Suppression of dendrite growth by cross-flow in microfluidics

Influence of Concentration Gradients on Electroconvection at a Cation-Exchange Membrane Surface

Membrane-based systems, such as electrodialysis, play an important role in desalination and industrial separation processes. Electrodialysis uses alternating anion- and cation-exchange membranes with a perpendicular electric field to generate concentrated and diluate streams from a feed solution. It is known that under overlimiting current conditions, reduced charge and mass transfer at the membrane interface leads to regions of high ion depletion generating instability and vortices termed electroconvection. While electroconvective mixing is known to directly impact the separation efficiency of electrodialysis, the influence of ion concentration gradients across the membrane experienced in a functional electrodialysis system is not known. Here, we report the influence of ion concentration gradients across a cation exchange membrane (Nafion) that is both aligned with and opposed to the applied electric field. Experiments were conducted by coflowing NaCl solutions of different concentrations (0.1-100 mM) on each side of the membrane, and electroconvection was visualized with a fluorescence dye (Rhodamine 6G). We obtained concentration profiles from fluorescence image data and systematically measured the thickness of the depletion boundary layer dBL under different conditions. We found smaller dBL values at a higher flow rate both with and without concentration gradients. Our results show that electroconvection is enhanced when the electric field is opposite to the direction of the concentration gradient.

Electroconvective viscous fingering in a single polyelectrolyte fluid on a charge selective surface

When a low-viscosity fluid displaces into a higher-viscosity fluid, the liquid-liquid interface becomes unstable causing finger-like patterns. This viscous fingering instability has been widely observed in nature and engineering systems with two adjoined fluids. Here, we demonstrate a hitherto-unrealizable viscous fingering in a single fluid-solid interface. In a single polyelectrolyte fluid on a charge selective surface, selective ion rejection through the surface initiates i) stepwise ion concentration and viscosity gradient boundaries in the fluid and ii) electroconvective vortices on the surface. As the vortices grow, the viscosity gradient boundary pushes away from the surface, resulting viscous fingering. Comparable to conventional one with two fluids, i) a viscosity ratio ([Formula: see text]) governs the onset of this electroconvective viscous fingering, and ii) the boundary properties (finger velocity and rheological effects) - represented by [Formula: see text], electric Rayleigh ([Formula: see text]), Schmidt ([Formula: see text]), and Deborah ([Formula: see text]) numbers - determine finger shapes (straight v.s. ramified, the onset length of fingering, and relative finger width). With controllable onset and shape, the mechanism of electroconvective viscous fingering offers new possibilities for manipulating ion transport and dendritic instability in electrochemical systems.

Electrode Materials for Enhancing the Performance and Cycling Stability of Zinc Iodide Flow Batteries at High Current Densities

Aqueous redox flow battery systems that use a zinc negative electrode have a relatively high energy density. However, high current densities can lead to zinc dendrite growth and electrode polarization, which limit the battery's high power density and cyclability. In this study, a perforated copper foil with a high electrical conductivity was used on the negative side, combined with an electrocatalyst on the positive electrode in a zinc iodide flow battery. A significant improvement in the energy efficiency (ca. 10% vs using graphite felt on both sides) and cycling stability at a high current density of 40 mA cm^-2 was observed. A long cycling stability with a high areal capacity of 222 mA h cm^-2 is obtained in this study, which is the highest reported areal capacity for zinc-iodide aqueous flow batteries operating at high current density, in comparison to previous studies. Additionally, the use of a perforated copper foil anode in combination with a novel flow mode was discovered to achieve consistent cycling at exceedingly high current densities of >100 mA cm^-2. In situ and ex situ characterization techniques, including in situ atomic force microscopy coupled with in situ optical microscopy and X-ray diffraction, are applied to clarify the relationship between zinc deposition morphology on the perforated copper foil and battery performance in two different flow field conditions. With a portion of the flow going through the perforations, a significantly more uniform and compact zinc deposition was observed compared to the case where all of the flow passed over the surface of the electrode. Results from modeling and simulation support the conclusion that the flow of a fraction of electrolyte through the electrode enhances mass transport, enabling a more compact deposit.

Regulating the Solvation Shell Structure of Lithium Ions for Smooth Li Metal Deposition in Quasi-Solid-State Batteries

Gel polymer electrolytes (GPE) are promising next-generation electrolytes for high-energy batteries, combining the multiple advantages of liquid and all-solid-state electrolytes. Herein, we a synthesized GPE using poly(ethylene glycol)acrylate (PEGDA) in order to understand how the GPE efficiently inhibits lithium dendrite formation and growth. The effects of PEGDA on the solvation shell structure of the lithium ion are investigated using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations, which are also supported by Raman spectroscopy. The GPE electrolytes with optimal PEGDA concentration exhibit high transference numbers (t Li + ${{_{{\rm Li}{^{+}}}}}$ =0.72) and ionic conductivity (σ=3.24 mS cm^-1 ). A symmetric lithium ion battery using GPE can be stably cycled for 1200 h in comparison to 320 h in a liquid electrolyte (LE), possibly owing to the high content of LiF (17.9 %) in the solid-electrolyte interphase film of the GPE cell. The observed concentration/electric field gradient observed through the finite element method also accounts for the good cycling performance. In addition, a LiCoO₂ |GPE|Li cell demonstrates excellent capacity retention of 87.09 % for 200 cycles; this approach could present promising guidelines for the design of high-energy lithium batteries.

Observation of Remote Electroconvection and Inert-Cation Concentration Valley within Supporting Electrolytes in a Microfluidic-Based Electrochemical Device

The electrochemical system is playing an increasingly important role in the advanced technology development for drinkable water and energy storage. While the binary electrolyte has been widely studied, such as the associated intriguing interfacial instabilities, multi-component electrolyte is by far less known. Here, based on the classic Cu|CuSO₄ |Cu electrochemical system, the effect of supporting electrolyte is systematically investigated by highlighting the inert cations. In an annulus microfluidic device, the suppression of a previously known electro-osmotic instability and the emergence of an array of the remote electroconvection along the azimuthal direction is found. A distinctive inert-cation concentration valley propagates radially outward at a speed limited by the electromigration velocity. Remarkably, the simultaneous visualization of spatiotemporal evolution demonstrates the correlation of the concentration valley and electroconvection at a microscopic level. The underlying physical mechanism of their correlation is discussed, and the scaling analysis agrees with experiments. This work might inspire more future work on the multi-component electrolyte, such as for the suppression of interfacial hydrodynamic instability and mitigation of dendrite growth, with the technological implications for water treatment and energy storage in batteries.

Production of fast-charge Zn-based aqueous batteries via interfacial adsorption of ion-oligomer complexes

Aqueous zinc batteries are attracting interest because of their potential for cost-effective and safe electricity storage. However, metallic zinc exhibits only moderate reversibility in aqueous electrolytes. To circumvent this issue, we study aqueous Zn batteries able to form nanometric interphases at the Zn metal/liquid electrolyte interface, composed of an ion-oligomer complex. In Zn||Zn symmetric cell studies, we report highly reversible cycling at high current densities and capacities (e.g., 160 mA cm⁻²; 2.6 mAh cm⁻²). By means of quartz-crystal microbalance, nuclear magnetic resonance, and voltammetry measurements we show that the interphase film exists in a dynamic equilibrium with oligomers dissolved in the electrolyte. The interphase strategy is applied to aqueous Zn||I₂ and Zn||MnO₂ cells that are charged/discharged for 12,000 cycles and 1000 cycles, respectively, at a current density of 160 mA cm⁻² and capacity of approximately 0.85 mAh cm⁻². Finally, we demonstrate that Zn||I₂-carbon pouch cells (9 cm² area) cycle stably and deliver a specific energy of 151 Wh/kg (based on the total mass of active materials in the electrode) at a charge current density of 56 mA cm⁻².

Aqueous zinc batteries attract interest because of their potential for cost-effective and safe electricity storage. Here, the authors develop an in situ formed ion-oligomer nanometric interphase strategy to enable fast-charge aqueous Zn cells.

High-performance all-solid-state electrolyte for sodium batteries enabled by the interaction between the anion in salt and Na3SbS4

All-solid-state sodium batteries with poly(ethylene oxide) (PEO)-based electrolytes have shown great promise for large-scale energy storage applications. However, the reported PEO-based electrolytes still suffer from a low Na⁺ transference number and poor ionic conductivity, which mainly result from the simultaneous migration of Na⁺ and anions, the high crystallinity of PEO, and the low concentration of free Na⁺. Here, we report a high-performance PEO-based all-solid-state electrolyte for sodium batteries by introducing Na₃SbS₄ to interact with the TFSI⁻ anion in the salt and decrease the crystallinity of PEO. The optimal PEO/NaTFSI/Na₃SbS₄ electrolyte exhibits a remarkably enhanced Na⁺ transference number (0.49) and a high ionic conductivity of 1.33 × 10⁻⁴ S cm⁻¹ at 45 °C. Moreover, we found that the electrolyte can largely alleviate Na⁺ depletion near the electrode surface in symmetric cells and, thus, contributes to stable and dendrite-free Na plating/stripping for 500 h. Furthermore, all-solid-state Na batteries with a 3,4,9,10-perylenetetracarboxylic dianhydride cathode exhibit a high capacity retention of 84% after 200 cycles and superior rate performance (up to 10C). Our work develops an effective way to realize a high-performance all-solid-state electrolyte for sodium batteries.

A high-performance all-solid-state PEO/NaTFSI/Na₃SbS₄ electrolyte for sodium batteries is realized owing to the electrostatic interaction between TFSI⁻ in the salt and Na₃SbS₄, which immobilizes TFSI⁻ anions and promotes the dissociation of NaTFSI.

An Ionic Liquid Electrolyte with Enhanced Li+ Transport Ability Enables Stable Li Deposition for High-Performance Li-O2 Batteries

Bis(trifluoromethanesulfonyl)imide-based ionic liquid (TFSI-IL) electrolyte could endow Li-O₂ batteries with low charging overpotential. However, their weak Li⁺ transport ability (LTA) leads to non-uniform Li deposition. Herein, guided by Sand formula, the LTA of TFSI-IL electrolyte is greatly enhanced to realize robust Li deposition through introducing hydrofluoroether (HFE) and optimizing electrolyte component ratios to regulate solvation environment. The solvation environment changes from Li(TFSI)₂ ^- ion pair into ionic aggregate clusters in the optimal electrolyte thanks to the slicing function of HFE toward ionic aggregate network. The transport parameters of Sand formula are synchronously enhanced, resulting in highly robust Li deposition behavior with greatly improved Coulombic efficiency (ca. 97.5 %) and cycling rate (1 mA cm^-2 ). Cycling stability of Li-O₂ batteries was greatly improved (a tiny overpotential rise of 64 mV after 75 cycles).