Polyolefin composite separator for lithium battery and preparation method and application thereof

Abstract

The application belongs to the technical field of lithium ion battery separators, and specifically discloses a polyolefin composite separator for lithium batteries, a preparation method and application thereof. The polyolefin composite separator provided by the application can form a bonding functional layer without a coating process during preparation, and the thickness of the film is reduced, thereby avoiding the adverse effects of the coating and thickening of the separator on its performance, and the presence of the bonding functional layer can improve the wettability of the polyolefin separator. In addition, the application can impart stable and firm polar sites to the separator by adding a coupling agent to the raw material, thereby enhancing the bonding force between the functional polymer material and the polyolefin matrix, allowing the formed bonding functional layer to be fixed on the surface of the separator and not easily peeled off. The preparation process of the separator provided by the application can realize the preparation of a type of coated film without needing to reform the existing wet polyolefin separator production line, thereby not only simplifying the process, but also greatly reducing the enterprise load and production cost, and having good application prospect and economic value.

Description

Technical Field

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

[0002] Currently, the wet-process polyolefin separators used in commercial lithium batteries are prepared through two main processes: a wet-process polyolefin separator production line and a coating production line. The wet-process polyolefin separator production line primarily uses polyolefins and white oil as raw materials, and dichloromethane as an extractant. The raw materials are processed through melt extrusion, cooling casting, sheet stretching (both longitudinal and transverse), extraction drying, and secondary stretching to prepare the polyolefin separator. After the separator is prepared, the coating production line first prepares a coating slurry (containing polymers such as polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), and polyvinyl alcohol (PVA), then uses a coating machine to coat the wet-process polyolefin separator, ensuring the slurry adheres evenly to the surface of the polyolefin separator. This increases the adhesion between the separator and the electrode, ensuring consistent cell thickness and improving the battery's interface performance. Finally, the separator is dried to obtain the coated separator.

[0003] To simplify the production process, new synchronous online coating equipment has been introduced to the market, adding a coating step directly after the secondary stretching process in a wet-process polyolefin separator production line. However, this requires significant modifications to existing wet-process polyolefin separator production lines, and the process debugging is complex, making it difficult to achieve the expected convenient and efficient coating effect. Secondly, while coating the separator can improve the liquid retention rate and battery safety to some extent, the coating itself increases the separator thickness, thereby increasing the battery's internal resistance. If inorganic materials are present in the coating, the interfacial compatibility between them and organic materials is poor, and problems such as coating powdering are prone to occur. To solve this problem, Chinese patent CN109860473B (Yinlong New Energy) provides a method for preparing a lithium-ion battery separator, which mixes a coating slurry with a polyethylene solution, first hot-presses it to obtain a flat sheet membrane, and then sequentially processes it through cooling casting, biaxial stretching, and extraction drying to obtain the battery separator. This method relies solely on a traditional wet-process polyolefin separator production line to prepare a composite separator, and the process is simple, but it also has problems such as the coating material affecting the separator performance. Therefore, there is an urgent need to develop a new polyolefin composite separator for lithium batteries and its preparation method. Summary of the Invention

[0004] (a) Technical problems to be solved

[0005] The technical problem to be solved by the present invention is to provide a polyolefin composite separator for lithium batteries and a method for preparing the same. The polyolefin composite separator with an adhesive layer can be prepared by a wet separator production line, which eliminates the complicated coating process and greatly reduces the production cost.

[0006] (II) Technical Solution

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A polyolefin composite separator for lithium batteries, the preparation method of the separator includes: adding a functional polymer material to the extractant in the extraction and drying process, drying after extraction to remove the extractant, and fixing the functional polymer material in the separator to form an adhesive functional layer; no coating process is performed after the separator is prepared.

[0009] In a preferred embodiment of the present invention, the functional polymer material includes, but is not limited to, one or more of polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), polyvinyl alcohol (PVA), cellulose triacetate (TCA), styrene-butadiene rubber (SBR), and polyvinylidene fluoride (PVD). When two or more are used, they can be mixed in any proportion.

[0010] Specifically, the mass ratio of the functional polymer material to the extractant is 5-20:80-95.

[0011] In a preferred embodiment of the present invention, the extractant includes, but is not limited to, dichloromethane. The extractant is mainly used to extract plasticizers (such as white oil) to form membrane micropores.

[0012] In a preferred embodiment of the present invention, the extraction and drying process involves multi-stage extraction, with functional polymer materials added in the later stages of extraction. For example, the extraction and drying process has eight extraction tanks, and functional polymer materials can be added to the extractant (such as dichloromethane) in the last or last two extraction tanks.

[0013] In a preferred embodiment of the present invention, the preparation method of the separator includes: mixing polyolefin and plasticizer and then melting and extruding the mixture, followed by sequential cooling casting, primary stretching, extraction and drying, and secondary stretching to obtain a polyolefin composite separator for lithium batteries.

[0014] Specifically, the mass ratio of the polyolefin to the plasticizer is 15-35:65-85.

[0015] Specifically, the polyolefin includes, but is not limited to, one or more of polyethylene, polypropylene, etc. The weight-average molecular weight of the polyethylene is 300,000 to 3,000,000.

[0016] Specifically, the plasticizers include, but are not limited to, white oil.

[0017] Specifically, the raw materials for preparing the diaphragm include polyolefins, plasticizers, and coupling agents, and the amount of coupling agent used is 0.5wt%-3wt% of the mass of the polyolefin.

[0018] Specifically, the coupling agent includes, but is not limited to, one or more of silane coupling agents, titanate coupling agents, and aluminate coupling agents. Among them, titanate coupling agents include isopropyltris(dioctylpyrophosphate)titanate, and aluminate coupling agents include distearate isopropoxyaluminate.

[0019] When using aluminate coupling agents, it is preferable to add them in the later stage of melt extrusion, and set the extruder process parameter Q / NS value to <4.5. Because aluminate coupling agents are prone to self-crosslinking under high-temperature conditions, this not only affects the uniform dispersion of the aluminate coupling agent but also causes pinholes in the separator, affecting the safety of lithium-ion batteries. Therefore, the key to adding aluminate coupling agents is to reduce their residence time in the (twin-screw) extruder. Adjustments need to be made in the following two aspects: (1) Add the aluminate coupling agent in the later stage of the extruder; (2) Set the extruder process parameter Q / NS value (extrusion rate / screw speed) to below 4.5. The smaller the Q / NS value, the shorter the residence time of the aluminate coupling agent in the extruder.

[0020] Specifically, the technical parameters for the melt extrusion are: temperature 180-220℃.

[0021] Specifically, the technical parameters of the cooling casting are: cooling roller temperature 5-35℃.

[0022] Specifically, the stretching process includes sequential longitudinal stretching and transverse stretching. The technical parameters for the longitudinal stretching are: preheating and stretching temperature 90-130℃, cooling temperature 30-70℃, and stretching ratio 3-20 times; the technical parameters for the transverse stretching are: preheating, stretching, and setting temperature 99-140℃, and stretching ratio 3-20 times.

[0023] Specifically, the secondary stretching is transverse stretching, and the technical parameters of the transverse stretching are: preheating, stretching, and setting temperature 99-140℃, and stretching ratio 3-20 times.

[0024] A method for preparing a polyolefin composite separator for lithium batteries includes the following steps:

[0025] Polyolefin and plasticizer are mixed and melt-extruded, and then passed through cooling casting, primary stretching, extraction drying and secondary stretching processes to obtain polyolefin composite separator for lithium batteries.

[0026] In the extraction and drying process, a functional polymer material is added to the extractant. After extraction, the extractant is removed by drying, and the functional polymer material is fixed in the diaphragm to form a bonding functional layer. No coating process is performed after the diaphragm is prepared.

[0027] In a preferred embodiment of the present invention, the mass ratio of the polyolefin to the plasticizer is 15-35:65-85.

[0028] Specifically, the polyolefin includes, but is not limited to, one or more of polyethylene, polypropylene, etc. The weight-average molecular weight of the polyethylene is 300,000 to 3,000,000.

[0029] Specifically, the plasticizers include, but are not limited to, white oil.

[0030] In a preferred embodiment of the present invention, the raw materials for preparing the diaphragm include polyolefin, plasticizer and coupling agent, and the amount of coupling agent is 0.5wt%-3wt% of the mass of polyolefin.

[0031] Specifically, the coupling agent includes, but is not limited to, one or more of silane coupling agents, titanate coupling agents, and aluminate coupling agents. Among them, titanate coupling agents include isopropyltris(dioctylpyrophosphate)titanate, and aluminate coupling agents include distearate isopropoxyaluminate.

[0032] When using aluminate coupling agents, it is preferable to add them in the later stage of melt extrusion, and set the extruder process parameters Q / NS value < 4.5.

[0033] In a preferred embodiment of the present invention, the technical parameters of the melt extrusion are: temperature 180-220℃.

[0034] In a preferred embodiment of the present invention, the technical parameters of the cooling casting sheet are: cooling roller temperature 5-35℃.

[0035] In a preferred embodiment of the present invention, the single stretching includes sequential longitudinal stretching and transverse stretching. The technical parameters for the longitudinal stretching are: preheating and stretching temperature 90-130℃, cooling temperature 30-70℃, and stretching ratio 3-20 times; the technical parameters for the transverse stretching are: preheating, stretching, and setting temperature 99-140℃, and stretching ratio 3-20 times.

[0036] In a preferred embodiment of the present invention, the functional polymer material includes, but is not limited to, one or more of polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), polyvinyl alcohol (PVA), cellulose triacetate (TCA), styrene-butadiene rubber (SBR), and polyvinylidene fluoride (PVDF). When two or more are used, they can be mixed in any proportion.

[0037] Specifically, the mass ratio of the functional polymer material to the extractant is 5-20:80-95.

[0038] In a preferred embodiment of the present invention, the extractant includes, but is not limited to, dichloromethane. The extractant is mainly used to extract plasticizers (such as white oil) to form membrane micropores.

[0039] In a preferred embodiment of the present invention, the extraction and drying process involves multi-stage extraction, with functional polymer materials added in the later stages of extraction. For example, the extraction and drying process has eight extraction tanks, and functional polymer materials can be added to the extractant (such as dichloromethane) in the last or last two extraction tanks.

[0040] In a preferred embodiment of the present invention, the technical parameters for the transverse stretching are: preheating, stretching, and shaping temperature 99-140℃, and stretching ratio 3-20 times.

[0041] Application of a polyolefin composite separator for lithium batteries in the preparation of lithium-ion batteries.

[0042] (III) Beneficial Effects

[0043] The improvement of the preparation method of polyolefin composite separator for lithium batteries provided by this invention lies in the extraction and drying process of the wet polyolefin separator production line. The oil film passes through multiple extraction tanks to extract plasticizers (such as white oil) with an extractant (such as dichloromethane). In this invention, a certain amount of functional polymer material (selected from one or more of PMMA, PAN, PVA, TCA, SBR, and PVDF) is added to the extractant in the later stage of extraction. Since these polymer materials can be dissolved by dichloromethane, dichloromethane containing polymer materials will remain inside and on the surface of the extracted membrane. After drying, the dichloromethane evaporates, and the polymer material will be uniformly fixed inside and on the surface of the membrane to form an adhesive functional layer. This functional layer can replace the adhesive coating prepared in the coating process, thus eliminating the complicated coating process and greatly reducing production costs. For manufacturing companies, simultaneous online coating of coating production lines and wet-process diaphragm production lines requires significant equipment investment and production costs. However, the preparation process provided by this invention can achieve the preparation of a type of coated membrane with almost no modification to existing wet-process polyolefin diaphragm production lines. This not only simplifies the process but also greatly reduces the burden on enterprises and production costs.

[0044] The polyolefin composite separator for lithium batteries provided by this invention can form an adhesive functional layer without a coating process during preparation, reducing film thickness and avoiding the adverse effects of coating thickness on separator performance. Simultaneously, the presence of the adhesive functional layer improves the wettability of the polyolefin separator. This invention also utilizes the flexibility and non-polarity of the long-chain alkane groups in the coupling agent, which are highly compatible with the long carbon chains of polyolefins. During the melt extrusion process, the mixing and shearing energy causes physical entanglement, forming a tight bond. This allows the coupling agent to be uniformly distributed in the polyolefin matrix, and its polar groups can also be uniformly distributed on the surface of the polyolefin separator, giving the separator stable and firm polar sites. This enhances the adhesion between the functional polymer material and the polyolefin matrix, making the formed adhesive functional layer firmly adhered to the separator surface and less prone to peeling off.

[0045] The modified polyolefin separator provided by this invention can be used to prepare lithium-ion batteries. The production process is simple, highly operable, and easy to industrialize, with good application prospects and high economic value. Detailed Implementation

[0046] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments, comparative examples, and experimental examples. Those skilled in the art should understand that the following embodiments and experimental examples are only used to illustrate the technical solution and technical effects of the present invention and should not be considered as any limitation on the scope of protection of the present invention. Based on the following embodiments, other technical solutions obtained by those skilled in the art without creative effort, such as technical solutions obtained through modifications, variations, or simple substitutions, are all within the scope of protection of the present invention.

[0047] Unless otherwise specified, the raw materials, reagents, equipment, etc. used in the examples, comparative examples, and experimental cases are all commercially available products.

[0048] Polyethylene powder, VH095, purchased from Daehan Oil & Chemical Co., Ltd.

[0049] White oil, purchased from Zhejiang Zhengxin Petroleum Technology Co., Ltd.

[0050] Titanate coupling agent, NDZ-201 (isopropyltris(dioctylpyrophosphoryloxy)titanate, molecular formula: C 51 H 112 O 22 P6Ti was purchased from Nanjing Shuguang Chemical Co., Ltd.

[0051] Polyvinyl alcohol, PVA44-88, purchased from Kuraray, Japan.

[0052] Example 1

[0053] This embodiment provides a polyolefin composite separator for lithium batteries, the preparation method of which includes the following steps:

[0054] (1) Raw material melting extrusion and cooling casting

[0055] By mass, 30 parts of polyethylene, 70 parts of white oil, and titanate coupling agent NDZ-201 (3% of the polyethylene mass) are mixed. The resulting mixture is heated and melted in a twin-screw extruder. The melt is extruded through a T-die and cooled to form a composite sheet.

[0056] The technical parameters for melt extrusion are: temperature 200℃; the technical parameters for cooling casting are: cooling roller temperature 25℃.

[0057] (2) One stretch

[0058] The composite sheet obtained in step (1) is longitudinally stretched using a longitudinal hot roller stretching method. The technical parameters are: preheating and stretching temperature 100℃, cooling temperature 50℃, stretching ratio 4 times, to obtain a longitudinally stretched film.

[0059] The longitudinally stretched film obtained above is then subjected to transverse uniform stretching using a chain clamp. The technical parameters are: preheating, stretching, and setting temperature 120℃, stretching ratio 4 times, to obtain a biaxially stretched film.

[0060] (3) Extraction and drying

[0061] The fully stretched film obtained in step (2) is immersed in a dichloromethane extraction tank for multi-stage extraction to remove the plasticizer white oil from the film. In the final extraction stage, a dichloromethane solution containing the functional polymer polyvinyl alcohol (the mass ratio of polyvinyl alcohol to dichloromethane is 5:95) is used. After drying, a composite film with a uniform adhesive functional layer is obtained.

[0062] (4) Secondary stretching

[0063] The composite film obtained in step (3) is stretched again in a horizontal stretching machine. The technical parameters are: preheating, stretching and shaping temperature 120℃, stretching ratio 4 times, to obtain a polyolefin composite separator for lithium batteries.

[0064] This embodiment also provides an application of a polyolefin composite separator for lithium batteries in the preparation of lithium-ion batteries.

[0065] Example 2-3

[0066] The methods for preparing polyolefin composite separators for lithium batteries in Examples 2-3 are basically the same as those in Example 1, except that the mass ratios of polyvinyl alcohol and dichloromethane in step (3) are 10:90 and 20:80, respectively.

[0067] In other embodiments of the present invention, the raw materials and their amounts used to prepare the polyolefin composite separator for lithium batteries can be arbitrarily selected or set within a given range. For example, polypropylene can be used instead of polyethylene, and the mass ratio of polyethylene to white oil can be set to 15:85; an aluminate coupling agent (such as distearate) can be used instead of a titanate coupling agent; and polymethyl methacrylate (PMMA) can be used instead of polyvinyl alcohol (PVA), which hardly affects the overall performance of the separator.

[0068] In other embodiments of the present invention, the parameters involved in the method for preparing polyolefin composite separators for lithium batteries can be arbitrarily selected within a given range. For example, the stretching ratios of both longitudinal and transverse stretching in a single stretching process are set to 3.5 times, which hardly affects the overall performance of the separator.

[0069] Comparative Example

[0070] The method for preparing lithium-ion battery separators in the comparative example is basically the same as in Example 1, except that: in step (3), the functional polymer material polyvinyl alcohol is not added to the extractant of the last stage of extraction, and only dichloromethane is used for extraction.

[0071] Experimental Example

[0072] Experimental methods: The thickness and water contact angle of the lithium-ion battery separators prepared in Examples 1-3 and the comparative examples were tested, and the peel force between the adhesive functional layer and the polyethylene separator was also tested. The test results are shown in Table 1 below.

[0073] Table 1 Test results of lithium-ion battery separator

[0074]

[0075]

[0076] As can be seen from Table 1, compared with existing composite separators, the thickness of the polyolefin composite separators for lithium batteries prepared in Examples 1-3 is significantly reduced, only slightly thicker than the comparative examples.

[0077] Compared with the comparative examples, the water contact angle of the polyolefin composite separators for lithium batteries prepared in Examples 1-3 was significantly reduced, indicating that the adhesive functional layer can improve the wettability of the polyethylene separator.

[0078] Compared to the comparative examples, the peel force between the adhesive functional layer and the polyethylene matrix in the polyolefin composite separators for lithium batteries prepared in Examples 1-3 increased, indicating that the functional layer is not easily peeled off.

[0079] Conclusion: The experimental results show that the polyolefin composite separator for lithium batteries provided by this invention can form an adhesive functional layer without a coating process during preparation. This reduces the film thickness and avoids the adverse effects of coating thickness on the separator's performance. Furthermore, the presence of the adhesive functional layer improves the wettability of the polyolefin separator. In addition, by adding a coupling agent to the raw materials, this invention imparts stable and robust polar sites to the separator, enhancing the adhesion between the functional polymer material and the polyolefin matrix. This ensures the formed adhesive functional layer is fixed to the separator surface, preventing peeling and detachment, thereby improving the overall stability of the separator.

[0080] The membrane preparation process provided by this invention can achieve the preparation of a type of coated membrane with almost no modification to the existing wet polyolefin membrane production line. It can not only simplify the process, but also greatly reduce the enterprise's burden and production costs, and has good application prospects and high economic value.

[0081] Although the technical solutions and effects of the present invention have been described in detail above with general descriptions, specific embodiments and experimental examples, modifications, substitutions or improvements made by those skilled in the art without departing from the spirit and scope of the present invention are all within the protection scope of the present invention.

Claims

1. A method for preparing a polyolefin composite separator for lithium batteries, characterized in that, Includes the following steps: Polyolefin and plasticizer are mixed and melt-extruded, and then passed through cooling casting, primary stretching, extraction drying and secondary stretching processes to obtain polyolefin composite separator for lithium batteries. Functional polymer materials are added to the extractant in the extraction and drying process. After extraction, the extractant is removed by drying, and the functional polymer materials are fixed in the diaphragm to form a bonding functional layer. No coating process is performed after the diaphragm is prepared. The raw materials for preparing the diaphragm also include a coupling agent, the amount of which is 0.5wt%-3wt% of the mass of the polyolefin; the coupling agent includes one or more of silane coupling agents, titanate coupling agents, and aluminate coupling agents. The extraction and drying process involves multi-stage extraction, with functional polymer materials added in the later stages of extraction.

2. The preparation method according to claim 1, characterized in that: The mass ratio of the polyolefin to the plasticizer is 15-35:65-85; And / or, the polyolefin includes one or both of polyethylene and polypropylene; And / or, the plasticizer includes white oil.

3. The preparation method according to claim 1, characterized in that: The technical parameters for the melt extrusion are: temperature 180-220℃; And / or, the technical parameters of the cooling casting are: cooling roller temperature 5-35℃; And / or, the first stretching includes sequential longitudinal stretching and transverse stretching; the technical parameters for the longitudinal stretching are: preheating and stretching temperature 90-130℃, cooling temperature 30-70℃, and stretching ratio 3-20 times; the technical parameters for the transverse stretching are: preheating, stretching, and setting temperature 99-140℃, and stretching ratio 3-20 times. And / or, the secondary stretching is transverse stretching, and the technical parameters of the transverse stretching are: preheating, stretching, and setting temperature 99-140℃, stretching ratio 3-20 times.

4. The preparation method according to claim 1, characterized in that: The functional polymer materials include one or more of polymethyl methacrylate, polyacrylonitrile, polyvinyl alcohol, cellulose triacetate, styrene-butadiene rubber, and polyvinylidene fluoride; And / or, the mass ratio of the functional polymer material to the extractant is 5-20:80-95; And / or, the extractant includes dichloromethane.

5. The application of a polyolefin composite separator for lithium batteries prepared by any one of claims 1-4 in the preparation of lithium-ion batteries.

Images