A dry single-pull diaphragm and its preparation method

Abstract

The invention discloses a preparation method of a dry-process single-pull separator, comprising the following steps: (1) feeding: after pre-treatment of the raw materials, the raw materials are transported to an extrusion system; (2) casting: the pre-treated raw materials are melt-plasticized in the extrusion system and then extruded from a die head to form a melt having a specific crystalline structure after casting; (3) heat treatment: the base film is heat-treated to obtain a hard elastic film; (4) compounding: the hard elastic film is cooled and then laminated and compounded; (5) stretching and shaping: the composite film is stretched, retracted and shaped to form a nanoporous membrane; (6) layering: the nanoporous membrane is layered through a layering device to obtain a nanoporous membrane of a target thickness, and the peeling angle during layering is 45-75 degrees; (7) slitting: the nanoporous membrane is cut into finished films. The preparation method can improve the stability of the layering process, so that the yield of the product is high, and the assembly short-circuit rate of the product is low when it is subsequently used for battery assembly.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery separators, and in particular relates to a dry-process single-draw separator and a preparation method thereof. Background Art

[0002] The diaphragm is one of the key internal components of lithium-ion batteries. Its main function is to separate the positive and negative electrodes and prevent electrons from passing through, while allowing ions to pass through, thereby achieving the rapid transfer of lithium ions between the positive and negative electrodes during the charge and discharge process. Among the four key materials in lithium batteries, the diaphragm material's primary function is to ensure battery safety. Controlling the stability of the diaphragm production process and preventing the existence of weak points in the diaphragm are important factors in ensuring battery safety. The dry single-draw process is a common process for preparing lithium battery diaphragms. One of the differences from other processes is that it involves a delamination step. Delamination is the process of separating the multi-layer diaphragm after stretching and shaping into independent diaphragm products of target thickness. The existing dry single-draw process has poor stability during delamination, which can easily cause the diaphragm to deform, resulting in problems such as wavy film drop, micro-damage and micro-tears, resulting in a low product yield. At the same time, the product's subsequent use in battery assembly leads to a high assembly short-circuit rate. Summary of the Invention

[0003] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention provides a dry-process single-stretch separator and a method for preparing the same. This method improves the stability of the delamination process, resulting in a higher product yield and a lower short-circuit rate when the product is subsequently used in battery assembly.

[0004] The above technical objectives of the present invention are achieved through the following technical solutions:

[0005] A method for preparing a dry-process single-drawn diaphragm comprises the following steps: (1) feeding: the raw materials are pretreated and then conveyed to an extrusion system;

[0006] (2) Casting: The pretreated raw materials are melted and plasticized in an extrusion system, and then a melt is extruded from a die head. The melt is cast to form a base film with a specific crystal structure;

[0007] (3) heat treatment: heat treating the base film to obtain a hard elastic film;

[0008] (4) Compounding: cooling the hard elastic film and then laminating the film;

[0009] (5) stretching and shaping: the composite film is cold stretched, then hot stretched, and finally shrunk and shaped to form a nanoporous membrane;

[0010] (6) Delamination: Delaminating the nanoporous membrane through a delamination device to obtain a nanoporous membrane of target thickness, wherein the peeling angle during delamination is 45-75 degrees;

[0011] (7) Cutting: Cut the nanoporous membrane into finished membranes.

[0012] Preferably, in step (1), the raw material is a high molecular weight polypropylene having an isotacticity of ≥97% and a melt index of 0.5-2.0 g / 10 min.

[0013] Preferably, in step (1), the pretreatment is dust removal and metal removal.

[0014] Preferably, in step (2), the temperature of the melt plasticization is 180-220°C.

[0015] Preferably, in step (2), the base film is a lamellar structure base film with high orientation and low crystallinity.

[0016] Preferably, in step (3), the heat treatment is performed by air drying at 120-140° C., and the heat treatment time is 10-20 hours.

[0017] Preferably, in step (4), the compounding is to first stack 3-5 pieces of the hard elastic films to form a first composite film, and then stack 8-10 pieces of the first composite films to form a second composite film.

[0018] Preferably, in step (5), the cold stretching ratio is 1.2-1.3, and the cold stretching temperature is 40-50°C.

[0019] Preferably, in step (5), the thermal stretching ratio is 2.8-3.2, and the thermal stretching temperature is 130-140°C.

[0020] Preferably, in step (5), the retraction refers to a retraction of 20%-30%.

[0021] Preferably, in step (5), the shaping temperature is 145-150°C.

[0022] Preferably, in step (6), the stratification is performed in a one-to-two manner.

[0023] Preferably, the delamination device includes, from left to right, an unwinding roller, a pressure roller group, a first tension roller group, a drive roller group, and a take-up roller group, wherein the pressure roller group includes two pressure rollers arranged adjacent to each other, the first tension roller group includes two tension rollers arranged symmetrically, the drive roller group includes two drive rollers arranged symmetrically, and the take-up roller group includes two take-up rollers arranged symmetrically. The intersection of the traction cross-section of the upper and lower tension rollers of the first tension roller group and the roller surfaces of the two pressure rollers of the pressure roller group forms the peeling angle. After being unwound by the unwinding roller, the nanoporous membrane passes through the pressure roller group and is separated into two nanoporous membranes. The membranes are then wound by the upper and lower tension rollers of the first tension roller group, the upper and lower drive rollers of the drive roller group, and the take-up rollers of the take-up roller group are wound.

[0024] Preferably, the peeling angle is 60 degrees.

[0025] Preferably, a second tension roller is disposed between the first tension roller group and the drive roller group. The second tension roller group includes two tension rollers symmetrically arranged vertically. The traction cross-sections of the upper and lower tension rollers of the first tension roller group and the upper and lower tension rollers of the second tension roller group form a balanced angle of 100-150 degrees. The two pressure rollers of the pressure roller group, the two tension rollers of the first tension roller group, the two tension rollers of the second tension roller group, the two drive rollers of the drive roller group, and the two winding rollers of the winding roller group are all symmetrically arranged along the same straight line.

[0026] Preferably, the equilibrium angle is 120 degrees.

[0027] A dry single-stretch separator is prepared by the above-mentioned preparation method.

[0028] Preferably, the thickness of the dry-process single-stretch separator is 8-20 μm.

[0029] The beneficial effects of the present invention are:

[0030] (1) The dry-process single-stretch separator preparation method of the present invention makes the delamination more stable by setting a specific peeling angle during delamination, thereby increasing the product yield and reducing the assembly short-circuit rate when the product is subsequently used in battery assembly;

[0031] (2) The dry-process single-pull separator preparation method of the present invention can keep the separation point in a stable and immobile state when the peeling angle is 60 degrees during layering, making the layering most stable and the product yield the highest. At the same time, the assembly short-circuit rate of the product is the lowest when it is subsequently used for battery assembly.

[0032] (3) The preparation method of the dry single-pull diaphragm of the present invention can set a specific equilibrium angle during stratification so that a smaller force can be used for stratification, thereby further improving the stability during stratification. When the equilibrium angle is 120 degrees, the minimum force can be used for stratification. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a schematic diagram of a layering device according to Example 1 of the present invention;

[0034] Figure 2 This is an electron microscope image of the finished film prepared in Example 1 of the present invention;

[0035] Figure 3 This is an electron microscope image of the finished film prepared in Example 2 of the present invention;

[0036] Figure 4 This is an electron microscope image of the finished film prepared in Comparative Example 1 of the present invention;

[0037] Figure 5 This is an electron microscope image of the finished film prepared in Comparative Example 2 of the present invention.

[0038] Reference numerals:

[0039] 101. Unwinding roller; 102. Pressing roller group; 103. First tension roller group; 104. Second tension roller group; 105. Driving roller group; 106. Winding roller group; 107. Peeling angle; 108. Balancing angle. DETAILED DESCRIPTION

[0040] The present invention will be further described below with reference to specific embodiments.

[0041] Example 1:

[0042] A method for preparing a dry-process single-stretch diaphragm comprises the following steps: (1) feeding: the raw material is a high molecular weight polypropylene with an isotacticity of 97.5 and a melt index of 2.0 g / 10 min, which is transported to an extrusion system after being separated from the dust removal device and metal;

[0043] (2) Casting: The pretreated raw materials are melted and plasticized at 200°C in an extrusion system and then extruded from a die head. The melt is cast to form a base film with a high degree of orientation and low crystallinity.

[0044] (3) Heat treatment: The base film is subjected to air drying at 126° C. for 18 h to obtain a hard elastic film after the lamellae are structured;

[0045] (4) Compounding: Cooling the hard elastic film, then compounding four hard elastic films into a first compound film, and then compounding eight first compound films into a second compound film;

[0046] (5) Stretching and shaping: The second composite film with a total of 32 layers of film is cold stretched at a ratio of 1.2 and a temperature of 45°C, and then hot stretched at a total ratio of 2.8 and a temperature of 136°C, and finally shrunk by 25% and shaped at a temperature of 148°C to form a nanoporous membrane;

[0047] (6) Layering: The nanoporous membrane is passed through a layering device for multiple cycles in a one-to-two manner to obtain a nanoporous membrane with a thickness of 18 μm.

[0048] like Figure 1 As shown, the layering device includes, from left to right, an unwinding roller 101, a pressure roller group 102, a first tension roller group 103, a second tension roller group 104, a driving roller group 105 and a winding roller group 106, wherein the pressure roller group 102 includes two pressure rollers adjacent to each other, the first tension roller group 103 includes two tension rollers symmetrically arranged in the upper and lower parts, the second tension roller group 104 includes two tension rollers symmetrically arranged in the upper and lower parts, the driving roller group 105 includes two driving rollers symmetrically arranged in the upper and lower parts, and the winding roller group 106 includes two winding rollers symmetrically arranged in the upper and lower parts. The traction section of the upper and lower tension rollers of the first tension roller group 103 is the same as that of the pressure roller group 10 The intersection line of the roller surfaces of the two pressure rollers 2 forms a peeling angle 107, and the peeling angle 107 is 60 degrees. The traction section of the upper and lower tension rollers of the first tension roller group 103 and the upper and lower tension rollers of the second tension roller group 104 forms a balanced angle 108, and the balanced angle 108 is 120 degrees. The two pressure rollers of the pressure roller group 102, the two tension rollers of the first tension roller group 103, the two tension rollers of the second tension roller group 104, the two driving rollers of the driving roller group 105 and the two winding rollers of the winding roller group 106 are all symmetrically arranged along the same straight line; (7) Cutting: Cut the nanoporous membrane into finished membranes. The electron microscope image of the finished membrane is as follows: Figure 2 As shown by Figure 2 It can be seen that the original appearance of the membrane surface remains intact and there is no sign of damage.

[0049] Example 2:

[0050] A dry single-pulled separator preparation method, the difference from Example 1 is that the thickness of the prepared nanoporous membrane is 10 μm. The electron microscope image of the finished membrane prepared in this example is as follows: Figure 3 As shown by Figure 3 It can be seen that the original appearance of the membrane surface remains intact and there is no sign of damage.

[0051] Comparative Example 1:

[0052] A method for preparing a dry single-stretch separator, which differs from Example 1 only in that the peeling angle 107 is 40 degrees and the equilibrium angle 108 is 95 degrees. No equilibrium state is formed during delamination. The tension fluctuation during delamination and the unstable peeling process cause the separator to deform and micro-damage. The electron microscope image of the finished film obtained in this comparative example is as follows: Figure 4 As shown by Figure 4 It can be seen that the membrane surface is not smooth and many micro damages have been formed.

[0053] Comparative Example 2:

[0054] A method for preparing a dry single-stretch separator, which differs from Example 2 only in that the peeling angle 107 is 80 degrees and the equilibrium angle 108 is 155 degrees. No equilibrium state is formed during delamination. The tension fluctuation during delamination and the unstable peeling process cause the separator to deform and micro-damage. The electron microscope image of the finished film obtained in this comparative example is as follows: Figure 5 As shown by Figure 5 It can be seen that the membrane surface is not smooth and many micro damages have been formed.

[0055] Test example:

[0056] The wave-fall ratio, product yield and battery assembly short-circuit rate of the dry-process single-pull separators prepared in Example 1-2 and Comparative Example 1-2 were statistically analyzed, and the statistical results are shown in Table 1 below.

[0057]

[0058] As shown in Table 1, the yield of the product prepared by the dry single-drawn separator preparation method of the present invention is above 96.3%, the battery assembly short-circuit rate is no higher than 0.23%, and no wave-like membrane drop occurs.

[0059] By comparing Example 1 with Comparative Example 1, it can be seen that when other conditions remain unchanged and the peeling angle is adjusted to 40 degrees and the equilibrium angle is 95 degrees, the yield of the final product is greatly reduced, and the proportion of diaphragm wave film drop and the battery assembly short circuit rate are greatly increased.

[0060] By comparing Example 2 with Comparative Example 2, it can be seen that when other conditions remain unchanged and the peeling angle is adjusted to 80 degrees and the equilibrium angle is 155 degrees, the yield of the final product is greatly reduced, and the proportion of diaphragm wave film drop and the battery assembly short-circuit rate are greatly increased.

[0061] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for preparing a dry single-pull diaphragm, characterized in that: The following steps are involved: (1) Feeding: After pretreatment, the raw materials are transported to the extrusion system; (2) Casting: The pretreated raw materials are melted and plasticized in an extrusion system, and then a melt is extruded from a die head. The melt is cast to form a base film; (3) Heat treatment: heat treating the base film to obtain a hard elastic film; (4) Compounding: cooling the hard elastic film and then laminating the film; (5) Stretching and shaping: The composite film is cold stretched, then hot stretched, and finally shrunk and shaped to form a nanoporous membrane; (6) Delamination: Delaminating the nanoporous membrane through a delamination device to obtain a nanoporous membrane of target thickness, wherein the peeling angle during delamination is 60 degrees; (7) Cutting: cutting the nanoporous membrane into finished membranes; the stratification device includes, from left to right, an unwinding roller, a pressure roller group, a first tension roller group, a driving roller group and a winding roller group, the pressure roller group includes two pressure rollers arranged close to each other up and down, the first tension roller group includes two tension rollers arranged symmetrically up and down, the driving roller group includes two driving rollers arranged symmetrically up and down, the winding roller group includes two winding rollers arranged symmetrically up and down, the traction section of the upper and lower tension rollers of the first tension roller group and the intersection line of the roller surfaces of the two pressure rollers of the pressure roller group form the peeling angle; a second tension roller group is also provided between the first tension roller group and the driving roller group, the second tension roller group includes two tension rollers arranged symmetrically up and down, the upper and lower tension rollers of the first tension roller group and the traction section of the upper and lower tension rollers of the second tension roller group form a balanced angle, and the balanced angle is 120 degrees.

2. The method for preparing a dry single-drawn separator according to claim 1, characterized in that: In step (3), the heat treatment is carried out by air drying at 120-140°C, and the heat treatment time is 10-20 hours.

3. The method for preparing a dry single-drawn separator according to claim 1, characterized in that: In step (5), the cold stretching ratio is 1.2-1.3, and the cold stretching temperature is 40-50°C.

4. The method for preparing a dry single-drawn separator according to claim 1, characterized in that: In step (5), the thermal stretching ratio is 2.8-3.2, and the thermal stretching temperature is 130-140°C.

5. A dry single-pull diaphragm, characterized by: The invention is prepared by the preparation method according to any one of claims 1 to 4.

6. The dry single-stretch diaphragm according to claim 5, characterized in that: The thickness of the dry-process single-drawn separator is 8-20 μm.

Images