Few-layer bismuth selenide cathode for low-temperature quasi-solid-state aqueous zinc metal batteries

How Thick Aqueous Alkali Should be Better for Aluminum-Air Batteries at Sub-Zero Temperatures: A Critical Anti-Freezing Concentration

The application of portable aluminum-air batteries (AABs) in extreme environments is an inevitable demand for future development. Aqueous electrolyte freezing is a major challenge for low-temperature operations. Conventionally, enlightened by the organic system in metal ion batteries, blindly increasing the concentration is regarded as an efficient technique to reduce the freezing point (FP). However, the underlying contradiction between the adjusting mechanism of the FP and OH- transportation is ignored. Herein, the aqueous alkali solution of CsOH is researched as a prototype to disclose the intrinsic conductive behavior and related solvent structure evolution. Different from these inorganic electrolyte systems, the concept of a critical anti-freezing concentration (CFC) is proposed based on a specific temperature. The relationship between hydrogen bond reconstruction and de-solvation behavior is analyzed. A high conductivity is obtained at -30 °C, which is also a recorded value in an intrinsic aqueous AAB. The homogenous dissolution of the Al anode is also observed. As a general rule, the CFC concept is also applied in both the KOH and NaOH systems.

Halogen-powered static conversion chemistry

Halogen-powered static conversion batteries (HSCBs) thrive in energy storage applications. They fall into the category of secondary non-flow batteries and operate by reversibly changing the chemical valence of halogens in the electrodes or/and electrolytes to transfer electrons, distinguishing them from the classic rocking-chair batteries. The active halide chemicals developed for these purposes include organic halides, halide salts, halogenated inorganics, organic-inorganic halides and the most widely studied elemental halogens. Aside from this, various redox mechanisms have been discovered based on multi-electron transfer and effective reaction pathways, contributing to improved electrochemical performances and stabilities of HSCBs. In this Review, we discuss the status of HSCBs and their electrochemical mechanism-performance correlations. We first provide a detailed exposition of the fundamental redox mechanisms, thermodynamics, conversion and catalysis chemistry, and mass or electron transfer modes involved in HSCBs. We conclude with a perspective on the challenges faced by the community and opportunities towards practical applications of high-energy halogen cathodes in energy-storage devices.

Bio-Inspired Trace Hydroxyl-Rich Electrolyte Additives for High-Rate and Stable Zn-Ion Batteries at Low Temperatures

High-rate and stable Zn-ion batteries working at low temperatures are highly desirable for practical applications, but are challenged by sluggish kinetics and severe corrosion. Herein, inspired by frost-resistant plants, we report trace hydroxyl-rich electrolyte additives that implement a dual remodeling effect for high-performance low-temperature Zn-ion batteries. The additive with high Zn absorbability not only remodels Zn2+ primary solvent shell by alternating H2 O molecules, but also forms a shielding layer thus remodeling the Zn surface, which effectively enhances fast Zn2+ de-solvation reaction kinetics and prohibits Zn anode corrosion. Taking trace α-D-glucose (αDG) as a demonstration, the electrolyte obtains a low freezing point of -55.3 °C, and the Zn//Zn cell can stably cycle for 2000 h at 5 mA cm-2 under -25 °C, with a high cumulative capacity of 5000 mAh cm-2 . A full battery that stably operates for 10000 cycles at -50 °C is also demonstrated.

In Situ Formed Amorphous Bismuth Sulfide Cathodes with a Self-Controlled Conversion Storage Mechanism for High Performance Hybrid Ion Batteries

Conversion-type electrodes offer a promising multielectron transfer alternative to intercalation hosts with potentially high-capacity release in batteries. However, the poor cycle stability severely hinders their application, especially in aqueous multivalence-ion systems, which can fundamentally impute to anisotropic ion diffusion channel collapse in pristine crystals and irreversible bond fracture during repeated conversion. Here, an amorphous bismuth sulfide (a-BS) formed in situ with unprecedentedly self-controlled moderate conversion Cu2+ storage is proposed to comprehensively regulate the isotropic ion diffusion channels and highly reversible bond evolution. Operando synchrotron X-ray diffraction and substantive verification tests reveal that the total destruction of the Bi─S bond and unsustainable deep alloying are fully restrained. The amorphous structure with robust ion diffusion channels, unique self-controlled moderate conversion, and high electrical conductivity discharge products synergistically boosts the capacity (326.7 mAh g-1 at 1 A g-1 ), rate performance (194.5 mAh g-1 at 10 A g-1 ), and long-lifespan stability (over 8000 cycles with a decay rate of only 0.02 ‰ per cycle). Moreover, the a-BS Cu2+ ‖Zn2+ hybrid ion battery can well supply a stable energy density of 238.6 Wh kg-1 at 9760 W kg-1 . The intrinsically high-stability conversion mechanism explored on amorphous electrodes provides a new opportunity for advanced aqueous storage.

A Layered Bi2 Te3 @PPy Cathode for Aqueous Zinc-Ion Batteries: Mechanism and Application in Printed Flexible Batteries

Low-cost, safe, and environmental-friendly rechargeable aqueous zinc-ion batteries (ZIBs) are promising as next-generation energy storage devices for wearable electronics among other applications. However, sluggish ionic transport kinetics and the unstable electrode structure during ionic insertion/extraction hamper their deployment. Herein, a new cathode material based on a layered metal chalcogenide (LMC), bismuth telluride (Bi2 Te3 ), coated with polypyrrole (PPy) is proposed. Taking advantage of the PPy coating, the Bi2 Te3 @PPy composite presents strong ionic absorption affinity, high oxidation resistance, and high structural stability. The ZIBs based on Bi2 Te3 @PPy cathodes exhibit high capacities and ultra-long lifespans of over 5000 cycles. They also present outstanding stability even under bending. In addition, here the reaction mechanism is analyzed using in situ X-ray diffraction, X-ray photoelectron spectroscopy, and computational tools and it is demonstrated that, in the aqueous system, Zn2+ is not inserted into the cathode as previously assumed. In contrast, proton charge storage dominates the process. Overall, this work not only shows the great potential of LMCs as ZIB cathode materials and the advantages of PPy coating, but also clarifies the charge/discharge mechanism in rechargeable ZIBs based on LMCs.

Oxygen Vacancy-Enriched Bi2SeO5 Nanosheets with Dual Mechanism for Ammonium-Ion Batteries

Ammonium ions feature a light molar mass and small hydrated radius, and the interesting interaction between NH4+ and host materials has attracted widespread attention in aqueous energy storage, while few studies focus on high-performance NH4+ storage anodes. Herein, we present a high-performance inset-type anode for aqueous ammonium-ion batteries (AIBs) based on Bi2SeO5 nanosheets. A reversible NH4+/H+ co-intercalation/deintercalation accompanied by hydrogen bond formation/breaking and a conversion reaction mechanism in layered Bi2SeO5 is proposed according to ex situ characterizations. Accordingly, the optimized Bi2SeO5 anode has a high reversible capacity of 341.03 mAh g-1 at 0.3 A g-1 in 1 M NH4Cl electrolyte and an impressive capacity retention of 86.7% after 7000 cycles at 3 A g-1, which is related to the existence of oxygen vacancies that enhance ion/electron transfer and promote the formation of hydrogen bonds between NH4+ and the host material. When the rocking-chair ammonium-ion battery is assembled using a MnO2 cathode, the device delivers an ultrahigh capacity of 140.73 mAh g-1 at 0.15 A g-1 and energy density of 207.13 Wh kg-1 at the power density of 2985.07 W kg-1. This work provides a promising strategy for designing high-performance anodes for next-generation AIBs.

Multifunction E-Skin Based on MXene-PA-Hydrogel for Human Behavior Monitoring

Hydrogels have attracted significant attention in various fields, such as smart sensing, human-machine interaction, and biomedicines, due to their excellent flexibility and versatility. However, current hydrogel electronic skins are still limited in stretchability, and their sensing functionality is often single-purpose, making it difficult to meet the requirements of complex environments and multitasking. In this study, we developed an MXene nanoplatelet and phytic acid-coreinforced poly(vinyl alcohol) (PVA) composite, denoted as MXene-PA-PVA. The strong hydrogen bonds formed by the interaction of the different components and the enhancement of chain entanglement result in a significant improvement in the mechanical properties of the PVA/PA/MXene composite hydrogel. This improvement is reflected in an increase of 271.43% in the maximum tensile strain and 35.29% in the maximum fracture stress. Moreover, the composite hydrogel exhibits excellent adhesion, water retention, heat resistance, and conductivity properties. The PVA/PA composite material combined with MXene demonstrates great potential for use as multifunctional sensors for strain and temperature detection with a strain-sensing sensitivity of 3.23 and a resistance temperature coefficient of 8.67. By leveraging the multifunctional characteristics of this composite hydrogel, electronic skin can accurately monitor human behavior and physiological reactions. This advancement opens up new possibilities for flexible electronic devices and human-machine interactions in the future.

Cation Defect-Engineered Boost Fast Kinetics of Two-Dimensional Topological Bi2 Se3 Cathode for High-Performance Aqueous Zn-Ion Batteries

The challenge with aqueous zinc-ion batteries (ZIBs) lies in finding suitable cathode materials that can provide high capacity and fast kinetics. Herein, two-dimensional topological Bi2 Se3 with acceptable Bi-vacancies for ZIBs cathode (Cu-Bi2-x Se3 ) is constructed through one-step hydrothermal process accompanied by Cu heteroatom introduction. The cation-deficient Cu-Bi2-x Se3 nanosheets (≈4 nm) bring improved conductivity from large surface topological metal states contribution and enhanced bulk conductivity. Besides, the increased adsorption energy and reduced Zn2+ migration barrier demonstrated by density-functional theory (DFT) calculations illustrate the decreased Coulombic ion-lattice repulsion of Cu-Bi2-x Se3 . Therefore, Cu-Bi2-x Se3 exhibits both enhanced ion and electron transport capability, leading to more carrier reversible insertion proved by in situ synchrotron X-ray diffraction (SXRD). These features endow Cu-Bi2-x Se3 with sufficient specific capacity (320 mA h g-1 at 0.1 A g-1 ), high-rate performance (97 mA h g-1 at 10 A g-1 ), and reliable cycling stability (70 mA h g-1 at 10 A g-1 after 4000 cycles). Furthermore, quasi-solid-state fiber-shaped ZIBs employing the Cu-Bi2-x Se3 cathode demonstrate respectable performance and superior flexibility even under high mass loading. This work implements a conceptually innovative strategy represented by cation defect design in topological insulator cathode for achieving high-performance battery electrochemistry.

Interlayer Engineering of VS2 Nanosheets via In Situ Aniline Intercalative Polymerization toward Long-Cycling Magnesium-Ion Batteries

Rechargeable magnesium batteries (RMBs) show great potential in large-scale energy storage systems, due to Mg2+ with high polarity leading to strong interactions within the cathode lattice, and the limited discovery of functional cathode materials with rapid kinetics of Mg2+ diffusion and desirable cyclability retards their development. Herein, we innovatively report the confined synthesis of VS2/polyaniline (VS2/PANI) hybrid nanosheets. The VS2/PANI hybrids with expanded interlayer spacing are successfully prepared through the exfoliation of VS2 and in situ polymerization between VS2 nanosheets and aniline. The intercalated PANI increases the interlayer spacing of VS2 from 0.57 to 0.95 nm and improves its electronic conductivity, leading to rapid Mg-ion diffusivity of 10-10-10-12 cm2 s-1. Besides, the PANI sandwiched between layers of VS2 is conducive to maintaining the structural integrity of electrode materials. Benefiting from the above advantages, the VS2/PANI-1 hybrids present remarkable performance for Mg2+ storage, showing high reversible discharge capacity (245 mA h g-1 at 100 mA g-1) and impressive long lifespan (91 mA h g-1 after 2000 cycles at 500 mA g-1). This work provides new perspectives for designing high-performance cathode materials based on layered materials for RMBs.

An Electrochemical Perspective of Aqueous Zinc Metal Anode

Based on the attributes of nonflammability, environmental benignity, and cost-effectiveness of aqueous electrolytes, as well as the favorable compatibility of zinc metal with them, aqueous zinc ions batteries (AZIBs) become the leading energy storage candidate to meet the requirements of safety and low cost. Yet, aqueous electrolytes, acting as a double-edged sword, also play a negative role by directly or indirectly causing various parasitic reactions at the zinc anode side. These reactions include hydrogen evolution reaction, passivation, and dendrites, resulting in poor Coulombic efficiency and short lifespan of AZIBs. A comprehensive review of aqueous electrolytes chemistry, zinc chemistry, mechanism and chemistry of parasitic reactions, and their relationship is lacking. Moreover, the understanding of strategies for suppressing parasitic reactions from an electrochemical perspective is not profound enough. In this review, firstly, the chemistry of electrolytes, zinc anodes, and parasitic reactions and their relationship in AZIBs are deeply disclosed. Subsequently, the strategies for suppressing parasitic reactions from the perspective of enhancing the inherent thermodynamic stability of electrolytes and anodes, and lowering the dynamics of parasitic reactions at Zn/electrolyte interfaces are reviewed. Lastly, the perspectives on the future development direction of aqueous electrolytes, zinc anodes, and Zn/electrolyte interfaces are presented.

Solid State Zinc and Aluminum ion batteries: Challenges and Opportunities

Solid-state zinc ion batteries (ZIBs) and aluminum-ion batteries (AIBs) are deemed as promising candidates for supplying power in wearable devices due to merits of low cost, high safety, and tunable flexibility. However, their wide-scale practical application is limited by various challenges, down to the material level. This Review begins with elaboration of the root causes and their detrimental effect for four main limitations: electrode-electrolyte interface contact, electrolyte ionic conductivity, mechanical strength, and electrochemical stability window of the electrolyte. Thereafter, various strategies to mitigate each of the described limitation are discussed along with future research direction perspectives. Finally, to estimate the viability of these technologies for wearable applications, economic-performance metrics are compared against Li-ion batteries.

Single-Ion-Conducting Hydrogel Electrolytes Based on Slide-Ring Pseudo-Polyrotaxane for Ultralong-Cycling Flexible Zinc-Ion Batteries

Flexible zinc-ion batteries (ZIBs) with high capacity and long cycle stability are essential for wearable electronic devices. Hydrogel electrolytes have been developed to provide ion-transfer channels while maintaining the integrity of ZIBs under mechanical strain. However, hydrogel matrices are typically swollen with aqueous salt solutions to increase ionic conductivity, which can hinder intimate contact with electrodes and reduce mechanical properties. To address this, a single-Zn-ion-conducting hydrogel electrolyte (SIHE) is developed by integrating polyacrylamide network and pseudo-polyrotaxane structure. The SIHE exhibits a high Zn2+ transference number of 0.923 and a high ionic conductivity of 22.4 mS cm-1 at room temperature. Symmetric batteries with SIHE demonstrate stable Zn plating/stripping performance for over 160 h, with a homogenous and smooth Zn deposition layer. Full cells with La-V2 O5 cathodes exhibit a high capacity of 439 mA h g-1 at 0.1 A g-1 and excellent capacity retention of 90.2% after 3500 cycles at 5 A g-1 . Moreover, the flexible ZIBs display stable electrochemical performance under harsh conditions, such as bending, cutting, puncturing, and soaking. This work provides a simple design strategy for single-ion-conducting hydrogel electrolytes, which could pave the way for long-life aqueous batteries.

Recent Advances in Structural Optimization and Surface Modification on Current Collectors for High-Performance Zinc Anode: Principles, Strategies, and Challenges

The last several years have witnessed the prosperous development of zinc-ion batteries (ZIBs), which are considered as a promising competitor of energy storage systems thanks to their low cost and high safety. However, the reversibility and availability of this system are blighted by problems such as uncontrollable dendritic growth, hydrogen evolution, and corrosion passivation on anode side. A functionally and structurally well-designed anode current collectors (CCs) is believed as a viable solution for those problems, with a lack of summarization according to its working mechanisms. Herein, this review focuses on the challenges of zinc anode and the mechanisms of modified anode CCs, which can be divided into zincophilic modification, structural design, and steering the preferred crystal facet orientation. The possible prospects and directions on zinc anode research and design are proposed at the end to hopefully promote the practical application of ZIBs.

Topological Insulator Bi2 Se3 -Assisted Heterostructure for Ultrafast Charging Sodium-Ion Batteries

The development of fast charging materials offers a viable solution for large-scale and sustainable energy storage needs. However, it remains a critical challenge to improve the electrical and ionic conductivity for better performance. Topological insulator (TI), a topological quantum material that has attracted worldwide attention, hosts unusual metallic surface states and consequent high carrier mobility. Nevertheless, its potential in promising high-rate charging capability has not been fully realized and explored. Herein, a novel Bi2 Se3 -ZnSe heterostructure as excellent fast charging material for Na+ storage is reported. Ultrathin Bi2 Se3 nanoplates with rich TI metallic surfaces are introduced as an electronic platform inside the material, which greatly reduces the charge transfer resistance and improves the overall electrical conductivity. Meanwhile, the abundant crystalline interfaces between these two selenides promote Na+ migration and provide additional active sites as well. As expected, the composite delivers the excellent high-rate performance of 360.5 mAh g-1 at 20 A g-1 and maintains its electrochemical stability of 318.4 mAh g-1 after 3000 long cycles, which is the record high for all reported selenide-based anodes. This work is anticipated to provide alternative strategies for further exploration of topological insulators and advanced heterostructures.

All-Round Enhancement in Zn-Ion Storage Enabled by Solvent-Guided Lewis Acid-Base Self-Assembly of Heterodiatomic Carbon Nanotubes

Designing zincophilic and stable carbon nanostructures is critical for Zn-ion storage with superior capacitive activity and durability. Here, we report solvent-guided Lewis acid-base self-assembly to customize heterodiatomic carbon nanotubes, triggered by the reaction between iron chloride and α,α'-dichloro-p-xylene. In this strategy, modulating the solvent-precursor interaction through the optimization of solvent formula stimulates differential thermodynamic solubilization, growth kinetics, and self-assembly behaviors of Lewis polymeric chains, thereby accurately tailoring carbon nanoarchitectures to evoke superior Zn-ion storage. Featured with open hollow interiors and porous tubular topologies, the solvent-optimized carbon nanotubes allow low ion-migration barriers to deeply access the built-in zincophilic sites by high-kinetics physical Zn²⁺/CF₃SO₃^- adsorption and robust chemical Zn²⁺ redox with pyridine/carbonyl motifs, which maximizes the spatial capacitive charge storage density. Thus, as-designed heterodiatomic carbon nanotube cathodes provide all-round improvement in Zn-ion storage, including a high energy density (140 W h kg^-1), a large current activity (100 A g^-1), and an exceptional long-term cyclability (100,000 cycles at 50 A g^-1). This study provides appealing insights into the solvent-mediated Lewis pair self-assembly design of nanostructured carbons toward advanced Zn-ion energy storage.

A Foldable Aqueous Zn-Ion Battery with Gear-Structured Composite as Freestanding Cathode

A foldable battery with high flexibility provides great potential in various wearable electronic devices for health and fitness tracking, chronic disease management, performance monitoring, navigation tracking, and portable gears for soldiers. We report a highly flexible, self-healing Zn-ion battery with a free-standing cathode that is composed of a 3D gear-like NH₄ V₄ O₁₀ @C composite on carbon paper. The battery retained a capacity of up to 102.4 mAh g^-1 even after being folded 60 times with a high angle of 180°. An aqueous hydrogel consisting polyvinyl alcohol, glycerin and Zn(CF₃ SO₃ )₂ was used as electrolyte, which showed as high as 580 % tensile strain under a loading weight of 78 N. The battery exhibited a better capacity retention of over 100 mAh g^-1 and Coulombic efficiency of over 99.8 % after cutting and twisting to 90°, thereby indicating a great self-healing performance. The gear-like geometry greatly improved the volume accommodation due to the increased interval space between the blades and the outward configuration. Meanwhile the Zn²⁺ ionic conductivity was improved by rapid re-binding of many existing hydroxy groups from the electrolyte and the enhanced contact surface area and diffusion route from the cathode material. The highly flexible, safe aqueous Zn-ion battery opens a practical way to power various carry-on electronics under mechanical agitation.

Rational Design of Flexible Zn-Based Batteries for Wearable Electronic Devices

The advent of 5G and the Internet of Things has spawned a demand for wearable electronic devices. However, the lack of a suitable flexible energy storage system has become the "Achilles' Heel" of wearable electronic devices. Additional problems during the transformation of the battery structure from conventional to flexible also present a severe challenge to the battery design. Flexible Zn-based batteries, including Zn-ion batteries and Zn-air batteries, have long been considered promising candidates due to their high safety, eco-efficiency, substantial reserve, and low cost. In the past decade, researchers have come up with elaborate designs for each portion of flexible Zn-based batteries to improve the ionic conductivities, mechanical properties, environment adaptabilities, and scalable productions. It would be helpful to summarize the reported strategies and compare their pros and cons to facilitate further research toward the commercialization of flexible Zn-based batteries. In this review, the current progress in developing flexible Zn-based batteries is comprehensively reviewed, including their electrolytes, cathodes, and anodes, and discussed in terms of their synthesis, characterization, and performance validation. By clarifying the challenges in flexible Zn-based battery design, we summarize the methodology from previous investigations and propose challenges for future development. In the end, a research paradigm of Zn-based batteries is summarized to fit the burgeoning requirement of wearable electronic devices in an iterative process, which will benefit the future development of Zn-based batteries.

Targeted Delivery of Zinc Ion Derived by Pseudopolyrotaxane Gel Polymer Electrolyte for Long-Life Zn Anode

Aqueous zinc ion battery is a potential alternative for a stationary energy storage system owing to the inherent properties of the Zn anode. However, the Zn anode suffers from serious Zn dendrite due to the uneven Zn plating. Thus, inspired by the nano-drug delivery to the target site of the tumor cell, it would be a promising strategy to introduce targeted delivery of zinc ion in the electrolyte for even Zn plating. Passive targeted transport plays an important role in nano-drug delivery, which presents the nano-drug would be released by the nano-drug carrier based on polymer to the particular target site. As a proof-of-concept, a pseudopolyrotaxane conducting the nano-drug carrier applied in targeted cancer therapy is employed as the gel polymer electrolyte (GPE) for long-life Zn anodes. The pseudopolyrotaxane is formed by the self-assembling of α-cyclodextrin (CD) and poly(ethylene oxide), where the zinc ion can be absorbed and delivered to the target site of the Zn anode benefiting from the hydrogen-bond. Impressively, even Zn plating can be induced by the hydroxyl groups of CD to inhibit Zn dendrite. Moreover, the hydrogen evolution reaction is suppressed by the GPE. Less produced H₂ is detected in the GPE, which is demonstrated by the online mass spectrometry. Thus, the Zn||Zn symmetrical cell based on the GPE exhibits a cycling life of 1370 h. Compared to the one based on aqueous electrolyte, Zn||MnO₂ battery based on the GPE shows a higher capacity retention. This work is expected to avail the development of the aqueous zinc ion battery.

Simultaneously Stabilizing Both Electrodes and Electrolytes by a Self-Separating Organometallics Interface for High-Performance Zinc-Ion Batteries at Wide Temperatures

Rechargeable aqueous zinc-ion batteries are of great potential as one of the next-generation energy-storage devices due to their low cost and high safety. However, the development of long-term stable electrodes and electrolytes still suffers from great challenges. Herein, a self-separation strategy is developed for an interface layer design to optimize both electrodes and electrolytes simultaneously. Specifically, the coating with an organometallics (sodium tricyanomethanide) evolves into an electrically responsive shield layer composed of nitrogen, carbon-enriched polymer network, and sodium ions, which not only modulates the zinc-ion migration pathways to inhibit interface side reactions but also adsorbs onto Zn perturbations to induce planar zinc deposition. Additionally, the separated ions from the coating can diffuse to the electrolyte to affect the Zn²⁺ solvation structure and maintain the cathode structural stability by forming a stable cathode-electrolyte interface and sodium ions' equilibrium, confirmed by in situ spectroscopy and electrochemical analysis. Due to these unique advantages, the symmetric zinc batteries exhibit an extralong cycling lifespan of 3000 h and rate performance at 20 mA cm^-2 at wide temperatures. The efficiency of the self-separation strategy is further demonstrated in practical full batteries with an ultralong lifespan over 10 000 cycles from -35 to 60 °C.

Phase-Dependent Energy Storage Performance of the NixSey Polymorphs for Supercapacitor-Battery Hybrid Devices

Transition-metal chalcogenides have emerged as a promising class of materials for energy storage applications due to their earth abundance, high theoretical capacity, and high electrical conductivity. Herein, we introduce a facile and one-pot electrodeposition method to prepare high-performance nickel selenide Ni_xSe_y (0.5 ≤ x/y ≤ 1.5) nanostructures (specific capacity = 180.3 mA h g^-1 at 1 A g^-1). The as-synthesized nickel selenide (NS) nanostructure is however converted to other polymorphs of nickel selenide including orthorhombic NiSe₂, trigonal Ni₃Se₂, hexagonal NiSe, and orthorhombic Ni₆Se₅ over cycling. Interestingly, NiSe₂ and Ni₃Se₂ polymorphs that display a more metallic character and superior energy storage performance are the predominant phases after a few hundred cycles. We fabricated a hybrid device using activated carbon (AC) as a supercapacitor-type negative electrode and NS as a high-rate battery-type positive electrode (AC||NS). This hybrid device provides a high specific energy of 71 W h kg^-1, an excellent specific power of up to 31 400 W kg^-1, and exceptional cycling stability (80% retention of the initial capacity after 20 000 cycles). The higher energy storage performance of the device is a result of the development of high-performance NiSe₂ and Ni₃Se₂ polymorphs. Moreover, the reduction of the critical dimension of the NS particles to the nanoscale partially induces an extrinsic pseudocapacitive behavior that improves the rate capability and durability of the device. We also explored the origin of the superior energy storage performance of the NS polymorphs using density functional theory calculations in terms of the computed density of states around the Fermi level, electrical conductivity, and quantum capacitance that follows the trend NiSe₂ > Ni₃Se₂ > NiSe > Ni₆Se₅. The present study thus provides an appealing approach for tailoring the phase composition of NS as an alternative to the commonly used templated synthesis methods.

Cellulose-Acetate Coating by Integrating Ester Group with Zinc Salt for Dendrite-Free Zn Metal Anodes

Zinc (Zn) metal is considered a potential anode owing to its high theoretical capacity, safety, and low cost. However, the dendrites and corresponding side reactions in aqueous electrolytes hinder their further development in environmentally-friendly energy storage. Herein, ion-affiliative cellulose acetate (CA) coating with Zn(CF₃ SO₃ )₂ is constructed on Zn anode (CAZ@Zn). Owing to the complexation effect between the polar ester group (CO) and Zn salt (Zn²⁺ ), the CAZ polymer coating enhances the hydrophilicity of the Zn anode and reduces the interfacial resistance, allowing the rapid Zn²⁺ diffusion and homogenizing the Zn deposition in an aqueous electrolyte to suppress zinc dendrite formation and growth. Therefore, the symmetric CAZ@Zn//CAZ@Zn battery achieves reversible plating/stripping over 2800 h at 1 mA cm^-2 with 1 mAh cm^-2 , about sevenfold higher than bare Zn. The full cell fabricated with an optimized Zn anode and the NH₄ V₄ O₁₀ cathode achieves substantially stable performance, superior to that of bare Zn. This work provides a straightforward, effective, and scalable method to suppress the zinc dendrites and corresponding side reactions for aqueous Zn-ions batteries.

Highly Reversible Zn Metal Anodes Enabled by Freestanding, Lightweight, and Zincophilic MXene/Nanoporous Oxide Heterostructure Engineered Separator for Flexible Zn-MnO2 Batteries

Aqueous zinc (Zn)-ion batteries are regarded as promising candidates for large-scale energy storage systems because of their high safety, low cost, and environmental benignity. However, the dendrite issue of Zn anode hinders their practical application. Herein, a freestanding, lightweight, and zincophilic MXene/nanoporous oxide heterostructure engineered separator is designed to stabilize a Zn metal anode. The nanoporous oxides prepared by a one-step vacuum distillation technique afford the advantages of large surface area, high porosity, and homogeneous porous structure. The zincophilic MXene@oxides layer can homogenize the electric field distribution, facilitate ion diffusion kinetics, reduce local current density, and promote even Zn ionic flux, which will regulate uniform Zn deposition and suppress side reactions. Accordingly, dendrite-free Zn anodes with stable cyclability are achieved for over 500 h at an ultrahigh area capacity of 10 mAh cm^-2. Besides, flexible, long-lifespan, and high-rate N/S-doped three-dimensional MXene@MnO₂||Zn full cells are constructed with the engineered separator. Moreover, this strategy can be successfully extended to lithium, sodium, potassium, and magnesium metal batteries, indicating that separator regulation is a universal approach to overcome the challenges of metal batteries.