Preparation method of heteropolyacid-aramid diaphragm and application of heteropolyacid-aramid diaphragm in lithium ion battery

Abstract

The invention discloses a preparation method of a heteropolyacid-aramid diaphragm and application of the heteropolyacid-aramid diaphragm in a lithium ion battery, and belongs to the technical field of electrode materials and lithium ion batteries. According to the invention, a solution blending in-situ synthesis method is adopted, heteropolyacid is dispersed in an aramid fiber solution, casting molding and heat treatment are carried out to form the diaphragm, and the heteropolyacid-aramid fiber diaphragm is obtained. According to the present invention, the heteropoly acid can form a certain interaction with the aramid fiber molecules, such that the aggregation state structure of the aramid fiber can be changed, the original compact aramid fiber structure can become relatively loose, the porosity can be increased, and the connectivity of the pores can be improved so as to improve the ionic conductivity; meanwhile, the heteropolyacid can provide additional transmission sites and channels for lithium ions, so that the charge-discharge performance of the battery is improved; the interaction of the heteropoly acid and the aramid fiber can limit the excessive movement of an aramid fiber molecular chain at a high temperature and reduce the occurrence of a thermal shrinkage phenomenon, so that the thermal stability of the aramid fiber diaphragm is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of electrode materials and lithium ion batteries, and in particular relates to a preparation method of a heteropolyacid-aramid diaphragm and application thereof in lithium ion batteries. Background Art

[0002] Battery separator refers to a layer of separator material between the positive and negative electrodes of the battery. It is a very critical part of the battery and has a direct impact on the safety and cost of the battery. Its main function is to isolate the positive and negative electrodes and prevent the electrons in the battery from passing freely, allowing the ions in the electrolyte to pass freely between the positive and negative electrodes. At present, the most commonly used commercial separators are polyethylene (PE) and polypropylene (PP), which are porous films formed by processes such as melt extrusion and biaxial stretching. The size of the micropores is usually between tens of nanometers and hundreds of nanometers, and is evenly distributed in the membrane. This structure enables lithium ions to be transmitted between the positive and negative electrodes through the micropores, while relying on the mechanical strength and chemical stability of the polyolefin material itself to prevent short circuits between the positive and negative electrodes and resist erosion by the electrolyte. However, its thermal stability is poor. Under high temperature conditions (generally over 130°C), the thermal motion of the molecular chain intensifies, which can easily cause the separator to shrink thermally, reduce the distance between the positive and negative electrodes, or even directly contact them, causing a short circuit in the battery, which seriously affects the safety of the battery. In addition, its chemical structure is relatively inert, its surface energy is low, and its affinity with the electrolyte is poor. This makes it difficult for the electrolyte to fully infiltrate the separator, resulting in increased ion transfer resistance and limited battery charge and discharge performance.

[0003] Aramid has the advantages of high strength and high heat resistance. Aramid fibers can be woven or formed into thin film materials through processes such as solution spinning. Aramid fibers are interwoven or stacked to form a certain pore structure, which provides channels for lithium ion transmission. However, due to the irregular pore structure and limited connectivity of the aramid diaphragm, the ion transmission path is complex and obstructed. At the same time, aramid itself lacks effective ion conduction functional groups and cannot efficiently promote lithium ion transmission like some materials with specific ion transmission channel structures, making it difficult to increase the battery's charge and discharge rate.

[0004] In addition, traditional preparation methods of aramid membranes, such as solution spinning, face problems such as difficulty in accurately controlling the fiber diameter and difficulty in regulating the pore structure during the preparation process. During solution spinning, factors such as the concentration, viscosity, and extrusion speed of the polymer solution will affect the formation of the fiber and the structure of the membrane, but these factors are interrelated and difficult to coordinate accurately, resulting in unstable structural performance of the prepared aramid membrane.

[0005] Therefore, how to use aramid to prepare a diaphragm with good thermal stability and high charge and discharge efficiency has become a technical problem that needs to be solved urgently in this field. Summary of the invention

[0006] In order to solve the above technical problems, the present invention proposes a preparation method of a heteropolyacid-aramid membrane and its application in lithium-ion batteries.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] The present invention provides a method for preparing a heteropolyacid-aramid membrane, comprising the following steps:

[0009] Aramid solution and heteropoly acid are mixed to obtain heteropoly acid-aramid composite solution; the heteropoly acid-aramid composite solution is cast and heat treated to obtain the heteropoly acid-aramid separator.

[0010] Preferably, the mass ratio of aramid to heteropoly acid in the aramid solution is (50-100):(10-50).

[0011] Preferably, the preparation method of the heteropolyacid-aramid composite solution comprises the following steps: mixing aramid and a solvent, stirring and heating to 60-80°C, and waiting for the aramid to be fully dissolved to obtain an aramid solution; mixing the heteropolyacid and the aramid solution, stirring and ultrasonicating to obtain the heteropolyacid-aramid composite solution.

[0012] Preferably, the water content of the aramid fiber is less than 0.5%; and / or,

[0013] The mass concentration of the aramid solution is 5-15%; and / or,

[0014] The power of the ultrasound is 200-500W, and the time of the ultrasound is 30-60 minutes.

[0015] Preferably, the heteropoly acid is selected from one or more of phosphotungstic acid, silicotungstic acid and phosphomolybdic acid; and / or,

[0016] The particle size of the heteropoly acid is 1-10 μm.

[0017] Preferably, the heteropolyacid further includes a drying step before being mixed with the aramid solution; the drying temperature is 60-100° C. and the time is 6-12 hours.

[0018] Preferably, the specific operation of the casting molding is: casting the heteropolyacid-aramid composite solution on a substrate, and then volatilizing the solvent; and / or,

[0019] The casting thickness is 20-50 μm; and / or,

[0020] The solvent evaporates at a temperature of 20-30° C., a relative humidity of 30-50%, and a time of 12-24 hours.

[0021] Preferably, the heat treatment is performed at a temperature of 100-150° C. and for a time of 2-6 hours.

[0022] The present invention provides a heteropolyacid-aramid membrane prepared by the preparation method described in the above technical solution, wherein the heteropolyacid-aramid membrane has a structure with aramid as a skeleton network, and the heteropolyacid fills the pores between the aramid or adheres to the surface of the aramid.

[0023] The present invention also provides the use of the heteropolyacid-aramid diaphragm described in the above technical solution in a lithium ion battery.

[0024] Technical principle:

[0025] The present invention adopts a solution blending in-situ synthesis method, by first uniformly dispersing the heteropoly acid in an aramid solution, and then forming a diaphragm through casting and heat treatment, and then obtaining a heteropoly acid-aramid diaphragm with a microstructure showing that the aramid fiber is a skeleton network, and the heteropoly acid is attached to the fiber surface or filled in the pores between the fibers. The heteropoly acid can form a certain interaction with the aramid molecule. From a structural point of view, the acidic group of the heteropoly acid can have an acid-base interaction or hydrogen bond with the functional groups such as the amino group on the aramid molecular chain, and change the aggregated structure of the aramid to a certain extent, so that the originally denser aramid structure becomes relatively loose, increase porosity and improve the connectivity of the pores, thereby improving ionic conductivity. At the same time, the heteropoly acid itself has a certain ion exchange and conduction capacity, and its presence in the aramid diaphragm can provide additional transmission sites and channels for lithium ions, further promote the migration of ions in the diaphragm, and improve the charge and discharge performance of the battery. In addition, the interaction between heteropolyacid and aramid can limit the excessive movement of aramid molecular chains at high temperatures, reduce the occurrence of thermal shrinkage, and thus improve the thermal stability of the aramid separator.

[0026] Compared with the prior art, the present invention has the following advantages and technical effects:

[0027] Traditional lithium-ion battery separators are mostly single materials (such as polyolefins) or simple coating composites (such as ceramic coated polyolefins), while the present invention constructs a deep composite system of heteropolyacids and aramid. Unlike simple coating, the heteropolyacids in the present invention are not just attached to the surface of aramid, but penetrate into the aramid fiber network through chemical bonding or strong interaction to form an organic whole. The acidic groups of the heteropolyacids form acid-base interactions with the amino groups of aramid, so that the two are closely combined at the molecular level. This composite method greatly improves the comprehensive performance of the separator and overcomes the limitations of the performance of single materials or simple composite separators.

[0028] The pore structure of pure aramid membranes has defects, such as poor pore connectivity and porosity that is difficult to meet the needs of efficient ion transmission. The present invention optimizes the aramid membrane structure by introducing heteropoly acids. The presence of heteropoly acids in the aramid fiber network changes the interaction and arrangement between fibers, making the originally relatively dense fiber network loose, increasing the porosity and improving the connectivity and regularity of the pores, forming a more efficient ion transmission channel. Structurally, heteropoly acids are like "opening agents", evenly distributed between aramid fibers, adjusting the spacing between fibers and the shape of the pores, and providing a smoother and more diverse transmission path for lithium ions. This is different from the structural form of traditional membranes and provides a new structural idea for the development of high-performance lithium-ion battery membranes. After 2000 charge and discharge cycles, the heteropoly acid-aramid membrane can maintain 349mAh g -1 The available reversible capacity is significantly higher than that of traditional diaphragm batteries.

[0029] Aramid itself has high thermal stability, and its molecular chain contains a rigid aromatic ring structure, and there is a strong hydrogen bond between molecules. The addition of heteropolyacid further strengthens this thermal stability structure. The heteropolyacid and aramid molecules are connected to each other through chemical bonding (such as acid-base action, hydrogen bonding, etc.), forming a more stable network structure. Heteropolyacid has certain chemical activity, and the acidic groups on its surface can interact with the active groups on the surface of the electrode material. In the battery system composed of heteropolyacid-aramid diaphragm and electrode, this interaction forms a stable transition layer at the interface between the diaphragm and the electrode. In addition, the fibrous structure of aramid also has a certain physical contact and entanglement with the electrode surface. The presence of heteropolyacid further enhances the stability of this contact, and the prepared heteropolyacid-aramid diaphragm has excellent interface stability. During the battery charge and discharge cycle, the electrode material will undergo volume changes. Due to its good interface stability with the electrode, the heteropolyacid-aramid diaphragm can better adapt to the volume change of the electrode and reduce the problem of increased interface impedance caused by interface peeling or gap generation. After 500 charge and discharge cycles, the increase in the interfacial impedance of the battery using the heteropolyacid-aramid diaphragm is only 40-50% of that of the battery using the traditional diaphragm (the impedance of the traditional PP diaphragm is about 160Ω), which extends the cycle life of the battery and enables the battery to maintain relatively stable performance during long-term use, reducing the battery's operating cost and improving the battery's economy and practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0031] Figure 1The SEM images and Mapping images of the heteropolyacid-aramid separators prepared in Examples 1-3, wherein a is the SEM image of Example 1, b is the SEM image of Example 2, c is the SEM image of Example 3, d is the Mapping image of Example 1, e is the Mapping image of Example 2, and f is the Mapping image of Example 3;

[0032] Figure 2 are XPS graphs of the heteropolyacid-aramid membranes prepared in Examples 1-3, wherein a is the XPS graph of Example 3, b is the C1s XPS graph of Example 3, c is the O1s XPS graph of Example 3, d is the Mo 3d XPS graph of Example 3, e is the W 4f XPS graph of Example 2, and f is the W 4f XPS graph of Example 1;

[0033] Figure 3 The charge and discharge curves of the LFP / membrane / Li battery assembled with the heteropolyacid-aramid membrane prepared in Examples 1-3 and the aramid membrane prepared in Comparative Example 1. DETAILED DESCRIPTION

[0034] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0036] The embodiment of the present invention provides a method for preparing a heteropolyacid-aramid membrane, comprising the following steps:

[0037] Aramid solution and heteropoly acid are mixed to obtain heteropoly acid-aramid composite solution; the heteropoly acid-aramid composite solution is cast and heat treated to obtain the heteropoly acid-aramid separator.

[0038] In a preferred embodiment, the mass ratio of aramid to heteropoly acid in the aramid solution is (50-100):(10-50).

[0039] In a preferred embodiment, the preparation method of the heteropoly acid-aramid composite solution includes the following steps: mixing aramid and a solvent, stirring and heating to 60-80°C, and waiting for the aramid to be fully dissolved to obtain an aramid solution; mixing the heteropoly acid and the aramid solution, stirring and ultrasonicating to obtain the heteropoly acid-aramid composite solution.

[0040] In a preferred embodiment, the water content of the aramid is less than 0.5%; the aramid is aramid fiber. The present invention controls the water content of the aramid to avoid adverse chemical reactions or structural defects caused by water in the subsequent preparation process.

[0041] In a preferred embodiment, the aramid fiber further includes a pretreatment step before mixing with the solvent; the pretreatment includes the following steps: removing impurities from the aramid fiber raw material, and then performing a drying treatment; the temperature of the drying treatment is 80-120°C, and the time is 12-24 hours; the environment of the drying treatment is vacuum or inert gas. The present invention removes impurities and agglomerates from the aramid fiber raw material to ensure the purity and uniformity of the raw material, and then removes the moisture adsorbed by the aramid fiber raw material through a drying treatment.

[0042] In a preferred embodiment, the stirring rate is 400-600 rpm. In the present invention, the aramid is fully dissolved by stirring and heating to 60-80° C. to form a uniform aramid solution.

[0043] In a preferred embodiment, the mass concentration of the aramid solution is 5-15%; and the solvent is N,N-dimethylformamide (DMF).

[0044] In a preferred embodiment, the heteropoly acid is selected from phosphotungstic acid (H 3 PW 12 O 40 ), silicotungstic acid (H 4 S W 12 O 40 ) and phosphomolybdic acid (H 3 PMo 12 O 40 ) or more. Heteropolyacids have a unique structure, which contains polyhedral structural units composed of central atoms (such as phosphorus, silicon, etc.) and coordinating atoms (such as molybdenum, tungsten, etc.). These units are connected by oxygen atoms to form a large anion structure and have exchangeable protons. This structure of heteropolyacids can provide abundant transport sites for lithium ions.

[0045] In a preferred embodiment, the particle size of the heteropoly acid is 1-10 μm. The heteropoly acid with a particle size within the above range is used to better disperse in the aramid solution in the subsequent steps.

[0046] In a preferred embodiment, the phosphotungstic acid (H 3 PW 12 O 40 ) comprises the following steps:

[0047] a. Add 10g Na 2 WO 4 ·2H 2O was dissolved in 10 mL of distilled water, and then heated to boiling until the solution was clear to obtain solution A; 1 mL of H 3 PO 4 , then slowly add 8 mL of concentrated HCl, cool, and obtain crystals containing tungstic acid;

[0048] b. The tungstic acid-containing crystals obtained in step a are dissolved in 12 mL of distilled water and transferred to a separatory funnel, 7 mL of ether and 4 mL of concentrated HCl are added, the lower layer solution is collected after extraction and separation, and then dissolved in water for recrystallization to obtain the phosphotungstic acid.

[0049] In a preferred embodiment, the silicotungstic acid (H 4 S W 12 O 40 ) comprises the following steps: 20.0 g Na 2 WO 4 ·2H 2 O was dissolved in 50 mL of double distilled water, heated to 90 °C, and 1.45 g of Na 2 SiO 3 9H 2 O, then add concentrated HCl at a slight boil to adjust the pH value to 1-2, continue stirring and heating for 30 minutes, then cool naturally, then place in a 60kHz ultrasonic field for ultrasonic oscillation for 30 minutes, let stand for 2 hours, filter with suction, transfer the filtrate into a separatory funnel, add 15mL of anhydrous ether, then add 15mL of sulfuric acid in 5 times, let stand for stratification, separate the lower layer of oil, add 3mL of distilled water, evaporate in a 60℃ water bath until a film is formed on the surface, and cool and place to obtain colorless, shiny, transparent crystals, which are the silicotungstic acid.

[0050] In a preferred embodiment, the phosphomolybdic acid (H 3 PMo 12 O 40 ) comprises the following steps: 1.8 g Na 2 HPO 4 12H 2 O was dissolved in 10 mL of deionized water and 14.5 g of Na 2 MoO 4 ·2H 2 O was dissolved in 25 mL of deionized water. After the two solutions were mixed, 10 mL of concentrated HCl was added dropwise at 65°C while stirring. The stirring was continued for 30 minutes. The mixture was cooled and filtered. Then, 10 mL of concentrated HCl was added dropwise for acidification. The mixture was extracted with 50 mL of ether. The lower layer of yellow oil was taken, which was the phosphomolybdic acid.

[0051] In a preferred embodiment, the heteropoly acid further comprises a drying step before being mixed with the aramid solution; the drying temperature is 60-100°C, the time is 6-12 hours, and the drying method is vacuum drying. The present invention removes moisture and other volatile impurities that may be adsorbed by the heteropoly acid through drying.

[0052] In a preferred embodiment, the ultrasonic power is 200-500W, the ultrasonic time is 30-60 minutes, and the stirring rate is 800-1000rpm. The present invention uses ultrasound to assist dispersion to ensure that the heteropoly acid is evenly dispersed in the aramid solution to form a heteropoly acid-aramid composite solution.

[0053] In a preferred embodiment, the specific operation of the casting molding is: casting the heteropolyacid-aramid composite solution on a substrate, and then volatilizing the solvent.

[0054] In a preferred embodiment, the substrate is selected from glass or polytetrafluoroethylene plate.

[0055] In a preferred embodiment, the casting thickness is 20-50 μm. In the present invention, the casting thickness is adjusted by controlling the amount of the heteropolyacid-aramid composite solution and the casting area.

[0056] In a preferred embodiment, the solvent evaporates at a temperature of 20-30°C, a relative humidity of 30-50%, and a time of 12-24 hours. The present invention places the cast film in a well-ventilated environment with stable temperature and humidity to allow the solvent to evaporate naturally, thereby forming an interaction between the aramid molecular chain and the heteropoly acid.

[0057] In a preferred embodiment, the heat treatment temperature is 100-150°C, the time is 2-6 hours, and the heat treatment equipment is a vacuum oven or an oven protected by inert gas. The present invention further removes residual solvents through heat treatment and forms a more stable interaction between the aramid molecular chain and the heteropoly acid, thereby solidifying the diaphragm structure and finally obtaining a heteropoly acid-aramid diaphragm.

[0058] The present invention provides a heteropolyacid-aramid membrane prepared by the preparation method described in the above technical solution, wherein the heteropolyacid-aramid membrane has a structure with aramid as a skeleton network, and the heteropolyacid fills the pores between the aramid or adheres to the surface of the aramid.

[0059] In the heteropolyacid-aramid diaphragm provided by the present invention, aramid is used as a matrix material and exists in a fibrous form. The fibers are interwoven to form a three-dimensional network structure. The polyhedral anionic structure of the heteropolyacid is combined with amino groups and the like on the aramid molecular chain through hydrogen bonds or electrostatic effects, thereby forming a tightly fixed structure with the aramid fibers.

[0060] The present invention also provides the use of the heteropolyacid-aramid diaphragm described in the above technical solution in a lithium ion battery.

[0061] Unless otherwise specified, the raw materials in the examples of the present invention were purchased from commercial sources.

[0062] Example 1

[0063] A method for preparing a heteropolyacid-aramid diaphragm, the specific steps are as follows:

[0064] (1) The aramid fibers are screened to remove impurities and agglomerates, and then dried under vacuum at a temperature of 120° C. for 20 hours to obtain pretreated aramid fibers with a moisture content of less than 0.5%.

[0065] (2) 10 g Na 2 WO 4 ·2H 2 O was dissolved in 10 mL of distilled water and then heated to boiling until the solution was clear to obtain solution A. 1 mL of 85% H 3 PO 4 , then slowly drop 8mL of concentrated HCl, cool, and obtain crystals containing tungstic acid; dissolve the above crystals containing tungstic acid in 12mL of distilled water, and transfer to a separatory funnel, add 7mL of ether and 4mL of concentrated HCl, collect the lower layer solution after extraction and separation, and then dissolve it in water for recrystallization to obtain phosphotungstic acid; grind the obtained phosphotungstic acid into fine powder, the particle size is controlled at 1-5μm, and then vacuum dry it at a temperature of 60°C for 6 hours to obtain phosphotungstic acid powder.

[0066] (3) Slowly adding 50 g of the pretreated aramid fiber obtained in step (1) into N,N-dimethylformamide, stirring continuously at a speed of 400 rpm and gradually heating to 60° C. to fully dissolve the aramid and obtain a uniform aramid solution, wherein the mass concentration of the aramid solution is controlled at 10%; slowly adding 20 g of the phosphotungstic acid powder obtained in step (2) into the above aramid solution, while increasing the stirring speed to 800 rpm, and using an ultrasonic device for auxiliary dispersion, with an ultrasonic power of 300 W and ultrasonication for 40 minutes to obtain a heteropolyacid-aramid composite solution.

[0067] (4) Casting the heteropolyacid-aramid composite solution obtained in step (3) onto a polytetrafluoroethylene plate, controlling the casting thickness to be 20-25 μm. After the casting is completed, placing the cast film in a well-ventilated environment at a temperature of 20-25° C. and a relative humidity of 30-40% to evaporate the solvent for 12 hours; and then transferring the film to a vacuum oven for heat treatment at a temperature of 120° C. for 3 hours to obtain a heteropolyacid-aramid diaphragm.

[0068] Example 2

[0069] The only difference from Example 1 is that the phosphotungstic acid powder in step (3) is replaced by silicotungstic acid powder, and the rest is the same as Example 1.

[0070] Example 3

[0071] The only difference from Example 1 is that the phosphotungstic acid powder in step (3) is replaced by phosphomolybdic acid powder, and the rest is the same as Example 1.

[0072] Figure 1 The SEM images and mapping images of the heteropolyacid-aramid diaphragms prepared in Examples 1-3, wherein a is the SEM image of Example 1, b is the SEM image of Example 2, c is the SEM image of Example 3, d is the mapping image of Example 1, e is the mapping image of Example 2, and f is the mapping image of Example 3. Figure 1 From the ac part in FIG. 1 , it can be seen that the microstructure of the heteropolyacid-aramid membrane prepared in the embodiment of the present invention presents a skeleton network composed of aramid fibers. The aramid fiber network structure is loose, and certain pore structures are formed between the fibers, and these pore structures are evenly distributed. Figure 1 It can be seen from the ef part that the heteropolyacid-aramid membranes prepared in the embodiments of the present invention contain C and O elements, and the P and W elements in Example 1, the Si and W elements in Example 2, and the Mo and P elements in Example 3 are all uniformly dispersed in the aramid membrane, indicating that the heteropolyacid particles or ion clusters are uniformly attached to the fiber surface or filled in the pores between fibers, just like the "opening agent" is uniformly distributed between the aramid fibers.

[0073] Figure 2 The XPS graphs of the heteropolyacid-aramid membranes prepared in Examples 1-3, wherein a is the XPS graph of Example 3, b is the C1s XPS graph of Example 3, c is the O1s XPS graph of Example 3, d is the Mo 3d XPS graph of Example 3, e is the W 4f XPS graph of Example 2, and f is the W 4f XPS graph of Example 1. Figure 2 As can be seen from the a part in the figure, the heteropolyacid-aramid membrane prepared in Example 3 is mainly composed of C, N, O and Mo elements. Figure 2 It can be seen from part b in that the C1s spectrum of the heteropolyacid-aramid membrane prepared in Example 3 has three peaks at binding energies of 288.0 eV, 285.1 eV and 284.6 eV, corresponding to C=O, CN / C=N and CO / C=O, respectively. Figure 2As can be seen from part c in FIG. 3 , the characteristic spectrum of element O of the heteropolyacid-aramid membrane prepared in Example 3 shows two obvious peaks at different binding energies, corresponding to C=O and CO, respectively. Figure 2 As can be seen from the d part, the heteropolyacid-aramid membrane prepared in Example 3 shows two obvious peaks at 235.2eV and 232.0eV, which are respectively attributed to Mo 6+ 3d 3 / 2 and Mo 6+ 3d 5 / 2 .from Figure 2 It can be seen from the d part in the figure that the binding energy of Mo3d in the heteropolyacid-aramid membrane prepared in Example 3 is lower than that of H 3 PMo 12 O 40 The binding energy of Mo 3d in 3 PMo 12 O 40 After that, part of the electrons in the heteropolyacid-aramid membrane are transferred to H 3 PMo 12 O 40 This results in an increase in the electron density of Mo. Figure 2 As can be seen from parts e and f in the figure, the characteristic spectra of the element W in the heteropolyacid-aramid membranes prepared in Examples 1 and 2 show two obvious peaks at different binding energies, corresponding to W 6+ 4f 5 / 2 andW 6+ 4f 7 / 2 Similarly, from Figure 2 As can be seen from the e and f parts in the figure, the binding energies of W 4f in Examples 1 and 2 are also lower than those of H 3 PW 12 O 40 and H 4 S W 12 O 40 This indicates that after the introduction of heteropoly acid, part of the electrons in the aramid membrane were transferred to the heteropoly acid, resulting in an increase in the W electron density.

[0074] Comparative Example 1

[0075] A method for preparing an aramid diaphragm, the specific steps are as follows:

[0076] (1) The aramid fibers are screened to remove impurities and agglomerates, and then dried under vacuum at a temperature of 120° C. for 20 hours to obtain pretreated aramid fibers with a moisture content of less than 0.5%.

[0077] (2) Slowly adding the pretreated aramid fiber obtained in step (1) into N,N-dimethylformamide, stirring continuously at a rotation speed of 400 rpm and gradually heating to 60° C. to fully dissolve the aramid and obtain a uniform aramid solution. The mass concentration of the aramid solution is controlled at 10%.

[0078] (3) Casting the aramid solution obtained in step (2) onto a polytetrafluoroethylene plate, controlling the casting thickness to be 20-25 μm. After the casting is completed, placing the cast film in a well-ventilated environment with a temperature of 20-25° C. and a relative humidity of 30-40% to evaporate the solvent for 12 hours; then transferring the film to a vacuum oven for heat treatment at a temperature of 120° C. for 3 hours to obtain an aramid membrane.

[0079] Comparative Example 2

[0080] The difference from Example 3 is that in step (2), 100 g of the phosphomolybdic acid powder obtained in step (2) is slowly added to the above-mentioned aramid solution, and the rest is the same as Example 3.

[0081] Comparative Example 3

[0082] The difference from Example 3 is that in step (3), 5 g of the phosphomolybdic acid powder obtained in step (2) is slowly added to the above-mentioned aramid solution, and the rest is the same as Example 3.

[0083] Comparative Example 4

[0084] The difference from Example 3 is that in step (4), the heat treatment temperature is 200°C, and the rest is the same as Example 3.

[0085] Comparative Example 5

[0086] The difference from Example 3 is that in step (4), the heat treatment temperature is 50°C, and the rest is the same as Example 3.

[0087] Comparative Example 6

[0088] The difference from Example 3 is that the heat treatment in step (4) is omitted, and the rest is the same as Example 3.

[0089] The porosity of the diaphragms prepared in Examples 1-3 and Comparative Examples 1-6 was tested using a mercury porosimeter, and the thermal shrinkage of the diaphragms prepared in Examples 1-3 and Comparative Examples 1-6 was tested using a battery diaphragm thermal shrinkage tester. The results are shown in Table 1.

[0090] Table 1 Porosity and thermal shrinkage of the diaphragms prepared in Examples 1-3 and Comparative Examples 1-6

[0091]

[0092] As can be seen from Table 1, the porosity of Examples 1-3 is higher than that of Comparative Example 1, indicating that the heteropoly acid can form a certain interaction with the aramid molecules, change the aggregated structure of the aramid, make the originally dense aramid structure relatively loose, and increase the porosity. The porosity of Comparative Examples 2 and 3 is significantly lower than that of Example 3, indicating that too much / little heteropoly acid will affect the porosity of the material. If the heteropoly acid content is too much, a large number of heteropoly acid molecules can not only fill the pores, but also may cause some smaller pores to be blocked, resulting in a decrease in porosity; if the heteropoly acid content is too little, the heteropoly acid molecules cannot fully enter the pores of the aramid diaphragm, and the change in the pore structure is small. The porosity of Comparative Examples 4-6 is significantly lower than that of Example 3, indicating that the heat treatment temperature is too high / too low or no heat treatment is performed, which will reduce the porosity. When the heat treatment temperature gradually increases, the thermal motion of the aramid molecules intensifies, causing some molecular segments to rearrange, and some originally blocked or smaller pores may be opened or expanded, increasing the porosity. If the temperature is too high, the aramid membrane may be thermally degraded, and the resulting fragments may fill the pores, causing the porosity to be greatly reduced.

[0093] By comparing the thermal shrinkage of the diaphragms prepared in Examples 1-3 and Comparative Examples 1-6, it can be seen that the thermal shrinkage of Examples 1-3 is lower than that of Comparative Example 1. The heteropolyacid molecules contain multiple metal atoms and oxygen atoms, which can form strong interactions with the polar groups on the aramid molecular chains, making it difficult for the molecular chains to move and shrink freely, thereby reducing the thermal shrinkage of the aramid diaphragm. Similarly, the thermal shrinkage of Comparative Examples 2 and 3 is significantly higher than that of Example 3, which may be due to the fact that the excessive amount of heteropolyacid introduced will damage the original structure of the aramid diaphragm, while the small amount of heteropolyacid introduced will have little effect on the original structure of the aramid diaphragm. The thermal shrinkage of Comparative Examples 4-6 is significantly higher than that of Example 3, indicating that at a lower heat treatment temperature, the thermal motion energy of the aramid molecules is relatively low, and the microstructure of the diaphragm cannot be fully optimized; when the heat treatment temperature is too high, the aramid molecular chains will undergo degradation reactions due to obtaining too much energy, which will increase the thermal shrinkage of the diaphragm.

[0094] Electrochemical measurements were performed using button cells. The positive electrode was made by mixing lithium iron phosphate, acetylene black, and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1. Lithium iron phosphate was used as the active material and dissolved in ethylene carbonate to prepare lithium iron phosphate slurry. 2The above lithium iron phosphate slurry was coated on copper foil, and then the copper foil was punched into small discs and dried in a vacuum environment at 120°C for 12 hours to serve as the negative electrode of the battery. The diaphragms prepared in Examples 1-3 and Comparative Examples 1-6 were also punched into discs with a diameter of 16 mm and then dried in a drying oven for 6 hours to serve as diaphragms; lithium hexafluorophosphate (LiPF 6 ) was dissolved in a mixed solvent of ethylene carbonate (EC) / diethyl carbonate (DEC) with a volume ratio of 1:1 to obtain LiPF with a concentration of 1 mol / L 6 solution, as an electrolyte.

[0095] The button cell was assembled in an argon-filled glove box, with copper foil coated with lithium iron phosphate slurry as the negative electrode and metallic lithium foil as the counter electrode. 6 The solution was used as an electrolyte and was respectively combined with the separators prepared in Examples 1-3 and Comparative Examples 1-6 to form LFP / separator / Li batteries.

[0096] The specific capacity, rate performance, cycle performance and coulombic efficiency of the assembled LFP / diaphragm / Li battery were tested. All LFP / diaphragm / Li batteries were left for 6 hours before testing and charged and discharged using a constant current in the range of 1.7-2.6V. The results are shown in Figure 3 and Table 2.

[0097] Figure 3 The charge and discharge curves of the LFP / membrane / Li battery assembled with the heteropolyacid-aramid membrane prepared in Examples 1-3 and the aramid membrane prepared in Comparative Example 1. Figure 3 It can be seen that at 2A g -1 At a current density of , the specific capacity of the LFP / membrane / Li battery assembled with the heteropoly acid-aramid membrane prepared in Examples 1-3 is higher than that of the LFP / membrane / Li battery assembled with the aramid membrane prepared in Comparative Example 1, indicating that the introduction of heteropoly acid may provide more active sites for the electrode, allowing lithium ions to be more fully embedded and extracted; in addition, the modification of the aramid membrane by heteropoly acid may optimize the microstructure of the membrane and form a channel that is more conducive to lithium ion transmission. By comparing the specific capacity of the LFP / membrane / Li battery assembled with the heteropoly acid-aramid membrane prepared in Examples 1-3, it can be found that the specific capacity of Example 3 (349 mA h g) after 2000 cycles is greater than that of Example 3 (349 mA h g -1 )>Example 2(270mAh g -1 )>Example 1(167mAh g -1), since phosphomolybdic acid has certain oxidizing and acidic properties, its conductivity is relatively good, and as an additive for the negative electrode material of lithium-ion batteries, it can better improve the conductivity of the battery. In addition, the coulombic efficiency (CE) of the LFP / membrane / Li battery assembled with the heteropolyacid-aramid membrane prepared in Example 3 is close to 100%, further indicating that the heteropolyacid-aramid membrane has excellent stability throughout the electrochemical process.

[0098] Table 2 Charge and discharge performance of LFP / diaphragm / Li batteries assembled with the diaphragms prepared in Examples 1-3 and Comparative Examples 1-6 at 1C

[0099]

[0100] As can be seen from Table 2, the coulombic efficiency and specific capacity after 2000 cycles of Comparative Examples 2 and 3 are significantly lower than those of Example 3, while the impedance of Example 3 (85Ω) is significantly lower than that of Comparative Example 2 (169Ω) and Comparative Example 3 (92Ω). This shows that the amount of heteropoly acid content has a great influence on battery performance. If the heteropoly acid content is too much, aggregates may be formed on the surface or inside the aramid membrane, and a large amount of heteropoly acid may block the ion channels in the aramid membrane. If the heteropoly acid content is too little, the ion conductivity of the aramid membrane will be affected to a certain extent, and the transmission and reaction of lithium ions cannot be effectively promoted. The coulombic efficiency and specific capacity after 2000 cycles of Comparative Examples 4-6 are significantly lower than those of Example 3. Similarly, the impedance of Example 3 is also significantly lower than that of Comparative Examples 4-6, indicating that the heat treatment temperature is too high or too low, which will have a great influence on the pore size distribution, porosity and structure of the material, thereby affecting the transmission resistance and polarization phenomenon of lithium ions in the membrane, resulting in changes in coulombic efficiency, interface impedance and specific capacity.

[0101] The above are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for preparing a heteropolyacid-aramid diaphragm, characterized in that: The following steps are involved: mixing the aramid solution and the heteropoly acid to obtain a heteropoly acid-aramid composite solution; The heteropoly acid-aramid composite solution is cast and heat treated to obtain the heteropoly acid-aramid separator.

2. The method for preparing the heteropolyacid-aramid membrane according to claim 1, characterized in that: The mass ratio of aramid to heteropoly acid in the aramid solution is (50-100):(10-50).

3. The method for preparing the heteropolyacid-aramid membrane according to claim 1, characterized in that: The preparation method of the heteropoly acid-aramid composite solution comprises the following steps: mixing aramid and a solvent, stirring and heating to 60-80° C., and waiting for the aramid to be fully dissolved to obtain an aramid solution; mixing a heteropoly acid and the aramid solution, stirring and ultrasonicating to obtain the heteropoly acid-aramid composite solution.

4. The method for preparing the heteropolyacid-aramid membrane according to claim 3, characterized in that: The water content of the aramid fiber is less than 0.5%; and / or, The mass concentration of the aramid solution is 5-15%; and / or, The power of the ultrasound is 200-500W, and the time of the ultrasound is 30-60 minutes.

5. The method for preparing the heteropolyacid-aramid diaphragm according to claim 1 or 3, characterized in that: The heteropoly acid is selected from one or more of phosphotungstic acid, silicotungstic acid and phosphomolybdic acid; and / or, The particle size of the heteropoly acid is 1-10 μm.

6. The method for preparing the heteropolyacid-aramid diaphragm according to claim 3, characterized in that: The heteropoly acid further comprises a drying step before being mixed with the aramid solution; the drying temperature is 60-100° C. and the time is 6-12 hours.

7. The method for preparing the heteropolyacid-aramid membrane according to claim 1, characterized in that: The specific operation of the casting molding is: casting the heteropoly acid-aramid composite solution on a substrate, and then volatilizing the solvent; and / or, The casting thickness is 20-50 μm; and / or, The solvent evaporates at a temperature of 20-30° C., a relative humidity of 30-50%, and a time of 12-24 hours.

8. The method for preparing the heteropolyacid-aramid membrane according to claim 1, characterized in that: The heat treatment is carried out at a temperature of 100-150° C. and for a time of 2-6 hours.

9. The heteropolyacid-aramid diaphragm prepared by the preparation method according to any one of claims 1 to 8, characterized in that: The heteropoly acid-aramid membrane has a structure in which aramid is used as a skeleton network, and the heteropoly acid is filled in the pores between the aramids or attached to the surface of the aramids.

10. Use of the heteropolyacid-aramid separator according to claim 9 in lithium ion batteries.

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