Hierarchical Structures Based on Two‐Dimensional Nanomaterials for Rechargeable Lithium Batteries

Two‐dimensional (2D) nanomaterials (i.e., graphene and its derivatives, transition metal oxides and transition metal dichalcogenides) are receiving a lot attention in energy storage application because of their unprecedented properties and great diversities. However, their re‐stacking or aggregation during the electrode fabrication process has greatly hindered their further developments and applications in rechargeable lithium batteries. Recently, rationally designed hierarchical structures based on 2D nanomaterials have emerged as promising candidates in rechargeable lithium battery applications. Numerous synthetic strategies have been developed to obtain hierarchical structures and high‐performance energy storage devices based on these hierarchical structure have been realized. This review summarizes the synthesis and characteristics of three styles of hierarchical architecture, namely three‐dimensional (3D) porous network nanostructures, hollow nanostructures and self‐supported nanoarrays, presents the representative applications of hierarchical structured nanomaterials as functional materials for lithium ion batteries, lithium‐sulfur batteries and lithium‐oxygen batteries, meanwhile sheds light particularly on the relationship between structure engineering and improved electrochemical performance; and provides the existing challenges and the perspectives for this fast emerging field.     

Structure Design and Composition Engineering of Carbon‐Based Nanomaterials for Lithium Energy Storage

Carbon‐based nanomaterials have significantly pushed the boundary of electrochemical performance of lithium‐based batteries (LBs) thanks to their excellent conductivity, high specific surface area, controllable morphology, and intrinsic stability. Complementary to these inherent properties, various synthetic techniques have been adopted to prepare carbon‐based nanomaterials with diverse structures and different dimensionalities including 1D nanotubes and nanorods, 2D nanosheets and films, and 3D hierarchical architectures, which have been extensively applied as high‐performance electrode materials for energy storage and conversion. The present review aims to outline the structural design and composition engineering of carbon‐based nanomaterials as high‐performance electrodes of LBs including lithium‐ion batteries, lithium–sulfur batteries, and lithium–oxygen batteries. This review mainly focuses on the boosting of electrochemical performance of LBs by rational dimensional design and porous tailoring of advanced carbon‐based nanomaterials. Particular attention is also paid to integrating active materials into the carbon‐based nanomaterials, and the structure–performance relationship is also systematically discussed. The developmental trends and critical challenges in related fields are summarized, which may inspire more ideas for the design of advanced carbon‐based nanostructures with superior properties.     

Recent Developments and Understanding of Novel Mixed Transition‐Metal Oxides as Anodes in Lithium Ion Batteries

Mixed transition‐metal oxides (MTMOs), including stannates, ferrites, cobaltates, and nickelates, have attracted increased attention in the application of high performance lithium‐ion batteries. Compared with traditional metal oxides, MTMOs exhibit enormous potential as electrode materials in lithium‐ion batteries originating from higher reversible capacity, better structural stability, and high electronic conductivity. Recent advancements in the rational design of novel MTMO micro/nanostructures for lithium‐ion battery anodes are summarized and their energy storage mechanism is compared to transition‐metal oxide anodes. In particular, the significant effects of the MTMO morphology, micro/nanostructure, and crystallinity on battery performance are highlighted. Furthermore, the future trends and prospects, as well as potential problems, are presented to further develop advanced MTMO anodes for more promising and large‐scale commercial applications of lithium‐ion batteries.     

Advanced Phosphorus‐Based Materials for Lithium/Sodium‐Ion Batteries: Recent Developments and Future Perspectives

High‐performance and lost‐cost lithium‐ion and sodium‐ion batteries are highly desirable for a wide range of applications including portable electronic devices, transportation (e.g., electric vehicles, hybrid vehicles, etc.), and renewable energy storage systems. Great research efforts have been devoted to developing alternative anode materials with superior electrochemical properties since the anode materials used are closely related to the capacity and safety characteristics of the batteries. With the theoretical capacity of 2596 mA h g^?1, phosphorus is considered to be the highest capacity anode material for sodium‐ion batteries and one of the most attractive anode materials for lithium‐ion batteries. This work provides a comprehensive study on the most recent advancements in the rational design of phosphorus‐based anode materials for both lithium‐ion and sodium‐ion batteries. The currently available approaches to phosphorus‐based composites along with their merits and challenges are summarized and discussed. Furthermore, some present underpinning issues and future prospects for the further development of advanced phosphorus‐based materials for energy storage/conversion systems are discussed.     

Graphene‐Containing Nanomaterials for Lithium‐Ion Batteries

Graphene‐containing nanomaterials have emerged as important candidates for electrode materials in lithium‐ion batteries (LIBs) due to their unique physical properties. In this review, a brief introduction to recent developments in graphene‐containing nanocomposite electrodes and their derivatives is provided. Subsequently, synthetic routes to nanoparticle/graphene composites and their electrochemical performance in LIBs are highlighted, and the current state‐of‐the‐art and most recent advances in the area of graphene‐containing nanocomposite electrode materials are summarized. The limitations of graphene‐containing materials for energy storage applications are also discussed, with an emphasis on anode and cathode materials. Potential research directions for the future development of graphene‐containing nanocomposites are also presented, with an emphasis placed on practicality and scale‐up considerations for taking such materials from benchtop curiosities to commercial products.     

Holey 2D Nanomaterials for Electrochemical Energy Storage

2D nanomaterials provide numerous fascinating properties, such as abundant active surfaces and open ion diffusion channels, which enable fast transport and storage of lithium ions and beyond. However, decreased active surfaces, prolonged ion transport pathway, and sluggish ion transport kinetics caused by self‐restacking of 2D nanomaterials during electrode assembly remain a major challenge to build high‐performance energy storage devices with simultaneously maximized energy and power density as well as long cycle life. To address the above challenge, porosity (or hole) engineering in 2D nanomaterials has become a promising strategy to enable porous 2D nanomaterials with synergetic features combining both 2D nanomaterials and porous architectures. Herein, recent important progress on porous/holey 2D nanomaterials for electrochemical energy storage is reviewed, starting with the introduction of synthetic strategies of porous/holey 2D nanomaterials, followed by critical discussion of design rule and their advantageous features. Thereafter, representative work on porous/holey 2D nanomaterials for electrochemical capacitors, lithium‐ion and sodium‐ion batteries, and other emerging battery technologies (lithium‐sulfur and metal‐air batteries) are presented. The article concludes with perspectives on the future directions for porous/holey 2D nanomaterial in energy storage and conversion applications.     

Transition Metal Sulfides Based on Graphene for Electrochemical Energy Storage

Transition metal sulfides, as an important class of inorganics, can be used as excellent electrode materials for various types of electrochemical energy storage, such as lithium‐ion batteries, sodium‐ion batteries, supercapacitors, and others. Recent works have identified that mixing graphene or graphene derivatives with transition metal sulfides can result in novel composites with better electrochemical performance. This review summarizes the latest advances in transition metal sulfide composites with graphene or graphene derivatives. The synthetic strategies and morphologies of these composites are introduced. The authors then discuss their applications in lithium‐ion batteries, sodium‐ion batteries, and supercapacitors. Finally, the authors give their personal viewpoints about the challenges and opportunities for the future development about this direction.     

Layered Oxide Cathodes for Sodium‐Ion Batteries: Phase Transition,Air Stability,and Performance

The increasing demand for replacing conventional fossil fuels with clean energy or economical and sustainable energy storage drives better battery research today. Sodium‐ion batteries (SIBs) are considered as a promising alternative for grid‐scale storage applications due to their similar “rocking‐chair” sodium storage mechanism to lithium‐ion batteries, the natural abundance, and the low cost of Na resources. Searching for appropriate electrode materials with acceptable electrochemical performance is the key point for development of SIBs. Layered transition metal oxides represent one of the most fascinating electrode materials owing to their superior specific capacity, environmental benignity, and facile synthesis. However, three major challenges (irreversible phase transition, storage instability, and insufficient battery performance) are known for cathodes in SIBs. Herein, a comprehensive review on the latest advances and progresses in the exploration of layered oxides for SIBs is presented, and a detailed and deep understanding of the relationship of phase transition, air stability, and electrochemical performance in layered oxide cathodes is provided in terms of refining the structure–function–property relationship to design improved battery materials. Layered oxides will be a competitive and attractive choice as cathodes for SIBs in next‐generation energy storage devices.     

An Efficient Separator with Low Li‐Ion Diffusion Energy Barrier Resolving Feeble Conductivity for Practical Lithium–Sulfur Batteries

Due to unprecedented features including high‐energy density, low cost, and light weight, lithium–sulfur batteries have been proposed as a promising successor of lithium‐ion batteries. However, unresolved detrimental low Li‐ion transport rates in traditional carbon materials lead to large energy barrier in high sulfur loading batteries, which prevents the lithium–sulfur batteries from commercialization. In this report, to overcome the challenge of increasing both the cycling stability and areal capacity, a metallic oxide composite (NiCo₂O₄@rGO) is designed to enable a robust separator with low energy barrier for Li‐ion diffusion and simultaneously provide abundant active sites for the catalytic conversion of the polar polysulfides. With a high sulfur‐loading of 6 mg cm^?2 and low sulfur/electrolyte ratio of 10, the assembled batteries deliver an initial capacity of 5.04 mAh cm^?2 as well as capacity retention of 92% after 400 cycles. The metallic oxide composite NiCo₂O₄@rGO/PP separator with low Li‐ion diffusion energy barrier opens up the opportunity for lithium–sulfur batteries to achieve long‐cycle, cost‐effective operation toward wide applications in electric vehicles and electronic devices.     

Iron‐Oxide‐Based Advanced Anode Materials for Lithium‐Ion Batteries

Iron oxides, such as Fe₂O₃ and Fe₃O₄, have recently received increased attention as very promising anode materials for rechargeable lithium‐ion batteries (LIBs) because of their high theoretical capacity, non‐toxicity, low cost, and improved safety. Nanostructure engineering has been demonstrated as an effective approach to improve the electrochemical performance of electrode materials. Here, recent research progress in the rational design and synthesis of diverse iron oxide‐based nanomaterials and their lithium storage performance for LIBs, including 1D nanowires/rods, 2D nanosheets/flakes, 3D porous/hierarchical architectures, various hollow structures, and hybrid nanostructures of iron oxides and carbon (including amorphous carbon, carbon nanotubes, and graphene). By focusing on synthesis strategies for various iron‐oxide‐based nanostructures and the impacts of nanostructuring on their electrochemical performance, novel approaches to the construction of iron‐oxide‐based nanostructures are highlighted and the importance of proper structural and compositional engineering that leads to improved physical/chemical properties of iron oxides for efficient electrochemical energy storage is stressed. Iron‐oxide‐based nanomaterials stand a good chance as negative electrodes for next generation LIBs.     

Covalent–Organic Frameworks: Advanced Organic Electrode Materials for Rechargeable Batteries

Covalent–organic frameworks (COFs), featuring structural diversity, framework tunability and functional versatility, have emerged as promising organic electrode materials for rechargeable batteries and garnered tremendous attention in recent years. The adjustable pore configuration, coupled with the functionalization of frameworks through pre‐ and post‐synthesis strategies, enables a precise customization of COFs, which provides a novel perspective to deepen the understanding of the fundamental problems of organic electrode materials. In this review, a summary of the recent research into COFs electrode materials for rechargeable batteries including lithium‐ion batteries, sodium‐ion batteries, potassium‐ion batteries, and aqueous zinc batteries is provided. In addition, this review will also cover the working principles, advantages and challenges, strategies to improve electrochemical performance, and applications of COFs in rechargeable batteries.     

Transition Metal Oxide Anodes for Electrochemical Energy Storage in Lithium‐ and Sodium‐Ion Batteries

Lithium‐ion batteries (LIBs) with outstanding energy and power density have been extensively investigated in recent years, rendering them the most suitable energy storage technology for application in emerging markets such as electric vehicles and stationary storage. More recently, sodium, one of the most abundant elements on earth, exhibiting similar physicochemical properties as lithium, has been gaining increasing attention for the development of sodium‐ion batteries (SIBs) in order to address the concern about Li availability and cost—especially with regard to stationary applications for which size and volume of the battery are of less importance. Compared with traditional intercalation reactions, conversion reaction‐based transition metal oxides (TMOs) are prospective anode materials for rechargeable batteries thanks to their low cost and high gravimetric specific capacities. In this review, the recent progress and remaining challenges of conversion reactions for LIBs and SIBs are discussed, covering an overview about the different synthesis methods, morphological characteristics, as well as their electrochemical performance. Potential future research directions and a perspective toward the practical application of TMOs for electrochemical energy storage are also provided.     

Metal Sulfide Hollow Nanostructures for Electrochemical Energy Storage

Metal sulfide hollow nanostructures (MSHNs) have received intensive attention as electrode materials for electrical energy storage (EES) systems due to their unique structural features and rich chemistry. Here, we summarize recent research progress in the rational design and synthesis of various metal sulfide hollow micro‐/nanostructures with controlled shape, composition and structural complexity, and their applications to lithium ion batteries (LIBs) and hybrid supercapacitors (HSCs). The current understanding of hollow structure control, including single‐shelled, yolk‐shelled, multi‐shelled MSHNs, and their hybrid micro‐/nanostructures with carbon (amorphous carbon nanocoating, graphene and hollow carbon), is focused on. The importance of proper structural and compositional control on the enhanced electrochemical properties of MSHNs is emphasized. A relationship between structural and compositional engineering with improved electrochemical activity of MSHNs is sought, in order to shed some light on future electrode design trends for next‐generation EES technologies.     

Na‐Ion Batteries for Large Scale Applications: A Review on Anode Materials and Solid Electrolyte Interphase Formation

The urgent need for optimizing the available energy through smart grids and efficient large‐scale energy storage systems is pushing the construction and deployment of Li‐ion batteries in the MW range which, in the long term, are expected to hit the GW dimension while demanding over 1000 ton of positive active material per system. This amount of Li‐based material is equivalent to almost 1% of current Li consumption and can strongly influence the evolution of the lithium supply and cost. Given this uncertainty, it becomes mandatory to develop an energy storage technology that depends on almost infinite and widespread resources: Na‐ion batteries are the best technology for large‐scale applications. With small working cells in the market that cannot compete in cost ($/W h) with commercial Li‐ion batteries, the consolidation of Na‐ion batteries mainly depends on increasing their energy density and stability, the negative electrodes being at the heart of these two requirements. Promising Na‐based negative electrodes for large‐scale battery applications are reviewed, along with the study of the solid electrolyte interphase formed in the anode surface, which is at the origin of most of the stability problems.     

Two‐Dimensional Materials for Beyond‐Lithium‐Ion Batteries

Lithium‐ion batteries (LIBs) have dominated the portable electronics industry and solid‐state electrochemical research and development for the past two decades. In light of possible concerns over the cost and future availability of lithium, sodium‐ion batteries (SIBs) and other new technologies have emerged as candidates for large‐scale stationary energy storage. Research in these technologies has increased dramatically with a focus on the development of new materials for both the positive and negative electrodes that can enhance the cycling stability, rate capability, and energy density. Two‐dimensional (2D) materials are showing promise for many energy‐related applications and particularly for energy storage, because of the efficient ion transport between the layers and the large surface areas available for improved ion adsorption and faster surface redox reactions. Recent research highlights on the use of 2D materials in these future ‘beyond‐lithium‐ion’ battery systems are reviewed, and strategies to address challenges are discussed as well as their prospects.     

Porous Electrode Materials for Lithium‐Ion Batteries – How to Prepare Them and What Makes Them Special

Numerous benefits of porous electrode materials for lithium ion batteries (LIBs) have been demonstrated, including examples of higher rate capabilities, better cycle lives, and sometimes greater gravimetric capacities at a given rate compared to nonporous bulk materials. These properties promise advantages of porous electrode materials for LIBs in electric and hybrid electric vehicles, portable electronic devices, and stationary electrical energy storage. This review highlights methods of synthesizing porous electrode materials by templating and template‐free methods and discusses how the structural features of porous electrodes influence their electrochemical properties. A section on electrochemical properties of porous electrodes provides examples that illustrate the influence of pore and wall architecture and interconnectivity, surface area, particle morphology, and nanocomposite formation on the utilization of the electrode materials, specific capacities, rate capabilities, and structural stability during lithiation and delithiation processes. Recent applications of porous solids as components for three‐dimensionally interpenetrating battery architectures are also described.     

Advanced Energy‐Storage Architectures Composed of Spinel Lithium Metal Oxide Nanocrystal on Carbon Textiles

Current battery technologies are known to suffer from kinetic problems associated with the solid‐state diffusion of Li⁺ in intercalation electrodes materials. Not only the use of nanostructure materials but also the design of electrode architectures can lead to more advanced properties. Here, advanced electrode architectures consisting of carbon textiles conformally covered by Li₄Ti₅O₁₂ nanocrystal are rationally designed and synthesized for lithium ion batteries. The efficient two‐step synthesis involves the growth of ultrathin TiO₂ nanosheets on carbon textiles, and subsequent conversion into spinel Li₄Ti₅O₁₂ through chemical lithiation. Importantly, this novel approach is simple and general, and it is used to successfully produce LiMn₂O₄/carbon composites textiles, one of the leading cathode materials for lithium ion batteries. The resulting 3D textile electrode, with various advantages including the direct electronic pathway to current collector, the easy access of electrolyte ions, the reduced Li⁺/e^? diffusion length, delivers excellent rate capability and good cyclic stability over the Li‐ion batteries of conventional configurations.     

Quantification of Pseudocapacitive Contribution in Nanocage‐Shaped Silicon–Carbon Composite Anode

Pseudocapacitive materials have been highlighted as promising electrode materials to overcome slow diffusion‐limited redox mechanism in active materials, which impedes fast charging/discharging in energy storage devices. However, previously reported pseudocapacitive properties have been rarely used in lithium‐ion batteries (LIBs) and evaluation methods have been limited to those focused on thin‐film‐type electrodes. Hence, a nanocage‐shaped silicon–carbon composite anode is proposed with excellent pseudocapacitive qualities for LIB applications. This composite anode exhibits a superior rate capability compared to other Si‐based anodes, including commercial silicon nanoparticles, because of the higher pseudocapacitive contribution coming from ultrathin Si layer. Furthermore, unprecedent 3D pore design in cage shape, which prevents the particle scale expansion even after full lithiation demonstrates the high cycling stability. This concept can potentially be used to realize high‐power and high‐energy LIB anode materials.     

Nanoarchitectured Array Electrodes for Rechargeable Lithium‐ and Sodium‐Ion Batteries

Rechargeable ion batteries have contributed immensely to shaping the modern world and been seriously considered for the efficient storage and utilization of intermittent renewable energies. To fulfill their potential in the future market, superior battery performance of high capacity, great rate capability, and long lifespan is undoubtedly required. In the past decade, along with discovering new electrode materials, the focus has been shifting more and more toward rational electrode designs because the performance is intimately connected to the electrode architectures, particularly their designs at the nanoscale that can alleviate the reliance on the materials' intrinsic nature. The utilization of nanoarchitectured arrays in the design of electrodes has been proven to significantly improve the battery performance. A comprehensive summary of the structural features and fabrications of the nanoarchitectured array electrodes is provided, and some of the latest achievements in the area of both lithium‐ and sodium‐ion batteries are highlighted. Finally, future challenges and opportunities that would allow further development of such advanced electrode configuration are discussed.     

Routes to High Energy Cathodes of Sodium‐Ion Batteries

Sodium‐ion batteries (SIBs) are now being actively developed as low cost and sustainable alternatives to lithium‐ion batteries (LIBs) for large‐scale electric energy storage applications. In recent years, various inorganic and organic Na compounds, mostly mimicked from their Li counterparts, have been synthesized and tested for SIBs, and some of them indeed demonstrate comparable specific capacity to the presently developed LIB electrodes. However, the lack of suitable cathode materials is still a major obstacle to the commercial development of SIBs. Here, we present a brief review on the recent developments of SIB cathodes, with a focus on low cost and high energy density materials (> 450 Wh kg^?1 vs Na) together with discussion of their Na‐storage mechanisms. The considerable differences in the structural requirements for Li‐ and Na‐storage reactions mean that it is not sufficient to design SIB cathode materials by simply mimicking LIB materials, and therefore great efforts are needed to discover new materials and reaction mechanisms to further develop variable cathodes for advanced SIB technology. Some directions for future research and possible strategies for building advanced cathode materials are also proposed here.