Mixed Metal Sulfides for Electrochemical Energy Storage and Conversion

Mixed metal sulfides (MMSs) have attracted increased attention as promising electrode materials for electrochemical energy storage and conversion systems including lithium‐ion batteries (LIBs), sodium‐ion batteries (SIBs), hybrid supercapacitors (HSCs), metal–air batteries (MABs), and water splitting. Compared with monometal sulfides, MMSs exhibit greatly enhanced electrochemical performance, which is largely originated from their higher electronic conductivity and richer redox reactions. In this review, recent progresses in the rational design and synthesis of diverse MMS‐based micro/nanostructures with controlled morphologies, sizes, and compositions for LIBs, SIBs, HSCs, MABs, and water splitting are summarized. In particular, nanostructuring, synthesis of nanocomposites with carbonaceous materials and fabrication of 3D MMS‐based electrodes are demonstrated to be three effective approaches for improving the electrochemical performance of MMS‐based electrode materials. Furthermore, some potential challenges as well as prospects are discussed to further advance the development of MMS‐based electrode materials for next‐generation electrochemical energy storage and conversion systems.     

Sodium‐Ion Batteries Paving the Way for Grid Energy Storage

The recent proliferation of renewable energy generation offers mankind hope, with regard to combatting global climate change. However, reaping the full benefits of these renewable energy sources requires the ability to store and distribute any renewable energy generated in a cost‐effective, safe, and sustainable manner. As such, sodium‐ion batteries (NIBs) have been touted as an attractive storage technology due to their elemental abundance, promising electrochemical performance and environmentally benign nature. Moreover, new developments in sodium battery materials have enabled the adoption of high‐voltage and high‐capacity cathodes free of rare earth elements such as Li, Co, Ni, offering pathways for low‐cost NIBs that match their lithium counterparts in energy density while serving the needs for large‐scale grid energy storage. In this essay, a range of battery chemistries are discussed alongside their respective battery properties while keeping metrics for grid storage in mind. Matters regarding materials and full cell cost, supply chain and environmental sustainability are discussed, with emphasis on the need to eliminate several elements (Li, Ni, Co) from NIBs. Future directions for research are also discussed, along with potential strategies to overcome obstacles in battery safety and sustainable recyclability.     

Textile‐Based Electrochemical Energy Storage Devices

In the past few years, insensitive attentions have been drawn to wearable and flexible energy storage devices/systems along with the emergence of wearable electronics. Much progress has been achieved in developing flexible electrochemical energy storage devices with high end‐use performance. However, challenges still remain in well balancing the electrochemical properties, mechanical properties, and the processing technologies. In this review, a specific perspective on the development of textile‐based electrochemical energy storage devices (TEESDs), in which textile components and technologies are utilized to enhance the energy storage ability and mechanical properties of wearable electronic devices, is provided. The discussion focuses on the material preparation and characteristics, electrode and device fabrication strategies, electrochemical performance and metrics, wearable compatibility, and fabrication scalability of TEESDs including textile‐based supercapacitors and lithium‐ion batteries.     

Rational Design of Nickel Hydroxide‐Based Nanocrystals on Graphene for Ultrafast Energy Storage

Compact, light, and powerful energy storage devices are urgently needed for many emerging applications; however, the development of advanced power sources relies heavily on advances in materials innovation. Here, the findings in rational design, one‐pot synthesis, and characterization of a series of Ni hydroxide‐based electrode materials in alkaline media for fast energy storage are reported. Under the guidance of density functional theory calculations and experimental investigations, a composite electrode composed of Co‐/Mn‐substituted Ni hydroxides grown on reduced graphene oxide (rGO) is designed and prepared, demonstrating capacities of 665 and 427 C g^?1 at current densities of 2 and 20 A g^?1, respectively. The superior performance is attributed mainly to the low deprotonation energy and the facile electron transport, as elaborated by theoretical calculations. When coupled with an electrode based on organic molecular‐modified rGO, the resulting hybrid device demonstrates an energy density of 74.7 W h kg^?1 at a power density of 1.68 kW kg^?1 while maintaining capacity retention of 91% after 10,000 cycles (20 A g^?1). The findings not only provide a promising electrode material for high‐performance hybrid capacitors but also open a new avenue toward knowledge‐based design of efficient electrode materials for other energy storage applications.     

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.     

Enhancing Energy Storage Devices with Biomacromolecules in Hybrid Electrodes

The development of energy storage devices with higher energy and power outputs, and long cycling stability is urgently required in the pursuit of the expanding challenges of electrical energy storage. The utilization of biologically renewable redox compounds holds a great potential in designing sustainable energy storage systems and contributes in reducing the dependence on fossil fuels for energy materials. Quinones are the principal redox centers in natural organic materials and play a key role as charge storage electrode materials because of their abundance, multiple forms and integration into the materials flow through the biosphere. Electrical energy storage devices and systems can be significantly improved by the combination of scalable quinone‐based biomaterials with good electronic conductors. This review uses recent examples to show how biopolymers are providing new directions in the development of renewable biohybrid electrodes for energy storage devices.     

The Role of Cation Vacancies in Electrode Materials for Enhanced Electrochemical Energy Storage: Synthesis,Advanced Characterization,and Fundamentals

The incorporation of atomic scale defects, such as cation vacancies, in electrode materials is considered an effective strategy to improve their electrochemical energy storage performance. In fact, cation vacancies can effectively modulate the electronic properties of host materials, thus promoting charge transfer and redox reaction kinetics. Such defects can also serve as extra host sites for inserted proton or alkali cations, facilitating the ion diffusion upon electrochemical cycling. Altogether, these features may contribute to improved electrochemical performance. In this review, the latest progress in cation vacancies‐based electrochemical energy storage materials, covering the synthetic approaches to incorporate cation vacancies and the advanced techniques to characterize such vacancies and identify their fundamental role, are provided from the chemical and materials point of view. The key challenges and future opportunities for cation vacancies‐based electrochemical energy storage materials are also discussed, particularly focusing on cation‐deficient transition metal oxides (TMOs), but also including newly emerging materials such as transition metal carbides (MXenes).     

Recent Advances in Rechargeable Magnesium‐Based Batteries for High‐Efficiency Energy Storage

Benefiting from higher volumetric capacity, environmental friendliness and metallic dendrite‐free magnesium (Mg) anodes, rechargeable magnesium batteries (RMBs) are of great importance to the development of energy storage technology beyond lithium‐ion batteries (LIBs). However, their practical applications are still limited by the absence of suitable electrode materials, the sluggish kinetics of Mg²⁺ insertion/extraction and incompatibilities between electrodes and electrolytes. Herein, a systematic and insightful review of recent advances in RMBs, including intercalation‐based cathode materials and conversion reaction‐based compounds is presented. The relationship between microstructures with their electrochemical performances is comprehensively elucidated. In particular, anode materials are discussed beyond metallic Mg for RMBs. Furthermore, other Mg‐based battery systems are also summarized, including Mg–air batteries, Mg–sulfur batteries, and Mg–iodine batteries. This review provides a comprehensive understanding of Mg‐based energy storage technology and could offer new strategies for designing high‐performance rechargeable magnesium batteries.     

Phase Boundary Derived Pseudocapacitance Enhanced Nickel‐Based Composites for Electrochemical Energy Storage Devices

Several strategies have been employed to improve the performance of energy storage devices through the development of new electrode materials. The construction of transition metal compound composite electrodes plays an important role in promoting the performance of energy storage devices. However, understandings of and insight into how to enhance the composites properties are rarely reported. Taking nickel‐based compounds as an example, Ni₃N@Ni₃S₂ hybrid nanosheets are reported as a high‐performance anode material for lithium‐ion batteries that delivers higher lithium storage properties than the pristine Ni₃N and Ni₃S₂ electrodes. This demonstrates that the phase boundaries between the Ni₃N and Ni₃S₂ may contribute additional lithium storage, which leads to a synergistic effect via the high pseudocapacitance contribution from the outstanding conductivity of Ni₃N and enhanced diffusion‐controlled capacity of Ni₃S₂. The use of composites prepared through sulfuration of hydrothermally annealed nickel hydroxide‐based precursor provides an enhancement of the energy storage properties. These results provide an important approach for increasing the electrochemical activity of composites by the combined effect of interfacial mismatch and pseudocapacitance, as well as understandings of the mechanism of the enhancement of the composite electrode properties.     

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.     

Design Considerations for Unconventional Electrochemical Energy Storage Architectures

Batteries have become fundamental building blocks for the mobility of modern society. Continuous development of novel battery chemistries and electrode materials has nourished progress in building better batteries. Simultaneously, novel device form factors and designs with multi‐functional components have been proposed, requiring batteries to not only integrate seamlessly to these devices, but to also be a multi‐functional component for a multitude of applications. Thus, in the past decade, along with developments in the component materials, the focus has been shifting more and more towards novel fabrication processes, unconventional configurations, and additional functionalities. This work attempts to critically review the developments with respect to emerging electrochemical energy storage configurations, including, amongst others, paintable, transparent, flexible, wire or cable shaped, ultra‐thin and ultra‐thick configurations, as well as hybrid energy storage‐conversion, or graphene‐incorporated batteries and supercapacitors. The performance requirements are elaborated together with the advantages, but also the limitations, with respect to established electrochemical energy storage technologies. Finally, challenges in developing novel materials with tailored properties that would allow such configurations, and in designing easier manufacturing techniques that can be widely adopted are considered.     

Highly Efficient Materials Assembly Via Electrophoretic Deposition for Electrochemical Energy Conversion and Storage Devices

Featuring pronounced controllability, versatility, and scalability, electrophoretic deposition (EPD) has been proposed as an efficient method for film assembly and electrode/solid electrolyte fabrication in various energy storage/conversion devices including rechargeable batteries, supercapacitors, and fuel cells. High‐quality electrodes and solid electrolytes have been prepared through EPD and exhibit advantageous performances in comparison with those realized with traditional methods. Recent advances in the application of EPD materials in electrochemical energy storage and conversion devices are summarized. In particular, the parameters that influence the efficiency of an EPD process from colloidal preparation to deposition are evaluated with the aim to provide insightful guidance for realizing high‐performance electrochemical energy conversion materials and devices.     

Hollow Carbon Spheres and Their Hybrid Nanomaterials in Electrochemical Energy Storage

Electrochemical energy storage is of extraordinary importance for fulfilling the utilization of renewable and sustainable energy sources. There is an increasing demand for energy storage devices with high energy and power densities, prolonged stability, safety, and low cost. In the past decade, numerous research efforts have been devoted to achieving these requirements, especially in the design of advanced electrode materials. Hollow carbon spheres (HCS) derived nanomaterials combining the advantages of 3D HCS and porous structures have been considered as alternative electrode materials for advanced energy storage applications, due to their unique features such as high surface‐to‐volume ratios, encapsulation capability, together with outstanding chemical and thermal stability. In this review, the authors first present a comprehensive overview of the synthetic strategies of HCS, and elucidate the design and synthesis of HCS‐derived nanomaterials including various types of HCS and their nanohybrids. Additionally, their significant roles as electrode materials for supercapacitors, lithium‐ion or sodium‐ion batteries, and sulfur hosts for lithium sulfur batteries are highlighted. Finally, current challenges in the synthesis of HCS and future directions in HCS‐derived nanomaterials for energy storage applications are proposed.     

Application of Synchrotron Radiation Technologies to Electrode Materials for Li‐ and Na‐Ion Batteries

The search for superior‐energy‐density electrode materials for rechargeable batteries is prompted by the continuously growing demand for new electric vehicles and large energy‐storage grids. The structural properties of electrode materials affect their electrochemical performance because their functionality is correlated to their structure at the atomic scale. Although challenging, a deeper and comprehensive understanding of the basic structural operating units of electrode materials may contribute to the advancement of new energy‐storage technologies and many other technologies. Therefore, we must strategically control both the structure and kinetics of electrode materials to achieve optimal electrochemical performance. In this contribution, advancements in synchrotron radiation techniques, specifically in situ/operando experiments on electrode materials for rechargeable batteries, are presented and discussed. Indeed, the latest synchrotron radiation methods offer deeper insights into pristine and chemically modified electrode materials, opening new opportunities to optimize these materials and exploit new technologies. In particular, the most recent results from in situ/operando synchrotron radiation measurements, which play a critical role in the fundamental understanding of the kinetics processes that occur in rechargeable batteries, are discussed.     

Nanoscale Elastic Changes in 2D Ti3C2Tx (MXene) Pseudocapacitive Electrodes

Designing sustainable electrodes for next generation energy storage devices relies on the understanding of their fundamental properties at the nanoscale, including the comprehension of ions insertion into the electrode and their interactions with the active material. One consequence of ion storage is the change in the electrode volume resulting in mechanical strain and stress that can strongly affect the cycle life. Therefore, it is important to understand the changes of dimensions and mechanical properties occurring during electrochemical reactions. While the characterization of mechanical properties via macroscopic measurements is well documented, in situ characterization of their evolution has never been achieved at the nanoscale. It is reported here with in situ imaging, combined with density functional theory of the elastic changes of a 2D titanium carbide (Ti₃C₂T_x) based electrode in direction normal to the basal plane (electrode surface) during alkaline cation intercalation/extraction. 2D carbides, known as MXenes, are promising new materials for supercapacitors and various kinds of batteries, and understanding the coupling between their mechanical and electrochemical properties is therefore necessary. The results show a strong correlation between the cations content and the out‐of‐plane elastic modulus. This strategy enables identifying the preferential intercalation pathways within a single particle, which is important for understanding ionic transport in these materials.     

Graphene‐Based Nanocomposites for Energy Storage

Since the first report of using micromechanical cleavage method to produce graphene sheets in 2004, graphene/graphene‐based nanocomposites have attracted wide attention both for fundamental aspects as well as applications in advanced energy storage and conversion systems. In comparison to other materials, graphene‐based nanostructured materials have unique 2D structure, high electronic mobility, exceptional electronic and thermal conductivities, excellent optical transmittance, good mechanical strength, and ultrahigh surface area. Therefore, they are considered as attractive materials for hydrogen (H₂) storage and high‐performance electrochemical energy storage devices, such as supercapacitors, rechargeable lithium (Li)‐ion batteries, Li–sulfur batteries, Li–air batteries, sodium (Na)‐ion batteries, Na–air batteries, zinc (Zn)–air batteries, and vanadium redox flow batteries (VRFB), etc., as they can improve the efficiency, capacity, gravimetric energy/power densities, and cycle life of these energy storage devices. In this article, recent progress reported on the synthesis and fabrication of graphene nanocomposite materials for applications in these aforementioned various energy storage systems is reviewed. Importantly, the prospects and future challenges in both scalable manufacturing and more energy storage‐related applications are discussed.     

From Ionogels to Biredox Ionic Liquids: Some Emerging Opportunities for Electrochemical Energy Storage and Conversion Devices

Ionic liquids (ILs) continue to receive attention for applications in electrochemistry because of their unique properties as charge carriers (electrolytes) and redox shuttles (solar cells) and their ability to promote energy storage either electrostatically (supercapacitors) or chemically (secondary batteries). More specifically, the confinement of ILs in nanopores or the adsorption at surfaces, are considered a promising strategy for all solid‐state energy storage and conversion devices. Upon such immobilization, one benefits from the specific properties of ILs (large electrochemical window, relatively high ionic conductivity, task‐specific functionalities, etc.) combined with surface and confinement effects that can be tuned by playing with the porosity and chemical nature of the host. Here, some emerging applications of ILs in electrochemistry are first discussed: silica‐based ionogels as solid electrolytes and systems that involve carbon host substrates, as typical electrode materials in electrical double layer capacitors and lithium secondary batteries. Also, a non‐exhaustive, yet a comprehensive picture of the confinement and surface effects at play in such applications is presented. Then, the confinement of task‐specific ILs such as protonic ILs, IL lithium salts, and biredox ILs, is discussed, which paves the way for promising perspectives. Finally, some concluding remarks are reported and directions for future work are suggested.     

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.     

Integration: An Effective Strategy to Develop Multifunctional Energy Storage Devices

Energy storage devices are arousing increasing interest due to their key role in next‐generation electronics. Integration is widely explored as a general and effective strategy aiming at high performances. Recent progress in integrating a variety of functions into electrochemical energy storage devices is carefully described. Through integration at the level of materials: flexible, stretchable, responsive, and self‐healing devices are discussed to highlight the state‐of‐the‐art multi‐functional electronics. Through the integration at the level of devices, the incorporation of photovoltaic and piezoelectric devices is detailed to reflect the advances in self‐powering electronics. Integrated energy storage devices are presented for wearable applications to indicate a new growth direction. The main challenges and important directions are summarized to offer some useful clues for future development.     

Emerging 3D‐Printed Electrochemical Energy Storage Devices: A Critical Review

Three‐dimensional (3D) printing, a layer‐by‐layer deposition technology, has a revolutionary role in a broad range of applications. As an emerging advanced fabrication technology, it has drawn growing interest in the field of electrochemical energy storage because of its inherent advantages including the freeform construction and controllable 3D structural prototyping. This article focuses on the topic of 3D‐printed electrochemical energy storage devices (EESDs), which bridge advanced electrochemical energy storage and future additive manufacturing. Basic 3D printing systems and material considerations are described to provide a fundamental understanding of printing technologies for the fabrication of EESDs. The performance metrics of 3D‐printed EESDs are then given and the related performance optimization strategies are discussed. Next, the recent advances of 3D‐printed EESDs, including sandwich‐type and in‐plane architectures, are summarized. Conclusions and future perspectives with some unique challenges and important directions are then discussed. It can be expected that, with the help of 3D printing technology, the development of advanced electrochemical energy storage systems will be greatly promoted.