Composite lithium ion battery diaphragm and preparation method and application thereof

Abstract

The application discloses a composite lithium ion battery diaphragm and a preparation method and application thereof, and belongs to the technical field of battery diaphragms, and the preparation method comprises the following steps: (1) mixing a high-strength polymer with a lithium salt, and then dissolving the mixture in a solvent to obtain a slurry; coating the slurry on a substrate to obtain a base film; (2) cutting the base film by using an array-arranged circular columnar cutter to obtain a base film containing arrayed circular cutting holes; (3) in a closed device, spraying a solvent to the cutting hole edges of the base film containing the arrayed circular cutting holes, then pouring a PP-PE solution into the arrayed circular cutting holes to fill the cutting holes, pressurizing the closed device during pouring, and drying after pouring, so as to obtain the composite lithium ion battery diaphragm. The composite lithium ion battery diaphragm prepared by the method can ensure the lithium ion transmission speed, avoid the shrinkage problem of the diaphragm, and further improve the safety of the battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery separators, and particularly relates to a composite lithium ion battery separator and a preparation method and application thereof. BACKGROUND

[0002] Lithium ion batteries have the advantages of high voltage, large energy density, good cycle performance, small self-discharge, no memory effect, and wide working temperature range. With the increase of the energy density of existing lithium ion batteries, the safety performance is greatly affected, and safety accidents occur frequently. The existing lithium ion battery separators are mostly made of polypropylene (PP) and / or polyethylene (PE) materials, but the melting point of the materials is low, and the separators are prone to shrinkage under heat, which may cause short circuit of the positive and negative electrodes and may cause the battery to catch fire or explode, resulting in low safety of the battery. In order to improve the safety of the lithium battery separator, the lithium battery separator is further treated. For example, a preparation method of a lithium ion battery composite separator is disclosed in patent CN104681762A, which coats an oxide coating layer and a PVDF-HFP copolymer layer on both sides of the base film; the oxide has high heat resistance and can reduce the shrinkage of the base film, and the PVDF-HFP copolymer layer can improve the strength of the base film. However, the above treatment scheme increases the number of coating layers, which increases the internal resistance of the separator and reduces the transmission rate of lithium ions, affecting the transmission of lithium ions. SUMMARY

[0003] In view of this, the purpose of the present application is to provide a composite lithium ion battery separator and a preparation method and application thereof

[0004] In a first aspect, the present application provides a preparation method of a composite lithium ion battery separator, comprising the following steps:

[0005] (1) mixing a high-strength polymer and a lithium salt, and then dissolving them in a solvent to obtain a slurry; coating the slurry on a substrate to obtain a base film; wherein: the high-strength polymer is one or both of PVDF and PVDF-HFP;

[0006] (2) cutting the base film by using an array-arranged circular column cutter to obtain a base film containing arrayed circular cutting holes;

[0007] (3) in a closed device, spraying a solvent to the edge of the cutting hole of the base film containing arrayed circular cutting holes, and then pouring a PP-PE solution into the arrayed circular cutting holes to fill the cutting holes; the closed device is pressurized during pouring, and then dried to obtain a composite lithium ion battery separator.

[0008] Preferably, in step (1), the lithium salt is one of LiTFSI, LiFSI and LiPF6; and the mass ratio of the high-strength polymer to the lithium salt is (1-2):1.

[0009] Preferably, in the step (1), the thickness of the base film is 20-40 μm.

[0010] Preferably, in the step (2), the area of the arrayed circular cutting hole is 50-80% of the area of the base film.

[0011] Preferably, in the step (2), the diameter of the circular cutting hole is 5-10 mm.

[0012] Preferably, in the steps (1) and (3), the solvent is one or more of NMP, DMF, and acetonitrile.

[0013] Preferably, in the step (3), the ratio of the spraying amount of the solvent to the area of the cutting hole is 0.4-0.6 mL / cm 2 .

[0014] Preferably, in the step (3), in the PP-PE solution, the mass ratio of PP to PE is (0.5-3):1; the solvent of the PP-PE solution is one or both of tetrahydronaphthalene and decahydronaphthalene; the mass concentration of the PP-PE solution is 27-28 g / L; and the ratio of the spraying amount of the PP-PE solution to the volume of the cutting hole is 2.04-2.30 μL / mm 3 .

[0015] Further preferably, the calculation formula of the volume V of the cutting hole is V = πr 2 h wherein the unit of V is mm 3 , r is the radius of the cutting hole, ranging from 2.5 to 5 mm; h is the thickness of the base film, ranging from 20 to 40 μm; and the volume V of a single cutting hole ranges from 0.3925 to 3.14 mm 3 .

[0016] Preferably, in the step (3), the pressure of the pressurization is 0.2-0.5 MPa.

[0017] Compared with the prior art, one or more of the above technical solutions can achieve at least one of the following beneficial effects:

[0018] The composite lithium ion battery diaphragm prepared in the application is based on a high-strength polymer base film spliced with a PP-PE film with an area of 50-80%. The base film has high mechanical strength and heat resistance, the PP-PE film has good lithium ion transmission performance, and the two are combined by splicing, which can ensure the lithium ion transmission speed while avoiding the shrinkage problem of the diaphragm, thereby improving the safety of the battery. Secondly, lithium salt is added to the base film of the application, which can increase the affinity of the base film and the electrolyte, avoid the large difference in lithium transmission capacity between the base film and the PP-PE film, and the introduction of lithium salt can reduce the formation of lithium dendrites. In addition, in the method of the application, the edge position of the base film cutting hole is sprayed with a solvent, which can soften part of the base film and produce strong adhesion. Therefore, when pouring the PP-PE solution, it can better fuse and splice, improve the adhesion strength of the splicing position, and ensure the stability of the diaphragm. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The structure of the base film with cutting holes of the application is shown in the figure.

[0020] Figure 2 The structure of the composite lithium ion battery diaphragm in the application is shown in the figure.

[0021] Figure 3 The cycle performance chart of the battery assembled by the diaphragm prepared in Examples 1-3 and Comparative Examples 1-4. DETAILED DESCRIPTION

[0022] In order to facilitate the understanding of the application, the application will be described in more detail and in a more comprehensive and detailed manner in combination with the drawings of the specification and the preferred embodiments, but the protection scope of the application is not limited to the following specific embodiments.

[0023] Unless otherwise defined, all professional terms used herein have the same meaning as generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the application.

[0024] As described above, in the first aspect, the application provides a preparation method of a composite lithium ion battery diaphragm, comprising the following steps:

[0025] (1) mixing a high-strength polymer with lithium salt and dissolving it in a solvent to obtain a slurry; coating the slurry on a substrate to obtain a base film; wherein: the high-strength polymer is one or both of PVDF and PVDF-HFP;

[0026] (2) using an array of circular column cutters to cut the base film to obtain a base film containing an array of circular cutting holes, the structure diagram of which is shown in Figure 1 ;

[0027] (3) in a closed device, the edge of the cutting hole of the base film containing the array of circular cutting holes is sprayed with a solvent, and then the cutting hole is filled with a PP-PE solution pouring, the closed device is pressurized during pouring, and drying is performed after pouring, to obtain a composite lithium ion battery separator, a structural diagram of which is shown in Figure 2

[0028] In the method of the application, the lithium salt is first introduced into the base film with high mechanical strength, which can improve the affinity of the base film to the electrolyte; then the array of circular holes is cut on the base film, and after spraying the solvent on the edge of the circular hole, the PP-PE solution is poured, which can improve the strength of the splicing position.

[0029] The electrolyte is difficult to enter the inside of the base film, and after splicing the PP-PE film, the electrolyte can first enter the PP-PE film, and then diffuse into the base film, and the lithium salt in the inside of the base film can be partially dissolved into the electrolyte, thereby guiding the electrolyte to enter the inside of the base film.

[0030] In the application, the base film has high strength, and the PP-PE has high lithium transmission performance, and the two are spliced by pouring, which can basically not affect the lithium transmission rate while avoiding the shrinkage of the film and improving the safety of the battery. In addition, the cutting hole is set to be circular in the application, which can reduce the stress of the splicing position and ensure the stability of the splicing part of the separator.

[0031] Preferably, in step (1), the lithium salt is one or more of LiTFSI, LiFSI, and LiPF6; and the mass ratio of the high-strength polymer to the lithium salt is (1-2):1.

[0032] In the application, the introduction of an appropriate amount of lithium salt into the base film can reduce the difference in lithium transmission capacity between the base film and the PP-PE film, and reduce the generation of lithium dendrites.

[0033] Preferably, in step (1), the thickness of the base film is 20-40 μm.

[0034] Preferably, in step (2), the area of the array of circular cutting holes is 50-80% of the area of the base film.

[0035] In the application, by controlling the area ratio of the base film to the PP-PE film, the separator can have better comprehensive performance.

[0036] Preferably, in step (2), the diameter of the circular cutting hole is 5-10 mm.

[0037] Preferably, in steps (1) and (3), the solvent is one or more of NMP, DMF, and acetonitrile.

[0038] ​Preferably, in the step (3), the ratio of the spraying amount of the solvent to the area of the cutting hole is 0.4-0.6 mL / cm 2 .

[0039] In the present application, by controlling the spraying amount of the solvent, the to-be-spliced part of the base film can be better softened, and thus the strength of the contact part between the base film and the PP-PE film can be effectively improved, and the stability of the separator can be improved.

[0040] Preferably, in the step (3), the mass ratio of PP to PE in the PP-PE solution is (0.5-3):1; the solvent of the PP-PE solution is one or both of tetrahydronaphthalene and decahydronaphthalene; the mass concentration of the PP-PE solution is 27-28 g / L; and the ratio of the filling amount of the PP-PE solution to the volume of the cutting hole is 2.04-2.30 μL / mm 3 .

[0041] In the present application, by controlling the concentration and filling amount of the PP-PE solution, the splicing strength of the PP-PE film and the base film can be improved, and the flatness of the separator can be ensured.

[0042] Further preferably, the calculation formula of the volume V of the cutting hole is V = πr 2 h , wherein the unit of V is mm 3 ; r is the radius of the cutting hole, ranging from 2.5 to 5 mm; h is the thickness of the base film, ranging from 20 to 40 μm; and the volume V of a single cutting hole ranges from 0.3925 to 3.14 mm 3 .

[0043] Preferably, in the step (3), the pressure of the pressurization is 0.2-0.5 MPa.

[0044] In the present application, by pressurization, the capillary action of the PP-PE solution in the hole can be reduced, and the PP-PE can be uniformly poured into a film, and the flatness of the whole separator can be improved.

[0045] Example 1

[0046] (1) 0.8 g of PVDF and 0.5 g of LiTFSI were added to 20 mL of DMF, and after ball milling for 5 h until they were uniformly dispersed, a slurry was obtained. The slurry was coated on a substrate to obtain a base film with a thickness of 30 μm.

[0047] (2) The base film was cut by an array-arranged circular column cutter (the diameter of the circular cutting hole of the cutter was 8 mm), and the area of the circular cutting hole was about 66% of the total area of the base film, to obtain a base film containing array-arranged circular cutting holes, and a structure diagram thereof is as shown inFigure 1 As shown, the diameter of the circular hole is 8mm.

[0048] (3) Dissolve 2.75g PP and 2.75g PE in 200mL of decahydronaphthalene solvent to obtain a PP-PE solution; in a closed device, spray DMF along the edge of the cut holes of the base film containing arrayed circular cut holes (the ratio of the amount of DMF sprayed to the area of ​​the cut holes is 0.5mL / cm²). 2 The sealed equipment was pressurized to 0.4 MPa, and then PP-PE solution was poured into the circular cut holes (3.32 μL per hole, with the PP-PE solution quantitatively poured into the cut holes by an array of micro-injectors controlled by a positioning injection pump). After pouring, it was dried to obtain a composite lithium-ion battery separator, the structural schematic of which is shown in the figure. Figure 2 As shown.

[0049] Comparative Example 1

[0050] The base film from step 1) of Example 1 is used as a separator for lithium-ion batteries.

[0051] Comparative Example 2

[0052] The PP-PE solution in step 3) is cast into a lithium battery separator with a thickness of 10 μm.

[0053] Comparative Example 3

[0054] It is basically the same as Example 1, except that LiTFSI is not added in step (1).

[0055] Comparative Example 4

[0056] It is basically the same as Example 1, except that in step (3), the edge of the cut hole is not sprayed with solvent.

[0057] Example 2

[0058] (1) Add 0.8g PVDF-HFP and 0.8g LiPF6 to 25mL DMF, and ball mill for 5h until uniformly dispersed to obtain a slurry. Coat the slurry onto a substrate to obtain a base film with a thickness of 20μm.

[0059] (2) The base film is cut using an array of circular columnar cutters (the diameter of the circular cut holes of the cutters is 10 mm). The area of ​​the circular cut holes is approximately 54% of the total area of ​​the base film, resulting in a base film containing an array of circular cut holes. The schematic diagram of its structure is shown below. Figure 1 As shown, the diameter of the circular hole is 10mm.

[0060] (3) 1.8 g of PP and 3.6 g of PE were dissolved in 200 mL of decalin solvent to obtain a PP-PE solution; DMF was sprayed along the edge of the cutting holes of the base film containing the array of circular cutting holes (the ratio of the spraying amount of DMF to the area of the cutting holes was 0.4 mL / cm 2 ), the closed device was pressurized to 0.5 MPa, then the PP-PE solution was poured into the circular cutting holes (the pouring amount of each hole was 3.61 μL, and the array of microinjectors was controlled by a positioning syringe pump to quantitatively pour the solution into the cutting holes), and after pouring, drying was performed to obtain a composite lithium ion battery separator, a schematic diagram of the structure of which is shown in Figure 2 .

[0061] Example 3

[0062] (1) 1 g of PVDF and 0.5 g of LiTFSI were added to 20 mL of DMF, and after being ball-milled for 5 h until uniformly dispersed, a slurry was obtained. The slurry was coated on a substrate to obtain a base film with a thickness of 40 μm.

[0063] (2) The base film was cut by an array of circular column-shaped cutters (the diameter of the circular cutting holes of the cutter was 5 mm), and the area of the circular cutting holes was about 76% of the total area of the base film, to obtain a base film containing an array of circular cutting holes, a schematic diagram of the structure of which is shown in Figure 1 , and the diameter of the circular holes was 5 mm.

[0064] (3) 4.2 g of PP and 1.4 g of PE were dissolved in 200 mL of tetralin solvent to obtain a PP-PE solution; DMF was sprayed along the edge of the cutting holes of the base film containing the array of circular cutting holes (the ratio of the spraying amount of DMF to the area of the cutting holes was 0.6 mL / cm 2 ), and after the closed device was pressurized to 0.2 MPa, the PP-PE solution was poured into the circular cutting holes (the pouring amount of each hole was 1.6 μL, and the array of microinjectors was controlled by a positioning syringe pump to quantitatively pour the solution into the cutting holes), and after pouring, drying was performed to obtain a composite lithium ion battery separator, a schematic diagram of the structure of which is shown in Figure 2 .

[0065] The NCM811 was used as the positive active material to prepare a positive electrode sheet with a thickness of 200 μm; the graphite was used as the negative active material to prepare a negative electrode sheet with a thickness of 200 μm; the positive electrode sheet, the negative electrode sheet and the battery separator (prepared in Examples 1-3 and Comparative Examples 1-4) were assembled into a battery cell by winding, and after injection of electrolyte (including solvent and lithium salt, wherein: the concentration of lithium salt was 1 mol / L (the type of lithium salt was the same as that added in the base film, and the solvent of the electrolyte was a mixed solvent of EC, DEC and DMC with a volume ratio of 1:1:1)) and vacuum packaging with an aluminum plastic film, the battery cell was aged for 12 h to obtain a battery.

[0066] Electrochemical performance test:

[0067] The battery was subjected to charge-discharge test at different potentials. The battery was activated for 3 cycles at a current density of 0.1 C under a voltage of 2.7-4.8 V, and then cycled for 100 cycles at a current density of 5 C; the test results are shown in Table 1 and Table 2. Figure 3 and Table 1.

[0068] Table 1

[0069]

[0070] The capacity retention rate of the composite lithium ion battery separator in Example 1 was 94% after 100 cycles at high rate; the cycle stability of the base film in Comparative Example 1 was obviously poorer than that of Example 1, indicating that the base film had poor effect as a lithium battery separator. The cycle stability of the PP-PE separator in Comparative Example 2 was better than that of the base film in Comparative Example 1, but was poorer than that of the composite lithium ion battery separator in Example 1, which may be because the composite of the base film and PP-PE in Example 1 effectively improved the high temperature resistance of the lithium battery separator, thereby improving the cycle stability. The cycle stability of the composite lithium ion battery separator in Comparative Example 3 was slightly improved compared with the base film in Comparative Example 1, but was obviously poorer than that of the composite lithium ion battery separator in Example 1, which may be because the base film did not contain lithium salt, resulting in a large difference in ion transport performance between the base film and the PP-PE film, thereby causing the cycle stability of the separator to decrease. The cycle stability of the composite lithium ion battery separator in Comparative Example 4 was further reduced compared with the base film in Comparative Example 1, which may be because the edge part of the base film in Comparative Example 4 was not sprayed with solvent, resulting in insufficient adhesion strength of the splicing part between the base film and the PP-PE, thereby causing poor cycle stability.

[0071] The cycle performance of the composite lithium ion battery separators prepared by adjusting the process parameters in Examples 2 and 3 had certain fluctuation compared with Example 1, but had good cycle stability as a whole.

[0072] Mechanical performance test:

[0073] The tensile strength of the separators prepared in Examples 1-3 and Comparative Examples 1-4 was tested using an electronic universal testing machine at 20°C, 0.5 MPa, and a test rate of 200 mm / min. The results are shown in Table 2.

[0074] Table 2

[0075]

[0076] The tensile strength of the separator prepared in Example 1 was between that of Comparative Example 1 and Comparative Example 2, and was improved compared to Comparative Example 2. In Comparative Example 3, no lithium salt was added to the separator, and the strength was slightly improved compared to Example 1. In Comparative Example 4, the edges of the holes were not sprayed with solvent, and the adhesion between the PP-PE film and the base film was not strong enough, resulting in a significant decrease in the tensile properties of the film. In Examples 2 and 3, the process parameters of the separator were adjusted, and the tensile strength of the prepared separator changed by a certain range.

[0077] Thermal stability test:

[0078] The thermal stability of the separators prepared in Examples 1-3 and Comparative Examples 1-4 was tested. A separator with an area of 25 cm 2 (5 cm x 5 cm) was placed in an oven and heat-treated at 120°C for 120 h. After removal, the area of the heat-treated separator was measured, and the shrinkage rate of the separator was calculated (shrinkage rate = (25 cm 2 - area after heat treatment) / 25 cm 2 ). The results are shown in Table 3.

[0079] Table 3

[0080]

[0081] The base film prepared in Comparative Example 1 had the best heat resistance, and there was almost no shrinkage after heat treatment. The PP-PE separator in Comparative Example 2 had relatively poor thermal stability, and there was obvious shrinkage. The composite lithium ion battery separator in Example 1 also had high thermal stability, and there was almost no shrinkage after heat treatment. This may be because the base film made it difficult for the PP-PE film to shrink, thereby improving the overall heat resistance of the separator. In Comparative Example 3, no lithium salt was added to the base film, and the heat resistance was slightly improved compared to Example 1. In Comparative Example 4, the splicing part was not sprayed with solvent during preparation, which may have resulted in insufficient strength at the splicing position of the PP-PE film, thereby reducing the heat resistance of the film. The composite lithium ion battery separators in Examples 2 and 3 also had good thermal stability.

[0082] The above examples are only for illustrating the technical concept and characteristics of the present application, and the purpose is to enable those skilled in the art to understand the content of the present application and to implement it, and cannot limit the protection scope of the present application. Any equivalent changes or modifications made according to the spirit and essence of the present application shall be covered within the protection scope of the present application.

Claims

1. A method for preparing a composite lithium-ion battery separator, characterized in that, The method comprises the following steps: (1) dissolving high-strength polymer and lithium salt in a solvent to obtain a slurry; coating the slurry on a substrate to obtain a base film; wherein: the high-strength polymer is one or both of PVDF and PVDF-HFP; (2) cutting the base film with an array of circular column-shaped cutters to obtain a base film containing an array of circular cut holes; (3) in a closed device, spraying a solvent to the edge of the cut hole of the base film containing an array of circular cut holes, and then pouring a PP-PE solution into the array of circular cut holes to fill the cut hole; pressurizing the closed device during pouring, and drying after pouring to obtain a composite lithium ion battery separator; In step (2), the area of the array of circular cut holes is 50-80% of the area of the base film. In steps (1) and (3), the solvent is one or more of NMP, DMF, and acetonitrile.

2. The method of claim 1, wherein the method is characterized by: In step (1), the lithium salt is one or more of LiTFSI, LiFSI, and LiPF6; the mass ratio of the high-strength polymer to the lithium salt is (1-2):

1.

3. The method of claim 1 or 2, wherein the method is characterized by, In step (1), the thickness of the base film is 20-40 μm.

4. The method of claim 1, wherein the method is characterized by: In step (2), the diameter of the circular cut hole is 5-10 mm.

5. The method of claim 1, wherein the method is characterized by: In step (3), the ratio of the spraying amount of the solvent to the area of the cut hole is 0.4-0.6 mL / cm 2 .

6. The method of claim 1 or 5, wherein the method is characterized by, In step (3), the mass ratio of PP to PE in the PP-PE solution is (0.5-3):1; The solvent of the PP-PE solution is one or both of tetrahydronaphthalene and decahydronaphthalene; The mass concentration of the PP-PE solution is 27-28 g / L; The ratio of the spraying amount of the PP-PE solution to the volume of the cutting hole is 2.04-2.30 μL / mm 3 .

7. The method of claim 6, wherein the method further comprises the step of: The calculation formula of the volume V of the cutting hole is V = πr 2 h wherein r is the radius of the cutting hole, and h is the thickness of the base film. ​ 8. The method for preparing the composite lithium-ion battery separator according to claim 1, characterized in that, In step (3), the pressure of the pressurization is 0.2-0.5 MPa.

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