A Single-Ion Conducting Borate Network Polymer as a Viable Quasi-Solid Electrolyte for Lithium Metal Batteries

A tetraphenylborate-based anionic metal-organic framework as a versatile solid electrolyte for fast Li+, Na+, K+, Mg2+, Ca2+, and Zn2+ transportation

Tetraphenylborate (BPh4 -) has been widely employed in the field of electrolytes and displayed better ionic conductivities in polymer solid-state Li+ conductors. However, the fabrication of tetraphenylborate monomers into metal-organic frameworks (MOFs) and the exploration of their potential in solid-state electrolytes have never been reported. In this work, carboxylic acid functionalized lithium tetraphenylborate was purposefully synthesized and employed to construct an anionic MOF as a solid electrolyte. The counter cation Li+ was encapsulated into the anionic channel to become the free mobile charge carrier that produced a lithium-ion solid electrolyte with outstanding ion conductivity (2.75 × 10-3 S cm-1 at 25 °C), an impressively high lithium-ion transference number (t Li+ = 0.89), and low activation energy (0.15 eV). Acting as a solid electrolyte, the anionic MOF-based lithium iron phosphate battery delivered an initial specific capacity of 135 mA h g-1 and retained 95% capacity after 220 charge-discharge cycles with a coulombic efficiency close to 100%. Moreover, by exchanging the free Li+ with Na+, K+, Mg2+, Ca2+, and Zn2+, our anionic MOF is also available for other types of solid electrolytes with the corresponding conductivities all above that of the functional battery electrolyte. Our work provided a convenient and tunable route to prepare conducting MOFs for alkali metal ions, alkaline earth metal ions, and other possible metal cations of interest, which could be used in solid-state electrolytic devices in the future.

Spatially Confined Engineering Toward Deep Eutectic Electrolyte in Metal-Organic Framework Enabling Solid-State Zinc-Ion Batteries

Uncontrollable interfacial side reactions generated from common aqueous electrolytes, just like the hydrogen evolution reaction (HER) and dendrite growth, have severely prevented the practical application of zinc-ion batteries (ZIBs). Solid-state ZIBs are considered to be an efficient strategy by adopting high-quality solid-state electrolytes (SSEs). Here, by confining deep eutectic electrolyte (DEE) into the nanochannels of metal-organic framework (MOF)-PCN-222, a stable DEE@PCN-222 SSE with internal Zn2+ transport channels was obtained. A distinctive ion-transport network composed of DEE and PCN-222 in the interior of DEE@PCN-222 realizes the efficient Zn2+ conduction, contributing to high ionic conductivity of 3.13×10-4 S cm-1 at room temperature, low activation energy of 0.12 eV, and a high Zn2+ transference number of 0.74. Furthermore, experimental and theoretical investigations demonstrate that DEE@PCN-222 with its unique channel structure could homogeneously regulate the Zn2+ distribution and effectively alleviate the side reactions. Highly reversible Zn plating/stripping performance of 2476 h can be realized by the SSE. The solid-state ZIBs show a specific capacity of 306 mAh g-1 and display cycling stability of 517 cycles. This unique design concept provides a new perspective in realizing the high-safety and high-performance ZIBs.

Ion transport mechanisms in covalent organic frameworks: implications for technology

Covalent organic frameworks (COFs) have emerged as promising materials for ion conduction due to their highly tunable structures and excellent electrochemical stability. This review paper explores the mechanisms of ion conduction in COFs, focusing on how these materials facilitate ion transport across their ordered structures, which is crucial for applications such as solid electrolytes in batteries and fuel cells. We discuss the design strategies employed to enhance ion conductivity, including pore size optimization, functionalization with ionic groups, and the incorporation of solvent molecules and salts. Additionally, we examine the various applications of ion-conductive COFs, particularly in energy storage and conversion technologies, highlighting recent advancements and future directions in this field. This review paper aims to provide a comprehensive overview of the current state of research on ion-conductive COFs, offering insights into their potential to design highly ion-conductive COFs considering not only fundamental studies but also practical perspectives for advanced electrochemical devices.

Functionally Modified Polymer Electrolyte Based on Noncovalent Interaction for Stable Lithium Metal Batteries

The charge transfer efficiency of the solid electrolyte depends on the number of lithium ions that can be effectively transported and participate in the electrode reaction. However, limited by the strong coupling relationship between Li+ and Lewis basic sites on the polymer chain, the Li+ transference number (tLi+) of the solid polymer electrolyte (SPE) based dual-ion conductor is typically low, resulting in excessive anion aggregation at the electrode side and inducing concentration polarization. In this study, we present a functionalized modified polymer electrolyte (FMPE) with selective cation transport, which was synthesized by embedding 4-(trifluoromethyl)styrene (TFS) functionalized groups onto the poly(diethylene glycol diacrylate) polymer chain. The TFS group formed noncovalent couplings with TFSI- anions through hydrogen bondings and dipole-dipole interactions, which effectively limited the migration of the anions and contributed to the elevated tLi+ of the FMPEs to 0.595 and 0.699 at 25 and 60 °C, respectively. Density functional theory (DFT) calculations were performed to verify the increased anion migration barriers for different noncovalent interactions and revealed that the conjugated system formed by the delocalized π electrons of the benzene ring and the C═O groups helped to disperse the electron distribution of the polymer chains. Consequently, the decrease in the degree of Li+ immobilization promotes the decoupling and migration of Li+ between the polymer chains. Benefiting from optimized Li+ transport behavior, the lithium metal batteries (LMBs) assembled by FMPEs and LiFePO4 exhibit excellent rate performance (discharge specific capacity of 88.8 mAh g-1 at 5 C) and stable long-term cycle performance (capacity decay rate of only 0.064% per cycle for 500 cycles at 25 °C and 0.5 C).

Accelerated Selective Li+ Transports Assisted by Microcrack-Free Anionic Network Polymer Membranes for Long Cyclable Lithium Metal Batteries

Rechargeable Li metal batteries have the potential to meet the demands of high-energy density batteries for electric vehicles and grid-energy storage system applications. Achieving this goal, however, requires resolving not only safety concerns and a shortened battery cycle life arising from a combination of undesirable lithium dendrite and solid-electrolyte interphase formations. Here, a series of microcrack-free anionic network polymer membranes formed by a facile one-step click reaction are reported, displaying a high cation conductivity of 3.1 × 10-5 S cm-1 at high temperature, a wide electrochemical stability window up to 5 V, a remarkable resistance to dendrite growth, and outstanding non-flammability. These enhanced properties are attributed to the presence of tethered borate anions in microcrack-free membranes, which benefits the acceleration of selective Li+ cations transport as well as suppression of dendrite growth. Ultimately, the microcrack-free anionic network polymer membranes render Li metal batteries a safe and long-cyclable energy storage device at high temperatures with a capacity retention of 92.7% and an average coulombic efficiency of 99.867% at 450 cycles.

A Thiol Branched 3D Network Quasi Solid-State Polymer Electrolyte Reinforced by Covalent Organic Frameworks for Lithium Metal Batteries

Quasi solid-state polymer electrolytes (QSPEs) are particularly attractive due to their high ionic conductivity and excellent safety for lithium metal batteries (LMBs). However, it is still a great challenge for QSPEs to achieve strong mechanical strength and high electrochemical performance simultaneously. Herein, a QSPE (SCOF-PEP-PEA) using a covalent organic framework (COF) containing abundant allyl groups (SCOF) as a rigid porous filler as well as a cross-linker to reinforce the polymer network is reported. Benefitting from the unique 3D nanonetwork structure and abundant lithiophilic functional groups, SCOF-PEP-PEA QSPE exhibits high ionic conductivity (4.0 × 10-4 S cm-1) and high lithium-ion transference number (0.82) at room temperature. Moreover, SCOF-PEP-PEA QSPE displays much improved mechanical strength compared to PEP-PEA QSPE (AFM Young's modulus: 453 vs 36 MPa). As a result, the Li/LFP full cell with SCOF-PEP-PEA QSPE shows great rate performance of 141 mAh g-1 at 1C and delivers a high specific capacity retention of 92% after 220 cycles at 0.5 C (60 °C). This work provides a new strategy to design and prepare high-performance QSPEs with COFs as porous organic filler, and further expand the application of COFs for energy storage applications.

Reduction of Li+ within a borate anion

Group 1 elements exhibit the lowest electronegativity values in the Periodic Table. The chemical reduction of Group 1 metal cations M+ to M(0) is extremely challenging. Common tetraaryl borates demonstrate limited redox properties and are prone to decomposition upon oxidation. In this study, by employing simple yet versatile bipyridines as ligands, we synthesized a series of redox-active borate anions characterized by NMR and X-ray single-crystal diffraction. Notably, the borate anion can realize the reduction of Li+, generating elemental lithium metal and boron radical, thereby demonstrating its potent reducing ability. Furthermore, it can serve as a powerful two-electron-reducing reagent and be readily applied in various reductive homo-coupling reactions and Birch reduction of acridine. Additionally, this borate anion demonstrates its catalytic ability in the selective two-electron reduction of CO2 into CO.

In Situ Formed Tribofilms as Efficient Organic/Inorganic Hybrid Interlayers for Stabilizing Lithium Metal Anodes

The practical application of Li metal anodes (LMAs) is limited by uncontrolled dendrite growth and side reactions. Herein, we propose a new friction-induced strategy to produce high-performance thin Li anode (Li@CFO). By virtue of the in situ friction reaction between fluoropolymer grease and Li strips during rolling, a robust organic/inorganic hybrid interlayer (lithiophilic LiF/LiC6 framework hybridized -CF2-O-CF2- chains) was formed atop Li metal. The derived interface contributes to reversible Li plating/stripping behaviors by mitigating side reactions and decreasing the solvation degree at the interface. The Li@CFO||Li@CFO symmetrical cell exhibits a remarkable lifespan for 5,600 h (1.0 mA cm-2 and 1.0 mAh cm-2) and 1,350 cycles even at a harsh condition (18.0 mA cm-2 and 3.0 mAh cm-2). When paired with high-loading LiFePO4 cathodes, the full cell lasts over 450 cycles at 1C with a high-capacity retention of 99.9%. This work provides a new friction-induced strategy for producing high-performance thin LMAs.

Multicomponent Covalent Organic Framework Solid Electrolyte Allowing Effective Li-Ion Dissociation and Diffusion for All-Solid-State Batteries

Organic solid electrolytes compatible with all-solid-state Li metal batteries (LMBs) are essential to ensuring battery safety, high energy density, and long-term cycling performance. However, it remains a challenge to develop an approach to provide organic solid electrolytes with capabilities for the facile dissociation of strong Li-ion pairs and fast transport of ionic components. Herein, a diethylene glycol-modified pyridinium covalent organic framework (DEG-PMCOF) with a well-defined periodic structure is prepared as a multicomponent solid electrolyte with a cationic moiety of high polarity, an additional flexible ion-transporter, and an ordered ionic channel for all-solid-state LMBs. The DEG-containing pyridinium groups of DEG-PMCOF allow a lower dissociation energy of Li salts and a smaller energy barrier of Li-ion transport, leading to high ion conductivity (1.71 × 10-4 S cm-1) and a large Li-ion transfer number (0.61) at room temperature in the solid electrolyte. The DEG-PMCOF solid electrolyte exhibits a wide electrochemical stability window and effectively suppresses the formation of Li dendrites and dead Li in all-solid-state LMBs. Molecular dynamics and density functional theory simulations provide insights into the mechanisms for the enhanced Li-ion transport driven by the integrated diffusion process based on hopping motion, vehicle motion, and free diffusion of DEG-PMCOF. The all-solid-state LMB assembled with a DEG-PMCOF solid electrolyte displays a high specific capacity with a retention of 99% and an outstanding Coulombic efficiency of 99% at various C-rates during long-term cycling. This DEG-PMCOF approach can offer an effective route to design various solid-state Li batteries.

Engineering Functionalized 2D Metal-Organic Frameworks Nanosheets with Fast Li+ Conduction for Advanced Solid Li Batteries

Solid-state batteries can ensure high energy density and safety in lithium metal batteries, while polymer electrolytes are plagued by slow ion kinetics and low selective transport of Li+ . Metal-organic frameworks (MOFs) are proposed as emerging fillers for solid-state poly(ethylene oxide)(PEO) electrolytes, however, developing functionalized MOFs and understanding their roles on ion transfer has proven challenging. Herein, combining computational and experimental results, the functional group regulation in MOFs can effectively change surficial charge distribution and limit anion movement is revealed, providing a potential solution to these issues. Specifically, functionalized 2D MOF sheets are designed through molecular engineering to construct high-performance composite electrolytes, where the electron-donating effect of substituents in 2D-MOFs effectively limits the movement of ClO4 - and promotes mechanical properties and ion migration numbers (0.36 up to 0.64) of PEO. As a result, Li/Li cells with composite electrolyte exhibit superior cyclability for 1000 h at a current density of 0.2 mA cm-2 . Meanwhile, the solid LiFePO4 /Li battery delivers highly reversible capacities of 148.8 mAh g-1 after 200 cycles. These findings highlight a new approach for anion confinement through the use of functional group electronic effects, leading to enhanced ionic conductivity, and a feasible direction for high-performance solid-state batteries.

Fast Li+ Transport Polyurethane-Based Single-Ion Conducting Polymer Electrolyte with Sulfonamide Side chains in the Hard Segment for Lithium Metal Batteries

Single-ion conducting polymer electrolytes (SICPEs) are considered as one of the most promising candidates for achieving lithium metal batteries (LMBs). However, the application of traditional SICPEs is hindered by their low ionic conductivity and poor mechanical stability. Herein, a self-standing and flexible polyurethane-based single-ion conductor membrane was prepared via covalent tethering of the trifluoromethanesulfonamide anion to polyurethane, which was synthesized using a facile reaction of diisocyanates with poly(ethylene oxide) and 3,5-diaminobenzoic acid (or 3,5-dihydroxybenzoic acid). The polymer electrolyte exhibited excellent ionic conductivity, mechanical properties, lithium-ion transference number, thermal stability, and a broad electrochemical window because of the bulky anions and unique two-phase structures with lithium-ion nanochannels in the hard domains. Consequently, the plasticized electrolyte membrane showed exceptional stability and reliability in a Li||Li symmetric battery. The assembled LiFePO4||Li battery exhibited an outstanding capacity (∼180 mA h g-1), Coulombic efficiency (>96%), and capacity retention. This research provides a promising polymer electrolyte for high-performance LMBs.

High-Efficiency Lithium-Ion Transport in a Porous Coordination Chain-Based Hydrogen-Bonded Framework

Fast and selective Li⁺ transport in solid plays a key role for the development of high-performance solid-state electrolytes (SSEs) of lithium metal batteries. Porous compounds with tunable Li⁺ transport pathways are promising SSEs, but the comprehensive performances in terms of Li⁺ transport kinetics, electrochemical stability window, and interfacial compatibility are difficult to be achieved simultaneously. Herein, we report a porous coordination chain-based hydrogen-bonded framework (NKU-1000) containing arrayed electronegative sites for Li⁺ transport, exhibiting a superior Li⁺ conductivity of 1.13 × 10^-3 S cm^-1, a high Li⁺ transfer number of 0.87, and a wide electrochemical window of 5.0 V. The assembled solid-state battery with NKU-1000-based SSE shows a high discharge capacity with 94.4% retention after 500 cycles and can work over a wide temperature range without formation of lithium dendrites, which derives from the linear hopping sites that promote a uniformly high-rate Li⁺ flux and the flexible structure that can buffer the structural variation during Li⁺ transport.

Advanced Nonflammable Organic Electrolyte Promises Safer Li-Metal Batteries: From Solvation Structure Perspectives

Batteries with a Li-metal anode have recently attracted extensive attention from the battery communities owing to their high energy density. However, severe dendrite growth hinders their practical applications. More seriously, when Li dendrites pierce the separators and trigger short circuit in a highly flammable organic electrolyte, the results would be catastrophic. Although the issues of growth of Li dendrites have been almost addressed by various methods, the highly flammable nature of conventional organic liquid electrolytes is still a lingering fear facing high-energy-density Li-metal batteries given the possibility of thermal runaway of the high-voltage cathode. Recently, various kinds of nonflammable liquid- or solid-state electrolytes have shown great potential toward safer Li-metal batteries with minimal detrimental effect on the battery performance or even enhanced electrochemical performance. In this review, recent advances in developing nonflammable electrolyte for high-energy-density Li-metal batteries including high-concentration electrolyte, localized high-concentration electrolyte, fluorinated electrolyte, ionic liquid electrolyte, and polymer electrolyte are summarized. Then, the solvation structure of different kinds of nonflammable liquid and polymer electrolytes are analyzed to provide insight into the mechanism for dendrite suppression and fire extinguishing. Finally, guidelines for future design of nonflammable electrolyte for safer Li-metal batteries are provided.

Single-Ion Conducting Polymeric Protective Interlayer for Stable Solid Lithium-Metal Batteries

With many reported attempts on fabricating single-ion conducting polymer electrolytes, they still suffer from low ionic conductivity, narrow voltage window, and high cost. Herein, we report an unprecedented approach on improving the cationic transport number (t_Li⁺) of the polymer electrolyte, i.e., single-ion conducting polymeric protective interlayer (SIPPI), which is designed between the conventional polymer electrolyte (PVEC) and Li-metal electrode. Satisfied ionic conductivity (1 mS cm^-1, 30 °C), high t_Li⁺ (0.79), and wide-area voltage stability are realized by coupling the SIPPI with the PVEC electrolyte. Benefiting from this unique design, the Li symmetrical cell with the SIPPI shows stable cycling over 6000 h at 3 mA cm^-2, and the full cell with the SIPPI exhibits stable cycling performance with a capacity retention of 86% over 1000 cycles at 1 C and 25 °C. This incorporated SIPPI on the Li anode presents an alternative strategy for enabling high-energy density, long cycling lifetime, and safe and cost-effective solid-state batteries.

Opportunities of Flexible and Portable Electrochemical Devices for Energy Storage: Expanding the Spotlight onto Semi-solid/Solid Electrolytes

The ever-increasing demand for flexible and portable electronics has stimulated research and development in building advanced electrochemical energy devices which are lightweight, ultrathin, small in size, bendable, foldable, knittable, wearable, and/or stretchable. In such flexible and portable devices, semi-solid/solid electrolytes besides anodes and cathodes are the necessary components determining the energy/power performances. By serving as the ion transport channels, such semi-solid/solid electrolytes may be beneficial to resolving the issues of leakage, electrode corrosion, and metal electrode dendrite growth. In this paper, the fundamentals of semi-solid/solid electrolytes (e.g., chemical composition, ionic conductivity, electrochemical window, mechanical strength, thermal stability, and other attractive features), the electrode-electrolyte interfacial properties, and their relationships with the performance of various energy devices (e.g., supercapacitors, secondary ion batteries, metal-sulfur batteries, and metal-air batteries) are comprehensively reviewed in terms of materials synthesis and/or characterization, functional mechanisms, and device assembling for performance validation. The most recent advancements in improving the performance of electrochemical energy devices are summarized with focuses on analyzing the existing technical challenges (e.g., solid electrolyte interphase formation, metal electrode dendrite growth, polysulfide shuttle issue, electrolyte instability in half-open battery structure) and the strategies for overcoming these challenges through modification of semi-solid/solid electrolyte materials. Several possible directions for future research and development are proposed for going beyond existing technological bottlenecks and achieving desirable flexible and portable electrochemical energy devices to fulfill their practical applications. It is expected that this review may provide the readers with a comprehensive cross-technology understanding of the semi-solid/solid electrolytes for facilitating their current and future researches on the flexible and portable electrochemical energy devices.

Ion-Dipole Interaction Regulation Enables High-Performance Single-Ion Polymer Conductors for Solid-State Batteries

Solid polymer electrolytes with large ionic conductivity, high ionic transference number, and good interfacial compatibility with electrodes are highly desired for solid-state batteries. However, unwanted polarizations and side reactions occurring in traditional dual-ion polymer conductors hinder their practical applications. Here, single-ion polymer conductors (SIPCs) with exceptional selectivity for Li-ion conduction (Li-ion transference number up to 0.93), high room-temperature ionic conductivity of about 10^-4 S cm^-1 , and a wide electrochemical stability window (>4.5 V, vs Li/Li⁺ ) are prepared by precisely regulating the ion-dipole interactions between Li⁺ and carbonyl/cyano groups. The resulting SIPCs show an excellent electrochemical stability with Li metal during long-term cycling at room temperature and 60 °C. LiFePO₄ -based solid-state cells containing the SIPCs exhibit good rate and cycling performance in a wide temperature range from -20 to 90 °C. By the same way of ion-dipole interaction regulation, sodium- and potassium-based SIPCs with both high ionic conductivity and high cationic transference numbers are also prepared. The findings in this work provide guidance for the development of high-performance SIPCs and other metal-ion systems beyond Li⁺ .

Dendrite-Free and Long-Cycling Lithium Metal Battery Enabled by Ultrathin, 2D Shield-Defensive, and Single Lithium-Ion Conducting Polymeric Membrane

Polymeric membranes are considered as promising materials to realize safe and long-life lithium metal batteries (LMBs). However, they are usually based on soft 1D linear polymers and thus cannot effectively inhibit piercing of lithium dendrites at high current density. Herein, single lithium-ion conducting molecular brushes (GO-g-PSSLi) are successfully designed and fabricated with a new 2D "soft-hard-soft" hierarchical structure by grafting hairy lithium polystyrenesulfonate (PSSLi) chains on both sides of graphene oxide (GO) sheets. The ultrathin GO-g-PSSLi membrane is further constructed by evaporation-induced layer-by-layer self-assembly of GO-g-PSSLi molecular brushes. Unlike conventional soft 1D linear polymeric structure, the rigid 2D extended aromatic structure of intralayer GO backbones can bear the shield effect of preventing the dendrites possibly generated at high current density from piercing. More importantly, such a shield effect can be significantly strengthened by layer-by-layer stacking of 2D molecular brushes. On the other hand, the 3D interconnected interlayer channels and the soft single lithium-ion conducting PSSLi side-chains on the surface of channels provide rapid lithium-ion transportation pathways and homogenize lithium-ion flux. As a result, LMBs with GO-g-PSSLi membrane possess long-term reversible lithium plating/striping (6 months) at high current density.

A Ni(COD)2-free approach for the synthesis of high surface area porous aromatic frameworks

Porous aromatic frameworks (PAFs) are attractive materials for applications where high surface area and material stability govern performance. Most of the highest surface area PAFs are synthesized using poorly scalable and costly methods involving super-stoichiometric bis(1,5-cyclooctadiene)Nickel(0) (Ni(COD)₂). This communication describes a general approach for the synthesis of high surface area PAFs that does not use isolated Ni(COD)₂. The method is general to at least seven microporous polymers and can be conducted on gram scales without the use of an inert atmosphere glovebox. This work is expected to improve the synthetic accessibility of these materials.

Transference Number Reinforced-Based Gel Copolymer Electrolyte for Dendrite-Free Lithium Metal Batteries

The progress of electric vehicles is highly inhibited by the limited energy density and growth of dendrite Li in current power batteries. Breakthroughs and improvements in electrolyte chemistry are highlighted to directly address the above issues, namely, the development of electrolytes with a high lithium-ion transference number (tLi+), enabling one to effectively restrict the concentration polarization during repetitious cycling. Herein, we propose a novel ether-based copolymer-based gel polymer electrolyte (ECP-based GPE) by in situ copolymerization as an intriguing strategy to achieve a high tLi+ of ∼0.64. Molecular dynamics simulations and finite element method analyses illustrate the enhanced Li+ diffusion process (DLi+, ∼1.76 × 10-10 m2 s-1) in ECP-based GPE with a homogeneous electric potential accommodated around the lithium metal anode. Therefore, such a high-tLi+-based electrolyte renders a high reversibility of dendrite-free lithium plating/stripping at a high areal capacity (5 mA cm-2/5 mA h cm-2) in an Li||Li symmetric cell and facilitates superior cycling performances (over 1000 cycles) at a high rate (5 C) with a capacity retention of ∼91.1% in Li||LiFePO4 batteries, promoting the practical application of solid-state lithium metal batteries.

Bridging Li7La3Zr2O12 Nanofibers with Poly(ethylene oxide) by Coordination Bonds to Enhance the Cycling Stability of All-Solid-State Lithium Metal Batteries

A solid-state composite polymer electrolyte comprising Li₇La₃Zr₂O₁₂ nanofibers (LLZO NFs) as fillers has the advantages of flexibility, ease of processing, and being low cost, thus being considered to be a promising electrolyte material for use in the next generation of highly safe lithium metal batteries. However, poor compatibility of organic parts and inorganic materials leads to quick capacity decay after long-term charge/discharging running because of inorganic/organic interface deterioration and thus, the related ineffective lithium-ion (Li⁺) conduction. Herein, a "Boston ivy-style" method is proposed to prepare a solid ceramic/polymer hybrid electrolyte that exhibits a dense interface structure. After grafting on Dynasylan IMEO (DI), the modified LLZO NFs are used as ligands to bond with coordinatively unsaturated metal centers of Ca²⁺. Furthermore, these Ca²⁺ bridge the modified LLZO NFs with poly(ethylene oxide) (PEO) via the ether oxygen atoms they possess. The bridges built between the two phases, PEO and LLZO NFs, are effective to interface strengthening and guarantee rapid Li⁺ conduction even after 900 cycles. The PEO/LLZO NFs-DI-Ca²⁺/LiTFSI electrolyte shows a high Li⁺ transference number of 0.72 (60 °C). The Li||LiFePO₄ cell delivers excellent cycling stability (capacity retention of 70.8% after 900 cycles, 0.5 C) and rate performance. The bridge strategy is proved to be effective and probably a promotion to the application of ceramic polymer-based solid-state electrolytes.

Designing Versatile Polymers for Lithium-Ion Battery Applications: A Review

Solid-state electrolytes are a promising family of materials for the next generation of high-energy rechargeable lithium batteries. Polymer electrolytes (PEs) have been widely investigated due to their main advantages, which include easy processability, high safety, good mechanical flexibility, and low weight. This review presents recent scientific advances in the design of versatile polymer-based electrolytes and composite electrolytes, underlining the current limitations and remaining challenges while highlighting their technical accomplishments. The recent advances in PEs as a promising application in structural batteries are also emphasized.

Solid electrolytes for solid-state Li/Na–metal batteries: inorganic, composite and polymeric materials

This feature article presents the electrolyte synthetic approaches, design strategies, and merging materials that may address the critical issues of solid electrolytes for solid-state Li/Na–metal batteries.

Single-ion conducting polymer electrolytes as a key jigsaw piece for next-generation battery applications

As lithium-ion batteries have been the state-of-the-art electrochemical energy storage technology, the overwhelming demand for energy storage on a larger scale has triggered the development of next-generation battery technologies possessing high energy density, longer cycle lives, and enhanced safety. However, commercial liquid electrolytes have been plagued by safety issues due to their flammability and instability in contact with electrodes. Efforts have focused on developing such electrolytes by covalently immobilizing anionic groups onto a polymer backbone, which only allows Li+ cations to be mobile through the polymer matrix. Such ion-selective polymers provide many advantages over binary ionic conductors in battery operation, such as minimization of cell polarization and dendrite growth. In this review, the design, synthesis, fabrication, and class are reviewed to give insight into the physicochemical properties of single-ion conducting polymer electrolytes. The standard characterization method and remarkable electrochemical properties are further highlighted, and perspectives on current challenges and future directions are also discussed.

Eutectic Crystallization Activates Solid-State Zinc-Ion Conduction

Solid-state zinc (Zn) batteries offer a new candidate for emerging applications sensitive to volume, safety and cost. However, current solid polymeric or ceramic electrolyte structures remain poorly conductive for the divalent Zn²⁺ , especially at room temperature. Constructing a heterogeneous interface which allows Zn²⁺ percolation is a viable option, but this is rarely involved in multivalent systems. Herein, we construct a solid Zn²⁺ -ion conductor by inducing crystallization of tailored eutectic liquids formed by organic Zn salts and bipolar ligands. High-entropy eutectic-networks weaken the ion-association and form interfacial Zn²⁺ -percolated channels on the nucleator surfaces, resulting in a solid crystal with exceptional selectivity for Zn²⁺ transport (t Zn 2 + =0.64) and appreciable Zn²⁺ conductivity (σ Zn 2 + =3.78×10^-5  S cm^-1 at 30 °C, over 2 orders of magnitude higher than conventional polymers), and finally enabling practical ambient-temperature Zn/V₂ O₅ metal solid cells. This design principle leveraged by the eutectic solidification affords new insights on the multivalent solid electrochemistry suffering from slow ion migration.

Ionic Liquid Functionalized Gel Polymer Electrolytes for Stable Lithium Metal Batteries

Metallic lithium (Li) is regarded as the ideal anode material in lithium-ion batteries due to its low electrochemical potential, highest theoretical energy density and low density. There are, however, still significant challenges to be addressed such as Li-dendrite growth and low interfacial stability, which impede the practical application of Li metal anodes. In order to circumvent these shortcomings, herein, we present a gel polymer electrolyte containing imidazolium ionic liquid end groups with a perfluorinated alkyl chain (F-IL) to achieve both high ionic conductivity and Li ion transference number by fundamentally altering the solubility of salt within the gel electrolyte through Lewis-acidic segments in the polymer backbone. Moreover, the presence of F-IL moieties decreased the binding affinity of Li cation towards the glycol chains, enabling a rapid transfer of Li cation within the gel network. These structural features enabled the immobilization of anions on the ionic liquid segments to alleviate the space-charge effect while promoting stronger anion coordination and weaker cation coordination in the Lewis-acidic polymers. Accordingly, we realized a high Li ion conductivity (9.16×10-3  S cm-1 ) and high Li ion transference number of 0.69 simultaneously, along with a good electrochemical stability up to 4.55 V, while effectively suppressing Li dendrite growth. Moreover, the gel polymer electrolyte exhibited stable cycling performance of the Li|Li symmetric cell of 9 mAh cm-2 for more than 1800 hours and retained 86.7 % of the original capacity after 250 cycles for lithium-sulfur (Li-S) full cell.

In-Situ Intermolecular Interaction in Composite Polymer Electrolyte for Ultralong Life Quasi-Solid-State Lithium Metal Batteries

Solid-state lithium metal batteries built with composite polymer electrolytes using cubic garnets as active fillers are particularly attractive owing to their high energy density, easy manufacturing and inherent safety. However, the uncontrollable formation of intractable contaminant on garnet surface usually aggravates poor interfacial contact with polymer matrix and deteriorates Li⁺ pathways. Here we report a rational designed intermolecular interaction in composite electrolytes that utilizing contaminants as reaction initiator to generate Li⁺ conducting ether oligomers, which further emerge as molecular cross-linkers between inorganic fillers and polymer matrix, creating dense and homogeneous interfacial Li⁺ immigration channels in the composite electrolytes. The delicate design results in a remarkable ionic conductivity of 1.43×10^-3  S cm^-1 and an unprecedented 1000 cycles with 90 % capacity retention at room temperature is achieved for the assembled solid-state batteries.

Stamping Flexible Li Alloy Anodes

Li metal holds great promise to be the ultimate anode choice owing to its high specific capacity and low redox potential. However, processing Li metal into thin-film anode with high electrochemical performance and good safety to match commercial cathodes remains challenging. Herein, a new method is reported to prepare ultrathin, flexible, and high-performance Li-Sn alloy anodes with various shapes on a number of substrates by directly stamping a molten metal solution. The printed anode is as thin as 15 µm, corresponding to an areal capacity of ≈3 mAh cm^-2 that matches most commercial cathode materials. The incorporation of Sn provides the nucleation center for Li, thereby mitigating Li dendrites as well as decreasing the overpotential during Li stripping/plating (e.g., <10 mV at 0.25 mA cm^-2 ). As a proof-of-concept, a flexible Li-ion battery using the ultrathin Li-Sn alloy anode and a commercial NMC cathode demonstrates good electrochemical performance and reliable cell operation even after repetitive deformation. The approach can be extended to other metal/alloy anodes such as Na, K, and Mg. This study opens a new door toward the future development of high-performance ultrathin alloy-based anodes for next-generation batteries.

Cathode/gel polymer electrolyte integration design based on continuous composition and preparation technique for high performance lithium ion batteries

Gel polymer electrolytes (GPEs) combine the high ionic conductivity of liquid electrolytes and good safety assurance of solid electrolytes. However, the poor interfacial contact between electrode materials and electrolyte is still a big obstacle to the high performance of solid-state batteries. Herein, an integrated cathode/GPE based on continuous composition and preparation technic is obtained by simple UV curing. The improved interfacial contact between cathode and GPE helps to facilitate the fast ions transfer at the interface. Compared with cells assembled with separated cathode and GPE, the cells with integrated cathode-GPE showed much lower interfacial impedance, lower potential polarization and more stable cycling property. This work provided a low-cost natural material gelatin and a simple UV irradiation method to prepare an integrated cathode and gel polymer electrolyte for solid-state lithium batteries. The capacity retention of the cells assembled from integrated structure was 91.4% which was much higher than that of the non-integrated cells (80.9%) after 200 cycles.

Macromolecular Design of Lithium Conductive Polymer as Electrolyte for Solid-State Lithium Batteries

In the development of solid-state lithium batteries, solid polymer electrolyte (SPE) has drawn extensive concerns for its thermal and chemical stability, low density, and good processability. Especially SPE efficiently suppresses the formation of lithium dendrite and promotes battery safety. However, most of SPE is derived from the matrix with simple functional group, which suffers from low ionic conductivity, reduced mechanical properties after conductivity modification, bad electrochemical stability, and low lithium-ion transference number. Appling macromolecular design with multiple functional groups to polymer matrix is accepted as a strategy to solve the problems of SPE fundamentally. In this review, macromolecular design based on lithium conducting groups is summarized including copolymerization, network construction, and grafting. Meanwhile, the construction of single-ion conductor polymer is also focused herein. Moreover, synergistic effects between the designed matrix, lithium salt, and fillers are reviewed with the objective to further improve the performance of SPE. At last, future studies on macromolecular design are proposed in the development of SPE for solid-state batteries with high energy density and durability.