1. INTRODUCTION

Lithium metal batteries (LMBs) have attracted significant attention as next-generation energy-storage systems owing to their high theoretical energy density^{[1, 2]}. However, conventional liquid-electrolyte-based LMBs suffer from safety concerns and rapid capacity degradation associated with continuous Li dendrite growth and undesirable electrolyte side reactions^{[1, 2, 6]}. To address these limitations, semi-solid-state electrolytes (SSSEs) based on metal–organic frameworks (MOFs) have emerged as a promising strategy^{[3- 6]}. Owing to their highly ordered porous structures, large specific surface areas, and tunable chemical environments, MOFs can serve as effective hosts for liquid electrolytes while providing ion-transport pathways^{[3, 6- 8]}. In particular, ZIF-based materials possess well-defined microporous structures with an average pore diameter of less than 0.5 nm, which is suitable for accommodating electrolyte species and regulating Li-ion transport^{[3- 5, 7]}. Despite these advantages, translating MOF-based SSSEs toward practical battery applications remains challenging because sufficiently large quantities of structurally uniform and highly porous MOFs must be produced without sacrificing material quality^{[8- 10]}.

The large-batch production of MOFs involves challenges that are distinct from those encountered in small-scale laboratory synthesis^{[9, 10]}. From a processing perspective, conventional MOF syntheses often rely on organic solvents, which can increase the requirements for solvent handling, recovery, and waste treatment as the production scale increases^{[9- 11]}. Aqueous synthesis is therefore particularly attractive for large-batch preparation as a green chemistry approach because it reduces dependence on organic solvents and offers a simpler processing route^{[11- 13]}. Meanwhile, increasing the batch size alone does not guarantee the formation of high-quality MOF crystals^{[9, 10, 13]}. Crystal formation is fundamentally governed by nucleation and subsequent crystal growth, which are sensitive to precursor concentration, mixing, local supersaturation, and precursor-delivery conditions^{[12- 17]}. As the batch size increases, these parameters can become increasingly difficult to maintain uniformly throughout the reaction system^{[9, 10]}. Therefore, understanding how nucleation and crystal growth proceed during large-batch synthesis is essential for obtaining uniform and highly crystalline MOFs.

In this study, we developed an aqueous large-batch synthesis strategy for ZIF-based MOFs by controlling the precursor feeding time as a key processing parameter governing the nucleation rate and subsequent crystal growth. By comparing long and short feeding times under large-batch conditions, we identified a clear relationship between the feeding condition and the resulting crystal quality. A short feeding time promoted rapid nucleation followed by more uniform crystal growth, whereas a long feeding time slowed nucleation and generated irregular particles, identifying the nucleation rate as the factor that preserves crystal uniformity during large-batch aqueous synthesis. Inspired by this finding, our synthesis strategy was further extended to a Cu/Zn bimetallic ZIF-based framework, which likewise produced uniform faceted crystals and confirmed that the nucleation–growth relationship established for ZIF-8 also holds for a compositionally modified MOF system. The resulting ZIF-8 and Cu/Zn MOFs retained their microporous structures after fabrication into separators, hosted the liquid electrolyte within their pores, and supported stable ion transport and reversible Li plating/stripping. These findings identify the precursor feeding rate as a simple and readily transferable design parameter for producing high-quality ZIF-based MOFs at scale without additional reactor modifications or organic solvents.

2. EXPERIMENTAL METHODS

3. RESULTS AND DISCUSSION

To understand how the nucleation rate governs crystal quality in large-batch synthesis, the precursor feeding conditions were systematically controlled to modulate the nucleation rate of ZIF-8. As illustrated in Figure 1a, the linker solution was fed into the Zn precursor solution over 30, 120, or 240 min, yielding ZIF-8 (DW-30), ZIF-8 (DW-120), and ZIF-8 (DW-240), respectively. The XRD patterns (Figure 1b) show that the synthesized ZIF-8 samples exhibit diffraction patterns consistent with the standard ZIF-8 pattern (JCPDS No. 062-1030), with characteristic reflections corresponding to the (011), (002), (112), (022), (013), and (222) planes^{[12- 14]}. The intense low-angle (011) reflection at approximately 7.3–7.4° is a representative diffraction feature of the crystalline ZIF-8 framework^{[12, 13, 21]}. In particular, ZIF-8 (DW-30) exhibited sharper and better-resolved reflections than ZIF-8 (DW-240), indicating the formation of a more highly ordered crystalline structure under the shorter feeding condition. In contrast, ZIF-8 (DW-240) exhibited relatively broad and poorly defined diffraction peaks. The SEM images (Figure 1c) further support this structural evolution. ZIF-8 (DW-240) and ZIF-8 (DW-120) consisted of irregular particles with poorly developed crystal facets, whereas ZIF-8 (DW-30) formed well-defined polyhedral particles with a more uniform morphology. SEM-based particle-size analysis yielded average particle sizes of 450 ± 45, 311 ± 61, and 343 ± 71 nm for ZIF-8 (DW-30), ZIF-8 (DW-120), and ZIF-8 (DW-240), respectively. The smaller standard deviation of ZIF-8 (DW-30) further indicates a narrower particle-size distribution under the shorter feeding condition. It is noteworthy that a more uniform MOF particle morphology can be beneficial for electrochemical performance, as previous studies have shown that controlled particle size and morphology promote more homogeneous ion-transport pathways and reduce transport heterogeneity in MOF-based battery systems^{[22- 24]}. This behavior is likely because a short feeding time promotes rapid nucleation, allowing the resulting nuclei to undergo a more uniform growth process and thereby producing more uniform and well-faceted ZIF-8 crystals^{[13- 17]}. Overall, the combined XRD and SEM results indicate that controlling the precursor feeding time is critical for recovering the characteristic crystallinity and morphology of ZIF-8 during large-batch aqueous synthesis.

Fig. 1. (a) Schematic representation of the large-batch aqueous synthesis of ZIF-8 and Cu/Zn MOF. (b) XRD patterns of the synthesized ZIF-8 samples and standard JCPDS card of ZIF-8. (c) SEM images of ZIF-8 prepared with different precursor feeding times. ZIF-8 (DW-30), ZIF-8 (DW-120), and ZIF-8 (DW-240) denote samples synthesized with dropwise feeding times of 30, 120, and 240 min, respectively.

Based on the understanding of nucleation behavior established for large-batch ZIF-8 synthesis, the optimized aqueous synthesis strategy was further extended to Cu/Zn MOF to examine its broad applicability. The XRD pattern of the Cu/Zn MOF closely matched the standard reference pattern (Figure 2a)^[7]. Consistent with this result, the corresponding SEM images revealed uniform, well-defined faceted particles with an average size of approximately 510 nm (Figure 2b). The relatively uniform particle size and well-developed crystal facets can be attributed to the controlled precursor feeding condition, which likely promotes rapid nucleation and a more uniform subsequent crystal-growth process^{[16, 17]}. These results indicate that the optimized synthesis condition established for ZIF-8 can also be applied to Cu/Zn MOF while preserving its characteristic crystalline structure and morphology. The N₂ adsorption–desorption isotherms of ZIF-8 and Cu/Zn MOF showed the characteristic low-pressure uptake of microporous materials, with Brunauer–Emmett–Teller (BET) specific surface areas of 1503 and 1402 m² g^-1, respectively (Figures 2c and 2d). These high specific surface areas primarily originate from the preserved microporous frameworks of the ZIF-based structures^[25]. These results indicate that the optimized aqueous synthesis maintained the characteristic porous structures of both MOFs.

Fig. 2. (a) XRD pattern and (b) SEM images of Cu/Zn MOF synthesized using the optimized large-batch aqueous process. (c-d) N₂ adsorption–desorption isotherms of (c) ZIF-8 and (d) Cu/Zn MOF.

To evaluate whether the high-quality MOFs obtained through large-batch aqueous synthesis retained the functionality required for MOF-based semi-solid-state electrolytes, the synthesized MOFs were fabricated into separators and electrochemically activated with a LiTFSI/PC electrolyte. The surface and cross-sectional SEM images (Figures 3a and 3b) show densely packed MOF particles forming continuous separator structures with thicknesses of approximately 50 μm for both the ZIF-8 and Cu/Zn MOF separators. The top-view images further reveal relatively uniform and continuous particle packing without obvious macroscopic defects, indicating successful fabrication of the large-batch MOFs into compact separator structures. To determine whether the porous MOF frameworks effectively accommodated the electrolyte during activation, XRD patterns were compared before and after electrochemical activation. After activation, the characteristic XRD peak positions of both ZIF-8 and Cu/Zn MOF remained largely unchanged, whereas the intensities of the low-angle characteristic peaks decreased noticeably (Figures 3c and 3d). In particular, the decrease in the low-angle (011) reflection is consistent with electrolyte incorporation into the MOF pores, which alters the electron-density contrast and consequently the scattering intensity within the porous framework^[7]. At the same time, the preservation of the characteristic peak positions indicates that the crystalline MOF frameworks were maintained during activation. The FT-IR spectra of the activated separators further showed additional bands associated with Li⁺–TFSI^-, Li⁺–solvent, and TFSI^- species (Figures 3e and 3f), supporting the incorporation and interaction of the LiTFSI/PC electrolyte within the MOF-based separators^{[6, 7]}. Gravimetric analysis showed retained liquid-electrolyte fractions of approximately 49.2 and 49.1 wt% for the activated ZIF-8 and Cu/Zn MOF separators, respectively, comparable to the liquid-electrolyte content reported for a ZIF-8-based quasi-solid electrolyte^[4]. Overall, these results indicate that the large-batch MOFs retain both their structural integrity and electrolyte-hosting functionality after electrochemical activation.

Fig. 3. (a-b) Surface and cross-sectional SEM images of (a) ZIF-8 and (b) Cu/Zn MOF separators. (c-d) XRD patterns of pristine and activated (c) ZIF-8 and (d) Cu/Zn MOF separators. (e-f) FT-IR spectra of pristine and activated (e) ZIF-8 and (f) Cu/Zn MOF separators.

The electrochemical properties of the activated separators were subsequently evaluated to determine whether the electrolyte-hosting functionality of the large-batch MOFs could be translated into effective ion transport and Li-metal compatibility. The LSV curves (Figure 4a) exhibited anodic onset potentials of approximately 4.61, 4.84, and 4.80 V for Celgard with liquid electrolyte, activated ZIF-8, and activated Cu/Zn MOF, respectively. The delayed anodic current onset of both MOF-based separators relative to Celgard indicates improved oxidative stability after electrochemical activation. This behavior can be attributed to the confined electrolyte environment within the MOF pores, which suppresses the oxidative decomposition of electrolyte species^{[6, 7, 26]}. The Nyquist plots (Figure 4b) yielded ionic conductivities of $7.98 \times 10^{-5}\text{ S cm}^{-1}$ for ZIF-8 and $1.2 \times 10^{-4}\text{ S cm}^{-1}$ for Cu/Zn MOF, confirming that the large-batch MOFs retained effective ion-transport capability after activation^[27]. The higher ionic conductivity of Cu/Zn MOF can be associated with the additional unsaturated metal sites introduced by Cu incorporation, which enhance interactions with TFSI^--containing electrolyte species and facilitate Li⁺ transport through the microporous channels^{[7, 26]}. Long-term Li||Li symmetric-cell cycling at 0.5 mA cm^-2 showed stable and reversible Li plating/stripping behavior for 200 h for both ZIF-8 and Cu/Zn MOF separators (Figures 4c and 4d), indicating good compatibility with Li metal. In the stepwise current-density tests (Figures 4e and 4f), the ZIF-8 and Cu/Zn MOF separators maintained reversible Li plating/stripping up to 1.5 and 2.0 mA cm^-2, respectively. The higher current-density tolerance of the Cu/Zn MOF separator is consistent with its higher ionic conductivity, which can alleviate ion-transport polarization under increased current demand. Overall, these results demonstrate that the MOFs prepared through the large-batch aqueous process retain the electrochemical functionality required for MOF-based semi-solid-state electrolytes.

Fig. 4. (a) LSV curves of activated ZIF-8 and Cu/Zn MOF separators and Celgard with liquid electrolyte. (b) Nyquist plots of activated ZIF-8 and Cu/Zn MOF separators. Long-term Li plating/stripping profiles of Li||Li symmetric cells using (c) ZIF-8 and (d) Cu/Zn MOF separators at 0.5 mA cm^-2. Stepwise current-density profiles of Li||Li symmetric cells using (e) ZIF-8 and (f) Cu/Zn MOF separators.

Based on these stable electrochemical characteristics, the practical applicability of the activated MOF separators as semi-solid-state electrolytes was further evaluated in Li||LFP cells. Figure 5 shows the electrochemical performance of Li||LFP cells employing the activated ZIF-8 and Cu/Zn MOF separators, in which the LiTFSI/PC electrolyte incorporated within the MOF pores provides the ion-conducting phase of the semi-solid electrolyte^{[6, 7]}. The galvanostatic charge–discharge profiles exhibited the characteristic LFP plateaus at approximately 3.4–3.5 V, and the overall profile shapes were largely maintained after prolonged cycling (Figures 5a and 5b). The corresponding cycling results show that the ZIF-8 and Cu/Zn MOF-based cells delivered initial discharge capacities of 122.3 and 130.0 mAh g^-1, respectively, and retained capacities above 110 mAh g^-1 after 200 cycles at 1 C (Figures 5c and 5d). In addition, the Coulombic efficiencies remained close to 100% throughout cycling, indicating highly reversible charge–discharge behavior. In contrast, cells assembled with statically soaked MOF separators without electrochemical activation showed rapid capacity loss during the initial cycles (Figures 5c and 5d), indicating that simple electrolyte wetting alone was insufficient to achieve the stable electrochemical behavior observed after activation. The Cu/Zn MOF-based cell maintained a slightly higher discharge capacity during cycling than the ZIF-8-based cell, consistent with its higher ionic conductivity observed in the EIS measurements. The improved ion-transport characteristics of Cu/Zn MOF, associated with Cu-induced unsaturated metal sites and enhanced electrolyte–framework interactions, can reduce transport polarization and facilitate Li⁺ transfer during repeated cycling^{[7, 26]}. Similar stable Li||LFP operation has previously been demonstrated for laboratory-scale MOF-based semi-solid electrolytes, supporting the suitability of these ZIF-based frameworks as electrolyte hosts^{[4, 5, 7]}.

Fig. 5. (a-b) Galvanostatic charge–discharge profiles of Li||LFP cells using activated (a) ZIF-8 and (b) Cu/Zn MOF separators for 200 cycles at 1 C and room temperature. (c-d) Cycling performance and Coulombic efficiency of Li||LFP cells using (c) ZIF-8 and (d) Cu/Zn MOF separators at 1 C and room temperature; cells using statically soaked separators without electrochemical activation are included for comparison.

Because the present separator thickness (50 μm) limits cell-level energy density, further thickness reduction while preserving separator integrity and ion transport is essential; optimization of key processing parameters, such as binder content, slurry viscosity, casting speed, and drying time and temperature, is the subject of ongoing work. Overall, these results indicate that the large-batch aqueous synthesis preserves not only the crystalline and porous characteristics of the MOFs but also their functional role as semi-solid-state electrolytes in Li-metal batteries.

4. CONCLUSION

In this study, the influence of nucleation and crystal-growth behavior on the large-batch aqueous synthesis of ZIF-based MOFs was systematically investigated by controlling the precursor feeding time. Simply increasing the batch size resulted in reduced crystallinity and irregular particle morphology, whereas shortening the precursor feeding time from 240 to 30 min promoted rapid nucleation and a more uniform subsequent growth process, recovering the characteristic crystalline structure and well-defined polyhedral morphology of ZIF-8. Under the optimized condition, ZIF-8 exhibited relatively uniform particles of approximately 450 nm, and the same synthesis strategy was successfully extended to Cu/Zn MOF, yielding uniform faceted particles of approximately 510 nm. The large-batch ZIF-8 and Cu/Zn MOF also retained high BET specific surface areas of 1503 and 1402 m² g^-1, respectively, confirming preservation of their characteristic microporous structures. After fabrication into separators and electrochemical activation, the decrease in low-angle XRD intensity together with the appearance of Li⁺–solvent, Li⁺–TFSI^-, and TFSI^--related FT-IR bands confirmed successful incorporation of electrolyte species into the MOF structures. The activated ZIF-8 and Cu/Zn MOF separators exhibited ionic conductivities of $7.98 \times 10^{-5}$ and $1.2 \times 10^{-4}\text{ S cm}^{-1}$, respectively, stable Li plating/stripping behavior for 200 h at 0.5 mA cm^-2, and sustained Li||LFP cycling for 200 cycles at 1 C. Taken together, these results show that the structural characteristics recovered through precursor feeding control were retained after separator fabrication and translated into effective electrolyte-hosting and electrochemical functionality. Importantly, the large-batch aqueous products maintained the key characteristics required for MOF-based semi-solid-state electrolytes, demonstrating that appropriate control of nucleation and crystal growth can mitigate the structural deterioration associated with increasing the batch size. This process therefore provides a practical strategy for extending aqueous synthesis of ZIF-based MOFs toward larger-batch preparation without substantial loss of their structural and electrochemical functionality.