Vanadium-Based Cathodes for Aqueous Zinc-Ion Batteries: Mechanisms, Challenges, and Strategies.

ConspectusZinc-ion batteries (ZIBs) are highly promising for large-scale energy storage because of their safety, high energy/power density, low cost, and eco-friendliness. Vanadium-based compounds are attractive cathodes because of their versatile structures and multielectron redox processes (+5 to +3), leading to high capacity. Layered structures or 3-dimensional open tunnel frameworks allow easy movement of zinc-ions without breaking the structure apart, offering superior rate-performance. However, challenges such as dissolution and phase transformation hinder the long-term stability of vanadium-based cathodes in ZIBs. Although significant research has been dedicated to understanding the mechanisms and developing high-performance vanadium-based cathodes, uncertainties still exist regarding the critical mechanisms of energy storage and dissolution, the actual active phase and the specific optimization strategy. For example, it is unclear whether materials such as α-V2O5, VO2, and V2O3 serve as the active phase or undergo phase transformations during cycling. Additionally, the root cause of V-dissolution and the role of byproducts such as Zn3(OH)2V2O7·2H2O in ZIBs are debated.In this account, we aim to outline a clear and comprehensive roadmap for V-based cathodes in ZIBs. On the basis of our studies, we analyzed intrinsic crystal structures and their correlation with performance to guide the design of V-based materials with high-capacity and high-stability for ZIBs. Then, we revealed the underlying mechanisms of energy storage and instability, enabling more effective design and optimization of V-based cathodes. After identifying the key challenges, we proposed effective design principles to achieve high cycling performance of V-based cathodes and outlined future development directions toward their practical application. Vanadium-based compounds include [VO4] tetrahedrons, [VO5] square pyramids, and [VO6] octahedra, which are connected through a cocorner, coedge and coplane. The [VO4] tetrahedron is inactive, and the [VO5] square pyramid is unstable in aqueous solutions because water attacks the exposed vanadium, whereas stable [VO6] octahedra are desirable because of their ability to reduce from +5 to +3 with minimal structural distortion. Therefore, high-performance vanadium-based oxides in ZIBs should maintain intact [VO6] octahedra while avoiding [VO4] tetrahedra or [VO5] square pyramids. The energy storage mechanism involves H2O/H+/Zn2+ coinsertion. The existence of interlayer water in V-based cathodes significantly improves the rate and cycling performance by expanding galleries, screening Zn2+ electrostatically via solvation, reducing ion diffusion energy barriers, and increasing layer flexibility. The insertion of H+/Zn2+ and the instability of V-based cathodes lead to the formation of byproducts such as basic zinc salts (i.e., Zn4SO4(OH)6·nH2O) and dead vanadium (Zn3(OH)2V2O7·2H2O), whose reversibility strongly affects long-term stability. To increase the cycling stability of vanadium-based cathodes, strategies such as electrolyte modulation and coating have been proposed to decrease water attack on the surface of V-oxides, thereby affecting the formation of byproducts. Additionally, in situ electrochemical transformation, ion preintercalation, and ion exchange were explored to prepare intrinsically stable V-based cathodes with enhanced performance. Furthermore, future research should focus on revealing atomic-scale mechanisms through advanced in situ characterization and theoretical calculations, enhancing rate-performance by facilitating ion/electron diffusion, promoting cycling stability by developing highly stable cathodes and refining interface engineering, and scaling up vanadium-based cathodes for practical ZIB applications.

Highly 002-Oriented Dendrite-Free Anode Achieved by Enhanced Interfacial Electrostatic Adsorption for Aqueous Zinc-Ion Batteries.

Rechargeable aqueous zinc-ion batteries (AZIBs) are gaining recognition as promising next-generation energy storage solution, due to their intrinsic safety and low cost. Nevertheless, the advancement of AZIBs is greatly limited by the abnormal growth of zinc dendrites during cycling. Electrolyte additives are effective at suppressing zinc dendrites, but there is currently no effective additive screening criterion. Herein, we propose employing the interfacial electrostatic adsorption strength of zinc ions for the initial screening of additives. Subsequently, dendrite-free plating is achieved by employing the anionic surfactant sodium dodecyl benzenesulfonate (SDBS) to enhance electrostatic adsorption. The cycled zinc anode exhibited a dense plating morphology and a high (002) orientation (I002/I101 = 22). The Zn||MnO2 full cell with SDBS exhibited a capacity retention of 85% after 1000 cycles at 1 A g-1. Furthermore, an instantaneous nucleation model and continuous nucleation model (CNM) are constructed to reveal the microscale plating/stripping dynamics under the scenarios of weak adsorption and strong adsorption. The CNM accurately explains the self-optimizing reconstruction of electrodes resulting from enhanced electrostatic adsorption. Our exploration was extended to other anionic surfactants (sodium dodecyl sulfate and disodium laureiminodipropionate), confirming the effectiveness of strong electrostatic adsorption in the screening of electrolyte additives.

Phosphorous and Nitrogen Dual-Doped Carbon as a Highly Efficient Electrocatalyst for Sodium-Oxygen Batteries.

Sodium-oxygen batteries have been regarded as promising energy storage devices due to their low overpotential and high energy density. Its applications, however, still face formidable challenges due to the lack of understanding about the influence of electrocatalysts on the discharge products. Here, a phosphorous and nitrogen dual-doped carbon (PNDC) based cathode is synthesized to increase the electrocatalytic activity and to stabilize the NaO2 superoxide nanoparticle discharge products, leading to enhanced cycling stability when compared to the nitrogen-doped carbon (NDC). The PNDC air cathode exhibits a low overpotential (0.36 V) and long cycling stability (120 cycles). The reversible formation/decomposition and stabilization of the NaO2 discharge products are clearly proven by in-situ synchrotron X-ray diffraction and ex-situ X-ray diffraction. Based on the density functional theory calculation, the PNDC has much stronger adsorption (-2.85 eV) for NaO2 than that of NDC (-1.80 eV), which could efficiently stabilize the NaO2 discharge products.

Adaptive healing of stress-induced dynamic cracks in a metal-organic framework membrane using nanoparticles.

Dewatering of aqueous azeotropes is crucial and pervasive in raw chemical refineries and solvent recovery in the chemical industry but is recognized as one of the most energy-intensive processes. Pervaporation using crystalline molecular sieve membranes provides an energy-efficient solution, but stress loads stemming from thermal and mechanical risks of pervaporation are most likely to cause membrane cracks, which greatly reduces reliability of membranes in real-world applications. Here, we propose adaptive healing of stress-induced dynamic cracks (AHSDC) in the membrane in a risk-responding manner before separation by using in situ-formed nanoparticles in the same chemical environment. These nanoparticles naturally filled in fissure gaps once cracks formed in the membrane, forming adaptive healing zones. Without loss of dewatering capacity, the separation durability of the membrane after AHSDC was improved by at least two orders of magnitude. The membrane also exhibited tolerance to industrial-grade azeotropes that epitomize industrial multisource nature and complexity.

Low-Overpotential Rechargeable Na-CO2 Batteries Enabled by an Oxygen-Vacancy-Rich Cobalt Oxide Catalyst.

Rechargeable sodium-carbon dioxide (Na-CO2) batteries have been proposed as a promising CO2 utilization technique, which could realize CO2 reduction and generate electricity at the same time. They suffer, however, from several daunting problems, including sluggish CO2 reduction and evolution kinetics, large polarization, and poor cycling stability. In this study, a rambutan-like Co3O4 hollow sphere catalyst with abundant oxygen vacancies was synthesized and employed as an air cathode for Na-CO2 batteries. Density functional theory calculations reveal that the abundant oxygen vacancies on Co3O4 possess superior CO2 binding capability, accelerating CO2 electroreduction, and thereby improving the discharge capacity. In addition, the oxygen vacancies also contribute to decrease the CO2 decomposition free energy barrier, which is beneficial for reducing the overpotential further and improving round-trip efficiency. Benefiting from the excellent catalytic ability of rambutan-like Co3O4 hollow spheres with abundant oxygen vacancies, the fabricated Na-CO2 batteries exhibit extraordinary electrochemical performance with a large discharge capacity of 8371.3 mA h g-1, a small overpotential of 1.53 V at a current density of 50 mA g-1, and good cycling stability over 85 cycles. These results provide new insights into the rational design of air cathode catalysts to accelerate practical applications of rechargeable Na-CO2 batteries and potentially Na-air batteries.

In situ electrochemical potential-induced synthesis of metal organic framework membrane on polymer support for H2/CO2 separation.

Metal-Organic Framework (MOF) membranes act as selective layers have offered unprecedented opportunities for energy-efficient and cost-effective gas separation. Searching for the green and sustainable synthesis method of dense MOF membrane has received huge attention in both academia and industry. In this work, we demonstrate an in situ electrochemical potential-induced synthesis strategy to aqueously fabricate Metal Azolate Framework-4 (MAF-4) membranes on polypropylene (PP) support. The constant potential can induce the heterogeneous nucleation and growth of MAF-4, resulting an ultrathin membrane with the thickness of only 390 nm. This high-quality membrane exhibits a high H2/CO2 separation performance with the H2 permeance as high as 1565.75 GPU and selectivity of 11.6. The deployment of this environment friendly one-step fabrication method under mild reaction conditions, such as low-cost polymer substrate, water instead of organic solvent, room temperature and ambient pressure shows great promise for the scale-up of MOF membranes.

Breaking the trade-off between capacity and stability in vanadium-based zinc-ion batteries.

Water in electrolytes is a double-edged sword in zinc-ion batteries (ZIBs). While it allows for proton insertion in the cathode, resulting in a significant increase in capacity compared to that of organic ZIBs, it also causes damage to electrodes, leading to performance degradation. To overcome the capacity-stability trade-off, organic solvents containing a small amount of water are proposed to mitigate the harmful effects of water while ensuring sufficient proton insertion. Remarkably, in a Zn(OTf)2 electrolyte using 8% H2O in acetonitrile as the solvent, Zn‖(NH4)0.5V2O5·0.5H2O exhibited a capacity as high as 490 mA h g-1 at a low current (0.3 A g-1), with a capacity retention of 80% even after 9000 cycles at high current (6 A g-1), simultaneously achieving the high capacity as in pure aqueous electrolytes and excellent stability as in organic electrolytes. We also found that the water content strongly impacts the kinetics and reversibility of ion insertion/extraction and zinc stripping/plating. Furthermore, compared to electrolytes with pure acetonitrile or H2O solvents, electrolytes with only 8% H2O in acetonitrile provide higher capacities at temperatures ranging from 0 to -50 °C. These discoveries enhance our understanding of the mechanisms involved in ZIBs and present a promising path toward enhancing electrolyte solutions for the creation of high-performance ZIBs.

Advancements in Achieving High Reversibility of Zinc Anode for Alkaline Zinc-Based Batteries.

Rechargeable alkaline zinc-based batteries (ZBBs) have attracted extensive research attention due to their advantages of low cost, high specific energy, and high safety. Although the investigation of cathodes for alkaline secondary ZBBs has reached a relatively advanced stage, the exploration of zinc anodes is still in its infancy. Zinc anodes in alkaline electrolytes encounter challenges such as dendrite formation, passivation, corrosion during periods of cell inactivity, and hydrogen evolution during cycling, thereby limiting their rechargeability and storability. Drawing upon the latest research on zinc anodes, six fundamental strategies that encompass a wide range of aspects are identified and categorized, from electrode modifications and electrolytes to charge protocols. Specifically, these strategies include 3D structures, coatings, alloying, additives, separators, and charge protocols. They serve as an insight summary of the current research progress on zinc anodes. Additionally, the complementary nature of these strategies allows for flexible combinations, enabling further enhancement of the overall performance of zinc anodes. Finally, several future directions for the advancement of practical alkaline Zn anode are proposed. This comprehensive review not only consolidates the existing knowledge but also paves the way for broader research opportunities in the pursuit of high-performance alkaline zinc anodes.

Modular Customization and Regulation of Metal-Organic Frameworks for Efficient Membrane Separations.

Metal-organic frameworks (MOFs) are considered ideal membrane candidates for energy-efficient separations. However, the MOF membrane amount to date is only a drop in the bucket compared to the material collections. The fabrication of an arbitrary MOF membrane exhibiting inherent separation capacity of the material remains a long-standing challenge. Herein, we report a MOF modular customization strategy by employing four MOFs with diverse structures and physicochemical properties and achieving innovative defect-free membranes for efficient separation validation. Each membrane fully displays the separation potential according to the MOF pore/channel microenvironment, and consequently, an intriguing H2 /CO2 separation performance sequence is achieved (separation factor of 1656-5.4, H2 permeance of 964-2745 gas permeation unit). Taking advantage of this strategy, separation performance can be manipulated by a non-destructive modification separately towards the MOF module. This work establishes a universal full-chain demonstration for membrane fabrication-separation validation-microstructure modification and opens an avenue for exclusive customization of membranes for important separations.

Energy-efficient extraction of linear alkanes from various isomers using structured metal-organic framework membrane.

Extraction of low concentration linear alkanes (C5-C7) from various isomers is critical for the petrochemical industry. At present, the separation of alkane isomers is mainly accomplished by distillation, which results in substantial energy expenditure. Metal-organic frameworks (MOFs) with well-tailored nanopores have been demonstrated to be capable of realizing molecule-level separation. In this study, oriented HKUST-1 membranes are formulated according to the morphology-biased principle and finally realized with a low dose synthesis method for terminating undesired crystal nucleation and growth. The fully exposed triangular sieving pore array of the membrane induces configuration entropic diffusion to split linear alkanes from mono-branched and di-branched isomers as well as their cyclical counterparts. Typically, the current separation technique consumes 91% less energy than vacuum distillation. Furthermore, our membranes can realize one-step extraction of normal-pentane, normal-hexane and normal-heptane from a ten-component alkane isomer solution that mimics light naphtha.