Insights into Nano-and Micro-Structured Scaffolds for Advanced Electrochemical Energy Storage

Adopting a nano-and micro-structuring approach to fully unleashing the genuine potential of electrode active material benefits in-depth understandings and research progress toward higher energy density electrochemical energy stor-age devices at all technology readiness levels.Due to various challenging issues,especially limited stability,nano-and micro-structured(NMS)electrodes undergo fast electrochemical performance degradation.The emerging NMS scaffold design is a pivotal aspect of many electrodes as it endows them with both robustness and electrochemical performance enhancement,even though it only occupies comple-mentary and facilitating components for the main mechanism.However,extensive efforts are urgently needed toward optimizing the stereoscopic geometrical design of NMS scaffolds to minimize the volume ratio and maximize their functionality to fulfill the ever-increasing dependency and desire for energy power source supplies.This review will aim at highlighting these NMS scaffold design strategies,summariz-ing their corresponding strengths and challenges,and thereby outlining the potential solutions to resolve these challenges,design principles,and key perspectives for future research in this field.Therefore,this review will be one of the earliest reviews from this viewpoint.

Recent advances in electrochemical performance of Mg-based electrochemical energy storage materials in supercapacitors:Enhancement and mechanism

The application of Mg-based electrochemical energy storage materials in high performance supercapacitors is an essential step to promote the exploitation and utilization of magnesium resources in the field of energy storage.Unfortunately,the inherent chemical properties of magnesium lead to poor cycling stability and electrochemical reactivity,which seriously limit the application of Mg-based materials in supercapacitors.Herein,in this review,more than 70 research papers published in recent 10 years were collected and analyzed.Some representative research works were selected,and the results of various regulative strategies to improve the electrochemical performance of Mg-based materials were discussed.The effects of various regulative strategies(such as constructing nanostructures,synthesizing composites,defect engineering,and binder-free synthesis,etc.)on the electrochemical performance and their mechanism are demonstrated using spinelstructured MgX_(2)O_(4) and layered structured Mg-X-LDHs as examples.In addition,the application of magnesium oxide and magnesium hydroxide in electrode materials,MXene's solid spacers and hard templates are introduced.Finally,the challenges and outlooks of Mg-based electrochemical energy storage materials in high performance supercapacitors are also discussed.

Recent advances in 3D printed electrode materials for electrochemical energy storage devices

Electrochemical energy storage(EES)systems like batteries and supercapacitors are becoming the key power sources for attempts to change the energy dependency from inadequate fossil fuels to sustainable and renewable resources.Electrochemical energy storage devices(EESDs)operate efficiently as a result of the construction and assemblage of electrodes and electrolytes with appropriate structures and effective materials.Conventional manufacturing procedures have restrictions on regulating the morphology and architecture of the electrodes,which would influence the performance of the devices.3D printing(3DP)is an advanced manufacturing technology combining computer-aided design and has been recognised as an artistic method of fabricating different fragments of energy storage devices with its ability to precisely control the geometry,porosity,and morphology with improved specific energy and power densities.The capacity to create mathematically challenging shape or configuration designs and high-aspect-ratio 3D architectures makes 3D printing technology unique in its benefits.Nevertheless,the control settings,interactive manufacturing processes,and protracted post-treatments will affect the reproducibility of the printed components.More intelligent software,sophisticated control systems,high-grade industrial equipment,and post-treatment-free methods are necessary to develop.3D printed(3DPd)EESDs necessitate dynamic printable materials and composites that are influenced by performance criteria and fundamental electrochemistry.Herein,we review the recent advances in 3DPd electrodes for EES applications.The emphasis is on printable material synthesis,3DP techniques,and the electrochemical performance of printed electrodes.For the fabrication of electrodes,we concentrate on major 3DP technologies such as direct ink writing(DIW),inkjet printing(IJP),fused deposition modelling(FDM),and stereolithography3DP(SLA).The benefits and drawbacks of each 3DP technology are extensively discussed.We provide an outlook on the integration of synthesis of emerging nanomaterials and fabrication of complex structures from micro to macroscale to construct highly effective electrodes for the EESDs.

Application and Progress of Confinement Synthesis Strategy in Electrochemical Energy Storage

Designing high-performance nanostructured electrode materials is the current core of electrochemical energy storage devices.Multi-scaled nanomaterials have triggered considerable interest because they effectively combine a library of advantages of each component on different scales for energy storage.However,serious aggregation,structural degradation,and even poor stability of nanomaterials are well-known issues during electrochemically driven volume expansion/contraction processes.The confinement strategy provides a new route to construct controllable internal void spaces to avoid the intrinsic volume effects of nanomaterials during the reaction or charge/discharge process.Herein,we discuss the confinement strategies and methods for energy storage-related electrode materials with a one-dimensional channel,two-dimensional interlayer,and three-dimensional space as reaction environments.For each confinement environment,the correlation between the confinement condition/structure and the behavioral characteristics of energy storage devices in the scope of metal-ion batteries(e.g.,Li-ion,Na-ion,K-ion,and Mg-ion batteries),Li-S batteries(LSBs),Zn-air batteries(ZIBs),and supercapacitors.Finally,we discussed the challenges and perspectives on future nanomaterial confinement strategies for electrochemical energy storage devices.

2D Metal-Organic Frameworks for Electrochemical Energy Storage

Metal-organic frameworks(MOFs)have been widely adopted in various fields(catalysis,sensor,energy storage,etc.)during the last decade owing to the trait of abundant surface chemistry,porous structure,easy-to-adjust pore size,and diverse functional groups.However,the limited active sites and the poor conductivity hinder the relative practical application.2D MOFs can shorten the ion transport path with the merit of layered structure.The large surface area can increase the number of active sites as well as effectively utilize the sufficient active sites,exhibiting enormous potential in the field of energy storage systems(EESs).In this review,the characteristics of the 2D MOFs have been introduced,and the systematic synthesis methods(top-down and bottom-up)of 2D MOFs are presented,providing fundamental understanding for the construction of 2D MOFs.Moreover,the applications of 2D MOFs in energy storage fields such as supercapacitors and batteries are demonstrated in detail.Finally,the future development prospects have been proposed,offering guidelines for the rational utilization of 2D MOFs and promoting the understanding of 2D MOFs in EESs.

MXene-based materials for electrochemical energy storage

Rechargeable batteries and supercapacitors are widely investigated as the most important electrochemical energy storage devices nowadays due to the booming energy demand for electric vehicles and hand-held electronics. The large surface-area-to-volume ratio and internal surface areas endow two-dimensional（2D） materials with high mobility and high energy density; therefore, 2D materials are very promising candidates for Li ion batteries and supercapacitors with comprehensive investigations. In 2011, a new kind of 2D transition metal carbides, nitrides and carbonitrides, MXene, were successfully obtained from MAX phases. Since then about 20 different kinds of MXene have been prepared. Other precursors besides MAX phases and even other methods such as chemical vapor deposition（CVD） were also applied to prepare MXene, opening new doors for the preparation of new MXene. Their 2D nature and good electronic properties ensure the inherent advantages as electrode materials for electrochemical energy storage. In this review, we summarize the recent progress in the development of MXene with emphasis on the applications to electrochemical energy storage. Also, future perspective and challenges of MXene-based materials are briefly discussed regrading electrochemical energy storage.

Opportunities and challenges of organic flow battery for electrochemical energy storage technology

For flow batteries(FBs), the current technologies are still expensive and have relatively low energy density, which limits their large-scale applications. Organic FBs(OFBs) which employ organic molecules as redox-active materials have been considered as one of the promising technologies for achieving lowcost and high-performance. Herein, we present a critical overview of the progress on the OFBs, including the design principles of key components(redox-active molecules, membranes, and electrodes) and the latest achievement in both aqueous and nonaqueous systems. Finally, future directions in explorations of the high-performance OFB for electrochemical energy storage are also highlighted.

3D Printing of Next-generation Electrochemical Energy Storage Devices: from Multiscale to Multimaterial

The increasing energy requirements to power the modern world has driven active research into more advanced electrochemical energy storage devices(EESD)with both high energy densities and power densities.Wide range of newly discovered materials with promising electrochemical properties has shown great potential for next-generation devices,but their performance is normally associated with contradicting demands of thin electrodes and high mass loading that can be hardly achieved for practical applications.Design of three-dimensional(3D)porous electrodes can increase the mass loading while maintaining the effective charge transport even with thick electrodes,which has proven to be efficient to overcome the limitations.3D structures have also been demonstrated excellent structural stability to withstand strong strains and stresses generated during charge/discharge cycle.3D printing,which can fabricate various delicate and complex structural designs,thus offering brand-new opportunities for the rational design and facile construction of next-generation EESDs.The recent developments in 3D printing of next-generation EESDs with high performance are reviewed.Advanced/multiscale electrode structures,such as hierarchically porous structure that can be constructed via high-resolution 3D printing or with post-treatment,are further emphasized.The ability of current 3D printing techniques to fulfill multimaterial printing to fulfill simple packaging will be covered.

Organic fast ion-conductor with ordered Li-ion conductive nano-pathways and high ionic conductivity for electrochemical energy storage

Solid electrolyte(SE)is the most crucial factor to fabricate safe and high-performance all-solid-state lithium-ion batteries.However,the most commonly reported SE,including solid polymer electrolyte(SPE)and inorganic oxides and sulfides,suffer problems of low ionic conductivity at room temperature for SPE and large interfacial impedance with electrodes for inorganic electrolytes.Here we for the first time demonstrate a novel ionic plastic crystal lithium salt solid electrolyte(OLiSSE)fast ion-conductor dilithium(1,3-diethyl-4,5-dicarboxylate)imidazole bromide with ordered Li-ion conductive nanopathways and an exceptional ionic conductivity of 4.4×10^(−3)Scm^(−1)at 30℃.The prepared OLiSSE exhibits apparent characters of typical ionic plastic crystals in the temperature range of−20 to 70℃,and shows remarkable thermal stability and electrochemical stability below 150℃ and 4.7 V,respectively.No lithium dendrite or short circuit behavior is detected for the Li|OLiSSE|Li cell after the galvanostatic charge-discharge test for 500 h.The fabricated Li|OLiSSE|LiFePO_(4) all-solid-state cell without using any separator and liquid plasticizer directly delivers an initial discharge capacity of 151.4 mAh g^(−1) at the discharge rate of 0.1 C,and shows excellent charge-discharge cycle stability,implying large potential application in the next generation of safe and flexible all-solid-state lithium batteries.

Preface to Special Issue on Electrochemical Energy Storage and Conversion

Energy utilization includes two aspects of storage and conversion. Both the density of energy storage and the efficiency of energy conversion are particularly considered in the application of energy. It is well known that chemical energy can be easily stored in chemical substances with high-energy density such as those containing hydrogen and lithium. Meanwhile, chemical energy can be highly converted into clean and efficient electrical energy through the systems of electrochemical energy storage and conversion, which include batteries, fuel cells, and electrochemical capacitors （also called supercapacitors）. Thus, the combination of chemical energy and electrochemical reactions makes full use of the advantages of chemical energy and electrical energy. Nowadays, systems of electrochemical energy storage and conversion have already played an important role in powering an increasingly diverse range of applications from electronic devices to cars.

Two-dimensional polymer-based nanosheets for electrochemical energy storage and conversion

Over the past decades, two-dimensional（2D） nanomaterials possessing planar layered architecture and unique electronic structures have been being quickly developed, due to their wide potential application in the fields of chemistry, physics, and materials science. As a new family of 2D nanomaterials, 2D polymerbased nanosheets, featuring excellent characters, such as tunable framework structures, light weight, flexibility, high specific surface, and good semiconducting properties, have been emerging as one kind of promising functional materials for optoelectronics, gas separation, catalysis and sensing, etc. In this review, the recent progress in synthetic approach and characterization of 2D polymer-based nanosheets were summarized, and their current advances in electrochemical energy storage and conversion including second batteries, supercapacitors, oxygen reduction and hydrogen evolution were discussed systematically.

Ni/Co bimetallic organic frameworks nanospheres for highperformance electrochemical energy storage

In addition to their many well-known advantages(e.g.,ultra-high porosity,good pore size distribution,easy functionalization,and structural tolerability),metal-organic frameworks(MOFs)are a new class of advanced functional materials.However,their backbones are highly susceptible to deformation after exposure to acidic or alkaline conditions.As a result of lithium-ion batteries embedding or detaching directly from MOFs,they irreversibly collapse.As a result,they fail to maintain their electrochemical performance.These factors have hindered the development of MOFs as direct electrode materials,making the design of MOF materials with controlled morphology and stable dimensions a new challenge.In this study,we adopted a versatile and effective method to synthesize a novel MOF material(NiCo-BP(BP=BTC/phen and BTC=1,3,5-benzenetricarboxylic acid))using the rigid ligands 1,10-phenanthroline and homobenzotrizoic acid,and the emergence of the Ni-O/N and Co-O/N coordination layers was observed by extended X-ray absorption fine structure(EXAFS)tests,indicating that Ni and Co were coordinated with heterocyclic N-given atoms to form a stable p-πconjugated structure.Meanwhile,the metal-ion is attached to the carboxylic acid ligand on the other side,making the metal-organic skeleton complete and robust.The nanosphere structure of NiCo-BP(~400 nm)allows for full exposure and utilisation of the active sites,especially the Ni,Co,and phenanthroline units,and exhibit impressively high specific capacity and cycling stability.At a high current density of 1.0 A·g^(−1),a high discharge specific capacity of 631.6 mAh·g^(−1)was obtained after 1000 cycles.The co-participation of two organic ligands in the coordination is in accordance with the theory of soft and hard acids and bases,which contributes to the ability of the material to maintain a high capacity in cycling as well as its cyclic stability.

Autonomous Chemistry Enabling Environment-Adaptive Electrochemical Energy Storage Devices

Next-generation electronics that are fused into the human body can play a key role in future intelligent communication,smart healthcare,and human enhancement applications.As a promising energy supply component for smart biointegrated electronics,environment-adaptive electrochemical energy storage(EES)devices with complementary adaptability and functions have garnered huge interest in the past decade.Owing to the advancements in autonomous chemistry,which regulate the constitutional dynamic networks in materials,EES devices have witnessed higher freedom of autonomous adaptability in terms of mechano-adaptable,biocompatibility,and stimuli-response properties for biointegrated and smart applications.In this mini-review,we summarize the recent progress in emerging environmentadaptive EES devices enabled by the constitutional dynamic network of mechanical adaptable materials,biocompatible materials,and stimuli-responsive supramolecular polymer materials.Finally,the challenges and perspectives of autonomous chemistry on the environment-adaptive EES devices are discussed.

From surface loading to precise confinement of polyoxometalates for electrochemical energy storage

Because of abundant redox activity,broad tunability,and specific atomic structure,polyoxometalates(POMs or POM)clusters have attracted burgeoning interests in electrochemical especially energy stor-age fields.Nevertheless,due to the high solubility and fully oxidized state,they often suffer from elec-trically insulation as well as chemical and electrochemical instability.Traditional noncovalent loading or covalent grafting of POMs on conductive substrates have been successfully performed to overcome this problem.However,severe shedding or agglomeration of POMs arising from weak interactions with substrates or excessive entrapment or weak destruction in conductive supports cause significantly re-duced availability and stability.To this end,precise confinement of POMs into conductive supports has been tried to improve their dispersibility and stability.Herein,recent progress of PoMs from surface loading to precise confinement in the electrochemistry energy storage field is reviewed.Firstly,we il-lustrate the typical non-confinement methods(viz.covalent and non-covalent)for supported POMs in energy storage applications.Secondly,different strategies for precise confinement of PoMs in organic and inorganic materials for related applications are also discussed.Finally,future research directions and opportunities for confined POMs,and derived ultrafine nanostructures are also proposed.This re-view seeks to point out future research directions of supported PoMs in the electrochemistry-related fields.

A stretchable fabric as strain sensor integrating electromagnetic shielding and electrochemical energy storage

Multifunctional intelligent fabric plays an integral role in health management,human–machine interaction,wireless energy storage and conversion,and many other artificial intelligence fields.Herein,we demonstrate a newly developed MXene/polyaniline(PANI)multifunctional fabric integrated with strain sensing,electrochemical energy storage,and electromagnetic shielding properties.The multifunctional fabric-based strain sensor possesses a real-time signal response at a sizeable tensile strain of 100%with a minute strain of 0.5%,maintaining a stable and consistent signal response even after 3000 stretch–release cycles.In addition,the multifunctional fabric exhibits excellent electromagnetic shielding capabilities,achieving a total shielding effectiveness value of up to 43 dB,and in the meantime shows attractive electrochemical energy storage performance as an electrode in a supercapacitor,offering a maximum specific capacity and energy density of 522.5 mF·cm^(−2)and 18.16μWh·cm^(−2),respectively.Such a multifunctional intelligent fabric offers versatile opportunities to develop smart clothes for various artificial intelligent applications.

Freestanding MXene-based macroforms for electrochemical energy storage applications

Freestanding MXene-based macroforms have gained significant attention as versatile components in electrochemical energy storage applications owing to their interconnected conductive network,strong mechanical strength,and customizable surface chemistries derived from MXene nanosheets.This comprehensive review article encompasses key aspects related to the synthesis of MXene nanosheets,strategies for structure design and surface medication,surface modification,and the diverse fabrication methods employed to create freestanding MXene-based macroform architectures.The review also delves into the recent advancements in utilizing freestanding MXene macroforms for electrochemical energy storage applications,offering a detailed discussion on the significant progress achieved thus far.Notably,the correlation between the macroform’s structural attributes and its performance characteristics is thoroughly explored,shedding light on the critical factors influencing efficiency and durability.Despite the remarkable development,the review also highlights the existing challenges and presents future perspectives for freestanding MXenebased macroforms in the realms of high-performance energy storage devices.By addressing these challenges and leveraging emerging opportunities,the potential of freestanding MXene-based macroforms can be harnessed to enable groundbreaking advancements in the field of energy storage.

Flexible Transparent Electrochemical Energy Conversion and Storage: From Electrode Structures to Integrated Applications

The rapid progress of flexible electronics tremendously stimulates the urgent demands for the matching power supply systems. Flexible transparent electrochemical energy conversion and storage devices (FT–EECSDs), with endurable mechanical flexibility, outstanding optical transmittance, excellent electrochemical performance, and additional intelligent functions, are considered as preferable energy supplies for future self-powered flexible electronic systems. A comprehensive review of the reasonable design of flexible transparent electrode and recent progress on the FT–EECSDs is presented herein. The manufacturing techniques of generally classified three types of flexible transparent electrodes are systematically summarized. Emphasis is given to the recent developments in the transparent solid-state electrolyte, flexible transparent energy conversion, and storage devices. The standard evaluation methods and reasonable evaluation parameters to evaluate the flexibility and transparency of FT–EECSDs are highlighted. Additionally, the typical integrated applications of FT–EECSDs are also described. Finally, the current challenges and a future perspective on the research and development direction are further outlined.

V_(2)CT_(x) MXene and its derivatives:synthesis and recent progress in electrochemical energy storage applications

With the continuous development of two-dimensional (2D) transition metal carbides and nitrides(collectively referred to as MXene).Nowadays,more than 70 MXene materials have been discovered,and the number is still increasing.Among them,the V_(2)CT_(x) MXene has attracted considerable attentions due to its outstanding physical and chemical properties.In this review,we mainly discussed the emerging V_(2)CT_(x) MXene and its derivative systems in various energy storage devices.Firstly,an introduction of the V-based MXene and its derivatives along with their synthetic methodologies is provided,then we summarize their applications in specific energy storage devices,such as metal (Li,Na,K,Mg,Zn and Al) ion batteries,lithium-sulfur batteries,supercapacitors and metal-ion capacitors.Finally,the main challenges and future perspectives existing in V-based MXene and its derivatives are reasonably put forward.

Interfacial structure design of MXene-based nanomaterials for electrochemical energy storage and conversion

2D transition metal carbides,carbonitrides,and nitrides known as MXenes possess high electrical conductivity,large redox active surface area,rich surface chemistry,and tunable structures.Benefiting from these exceptional chemical and physical properties,the applications of MXenes for electrochemical energy storage and conversion have attracted increasing research interests around the world.Notably,the electrochemical performances of MXenes are directly dependent on their synthesis conditions,interfacial chemistries and structural configurations.In this review,we summarize the synthesis techniques of MXenes,as well as the recent advances in the interfacial structure design of MXene-based nanomaterials for electrochemical energy storage and conversion applications.Additionally,we provide an in-depth discussion on the relationship between interfacial structure and electrochemical performance from the perspectives of energy storage and electrocatalysis mechanisms.Finally,the challenges and insights for the future research of interfacial structure design of MXenes are outlined.

Supercapacitor and nanoscale research towards electrochemical energy storage

Electrochemical capacitors,also known as supercapacitors or ultracapacitors,have received much attention from research and development to industrialization,owing to their promise to deliver high levels of electrical power and offer long operating lifetimes.They are considered ideal candidates for energy storage in high-power applications.Benefiting from intensive nanoscale research in recent decades,remarkable improvements and development of supercapacitive energy storage systems have been achieved.Both the energy density and power density for supercapacitors have been substantially improved.In this review article,we endeavor to assess the profound impacts of nanoscale research on the development of supercapacitors,in terms of the substantial improvement of capacitive performance for electrode materials,and revolutionary advances in electrode and device configurations.In addition,recent progress in basic energy storage mechanisms and prototypes of supercapacitors are also reviewed,including a new kinetically-favored intercalation mechanism introduced for the first time.The review concludes with descriptions of the demonstration of already-realized practical applications of commercially-available supercapacitor devices,especially focusing on real usage in vehicles that are highly anticipated by future communities to further heighten the wide attention on clean energy storage systems.