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.     

Recent Progress in High Donor Electrolytes for Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries continue to be considered promising post‐lithium‐ion batteries owing to their high theoretical energy density. In pursuit of a Li–S cell with long‐term cyclability, most studies thus far have relied on using ether‐based electrolytes. However, their limited ability to dissolve polysulfides requires a high electrolyte‐to‐sulfur ratio, which impairs the achievable specific energy. Recently, the battery community found high donor electrolytes to be a potential solution to this shortcoming because their high solubility toward polysulfides enables a cell to operate under lean electrolyte conditions. Despite the increasing number of promising outcomes with high donor electrolytes, a critical hurdle related to stability of the lithium‐metal counter electrode needs to be overcome. This review provides an overview of recent efforts pertaining to high donor electrolytes in Li–S batteries and is intended to raise interest from within the community. Furthermore, based on analogous efforts in the lithium‐air battery field, strategies for protecting the lithium metal electrode are proposed. It is predicted that high donor electrolytes will be elevated to a higher status in the field of Li–S batteries, with the hope that either existing or upcoming strategies will, to a fair extent, mitigate the degradation of the lithium–metal interface.     

High‐Abundance and Low‐Cost Metal‐Based Cathode Materials for Sodium‐Ion Batteries: Problems,Progress, and Key Technologies

Recently, room‐temperature stationary sodium‐ion batteries (SIBs) have received extensive investigations for large‐scale energy storage systems (EESs) and smart grids due to the huge natural abundance and low cost of sodium. The SIBs share a similar “rocking‐chair” sodium storage mechanism with lithium‐ion batteries; thus, selecting appropriate electrodes with a low cost, satisfactory electrochemical performance, and high reliability is the key point for the development for SIBs. On the other hand, the carefully chosen elements in the electrodes also largely determine the cost of SIBs. Therefore, earth‐abundant‐metal‐based compounds are ideal candidates for reducing the cost of electrodes. Among all the high‐abundance and low‐cost metal elements, cathodes containing iron and/or manganese are the most representative ones that have attracted numerous studies up till now. Herein, recent advances on both iron‐ and manganese‐based cathodes of various types, such as polyanionic, layered oxide, MXene, and spinel, are highlighted. The structure–function property for the iron‐ and manganese‐based compounds is summarized and analyzed in detail. With the participation of iron and manganese in sodium‐based cathode materials, real applications of room‐temperature SIBs in large‐scale EESs will be greatly promoted and accelerated in the near future.     

Exploration of Advanced Electrode Materials for Rechargeable Sodium‐Ion Batteries

As the rapid growth of the lithium‐ion battery (LIB) market raises concerns about limited lithium resources, rechargeable sodium‐ion batteries (SIBs) are attracting growing attention in the field of electrical energy storage due to the large abundance of sodium. Compared with the well‐developed commercial LIBs, all components of the SIB system, such as the electrode, electrolyte, binder, and separator, need further exploration before reaching a practical industrial application level. Drawing lessons from the LIB research, the SIB electrode materials are being extensively investigated, resulting in tremendous progress in recent years. In this article, the progress of the research on the development of electrode materials for SIBs is summarized. A variety of new electrode materials for SIBs, including transition‐metal oxides with a layered or tunnel structure, polyanionic compounds, and organic molecules, have been proposed and systematically investigated. Several promising materials with moderate energy density and ultra‐long cycling performance are demonstrated. Appropriate doping and/or surface treatment methodologies are developed to effectively promote the electrochemical properties. The challenges of and opportunities for exploiting satisfactory SIB electrode materials for practical applications are outlined.     

Challenges in Developing Electrodes,Electrolytes, and Diagnostics Tools to Understand and Advance Sodium‐Ion Batteries

Considering the natural abundance and low cost of sodium resources, sodium‐ion batteries (SIBs) have received much attention for large‐scale electrochemical energy storage. However, smart structure design strategies and good mechanistic understanding are required to enable advanced SIBs with high energy density. In recent years, the exploration of advanced cathode, anode, and electrolyte materials, as well as advanced diagnostics have been extensively carried out. This review mainly focuses on the challenging problems for the attractive battery materials (i.e., cathode, anode, and electrolytes) and summarizes the latest strategies to improve their electrochemical performance as well as presenting recent progress in operando diagnostics to disclose the physics behind the electrochemical performance and to provide guidance and approaches to design and synthesize advanced battery materials. Outlook and perspectives on the future research to build better SIBs are also provided.     

Carbon Anodes for Nonaqueous Alkali Metal‐Ion Batteries and Their Thermal Safety Aspects

Since their commercialization by Sony in 1991, graphite anodes in combination with various cathodes have enabled the widespread success of lithium‐ion batteries (LIBs), providing over 10 billion rechargeable batteries to the global population. Next‐generation nonaqueous alkali metal‐ion batteries, namely sodium‐ion batteries (SIBs) and potassium‐ion batteries (PIBs), are projected to utilize intercalation‐based carbon anodes as well, due to their favorable electrochemical properties. While traditionally graphite anodes have dominated the market share of LIBs, other carbon materials have been investigated, including graphene, carbon nanotubes, and disordered carbons. The relationship between carbon material properties, electrochemical performance, and charge storage mechanisms is clarified for these alkali metal‐ion batteries, elucidating possible strategies for obtaining enhanced cycling stability, specific capacity, rate capability, and safety aspects. As a key component in determining cell performance, the solid electrolyte interphase layer is described in detail, particularly for its dependence on the carbon anode. Finally, battery safety at extreme temperatures is discussed, where carbon anodes are susceptible to dendrite formation, accelerated aging, and eventual thermal runaway. As society pushes toward higher energy density LIBs, this review aims to provide guidance toward the development of sustainable next‐generation SIBs and PIBs.     

All‐Solid‐State Printed Bipolar Li–S Batteries

Despite their potential advantages over currently widespread lithium‐ion batteries, lithium–sulfur (Li–S) batteries are not yet in practical use. Here, for the first time bipolar all‐solid‐state Li–S batteries (ASSLSBs) are demonstrated that exhibit exceptional safety, flexibility, and aesthetics. The bipolar ASSLSBs are fabricated through a solvent‐drying‐free, ultraviolet curing‐assisted stepwise printing process at ambient conditions, without (high‐temperature/high‐pressure) sintering steps that are required for inorganic electrolyte‐based all‐solid‐state batteries. Two thermodynamically immiscible and nonflammable gel electrolytes based on ethyl methyl sulfone (EMS) and tetraethylene glycol dimethyl ether (TEGDME) are used to address longstanding concerns regarding the grain boundary resistance of conventional inorganic solid electrolytes, as well as the polysulfide shuttle effect in Li–S batteries. The EMS gel electrolytes embedded in the sulfur cathodes facilitate sulfur utilization, while the TEGDME gel composite electrolytes serve as polysulfide‐repelling separator membranes. Benefiting from the well‐designed cell components and printing‐driven facile processability, the resulting bipolar ASSLSBs exhibit unforeseen advancements in bipolar cell configuration, safety, foldability, and form factors, which lie far beyond those achievable with conventional Li–S battery technologies.     

Recent Progress on Spray Pyrolysis for High Performance Electrode Materials in Lithium and Sodium Rechargeable Batteries

Advanced electrode materials have been intensively explored for next‐generation lithium‐ion batteries (LIBs), and great progresses have been achieved for many potential candidates at the lab‐scale. To realize the commercialization of these materials, industrially‐viable synthetic approaches are urgently needed. Spray pyrolysis (SP), which is highly scalable and compatible with on‐line continuous production processes, offers great fidelity in synthesis of electrode materials with complex architectures and chemistries. In this review, motivated by the rapid advancement of the given technology in the battery area, we have summarized the recent progress on SP for preparing a great variety of anode and cathode materials of LIBs with emphasis on their unique structures generated by SP and how the structures enhanced the electrochemical performance of various electrode materials. Considering the emerging popularity of sodium‐ion batteries (SIBs), recent electrode materials for SIBs produced by SP will also be discussed. Finally, the powerfulness and limitation along with future research efforts of SP on preparing electrode materials are concisely provided. Given current worldwide interests on LIBs and SIBs, we hope this review will greatly stimulate the collaborative efforts among different communities to optimize existing approaches and to develop innovative processes for preparing electrode materials.     

Low-Temperature Sodium-Ion Batteries: Challenges and Progress

As an ideal candidate for the next generation of large-scale energy storage devices, sodium-ion batteries (SIBs) have received great attention due to their low cost. However, the practical utility of SIBs faces constraints imposed by geographical and environmental factors, particularly in high-altitude and cold regions. In these areas, the low-temperature (LT) performance of SIBs presents a pressing technological challenge that requires significant breakthroughs. In LT environments, the electrochemical reaction kinetics of SIBs are sluggish, the electrode/electrolyte interface is unstable, and the diffusion of sodium ions in electrode materials is slow, leading to a decrease in battery performance. Therefore, the reasonable design of electrolyte and electrode materials is of great significance for optimizing the LT performance of SIBs. In this review, the research progress of LT SIBs electrolytes, cathode, and anode materials, as well as sodium metal batteries and solid-state electrolytes is systematically summarized in recent years, aiming to understand the design principles of LT SIBs, clarify the basic research and development of high-performance SIBs in practical applications, and promote the development of SIBs technology in the full temperature range.     

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.     

Hybrid and Aqueous Lithium‐Air Batteries

Lithium‐air (Li‐air) batteries have become attractive because of their extremely high theoretical energy density. However, conventional Li‐air cells operating with non‐aqueous electrolytes suffer from poor cycle life and low practical energy density due to the clogging of the porous air cathode by insoluble discharge products, contamination of the organic electrolyte and lithium metal anode by moist air, and decomposition of the electrolyte during cycling. These difficulties may be overcome by adopting a cell configuration that consists of a lithium‐metal anode protected from air by a Li⁺‐ion solid electrolyte and an air electrode in an aqueous catholyte. In this type of configuration, a Li⁺‐ion conducting “buffer” layer between the lithium‐metal anode and the solid electrolyte is often necessary due to the instability of many solid electrolytes in contact with lithium metal. Based on the type of buffer layer, two different battery configurations are possible: “hybrid” Li‐air batteries and “aqueous” Li‐air batteries. The hybrid and aqueous Li‐air batteries utilize the same battery chemistry and face similar challenges that limit the cell performance. Here, an overview of recent developments in hybrid and aqueous Li‐air batteries is provided and the factors that influence their performance and impede their practical applications, followed by future directions are discussed.     

Regulating the Hidden Solvation‐Ion‐Exchange in Concentrated Electrolytes for Stable and Safe Lithium Metal Batteries

Lithium–sulfur batteries are attractive for automobile and grid applications due to their high theoretical energy density and the abundance of sulfur. Despite the significant progress in cathode development, lithium metal degradation and the polysulfide shuttle remain two critical challenges in the practical application of Li–S batteries. Development of advanced electrolytes has become a promising strategy to simultaneously suppress lithium dendrite formation and prevent polysulfide dissolution. Here, a new class of concentrated siloxane‐based electrolytes, demonstrating significantly improved performance over the widely investigated ether‐based electrolytes are reported in terms of stabilizing the sulfur cathode and Li metal anode as well as minimizing flammability. Through a combination of experimental and computational investigation, it is found that siloxane solvents can effectively regulate a hidden solvation‐ion‐exchange process in the concentrated electrolytes that results from the interactions between cations/anions (e.g., Li⁺, TFSI^?, and S^2?) and solvents. As a result, it could invoke a quasi‐solid‐solid lithiation and enable reversible Li plating/stripping and robust solid‐electrolyte interphase chemistries. The solvation‐ion‐exchange process in the concentrated electrolytes is a key factor in understanding and designing electrolytes for other high‐energy lithium metal batteries.     

Metal‐Ion Coupled Electron Transfer Kinetics in Intercalation‐Based Transition Metal Oxides

Electrochemical metal‐ion intercalation systems are acknowledged to be a critical energy storage technology. The kinetics of the intercalation processes in transition‐metal based oxides determine the practical characteristics of metal‐ion batteries, such as the energy density, power, and cyclability. With the emergence of post lithium‐ion batteries, such as sodium‐ion and potassium‐ion batteries, which function predominately in nonaqueous electrolytes of special formulation and exhibit quite varied material stability with regard to their surface chemistries and reactivity with electrolytes, the practical routes for the optimization of metal‐ion battery performance become essential. Electrochemical methods offer a variety of means to quantitatively study the diffusional, charge transfer, and phase transformation rates in complex systems, which are, however, rather rarely fully adopted by the metal‐ion battery community, which slows down the progress in rationalizing the rate‐controlling factors in complex intercalation systems. Herein, several practical approaches for diagnosing the origin of the rate limitations in intercalation materials based on phenomenological models are summarized, focusing on the specifics of charge transfer, diffusion, and nucleation phenomena in redox‐active solid electrodes. It is demonstrated that information regarding rate‐determining factors can be deduced from relatively simple analysis of experimental methods including cyclic voltammetry, chronoamperometry, and impedance spectroscopy.     

Sodium‐Ion Batteries: From Academic Research to Practical Commercialization

Sodium‐ion batteries (SIBs) have been considered as the most promising candidate for large‐scale energy storage system owing to the economic efficiency resulting from abundant sodium resources, superior safety, and similar chemical properties to the commercial lithium‐ion battery. Despite the long period of academic research, how to realize sodium‐ion battery commercialization for market applications is still a great challenge. Thus, from the perspective of future practical application, this review will identify the factors that are restricting commercialization, and evaluate the existing active materials and sodium‐ion‐based full‐cell system. The design and development trends that are needed for SIBs to meet the requirements of practical applications in large‐scale energy storage will also be discussed in detail.     

Thick Electrode Batteries: Principles,Opportunities, and Challenges

The ever‐growing portable electronics and electric vehicle markets heavily influence the technological revolution of lithium batteries (LBs) toward higher energy densities for longer standby times or driving range. Thick electrode designs can substantially improve the electrode active material loading by minimizing the inactive component ratio at the device level, providing a great platform for enhancing the overall energy density of LBs. However, extensive efforts are still needed to address the challenges that accompany the increase in electrode thickness, not limited to sluggish charge kinetics and electrode mechanical instability. In this review, the principles and the recent developments in the fabrication of thick electrodes that focus on low‐tortuosity structural designs for rapid charge transport and integrated cell configuration for improved energy density, cell stability, and durability are summarized. Advanced thick electrode designs for application in emerging battery chemistries such as lithium metal electrodes, solid state electrolytes, and lithium–air batteries are also discussed with a perspective on their future opportunities and challenges. Finally, suggestions on the future directions of thick electrode battery development and research are suggested.     

Biomaterials for High‐Energy Lithium‐Based Batteries: Strategies,Challenges, and Perspectives

Developing high‐performance batteries through applying renewable resources is of great significance for meeting ever‐growing energy demands and sustainability requirements. Biomaterials have overwhelming advantages in material abundance, environmental benignity, low cost, and more importantly, multifunctionalities from structural and compositional diversity. Therefore, significant and fruitful research on exploiting various natural biomaterials (e.g., soy protein, chitosan, cellulose, fungus, etc.) for boosting high‐energy lithium‐based batteries by means of making or modifying critical battery components (e.g., electrode, electrolyte, and separator) are reported. In this review, the recent advances and main strategies for adopting biomaterials in electrode, electrolyte, and separator engineering for high‐energy lithium‐based batteries are comprehensively summarized. The contributions of biomaterials to stabilizing electrodes, capturing electrochemical intermediates, and protecting lithium metal anodes/enhancing battery safety are specifically emphasized. Furthermore, advantages and challenges of various strategies for fabricating battery materials via biomaterials are described. Finally, future perspectives and possible solutions for further development of biomaterials for high‐energy lithium‐based batteries are proposed.     

Commercialization of Lithium Battery Technologies for Electric Vehicles

The currently commercialized lithium‐ion batteries have allowed for the creation of practical electric vehicles, simultaneously satisfying many stringent milestones in energy density, lifetime, safety, power, and cost requirements of the electric vehicle economy. The next wave of consumer electric vehicles is just around the corner. Although widely adopted in the vehicle market, lithium‐ion batteries still require further development to sustain their dominating roles among competitors. In this review, the authors survey the state‐of‐the‐art active electrode materials and cell chemistries for automotive batteries. The performance, production, and cost are included. The advances and challenges in the lithium‐ion battery economy from the material design to the cell and the battery packs fitting the rapid developing automotive market are discussed in detail. Also, new technologies of promising battery chemistries are comprehensively evaluated for their potential to satisfy the targets of future electric vehicles.     

Sulfide Solid Electrolytes for Lithium Battery Applications

The use of solid electrolytes is a promising direction to improve the energy density of lithium‐ion batteries. However, the low ionic conductivity of many solid electrolytes currently hinders the performance of solid‐state batteries. Sulfide solid electrolytes can be processed in a number of forms (glass, glass‐ceramic, and crystalline) and have a wide range of available chemistries. Crystalline sulfide materials demonstrate ionic conductivity on par with those of liquid electrolytes through the utilization of near ideal conduction pathways. Low‐temperature processing is also possible for these materials due to their favorable mechanical properties. The main drawback of sulfide solid electrolytes remains their electrochemical stability, but this can be addressed through compositional tuning or the use of artificial solid electrolyte interphase (SEI). Implementation of sulfide solid electrolytes, with proper treatment for stability, can lead to substantial improvements in solid‐state battery performance leading to significant advancement in electric vehicle technology.     

High‐Energy Aqueous Lithium Batteries

Owing to the high voltage of lithium‐ion batteries (LIBs), the dominating electrolyte is non‐aqueous. The idea of an aqueous rechargeable lithium battery (ARLB) dates back to 1994, but it had attracted little attention due to the narrow stable potential window of aqueous electrolytes, which results in low energy density. However, aqueous electrolytes were employed during the 2000s for the fundamental studies of electrode materials in the absence of side reactions such as the decomposition of organic species. The high solubility of lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in water has introduced new opportunities for high‐voltage ARLBs. Nonetheless, these ideas are somehow overshadowed by the common perception about the essential limitation of the aqueous electrolyte. The electrochemical behaviour of conventional electrode materials can be substantially tuned in the water‐in‐salt electrolytes. The latest idea of utilising a graphite anode in the aqueous water‐in‐salt electrolytes has paved the way towards not only 4‐V ARLB but also a new generation of Li?S batteries with a higher operating voltage and energy efficiency. Furthermore, aqueous electrolytes can provide a cathodically stable environment for Li?O₂ batteries. The present paper aims to highlight these emerging opportunities possibly leading to a new generation of LIBs, which can be substantially cheaper and safer.     

Advanced Organic Electrode Materials for Rechargeable Sodium‐Ion Batteries

Benefiting from the high abundance and low cost of sodium resource, rechargeable sodium‐ion batteries (SIBs) are regarded as promising candidates for large‐scale electrochemical energy storage and conversion. Due to the heavier mass and larger radius of Na⁺ than that of Li⁺, SIBs with inorganic electrode materials are currently plagued with low capacity and insufficient cycling life. In comparison, organic electrode materials display the advantages of structure designability, high capacity and low limitation of cationic radius. However, organic electrode materials also encounter issues such as high‐solubility in electrolyte and low conductivity. Here, recently reported organic electrode materials, which mainly include the reactions based on either carbon‐oxygen double bond or carbon‐nitrogen double bond, and doping reactions, are systematically reviewed. Furthermore, the design strategies of organic electrodes are comprehensively summarized. The working voltage is regulated through controlling the lowest unoccupied molecular orbital energies. The theoretical capacity can be enhanced by increasing the active groups. The dissolution is inhibited with elevating the intermolecular forces with proper molecular weight. The conductivity can be improved with extending conjugated structures. Future research into organic electrodes should focus on the development of full SIBs with aqueous/aprotic electrolytes and long cycling stability.