An ultrathin and crack‐free metal‐organic framework film for effective polysulfide inhibition in lithium–sulfur batteries

Due to their extensive microporous structure, metal‐organic frameworks (MOFs) find widespread application in constructing modification layers, functioning as ion sieves. However, the modification layers prepared by existing methods feature gaps between MOFs that are noticeably larger than the inherent MOF pore dimensions. Polysulfides and lithium ions unavoidably permeate through these gaps, hindering the full exploitation of the structural advantages. Herein, an ultrathin (20 nm) and crack‐free MOF film is formed on the separator by atomic layer deposition for the first time. Based on the separator, the mechanism of different MOF layers has been verified by phase field simulation and in situ Raman spectroscopy. The results accurately prove that the MOF particle layer can relieve the shuttle of polysulfides, but it does not have the effect of homogenizing lithium ions. Only the ultrathin and crack‐free MOF film with proper pore size can act as the ion sieve for both polysulfides and lithium ions. As a result, under the test condition of 2 mA cm−2–2 mAh cm−2, the overpotential of the Li/Li symmetric battery is only 18 mV after 2500 h. The capacity retention rate of the lithium–sulfur battery is 95.6% after 500 cycles and 80% after 1000 cycles at 2 C.

As porous materials, MOFs exhibit numerous onedimensional pores that can accommodate electrolytes and act as pathways for lithium-ion diffusion.33] Bai et al. fabricated the separator using HKUST-1 through a combination of vacuum filtration and an in situ growth method. [34]In this work, it postulates that the pore diameter of HKUST-1 is smaller than that of polysulfides, rendering it suitable for serving as an ion sieve to efficiently impede the passage of polysulfides.Nevertheless, the separator typically exhibits considerable thickness, along with noticeable gaps between the MOF particles.To some extent, the MOF layer impacts the migration pathways of polysulfides and lithium ions, primarily through physical barrier effects and chemical adsorption of polysulfides.However, the unique pore characteristics of MOF materials have not been fully harnessed, hindering our ability to accurately elucidate the working mechanisms of MOF materials on polysulfides and lithium ions.Li et al. employed four types of MOFs for separator modification. [35]It was found that the separator modified with MOF did improve the cycle stability, but it was related to the bulk density of the MOF layer, not directly related to the pore size.However, this conclusion is only right under the circumstances that the MOF layer is composed of stacked MOF particles, because the gap dimensions between particles in the densest MOF modification layers are significantly larger than the MOF pore sizes.As a result, there is an ongoing debate surrounding the working mechanism of MOF modification layers in Li-S battery.Only through the construction of an ultrathin and crack-free MOF film can we eliminate the influence of these factors and gain a precise understanding of the migration patterns of polysulfides and lithium ions within the MOF pore structure.However, in the conventional approach to preparing modification layer on porous substrates, materials invariably infiltrate into the pores, preventing the formation of a continuous thin film and leading to increased film thickness.A thicker film can also elevate the ion migration barrier, resulting in heightened battery interface impedance and polarization effects.Hence, the design and construction of a seamless and continuous MOF layer to function as an ion sieve hold paramount importance.Atomic layer deposition (ALD) holds significant potential for applications in nanomaterials due to its precise control over deposition parameters, exceptional deposition uniformity, and consistency.8]

| RESULTS AND DISCUSSION
Herein, in this work, the ALD method was used to prepare ultrathin and continuous crack-free ZIF-8 films (Supporting Information S1: Figure S1).The average pore size of ZIF-8 is 0.34 nm, which is just between lithium-ion (0.152 nm) and polysulfides (from 1.2 to 1.7 nm), so it can be used as an ionic sieve to block polysulfides and homogenize lithium ions.Since ALD is conformal, to prevent the formation of the MOF layer inside the separator, the polypropylene (PP) separator was first modified with a thin layer of hydrophilic and flexible graphene oxide (GO) film.Through contact angle test, it can be found that the hydrophilicity of the GOmodified separator (G-PP) has been significantly improved (Supporting Information S1: Figure S2).As shown in Figure 1A, compared to the bare PP separator (Supporting Information S1: Figure S3), the GO layer completely covers the pores on the surface of PP separator, forming a complete modification layer, which provides necessary preparation conditions and relatively flat substrate for the subsequent ALD process.Then, the ZnO layer was prepared on G-PP separator based on ALD method to obtain ZnO/GO-PP (Z/ G-PP separator) (Figure 1B).Finally, the ultrathin ZIF-8 film was formed by coordinating the ZnO layer with 2-MeIm to obtain the designed separator (F/ZIF-PP).As shown in Figure 1C, after the transformation from ZnO to ZIF-8, the modification layer becomes more compact and smoother.The cross-sectional scanning electron microscope (SEM) image of the F/ZIF-PP separator also shows that an ultrathin modification layer was formed on the PP separator (Figure 1D).The uniform thickness of the ZIF film is measured to be about 20 nm by the cross-sectional transmission electron microscope (TEM) image (Figure 1E).The formation of ZIF-8 film was first proved by X-ray diffractometer (XRD).As shown in Figure 1F, the XRD pattern of F/ZIF-PP shows obvious characteristic peaks of ZIF-8.Compared to Z/G-PP separator, the EDS mappings of F/ZIF-PP separator show uniform distribution of Zn and N elements, which also confirms that the coordination between 2-MeIm and zinc ion forms ZIF-8 (Supporting Information S1: Figure S4).Furthermore, as shown in Supporting Information S1: Figure S5, the bare PP separator shows a contact angle of 45°, which indicates poor wettability to electrolyte.After modified with GO layer, the electrolyte is completely spread on the separator.The wettability of the separator to the electrolyte decreases after ALD process, and it shows complete wettability again after the combination with 2-MeIm, which also confirm the formation of the ZIF-8 film.For comparation, the ZIF-8 particles modified PP separator (P/ZIF-PP) was also prepared by vacuum filtration method.As shown in Supporting Information S1: Figure S6, the thickness of the ZIF-8 layer reaches 15 μm and obvious gaps can be clearly observed between ZIF-8 particles.
The simplest visual experiment was first employed to observe the blocking effect of ZIF-8 film on polysulfides.The permeation tests with double-L devices separated by F/ZIF-PP, G-PP separators are shown in Supporting Information S1: Figure S7.As for the device with G-PP separator, at the beginning, due to the physical blocking effect of ZIF particle layer, there is no obvious polysulfide penetration.After 6 h, the color of electrolytes on the right side of the device begins to turn yellow and gradually deepens, which means that the polysulfide shuttle gradually intensified.For the device with F/ZIF-PP separator, there is almost no polysulfide penetration even after 48 h.Furthermore, the UV-Vis spectroscopy of the electrolyte in the right side with different separator after 3 h are shown in Supporting Information S1: Figure S8.It can be seen that the electrolyte with G-PP separator show typical peak of Li 2 S 6 , which is more obvious than that of F/ZIF-PP separator.This result directly proves the effective limitation of ZIF-8 film on soluble polysulfide.
Subsequently, the electrochemical properties of the different separator were investigated.Our primary objective was to demonstrate the full coverage of the resulting ZIF thin film on PP separator, ensuring its crack-free nature.To achieve this objective, the Co tetraphenylporphyrin (CoTPP) with a diameter of 1.9 nm was used as a probe redox specie, which is larger than the pore size of ZIF-8 (0.34 nm).As shown in Figure 2A, the cyclic voltammetry (CV) curves with PP separator show obvious redox peaks, which belong to the reversible conversion of Co I ↔ Co II and Co II ↔Co III .As for the P/ ZIF-PP separator, the response current significantly decreases, but still exhibits reactivity, indicating that the CoTPP will pass through the separator.However, the F/ZIF-PP separator exhibits the CV curves electrochemical characteristics, which indicate the ZIF film is crack-free and electronically insulating.To explore the influence of the crack-free ZIF-8 film to the electrochemical performance, the Li-S cells were assembled with different separators.
The S/rGO is used as the cathode and the content of sulfur in S/rGO for routine electrochemical test is around 60% (Supporting Information S1: Figure S9 CV curves of Li-S battery, and the anodic peak of F/ZIF-PP is lower than that of P/ZIF-PP separator, which indicates smaller polarization in the battery.First, the electrochemical impedance spectroscopy tests of Li-S battery with different separators before and after cycling are shown in Supporting Information S1: Figure S11.It can be seen that the F/ZIF-PP exhibits the lower charge transfer resistance, implying its fast charge transfer kinetics at the interface.As shown in Figure 2B, it can be seen that the battery assembled with PP separator shows continuous capacity decline, which is because polysulfides can easily pass through the pores of PP separator.However, the P/ZIF-PP separator only shows a rapid capacity decline at the initial stage, and maintains a stable cycle after a certain number of cycles.In contrast, the Li-S battery assembled with the F/ZIF-PP separator maintains a stable cycle from the initial cycle at 0.5 C, with a capacity retention rate of 90% for 100 cycles.The SEM image of F/ZIF-PP and P/ZIF-PP separator after cycling are shown in Supporting Information S1: Figure S12, it can be seen that the F/ZIF-PP separator maintains a complete modification layer structure, while the ZIF-8 particles in P/ZIF-PP separator have peeled off, and the structure of the modified layer has been destroyed.This result further proves the stability of the crack-free ZIF-8 film.To prove the role of the ZIF film, Z/G-PP and G-PP separators are also used as the references (Supporting Information S1: Figure S13).It can be seen that due to the limitation of GO on polysulfides, the stability of the battery assembled with the G-PP separator is improved compared with that of the PP separator, but is obviously inferior to that of the F/ZIF-PP separator.For the Z/G-PP separator, due to the strong adsorption of ZnO on polysulfides, it will also cause the deposition of polysulfides toward the separator, leading to the loss of active sulfur, so it also shows a significant capacity decline at the initial stage.The rate performance of batteries assembled with different separators also shows similar results.
As shown in Figure 2C, the F/ZIF-PP separator shows the highest discharge capacities of 1314, 1172, 984, 763, and 545 mAh g −1 at 0.2, 0.5, 1, 2, and 3 C, respectively.However, the Z/G-PP and G-PP separators show poor rate performances, which display the capacities of only 466 and 346 mAh g −1 at 3 C, respectively.The galvanostatic charge/discharge curves at different rates are shown in Supporting Information S1: Figure S14.Compared with Z/G-PP and G-PP separators, the Li-S cell assembled with F/ZIF-PP shows a distinct charge/discharge voltage plateau even at 3 C, which indicates fast reaction kinetics.These results all prove that the dense ZIF-8 layer can give full play to the role of ion sieve, that is, effectively block polysulfides and homogenize the deposition of lithium ions.To further verify the stability of F/ZIF-PP separator during long-term cycle, the cycle performance of Li-S battery at 2 C was tested.As shown in Figure 2D, an initial capacity of 681 mAh g −1 was achieved.After 500 cycles, the discharge capacity is kept at 651 mAh g −1 , the capacity retention rate is as high as 95.6%, and after 1000 cycles, the fading rate is only 0.02% per cycle.
As we know, the cathode with high sulfur loading will produce more polysulfides, and the shuttle effect will be more serious, so the requirements for the separator will be higher.As a result, the Li-S cell was assembled with a high sulfur loading cathode.As the areal sulfur loading increased to 5.3 mg cm −2 , the Li-S cell assembled with F/ZIF-PP separator also delivers a high initial capacity of 1196 mAh g −1 (6.3 mAh cm −2 ) at 0.2 C, and maintained at 976 mAh g −1 (5.2 mAh cm −2 ) after 80 cycles (Figure 2E).This result shows that F/ZIF-PP separator can still play a stable role as ion sieve under the condition of high sulfur loadings.To verify the ability of the crack-free ZIF film to block polysulfides in real conditions, the insitu Raman spectroscopy was tested.A battery mold with a window on the negative electrode is used for in situ Raman testing.At the same time, to make the Raman signal reach the surface of the separator directly, a hole is punched in the middle of the lithium metal anode.The in-situ Raman signals of Li-S batteries with F/ZIF-PP and G-PP separators were collected at 0.5 C. The time-resolved Raman spectra of the Li-S batteries with different separators are shown in Figure 2F-I.As for the G-PP separator, the typical Raman peaks of Li 2 S 8 at 158, 224, and 478 cm −1 appear in the early stage of discharge process.With the discharge process, the Raman characteristic peaks of Li 2 S 8 gradually disappear, and the Raman peaks belonging to Li 2 S 4 at 435 and 454 cm −1 gradually become stronger, which indicates that a large number of polysulfides produced in the discharge process shuttle to the anode side.For F/ZIF-PP separator, no significant polysulfide Raman signals were observed, which further indicate that the crack-free ZIF-8 film can effectively limit polysulfides and inhibit the shuttle effect of Li-S battery.
At present, there is basically no model for the diffusion of polysulfides in separators.By establishing geometric models of PP (Supporting Information S1: Figure S15), P/ZIF-PP (Supporting Information S1: Figure S16), and F/ZIF-PP (Supporting Information S1: Figure S17), the phase field simulation was employed to analyze the diffusion process of polysulfides from transient to quasisteady state.The concentration distribution profiles of polysulfides from cathode to anode in Li-S batteries with different separators are shown in Figure 3A.Notably, compared to the higher concentration of polysulfides in PP and P/ZIF-PP separators, the polysulfides in F/ZIF-PP separators can be almost negligible.Moreover, Figure 3B shows the Li 2 S 6 concentration below the 200 nm of PP separator for different composite separators at the initial 10 s.It can be found that the Li 2 S 6 concentration is only 115 μM for F/ZIF-PP separator, which is much lower than that of PP and P/ZIF-PP separator.And as shown in the cross-sectional Li 2 S 6 concentration distribution of different separators (Figure 3C), the Li 2 S 6 exhibits a rapid concentration decrease when passing through the ZIF film.These simulation results further indicate that the crack-free MOF film can effectively block polysulfides.
The cycling performance of Li/Li symmetric batteries can directly reflect the stability of the lithium metal anode.The Li/Li cells assembled with F/ZIF-PP, P/ZIF-PP, and G-PP were tested at a current density of 2 mA cm −2 with a capacity of 2 mAh cm −2 .As show in Figure 4A, the symmetric cell with F/ZIF-PP separator exhibits stable cycling with an overpotential only around 18 mV over 2500 h.Due to the modification of GO layer, the cell with G-PP separator also shows a stable cycling at the initial cycles, but the overpotential suddenly increases after less than 500 h.However, the cell with P/ZIF-PP separator shows severe voltage hysteresis at the initial cycles, and this phenomenon quickly increases and eventually leads to the short circuit in the battery.To verify the homogenization effect of crack-free ZIF film on lithium ions, the Li/Li symmetric cells cycling at current densities varying from 1 to 10 mA cm −2 for F/ ZIF-PP, Z/G-PP, and G-PP were also investigated (Figure 4B).As the current density increases, the G-PP and Z/G-PP separator-based cells show drastically rising polarizations.Especially for Z/G-PP separator, when the current rises to 10 mA cm −2 , the short circuit even occurs in the battery.In the same condition, the voltage hysteresis of the F/ZIF-PP separator-based cell is much lower comparing to the other two cells, which can be stable cycled even at the high current density of 10 mA cm −2 .This result indicates that it is not a dense film that can homogenize ions, and it still depends on the structural characteristics of ZIF materials.According to the lithium dendrite growth model, it is easier to grow under the conditions of faster charge/discharge and higher capacity.As a result, the protective effect of F/ ZIF-PP separator on lithium metal anode is further verified under higher current density and capacity.The Li/Li symmetric cell assembled with F/ZIF-PP separator was tested under the condition of 2 mA cm −2 -10 mAh cm −2 .As shown in Figure 4C, the stable polarization can be well maintained for 1000 h without distinct fluctuation.This result also verifies that the F/ZIF-PP can still realize the homogenization of lithium-ion deposition under rapid and deep charge/discharge conditions, thereby inhibiting the growth of lithium dendrites.To explore the reasons for this result, the morphology of the lithium metal anode after cycling was observed.The SEM images of the lithium metal anodes with different separators after cycling for 100 h are shown in Supporting Information S1: Figure S18.The anode with F/ZIF-PP separator displays a flat surface and has no obvious change (Supporting Information S1: Figure S18a).As for the anode with G-PP separator, a small amount of dead lithium forms on the electrode surface, which is because GO can regulate lithium-ion deposition to a certain extent (Supporting Information S1: Figure S18b).And a large number of lithium dendrites generated on the anode with Z/G-PP separator (Supporting Information S1: Figure S18c).This result may be due to the lithiophilic ZnO on the separator, which makes the lithium metal tend to deposit toward the separator, resulting in the uneven deposition of lithium metal on the anode surface and the growth of lithium dendrites.At the meanwhile, consistent with the electrochemical performance test results, a large number of lithium dendrites generated on the anode with P/ZIF-PP separator (Supporting Information S1: Figure S18d).This result indicates that the modified layer composed of ZIF particles, although able to block polysulfides to a certain extent, cannot effectively homogenize lithium-ion deposition and is even less effective than GO thin films.Based on performance tests and characterizations, the working mechanisms of the separator modified with different types of ZIF-8 layers can be simplified as Figure 4D,E.As shown in Figure 4D, since the ZIF-8 layer prepared by filtration is thick and has many gaps, the polysulfides will not only shuttle through the gaps of ZIF-8 particles, but also be adsorbed by the ZIF particles and become "dead sulfur," resulting in rapid fading of capacity.Subsequently, due to the blocking effect of ZIF-8 particles and the electrostatic repulsion of the adsorbed polysulfides, the shuttle effect is alleviated, so that the capacity can be maintained stable.At the same time, due to the existence of polysulfides, lithium ions will also migrate to the anode along with the polysulfides, so that the ZIF-8 pores will lose their effect on lithium-ion homogenization.The lithium dendrite growth caused by the uneven deposition of lithium ions in Li-S battery are more serious and complex than other lithium metal battery systems.In contrast, the ZIF-8 layer constructed by ALD is an ultrathin and crack-free film, so that the polysulfide anions are effectively blocked (Figure 4E).The interaction between lithium ions solvents is cut off when lithium ions are in the channels of ZIF-8, leading to rapid lithium-ion migration and homogenization, and finally realizing the uniform deposition on the anode surface.
To demonstrate the effect of crack-free ZIF film on lithium metal anode, phase field simulation models were also constructed to further demonstrate the mechanism of ZIF film on the behaviors of lithiumion deposition.The original simulated models are shown in Supporting Information S1: Figure S19, and the lithium deposition is carried out at a constant current of 1 mA cm −2 .As shown in Figure 5A electrode interface, and the corresponding local current density is also uneven.As a result, tip deposition is formed on the current collector, which will finally form lithium dendrites (Figure 5C).As for the separator modified with ZIF-8 film, the Li-ion concentration is even (Figure 5B), which indicates that continuous and uniform lithium ions can be provided through the separator.The corresponding uniform current density distribution also leads to homogeneous Li nucleation and eventually smooth surface of the anode (Figure 5D).

| CONCLUSION
In conclusion, to accurately reveal the role of MOF as an ion sieve in Li-S batteries for both cathode and andoe, an ultrathin (20 nm) and crack-free ZIF-8 film is constructed by the surface modification and ALD method for the time.When the separator is used for Li-S battery, it also shows good cycle stability.After 500 cycles at 2 C, the capacity retention rate is 95.6%, and the capacity fading rate is only 0.02% per cycle after 1000 cycles.At the same time, the Li-S cell can provide a discharge capacity of 545 mAh g −1 at 3 C, which also indicates the good rate performance.Even under the high sulfur loading of 5.3 mg cm −2 , the initial area is about 6.3 and 5.2 mAh cm −2 maintains after 80 cycles.The Li/Li symmetrical battery with F/ZIF-PP separator also shows a stable cycle with only 18 mV for 2500 h.Moreover, by comparing with P/ZIF-PP separator, the working mechanism of the different ZIF layer for and polysulfides blocking and lithium-ion homogenization is also analyzed by phase field simulation, visual experiments, and in situ Raman spectroscopy, respectively.These results accurately prove that the modification layer composed of MOF particles can indeed inhibit the shuttle of polysulfides to a certain extent, but it does not have the effect of homogenizing lithium ions.Only the crack-free MOF film with proper pore size can really act as the ion sieve for both polysulfides and lithium ions.This work provides a new idea for accurately exploring the ion sieve mechanism of MOF in Li-S batteries, and the relevant results ).The CV curves of Li-S batteries assembled with different separators are shown in Supporting Information S1: Figure S10.Both F/ZIF-PP and P/ZIF-PP separators exhibit typical F I G U R E 1 Morphological and structural characterization.The SEM images of (A) G-PP, (B) Z/G-PP, and (C) ZIF film modified separator (F/ZIF-PP) separator.The cross-sectional (D) SEM and (E) TEM images of F/ZIF-PP separator.(F) XRD patterns of different separators.

F I G U R E 2
Electrochemical performance of the Li-S cell with different separators.(A) Cyclic voltammograms with 1 mm Co tetraphenylporphyrin. (B) Cycling test at 0.5 C. (C) Rate performance.(D) Long-term performance of ZIF film modified separator (F/ZIF-PP) separator at a current density of 2 C. (E) Cycling stability under high sulfur loading at 0.2 C. In situ Raman spectrum results of (F, G) G-PP and (H, I) F/ZIF-PP separator.

F I G U R E 3
Simulations of polysulfide diffusion in different separators.(A) Representative contour map of Li 2 S 6 concentration distribution at 10 s. (B) The Li 2 S 6 concentration distribution curve at 200 nm under PP separator.(C) The cross-sectional concentration distribution of Li 2 S 6 from cathode to anode.F/ZIF-PP, ZIF film modified separator; P/ZIF-PP, ZIF particles modified separator.
, the simulated Li ions of the model with PP separator display obvious concentration polarization at the separator/ F I G U R E 4 Electrochemical performance of the Li/Li symmetrical battery with different separators.(A) Galvanostatic cycling performance of symmetric cells at 2 mA cm −2 with an areal capacity of 2 mAh cm −2 , (B) rate performance, and (C) galvanostatic cycling performance of symmetric cells at 2 mA cm −2 with an areal capacity of 10 mAh cm −2 .Schematic of the reaction mechanism of different polysulfides and lithium-ion.(D) ZIF particles modified separator (P/ZIF-PP) and (E) ZIF film modified separator (F/ZIF-PP).

F I G U R E 5
Phase field simulation of different lithium metal anode.Simulations of lithium-ion concentration for the lithium metal anode with (A) PP and (B) ZIF film modified separator (F/ZIF-PP) separator.Simulations of the electric field distribution for the lithium metal anode with (C) PP and (D) F/ZIF-PP separator.