Zen-ichiro Takehara

Gas Permeation in SPE Method I . Oxygen Permeation Through Nafion and NEOSEPTA

The permeation of oxygen at atmospheric pressure through Nafion® 120 and NEOSEPTA® ACH‐45T ion‐exchange membranes was investigated by an electrochemical monitoring technique, which utilizes SPE composite electrodes prepared by an electroless plating method. The oxygen diffusion coefficients were almost the same for each material, but the oxygen solubility was much higher in Nafion than in NEOSEPTA. The oxygen solubility in NEOSEPTA could be explained in terms of dissolution in the aqueous component of the membrane, but the oxygen solubility in Nafion was too high for such an explanation, and was postulated to involve the role of the polytetrafluoroethylene backbone.

Nonaqueous lithium/titanium dioxide cell

The cell performance and the reaction mechanism of the Li/1 M LiClO4 propylene carbonate/TiO2 cell, which has an open circuit voltage of 2.85–2.90 V and 950 Wh/kg of theoretical energy density, are presented. The open circuit voltage and the working voltage exceeded the theoretical open circuit voltage (1.1 V) predicted from thermodynamic data assuming 1 electron transfer per molecule of TiO2 and the products to be Ti2O3 and Li2O. The working voltage of Li/anatase TiO2 cell is higher than that of Li/rutile TiO2 cell. The reaction mechanism has been shown by the electrochemical and X-ray diffractional examination to be The self-discharge test was also done and proved the capacity loss to be negligible during a month of storage. storage.

XPS analysis of a lithium surface immersed in propylene carbonate solution containing various salts

Abstract The film resistances of lithium electrodes immersed in propylene carbonate (PC) solution containing various salts were measured using cyclic voltammetry. The changes in the film resistance depended on the type of electrolyte used. The film formed on the lithium electrode surface was analyzed by X-ray photoelectron spectroscopy (XPS). Though the surface of the lithium electrode immersed in PC containing 1.0 mol dm−3 LiAsF6 or LiClO4 reacted with the electrolyte to form small amounts of LiF or LiCl respectively, the native film originally covering the electrode surface was still present after immersion in the electrolyte. However, the native film on the lithium electrode immersed in PC containing 1.0 mol dm−3 LiPF6 or LiBF4 reacted to form a large amount of LiF. This reaction was related to the stability of the salt and the impurities in the electrolyte. The dependence of the film resistance of the lithium electrode on the type of salt used was explained by changes in the composition of the film caused by the reaction between the native film and the salt.

Electrochemical Deposition of Very Smooth Lithium Using Nonaqueous Electrolytes Containing HF

X-ray photoelectron spectroscopy and scanning electron microscopy methods were used for analysis of the surface layers of lithium deposited at various current densities from propylene carbonate containing 1.0 ml/dm{sup 3} LiClO{sub 4} and various amounts of HF, to investigate the effect of HF in electrolytes on the surface reaction of lithium during electrochemical deposition. The analyses indicate that the surface state of lithium and the morphology of lithium deposits are influenced by both the concentration of HF and the electrodeposition current. The first parameter for the electrodeposition of lithium is related to the chemical reaction rate of the lithium surface with HF and second to the electrodeposition rate of lithium. These results suggest that surface modification is effective in suppressing lithium dendrite formation when the chemical reaction rate with HF is greater than the electrochemical deposition rate of lithium.

Morphology and chemical compositions of surface films of lithium deposited on a Ni substrate in nonaqueous electrolytes

The chemical compositions of surface films of lithium deposited on a Ni substrate in γ-butyrolactone (γ-BL) or tetrahydrofuran (THF) containing 1.0 mol dm−3 LiClO4, LiAsF6, LiBF4, or LiPF6 were analyzed by X-ray photoelectron spectroscopy. The morphology of lithium deposited on these electrolytes was examined with a scanning electron microscope. The relationship between the lithium surface film formed during electrochemical deposition in γ-BL electrolytes and the morphology of the lithium were deduced. Dendrite formation was suppressed when LiPF6 + γ-BL, which includes a small amount of HF as the decomposition product of PF−6 ions, was used as the electrolyte. A similar surface film was obtained when a small amount of HF was added to LiClO4 + γ-BL. This suggests that lithium dendrite formation is suppressed in the presence of a small amount of HF which may provide a thin compact surface film. Suppression of lithium dendrites was also observed when LiAsF6 + THF was used as the electrolyte. However, it did not suppress dendrite formation completely.

Surface Condition Changes in Lithium Metal Deposited in Nonaqueous Electrolyte Containing HF by Dissolution‐Deposition Cycles

The dissolution‐deposition cycle behavior of Li metal electrodeposited in nonaqueous electrolyte containing a small amount of HF was investigated. In the first deposition process, Li particles with a smooth hemispherical shape were deposited on Ni in 1.0 M carbonate containing HF. The morphology of these fine Li particles is due to electrodeposition via migration of ions through a thin and compact surface film consisting of a bilayer, which was produced via surface modification by HF. After the first dissolution process, a residual film was observed on the entire surface of the Ni substrate. This residual film is derived from the surface film on the Li particles. Moreover, the residual film continuously accumulated on the electrode during the cycling. On the other hand, it was found that the coulombic efficiency of Li deposition‐dissolution during cycling was much improved by the addition of HF. Unfortunately, the formation of dendritic Li was observed after the 45th cycle, suggesting that the accumulated thick residual film on the Li surface inhibits the supply of HF to the Li surface during the deposition process.

Influence of Nafion® film on the kinetics of anodic hydrogen oxidation

The influence of Nafion® film on the kinetics of anodic oxidation of hydrogen was investigated on Nafion®-coated platinum electrodes. Hydrodynamic voltammetry was used to obtain the kinetic current for the H2 oxidation reaction, and the exchange current densities at bare and Nafion®-coated Pt rotating disk electrodes were determined using the modified Koutecky–Levich equations. The exchange current density at Nafion®-coated Pt was higher by 20% than that at bare Pt in 0.1 M HClO4. The enhancement in exchange current density was attributed to a higher H2 solubility in recast Nafion® than in the solution. The hydrogen solubility in recast Nafion® determined by potential step chronoamperometry (PSCA) was 1.4×10−6 mol cm−3, which was 1.8 times higher than that in the solution (0.78×10−6 mol cm−3). The difference in H2 solubility determined by potential step chronoamperometry was larger than that estimated from the difference in exchange current density. The discrepancy was explained by the peculiar multiphase-structure of Nafion®, where H2 solubility in the electrochemically inactive fluorocarbon region is higher than that in the ionic cluster region.

Chemical Reaction of Lithium Surface during Immersion in LiClO4 or LiPF6 / DEC Electrolyte

Chemical reactions of lithium with diethyl carbonate (DEC) containing 1.0 mol dm -3 LiClO 4 or LiPF 6 (LiClO 4 /DEC or LiPF 6 /DEC) were studied by using in situ Fourier transform infrared (FTIR) and x-ray photoelectron spectroscopies. The in situ FTIR spectra for both electrolytes show a penetration of the DEC electrolyte into the native surface film on lithium foils at the initial period of immersion. In the case of LiClO 4 /DEC, the DEC solvent contacts the lithium metal, and then reacts directly with lithium metal to form reductive decomposition products of DEC, such as lithium alkylcarbonate, lithium alkoxide, and LiC0 3 . When LiPF 6 /DEC was used as the electrolyte, the native surface film was gradually etched and then changed to a LiF/Li 2 O bilayer surface film. The in situ FTIR spectra showed no formation of decomposition products of DEC. This means that the surface film consisting of LiF/Li 2 O was highly effective in suppressing the direct chemical reactions of DEC with lithium metal.

Studies on Electrochemical Oxidation of Nonaqueous Electrolytes Using In Situ FTIR Spectroscopy I . The Effect of Type of Electrode on On‐Set Potential for Electrochemical Oxidation of Propylene Carbonate Containing 1.0 mol dm−3

The electrochemical oxidation of propylene carbonate containing 1.0 mol dm -3 LiClO 4 was investigated with the aid of in situ Fourier transform infrared spectroscopy. The subtractively normalized interfacial Fourier transform infrared spectra were obtained for potentials ranging from 4.0 V vs. Li/Li + to 5.0 V vs. Li/Li + . From these spectra it is concluded that propylene carbonate decomposes at more positive potentials than does 4.2 V vs. Li/Li + on an Ni electrode. The decomposition products adsorbed on the electrode surface and then gradually dissolved in the electrolyte. From the spectral change for carbonyl groups, it can be seen that the ring opening reaction of propylene carbonate is included in the decomposition process of propylene carbonate electrolytes. On the other hand, the oxidation of propylene carbonate on Al, Pt, and Au electrodes was not observed in the range of potentials investigated. Thus, the oxidation of propylene carbonate containing 1.0 mol dm -3 LiClO 4 must depend on the electrode material. When the electrode surfaces were analyzed by x-ray photoelectron spectroscopy, those of the Ni and Al electrodes were found to be covered with their oxides, but oxides were not observed on the Pt or Au electrodes. It is therefore concluded that Ni oxide probably contributes to the decomposition of propylene carbonate

Studies on electrochemical oxidation of non-aqueous electrolyte on the LiCoO2 thin film electrode

In this study, we demonstrated an in situ FTIR measurement for an electrochemical oxidation of propylene carbonate with 1.0 mol dm−3 LiClO4 on LiCoO2 cathode active material used in rechargeable lithium batteries. A thin film electrode of LiCoO2 was prepared by an r.f. sputtering method. The prepared LiCoO2 film had high quality as an electrode for the in situ FTIR measurement as well as a cathode material. The FTIR spectra were obtained at various electrode potentials ranging from 4.1 to 4.8 V vs. Li/Li+. Peaks corresponding to decomposition products of propylene carbonate show that the electrochemical oxidation of propylene carbonate was assigned to some compounds having carboxylic groups and carboxylic acid anhydrides. Several peaks attributed to propylene carbonate were also observed, indicating that propylene carbonate was adsorbed on the LiCoO2 thin film electrode surface before the anodic polarization.