Composite nanofiber separator, method for manufacturing the same, and secondary battery

Abstract

The application provides a composite nanofiber diaphragm, a preparation method thereof and a secondary battery. The method comprises the following steps: preparing a cellulose spinning solution and a perfluorosilane solution to obtain a cellulose spinning solution, and preparing a polymer, a lithium salt and modified inorganic nanoparticles to obtain a polymer spinning solution; performing double-needle electrostatic spinning on at least one side of a base film with the cellulose spinning solution and the polymer spinning solution to obtain the composite nanofiber diaphragm. The application provides a composite nanofiber diaphragm with high heat resistance, high membrane breaking temperature and high liquid absorption rate. The composite nanofiber diaphragm can still maintain a small heat shrinkage rate at 180 DEG C, the membrane breaking temperature can reach above 250 DEG C, and the liquid absorption rate of the electrolyte is obviously improved. More importantly, the battery prepared by using the composite nanofiber diaphragm has good safety performance.

Description

Technical Field

[0001] This invention belongs to the field of membrane material technology, specifically relating to a composite nanofiber membrane, its preparation method, and a secondary battery. Background Technology

[0002] Lithium-ion batteries are playing an increasingly important role as a crucial energy storage device. Electric vehicles inevitably encounter various unexpected situations during operation, such as mechanical compression, impact, or puncture by sharp objects. These situations can cause a short circuit and fire in a single cell within the battery pack. To prevent excessive heat from spreading to other cells and causing large-scale thermal runaway, increasingly stringent requirements are being placed on battery safety performance.

[0003] In lithium-ion battery systems, the separator, as a porous medium between the positive and negative electrodes, plays a crucial role in ion conduction and preventing short circuits by isolating the electrodes. Although the separator does not participate in the electrochemical reaction process, it plays a vital role in protecting the safety performance of the battery cell. Currently, most separators produced by wet processes use polyethylene as the base material. Because polyethylene has a low melting point and its pore-closing temperature is around 140°C, which is very close to its rupture temperature (less than 15°C), the close proximity of the separator's pore-closing and rupture temperatures can lead to melting and rupture of the separator before it can close its pores when the battery overheats, resulting in a short circuit and safety hazard. Therefore, it is often necessary to lower the pore-closing temperature and increase the rupture temperature of the separator to further improve battery safety. However, existing technologies for increasing the separator rupture temperature are not very effective and cannot completely eliminate the thermal runaway phenomenon caused by thermal propagation. Furthermore, the raw materials for increasing the separator rupture temperature in existing technologies are difficult to process, and the process flow is relatively complex.

[0004] Therefore, there is an urgent need in this field to develop a membrane material to solve the above problems. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a composite nanofiber separator, its preparation method, and a secondary battery. The present invention provides a composite nanofiber separator that combines high heat resistance, high membrane rupture temperature, and high electrolyte absorption rate. The composite nanofiber separator maintains a low thermal shrinkage rate even at 180°C, and its membrane rupture temperature can reach above 250°C, while significantly improving the electrolyte absorption rate. More importantly, the battery prepared using the above-mentioned composite nanofiber separator exhibits excellent safety performance.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing a composite nanofiber membrane, the method comprising the following steps:

[0008] A cellulose spinning solution was prepared by combining a cellulose solution and a perfluorosilane solution, and a polymer spinning solution was prepared by combining a polymer, a lithium salt, and modified inorganic nanoparticles.

[0009] The cellulose spinning solution and the polymer spinning solution are electrospun in a double-needle manner on at least one side of the base membrane to obtain a composite nanofiber membrane.

[0010] This invention employs a dual-needle synchronous electrospinning technique. On at least one side of the base membrane, a dual-needle electrospinning technique is used, with one spinneret electrospinning cellulose nanofibers and the other spinneret electrospinning polymer nanofibers. This allows the two different types of nanofibers to interweave, forming an interpenetrating network coating with a three-dimensional porous structure. This coating not only improves the heat resistance of the membrane, such as reducing the thermal shrinkage rate and increasing the membrane rupture temperature, but also significantly enhances the electrolyte absorption rate.

[0011] On the one hand, this invention adds perfluorosilane to the cellulose spinning solution, giving the spun cellulose nanofibers hydrophobic and oleophilic properties, thereby improving the wetting performance between the separator and the electrolyte and greatly increasing the separator's electrolyte absorption rate. Furthermore, the use of modified inorganic nanoparticles enhances the dispersion and bonding ability with the polymer, thus helping to maintain spinning stability. Simultaneously, during the double-needle spinning process, when polymer nanofibers and cellulose nanofibers are combined, the presence of modified inorganic nanoparticles allows for the formation of more cross-linking points, evolving from a single line-to-line cross-linking mode to a combination of point-to-line and line-to-line cross-linking modes. The more cross-linking points formed, the tighter the interpenetrating network formed by the coating, resulting in a composite separator with excellent heat resistance and a high membrane rupture temperature, thereby improving battery safety.

[0012] On the other hand, adding lithium salts to the polymer spinning solution not only facilitates the dissolution of the polymer in the solution, thereby improving the conductivity and spinnability of the spinning solution, but also enables the separator to play a lithium replenishment role during battery cycling, maintaining sufficient lithium ion transport and thus improving the cycle performance of the battery.

[0013] Preferably, the cellulose solution comprises lignocellulose and a first solvent.

[0014] Preferably, the first solvent includes any one of a mixed solvent of urea and choline, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, or methylpyrrolidine.

[0015] Preferably, the perfluorosilane solution comprises a perfluorosilane and a second solvent.

[0016] Preferably, the perfluorosilane includes any one or a combination of at least two of perfluorodecyltrimethoxysilane, perfluorooctyltrimethoxysilane, perfluorodecyltriethoxysilane, perfluorooctyltriethoxysilane, perfluorodecyltrichlorosilane, or perfluorooctyltrichlorosilane, with perfluorodecyltrimethoxysilane being the most preferred.

[0017] Preferably, the second solvent includes ethanol.

[0018] Preferably, the mass concentration of the perfluorosilane solution is 1.8-2.4 mg / mL, for example, it can be 1.8 mg / mL, 1.9 mg / mL, 2.0 mg / mL, 2.1 mg / mL, 2.2 mg / mL, 2.3 mg / mL, 2.4 mg / mL, etc.

[0019] In this invention, by adjusting the mass concentration of the perfluorosilane solution, the spun cellulose nanofibers are made hydrophobic, thereby reducing the water absorption of the membrane. If the mass concentration is too low, the hydrophobic effect will be insignificant, while if the concentration is too high, the affinity of the cellulose nanofibers for the electrolyte will decrease, and the wettability of the membrane will deteriorate.

[0020] Preferably, the process of preparing the cellulose spinning solution includes stirring the cellulose solution and the perfluorosilane solution at 20-30°C for 6-10 hours.

[0021] Preferably, the cellulose mass concentration in the cellulose spinning solution is 2.5-4%, for example, it can be 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, etc.

[0022] In this invention, by controlling the mass concentration of cellulose in the cellulose spinning solution, fibers can be ejected during the spinning process. If the mass concentration is too low, the viscosity of the spinning solution will be too low, and fibers cannot be formed during the spinning process. Conversely, if the concentration is too high, the spinning solution is prone to agglomeration and clumping, which affects the stability of the spinning process.

[0023] Preferably, the polymer comprises any one or a combination of at least two of meta-aramid, para-aramid, or polyimide.

[0024] Preferably, the lithium salt comprises any one or a combination of at least two of lithium chloride, lithium bromide, lithium fluoride, or lithium iodide.

[0025] Preferably, the modified inorganic nanoparticles are obtained by modifying inorganic nanoparticles with alkylsiloxane compounds.

[0026] Preferably, the inorganic nanoparticles include any one or a combination of at least two of nano-titanium dioxide, nano-alumina, nano-magnesium oxide, nano-tungsten oxide, nano-zinc oxide, or nano-zirconia.

[0027] Preferably, the alkylsiloxane compound includes any one or a combination of at least two of hexadecyltrimethoxysilane, dodecyltrimethoxysilane, octadecyltrimethoxysilane, hexadecyltriethoxysilane, dodecyltriethoxysilane, or octadecyltriethoxysilane, with hexadecyltrimethoxysilane being the most preferred.

[0028] Preferably, the preparation method of the modified inorganic nanoparticles includes the following steps:

[0029] The modified inorganic nanoparticles are obtained by stirring and dispersing inorganic nanoparticles, alkylsiloxane compounds, and organic solvents.

[0030] Preferably, the mass concentration of the alkylsiloxane compound is 1.2-1.6 mg / mL, for example, it can be 1.2 mg / mL, 1.3 mg / mL, 1.4 mg / mL, 1.5 g / mL, 1.6 mg / mL, etc.

[0031] In this invention, by adjusting the mass concentration of alkylsiloxane compounds, the inorganic nanoparticles are made hydrophobic, thereby reducing the water absorption of the coating. If the mass concentration is too low, the hydrophobic modification effect will be poor. Conversely, if the concentration is too high, the affinity of the polymer coating to the electrolyte will decrease, and the wettability of the membrane will deteriorate.

[0032] Preferably, the stirring temperature is 20-30℃, for example, 20℃, 22℃, 25℃, 28℃, 30℃, etc.; the stirring time is 1-3h, for example, 1h, 1.5h, 2h, 2.5h, 3h, etc.

[0033] Preferably, the dispersion process further includes washing and drying.

[0034] Preferably, the third solvent in the polymer spinning solution includes any one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, or N-methylpyrrolidone.

[0035] Preferably, the process of preparing the polymer spinning solution is carried out at room temperature.

[0036] Preferably, the mass ratio of the polymer, lithium salt and modified inorganic nanoparticles is (15-20):(1-3):(3-7), for example, it can be 15:1:3, 16:2:4, 17:2:5, 18:2:6, 19:2:7, 19:3:7, 20:3:7, etc.

[0037] In this invention, by adjusting the mass ratio of polymer, lithium salt and modified inorganic nanoparticles, the three are evenly dispersed, which is beneficial to the stability of spinning. If the mass ratio is too low, the viscosity of the spinning solution is low, and nanofibers cannot be spun. Conversely, if the mass ratio is too high, the spinning solution is prone to agglomeration and clumping, which affects the stability of the spinning process.

[0038] Preferably, the thickness of the base film is 5-14 micrometers, for example, it can be 5 micrometers, 8 micrometers, 10 micrometers, 12 micrometers, 14 micrometers, etc.

[0039] Preferably, the electrospinning conditions of the cellulose spinning solution are as follows: voltage of 15-20KV, for example, 15KV, 16KV, 17KV, 18KV, 19KV, 20KV, etc.; speed of 0.7-1.2mL / h, for example, 0.7mL / h, 0.8mL / h, 0.9mL / h, 1.0mL / h, 1.2mL / h, etc.; and receiving distance of 10-16cm, for example, 10cm, 11cm, 12cm, 13cm, 14cm, 15cm, 16cm, etc.

[0040] Preferably, the electrospinning conditions of the polymer spinning solution are as follows: voltage of 12-15KV, for example, 12KV, 13KV, 14KV, 15KV, etc.; speed of 0.5-0.8mL / h, for example, 0.5mL / h, 0.6mL / h, 0.7mL / h, 0.8mL / h, etc.; and receiving distance of 10-16cm, for example, 10cm, 11cm, 12cm, 13cm, 14cm, 15cm, 16cm, etc.

[0041] Preferably, the temperature of the double-needle electrospinning is 25-30℃, for example, 25℃, 28℃, 30℃, etc.; the humidity is 50-60%, for example, 50%, 52%, 55%, 58%, 60%, etc.

[0042] In a second aspect, the present invention provides a composite nanofiber membrane, which is prepared by the method for preparing a composite nanofiber membrane according to the first aspect.

[0043] Preferably, the composite nanofiber membrane includes a base membrane and a composite nanofiber layer disposed on at least one side of the base membrane.

[0044] Preferably, the thickness of one side of the composite nanofiber layer is 1-3 micrometers, for example, it can be 1 micrometer, 1.2 micrometers, 1.5 micrometers, 1.8 micrometers, 2 micrometers, 2.2 micrometers, 2.5 micrometers, 2.8 micrometers, 3 micrometers, etc.

[0045] Preferably, the thickness of the composite nanofiber membrane is 6-20 micrometers, for example, it can be 6 micrometers, 8 micrometers, 10 micrometers, 12 micrometers, 14 micrometers, 16 micrometers, 18 micrometers, 20 micrometers, etc.

[0046] Thirdly, the present invention provides a secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the separator comprises a composite nanofiber separator according to the second aspect.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] This invention provides a method for preparing a composite nanofiber membrane. The method employs a dual-needle synchronous electrospinning technique, where cellulose nanofibers are electrospun on at least one side of a base membrane using a dual-needle electrospinning technique. One spinneret electrospins cellulose nanofibers, while the other spinneret electrospins polymer nanofibers, causing the two different types of nanofibers to interweave and form an interpenetrating network coating with a three-dimensional porous structure. This coating not only improves the membrane's heat resistance (e.g., reducing thermal shrinkage and increasing rupture temperature), but also significantly enhances the electrolyte absorption rate.

[0049] On the one hand, this invention adds perfluorosilane to the cellulose spinning solution, giving the spun cellulose nanofibers hydrophobic and oleophilic properties, thereby improving the wetting performance between the separator and the electrolyte and greatly increasing the separator's electrolyte absorption rate. Furthermore, the use of modified inorganic nanoparticles enhances the dispersion and bonding ability with the polymer, thus helping to maintain spinning stability. Simultaneously, during the double-needle spinning process, when polymer nanofibers and cellulose nanofibers are combined, the presence of modified inorganic nanoparticles allows for the formation of more cross-linking points, evolving from a single line-to-line cross-linking mode to a combination of point-to-line and line-to-line cross-linking modes. The more cross-linking points formed, the tighter the interpenetrating network formed by the coating, resulting in a composite separator with excellent heat resistance and a high membrane rupture temperature, thereby improving battery safety.

[0050] On the other hand, adding lithium salts to the polymer spinning solution not only facilitates the dissolution of the polymer in the solution, thereby improving the conductivity and spinnability of the spinning solution, but also enables the separator to play a lithium replenishment role during battery cycling, maintaining sufficient lithium ion transport and thus improving the cycle performance of the battery. Detailed Implementation

[0051] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0052] Example 1

[0053] This embodiment provides a composite nanofiber membrane and its preparation method, which includes the following steps:

[0054] Preparation of cellulose nanofiber spinning solution: First, lignocellulose was pulverized into cellulose powder using a high-speed crusher. Then, the cellulose powder was dissolved in N,N-dimethylacetamide and stirred at 120°C for 3 hours to activate the cellulose. Subsequently, an ethanol solution of perfluorodecyltrimethoxysilane with a concentration of 2.3 mg / mL was added to the solution, and the mixture was stirred at 25°C for 8 hours to obtain a modified cellulose spinning solution. The mass concentration of cellulose in the cellulose spinning solution was 3%.

[0055] Preparation of aramid nanofiber spinning solution: First, titanium dioxide nanoparticles were pretreated by dispersion and stirring in an ethanol solution of hexadecyltrimethoxysilane (1.4 mg / mL) for 2 hours at 25°C. Then, the titanium dioxide nanoparticles were ultrasonically dispersed, centrifuged, washed, and dried to obtain modified titanium dioxide particles. Next, aramid polymer and lithium chloride were dissolved in N,N-dimethylacetamide and stirred at room temperature for 4 hours to obtain a homogeneous solution. The modified titanium dioxide particles were then added to this solution and stirred at room temperature for 6 hours to obtain the spinning solution. The mass ratio of aramid polymer, lithium chloride, and modified titanium dioxide particles was 17:2:5.

[0056] Double-needle electrospinning: A commercially available polyethylene film with a thickness of 9 micrometers was used as the substrate for electrospinning. The film was fixed on a receiving device, and cellulose nanofibers and aramid nanofibers were electrospun onto it simultaneously, resulting in a 1-micrometer-thick composite nanofiber layer on each side of the film. The electrospinning voltage for the cellulose nanofibers was 17 kV, the speed was 1.0 mL / h, and the receiving distance was 13 cm; the electrospinning voltage for the aramid nanofibers was 13.5 kV, the speed was 0.65 mL / h, and the receiving distance was 13 cm. The spinning temperature and relative humidity were 25℃ and 55%, respectively. After spinning, the membrane was baked in a 60℃ oven for 10 hours to obtain the composite nanofiber membrane.

[0057] This embodiment also provides a battery assembled using the above-mentioned composite nanofiber separator, the method of which is as follows:

[0058] The positive electrode material is lithium iron phosphate, which is mixed with a conductive agent, binder, and solvent to form a slurry, with the main material accounting for 96.9%. This slurry is then coated onto aluminum foil to obtain the positive electrode sheet. The negative electrode material is graphite, which is mixed with a conductive agent, binder, and solvent to form a slurry, with the main material accounting for 96.1%. This slurry is then coated onto copper foil to obtain the negative electrode sheet. The electrode sheets and composite nanofiber separator are shaped through stacking and hot pressing processes to obtain an electrode assembly with a certain degree of hardness. Electrolyte is then injected. After settling, the cell is pre-charged and formed to obtain the prepared battery.

[0059] Example 2

[0060] This embodiment provides a composite nanofiber membrane and its preparation method, which includes the following steps:

[0061] Preparation of cellulose nanofiber spinning solution: First, lignocellulose was pulverized into cellulose powder using a high-speed crusher. Then, the cellulose powder was dissolved in N,N-dimethylacetamide and stirred at 120°C for 3 hours to activate the cellulose. Subsequently, an ethanol solution of perfluorodecyltrimethoxysilane with a concentration of 1.8 mg / mL was added to the solution, and the mixture was stirred at 25°C for 8 hours to obtain a modified cellulose spinning solution. The mass concentration of cellulose in the cellulose spinning solution was 2.5%.

[0062] Preparation of aramid nanofiber spinning solution: First, titanium dioxide nanoparticles were pretreated by dispersion and stirring in an ethanol solution of hexadecyltrimethoxysilane (1.2 mg / mL) for 2 hours at 25°C. Then, the titanium dioxide nanoparticles were ultrasonically dispersed, centrifuged, washed, and dried to obtain modified titanium dioxide particles. Next, aramid polymer and lithium chloride were dissolved in N,N-dimethylacetamide and stirred at room temperature for 4 hours to obtain a homogeneous solution. The modified titanium dioxide particles were then added to this solution and stirred at room temperature for 6 hours to obtain the spinning solution. The mass ratio of aramid polymer, lithium chloride, and modified titanium dioxide particles was 15:1:3.

[0063] Double-needle electrospinning: A commercially available polyethylene film with a thickness of 9 micrometers was used as the substrate for electrospinning. The film was fixed on a receiving device, and cellulose nanofibers and aramid nanofibers were electrospun onto it simultaneously, resulting in a 2-micrometer-thick composite nanofiber layer on each side of the film. The electrospinning voltage for the cellulose nanofibers was 17 kV, the speed was 1.0 mL / h, and the receiving distance was 13 cm; the electrospinning voltage for the aramid nanofibers was 13.5 kV, the speed was 0.65 mL / h, and the receiving distance was 13 cm. The spinning temperature and relative humidity were 25℃ and 55%, respectively. After spinning, the membrane was baked in a 60℃ oven for 10 hours to obtain the composite nanofiber membrane.

[0064] The method of assembling the battery in this embodiment is the same as that in Embodiment 1.

[0065] Example 3

[0066] This embodiment provides a composite nanofiber membrane and its preparation method, which includes the following steps:

[0067] Preparation of cellulose nanofiber spinning solution: First, lignocellulose was pulverized into cellulose powder using a high-speed crusher. Then, the cellulose powder was dissolved in N,N-dimethylacetamide and stirred at 120°C for 3 hours to activate the cellulose. Subsequently, an ethanol solution of perfluorodecyltrimethoxysilane with a concentration of 2.4 mg / mL was added to the solution, and the mixture was stirred at 25°C for 8 hours to obtain a modified cellulose spinning solution. The mass concentration of cellulose in the cellulose spinning solution was 4%.

[0068] Preparation of aramid nanofiber spinning solution: First, titanium dioxide nanoparticles were pretreated by dispersion and stirring in an ethanol solution of hexadecyltrimethoxysilane (1.6 mg / mL) for 2 hours at 25°C. Then, the titanium dioxide nanoparticles were ultrasonically dispersed, centrifuged, washed, and dried to obtain modified titanium dioxide particles. Next, aramid polymer and lithium chloride were dissolved in N,N-dimethylacetamide and stirred at room temperature for 4 hours to obtain a homogeneous solution. The modified titanium dioxide particles were then added to this solution and stirred at room temperature for 6 hours to obtain the spinning solution. The mass ratio of aramid polymer, lithium chloride, and modified titanium dioxide particles was 20:3:7.

[0069] Double-needle electrospinning: A commercially available polyethylene film with a thickness of 9 micrometers was used as the substrate for electrospinning. The film was fixed on a receiving device, and cellulose nanofibers and aramid nanofibers were electrospun onto it simultaneously, resulting in a 3-micrometer-thick composite nanofiber layer on each side of the film. The electrospinning voltage for the cellulose nanofibers was 17 kV, the speed was 1.0 mL / h, and the receiving distance was 13 cm; the electrospinning voltage for the aramid nanofibers was 13.5 kV, the speed was 0.65 mL / h, and the receiving distance was 13 cm. The spinning temperature and relative humidity were 25℃ and 55%, respectively. After spinning, the membrane was baked in a 60℃ oven for 10 hours to obtain the composite nanofiber membrane.

[0070] The method for assembling the battery in this embodiment is the same as that in Embodiment 1.

[0071] Example 4

[0072] The difference between this embodiment and Example 1 is that the concentration of the ethanol solution of perfluorodecyltrimethoxysilane is 1 mg / mL, while all other aspects are the same as in Example 1.

[0073] Example 5

[0074] The difference between this embodiment and Example 1 is that the concentration of the ethanol solution of perfluorodecyltrimethoxysilane is 5 mg / mL, while all other aspects are the same as in Example 1.

[0075] Example 6

[0076] The difference between this embodiment and Embodiment 1 is that the mass concentration of cellulose in the cellulose spinning solution is 1%, while all other aspects are the same as in Embodiment 1.

[0077] Example 7

[0078] The difference between this embodiment and Embodiment 1 is that the mass concentration of cellulose in the cellulose spinning solution is 8%, while all other aspects are the same as in Embodiment 1.

[0079] Example 8

[0080] The difference between this embodiment and Example 1 is that the mass concentration of hexadecyltrimethoxysilane is 0.6 mg / mL, while all other aspects are the same as in Example 1.

[0081] Example 9

[0082] The difference between this embodiment and Example 1 is that the mass concentration of hexadecyltrimethoxysilane is 3 mg / mL, while all other aspects are the same as in Example 1.

[0083] Example 10

[0084] The difference between this embodiment and Example 1 is that the mass ratio of aramid polymer, lithium chloride and modified titanium dioxide particles is 10:0.5:1, while all other aspects are the same as in Example 1.

[0085] Example 11

[0086] The difference between this embodiment and Example 1 is that the mass ratio of aramid polymer, lithium chloride and modified titanium dioxide particles is 30:5:10, while all other aspects are the same as in Example 1.

[0087] Example 12

[0088] The difference between this embodiment and Example 1 is that perfluorodecyltrimethoxysilane is replaced with decyltrimethoxysilane of equal concentration; otherwise, they are the same as in Example 1.

[0089] Comparative Example 1

[0090] This comparative example provides a polyethylene separator, and the method for assembling the battery in this comparative example is the same as that in Example 1.

[0091] Comparative Example 2

[0092] This comparative example provides a commercial ceramic-coated separator, with a base film of 9 micrometers thickness, a ceramic coating thickness of 2 micrometers, and a total thickness of 11 micrometers for the commercial ceramic-coated separator. The method for assembling the battery in this comparative example is the same as that in Example 1.

[0093] Comparative Example 3

[0094] This comparative example provides an aramid electrospun nanofiber membrane, and the method for assembling the battery in this comparative example is the same as that in Example 1.

[0095] Test conditions

[0096] The composite nanofiber membranes provided in Examples 1 to 12 and Comparative Examples 1 to 3 were tested using the following methods:

[0097] The composite nanofiber membrane was tested for heat shrinkage, membrane rupture temperature, moisture content, and electrolyte absorption rate. The heat shrinkage test method was as follows: before baking in an oven, the dimensions in the MD and TD directions were measured and recorded as L1 and H1, respectively. After baking at a certain temperature for 1 hour, the dimensions in the MD and TD directions were measured again and recorded as L2 and H2, respectively. The baking temperatures were 130 / 150 / 180℃. Heat shrinkage rates were calculated as follows: MD heat shrinkage rate = (L1-L2) / L1 × 100%, TD heat shrinkage rate = (H1-H2) / H1 × 100%.

[0098] The membrane rupture temperature was measured using the TMA test method.

[0099] Moisture content was measured using a Karl Fischer moisture analyzer under the following conditions: baking at 150°C, gas flow rate of 50 ml / min, baking time of at least 300 s, and the drift value was stopped when it stabilized.

[0100] The electrolyte absorption rate test method is as follows: Before immersing the diaphragm in the electrolyte, measure the weight of a fixed area of ​​diaphragm and record it as M1. Then, immerse the diaphragm in the electrolyte for 0.5 hours, remove it, wipe off the free electrolyte on the surface of the diaphragm, and weigh it again, recording it as M2. Absorption rate = (M2 - M1) / M1 × 100%.

[0101] Multi-cell thermal runaway tests were conducted on high-safety batteries at ambient temperature (25±2℃) and high temperature (60±5℃), verifying that the cells had a state of charge (SOC) greater than 95%. First, the target cell was heated using a heating device to induce thermal runaway, and then adjacent cells were observed to see if thermal runaway would occur. The thermal runaway trigger condition was determined when a) + b) or c) occurred.

[0102] a) The triggering object generates a voltage drop that exceeds 25% of the initial voltage;

[0103] b) The temperature rise rate at the monitoring point, dT / dt, is ≥ 1℃ / s;

[0104] c) Battery cell fire and explosion.

[0105] The test results are shown in Table 1:

[0106] Table 1

[0107]

[0108]

[0109] As can be seen from Table 1, the composite nanofiber membrane prepared by the present invention has low thermal shrinkage, high membrane rupture temperature and excellent electrolyte absorption capacity. At the same time, the prepared battery can pass the thermal spread test at 25℃ and 60℃.

[0110] The data from Examples 1-3 show that, within the given preferred parameter range, the prepared composite nanofiber membranes all exhibit low shrinkage, a membrane rupture temperature greater than 250°C, a moisture content less than 1000 ppm, and good electrolyte wettability. The prepared batteries all pass thermal spread tests at both room temperature and high temperature. Furthermore, the higher the content of surface cellulose and aramid fibers, and the thicker the coating, the better the heat resistance of the membrane.

[0111] The data from Examples 1 and 4-5 show that if the concentration of perfluorodecyltrimethoxysilane is too low, the hydrophobic modification effect of cellulose nanofibers is not obvious, leading to easy water absorption by the coating and high moisture content in the separator. This poses a risk of gas generation in the prepared battery. If the concentration of perfluorodecyltrimethoxysilane is too high, the affinity of the separator for the electrolyte decreases, resulting in a poorer electrolyte absorption rate.

[0112] Because a low cellulose concentration prevents the formation of nanofibers during spinning, while a high concentration leads to uneven mixing of the cellulose spinning solution and instability during spinning, aramid / titanium dioxide nanofiber coated membranes were prepared in Examples 6-7. The data shows that the lack of cross-linking between cellulose and aramid / titanium dioxide nanofibers significantly increases the membrane's thermal shrinkage rate, lowers the membrane rupture temperature, and deteriorates its heat resistance. The prepared battery only passed the room-temperature thermal runaway test and failed the high-temperature thermal runaway test.

[0113] The data from Examples 1 and 8-9 show that if the concentration of hexadecyltrimethoxysilane is too low, the hydrophobic modification effect on titanium dioxide nanoparticles is not significant, leading to easy water absorption in the coating and high moisture content in the separator. This poses a risk of gas generation in the prepared battery. If the concentration of hexadecyltrimethoxysilane is too high, the separator's affinity for the electrolyte decreases, and the electrolyte absorption rate of the separator deteriorates.

[0114] Because the mass ratio of aramid polymer, lithium chloride, and modified titanium dioxide particles was too low, nanofibers could not be formed during spinning; and because the concentration was too high, the spinning solution was unevenly agitated, leading to instability during spinning. Therefore, in Examples 10-11, a modified cellulose nanofiber coated membrane was prepared. Data from Examples 1 and 10-11 show that due to the lack of the aramid coating, the membrane rupture temperature dropped to 215.3°C, and the battery cell failed the thermal propagation test at both room temperature and high temperature.

[0115] The data from Examples 1 and 12 show that replacing the membrane with decyltrimethoxysilane of the same concentration results in a moisture content more than 800 ppm higher than that with perfluorodecyltrimethoxysilane, which could lead to cell bulging due to gas buildup. This is because the F atom has a low surface energy, resulting in better hydrophobic modification. At the same concentration, decyltrimethoxysilane is less effective than perfluorodecyltrimethoxysilane as a modifier.

[0116] The data from Example 1 and Comparative Example 1 show that, without a heat-resistant coating, the polyethylene film exhibits poor thermal shrinkage properties. Above 150°C, the film melts, and the film rupture temperature is around 150°C, resulting in poor wettability to the electrolyte. Consequently, the battery fails the thermal spread test.

[0117] As can be seen from the data results of Example 1 and Comparative Example 2, the composite nanofiber diaphragm proposed in this invention has better performance than the ceramic-coated diaphragm, and the ceramic-coated diaphragm disclosed in the prior art cannot achieve all the technical effects.

[0118] The data results from Example 1 and Comparative Example 3 show that, due to the lack of ceramic particles as a heat-resistant skeleton and cross-linking points, and the absence of an interpenetrating network formed by the composite fiber coating, the thermal shrinkage of the pure aramid electrospun nanofiber membrane will increase, and the membrane rupture temperature will drop to about 242°C. The battery can pass the thermal spread test under normal temperature conditions, but it cannot meet the requirements under high temperature conditions.

[0119] The applicant declares that the present invention is illustrated by the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing a composite nanofiber membrane, characterized in that, The method includes the following steps: A cellulose spinning solution was prepared by combining a cellulose solution and a perfluorosilane solution, and a polymer spinning solution was prepared by combining a polymer, a lithium salt, and modified inorganic nanoparticles. The cellulose spinning solution and the polymer spinning solution are electrospun on at least one side of the base membrane to obtain a composite nanofiber membrane. The perfluorosilane solution includes perfluorosilane, which includes any one or a combination of at least two of the following: perfluorodecyltrimethoxysilane, perfluorooctyltrimethoxysilane, perfluorodecyltriethoxysilane, perfluorooctyltriethoxysilane, perfluorodecyltrichlorosilane, or perfluorooctyltrichlorosilane. The mass concentration of the perfluorosilane solution is 1.8-2.4 mg / mL; The cellulose spinning solution has a cellulose mass concentration of 2.5-4%; The modified inorganic nanoparticles are obtained by modifying inorganic nanoparticles with an alkylsiloxane compound at a mass concentration of 1.2-1.6 mg / mL; The mass ratio of the polymer, lithium salt, and modified inorganic nanoparticles is (15-20):(1-3):(3-7).

2. The method according to claim 1, characterized in that, The cellulose solution comprises lignocellulose and a first solvent.

3. The method according to claim 2, characterized in that, The first solvent includes any one of the following: a mixed solvent of urea and choline, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, or methylpyrrolidine.

4. The method according to claim 1, characterized in that, The perfluorosilane solution also includes a second solvent.

5. The method according to claim 4, characterized in that, The second solvent includes ethanol.

6. The method according to claim 1, characterized in that, The perfluorosilane is perfluorodecyltrimethoxysilane.

7. The method according to claim 1, characterized in that, The process of preparing the cellulose spinning solution includes stirring the cellulose solution and the perfluorosilane solution at 20-30°C for 6-10 hours.

8. The method according to claim 1, characterized in that, The polymer includes any one or a combination of at least two of meta-aramid, para-aramid, or polyimide; The lithium salt includes any one or a combination of at least two of lithium chloride, lithium bromide, lithium fluoride or lithium iodide. The inorganic nanoparticles include any one or a combination of at least two of nano-titanium dioxide, nano-alumina, nano-magnesium oxide, nano-tungsten oxide, nano-zinc oxide, or nano-zirconia. The alkylsiloxane compound includes any one or a combination of at least two of hexadecyltrimethoxysilane, dodecyltrimethoxysilane, octadecyltrimethoxysilane, hexadecyltriethoxysilane, dodecyltriethoxysilane, or octadecyltriethoxysilane.

9. The method according to claim 8, characterized in that, The alkylsiloxane compound is hexadecyltrimethoxysilane.

10. The method according to claim 1, characterized in that, The preparation method of the modified inorganic nanoparticles includes the following steps: The modified inorganic nanoparticles are obtained by stirring and dispersing inorganic nanoparticles, alkylsiloxane compounds, and organic solvents.

11. The method according to claim 10, characterized in that, The stirring temperature is 20-30℃, and the stirring time is 1-3 hours.

12. The method according to claim 10, characterized in that, The dispersion process further includes washing and drying.

13. The method according to claim 1, characterized in that, The third solvent in the polymer spinning solution includes any one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, or N-methylpyrrolidone. The process of preparing the polymer spinning solution is carried out at room temperature.

14. The method according to claim 1, characterized in that, The thickness of the base film is 5-14 micrometers.

15. The method according to claim 1, characterized in that, The electrospinning conditions for the cellulose spinning solution are: voltage of 15-20KV, speed of 0.7-1.2mL / h, and receiving distance of 10-16cm.

16. The method according to claim 1, characterized in that, The electrospinning conditions for the polymer spinning solution are: voltage of 12-15KV, speed of 0.5-0.8mL / h, and receiving distance of 10-16cm.

17. The method according to claim 1, characterized in that, The temperature of the double needle electrospinning is 25-30℃, and the humidity is 50-60%.

18. A composite nanofiber membrane, characterized in that, The composite nanofiber membrane is prepared by the method for preparing a composite nanofiber membrane according to any one of claims 1-17.

19. The composite nanofiber membrane according to claim 18, characterized in that, The composite nanofiber membrane includes a base membrane and a composite nanofiber layer disposed on at least one side of the base membrane.

20. The composite nanofiber membrane according to claim 19, characterized in that, The thickness of the composite nanofiber layer on one side is 1-3 micrometers.

21. The composite nanofiber membrane according to claim 18, characterized in that, The thickness of the composite nanofiber membrane is 6-20 micrometers.

22. A secondary battery, characterized in that, The secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the separator includes a composite nanofiber separator according to any one of claims 18-21.

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