Dry-process single-pull diaphragm and preparation method thereof

Abstract

The invention discloses a preparation method of a dry-process single-pull diaphragm, which comprises the following steps: (1) feeding: pretreating raw materials, and conveying the pretreated raw materials to an extrusion system; (2) tape casting: melting and plasticizing the pretreated raw materials in an extrusion system, extruding a melt from a die head, and carrying out tape casting on the melt to form a base film with a specific crystal structure; (3) heat treatment: carrying out heat treatment on the base film to obtain a hard elastic film; (4) compounding: cooling the hard elastic film, and then carrying out superposition compounding; (5) stretching and shaping: stretching, retracting and shaping the compounded film to form a nano microporous film; (6) layering: layering the nano microporous membrane through a layering device to obtain a nano microporous membrane with a target thickness, wherein a stripping included angle during layering is 45-75 degrees; and (7) slitting: cutting the nano microporous membrane into a finished product membrane. According to the preparation method, the stability of the layering process can be improved, so that the product yield is relatively high, and meanwhile, the assembly short-circuit rate is relatively low when the product is subsequently used for battery assembly.

Description

Technical Field

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

[0002] The diaphragm is one of the key inner components of lithium-ion batteries. Its main function is to isolate the positive and negative electrodes and prevent electrons from passing through, while allowing ions to pass through, thereby completing the rapid transmission of lithium ions between the positive and negative electrodes during the charging and discharging process. Among the four key materials of lithium batteries, the main function of the diaphragm material is to ensure the safety of the battery. Controlling the stability of the diaphragm production process and preventing the existence of weak points in the diaphragm are one of the important factors in ensuring battery safety. The dry single-pull process is a common process in the preparation of lithium battery diaphragms. One of the differences from other processes is that it has a stratification link. The stratification is to separate the multi-layer diaphragms after stretching and shaping into independent diaphragm products of target thickness. The existing dry single-pull process has poor stability during stratification, which can easily cause the diaphragm to deform, resulting in wave-like film falling, micro-damage and micro-tearing, resulting in a low product yield. At the same time, the product leads to a high assembly short-circuit rate when used for battery assembly. 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 single-pull separator and a preparation method thereof, which can improve the stability of the stratification process, so that the product yield is higher, and the assembly short circuit rate of the product is lower when it is subsequently used for 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 single-drawn diaphragm comprises the following steps: (1) feeding: after pre-treatment of the raw materials, the raw materials are transported 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 compounding it;

[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) stratification: the nanoporous membrane is stratified by a stratification device to obtain a nanoporous membrane of target thickness, wherein the peeling angle during stratification 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 carried out 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 firstly stack 3-5 sheets of the hard elastic films to form a first composite film, and then stack 8-10 sheets 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 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 that are adjacent to each other, the first tension roller group includes two tension rollers that are symmetrically arranged, the driving roller group includes two driving rollers that are symmetrically arranged, and the winding roller group includes two winding rollers that are symmetrically arranged. The traction section of the upper and lower tension rollers of the first tension roller group and the intersection line of the two roller surfaces of the pressure roller group form the peeling angle. After the nanoporous membrane is unwound by the unwinding roller, it passes through the pressure roller group and is divided into two nanoporous membranes, which are respectively wound by the upper and lower tension rollers of the first tension roller group, and then respectively wound by the upper and lower driving rollers of the driving roller group, and then wound by the upper and lower winding rollers of the winding roller group.

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

[0025] Preferably, a second tension roller is further arranged between the first tension roller group and the driving roller group, the second tension roller group includes two tension rollers symmetrically arranged up and down, the traction 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, and the balanced angle is 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 driving rollers of the driving 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-pull separator is prepared by the preparation method as described above.

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

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

[0030] (1) The dry single-pull separator preparation method of the present invention sets a specific peeling angle during layering to make the layering smoother, so that the product yield is higher and the assembly short-circuit rate is lower when the product is subsequently used for battery assembly;

[0031] (2) The preparation method of the dry single-pull separator 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. Balance angle. DETAILED DESCRIPTION

[0040] The present invention will be further described below in conjunction with specific embodiments.

[0041] Embodiment 1:

[0042] A method for preparing a dry single-drawn 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 the metal;

[0043] (2) Casting: The pretreated raw material is melted and plasticized at 200°C in an extrusion system and then extruded from a die head to form a base film with a high degree of orientation and low crystallinity lamellar structure after casting;

[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 thin films 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 up and down, the second tension roller group 104 includes two tension rollers symmetrically arranged up and down, the driving roller group 105 includes two driving rollers symmetrically arranged up and down, and the winding roller group 106 includes two winding rollers symmetrically arranged up and down, and 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 sections 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 form 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: cutting the nanoporous membrane into finished membranes, and the electron microscope image of the finished membrane is as shown in 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] Embodiment 2:

[0050] A method for preparing a dry single-pulled separator, which differs from Example 1 only in 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-pull separator, which is different from Example 1 only in that the peeling angle 107 is 40 degrees, the equilibrium angle 108 is 95 degrees, and 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-pull separator, which differs from Example 2 only in that the peeling angle 107 is 80 degrees, the equilibrium angle 108 is 155 degrees, and no equilibrium state is formed during delamination. The tension fluctuation during delamination and the instability of the peeling process cause deformation and micro-damage of the separator. 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 film ratio, product yield rate 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 respectively, and the statistical results are shown in Table 1 below.

[0057]

[0058] As can be seen from Table 1, the yield of the product prepared by the dry single-pull separator preparation method of the present invention is above 96.3%, the battery assembly short-circuit rate is not higher than 0.23%, and there is no wave film falling.

[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 falling film and the short-circuit rate of battery assembly 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 falling film and the short-circuit rate of battery assembly 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 equivalent replacement methods and are included in the protection scope 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 with a specific crystal structure; (3) heat treatment: heat treating the base film to obtain a hard elastic film; (4) Compounding: cooling the hard elastic film and then compounding it; (5) stretching and shaping: the composite film is cold stretched, then hot stretched, and finally shrunk and shaped to form a nanoporous membrane; (6) stratification: the nanoporous membrane is stratified by a stratification device to obtain a nanoporous membrane of target thickness, wherein the peeling angle during stratification is 45-75 degrees; (7) Cutting: Cut the nanoporous membrane into finished membranes.

2. The method for preparing a dry single-pulled diaphragm 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. The method for preparing a dry single-drawn separator according to claim 1, characterized in that: The stratification device includes, from left to right, a reeling roller, a pressure roller group, a first tension roller group, a driving roller group and a winding roller group, which are arranged in sequence. The pressure roller group includes two pressure rollers that are adjacent to each other up and down. The first tension roller group includes two tension rollers that are symmetrically arranged up and down. The driving roller group includes two driving rollers that are symmetrically arranged up and down. The winding roller group includes two winding rollers that are symmetrically arranged 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.

6. The method for preparing a dry single-drawn separator according to claim 5, characterized in that: The peeling angle is 60 degrees.

7. The method for preparing a dry single-drawn separator according to claim 5, characterized in that: A second tension roller is also arranged between the first tension roller group and the driving roller group. The second tension roller group includes two tension rollers symmetrically arranged up and down. The traction 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, and the balanced angle is 100-150 degrees.

8. The method for preparing a dry single-drawn separator according to claim 7, characterized in that: The equilibrium angle is 120 degrees.

9. A dry single-pull diaphragm, characterized in that: The invention is prepared by the preparation method according to any one of claims 1 to 8.

10. A dry single-pull separator according to claim 9, characterized in that: The thickness of the dry single-drawn separator is 8-20 μm.

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