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.     

Novel Design Concepts of Efficient Mg‐Ion Electrolytes toward High‐Performance Magnesium–Selenium and Magnesium–Sulfur Batteries

Developing high‐voltage Mg‐compatible electrolytes (>3.0 V vs Mg) still remains to be the biggest R&D challenge in the area of nonaqueous rechargeable Mg batteries. Here, the key design concepts toward exploring new boron‐based Mg salts in a specific way of highlighting the implications of anions are proposed for the first time. The well‐defined boron‐centered anion‐based magnesium electrolyte (BCM electrolyte) is successfully presented by facile one‐step mixing of tris(2H‐hexafluoroisopropyl) borate and MgF₂ in 1,2‐dimethoxyethane, in which the structures of anions have been thoroughly investigated via mass spectrometry accompanied by NMR and Raman spectra. The first all‐round practical BCM electrolyte fulfills all requirements of easy synthesis, high ionic conductivity, wide potential window (3.5 V vs Mg), compatibility with electrophilic sulfur, and simultaneously noncorrosivity to coin cell assemblies. When utilizing the BCM electrolyte, the fast‐kinetics selenium/carbon (Se/C) cathode achieves the best rate capability and the sulfur/carbon (S/C) cathode exhibits an impressive prolonged cycle life than previously published reports. The BCM electrolyte offers the most promising avenue to eliminate the major roadblocks on the way to high‐voltage Mg batteries and the design concepts can shed light on future exploration directions toward high‐voltage Mg‐compatible electrolytes.     

Designing Safe Electrolyte Systems for a High‐Stability Lithium–Sulfur Battery

Safety, nontoxicity, and durability directly determine the applicability of the essential characteristics of the lithium (Li)‐ion battery. Particularly, for the lithium–sulfur battery, due to the low ignition temperature of sulfur, metal lithium as the anode material, and the use of flammable organic electrolytes, addressing security problems is of increased difficulty. In the past few years, two basic electrolyte systems are studied extensively to solve the notorious safety issues. One system is the conventional organic liquid electrolyte, and the other is the inorganic solid‐state or quasi‐solid‐state composite electrolyte. Here, the recent development of engineered liquid electrolytes and design considerations for solid electrolytes in tackling these safety issues are reviewed to ensure the safety of electrolyte systems between sulfur cathode materials and the lithium‐metal anode. Specifically, strategies for designing and modifying liquid electrolytes including introducing gas evolution, flame, aqueous, and dendrite‐free electrolytes are proposed. Moreover, the considerations involving a high‐performance Li⁺ conductor, air‐stable Li⁺ conductors, and stable interface performance between the sulfur cathode and the lithium anode for developing all‐solid‐state electrolytes are discussed. In the end, an outlook for future directions to offer reliable electrolyte systems is presented for the development of commercially viable lithium–sulfur batteries.     

Toward a Reversible Calcium‐Sulfur Battery with a Lithium‐Ion Mediation Approach

Calcium represents a promising anode for the development of high‐energy‐density, low‐cost batteries. However, a lack of suitable electrolytes has restricted the development of rechargeable batteries with a Ca anode. Furthermore, to achieve a high energy density system, sulfur would be an ideal cathode to couple with the Ca anode. Unfortunately, a reversible calcium‐sulfur (Ca‐S) battery has not yet been reported. Herein, a basic study of a reversible nonaqueous room‐temperature Ca‐S battery is presented. The reversibility of the Ca‐S chemistry and high utilization of the sulfur cathode are enabled by employing a Li⁺‐ion‐mediated calcium‐based electrolyte. Mechanistic insights pursued by spectroscopic, electrochemical, microscopic, and theoretical simulation (density functional theory) investigations imply that the Li⁺‐ions in the Ca‐electrolyte stimulate the reactivation of polysulfide/sulfide species. The coordination of lithium to sulfur reduces the formation of sturdy Ca‐S ionic bonds, thus boosting the reversibility of the Ca‐S chemistry. In addition, the presence of Li⁺‐ions facilitates the ionic charge transfer both in the electrolyte and across the solid electrolyte interphase layer, consequently reducing the interfacial and bulk impedance of Ca‐S batteries. As a result, both the utilization of active sulfur in the cathode and the discharge voltage of Ca‐S batteries are significantly improved.     

Alloy Anode Materials for Rechargeable Mg Ion Batteries

Rechargeable magnesium ion batteries are interesting as one of the alternative metal ion battery systems to lithium ion batteries due to the wide availability and accessibility of magnesium in the earth's crust. On the one hand, electrolyte solutions in which Mg metal anodes are fully reversible are not suitable for the use of high voltage/high capacity transition metal oxide cathodes due to complex surface phenomena. On the other hand, Mg metal anodes cannot work reversibly in conventional electrolyte solutions in which high voltage/high capacity Mg insertion cathodes can work because of passivation phenomena that fully block them. Replacing Mg metal with alternative anodes that can work reversibly in conventional electrolyte solutions could provide a promising route to elaborate high voltage and high capacity rechargeable Mg battery systems. Herein, the recent progress in alloy anodes based on group IIIA, IVA, VA elements is summarized. The theoretical evaluations, achievable capacities, synthetic strategies, battery test configurations, electrochemical properties, and underlying reaction mechanisms are systematically summarized and discussed. The key issues and challenges impeding their current use are identified and some valuable suggestions for their future development as practical reversible anodes for Mg batteries are provided.     

Dynamic Visualization of Cathode/Electrolyte Evolution in Quasi‐Solid‐State Lithium Batteries

Solid‐state lithium–sulfur batteries (SSLSBs) are highly appealing for electrochemical energy devices because of their promising theoretical energy density. An intensive acquaintance of SSLS interfacial behavior is of importance in gaining fundamental knowledge of working/failure mechanisms and clarifying further optimized design of advanced batteries. Herein, a direct visualization of the evolution of both component and structure is present inside a working SSLSB. In situ Raman spectroscopy clearly sheds light on the potential‐dependent evolution of sulfur speciation via subtly fabricating the electrochemical cell. Moreover, the real‐time optical microscopic views show that the irreversible structure deformation of solid‐state electrolytes (SSEs), which results from the decomposition of dissolved polysulfides (PSs) and gas generation inside the SSE, directly causes the fracture of sulfur cathode with the cycling times increasing. Furthermore, by an atomic force microscopy study, the evolving structure and dynamic behavior of SSEs are directly captured at the nano/microscale and further elucidate the PS shuttling determining the mechanism stability of electrolyte. This work provides a straightforward monitoring of the compositional and morphological evolution, which contributes one to exploring the failure mechanisms and interfacial reactions for the cell performance enhancement.     

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.     

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.     

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 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.     

Hard Carbon Anodes and Novel Electrolytes for Long‐Cycle‐Life Room Temperature Sodium‐Sulfur Full Cell Batteries

A novel combination of hard carbon anode sodium pre‐loading and a tailored electrolyte is used to prepare room temperature sodium‐sulfur full cell batteries. The electrochemical loading with sodium ions is realized in a specific mixture of diethyl carbonate, ethylene carbonate, and fluoroethylene carbonate electrolyte in order to create a first solid electrolyte interface (SEI) on the anode surface. Combining such anodes with a porous carbon/sulfur composite cathode results in full cells with a significantly decreased polysulfide shuttle when compared to half cells combined with metallic sodium anodes. Further optimization involves the use of Na₂S/P₂S₅ doped tetraethylene glycol dimethyl ether based electrolyte in the full cell for the formation of a second SEI, reducing polysulfide shuttle even further. More importantly, the electrochemical discharge processes in the cell are improved by adding this dissolved complexation agent to the electrolyte. As a result of this combination sodium‐sulfur cells with tailored cathode materials and electrolytes can achieve high discharge capacities up to 980 mAh g^?1_sulfur and 1000 cycles with 200 mAh g^?1_sulfur remaining capacity, at room temperature.     

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.     

Ethers Illume Sodium‐Based Battery Chemistry: Uniqueness,Surprise, and Challenges

Sodium‐ion batteries (SIBs) have the potential to be practically applied in large‐scale energy storage markets. The rapid progress of SIBs research is primarily focused on electrodes, while electrolytes attract less attention. Indeed, the improvement of electrode performance is arguably correlated with the electrolyte optimization. In conventional lithium‐ion batteries (LIBs), ether‐based electrolytes are historically less practical owing to the insufficient passivation of both anodes and cathodes. As an important class of aprotic electrolytes, ethers have revived with the emerging lithium‐sulfur and lithium‐oxygen batteries in recent years, and are even booming in the wave of SIBs. Ether‐based electrolytes are unique to enabling these new battery chemistries in terms of producing stable ternary graphite intercalation compounds, modifying anode solid electrolyte interphases, reducing the solubility of intermediates, and decreasing polarization. Better still, ether‐based electrolytes are compatible with specific inorganic cathodes and could catalyze the assembly of full SIBs prototypes. This Research News article aims to summarize the recent critical reports on ether‐based electrolytes in sodium‐based batteries, to unveil the uniqueness of ether‐based electrolytes to advancing diverse electrode materials, and to shed light on the viability and challenges of ether‐based electrolytes in future sodium‐based battery chemistries.     

Theoretical versus Practical Energy: A Plea for More Transparency in the Energy Calculation of Different Rechargeable Battery Systems

Electrochemical energy storage at a large scale poses one of the main technological challenges of this century. The scientific community in academia and industry worldwide intensively is exploring various alternative rechargeable battery concepts beside state‐of‐the‐art lithium ion batteries (LIBs), for example, all‐solid‐state batteries, lithium/sulfur batteries, magnesium/sulfur batteries or dual‐ion batteries that could outperform LIBs in different aspects. Often, these concepts also promise very high theoretical energies per mass or volume. However, as theoretical values exclude numerous relevant parameters, they do not translate directly into practically achievable energy values: The gaps between practical capacities and voltages compared to the theoretical values differ for each system. In order to provide high transparency and to illustrate which cell components are most important in the limitation of the practical energy values, in this study, the specific energies and energy densities are calculated in six subsequent steps—from the theoretical energy values of the active materials alone to the practical energy values in an 18650 cylindrical cell. By providing a tool to calculate the energy values of six different battery technologies with different assumptions made evident, this study aims for more transparency and reliability in the comparison of different cell chemistries.     

Highly Solvating Electrolytes for Lithium–Sulfur Batteries

There is a critical need to evaluate lithium–sulfur (Li–S) batteries with practically relevant high sulfur loadings and minimal electrolyte. Under such conditions, the concentration of soluble polysulfide intermediates in the electrolyte drastically increases, which can alter the fundamental nature of the solution‐mediated discharge and thereby the total sulfur utilization. In this work, an investigation into various high donor number (DN) electrolytes that allow for increased polysulfide dissolution is presented, and the way in which this property may in fact be necessary for increasing sulfur utilization at low electrolyte and high loading conditions is demonstrated. The solvents dimethylacetamide, dimethyl sulfoxide, and 1‐methylimidazole are holistically evaluated against dimethoxyethane as electrolyte co‐solvents in Li–S cells, and they are used to investigate chemical and electrochemical properties of polysulfide species at both dilute and practically relevant conditions. The nature of speciation exhibited by lithium polysulfides is found to vary significantly between these concentrations, particularly with regard to the S₃^?? species. Furthermore, the extent of the instability in conventional electrolyte solvents and high DN solvents with both lithium metal and polysulfides is thoroughly investigated. These studies establish a basis for future efforts into rationally designing an optimal electrolyte for a lean electrolyte, high energy density Li–S battery.     

Rechargeable Solid‐State Li–Air and Li–S Batteries: Materials,Construction, and Challenges

Nonaqueous Li–air and Li–S batteries are attracting considerable interest because of their outstanding theoretical capacities and energy densities. However, despite the substantial progress in their development, safety remains an issue because of the flammability of their organic electrolytes. Moreover, the electrolyte volatilization of Li–air batteries and “shuttle effect” in Li–S batteries seriously hinder their development. The use of solid‐state Li–air and Li–S batteries is one of the best solutions. Nevertheless, many challenges remain in solid electrolytes, electrodes, and interfaces. In this review, a comprehensive discussion on the development of solid‐state Li–air and Li–S batteries is provided. The discussion begins with introduction of the progress in solid electrolytes, including their ionic conductivities and chemical stabilities. It then moves on to the cathodes of both batteries and the interface between electrolytes and electrodes. The reaction process inside the cathode is also presented. Suggestions for the optimization of composite cathodes and modification of the electrode–electrolyte interface are provided in the end. Intensive effort is required for the development of solid‐state Li–air and Li–S batteries in the future.     

High‐Performance All‐Solid‐State Lithium–Sulfur Batteries Enabled by Amorphous Sulfur‐Coated Reduced Graphene Oxide Cathodes

Safety and the polysulfide shuttle reaction are two major challenges for liquid electrolyte lithium–sulfur (Li–S) batteries. Although use of solid‐state electrolytes can overcome these two challenges, it also brings new challenges by increasing the interface resistance and stress/strain. In this work, the interface resistance and stress/strain of sulfur cathodes are significantly reduced by conformal coating ≈2 nm sulfur (S) onto reduced graphene oxide (rGO). An Li–S full cell consisting of an rGO@S‐Li₁₀GeP₂S₁₂‐acetylene black (AB) composite cathode is evaluated. At 60 °C, the all‐solid‐state Li–S cell demonstrates a similar electrochemical performance as in liquid organic electrolyte, with high rate capacities of 1525.6, 1384.5, 1336.3, 903.2, 502.6, and 204.7 mA h g^?1 at 0.05, 0.1, 0.5, 1.0, 2.0, and 5.0 C, respectively. It can maintain a high and reversible capacity of 830 mA h g^?1 at 1.0 C for 750 cycles. The uniform distribution of the rGO@S nanocomposite in the Li₁₀GeP₂S₁₂‐AB matrix generates uniform volume changes during lithiation/delithiation, significantly reducing the stress/strain, thus extending the cycle life. Minimization of the stress/strain of solid cells is the key for a long cycle life of all‐solid‐state Li–S batteries.     

Solvent‐Mediated Li2S Electrodeposition: A Critical Manipulator in Lithium–Sulfur Batteries

Controlling electrochemical deposition of lithium sulfide (Li₂S) is a major challenge in lithium–sulfur batteries as premature Li₂S passivation leads to low sulfur utilization and low rate capability. In this work, the solvent's roles in controlling solid Li₂S deposition are revealed, and quantitative solvent‐mediated Li₂S growth models as guides to solvent selection are developed. It is shown that Li₂S electrodeposition is controlled by electrode kinetics, Li₂S solubility, and the diffusion of polysulfide/Li₂S, which is dictated by solvent's donicity, polarity, and viscosity, respectively. These solvent‐controlled properties are essential factors pertaining to the sulfur utilization, energy efficiency and reversibility of lithium–sulfur batteries. It is further demonstrated that the solvent selection criteria developed in this study are effective in guiding the search for new and more effective electrolytes, providing effective screening and design criteria for computational and experimental electrolyte development for lithium–sulfur batteries.     

Electrochemical Stability of Li10GeP2S12 and Li7La3Zr2O12 Solid Electrolytes

The electrochemical stability window of solid electrolyte is overestimated by the conventional experimental method using a Li/electrolyte/inert metal semiblocking electrode because of the limited contact area between solid electrolyte and inert metal. Since the battery is cycled in the overestimated stability window, the decomposition of the solid electrolyte at the interfaces occurs but has been ignored as a cause for high interfacial resistances in previous studies, limiting the performance improvement of the bulk‐type solid‐state battery despite the decades of research efforts. Thus, there is an urgent need to identify the intrinsic stability window of the solid electrolyte. The thermodynamic electrochemical stability window of solid electrolytes is calculated using first principles computation methods, and an experimental method is developed to measure the intrinsic electrochemical stability window of solid electrolytes using a Li/electrolyte/electrolyte‐carbon cell. The most promising solid electrolytes, Li₁₀GeP₂S₁₂ and cubic Li‐garnet Li₇La₃Zr₂O₁₂, are chosen as the model materials for sulfide and oxide solid electrolytes, respectively. The results provide valuable insights to address the most challenging problems of the interfacial stability and resistance in high‐performance solid‐state batteries.