Michel Rosso

Dendritic growth mechanisms in lithium/polymer cells

Direct in situ observation of dendritic electrodeposition of lithium has been performed in symmetrical lithium/PEO-LiTFSI cells under galvanostatic conditions. Our experimental set-up allows us to measure simultaneously the variation of the cell potential, the evolution of the dendrites, and the variation of the ionic concentration in the electrolyte around the dendrites. Depending on current density, we observe two different regimes for the dendritic growth: at high current density, dendrites start when the ionic concentrations drop to zero at the negative electrode, whereas at low current density, local inhomogeneities seem to play a major role.

Onset of dendritic growth in lithium/polymer cells

We have studied the onset of dendritic growth in symmetrical lithium/polymer/lithium cells, at current densities ranging from 2 x 10 -2 to 3 x 10 -1 mA cm -2 . We observe that dendritic growth starts at a time t cc which follows a power law as a function of the current density, very close to Sands law. However, in our experimental conditions no Sand behavior is expected. We attribute this surprising result to the existence and/or formation of local inhomogeneities on the surface of the electrodes. We discuss a preliminary model accounting for the observed behavior.

In situ study of dendritic growth inlithium/PEO-salt/lithium cells

Abstract Direct in situ observation of dendritic growth in lithium/PEO-LiTFSI/lithium cells shows that dendrites grow at a velocity close to the anionic drift velocity. In cycling experiments memory effects are observed, which limit the dendrite length. We have also observed a global motion of the electrolyte, related to concentration changes at the electrodes.

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 .

In Situ Concentration Cartography in the Neighborhood of Dendrites Growing in Lithium/Polymer‐Electrolyte/Lithium Cells

We report on three different in situ and ex situ concentration measurement methods in symmetric lithium/polymer‐electrolyte/lithium cells. We examine our results on the basis of a simple calculation of ionic concentration within the electrolyte; in the case where no dendrite is observed, this calculation accounts quantitatively for all experimental results. In the case of dendritic growth, we can measure the concentration distribution around the dendrites; this permits correlation of the active parts of the electrodes and of the growing dendrites with local ionic depletion in the vicinity of these active parts.

An EIS Study of the Anode Li/PEO-LiTFSI of a Li Polymer Battery

In this work, an industrially produced polymer electrolyte based on the poly(ethylene oxide) PEO-LiTFSI is studied. The evolution of the impedance spectra of symmetric cells Li/PEO-LiTFSI/Li with the aging time at 90°C is presented. The variation of impedance spectra as a function of temperature and electrolyte geometry is also shown, especially the low frequency part (up to 0.5 mHz). An equivalent electrical circuit is proposed to describe the whole spectra. The major contributions to the impedance of this interface are identified. From the low frequency contribution, the salt diffusion coefficient is determined. Finally, we use a simple model of the surface layer to gain some insight into its properties.

Experimental evidence for gravity induced motion in the vicinity of ramified electrodeposits

Abstract Electrodeposition of copper from CuSO 4 solutions under high electric field gives rise to ramified deposits. We present optical measurements of the concentration maps in the electrolyte during copper electrodeposition, showing the role of gravity driven motion in the solution. Together with cell potential measurements, our experiments permit to give a semi-quantitative description of the onset of ramified electrodeposition.

Viscosity Effects in Thin-Layer Electrodeposition

We present experimental results and a theoretical macroscopic model on the effects of viscosity in thin-layer electrochemical growth. The viscosity was changed through glycerol additions; simultaneous use was made of optical and schlieren techniques for tracking concentration and convective fronts, while pH indicators were used for migratory fronts. The theoretical model describes diffusive, migratory, and convective ion transport in a fluid subject to an electric field, The equations are written in terms of dimensionless quantities, in particular, the Migration, Peclet, Poisson, Reynolds, and electrical Grashof numbers, which are found to depend on viscosity, Experiments reveal that with increasing viscosity, convection decreases, concentration profiles are less pronounced, while electric resistance and voltage increase. Concentration and convective fronts slow down with viscosity, but their time scaling follows the Same law as for solutions without glycerol, only differing by a constant. Moreover, under constant electrical current, an increase in viscosity yields slower deposit front velocities, a more uniform deposit with smaller separation between branches, i.e., a change in morphology from more separated compact trees to a more dense, fractal-like structure.

Effect of Lithiation Potential and Cycling on Chemical and Morphological Evolution of Si Thin Film Electrode Studied by ToF-SIMS

Si thin films obtained by plasma enhanced chemical vapor deposition (PECVD) were used to investigate chemical and morphological modifications induced by lithiation potential and cycling. These modifications were thoughtfully analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling, which allows to distinguish the surface and bulk processes related to the formation of the solid electrolyte interphase (SEI) layer, and Li-Si alloying, respectively. The main results are a volume expansion/shrinkage and a dynamic behavior of the SEI layer during the single lithiation/delithiation process and multicycling. Trapping of lithium and other ions corresponding to products of electrolyte decomposition are the major reasons of electrode modifications. It is shown that the SEI layer contributes to 60% of the total volume variation of Si electrodes (100 nm). The apparent diffusion coefficient of lithium (DLi) calculated from the Ficks second law directly from Li-ion ToF-SIMS profiles is of the order of ∼5.9 × 10(-15) cm(2).s(-1). This quite low value can be explained by Li trapping in the bulk of electrode material, at the interfaces, continuous growth of the SEI layer and increase of SiO2 quantity. These modifications can result in limitation the ionic transport of Li.

Concentration measurements in lithium/polymer–electrolyte/lithium cells during cycling

Abstract We report on in situ and ex situ concentration measurements in lithium/polymer–electrolyte/lithium cells during cycling. We have used three different methods which give complementary results, in good agreement with theoretical predictions and previous concentration measurements by Raman confocal microspectroscopy. Our methods allow to obtain concentration maps in the electrolyte, in particular, when dendrites are observed: from these measurements, we can correlate the onset of dendritic growth with local concentration gradients.