PVDF/Palygorskite Nanowire Composite Electrolyte for 4 V Rechargeable Lithium Batteries with High Energy Density

Super-Ionic Conductor Soft Filler Promotes Li+ Transport in Integrated Cathode-Electrolyte for Solid-State Battery at Room Temperature

Composite polymer solid electrolytes (CPEs), possessing good rigid flexible, are expected to be used in solid-state lithium-metal batteries. The integration of fillers into polymer matrices emerges as a dominant strategy to improve Li+ transport and form a Li+-conducting electrode-electrolyte interface. However, challenges arise as traditional fillers: 1) inorganic fillers, characterized by high interfacial energy, induce agglomeration; 2) organic fillers, with elevated crystallinity, impede intrinsic ionic conductivity, both severely hindering Li+ migration. Here, a concept of super-ionic conductor soft filler, utilizing a Li+ conductivity nanocellulose (Li-NC) as a model, is introduced which exhibits super-ionic conductivity. Li-NC anchors anions, and enhances Li+ transport speed, and assists in the integration of cathode-electrolyte electrodes for room temperature solid-state batteries. The tough dual-channel Li+ transport electrolyte (TDCT) with Li-NC and polyvinylidene fluoride (PVDF) demonstrates a high Li+ transfer number (0.79) due to the synergistic coordination mechanism in Li+ transport. Integrated electrodes' design enables stable performance in LiNi0.5Co0.2Mn0.3O2|Li cells, with 720 cycles at 0.5 C, and 88.8% capacity retention. Furthermore, the lifespan of Li|TDCT|Li cells over 4000 h and Li-rich Li1.2Ni0.13Co0.13Mn0.54O2|Li cells exhibits excellent performance, proving the practical application potential of soft filler for high energy density solid-state lithium-metal batteries at room temperature.

Improving the Ionic Conductivity and Anode Interface Compatibility of LLZO/PVDF Composite Polymer Electrolytes by Compositional Tuning

Utilizing aluminum-doped nano LLZO (Li6.28La3Zr2Al0.24O12) as the ceramic filler, we synthesized and optimized LLZO/PVDF/LiClO4 composite polymer electrolytes (CPEs) to achieve high ionic conductivity and good interfacial stability with metallic lithium. The research examines how the PVDF grade and the mass ratio of PVDF to LiClO4 affect the ionic conductivity, lithium metal compatibility, and overall performance of CPEs. The CPE using Kynar PVDF 741 and a PVDF-to-LiClO4 mass ratio of 2:1 emerged as superior, displaying a high ionic conductivity at room temperature (0.12 mS/cm), the lowest activation energy (0.247 eV), an extensive electrochemical stability window (approximately 4.9 V), and robust mechanical strength. In tests with lithium metal symmetric cells, the membrane facilitated over 1000 h of stable cycling at 0.1 mA cm-2 and 0.1 mAh cm-2. Furthermore, when integrated into full solid-state lithium-metal batteries with LiFePO4 cathodes, it sustained more than 80% capacity retention across 500 charge/discharge cycles at a rate of 0.5 C with constantly high Coulombic efficiencies above 99.8%, underscoring its exceptional durability and efficiency. This research provides a practical framework and benchmarks for developing LLZO/PVDF-based CPEs with high ionic conductivity and enhanced stability against lithium metals.

Solvation-Tailored PVDF-Based Solid-State Electrolyte for High-Voltage Lithium Metal Batteries

Poly(vinylidene fluoride) (PVDF)-based polymer electro-lytes are attracting increasing attention for high-voltage solid-state lithium metal batteries because of their high room temperature ionic conductivity, adequate mechanical strength and good thermal stability. However, the presence of highly reactive residual solvents, such as N, N-dimethylformamide (DMF), severely jeopardizes the long-term cycling stability. Herein, we propose a solvation-tailoring strategy to confine residual solvent molecules by introducing low-cost 3 Å zeolite molecular sieves as fillers. The strong interaction between DMF and the molecular sieve weakens the ability of DMF to participate in the solvation of Li+, leading to more anions being involved in solvation. Benefiting from the tailored anion-rich coordination environment, the interfacial side reactions with the lithium anode and high-voltage NCM811 cathode are effectively suppressed. As a result, the solid-state Li||Li symmetrical cells demonstrates ultra-stable cycling over 5100 h at 0.1 mA cm-2, and the Li||NCM811 full cells achieve excellent cycling stability for more than 1130 and 250 cycles under the charging cut-off voltages of 4.3 V and 4.5 V, respectively. Our work is an innovative exploration to address the negative effects of residual DMF in PVDF-based solid-state electrolytes and highlights the importance of modulating the solvation structures in solid-state polymer electrolytes.

PDOL-Based Solid Electrolyte Toward Practical Application: Opportunities and Challenges

Polymer solid-state lithium batteries (SSLB) are regarded as a promising energy storage technology to meet growing demand due to their high energy density and safety. Ion conductivity, interface stability and battery assembly process are still the main challenges to hurdle the commercialization of SSLB. As the main component of SSLB, poly(1,3-dioxolane) (PDOL)-based solid polymer electrolytes polymerized in-situ are becoming a promising candidate solid electrolyte, for their high ion conductivity at room temperature, good battery electrochemical performances, and simple assembly process. This review analyzes opportunities and challenges of PDOL electrolytes toward practical application for polymer SSLB. The focuses include exploring the polymerization mechanism of DOL, the performance of PDOL composite electrolytes, and the application of PDOL. Furthermore, we provide a perspective on future research directions that need to be emphasized for commercialization of PDOL-based electrolytes in SSLB. The exploration of these schemes facilitates a comprehensive and profound understanding of PDOL-based polymer electrolyte and provides new research ideas to boost them toward practical application in solid-state batteries.

Dielectric Filler-Induced Hybrid Interphase Enabling Robust Solid-State Li Metal Batteries at High Areal Capacity

The fillers in composite solid-state electrolyte are mainly responsible for the enhancement of the conduction of Li ions but barely regulate the formation of solid electrolyte interphase (SEI). Herein, a unique filler of dielectric NaNbO3 for the poly(vinylidene fluoride) (PVDF)-based polymer electrolyte, which is subjected to the exchange of Li+ and Na+ during cycling, is reported and the substituted Na+ is engaged in the construction of a fluorinated Li/Na hybrid SEI with high Young's modulus, facilitating the fast transport of Li+ at the interface at a high areal capacity and suppressing the Li dendrite growth. The dielectric NaNbO3 also induces the generation of high-dielectric β phase of PVDF to promote the dissociation of Li salt. The Li/Li symmetrical cell exhibits a long-term dendrite-free cycling over 600 h at a high areal capacity of 3 mA h cm-2 . The LiNi0.8 Mn0.1 Co0.1 O2 /Li solid-state cells with NaNbO3 stably cycle 2200 times at 2 C and that paired with high-loading cathode (10 mg cm-2 ) can stably cycle for 150 times and exhibit excellent performances at -20 °C. This work provides a novel design principle of fillers undertaking interfacial engineering in composite solid-state electrolytes for developing the safe and stable solid-state lithium metal battery.

Polyvinylidene Fluoride-Based Gel Polymer Electrolytes for Calcium Ion Conduction: A Study of the Influence of Salt Concentration and Drying Temperature on Coordination Environment and Ionic Conductivity

Calcium-ion batteries emerged as a potential sustainable alternative energy storage system; however, there remains the need to further develop electrolytes to improve their performance. We report a gel polymer electrolyte (GPE)-based on polyvinylidene fluoride (PVDF) for calcium ion conduction. The gel electrolyte was synthesized by combining a PVDF polymer host, Ca(TFSI)2 salt, and N-methyl-2-pyrrolidone (NMP) solvent. Using Fourier transform infrared spectroscopy, we analyze the effect of salt concentration and drying temperature on the degree of salt dissociation in the electrolyte. Our results show that the concentration of free cations in the electrolyte is primarily coordinated with NMP as well as PVDF, generating a suitable network for ion transport, i.e., a liquid electrolyte encompassed within a polymer matrix. We find that processing conditions such as drying temperature, which varies solvent content, play a critical role in developing polymer electrolytes that demonstrate optimal electrochemical performance. The GPEs are semicrystalline and stable up to 120 °C, which is critical for their use in applications such as in electric vehicles and renewable energy storage systems. The ionic conductivity of the GPEs exhibit Arrhenius-type behavior, and the total ionic conductivity at room temperature is suitable for applications, with values of 0.85 × 10-4 S/cm for 0.5 M and 3.56 × 10-4 S/cm for 1.0 M concentrations. The results indicate that the GPE exhibits high conductivity and good stability, making it a promising candidate for use in high-performance calcium ion batteries.

Plant Leaf-Inspired Separators with Hierarchical Structure and Exquisite Fluidic Channels for Dendrite-Free Lithium Metal Batteries

Lithium (Li) metal batteries are among the most promising devices for high energy storage applications but suffer from severe and irregular Li dendrite growth. Here, it is demonstrated that the issue can be well tackled by precisely designing the leaf-like membrane with hierarchical structure and exquisite fluidic channels. As a proof of concept, plant leaf-inspired membrane (PLIM) separators are prepared using natural attapulgite nanorods. The PLIM separators feature super-electrolyte-philicity, high thermal stability and high ion-selectivity. Thus, the separators can guide uniform and directed Li growth on the Li anode. The Li//PLIM//Li cell with limited Li anode shows high Coulombic efficiency and cycling stability over 1500 h with small overpotential and interface impedance. The Li//PLIM//S battery exhibits high initial capacity (1352 mAh g-1 ), cycling stability (0.019% capacity decay per cycle at 1 C over 500 cycles), rate performance (673 mAh g-1 at 4 C), and high operating temperature (65 °C). The separators can also effectively improve reversibility and cycling stability of the Li/Li cell and Li//LFP battery with carbonate-based electrolyte. As such, this work provides fresh insights into the design of bioinspired separators for dendrite-free metal batteries.

Solid-State Electrolytes in Lithium-Sulfur Batteries: Latest Progresses and Prospects

Solid-state lithium-sulfur batteries (SSLSBs) have attracted tremendous research interest due to their large theoretical energy density and high safety, which are highly important indicators for the development of next-generation energy storage devices. Particularly, safety and "shuttle effect" issues originating from volatile and flammable liquid organic electrolytes can be fully mitigated by switching to a solid-state configuration. However, their road to thecommercial application is still plagued with numerous challenges, most notably the intrinsic electrochemical instability of solid-state electrolytes (SSEs) materials and their interfacial compatibility with electrodes and electrolytes. In this review, a critical discussion on the key issues and problems of different types of SSEs as well as the corresponding optimization strategies are first highlighted. Then, the state-of-the-art preparation methods and properties of different kinds of SSE materials, and their manufacture, characterization and performance in SSLSBs are summarized in detail. Finally, a scientific outlook for the future development of SSEs and the avenue to commercial application of SSLSBs is also proposed.

Ionic Conduction in Polymer-Based Solid Electrolytes

Good safety, high interfacial compatibility, low cost, and facile processability make polymer-based solid electrolytes promising materials for next-generation batteries. Key issues related to polymer-based solid electrolytes, such as synthesis methods, ionic conductivity, and battery architecture, are investigated in past decades. However, mechanistic understanding of the ionic conduction is still lacking, which impedes the design and optimization of polymer-based solid electrolytes. In this review, the ionic conduction mechanisms and optimization strategies of polymer-based solid electrolytes, including solvent-free polymer electrolytes, composite polymer electrolytes, and quasi-solid/gel polymer electrolytes, are summarized and evaluated. Challenges and strategies for enhancing the ionic conductivity are elaborated, while the ion-pair dissociation, ion mobility, polymer relaxation, and interactions at polymer/filler interfaces are highlighted. This comprehensive review is especially pertinent for the targeted enhancement of the Li-ion conductivity of polymer-based solid electrolytes.

Rational Design of High-Performance PEO/Ceramic Composite Solid Electrolytes for Lithium Metal Batteries

Composite solid electrolytes (CSEs) with poly(ethylene oxide) (PEO) have become fairly prevalent for fabricating high-performance solid-state lithium metal batteries due to their high Li+ solvating capability, flexible processability and low cost. However, unsatisfactory room-temperature ionic conductivity, weak interfacial compatibility and uncontrollable Li dendrite growth seriously hinder their progress. Enormous efforts have been devoted to combining PEO with ceramics either as fillers or major matrix with the rational design of two-phase architecture, spatial distribution and content, which is anticipated to hold the key to increasing ionic conductivity and resolving interfacial compatibility within CSEs and between CSEs/electrodes. Unfortunately, a comprehensive review exclusively discussing the design, preparation and application of PEO/ceramic-based CSEs is largely lacking, in spite of tremendous reviews dealing with a broad spectrum of polymers and ceramics. Consequently, this review targets recent advances in PEO/ceramic-based CSEs, starting with a brief introduction, followed by their ionic conduction mechanism, preparation methods, and then an emphasis on resolving ionic conductivity and interfacial compatibility. Afterward, their applications in solid-state lithium metal batteries with transition metal oxides and sulfur cathodes are summarized. Finally, a summary and outlook on existing challenges and future research directions are proposed.

Insight into the Key Factors in High Li+ Transference Number Composite Electrolytes for Solid Lithium Batteries

Solid lithium batteries (SLBs) have received much attention due to their potential to achieve secondary batteries with high energy density and high safety. The solid electrolyte (SE) is believed to be the essential material for SLBs. Among the recent SEs, composite electrolytes have good interfacial compatibility and customizability, which have been broadly investigated as promising contenders for commercial SLBs. The high Li⁺ transference number (t Li + ${{_{{\rm Li}{^{+}}}}}$ ) of composite electrolytes is critically important concerning the power/energy density and cycling life of SLBs, however, which is often overlooked. This Review presents a current opinion on the key factors in high t Li + ${{_{{\rm Li}{^{+}}}}}$ composite electrolytes, including polymers, Li-salts, inorganic fillers, and additives. Various strategies concerning providing a continuous pathway for Li-ions and immobilizing anions via component interaction are discussed. This Review highlights the major obstacles hindering the development of high t Li + ${{_{{\rm Li}{^{+}}}}}$ composite electrolytes and proposes future research directions for developing composite electrolytes with high t Li + ${{_{{\rm Li}{^{+}}}}}$ .

Bacteria cellulose framework-supported solid composite polymer electrolytes for ambient-temperature lithium metal batteries

Composite polymer electrolyte (CPE) films with high room temperature ionic conductivity are urgently needed for the practical application of high-safety solid-state batteries (SSBs). Here, a flexible polymer-polymer CPE thin film reinforced by a three-dimensional (3D) bacterial cellulose (BC) framework derived from natural BC hydrogel was prepared via thein situphoto-polymerization method. The BC film was utilized as the supporting matrix to ensure high flexibility and mechanical strength. The BC-CPE attained a high room temperature ionic conductivity of 1.3 × 10^-4S cm^-1. The Li∣BC-CPE∣Li symmetric cell manifested stable cycles of more than 1200 h. The LCO∣BC-CPE∣Li full cell attained an initial discharge specific capacity of 128.7 mAh g^-1with 82.6% discharge capacity retention after 150 cycles at 0.2 C under room temperature. The proposed polymer-polymer CPE configuration represents a promising route for manufacturing environmental SSBs, especially since cellulose biomaterials are abundant in nature.

Design of advanced separators for high performance Li-S batteries using natural minerals with 1D to 3D microstructures

Lithium-sulfur (Li-S) batteries are of great interest due to their high energy density. However, polysulfides shuttle and low S loading severely impede their practical applications. Here, we report design of advanced separators for Li-S batteries using natural minerals with 1D to 3D microstructures. Four natural minerals with different microstructures including 1D halloysite nanotubes, 1D attapulgite nanorods, 2D Li⁺-montmorillonite (Mmt) nanosheets and 3D porous diatomite were used together with carbon black (CB) for preparation of the mineral/CB-Celgard separators. The Si-OH groups of the minerals act as Lewis acid sites, which could effectively absorb polysulfides by forming LiO and OS bonds with polysulfides. Among all the separators, the Mmt/CB-Celgard separator endowed the Li-S battery with the highest upper plateau discharge capacity (369 mA h g^-1), initial reversible capacity (1496 mA h g^-1 at 0.1 C), rate performance and cycling stability (666 mA h g^-1 after 500 cycles at 1.0 C with 0.046% capacity decay per cycle). The Mmt/CB-Celgard separator also enabled stable cycling of the Li-S battery with high S loading (8.3 mg cm^-2) cathode. This work will provide inspiration for future development of advanced separators for high-energy-density Li-S batteries.

Enhanced ionic conductivity and interface compatibility of PVDF-LLZTO composite solid electrolytes by interfacial maleic acid modification

Improving conductivity and optimizing interface contact are two primary targets to promote the development of solid-state electrolytes. Herein, maleic acid (MA) is introduced into normal PVDF-LLZTO(Li_6.75La₃Zr_1.75Ta_0.25O₁₂) based composite polymer-ceramic electrolytes (CPEs). Benefiting the self-polymerization of MA, a core-shell structure is spontaneously formed with LLZTO as core and MA nano-film as shell, the MA shell builds a bridge to link LLZTO and PVDF. In addition, carboxyl groups in MA provide extra channels for Li⁺ transmission. As a proof, the optimized 25MA-75PVDF-LLZTO CPEs demonstrates an enhanced conductivity as high as 1.15 × 10^-3 S cm^-1 at 30 °C, an extended electrochemical window up to < 5.0 V, a raised Li⁺ transfer number of 0.596, and an improved compatibility with Li metal anode. The as-prepared Li‖25MA-75PVDF-LLZTO CPEs‖LiFePO₄ full cell delivers an initial specific discharge capacity of 170.5 mAh g^-1 at 0.2C, a high rate capability up to 1.0C with 138 mAh g^-1 and an excellent long-term cycling stability of over 180cycles without capacity attenuation. The work provides a new strategy to optimize solid state lithium batteries by introducing unsaturated small organic molecules.

High-performance composite separators based on the synergy of vermiculite and laponite for lithium-ion batteries

The electrochemical performance and safe operation of the separator plays an important role in lithium-ion batteries. The introduction of inorganic nanoparticles into the separators with organic matter as the matrix effectively improves the thermal stability and wettability of the composite separators, but it also blocks some pores and adversely affects the electrochemical performance. Herein, vermiculite and laponite nanoparticles are introduced into a poly(vinylidene fluoride) matrix to prepare organic-inorganic composite separators for lithium-ion batteries and the synergistic effect of the two inorganic nanofillers is explored. By adding the same amount of the two nanoparticles into the polymer matrix, the prepared separator has the highest ionic conductivity (0.72 mS cm^-1) at room temperature and the lowest interfacial impedance (283 Ω). It has an initial discharge capacity of 161.2 mA h g^-1 at a rate of 0.5C, a coulombic efficiency of 99.5% after 100 cycles, and a high capacity retention rate of 98.4%, which shows excellent rate performance. The results show that the two clay nanoparticles exert their respective advantages and exhibit a synergistic enhancement effect on the battery performance, which inspires new ideas for the preparation of new organic-inorganic composite separators.

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.

Elastomeric electrolytes for high-energy solid-state lithium batteries

The use of lithium metal anodes in solid-state batteries has emerged as one of the most promising technologies for replacing conventional lithium-ion batteries^1,2. Solid-state electrolytes are a key enabling technology for the safe operation of lithium metal batteries as they suppress the uncontrolled growth of lithium dendrites. However, the mechanical properties and electrochemical performance of current solid-state electrolytes do not meet the requirements for practical applications of lithium metal batteries. Here we report a class of elastomeric solid-state electrolytes with a three-dimensional interconnected plastic crystal phase. The elastomeric electrolytes show a combination of mechanical robustness, high ionic conductivity, low interfacial resistance and high lithium-ion transference number. The in situ-formed elastomer electrolyte on copper foils accommodates volume changes for prolonged lithium plating and stripping processes with a Coulombic efficiency of 100.0 per cent. Moreover, the elastomer electrolytes enable stable operation of the full cells under constrained conditions of a limited lithium source, a thin electrolyte and a high-loading LiNi_0.83Mn_0.06Co_0.11O₂ cathode at a high voltage of 4.5 volts at ambient temperature, delivering a high specific energy exceeding 410 watt-hours per kilogram of electrode plus electrolyte. The elastomeric electrolyte system presents a powerful strategy for enabling stable operation of high-energy, solid-state lithium batteries.

Double-Layer Solid Composite Electrolytes Enabling Improved Room-Temperature Cycling Performance for High-Voltage Lithium Metal Batteries

The development of solid-state electrolytes (SSEs) for high energy density lithium metal batteries (LMBs) usually needs to take into account of the interfacial compatibility against lithium metal and the electrolyte stability suitable for a high-potential cathode. In this study, through a facile two-step coating process, novel double-layer solid composite electrolytes (SCEs) with Janus characteristics are customized for the high-voltage LMBs with improved room-temperature cycling performance. Among which, high-voltage resistant poly(vinylidene fluoride) (PVDF) is adopted here for the construction of an electrolyte layer facing the cathode, while the other layer against the lithium anode is composed of the polymer matrix of poly(ethylene oxide) (PEO) blended with PVDF to obtain a lithium metal-friendly interface. With the further incorporation of Laponite clay, the PVDF/(PEO+PVDF)-L SCEs not only exhibit improved mechanical properties, but also achieve a highly increased ionic conductivity (5.2 × 10^-4 S cm^-1) and lithium ion migration number (0.471) at room temperature. The assembled NCM523|PVDF/(PEO+PVDF)-L SCEs|Li cells thus are able to deliver the initial discharge capacity of 153.9 mAh g^-1 with 80.8% capacity retention after 200 cycles at 0.3 C. Such easily manufactured double-layer SCEs capable of operating steadily at room temperature provide a competitive electrolyte option for high-voltage solid-state LMBs.

Ionic Covalent Organic Frameworks for Energy Devices

Covalent organic frameworks (COFs) are a class of porous crystalline materials whose facile preparation, functionality, and modularity have led to their becoming powerful platforms for the development of molecular devices in many fields of (bio)engineering, such as energy storage, environmental remediation, drug delivery, and catalysis. In particular, ionic COFs (iCOFs) are highly useful for constructing energy devices, as their ionic functional groups can transport ions efficiently, and the nonlabile and highly ordered all-covalent pore structures of their backbones provide ideal pathways for long-term ionic transport under harsh electrochemical conditions. Here, current research progress on the use of iCOFs for energy devices, specifically lithium-based batteries and fuel cells, is reviewed in terms of iCOF backbone-design strategies, synthetic approaches, properties, engineering techniques, and applications. iCOFs are categorized as anionic COFs or cationic COFs, and how each of these types of iCOFs transport lithium ions, protons, or hydroxides is illustrated. Finally, the current challenges to and future opportunities for the utilization of iCOFs in energy devices are described. This review will therefore serve as a useful reference on state-of-the-art iCOF design and application strategies focusing on energy devices.

Stable Interface Chemistry and Multiple Ion Transport of Composite Electrolyte Contribute to Ultra-long Cycling Solid-State LiNi0.8 Co0.1 Mn0.1 O2 /Lithium Metal Batteries

Severe interfacial side reactions of polymer electrolyte with LiNi_0.8 Co_0.1 Mn_0.1 O₂ (NCM811) cathode and Li metal anode restrict the cycling performance of solid-state NCM811/Li batteries. Herein, we propose a chemically stable ceramic-polymer-anchored solvent composite electrolyte with high ionic conductivity of 6.0×10^-4  S cm^-1 , which enables the solid-state NCM811/Li batteries to cycle 1500 times. The Li_1.4 Al_0.4 Ti_1.6 (PO₄ )₃ nanowires (LNs) can tightly anchor the essential N, N-dimethylformamide (DMF) in poly(vinylidene fluoride) (PVDF), greatly enhancing its electrochemical stability and suppressing the side reactions. We identify the ceramic-polymer-liquid multiple ion transport mechanism of the LNs-PVDF-DMF composite electrolyte by tracking the ⁶ Li and ⁷ Li substitution behavior via solid-state NMR. The stable interface chemistry and efficient ion transport of LNs-PVDF-DMF contribute to superior performances of the solid-state batteries at wide temperature range of -20-60 °C.

High Energy Density Solid State Lithium Metal Batteries Enabled by Sub-5 µm Solid Polymer Electrolytes

Solid-state batteries (SSBs) are considered as the most promising next-generation high-energy-density energy storage devices due to their ability in addressing the safety concerns from organic electrolytes and enabling energy dense lithium anodes. To ensure the high energy density of SSBs, solid-state electrolytes (SSEs) are required to be thin and light-weight, and simultaneously offer a wide electrochemical window to pair with high-voltage cathodes. However, the decrease of SSE thickness and delicate structure may increase the cell safety risks, which is detrimental for the practical application of SSBs. Herein, to demonstrate a high-energy-density SSB with sufficient safety insurance, an ultrathin (4.2 µm) bilayer SSE with porous ceramic scaffold and double-layer Li⁺ -conducting polymer, is proposed. The fire-resistant and stiff ceramic scaffold improves the safety capability and mechanical strength of the composite SSE, and the bilayer polymer structure enhances the compatibility of Li metal anode and high-voltage cathodes. The 3D ceramic facilitates Li-ion conduction and regulates Li deposition. Thus, high energy density of 506 Wh kg^-1 and 1514 Wh L^-1 is achieved based on LiNi_0.8 Co_0.1 Mn_0.1 O₂ (NCM811) cathodes with a low N/P ratio and long lifespan over 3000 h. High-energy-density anode-free cells are further demonstrated.

2D Silicate Materials for Composite Polymer Electrolytes

Two-dimensional (2D) silicate materials have become one of the promising candidates for constructing composite polymer electrolytes due to their advantages of low cost, high stability, good mechanical property, high ionic conductivity and potential to inhibit the growth of lithium dendrites. However, the application of 2D silicate materials in composite polymer electrolytes (CPEs) is still at the infancy stage and facing a lot of challenges. In this minireview, we summarize the structures and properties of 2D silicate materials that have been applied in CPEs, the processing methods of composite electrolytes based on 2D silicates, and the recent process of 2D silicate materials in CPEs. We hope this review could present a general overview of the 2D silicates for CPEs and promote the further study for potential applications.

Engineering Two-Dimensional Metal-Organic Framework on Molecular Basis for Fast Li+ Conduction

Metal-organic frameworks (MOFs) have been proposed as emerging fillers for composite polymer electrolytes (CPEs). However, MOF particles are usually served as passive fillers that yield limited ionic conductivity improvement. Building continuous MOF reinforcements and exploiting their active roles remain challenging. Here we demonstrate the feasibility of engineering fast Li⁺ conduction within MOF on molecule conception. Two-dimensional Cu(BDC) MOF is selected as an active filler due to its sufficient accessible open metal sites for perchlorate anion anchoring to release free Li⁺, verified by theoretical calculations and measurements. A novel Cu(BDC)-scaffold-reinforced CPE is developed via in situ growth of MOF, which provides fast Li⁺ channels inside MOF and continuous Li⁺ paths along the MOF/polymer interface for high Li⁺ conductivity (ambient 0.24 mS cm^-1) and enables high mechanical strength. Stable cycling is achieved in solid-state Li-NCM811 full cell using the MOF-reinforced CPE. This molecule-basis Li⁺ conduction strategy brings new ideas for designing advanced CPEs.

Quasi-Ionic Liquid Enabling Single-Phase Poly(vinylidene fluoride)-Based Polymer Electrolytes for Solid-State LiNi0.6 Co0.2 Mn0.2 O2 ||Li Batteries with Rigid-Flexible Coupling Interphase

Poly(vinylidene fluoride)-based polymer electrolytes are being intensely investigated for solid-state lithium metal batteries. However, phase separation and porous structures are still pronounced issues in traditional preparing procedure. Herein, a bottom-to-up strategy is employed to design single-phase and densified polymer electrolytes via incorporating quasi-ionic liquid with poly(vinylidene fluoride-co-hexafluoropropylene). Due to strong ion/dipole-dipole interaction, the optimized polymer electrolyte delivers high room-temperature ionic conductivity of 1.55 × 10^-3 S cm^-1 , superior thermal and oxidation stability of 4.97 V, excellent stretchability of over 1500% and toughness of 43 MJ cm^-3 as well as desirable self-extinguishing ability. Furthermore, the superb compatibility toward Li anode enables over 3000 h cycling of Li plating/stripping and ≈98% Coulombic efficiency in Li||Cu test at 0.1 mA cm^-2 . In particular, lithium metal battery Li||LiNi_0.6 Co_0.2 Mn_0.2 O₂ exhibits a room-temperature discharge retention rate of 96% after 500 cycles under a rate of 0.1 C, which is associated with the rigid-flexible coupling electrodes/electrolytes interphase. This investigation demonstrates the potential application of quasi-ionic liquid/polymer electrolytes in safe lithium metal batteries.

Revealing the Superiority of Fast Ion Conductor in Composite Electrolyte for Dendrite-Free Lithium-Metal Batteries

Composite electrolytes composed of a nanoceramic and polymer have been widely studied because of their high ionic conductivity, good Li-ion transference number, and excellent machinability, whereas the intrinsic reason for the improvement of performance is ambiguous. Herein, we have designed a functional polymer skeleton with different types of nanofiller to reveal the superiority of fast ion conductors in composite electrolyte. Three types of ceramics with different dielectric constants and Li-ion transfer ability were selected to prepare composite electrolytes, the composition, structure, and electrochemical performances of which were systematically investigated. It was found that the addition of fast ion conductive ceramics could provide a high Li-ion transference ability and decreased diffusion barrier because the additional pathways existed in the ceramic, which are revealed by experiment and density functional theory calculations. Benefiting from the superiority of fast ion conductor, Li-metal batteries with this advanced composite electrolyte exhibit an impressive cycling stability and enable a dendrite-free Li surface after cycling. Our work enriches the understanding of the function of fast ion conductors in composite electrolyte and guides the design for other high-performance composite electrolytes in rechargeable solid batteries.

Nanophase-Separated, Elastic Epoxy Composite Thin Film as an Electrolyte for Stable Lithium Metal Batteries

The design of solid polymer electrolytes (SPE) with high ionic conductivity and excellent mechanical properties is challenging because these two properties are often conflicting. To achieve both, a reaction-controlled strategy is proposed based on the nanophase separation of an ionic transport pathway and a supporting matrix to balance ionic mobility and mechanical properties. Specifically, an elastic epoxy polymer electrolyte (eEPE), synthesized via two-step polymerization, combines outstanding mechanical strength (toughness of 3.4 MJ m^-3) and high ionic conductivity (3.5 × 10^-4 S cm^-1 at 25 °C). The nanostructured eEPE is both tough and flexible, therefore promotes uniform deposition of Li even under a high current density (2 mA cm^-2 and 2 mAh cm^-2). Importantly, eEPE composite films greatly improve the safety performance of the LiFePO₄/Li pouch cells: safe operations are achieved under several abusive conditions. This work highlights an alternative route for high-safety solid-state lithium metal batteries of the next generation.

Homogenously dispersed ultrasmall niobium(V) oxide nanoparticles enabling improved ionic conductivity and interfacial compatibility of composite polymer electrolyte

Composite polymer electrolytes (CPEs) decorated with ceramic fillers have emerged as appealing structures that exhibit coalesced merits of both inorganic and polymer solid electrolytes, but are currently challenged by the particle agglomeration that weakens ionic conductivity and electrochemical performances. Herein, a facile solvothermal method is proposed to fabricate the ultrasmall niobium(V) oxide (Nb₂O₅) nanoparticle of average size being less than 3 nm, enabling the composite polymer electrolyte with homogenous dispersity (nano-CPE). Owning to the superior dispersity of ultrasmall Nb₂O₅ nanoparticles, the polymer chains can be effectively disordered to enhance the local segmental motion through the physical interruption. Moreover, strong Lewis acid-based interactions between Nb₂O₅ nanoparticles and lithium salts are formed, resulting in accelerating the dissociation of lithium salt and releasing more free charge carriers. Therefore, the 3D connected Li⁺ fast pathways along the amorphous region between the Nb₂O₅ nanoparticles and polymer chains are constructed, ensuring the improved ionic conductivity. In addition, the homogenous Li deposition can also be simultaneously achieved through the intimate interfacial contact, which can efficiently suppress the growth of lithium dendrite in the metal anode. The fabricated nano-CPE presents a high ionic conductivity of 6.6 × 10^-5 S/cm at room temperature and wide anti-oxidative potential of 5.1 V. The lithium symmetric battery using nano-CPE delivers a decent lithium plating/stripping performance for 200 h at 0.5 mA/cm². The solid-sate LiFePO₄ battery achieves long stable cycling performances (151mAh/g and 140 mAh/g after 230 cycles at 0.5C and 1.0C, respectively). This work may offer a facile and efficient synthesized method of highly dispersed ultrasmall nanoparticles for advancing the CPE with improved ionic conductivity, interfacial contact and cell performances.

Gel-polymer electrolytes based on polyurethane ionomers for lithium power sources

Polyurethanes based on the aminoethers of ortho-phosphoric acid and polyisocyanates of an aliphatic nature were studied as a substrate for the preparation of new polymer electrolyte. The conductivity of polyurethane ionomer samples obtained using the optimal amount of aliphatic polyisocyanates and after keeping them in a 1 M LiBF₄ solution in γ-butyrolactone reaches 0.62 mS cm⁻¹. It has been established that the transport of positively charged ions through the polymer matrix is due to the formation of clusters of phosphate ions and their association into the conducting channels. The introduction of carboxylate ions into the conducting channels by modifying the aminoethers of ortho-phosphoric acid with phthalic anhydride leads to an increase in their size and rise in the mobility of cations. As a result, the conductivity of polyurethane gel electrolytes increased to 2.1 mS cm⁻¹.

Polyurethanes based on the aminoethers of ortho-phosphoric acid and polyisocyanates of an aliphatic nature were studied as a substrate for the preparation of a new polymer electrolyte.

The effect of thermal treatment on ac/dc conductivity and current fluctuations of PVDF/NMP/[EMIM][TFSI] solid polymer electrolyte

The experimental study deals with the investigation of the effect of diverse crystallinity of imidazolium ionic-liquid-based SPE on conductivity and current fluctuations. The experimental study was carried out on samples consisting of [EMIM][TFSI] as ionic liquid, PVDF as a polymer matrix and NMP as a solvent. After the deposition, the particular sample was kept at an appropriate temperature for a specific time in order to achieve different crystalline forms of the polymer in the solvent, since the solvent evaporation rate controls crystallization. The ac/dc conductivities of SPEs were investigated across a range of temperatures using broadband dielectric spectroscopy in terms of electrical conductivity. In SPE samples of the higher solvent evaporation rate, the real parts of conductivity spectra exhibit a sharper transition during sample cooling and an increase of overall conductivity, which is implied by a growing fraction of the amorphous phase in the polymer matrix in which the ionic liquid is immobilized. The conductivity master curves illustrate that the changing of SPEs morphology is reflected in the low frequency regions governed by the electrode polarization effect. The dc conductivity of SPEs exhibits Vogel–Fulcher–Tammann temperature dependence and increases with the intensity of thermal treatment. Spectral densities of current fluctuations showed that flicker noise, thermal noise and shot noise seems to be major noise sources in all samples. The increase of electrolyte conductivity causes a decrease in bulk resistance and partially a decrease in charge transfer resistance, while also resulting in an increase in shot noise. However, the change of electrode material results in a more significant change of spectral density of current fluctuations than the modification of the preparation condition of the solid polymer electrolyte. Thus, the contact noise is considered to contribute to overall current fluctuations across the samples.

3D Coral-like LLZO/PVDF Composite Electrolytes with Enhanced Ionic Conductivity and Mechanical Flexibility for Solid-State Lithium Batteries

Composite polymer electrolytes (CPEs) are very promising for high-energy lithium-metal batteries as they combine the advantages of polymeric and ceramic electrolytes. The dimensions and morphologies of active ceramic fillers play critical roles in determining the electrochemical and mechanical performances of CPEs. Herein, a coral-like LLZO (Li_6.4La₃Zr₂Al_0.2O₁₂) is designed and used as a 3D active nanofiller in a poly(vinylidene difluoride) polymer matrix. Building 3D interconnected frameworks endows the as-made CPE membranes with an enhanced ionic conductivity (1.51 × 10^-4 S cm^-1) at room temperature and an enlarged tensile strength up to 5.9 MPa. As a consequence, the flexible 3D-architectured CPE enables a steady lithium plating/stripping cycling over 200 h without a short circuit. Moreover, the assembled solid-state Li|LiFePO₄ cells using the electrolyte exhibit decent cycling performance (95.2% capacity retention after 200 cycles at 1 C) and excellent rate capability (120 mA h g^-1 at 3 C). These results demonstrate the superiority of 3D interconnected garnet frameworks in developing CPEs with excellent electrochemical and mechanical properties.

A separator based on natural illite/smectite clay for highly stable lithium-sulfur batteries

In spite of high theoretical specific capacity and specific energy of lithium-sulfur (Li-S) batteries, the poor cycle stability caused by polysulfides shuttle severely hinders their real-world applications. Here, a natural clay mineral (illite/smectite, ISC) and carbon black (C) coated Celgard^@2400 (ISC/C@Celgard) separator is reported. The separator shows super-electrolyte-philicity and good mechanical stability. The low-cost and eco-friendly ISC with abundant -OH groups can quickly trap a lot of polysulfides by Li-O and Li-S bonding with polysulfides. The ISC/C layer with uniform nanopores can also inhibit polysulfides shuttle by physical shield. Moreover, good electrical conductivity of the ISC/C layer can reactivate the adsorbed polysulfides and thus enhance S utilization. So, the separator endows the Li-S battery with very high initial reversible capacity (1322 mA h g^-1) at 0.1 C and excellent cycle stability with low capacity decay rate (0.054% per cycle) during 500 cycles at 1.0 C. Furthermore, a very high areal capacity (5.9 mAh cm^-2) is achieved for the battery composed of the separator and the self-supporting high S loading (8.9 mg cm^-2) CNT/S cathode at 0.32 mA cm^-2. This study opens the possibility of developing advanced separators using natural clay minerals for highly stable Li-S batteries.

Gelation, Liquid Crystalline Behavior, and Ionic Conductivity of Nanocomposite Ionogel Electrolytes Based On Attapulgite Nanorods

Anisotropic nanoparticles and their dispersions have attracted much attention because of their distinguished characteristics and promising applications. In this study, the novel liquid crystalline nanocomposite ionogel electrolyte materials based on anisotropic nanoparticles of attapulgite (ATP) have been prepared. The gelation, liquid crystalline (LC) behavior, thermal stability, and ionic conductivity were systematically investigated. Rheological, polarized optical microscopy (POM), and small-angle X-ray scattering (SAXS) measurements demonstrated that these liquid crystalline ionogels showed a two-step mechanism consisting of gelation and subsequent reorganization of the gel. Interestingly, the obtained ionogel electrolytes were very stable and LC gel structures were not destroyed even though the temperature was as high as 200 °C. Furthermore, these ionogels possessed outstanding thermal stability and the decomposition temperature exceeded 400 °C. Remarkably, the LC nanocomposite ionogel electrolytes exhibited high room temperature ionic conductivity and the value still exceeded 1.0 × 10^-3 S/cm even when the ATP concentration up to 30 wt %. These novel findings are very useful for the fabrication of high temperature resistant electrochemical devices and liquid crystalline nanocomposite materials.

Progress on Lithium Dendrite Suppression Strategies from the Interior to Exterior by Hierarchical Structure Designs

Lithium (Li) metal is promising for high energy density batteries due to its low electrochemical potential (-3.04 V) and high specific capacity (3860 mAh g^-1 ). However, the safety issues impede the commercialization of Li anode batteries. In this work, research of hierarchical structure designs for Li anodes to suppress Li dendrite growth and alleviate volume expansion from the interior (by the 3D current collector and host matrix) to the exterior (by the artificial solid electrolyte interphase (SEI), protective layer, separator, and solid state electrolyte) is concluded. The basic principles for achieving Li dendrite and volume expansion free Li anode are summarized. Following these principles, 3D porous current collector and host matrix are designed to suppress the Li dendrite growth from the interior. Second, artificial SEI, the protective layer, and separator as well as solid-state electrolyte are constructed to regulate the distribution of current and control the Li nucleation and deposition homogeneously for suppressing the Li dendrite growth from exterior of Li anode. Ultimately, this work puts forward that it is significant to combine the Li dendrite suppression strategies from the interior to exterior by 3D hierarchical structure designs and Li metal modification to achieve excellent cycling and safety performance of Li metal batteries.

Organic-Organic Composite Electrolyte Enables Ultralong Cycle Life in Solid-State Lithium Metal Batteries

Ionic conducting polymer electrolytes for solid-state lithium-ion batteries have attracted ever-increasing attention because of their decent ionic conductivity, flexibility, no liquid leakage, and good processability. Poly(vinylidene fluoride) (PVDF)-based polymer electrolytes have recently stood out among the polymer electrolytes due to their high room temperature ionic conductivity. However, the interface between PVDF-based polymer electrolytes and lithium metal decays over time until the batteries break down. Here, we introduce a small amount of poly(acrylic acid) (PAA) into a PVDF-based polymer electrolyte and synthesize an organic-organic composite electrolyte that alleviates the interfacial reaction with lithium metal, which shows great superiority over other modification methods such as coating. The cycle life of lithium symmetric cells is prolonged from 130 to 850 h at 0.44 mA cm^-2 due to the effective suppression of interfacial reaction. The much more stable interface also enables excellent cycle performance in a solid-state LiCoO₂||Li cell at 30 °C with a capacity decay of 0.03% per cycle for 1000 cycles, which is much lower than that of a cell without blending PAA (0.13% per cycle for only 450 cycles). The results would shed light on the applications of PVDF-based polymer electrolytes in solid-state lithium metal batteries.