ion-conductive ceramic and the polymer composite electrolytes is discussed based on the results.

Kinetics of Electrochemical Insertion and Extraction of Lithium Ion at SiO

The kinetics of the electrochemical insertion and extraction of lithium ion at silicon monoxide (SiO) were investigated by ac impedance spectroscopy. The resultant Nyquist plots showed two semicircles at high and middle frequency regions. These two semicircles were attributed to lithium-ion transport resistance in a surface film and alloying reaction resistance (charge-transfer resistance), respectively. We evaluated the activation energies of the charge-transfer reaction from the temperature dependences of the interfacial conductivities. When an ethylene-carbonate-based electrolyte was used, the activation energy of the charge transfer was 32 kJ mol ―1 . This activation energy was much smaller than those at graphite electrode or positive electrode materials (around 50 kJ mol ―1 or more). Based on these results, the charge transfer at SiO is exceptionally fast compared to those at other insertion materials. Furthermore, the activation energies of the charge transfer at SiO remained unchanged in various electrolytes. These results suggest that the charge-transfer kinetics at SiO is not influenced by the desolvation of lithium ion from solvent molecules.

Suppression of an Alkyl Dicarbonate Formation in Li-Ion Cells

Two methods were developed to suppress the formation of alkyl dicarbonate in Li-ion cells. One is the addition of vinylene carbonate (VC), which is vulnerable to nucleophilic attack, into the electrolyte to trap alkoxide anions that promote an alkyl dicarbonate formation. The addition of VC into the electrolyte of practical graphite/LiCoO 2 prismatic cells effectively suppressed an alkyl dicarbonate formation and gave better cycle and power performance. The other method is a surface treatment of graphite to reduce the concentration of phenolic groups that were directly bonded to the graphite edge plane. The mild burn-off treatment successfully decreased the oxygen concentration on graphite surface and effectively suppressed an alkyl dicarbonate formation in the electrolyte. However, the irreversible capacity increased significantly, which was considered to be due to a change in the surface morphology of graphite by the burn-off treatment.