Soshi Shiraishi

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.

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.

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.

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.

Influence of initial surface condition of lithium metal anodes on surface modification with HF

Surface modification of as-received lithium foils was carried out using acid-base reactions of the native surface films on lithium metal with HF. Two types of as-received lithium foils covered with different native films were used as samples for this surface modification. One was a lithium foil having a very thin native surface film and the other one had a thicker native surface film. The surface condition of the lithium metal was analysed by X-ray photoelectron spectroscopy before and after the surface modification using HF, and the coulombic efficiency was measured electrochemically. The thickness of the surface film on the modified lithium foils was related to the Li2O layer thickness in the native film on the as-received lithium foils. The modified lithium foil which had the thinner native surface film provided more uniform deposition of lithium and a higher coulombic efficiency during charge and discharge cycles when propylene carbonate electrolyte with 1.0 m LiPF6 was used as the electrolyte. These results show that the initial condition of the native surface film plays an important role in surface modification with HF.

Effect of surface modification using various acids on electrodeposition of lithium

Sulface modification of lithium was carried out using the chemical reaction of the native film with acids (HF, H3PO4, HI, HCl) dissolved in propylene carbonate (PC). The chemical composition change of the lithium surface was detected using X-ray photoelectron spectroscopy. The electrodeposition of lithium on the as-received lithium or the modified lithium was conducted in PC containing 1.0 mol dm−3 LiClO4 or LiPF6 under galvanostatic conditions. The morphology of electrodeposited lithium particles was observed with scanning electron microscopy. The lithium dendrites were observed when lithium was deposited on the as-received lithium in both electrolytes. Moreover the dendrites were also formed on the lithium surface modified with H3PO4, HI, or HCl. On the other hand, spherical lithium particles were produced, when lithium was electrodeposited in PC containing 1.0 mol dm−3 LiPF6 on the lithium surface modified with HE However spherical lithium particles were not obtained, when PC containing 1.0 mol dm−3 LiClO4 was used as the electrolyte. The lithium surface modified by H3PO4, HI, or HCl was covered with a thick film consisting of Li3PO4, Li2CO3, LiOH, or Li2O. The lithium surface modified with HF was covered with a thin bilayer structure film consisting of LiF and Li2O. These results clearly show that the surface film having the thin bilayer structure (LiF and Li2O) and the use of PC containing 1.0 mol dm−3 LiPF6 enhance the suppression of dendrite formation of lithium.