Achieving Planar Zn Electroplating in Aqueous Zinc Batteries with Cathode-Compatible Current Densities by Cycling under Pressure.

The value of aqueous zinc-ion rechargeable batteries is held back by the degradation of the Zn metal anode with repeated cycling. While raising the operating current density is shown to alleviate this anode degradation, such high cycling rates are not compatible with full cells, as they cause Zn-host cathodes to undergo capacity decay. A simple approach that improves anode performance while using more modest cathode-compatible current densities is required. This work reports reversible planar Zn deposition under cathode-compatible current densities can instead be achieved by applying external pressure to the cell. Employing multiscale characterization, this work illustrates how cycling under pressure results in denser and more uniform Zn deposition, analogous to that achieved under high cycling rates, even at low areal current densities of 1 to 10 mA cm-2. Microstructural mechanical measurements reveal that Zn structures plated under lower current densities are particularly susceptible to pressure-induced compression. The ability to achieve planar Zn plating at cathode-compatible current densities holds significant promise for enabling high-capacity Zn-ion battery full cells.

An Electrochemically Initiated Self-Limiting Hydrogel Electrolyte for Dendrite-Free Zinc Anode.

The zinc dendrite growth generally relies upon a "positive-feedback" mode, where the fast-grown tips receive higher current densities and ion fluxes. In this study, a self-limiting polyacrylamide (PAM) hydrogel that presents negative feedback to dendrite growth is developed. The monomers are purposefully polymerized at the dendrite tips, then the hydrogel reduces the local current density and ion flux by limiting zinc ion diffusion with abundant functional groups. As a consequence, the accumulation at the dendrite tips is restricted, and the (002) facets-oriented deposition is achieved. Moreover, the refined porous structure of the gel enhances Coulombic Efficiency by reducing water activity. Due to the synergistic effects, the zinc anodes perform an ultralong lifetime of 5100 h at 0.5 mA cm-2 and 1500 h at 5 mA cm-2, which are among the best records for PAM-based gel electrolytes. Further, the hydrogel significantly prolongs the lifespan of zinc-ion batteries and capacitors by dozens of times. The developed in situ hydrogel presents a feasible and cost-effective way to commercialize zinc anodes and provides inspiration for future research on dendrite suppression using the negative-feedback mechanism.

In situ BaSO4 coating enabled activation-free and ultra-stable δ-MnO2 for aqueous Zn-ion batteries.

In situ BaSO4 coating, generated in the first discharging of Ba2+ pre-intercalated δ-MnO2, shortens the activation process by inducing fast proton intercalation and stabilizes the MnO2 crystal by suppressing Mn dissolution. The cathode delivers a decent electrochemical performance of 210 mA h g-1 at 1C with a 98% retention after 200 cycles.

Nanoscale Ultrafine Zinc Metal Anodes for High Stability Aqueous Zinc Ion Batteries.

Aqueous Zn batteries (AZBs) are a promising energy storage technology, due to their high theoretical capacity, low redox potential, and safety. However, dendrite growth and parasitic reactions occurring at the surface of metallic Zn result in severe instability. Here we report a new method to achieve ultrafine Zn nanograin anodes by using ethylene glycol monomethyl ether (EGME) molecules to manipulate zinc nucleation and growth processes. It is demonstrated that EGME complexes with Zn2+ to moderately increase the driving force for nucleation, as well as adsorbs on the Zn surface to prevent H-corrosion and dendritic protuberances by refining the grains. As a result, the nanoscale anode delivers high Coulombic efficiency (ca. 99.5%), long-term cycle life (over 366 days and 8800 cycles), and outstanding compatibility with state-of-the-art cathodes (ZnVO and AC) in full cells. This work offers a new route for interfacial engineering in aqueous metal-ion batteries, with significant implications for the commercial future of AZBs.

Structure evolution and energy storage mechanism of Zn3V3O8 spinel in aqueous zinc batteries.

Spinel-type materials are promising for the cathodes in rechargeable aqueous zinc batteries. Herein, Zn3V3O8 is synthesized via a simple solid-state reaction method. By tuning the Zn(CF3SO3)2 concentration in electrolytes and the cell voltage ranges, improved electrochemical performance of Zn3V3O8 can be achieved. The optimized test conditions give rise to progressive structure evolution from bulk to nano-crystalline spinel, which leads to capacity activation in the first few cycles and stable cycling performance afterward. Furthermore, the energy storage mechanism in this nano-crystalline spinel is interpreted as the co-intercalation of zinc ions and protons with some water. This work provides a new viewpoint of the structure evolution and correlated energy storage mechanism in spinel-type host materials, which would benefit the design and development of next-generation batteries.

Oxygen-Deficient ß-MnO2@Graphene Oxide Cathode for High-Rate and Long-Life Aqueous Zinc Ion Batteries.

Recent years have witnessed a booming interest in grid-scale electrochemical energy storage, where much attention has been paid to the aqueous zinc ion batteries (AZIBs). Among various cathode materials for AZIBs, manganese oxides have risen to prominence due to their high energy density and low cost. However, sluggish reaction kinetics and poor cycling stability dictate against their practical application. Herein, we demonstrate the combined use of defect engineering and interfacial optimization that can simultaneously promote rate capability and cycling stability of MnO2 cathodes. ß-MnO2 with abundant oxygen vacancies (VO) and graphene oxide (GO) wrapping is synthesized, in which VO in the bulk accelerate the charge/discharge kinetics while GO on the surfaces inhibits the Mn dissolution. This electrode shows a sustained reversible capacity of ~ 129.6 mAh g-1 even after 2000 cycles at a current rate of 4C, outperforming the state-of-the-art MnO2-based cathodes. The superior performance can be rationalized by the direct interaction between surface VO and the GO coating layer, as well as the regulation of structural evolution of ß-MnO2 during cycling. The combinatorial design scheme in this work offers a practical pathway for obtaining high-rate and long-life cathodes for AZIBs.

Optimizing the sulfonic groups of a polymer to coat the zinc anode for dendrite suppression.

The zinc metal anodes in aqueous zinc-ion batteries suffer from low cycling performance caused by uncontrolled dendrite. We have designed sulfonated poly-ether-ether-ketone (SPEEK) polymers as a surface coating layer on the zinc anode for dendrite suppression, in which the sulfonic groups in polymers act as effective active sites for zinc-ion diffusion. In SPEEK, the un-sulfonated domain serves as the framework and the sulfonated domain serves as the functional part to re-distribute the zinc ions. By optimizing the degree of SPEEK sulfonation, the best zinc anode coating has been achieved to present a high reversibility of over 1600 hours in symmetric cells and improved performance in full cells.

Progressive "Layer to Hybrid Spinel/Layer" Phase Evolution with Proton and Zn2+ Co-intercalation to Enable High Performance of MnO2-Based Aqueous Batteries.

Manganese oxides are promising host materials in rechargeable aqueous batteries due to their low cost and high capacity; however, their practical applications have long been restricted by their sluggish reaction kinetics and poor cycling stability. Herein, the layered K0.36H0.26MnO2·0.28H2O (K36) with a proton and Zn2+ cointercalation mechanism leads to a progressive phase evolution from layer-type K36 to hybrid layer-type KxHyZnzMnO2·nH2O and spinel-type ZnMn2O4 nanocrystal after a long-term cycle. Accordingly, K36 shows a high specific capacity (∼329.8 mAh g-1 at 0.1C), a superior rate performance (∼100.1 mAh g-1 at 10C), and a remarkable cycling stability (capacity retention of ∼93.4% over 3000 cycles at 4C). This work provides a new viewpoint of enhancing electrode performance via generating hybrid phases under electrochemical driving and will be a benefit to developing the next-generation aqueous batteries.

Boosting the Energy Density of Aqueous Batteries via Facile Grotthuss Proton Transport.

The recent developments in rechargeable aqueous batteries have witnessed a burgeoning interest in the mechanism of proton transport in the cathode materials. Herein, for the first time, we report the Grotthuss proton transport mechanism in α-MnO2 which features wide [2×2] tunnels. Exemplified by the substitution doping of Ni (≈5 at.%) in α-MnO2 that increases the energy density of the electrode by ≈25 %, we reveal a close link between the tetragonal-orthorhombic (TO) distortion of the lattice and the diffusion kinetics of protons in the tunnels. Experimental and theoretical results verify that Ni dopants can exacerbate the TO distortion during discharge, thereby facilitating the hydrogen bond formation in bulk α-MnO2 . The isolated direct hopping mode of proton transport is switched to a facile concerted mode, which involves the formation and concomitant cleavage of O-H bonds in a proton array, namely via Grotthuss proton transport mechanism. Our study provides important insight towards the understanding of proton transport in MnO2 and can serve as a model for the compositional design of cathode materials for rechargeable aqueous batteries.

Preintercalation Strategy in Manganese Oxides for Electrochemical Energy Storage: Review and Prospects.

Manganese oxides (MnO2 ) are promising cathode materials for various kinds of battery applications, including Li-ion, Na-ion, Mg-ion, and Zn-ion batteries, etc., due to their low-cost and high-capacity. However, the practical application of MnO2 cathodes has been restricted by some critical issues including low electronic conductivity, low utilization of discharge depth, sluggish diffusion kinetics, and structural instability upon cycling. Preintercalation of ions/molecules into the crystal structure with/without structural reconstruction provides essential optimizations to alleviate these issues. Here, the intrinsic advantages and mechanisms of the preintercalation strategy in enhancing electronic conductivity, activating more active sites, promoting diffusion kinetics, and stabilizing the structural integrity of MnO2 cathode materials are summarized. The current challenges related to the preintercalation strategy, along with prospects for the future research and development regarding its implementation in the design of high-performance MnO2 cathodes for the next-generation batteries are also discussed.

An Interface-Bridged Organic-Inorganic Layer that Suppresses Dendrite Formation and Side Reactions for Ultra-Long-Life Aqueous Zinc Metal Anodes.

Aqueous zinc (Zn) batteries (AZBs) are widely considered as a promising candidate for next-generation energy storage owing to their excellent safety features. However, the application of a Zn anode is hindered by severe dendrite formation and side reactions. Herein, an interfacial bridged organic-inorganic hybrid protection layer (Nafion-Zn-X) is developed by complexing inorganic Zn-X zeolite nanoparticles with Nafion, which shifts ion transport from channel transport in Nafion to a hopping mechanism in the organic-inorganic interface. This unique organic-inorganic structure is found to effectively suppress dendrite growth and side reactions of the Zn anode. Consequently, the Zn@Nafion-Zn-X composite anode delivers high coulombic efficiency (ca. 97 %), deep Zn plating/stripping (10 mAh cm-2 ), and long cycle life (over 10 000 cycles). By tackling the intrinsic chemical/electrochemical issues, the proposed strategy provides a versatile remedy for the limited cycle life of the Zn anode.

Unravelling H+ /Zn2+ Synergistic Intercalation in a Novel Phase of Manganese Oxide for High-Performance Aqueous Rechargeable Battery.

Aqueous Zn-MnO2 batteries using mild electrolyte show great potential in large-scale energy storage (LSES) application, due to high safety and low cost. However, structure collapse of manganese oxides upon cycling caused by the conversion mechanism (e.g., from tunnel to layer structures for α-, ß-, and γ-phases) is one of the most urgent issues plaguing its practical applications. Herein, to avoid the phase conversion issue and enhance battery performance, a structurally robust novel phase of manganese oxide MnO2 H0.16 (H2 O)0.27 (MON) nanosheet with thickness of ≈2.5 nm is designed and synthesized as a promising cathode material, in which a nanosheet structure combined with a novel H+ /Zn2+ synergistic intercalation mechanism is demonstrated and evidenced. Accordingly, a high-performance Zn/MON cell is achieved, showing a high energy density of ≈228.5 Wh kg-1 , impressive cyclability with capacity retention of 96% at 0.5 C after 300 cycles, as well as exhibiting rate performance of 115.1 mAh g-1 at current rate of 10 C. To the best current knowledge, this H+ /Zn2+ synergistic intercalation mechanism is first reported in an aqueous battery system, which opens a new opportunity for development of high-performance aqueous Zn ion batteries for LSES.

Anodic Polarization Behavior of X80 Steel in Na2SO4 Solution under High Potential and Current Density Conditions.

X80 steel has great risk of corrosion in high voltage direct current (HVDC) interference cases. In this study, the anodic polarization behavior of X80 steel under high potential and current density in Na2SO4 solution was investigated. The I × R drop was eliminated using current interrupt technique during the potentiodynamic measurement. Therefore, the real polarization curve was obtained. The corrosion behavior was investigated by galvanostatic polarization, scanning electron microscopy, and X-ray photoelectron spectroscopy. The results show a new form of passivation route. The steel dissolved actively below -0.388 VSCE, then became partly passivated from -0.388 to 1.448 VSCE, and fully passivated above 1.448 VSCE. The passive film was formed containing Fe2O3 and FeOOH, and resistant to SO42- ions. It not only blocked the direct dissolution of steel, but also facilitated oxygen evolution. The corrosion rates of steel samples decreased after the passivation.

Influence of Cl- ions on anodic polarization behaviour of API X80 steel in high potential/current density conditions in Na2SO4 solution.

Pipeline steel has considerable risk of corrosion in the high voltage direct current interference cases. Thus, under high potential/current density conditions, the anodic polarization behaviour of X80 steel in Na2SO4 solution and the influence of Cl- ions were investigated using reversed potentiodynamic polarization, the current interrupt method, galvanostatic polarization, scanning electron microscopy, and X-ray photoelectron spectroscopy. In the Na2SO4 solution free of Cl- ions, steel was passivated above 0.120 A cm-2 and the potential increased from -0.32 V to 1.43 V. The passive film was composed of Fe3O4, γ-Fe2O3, and FeOOH. The addition of Cl- ions observably influenced the passivation by attacking the passivate film. Low concentration of Cl- ions (<5 mg L-1 NaCl) could set higher demands of current density to achieve passivation and increase the requirement of potential to maintain passivation. A high concentration of Cl- ions (>5 mg L-1 NaCl) completely prevented passivation, showing strong corrosiveness. Thus, the X80 steel was corroded even under high-current-density conditions. The corrosion products were mainly composed of Fe3O4, α-Fe2O3, and FeOOH.