A chelating membrane, its preparation method and application

Abstract

This invention discloses a chelating membrane, its preparation method, and its application. The chelating membrane comprises a polyolefin membrane and a pyrrolidone-containing cyanoethyl polyvinyl alcohol composite gel coating disposed on at least one surface of the polyolefin membrane. The pyrrolidone-containing cyanoethyl polyvinyl alcohol composite gel coating is formed by applying a slurry to the surface of the polyolefin membrane, the slurry being prepared from pyrrolidone-containing cyanoethyl polyvinyl alcohol and carbonate. This invention protects the electrolyte from contamination and stabilizes the SEI membrane through the pyrrolidone-containing cyanoethyl polyvinyl alcohol composite gel coating. The pyrrolidone in the coating has a very strong chelating effect on transition metal ions, limiting the shuttle movement of transition metals dissolved in the electrolyte. Furthermore, the polar cyano bonds in the cyanoethyl polyvinyl alcohol interact with the carbonyl dipole-dipole interaction in the carbonate molecules, stabilizing the SEI membrane and thus protecting the battery negative electrode.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a separator with chelating function, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, due to their high energy density, high operating voltage, and long cycle life, have been widely adopted in mainstream fields such as consumer electronics, transportation, and electrochemical energy storage. The cathode, as one of the key components of a battery, determines its voltage, energy density, and safety. Among these, nickel-cobalt-manganese ternary materials, lithium manganese oxide, lithium manganese iron phosphate, and lithium-rich manganese-based cathode materials are currently the mainstream research materials. However, the transition metals in these cathode materials, especially manganese ions, can dissolve from the surface and diffuse through the electrolyte, depositing on the anode. This results in the loss of active material in the cathode and passivates the anode, forming an unstable SEI film, leading to lithium ion loss and increased interfacial resistance.

[0003] The separator is one of the most critical components inside a lithium-ion battery. Its basic function is to separate the positive and negative electrodes, preventing short circuits caused by direct contact between the two electrodes, while allowing lithium ions to migrate between the electrodes to ensure a complete electrochemical reaction. On the other hand, transition metal ions dissolved in the electrolyte can also shuttle to the negative electrode side through the separator pores. Therefore, it is particularly important to give the separator a chelating function to block transition metal ions while ensuring lithium ion migration. Currently, a common method is to directly add transition metal chelating agents to the ceramic system and coat them onto the separator surface. However, the chelating agent is only bonded to the ceramic through an adhesive, which carries the risk of chelating agent detachment and electrolyte contamination. Summary of the Invention

[0004] To address the existing problems of chelating agent shedding and SEI membrane instability, this invention provides a chelating membrane, its preparation method, and its application.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A chelating membrane includes a polyolefin membrane and a pyrrolidone-containing cyanoethyl polyvinyl alcohol composite gel coating (PCA-PVA-CN) disposed on at least one surface of the polyolefin membrane.

[0007] The pyrrolidone-containing cyanoethyl polyvinyl alcohol composite gel coating is formed by applying a slurry to the surface of a polyolefin membrane. The slurry is prepared from pyrrolidone-containing cyanoethyl polyvinyl alcohol (PCA-PVA-CN) and carbonate.

[0008] In the above technical solution, the present invention protects the electrolyte from contamination and stabilizes the SEI film through a cyanoethyl polyvinyl alcohol composite gel coating (PCA-PVA-CN) containing pyrrolidone. The pyrrolidone in this coating has a very strong chelating effect on transition metal ions, limiting the shuttle movement of transition metals dissolved in the electrolyte. Secondly, the polar cyano bond (-C≡N) in the cyanoethyl polyvinyl alcohol interacts with the carbonyl group (C=O) in the carbonate molecule via a dipole-dipole interaction, stabilizing the SEI film and thus protecting the battery's negative electrode.

[0009] Furthermore, the polyolefin separator is selected from one or a combination of two of polyethylene separators and polypropylene separators.

[0010] Furthermore, the slurry, by weight, contains the following components: 0.5 to 30 parts PCA-PVA-CN and 100 parts carbonate.

[0011] Furthermore, the slurry, by weight, contains the following components: 0.5 to 5 parts PCA-PVA-CN and 100 parts carbonate.

[0012] According to one embodiment of the present invention, the slurry, by weight, has the following components added: 2 parts PCA-PVA-CN and 100 parts carbonate.

[0013] According to one embodiment of the present invention, the slurry, by weight, comprises the following components: 0.5 parts PCA-PVA-CN and 100 parts carbonate.

[0014] According to one embodiment of the present invention, the slurry, by weight, has the following components added: 5 parts PCA-PVA-CN and 100 parts carbonate.

[0015] Furthermore, the PCA-PVA-CN is prepared by an esterification reaction method:

[0016] Cyanoethyl polyvinyl alcohol (PVA-CN), dicyclohexylcarbodiimide (DCC) as a dehydrating agent, and 2-pyrrolidone-5-carboxylic acid (PCA) were sequentially added to N,N-dimethylformamide (DMF). Under the action of the catalyst 4-dimethylaminopyridine (DMAP), the mixture was stirred at room temperature for 36-48 hours. The supernatant was filtered, and water was added to precipitate the polymerization product. The product was redissolved in DMF, precipitated again in ethanol, and dried to obtain PCA-PVA-CN.

[0017] In the above-mentioned PCA-PVA-CN preparation method, the mass ratio of cyanoethyl polyvinyl alcohol (PVA-CN), dehydrating agent dicyclohexylcarbodiimide (DCC), 2-pyrrolidone-5-carboxylic acid (PCA), and 4-dimethylaminopyridine is (7-10):(16-20):(10-20):(0.4-0.8), specifically 7:16:(10-20):0.4.

[0018] The relative molecular weight of the cyanoethyl polyvinyl alcohol (PVA-CN) is 13,000-25,000, and the nitrogen content is 10-15%.

[0019] Further, the carbonate is selected from at least two of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dipropyl carbonate (DPC), diphenyl carbonate (DPhC), dibutyl carbonate (DBC), and butylene carbonate (BC).

[0020] According to one embodiment of the present invention, the carbonate is selected from a mixed solvent formed by ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 1:2.

[0021] Furthermore, the thickness of the polyolefin membrane can be 5-20 μm.

[0022] Furthermore, the thickness of the composite gel coating can be 1-5 μm, and the coating surface density can be 0.1-10 g / m³. 2 .

[0023] The present invention also provides a method for preparing the above-mentioned chelating membrane.

[0024] The method for preparing the chelating membrane provided by the present invention includes the following steps:

[0025] PCA-PVA-CN is added to carbonate to form a uniform slurry. The slurry is then coated on the surface of the polyolefin membrane and subjected to a thermal crosslinking reaction to obtain the modified polyolefin membrane.

[0026] In the above method, the reaction temperature of the thermal crosslinking reaction is 60-80℃, and the reaction time is 2-3 hours.

[0027] Furthermore, the coating can be applied using roller coating, dot coating, spray coating, or strip coating processes.

[0028] The present invention also provides the application of the above-mentioned separator with chelating function in the preparation of lithium-ion batteries.

[0029] The present invention also protects a lithium-ion battery comprising a separator with chelating function provided by the present invention.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] In the 2-pyrrolidone-5-carboxylic acid (PCA) involved in this invention, -NH and -C=O are relatively stable metal chelate conformations, wherein Mn 2+ It exhibits strong ionic dipole interaction with PCA, enabling better coupling of Mn. 2+ .

[0032] This invention relates to polar cyanoethyl polyvinyl alcohol (CEV). The cyanoethyl polyvinyl alcohol effectively reduces the reactivity of free carbonate molecules in the electrolyte through dipole-dipole interactions between the cyano group (-C≡N) in CEV and the carbonyl group (C=O) in the carbonate molecule. Furthermore, the polyvinyl alcohol segments participate in the formation of a polyvinyl alcohol-modified SEI film at the negative electrode, effectively protecting the negative electrode and reducing corrosion from transition metal ions at the positive electrode. The chelating performance of the chelating agent can be controlled by altering the ratio of PCA to PVA-CN. Attached Figure Description

[0033] Figure 1 This is a scanning electron microscope image of PCA-PVA-CN on the membrane surface of Example 1;

[0034] Figure 2 The infrared spectrum of the PCA-PVA-CN coated diaphragm in Example 1 is shown. Detailed Implementation

[0035] To facilitate understanding of the present invention, a more comprehensive description of the invention will be provided below in conjunction with specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to enable a more thorough and complete understanding of the disclosure of the present invention.

[0036] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0037] The first aspect of the present invention discloses a chelating membrane comprising a polyolefin membrane, wherein a pyrrolidone-containing cyanoethyl polyvinyl alcohol composite gel coating (PCA-PVA-CN) is provided on the surface of the polyolefin membrane; the pyrrolidone-containing cyanoethyl polyvinyl alcohol composite gel coating is formed by applying a slurry to the surface of the polyolefin membrane, wherein the slurry is prepared by pyrrolidone-containing cyanoethyl polyvinyl alcohol and carbonate.

[0038] The polyolefin separator in this invention is a conventional choice for separators in the field of lithium-ion batteries. Specific embodiments include, but are not limited to, composite separators consisting of one or a combination of two of polyethylene separators and polypropylene separators.

[0039] Specifically, PCA-PVA-CN is prepared using the following method:

[0040] Cyanoethyl polyvinyl alcohol (PVA-CN), the dehydrating agent dicyclohexylcarbodiimide (DCC), and 2-pyrrolidone-5-carboxylic acid (PCA) were sequentially added to N,N-dimethylformamide (DMF). Separately, under ice bath conditions, the catalyst 4-dimethylaminopyridine (DMAP) was added to the DMF over 10 minutes. The reaction mixture was stirred at room temperature for 40 hours, filtered, and the supernatant was precipitated with water. The product was redissolved in DMF and then precipitated again in ethanol. Finally, the product was dried under vacuum at 100°C for 24 hours to obtain PCA-PVA-CN.

[0041] Further, the carbonate is one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), dipropyl carbonate (DPC), diphenyl carbonate (DPhC), dibutyl carbonate (DBC), and butylene carbonate (BC).

[0042] Furthermore, the synthesis of PCA-PVA-CN and the amount of each component added to the slurry have a certain impact on the performance of the chelating membrane. Those skilled in the art can adjust it as needed. Preferably, in some specific embodiments of the present invention, in the synthesis of PCA-PVA-CN, the ratio of cyanoethyl polyvinyl alcohol (PVA-CN), dehydrating agent dicyclohexylcarbodiimide (DCC), 2-pyrrolidone-5-carboxylic acid (PCA), and 4-dimethylaminopyridine can be (7-10):(16-20):(10-20):(0.4-0.8); the amount of each component added to the mixed slurry, by mass, is: 0.5-30 parts PCA-PVA-CN and 100 parts carbonate.

[0043] Furthermore, the slurry is coated onto the surface of the diaphragm to form a coating. This surface can be one side or both sides of the polyolefin diaphragm, depending on the requirements. The coating thickness should not be too thick or too thin, as both excessive thickness and thinness will affect the diaphragm's performance. Therefore, preferably, the coating thickness is 1-5 μm. Similarly, the areal density of the coating also has a preferred range. In some specific embodiments of the present invention, the areal density of the coating is 0.1–10 g / m³. 2 Within the preferred range, the diaphragm's performance can reach its optimal level.

[0044] The second aspect of the present invention discloses a method for preparing the polyolefin separator disclosed in the first aspect of the present invention, comprising the following steps: dissolving PCA-PVA-CN in carbonate, then coating the slurry onto the surface of the polyolefin separator, and thermally crosslinking at 60-80°C to obtain the modified polyolefin separator.

[0045] The coating method in this invention is a conventional choice in the art, and specific examples include, but are not limited to, roller coating, dot coating, spray coating, or strip coating processes.

[0046] The third aspect of the present invention discloses a lithium-ion battery comprising a separator with chelating function as described in the first aspect of the present invention. It is understood that the lithium-ion battery here also includes a positive electrode, a negative electrode and an electrolyte, all of which are conventional choices in the art and will not be described in detail here. The assembly method of the lithium-ion battery can adopt conventional methods in the art and will not be described in detail here.

[0047] The technical solution of the present invention will be described more clearly and completely below with reference to specific embodiments.

[0048] The “cyanoethyl polyvinyl alcohol (PVA-CN)” used in the following examples is prepared by cyanoethylating polyvinyl alcohol (PVA), with a relative molecular weight of 13,000-25,000 and a nitrogen content of 10-15%.

[0049] Specifically, the preparation method of cyanoethyl polyvinyl alcohol (PVA-CN) is as follows:

[0050] PVA and NaOH (1 mol / L) were mixed thoroughly at a [OH] molar mass ratio of 1:1. An equal volume of 3 mol / L acrylonitrile-acetone solution was added, followed by 0.5% tetrabutylene ammonium bromide (TBAB) as a phase transfer catalyst. The reaction was carried out at room temperature for 4-5 hours. After the reaction, the supernatant was poured off, and the precipitate was neutralized with acetic acid. Finally, the mixture was washed with acetone and deionized water to obtain cyanoethyl polyvinyl alcohol (PVA-CN), with a relative molecular weight of 13,000-25,000 and a nitrogen content of 10-15%.

[0051] Example 1

[0052] Step 1: 7 parts of cyanoethyl polyvinyl alcohol (PVA-CN), 16 parts of the dehydrating agent dicyclohexylcarbodiimide (DCC), and 10 parts of 2-pyrrolidone-5-carboxylic acid (PCA) were sequentially added to 175 parts of N,N-dimethylformamide (DMF). Separately, under ice bath conditions, 0.4 parts of the catalyst 4-dimethylaminopyridine (DMAP) were added to 25 parts of DMF over 10 minutes. The reaction mixture was stirred at room temperature for 40 hours, filtered, and the supernatant was collected. Water was added to precipitate the polymerization product, which was then redissolved in DMF and precipitated again in ethanol. Finally, the product was dried under vacuum at 100°C for 24 hours to obtain PCA-PVA-CN.

[0053] Step 2: Add 2 parts by weight of PCA-PVA-CN powder to 100 parts by weight of a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), wherein the volume ratio of EC to EMC is 1:2. After complete dissolution, a slurry is obtained.

[0054] Step 3: The obtained slurry is coated onto a PE membrane with a thickness of 12μm using a micro-grooving roller coating method. The coating thickness is controlled to be approximately 2μm (coating areal density is 2.4g / m²). 2 By thermally crosslinking the coating layer at 60°C for 3 hours, a modified PE membrane with chelating function can be obtained.

[0055] Figure 1 Here is a scanning electron microscope image of PCA-PVA-CN on the membrane surface; by Figure 1 It can be seen that PCA-PVA-CN is uniformly coated on the surface of the PE base film, with no obvious missed coating.

[0056] Figure 2 Infrared spectrum of PCA-PVA-CN coated diaphragm; by Figure 2 It can be seen that 2919, 2850, and 1465cm -1 The absorption peak is mainly CH on the PE membrane, at 2208 cm⁻¹. -1 The absorption peaks for -C≡N in cyanoethyl polyvinyl alcohol are 1743 and 1704 cm⁻¹. -1 It is characterized by a C=O absorption peak.

[0057] Example 2

[0058] Step 1: 7 parts of cyanoethyl polyvinyl alcohol (PVA-CN), 16 parts of the dehydrating agent dicyclohexylcarbodiimide (DCC), and 20 parts of 2-pyrrolidone-5-carboxylic acid (PCA) were sequentially added to 175 parts of N,N-dimethylformamide (DMF). Separately, under ice bath conditions, 0.4 parts of the catalyst 4-dimethylaminopyridine (DMAP) were added to 25 parts of DMF over 10 minutes. The reaction mixture was stirred at room temperature for 40 hours, filtered, and the supernatant was collected. Water was added to precipitate the polymerization product, which was then redissolved in DMF and precipitated again in ethanol. Finally, the product was dried under vacuum at 100°C for 24 hours to obtain PCA-PVA-CN.

[0059] Step 2: Add 2 parts by weight of PCA-PVA-CN powder to 100 parts by weight of a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), wherein the volume ratio of EC to EMC is 1:2. After complete dissolution, a slurry is obtained.

[0060] Step 3: The obtained slurry is coated onto a PE membrane with a thickness of 12μm using a micro-grooving roller coating method. The coating thickness is controlled to be approximately 2μm (coating areal density is 2.4g / m²). 2 By thermally crosslinking the coating layer at 60°C for 3 hours, a modified PE membrane with chelating function can be obtained.

[0061] Example 3

[0062] Step 1: 7 parts of cyanoethyl polyvinyl alcohol (PVA-CN), 16 parts of the dehydrating agent dicyclohexylcarbodiimide (DCC), and 10 parts of 2-pyrrolidone-5-carboxylic acid (PCA) were sequentially added to 175 parts of N,N-dimethylformamide (DMF). Separately, under ice bath conditions, 0.4 parts of the catalyst 4-dimethylaminopyridine (DMAP) were added to 25 parts of DMF over 10 minutes. The reaction mixture was stirred at room temperature for 40 hours, filtered, and the supernatant was collected. Water was added to precipitate the polymerization product, which was then redissolved in DMF and precipitated again in ethanol. Finally, the product was dried under vacuum at 100°C for 24 hours to obtain PCA-PVA-CN.

[0063] Step 2: Add 0.5 parts by weight of PCA-PVA-CN powder to 100 parts by weight of a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), wherein the volume ratio of EC to EMC is 1:2. After complete dissolution, a slurry is obtained.

[0064] Step 3: The obtained slurry is coated onto a PE membrane with a thickness of 12μm using a micro-grooving roller coating method. The coating thickness is controlled to be around 2μm (coating areal density is 1.8g / m²). 2By thermally crosslinking the coating layer at 60°C for 2 hours, a modified PE membrane with chelating function can be obtained.

[0065] Example 4

[0066] Step 1: 7 parts of cyanoethyl polyvinyl alcohol (PVA-CN), 16 parts of the dehydrating agent dicyclohexylcarbodiimide (DCC), and 10 parts of 2-pyrrolidone-5-carboxylic acid (PCA) were sequentially added to 175 parts of N,N-dimethylformamide (DMF). Separately, under ice bath conditions, 0.4 parts of the catalyst 4-dimethylaminopyridine (DMAP) were added to 25 parts of DMF over 10 minutes. The reaction mixture was stirred at room temperature for 40 hours, filtered, and the supernatant was collected. Water was added to precipitate the polymerization product, which was then redissolved in DMF and precipitated again in ethanol. Finally, the product was dried under vacuum at 100°C for 24 hours to obtain PCA-PVA-CN.

[0067] Step 2: Add 5 parts by weight of PCA-PVA-CN powder to 100 parts by weight of a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), wherein the volume ratio of EC to EMC is 1:2. After complete dissolution, a slurry is obtained. Step 3: Coat the obtained slurry onto a 12μm thick PE membrane using a micro-gravure roller coating method. The coating thickness is controlled to be approximately 2μm (coating areal density is 2.4g / m²). 2 By thermally crosslinking the coating layer at 60°C for 3 hours, a modified PE membrane with chelating function can be obtained.

[0068] Example 5

[0069] Step 1: 7 parts of cyanoethyl polyvinyl alcohol (PVA-CN), 16 parts of the dehydrating agent dicyclohexylcarbodiimide (DCC), and 10 parts of 2-pyrrolidone-5-carboxylic acid (PCA) were sequentially added to 175 parts of N,N-dimethylformamide (DMF). Separately, under ice bath conditions, 0.4 parts of the catalyst 4-dimethylaminopyridine (DMAP) were added to 25 parts of DMF over 10 minutes. The reaction mixture was stirred at room temperature for 40 hours, filtered, and the supernatant was collected. Water was added to precipitate the polymerization product, which was then redissolved in DMF and precipitated again in ethanol. Finally, the product was dried under vacuum at 100°C for 24 hours to obtain PCA-PVA-CN.

[0070] Step 2: Add 30 parts by weight of PCA-PVA-CN powder to 100 parts by weight of a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), wherein the volume ratio of EC to EMC is 1:2. After complete dissolution, a slurry is obtained.

[0071] Step 3: The obtained slurry is coated onto a PE membrane with a thickness of 12μm using a micro-grooving roller coating method. The coating thickness is controlled to be around 2μm (coating areal density is 2.6g / m³). 2 By thermally crosslinking the coating layer at 60°C for 3 hours, a modified PE membrane with chelating function can be obtained.

[0072] Comparative Example 1

[0073] The difference between this comparative example and Example 1 is that step 1 is omitted. In step 2, polyvinyl alcohol (PVA) is added to 100 parts of a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), wherein the volume ratio of EC to EMC is 1:2. After complete dissolution, a slurry is obtained. The other composition and preparation are the same as in Example 1.

[0074] Comparative Example 2

[0075] The difference between this comparative example and Example 1 is that step 1 is omitted. In step 2, 2-pyrrolidone-5-carboxylic acid (PCA) is added to 100 parts of a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). The other composition and preparation are the same as in Example 1.

[0076] Comparative Example 3

[0077] Step 1: Grind 7 parts of cyanoethyl polyvinyl alcohol (PVA-CN) and 10 parts of 2-pyrrolidone-5-carboxylic acid (PCA) into a mixed powder.

[0078] Step 2: Add 2 parts of the mixed powder to 100 parts of a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC to EMC volume ratio of 1:2) according to the mass ratio, and stir thoroughly to obtain a mixed slurry.

[0079] Step 3: The obtained mixed slurry is coated onto a PE membrane with a thickness of 12μm using a micro-grooved roller coating method. The coating thickness is controlled at about 2μm. After drying, the modified PE membrane can be obtained.

[0080] The membranes prepared in Examples 1, 2, 3, 4, 5 and the comparative examples were subjected to lithium-ion transference number tests, and the test results are shown in Table 1.

[0081] Lithium-ion transference number (t) Li+ The test uses the steady-state current method, and the specific steps are as follows:

[0082] A lithium metal symmetric battery with a test separator was assembled using CR2016. The electrolyte was 1M LiPF6 EC / DEC (v:v = 1:1). First, the battery underwent an AC impedance test (conditions: frequency 100kHz - 0.1Hz, voltage perturbation 10mV; initial interface impedance R0 was obtained by fitting the equivalent circuit diagram). Then, a DC polarization test was performed (conditions: voltage (ΔV) 10mV, time 2000s), obtaining the initial current I0 and steady-state current I0. ss Finally, an AC impedance test is performed again to obtain the steady-state interface impedance R. ss The lithium-ion transport number is calculated using the formula: t Li+ =I ss (ΔV-R0I0) / I0(ΔV-R ss I ss ).

[0083] Table 1 shows the lithium-ion transference number test results of the membranes prepared in Examples 1, 2, 3, 4, 5 and Comparative Example 3.

[0084] Example Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 3 <![CDATA[t Li+ ]]> 0.65 0.63 0.60 0.55 0.57 0.43

[0085] As can be seen from Examples 1, 2, 3, 4, 5 and Comparative Example 3 in Table 1, the PCA-PVA-CN modified separator has a high lithium-ion migration number, and the ratio of PCA to PVA-CN has little effect on the lithium-ion migration number. When the mass ratio of PCA-PVA-CN increases, the slurry viscosity increases, and lithium-ion migration is hindered after thermal cross-linking, thus reducing the lithium-ion migration number. PCA and PVA-CN that do not undergo cross-linking have almost no ability to improve lithium-ion migration.

[0086] Using the separators from Examples 1 and 2 above, along with lithium iron manganese phosphate positive electrode sheets and graphite negative electrode sheets, a 3.0Ah soft-pack lithium-ion battery was fabricated using a stacking process. After being fully charged, the battery was placed in an environment of 25°C and 50% humidity for voltage and internal resistance testing. The results are shown in Table 2.

[0087] Table 2 shows the voltage and internal resistance test results of the lithium-ion batteries in Example 1 and Comparative Examples 1 and 2.

[0088] Example Example 1 Comparative Example 1 Comparative Example 2 Average voltage (V) 4.251 4.252 4.248 Average internal resistance (mΩ) 6.296 6.271 6.365

[0089] In Example 1, the PCA-PVA-CN modified separator was used to assemble a lithium-ion battery. According to the data in Table 2, after capacity testing, the lithium-ion batteries prepared in Example 1 and Comparative Examples 1 and 2 showed normal voltage and little difference in internal resistance.

[0090] The batteries in Table 2 were placed in an environment of 45°C and subjected to charge-discharge cycle tests at a current of 1C. After 300 cycles, the capacity retention rate of the battery prepared in Example 1 was 92.73%, and the capacity retention rate of the battery prepared in Comparative Example 1 was 88.94%. The negative electrode materials from both batteries were taken after cycling, and a quantitative amount of the negative electrode active material was digested and diluted with aqua regia. The Mn content in the digestion solution was detected using an ICP instrument, and the results are shown in Table 3.

[0091] Table 3. Results of Fe and Mn content tests after cycling of lithium-ion batteries in Example 1 and Comparative Example 1.

[0092] Example Example 1 Comparative Example 1 Fe (ppm) 78.3 148.9 Mn (ppm) 43.8 154.6

[0093] As shown in Table 3, the Fe and Mn contents in Example 1 are much lower than those in Comparative Example 1. This is because the PCA in the PCA-PVA-CN coating in Example 1 has a chelating effect on Fe and Mn, which can effectively capture ions dissolved from the surface of the positive electrode active material. In contrast, the membrane surface in Comparative Example 1 is only modified with PVA-CN and does not have a chelating function, so the Mn content in the negative electrode active material is higher.

[0094] Based on the above test results, it can be seen that the preparation method of the PCA-PVA-CN modified separator in this invention is better than that of the comparative example.

[0095] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0096] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A chelating membrane comprising a polyolefin membrane and a pyrrolidone-containing cyanoethyl polyvinyl alcohol composite gel coating disposed on at least one surface of the polyolefin membrane; The pyrrolidone-containing cyanoethyl polyvinyl alcohol composite gel coating is formed by applying a slurry to the surface of a polyolefin membrane. The slurry is prepared from pyrrolidone-containing cyanoethyl polyvinyl alcohol and carbonate. The pyrrolidone-containing cyanoethyl polyvinyl alcohol is designated as PCA-PVA-CN. The slurry, by weight, has the following components added: 0.5-30 parts PCA-PVA-CN and 100 parts carbonate.

2. The chelating membrane according to claim 1, characterized in that: The PCA-PVA-CN was prepared by an esterification reaction, specifically as follows: cyanoethyl polyvinyl alcohol, dicyclohexylcarbodiimide (a dehydrating agent), and 2-pyrrolidone-5-carboxylic acid were sequentially added to N,N-dimethylformamide. Under the action of the catalyst 4-dimethylaminopyridine, the mixture was stirred at room temperature for 36-48 h. The supernatant was filtered, and water was added to precipitate the polymerization product. The product was then redissolved in DMF and precipitated again in ethanol. After drying, PCA-PVA-CN was obtained.

3. The chelating membrane according to claim 2, characterized in that: The mass ratio of cyanoethyl polyvinyl alcohol, dehydrating agent dicyclohexylcarbodiimide, 2-pyrrolidone-5-carboxylic acid, and 4-dimethylaminopyridine is (7-10):(16-20):(10-20):(0.4-0.8).

4. The chelating membrane according to any one of claims 1-3, characterized in that: The polyolefin separator is selected from one or a combination of two of the following: polyethylene separator and polypropylene separator.

5. The chelating membrane according to any one of claims 1-3, characterized in that: The carbonate is selected from at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, diphenyl carbonate, dibutyl carbonate, and butene carbonate.

6. The chelating membrane according to any one of claims 1-3, characterized in that: The thickness of the polyolefin separator is 5-20 μm; or, the thickness of the composite gel coating is 1-5 μm, and the areal density of the coating is 0.1-10 g / m 2 .

7. A method for preparing a chelating membrane according to any one of claims 1-6, comprising the following steps: The PCA-PVA-CN is added to carbonate to form a uniform slurry. The slurry is then coated on the surface of the polyolefin membrane and subjected to a thermal crosslinking reaction to obtain the modified polyolefin membrane.

8. The preparation method according to claim 7, characterized in that: The thermal crosslinking reaction is carried out at a temperature of 60-80℃ for 2-3 hours. Alternatively, the coating may be applied using roller coating, dot coating, or spray coating processes.

9. A lithium-ion battery comprising a separator with chelating function as described in any one of claims 1-6.

Images