Morphological evolution of mossy structures during the electrodeposition of zinc from an alkaline zincate solution

New Alkalescent Electrolyte Chemistry for Zinc-Ferricyanide Flow Battery

Alkaline zinc-ferricyanide flow batteries are efficiency and economical as energy storage solutions. However, they suffer from low energy density and short calendar life. The strongly alkaline conditions (3 mol L-1 OH-) reduce the solubility of ferri/ferro-cyanide (normally only 0.4 mol L-1 at 25 °C) and induce the formation of zinc dendrites at the anode. Here, we report a new zinc-ferricyanide flow battery based on a mild alkalescent (pH 12) electrolyte. Using a chelating agent to rearrange ferri/ferro-cyanide ion-solvent interactions and improve salt dissociation, we increased the solubility of ferri/ferro-cyanide to 1.7 mol L-1 and prevented zinc dendrites. Our battery has an energy density of ~74 Wh L-1 catholyte at 60 °C and remains stable for 1800 cycles (1800 hours) at 0 °C and for >1400 cycles (2300 hours) at 25 °C. An alkalescent zinc-ferricyanide cell stack built using this alkalescent electrolyte stably delivers 608 W of power for ~40 days.

Alkaline Ni-Zn Rechargeable Batteries for Sustainable Energy Storage: Battery Components, Deterioration Mechanisms, and Impact of Additives

The demand for long-term, sustainable, and low-cost battery energy storage systems with high power delivery capabilities for stationary grid-scale energy storage, as well as the necessity for safe lithium-ion battery alternatives, has renewed interest in aqueous zinc-based rechargeable batteries. The alkaline Ni-Zn rechargeable battery chemistry was identified as a promising technology for sustainable energy storage applications, albeit a considerable investment in academic research, it still fails to deliver the requisite performance. It is hampered by a relatively short-term electrode degradation, resulting in a decreased cycle life. Dendrite formation, parasitic hydrogen evolution, corrosion, passivation, and dynamic morphological growth are all challenging and interrelated possible degradation processes. This review elaborates on the components of Ni-Zn batteries and their deterioration mechanisms, focusing on the influence of electrolyte additives as a cost-effective, simple, yet versatile approach for regulating these phenomena and extending the battery cycle life. Even though a great deal of effort has been dedicated to this subject, the challenges remain. This highlights that a breakthrough is to be expected, but it will necessitate not only an experimental approach, but also a theoretical and computational one, including artificial intelligence (AI) and machine learning (ML).

Revealing the missing puzzle piece of concentration in regulating Zn electrodeposition

Despite achievements in suppressing dendrites and regulating Zn crystal growth, secondary aqueous Zn batteries are still rare in the market. Existing strategies mainly focus on electrode modification and electrolyte optimization, while the essential role of ion concentration in liquid-to-solid electrodeposition is neglected for a long time. Herein, the mechanism of concentration regulation in Zn electrodeposition is investigated in depth by combining electrochemical tests, post hoc characterization, and multiscale simulations. First, initial Zn electrodeposition is thermodynamically controlled epitaxial growth, whereas with the rapid depletion of ions, the concentration overpotential transcends the thermodynamic influence to kinetic control. Then, the evolution of the morphology from 2D sheets to 1D whiskers due to the concentration change is insightfully revealed by the morphological characterization and phase-field modeling. Furthermore, the depth of discharge (DOD) results in large concentration differences at the electrode-electrolyte interface, with a mild concentration distribution at lower DOD generating (002) crystal plane 2D sheets and a heavily varied concentration distribution at higher DOD yielding arbitrarily oriented 3D blocks. As a proof of concept, relaxation is introduced into two systems to homogenize the concentration distribution, revalidating the essential role of concentration in regulating electrodeposition, and two vital factors affecting the relaxation time, i.e., current density and electrode distance, are deeply investigated, demonstrating that the relaxation time is positively related to both and is more sensitive to the electrode distance. This work contributes to reacquainting aqueous batteries undergoing phase transitions and reveals a missing piece of the puzzle in regulating Zn electrodeposition.

In Situ Synthesis of NiFeLDH/A-CBp from Pyrolytic Carbon as High-Performance Oxygen Evolution Reaction Catalyst for Water Splitting and Zinc Hydrometallurgy

Nickel-iron-layered double hydroxide (NiFeLDH) is one of the promising catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes, but its conductivity limits its large-scale application. The focus of current work is to explore low-cost, conductive substrates for large-scale production and combine them with NiFeLDH to improve its conductivity. In this work, purified and activated pyrolytic carbon black (CBp) is combined with NiFeLDH to form an NiFeLDH/A-CBp catalyst for OER. CBp not only improves the conductivity of the catalyst but also greatly reduces the size of NiFeLDH nanosheets to increase the activated surface area. In addition, ascorbic acid (AA) is introduced to enhance the coupling between NiFeLDH and A-CBp, which can be evidenced by the increase of Fe-O-Ni peak intensity in FTIR measurement. Thus, a lower overvoltage of 227 mV and larger active surface area of 43.26 mF·cm^-2 are achieved in 1 M KOH solution for NiFeLDH/A-CBp. In addition, NiFeLDH/A-CBp shows good catalytic performance and stability as the anode catalyst for water splitting and Zn electrowinning in alkaline electrolytes. In Zn electrowinning with NiFeLDH/A-CBp, the low cell voltage of 2.08 V at 1000 A·m^-2 results in lower energy consumption of 1.78 kW h/Kg_Zn, which is nearly half of the 3.40 kW h/Kg_Zn of industrial electrowinning. This work demonstrates the new application of high-value-added CBp in hydrogen production from electrolytic water and zinc hydrometallurgy to realize the recycling of waste carbon resources and reduce the consumption of fossil resources.

Effect of Pulse Frequency on the Microstructure and the Degradation of Pulse Electroformed Zinc for Fabricating the Shell of Biodegradable Dosing Pump

In this work, we applied single-pulse electrodeposition method to prepare biodegradable zinc coating for the shell of an implantable dosing pump, and explored the effect of pulse frequency on microstructures and degradation behavior of electroformed zinc. Samples were produced by single-pulse electro-deposition technique with different pulse frequencies (50 Hz, 100 Hz, and 1000 Hz). By controlling the pulse frequency, the thickness of the zinc coating can be adjusted. The 50 Hz produced zinc film possesses strong (11.0) grain orientation, 100 Hz produced zinc film possesses clear ((11.0) and (10.0)) grain orientations, yet 1000 Hz produced zinc film shows more random grain orientations of (10.0), (10.1), and (11.0), which provides a possible way to design a controllable nanometer surface microtopography. Although thermodynamic degradation tendency implied from open current corrosion voltage were similar, the kinetic corrosion rate showed a clear increasing trend as pulse frequency increased from 50 Hz to 1000 Hz, which corresponded with the electrochemical impedance spectroscopy and long-term soaking test in hanks solution. According to ISO 10993-5:2009 and ISO 10993-4:2002, electrodeposited zinc materials produced in this study showed a benign hemolysis ratio and no toxicity for cell growth. Zinc prepared under 50 Hz shows the best blood compatibility. Electrodeposited zinc materials are expected to be used for the shell of a degradable dosing pump.

Separator Effect on Zinc Electrodeposition Behavior and Its Implication for Zinc Battery Lifetime

Uncontrolled zinc electrodeposition is an obstacle to long-cycling zinc batteries. Much has been researched on regulating zinc electrodeposition, but rarely are the studies performed in the presence of a separator, as in practical cells. Here, we show that the microstructure of separators determines the electrodeposition behavior of zinc. Porous separators direct zinc to deposit into their pores and leave "dead zinc" upon stripping. In contrast, a nonporous separator prevents zinc penetration. Such a difference between the two types of separators is distinguished only if caution is taken to preserve the attachment of the separator to the zinc-deposited substrate during the entire electrodeposition-morphological observation process. Failure to adopt such a practice could lead to misinformed conclusions. Our work reveals the mere use of porous separators as a universal yet overlooked challenge for metal anode-based rechargeable batteries. Countermeasures to prevent direct exposure of the metal growth front to a porous structure are suggested.

Controlling electrochemical growth of metallic zinc electrodes: Toward affordable rechargeable energy storage systems

Scalable approaches for precisely manipulating the growth of crystals are of broad-based science and technological interest. New research interests have reemerged in a subgroup of these phenomena-electrochemical growth of metals in battery anodes. In this Review, the geometry of the building blocks and their mode of assembly are defined as key descriptors to categorize deposition morphologies. To control Zn electrodeposit morphology, we consider fundamental electrokinetic principles and the associated critical issues. It is found that the solid-electrolyte interphase (SEI) formed on Zn has a similarly strong influence as for alkali metals at low current regimes, characterized by a moss-like morphology. Another key conclusion is that the unique crystal structure of Zn, featuring high anisotropy facets resulting from the hexagonal close-packed lattice with a c/a ratio of 1.85, imposes predominant influences on its growth. In our view, precisely regulating the SEI and the crystallographic features of the Zn offers exciting opportunities that will drive transformative progress.

Non-Dendritic Zn Electrodeposition Enabled by Zincophilic Graphene Substrates

Rechargeable zinc (Zn) batteries suffer from poor cycling performance that can be attributed to dendrite growth and surface-originated side reactions. Herein, we report that cycling performance of Zn metal anode can be improved significantly by utilizing monolayer graphene (Gr) as the electrodeposition substrate. Utilizing microscopy and X-ray diffraction techniques, we demonstrate that electrodeposited Zn on Gr substrate has a compact, uniform, and nondendritic character. The Gr layer, due to its high lattice compatibility with Zn, provides low nucleation overpotential sites for Zn electrodeposition. Atomistic calculations indicate that Gr has strong affinity to Zn (binding energy of 4.41 eV for Gr with four defect sites), leading to uniform distribution of Zinc adatoms all over the Gr surface. This synergistic compatibility between Gr and Zn promotes subsequent homogeneous and planar Zn deposits with low interfacial energy (0.212 J/m²) conformal with the current collector surface.

Morphology control of zinc electrodeposition by surfactant addition for alkaline-based rechargeable batteries

The development of Zn-air batteries with a high energy density of 1350 W h kg-1 is one of the breakthroughs required to achieve a low carbon society. However, morphology control of the Zn negative electrode during charge/discharge (Zn-deposition/stripping) is essential for practical application. Considering the manufacturing process, a simple strategy is preferable. Herein, we employed surfactants as an inhibitor of the formation of mossy and dendrite Zn structures, and studied electrochemical Zn growth from the perspective of the electric charge of the surfactant. Even by using an additive free electrolyte of 0.25 M ZnO + 4 M KOH and with 1 mM sodium dodecyl sulfate (SDS: anionic surfactant) or polyacrylic acid (PAA: non-ionic surfactant), mossy and dendrite formations were unavoidable irrespective of the current density. On the other hand, a cationic surfactant, trimethyloctadecylammonium chloride (STAC), suppressed the shape change and resulted in a smooth and dense morphology. Zeta potential measurements, kinetic current densities observed from Tafel plots, and constant potential electrolysis indicate that quaternary ammonium cations (STAC) with bulky size adsorb onto protrusions which are the cause of shape change and suppress Zn deposition in the region to promote lateral growth. Although the adsorption of STAC increased the average overvoltage for Zn-deposition/stripping in a symmetric Zn|Zn cell under a current density of 10 mA cm-2, significantly stable behavior continued for 200 h. In contrast, the overvoltage of the additive free system suddenly increased after 156 h, associated with the accumulation of insulating ZnO and Zn(OH)2 formed on the Zn surface. In charge-discharge tests using an asymmetric Cu|Zn cell, the coulombic efficiency in the additive free electrolyte was less than 95%, whereas the addition of STAC at 1 mM achieved superior cycling performance without any capacity loss originating from the generation of dead Zn (electrical isolation). These results demonstrate that the addition of STAC is a promising method of controlling the Zn morphology.