Theory and experimental evidence of electroconvection around electrochemical deposits

Review of the Real-Time Monitoring Technologies for Lithium Dendrites in Lithium-Ion Batteries

Lithium-ion batteries (LIBs) have the advantage of high energy density, which has attracted the wide attention of researchers. Nevertheless, the growth of lithium dendrites on the anode surface causes short life and poor safety, which limits their application. Therefore, it is necessary to deeply understand the growth mechanism of lithium dendrites. Here, the growth mechanism of lithium dendrites is briefly summarized, and the real-time monitoring technologies of lithium dendrite growth in recent years are reviewed. The real-time monitoring technologies summarized here include in situ X-ray, in situ Raman, in situ resonance, in situ microscopy, in situ neutrons, and sensors, and their representative studies are summarized. This paper is expected to provide some guidance for the research of lithium dendrites, so as to promote the development of LIBs.

Self-assembly growth of electrolytic silver dendrites

The atomic level assembly of silver dendrite has never been disclosed despite the numerous studies published on fractal dendrite structures. We report for the first time an HRTEM investigation of the formation of atomic embryos (< 5 nm) and the self-assembly of atoms on an atomic plane of the tip of a dendrite arm. The mechanism of dendrite formation proceeds via the sequence of amorphous embryos aggregates (5–10 nm), nuclei, crystallites (10–20 nm), dendritelets (50–100 nm) and submicron dendrite protypes. The atomic plane is an entirely atomic-level zig-zag structure with d-spacing kink steps. The zig-zag structure triggers the self-assembly of atoms and thus directional growth to produce a dendrite arm with a high aspect ratio.

Formation of magnetic nanowire arrays by cooperative lateral growth

Nanowires typically grow along their longitudinal axis, and the long-range order among wires sustains only when a template exists. Here, we report an unprecedented electrochemical growth of ordered metallic nanowire arrays from an ultrathin electrolyte layer, which is achieved by solidifying the electrolyte solution below the freezing temperature. The thickness of the electrodeposit is instantaneously tunable by the applied electric pulses, leading to parallel ridges on webbed film without using any template. An array of metallic nanowires with desired separation and width determined by the applied pulses is formed on the substrate with arbitrary surface patterns by etching away the webbed film thereafter. This work demonstrates a previously unrecognized fabrication strategy that bridges the gap of top-down lithography and bottom-up self-organization in making ordered metallic nanowire arrays over a large area with low cost.

Nanoscaled Lithium Powders with Protection of Ionic Liquid for Highly Stable Rechargeable Lithium Metal Batteries

To suppress the dendrite formation and alleviate volume expansion upon striping/platting is a key challenge for developing practical lithium metal anodes. Lithium metal in powder form possesses great potential to address this issue due to large specific surface area. However, the fabrication of powdery metallic lithium is largely restricted because of its unique softness, stickiness, and high reactivity. Here, a safe and readily accessible cryomilling process toward lithium powders is reported. Nanoscaled lithium powders (<500 nm) are successfully prepared from lithium foils with the assistance of a high-melting-point ionic liquid under cryogenic temperature. The prepared lithium powder anode exhibits superior electrochemical properties in symmetric cells, including extraordinarily low yet stable overpotential (≈50 mV), ultrahigh area capacity (30 mAh cm-2), and good long-term cyclability (1200 h) even cycling at high current density (10 mA cm-2). The powdery form of lithium also functions as a favorable prelithiation reagent for lithium-free anodes (e.g., Si, SiO, and SnO2). The findings open up a new avenue for the real-world application of lithium metal anodes for next-generation lithium batteries.

Developing High-Performance Lithium Metal Anode in Liquid Electrolytes: Challenges and Progress

Lithium metal anodes are potentially key for next-generation energy-dense batteries because of the extremely high capacity and the ultralow redox potential. However, notorious safety concerns of Li metal in liquid electrolytes have significantly retarded its commercialization: on one hand, lithium metal morphological instabilities (LMI) can cause cell shorting and even explosion; on the other hand, breaking of the grown Li arms induces the so-called "dead Li"; furthermore, the continuous consumption of the liquid electrolyte and cycleable lithium also shortens cell life. The research community has been seeking new strategies to protect Li metal anodes and significant progress has been made in the last decade. Here, an overview of the fundamental understandings of solid electrolyte interphase (SEI) formation, conceptual models, and advanced real-time characterizations of LMI are presented. Instructed by the conceptual models, strategies including increasing the donatable fluorine concentration (DFC) in liquid to enrich LiF component in SEI, increasing salt concentration (ionic strength) and sacrificial electrolyte additives, building artificial SEI to boost self-healing of natural SEI, and 3D electrode frameworks to reduce current density and delay Sand's extinction are summarized. Practical challenges in competing with graphite and silicon anodes are outlined.

Confined Electroconvective Vortices at Structured Ion Exchange Membranes

In this paper, we investigate electroconvective ion transport at cation exchange membranes with different geometry square-wave structures (line undulations) experimentally and numerically. Electroconvective microvortices are induced by strong concentration polarization once a threshold potential difference is applied. The applied potential required to start and sustain electroconvection is strongly affected by the geometry of the membrane. A reduction in the resistance of approximately 50% can be obtained when the structure size is similar to the mixing layer (ML) thickness, resulting in confined vortices with less lateral motion compared to the case of flat membranes. From electrical, flow, and concentration measurements, ion migration, advection, and diffusion are quantified, respectively. Advection and migration are dominant in the vortex ML, whereas diffusion and migration are dominant in the stagnant diffusion layer. Numerical simulations, based on Poisson-Nernst-Planck and Navier-Stokes equations, show similar ion transport and flow characteristics, highlighting the importance of membrane topology on the resulting electrokinetic and electrohydrodynamic behavior.

Suppression of dendritic lithium growth in lithium metal-based batteries

We describe the challenges of high-energy lithium-metal batteries and outline the future directions that are expected to drive their progress.

Electrochemical migration of Sn and Sn solder alloys: a review

The schematic diagram of electrochemical migration of Sn solder alloys joints.

Fractal growth of platinum electrodeposits revealed by in situ electron microscopy

Fractals are commonly observed in nature and elucidating the mechanisms of fractal-related growth is a compelling issue for both fundamental science and technology. Here we report an in situ electron microscopy study of dynamic fractal growth of platinum during electrodeposition in a miniaturized electrochemical cell at varying growth conditions. Highly dendritic growth - either dense branching or ramified islands - are formed at the solid-electrolyte interface. We show how the diffusion length of ions in the electrolyte influences morphology selection and how instability induced by initial surface roughness, combined with local enhancement of electric field, gives rise to non-uniform branched deposition as a result of nucleation/growth at preferred locations. Comparing the growth behavior under these different conditions provides new insight into the fundamental mechanisms of platinum nucleation.

Competition between diffusion and electroconvection at an ion-selective surface in intensive current regimes

Considering diffusion near a solid surface and simplifying the shape of concentration profile in diffusion-dominated layer allowed Nernst and Brunner to propose their famous equation for calculating the solute diffusion flux. Intensive (overlimiting) currents generate electroconvection (EC), which is a recently discovered interfacial phenomenon produced by the action of an external electric field on the electric space charge formed near an ion-selective interface. EC microscale vortices effectively mix the depleted solution layer that allows the reduction of diffusion transport limitations. Enhancement of ion transport by EC is important in membrane separation, nano-microfluidics, analytical chemistry, electrode kinetics and some other fields. This paper presents a review of the actual understanding of the transport mechanisms in intensive current regimes, where the role of diffusion declines in the profit of EC. We analyse recent publications devoted to explore the properties of different zones of the diffusion layer. Visualization of concentration profile and fluid current lines are considered as well as mathematical modelling of the overlimiting transfer.

Sharp-interface model of electrodeposition and ramified growth

We present a sharp-interface model of two-dimensional ramified growth during quasisteady electrodeposition. Our model differs from previous modeling methods in that it includes the important effects of extended space-charge regions and nonlinear electrode reactions. The electrokinetics is described by a continuum model, but the discrete nature of the ions is taken into account by adding a random noise term to the electrode current. The model is validated by comparing its behavior in the initial stage with the predictions of a linear stability analysis. The main limitations of the model are the restriction to two dimensions and the assumption of quasisteady transport.

Nanometer-thick lateral polyelectrolyte micropatterns induce macrosopic electro-osmotic chaotic fluid instabilities

Electro-convective vortices in ion concentration polarization under shear flow have been of practical relevance for desalination processes using electrodialysis. The phenomenon has been scientifically disregarded for decades, but is recently embraced by a growing fluid dynamics community due its complex superposition of multi-scale gradients in electrochemical potential and space charge interacting with emerging complex fluid momentum gradients. While the visualization, quantification and fundamental understanding of the often-chaotic fluid dynamics is evolving rapidly due to sophisticated simulations and experimentation, little is known whether these instabilities can be induced and affected by chemical topological heterogeneity in surface properties. In this letter, we report that polyelectrolyte layers applied as micropatterns on ion exchange membranes induce and facilitate the electro-osmotic fluid instabilities. The findings stimulate a variety of fundamental questions comparable to the complexity of today's turbulence research.

Template-free and filamentary growth of silver nanowires: application to anisotropic conductive transparent flexible electrodes

Silver nanowires (NWs) are currently fabricated via template-free or template-assisted methods. The former is based on a medium-mediated anisotropic synthesis, which enables precursor atoms to be selectively adsorbed onto specific crystallographic planes, and the latter is performed via directional growth guided by preformed templates. These methods are costly and complicated. We outline a facile and low-cost approach for the electrochemical synthesis of silver NWs in a manner that is template- and surfactant-free and that provides control over the NW diameter in the range of 80 to 800 nm by the repetition of nucleation and dissolution. The nanowires vertically grow with the help of the interface anisotropy driven by a field enhancement at the tips of the islands nucleated on the substrate in ultra-dilute electrolytes (ca. 10(-5) M), which is similar to a lightning-rod effect. The silver nanowires of vertical configuration are utilized for fabrication of anisotropic conducting, transparent, and flexible films.

Recent Progress in Electrochemical Deposition without Supporting Electrolyte

Electrochemical deposition (ECD) of metals is a very old subject[l], which has considerable applications in the context of electroshaping or electroplating. Electrochemists and chemical engineers have long known the different growth conditions of these metal aggregates and the different parameters which drive morphological changes, at least empirically [2-4]. However, in the recent years, after the introduction of the concept of fractal aggregation[5,6], in the field of non-linear pattern formation[7,8], a lot of work has been devoted to the specific problem of growth of electrodeposits from binary electrolytes [9-51] (i.e. without supporting electrolyte). These studies aimed at understanding the morphology, on the large scale (∼1cm) of the deposits and, more specifically, the transitions between morphologies. It is the aim of this paper to review the progress which has been achieved in the past five years on this question.

Electrokinetic instability in microchannels

The effect of geometric confinement on electroconvective instability due to nonequilibrium electro-osmotic slip at the interface of an electrolytic fluid and charge-selective solid is studied. It is shown that the topology of the marginal stability curves and the behavior of the critical parameters depend strongly on both channel geometry and dimensionless Debye length at low voltages for sufficiently deep channels, corresponding to the Rubinstein-Zaltzman instability mechanism, but that stability is governed almost entirely by channel depth for narrow channels at higher voltages. For shallow channels, it is shown that above a transition threshold, determined by both channel depth and Debye length, the low-voltage instability is completely suppressed.

A Growth Model For Ramified Electrochemical Deposition

We introduce a macroscopic model for the description of growth pattern formation in ramified electrochemical deposition. The theoretical model is formulated as a 2D time-dependent problem consisting in the Nernst-Planck equations for the concentration of the solute (cations and anions), coupled to a Poisson equation for the electrostatic potential and the Navier-Stokes equations for the solvent, with a moving boundary. A dimensional analysis is performed and a new set of dimensionless numbers governing the flow regime is derived. A 2D discrete version of these equations in a DBM scheme with a random moving boundary constitutes the computational model. We present numerical results which show that our growth model, with a proper variation of the set of dimensionless numbers, gives a reasonable picture of the interplay of the electroconvective, migration and diffusive motion of the ions near the growing tips.

Simulations of transport regime in electrodeposition in different viscosity scenarios

In this work we study the effects of viscosity variations in thin-layer electrochemical deposition (ECD) under galvanostatic conditions through experimental measurements and theoretical modeling. The theoretical model, written in terms of dimensionless quantities, describes diffusive, migratory and convective ion transport in a fluid under galvanostatic conditions. Experiments reveal that as viscosity increases, convection decreases when the cell resistance remains constant. Our numerical model predicts that as viscosity increases, electroconvection becomes less relevant and concentration and convective fronts slow down. The time scaling of this phenomenon is studied and compared to previously reported low viscosity solution studies.

Direct observation of a nonequilibrium electro-osmotic instability

We present a visualization of the predicted instability in ionic conduction from a binary electrolyte into a charge selective solid. This instability develops when a voltage greater than critical is applied to a thin layer of copper sulfate flanked by a copper anode and a cation selective membrane. The current-voltage dependence exhibits a saturation at the limiting current. With a further increase of voltage, the current increases, marking the transition to the overlimiting conductance. This transition is mediated by the appearing vortical flow that increases with the applied voltage.

Transition between two dendritic growth mechanisms in electrodeposition

We report in this paper the observation of a transition between two different dendritic growth mechanisms in the electrodeposition of a metal from a binary electrolyte. Our results, in particular concerning the dendritic growth velocities, enable us to explain this behavior in terms of models previously proposed in the literature.

Colloidal electroconvection in a thin horizontal cell. II. Bulk electroconvection of water during parallel-plate electrolysis

We recently have reported [J. Chem. Phys. 122, 164701 (2005)] a family of electroconvective patterns that arise when charge-stabilized colloidal dispersions are driven by constant (dc) vertical electric fields. Competition between gravity and electrokinetic forces acting on the individual spheres in this system leads to the formation of highly organized convective instabilities involving hundreds of spheres. Here, we report a distinct class of electroconvective patterns that emerge in confined aqueous dispersions at higher biases. These qualitatively resemble the honeycomb and labyrinthine patterns formed during thermally driven Rayleigh-Benard convection, but arise from a distinct mechanism. Unlike the localized colloidal electroconvective patterns observed at lower biases, moreover, these system-spanning patterns form even without dispersed colloidal particles. Rather, they appear to result from an underlying electroconvective instability during electrolysis in the parallel plate geometry. This contrasts with recent theoretical results suggesting that simple electrolytes are linearly stable against electroconvection.

An elasto-plastic model of avian gastrulation

The motions observed during avian gastrulation may be simply interpreted in terms of elasto-plastic flow of sheets. Such a model allows one to calculate the flow map inside the blastodisc, hence the evolution of its shape. In addition, the model predicts that there exists a region of high stress oriented radially from the caudal pole towards the center of the blastodisc, with a tensile component oriented orthoradially. If the stress generated by cellular motion is enough to provoke a crack in the extra cellular matrix, then mesoderm ingression proceeds through a "streak" (the primitive streak) oriented from the caudal pole inwards, which relieves the stress while it creates the three germ layers. The model predicts that crack opening is next followed by crack retreat (primitive streak retreat), as mesoderm ingression continues. As mesoderm ingression proceeds around the area pellucida, similar phenomena in the anterior pole may contribute to formation of the embryo. This gives a mechanical description of avian gastrulation which complements the biochemical approach. In addition, the model provides a simple explanation to the shape of the embryo at very early stages, and possibly an explanation of the entry point of the vitteline arteries into the mesoderm.

Chirality of electrodeposits grown in a magnetic field

Electrodeposits grown around a point cathode in a flat, horizontal electrochemical cell have fractal form. When grown in the presence of a perpendicular applied magnetic field, the deposits develop a spiral structure with chirality which reverses on switching the field direction. These structures are modeled numerically using biased variants of the diffusion limited aggregation (DLA) model. The effects of electric and magnetic fields are modeled successfully by varying the probabilities that a random walker will move in a given direction as a result of a Coulomb force and the Lorentz force-induced flow of electrolyte past the deposit surface. By contrast, a numerical model which considers only the effect of the Lorentz force on individual ions, without reference to the surface of the growing deposit, produces spiral structures with incorrect chirality. The modified DLA model is related to the differential equations for diffusion, migration, and convection. Length scales in the problem are understood by associating the step length of the random walker with the diffusion layer thickness, the lookup radius with the hydrodynamic boundary layer thickness and a point on the numerical deposit with a nucleation center for growth of a crystallite.

Three-dimensional nature of ion transport in thin-layer electrodeposition

A generalized three-dimensional model for ion transport in electrodeposition is introduced. Ion transport is mainly governed by diffusion, migration, and convection. When convection prevails, in particular, in the limiting case of gravity-driven convection, the model predicts concentration shells and convection rolls and their interaction mode with a deposit tip: shell and roll bend and surround the tip forming a three-dimensional envelope tube squeezed at the deposit tip. In the limiting case of electrically driven convection, a vortex ring and an electric spherical drop crowning the deposit tip are predicted. When gravity and electric convection are both relevant, the interaction of ramified deposits, vortex tubes and rings, and electric spherical drops, leading to complex helicoidal flow, is predicted. Many of these predictions are experimentally observed, suggesting that ion transport underlying dendrite growth is remarkably well captured by our model.

Influence of ohmic heating on the flow field in thin-layer electrodeposition

In thin-layer electrodeposition the dissipated electrical energy leads to a substantial heating of the ion solution. We measured the resulting temperature field by means of an infrared camera. The properties of the temperature field correspond closely with the development of the concentration field. In particular, we find that the thermal gradients at the electrodes act similar to a weak additional driving force to the convection rolls driven by concentration gradients.

Transition from gravito- to electroconvective regimes in thin-layer electrodeposition

The transition from gravitoconvective to electroconvective prevailing regimes in thin-layer electrochemical deposition is analyzed through variations of electrolyte viscosity at constant cell thickness. The distribution of velocity directions at the deposit front is a measure of the relative weight of electroconvection versus gravitoconvection, and a signature of that transition. The experiments are carried out under galvanostatic conditions in convection prevailing regimes. Particle image velocimetry reveals that at low viscosities, buoyancy driven convection dominates; as viscosity increases, electrically driven convection becomes more important, eventually prevailing. The transition is observed at 1.5 times the viscosity of water. The theoretical model presented reveals that an increase of the Poisson and Reynolds numbers and a decrease of the Peclet and electric Grashof numbers, when viscosity increases, makes the electroconvective motion relatively more important. The model predicts a transition at approximately two times the viscosity of water. We may conclude that, in a physicochemical hydrodynamic flow involving ions, under galvanostatic conditions, increasing viscosity damps gravitoconvection and enhances electroconvection.