Architectural design and electrochemical performance of MOF‐based solid‐state electrolytes for high‐performance secondary batteries

Nowadays solid‐state batteries have become a hot spot in the research of batteries and a significant candidate for commercial batteries for the increasing demands for good safety and excellent energy density. Metal‐organic frameworks (MOFs) have been considered as suitable materials for solid‐state electrolytes (SSEs) for the merits of regular channels and large specific surface areas, which can provide a promising structural platform for fast‐ion conduction. Therefore, numerous kinds of MOF‐based SSEs with enhanced electrochemical performance have been successfully synthesized and studied in recent years. In this review, the recent progress (synthesis methods, physical and chemical characteristics) of MOF‐based SSEs for secondary batteries have been summarized. Finally, the challenges and opportunities faced by the future development in this field are put forward, hoping to provide some enlightenment for the synthesis of MOF‐based SSEs, so as to create more efficient, long‐lasting, and safe SSE‐based secondary batteries.


| INTRODUCTION
The troublesome problems created by the combustion of fossil fuels, for example, serious environmental pollution and energy shortage, force humans to focus on the development of clean energy (wind, tidal, geothermal, solar energy, and so on). [1][2][3] However, their universal inherent problem is the discontinuity of time and space, which prevent their further extensive application. Therefore, exploring effective and sustainable energy storage systems (ESSs) to store the intermittent energy has become the research direction of most scientific and technological staff. Among various ESSs, electrochemical ESSs, especially secondary batteries, have received much attention for their advantages of no geographical restrictions, high efficiency, and intelligent management. Nowadays, various kinds of secondary batteries, including lithium-ion batteries (LIBs), lithium-sulfur batteries (LSBs), sodium-ion batteries (SIBs), and so on, have been employed and vigorously developed. [4][5][6] For the next-generation batteries, they are usually composed of metal or metallic (Li, Na, K, Mg, Zn, etc.) anodes, graphite cathode, and electrolyte. Among these components, electrolytes can be regarded as "blood," because it not only plays a significant part in the transformation of ions but also is responsible for stabilizing the electrodes and avoiding irreversible structural damage. [7,8] Furthermore, organic solutions, for example, dimethyl ether (DME), dimethyl formamide (DMF), dimethyl phthalate, diethyl carbonate (DEC), ethylene carbonate (EC), ethyl methyl carbonate, propylene carbonate (PC) as well as other compounds, have been selected as electrolyte for batteries. [9,10] However, the liquid organic electrolytes mentioned above have the risk of leakage and inflammation, which leads to lower safety and restricts their further application in secondary batteries. [11,12] To solve these problems, solid-state electrolytes (SSEs) has emerged in recent years and attracted numerous attention. Compared with common liquid electrolytes, SSEs possess numerous superior advantages, which can be concluded as follows. Firstly, SSEs commonly have a lower risk of leakage, which ensures improved safety of batteries. Secondly, due to the excellent electrochemical and thermochemical stability, the obtained batteries with SSEs as electrolyte can exhibit higher operating temperatures and larger voltage window. Thirdly, the high modulus and appropriate mechanical rigidity of SSEs can further reduce the side reaction and production of Li dendrites, thus avoiding the microcosmic short circuit and loss of moveable cations. Finally, SSEs, with the characteristics of low electronic conductivity and high-ionic conductivity, can inhibit the self-discharge phenomenon and enhance the capacity retention of battery to a certain degree. [13][14][15][16][17][18] Until now, it is generally believed that SSEs can be divided into three categories, including solid polymer electrolytes (SPEs), solid inorganic electrolytes (SIEs), and solid composite electrolytes (SCEs). [19,20] Although SSEs possess many superiorities, there are still some potential drawbacks when employing SSEs in energy storage devices. For instance, the existence of a little bit of organic substance used in SSEs will still lead to the risk of leakage and flammability. SSEs with polymer substrate always suffer from the problem of lower ionic conductivity and the risk of puncturing membrane caused by Li/Na dendrites, while SSEs with inorganic substrate are hard to adapt to the volumetric change in the charge and discharge reaction, resulting a short cycling life. More importantly, unlike the solid-liquid interface reactions, solid-solid interface reactions often possess complex and slow reaction kinetics, which hinders the further development of SSEs in energy storage/conversion systems. [21][22][23][24] A lot of work has been done to enhance the electrochemical performance of SSE-based energy storage and conversion systems and have achieved some progress. For instance, the enhanced SSE-based batteries can exhibit high-ionic conductivity of 2 × 10 −3 S cm −1 as well as long cycling life of more than 3000 cycles for LIBs, [25] excellent discharge capacity of 9340 mA h g −1 in lithium-oxygen battery, [26] quick synthesis method of 5 min, [27] and so on. Taking the SIEs as an example, which suffer from weak wetting behavior and large interface resistance, the interface reaction can be enhanced effectively through a cover of appropriate coating on the surface of electrode. For SPEs, with high flexibility but lower ionic conductivity as well as lower elastic molding, many strategies have been introduced, such as introducing a little bit of polarity organic solvent or inorganic particles, designing "polymer-in-salt" electrolyte or using various kinds of the polymer membrane. Moreover, more in-depth understanding and exploration should be continued to pursue the long-term development of SSEs. [28,29] Metal-organic frameworks (MOFs), assembled by metal ions/clusters and organic ligands, have been regarded as the ideal candidates for many areas (energy storage devices, gas adsorption, drug transportation, catalytic action, and other applications) for the characteristics of the porous structure, large specific surface, high designability, and other properties. [30][31][32][33][34] Recently, MOFs have been also widely applied in new type SSEs with high safety, high energy density, and high operating temperature. Furthermore, the merits of MOFs on SSEs can be summarized as follows. (1) The homogeneous porous structure provides uniform sites and specific pathways for the transportation of ions, thus promoting the ion transport dynamics and compensate for the low ionic conductivity of SSEs. (2) The large surface area of MOFs, which can even reach more than 1000 m 2 g −1 , can provide the essential condition for the accommodation of metal ions and hopping sites, which can bring about a high-power density. (3) Thanks to the Lewis acid-base interaction (LABI) between metal ions in MOF and anions in solution, the cation transfer number in the solution is significantly increased, and the concentration polarization problem during charging and discharging is also improved. (4) The high designability of MOFs means an enhanced performance of interfacial stability and ionic mobility through the functionalization and modification of ligands. [35][36][37][38] In recent years, more and more invigorative publications related to MOF-based SSEs are showing up (Figures 1-3).
MOFs have been widely applied in many fields, especially in energy storage/conversion system, many reviews have summarized the synthesis methods, structural advantages, and electrochemical performance of MOFs and their related materials. For instance, Zhao et al. [39] mainly focus on the synthesis and application of MOF-based materials in batteries, including pristine MOF, MOF composites, and MOF derivatives. Additionally, MOF-based materials can not only be directly used as electrode materials but also employed to modify membranes and electrolytes. In 2020, MOF-based SSEs have been classified into three categories according to the different functions of MOFs in SSEs and the synthesis methods and corresponding applications of MOF-based SSEs have also been discussed in Zhao et al.'s [40] report. More in detail, Liu et al. [41] summarized the various fillers of composite polymer electrolytes (CPEs) for LIBs and the reaction mechanisms of the enhanced performance.
Although many reviews about MOF-based SSEs have been published, there are few publications to summarize the reaction mechanism and synthesis methods of MOFbased SSEs in LIBs. Furthermore, with the rapid development and hard lucubration of solid-state batteries, the latest research progress should further be summarized. In this review, firstly, according to the different functions of MOFs for SSEs, MOF-based SSEs can be divided into three sorts: (1) MOF-incorporated polymer composites as SSEs.
Moreover, the corresponding preparation methods and characteristics of each MOF-based SSEs have also been summarized in Section 2. Then, high-performance batteries with MOF-based SSEs are classified and summarized, and the principles of modification as well as the possible universal rule are focused on. Finally, we highlighted the major development bottlenecks and current improvement measures, and pointed out our personal perspectives about the challenges and opportunities for the further development of SSEs. We hope this review can provide assistance and guidance on the development of MOFsbased SSEs for high-performance secondary batteries to some extent.

| SYNTHESIS OF MOF-BASED SSEs
As we all know, electrolytes, as one of the key components of batteries, will affect the electrochemical process and further determine the overall physical and electrochemical performance. Unfortunately, conventional liquid electrolytes are mainly faced with the problems of irreversible decomposition, high flammability as well as leakage issues. [42,43] Therefore, it is urgent to explore solving strategies to overcome the drawbacks and provide possible development direction for batteries. SEEs have attracted more and more attention because of the high modulus and excellent stability. In comparison with conventional liquid electrolytes, the advantages of SEEs could be summarized as follows: (1) excellent electrochemical stability and mechanical rigidity; (2) no leakage problems; (3) suitable transference number of metal ions. [44][45][46] Up until now, several kinds of SSEs have been developed, of which inorganic ceramic electrolytes (ICEs) and CPEs are the most widely researched. For instance, ICEs often possess excellent mechanical robustness and reasonable contact. Furthermore, sulfide-based SSEs, one of the ICEs, can show higher ionic conductivity because of the weak interactions between Li + and anionic sublattice caused by the polarization of sulfur anions. However, generic ICEs face the problems of poor interfacial contact hard volume charge as well as sulfidebased SSEs have the disadvantages of poor stability and narrow voltage window, which hinder further application. [47,48] Compared with ICEs, CPEs have higher flexibility and are easier to process, but the low elastic moduli and insufficient ionic conductivity at room temperature restrict further application. To obtain excellent electrical energy storage performance, more and more efforts have been made to optimize the composition/structure of the electrolytes system and seeking new conceptions of electrolytes. [49][50][51] In recent years, MOFs have been applied in the synthesis of SSEs due to their insulating characteristics and inherent properties. MOFs, as a three-dimensional (3D) porous network structure with rich porosity, could provide evenly distributed sites and diffusion paths for ion diffusion. Moreover, the pores inside MOFs can restrict the movement of relatively large ions, so the transference of small ions (Li + and Na + ) can be facilitated further. Moreover, the excellent surface properties can also ensure the excellent electrochemical performance of MOFs-based SSEs. Firstly, the large surface area of MOFs can provide more contact with active constituents. Secondly, the controllable surface polarity can provide more opportunities for the enhancement of ionic conductivity through the acid-base interaction in Lewis's modulation mixing system. More importantly, MOFs exhibit a highly adjustable property, which includes the components, the size of pores, the surface polarity, and microstructure of MOFs. Moreover, the high surface energy and strong adsorption ability of MOFs, can trap the impurities and by-products and thus inhibit the side reactions. [52][53][54] In this part, we mainly summarize the significant achievements of MOF-based SSEs, including the common synthesis methods and their advantages and disadvantages. Hence, three categories of MOF-based SSEs are divided according to the role of MOFs and the components of SSEs. There are (1) MOF-incorporated F I G U R E 2 Diagrammatic sketch of the advantageous properties of MOFs and some characteristics of SSEs. MOF, metal-organic framework; SSE, solid-state electrolyte. polymer composites as SSEs; (2) ILs-incorporated MOFs composites as SSEs; (3) MOFs for single-ionic conducting SSEs.

| MOF-incorporate polymer composites as SSEs
Polymer has attracted much attention to prepare SSEs due to its flexibility as well as complete contact with electrodes. However, the relatively poor thermostability and ionic conductivity restrict further application in electrical energy storage equipment. [55,56] In recent years, through introducing lithium salts and metal-oxide particles, polymer composite electrolytes exhibit improved thermal stability and a wider operating temperature range. Additionally, Lewis-acid sites and pores in inorganic fillers can effectively facilitate the segmental movement of polymer chains as well as the transportation of metal ions, thereby improving the ionic conductivity as well as the electrochemical performance of the SSEs. [57,58] Among various of CPEs, MOF-incorporated polymer compounds are composed of metal salts, polymer matrices, MOFs fillers, and occasionally liquid plasticizers. More interestingly, MOF-based SSEs have been widely synthesized by combining different types of MOFs with polymers, including polystyrene sulfonate, polyethylene oxide (PEO), poly(vinyl alcohol), poly acrylonitrile (PAN), polymethyl methacrylate (PMMA), as well as poly(vinylidene fluoride) (PVDF). The MOF-based SSEs not only inherit the characteristics of MOFs, including F I G U R E 3 The diagrammatic sketch of MOF-based SSEs and their corresponding properties (the ribbon: polymer chains; green balls: ILs ions; blue balls: organic molecules; red balls: grafted anions/groups). IL, ionic liquid; MOF, metal-organic framework; SSE, solid-state electrolyte. regular channels, high porosity, good stability, and active open sites, but also exhibit good flexibility and interface contact. [59][60][61][62][63] Research has proved that the interaction or the binding between polymer matrixes and MOFs plays an important role in the enhancement of the performance for the MOF-incorporate polymer composites SSEs. In the porous structures, the monomers with small size are introduced first, and then the polymerization occurs in the channels. Adjustable surface polarity can promote polymerizable ligands to polymerize in a specific location and achieve polymerization in the inside of MOFs. More in detail, the common synthesis methods of MOFincorporate polymer composites can be divided into two groups: (1) physical mixing polymer and MOF in SSE film. Through the adding of MOFs and other additives, intereactive force can be formed between polymer chains and MOFs. After the grinding/stirring and vaccum treatment, the excessive polymer can be removed and the liquid electrolytes turn into SSEs. For instance, Shen et al. [64] prepared a series of SSEs (LPC@MOFs) through dispreading the MOFs (HKUST-1, MIL-100-Al, MIL-100-Cr, MIL-100-Fe, UiO-66, and UiO-67) into the ethanol and adding polytetrafluoroethylene as binder. A membrane can be obtained after the evaporation of the solvent and excessive liquid electrolyte can be further removed through pressing at 200 MPa. Similarly, Wen et al. [65] designed across-linked network polyethylene glycol (PEG)-hyperbranched poly(ethyleneimine) (HPEI) as polymer substrate, then added the mixed solution of UIO-66-(F) and methanol, the SSE can be obtained after the solvent evaporation and vacuum for 24 h. In this method, volatile polar solvents and appropriate polymers are often selected to strengthen the connection of MOFs and polymer substrates, so that the transference of ions can be accelerated and the stability of SSEs can be enhanced. (2) Through the modification after synthesis, the polymer monomer was chemically grafted onto MOF nanoparticles and further photopolymerized into a film. [66] Furthermore, research has proved that the effect between polymers and functional MOFs can achieve the enhanced performance of the obtained SSEs. But in normal conditions, the connection between polymer chains and MOFs is unordered. The large aperture of MOFs provides the possibility of purposive assembly of polymers and MOFs.
However, the MOF incorporate polymer composites, which are prepared through the physical mixing process, always have the disadvantages of weak interaction between MOFs and polymers, which may lead to agglomeration of nanofillers as well as inadequate contact of multiple interfaces in the composite. Therefore, more and more efforts have been devoted to enhancing the chemical and physical interaction between MOFs and other constituent parts. Numerous approaches have been applied in the preparation of functional MOFs, which possess various structures (nanofibers, nanosheets, 3D aerogels, and others) and different surface polarities (metal sites, organic groups, and MOF loading content). Among them, the structural design of the organic linkers in MOFs is a promising approach for improving the chemical interactions of MOFs with other components. [67][68][69][70] 2.2 | ILs-incorporated composites as SSEs As a fascinating type of green solvents, ILs are mainly composed of point-charged electrons and transient-ion pairs, the melting point of which is less than 100°C. In comparison with traditional organic solvents, ILs exhibit numerous significant advantages, such as high solubility, negligible volatility, excellent ionic conductivity, nonflammability as well as good electrochemical stability. [71][72][73] However, ILs are still faced with the problem of fluidity. The concept of "supported IL phase" which means that impregnate ILs into porous materials to harmonize the characteristics, have been accepted. [74] MOFs are appropriate materials to host ILs for the high porosity and large specific surface area, and the performance can be improved greatly due to the synergistic effect. Moreover, the open channels of MOFs could promote the contact with electrode interface and further accelerate the ion transport. [75][76][77] Generally, the synthesis methods of ILs-incorporated composites can be grouped into three categories: (1) insert the preobtained MOFs into ILs or solutions containing ILs, which is called the postsynthetic modification. The target materials can be obtained through stirring the mixed solution and be further purified by the solvent evaporation. The method is used widely due to the easy operation. For instance, Luo et al. [78] prepared amino-functionalized basic-ionic liquid (ABIL-OH and ABIL-Br) and HKUST-1, respectively. Then, with the solvent of ethanol, ABIL-OH or ABIL-Br and HKUST-1 are stirred for 24 h at room temperature. The designed ABIL-OH/HKUST-1 and ABIL-Br/HKUST-1 can be obtained after the filtration and washing. Notably, the appropriate size of MOFs particles and ILs are important issues to obtain stable structures and enhanced performance. (2) Introduce IL into MOF pores to obtain the structure (ILs inside MOF pores), which can be named as the "ship-in-bottle" method. The method can effectively restrict the ILs into MOFs structures and provide the possibility of incorporating the ILs with larger ions than pore aperture diameter into the pores of MOFs. [79,80] In this strategy, the choice of MOFs and ILs is crucial for the reason that the guest molecules can separate from the large pores of MOFs while the over-small size prevents the entrance of ILs. On the other hand, a well-selective ligand can provide Lewis acid-base sites, so that a powerful connection between ILs and MOFs can be achieved. For example, Sun et al. [79] prepared IL@ZIF-8 through the grinding method which use 1-methyl-3propylimidazolium bromide (MPImBr) as ILs. Benefit the narrow crystallographic pore aperture and the halide anions, the as-prepared catalysis can achieve sturdy structures and enhanced stability of without obvious reduction for catalytic performance. (3) For some MOFs without coordinatively unsaturated sites, ILs can be introduced into MOFs through capillary action, including the procedure of physically mixing and annealing. After the annealing treatment, the guest small molecules of MOFs can be removed and apertures are reserved. Then the mixture of the ILs and MOFs is heated or grinded subsequently to accelerate the diffusion of ILs into the micropores of MOFs through capillary action. [81,82]

| MOFs for single-ionic conducting SSEs
Single-ionic conducting SSE is a type of SSE in which only one type of ions (cations in most cases) is responsible for the apparent electric conductivity, which can be achieved through restricting the anions by intermolecular interactions or steric effect. Single-ionic conducting SSEs have drawn widespread attention due to the advantages of high-ionic conductivity, excellent stability, and superior safety. MOFs are suitable to construct single-ionic conducting SSEs due to the properties of abundant cavity structure, large surface area and highly adjustable in composition and structure. On the one hand, the rational design of pores can prompt charged species to store into suitable geometric volume and form packed cation hopping sites which can accelerate the transport of target ions. On the other hand, through the decoration of MOFs, plentiful cationic sites are introduced and anions can be docked, thus the relatively high cation transference number can be achieved.
Common methods of designing MOFs-based singleionic conducting SSEs can be described as follows: (1) soak MOFs into salt solutions for subpackage of metal ions, subsequent by filtrating and drying. Normally, solvent molecules should be removed from MOFs to expose active sites and promote the entrance of anions, which are called the acitivation process of MOF. Subsequently, some types of binders (PVDF, PAN, and others) should be introduced to avoid the separation of SSE. For instance, MOF-74 prepared by Yuan et al. [83] was heated at 200°C to remove the solvent molecules to expose more active sites for the introduction of anions. Then, at the thermal condition, LiClO 4 is dissolved in several kinds of organic solvents (PC, DME/ 1,2-Dimethoxyethane/1,3-Dioxolane [DOL], and Tetraethylene glycol dimethyl ether) to make ClO 4 − enter into the cavity of activated MOF-74. Due to the gathering of ClO 4 − in the pores of MOF-74, Li + are preferentially adsorbed into the pores by Grotthuss-like mechanism (proton transport) and realize transportation inside the pores.
(2) Modify as-prepared MOFs to form an anionic skeleton with charge-balancing mobile cations, which may need cumbersome steps and experimental conditions. [84,85] More detailedly, these metal salts consist of small ions, which are usually smaller than the pore size of MOFs. Therefore, the uptake number of salts is largely determined by the pore confinement effect, while the ion mobility rate and transference number are decided by the chemical interactions between MOFs and salts. More and more efforts have been devoted to reducing pore confinement effect and enhancing chemical interaction between MOF and existing salts, which are summarized as follows: (1) change the surface polarity through ligand functionalization.

| Lithium-ion batteries
As the most popular rechargeable batteries, LIBs have drawn much attention because of the high energy density, no memory effect as well as excellent electrochemical performance. Therefore, LIBs have been extensively applied in electric equipment and flexible electrodes, such as electric vehicles, mobile phones, and watches. The complete LIBs device mainly includes anode, cathode, electrolyte, separator, and enclosure. Although the decisive electrochemical performances of LIBs mainly depend on the electrodes, the electrolytes can also influence the physics as well as chemical properties of the batteries such as the operating temperature, internal resistance, and working life. [89,90] Therefore, the transport rate of Li + is largely determined by the electrolyte and a slower transmission speed may induce poorer rate capacity, which can impute to a lengthy chargingdischarging reaction time. Unfortunately, for the most of LIBs which choose the liquid electrolyte, they are often faced with controversial issues of leakage and combustion problems. It is extremely urgent to explore new types of electrolytes, and some electrolytes, such as aqueous electrolyte, solid electrolyte, and other multicomponent liquid electrolytes have emerged. [91][92][93][94] Solid-state LIBs, with SSEs can not only avoid the security problem but also improve the energy density, are appropriate development trends of LIBs.
There are several significant parameters to measure the electrochemical performance of Solid state lithium-ion batteries (SSLIBs). (1) Li + ionic conductivity. Li + ionic conductivity is an important parameter for SSLIBs, which determines the resistance and rate capacity. Normally, LIBs with liquid electrolyte can obtain a high lithium ionic conductivity of 10 −2 to 10 −3 S cm −1 , while the SSEs possess a relatively lower ionic conductivity. Notably, the SSEs for LIBs should reach a Li + conductivity of 10 −4 S cm −1 to meet the practical application. (2) Li + transference number. As the only cation that participated in the migration process, the high Li + transference number means less lithium deposition, lower polarization concentration, which provides a foundation for rapid charging and discharging process. When the movement of Li + is limited, excessive anions will gather at cathode and result in ion concentration polarization, which can prevent the improvement of energy density and power density. (3) Mechanical property. Mechanical properties, including strength, toughness, brittleness, and resilience, can show a serious influence on the energy density, voltage windows, and their large-scale applications. (4) Glass transition temperature (T g ). The lower T g indicates the accelerated transportation of Li + and corresponding higher ionic conductivity as well as the possible applications at lower temperature. For SSLIBs, the interfacial resistance between electrodes and electrolytes, the formation of solid electrolyte interface (SEI), and other electrochemical behaviors can be understood through the electrochemical test. The above reaction can be measured through electrochemical impedance spectroscopy (EIS), density functional theory simulations, linear sweep voltammetry, and other methods.

| MOF-incorporate polymer composites as SSEs
As an important part of SSEs, SPEs possess the advantages of good safety, high electrochemical stability, wide voltage windows, and suitable operating temperature. However, the relatively poorer ionic conductivity compared with liquid electrolytes restricts their practical application in LIBs. Many studies have demonstrated that through the appropriate addition, which is also known as fillers, the comprehensive characteristics of SPEs, including mechanical strength, stability, and electrochemical properties, can be effectively enhanced. In recent years, MOFs with porous structures and large surface areas have been used as fillers for SSEs. [95][96][97][98] Moreover, the ordered channels of MOFs could offer convenient pathways for the transference of Li + , which brings about a higher ionic conductivity. More importantly, the characteristic of surface adjustability offers opportunities to reinforce the interaction of MOFs and PEO/Li + through the surface functionalization. [99,100] Normally, a conventional SPE mainly consists of polymer, Li salts (LiClO 4 , LiPF 6 , LiBF 4 , etc.), and additives (ILs, graphene oxide, clays, and MOFs, etc.). [101][102][103] As for polymers, they should have several advantageous characteristics, such as low density, high thermal stability, nonflammability, and high-ionic conductivity. There are many polymers that have been employed for SSEs, such as PAN, PMMA, PVDF, PEO, and other hybrids. Among them, PEO with oligoether (-CH 2 -CH 2 -O-) n constitute are used most extensively for the benefit of excellent mechanical flexibility, low interface resistance, as well as good stability with Li + . However, compared with liquid electrolytes, PEO-based SPEs still exist some drawbacks, such as lower ionic conductivity of 10 −7 S cm −1 at room temperature, which hides the broad application in LIBs. By the introduction of MOFs as additives, the performance of SSLIBs can be improved. The enhancement could be ascribed to the porous structures, large surface area, as well as ordered pores of MOF materials, which can provide reaction sites for metal ions and polymers as well as accelerate the ion transportation. [104][105][106] Moreover, the surface properties of MOFs are adjustable, indicating the flexible functionalization. Through immobilizing function groups on MOFs, the interaction between MOFs and Li + can be enhanced, which can enhance the thermal and electrochemical stability further. Generally, the preparation of MOF-incorporate polymer composites mainly includes mechanically blending method and in situ growth method. [107] Although the SSEs prepared by in situ growth method show the advantages of good homogeneity and high bonding strength of product, the exploration and application still have limitations due to the complex mechanism. [108][109][110] The mechanical blending method has been used widely for the reason of easy operation and high controllability. Through different raw materials and various methods, SSEs can be prepared and assembled for high-performance solid-state LIBs, the corresponding data can be seen in Table 1.
Conventional technological procedure prepared MOFs-based SSEs including solvent evaporation method, hot-pressing method, solution casting method, electrospinning method, and others. With the T A B L E 1 Summary of the MOF-incorporate polymer composites for SSEs. appropriate pressure and temperature condition, the hotpressing method can shorten the reaction time, but the stability of the sample is required. Through covering the composites of polymer, fillers, and solvents on a horizontal Teflon plate uniformly, the as-prepared SSEs can be obtained after the evaporation of solvents through solution casting method. In this section, several different MOFs are synthesized and then prepared SSEs through solution casting method. In 2013, Yuan et al. [125] introduce MOF-5 (Zn-BDC) ( Figure 4A) to the SPE for the first time. [118] With the CH 2 Cl 2 -CH 3 CN as solvent, the homogeneous colloidal mixture is cast and dried into a film at 55°C for 48 h until the weight does not change. In contrast with the sample without MOF-5, the assembled MOF-based SSEs can exhibit enhanced ionic conductivity of 3.16 × 10 −5 S cm −1 . Different from the major perspective that the Lewis-acidic sites of MOF-5 interacted with Li salts and PEO chain segments, which can prevent the crystallization of PEO chains and is beneficial to the transmission of Li + in the conducting passages similar to the structure of PEO 6 :Li + . The researchers provided another possible assumption that the absorbed trace solvent in SSEs can form facilitated and isotropic open channels in the frameworks of MOF-5, which is benefit for the ionic transportation ( Figure 4B). The other important contribution of this work is to investigate the effect of the molar ratio of Li and EO on ionic conductivity.

MOFs
The results show that the highest ionic conductivity is achieved when the molar ratio of EO and Li is 10:1, which is close to the eutectic composition of the PEO-LiTFSI system. [137,138] The result can be ascribed to that the activation energy used for different phase formation possesses just a small difference and the energy barrier of phase transformation is smaller at the eutectic point. [139] Different from the Zn-based MOFs Yuan et al. prepared, various kinds of MOFs with the same nanoparticles structure such as ZIF-67, [136] UIO-66, [122] and Al-NDC ( Figure 4C) [113] are synthesized, and employed for SSEs with PEO and LiTFSI. Through the reaction between nanostructures and PEO membrane, the 3D networks can be formed and play an important role in the transference of Li + . For example, ZIF-67 prepared by Xu et al. [136] exhibit cage-like micropores structure as well as a large surface area of 1420 m 2 g −1 . Benefiting from the ordered channels and porous structure, the long-columnar TFSI − anions can be immobilized and thus improve the transmission of Li ions. At the same time, the large surface can provide more contact between PEO and MOFs so that the alignment of the PEO chains can be alleviated and thus decrease the crystallinity. The composite SSE prepared by Zhang et al. [113] through solvent evaporation method ( Figure 4D) can exhibit enhanced electrochemical performance. For instance, the electrochemical window can be broadened to 5.2 V as well as the ASLMB can show stability of 135.4 mA h g −1 more than 100 cycles. From another group, Xu et al. [129] designed MOFs with Bi 3+ as metal centers and HMT (hexamine) as ligand, and the Bi-HMT show a structure of nonporous nanowires ( Figure 4E). Although the Bi-HMT nanowires exhibit a specific surface area of 17.13 m 2 g −1 , which is much lower than traditional MOFs, the nanowires can disperse into the PEO matrix as well as generate strong coupling interfaces with PEO/LiTFSI to form quick transportation pathways for Li ions and thus realize the enhanced ionic conductivity. In this article, the authors explained the reason for using the Bi-MOF. As a typical borderline Lewis acid in accordance with Pearson's hard and soft acid and base theory, Bi 3+ can combine with both soft (TFSI − , FSI − , and PF 6 − ) and hard bases (polymers with O and N groups) and thus form interfacial channels through LABI ( Figure 4F). Benefiting from the coupling interfaces and strong LABI, the CPE can exhibit improved ionic conductivity of 3.06 × 10 −5 S cm −1 and Li + transference number of 0.531, which are higher than SPE of 2.57 × 10 −6 S cm −1 and 0.202 correspondingly. Similarly, considering the strong LABI, Wu et al. [114] designed 3D nanostructured Ce-MOF with plenty of open metal sites (OMSs) inspired by defective or catalytic sites that can improve the ionic conductivity and prepared solid polymer electrodes through the solution casting method ( Figure 4G). The abundant OMSs enable the strong LABIs with the PEO fragments as well as anions of Li salts, and thus promote the transportation of Li + and reach to enhanced ionic conductivity and Li + transference number. The mechanism could be concluded that the metal centers trap TFSI − and thus release plentiful free Li + , as well as the TFSI − anions are restricted into the nanocages of CE-MOFs and form a convenient access way for Li + . At the same time, the dispersive 3D Ce-MOF in the PEO matrix can work as crosslinking centers so that it plays a significant role in preventing aggregation and reorganization as well as improving the movement of polymer segments ( Figure 4H). Benefiting from the above factors, the composite SPE with 10 wt% Ce-MOF exhibit ionic conductivity of 3.1 × 10 −4 S cm −1 at 60°C and Li + transference number of 0.75. When assembled for SSLIBs with LiFePO 4 cathodes, the battery can exhibit excellent cycling stability of 120 mA h g −1 after 3800 cycles.
Stephan et al.'s [119] group, from the Central Electrochemical Research Institute (CSIR-CECRI), prepared numerous MOFs including Mg-TPA ( Figure 5A), Mg-TMA, [119] Mg-BTC, [118] Ni-BTC ( Figure 5B), [120] Al-BTC ( Figure 5C), [111] and Cu-BDC ( Figure 5D) [115] and assembled SSEs through hot-pressing method. As hard acids, strong LABI can be generated between Al 3+ / Mg 2+ and PEO chains with hard bases of N and O, hence reducing the ratio of crystalline phase in PEO and facilitating the movement of fragments. For instance, the highest ionic conductivity of the PEO-LiTFSI with 10 wt% Mg-TPA is 7.02 × 10 −4 S cm −1 and Mg-TMA is a little bit lower ( Figure 5F,G). Similarly, the nanoCPEs with Al-BTC can also show ionic conductivity from 10 −5 to 10 −3 between 20°C and 80°C. The effect that MOFs put on PEO can be also reflected through the T g for the reason that the relatively restricted polymer segments mean a higher T g . For instance, the Al-BTC-based SSEs have an improved T g of −49°C ( Figure 5H), which is higher than PEO-LiTFSI SSEs. The enhancement could be ascribed to the amorphization in PEO with the existence of LiTFSI as well as interaction within the PEO matrix provided by the MOF filler. For SSLIBs, the compatibility between electrolytes and electrodes determines the performance of SSEs to a large extent, which mainly manifest as the production of SEI. Although possess complex composition and intricate formation mechanisms, the formation of SEI plays a significant role in the electrochemical performance of batteries. [140,141] Figure 5E exhibits the formation of SEI at the interface of the Li electrolyte. The formation of SEI film consumes partial Li + and results in the capacity fading at higher rates and the interface resistance can reveal the influence of SEI film to SSEs. Except for ions transport performance, the enhancement of physical property such as thermal stability is obvious after the addition of MOFs fillers. For instance, the melting point of complex PEO/ LiTFSI electrolyte increase to 66°C with the addition of Al-BTC which can be ascribed to the intercalation/ exfoliation of the polymeric matrix with MOF filler particles that can enhance the thermal stability of designed SSEs. [142,143] Benefiting from the above improvement of ions transference and stability provided by MOFs filler, the electrochemical performance can achieve substantial promotion. For the composite electrolyte of PEO, LiTFSI, and Mg-TPA, the assembled SSLIB with LiFePO 4 cathode can show an enhanced first discharge capacity of 140 mA h g −1 with 0.1 C at 60°C ( Figure 5I). When replace Mg-TPA with Al-BTC and increase the temperature to 70°C while keep the other factors the same, the prepared SSLIB can also exhibit 135 mA h g −1 at 0.1 C as well as 118 mA h g −1 after 80 cycles at 1 C ( Figure 5J). The difference can be attributed to the high temperature accelerating the transference of ions, corresponding to the enhanced ionic conductivity and Li + transference number.
Except for ordered channels and porous structure, another advantage that MOFs possess is the character of the adjustability and tunability of the surface. Among various MOFs, Zr-based MOFs have been paid a lot of attention for SSEs due to their thermal stability, chemical stability, and tunability. Qing et al. [123] prepared functionalized UIO-66 laden on PEO and LiTFSI (UIO-66-NH 2 @P and UIO-66-NO 2 @P) as S.SE for LIBs. Compared with UIO-66-NO 2 @P which showed capacity retention of 50% after 50 cycles and was totally short-circuited after 100 cycles, SSLIB with UIO-66-NH 2 @P can exhibit excellent cycling performance of keeping stable even after 1500 h at 0.1 mA cm −2 before being short-circuited and superior Coulombic efficiency of around 99% after 110 cycles. The author holds the assumption that the electronic effect of the substituent groups can competitively bind with the Li + through forming more polar MOF bridges ( Figure 6A). Meanwhile, the electronic effect can promote the dissociation of Li + from PEO chains as well as induce uniform lithium plating, and thus prevent lithium dendrites propagation and extend cyclability. Similar to Qing's work, Hou and his coworkers designed anionimmobilized UIO-66 (cationic metal organic frameworks  [125] Copyright 2013, Elsevier. (C) Reproduced with permission. [113] Copyright 2020, Wiley. (D) Reproduced with permission. [122] Copyright 2019, Elsevier. (E, F) Reproduced with permission. [129] Copyright 2022, Wiley. (G, H) Reproduced with permission. [114] Copyright 2021, Elsevier. ASLMBs, All solid lithium metal batteries; CPE, composite polymer electrolyte; CSEs, composite solid electrolytes; MOF, metal-organic framework; PEO, polyethylene oxide; SPE, solid polymer electrolyte.
[CMOF]) ( Figure 6B) based on UIO-66-NH 2 and the designed SSE exhibited enhanced cycling performance. The as-prepared CMOF exhibit a large surface area of 1082 m 2 g −1 and abundant cationic graft, and thus anions can be confined effectively through the electrostatic interaction of charge carriers. On the other hand, the functional groups of -NH 2 can protect the ether oxygen of PEO chains through the generation of hydrogen bonds. Benefiting from the above synergistic effect, the designed SSEs can exhibit no dendrite generation during the lithium deposition process ( Figure 6C). The published research has proved that the formation of Li dendrites is the major cause of poor cycling performance. [144] Hence, when assembled for SSLIBs with lithium-iron phosphate (LFP) cathode, the battery can show a high capacity retention of 85.4% at 1 C ( Figure 6D) and enhanced discharge capacity ( Figure 6E). Except for work as effective filler for SSEs, anion-immobilized MOFs can function as a bridge to obtain functional MOFs through the method of postsynthetic modification. For instance, considering the strong coordination between oxygen atoms of PEO and cations of ILs, which can exhibit good compatibility and excellent synergistic effect, Lei et al. [130] designed ZIF-90 grafted with imidazole IL containing siloxane groups (ZIF-90-g-IL) through the ZIF-90-NH 2 ( Figure 6F). Notably, the PEO/ZIF-90-g-IL SPE can exhibit enhanced ionic conductivity (7.2 × 10 −5 S cm −1 -2.1 × 10 −3 S cm −1 ) than PEO/ZIF-90 electrolyte (4.3 × 10 −5 S cm −1 -8.0 × 10 −4 S cm −1 ) from 20°C to 80°C and relatively low activate energy of 0.21 eV. The dispersive rigid MOFs fillers can reduce the crystallinity of PEO chains and release more amorphous regions for the transportation of Li + , and thus realize the improved ionic transference performance. When assembled for SSLIBs, the coordination and synergistic effect between PEO and ILs are shown as the wide steady voltages of 4.8 V and 71% capacity retention (101 mA h g −1 ) after 500 cycles ( Figure 6G).  [123] Copyright 2020, The Royal Society of Chemistry. (B-E) Reproduced with permission. [144] Copyright 2019, Elsevier. (F, G) Reproduced with permission. [130] Copyright 2021, Elsevier. (H, I) Reproduced with permission. [128] Copyright 2022, Elsevier. CMOF, cationic metal organic frameworks; CPE, composite polymer electrolyte; IL, ionic liquid; MOF, metalorganic framework; PAN, poly acrylonitrile; PEO, polyethylene oxide; SEM, scanning electron microscopy.
For the above work which only adds MOF nanofillers to the PEO matrixes to prepare MOF-PEO-Li salts SSEs, the additive of MOFs fillers could exhibit a limited effect. The inadequate distribution of MOFs filler, on the one hand, cannot provide continuous channels for the transference of Li + . On the other hand, the agglomeration effect of nanoparticles enhances the inhomogeneous distribution.
To solve the problems, Li et al.'s [62] group designed two kinds of MOFs@PAN of UIO-66@PAN and ZIF-8@PAN [121] through electrospinning method and in situ grown method, respectively, and thus prepared SSEs with PEO and LiTFSI through vacuum-assisted liquid filling method. For the ZIF-8@PAN SSEs (Figure 6H Although PEO-based SSEs are used the most extensively, the intrinsic properties still cause poor electrochemical performance and restrict its application. [145] Many research about other polymers such as poly (tetrahydrofuran), [146] poly(ethylene carbonate), [147] tetra (ethylene oxide), [148] and others have been published. Wen et al. [124] designed P(TFEMA-ran-PEGMA) through the free radical copolymerization of tri-fluoroethyl methacrylate (TFEMA) and poly(ethylene glycol) methacrylate (PEGMA). When adding MOF-5 and LiTFSI to P(TFEMAran-PEGMA), the SSE exhibits an enhanced ionic conductivity of 1.44 × 10 −5 S cm −1 at 30°C and high Li + transference number of 0.512. Compared with PEO chains, PPEGMA segments possess a more rigid structure, which can keep stable in the transport process of charge carries. Hence, the as-prepared MOF-5 can dispera into the electrolyte of LiTFSI and P(TFEMA-ran-PEGMA), which ensures good compatibility. When tested for SSLIB, the cells can show the first discharge capacity of 116 and 107 mA h g −1 after 25 cycles at 60°C at the current density of 0.1 C, corresponding the capacity retention of 92.2%. Except for the enhancement of ionic conductivity, an appropriate design can alleviate the aggregation of MOFs fillers. Wen et al. [65] designed cross-linked hyperbranched polymer electrolyte composed of aldehyde-terminated PEG and HPEI for SSLIBs. When UIO-66 and LiTFSI were added into the composite polymer matrix, the CSPE can show considerable enhancement of Li + transference number from 0.23 to 0.54.
In this section, different kinds of MOFs and their merits for polymer-based SSEs are discussed. First, many kinds of MOFs with 3D open structures can show OMSs, which can attract anions through charge interaction, and thus improve the transference number. Then, the large surface and ordered channels provide plentiful active sites and fast pathways for Li + . Third, the high-tunability of the structures and functional groups provide more space for further study. Previous studies have shown that MOF-based SSEs still have room for improvement and the modification strategies can be concluded as follows: (1) the stability and thermal endurance of MOFs should be guaranteed because they influence the operating temperature and service life of SSEs. (2) The size and the surface characteristics of MOFs should be designed elaborately because they affect the electrochemical characteristics including ionic conductivity, Li + transference number, and thus cell's performance. (3) The interfacial reaction between MOFs and polymers has a great relationship with ions transportation and determines the electrochemical performance. Through a modification process such as carboxylation and amination, the LABI between MOFs and polymer matrix as well as ions can be enhanced so as to enhance the diffusion ability of Li + . (4) As the most abundant materials, the choice of polymer can also show a great effect on the comprehensive properties of SPEs. Polymers or their hybrids with high-ionic conductivity, excellent stability, and wide operation temperature should be designed. (5) The proportion of MOFs, polymer, and Li salts should also be controlled carefully. Results have proved that moderate content of MOFs fillers (around 10 wt%) and Li salts (10-15 wt%) is advisable to obtain superior performance. [149] 3.  [150][151][152] Different from traditional organic solvents, ILs have been regarded as "green solvent" because the characteristics of nontoxic, low melting point of below 100°C, high-ionic conductivity, negligible volatility, nonflammability, as well as high thermal/electrochemical stability. Nowadays, research have proven that ILs can be employed for catalysis, electrolysis, electroplate, supercapacitors, and high-performance batteries. [153][154][155] Meanwhile, many studies have reported that ILs with higher ionic conductivity are promising materials for SSEs. MOFs, with porous and strong skeleton structures, are suitable materials to provide space for ILs through the absorptive reaction and change the phase behavior of fixed ILs, and thus design highperformance batteries with SSEs. More importantly, the large surface of MOFs can enhance the interfacial contact between electrolyte and electrode, as well as enhance the ionic conductivity, the corresponding data can be seen as Table 2. [156][157][158] Different from the liquid electrolyte system, which can exhibit excellent infiltration in the electrodes through wetting action, solid electrolytes often possess poor interfacial compatibility to electrolytes thus the ions transport and battery performance. ILs, with high flowability and excellent ionic conductivity, can effectively improve the interface transport dynamics and enhance the performance of SSEs. Wang et al. [162] designed Li-ILs (0. 8

M LiTFSI dissolved in [EMIM] [TFSI]) for MOF-based SSLIBs and exhibited the ionic transference mechanism of nanowetted interfaces. Firstly, the Li-IL@MOF (LIM) composed of UIO-67 and
Li-IL as well as the LLZO-based SSE (LLZO: Li 7 La 3 Zr 2 O 12 ) can be prepared through the process of Figure 7A. As the host of ILs, UIO-67 possess large porosity and appropriate size of 12 Å for octahedral cage, and the guest of [EMIM] + (size: 7.9 Å) as well as [TFSI] − (size: 7.6 Å) have high-ionic conductivity, low vapor pressure, low viscosity, and wide electrochemical window. A good absorbing behavior can be seen from the test of N 2 adsorption-desorption obviously, which the surface area drops to 8 m 2 g −1 from 2169 m 2 g −1 . The ionic conductivity exhibits a rising trend with the content of Li-IL increases, but the as prepared electrode with 2.0 mL Li-IL guest and 1.0 g MOF cannot retain even gel-state, which indicates MOFs cannot immobilize excess ILs. The enhanced ionic conductivity indicates modified interfaces transference, which can also be seen from the EIS ( Figure 7G), A smaller diagram of the semicircle of the LIM-L hybrid SSEs prove a smaller interface resistance. Besides, a solid-state lithium battery with SSE of MOF-525 (Cu) encapsulated the same Li-IL was designed and exhibit improved performance. [159] The transfer behavior of Li + can be seen from ( Figure 7B), the movement of [EMIM] + and [TFSI] − is limited to the pores of MOF-525 (size of apertures: ∼12 × 7 Å), so the Li + transference number can be increased to 0.36 from 0.14 of pristine Li-IL. The excellent transmittability and small interface resistance can be shown from the direct current Li plating/stripping of Li|Li-IL@MOF|Li. Notably, the total resistance reduces to 120 Ω from 130 Ω after cycling ( Figure 7C). The potential mechanism can be represented from Figure 7D. Compared with the rough and mismatched interface between electrolyte and Li electrode, a smooth and dense interface can be formed after the Li deposition and the resistance is reduced. In fact, the fillers are Li dendrite generated during the Li plating/stripping reaction. Rather than form dendritic thorns, the metal Li deposits uniformly and filled in the blanks. The advantages of stable resistance and dendrite-free provide possible for high active loading and high energy density. When increasing the active loading to 25 mg cm −2 , the SSLIBs with LFP cathode can exhibit the first discharge capacity of 145 mA h g −1 at 0.1 C and an acceptable discharge capacity 67 mA h g −1 with 0.05 C even at −20°C.
It can be believed that, as the host of ILs, MOFs should possess porous structures and appropriate size of cores so that the ions can be anchored. At the same time, the ordered channels of MOFs can provide quick pathways, so that enhanced ionic transportation can be achieved. Wu et al. [163] prepared UIO/Li-IL SEs through hot-pressing method. The results show that UIO/Li-IL can maintain the same cuboid structure of UIO and the ILs distributed uniformly ( Figure 7E,F). The author believes that the nanostructured UIO/Li-IL possesses high surface tension, which prompts a tight interface with electrode and provides fast kinetics of ionic transference. Thanks to the advantages, the SSE can show enhanced ionic conductivity of 3.2 × 10 −4 S cm −1 as well as Li + transference number of 0.33 at 25°C. Furtherly, the as-prepared SSLIBs with LiFePO 4 cathode can exhibit a capacity of 130.2 mA h g −1 and retain 130.4 mA h g −1 even after 100 cycles at 0.2 C, as well as 119 mA h g −1 after 380 cycles at 1 C. ( Figure 7K). Differently, Abdelmaoula et al. [160] designed core-shell MOF-in-MOF nanopores UIO-66@67 ( Figure 7H) as the host of ILs and the UIO-66 with core structures can show selection effect for the transportation of ions. Although the UIO-66@67 show a cubic and octahedral nanostructure, adjacent framework channels are connected with each other after the hot-pressing process and thus form 3D conductive pathways through the interface of electrolyte and electrodes. Notably, the size of is 1.6-2.1 nm, which can allow the Li + move freely and limit the movement of ions of ILs with large size. On the other hand, due to the face-sharing of every open framework structure, Li ions can transmit throughout the contiguous crystals easily and enhance the transference kinetics ( Figure 7I). Hence, the as-prepared composite SSE can show a high Li + transference number of 0.63, high-ionic conductivity of 2.1 × 10 −3 S cm −1 as well as super low activation energy of 0.086 eV. When assembled for SSLIBs, the Li|CSIL|LFP can exhibit superior performance of specific capacity of 158 mA h g −1 as well as excellent capacity retention of 99% after 100 cycles correspondingly.
Although SSEs can reduce the generation of Li dendrites, a small amount of dendrite can still be produced during the battery reaction and thus damage the electrochemical performance. A lot of efforts have been done to search for appropriate materials and effective strategies to achieve dendrite-free lithium batteries. Shang et al. [165] designed composite electrolyte through the in situ polymerization of UiO-66-NH-M, DDC, AIBN, and LiTFSI ( Figure 7L). Benefiting from the unsaturated metal cation sites and the ordered channels of MOFs, the anions of TFSI − can be anchored and amount of Li + can move at freedom, which avoid the Li deposition as well as the generation of dendrite. The different Li + transference behaviors of LIBs with liquid electrolyte and SSE can be seen as ( Figure 7J). Due to the stability of electrode and electrolyte, the assembled battery can exhibit cycling performance of 95.75%  [159] Copyright 2018, Elsevier. (E, F, and K) Reproduced with permission. [163] Copyright 2019, Wiley. (H, I) Reproduced with permission. [160] Copyright 2021, Wiley. (J, L, and M) Reproduced with permission. [165] Copyright 2020, American Chemical Society. EIS, electrochemical impedance spectroscopy; LIM, Li-IL@MOF; MOF, metal-organic framework; SSE, solid-state electrolyte; TEM, transmission electron microscopy. capacity retention and 99.4% Coulombic efficiency after 200 cycles at 0.2 C ( Figure 7M).
For the SSEs with MOFs encapsulated ILs, the porous structures and the composition of ILs play a significant role in the electrochemical performance. Although the addition of ILs can enhance the ionic conductivity of electrolytes, the excessive ILs can still break the solid state of electrolytes. Hence, the widespread use of ILs will increase the costs and restrict the practical application. In fact, the optimal performance can be achieved when the content of ILs is between 10% and 15%. For MOFs, an appropriate size of pores can limit the movement of large anions of ILs, expedite the transportation of Li + , improve the Li + transference number, and enhance the electrochemical performance of SSLIBs. Meanwhile, the OMSs in MOFs can attract anions according to the Lewis acids-bases, and the introduced functional groups with positive charges can also attract anions further. Therefore, Zr-based MOFs, such as UIO-66 and UIO-67, with 3D open structures and suitable size of pores (around 12 Å) have been proved to be popular substrates for ILs and been widely studied.

| MOFs for single-ionic conducting SSEs
Although MOF-based SPEs have achieved enhanced performance through the constant efforts, they still suffer from inherent problems. For example, the anions and cations from the Li salts can move freely and the anions move at least four times faster than lithium cations generally, which can induce the small Li + transference number. The result can be ascribed to that lithium cations suffer from the less mobility in polymer matrix in comparison to the movement of Lewis-base sites. Moreover, the accumulation of the unreacted anions can cause polarization, voltage loss, high internal impedance as well as thus poor battery life. [166,167] Single-ionic conducting SSEs, which possess a superior high Li + transference number, have gotten researchers' attention. For single-ionic conducting SSEs, the anions are immobilized through attaching anions to the inorganic/polymer backbone or employing trapping agents for anions, and leave free cations. Many research have proved that the single-ionic conducting SSEs can not only modify the cations transference kinetics but also suppress the generation of Li dendrite through reducing the deposition of Li + . MOFs, with numerous active sites, strong skeletons and high tunability of functional groups, have been employed for single-ionic conducting SSEs. The corresponding information is listed as Table 3. Through the elaborate design of MOFs structures, such as postsynthetic modification, outstanding SSEs, and high-performance batteries can be obtained. [168][169][170] In 2011, Wiers et al. [175] firstly introduce MOFs to SSEs through designing lithium isopropoxide (LiO i Pr) grafted Mg-MOF (Mg 2 (dobdc)) by a postsynthetic modification ( Figure 8A). [169] In the presence of OMSs, alkoxide anions might preferentially link with the Mg 2+ ions and be immobilized. The sample of Mg 2 (dobdc) 3 · 0.35LiO i Pr·0.25LiBF 4 ·EC·DEC, prepared through soaking the as-prepared Mg 2 (dobdc)·0.5LiO i Pr into 1 M mixed solution of LiBF 4 in 1:1 EC/DEC, can exhibit low activated energy of 0.15 eV and almost ionic conductivity 0.1 S cm −1 at the temperature of 30°C. As one of the MOFs with the highest density of OMS, Cu-MOF-74 (Cu) can also be used to design single-ionic conducting SSEs. Yuan et al. [83] prepared single-ionic conducting SSEs through thermal treating of MOF-74 and 1 M LiClO 4 in the PC at 80°C ( Figure 8B). The as-prepared SSEs have ionic conductivity of 10 −5 S cm −1 as well as relatively low activated energy of 0.29 eV. The author holds views that the ions transference kinetics of single-ionic conducting MOF-74−based SSEs is similar to Grotthuss-like mechanism, of which ions transported by the coordinated hopping of solvated Li + between the oxygen groups in the pores of Cu-MOF-74 (O containing groups in the MOF-71 are electronegative and highly lithiophilic) ( Figure 8C). [177][178][179] By a postsynthetic modification process, Yang et al. [174] prepared UiO-66-LiSS ( Figure 8I), which the UiO-66-NaSS is obtained through the link of sulfonated side chains and sodium p-styrene sulfonate to UiO-66-Br and then exchange Na + with Li + . A super high Li + transference number of 0.88 and enhanced ionic conductivity 6.0 × 10 −5 S cm −1 at 25°C can be obtained through the introduction of PC and EC to the single-ionic conducting SSEs. The results can be ascribed to the existence of EC and PC can form Li +solvent properties such as Li + -EC and the Li + on the styrene sulfonate skeleton free. [180,181] In fact, the OMSs play a significant role in the ionic transportation of single-ionic conducting SSEs, which can produce more links and attract anions. Shen et al. [64] prepared a series of MOFs with/without OMSs, including HKUST-1 ( Figure 8D), MIL-100 ( Figure 8E), and UIO-66/67 ( Figure 8F). Firstly, HKUST-1 with plenty of OMSs are prepared due to the deep study previously. After an immersion process into the hybrids of LiClO 4    Constructing single-ionic conducting SSEs is the effective strategy to reduce polarization, increase the transference number and enhance ionic conductivity. The ordered channels, adjustable pores, and controllable surface polarity make MOFs are excellent candidates to prepare single-ionic conducting SSEs. However, there are still some potential problems that restrict further development. Although the ionic conductivity of singleionic conducting SSEs is superior, the commercial applications are still hard to meet. Hence, conducting MOFs should be introduced to form single-ionic conducting SSEs. On the other hand, to enhance the ionic conductivity further, lithium salts should be grafted on the MOFs, but the SSEs with different lithium salts often show various performances. Hence, more details should be investigated through constructing models and developing theoretical calculations.

| Lithium-sulfur batteries
Considered as the possible candidate for the nextgeneration energy storage devices, LSBs have become a research hot spot because of the high theoretical energy density (∼2600 Wh kg −1 ), outstanding energy density, low cost as well as minimal memory effect. [182][183][184] However, there are some drawbacks that still restrict the further application of LSBs, such as the shuttle effect caused by the dissolution of polysulfide (Li 2 S x ), large volumetric change of sulfur electrode and the insulating property of S and Li 2 S. [185][186][187] Nowadays, the research on LSBs mainly focuses on the modification of electrodes, the upgrade of electrolyte, and the design of separator. It has been a great success in the aspect of high energy density, long cycling life and high sulfur loadings. LSBs with high-performance such as superior energy density of 916 W h kg −1 , [188] high initial discharge capacity of 1068.4 mA h g −1 , [189] long cycling life of more than 4000 cycles, [190] high load of S which over 12 mg cm −2 , and others. [191] Notably, replacing the liquid electrolytes can not only improve security but also reduce the use of separators for the reason that SSEs can work as separators and thus release more space as well as enhance the energy density. [192][193][194][195][196] At present, the SSEs used for SSLSBs mainly including polymer-based electrolytes, ceramic electrolytes, and hybrid electrolytes. Although ceramic electrolytes which mainly including oxide-based and sulfide-based often possess superior ionic conductivity and good stability, the unsatisfactory interfacial resistance caused by poor contact needs to be alleviated. With good flexibility and compatibility, polymer-based electrolytes can exhibit low interface resistance. But on the other hand, polymer-based electrolytes suffer from poor mechanical properties which fail to work for the suppression of Li dendrite. [197][198][199] Through the introduction of fillers such as LiN 3 , In 2 O 3 , and MOFs, the comprehensive proprieties can be enhanced. [200,201] For instance, fillers with large specific surface areas can form more interfacial areas with the polymer matrix so that increases the mechanical property and fillers with abundant active groups on the surface can interact with polymer matrix and enhance ionic conductivity. Among various fillers, MOFs have received attention because of their porous structures, large surfaces, and the adjustability of functional groups. Firstly, the holes in MOFs can retain the Li 2 S x and strong interaction force can prevent the permeation of Li 2 S x further. Secondly, the favorable contact of MOFs and S can provide a good conductivity and a good rate capacity corresponding. [202][203][204] Except for SSLIBs, Suriyakumar et al.'s [121] group also explored and designed SSLSBs. For instance, the Al-MOF-based SSE was prepared and the assembled batteries can exhibit an initial discharge capacity of nearby 1500 mA h g −1 and a stable capacity of 800 mA h g −1 (Figure 9A-C). Han et al. [205] obtained MOF-74 through the thermal reaction of Mg 2 O 2 (CO 2 ) 2 and 2,5dioxido-1,4-benzenedicarboxylate, then dispersed MOF-74 into PVDF to prepare gel polymer electrolyte (GPE). When LiTFSI and other organic solutions are introduced to the GPE, the quasi-solid-state LSBs with MOF-PVDF GPE can exhibit an enhanced initial discharge capacity of 1383 mA h g −1 . Notably, the as-prepared MOF-modified PVDF-based GPE possess ionic transference number of 0.66 and ionic conductivity of 6.72 × 10 −4 S cm −1 , which is higher than PVDF electrolyte (4.26 × 10 −4 S cm −1 ) and commercial separator (3.43 × 10 −4 S cm −1 ). The improved performance, can be ascribed to not only the Lewis acid-base effect between TFSI − anions and abundant of Mg 2+ , but also the constraint of TFSI − anions given by MOFs' porous structure. More important, Han et al. [205] explored the formation of SEI and made a comparison further ( Figure 9H,I). The result shows that the MOFmodified GPE can generate smooth SEI film of 5 µm thickness on the anode, while the battery with separator shows SEI films with various thickness of 4-9 µm and the battery with PVDF exhibit patchy SEI film. SEI films with neat and dense structures can inhibit the decomposition of electrolytes and improve the cycle life of batteries to a great extent.
To mitigate the impact of shuttle effect, Chiochan et al. [206] designed lithium sulfonate group grafted MOF (UIOSLi) and then treated with Li-IL (LiTFSI in EMIM [TFSI]) to improve the ionic conductivity of SSE further ( Figure 9D-G). As the schematic structures show, the pore with size of 0.6-1.2 nm which can inhibit the movement of polysulfide. Meanwhile, the existence of SO 3− can introduce repulsion effect on the S n 2− as well as attraction effect on the Li + so that the transport of Li + can be improved. Benefiting from the structural design, the LSB with the Li-IL/UIOSLi solid electrolyte can exhibit discharge capacity of 1095 mA h g −1 at 0.1 C and 749 mA h g −1 at 1 C. When the current density comes back to 0.1 C, the cell can still maintain a discharge capacity of 1033 mA h g −1 , which mean an excellent electrochemical stability. Compared with the commercial separator, the designed SSE manifests longer cycling performance and enhanced discharge capacity, which can be ascribed to the polysulfide restriction provided by Li-IL/UIOSLi. Up until now, the development of MOF-based SSLSBs is at the beginning stage and there is still plenty of scope for development. (1) Effective solid electrolytes need to be designed. Ideal SSEs for LSBs should have a high-ionic conductivity of around 10 −3 S cm −1 , physical and chemical stability, and wide electrochemical window. (2) The interface reaction between electrodes and SSE should be explored further. The insufficient solid-solid phase contact and sluggish interface reaction kinetics can bring about inferior electronic transport and poor electrochemical performance. [207][208][209]

| Sodium-ion batteries
Sodium-ion batteries have been regarded as the possible substitution of LIBs, due to the abundance of sodium resource, acceptable specific capacity of around 1166 mA h g −1 , relatively high standard reduction potential (−2.71 V vs. standard hydrogen electrode [SHE]), and similar electrochemical reaction mechanisms as LIBs. However, due to the larger ionic radius of 1.02 Å, SIBs often suffer from large volume changes of electrodes and sluggish reaction mechanisms. On the other hand, a higher molar mass of 23 g mol −1 as well as larger reduction potential could lead to low energy density and low operating voltage. More importantly, the growth and accumulation of sodium dendrite in the battery reaction may puncture the separator and induce the short circuit. [210][211][212] Hence, many efforts have been made to solve the present situation and explore high-performance SIBs. Nowadays, the directions mainly include the exploration of electrodes, the design of electrolytes, and the construction of batteries' structures and there have been some success including high working voltage of ∼4 V, [213] excellent stability of more than 5000 cycles, [214] rational design of electrode materials in theory and so on. [215] Similar to SSLIBs, the SSEs for SIBs can be divided into three categories of CPEs, SIEs, and SCEs of which CPEs are researched widely due to their good flexibility and high security. The ions transference of CPEs can be attributed to the alkali ions solvated by polymer chains that can move along with the movement of molecular chains. However, the simplex existence of polymers such as PEO, PMMA, cannot support an acceptable ionic conductivity. Therefore, some strategies are chosen to solve the dilemma which mainly include the following: (1) employ other kinds of Na salts such as NaPF 6 , NaClO 4 , NaCF 3 SO 3 , NaSCN, NaBF 4 , sodium 2,3,4,5tetracyano-pirolate, sodium 2,4,5-tricyanoimidazolate.
Inspired by the structure of red blood cells, Zhang et al. [222] prepared pancake-like MOF (PLM) for SSSIBs to optimize the ion transport and interfacial impedance. Firstly, pancake-like MIL-125 is prepared through the hydrothermal reaction of titanium isopropoxide (Ti(OCH (CH 3 ) 2 ) 4 ) and p-phthalic acid (HOOC(C 6 H 4 )COOH), then decanedron-like MIL-125 and NH 2 -MIL-125 were designed through the PLM precursor ( Figure 10A,B). Notably, NH 2 -MIL-125 still maintain a pancake-like structure as same as MIL-125, which mean that the NH 2 functional group would not change the structure of precursor. When assembled for SSSIBs, a small quantity of liquid electrolyte (LE: NaClO 4 -PC with 5% FEC) is added to propel the ion transmission. Without the MOFs, the initial LE possess Na + transference number of 0.16, which ascribed to the transportation of ClO 4 − rather than Na + ( Figure 10C). However, the PLM@LE can show an enhanced Na + transference number of 0.38. Different from the previous report that the limitation of solvent molecules migration will increase the desolvated process of Na + and the Na + -ion transference number. [223] Zhang et al. provide a hypothesis that the increase of the density of the electron cloud brought from some special functional groups can produce repulsive force against the electron-rich material. ILs can be employed for SSSIBs to improve the ionic conductivity and stabilize interfacial reaction due to the merits of wide electrochemical window, excellent stability, and high-ionic conductivity. Many research have been done to explore different kinds of Na-ILs and received improvement. Feng et al. [224] designed sodium ILs through the hydrothermal reaction between NaTFSI and 1-(1-ethyl-3-imidazolio)propane-3-sulfonate (EIMS) which donated as EN for SSE. By an impregnation-evaporation method, EN-n@UiO-6 7 −MIMS with the reaction of activated UiO-67-MIMS and EN-1 (EN-n: the molar ratio of NaTFSI and EIMS is 1:n) ( Figure 10D). Notably, the EN-1@UiO-67-MIMS shows a high-ionic conductivity of 1.24 × 10 −4 S cm −1 , which is much higher than that of EN-2.3@UiO-67-MIMS. The reason can be ascribed to the ordered abundant pathways in EN-1@UiO-67-MIMS, produced by zwitterion MIMS groups on the UiO-67-MIMS framework which can contact and pair with NaTFSI as that for EIMS through H-bonding and electrostatic interaction. Employed similar Zr-MOF, Yu et al. [221] prepared SSSIBs of Na|Na-IL/UIOSNa|Na 3 Ni 1.5 TeO 6  [222] Copyright 2021, Springer. (D) Reproduced with permission. [224] Copyright 2022, The Royal Society of Chemistry. (E) Reproduced with permission. [221] Copyright 2021, The Royal Society of Chemistry. (F, G) Reproduced with permission. [225] Copyright 2020, Wiley. IL, ionic liquid; MOF, metal-organic framework; PLM, pancake-like MOF.

| Other batteries
Owing to the inflammability of Li metal and high cost, other alternative metals (Zn, Mg, Al) are considered to design newtype high-performance batteries, with the characteristics of nontoxic, lower cost, and excellent safety. Meanwhile, replacing the liquid electrolytes with SSEs can enhance the safety and energy density of the battery itself, which can be applied in many fields, such as wearable devices, wireless sensors, implantable medical devices, electromobile, and others. [226][227][228][229][230] In this section, SSEs applied in other highperformance batteries, including magnesium-ion batteries, zinc-ion batteries as well as aluminum-sulfur batteries, are mainly discussed.
Solid-state Zn batteries, with the merits of good biocompatibility, high volumetric capacity of 5855 Ah L −1 , low redox potential (−0.76 vs. the SHE), low cost of $0.9 b −1, and excellent processing ability. [231,232] However, the inherent poorer ionic conductivity (around 10 −7 S cm −1 ) as well as strong bonding with solvating polymer chains limit the ion transference kinetics. As the fillers, MOFs can modify the ionic transportation and improve the mobility of polymer chains through providing conductive networks Duan et al. [233] designed carboxyl-decorated anionic In-MOF through a solvothermal reaction, as Figure 11A shows. Notably, the as-prepared MOFs can exhibit 12 negative sites of one unit cell Figure 11B. The high-density functional sites can attract Li + and accelerate the transference rate, so the In-MOF can show enhanced Zn 2+ ionic conductivity of 1.22 × 10 −3 S cm −1 at 25°C.
Wang et al. [172] are dedicated to exploring highperformance solid batteries, especially solid-state Zn batteries. In 2019, his group successfully prepared single-ion Zn 2+ MOF (ZnMOF-808) SSEs through the postsynthetic modification of MOF-808 ( Figure 11C,D) Figure 11F) and around 125 mA h g −1 even at high current density of 1 A g −1 ( Figure 11G). Similar to LSBs, Al-S batteries, which replace expensive metal Li with low-cost metal Al, can also show an excellent theoretical capacity of up to 1340 Wh kg −1 . As mentioned above, SSEs could effectively refrain from the shuttle effect of S and enhance stability for the reason that the shuttle effect can lead to severe contamination on cathode and short cycle life. However, the reaction of Al and S involve a multielectron transfer process, which means a slow reaction kinetic. Meanwhile, the sluggish ionic transference in the SSEs reduce the ionic conductivity further. The introduction of electrocatalysts for SSEs, can not only improve the reaction kinetics but also can stabilize the battery system. Huang et al. [234] designed cobalt-nitrogen codoped graphene (CoNG) for IL@MOF SSE and assembled solid-state Al-S battery ( Figure 11H,I). These results show that ordered channels of MOFs can effectively transport active ions (AlCl 4 − And Al 2 Cl 7 − ) as well as constraint the shuttle effect. Hence, the as-prepared battery can exhibit excellent stability of 820 mA h g −1 and the capacity of which can remain 78% of the initial capacity even after 300 cycles ( Figure 11J). At the same time, the composites of CoNG on the S cathode, which could effectively decrease the inherent energy barrier and prompt the dissociation of Al 3+ , as well as the higher ionic conductivity (4.2 × 10 −4 S cm −1 ) and relative lower activation energy of about 0.233 V, the improved electrochemical performance could be achieved. The assembled Al|IL@MOF|S@CoNG can show an initial discharge capacity of about 820/202 mA h g −1 at current density of 50/300 mA g −1, respectively ( Figure 11K,L).

| CONCLUSIONS AND OUTLOOK
Recently, SSEs have become a hot research topic and has been widely employed in high-performance batteries because of the excellent characteristics the advantages of high security as well as excellent stability. However, some drawbacks, such as severe interfacial transportation problem, low ionic conductivity, complicated F I G U R E 11 (A) Solvothermal reaction scheme. (B) X-ray crystal structure of In-MOF as well as the crystal structure viewed along the crystallographic [117] direction.  [233] Copyright 2021, American Chemical Society. (C-G) Reproduced with permission. [172] Copyright 2019, Elsevier. (H-L) Reproduced with permission. [234] Copyright 2022, Wiley. Al-S battery, aluminum-sulfur battery; MOF, metal-organic framework; WZM, water@ZnMOF-808. interface reaction and poor cycling performance still restrict the further development of SSEs in secondary batteries. To enhance the electrochemical performance to meet the practical application, many strategies have been implement including the employment of fillers, the design of the component and the exploration of construction for SSEs. MOF-based SSEs, which can inherent the merits of MOFs such as high stability, tunable size of pores and structures, and adjustable polarity, are promising candidates for high-performance secondary batteries.
In this review, different kinds types of MOF-based SSEs and their corresponding application in highperformance secondary batteries. According to the different role of MOFs, the MOF-based SSEs could be divided into three classes: MOFs-incorporated polymer as SSEs; ILs-incorporated MOFs as SSEs and singleionic conducting MOFs as SSEs. In the first case, MOFs work as fillers and play a significant role in adjusting the comprehensive properties including ionic conductivity, stability, and assemble of SSEs. To enhance the ionic transference performance further, ILs are often employed for SSEs due to the abundant anions/cations at room temperature. For ILs incorporated MOFs composites, MOFs play a role of host for ILs and often restrict the fast movement of anions, and thus promote the transference of Na + /Li + . Among the three categories, SSEs with single-ionic conducting MOFs can exhibit a more superior ionic conductivity and ionic transference performance. Through the postsynthetic modification, MOFs can graft anion or groups, and thus form quick ionic channels for the transportation of cations.
However, the development of solid-state batteries, as new research, is currently in its infancy and there are still numerous difficulties limit the further application. For polymer-based SSEs, the lower ionic conductivity hinders the high performance. Although the inorganic SSEs possess a higher ionic conductivity to fulfill the requirement, the hardness properties increase the difficulty of processing. As for MOF-based SSEs, although the single-ionic conducting SSEs with MOFs can show high-ionic conductivity to meet the practical application, the complex multistep prepare reactions limit the implement on the large scale. Hence, efforts need to be done to achieve the accomplishment. To further explore SSEs with higher electrochemical performance, the future research direction could be conducted on the basis of the following fields: (1) The ability of ionic transportation is an important parameter of SSEs and plays an important role in the enhancement of electrochemical performance for solid-state batteries. Generally speaking, the ionic conductivity of SSEs should be greater than 10 −4 S cm −1 to fulfill the operation of secondary batteries, while the polymer-based SSEs exhibits a lower ionic conductivity. The introduction of inorganic materials, especially the MOFs, could enhance the ionic conductivity through preventing the aggregation of polymer chains and improving the movement of ions. Moreover, MOFs could enhance the ionic conductivity for MOF-based SSEs, the ionic conductivity can be further enhanced through the decoration of surface functional groups as well as the diverse coordination modes of MOFs. Especially, MOFs with OMSs exhibits better ionic conductivity through the strong interacting strength between metal centers and anions. Moreover, the introduction of additives, especially ILs and organic solution, can provide more free ions and improve the transportation, thus enhancing the transference number and ionic conductivity. (2) The problem of serious interface is one of the most important problem limiting the performance of SSEs, including the stability of interface and transference of ions between the electrodes and electrolytes. The resulting structural stress can accumulate with the battery reaction progress and finally influence the performance. On the other hand, the poor interface wettability and interfacial stability limit the transmission of ions and thus affecting the ionic conductivity. Numerous strategies, such as reducing the size of material, constructing the 3D interface as well as transforming the rigid interface into flexible interface, can improve the interfacial behavior. Moreover, a little bit of liquids can effectively improve the interfacial transference and the safety is not compromised too much. (3) Although numerous research about SSEs and their application in high-performance batteries have been published, the ionic transference mechanism and interface behavior are not clear in detail. At the same time, the role of MOFs and other additives such as ILs for SSEs should be further investigated to provide the theoretical foundation for the improvement of ionic transference performance and enhancement of electrochemical performance for solid-state batteries. Some advanced characterization technologies, such as in situ characterization, should be employed to analyze the structural evolution, ionic transportation, and interfacial behavior. (4) The rational design of SSEs and solid-state batteries should be put into consideration. Except for the materials of SSEs, the configuration can also play a significant influence in the safety and electrochemical performance of solid-state batteries.
(5) For MOFs, at present, only several kinds of MOFs, especially Zr-and Mg-based are employed for SSEs, other MOFs should also be explored for the SSEs. Meanwhile, in the view of the flexibility of the porous structures, the ordered channels, the adjustable of the size for pores and channels, the tunability of the surficial polarity, more research should be done to investigate the role of MOFs and summarize the universal rules for the preparation of high-performance batteries. More importantly, for most MOFs, those whose preparation needs the condition of high temperature and pressure, a long reaction time, and tedious steps. The demanding reaction conditions and high cost restrict the largescale commercial applications. Hence, large-scale synthesis approaches under atmospheric pressure as well as room temperature should be widely exploited. Nowadays, some strategies, such as reflux-assisted postsynthesis method and flow-synthesis method, have shown the rudiments.