Advanced Electrolyte Additives for Enhanced Homogeneous Sulfur Fixation in Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are regarded as leading contenders for next-generation energy storage owing to their exceptional theoretical energy density. However, severe sulfur electrode depletion causes rapid capacity fading and compromised cycling stability. Electrolyte engineering effectively enables homogeneous sulfur fixation, improving battery performance. The study investigates the mechanisms behind these homogeneous reactions, focusing on sulfur fixation processes. Sulfur fixation is explored through multiple perspectives, including the inhibition of polysulfide shuttling, mitigation of electrode passivation, and the combined application of both strategies. Regarding polysulfide shuttling inhibition, three distinct mechanisms for sulfur fixation are identified: 1) chemisorption-based sulfur fixation, involving the formation of chemical bonds with polysulfides; 2) redox-mediated sulfur fixation, which accelerates the kinetics of sulfur species; and 3) hybrid sulfur fixation, which combines elements of both approaches. Furthermore, the review analyzes current methods for homogeneous sulfur fixation, focusing on electrolyte designs that enable homogeneous sulfur fixation under limited conditions. It provides insights to optimize electrolytes, advancing Li-S battery performance and commercialization.

Polysulfide chemistry in metal-sulfur batteries

Renowned for their high theoretical energy density and cost-effectiveness, metal-sulfur (M-S) batteries are pivotal in overcoming the current energy storage bottlenecks and accelerating the transition toward a cleaner society. Polysulfides (PSs) serve as essential intermediates in M-S batteries and bridge the electrochemical redox processes of sulfur, playing a decisive role in controlling the electrode behaviors and regulating the battery performances. Understanding PS chemistry across diverse battery environments is key to advancing M-S batteries. This review aims to provide a comprehensive overview of the PS chemistry in high-energy-density battery systems and outline future research directions. The compositions, properties, and characterization methods of PSs are introduced to facilitate a fundamental understanding of the PS chemistry in working batteries. Following this, a thorough examination of the chemical and electrochemical behaviors of PSs and their impacts on electrode performances is conducted to deepen the insights into the PS reactions in batteries. Building on this foundation, representative PS regulation strategies are discussed, focusing on molecular modification, solvation optimization, and interfacial regulation, to achieve superior M-S battery performances. Challenges of PSs in practical M-S batteries are finally analyzed, and perspectives on the future research trends of PS chemistry are presented.

Dynamic D-p-π Orbital Coupling of FeN4-Spπ Atomic Centers on Graphitized Carbon Toward Invigorated Sulfur Kinetic Chemistry

Precisely modulating d-p orbital coupling of single-atom electrocatalysts for sulfur reduction reactions in lithium-sulfur batteries maintains tremendous challenges. Herein, a dynamic d-p-π orbital coupling modulation is elucidated by unsaturated Fe centers on nitrogen-doped graphitized carbon (NG) coordinated with trithiocyanuric acid featuring with p-π conjugation to engineer Fe single atom architecture (FeN4-Spπ-NG). Intriguingly, this coordination microenvironment of the Fe center is dynamically reconstituted during charge/discharge processes, because of the formation of trilithium salts rooted from the departed axial ligands to engineer interfacial coating on the sulfur cathode, and then it recovers to the initial coordination configuration. Theoretical and experimental results unravel that the axial p-π conjugated ligand reinforcing d-p orbital coupling enables the interfacial charge interaction, thereby strengthening LiPSs adsorption, and reducing the Li2S decomposition barrier by formation of Fe─S and S─Li bonds. Thus, dynamic d-p-π orbital coupling modulation of FeN4-Spπ endow lithium-sulfur batteries with considerable electrochemical performances, highlighting an intriguingly dynamic orbital coupling modulation strategy for single atom electrocatalysts.

Revitalizing Li-S batteries: the power of electrolyte additives

Lithium-sulfur (Li-S) batteries have garnered significant attention as promising next-generation energy storage solutions due to their high energy density and cost efficiency. However, the broad adoption of Li-S batteries is impeded by several critical issues. These include the intrinsically low conductivities of sulfur (S) and lithium sulfide (Li2S), the polysulfide shuttle effect, and dendrite formation on the lithium (Li) electrode, among other challenges. Overcoming these obstacles is crucial to realizing the full potential of Li-S batteries. A key step towards improving Li-S battery performance is the optimization of electrolytes, with a particular focus on enhancing cell cyclability, rate capability, safety, and lifespan. This review examines the current advancements in various electrolyte additive options, including their concepts, designs, and materials, and how the electrolyte's final chemical and physical properties influence the overall performance of Li-S batteries. The aim is to provide a comprehensive framework for the rational selection of future electrolyte additives for Li-S batteries, based on the available concepts, and to evaluate the existing electrolyte additives.

Advances in Functional Organosulfur-Based Mediators for Regulating Performance of Lithium Metal Batteries

Rechargeable lithium metal batteries (LMBs) are promising next-generation energy storage systems due to their high theoretical energy density. However, their practical applications are hindered by lithium dendrite growth and various intricate issues associated with the cathodes. These challenges can be mitigated by using organosulfur-based mediators (OSMs), which offer the advantages of abundance, tailorable structures, and unique functional adaptability. These features enable the rational design of targeted functionalities, enhance the interfacial stability of the lithium anode and cathode, and accelerate the redox kinetics of electrodes via alternative reaction pathways, thereby effectively improving the performance of LMBs. Unlike the extensively explored field of organosulfur cathode materials, OSMs have garnered little attention. This review systematically summarizes recent advancements in OSMs for various LMB systems, including lithium-sulfur, lithium-selenium, lithium-oxygen, lithium-intercalation cathode batteries, and other LMB systems. It briefly elucidates the operating principles of these LMB systems, the regulatory mechanisms of the corresponding OSMs, and the fundamentals of OSMs activity. Ultimately, strategic optimizations are proposed for designing novel OSMs, advanced mechanism investigation, expanded applications, and the development of safe battery systems, thereby providing directions to narrow the gap between rational modulation of organosulfur compounds and their practical implementation in batteries.

Chip-Inspired Design of High-Performance Lithium-Sulfur Batteries by Integrating Monodisperse Sulfur Nanoreactors on Graphene

For practical application of lithium-sulfur batteries (LSBs), designing devices with an overall optimal structure instead of modifying electrode materials is significant. Herein, we report a chip-inspired design of a vertically integrated structure as an LSB cathode by implanting Mo2C nanoparticles and nanosulfur into the reduced graphene oxide (rGO) matrix. This configuration enabled the synthesis of isolated sulfur nanoreactors (S-NRs) integrated in a tandem array on the rGO, generating chip-like integrated LSBs. The spatial confinement/protection and concentration gradient of the S-NRs effectively avoided the dissolution, diffusion, and loss of polysulfides, thereby enhancing the sulfur utilization and redox reaction kinetics. Additionally, the adaptive storage energy can be improved by utilizing the tandem, isolation, and synergistic multiplicative effect among the nanoreactor units. As a result, the integrated LSB cathode obtained excellent electrochemical performances with an initial capacity of 1392 mAh g-1 at 0.1C, a low capacity decay rate of 0.017% per cycle during 1500 cycles of operation at 0.5C, and a superior rate performance. This work provides a rational design idea and method of further advancing the precise preparation of high-performance energy storage devices.

Alveoli-Inspired Carbon Cathodes with Interconnected Porous Structure and Asymmetric Coordinated Vanadium Sites for Superior Li-S Batteries

Accelerating sulfur conversion catalysis to alleviate the shuttle effect has become a novel paradigm for effective Li-S batteries. Although nitrogen-coordinated metal single-atom (M-N4) catalysts have been investigated, further optimizing its utilization rate and catalytic activities is urgently needed for practical applications. Inspired by the natural alveoli tissue with interconnected structure and well-distributed enzyme catalytic sites on the wall for the simultaneously fast diffusion and in situ catalytic conversion of substrates, here, we proposed the controllable synthesis of bioinspired carbon cathode with interconnected porous structure and asymmetric coordinated V-S1N3 sites for efficient and stable Li-S batteries. The enzyme-mimetic V-S1N3 shows asymmetric electronic distribution and high tunability, therefore enhancing in situ polysulfide conversion activities. Experimental and theoretical results reveal that the high charge asymmetry degree and large atom radius of S in V-S1N3 result in sloping adsorption for polysulfide, thereby exhibiting low thermodynamic energy barriers and long-range stability (0.076 % decay over 600 cycles).

Rechargeable Metal-Sulfur Batteries: Key Materials to Mechanisms

Rechargeable metal-sulfur batteries are considered promising candidates for energy storage due to their high energy density along with high natural abundance and low cost of raw materials. However, they could not yet be practically implemented due to several key challenges: (i) poor conductivity of sulfur and the discharge product metal sulfide, causing sluggish redox kinetics, (ii) polysulfide shuttling, and (iii) parasitic side reactions between the electrolyte and the metal anode. To overcome these obstacles, numerous strategies have been explored, including modifications to the cathode, anode, electrolyte, and binder. In this review, the fundamental principles and challenges of metal-sulfur batteries are first discussed. Second, the latest research on metal-sulfur batteries is presented and discussed, covering their material design, synthesis methods, and electrochemical performances. Third, emerging advanced characterization techniques that reveal the working mechanisms of metal-sulfur batteries are highlighted. Finally, the possible future research directions for the practical applications of metal-sulfur batteries are discussed. This comprehensive review aims to provide experimental strategies and theoretical guidance for designing and understanding the intricacies of metal-sulfur batteries; thus, it can illuminate promising pathways for progressing high-energy-density metal-sulfur battery systems.

All-Round Ionic Liquids for Shuttle-Free Zinc-Iodine Battery

The practical implementation of aqueous zinc-iodine batteries (ZIBs) is hindered by the rampant Zn dendrites growth, parasite corrosion, and polyiodide shuttling. In this work, ionic liquid EMIM[OAc] is employed as an all-round solution to mitigate challenges on both the Zn anode and the iodine cathode side. First, the EMIM+ embedded lean-water inner Helmholtz plane (IHP) and inert solvation sheath modulated by OAc- effectively repels H2 O molecules away from the Zn anode surface. The preferential adsorption of EMIM+ on Zn metal facilitates uniform Zn nucleation via a steric hindrance effect. Second, EMIM+ can reduce the polyiodide shuttling by hindering the iodine dissolution and forming an EMIM+ -I3 - dominated phase. These effects holistically enhance the cycle life, which is manifested by both Zn || Zn symmetric cells and Zn-I2 full cells. ZIBs with EAc deliver a capacity decay rate of merely 0.01 ‰ per cycle after over 18,000 cycles at 4 A g-1 , and lower self-discharge and better calendar life than the ZIBs without ionic liquid EAc additive.

Kinetic Promoters for Sulfur Cathodes in Lithium-Sulfur Batteries

ConspectusLithium-sulfur (Li-S) batteries have attracted worldwide attention as promising next-generation rechargeable batteries due to their high theoretical energy density of 2600 Wh kg-1. The actual energy density of Li-S batteries at the pouch cell level has significantly exceeded that of state-of-the-art Li-ion batteries. However, the overall performances of Li-S batteries under practical working conditions are limited by the sluggish conversion kinetics of the sulfur cathodes. To overcome the above challenge, various kinetic promotion strategies have been proposed to accelerate the multiphase and multi-electron cathodic redox reactions between sulfur, lithium polysulfides (LiPSs), and lithium sulfide. Nowadays, kinetic promoters have been massively employed in sulfur cathodes to achieve Li-S batteries with high energy densities, high rates, and long lifespans. A comprehensive and timely summary of cutting-edge kinetic promoters for sulfur cathodes is of great essence to afford an in-depth understanding of the unique Li-S electrochemistry.In this Account, we outline the recent efforts on the design of sulfur cathode kinetic promoters for advanced Li-S batteries. The latest progress is discussed in detail regarding heterogeneous, homogeneous, and semi-immobilized kinetic promoters. Heterogeneous promoters, representatively known as electrocatalysts, function mainly by reducing the energy barriers of the interfacial electrochemical reactions. The working mechanism, activity regulation strategies, and reconstitution/deactivation processes of the heterogeneous promoters are reviewed to provide guiding principles for rational design. In comparison, homogeneous promoters are able to fully contact with the reaction interfaces and regulate the electron/ion-inaccessible reactants in working Li-S batteries. Redox mediators and redox comediators are typical homogeneous promoters. The former establishes extra chemical reaction pathways to circumvent the originally sluggish steps and boost the overall kinetics, while the latter fundamentally modifies the LiPS molecules to enhance their redox kinetics. For semi-immobilized promoters, the active units are generally anchored on the cathode substrate through flexible chains with mobile characteristics. Such a design endows the promoter with both heterogeneous and homogeneous characteristics to comprehensively regulate the multiphase sulfur redox reactions involving both mobile and immobile reactants.Overall, this Account summarizes the fundamental electrochemistry, design principles, and practical promotion effects of the various kinetic promoters used for the sulfur cathodes in Li-S batteries. We believe that this Account will provide an in-depth and cutting-edge understanding of the unique sulfur electrochemistry, thereby providing guidance for further development of high-performance Li-S batteries and analogous rechargeable battery systems.

Design principles for 2D transition metal dichalcogenides toward lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries are regarded as a promising candidate for next-generation energy storage systems owing to their remarkable energy density, resource availability, and environmental benignity. Nevertheless, severe shuttling effect, sluggish redox kinetics, large volumetric expansion, and uncontrollable dendrite growth hamper the practical applications. To address these intractable issues, two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged expeditiously as an essential material strategy. Herein, this review emphasizes the development and application of 2D TMDs in Li-S batteries. It starts with introducing the fundamentals of Li-S batteries and common synthetic routes of TMDs, followed by summarizing the employment of pristine, hybrid, and defective TMDs in the realm of expediting sulfur chemistry and stabilizing lithium anode. Finally, the development roadmap and possible research directions of TMDs are proposed to offer guidance for the future design of high-performance Li-S batteries.

In Situ Construction Channels of Lithium-Ion Fast Transport and Uniform Deposition to Ensure Safe High-Performance Solid Batteries

Solid-state lithium-ion batteries (SLIBs) are the promising development direction for future power sources because of their high energy density and reliable safety. To optimize the ionic conductivity at room temperature (RT) and charge/discharge performance to obtain reusable polymer electrolytes (PEs), polyvinylidene fluoride (PVDF), and poly(vinylidene fluoride-hexafluoro propylene) (P(VDF-HFP)) copolymer combined with polymerized methyl methacrylate (MMA) monomers are used as substrates to prepare PE (LiTFSI/OMMT/PVDF/P(VDF-HFP)/PMMA [LOPPM]). LOPPM has interconnected lithium-ion 3D network channels. The organic-modified montmorillonite (OMMT) is rich in the Lewis acid centers, which promoted lithium salt dissociation. LOPPM PE possessed high ionic conductivity of 1.1 × 10-3 S cm-1 and a lithium-ion transference number of 0.54. The capacity retention of the battery remained 100% after 100 cycles at RT and 0.5 C. The initial capacity of one with the second-recycled LOPPM PE is 123.9 mAh g-1 . This work offered a feasible pathway for developing high-performance and reusable LIBs.

A Low-Strain Cathode by sp-Carbon Induced Conversion in Multi-Level Structure of Graphdiyne

A multi-level architecture formed alternatively by the conformal graphdiyne (GDY) and CuS is well engineered for Li-free cathode. Such a proof-of-concept architecture efficiently integrates the advantages of GDY and produces new functional heterojunctions (sp-C-S-Cu hybridization bond). The layer-by-layer 2D confinement effect successfully avoids structural collapse, the selective transport inhibits the shuttling of active components, and the interfacial sp-C-S-Cu hybridization bond significantly regulates the phase conversion reaction. Such new sp-C-S-Cu hybridization of GDY greatly improves the reaction dynamics and reversibility, and the cathode delivers an energy density of 934 Wh kg-1 and an unattenuated lifespan of 3000 cycles at 1 C. Our results indicate that the GDY-based interface strategy will greatly promote the efficient utilization of the conversion-type cathodes.

Organosulfur Materials for Rechargeable Batteries: Structure, Mechanism, and Application

Lithium-ion batteries have received significant attention over the last decades due to the wide application of portable electronics and increasing deployment of electric vehicles. In order to further enhance the performance of the batteries and overcome the capacity limitations of inorganic electrode materials, it is imperative to explore new cathode and functional materials for rechargeable lithium batteries. Organosulfur materials containing sulfur-sulfur bonds as a kind of promising organic electrode materials have the advantages of high capacities, abundant resources, tunable structures, and environmental benignity. In addition, organosulfur materials have been widely used in almost every aspect of rechargeable batteries because of their multiple functionalities. This review aims to provide a comprehensive overview on the development of organosulfur materials including the synthesis and application as cathode materials, electrolyte additives, electrolytes, binders, active materials in lithium redox flow batteries, and other metal battery systems. We also give an in-depth analysis of structure-property-performance relationship of organosulfur materials, and guidance for the future development of organosulfur materials for next generation rechargeable lithium batteries and beyond.

Understanding the lithium-sulfur battery redox reactions via operando confocal Raman microscopy

The complex interplay and only partial understanding of the multi-step phase transitions and reaction kinetics of redox processes in lithium–sulfur batteries are the main stumbling blocks that hinder the advancement and broad deployment of this electrochemical energy storage system. To better understand these aspects, here we report operando confocal Raman microscopy measurements to investigate the reaction kinetics of Li–S redox processes and provide mechanistic insights into polysulfide generation/evolution and sulfur deposition. Operando visualization and quantification of the reactants and intermediates enabled the characterization of potential-dependent rates during Li–S redox and the linking of the electronic conductivity of the sulfur-based electrode and concentrations of polysulfides to the cell performance. We also report the visualization of the interfacial evolution and diffusion processes of different polysulfides that demonstrate stepwise discharge and parallel recharge mechanisms during cell operation. These results provide fundamental insights into the mechanisms and kinetics of Li–S redox reactions.

The complex redox processes in lithium–sulfur batteries are not yet fully understood at the fundamental level. Here, the authors report operando confocal Raman microscopy measurements to provide mechanistic insights into polysulfide evolution and sulfur deposition during battery cycling.

Biomass-Derived Lenthionine Enhanced by Radical Receptor for Rechargeable Lithium Battery

Organic compounds with tunable structures and high capacities are promising electrode materials for batteries. Cyclic organosulfide (i. e., lenthionine), as a natural material that can provide excellent ratio of effective atoms (S) and non-efficient atoms (C, H, and others), has a high theoretical specific capacity of 853.6 mAh g^-1 . However, the multiphase transformation causes rapid capacity decay and hysteresis of charge/discharge voltage plateaus. To overcome these issues, a receptor, phenyl disulfide (PDS), was introduced to truncate subsequent transformations directly from the source and change the reaction path, inhibit the capacity decay, and improve the cycling stability. After 500 cycles, the capacity retention was 81.1 % with PDS, which was in sharp contrast to that (35.6 %) of the control cell. This study helps to understand the electrochemistry mechanism of biomass-derived lenthionine used as a high-capacity cathode material for rechargeable lithium batteries, also offering a strategy to overcome its inherent issues.

A graphene oxide scaffold-encapsulated microcapsule for polysulfide-immobilized long life lithium-sulfur batteries

Engineering high-performance cathodes for high energy-density lithium-sulfur (Li-S) batteries is quite significant to achieve commercialization. Here, we develop a graphene oxide scaffold/sulfur composite-encapsulated microcapsule (GSM) for high-performance Li-S batteries, which is prepared through the co-flow focusing (CFF) approach. The GSM-based cathode displays a high capacity of 1004 mA h g^-1 at 0.2C after cycling 200 times, a long-term cycling stability after 1000 cycles at 2C, and a good rate-performance. At temperatures of -5 °C and 45 °C, the electrochemical performance is also excellent. The computational calculations based on density functional theory (DFT) verify the high adsorption energies of the microcapsules towards polysulfides, suppressing the shuttle effect efficiently. It is expected that the GSM system developed based on the CFF method here and its high electrochemical performance will enable it to be applicable for preparing many other emerging energy-storage materials and secondary batteries.

Advances of Organosulfur Materials for Rechargeable Metal Batteries

Battery materials have become a hotspot in the academic research. Organosulfur compounds are considered as a promising class of cathode materials for rechargeable metal batteries. They have attracted increasing attention in recent years after a long-term stagnancy since 1980s. Recent studies have focused on the understanding of redox mechanism of linear organosulfur molecules R-Sn -R with defined structures. In addition, some new organosulfur compounds are developed. The reversible sulfursulfur (SS) bond breakage/formation of organosulfur in batteries makes them applicable as functional materials in batteries. In this review, new organosulfur materials including molecules, polymers, and composites are introduced. In the following, organosulfur-inorganic hybrid materials are discussed, which have shown unique redox process and enhanced battery performance. In the third part, organosulfur additives are used in Li-S batteries, which can improve the formation of solid-electrolyte interphase (SEI) and alter the redox pathways of sulfur cathodes. In the fourth part, organosulfur materials used in other metal batteries are introduced. Lastly, a summary and some perspectives are given. This review presents an overview of the recent advances of organosulfur materials in batteries and provides guidance for the future development of these materials.

Polymers in Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) hold great promise as one of the next-generation power supplies for portable electronics and electric vehicles due to their ultrahigh energy density, cost effectiveness, and environmental benignity. However, their practical application has been impeded owing to the electronic insulation of sulfur and its intermediates, serious shuttle effect, large volume variation, and uncontrollable formation of lithium dendrites. Over the past decades, many pioneering strategies have been developed to address these issues via improving electrodes, electrolytes, separators and binders. Remarkably, polymers can be readily applied to all these aspects due to their structural designability, functional versatility, superior chemical stability and processability. Moreover, their lightweight and rich resource characteristics enable the production of LSBs with high-volume energy density at low cost. Surprisingly, there have been few reviews on development of polymers in LSBs. Herein, breakthroughs and future perspectives of emerging polymers in LSBs are scrutinized. Significant attention is centered on recent implementation of polymers in each component of LSBs with an emphasis on intrinsic mechanisms underlying their specific functions. The review offers a comprehensive overview of state-of-the-art polymers for LSBs, provides in-depth insights into addressing key challenges, and affords important resources for researchers working on electrochemical energy systems.

Understanding the modulation effect and surface chemistry in a heteroatom incorporated graphene-like matrix toward high-rate lithium-sulfur batteries

The underlying interface effects of sulfur hosts/polysulfides at the molecular level are of great significance to achieve advanced lithium-sulfur batteries. Herein, we systematically study the polysulfide-binding ability and the decomposition energy barrier of Li₂S enabled by different kinds of nitrogen (pyridinic N, pyrrolic N and graphitic N) and phosphorus (P-O, PO and graphitic P) doping and decipher their inherent modulation effect. The doping process helps in forming a graphene-like structure and increases the micropores/mesopores, which can expose more active sites to come into contact with polysulfides. First-principles calculations reveal that the PO possesses the highest binding energies with polysulfides due to the weakening of the chemical bonds. Besides, PO as a promoter is beneficial for the free diffusion of lithium ions, and the pyridinic N and pyrrolic N can greatly reduce the kinetic barrier and catalyze the polysulfide conversion. The synergetic effects of nitrogen and phosphorus as bifunctional active centers help in achieving an in situ adsorption-diffusion-conversion process of polysulfides. Benefiting from these features, the graphene-like network achieves superior rate capability (a high reversible capacity of 954 mA h g^-1 at 2C) and long-term stability (an ultralow degradation rate of 0.009% around 800 cycles at 5C). Even at a high sulfur loading of 5.6 mg cm^-2, the cell can deliver an areal capacity of 4.6 mA h cm^-2 at 0.2C.

Advanced Electrolyte Design for High-Energy-Density Li-Metal Batteries under Practical Conditions

Given the limitations inherent in current intercalation-based Li-ion batteries, much research attention has focused on potential successors to Li-ion batteries such as lithium-sulfur (Li-S) batteries and lithium-oxygen (Li-O₂ ) batteries. In order to realize the potential of these batteries, the use of metallic lithium as the anode is essential. However, there are severe safety hazards associated with the growth of Li dendrites, and the formation of "dead Li" during cycles leads to the inevitable loss of active Li, which in the end is undoubtedly detrimental to the actual energy density of Li-metal batteries. For Li-metal batteries under practical conditions, a low negative/positive ratio (N/P ratio), a electrolyte/cathode ratio (E/C ratio) along with a high-voltage cathode is prerequisite. In this Review, we summarize the development of new electrolyte systems for Li-metal batteries under practical conditions, revisit the design criteria of advanced electrolytes for practical Li-metal batteries and provide perspectives on future development of electrolytes for practical Li-metal batteries.