Emerging trends in anion storage materials for the capacitive and hybrid energy storage and beyond

Decoupling Self-Matching Effect Between Cathode and Anode in Hybrid Electrochemical Capacitors

Hybrid electrochemical capacitors (HECs) are advanced energy storage devices that offer high energy density, high power density, and long cycle life by integrating the energy storage mechanisms of both batteries and supercapacitors. The electrochemical coupling resulting from the cathode-anode kinetic differences severely restricts the accuracy of predicting the performance of HECs based on electrode performance. However, no general method can decouple the effects of cathode-anode kinetics matching on electrochemical performance by integrating electrochemical coupling from an electrochemical perspective. Here, using an integrated method that combines two distinct three-electrode testing modes in the typical sodium-ion hybrid capacitors, the self-matching effect of cathode-anode working potential ranges is first refined as the foundation for kinetic matching and electrochemical coupling. The discrepancies and interdependencies in cathode-anode electrochemical coupling for an individual kinetic match are decoupled by examining the impact of the self-matching effect on the electrochemical results. The critical matching current density is introduced as a key kinetic parameter for evaluating the degree of cathode-anode matching in complex electrochemical systems. This advancement provides an innovative design principle for electrode matching and coupling in high-performance HECs.

Emerging Issues and Opportunities of 2D Layered Transition Metal Dichalcogenide Architectures for Supercapacitors

Two-dimensional layered transition metal dichalcogenides (2D TMDs) have emerged as promising candidates for supercapacitor (SCs) owing to their tunable electronic properties, layered structures, and effective ion intercalation capabilities. Despite these advantages, challenges such as low electrical conductivity, the interlayer restacking, oxidation and structural collapse hinder their practical implementation. This review provides a comprehensive overview of recent advances in the development of 2D TMDs for SCs. We begin by outlining the charge storage mechanisms and design principles for SCs, followed by an in-depth discussion of the synthesis methods and the associated challenges in fabricating 2D TMD architectures. The subsequent sections explore their crystal structures and reaction mechanisms, illustrating their electrochemical potential in SCs. Furthermore, we highlight material modification strategies, including nanostructuring, defect engineering, phase control, and surface/interface modulation, which have been proposed to overcome existing challenges. Finally, we address critical issues and emerging opportunities for 2D TMDs to inspire the development of SC technologies.

MOFs-Derived Nanoarrays: A Promising Strategy for Next-Generation Supercapacitors

Developing high-performance electrode materials for supercapacitors is one of the keys to improving their overall performance. Metal-organic framework (MOF) is a kind of crystalline porous material with periodic network structure, which is connected by inorganic metal centres and bridged organic ligands through self-assembly. It has the advantages of a large specific surface area, controllable pore size, excellent stability and ordered crystal structure. MOF-derived nanoarrays exhibit excellent electrochemical performance due to their unique structure, rich activation points, close interface contact, and easy electron migration and mass transfer, which have attracted extensive attention in supercapacitor applications. This study mainly reviews the synthesis methods of MOF array electrodes and their applications in supercapacitors. In addition, we also described the challenges and prospects of MOF-derived array electrodes in the application of supercapacitors. This paper has important reference value for the design of MOF-derived array electrodes and advanced energy storage systems. The progress of advanced energy storage systems will further promote the development of sustainable renewable energy, avoid adverse climate and greenhouse effect caused by excessive use of fossil fuels, and achieve a green energy future.

Localized High-Concentration Electrolyte for All-Carbon Rechargeable Dual-Ion Batteries with Durable Interfacial Chemistry

Lithium-based rechargeable dual-ion batteries (DIBs) based on graphite anode-cathode combinations have received much attention due to their high resource abundance and low cost. Currently, the practical realization of the batteries is hindered by easy oxidation of the electrolyte at the cathode interface, and solvent co-intercalation at the anode-electrolyte interface. Configuration of a "solvent-in-salt" electrolyte with a high concentration of Li salt is expected to stabilize the electrolyte chemistry versus both electrodes, yet inevitably reduces the mobility of the solvated working ions and increases the cost of the electrolyte. Herein, we propose to build a localized high-concentration electrolyte by adding hydrofluoroether as the diluent to reduce the salt content while improving the solvation structure, allowing more anions to enter the inner solvation sheath. The new electrolyte helps to form uniform and thin interfaces, with elevated contents of inorganic fluorides, on both electrodes, which effectively suppress electrolyte oxidation at the cathode and optimize electrolyte-electrode compatibility at the anode while facilitating charge transfer across the interface. Consequently, the DIBs with graphite as anode and cathode operate for 3000 cycles and retain a high-capacity retention of 95.7 %, highlighting the importance of stable interfacial chemistry in boosting the electrochemical performance of DIBs.

Functional metal-organic frameworks derived electrode materials for electrochemical energy storage: a review

Pristine metal-organic frameworks (MOFs) are built through self-assembly of electron rich organic linkers and electron deficient metal nodes via coordinate bond. Due to the unique properties of MOFs like highly tunable frameworks, huge specific surface areas, flexible chemical composition, flexible structures and a large volume of pores, they are being used to design the electrode materials for electrochemical energy storage devices. As per the literature, MOFs (including manganese, nickel, copper, and cobalt-based zeolitic imidazolate frameworks (ZIFs), University of Oslo (UiO) MOFs, Hong Kong University of Science and Technology (HKUST) MOFs and isoreticular MOFs (IRMOFs)) have attracted much attention in the field of supercapacitors (SCs)/batteries. According to their dimensionality such as 1D, 2D and 3D, pristine MOFs are mainly used as SC materials. Highly porous materials and their composites are capable for intercalation of metal ions (Na+/Li+). Moreover, the supramolecular features (π⋯π, C-H⋯π, hydrogen bond interactions) of redox stable MOFs provide better insight for electrochemical stability. So, this review provides an in-depth analysis of pure MOFs and MOF derived composites (MOF composites and MOF derived porous carbon) as electrode materials and also discusses their metal ion charge storage mechanism. Finally, we provide our perspectives on the current issues and future opportunities for supercapacitor materials.

Anion Storing Boron Nitride Hybrid Nanosheets for High-Performance Dual Ion and Zinc Alkaline Full Cells

Hexagonal boron nitride (BN), a well-known member of 2D materials, has a structure similar to graphene and is often referred to as white graphene. Despite its unique physical and chemical properties for energy storage applications, there have been very few studies on how BN stores anion carriers. Herein, the hybrid architecture and anion storage mechanism of BN nanosheets for high-performance hybrid energy storage full cells based on dual-ion and Zinc (Zn) alkaline systems is demonstrated. The chemical bonding between BN and reduced graphene oxide (rGO) is attributed to the formation of the heterointerface, which facilitates the charge transfer kinetics during an OH storing process. Based on the reversible surface redox reaction of BN and rGO hybrid (BN@rGO) confirmed by computational and spectroscopic analyses, the BN@rGO electrode is applied to both Na and OH dual-ion and Zn alkaline full cells. In the dual-ion system, Ti3C2‖BN@rGO full cells extended the operating voltage range up to 1.7 V, delivering a cell capacity of 49.4 mAh g-1 at 1000 mA g-1 and retaining 80% of its initial capacity after 40 000 cycles. In the Zn alkaline system, Zn‖BN@rGO full cells achieved a cell capacity of 58.1 mAh g-1 at 1000 mA g-1 and retained 80% capacity over 90 000 cycles.

Rechargeable Seawater-Based Chloride-Ion Batteries Enabled by Covalent Surface Chemistry in MXenes

Rechargeable aqueous chloride-ion batteries (ACIBs) using Cl- ions as charge carriers represent a promising energy-storage technology, especially when natural seawater is introduced as the electrolyte, which can bring remarkable advantages in terms of cost-effectiveness, safety, and environmental sustainability. However, the implementation of this technology is hindered by the scarcity of electrodes capable of reversible chloride-anion storage. Here, we show that a Ti3C2Clx MXene with Cl surface terminations enables reversible Cl- ion storage in aqueous electrolytes. Further, we developed seawater-based ACIBs that show a high specific capacity and an exceptionally long lifespan (40000 cycles, more than 1 year) in natural seawater electrolyte. The pouch-type cells achieve a high energy density (50 Wh Lcell-1) and maintain stable performance across a broad temperature range (-20 to 50 °C). Our investigations reveal that the covalent interaction between Cl- ions and Cl-terminated MXene facilitates Cl- ion intercalation into the MXene interlayer, promoting rapid ion migration with a low energy barrier (0.10 eV). Moreover, this MXene variant also enables the reversible storage of Br- ions in an aqueous electrolyte with a long cycle life. This study may advance the design of anion storage electrodes and enable the development of sustainable aqueous batteries.

Advancements in Asymmetric Supercapacitors: From Historical Milestones to Challenges and Future Directions

Numerous challenges, like the uninterrupted supply of electricity, stable and reliable power, and energy storage during non-operational hours, arise across various industries due to the absence of advanced energy storage technologies. With the continual technological advancements in portable electronics, green energy, and transportation, there are inherent limitations in their innovative production. Thus, ongoing research is focused on pursuing sustainable energy storage technologies. An emerging solution lies in the development of asymmetric supercapacitors (ASCs), which offer the potential to extend their operational voltage limit beyond the thermodynamic breakdown voltage range of electrolytes. This is achieved by employing two distinct electrode materials, presenting an effective solution to the energy storage limitations faced by ASCs. The current review concentrates on the progression of working materials to develop authentic pseudocapacitive energy storage systems (ESS). Also, evaluates their ability to exceed energy storage constraints. It provides insights into fundamental energy storage mechanisms, performance evaluation methodologies, and recent advancements in electrode material strategies. The review approaches developing high-performance electrode materials and achieving efficient ASC types. It delves into critical aspects for enhancing the energy density of ASCs, presenting debates and prospects, thereby offering a comprehensive understanding and design principles for next-generation ASCs in diverse applications.

Operando Studies of Bismuth Nanoparticles Embedded in N, O-Doped Porous Carbon for High-Performance Potassium-Ion Hybrid Capacitor

A highly viable alternative to lithium-ion batteries for stationary electrochemical energy-storage systems is the potassium dual-ion hybrid capacitor (PIHC), especially toward fast-charging capability. However, the sluggish reaction kinetics of negative electrode materials seriously impedes their practical implementation. In this paper, a new negative electrode Bi@RPC (Nano-bismuth confined in nitrogen- and oxygen-doped carbon with rationally designed pores, evidenced by advanced characterization) is developed, leading to a remarkable electrochemical performance. PIHCs building with the active carbon YP50F positive electrode result in a high operation voltage (0.1-4 V), and remarkably well-retained energy density at a high-power density (11107 W kg-1 at 98 Wh kg-1). After 5000 cycles the proposed PHICs still show a superior capacity retention of 92.6%. Moreover, a reversible mechanism of "absorption-alloying" of the Bi@RPC nanocomposite is revealed by operando synchrotron X-ray diffraction and Raman spectroscopy. With the synergistic potassium ions storage mechanism arising from the presence of well-structured pores and nano-sized bismuth, the Bi@RPC electrode exhibits an astonishingly rapid kinetics and high energy density. The results demonstrate that PIHCs with Bi@RPC-based negative electrode is the promising option for simultaneously high-capacity and fast-charging energy storage devices.

Crucial Roles of Ethyl Methyl Carbonate in Lithium-Ion and Dual-Ion Batteries: A Review

The essential role of electrolyte solutions in traditional electrochemical energy storage devices is crucial to enhancing their performance. Consequently, a wide array of electrolyte mixtures along with diverse electrodes have been extensively explored across different models of secondary batteries. Fascinatingly, the role of ethyl methyl carbonate (EMC) as a key cosolvent in the electrolyte mixture of commercial lithium-ion batteries with a graphite anode is garnering growing attention in alternative rechargeable dual-ion batteries utilizing graphite cathodes. In this context, the advancement and function of EMC as a solvent in electrolyte mixtures for lithium-ion and dual-ion batteries were extensively and thoroughly examined in this analysis, encompassing the genesis, synthesis process, and diverse characteristics for the practical uses of these batteries. Here, the review aims to guide readers in understanding EMC's function and impact as a cosolvent in electrolyte mixtures for both major secondary lithium-ion and dual-ion batteries, considering their distinct physicochemical characteristics.

Super-Stretchable and High-Energy Micro-Pseudocapacitors Based on MXene Embedded Ag Nanoparticles

The advancement of aqueous micro-supercapacitors offers an enticing prospect for a broad spectrum of applications, spanning from wearable electronics to micro-robotics and sensors. Unfortunately, conventional micro-supercapacitors are characterized by low capacity and slopy voltage profiles, limiting their energy density capabilities. To enhance the performance of these devices, the use of 2D MXene-based compounds has recently been proposed. Apart from their capacitive contributions, these structures can be loaded with redox-active nanowires which increase their energy density and stabilize their operation voltage. However, introducing rigid nanowires into MXene films typically leads to a significant decline in their mechanical properties, particularly in terms of flexibility. To overcome this issue, super stretchable micro-pseudocapacitor electrodes composed of MXene nanosheets and in situ reconstructed Ag nanoparticles (Ag-NP-MXene) are herein demonstrated, delivering high energy density, stable operation voltage of ≈1 V, and fast charging capabilities. Careful experimental analysis and theoretical simulations of the charging mechanism of the Ag-NP-MXene electrodes reveal a dual nature charge storage mechanism involving ad(de)sorption of ions and conversion reaction of Ag nanoparticles. The superior mechanical properties of synthesized films obtained through in situ construction of Ag-NP-MXene structure show an ultra stretchability, allowing the devices to provide stable voltage and energy output even at 100% elongation.

Multifunctional Iron Selenate Sheath of Fe-Based Anode Achieving High-Rate Capacity-Durability Combination of Aqueous Hybrid Energy Storage Devices

The introduction of battery-type cathode has been commonly considered a preferred approach to boost the energy density of aqueous hybrid energy storage devices (AHESDs) in alkalic systems, but AHESDs with both high energy density and power density are rare due to the great challenge in designing battery-type anode materials with high rate and durability comparable to capacitive-type carbon anodes. In this paper, a well-hydrated iron selenate (FeSeO) sheath is constructed around FeOOH nanorods by a facile electrochemical activation, demonstrating the unique multifunction in fasting charge diffusion, promoting the dissociation of H2O, and inhibiting the irreversible phase transition of FeOOH to inert γ-Fe2O3, which endow the hydrated sheath coated Fe-based anodes with an impressive rate capability and superior durability. Thanks to the comprehensive performance of this Fe-based anode, the assembled AHESD delivered a high energy density of 117 Wh kg-1 with the extraordinary durability of almost 100% capacity retention after 40 000 cycles. Even at an ultrahigh power density of 27 000 W kg-1, an impressive energy density of 65 Wh kg-1 can be achieved, which rivals previously reported energy-storage devices.

Rational Design of High-Loading Electrodes with Superior Performances Toward Practical Application for Energy Storage Devices

High-loading electrodes play a crucial role in designing practical high-energy batteries as they reduce the proportion of non-active materials, such as current separators, collectors, and battery packaging components. This design approach not only enhances battery performance but also facilitates faster processing and assembly, ultimately leading to reduced production costs. Despite the existing strategies to improve rechargeable battery performance, which mainly focus on novel electrode materials and high-performance electrolyte, most reported high electrochemical performances are achieved with low loading of active materials (<2 mg cm-2). Such low loading, however, fails to meet application requirements. Moreover, when attempting to scale up the loading of active materials, significant challenges are identified, including sluggish ion diffusion and electron conduction kinetics, volume expansion, high reaction barriers, and limitations associated with conventional electrode preparation processes. Unfortunately, these issues are often overlooked. In this review, the mechanisms responsible for the decay in the electrochemical performance of high-loading electrodes are thoroughly discussed. Additionally, efficient solutions, such as doping and structural design, are summarized to address these challenges. Drawing from the current achievements, this review proposes future directions for development and identifies technological challenges that must be tackled to facilitate the commercialization of high-energy-density rechargeable batteries.

Microcrystalline Nanofiber Electrode with Adaptive Intrinsic Structure and Microscopic Interface

A strategy of microcrystalline aggregation is proposed to fabricate energy storage electrode with outstanding capacity and stability. Carbon-rich electrode (BDTG) functionalized with benzo[1,2-b:4,5-b']dithiophene units and butadiyne segments are prepared. The linear conjugate chains pack as microcrystalline nanofibers on nanoscale, which further aggregates to form a porous interpenetrating network. The microcrystalline aggregation feature of BDTG exhibit stable structure during long cycling test, revealing the following advantage in structure and property. The stretchable butadiyne linker facilitates reversible adsorption and desorption of Li with the aid of adjacent sulfur heteroatom. The alkyne-alkene transition exhibits intrinsic structural stability of microcrystalline region in BDTG electrodes. Meanwhile, alkynyl groups and sulfur heteroatoms on the surface of BDTG nanofibers participate in the formation of microscopic interface, providing a stable interfacial contact between BDTG electrodes and adjacent electrolyte. As a proof-of-concept, BDTG-based electrode shows high capacity (1430 mAh g-1 at 50 mA g-1) and excellent cycle performance (8000 cycles under 5 A g-1) in half-cell of lithium-ion batteries, and a reversible capacity of 120 mAh g-1 is obtained under the current density of 2 C in full-cell. This work shows microcrystalline aggregation is beneficial to realize adaptive intrinsic structure and interface contact during the charge-discharge process.

Higher hydrogen fractions in dielectric polymers boost self-healing in electrical capacitors

Electrical capacitors are omnipresent in modern electronic devices, in which they swiftly release large portions of energy on demand. The capacitors may suffer from arc discharges due to local structural heterogeneities in their components and inappropriate exploitation practices. High energies of the arc discharge are transferred as phonons to the electrode and dielectric film, which burn out locally. The dielectric breakdown takes place. The complete burnout leads to the isolation of the failed region and the capacitor's self-healing. The emerging soot can form a semiconducting channel and damage the capacitor. The efficiency of self-healing depends on the dielectric properties of the soot and its amount. We employ reactive molecular dynamics simulations to reveal the regularities of the high-temperature polymer destruction and record by-products emerging during this process. We found the formation of multiple volatile low-molecular compounds and contaminated quantum carbon dots (CQD) designated as soot. The percentage of carbon in soot is higher compared to the polymer. Furthermore, the CQD contains numerous unsaturated C-C bonds and aromatic C6-rings suggesting an enhanced electrical conductivity. The size of the CQD depends on the available volume, i.e., on the spatial scale of the dielectric breakdown. The elemental composition of the soot is unique for each polymer. Polypropylene undergoes the most efficient self-healing thanks to containing a large molar fraction of hydrogen atoms. The results are addressed to the experts in electrical engineering and polymer fine-tuning.

Robust Interfacial Effect in Multi-interface Environment through Hybrid Reconstruction Chemistry for Enhanced Energy Storage

Electrochemical-oxidation-driven reconstruction has emerged as an efficient approach for developing advanced materials, but the reconstructed microstructure still faces challenges including inferior conductivity, unsatisfying intrinsic activity, and active-species dissolution. Herein, we present hybrid reconstruction chemistry that synergistically couples electrochemical oxidation with electrochemical polymerization (EOEP) to overcome these constraints. During the EOEP process, the metal hydroxides undergo rapid reconstruction and dynamically couple with polypyrrole (PPy), resulting in an interface-enriched microenvironment. We observe that the interaction between PPy and the reconstructed metal center (i.e., Mn > Ni, Co) is strongly correlated. Theoretical calculation results demonstrate that the strong interaction between Mn sites and PPy breaks the intrinsic limitation of MnO2, rendering MnO2 with a metallic property for fast charge transfer and enhancing the ion-adsorption dynamics. Operando Raman measurement confirms the promise of EOEP-treated Mn(OH)2 (E-MO/PPy) to stably work under a 1.2 V potential window. The tailored E-MO/PPy exhibits a high capacitance of 296 F g-1 at a large current density of 100 A g-1. Our strategy presents breakthroughs in upgrading the electrochemical reconstruction technique, which enables both activity and kinetics engineering of electrode materials for better performance in energy-related fields.

Ultrafast Synthesis of Graphene-Embedded Cyclodextrin-Metal-Organic Framework for Supramolecular Selective Absorbency and Supercapacitor Performance

Limited by preparation time and ligand solubility, synthetic protocols for cyclodextrin-based metal-organic framework (CD-MOF), as well as subsequent derived materials with improved stability and properties, still remains a challenge. Herein, an ultrafast, environmentally friendly, and cost-effective microwave method is proposed, which is induced by graphene oxide (GO) to design CD-MOF/GOs. This applicable technique can control the crystal size of CD-MOFs from macro- to nanocrystals. CD-MOF/GOs are investigated as a new type of supramolecular adsorbent. It can selectively adsorb the dye molecule methylene green (MG) owing to the synergistic effect between the hydrophobic nanocavity of CDs, and the abundant O-containing functional groups of GO in the composites. Following high temperature calcination, the resulting N, S co-doped porous carbons derived from CD-MOF/GOs exhibit a high capacitance of 501 F g-1 at 0.5 A g-1 , as well as stable cycling stability with 90.1% capacity retention after 5000 cycles. The porous carbon exhibits good electrochemical performance due to its porous surface containing numerous electrochemically active sites after dye adsorption and carbonization. The design strategy by supramolecular incorporating a variety of active molecules into CD-MOFs optimizes the properties of their derived materials, furthering development toward the fabrication of zeitgeisty and high-performance energy storage devices.

Hierarchical bismuthyl bromide microspheres assembled by laminas as efficient negative material for aqueous alkali battery

Bismuth (Bi) based compounds are promising negative materials in aqueous alkali batteries (AABs) for the 3-electron redox chemistry of Bi element within low potentials, the exploration of new Bi-based negative materials is still a meaningful work in this field. Herein, a hierarchical bismuthyl bromide (BiOBr) microspheres material assembled by laminas was prepared via solvothermal reaction and attempted as negative battery material for AAB. The pronounced redox reactions of Bi species in low potential enable high battery capacity, and the porous texture with high hydrophilicity facilitates diffusion of OH- and participation in faradaic reactions. When used as negative battery electrode, the BiOBr could offer decent specific capacity (Cs, 190 mAh g-1 at 1 A g-1), rate capability (Cs remained to 163 mAh g-1 at 8 A g-1) and cycleability (85% Cs retention after 1000 charge-discharge cycles). The AAB based on BiOBr negative electrode could export an energy density (Ecell) of 61.5 Wh kg-1 at power density (Pcell) of 558 W kg-1 and good cycleability. The current work showcases valuable application expansion of a traditional BiOBr photocatalyst in battery typed charge storage.

Advanced Self-Powered Biofuel Cells with Capacitor and Nanogenerator for Biomarker Sensing

Self-powered biofuel cells (BFCs) have evolved for highly sensitive detection of biomarkers such as noncodon micro ribonucleic acids (miRNAs) in the presence of interfering substrates. Self-charging supercapacitive BFCs for in vivo and in vitro cellular microenvironments represent the most prevalent sensing mechanism for diagnosis. Therefore, self-powered biosensing (SPB) with a capacitor and contact separation with a triboelectric nanogenerator (TENG) offers electrochemical and colorimetric dual-mode detection via improved electrical signal intensity. In this review, we discuss three major components: stretchable self-powered BFC design, miRNA sensing, and impedance spectroscopy. A specific focus is given to 1) assembling of sensors for biomarkers, 2) electrical output signal intensification, and 3) role of supercapacitors and nanogenerators in SPBs. We outline the key features of stretchable SPBs and the sequence of miRNA sensing by SPBs. We have emphasized the need of a supercapacitor and nanogenerator for SPBs in the context of advanced assembly of the sensing unit. Finally, we outline the role of impedance spectroscopy in the detection and estimation of biomarkers. We highlight key challenges in SPBs for biomarker sensing, which needs improved sensing accuracy, integration strategies of electrochemical biosensing for in vitro and in vivo microenvironments, and the impact of miRNA sensing on cancer diagnostics. This article attempts a specific focus on the accuracy and limitations of sensing unit for miRNA biomarkers and associated tool for boosting electrical signal intensity for a potential big step further.

Anion chemistry in energy storage devices

Anions serve as an essential component of electrolytes, whose effects have long been ignored. However, since the 2010s, we have seen a considerable increase of anion chemistry research in a range of energy storage devices, and it is now understood that anions can be well tuned to effectively improve the electrochemical performance of such devices in many aspects. In this Review, we discuss the roles of anion chemistry across various energy storage devices and clarify the correlations between anion properties and their performance indexes. We highlight the effects of anions on surface and interface chemistry, mass transfer kinetics and solvation sheath structure. Finally, we conclude with a perspective on the challenges and opportunities of anion chemistry for enhancing specific capacity, output voltage, cycling stability and anti-self-discharge ability of energy storage devices.

Inorganic-Organic Double Network Ionogels Based on Silica Nanoparticles for High-Temperature Flexible Supercapacitors

Herein, we demonstrate an inorganic-organic double network gel electrolyte consisting of a silica particle network and a poly-2-hydroxyethyl methacrylate network in which 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquids are confined. The as-synthesized double network ionogel electrolytes exhibited high ion conductivity of 3.8 to 12.8 mS cm-1 over a wide temperature range of 30 to 150 °C and mechanical integrity with a maximum toughness of 1.8 MJ m-3 at 30 °C. These remarkable properties of the ionogel were associated with the formation of an optimal physical network of the silica nanoparticles in the colloidal dispersion. Accordingly, a flexible supercapacitor using ionogel electrolytes and reduced graphene oxide electrodes delivered energy and power densities of 48 Wh kg-1 and 4 kW kg-1, respectively, even at a high temperature of 120 °C, demonstrating excellent long-term stability that retains 93% of the initial capacitance even over 10,000 charge/discharge cycles at 120 °C.

Anion Storage of MXenes

Recently, anion storage materials have gained significant attention owing to the widened cell voltage and additional anion storing capacity for a large energy density. MXenes are considered as the emerging anion storing materials owing to their sufficient interlayer spacing, rich surface chemistries, tunable structures, remarkable electrochemical properties, and mechanical integrity. Herein, a comprehensive review on the anion storage of MXenes covering their anion storage mechanism and state-of-the-art chemical strategies for the improved anion storage performances is reported. The recent progress of MXenes on aluminum ion batteries, metal halogen batteries, halogen ion batteries, and electrochemical electrode deionization is addressed. The scientific and technical challenges and the research direction into the anion storage of MXenes are also addressed and finally the authors' perspective on anion storage of MXenes is provided. Therefore, this review offers an insight into the rational design of MXenes for anion storage materials and the correlation of surface chemistries and structural modifications with anion storage properties for the applications into electrochemical energy storage and water purification.

Phosphorus Doping Strategy-Induced Synergistic Modification of Interlayer Structure and Chemical State in Ti3C2Tx toward Enhancing Capacitance

Heteroatom doping is considered an effective method to substantially improve the electrochemical performance of Ti₃C₂T_x MXene for supercapacitors. Herein, a facile and controllable strategy, which combines heat treatment with phosphorous (P) doping by using sodium phosphinate (NaH₂PO₂) as a phosphorus source, is used to modify Ti₃C₂T_x. The intercalated ions from NaH₂PO₂ act as "pillars" to expand the interlayer space of MXene, which is conducive to electrolyte ion diffusion. On the other hand, P doping tailors the surface electronic state of MXene, optimizing electronic conductivity and reducing the free energy of H⁺ diffusion on the MXene surface. Meanwhile, P sites with lower electronegativity owning good electron donor characteristics are easy to share electrons with H⁺, which is beneficial to charge storage. Moreover, the adopted heat treatment replaces -F terminations with O-containing groups, which enhances the hydrophilicity and provides sufficient active sites. The change in surface functional groups increases the content of high valence-stated Ti with a high electrochemical activity that can accommodate more electrons during discharge. Synergistic modification of interlayer structure and chemical state improves the possibility of Ti₃C₂T_x for accommodating more H⁺ ions. Consequently, the modified electrode delivers a specific capacitance of 510 F g^-1 at 2 mV s^-1, and a capacitance retention of 90.2% at 20 A g^-1 after 10,000 cycles. The work provides a coordinated strategy for the rational design of high-capacitance Ti₃C₂T_x MXene electrodes.

Ion Co-storage in Porous Organic Frameworks through On-site Coulomb Interactions for High Energy and Power Density Batteries

Fast and continuous ion insertion is blocked in the common electrodes operating with widely accepted single-ion storage mechanism, primarily due to Coulomb repulsion between the same ions. It results in an irreconcilable conflict between capacity and rate performance. Herein, we designed a porous organic framework with novel multiple-ion co-storage modes, including PF₆ ^- /Li⁺ , OTF^- /Mg²⁺ , and OTF^- /Zn²⁺ co-storage. The Coulomb interactions between cationic and anionic carriers in the framework can significantly promote electrode kinetics, by rejuvenating fast ion carrier migration toward framework interior. Consequently, the framework via PF₆ ^- /Li⁺ co-storage mode shows a high energy density of 878 Wh kg^-1 cycled more than 20 000 cycles, with an excellent power density of 28 kW kg^-1 that is already comparable to commercial supercapacitors. The both greatly improved energy and power densities via the co-storage mode may pave a way for exploring new electrodes that are not available from common single-ion electrodes.

Molten salt-confined pyrolysis towards heteroatom-doped porous carbon nanosheets for high-energy-density Zn-ion hybrid supercapacitors

Carbon nanosheets with heteroatom doping and well-developed porosity exhibit broad application foreground for Zn-ion hybrid supercapacitors (ZHSCs), but the simple and controllable preparation is still of great challenge. In this study, by using LiCl-KCl as in-built templates, histidine as carbon and nitrogen sources, and KNO₃, K₂SO₄, KOH or Na₂S₂O₃ as active agent, a series of N and NS doped porous carbon nanosheets are developed. Results indicate that, with the activator introduction, pore structures of the carbonized products are notably boosted, showing an astounding 30-244 % increase in BET specific surface area, and meanwhile, heteroatom with a content of ca. 12 % can be doped into the resultant carbon skeletons. Specifically, the NSPCN-800 (activated by Na₂S₂O₃) with a large specific surface area of 1297 m²/g, a hierarchically porous structure composed of abundant micropores and mesopores, and a suitable heteroatom content (N: 11.9 wt%; S: 0.6 wt%) presents an impressive energy storage behavior as cathode for ZHSCs, including a specific capacitance of 165.8F/g, a specific capacity of 95.2 mAh/g, an energy density of 59.0 Wh kg^-1 and a cyclic stability with a 82.6 % capacity retention after 5000 cycles. These performance parameters surpass numerous reported ZHSCs, making NSPCN-800 a very promising cathode for practical use.

Advances in Low-Temperature Dual-Ion Batteries

Fabricating rechargeable batteries for low-temperature (LT) applications is highly desired at high altitudes/latitudes, aerospace/subsea exploration, and defense. Lithium-ion batteries (LIBs) suffer from severe loss of capacity and energy/power density at sub-zero temperatures caused by the sluggish kinetics. By utilizing both cations and anions as charge carriers, dual-ion batteries (DIBs) become a nascent battery system for LT tolerance by overcoming ion-desolvation during discharge. Here, we summarize recent advances in LT DIBs. To begin with, distinctive advantages of DIBs at LTs are highlighted compared to LIBs, with a special attention to anion (de-)intercalation, and the in-depth understanding of key challenges for LT operation is discussed. The next major section deals with the exciting progress on the advanced strategies to improve the LT performance of DIBs, including alternative electrode materials, reliable electrolyte formulations, and construction of interphase protective layers. Finally, prospects and future developments in this exciting field of LT DIBs are suggested.

Construction of Fluorine- and Piperazine-Engineered Covalent Triazine Frameworks Towards Enhanced Dual-Ion Positive Electrode Performance

Organic positive electrodes featuring lightweight and tunable energy storage modes by molecular structure engineering have promising application prospects in dual-ion batteries. Herein, a series of highly porous covalent triazine frameworks (CTFs) were synthesized under ionothermal conditions using fluorinated aromatic nitrile monomers containing a piperazine ring. Fluorinated monomers can result in more defects in CTFs, leading to a higher surface area up to 2515 m²  g^-1 and a higher N content of 11.34 wt % compared to the products from the non-fluorinated monomer. The high surface area and abundant redox sites of these CTFs afforded high specific capacities (up to 279 mAh g^-1 at 0.1 A g^-1 ), excellent rate performance (89 mAh g^-1 at 5 A g^-1 ), and durable cycling performance (92.3 % retention rate after 500 cycles at 2.0 A g^-1 ) as dual-ion positive electrodes.

Design of Ni(OH)2/M-MMT Nanocomposite With Higher Charge Transport as a High Capacity Supercapacitor

Nano-petal nickel hydroxide was prepared on multilayered modified montmorillonite (M-MMT) using one-step hydrothermal method for the first time. This nano-petal multilayered nanostructure dominated the ion diffusion path to be shorted and the higher charge transport ability, which caused the higher specific capacitance. The results showed that in the three-electrode system, the specific capacitance of the nanocomposite with 4% M-MMT reached 1068 F/g at 1 A/g and the capacity retention rate was 70.2% after 1,000 cycles at 10 A/g, which was much higher than that of pure Ni(OH)2 (824 F/g at 1 A/g), indicating that the Ni(OH)2/M-MMT nanocomposite would be a new type of environmentally friendly energy storage supercapacitor.

Mesoporous vanadium nitride as anion storage electrode for reverse dual-ion hybrid supercapacitor

In traditional dual-ion systems, the cathode usually is employed as anion-storage materials. Herein, we propose a new dual-ion hybrid supercapacitor with reverse anion/cation-storage mechanism, consisting of a mesoporous (MPs) VN anode as a pivotal anion-storage material and K_2-xMn₈O₁₆ nanosheet arrays grown on carbon cloth (NSs/CC) as (K-storage) cathode. During charge/discharge, the anode and cathode reversibly store/release OH⁻ ions and K⁺ ions, respectively. Herein, the MPs VN as anion-storage electrode can operate in an alkaline condition and deliver a high capacitance of 251 mF cm⁻² with desired low-voltage plateau. More importantly, benefiting from unique reverse dual-ion mechanism, the (MPs VN-K_2-xMn₈O₁₆ NSs/CC) hybrid device displays excellent rate performance and satisfying area capacitance along with good durability of 92.2% after 10,000 cycles at a scan rate of 100 mV s⁻¹. It offers new ideas to expand the range of anion-storage materials in dual-ion hybrid supercapacitors.

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The MPs VN anode as anion storage material was firstly proposed

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The dual-ion supercapacitors were firstly constructed by employing the MPs VN anode

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The reverse storage mechanism was firstly applied to hybrid supercapacitors

Graphitic Aza-Fused π-Conjugated Networks: Construction, Engineering, and Task-Specific Applications

2D π-conjugated networks linked by aza-fused units represent a pivotal category of graphitic materials with stacked nanosheet architectures. Extensive efforts have been directed at their fabrication and application since the discovery of covalent triazine frameworks (CTFs). Besides the triazine cores, tricycloquinazoline and hexaazatriphenylene linkages are further introduced to tailor the structures and properties. Diverse related materials have been developed rapidly, and a thorough outlook is necessitated to unveil the structure-property-application relationships across multiple subcategories, which is pivotal to guide the design and fabrication toward enhanced task-specific performance. Herein, the structure types and development of related materials including CTFs, covalent quinazoline networks, and hexaazatriphenylene networks, are introduced. Advanced synthetic strategies coupled with characterization techniques provide powerful tools to engineer the properties and tune the associated behaviors in corresponding applications. Case studies in the areas of gas adsorption, membrane-based separation, thermo-/electro-/photocatalysis, and energy storage are then addressed, focusing on the correlation between structure/property engineering and optimization of the corresponding performance, particularly the preferred features and strategies in each specific field. In the last section, the underlying challenges and opportunities in construction and application of this emerging and promising material category are discussed.