Zeolite imidazate framework porous material low-temperature melting-quenching vitrification method and application

Abstract

The invention relates to a low-temperature melting-quenching vitrification method and application of a zeolite imidazate framework porous material, and the method comprises the following steps: grinding powder ZIF and powdery iodine into mixed powder, and adding the mixed powder into a sealed container; putting the container into a drying oven which is heated to a constant temperature of 70-200 DEG C, and carrying out heat preservation until a melting phenomenon is observed; the container is taken out of the drying oven and cooled to the room temperature, and it is observed that the molten substance is solidified after being cooled; adding ethanol to soak the cured substance, and shaking uniformly; after standing, observing that the material is settled and layered, sucking out supernate, continuously adding ethanol, and oscillating; repeating the process of settling, absorbing supernate, adding ethanol and oscillating, and cleaning the material until the supernate is colorless; and sucking the lower turbid liquid, centrifuging, and drying by using a vacuum drying oven to obtain the powdery glassy zeolite imidazate framework porous material. After the battery is assembled and operates for 520 circles, the specific capacity is 149mAh g <-1 >, and the capacity retention ratio is 89%.

Description

Technical Field

[0001] The present invention belongs to the field of preparation of porous materials, and particularly relates to a synthesis method and use of solid electrolyte materials; particularly relates to a low-temperature melting-quenching vitrification method and application of zeolitic imidazolate framework porous materials. Background Art

[0002] Metal-Organic Frameworks (MOFs) are crystalline porous materials with a periodic network structure formed by the self-assembly of inorganic metal centers (metal ions or metal clusters) and bridging organic ligands. As a new type of porous crystalline material, MOFs have the advantages of high porosity, large specific surface area, and strong tunability of composition and structure. The properties required for solid electrolytes include insulation, high voltage stability, high ionic conductivity, and high ionic transference number. The adjustable and customizable structure of MOFs can regulate this series of properties, making the optimized MOFs and their derivative materials potential candidates for the new generation of solid electrolyte materials. When current MOFs and derived crystalline materials are used as ion conductors, the powder polycrystals often aggregate and arrange with different orientations. This disordered orientation of crystals not only may conflict with the concept of ordered microchannel design but also has a large number of grain boundaries. In polycrystalline materials as ion conductors, grain boundary impedance is a negative factor. When ions pass through grain boundaries, they will separate from anions and recombine, bind to and separate from the solid electrolyte within a short distance, resulting in high energy absorption and release. This process not only hinders ion transport, but the generated energy will also cause uncontrollable side reactions, forming dead lithium and interfaces. These factors lead to instability during long cycles. For solid electrolyte membranes prepared from unmodified and unvitrified original MOFs, the ionic conductivity is difficult to reach 1*10 -3 Scm -1 or above.

[0003] Zeolitic imidazolate framework porous materials (ZIF) belong to the subclass of MOF materials and have potential application value as solid electrolyte materials. The framework of ZIF is composed of tetrahedrally coordinated transition metal ions and imidazole-based linkers, and the metal center is surrounded by nitrogen atoms in the five-membered imidazole ring, such as Figure 1As shown; the metal center of ZIF can be formed by various metal ions, such as Zn, Cu, Co, Fe, etc., and the organic molecules are mainly imidazole and its derivatives. The resulting coordination polymer has a tetrahedral spatial structure. Since the metal-imidazole-metal angle is similar to the 145° Si-O-Si bond angle in zeolites, ZIF thus has a zeolite-like topological structure and is called a zeolitic imidazolate framework porous material. However, when the original crystalline ZIF is used as a solid electrolyte, the existence of grain boundaries results in the material's properties still not meeting the application standards. To eliminate the existence of grain boundaries, melt-quenching vitrification is an effective method.

[0004] Under the action of high temperature or other conditions, the short-range ordered coordination bonds are maintained, and the bond angles change widely, eliminating the long-range order of the material. This process is called vitrification. Classical glasses are divided into organic, inorganic, and metallic glasses. ZIF materials have the potential for vitrification. As metal-organic coordination polymers, ZIFs can be fired into a new type of glass, such as Figure 2 shown. Currently, there are many methods for vitrifying ZIF materials, including melt-quenching, direct synthesis, ball milling, irradiation, etc. Among these methods, the methods based on melt-quenching are the most representative and can eliminate grain boundaries. In 2015, Bennett et al. first discovered ZIFs obtained by melt quenching. The melt-quenching vitrification method melts the material to a liquid state and then cools and solidifies it. Currently, the melting points of all known ZIFs are above 400 °C. For example, the melting point of ZIF-4 (Zn(Im)2) is 590 °C, and the melting point of ZIF-62 (Zn(Im) 1.75 (bIm) 0.25 ) is 435 °C. In addition, some classical ZIFs, such as ZIF-8 (Zn(MeIm)2) and ZIF-14 (Zn(EtIm)2), are considered not directly meltable because they decompose before melting. By eliminating grain boundaries, the grain boundary impedance of the material is eliminated, thus assisting the transport of substances therein. Therefore, ZIF-glass materials have the potential to be used as ion conductors. Figure 3 shows the process of ion transport in the grain-boundary-free glassy material. Compared with polycrystalline powder materials, ions do not need to pass through grain boundaries frequently. Summary of the Invention

[0005] Through the iodine-assisted melting-quenching method and its application, the present invention significantly reduces the glass transition temperature of classical ZIFs, widely improves the operability of vitrification, and broadens the types of materials that can be vitrified. During the vitrification process, crystalline ZIFs are converted into amorphous states, and the crystal boundaries are eliminated through melting, thereby reducing the crystal boundary impedance existing during the transport of substances in porous materials. When the glassy ZIF material obtained by applying this method is used as a solid electrolyte for lithium-ion batteries, high conductivity and high transference number can be achieved. This study solves the following problems: (1) The high glass transition temperature of ZIFs makes the synthesis difficult; (2) The types of ZIFs that can be vitrified are limited; (3) The existing MOF solid electrolyte materials have high crystal boundary impedance and poor cycle stability. The present invention provides a new idea for the vitrification of crystalline materials and proves the potential of such materials for use as solid electrolytes in lithium-ion batteries.

[0006] The technical solution of the present invention:

[0007] A low-temperature melting-quenching vitrification method for zeolitic imidazolate framework porous materials, comprising the following steps:

[0008] 1) Grind the powdered ZIF and powdered iodine into a mixed powder, and add it to a sealed container; place the container in an oven that has been heated to a constant temperature of 70 - 200 °C and keep it warm until melting is observed; take the container out of the oven and cool it to room temperature, and observe that the melted substance solidifies after cooling;

[0009] 2) Add ethanol to soak the solidified substance, and shake it well; after standing, observe that the material settles and layers, suck out the supernatant and continue to add ethanol and shake; repeat the above process of sedimentation - sucking out the supernatant - adding ethanol - shaking to wash the material until the supernatant is colorless;

[0010] 3) Suck out the lower turbid liquid, centrifuge it and dry it in a vacuum oven to obtain a powdered glassy zeolitic imidazolate framework porous material.

[0011] The volume of the mixed powder in step 1) accounts for 1 / 4 - 1 / 2 of the volume of the container.

[0012] The mass ratio of ZIF to iodine in step 1) is 1:1 / 3 - 3.

[0013] The soaking temperature of the solid substance in step 2) is 50 °C - 70 °C.

[0014] In step 2), the process of sedimentation - sucking out the supernatant - adding ethanol - shaking is repeated 6 - 8 times.

[0015] The drying temperature of the vacuum oven in step 3) is 80 °C - 120 °C.

[0016] The ZIF-glass prepared by the low-temperature melting-quenching vitrification method of the zeolitic imidazolate framework porous material of the present invention does not have X-ray diffraction peaks; the positions of the infrared FTIR and X-ray photoelectron spectroscopy XPS signal peaks are the same as those of the original ZIF.

[0017] The performance of existing MOF-based solid electrolytes generally does not exceed the following parameters: the specific capacity is 140 mAh g after the full cell runs for 200 cycles -1 , and the capacity retention rate is 85%. When the ZIF-glass of the present invention is used as a solid electrolyte material, the specific capacity is 149 mAh g after the full cell runs for 520 cycles -1 , and the capacity retention rate is 89%.

[0018] The synthesized ZIF-glass has the following structural characteristics:

[0019] (1) It does not have X-ray diffraction peaks, indicating that the material has been transformed into an amorphous state;

[0020] (2) The infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) signals can correspond to those of the original ZIF, indicating that the coordination structure is the same as that of the original ZIF;

[0021] The material can be used as a solid electrolyte material. The specific capacity is 159 mAh g after the full cell runs for 220 cycles -1 , and the capacity retention rate is 96%.

[0022] The advantages and beneficial effects of the present invention are:

[0023] (1) The glass transition temperature is low and the operation is simple, which is reduced from more than 400 °C to 120 °C;

[0024] (2) It broadens the range of vitrified materials, and for the first time synthesizes the glassy ZIF-14 that has not been synthesized before;

[0025] (3) When applied as a solid electrolyte for lithium-ion batteries, compared with the existing ZIF-based lithium-ion solid electrolyte materials, this method has high ionic conductivity (exceeding 1×10 -3 S cm -1 ), high ion transference number (exceeding 0.50), high voltage stability (exceeding 4.5 V), and good cycle stability (the full cell can operate stably for more than 200 cycles, the specific capacity is higher than 140 mAh g -1 , and the capacity retention rate exceeds 85%).

[0026] After vitrification, the coordination structure of the ZIF material is well maintained; the range of materials that can be melt-quenched vitrified is broadened. Description of the Drawings

[0027] Figure 1 It is a schematic diagram of the ZIF material structure.

[0028] Figure 2 It is a schematic diagram of the glass transition of ZIF.

[0029] Figure 3 It is a schematic diagram of the ion transport of glassy ZIF and crystalline ZIF.

[0030] Figure 4 It is the X-ray powder diffraction pattern of ZIF-4-glass and ZIF-4.

[0031] Figure 5 It is the X-ray powder diffraction pattern of ZIF-8-glass and ZIF-8.

[0032] Figure 6 It is the X-ray powder diffraction pattern of ZIF-14-glass and ZIF-14.

[0033] Figure 7 It is the X-ray powder diffraction pattern of ZIF-7-glass and ZIF-7.

[0034] Figure 8 It is the X-ray powder diffraction pattern of ZIF-67-glass and ZIF-67.

[0035] Figure 9 It is the Fourier transform infrared spectrum of ZIF-4-glass and ZIF-4.

[0036] Figure 10 It is the Fourier transform infrared spectrum of ZIF-8-glass and ZIF-8.

[0037] Figure 11 It is the Fourier transform infrared spectrum of ZIF-14-glass and ZIF-14.

[0038] Figure 12 It is the Fourier transform infrared spectrum of ZIF-7-glass and ZIF-7.

[0039] Figure 13 It is the Fourier transform infrared spectrum of ZIF-67-glass and ZIF-67.

[0040] Figure 14 It is a bar chart of the room temperature ionic conductivity.

[0041] Figure 15 It is a bar chart of the ion transference number.

[0042] Figure 16 It is a bar chart of the electrochemical stability window.

[0043] Figure 17 It is the specific capacity - cycle diagram of the full - cell cycle. Detailed implementation manners

[0044] To facilitate the understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with the drawings and embodiments. However, the protection scope of the present invention is not limited to the following specific embodiments.

[0045] Common ZIFs currently can be classified according to whether they can be directly melt - quenched and the metal center. ZIFs that can be directly melt - quenched to vitrify include ZIF - 4 and ZIF - 62; Zn - based ZIFs that cannot be directly melt - quenched to vitrify include ZIF - 8, ZIF - 7, ZIF - 11, and ZIF - 14; Co - based ZIFs that cannot be directly melt - quenched to vitrify include ZIF - 9 and ZIF - 67. By introducing iodine into the ZIF framework and heating, iodine molecules with low intermolecular forces will move in the cavities of the material and impact the framework structure, widely changing the bond angles, assisting vitrification and significantly reducing the glass transition temperature.

[0046] The following examples are just representative:

[0047] ZIF - 4 represents the ZIF that can be directly melt - quenched to vitrify;

[0048] ZIF - 8, ZIF - 7, and ZIF - 14 represent Zn - based ZIFs that cannot be directly melt - quenched to vitrify;

[0049] ZIF - 67 represents the Co - based ZIF that cannot be directly melt - quenched to vitrify.

[0050] I. Preparation of ZIF - glass

[0051] Example 1: Preparation of ZIF - 4 - glass

[0052] 1) Mix ZIF - 4 (1.0 g) and iodine (1.5 g), add them into a 20 - mL glass vial with good airtightness, and then place it in an oven heated to 120 °C and keep it warm for 20 min. Then take out the vial from the oven and cool it to room temperature.

[0053] 2) Add ethanol to the original container, soak the obtained solid material at 60 °C, and shake well. After standing, it can be observed that the material settles and layers. Suck out the supernatant and continue to add ethanol and shake. Repeat the process of "settling - sucking out the supernatant - adding ethanol - shaking" 7 times.

[0054] 3) Suck out the lower turbid liquid, centrifuge it and dry it in a vacuum oven at 80 °C for 12 h to obtain powdery ZIF - 4 - glass.

[0055] Example 2: Preparation of ZIF-4-glass

[0056] 1) Mix ZIF-4 (1.0 g) and iodine (3.0 g), add them into a 40 mL glass vial with good airtightness, then place it in an oven heated to 200 °C and keep it warm for 20 min. After that, take the vial out of the oven and cool it to room temperature.

[0057] 2) Add ethanol to the original container, soak the obtained solid material at 70 °C, and shake well. After standing, it can be observed that the material settles and stratifies. Suck out the supernatant and continue to add ethanol and shake. Repeat the process of "settling - sucking out the supernatant - adding ethanol - shaking" 8 times.

[0058] 3) Suck out the lower turbid liquid, centrifuge it and dry it in a vacuum oven at 80 °C for 12 h to obtain powdery ZIF-4-glass.

[0059] Example 3: Preparation of ZIF-4-glass

[0060] 1) Mix ZIF-4 (3.0 g) and iodine (1.0 g), add them into a 40 mL glass vial with good airtightness, then place it in an oven heated to 70 °C and keep it warm for 20 min. After that, take the vial out of the oven and cool it to room temperature.

[0061] 2) Add ethanol to the original container, soak the obtained solid material at 50 °C, and shake well. After standing, it can be observed that the material settles and stratifies. Suck out the supernatant and continue to add ethanol and shake. Repeat the process of "settling - sucking out the supernatant - adding ethanol - shaking" 6 times.

[0062] 3) Suck out the lower turbid liquid, centrifuge it and dry it in a vacuum oven at 120 °C for 12 h to obtain powdery ZIF-4-glass.

[0063] Example 4: Preparation of ZIF-8-glass

[0064] 1) Mix ZIF-8 (1.0 g) and iodine (1.5 g), add them into a 20 mL glass vial with good airtightness, then place it in an oven heated to 130 °C and keep it warm for 20 min. After that, take the vial out of the oven and cool it to room temperature.

[0065] 2) Add ethanol to the original container, soak the obtained solid material at 60 °C, and shake well. After standing, it can be observed that the material settles and stratifies. Suck out the supernatant and continue to add ethanol and shake. Repeat the process of "settling - sucking out the supernatant - adding ethanol - shaking" 6 times.

[0066] 3) Pipette the lower turbid liquid, centrifuge it, and dry it in a vacuum oven at 100 °C for 12 h to obtain powdered ZIF-4-glass.

[0067] Example 5: Preparation of ZIF-14-glass

[0068] 1) Mix ZIF-14 (1.0 g) and iodine (1.2 g), add them to a 20 mL glass vial with good airtightness, and then place it in an oven heated to 100 °C and keep it warm for 40 min. Then take the vial out of the oven and cool it to room temperature.

[0069] 2) Add ethanol to the original container, soak the obtained solid material at 70 °C, and shake it well. After standing, it can be observed that the material settles and layers. Pipette out the supernatant and continue to add ethanol and shake. Repeat the process of "settling - pipetting out the supernatant - adding ethanol - shaking" 6 times.

[0070] 3) Pipette the lower turbid liquid, centrifuge it, and dry it in a vacuum oven at 80 °C for 12 h to obtain powdered ZIF-14-glass.

[0071] Example 6: Preparation of ZIF-7-glass

[0072] 1) Mix ZIF-7 (2.0 g) and iodine (3 g), add them to a 40 mL glass vial with good airtightness, and then place it in an oven heated to 140 °C and keep it warm for 20 min. Then take the vial out of the oven and cool it to room temperature.

[0073] 2) Add ethanol to the original container, soak the obtained solid material at 70 °C, and shake it well. After standing, it can be observed that the material settles and layers. Pipette out the supernatant and continue to add ethanol and shake. Repeat the process of "settling - pipetting out the supernatant - adding ethanol - shaking" 8 times.

[0074] 3) Pipette the lower turbid liquid, centrifuge it, and dry it in a vacuum oven at 110 °C for 12 h to obtain powdered ZIF-7-glass.

[0075] Example 7: Preparation of ZIF-67-glass

[0076] 1) Mix ZIF-67 (2.0 g) and iodine (3.0 g), add them to a 50 mL flask with good airtightness, and then place it in an oven heated to 120 °C and keep it warm for 20 min. Then take the vial out of the oven and cool it to room temperature.

[0077] 2) Add ethanol to the original container, soak the obtained solid material at 50 °C, and shake well. After standing, it can be observed that the material settles and layers. The supernatant is aspirated and ethanol is added again for shaking. Repeat the process of "settling - aspirating the supernatant - adding ethanol - shaking" 8 times.

[0078] 3) Aspirate the lower turbid liquid, centrifuge it, and dry it in a vacuum oven at 90 °C for 12 h to obtain powdered ZIF-67-glass.

[0079] II. Structure Characterization of ZIF-glass

[0080] 1) Take the powdered sample in the above example and collect the X-ray powder diffraction pattern using a Rigaku SmartLab SE X-ray diffractometer equipped with Cu-Kα radiation. Figures 4 - 8 They are the X-ray powder diffraction (PXRD) patterns of ZIF-4, ZIF-8, ZIF-14, ZIF-7, ZIF-67 and their glassy states respectively. The PXRD pattern shows that compared with the original crystalline material, the glassy material has no diffraction peaks, indicating that there is no long-range ordered structure in the glassy material.

[0081] 2) Take the powdered sample in the above example and collect the Fourier transform infrared spectrum using a Bruker Alpha II spectrometer. Figures 9 - 13 They are the Fourier transform infrared spectra (FTIR) of ZIF-4, ZIF-8, ZIF-14, ZIF-7, ZIF-67 and their glassy states respectively. FTIR shows that the original material and the glassy material have the same molecular vibration and rotation signals, such as the overall stretching and out-of-plane bending of the imidazole ring marked on the figure, further indicating that the coordination structure of the material is maintained.

[0082] III. Electrochemical Performance Characterization of ZIF-glass

[0083] Add 90 mg of activated ZIF-glass to 50 mg of PTFE aqueous solution (m (PVDF-HFP) :m (H2O) = 1:4), mix well and press it into a thin sheet with a thickness of about 0.09 - 0.11 mm, and cut it into a disc with a diameter of 12 mm. This disc is used as a solid-state electrolyte (SSE) and assembled with a stainless steel sheet (SS), a lithium sheet (Li) into SS|SSE|SS battery, Li|SSE|Li battery and Li|SSE|SS battery. The following electrochemical performance characterizations are all tested on a Shanghai Chenhua CHI 660E electrochemical workstation.

[0084] A high ionic conductivity indicates that the material has better ion-conducting properties. The ionic conductivity was measured using an SS|SSE|SS cell and EIS testing, with a test frequency ranging from 0.1 Hz to 1*10 5 Hz, and the test temperature was 25 °C. The ionic conductivity was calculated according to the formula σ = L / (R*S), where L (cm) is the thickness of the solid electrolyte, S (cm 2 ) is the area of the electrolyte, and R (Ω) is the resistance. The ionic conductivities of the glassy materials all exceeded those of the crystalline substances, and among them, ZIF-14-glass had the highest ionic conductivity, up to 1.51*10 -3 S cm -1 , as shown in Figure 14 .

[0085] The ion transference number is the ratio of the charge carried by ions to the total charge. The higher the ion transference number, the more cations and fewer anions there are among the ions transported in the framework. The ion transference number was obtained using a Li|SSE|Li cell through EIS testing and potentiostatic polarization testing, with a test temperature of 25 °C. The test frequency of EIS was from 0.1 Hz to 1*10 5 Hz, and for the potentiostatic polarization test, a voltage of 10 mV was used, and the polarization time was 3000 s. The ion transference number was calculated according to the formula t Li+ =(I S (ΔV - I0R0)) / (I0(ΔV - I S R S ). Among them, I0 and I S are the initial current and the final current during the polarization process, respectively, ΔV is the polarization voltage, and R0 and R S are the impedances before and after polarization, respectively. The ion transference numbers of the glassy materials all exceeded those of the crystalline substances, and among them, ZIF-14-glass had the highest ion transference number, up to 0.82, as shown in Figure 15 .

[0086] The electrochemically stable window indicates the high-voltage resistance performance of the material. The higher the stable window, the higher the voltage at which the material can operate. The electrochemically stable window was obtained using a Li|SSE|SS cell through LSV testing. The test rate was 1 mVs -1 , and it was boosted to 6 V. The critical voltage of the LSV curve was the value of the electrochemically stable window. Among them, ZIF-14-glass had an electrochemically stable window as high as 5.2 V, as shown in Figure 16 .

[0087] The full-cell data were collected using a Li|SSE|LFP cell. The loading of LFP was 3.4 mg cm -2, assembled into a CR2032 battery, tested in the range of 2.0 - 4.0V using a LanDian CT2001A battery tester. The ratio of lithium iron phosphate, Super, and PVDF is 8:1:1. After the full battery runs 520 cycles, the specific capacity is 149 mAh g -1 , and the capacity retention rate is 89%, as Figure 17 .

[0088] For the technical solutions disclosed and proposed in the present invention, those skilled in the art can achieve them by referring to the content herein and appropriately changing conditions, routes and other links. Although the methods and preparation techniques of the present invention have been described through preferred embodiments, it is obvious that relevant technical personnel can make changes or recombinations to the methods and technical routes described herein without departing from the content, spirit and scope of the present invention to achieve the final preparation technique. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art, and they are all regarded as being included in the spirit, scope and content of the present invention.

Claims

1. A method for low-temperature melting-quenching vitrification of zeolitic imidazolate framework porous materials, characterized in that It includes the following steps: 1) Grind the powdered ZIF and powdered iodine into a mixed powder, and add it to a sealed container; place the container in an oven that has been heated to a constant temperature of 70 - 200 °C and keep it warm until the melting phenomenon is observed; take the container out of the oven and cool it to room temperature, and observe that the molten substance solidifies after cooling; 2) Add ethanol to soak the solidified substance and shake it well; after standing, observe that the material settles and stratifies, suck out the supernatant and continue to add ethanol and shake; repeat the above process of sedimentation - sucking out the supernatant - adding ethanol - shaking to wash the material until the supernatant is colorless; 3) Suck out the lower turbid liquid, centrifuge it and dry it in a vacuum oven to obtain a powdered glassy zeolitic imidazolate framework porous material.

2. The cryogenic melting-quenching vitrification method according to claim 1, characterized in that, The volume of the mixed powder in step 1) accounts for 1 / 4 - 1 / 2 of the volume of the container.

3. The cryogenic melting-quenching vitrification method according to claim 1, characterized in that, The mass ratio of ZIF to iodine in step 1) is 1:1 / 3 - 3.

4. The cryogenic melting-quenching vitrification method according to claim 1, characterized in that, The soaking temperature of the solid substance in step 2) is 50 - 70 °C.

5. The cryogenic melting-quenching vitrification method according to claim 1, characterized in that In step 2), repeat the process of sedimentation - sucking out the supernatant - adding ethanol - shaking 6 - 8 times.

6. The cryogenic melting-quenching vitrification method according to claim 1, characterized in that, The drying temperature of the vacuum oven in step 3) is 80 - 120 °C.

7. The ZIF-glass prepared by the low-temperature melting - quenching vitrification method of the zeolitic imidazolate framework porous material of claim 1 does not have an X-ray diffraction peak; the positions of the infrared FTIR and X-ray photoelectron spectroscopy XPS signal peaks are the same as those of the original ZIF.

8. The application of the ZIF-glass of claim 7 as a solid electrolyte material.

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