Status and Prospects of MXene-Based Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries with a theoretical energy density of 2567 Wh kg⁻¹ are very promising next-generation energy storage systems, but suffer from the insulativity of sulfur and Li₂S, the shuttle effect due to the dissolution and migration of polysulfides, and the lithium dendrite issue. MXenes, a family of 2D transition metal carbides/nitrides, which have metallic conductivity, structural variety, strong chemical adsorption ability to polysulfides, effective catalytic effect for fast kinetics, and inducing effect for uniform growth of Li, exhibit promising potential for high-performance Li–S batteries. In this review, the recent progress and achievements of MXene-based Li–S batteries are summarized, including the use of MXenes in sulfur cathode, interlayer between cathode and separator, and Li anode. The architecture construction and chemical modification of MXenes, as well as hybridization with other materials are demonstrated. The enhancement on electrochemical performance and the related mechanisms of MXenes and MXene-based composites are discussed. Finally, challenges and perspectives of MXenes for Li–S battery application are also given.     

Critical Current Density in Solid-State Lithium Metal Batteries: Mechanism,Influences, and Strategies

Solid-state lithium (Li) metal batteries (SSLMBs) have become a research hotspot in the energy storage field due to the much-enhanced safety and high energy density. However, the SSLMBs suffer from failures including dendrite-induced short circuits and contact-loss-induced high impedance, which are highly related to the Li plating/stripping kinetics and hinder the practical application of SSLMBs. The maximum endurable current density of lithium battery cycling without cell failure in SSLMB is generally defined as critical current density (CCD). Therefore, CCD is an important parameter for the application of SSLMBs, which can help to determine the rate-determining steps of Li kinetics in solid-state batteries. Herein, the theoretical and practical meanings for CCD from the fundamental thermodynamic and kinetic principles, failure mechanisms, CCD identifications, and influence factors for improving CCD performances are systematically reviewed. Based on these fundamental understandings, a series of strategies and outlooks for future researches on SSLMB are presented, endeavoring on increasing CCD for practical SSLMBs.     

Toward the Advanced Next-Generation Solid-State Na-S Batteries: Progress and Prospects

Although batteries fitted with sodium metal anodes and sulfur cathodes are attractive for their higher energy density and lower cost, the threat of polysulfide migration in organic liquid electrolytes, uncontrollable dendrites, and corresponding safety issues has locked the deployment of the battery system. Introduction of solid-state electrolytes to replace conventional liquid-based electrolytes has been considered an effective approach to address these issues and further render solid-state sodium-sulfur battery (SSSSB) systems with higher safety and improved energy density. Nevertheless, the practical applications of SSSSB are still hampered by grand challenges, such as poor interfacial contact, sluggish redox kinetics of sulfur conversion, and Na dendrites. Currently, various strategies have been proposed and utilized to negate the problems within the solid-state battery. Herein, a timely and comprehensive review of emerging strategies to promote the development of SSSSB is presented. The critical challenges that prevent the real application of the SSSSB technique are analyzed initially. Subsequently, various strategies for boosting the development of SSSSB are comprehensively summarized, containing the developing of the advanced cathode and cathode/electrolyte interface, tailoring the solid electrolyte, and designing the stable anode and anode/electrolyte interface. Finally, further perspectives on stimulating the practical application of SSSSB technology are provided.     

Vanadium‐Based Nanomaterials: A Promising Family for Emerging Metal‐Ion Batteries

The emerging electrochemical energy storage systems beyond Li‐ion batteries, including Na/K/Mg/Ca/Zn/Al‐ion batteries, attract extensive interest as the development of Li‐ion batteries is seriously hindered by the scarce lithium resources. During the past years, large amounts of studies have focused on the investigation of various electrode materials toward emerging metal‐ion batteries to realize high energy density, high power density, and a long cycle life. In particular, vanadium‐based nanomaterials have received great attention. Vanadium‐based compounds have a big family with different structures, chemical compositions, and electrochemical properties, which provide huge possibilities for the development of emerging electrochemical energy storage. In this review, a comprehensive overview of the recent progresses of promising vanadium‐based nanomaterials for emerging metal‐ion batteries is presented. The vanadium‐based materials are classified into four groups: vanadium oxides, vanadates, vanadium phosphates, and oxygen‐free vanadium‐based compounds. The structures, electrochemical properties, and modification strategies are discussed. The structure–performance relationships and charge storage mechanisms are focused on. Finally, the perspectives about future directions of vanadium‐based nanomaterials for emerging energy storage devices are proposed. This review will provide comprehensive knowledge of vanadium‐based nanomaterials and shed light on their potential applications in emerging energy storage.     

High Energy Density Solid-State Lithium Metal Batteries Enabled by In Situ Polymerized Integrated Ultrathin Solid Electrolyte/Cathode

Solid-state batteries (SSBs) are regarded as the most promising next-generation energy storage devices due to their potential to achieve higher safety performance and energy density. However, the troubles in the preparation of ultrathin solid-state electrolytes (SEs) as well as the resultant compromise in mechanical strength greatly limit the safety application of SSBs. Herein, a novel in situ polymerized integrated ultrathin SE/cathode design is developed. The ultrathin ceramic layer supported on the cathode serves not only as a rigid scaffold to prevent direct contact between the cathode and anode but also as active inorganic fillers to enhance the mechanical properties of in situ polymerized SE film. The unique Li-ion coordination environments as well as the Li hopping mechanism profoundly promote fast ion transport in composite SEs. The in situ polymerized SEs simultaneously achieve the balance in ultrathin thickness (10 µm), fast ion transport (0.65 mS cm⁻¹), superior Young's modulus (66.8 GPa), and excellent interface contact. The pouch cells with practical Li||LiNi_0.8Co_0.1Mn_0.1O₂ configuration achieve an ultrahigh volumetric energy density of 1018 Wh L⁻¹ and safety performance. The in situ polymerized integrated ultrathin SE/cathode design exhibits great promise for the practical application of SSBs with high energy density and safety performance.     

In-Built Quasi-Solid-State Poly-Ether Electrolytes Enabling Stable Cycling of High-Voltage and Wide-Temperature Li Metal Batteries

Developing solid-state electrolytes with good compatibility for high-voltage cathodes and reliable operation of batteries over a wide-temperature-range are two bottleneck requirements for practical applications of solid-state metal batteries (SSMBs). Here, an in situ quasi solid-state poly-ether electrolyte (SPEE) with a nano-hierarchical design is reported. A solid-eutectic electrolyte is employed on the cathode surface to achieve highly-stable performance in thermodynamic and electrochemical aspects. This performance is mainly due to an improved compatibility in the electrode/electrolyte interface by nano-hierarchical SPEE and a reinforced interface stability, resulting in superb-cyclic stability in Li || Li symmetric batteries ( > 4000 h at 1 mA cm⁻²/1 mAh cm⁻²; > 2000 h at 1 mA cm⁻²/4 mAh cm⁻²), which are the same for Na, K, and Zn batteries. The SPEE enables outstanding cycle-stability for wide-temperature operation (15–100 ° C) and 4 V-above batteries (Li || LiCoO₂ and Li || LiNi_0.8Co_0.1Mn_0.1O₂). The work paves the way for development of practical SSMBs that meet the demands for wide-temperature applicability, high-energy density, long lifespan, and mass production.     

Progress and Perspectives of Lithium Aluminum Germanium Phosphate-Based Solid Electrolytes for Lithium Batteries

Solid-state lithium batteries are considered promising energy storage devices due to their superior safety and higher energy density than conventional liquid electrolyte-based batteries. Lithium aluminum germanium phosphate (LAGP), with excellent stability in air and good ionic conductivity, has gained tremendous attention over the past decades. However, the poor interface compatibility with Li anode, slow Li-ion conduction in thick pellets, and high-temperature sintering procedure limit the further development of LAGP solid electrolytes in practical applications. This review comprehensively summarizes the crystal structure, Li-ion conducting mechanism, and various synthesis methods, especially the latest thin-film preparation approach. The underlying reason for Li/LAGP interfacial instability is identified, followed by several advanced interface engineering strategies, for example, introducing a functional interlayer. The integration design of LAGP-based solid electrolytes and cathode is also highlighted to enable high-loading cathodes. Additionally, recent progress of lithium-oxygen and lithium-sulfur batteries with LAGP-based solid electrolytes is discussed. Moreover, the different Li-ion migration pathways, preparation procedures, and electrochemical performance of polymer-LAGP composite solid electrolytes in Li-ion batteries are introduced. Lastly, the remaining challenges and opportunities are proposed to encourage more efforts in this field. This review aims to provide fundamental insights and promising directions toward practical LAGP-based solid-state batteries.     

过渡金属碳氮化合物(MXenes)在能量储存装置的研究进展

层状过渡金属碳化物和/或氮化物(MXenes)凭借其特有的结构和性能成为了新型储能装置电极催化剂的重要候选材料。MXenes的表达式一般为Mn+1 Xn Tx,其中“M”代表早期的过渡金属元素,“x”表示C、N或CN,而“Tx”为“-OH”、“-F”等基团,n的值一般为1,2或3。MXenes具有比表面积高、离子电导率大、亲水性好等优良特性,有利于其在离子电池、超级电容器等储能装置中应用推广。本文综述了近几年MXenes在能源储存装置中的应用及研究现状,归纳总结了MXenes的结构及能量储存机制,指出了目前研究过程中存在的短板,并对其发展方向进行展望。     

Strategies in Structure and Electrolyte Design for High-Performance Lithium Metal Batteries

Lithium metal is the “holy grail” anode for next-generation high-energy rechargeable batteries due to its high capacity and lowest redox potential among all reported anodes. However, the practical application of lithium metal batteries (LMBs) is hindered by safety concerns arising from uncontrollable Li dendrite growth and infinite volume change during the lithium plating and stripping process. The formation of stable solid electrolyte interphase (SEI) and the construction of robust 3D porous current collectors are effective approaches to overcoming the challenges of Li metal anode and promoting the practical application of LMBs. In this review, four strategies in structure and electrolyte design for high-performance Li metal anode, including surface coating, porous current collector, liquid electrolyte, and solid-state electrolyte are summarized. The challenges, opportunities, perspectives on future directions, and outlook for practical applications of Li metal anode, are also discussed.     

3D Porous Cu Current Collector/Li‐Metal Composite Anode for Stable Lithium‐Metal Batteries

Lithium‐metal batteries are of particular interest for next‐generation electrical energy storage because of their high energy density on both volumetric and gravimetric bases. Effective strategies to stabilize the Li‐metal anode are the prerequisite for the progress of these exceptional storage technologies, such as Li–S and Li–O₂ batteries. Various challenges, such as uneven Li electrodeposition, anode volume expansion, and dendrite‐induced short‐circuit have hindered the practical application of rechargeable Li‐metal batteries. Herein, a one‐step facile and cost‐effective strategy for stabilizing lithium‐metal batteries via 3D porous Cu current collector/Li‐metal composite anode is reported. The porous structure of the composite electrode provides a “cage” for the redeposition of “hostless” lithium and accommodates the anode volume expansion during cycling. Compared with planar Cu foil, its high specific surface area favors the electrochemical reaction kinetics and lowers the local current density along the anode. It leads to low interfacial resistance and stabilizes the Li electrodeposition. On this basis, galvanostatic measurements are performed on both symmetric cells and Li/Li₄Ti₅O₁₂ cells and it is found that the electrodes exhibit exceptional abilities of promoting cell lifetime and stabilizing the cycling behavior. Although this work focuses on lithium metal, this novel tactic is easy to generalize to other metal electrodes.     

Facile Design of Sulfide-Based all Solid-State Lithium Metal Battery: In Situ Polymerization within Self-Supported Porous Argyrodite Skeleton

All solid-state batteries holds great promise for superiorly safe and high energy electrochemical energy storage. The ionic conductivity of electrolytes and its interfacial compatibility with the electrode are two critical factors in determining the electrochemical performance of all solid-state batteries. It is a great challenge to simultaneously demonstrate fantastic ionic conductivity and compatible electrolyte/electrode interface to acquire a well-performed all solid-state battery. By in situ polymerizing poly(ethylene glycol) methyl ether acrylate within a self-supported 3D porous Li-argyrodite (Li₆PS₅Cl) skeleton, the two bottlenecks are tackled successfully at once. As a result, all solid-state lithium metal batteries with a 4.5 V LiNi_0.8Mn_0.1Co_0.1O₂ cathode designed by this integrated strategy demonstrates a high Coulombic efficiency exceeding 99% at room temperature. Solid-state nuclear magnetic resonance data suggest that Li⁺ mainly migrates along the continuous Li₆PS₅Cl phase to result in a room temperature conductivity of 4.6 × 10⁻⁴ S cm⁻¹, which is 128 times higher than that of the corresponding polymer. Meanwhile, the inferior solid–solid electrolyte/electrode interface is integrated via in situ polymerization to lessen the interfacial resistance significantly. This study thereby provides a very promising strategy of solid electrolyte design to simultaneously meet both high ionic conductivity and good interfacial compatibility towards practical high-energy-density all solid-state lithium batteries.     

Polyether-b-Amide Based Solid Electrolytes with Well-Adhered Interface and Fast Kinetics for Ultralow Temperature Solid-State Lithium Metal Batteries

Solid-state lithium metal batteries (SSLMBs) are highly desirable for energy storage because of the urgent need for higher energy density and safer batteries. However, it remains a critical challenge for stable cycling of SSLMBs at low temperature. Here, a highly viscoelastic polyether-b-amide (PEO-b-PA) based composite solid-state electrolyte is proposed through a one-pot melt processing without solvent to address this key process. By adjusting the molar ratio of PEO-b-PA to lithium bis(trifluoromethanesulphonyl)imide (ethylene oxide:Li = 6:1) and adding 20 wt.% succinonitrile, fast Li⁺ transport channel is conducted within the homogeneous polymer electrolyte, which enables its application at ultra-low temperature (−20 to 25 °C). The composite solid-state electrolyte utilizes dynamic hydrogen-bonding domains and ion-conducting domains to achieve a low interfacial charge transfer resistance (<600 Ω) at −20 °C and high ionic conductivity (25 °C, 3.7 × 10⁻⁴ S cm⁻¹). As a result, the LiFePO₄|Li battery based on composite electrolyte exhibits outstanding electrochemical performance with 81.5% capacity retention after 1200 cycles at −20 °C and high discharge specific capacities of 141.1 mAh g⁻¹ with high loading (16.1 mg cm⁻²) at 25 °C. Moreover, the solid-state SNCM811|Li cell achieves excellent safety performance under nail penetration test, showing great promise for practical application.     

Largely Promoted Mechano-Electrochemical Coupling Properties of Solid Polymer Electrolytes by Introducing Hydrogen Bonds-Rich Network

Solid polymer electrolytes (SPEs) and their composites are the most promising spices to access the commercial application in all-solid-state lithium batteries, where definite requirements for SPEs should be satisfied including moderate mechanical strength, high Li-ion conductivity, and stable electrode/electrolyte interface. Herein, polyurethane-based polymer (PNPU) is designed to further construct the hybrid solid polymer electrolyte (named as PNPU-PVDF-HFP) with poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) for high energy density solid-state lithium metal batteries. The theoretical calculation and characterization demonstrate that PNPU-PVDF-HFP SPEs still maintain the multiple hydrogen bonding modes of PNPU, which contributes a significantly improved mechanical properties of the polymer membrane with compact structure. Moreover, it is corroborated that PNPU is involved to form the double Li⁺ transport paths in the hybrid electrolyte, accelerating the migration of lithium ions. Therefore, PNPU-PVDF-HFP SPEs are achieved with suitable tensile strength of 5.16 MPa and high elongation of 140.8%, high ambient ionic conductivity of 4.13 × 10⁻⁴ S cm⁻¹, excellent ductile, and stability on the interface of lithium metal anode. The Li/ LiFePO₄ and Li/Li[Ni_0.8Co_0.1Mn_0.1]O₂ solid-state batteries using PNPU-PVDF-HFP SPEs present a stable cycling performance at 30 °C. This study provides a feasible strategy to achieve mechano-electrochemical coupling stable SPEs for solid-state batteries.     

Integrated Configuration Design Strategy Via Cathode-Gel Electrolyte With Forged Solid Electrolyte Interface Toward Advanced Lithium-Sulfurized Polyacrylonitrile Batteries

Li-SPAN batteries are a promising energy storage system, providing remarkable energy density and high Coulomb efficiency. However, the inherent sluggishness of the cathode's electrochemical kinetics and the instability of the Li anode hamper their cycle lifespan. In this study, a novel design of integrated configuration between cathode and electrolyte that addresses the challenges and promises to reshape the landscape of Li-SPAN, significantly enhancing the cycling stability, is presented. An artificial solid electrolyte interface (ASEI) is forged to simultaneously stabilize the Li anode and improve the interfacial compatibility, enabling an all-in-one battery system. A vertically aligned cathode structure is achieved using directional ice templating, enabling efficient Li-ion diffusion and enhancing electrochemical kinetics. The Li metal anode is coated with a MOF-on-COF ASEI, ensuring uniform Li⁺ deposition and high Li-ion transference number (0.86). Dual surface engineering further enhances the Li-SPAN cell, exhibiting a low capacity decay rate of 0.037% per cycle after 1000 cycles and superior C-rate performance. This study introduces promising strategies for effectively overcoming the challenges associated with the SPAN cathode and Li anode and paves the way for the design of high-performance Li-SPAN batteries, unlocking their full potential in the field of advanced energy storage systems.     

Ultrafine Titanium Nitride Sheath Decorated Carbon Nanofiber Network Enabling Stable Lithium Metal Anodes

Nonuniform local electric field and few nucleation sites on the reactive interface tend to cause detrimental lithium (Li) dendrites, which incur severe safety hazards and hamper the practical application of Li metal anodes in batteries. Herein, a carbon nanofiber (CNF) mat decorated with ultrafine titanium nitride (TiN) nanoparticles (CNF‐TiN) as both current collector and host material is reported for Li metal anodes. Uniform Li deposition is achieved by a synergetic effect of lithiophilic TiN and 3D CNF configuration with a highly conductive network. Theoretical calculations reveal that Li prefers to be adsorbed onto the TiN sheath with a low diffusion energy barrier, leading to controllable nucleation sites and dendrite‐free Li deposits. Moreover, the pseudocapacitive behavior of TiN identified through kinetics analysis is favorable for ultrafast Li⁺ storage and the charge transfer process, especially under a high plating/stripping rate. The CNF‐TiN‐modified Li anodes deliver lower nucleation overpotential for Li plating and superior electrochemical performance under a large current density (200 cycles at 3 mA cm^?2) and high capacity (100 cycles with 6 mAh cm^?2), as well as a long‐running lifespan (>600 h). The CNF‐TiN‐based full cells using lithium iron phosphate and sulfur cathodes exhibit excellent cycling stability.     

In Situ Surface Film Formed by Solid-State Anodic Oxidation for Stable Lithium Metal Anodes

Surface protection has drawn increasing attention in lithium (Li) metal batteries by simultaneously taking advantage of the uniform potential distribution and high conductivity, whereas tuning the composition of the protective film and improving the resistance at the interface of protective film/Li metal remain the challenges. Herein, a solid-state anodic oxidation strategy for preparing an in situ protective film for enhanced cycle life of Li metal anode is illustrated. The solid-state anodic oxidation of lithium metal is realized for the first time, and the method can be further modified for tuning the composition of the protective film for highly ionic conductive fluorine-rich interface. Surface electrodeposition simulations indicate the important role of the conductive protective film in increasing the uniform potential distribution on the Li surface. The as-prepared in situ fluorinated protective film efficiently suppresses the dendrite growth and promotes the cycling performance of the Li metal full cell in commercial ester electrolyte without any additive. This work opens up a new avenue for fabricating unique in situ protective films with controlled composition on Li surface for energy storage applications.     

From Lithium-Metal toward Anode-Free Solid-State Batteries: Current Developments,Issues, and Challenges

The development of rechargeable batteries with high-energy density is critical for future decarbonization of transportation. Anode-free Li-ion batteries, using a bare current collector at the anode side without any excess of Li, provide the highest volumetric energy density ( > 1500 Wh L⁻¹) among all possible cell configurations. Furthermore, elimination of the anode material coating reduces material consumption and greatly simplifies cell production, which in turn lowers costs. Although significant progress has been made recently by the application of modified current collectors, optimized cycling parameters and improved liquid electrolytes, insufficient efficiencies, and dendritic growth during lithium plating lead to poor cycle life of typically less than 100 cycles as well as safety issues. Alternatively, very recent studies have demonstrated anode-free solid-state batteries that combine the benefits of high energy anode-free cell configuration and solid-state systems with high safety, exceeding 1000 cycles. This review provides an overview of recent developments toward anode-free solid-state batteries and highlights the current issues and challenges in this nascent field. It is concluded that, although major challenges remain at the present, the lessons learned in the fields of liquid electrolytes and solid-state lithium metal batteries can accelerate the development of anode-free solid-state batteries of practical relevance.     

Alloyable Viscous Fluid for Interface Welding of Garnet Electrolyte to Enable Highly Reversible Fluoride Conversion Solid State Batteries

The garnet-type solid-state Li-metal batteries are promising to develop into the high-energy-density system when coupled with the high-capacity conversion reaction cathodes. However, the high interfacial resistance and poor contact between garnet electrolyte and Li anode are still a challenge. Here, an alloyable viscous fluid strategy is proposed for Li/garnet interface welding to enable highly reversible fluoride conversion solid-state batteries. The super-assembled phenide polymer with liquid metal property can serve as “oily” interlayer to in situ construct an ionic/electronic mixed conduction network by thermal and electrochemical lithiation. The resultant healing effect of contact voids between garnet and Li enables a dramatic reduction of interfacial resistance to 6 Ω cm². The confinement and compaction of conversion products by garnet electrolyte endow the FeF₃ based batteries with long-cycling and high-rate performance (520 and 330 mAh g⁻¹ at 0.2 and 2 C respectively). This ceramic configuration also endows the CuF₂ conversion battery with much better rechargeability (instead as widely known primary battery).     

Constructing a Multifunctional Interlayer toward Ultra-High Critical Current Density for Garnet-Based Solid-State Lithium Batteries

All-solid-state lithium batteries (ASSLBs), exhibiting great advantages of high energy density and safety, are proposed to be the next generation energy storage system. However, the successful commercialization of garnet-based ASSLBs is hindered by the poor contact between solid-state electrolytes (Li_6.25Ga_0.25La₃Zr₂O₁₂, LGLZO) and lithium anode, as well as low critical current density (CCD). Herein, an indium tin oxide (ITO) layer is prepared on LGLZO by ultrasonic spraying technique, where ITO reacts with molten lithium to form a composite interlayer, consisting of Li₁₃In₃, Li₂O, and LiInSn. Experiments and density functional theory calculations demonstrate that such a unique interlayer plays a multifunctional role in achieving simultaneously better interface wettability, uniform Li deposition, and dendrite suppression at Li/LGLZO interface. Consequently, the CCD of ITO-treated symmetric cell is increased to a record-high value of 12.05 mA cm⁻² at room temperature, which is expected to promote practical application of ASSLBs. Moreover, the Li/ITO@LGLZO/Li cell exhibits a low interfacial resistance of only 5.9 Ω cm² and performs stable electrochemical operations for over 2000 h at 2 mA cm⁻². The Li/ITO@LGLZO/LiFePO₄ full cell also delivers superior electrochemical performances, demonstrating the efficiency of the ITO layer.     

Recent Advances in Electrolytes for Potassium-Ion Batteries

Potassium-ion batteries (KIBs) are considered as the potential energy storage devices due to the abundant reserves and low cost of potassium. In the past decade, research on KIBs has generally focused on electrode materials. However, since electrolytes also play a key role in determining the cell performance, this review summarizes recent advances in KIB electrolytes and design strategies. Specifically, the review includes five parts. First, the organic liquid electrolyte is the most widely used type for KIBs. Its two major components, salts and solvents, have a huge impact on the formation of the solid electrolyte interphase and the performance of KIBs. Changes in salts/solvents, the introduction of additives, and the concentration increase all have a positive effect on organic liquid electrolytes. Second, the design of water-in-salt electrolytes can effectively widen the narrow electrochemical stability window of aqueous electrolytes. Third, despite the appealing properties, the ionic liquid electrolytes have not been widely applied due to its high cost. Fourth, the solid-state electrolytes have drawn much attention due to high safety, and current research has been working on improving their ionic conductivity at room temperature. Lastly, perspectives are provided to support the future development of suitable electrolytes for high-performance KIBs.