High-Voltage and Wide-Temperature Lithium Metal Batteries Enabled by Ultrathin MOF-Derived Solid Polymer Electrolytes with Modulated Ion Transport

Mechanisms of the Accelerated Li+ Conduction in MOF-Based Solid-State Polymer Electrolytes for All-Solid-State Lithium Metal Batteries

Solid polymer electrolytes (SPEs) for lithium metal batteries have garnered considerable interests owing to their low cost, flexibility, lightweight, and favorable interfacial compatibility with battery electrodes. Their soft mechanical nature compared to solid inorganic electrolytes give them a large advantage to be used in low pressure solid-state lithium metal batteries, which can avoid the cost and weight of the pressure cages. However, the application of SPEs is hindered by their relatively low ionic conductivity. In addressing this limitation, enormous efforts are devoted to the experimental investigation and theoretical calculations/simulation of new polymer classes. Recently, metal-organic frameworks (MOFs) have been shown to be effective in enhancing ion transport in SPEs. However, the mechanisms in enhancing Li+ conductivity have rarely been systematically and comprehensively analyzed. Therefore, this review provides an in-depth summary of the mechanisms of MOF-enhanced Li+ transport in MOF-based solid polymer electrolytes (MSPEs) in terms of polymer, MOF, MOF/polymer interface, and solid electrolyte interface aspects, respectively. Moreover, the understanding of Li+ conduction mechanisms through employing advanced characterization tools, theoretical calculations, and simulations are also reviewed in this review. Finally, the main challenges in developing MSPEs are deeply analyzed and the corresponding future research directions are also proposed.

5.1 µm Ion-Regulated Rigid Quasi-Solid Electrolyte Constructed by Bridging Fast Li-Ion Transfer Channels for Lithium Metal Batteries

An ultra-thin quasi-solid electrolyte (QSE) with dendrite-inhibiting properties is a requirement for achieving high energy density quasi-solid lithium metal batteries (LMBs). Here, a 5.1 µm rigid QSE layer is directly designed on the cathode, in which Kevlar (poly(p-phenylene terephthalate)) nanofibers (KANFs) with negatively charged groups bridging metal-organic framework (MOF) particles are served as a rigid skeleton, and non-flammable deep eutectic solvent is selected to be encapsulated into the MOF channels, combined with in situ polymerization to complete safe electrolyte system with high rigidness and stability. The QSE with constructed topological network demonstrates high rigidity (5.4 GPa), high ionic conductivity (0.73 mS cm-1 at room temperature), good ion-regulated properties, and improved structural stability, contributing to homogenized Li-ion flux, excellent dendrite suppression, and prolonged cyclic performance for LMB. Additionally, ion regulation influences the Li deposition behavior, exhibiting a uniform morphology on the Li-metal surface after cycling. According to density-functional theory, KANFs bridging MOFs as hosts play a vital function in the free-state and fast diffusion dynamics of Li-ions. This work provides an effective strategy for constructing ultrathin robust electrolytes with a novel ionic conduction mode.

Strategies to improve the ionic conductivity of quasi-solid-state electrolytes based on metal-organic frameworks

The low ionic conductivity of quasi-solid-state electrolytes (QSSEs) at ambient temperature is a barrier to the development of solid-state batteries (SSBs). Conversely, metal-organic frameworks (MOFs) with porous structure and metal sites show great potential for the fabrication of QSSEs. Numerous studies have proven that the structure and functional groups of MOFs could significantly impact the ionic conductivity of QSSEs based on MOFs (MOFs-QSSEs). This review introduces the transport mechanism of lithium ions in various MOFs-QSSEs, and then analyses how to construct an effective and consistent lithium ions pathway from the perspective of MOFs modification. It is shown that the ion conductivity could be enhanced by modifying the morphology and functional groups, as well as applying amorphous MOFs. Lastly, some issues and future perspectives for MOFs-QSSEs are examined. The primary objective of this review is to enhance the comprehension of the mechanisms and performance optimization methods of MOFs-QSSEs. Consequently, this would guide the design and synthesis of QSSEs with high ionic conductivity, and ultimately enhance the performance of commercial SSBs.

Unveiling Confinement Engineering for Achieving High-Performance Rechargeable Batteries

The confinement effect, restricting materials within nano/sub-nano spaces, has emerged as an innovative approach for fundamental research in diverse application fields, including chemical engineering, membrane separation, and catalysis. This confinement principle recently presents fresh perspectives on addressing critical challenges in rechargeable batteries. Within spatial confinement, novel microstructures and physiochemical properties have been raised to promote the battery performance. Nevertheless, few clear definitions and specific reviews are available to offer a comprehensive understanding and guide for utilizing the confinement effect in batteries. This review aims to fill this gap by primarily summarizing the categorization of confinement effects across various scales and dimensions within battery systems. Subsequently, the strategic design of confinement environments is proposed to address existing challenges in rechargeable batteries. These solutions involve the manipulation of the physicochemical properties of electrolytes, the regulation of electrochemical activity, and stability of electrodes, and insights into ion transfer mechanisms. Furthermore, specific perspectives are provided to deepen the foundational understanding of the confinement effect for achieving high-performance rechargeable batteries. Overall, this review emphasizes the transformative potential of confinement effects in tailoring the microstructure and physiochemical properties of electrode materials, highlighting their crucial role in designing novel energy storage devices.

Recent progress and perspectives on metal-organic frameworks as solid-state electrolytes for lithium batteries

Solid-state electrolytes (SSEs) are the key materials in the new generation of all-solid-state lithium ion/metal batteries. Metal-organic frameworks (MOFs) are ideal materials for developing solid electrolytes because of their structural diversity and porous properties. However, there are several significant issues and obstacles involved, such as lower ion conductivity, a smaller ion transport number, a narrower electrochemical stability window and poor interface contact. In this review, a comprehensive analysis and summary of the unique ion-conducting behavior of MOF-based electrolytes in rechargeable batteries are presented, and the different design principles of MOF-based SSEs are classified and emphasized. Accordingly, four design principles for achieving these MOF-based SSEs are presented and the influence of SSEs combined with MOFs on the electrochemical performance of the batteries is described. Finally, the challenges in the application of MOF materials in lithium ion/metal batteries are explored, and directions for future research on MOF-based electrolytes are proposed. This review will deepen the understanding of MOF-based electrolytes and promote the development of high-performance solid-state lithium ion/metal batteries. This review not only provides theoretical guidance for research on new MOF-based SSE systems, but also contributes to further development of MOFs applied to rechargeable batteries.

Electrolytes Design for Extending the Temperature Adaptability of Lithium-Ion Batteries: from Fundamentals to Strategies

With the continuously growing demand for wide-range applications, lithium-ion batteries (LIBs) are increasingly required to work under conditions that deviate from room temperature (RT). However, commercial electrolytes exhibit low thermal stability at high temperatures (HT) and poor dynamic properties at low temperatures (LT), hindering the operation of LIBs under extreme conditions. The bottleneck restricting the practical applications of LIBs has promoted researchers to pay more attention to developing a series of innovative electrolytes. This review primarily covers the design of electrolytes for LIBs from a temperature adaptability perspective. First, the fundamentals of electrolytes concerning temperature, including donor number (DN), dielectric constant, viscosity, conductivity, ionic transport, and theoretical calculations are elaborated. Second, prototypical examples, such as lithium salts, solvent structures, additives, and interfacial layers in both liquid and solid electrolytes, are presented to explain how these factors can affect the electrochemical behavior of LIBs at high or low temperatures. Meanwhile, the principles and limitations of electrolyte design are discussed under the corresponding temperature conditions. Finally, a summary and outlook regarding electrolytes design to extend the temperature adaptability of LIBs are proposed.

Frameworked Electrolytes: A Pathway Towards Solid Future of Batteries

All-solid-state batteries (ASSBs) represent a highly promising next-generation energy storage technology owing to their inherently high safety, device reliability, and potential for achieving high energy density in the post-ara of lithium-ion batteries, and therefore extensive searches are ongoing for ideal solid-state electrolytes (SSEs). Though promising, there is still a huge barrier that limits the large-scale applications of ASSBs, where there are a couple of bottleneck technical issues. In this perspective, a novel category of electrolytes known as frameworked electrolytes (FEs) are examined, where the solid frameworks are intentionally designed to contain 3D ionic channels at sub-nano scales, rendering them macroscopically solid. The distinctive structural design of FEs gives rise to not only high ionic conductivity but also desirable interfaces with electrode solids. This is achieved through the presence of sub-nano channels within the framework, which exhibit significantly different ion diffusion behavior due to the confinement effect. This perspective offers a compelling insight into the potential of FEs in the pursuit of ASSBs, where FEs offer an exciting opportunity to overcome the limitations of traditional solid-state electrolytes and propel the development of ASSBs as the holy grail of energy storage technology.

Progress and Perspectives on the Development of Pouch-Type Lithium Metal Batteries

Lithium (Li) metal batteries (LMBs) are the "holy grail" in the energy storage field due to their high energy density (theoretically >500 Wh kg-1 ). Recently, tremendous efforts have been made to promote the research & development (R&D) of pouch-type LMBs toward practical application. This article aims to provide a comprehensive and in-depth review of recent progress on pouch-type LMBs from full cell aspect, and to offer insights to guide its future development. It will review pouch-type LMBs using both liquid and solid-state electrolytes, and cover topics related to both Li and cathode (including LiNix Coy Mn1-x-y O2 , S and O2 ) as both electrodes impact the battery performance. The key performance criteria of pouch-type LMBs and their relationship in between are introduced first, then the major challenges facing the development of pouch-type LMBs are discussed in detail, especially those severely aggravated in pouch cells compared with coin cells. Subsequently, the recent progress on mechanistic understandings of the degradation of pouch-type LMBs is summarized, followed with the practical strategies that have been utilized to address these issues and to improve the key performance criteria of pouch-type LMBs. In the end, it provides perspectives on advancing the R&Ds of pouch-type LMBs towards their application in practice.

Studies on Composite Solid Electrolytes with a Dual Selective Confinement Interface Structure of Anions for High-Performance Lithium Metal Batteries

Solid-state lithium batteries (SSLBs) have attracted much attention due to their good thermal stability and high energy density. However, solid-state electrolytes with low conductivity and prominent interfacial issues have hindered the further development of SSLBs. In this research, inspired from a selective confinement structure of anions, a novel HMOF-DNSE composite solid electrolyte with a dual selective confinement interface structure is proposed based on the semi-interpenetrating structure generated by poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), poly(di-n-butylmethylammonium) bis(trifluoromethanesulfonyl)imide (PDADMATFSI), and a metal-organic frameworks MOF derivative (HMOF) as a filler. The dual-network structure of PVDF-HFP/PDADMATFSI combined with HMOF formed a dual selective confinement interface structure to confine out the movement of large anions TFSI-, thereby enhancing the transfer ability of Li+. Subsequently, the addition of HMOF further improves the transfer of Li+ by binding up TFSI- through its crystal structure. The results show that HMOF-DNSE possesses a high room-temperature ionic conductivity (0.7 mS cm-1), a wide electrochemical window (up to 4.5 V), and a high Li+ transfer number (tLi+) (0.56). LiFePO4/HMOF-DNSE/Li cell shows an excellent capacity of 141.5 mAh g-1 at 1C rate under room temperature, with a high retention of 80.1% after 500 cycles. The material design strategy, which is based on selective confinement interface structures of anions, offers valuable insights into enhancing the electrochemical performance of solid-state lithium batteries.

The ionic liquid-based electrolytes during their charging process: Movable endpoints of overscreening effect near the electrode interface

HYPOTHESIS

Adding solvents to ionic liquids (ILs) can lead to the suppression of the overscreening effect near an electrode interface. Also, this suppression can be observed in neat ILs by elongating the length of the nonpolar chains on their ions. Most neat ILs, unlike the ideal model, do not exhibit a crowding effect in experiments. Through molecular dynamics (MD) simulations, researchers can model and analyze these systems in order to understand them.

SIMULATIONS

In this study, the dynamic change near the electrode interface of ILs-based electrolytes was investigated using MD simulations. The phenomena observed in MD simulations are generally understandable because factors can attenuate charge densities calculated from these simulations.

FINDINGS

The study findings reveal that both the solvents or nonpolar chains contributed to the formation of nonpolar domains. Also, the microscopic mechanisms and influences of these nonpolar domains were clearly identified. The results are important for real life applications. Some ions form a "point to surface" layer near the electrode of neat ILs. When ILs contain long nonpolar chains, they can suppress the crowding effect through self-assembly behavior. However, when they do not have any chains or short nonpolar chains, it can be difficult to stop the overscreening effect. This means it can become challenging to begin the next stage of the crowding effect.

Ionic Liquids Functionalized MOFs for Adsorption

Metal-organic frameworks (MOFs) and ionic liquids (ILs) represent promising materials for adsorption separation. ILs incorporated into MOF materials (denoted as IL/MOF composites) have been developed, and IL/MOF composites combine the advantages of MOFs and ILs to achieve enhanced performance in the adsorption-based separation of fluid mixtures. The designed different ILs are introduced into the various MOFs to tailor their functional properties, which affect the optimal adsorptive separation performance. In this Perspective, the rational fabrication of IL/MOF composites is presented, and their functional properties are demonstrated. This paper provides a critical overview of an emergent class of materials termed IL/MOF composites as well as the recent advances in the applications of IL/MOF composites as adsorbents or membranes in fluid separation. Furthermore, the applications of IL/MOF in adsorptive gas separations (CO2 capture from flue gas, natural gas purification, separation of acetylene and ethylene, indoor pollutants removal) and liquid separations (separation of bioactive components, organic-contaminant removal, adsorptive desulfurization, radionuclide removal) are discussed. Finally, the existing challenges of IL/MOF are highlighted, and an appropriate design strategy direction for the effective exploration of new IL/MOF adsorptive materials is proposed.

Advanced Composite Solid Electrolytes for Lithium Batteries: Filler Dimensional Design and Ion Path Optimization

Composite solid electrolytes are considered to be the crucial components of all-solid-state lithium batteries, which are viewed as the next-generation energy storage devices for high energy density and long working life. Numerous studies have shown that fillers in composite solid electrolytes can effectively improve the ion-transport behavior, the essence of which lies in the optimization of the ion-transport path in the electrolyte. The performance is closely related to the structure of the fillers and the interaction between fillers and other electrolyte components including polymer matrices and lithium salts. In this review, the dimensional design of fillers in advanced composite solid electrolytes involving 0D-2D nanofillers, and 3D continuous frameworks are focused on. The ion-transport mechanism and the interaction between fillers and other electrolyte components are highlighted. In addition, sandwich-structured composite solid electrolytes with fillers are also discussed. Strategies for the design of composite solid electrolytes with high room temperature ionic conductivity are summarized, aiming to assist target-oriented research for high-performance composite solid electrolytes.

Phosphonate-Functionalized Ionic Liquid Gel Polymer Electrolyte with High Safety for Dendrite-Free Lithium Metal Batteries

The conventional lithium-ion battery technology relies on the liquid carbonate-based electrolyte solution, which causes excessive side reactions, serious risk of electrolyte leakage, high flammability, and significant safety hazards. In this work, phosphonate-functionalized imidazolium ionic liquid (PFIL) is synthesized and used as a gel polymer electrolyte (GPE) to replace the organic carbonate-based electrolyte solution. The as-prepared ionic liquid-based gel polymer electrolyte (IL-GPE) shows low crystallinity, flame retardance, and excellent electrochemical performance. Thanks to the fast double channel transport of lithium ions in the IL-GPE electrolyte, a high ionic conductivity of 0.48 mS cm^-1 and a lithium-ion transference number of 0.37 are exhibited. Symmetrical lithium cells with IL-GPE retain stable cycling even after 3000 h under 0.1 mA cm^-2. IL-GPE exhibits good compatibility toward lithium metal, yielding excellent long-term electrochemical kinetic stability. IL-GPE induces the formation of a uniform and robust SEI layer, inhibiting the growth of lithium dendrites and improving the rate performance and cycle stability. Furthermore, Li/LiFePO₄ cells exhibit a specific capacity of 63 mA h g^-1 after 150 cycles at 5.0 C, with a capacity retention of 90.2%. It is foreseen that this GPE is a promising candidate to enhance the safety of high-performance lithium metal batteries.

Bifunctional MOF Doped PEO Composite Electrolyte for Long-Life Cycle Solid Lithium Ion Battery

A highly stable composite electrolyte was developed in this research to address the performance decline over time in a solid lithium ion battery (SLIB). It involved the synthesis of bifunctional MOF material (MOF-2) from two different functionalized UiO-66 materials containing carboxyl groups and amine groups, respectively, and the subsequent blending of PEO (polyethylene oxide) with the MOF-2 to form the novel composite solid electrolyte (PEO-MOF-2). The composite electrolytes showed higher ionic conductivity (5.20 × 10^-4 S/cm) than that of pristine PEO. The LiFePO₄||Li cells constructed with PEO-MOF-2 exhibited 98.45% capacity retention with 149.92 mA h/g after 100 cycles operation at 1.0 C, which was higher than those cells prepared with pristine PEO electrolyte or with PEO-based electrolytes that were only doped by aminated MOF or carboxylated MOF. Furthermore, our experiments showed that there was about a 40% increase in the potential window (from 3.5 to 5.0 V) and 80% increase in the lithium ion transfer number (from 0.20 to 0.36 at 60 °C) as a result of replacing pristine PEO electrolyte with PEO-MOF-2.

Toward Sustainable Solid Polymer Electrolytes for Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are the most widely used energy storage system because of their high energy density and power, robustness, and reversibility, but they typically include an electrolyte solution composed of flammable organic solvents, leading to safety risks and reliability concerns for high-energy-density batteries. A step forward in Li-ion technology is the development of solid-state batteries suitable in terms of energy density and safety for the next generation of smart, safe, and high-performance batteries. Solid-state batteries can be developed on the basis of a solid polymer electrolyte (SPE) that may rely on natural polymers in order to replace synthetic ones, thereby taking into account environmental concerns. This work provides a perspective on current state-of-the-art sustainable SPEs for lithium-ion batteries. The recent developments are presented with a focus on natural polymers and their relevant properties in the context of battery applications. In addition, the ionic conductivity values and battery performance of natural polymer-based SPEs are reported, and it is shown that sustainable SPEs can become essential components of a next generation of high-performance solid-state batteries synergistically focused on performance, sustainability, and circular economy considerations.