Zempachi Ogumi

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.

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.

Stability of Pt-catalyzed highly oriented pyrolytic graphite against hydrogen peroxide in acid solution

Aiming at clarifying the effects of hydrogen peroxide on the degradation of Pt/C catalyst in polymer electrolyte fuel cells (PEFCs), a model electrode for the Pt/C catalyst was prepared by electrodeposition of Pt on highly oriented pyrolytic graphite (HOPG) and its durability against hydrogen peroxide investigated. After storage in acidic 1.0 mol dm - 3 H 2 O 2 solution at room temperature, many scars were formed by oxidative etching of HOPG and Pt-particle catalysts aggregated on the surface. In contrast, no surface change was observed without hydrogen peroxide. These results showed that hydrogen peroxide significantly deteriorates the Pt/HOPG electrode, and the active surface area of the Pt particles, which was determined by cyclic voltammetry, was reduced after immersion in the H 2 O 2 solution. The influence of applied potential was also investigated, and the deterioration of the electrode was accelerated at potentials both more positive and negative than open-circuit potential (ca. 0.8 V). Moreover, similar degradation phenomenon was observed when the potential was set at 0.1 V, at which a large amount of H 2 O 2 is formed in oxygen reduction, in O 2 -saturated H 2 O 2 -free acid solution for 5 days.

Electrocatalytic Oxidation of Ethylene Glycol in Alkaline Solution

The electrochemical oxidation of ethylene glycol in alkaline solution was investigated using Pt-Ru/C, Pt-Pb/C and ultrafine gold deposited on a-Fe 2 O 3 /Pt/C (Au/α-Fe 2 O 3 /Pt/C) electrodes. The ultrafine gold particles prepared in the present study were ∼3 nm in diameter. These catalysts enhanced the activity in ethylene glycol oxidation in lower potential regions. In particular, the Aula-Fe 2 O 3 /Pt/C electrode showed the highest catalytic activity in ethylene glycol oxidation in a less noble potential region (<200 mV vs. RHE).

Charge-Transfer Reaction at the Lithium Phosphorus Oxynitride Glass Electrolyte/Lithium Manganese Oxide Thin-Film Interface and Its Stability on Cycling

Charge-transfer reaction at a lithium manganese oxide (LiMn 2 O 4 ) thin-film electrode/lithium phosphorus oxynitride glass electrolyte (LiPON) interface was investigated using all-solid-state thin-film batteries (Li/LiPON/LiMn 2 O 4 ). X-ray diffraction measurements revealed that the crystal structure of the thin-film LiMn 2 O 4 electrode changed on depositing the LiPON thin-film electrode, but a thermal treatment at 498 K for 60 min re-formed the original crystal structure. The potential sweep curve of the thermally treated film battery was identical to the cyclic voltammogram of a LiMn 2 O 4 thin-film electrode in a conventional organic electrolyte (1 mol dm -3 LiClO 4 dissolved in propylene carbonate). In contrast to a LiPON/LiCoO 2 interface, the charge-transfer resistance at the LiPON/LiMn 2 O 4 interface did not decrease sufficiently after the thermal treatment relative to the charge-transfer resistance of the organic electrolyte/LiMn 2 O 4 interface. This indicates that there should be a compatible electrode and LiPON film electrolyte combination to obtain an effective decrease in the charge-transfer resistance. Charge-discharge tests revealed that the resultant film battery repeated stable charge-transfer reaction on its cycling compared with the organic electrolyte system. Also, this electrochemical stability was maintained at a high temperature (333 K), which is probably because the formation of the LiMn 2 O 4 /LiPON interface inhibited Mn dissolution from the LiMn 2 O 4 thin-film electrode.

Electro-oxidation of methanol on gold nanoparticles supported on Pt/MoOx/C

Gold nanoparticles supported on Pt/MoO x /C were prepared using the chemical vapor deposition method, and their electrocatalytic activities for methanol oxidation in acid solution were evaluated. The gold nanoparticles on Pt/MoO x /C were about 4 nm in diameter. The catalytic properties of such catalysts containing gold nanoparticles were studied in 1 mol dm - 3 HClO 4 with 1 M CH 3 OH by the steady-state polarization method using a conventional three-electrode cell. Steady-state currents were measured after 1000 s from the beginning of electro-oxidation at potentials from 350 to 700 mV. The catalytic activities of Pt/MoO x /C and Pt/C were also studied for comparison. As a result, gold nanoparticles enhanced the electrocatalytic activities for methanol oxidation in the potential region below 450 mV (vs reversible hydrogen electrode) compared with the catalysts Pt/MoO x /C and Pt/C.

Effect of an Alkyl Dicarbonate on Li-Ion Cell Performance

The effect of an alkyl dicarbonate, diethyl 2,5-dioxahexane dicarboxylate (DEDOHC, C 2 H 5 OCO 2 C 2 H 4 OCO 2 C 2 H 5 ), which is formed by an ester exchange reaction in a mix solvent system of ethylene carbonate (EC) and diethyl carbonate (DEC), on the performance of graphite/LiCoO 2 lithium-ion cells was investigated. In the presence of 10 vol % DEDOHC in 1 M LiClO 4 /EC + DEC(1:1 v/v), discharge capacity decreased, in particular at a high charge/discharge rate. Capacity retention at a high charge/ discharge rate also decreased during repeated charge and discharge cycles. These deteriorations in performance were attributed to an increase in cell impedance and deficiency of mobile lithium ion, which is caused by continuous consumption of lithium ion on graphite negative electrode. The results of half-cell tests revealed that the addition of DEDOHC causes serious degradation for the graphite half-cell. From ac impedance measurements it was found that surface film resistance on graphite electrode was fourfold higher in the solution containing DEDOHC, whereas charge-transfer resistance scarcely changed. Atomic force microscopy observation revealed the formation of dense and highly adhesive surface precipitate layer on graphite electrode in the presence of DEDOHC. Consequently, it was concluded that the increase in cell impedance, which leads to deterioration of charge and discharge performance, is mainly due to the increase in the resistance of the surface film formed from the reduction of DEDOHC on the graphite negative electrode.

Amorphous Li–V–Si–O Thin Films as High-Voltage Negative Electrode Materials for Thin-Film Rechargeable Lithium-Ion Batteries

A new kind of high-voltage negative electrode material for thin-film rechargeable lithium-ion batteries was prepared through electrochemically reductive decomposition of an amorphous Li-V-Si-O thin solid electrolyte film. The thin solid electrolyte film was prepared by pulsed laser deposition, showing 1.3 X 10 -7 S cm -1 in ionic conductivity with negligible electronic conductivity. Electrochemical lithium insertion into the thin solid electrolyte film was observed at ca. 1.7 V (vs Li/Li + ), which was the potential window of the reduction side for the film electrolyte. When the lithium insertion/extraction reaction was repeated over the reductive-side potential window (1.0-4.0 vs Li/Li + ) at low current density, the charge/discharge capacity gradually increased with the repetition of the reaction. Consequently, large charge/discharge capacity (350 mAh g -1 ) was achieved under quasi-open-circuit voltage conditions between 1.2 and 2.7 V (vs Li/Li + ). X-ray photoelectron spectroscopy analysis revealed that the redox reaction of vanadium ions in the film compensated the electrical charge balance for the lithium insertion/extraction reaction of the film electrolyte. The apparent lithium diffusion coefficient of the film was 1 X 10 -15 cm 2 s -1 < D app Li < 2 X 10 -13 cm 2 s -1 .