Kei Nishikawa

In Situ Observation of Dendrite Growth of Electrodeposited Li Metal

The dendrite growth behavior of Li metal galvanostatically electrodeposited on Ni substrate in a LiClO 4 -propylene carbonate electrolyte solution was in situ observed by a laser scanning confocal microscope with a metallographic microscope. A Li dendrite precursor is stochastically evolved on Ni substrate probably through a solid electrolyte interphase layer produced by the surface chemical reaction between a reduced Li metal and an organic electrolyte. The measured length of randomly growing Li dendrite arms was statistically analyzed. The initiation period of the dendrite precursor becomes shorter with increasing current density and decreasing LiClO 4 concentration. Once it has been initiated, the ionic mass transfer rate starts to govern the growth process of the dendrite arm length, exceeding over the surface chemistry controlling step. The dendrite arm length averaged over the substrate surface grows linearly proportional to the square root of time. The lower the concentration of LiClO 4 , the steeper the inclination of the line at 5 mA cm -2 , whereas the concentration dependence of inclination is not evident at 0.5 mA cm -2 .

Diffusivity Measurement of LiPF6, LiTFSI, LiBF4 in PC

The molecular diffusion coefficients of LiPF6, LiTFSI and LiBF4 in PC were measured by Moire pattern technique within the salt concentration range from 0.5 M to 1.0 M and at the temperature range from 258 K to 298 K. The diffusion coefficient of LiPF6 and LiTFSI are around 4×10 cm s and that of LiBF4 is around 3×10 cm s at 298 K. The activation energy of LiPF6-PC, LiTFSI-PC and LiBF4-PC system was calculated from an Arrhenius plots. They were 14, 18 and 21 kJ mol, respectively.

Measurement of Concentration Profiles during Electrodeposition of Li Metal from LiPF6-PC Electrolyte Solution The Role of SEI Dynamics

During Li metal electrodeposition from a 0.5 M LiPF 6 -PC electrolyte solution onto a horizontal Li metal electrode, the refractive index profile corresponding to the concentration profile of Li + ion near the cathode was measured in situ by holographic interferometry. The Li + concentration gradient around the rapidly growing dendrite arms is steeper than at the cathode plane, clearly reflecting the local current density convergence at the dendrite tips and arms. As in a LiCl0 4 -PC solution, an incubation period was observed between the start of current passage and the onset of the refractive index fringe shift. It increases with decreasing applied current density. The incubation period in LiPF 6 -PC is shorter than that in LiClO 4 -PC at current densities greater than 1.0 mA cm -2 ; however, at 0.5 mA cm -2 in LiPF 6 -PC electrolyte solution it is appreciably longer than in LiClO 4 -PC. This complicated behavior is apparently due to the solution chemistry of LiPF 6 -PC electrolyte, which produces HF and oxyfluoride impurities that are lacking in the LiClO 4 -PC system. Thus, the different current dependence of the incubation time in these two systems may yield clues for an elucidation of the dynamics of solid electrolyte interphase formation.

Measurement of LiClO4 Diffusion Coefficient in Propylene Carbonate by Moiré Pattern

The binary (molecular) diffusion coefficients of LiClO 4 in propylene carbonate (PC) solution was measured by the Moire pattern method. The Moire pattern method has the advantage over standard electrochemical techniques in that it allows simultaneous measurements of local, i.e., concentration-dependent, diffusivities over the concentration range used in one single experiment, whereas electrochemical methods yield a single effective (i.e., integral) diffusivity over the same concentration range. At 298 K, the variation of the binary diffusion coefficient with concentration is from 1.9 × 10 -6 (cm 2 /s) at 0.5 M to 1.2 X 10 -6 (cm 2 /s) at 1.0 M. Linear extrapolation to the dilute concentration region yields a diffusion coefficient of 2.6 × 10 -6 (cm 2 /s) (at 298 K), in reasonable agreement with the self-diffusion coefficient of Li + ion in nonaqueous solutions measured by the pulsed-gradient spin echo nuclear magnetic resonance technique. The Stokes-Einstein parameter of the LiClO 4 -PC diffusivity is independent of concentration within 9% over the 0.1-0.9 M range and yields a Stokes radius for the Li + ClO - 4 pair of approximately 0.36 nm.

Flux growth of hexagonal cylindrical LiCoO2 crystals surrounded by Li-ion conducting preferential facets and their electrochemical properties studied by single-particle measurements

In this study, we demonstrate the template-mediated flux growth of one-dimensional LiCoO2 single crystals surrounded by {104} faces in a hot solution of LiCl–KCl. The reaction and growth processes were characterized by time-dependent X-ray diffraction and scanning electron microscopy. The transformation in the crystal shape from rectangular to hexagonal cylindrical was considered to be directly related to the gradual lithiation of the starting CoO whiskers. Single particle galvanostatic tests of the single-strand LiCoO2 crystals were carried out. The LiCoO2 crystals exhibited excellent rate performance and more than 65% of the full capacity was maintained under ca. 370C. These characteristics likely resulted from the exposure of the {104} faces, since they were electrochemically active in layered LiCoO2 with an α-NaFeO2 structure and favored fast Li+ transportation. This finding will facilitate the development of new materials for advanced lithium ion rechargeable batteries.

Morphological Variation of Electrodeposited Li in Ionic Liquid

Li metal negative electrode is expected for the next-generation Li battery electrode material, and the control of morphological variation of the Li metal electrode surface is very important issue. In-situ observation of the Li dendrite growth in an ionic liquid by an optical microscope was done in order to discuss the relationship between the Li dendrite growth and the Li + ionic mass transfer rate. Li dendrite growth in the ionic liquid starts before the surface Li + ion concentration which is calculated by a simple model become zero. The development of the dendrite length shows the linearity with the square root of time, and this slope changes just after the Li + ion is depleted at the electrode surface. These phenomena mean the Li + ionic mass transfer rate is also an important parameter for the Li dendrite growth, as well as the surface state of the Li metal electrode.

Electrodeposition experiments in microgravity conditions

Metal electrodeposition may produce irregular deposits with various morphologies. For liquid electrolytes a precise study of these deposits may be complicated by convective motion due to buoyancy. Zero-gravity (0g) conditions provided by parabolic flights give a straightforward mean to avoid this effect: we present here 0g electrodeposition experiments, that we compare to ground experiments (1g). Two electrochemical systems were studied by laser interferometry, allowing to measure concentration variations in the electrolyte: copper deposition from copper sulfate aqueous solution and lithium deposition from an ionic liquid containing LiTFSI. For copper, concentration variations were in good agreement with theory. For lithium, an induction time was observed for the concentration evolution at 1g: due to this induction time and to the low diffusion coefficient in the ionic liquid, concentration variations were hardly measurable in the parabolic flight 0g periods of 20 seconds.