Disordered coordination networks are regarded as one of the defining structural characteristics of MOF glasses and are fundamentally distinct from the highly periodic frameworks observed in crystalline MOFs. Unlike crystalline counterparts that rely on long-range periodic ordering, MOF glasses generally preserve only short-range ordering around the metal coordination environment while exhibiting substantial long-range disorder throughout the extended framework. For example, in ZIF-derived glasses, Zn-N coordination and tetrahedral Zn-imidazolate local structures are often preserved after glass formation, whereas longer-range Zn-Zn or framework correlation peaks become broadened or weakened^[31]. This suggests that the periodic framework structure is transformed into an isotropic disordered network.

Figure 1 further summarizes this local preservation-long-range disorder structural feature. Single-crystal X-ray diffraction (XRD) indicates that certain coordination bonds in crystalline coordination frameworks become weakened before melting and may act as initiation sites for crystal-structure collapse and melting^[32] [Figure 1A]. The differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) results show that MOF glass formation is accompanied by clear melting, glass transition, and temperature-dependent viscosity changes, suggesting that these materials are not simply amorphized products but undergo an identifiable crystal-liquid-glass phase transition [Figure 1B]. Variable-temperature solid-state nuclear magnetic resonance (NMR) and molecular simulations reveal the presence of dynamically heterogeneous coordination environments during glass formation [Figure 1C]. Furthermore, reverse Monte Carlo (RMC) modeling, combined with pair distribution function (PDF), extended X-ray absorption fine structure (EXAFS), and magic-angle spinning nuclear magnetic resonance (MAS NMR) data, reconstructs the atomistic glass structure and further demonstrates that short-range coordination motifs can be retained within the glassy network, while medium- and long-range structures become significantly disordered [Figure 1D].

Compared with crystalline MOFs, such structural heterogeneity is particularly important because it introduces locally flexible coordination sites, that are difficult to achieve in ordered crystalline frameworks. The resulting disordered coordination networks therefore provide a structurally adaptive platform for ion transport, where local coordination fluctuations and dynamically rearrangeable environments may facilitate ion hopping and reduce migration barriers^[33]. These characteristics increasingly suggest that the functional advantages of MOF glasses are not derived from preserving crystalline order, but rather from exploiting controlled disorder within coordination networks to enable emergent transport behaviors inaccessible in rigid crystalline MOFs.

MOF glasses are the product of certain crystalline MOFs that undergo melting, quenching, and finally vitrify^[34-36]. The thermodynamic characteristics of MOF glasses are fundamentally governed by the interplay between framework mobility, coordination dynamics, and temperature-dependent structural rearrangements during melting and vitrification. Unlike conventional inorganic glasses, MOF glasses originate from coordination networks in which metal-ligand bonds remain partially dynamic even in the liquid state, enabling a unique form of thermodynamic softening prior to complete structural collapse.

Among them, zeolitic imidazolate frameworks (ZIFs) family, especially ZIF-62, has received considerable interest due to its excellent glass-forming performance. Figure 2 illustrates the glass-forming process of ZIF-62^[37]. Adopting similar structures to zeolites, the ZnN₄ tetrahedra in ZIF-62 replace SiO₄ tetrahedra and are linked by rigid organic ligands rather than less-constrained bridging oxygens [Figure 2A]. This zeolite-like tetrahedral topology reduces the angular constraints, and enables continuous framework reorganization upon heating, thereby fundamentally underpinning its ability to undergo vitrification prior to decomposition. The dominant prerequisite for MOF glass formation is the existence of a well-defined melting regime prior to framework decomposition, namely that the melting temperature (T_m) lies below the decomposition temperature (T_d), resulting in a sufficiently wide melting window^[31] (ΔT = T_d - T_m). Upon rapid cooling from the molten state, the disordered coordination network becomes kinetically arrested, suppressing crystal nucleation and growth, and yielding an amorphous glassy solid^[37] [Figure 2B and C]. The melt-quench nature of MOF glasses allows shaping and device fabrication (e.g., hot imprinting and membrane processing). Compared with crystalline MOFs, bulk MOF glasses have been shown to possess mechanical robustness, which enables membrane fabrication and thermal processability, thereby enhancing their potential for device integration^[22,38-41].

In the case of ZIF-4, molecular dynamics simulations provide mechanistic insight into the microscopic origin of MOF melting. As shown in Figure 3A, crystalline ZIF-4 transforms into a disordered melt at 1,500 K and can subsequently be quenched into a glassy state. The corresponding PDFs in Figure 3B show that the structural peaks broaden upon melting, indicating the loss of long-range order while short-range correlations remain partially preserved. Figure 3C further shows that the N-Zn-N bond-angle distribution becomes significantly broader in the molten state, suggesting distortion of the local ZnN₄ tetrahedral coordination environment. These changes indicate that ZIF-4 melting is associated with Zn-N coordination lability and local network rearrangement rather than complete collapse of the metal-ligand framework. Meanwhile, Figure 3D shows that the pore-size distribution becomes broader after melt-quenching, reflecting the formation of a more disordered glassy network. Overall, ZIF-4 melting produces a liquid-like disordered state in which local Zn-N connectivity and partial framework memory are retained, while medium- and long-range order is progressively lost^[42].

Given the melting behavior of certain MOFs, the possible role of framework mobility in MOF-glass ion transport can be discussed with reference to soft matter electrolytes based on this broader principle: structural mobility and transient free-volume formation can influence ion migration in disordered solid electrolytes. Figure 4A shows the tensile deformation of polyethylene oxide (PEO)/LiClO₄ polymer electrolyte films, while both the in-plane and out-of-plane ionic conductivities increase with deformation/strain [Figure 4B]. This enhancement is attributed to cavitation-induced tiny voids, particularly in the amorphous regions of the semicrystalline polymer matrix [Figure 4C]. These voids increase free volume and may facilitate cooperative rearrangements of polymer chains, thereby improving ionic conductivity. In MOF-derived glass electrolytes, an analogous broad principle may apply, although through a distinct structural mechanism: glass-state framework mobility may promote ion migration through transient free-volume generation, local bond-angle fluctuations, and cooperative rearrangement of coordination motifs, rather than through polymer-chain segmental motion or a purely static hopping pathway.

In addition, it is important to note that MOF-glass conductivity is not governed by free volume or viscosity alone both of which are influenced by the vitrification process. Figure 5 further shows that, in Zn-H_nPO₄-based MOF glasses, changes in microdomain size and coordination-network connectivity alter bulk-phase viscosity but have only limited influence on ion conductivity. This suggests that local coordination chemistry, especially metal coordination number and ligand-based hopping-site density, can be more decisive than macroscopic viscosity. Therefore, MOF glasses should not be viewed simply as ordered crystals transformed into static amorphous solids, but as dynamically disordered yet partially connected coordination networks, in which framework mobility, free-volume evolution, thermodynamic relaxation, and local coordination motifs collectively regulate both vitrification and ion-transport properties.

Porosity and pore behavior in MOF glasses are fundamentally governed by the competition between structural densification during vitrification and the retention of accessible free volume inherited from the parent crystalline framework. Early studies on melt-quenched ZIF glasses showed that vitrification is often accompanied by substantial densification and loss of gas-accessible porosity^[45]. For example, melt-quenched ZIF-4 was reported to form a nearly non-porous glass with a Brunauer-Emmett-Teller surface area below 5 m²·g^-1, indicating severe collapse or contraction of the originally porous framework^[35]. Subsequent positron annihilation lifetime spectroscopy studies further showed that a_gZIF-4 retains internal free volume, but much of this porosity is inaccessible to conventional gas sorption probes, highlighting the distinction between intrinsic free volume and gas-accessible porosity^[46,47]. Similar reductions in accessible pore volume and surface area have also been observed in pressure-induced and mechanically amorphized ZIFs, suggesting that framework collapse, densification, and loss of long-range order commonly reduce the accessible porosity of amorphous MOF phases^[48].

However, subsequent studies showed that porosity in MOF glasses is not completely lost, but can evolve into a partially retained and dynamically heterogeneous microporous network. As Figure 6 shows, porous glasses derived from ZIF-76 and ZIF-76-mbIm demonstrate that bulky benzimidazolate-based linkers can inhibit dense packing in the liquid state and help preserve free volume after vitrification. The PDFs of crystalline and glassy samples remain nearly identical below ~6 Å, indicating that local C-C/C-N and Zn-N correlations are preserved, while the remaining features beyond 6 Å, especially around 7.5 and 10.8 Å, further suggest retained medium-range Zn-imidazolate-Zn-benzimidazolate connectivity in the glass state [Figure 6A]. This retained structure is confirmed to lead to functional pore accessibility, as a_gZIF-76-mbIm reversibly adsorbs CO₂ and CH₄ [Figure 6B]. Meanwhile, vitrification contracts the pore-size distribution but still preserves microporous voids, consistent with positron annihilation lifetime spectroscopy (PALS)-detected pores of ~4.8 and 7.2 Å [Figure 6C]. But the slower CO₂ uptake in the glass indicates there are more tortuous and constricted diffusion pathways in glass state [Figure 6D]. Together, these results suggest that transport accessibility in MOF glasses arises from interconnected disordered cavities rather than conventional crystalline pore channels.

Recent studies have therefore increasingly explored pore engineering strategies to overcome vitrification-induced densification, including linker steric modulation, mixed-linker design, crystal-glass composites, and porogen-assisted approaches inspired by traditional porous glass fabrication^[50]. In particular, porogen strategies involving thermally decomposable salts, polymers, or sacrificial fillers have been proposed as promising routes for introducing hierarchical porosity and tunable pore architectures into MOF glasses. The introduction of hierarchical porosity may be especially important because ion and gas transport in dense glassy networks is frequently limited by diffusion bottlenecks and restricted pore interconnectivity. Moreover, the macroscopically grain-boundary-free nature of MOF glasses provides an additional advantage over crystalline MOFs, as continuous isotropic transport pathways may emerge when pore connectivity is successfully retained. Consequently, pore behavior in MOF glasses should not be interpreted simply as a loss of crystallinity-induced porosity, but rather as the evolution of adaptive disordered free-volume networks whose accessibility, connectivity, and transport functionality can potentially be engineered through controlled liquid-state structural design and vitrification processes.

For alkali metal cations, transport in MOF glasses is not simply governed by rigid pore diffusion^[52]. Instead, it is strongly affected by the coupled effects of free volume, coordination disorder, linker dynamics, and local ion-framework interactions.

The recent ZIF-62 glass electrolyte study showed that Li⁺ migration in glassy ZIF-62 occurs through a “relay-race” mechanism, where Li⁺ is dynamically delivered between N sites from imidazole and benzimidazole ligands. Compared with crystalline ZIF-62, the glass phase provides a larger Li⁺ migration space, higher calculated Li⁺ diffusivity, and more continuous homogeneous transport channels. In crystalline ZIF-62, Li⁺ motion is confined to localized regions, whereas glassy ZIF-62 provides a larger spatial distribution for Li⁺ migration [Figure 7A and B]. The mean square displacement (MSD) analysis further shows that the calculated Li⁺ diffusivity in glassy ZIF-62 is 3.97 × 10^-9 m²·s^-1, which is higher than that in crystalline ZIF-62 (1.43 × 10^-9 m²·s^-1) [Figure 7C]. Within the same simulation time frame, the Li⁺ migration distance is approximately 1.49 Å in crystalline ZIF-62, but increases to approximately 3.92 Å in glassy ZIF-62, indicating faster Li⁺ migration and a more open transport space in the glassy phase [Figure 7D and E]. In addition, Li⁺ trajectories in crystalline ZIF-62 are relatively localized and mainly distributed around N species, whereas in glassy ZIF-62, the Li⁺ trajectories become more continuous along N atoms [Figure 7F and G]. The radial distribution function further reveals that the Li-N distance increases from approximately 2.14 Å in the crystalline phase to 2.22 Å in the glassy phase. This longer Li-N distance suggests weaker interactions between Li⁺ and N sites in glassy ZIF-62, making Li⁺ easier to detach from one N site and migrate to another [Figure 7H]. Consistently, the Li⁺ migration activation energy calculated from the Arrhenius relationship was 0.17 eV for glassy ZIF-62 quasi-solid-state electrolyte (GZ-62-QSSE), lower than that of crystalline ZIF-62 quasi-solid-state electrolyte (CZ-62-QSSE), 0.20 eV. In contrast, the binding energy between Li⁺ and GZ-62-QSSE was only 0.21 eV, markedly lower than that between Li⁺ and CZ-62-QSSE, 2.91 eV^[53]. These results demonstrate that incompletely coordinated Zn²⁺ sites and distorted coordination environments generated by vitrification can immobilize anions, weaken Li⁺ binding, and thereby promote Li⁺ migration.

Another important Li⁺-related mechanism is modifier-induced network depolymerization. In lithium benzimidazolate-modified ZIF-62 glasses, Li(bIm) acts as a compatible glass modifier. Rather than merely occupying the residual pores, Li(bIm) becomes incorporated into the disordered glass network and disrupts the original Zn-Im/bIm-Zn connectivity, generating non-Zn-bridging linker environments and partially depolymerizing the framework. This structural modification lowers the glass-transition temperature and increases the configurational freedom of the network. These results suggest that Li⁺ can influence ion transport not only as a potential mobile charge carrier but also by modifying the local coordination network and increasing structural flexibility^[54].

Porosity and free volume also play a central role. MOF glass formation often causes partial pore collapse, which may impede Li⁺ transport by generating dead-end pathways. Therefore, alkali-ion transport in MOF glasses should be understood as a coupled structure-dynamics process. The most important mechanisms include: N-site-assisted hopping, free-volume-mediated diffusion, modifier-induced network depolymerization, coordination disorder, and glass-dynamics-assisted ion migration. Future MOF-glass-based SSEs may be rationally designed by combining alkali salt incorporation, defect engineering, pore accessibility control, and thermodynamic softening to create continuous, isotropic, and dynamically adaptive ion-transport networks^[26].

In disordered or amorphous solid electrolytes, Na⁺ transport is strongly affected by the topology of the local free-volume network, the distribution of transient coordination sites, and the connectivity of ion-hopping pathways. Recent studies on amorphous and nanocrystalline Na-Y-Zr-Cl halide electrolytes showed that the introduction of amorphous domains can enhance Na⁺ conductivity by reducing grain-size-related transport barriers and generating more continuous ion-transport pathways^[55]. Similarly, molecular-dynamics studies on amorphous alkali oxyhalides indicate that alkali-ion migration is governed by local anion coordination, residence time near framework-forming units, and ion-ion correlation effects rather than by rigid crystallographic channels alone^[56]. These findings suggest that, in MOF glasses, Na⁺ migration should be understood as transport through a dynamically connected disordered free-volume network, where both bottleneck size and the flexibility of local coordination environments are critical.

MOF-based Na⁺ solid electrolytes further highlight the importance of pore size and channel chemistry. Nozari et al. introduced the sodium-salt-containing ionic liquid (Na_0.1EMIM_0.9) (Na0.1[1-ethyl-3-methylimidazolium]0.9) bis(trifluoromethanesulfonyl)imide (TFSI) into ZIF-8 and demonstrated that MOF-ionic liquid host-guest interactions can promote high Na⁺ conductivity, while structural disorder improves the stability of the composite electrolyte^[57]. In MOF-74 systems, the one-dimensional hexagonal channels and open metal sites provide larger ion-conducting pathways and sites for anion immobilization. For example, Na/Mg-MOF-74 exhibited an ionic conductivity of 8.53 × 10^-4 S·cm^-1 at 70 °C, which was attributed to its stable ion-conducting channels and exposed Mg²⁺ sites^[58]. Similarly, Na[EMIm][TFSI]@Zn-MOF-74 reached 5 × 10^-4 S·cm^-1 at room temperature and enabled stable sodium plating for over 100 h^[59].

Therefore, the design principles established for Li⁺ conducting MOF glasses cannot be directly transferred to Na⁺ systems. Because Na⁺ has a larger ionic radius and different coordination preferences, Na⁺ conductors generally require wider bottlenecks, larger accessible free volume, weaker ion-framework binding, and more continuous percolated pathways. However, current studies remain limited: most Na⁺ MOF electrolytes rely on crystalline MOFs or ionic-liquid-filled composites, while direct evidence for Na⁺ migration in genuine MOF glass networks is still scarce. Future work should therefore combine structural characterization, ion-dynamics simulations, and electrochemical measurements to clarify how glass topology, free-volume distribution, and anion immobilization jointly regulate Na⁺ transport.

Compared with Li⁺ transport, K⁺ conduction in MOF glasses remains largely unexplored. To date, most reported alkali-metal-ion-conducting MOF glasses and MOF-glass-containing electrolytes have focused on Li⁺ transport, as exemplified by glassy ZIF-based quasi-solid-state electrolytes for lithium-metal batteries^[53]. In contrast, K⁺ conducting MOF systems reported so far are mainly based on crystalline single-ion-conducting frameworks or MOF/polymer composite electrolytes rather than melt-quenched MOF glasses^[60,61]. For example, MOF-808-SO₃K has been reported as a single potassium-ion conducting crystalline MOF, in which immobilized sulfonate groups provide fixed anionic sites and K⁺ serves as the mobile charge carrier^[61]. More recently, MOF-containing composite polymer electrolytes have also been developed for solid-state potassium batteries, where UiO-66 is incorporated into a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)-based polymer matrix to enhance mechanical stability, increase polymer disorder, and provide K⁺ migration channels^[60]. These examples demonstrate the feasibility of K⁺ transport in MOF-based electrolyte systems, but they also highlight that the structure-transport relationships established in crystalline MOFs or MOF/polymer composites cannot be directly transferred to MOF glasses.

The scarcity of K⁺ conducting MOF glasses likely arises from several coupled challenges. K⁺ has a larger ionic radius and lower charge density than Li⁺, making its transport more sensitive to pore aperture, free-volume size, and the availability of weakly coordinating sites^[62,63]. Besides, many glass-forming MOFs are charge-neutral frameworks without intrinsic fixed anionic groups; therefore, K⁺ transport generally requires the incorporation of external potassium salts, ionic liquids, or framework-bound anionic functionalities^[54]. The preparation of homogeneous K⁺ containing MOF glasses is also synthetically challenging, as the potassium salts may crystallize, phase-separate, or decompose during high-temperature melting and quenching. This issue is particularly relevant because MOF glasses are generally produced by heating/melting followed by rapid quenching^[64-66]. These factors explain why K⁺ conduction in MOF glasses remains an important research gap and suggest that future studies should focus on designing glass-forming frameworks with sufficiently large and connected free-volume elements, introducing fixed anionic sites or weakly coordinating ligands, and suppressing salt crystallization to enable continuous K⁺ transport.

Different from additive modulation, transient flux-assisted processing relies on the presence of an external molten medium during heating, which facilitates mass transport and framework rearrangement without necessarily becoming a permanent component of the final glass^[97]. In MOF systems, meltable frameworks phases can act as transient fluxes, enabling the co-melting of otherwise non-meltable MOFs and expanding the accessible vitrification window. By lowering kinetic barriers associated with solid-state diffusion and coordination rearrangement, flux-assisted pathways allow melting to occur at temperatures below the intrinsic decomposition threshold. Previous research used meltable ZIF-62 to dissolve non-fusible ZIF-8 in a liquid phase effectively lowering the overall melting threshold, and the whole composite achieved co-melting at 430-450 °C^[98] [Figure 10A and B]. Flux melting also provides a promising pathway for constructing ion-conductive MOF glasses through the formation of newly accessible disordered pore networks. Although the original crystalline pore structure of ZIF-8 collapses during vitrification, the incorporation of ZIF-8 into the liquid ZIF-62 matrix disrupts dense liquid-state packing and generates a less compact disordered framework with enlarged pore-limiting apertures and enhanced free volume [Figure 10C-E]. The resulting flux-melted glass exhibits significantly improved H₂ accessibility and larger transport apertures compared with pure a_gZIF-62, indicating the emergence of more continuous and accessible diffusion pathways within the disordered coordination network [Figure 10F]. Such dynamically disordered pore environments may be particularly beneficial for ionic transport because they can reduce steric constraints and facilitate ion hopping through interconnected free-volume domains. In contrast to rigid crystalline pore channels, these isotropic disordered networks may also provide macroscopically grain-boundary-free transport pathways and adaptable coordination environments capable of accommodating alkali ions during migration. Therefore, flux-mediated pore restructuring not only expands the vitrification accessibility of non-melting MOFs, but also suggests a viable strategy for engineering transport-accessible MOF glasses with potential ionic conduction functionality.

At a higher structural level, topology and network engineering offer a powerful means of thermodynamic regulation by modifying the global constraint landscape of MOFs. Reductions in framework connectivity, changes in network topology, or cooperative interactions with confined guests can increase configurational freedom and defect density, thereby lowering the energetic barriers associated with melting and vitrification. These topology-driven strategies highlight that glass formation in MOFs is governed not only by local bond chemistry but also by collective network constraints. In ZIFs, works have shown that partial substitution or mixing of metal nodes (e.g., Zn/Co) enables continuous modulation of metal-ligand bond strength thereby systematically lowering the melting temperature and expanding the glass-forming window^[99,100]. There have been studies clearly demonstrating that simply changing the metal center can transform an otherwise non-meltable framework into one exhibiting reversible melting behavior^[101]. At the microscopic level, metal-node engineering alters local coordination geometry, electronic structure, and metal-organic bond energetics.

Early work by Bennett et al. targeted linker substitution in ZIFs - where a fraction of imidazolate linkers in ZIF-4 (Zn(Im)₂) is replaced during synthesis by bulkier benzimidazolate or methylbenzimidazolate ligands to form TIF-4 and ZIF-62 - systematically lowers the melting temperature from ~863 K (ZIF-4) to ~740 K (TIF-4) and ~710 K (ZIF-62), suppresses reconstructive recrystallization, and enables direct melt-quenching into a glass while preserving local Zn-N coordination^[31]. Building on this foundation, recent studies by Xue et al. demonstrated that even highly stable carboxylate-based frameworks, such as UiO-67 and DUT-5, can be rendered meltable through reductions in network connectivity, or cooperative interactions with pore-confined guests^[92]. From a thermodynamic perspective, topology-driven strategies effectively reduce the rigidity of the coordination network, increase structural degrees of freedom, and enhance defect tolerance, thereby lowering the energetic barriers associated with melting or vitrification^[12].

Building on the diverse strategies developed for ion-conducting MOF glasses, we further propose that molten-salt chemistry may offer a useful conceptual framework for designing and understanding ion-conducting MOF glasses. This perspective is inspired by molten-salt strategies widely used in carbon and oxide chemistry, where molten salts can act not only as ion reservoirs, but also as reaction media, fluxes, structure-directing agents, and thermal stabilizers. When translated to MOF-glass systems, molten salts may play multiple roles: they can introduce mobile alkali-metal carriers, weaken or screen metal-ligand interactions, lower the effective melting or softening temperature, suppress premature framework decomposition, and facilitate vitrification during quenching. In our view, four coupled mechanisms are particularly relevant when translating these approaches to salt-assisted MOF systems, as demonstrated in Figure 11.

Halide or mixed chloride salts behave as low-viscosity ionic fluxes. They provide a continuous liquid environment that dramatically accelerates mass transport and lowers kinetic barriers for bond exchange and framework rearrangement. When a crystalline MOF is immersed in such a molten-salt medium, the framework no longer needs to reach its intrinsic melting point to undergo large-scale reorganization; instead, depolymerization-repolymerization events can be triggered at significantly reduced temperatures within the salt flux. This is mechanistically analogous to flux-melting using a low-T_m MOF glass, but the role of the liquid phase is now played by an inorganic salt.

Second, molten salts are not inert solvents. Their highly polarizable anions (e.g., Cl^-) and small, strongly coordinating cations can directly interact with metal-ligand bonds in the MOF^[102]. At the microscopic level, these ions compete with the original linkers for coordination to the metal nodes and screen metal-ligand electrostatics. This weakens metal-ligand coordination, promotes linker cleavage, and locally depolymerizes the network, effectively softening the framework. In ZIFs, for example, partial replacement of Zn-imidazolate coordination by halide or alkali-ion coordination can facilitate Zn-N bond rearrangement, thereby lowering the thermal transition temperatures and enabling access to a viscous, melt-like state before decomposition^[54,68,97].

Besides, the molten-salt environment also exerts a spatial confinement effect. In a salt-filled pore space, the surrounding ionic medium acts as a liquid matrix that limits crystallite growth, suppresses sintering, and stabilizes highly defective or partially depolymerized states that would be unstable in the absence of the salt. Once incorporated into the softened coordination network, ionic species such as Li⁺, Na⁺, and Cl^- can act as network modifiers and become integrated into the local coordination structure rather than simply occupying residual pore space^[54]. Their incorporation creates chemically heterogeneous coordination environments, including undercoordinated alkali-ion sites and mixed metal-linker/halide coordination around the metal nodes^[68]. Meanwhile, the dynamic disruption and reformation of metal-linker-metal bridges during vitrification increases the population of non-bridging or dangling linkers and undercoordinated metal sites, resulting in a more depolymerized and structurally heterogeneous glass network^[103].

Once vitrification occurs, the same ions that acted as flux now play a second role: they form nanoconfined ionic domains within the amorphous framework. In pores, interstitial regions, and defect-rich zones, the molten salt behaves as a highly dissociated ionic medium. Under nanoconfinement, its structure and dynamics differ from the bulk, but the effective ion mobility can remain high^[104,105]. The result is a percolating network of salt-rich pathways threaded through a mechanically robust MOF glass matrix. From the perspective of ionic conductivity, this hybrid state combines (i) the isotropic, macroscopically grain-boundary-free transport of a glass with (ii) continuous, liquid-like ion highways provided by the nanoconfined salt.

The structural programmability of MOF glasses provides a promising platform for developing next-generation solid-state ion conductors. MOF glasses exhibit features including disorder in long-range yet remain chemically tunable coordination networks, sub-nanometer free volume, dynamic metal-ligand environments, and macroscopically grain-boundary-free monolithic architectures, which create opportunities to design ion-transport pathways that are not limited to intrinsic crystallographic pores, but can instead be regulated through glass composition, coordination chemistry, defect concentration, free-volume distribution, and thermal history.

One particularly attractive direction is the development of salt-assisted MOF-glass electrolytes. The incorporation of light-metal halide salts, such as LiCl or NaCl, may serve a dual function: reducing the effective vitrification barrier of rigid coordination frameworks and simultaneously introducing mobile ionic carriers. This approach may expand MOF-derived glasses beyond intrinsically meltable ZIF systems to include frameworks that are traditionally difficult to vitrify. If the salt-framework interaction is properly controlled, such modified glasses have the potential to exhibit even higher ion conductivity than pristine MOF glass under near-room-temperature or moderately elevated-temperature conditions, thereby occupying an important materials space between polymer electrolytes and ceramic solid electrolytes.

Hybrid MOF glasses are also promising for ion conduction and may be deployed in intermediate-temperature solid-state batteries, particularly sodium- and lithium-based stationary energy-storage systems. Their amorphous and isotropic nature can suppress grain-boundary resistance, improve interfacial conformity with electrodes, and provide mechanically adaptable electrolyte layers. At temperatures approximating the glass transition point, partial softening of the coordination network may enhance local structural dynamics, promote ion mobility, and improve electrode reaction kinetics, while avoiding the severe thermal-management requirements associated with conventional high-temperature ceramic electrolytes. These advantages are especially relevant for stationary batteries, where long-term durability, processability, and interfacial stability are often more important than maximum gravimetric energy density.

Moreover, as demonstrated by Ogawa et al., beyond alkali-metal-ion conduction, MOF glasses offer considerable potential as anhydrous or low-humidity proton conductors^[39]. The combination of coordinatively unsaturated metal sites, hydrogen-bonding ligands, phosphate- or sulfonate-containing units, and nanoconfined ionic domains could generate proton-transfer pathways that remain active under conditions where hydrated polymer membranes lose performance. This makes MOF-glass electrolytes attractive for intermediate-temperature proton-conducting fuel cells, membrane reactors, and electrochemical hydrogen-separation devices. Compared with conventional polymer membranes, MOF glasses may provide improved thermal robustness, reduced dehydration sensitivity, and more chemically programmable proton-transfer environments.

Another important application area is ion-selective membranes and electrochemical separation devices. Wang et al. have initiated investigations into MOF glass composites for lithium enrichment^[125]. Because the ionic species, coordination sites, and free-volume architecture can be independently tuned, MOF glasses could be engineered for selective transport of Li⁺, Na⁺, K⁺, Ag⁺, H⁺, or multivalent ions. The absence of grain boundaries may also reduce non-selective leakage pathways, which is a major limitation in many polycrystalline ion-conducting membranes.

MOF-glass ion conductors may further contribute to solid-state sensors and harsh-environment ionic devices. Their compositional flexibility, thermal stability, and resistance to grain-boundary degradation make them suitable for gas sensors, humidity sensors, electrochemical probes, and high-temperature ionic devices. By selecting appropriate metal nodes, ligands, and incorporated salts, the transport response could be coupled with specific chemical stimuli, enabling ion-conducting glasses with sensing functions.

More broadly, the key opportunity lies in transforming meltability from a rare intrinsic property into a programmable design parameter. Through flux-assisted vitrification, coordination-environment modulation, and compositing or interface-mediated transport design, MOF glasses could evolve from a small family of meltable ZIF-derived materials into a broader class of processable ionic glasses.

Despite these opportunities, several fundamental and technological challenges must be addressed. The first challenge is to establish reliable structure-transport relationships. Ion conduction in MOF glasses is expected to involve multiple coupled factors, including free-volume size, defect concentration, metal-ligand bond dynamics, salt distribution, local coordination disorder, and segmental motion near the glass transition temperature. However, these factors are difficult to decouple experimentally. A high apparent conductivity alone is insufficient to prove efficient ionic transport; it must be supported by activation-energy analysis, isotope-effect measurements, DC polarization, impedance spectroscopy using appropriate blocking and non-blocking electrodes, and structural probes such as PDF, EXAFS, PALS, and variable-temperature spectroscopy as discussed in Section "CHARACTERIZATION OF MOF-GLASS-BASED SSEs".

Long-term stability is another central concern. Salt-assisted vitrification and ion incorporation may create metastable structures that are beneficial for transport but vulnerable to structural relaxation, salt redistribution, phase segregation, or crystallization during extended operation. Repeated thermal cycling, operation near or above T_g, electrochemical bias, and contact with reactive electrodes may progressively alter the defect population, free-volume distribution, and local coordination environment. Understanding how ionic mobility couples with glass relaxation will therefore be essential for predicting lifetime performance.

Device integration also remains nontrivial. Although the amorphous and isotropic nature of MOF glasses may improve physical contact with electrodes compared with rigid crystalline ceramics, the formation of interphases at electrode-electrolyte boundaries remains poorly understood. Interfacial resistance, electrode compatibility, redox stability, dendrite suppression in metal batteries, and mechanical integrity under cycling must be systematically evaluated. The electrochemical stability window of each MOF-glass electrolyte should also be carefully determined, since organic ligands, redox-active metal nodes, and incorporated salts may introduce decomposition pathways under bias.

Scalable synthesis and processing are also key barriers. Electrolytes typically require homogeneous composition and compatibility with established workflows. Achieving these requirements for fabricating MOF glass ion conductor at large scale is challenging because small variations in precursor crystallinity, particle size, salt loading, heating rate, pressure, or quenching conditions may strongly influence glass structure and ionic conductivity. Future work should therefore move beyond small bulk pellets toward thin films, supported membranes, laminated electrolytes, and prototype cells.

For solid-state battery applications, the practical viability of MOF glass electrolytes should be evaluated beyond ionic conductivity. Key issues include the electrochemical stability window, chemical compatibility with metal anodes, oxidative stability against high-voltage cathodes, interfacial decomposition, and dendrite suppression. Previous studies have shown that many solid electrolytes are not thermodynamically stable against alkali-metal electrodes and may form resistive decomposition interphases during cycling^[126,127]. Moreover, interfacial instability can increase cell resistance, promote uneven metal deposition, and accelerate dendrite growth^[128]. Therefore, future MOF glass electrolytes should be designed with stable electrode interfaces, sufficient mechanical robustness, and suppressed parasitic reactions.

Finally, the field requires clearer performance benchmarks and standardized testing protocols. Conductivity values should be reported together with activation energy, temperature and humidity conditions, sample geometry, electrode configuration, electronic transference number, cycling stability, and post-mortem structural analysis. For battery-related applications, symmetric-cell cycling, full-cell testing, critical current density, and interfacial impedance evolution should be included.

Overall, salt-assisted and structurally modulated MOF glasses offer a promising route to modulate glass formation, coordination chemistry, and ion transport within a single design framework. Their greatest promise lies not simply in achieving ion conductivity, but in enabling processable, chemically programmable, and interface-adaptable ion conductors. If the challenges of mechanistic verification, long-term stability, interfacial compatibility, and scalable processing can be addressed, MOF-glass ion conductors could become a versatile platform for solid-state batteries, proton-conducting devices, ion-selective membranes, and even integrated electrochemical energy systems.