A gradient pore structure battery separator, a preparation method and application thereof

Abstract

The application relates to the technical field of battery materials, and discloses a gradient pore structure battery diaphragm as well as a preparation method and application thereof. The preparation method comprises the following steps: uniformly mixing polyvinylpyrrolidone, acetone, N-methylpyrrolidone and polyphenyl ether to obtain a diaphragm forming solution; forming a wet film on a bottom plate by using the diaphragm forming solution, standing for 5-10 min at 20-25 DEG C, sequentially immersing in multiple ethanol-water mixed coagulation baths with decreasing ethanol content and increasing temperature, and then drying. By using the preparation method, the battery diaphragm with small pore diameter on the surface layer and large pore diameter on the inner layer can be prepared, the pore diameter presents a continuous gradient change from the surface layer to the inner layer, the pore diameter mutation is reduced, and therefore the liquid absorption rate and the ion conductivity of the diaphragm are improved.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and in particular to a gradient pore structure battery separator, its preparation method, and its application. Background Technology

[0002] With the widespread adoption of new energy vehicles and lithium-ion electric bicycles, the use of lithium-ion batteries has increased significantly. The separator, as a crucial component of lithium-ion batteries, prevents contact between the positive and negative electrodes and facilitates the movement of lithium ions between them. Separator performance is a key factor determining battery performance and safety. The diverse application scenarios of lithium batteries place higher demands on the separator, which influences battery safety and overall performance.

[0003] Patent CN212147767U discloses a multilayer separator for lithium-ion batteries. This separator comprises, from top to bottom, a first surface layer, an intermediate layer, and a second surface layer, all three layers being porous structures. The average pore size of the intermediate layer is larger than that of the first and second surface layers. In this structure, the small pore size design of the two surface layers improves the separator's heat resistance and reduces the risk of electrode particles entering the separator, thereby improving battery cycle performance and safety. Simultaneously, the large pore size design of the intermediate layer facilitates the entry of electrolyte and the passage of lithium ions, thus improving the electrolyte absorption rate and ionic conductivity. However, when preparing this structure using existing methods, there is a problem of abrupt pore size changes between the surface and intermediate layers (the pore size is difficult to exhibit a continuous gradient), resulting in poor improvement in electrolyte absorption rate and ionic conductivity. Summary of the Invention

[0004] To address the aforementioned technical problem—namely, the abrupt change in pore size between the surface and middle layers during the fabrication of a separator with small pores on the surface and large pores in the middle layer—which leads to poor improvements in liquid absorption rate and ionic conductivity, this invention provides a method for preparing a gradient pore structure battery separator. Using this method, a battery separator with smaller pores on the surface and larger pores in the inner layer can be fabricated, creating a continuous gradient in pore size from the surface to the inner layer, reducing abrupt changes in pore size, and thereby improving the liquid absorption rate and ionic conductivity of the separator.

[0005] The specific technical solution of this invention is as follows:

[0006] In a first aspect, the present invention provides a method for preparing a gradient pore structure battery separator, comprising:

[0007] S1: Mix polyvinylpyrrolidone (PVP), acetone, N-methylpyrrolidone (NMP) and polyphenylene ether (PPO) to obtain a film-forming solution;

[0008] S2: Form a wet film on the substrate with the film-forming solution, let it stand at 20~25℃ for 5~10 minutes, immerse it in multiple ethanol-water mixed coagulation baths with decreasing ethanol content and increasing temperature, and then dry it.

[0009] The mechanism of the gradient pore structure with small pores on the surface and large pores in the inner layer in this invention is as follows: When the wet membrane is left to stand at 20-25°C for 5-10 minutes, the acetone in the membrane-forming solution initially evaporates, forming a surface skin on the exposed surface (upper surface) of the wet membrane. When immersed in an ethanol-water mixed coagulation bath, on the surface, the skin formed by the evaporation of acetone in the early stage has a certain hindering effect on solvent diffusion, resulting in relatively slow phase separation and small pore size; while in the interior, the NMP content is high and its polarity difference with water is large, resulting in intense solvent exchange and the formation of a large pore structure, ultimately achieving a gradient distribution from small pores on the surface to large pores in the inner layer; furthermore, in the coagulation bath, the adhesion between the substrate and the wet membrane is separated by the penetration of the bath solution, and the water in the coagulation bath can fully contact the lower surface of the wet membrane, exchange with the solvent inside the membrane, drive the rapid precipitation of polymer, and thus form a lower skin with smaller pore size.

[0010] This invention employs a dual-solvent system composed of acetone and NMP in the membrane-forming solution. Acetone rapidly evaporates to form a surface skin, while NMP delays internal phase separation. Combined with the gradual increase in water content and temperature in the coagulation bath, the diffusion rate of the solvent (acetone + NMP) within the membrane exhibits a continuous change from "slow → fast → stable" from the surface to the inner layer. The phase separation process proceeds from rapid phase separation at the surface (forming dense pores) to gradual phase separation at the inner layer (forming gradually larger pores). This allows for the formation of a structure with smaller pore sizes at the surface and larger pore sizes at the inner layer, while helping to avoid abrupt changes in pore size. This results in a more ideal continuous gradient change in pore size from the surface skin to the inner layer of the membrane, thereby improving the membrane's liquid absorption rate and ionic conductivity.

[0011] As an optional implementation, in step S1, hydroxylated nanoparticles are also added to the film-forming solution.

[0012] Hydroxylated nanoparticles can act as "heterogeneous nucleation sites" during the formation of the gradient pore structure in this invention, promoting the uniform distribution of the solvent phase region during phase separation and reducing pore structure aggregation. Simultaneously, the hydroxyl groups on their surface interact with the polar groups of the PPO matrix and PVP, stabilizing the pore wall structure. The synergistic effect of differential surface pre-construction through dual-solvent evaporation, phase separation rate regulation through a gradient coagulation bath, and pore structure stabilization by nanoparticles better avoids abrupt changes in pore size between the surface and inner layers, thereby significantly improving liquid absorption and ionic conductivity.

[0013] As an optional implementation, in step S1, the volume ratio of acetone to N-methylpyrrolidone is 3~5:1.

[0014] As an optional implementation, in step S2, during the process of sequentially immersing in multiple ethanol-water mixed coagulation baths with successively decreasing ethanol content and successively increasing temperature, the number of ethanol-water mixed coagulation baths is 2 to 4, the ethanol content is 85 to 95% vol, the temperature is 19 to 29°C, and the immersion time in each ethanol-water mixed coagulation bath is 3 to 4 hours.

[0015] As an optional implementation, in step S2, the specific process of sequentially soaking in multiple ethanol-water mixed coagulation baths with successively decreasing ethanol content and successively increasing temperature is as follows: soaking in an ethanol-water mixed coagulation bath with an ethanol content of 91~95%vol and a temperature of 19~21℃ for 3~4 hours, then soaking in an ethanol-water mixed coagulation bath with an ethanol content of 87~90%vol and a temperature of 23~25℃ for 3~4 hours, and finally soaking in an ethanol-water mixed coagulation bath with an ethanol content of 85~86%vol and a temperature of 27~29℃ for 3~4 hours.

[0016] With the above-mentioned volume ratio design of acetone to N-methylpyrrolidone and gradient coagulation bath design, the difference in volatility between acetone and N-methylpyrrolidone, as well as the control of phase separation rate by the gradient coagulation bath, can be better utilized to form a structure with a continuous gradient change in pore size from the surface to the inner layer, giving the membrane higher liquid absorption rate and ionic conductivity.

[0017] As an optional implementation, in step S2, after the drying is completed, the sample is soaked in a 3-8 wt% polyethylene glycol (PEG) solution for 0.5-1 h, and then washed and dried.

[0018] PEG treatment can complex with lithium ions in the electrolyte, forming a lithium-ion-rich adsorption layer on the surface of hydroxylated nanoparticles. The PPO matrix itself has a certain degree of molecular chain flexibility and microphase separation structure. When this structure works synergistically with hydroxylated nanoparticles, it can build a continuous and efficient ion transport channel inside the material, shorten the conduction distance of lithium ions in the membrane, reduce ion transport resistance, and thus improve ion conductivity.

[0019] As an optional implementation, the specific process of step S1 includes: dissolving polyvinylpyrrolidone in a mixed solvent of acetone and N-methylpyrrolidone, then adding polyphenylene ether, mixing well, adding hydroxylated nanoparticles, dispersing evenly, and allowing to stand at 25~30℃ for 2~4 hours to degas, thereby obtaining a film-forming solution.

[0020] As an optional implementation, in step S1, the mass ratio of the polyvinylpyrrolidone to the total mass of acetone and N-methylpyrrolidone is 1:5~10; the weight-average molecular weight of the polyphenylene ether is 800~1500 g / mol, and the mass ratio of the polyvinylpyrrolidone to the polyvinylpyrrolidone is 5~9:1; the hydroxylated nanoparticles include hydroxylated silica and / or hydroxylated alumina, and the mass ratio of the hydroxylated nanoparticles to the polyphenylene ether is 0.03~0.08:1.

[0021] As an optional implementation, in step S2, the drying process includes: drying at room temperature for 2-3 hours, followed by drying at 60-65°C for 10-15 hours.

[0022] Secondly, the present invention provides a gradient pore structure battery separator prepared by the preparation method described above, comprising an inner layer and a skin layer disposed on both sides of the inner layer; the average pore sizes of the skin layer and the inner layer are 20~40nm and 41~100nm, respectively, and the average pore size of the inner layer is 1.5~3 times that of the average pore size of the skin layer.

[0023] As an optional implementation, the total thickness of the skin layer disposed on both sides of the inner layer is 4~10μm; the thickness of the inner layer is 10~40μm.

[0024] As an optional implementation, the porosity of the gradient pore structure battery separator is 60-75%.

[0025] Thirdly, the present invention provides the application of the gradient pore structure battery separator in lithium-ion batteries.

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

[0027] (1) In the preparation process of the diaphragm of the present invention, by using a dual solvent system composed of acetone and NMP, combined with a coagulation bath with varying ratios of ethanol and water and temperature gradient, it is possible to form a structure with smaller surface pore size and larger inner pore size, while helping to avoid abrupt changes in pore size, so that the diaphragm exhibits a more ideal continuous gradient change from the surface skin layer to the inner layer pore size, thereby improving the liquid absorption rate and ionic conductivity of the diaphragm.

[0028] (2) In the membrane preparation process of the present invention, by adding hydroxylated nanoparticles to the membrane preparation solution, the interaction between them and PPO and PVP can be utilized to stabilize the pore wall structure. When combined with the dual solvent system and gradient coagulation bath, a structure with a continuous gradient change in pore size from the surface skin layer to the inner layer can be better formed, further improving the liquid absorption rate and ionic conductivity.

[0029] (3) In the preparation process of the diaphragm of the present invention, the PEG solution soaking treatment can be used to combine with hydroxylated nanoparticles to give the diaphragm a higher liquid absorption rate and ionic conductivity. Detailed Implementation

[0030] The present invention will be further described below with reference to embodiments.

[0031] A method for preparing a gradient pore structure battery separator, comprising:

[0032] S1: Mix polyvinylpyrrolidone (PVP), acetone, N-methylpyrrolidone (NMP) and polyphenylene ether (PPO) to obtain a film-forming solution;

[0033] S2: Form a wet film on the substrate with the film-forming solution, let it stand at 20~25℃ for 5~10 minutes, immerse it in multiple ethanol-water mixed coagulation baths with decreasing ethanol content and increasing temperature, and then dry it.

[0034] In some specific embodiments, in step S1, the volume ratio of acetone to N-methylpyrrolidone is 3~5:1.

[0035] In some specific embodiments, hydroxylated nanoparticles are also added to the film-forming solution in step S1.

[0036] In some specific embodiments, the specific process of step S1 includes: dissolving polyvinylpyrrolidone in a mixed solvent of acetone and N-methylpyrrolidone, then adding polyphenylene ether, mixing well, adding hydroxylated nanoparticles, dispersing evenly, and allowing to stand at 25~30℃ for 2~4 hours to degas, thereby obtaining a film-forming solution.

[0037] In some specific embodiments, in step S1, the mass ratio of the polyvinylpyrrolidone to the total mass of acetone and N-methylpyrrolidone is 1:5~10; the weight-average molecular weight of the polyphenylene ether is 800~1500 g / mol, and the mass ratio of the polyvinylpyrrolidone to the polyvinylpyrrolidone is 5~9:1; the hydroxylated nanoparticles include hydroxylated silica and / or hydroxylated alumina, and the mass ratio of the hydroxylated nanoparticles to the polyphenylene ether is 0.03~0.08:1.

[0038] In some specific embodiments, during step S2, the process of sequentially immersing in multiple ethanol-water mixed coagulation baths with progressively decreasing ethanol content and progressively increasing temperature involves 2 to 4 ethanol-water mixed coagulation baths, each with an ethanol content of 85 to 95% vol and a temperature of 19 to 29°C. The immersion time in each ethanol-water mixed coagulation bath is 3 to 4 hours. As one specific embodiment, the specific process of sequentially immersing in multiple ethanol-water mixed coagulation baths with progressively decreasing ethanol content and progressively increasing temperature is as follows: immersion in an ethanol-water mixed coagulation bath with an ethanol content of 91 to 95% vol and a temperature of 19 to 21°C for 3 to 4 hours; then immersion in an ethanol-water mixed coagulation bath with an ethanol content of 87 to 90% vol and a temperature of 23 to 25°C for 3 to 4 hours; and finally immersion in an ethanol-water mixed coagulation bath with an ethanol content of 85 to 86% vol and a temperature of 27 to 29°C for 3 to 4 hours.

[0039] In some specific embodiments, in step S2, the drying process includes: drying at room temperature for 2-3 hours, followed by drying at 60-65°C for 10-15 hours.

[0040] In some specific embodiments, in step S2, after the drying is completed, the sample is soaked in a 3-8 wt% polyethylene glycol (PEG) solution for 0.5-1 h, and then washed and dried.

[0041] Secondly, the present invention provides a gradient pore structure battery separator prepared by the preparation method described above, comprising an inner layer and a skin layer disposed on both sides of the inner layer; the average pore sizes of the skin layer and the inner layer are 20~40nm and 41~100nm, respectively, and the average pore size of the inner layer is 1.5~3 times that of the average pore size of the skin layer.

[0042] In some specific embodiments, the total thickness of the skin layer disposed on both sides of the inner layer is 4~10μm; the thickness of the inner layer is 10~40μm.

[0043] In some specific embodiments, the porosity of the gradient pore structure battery separator is 60-75%.

[0044] Thirdly, the present invention provides the application of the gradient pore structure battery separator in lithium-ion batteries.

[0045] The present invention will now be described with reference to specific embodiments and comparative examples. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0046] Example 1

[0047] The gradient pore structure battery separator is prepared by the following steps:

[0048] S1: Preparation of PVP dual-solvent solution

[0049] PVP-K90 was added to a mixed solvent of acetone and NMP at a mass ratio of 1:7 (where the volume ratio of acetone to NMP was 5:1), and stirred at 1000 rpm for 80 min at 25 °C to obtain a clear PVP dual-solvent solution.

[0050] S2: Preparation of film-forming solution

[0051] PPO with a weight-average molecular weight of 1200 g / mol was divided into three batches and added to a PVP dual-solvent solution with a mass ratio of PPO to PVP of 87:13. The mixture was stirred at 1200 rpm for 120 min, and then 5% of the mass of PPO hydroxylated silica nanoparticles (average particle size of 40 nm) were added. After ultrasonic dispersion for 30 min, the mixture was allowed to stand at 25 °C for 3 h to remove bubbles, resulting in a homogeneous film-forming solution.

[0052] S3: Film Formation

[0053] Using a 70 μm thick film scraper, the film-forming solution was scraped onto a glass plate to form a wet film, which was immediately transferred to an environment at 25°C and allowed to stand for 10 minutes to allow the acetone to initially evaporate, forming a surface skin precursor. Then, the film was sequentially immersed in three series-connected coagulation baths, each containing a mixture of ethanol and water at volume ratios of 95:5, 90:10, and 85:15, respectively, at temperatures of 20±1°C, 24±1°C, and 28±1°C, for 3 hours in each bath. After removal from the coagulation baths, the film was dried at room temperature for 2 hours, and then dried in a vacuum oven at 60°C for 12 hours to obtain the formed film.

[0054] S4: Post-processing

[0055] The molded membrane was immersed in a 5 wt% PEG-2000 solution for 30 min, then rinsed with deionized water and vacuum dried to obtain a gradient pore structure battery separator.

[0056] Example 2

[0057] The only difference between this embodiment and Example 1 is that in step S1, the volume ratio of acetone to NMP is changed to 4:1; all other raw materials and steps are the same as in Example 1. Specifically, this embodiment prepares a gradient pore structure battery separator through the following steps:

[0058] S1: Preparation of PVP dual-solvent solution

[0059] PVP-K90 was added to a mixed solvent of acetone and NMP at a mass ratio of 1:7 (where the volume ratio of acetone to NMP was 4:1), and stirred at 1000 rpm for 80 min at 25 °C to obtain a clear PVP dual-solvent solution.

[0060] S2: Preparation of film-forming solution

[0061] PPO with a weight-average molecular weight of 1200 g / mol was divided into three batches and added to a PVP dual-solvent solution with a mass ratio of PPO to PVP of 87:13. The mixture was stirred at 1200 rpm for 120 min, and then 5% of the mass of PPO hydroxylated silica nanoparticles (average particle size of 40 nm) were added. After ultrasonic dispersion for 30 min, the mixture was allowed to stand at 25 °C for 3 h to remove bubbles, resulting in a homogeneous film-forming solution.

[0062] S3: Film Formation

[0063] Using a 70 μm thick film scraper, the film-forming solution was scraped onto a glass plate to form a wet film, which was immediately transferred to an environment at 25°C and allowed to stand for 10 minutes to allow the acetone to initially evaporate, forming a surface skin precursor. Then, the film was sequentially immersed in three series-connected coagulation baths, each containing a mixture of ethanol and water at volume ratios of 95:5, 90:10, and 85:15, respectively, at temperatures of 20±1°C, 24±1°C, and 28±1°C, for 3 hours in each bath. After removal from the coagulation baths, the film was dried at room temperature for 2 hours, and then dried in a vacuum oven at 60°C for 12 hours to obtain the formed film.

[0064] S4: Post-processing

[0065] The molded membrane was immersed in a 5 wt% PEG-2000 solution for 30 min, then rinsed with deionized water and vacuum dried to obtain a gradient pore structure battery separator.

[0066] Example 3

[0067] The only difference between this embodiment and Example 1 is that in step S1, the volume ratio of acetone to NMP is changed to 3:1; all other raw materials and steps are the same as in Example 1. Specifically, this embodiment prepares a gradient pore structure battery separator through the following steps:

[0068] S1: Preparation of PVP dual-solvent solution

[0069] PVP-K90 was added to a mixed solvent of acetone and NMP at a mass ratio of 1:7 (where the volume ratio of acetone to NMP was 3:1), and stirred at 1000 rpm for 80 min at 25 °C to obtain a clear PVP dual-solvent solution.

[0070] S2: Preparation of film-forming solution

[0071] PPO with a weight-average molecular weight of 1200 g / mol was divided into three batches and added to a PVP dual-solvent solution with a mass ratio of PPO to PVP of 87:13. The mixture was stirred at 1200 rpm for 120 min, and then 5% of the mass of PPO hydroxylated silica nanoparticles (average particle size of 40 nm) were added. After ultrasonic dispersion for 30 min, the mixture was allowed to stand at 25 °C for 3 h to remove bubbles, resulting in a homogeneous film-forming solution.

[0072] S3: Film Formation

[0073] Using a 70 μm thick film scraper, the film-forming solution was scraped onto a glass plate to form a wet film, which was immediately transferred to an environment at 25°C and allowed to stand for 10 minutes to allow the acetone to initially evaporate, forming a surface skin precursor. Then, the film was sequentially immersed in three series-connected coagulation baths, each containing a mixture of ethanol and water at volume ratios of 95:5, 90:10, and 85:15, respectively, at temperatures of 20±1°C, 24±1°C, and 28±1°C, for 3 hours in each bath. After removal from the coagulation baths, the film was dried at room temperature for 2 hours, and then dried in a vacuum oven at 60°C for 12 hours to obtain the formed film.

[0074] S4: Post-processing

[0075] The molded membrane was immersed in a 5 wt% PEG-2000 solution for 30 min, then rinsed with deionized water and vacuum dried to obtain a gradient pore structure battery separator.

[0076] Example 4

[0077] The only difference between this embodiment and Embodiment 1 is that in step S3, the volume ratio of ethanol to water in the three coagulation baths is changed to 93:7, 88:12, and 85:15 respectively; all other raw materials and steps are the same as in Embodiment 1. Specifically, this embodiment prepares a gradient pore structure battery separator through the following steps:

[0078] S1: Preparation of PVP dual-solvent solution

[0079] PVP-K90 was added to a mixed solvent of acetone and NMP at a mass ratio of 1:7 (where the volume ratio of acetone to NMP was 5:1), and stirred at 1000 rpm for 80 min at 25 °C to obtain a clear PVP dual-solvent solution.

[0080] S2: Preparation of film-forming solution

[0081] PPO with a weight-average molecular weight of 1200 g / mol was divided into three batches and added to a PVP dual-solvent solution with a mass ratio of PPO to PVP of 87:13. The mixture was stirred at 1200 rpm for 120 min, and then 5% of the mass of PPO hydroxylated silica nanoparticles (average particle size of 40 nm) were added. After ultrasonic dispersion for 30 min, the mixture was allowed to stand at 25 °C for 3 h to remove bubbles, resulting in a homogeneous film-forming solution.

[0082] S3: Film Formation

[0083] Using a 70 μm thick film scraper, the film-forming solution was scraped onto a glass plate to form a wet film, which was immediately transferred to an environment at 25°C and allowed to stand for 10 minutes to allow the acetone to initially evaporate, forming a surface skin precursor. Then, the film was sequentially immersed in three series-connected coagulation baths, each containing a mixture of ethanol and water at volume ratios of 93:7, 88:12, and 85:15, respectively, at temperatures of 20±1°C, 24±1°C, and 28±1°C, for 3 hours in each bath. After removal from the coagulation baths, the film was dried at room temperature for 2 hours, and then dried in a vacuum oven at 60°C for 12 hours to obtain the formed film.

[0084] S4: Post-processing

[0085] The molded membrane was immersed in a 5 wt% PEG-2000 solution for 30 min, then rinsed with deionized water and vacuum dried to obtain a gradient pore structure battery separator.

[0086] Example 5

[0087] The only difference between this embodiment and Embodiment 1 is that in step S3, the volume ratio of ethanol to water in the three coagulation baths is changed to 91:9, 87:13, and 85:15 respectively; all other raw materials and steps are the same as in Embodiment 1. Specifically, this embodiment prepares a gradient pore structure battery separator through the following steps:

[0088] S1: Preparation of PVP dual-solvent solution

[0089] PVP-K90 was added to a mixed solvent of acetone and NMP at a mass ratio of 1:7 (where the volume ratio of acetone to NMP was 5:1), and stirred at 1000 rpm for 80 min at 25 °C to obtain a clear PVP dual-solvent solution.

[0090] S2: Preparation of film-forming solution

[0091] PPO with a weight-average molecular weight of 1200 g / mol was divided into three batches and added to a PVP dual-solvent solution with a mass ratio of PPO to PVP of 87:13. The mixture was stirred at 1200 rpm for 120 min, and then 5% of the mass of PPO hydroxylated silica nanoparticles (average particle size of 40 nm) were added. After ultrasonic dispersion for 30 min, the mixture was allowed to stand at 25 °C for 3 h to remove bubbles, resulting in a homogeneous film-forming solution.

[0092] S3: Film Formation

[0093] Using a 70 μm thick film scraper, the film-forming solution was scraped onto a glass plate to form a wet film, which was immediately transferred to an environment at 25°C and allowed to stand for 10 minutes to allow the acetone to initially evaporate, forming a surface skin precursor. Then, the film was sequentially immersed in three series-connected coagulation baths, each containing a mixture of ethanol and water at volume ratios of 91:9, 87:13, and 85:15, respectively, at temperatures of 20±1°C, 24±1°C, and 28±1°C, for 3 hours in each bath. After removal from the coagulation baths, the film was dried at room temperature for 2 hours, and then dried in a vacuum oven at 60°C for 12 hours to obtain the formed film.

[0094] S4: Post-processing

[0095] The molded membrane was immersed in a 5 wt% PEG-2000 solution for 30 min, then rinsed with deionized water and vacuum dried to obtain a gradient pore structure battery separator.

[0096] Example 6

[0097] The only difference between this embodiment and Example 1 is that hydroxylated silica nanoparticles are not added in step S2; all other raw materials and steps are the same as in Example 1. Specifically, this embodiment prepares a gradient pore structure battery separator through the following steps:

[0098] S1: Preparation of PVP dual-solvent solution

[0099] PVP-K90 was added to a mixed solvent of acetone and NMP at a mass ratio of 1:7 (where the volume ratio of acetone to NMP was 5:1), and stirred at 1000 rpm for 80 min at 25 °C to obtain a clear PVP dual-solvent solution.

[0100] S2: Preparation of film-forming solution

[0101] PPO with a weight-average molecular weight of 1200 g / mol was divided into three batches and added to a PVP dual-solvent solution with a mass ratio of PPO to PVP of 87:13. After stirring at 1200 rpm for 120 min, the solution was allowed to stand at 25 °C for 3 h to remove bubbles, resulting in a homogeneous film-forming solution.

[0102] S3: Film Formation

[0103] Using a 70 μm thick film scraper, the film-forming solution was scraped onto a glass plate to form a wet film, which was immediately transferred to an environment at 25°C and allowed to stand for 10 minutes to allow the acetone to initially evaporate, forming a surface skin precursor. Then, the film was sequentially immersed in three series-connected coagulation baths, each containing a mixture of ethanol and water at volume ratios of 95:5, 90:10, and 85:15, respectively, at temperatures of 20±1°C, 24±1°C, and 28±1°C, for 3 hours in each bath. After removal from the coagulation baths, the film was dried at room temperature for 2 hours, and then dried in a vacuum oven at 60°C for 12 hours to obtain the formed film.

[0104] S4: Post-processing

[0105] The molded membrane was immersed in a 5 wt% PEG-2000 solution for 30 min, then rinsed with deionized water and vacuum dried to obtain a gradient pore structure battery separator.

[0106] Example 7

[0107] The only difference between this embodiment and Embodiment 1 is that the post-processing in step S4 is omitted; all other raw materials and steps are the same as in Embodiment 1. Specifically, this embodiment prepares a gradient pore structure battery separator through the following steps:

[0108] S1: Preparation of PVP dual-solvent solution

[0109] PVP-K90 was added to a mixed solvent of acetone and NMP at a mass ratio of 1:7 (where the volume ratio of acetone to NMP was 5:1), and stirred at 1000 rpm for 80 min at 25 °C to obtain a clear PVP dual-solvent solution.

[0110] S2: Preparation of film-forming solution

[0111] PPO with a weight-average molecular weight of 1200 g / mol was divided into three batches and added to a PVP dual-solvent solution with a mass ratio of PPO to PVP of 87:13. The mixture was stirred at 1200 rpm for 120 min, and then 5% of the mass of PPO hydroxylated silica nanoparticles (average particle size of 40 nm) were added. After ultrasonic dispersion for 30 min, the mixture was allowed to stand at 25 °C for 3 h to remove bubbles, resulting in a homogeneous film-forming solution.

[0112] S3: Film Formation

[0113] Using a 70 μm thick film scraper, the film-forming solution was scraped onto a glass plate to form a wet film, which was immediately transferred to an environment at 25°C and allowed to stand for 10 minutes to allow the acetone to initially evaporate, forming a surface skin precursor. Then, the film was sequentially immersed in three series-connected coagulation baths, each containing a mixture of ethanol and water at volume ratios of 95:5, 90:10, and 85:15, respectively, at temperatures of 20±1°C, 24±1°C, and 28±1°C, for 3 hours in each bath. After removal from the coagulation baths, the film was dried at room temperature for 2 hours, then dried in a vacuum oven at 60°C for 12 hours, and finally immersed in water for 30 minutes. After removal, the film was rinsed with deionized water and vacuum dried to obtain a gradient pore structure battery separator.

[0114] Comparative Example 1

[0115] This comparative example is a commercially available 25μm wet-process PE membrane prepared by the phase transition method.

[0116] Comparative Example 2

[0117] The only difference between this comparative example and Example 7 is that in step S1, only N,N-dimethylacetamide (DMAC) is used as the solvent, and in step S3, the evaporation temperature and time corresponding to DMAC are used; the remaining raw materials and steps are the same as in Example 7. Specifically, this comparative example prepares the battery separator through the following steps:

[0118] S1: Preparation of PVP solution

[0119] PVP-K90 was added to DMAC at a mass ratio of 1:7, and stirred at 1000 rpm for 80 min at 25 °C to obtain a clear PVP solution.

[0120] S2: Preparation of film-forming solution

[0121] PPO with a weight-average molecular weight of 1200 g / mol was divided into three batches and added to the PVP solution. The mass ratio of PPO to PVP was 87:13. The mixture was stirred at 1200 rpm for 120 min. Then, 5% of the mass of PPO hydroxylated silica nanoparticles (average particle size 40 nm) were added. After ultrasonic dispersion for 30 min, the mixture was allowed to stand at 25 °C for 3 h to remove bubbles, resulting in a homogeneous film-forming solution.

[0122] S3: Film Formation

[0123] Using a 70 μm thick film scraper, the film-forming solution was scraped onto a glass plate to form a wet film, which was immediately transferred to an environment at 90 °C and allowed to stand for 1 hour to allow the DMAC to evaporate initially, forming a surface skin precursor. Then, the film was sequentially immersed in three series-connected coagulation baths, each containing a mixture of ethanol and water at volume ratios of 95:5, 90:10, and 85:15, respectively, at temperatures of 20±1 °C, 24±1 °C, and 28±1 °C, for 3 hours in each bath. After removal from the coagulation baths, the film was dried at room temperature for 2 hours, then dried in a vacuum oven at 60 °C for 12 hours, and finally immersed in water for 30 minutes. After removal, the film was rinsed with deionized water and vacuum dried to obtain a gradient pore structure battery separator.

[0124] Comparative Example 3

[0125] The only difference between this comparative example and Example 7 is that in step S3, the volume ratio of ethanol to water in all three coagulation baths is 90:10; all other raw materials and steps are the same as in Example 7. Specifically, this comparative example prepares a battery separator through the following steps:

[0126] S1: Preparation of PVP dual-solvent solution

[0127] PVP-K90 was added to a mixed solvent of acetone and NMP at a mass ratio of 1:7 (where the volume ratio of acetone to NMP was 5:1), and stirred at 1000 rpm for 80 min at 25 °C to obtain a clear PVP dual-solvent solution.

[0128] S2: Preparation of film-forming solution

[0129] PPO with a weight-average molecular weight of 1200 g / mol was divided into three batches and added to a PVP dual-solvent solution with a mass ratio of PPO to PVP of 87:13. The mixture was stirred at 1200 rpm for 120 min, and then 5% of the mass of PPO hydroxylated silica nanoparticles (average particle size of 40 nm) were added. After ultrasonic dispersion for 30 min, the mixture was allowed to stand at 25 °C for 3 h to remove bubbles, resulting in a homogeneous film-forming solution.

[0130] S3: Film Formation

[0131] Using a 70 μm thick film scraper, the film-forming solution was scraped onto a glass plate to form a wet film, which was immediately transferred to an environment at 25 °C and allowed to stand for 10 min to allow the acetone to initially evaporate, forming a surface skin precursor. Then, the film was sequentially immersed in three series-connected coagulation baths, each containing a mixture of ethanol and water at a volume ratio of 90:10, at temperatures of 20±1 °C, 24±1 °C, and 28±1 °C, for 3 h in each bath. After removal from the coagulation baths, the film was dried at room temperature for 2 h, then dried in a vacuum oven at 60 °C for 12 h, and finally immersed in water for 30 min. After removal, the film was rinsed with deionized water and vacuum dried to obtain a gradient pore structure battery separator.

[0132] Comparative Example 4

[0133] The only difference between this comparative example and Example 7 is that in step S3, the temperature of all three coagulation baths is 24±1℃; the remaining raw materials and steps are the same as in Example 7. Specifically, this comparative example prepares the battery separator through the following steps:

[0134] S1: Preparation of PVP dual-solvent solution

[0135] PVP-K90 was added to a mixed solvent of acetone and NMP at a mass ratio of 1:7 (where the volume ratio of acetone to NMP was 5:1), and stirred at 1000 rpm for 80 min at 25 °C to obtain a clear PVP dual-solvent solution.

[0136] S2: Preparation of film-forming solution

[0137] PPO with a weight-average molecular weight of 1200 g / mol was divided into three batches and added to a PVP dual-solvent solution with a mass ratio of PPO to PVP of 87:13. The mixture was stirred at 1200 rpm for 120 min, and then 5% of the mass of PPO hydroxylated silica nanoparticles (average particle size of 40 nm) were added. After ultrasonic dispersion for 30 min, the mixture was allowed to stand at 25 °C for 3 h to remove bubbles, resulting in a homogeneous film-forming solution.

[0138] S3: Film Formation

[0139] Using a 70 μm thick film scraper, the film-forming solution was scraped onto a glass plate to form a wet film, which was immediately transferred to an environment at 25°C and allowed to stand for 10 minutes to allow the acetone to initially evaporate, forming a surface skin precursor. Then, the film was sequentially immersed in three series-connected coagulation baths, each containing a mixture of ethanol and water at volume ratios of 95:5, 90:10, and 85:15, respectively, at a temperature of 24 ± 1°C, for 3 hours in each bath. After removal from the coagulation baths, the film was dried at room temperature for 2 hours, then placed in a vacuum oven at 60°C for 12 hours, and finally immersed in water for 30 minutes. After removal, the film was rinsed with deionized water and vacuum dried to obtain a gradient pore structure battery separator.

[0140] Test case

[0141] The battery separators prepared in each embodiment and comparative example were tested for pore size, porosity, liquid absorption rate, and air permeability, and then assembled into battery cells for ionic conductivity testing. The testing methods are as follows:

[0142] (1) Porosity test: Refer to GB / T36363-2018 and use the density method to test porosity.

[0143] (2) Liquid absorption rate test: The liquid absorption rate of the diaphragm is tested by weighing method.

[0144] (3) Air permeability test: The air permeability of the diaphragm is tested using an air permeability tester.

[0145] (4) Ionic conductivity test: The separator was assembled into a 2032 coin cell lithium-ion battery. The positive electrode material was lithium iron phosphate, the negative electrode material was lithium metal, and the electrolyte was a 1 mol / L LiPF6 ethylene carbonate-dimethyl carbonate solution. The ionic conductivity was tested using an electrochemical workstation.

[0146] The test results are shown in Table 1. In Table 1, the cortical thickness is the total thickness of the upper and lower cortical layers, and the cortical pore size is the average pore size of the upper and lower cortical layers.

[0147] Table 1. Battery separator performance test results

[0148]

[0149] Analyzing the data in Table 1, we can see that:

[0150] (1) In the separators prepared in Examples 1 to 7, the pore size of the outer layer is smaller than that of the inner layer, and the liquid absorption rate and ionic conductivity are significantly higher than those of Comparative Example 1. This indicates that the method of the present invention can produce battery separators with smaller outer layer pore size and larger inner layer pore size, and can effectively improve the liquid absorption rate and ionic conductivity of the separator.

[0151] (2) Compared with Comparative Example 2, Example 7 has a higher liquid absorption rate and ionic conductivity. The reason for this is that Example 7 uses a dual solvent system composed of acetone and NMP. Acetone can be used to quickly evaporate and form a skin layer, while NMP can be used to delay the internal phase separation. Combined with an ethanol-water mixed coagulation bath, it can reduce the abrupt change in pore size from the surface skin layer to the inner layer, which helps to make the membrane exhibit a more ideal continuous gradient change in pore size from the surface skin layer to the inner layer.

[0152] (3) Compared with Comparative Examples 3 and 4, Example 7 has a higher liquid absorption rate and ionic conductivity. The reason for this is that, in combination with the dual solvent system composed of acetone and NMP, the use of a gradient ethanol-water mixed coagulation bath with gradually increasing water content and temperature allows the diffusion rate of the solvent (acetone + NMP) in the membrane to change continuously from the surface to the inner layer in a "slow → fast → stable" manner. The phase separation process proceeds from rapid phase separation in the surface layer (forming dense pores) to gradual phase separation in the inner layer (forming gradually increasing macropores), which helps to avoid abrupt changes in pore size and allows the pore size to show a continuous gradient change from the surface layer to the inner layer.

[0153] (4) Compared with Example 6, Example 1 has a higher liquid absorption rate and ionic conductivity. The reason for this is that: in Example 1, hydroxylated nanoparticles were added to the film-forming solution, which can act as "heterogeneous nucleation points" during the formation of gradient pore structure, promote the uniform distribution of the solvent phase region during phase separation, reduce pore structure aggregation, and at the same time, the hydroxyl groups on their surface interact with the polar groups of PPO matrix and PVP to stabilize the pore wall structure. This, combined with the design of pre-forming the surface layer by differential evaporation of dual solvents and controlling the phase separation rate by gradient coagulation bath, better avoids abrupt changes in pore size between the surface layer and the inner layer.

[0154] (5) Compared with Example 7, Example 1 has a higher liquid absorption rate and ionic conductivity. The reason for this is that Example 1 uses PEG solution for soaking treatment, which can complex with lithium ions in the electrolyte to form a lithium ion-rich adsorption layer on the surface of hydroxylated nanoparticles. The PPO matrix itself has a certain degree of molecular chain flexibility and microphase separation structure. When this structure works synergistically with hydroxylated nanoparticles, it can build a continuous and efficient ion transport channel inside the material, shorten the conduction distance of lithium ions in the membrane, and reduce ion transport resistance.

Claims

1. A method for preparing a gradient pore structure battery separator, characterized in that, The method comprises the following steps: S1: mixing polyvinylpyrrolidone, acetone, N-methylpyrrolidone and polyphenyl ether to obtain a film forming solution; the volume ratio of the acetone to the N-methylpyrrolidone is 3-5:1; S2: forming a wet film on a base plate by using the film forming solution, standing for 5-10 minutes at 20-25 DEG C, sequentially immersing in multiple ethanol-water mixed coagulation baths with decreasing ethanol content and increasing temperature, and then drying.

2. The production method according to claim 1, characterized by, In step S1, hydroxylated nanoparticles are further added to the film forming solution.

3. The preparation method according to claim 1, characterized in that, In step S2, during the process of sequentially immersing in multiple ethanol-water mixed coagulation baths with decreasing ethanol content and increasing temperature, the number of the ethanol-water mixed coagulation baths is 2-4, the ethanol content is 85-95 vol%, the temperature is 19-29 DEG C, and the immersion time in each ethanol-water mixed coagulation bath is 3-4 hours.

4. The production method according to claim 3, characterized by, In step S2, during the process of sequentially immersing in multiple ethanol-water mixed coagulation baths with decreasing ethanol content and increasing temperature, the number of the ethanol-water mixed coagulation baths is 2-4, the ethanol content is 85-95 vol%, the temperature is 19-29 DEG C, and the immersion time in each ethanol-water mixed coagulation bath is 3-4 hours.

5. The production method according to claim 1 or 2, characterized by, In step S2, during the process of sequentially immersing in multiple ethanol-water mixed coagulation baths with decreasing ethanol content and increasing temperature, the number of the ethanol-water mixed coagulation baths is 2-4, the ethanol content is 85-95 vol%, the temperature is 19-29 DEG C, and the immersion time in each ethanol-water mixed coagulation bath is 3-4 hours.

6. The preparation method according to claim 2, characterized in that, In step S2, after the drying, the film is immersed in a 3-8 wt% polyethylene glycol solution for 0.5-1 hour, and then washed with water and dried.

7. The production method according to claim 2 or 6, characterized by, The specific process of step S1 comprises: dissolving polyvinylpyrrolidone in a mixed solvent of acetone and N-methylpyrrolidone, adding polyphenyl ether, uniformly mixing, adding hydroxylated nanoparticles, uniformly dispersing, standing for 2-4 hours at 25-30 DEG C to remove bubbles, and obtaining the film forming solution.

8. A battery separator having a gradient pore structure prepared by the method of any one of claims 1 to 7, characterized in that, In step S1, the mass ratio of the polyvinylpyrrolidone to the total mass of the acetone and the N-methylpyrrolidone is 1:5-10; the weight average molecular weight of the polyphenyl ether is 800-1500 g / mol, and the mass ratio of the polyphenyl ether to the polyvinylpyrrolidone is 5-9:1; the hydroxylated nanoparticles comprise hydroxylated silicon dioxide and / or hydroxylated aluminum oxide, and the mass ratio of the hydroxylated nanoparticles to the polyphenyl ether is 0.03-0.08:

1. The gradient pore structure battery separator comprises an inner layer and skin layers arranged on both sides of the inner layer; the average pore diameters of the skin layers and the inner layer are 20-40 nm and 41-100 nm respectively, and the average pore diameter of the inner layer is 1.5-3 times that of the skin layer.

9. Use of the gradient pore structure battery separator according to claim 8 in a lithium ion battery.

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