Kiyoshi Kanamura

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.

Anodic oxidation of nonaqueous electrolytes on cathode materials and current collectors for rechargeable lithium batteries

Abstract Oxidation of propylene carbonate on Al and LiCoO 2 electrodes was observed by using cyclic voltammetry and in situ FTIR spectroscopy combined with potential step experiment. From these measurements, two points were discussed in this study. The oxidation of solvents on LiCoO 2 occurred at 4.2 V vs. Li/Li + that corresponds to the cut-off potential of rechargeable lithium ion batteries, because of high catalytic activity of transition metal oxide materials. On the other hand, the oxidation on Al as a current collector was strongly influenced by passivation phenomena in nonaqueous electrolyte. The passivation phenomena depend on a kind of electrolyte salt. Among four electrolyte salts used in this study [LiClO 4 , LiPF 6 , Li(CF 3 SO 2 ) 2 N, and Li(CF 3 SO 2 )(C 4 F 9 SO 2 )N], Li(CF 3 SO 2 )(C 4 F 9 SO 2 )N exhibited several interesting features which were useful to suppress the anodic oxidation of nonaqueous electrolytes and dissolution of Al. Furthermore, a mixed solvent of ethylene carbonate and dimethoxyethane with Li(CF 3 SO 2 )(C 4 F 9 SO 2 )N was not oxidized at 4.8 V vs. Li/Li + on both LiCoO 2 and Al electrodes. In addition, the anodic corrosion of Al in this electrolyte was suppressed.

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.

XPS analysis for the lithium surface immersed in γ-butyrolactone containing various salts

Lithium surfaces immersed in γ-butyrolactone solutions containing salts were analyzed by X-ray photoelectron spectroscopy. The lithium surface before immersion in electrolyte was covered with the native film that consists of Li2CO3, Li2O and LiOH. During the immersion of lithium in electrolyte, the native film reacted with acid in the electrolyte to form lithium halide on the lithium surface. The formation of lithium halide strongly depended on the kind of salt. The reaction rate of the native film in γ-butyrolactone containing 1.0 mol dm−3 LiClO4 or LiAsF6 was much less than that in γ-butyrolactone containing 1.0 mol dm−3 LiBF4 or LiPF6. The morphology of lithium deposited on the lithium surface immersed in electrolyte for three days was influenced by the surface state of lithium. The film formed on the lithium surface immersed in γ-butyrolactone containing 1.0 mol dm−3 LiPF6 was thinner than those immersed in any other electrolytes. The chemical compositions of the lithium surface immersed in γ-butyrolactone containing 1.0 mol dm−3 LiPF6 were different from those of the lithium surface immersed in γ-butyrolactone containing 1.0 mol dm−3 LiAsF6, LiClO4, or LiBF4. When γ-butyrolactone containing 1.0 mol dm−3 LiPF6 was used as electrolyte, dendrite formation was not observed on lithium immersed in the electrolyte for three days.

Effect of morphology of polyaniline on its discharge characteristics in nonaqueous electrolyte

Various conducting organic polymers, such as polyaniline, polypyrrole, and polyacetylene have been investigated as functional materials for many kinds of electrochemical applications. For example, polyaniline has been used as a cathode material in rechargeable nonaqueous batteries. The discharge and charge performance of polyaniline which has a fibrous morphology was investigated from microscopic current density and anion diffusion time constant measurements. The microscopic discharge current density is calculated from the radius of the polyaniline fibril, assuming the true electrode surface area corresponds to the surface area of the long polyaniline fibril (2 Al, L:the sum of the length of the polyaniline fibril). The time constant for the diffusion of anions in the polyaniline fibril was calculated from a{sup 2}/D, where D is the diffusion coefficient and a is the polyaniline fibril radius. The diffusion coefficient was measured using an impedance method, and the a{sup 2}/D parameter was calculated. Galvanostatic discharge and charge cycle tests were carried out to decide the performance of polyaniline cathodes with different fibril radii. From these experiments, it can be seen that the dependence of the discharge characteristics on the fibril radius is diminished by the positive dependencies of the diffusion coefficient and roughness of the polyanilinemorexa0» fibril on the fibril radius.«xa0less

Electrochemical Deposition of Uniform Lithium on an Ni Substrate in a Nonaqueous Electrolyte

The electrochemical deposition of lithium on an Ni substrate was conducted in propylene carbonate (PC) containing 1.0 mol dm[sup [minus]3] LiClO[sub 4] (LiClO[sub 4]/PC). The morphology of the lithium deposited on the Ni substrate had the typical dendrite form. The electrodeposition of lithium was then performed in LiClO[sub 4]/PC containing 5 [times] 10[sup [minus]3] HF. The lithium deposited on the Ni substrate in this electrolyte had a hemispherical form, and irregular shapes were not observed. The color of the Ni electrodes surface turned to brilliant blue during the electrodeposition of lithium. This indicates that the lithium surface is very smooth and uniform. After five discharge and charge cycles, there were no lithium dendrites on the electrode surface. From these results, it can be concluded that the addition of a small amount of HF to the electrolyte is significantly effective for the suppression to the lithium dendrite formation.

Electrochemical oxidation of propylene carbonate (containing various salts) on aluminium electrodes

Electrochemical reactions taking place on aluminium electrodes in propylene carbonate, with 1.0 M LiClO4, LiBF4, or LiPF6, are investigated in terms of the stability of non-aqueous electrolytes. The techniques used are potential sweep method, X-ray photoelectron spectroscopy, and in situ Fourier-transform infrared (FT-IR) spectroscopy. From these analysis, it is found that the surface of the aluminium electrode is covered with aluminium oxides and fluorides as a passivation layer when it is polarized at 5.5 V versus LiLi+. Moreover, the surface state of the aluminium electrode depends on the type of electrolyte salt, as do the current-potential curves. On the other hand, in situ FT-IR studies indicate that the oxidation products of propylene carbonate are independent of the type of electrolyte salt. From these results, it is concluded that the stability of propylene carbonate electrolyte depends on the surface state of the aluminium electrode which is strongly combined with the stability of anions.