A separator, a method of manufacturing the same, a secondary battery, and an electric device

Abstract

This invention provides a separator and its preparation method, a secondary battery, and an electrical device, comprising a base membrane, wherein the thickness of the base membrane varies along the direction from the center of the base membrane to its edge, satisfying the following trend: 1×10⁻⁶. ‑4 ≤β≤2×10 ‑2 , β=(H x -γ) / L x 2 ; where H x The thickness of the base film at any location X between the center and the edge of the base film, expressed in μm, L x The distance from position X to the center of the base film is expressed in cm, and γ is the thickness of the base film at its center in μm. This invention helps to improve the problem of uneven current density distribution in the separator during charging and discharging.

Description

Technical Field

[0001] This invention relates to the field of battery materials, specifically to a separator and its preparation method, a secondary battery, and an electrical device. Background Technology

[0002] Limited by existing processes, uneven current density distribution exists within the battery, which exacerbates battery polarization, increases DC internal resistance, and consequently leads to a significant decrease in overall battery cycle performance, affecting the overall battery performance. Therefore, solving the aforementioned problem of uneven current density is a research hotspot in this field. Summary of the Invention

[0003] This invention provides a separator and its preparation method, a secondary battery, and an electrical device, which helps to solve the problem of uneven current density in batteries.

[0004] This invention provides a diaphragm, comprising a base membrane, wherein the thickness of the base membrane varies along the direction from the center of the base membrane to its edge, satisfying the following trend: 1×10⁻⁶. -4 ≤β≤2×10 -2 , β=(H x -γ) / L x 2 ; where H x The thickness of the base film at any location X between the center and the edge of the base film, expressed in μm, L x The distance from position X to the center of the base film is expressed in cm, and γ is the thickness of the base film at its center in μm.

[0005] Optionally, 1μm≤γ≤50μm, preferably, 2μm≤γ≤30μm.

[0006] Optionally, the distance between the center of the base film and the edge of the base film in the direction from the center of the base film to the edge of the base film is 3cm to 500cm, preferably 3cm to 105cm.

[0007] Optionally, the thickness of the base film in the width direction satisfies the aforementioned variation trend.

[0008] Optionally, the diaphragm is a single base membrane or includes multiple sub-base membranes.

[0009] Optionally, the base film comprises a polymer, which includes one or more of polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, polyurethane, polymethylpentene, polyethylene terephthalate, polycarbonate, polyester, polyimide, polyvinyl alcohol, polyacrylonitrile, polyoxymethylene, polymethyl methacrylate, polyoxyethylene, and cellulose.

[0010] Optionally, the diaphragm further includes a coating disposed on at least one surface of the base membrane; optionally, the coating has a uniform thickness; optionally, the coating has a non-uniform thickness.

[0011] This invention provides a method for preparing a diaphragm as described above, comprising: molding a raw material for forming a base membrane to obtain a base membrane precursor; wherein the thickness of the base membrane precursor increases in the direction from the center of the base membrane precursor to its edge; subjecting the base membrane precursor to a stretching treatment and a heat-setting treatment in sequence, wherein the stretching treatment includes a first stretching treatment and a second stretching treatment in sequence, wherein the temperature of the first stretching treatment is 40°C to 130°C, the temperature of the second stretching treatment is 120°C to 170°C, and the product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment is 1.2 to 40.0, so that the base membrane precursor forms the base membrane to obtain the diaphragm.

[0012] Optionally, the thickness of the base film precursor increases in a gradient direction from the center of the base film precursor to its edge; and / or, the direction from the center of the base film precursor to its edge is parallel to the width direction of the base film precursor.

[0013] Optionally, the gradient increasing trend satisfies the following: the base film precursor includes at least three partitions distributed sequentially along the direction from the center of the base film precursor to the edge of the base film precursor, wherein in any two adjacent partitions, the thickness of the partition closer to the edge of the base film precursor is greater than the thickness of the partition closer to the center of the base film precursor.

[0014] Optionally, the molding process includes: melting and extruding the raw material used to form the base film, and casting it into a film to obtain the base film precursor.

[0015] Optionally, the process of melting and extruding the raw material for forming the base film includes: melting at least a portion of the raw material and extruding it through a first die, wherein the opening of the first die tends to increase along the direction from the preset center of the base film precursor to the preset edge of the base film precursor;

[0016] Preferably, the first die head includes at least three die head areas distributed sequentially along a predetermined direction from the predetermined center of the base film precursor to the predetermined edge of the base film precursor, wherein in any two adjacent die head areas, the opening of the die head area closer to the predetermined edge of the base film precursor is greater than the opening of the die head area closer to the predetermined center of the base film precursor.

[0017] Optionally, the raw materials are melt-extruded using a co-extrusion method to obtain the base film precursor comprising a multilayer sub-base film precursor; wherein, of the raw materials, the raw material used to form at least one of the multilayer sub-base films is extruded through the first die.

[0018] Optionally, the direction of the stretching process intersects with the direction from the center of the base film precursor to the edge of the base film precursor; and / or, the stretching ratio of the first stretching process is less than the stretching ratio of the second stretching process.

[0019] The present invention provides a secondary battery, comprising the separator as described above or the separator prepared according to the preparation method described above.

[0020] Optionally, it also includes a positive electrode and a negative electrode, with the separator located between the positive electrode and the negative electrode.

[0021] Optionally, the positive electrode sheet is provided with a positive electrode tab, which is disposed at a position corresponding to the edge of the positive electrode sheet and the edge of the separator in the direction from the center of the separator to the edge of the separator; preferably, the positive electrode tab is disposed on at least one side in the width direction of the positive electrode sheet.

[0022] Optionally, the negative electrode sheet is provided with a negative electrode tab, which is disposed at a position corresponding to the edge of the negative electrode sheet in the direction from the center of the diaphragm to the edge of the diaphragm; preferably, the negative electrode tab is disposed on at least one side in the width direction of the negative electrode sheet.

[0023] The present invention provides an electrical device, including a secondary battery as described above, wherein the secondary battery serves as the power supply for the electrical device.

[0024] This invention provides a separator and its preparation method, a secondary battery, and an electrical device. The thickness of the base film has the aforementioned variation trend in the direction from the center to the edge of the base film, so that the ionic conductivity of the central region of the base film is higher than that of the edge region. This helps to compensate for the problem of uneven current density caused by the impedance difference between the central and edge regions of the base film, thereby improving the problem of uneven current density distribution in the battery and improving the DC internal resistance and cycle life of the battery. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a longitudinal cross-sectional view of the base film in some embodiments;

[0027] Figure 2 This is a longitudinal cross-sectional view of the base film in some embodiments;

[0028] Figure 3 This is a schematic diagram of the battery cell stacked structure.

[0029] Explanation of reference numerals in the attached figures:

[0030] 1-Negative electrode; 1a-Negative electrode tab; 2-Separator; 3-Positive electrode; 3a-Positive electrode tab. Detailed Implementation

[0031] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below. The specific embodiments listed below are merely descriptions of the principles and features of the present invention, and the examples are only for explaining the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] With the rapid development of the new energy vehicle industry, the market has placed higher demands on the energy density and fast-charging performance of power batteries. Against this backdrop, the inherent challenges of blade batteries, as a mainstream technology, in long-cycle and fast-charging scenarios are becoming increasingly prominent. During charging and discharging, the uneven distribution of electrode current density along the length of the blade battery is a key technical challenge. Specifically, the current density in the tab connection area is significantly higher than that in the middle area of ​​the electrode. This uneven current distribution leads to an imbalance in ion flux, and under long-cycle conditions, especially high-rate fast charging, it easily triggers lithium metal deposition (lithium plating) in areas of high current density, ultimately causing rapid capacity decay and a significant drop in battery capacity.

[0033] In existing technologies, one improvement approach is to increase the thickness of the current collector at both ends to optimize the electronic conductivity distribution, thereby improving current uniformity. However, this method mainly targets and improves the distribution of electronic impedance, but fails to solve the problem of local ion impedance differences caused by uneven current distribution. As a key component for ion transport in a battery, the separator's core function is to block the active materials of the positive and negative electrodes to prevent short circuits and excessive self-discharge, while providing a smooth channel for ion migration in the electrolyte. Currently, commercially available separators cannot effectively solve the aforementioned technical problems. The inventors discovered that the fundamental reason is that existing separators are usually designed as isotropic homogeneous structures, resulting in a homogeneous ion transport channel inside the battery. For example, the most commonly used polyolefin separators (such as PP and PE), although possessing excellent matrix properties, typically have a uniform thickness and pore size. This homogeneous structure cannot compensate for the uneven current distribution caused by the electrodes themselves, thus making it difficult to solve the resulting impedance unevenness and lithium plating problems. This disadvantage is even more pronounced under fast charging conditions. Furthermore, existing technologies often involve coating the separator (using inorganic or organic materials), which, while enhancing its resistance to foreign object puncture, also increases ion transport resistance and leads to increased separator thickness, thus sacrificing battery energy density. Other solutions employ thinner separators or separators with larger pore sizes to reduce overall ion impedance, but this also fails to address the fundamental problem of uneven current distribution. Instead, it weakens the separator's mechanical strength and heat resistance, introducing new risks of short circuits. In short, existing technologies fail to fundamentally and effectively solve the problems of lithium plating and lifespan degradation caused by uneven current density within the battery, especially in blade batteries during fast charging.

[0034] Through long-term research and practice, the inventors observed an inherent and unfavorable electrochemical behavior in the internal separator of lithium-ion batteries during actual operation: due to insufficient structural uniformity and ion transport pathways of the separator material at the macroscopic scale, a systematic impedance difference often occurs between the central and peripheral regions of the separator during charging and discharging. Specifically, ion current is excessively concentrated in the peripheral region (higher current density), while transport is impeded in the central region (lower current density). Since battery performance, especially fast-charging performance, highly depends on efficient ion transport within the system, this uneven current distribution determined by the internal structure of the separator directly leads to non-uniform utilization of active materials and exacerbates side reactions, thus becoming a key bottleneck restricting the overall performance and cycle life of the battery.

[0035] To overcome the shortcomings of the prior art, embodiments of the present invention provide a diaphragm, including a base membrane, wherein the thickness of the base membrane varies along the direction from the center to the edge of the base membrane, satisfying the following trend: 1×10⁻⁶. -4 ≤β≤2×10 -2 , β=(Hx -γ) / L x 2 ; where H x L represents the thickness at any position X between the center and the edge of the base film, expressed in μm. x The distance from position X to the center of the base film is expressed in cm, and γ is the thickness of the base film at its center, expressed in μm.

[0036] According to research and analysis, the thickness of the base film exhibits the aforementioned variation trend along the direction from the center to the edge of the base film. This results in the ionic conductivity of the central region of the base film being higher than that of the edge region. This helps to compensate for the problem of uneven current density caused by the impedance difference between the central and edge regions of the base film, which helps to improve the problems of lithium plating and capacity drop, thereby improving the problem of uneven current density distribution in the battery and improving the DC internal resistance and cycle life of the battery.

[0037] Compared to solutions that simply use high-porosity or ultra-thin membranes, the membrane in this invention is more targeted and does not sacrifice overall performance. Furthermore, while existing technologies that add inorganic / organic layers to the membrane's edge region or both ends can increase its adhesion and resistance to foreign matter, their effectiveness in improving uneven current distribution is unclear, and differences in coating height can easily lead to coating powder shedding, causing short-circuit risks and safety issues. Compared to these solutions, the membrane in this invention has an integrated structure, resulting in higher reliability and better safety.

[0038] The thickness of the base film varies along the direction from the center to the edge of the base film, satisfying the following trend: 1×10 -4 ≤β≤2×10 -2 , β=(H x -γ) / L x 2 H represents the thickness H of the base film at any position X along the direction from the center to the edge of the base film. x And the distance L from position X to the center of the base film. x All of the above conditions are met.

[0039] Understandably, the above H x >γ.

[0040] For a specific diaphragm, the following method can be used to determine whether the above technical solution is met: Measure the thickness γ (unit: μm) of the base membrane at its center position, and take any position X between the center and the edge of the base membrane, and measure the thickness H of the base membrane at position X. x (Unit: μm) Distance L from location X to the center of the base film x (Unit: cm), according to β=(H x -γ) / Lx 2 Calculate β, if β satisfies 1×10 -4 ≤β≤2×10 -2 If the diaphragm meets the above technical requirements, then the diaphragm satisfies the above technical requirements; otherwise, it does not.

[0041] Specifically, along the width direction of the base film, the location of the centerline in the width direction is defined as the center, and the average thickness of the base film at this center is measured as γ (unit: μm); along the width direction of the base film, an arbitrary point is selected between the center and the edge of the base film as position X, and the average thickness of the base film on the strip perpendicular to the width direction at position X is measured and denoted as H. x (Unit: μm); Simultaneously, the straight-line distance from measurement position X, parallel to the width direction, to the center of the base film is denoted as L. x (Unit: cm). According to β=(H x -γ) / L x 2 Calculate β, if β satisfies 1×10 -4 ≤β≤2×10 -2 If the membrane meets the above technical requirements, then it satisfies the above technical solution; otherwise, it does not. For example, L... x The value of L can satisfy the following: Let L be the total length of the base film in the width direction, and the above L... x It can be L / 6, L / 3, or L / 2.

[0042] Understandably, L x Less than or equal to the maximum distance from the center of the base film to the edge of the base film.

[0043] In specific implementation, the distance between the center and the edge of the base film along the direction from the center to the edge of the base film is 3cm to 500cm, for example, 3cm, 5cm, 10cm, 20cm, 30cm, 40cm, 45cm, 50cm, 60cm, 70cm, 80cm, 90cm, 100cm, 105cm, 110cm, 150cm, 200cm, 250cm, 300cm, 400cm, 500cm or any combination thereof, preferably 45cm to 105cm.

[0044] Generally, in the direction from the center to the edge of the base film, the total length of the base film, that is, the distance between the edges of the base film on both sides of its center, is twice the distance between the center and the edge of the base film. In other words, in the direction from the center to the edge of the base film, the total length of the base film can be 6cm to 1000cm, preferably 6cm to 210cm.

[0045] The length of the base film in the direction from the center to the edge of the base film meets the above range, covering the manufacturing needs of all electrical equipment from micro-devices to large-scale energy storage. More importantly, within the above size range, by limiting the thickness trend of the base film as described above, the problem of uneven current density distribution in the separator during charging and discharging can be effectively improved, thereby improving the problem of uneven current density distribution in the battery, which helps to reduce the DC internal resistance of the battery and improve the cycle performance of the battery.

[0046] In some embodiments, the direction from the center of the base film to the edge of the base film is the width direction of the base film, that is, the width of the base film can be 6cm to 1000cm, preferably 6cm to 210cm.

[0047] The width of the base film meets the above-mentioned range, covering the manufacturing needs of all electrical equipment from micro-devices to large-scale energy storage. More importantly, within the width range of the base film, by limiting the thickness trend of the base film as described above, the problem of uneven current density distribution in the separator during charging and discharging can be effectively improved, thereby improving the problem of uneven current density distribution in the battery, which helps to reduce the DC internal resistance of the battery and improve the cycle performance of the battery.

[0048] The base film of this invention is designed and manufactured with full consideration for the flexibility of practical applications, and can be precisely cut according to specific cell specifications and assembly process requirements. For example, for a base film with the aforementioned thickness variation trend in the width direction, after cutting with the original centerline of the base film as a reference, the resulting new base film can still maintain the same thickness variation trend as the raw material. This feature allows a single specification of base film to adapt to various battery size designs, while ensuring that the cut edge area still has good mechanical integrity and interface stability, thereby meeting customized needs without compromising the core function of the base film and the overall cycle reliability of the battery.

[0049] In one specific embodiment, 1 μm ≤ γ ≤ 50 μm. That is, the thickness at the center of the base film is 1 μm to 50 μm. Exemplarily, γ can be a range of 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 20 μm, 30 μm, 40 μm, 50 μm, or any combination thereof. In some preferred embodiments, 2 μm ≤ γ ≤ 30 μm.

[0050] The thickness γ at the center of the base film meets the above range, which helps to optimize the current density distribution of the separator during charging and discharging, thereby improving the problem of uneven current density distribution in the battery, reducing the DC internal resistance of the battery, and improving the cycle performance of the battery.

[0051] In this embodiment of the invention, 1×10 -4 ≤β≤2×10 -2 For example, β can be 1×10 -4 2×10 -4 3×10 -4 4×10 -4 5×10 -4 6×10 -4 1×10 -4 7×10 -4 8×10 -4 9×10 -4 1×10 -3 2×10 -3 3×10 -3 4×10 -3 5×10 -3 6×10 -3 7×10 -3 8×10 -3 9×10 -3 1×10 -2 2×10 -2 Or a range consisting of any two of them.

[0052] In some specific embodiments, the thickness of the base film follows the aforementioned variation trend in its width direction. This variation trend in the thickness of the base film ensures that the ionic conductivity of the central region is higher than that of the edge region. This helps to compensate for the uneven current density caused by the impedance difference between the central and edge regions of the base film, thereby improving the uneven current density distribution of the battery and enhancing its DC internal resistance and cycle life.

[0053] Specifically, in the width direction of the base film, the thickness of the base film satisfying the above-mentioned variation trend means that, in the width direction of the base film, the location of the centerline in the width direction is defined as the center, and the average thickness of the base film at this center is denoted as γ (unit: μm); along the width direction of the base film, any point between the center and the edge of the base film is taken as position X, and the average thickness of the base film on the strip perpendicular to the width direction at position X is denoted as H. x (Unit: μm); Let L be the straight-line distance from position X along the width direction to the center. x (Unit: cm). According to β=(H x -γ) / L x 2 β is calculated, and β satisfies 1×10 -4 ≤β≤2×10 -2 .

[0054] If β satisfies the above range in the width direction of the base film, it indicates that the thickness of the base film tends to increase along the direction from the center to the edge, for example, a gradient increase trend or a gradual increase trend.

[0055] Typically, the width direction of the base film is the same as the current direction. The thickness of the base film in the width direction follows the aforementioned trend, which compensates for the impedance difference in the width direction. This results in the central region of the base film having relatively high ionic conductivity and relatively low transport resistance, thereby accelerating ion migration. Conversely, the edge regions of the base film have relatively low ionic conductivity and relatively high transport resistance, thereby slowing down ion migration. This regional difference in ion migration rate balances the problem of uneven current density distribution in the battery, improves the ionic impedance difference caused by current concentration, and helps to improve the problem of uneven current density distribution in the battery. This effectively improves the problems of interface lithium plating and capacity drop, and improves the DC internal resistance and cycle life of the battery.

[0056] In addition, the aforementioned diaphragm can be a single base membrane or may include multiple sub-base membranes.

[0057] Specifically, the separator is a single base film. This separator not only improves the problem of uneven current density distribution inside the separator, but also has a simple and low-cost manufacturing process. The separator includes multiple sub-base films, which can be controlled by the composite and independent processes of different functional layers. While ensuring the uniform current density distribution inside the separator, the performance of the battery can be specifically improved by further optimizing the pore structure and ion transport dynamics.

[0058] In specific implementations, the porosity of the aforementioned separator can be 20% to 80%. Separators with different porosities can be selected according to the battery performance requirements. In the separator system of this invention embodiment, by further controlling its porosity, it helps to improve the problem of uneven current density distribution inside the separator, which in turn helps to improve the problem of uneven current density distribution in the battery, thereby improving the battery's DC internal resistance and cycle life.

[0059] For example, the porosity of the membrane is in the range of 20%, 30%, 40%, 50%, 60%, 70%, 80%, or any combination thereof.

[0060] The embodiments of the present invention do not impose any particular limitation on the material of the base membrane, and the base membrane can be any membrane material commonly used in the art.

[0061] In specific implementations, the base film may include a polymer with a weight-average molecular weight of 50,000 to 2,000,000, for example, 50,000, 100,000, 200,000, 300,000, 500,000, 600,000, 700,000, 800,000, 900,000, 1,000,000, 1,100,000, 1,200,000, 1,300,000, 1,500,000, 1,600,000, 1,700,000, 1,800,000, 1,900,000, 2,000,000, or any combination thereof, preferably 200,000 to 800,000. A weight-average molecular weight of the polymer within the above range optimizes the microstructure and interfacial compatibility of the base film, helps reduce the DC internal resistance of the battery, and improves the battery's cycle performance.

[0062] In some embodiments, the polymer includes one or more of polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, polyurethane, polymethylpentene, polyethylene terephthalate, polycarbonate, polyester, polyimide, polyvinyl alcohol, polyacrylonitrile, polyoxymethylene, polymethyl methacrylate, polyoxyethylene, and cellulose. These polymers can help optimize the microstructure and interfacial compatibility of the base film, contributing to a reduction in the battery's DC internal resistance and improving its cycle performance.

[0063] In specific implementations, the separator may further include a coating disposed on at least one surface of the base membrane. The embodiments of the present invention do not particularly limit the material of the coating. For example, the material of the coating may include at least one of inorganic particles and polymers. Further, the inorganic particles may include at least one of perovskite, metal oxides, boehmite, barium sulfate, magnesium hydroxide, aluminum hydroxide, silica, and calcium carbonate; the polymers may include at least one of acrylate polymers, acrylamide polymers, acrylonitrile polymers, methacrylic acid polymers, polystyrene polymers, polymaleic anhydride polymers, fluoropolymers, and aromatic polyamides. Methacrylic acid polymers may include PMMA (polymethyl methacrylate), fluoropolymers may include PVDF (polyvinylidene fluoride) and its copolymers (such as PVDF-HFP), and aromatic polyamides may include polyaramid. The above coating helps improve the adhesion, mechanical properties, and heat resistance of the separator, thereby helping to improve the cycle performance of the battery.

[0064] In one embodiment, the coating has a uniform thickness, which largely solves the problem of uneven current distribution while meeting other requirements such as battery manufacturing process.

[0065] In another embodiment, the coating thickness is uneven. Further, the thickness of the separator can be made uniform by controlling the thickness of the coating; that is, the thickness of the coating is complementary to the thickness of the base film, thereby making the separator thickness uniform. This helps to improve the problem of uneven current distribution and is also more conducive to battery shaping and bonding.

[0066] This invention also provides a method for preparing the above-mentioned diaphragm, comprising: molding a raw material for forming a base membrane to obtain a base membrane precursor; wherein the thickness of the base membrane precursor increases in the direction from the center of the base membrane precursor to its edge; and sequentially performing a stretching treatment and a heat setting treatment on the base membrane precursor, wherein the temperature of the first stretching treatment is 40°C to 130°C, the temperature of the second stretching treatment is 120°C to 170°C, and the product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment is 1.2 to 40.0, so that the base membrane precursor forms a base membrane to obtain a diaphragm.

[0067] In the above preparation method, a base film precursor is first prepared. The thickness of the base film precursor increases in the direction from the center of the base film precursor to the edge of the base film precursor. By co-designing the thickness distribution trend of the base film precursor with the temperature parameters of the subsequent stretching process, the base film precursor is then subjected to stretching and heat setting processes in sequence to form a base film, thereby obtaining the diaphragm of the present invention.

[0068] The method for preparing the diaphragm provided in this embodiment of the invention can control the shape of the base membrane, and the process is simple and easy to implement.

[0069] In some embodiments, the direction from the center of the basement membrane precursor to the edge of the basement membrane precursor is parallel to the width direction of the basement membrane precursor.

[0070] Correspondingly, the thickness of the prepared base film varies along the direction from the center to the edge of the base film, satisfying the following trend: 1×10 -4 ≤β≤2×10 -2 , β=(H x -γ) / L x 2 .

[0071] In some specific embodiments, the thickness of the base film precursor increases in a gradient direction from the center of the base film precursor to its edge.

[0072] Specifically, the aforementioned gradient increasing trend can be a segmented increasing trend.

[0073] In some embodiments, the gradient increasing trend satisfies the following: the basement membrane precursor includes at least three partitions distributed sequentially along the direction from the center of the basement membrane precursor to the edge of the basement membrane precursor, wherein in any two adjacent partitions, the thickness of the partition closer to the edge of the basement membrane precursor is greater than the thickness of the partition closer to the center of the basement membrane precursor.

[0074] In practice, the width of each partition can be basically the same. The width direction of each partition is parallel to its distribution direction, specifically parallel to the width direction of the base film precursor.

[0075] Since the basement membrane precursor includes at least three partitions distributed sequentially along the direction from the center of the basement membrane precursor to its edge, the basement membrane precursor as a whole includes at least six partitions. The following is in conjunction with... Figure 1 This will be explained in detail.

[0076] and Figure 1 Correspondingly, the basement membrane precursor comprises N partitions distributed sequentially along the direction from the center of the basement membrane precursor to its edge, where N can be greater than or equal to 3. The basement membrane precursor as a whole comprises 2N partitions, and each partition satisfies the aforementioned variation trend.

[0077] and Figure 2 Correspondingly, in some embodiments, the basement membrane precursor includes three partitions sequentially distributed along the direction from the center to the edge of the basement membrane precursor, namely, a first partition, a second partition, and a third partition, and the direction from the center to the edge of the basement membrane precursor is parallel to the width direction of the basement membrane precursor; wherein, the width of the first partition, the second partition, and the third partition are all equal, and the width direction of the three partitions is parallel to the distribution direction of these partitions, specifically parallel to the width direction of the basement membrane precursor. The basement membrane precursor as a whole includes six partitions sequentially distributed along the width direction of the basement membrane precursor.

[0078] The membrane preparation methods of the embodiments of the present invention include, but are not limited to, dry methods and wet methods.

[0079] In practice, the above-mentioned molding process includes: melting and extruding the raw material used to form the base film, and casting it into a film to obtain the base film precursor.

[0080] In some specific embodiments, the process of melting and extruding the raw material used to form the base film includes: melting at least a portion of the raw material and extruding it through a first die, wherein the opening of the first die tends to increase along the direction from the preset center of the base film precursor to the preset edge of the base film precursor.

[0081] By controlling the opening of the first die head to increase along the direction from the preset center of the base film precursor to the preset edge of the base film precursor, the desired gradient thickness trend of the base film precursor is accurately and controllably achieved, and the process is stable and reliable.

[0082] In some embodiments, the first mold head includes at least three mold head regions sequentially distributed along a direction from a preset center of the base film precursor to a preset edge of the base film precursor, wherein in any two adjacent mold head regions, the opening of the mold head region closer to the preset edge of the base film precursor is greater than the opening of the mold head region closer to the preset center of the base film precursor.

[0083] By controlling the first mold head to meet the above-mentioned limitations, a base film precursor with at least three partitions distributed sequentially along the direction from the center of the base film precursor to the edge of the base film precursor can be formed.

[0084] Understandably, the overall structure of the first mold head includes at least three mold head regions symmetrically distributed along the preset center of the base film precursor, which means that the overall structure of the first mold head includes at least six mold head regions.

[0085] In some embodiments, the first mold head includes three mold head regions distributed sequentially along a predetermined direction from a predetermined center of the base film precursor to a predetermined edge of the base film precursor, namely, the first mold head region, the second mold head region, and the third mold head region.

[0086] It is understandable that the overall structure of the first mold head includes two sets of first mold head areas, second mold head areas, and third mold head areas symmetrically distributed along the preset center of the base film precursor, that is, the overall structure of the first mold head includes 6 mold head areas.

[0087] The width of different areas of the first mold head can be adjusted in a conventional manner in the embodiments of the present invention, and there are no special limitations on this.

[0088] In the specific implementation process, the melt extruded liquid flows onto the cooling roller to form a film. The formed film is pulled and moved by the guide roller. The direction in which the film is pulled and moved by the guide roller is basically perpendicular to the direction from the preset center of the base film precursor to the preset edge of the base film precursor. The first die head includes at least three die head areas distributed sequentially along the direction from the preset center of the base film precursor to the preset edge of the base film precursor. That is, the direction in which the film is pulled and moved by the guide roller is basically perpendicular to the distribution direction of these die head areas in the first die head.

[0089] In this embodiment of the invention, melt extrusion and casting are conventional operations in the art, and the extruder, cooling roller, guide roller, etc. used are conventional structures in the art, and no special limitations are imposed on them.

[0090] In some specific embodiments, the above-mentioned raw materials are melt-extruded by co-extrusion to obtain a base film precursor including a multilayer base film precursor; wherein, of the above-mentioned raw materials, the raw material used to form at least one of the multilayer base films is extruded through a first die.

[0091] In some embodiments, the multiple raw materials forming the multilayer sub-base film can be extruded through different dies of a composite die head, wherein the opening of at least one die head of the composite die head satisfies the increasing trend in the direction from the preset center of the base film precursor to the preset edge of the base film precursor, thereby obtaining a base film precursor including the multilayer sub-base film precursor by melt extrusion of the raw materials using a co-extrusion method.

[0092] Furthermore, the sum of the openings of the different dies in the composite die head can satisfy the increasing trend in the direction from the preset center of the base film precursor to the preset edge of the base film precursor, so that the above raw materials can be melt-extruded by co-extrusion to obtain a base film precursor including multiple sub-base film precursors.

[0093] Furthermore, the embodiments of the present invention do not impose specific numerical limits on the opening of each of the above-mentioned die head areas. Those skilled in the art know that, based on the specific equipment, raw materials and process conditions used, the appropriate die head opening can be determined through conventional adjustments under the guidance of quantitative relationships.

[0094] The embodiments of the present invention do not impose special limitations on the temperature and other conditions of melt extrusion, and can be adjusted conventionally according to the characteristics of the raw materials.

[0095] In specific implementation, by further limiting the product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment, the base film precursor can be better formed into the aforementioned base film after stretching treatment.

[0096] As is known in the art, the stretching ratio of a stretching treatment is the ratio of the length of the base film precursor after stretching treatment to its initial length before stretching treatment in the stretching direction. For example, the stretching ratio of a first stretching treatment is the ratio of the length of the base film precursor after the first stretching treatment to its initial length before the first stretching treatment, and the stretching ratio of a second stretching treatment is the ratio of the length of the base film precursor after the second stretching treatment to its initial length before the second stretching treatment.

[0097] In this embodiment of the invention, the product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment is equal to the stretching ratio of the stretching treatment.

[0098] Specifically, the product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment is 1.2 to 40.0. That is, in the stretching direction, the ratio of the length of the base film precursor after stretching treatment to its initial length before stretching treatment is 1.2 to 40.0.

[0099] For example, the product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment can be a range of 1.2, 1.3, 1.4, 1.5, 1.7, 1.9, 2.0, 2.5, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 10, 11, 12, 13, 14, 15, 20, 30, 40 or any two of these.

[0100] After the base film precursor is subjected to the above stretching treatment, a base film can be obtained. The thickness of the base film varies along the direction from the center to the edge of the base film, satisfying the following trend: 1×10 -4 ≤β≤2×10 -2 , β=(H x -γ) / L x 2 .

[0101] In one specific embodiment, the direction of the stretching process intersects with the direction from the center of the base film precursor to the edge of the base film precursor.

[0102] Furthermore, the direction of the stretching process is perpendicular to the direction from the center of the base membrane precursor to the edge of the base membrane precursor.

[0103] In practice, the stretching ratio of the first stretching treatment is greater than that of the second stretching treatment, which helps to obtain a base film with a thickness that conforms to the aforementioned trend.

[0104] Furthermore, the stretching ratio of the first stretching treatment is not particularly limited in the embodiments of the present invention. For example, it can be 1.02 to 8.0, preferably 1.2 to 4.5, and more preferably 1.2 to 1.6. The stretching ratio of the second stretching treatment is also not particularly limited in the embodiments of the present invention. For example, it can be 1.10 to 9.80, preferably 1.14 to 9.11.

[0105] In some embodiments, the direction of the stretching process is perpendicular to the width direction of the base film precursor, that is, parallel to the length direction of the base film precursor.

[0106] The above-mentioned stretching treatment can be a longitudinal stretching treatment, which includes performing a first stretching treatment and a second stretching treatment in sequence. The temperature of the first stretching treatment is 40℃~130℃, the temperature of the second stretching treatment is 120℃~170℃, and the product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment is 1.2~40.

[0107] The heat setting temperature is 100℃~160℃, and the time is 10h~24h.

[0108] In this embodiment of the invention, the width direction of the base film is perpendicular to the length direction or thickness direction of the base film, and the length direction of the base film is the longitudinal direction.

[0109] This invention also provides a secondary battery, comprising the above-described separator or a separator prepared according to the above-described method.

[0110] This separator significantly improves the performance of secondary batteries under high-rate cycling conditions by balancing current distribution, specifically manifested in higher capacity retention and stronger resistance to lithium plating. Batteries using this separator exhibit simultaneously optimized cycle life and fast-charging performance.

[0111] In addition, the aforementioned secondary batteries can be selected from lithium-ion power batteries and new energy storage batteries, which helps to meet the battery's requirements for high power and long cycle life, and have broad application prospects.

[0112] like Figure 3 As shown, the above-mentioned secondary battery also includes a positive electrode 3, a negative electrode 1, and a separator 2 located between the positive electrode 3 and the negative electrode 1.

[0113] In one specific embodiment, the positive electrode sheet is provided with a positive electrode tab 3a, and the positive electrode tab is disposed at a position corresponding to the edge of the positive electrode sheet and the edge of the separator in the direction from the center of the separator to the edge of the separator.

[0114] Based on the aforementioned variation trend in the thickness of the base film along the direction from the center to the edge of the base film, the ionic conductivity of the base film generally shows that the ionic conductivity in the central region of the base film is higher than that in the edge region. This effectively suppresses the current concentration effect at both ends of the positive electrode tab and effectively prevents surface lithium deposition during cycling. This significantly improves the long-term cycle stability and capacity retention of the battery, overcomes the problem of early capacity drop caused by uneven current distribution in traditional uniform electrode structures, and effectively improves the cycle life of the battery.

[0115] In some embodiments, the positive electrode tab is disposed on at least one side of the positive electrode sheet in the width direction. The above-described technical solution helps to precisely compensate for uneven current distribution caused by the location of the positive electrode tab by varying the thickness of the base film in the width direction. Specifically, in the edge region near the positive electrode tab, the base film is relatively thick to increase ion transport resistance in that region and slow down excessively high ion flow; in the central region away from the positive electrode tab, the base film is relatively thin to reduce ion transport resistance and promote ion flow. This gradient design effectively balances the current density in the width direction of the battery, thereby optimizing high-rate charge / discharge performance and suppressing localized lithium plating, thus improving the battery's cycle life.

[0116] In one specific implementation, such as Figure 3 As shown, the negative electrode sheet is provided with a negative electrode tab 1a. In the direction from the center of the diaphragm to the edge of the diaphragm, the negative electrode tab is provided at a position corresponding to the edge of the negative electrode sheet and the diaphragm.

[0117] Based on the aforementioned variation trend in the thickness of the base film along the direction from the center to the edge of the base film, the ionic conductivity of the base film generally shows that the ionic conductivity in the central region of the base film is higher than that in the edge region. This effectively suppresses the current concentration effect at both ends of the negative electrode tab and effectively prevents surface lithium deposition during cycling. This significantly improves the long-term cycle stability and capacity retention of the battery, overcomes the problem of early capacity drop caused by uneven current distribution in traditional uniform electrode structures, and effectively improves the cycle life of the battery.

[0118] In some embodiments, the negative electrode tab is disposed on at least one side of the negative electrode sheet in the width direction. The above-described technical solution helps to precisely compensate for uneven current distribution caused by the location of the negative electrode tab by varying the thickness of the base film in the width direction. Specifically, in the edge region near the negative electrode tab, the base film is relatively thick to increase ion transport resistance in that region and slow down excessively high ion flow; in the central region away from the negative electrode tab, the base film is relatively thin to reduce ion transport resistance and promote ion flow. This gradient design effectively balances the current density in the width direction of the battery, thereby optimizing high-rate charge / discharge performance and suppressing localized lithium plating, thus improving the battery's cycle life.

[0119] This invention also provides an electrical device, including the aforementioned secondary battery, which serves as the power supply for the electrical device.

[0120] The aforementioned electrical equipment covers a wide range of categories, including not only consumer electronics such as smartphones, laptops, tablets, smartwatches, wireless headphones, digital cameras, and drones, but also vehicles such as electric vehicles, electric bicycles, and electric scooters, as well as home energy storage systems, uninterruptible power supplies, power tools, garden machinery, medical equipment, and various security monitoring equipment. It is applicable to all mobile or fixed electrical devices that rely on stable, efficient, and rechargeable power sources.

[0121] The present invention will be further described below through specific embodiments and comparative examples. Unless otherwise specified, the reagents, materials and instruments used below are all conventional reagents, materials and instruments, all of which are commercially available, and the reagents and materials involved can also be synthesized by conventional synthetic methods.

[0122] Example 1

[0123] 1) Preparation of the diaphragm, including;

[0124] Polypropylene (weight-average molecular weight of 250,000) was used as the base film material;

[0125] The above-mentioned base film raw material is fed into an extruder and melted at 240°C. The molten base film raw material is then extruded and cast through a first die, wherein the extrusion temperature is 240°C and the temperature of the first die is 240°C.

[0126] The first mold head is divided into a first mold head area, a second mold head area, and a third mold head area along the length of its opening. The opening width (h1) of the first mold head area is 1.20 mm, the opening width (h2) of the second mold head area is 1.25 mm, and the opening width (h3) of the third mold head area is 1.30 mm. Figure 2As shown, the base film precursor formed by extrusion using the first die has a gradient thickness distribution characteristic. Specifically, the base film precursor has a first partition corresponding to the first die area of ​​the first die, a second partition corresponding to the second die area of ​​the first die, and a third partition corresponding to the third die area of ​​the first die. The first, second, and third partitions are distributed sequentially along the width direction of the base film precursor. The width of the first partition is d1 = 15 cm, the width of the second partition is d2 = 15 cm, and the width of the third partition is d3 = 15 cm. The length direction of the opening of the first die is parallel to the width direction of the base film precursor formed by extrusion using the first die.

[0127] The base film precursor with a gradient thickness distribution of thick at both ends and thin in the middle is subjected to stretching treatment. The stretching treatment includes a first stretching treatment and a second stretching treatment in sequence. The temperature of the first stretching treatment is 85°C and the stretching ratio of the first stretching treatment is 1.20. The temperature of the second stretching treatment is 145°C and the stretching ratio of the second stretching treatment is 1.83. The product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment is 2.2. The stretching direction of the stretching treatment is parallel to the length direction of the base film precursor. Then, heat setting treatment is performed at 150°C for 5 minutes to form the base film precursor into a base film, thereby obtaining the diaphragm of this embodiment.

[0128] A schematic diagram of the base membrane is shown below. Figure 2 As shown, its thickness distribution trend is symmetrical about the centerline position in the width direction of the base film.

[0129] 2) Preparation of negative electrode sheet

[0130] Graphite, conductive carbon black, sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed at a mass ratio of 100:1:1.6:3.3 at 25℃. Deionized water and N-methylpyrrolidone (NMP) solvent were added to the mixed dry powder, and the mixture was stirred thoroughly to prepare a uniform negative electrode slurry. Subsequently, the negative electrode slurry was continuously coated onto the surface of a copper foil current collector. After coating, the mixture was dried and rolled to obtain a compaction density of 1.6 g / cm³. 3 The negative electrode sheet is then coated. After the negative electrode sheet cools, the coating, drying, and rolling operations are repeated on the uncoated side to produce a double-sided negative electrode sheet. The compaction density of the negative electrode sheet is obtained through the following process: the average thickness of the negative electrode sheet is measured and calculated using a micrometer, and the compaction density of the negative electrode sheet is obtained by calculating the ratio of the areal density to the thickness, in g / cm³. 3 (The above method is applicable to the testing of the compaction density of the electrode sheets in the embodiments of the present invention.)

[0131] The prepared double-sided negative electrode sheet was cut to obtain a surface density of 230 g / m². 2The negative electrode sheet is for later use.

[0132] 3) Preparation of positive electrode sheet

[0133] Lithium iron phosphate, carbon nanotubes (CNTs), conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 100:0.3:0.5:2.5. N-methylpyrrolidone (NMP) solvent was added to the mixture, and the mixture was stirred thoroughly to prepare a uniform positive electrode slurry. This slurry was then coated onto an aluminum foil current collector. After drying, rolling, and cutting, a negative electrode sheet with a surface density of 500 g / m² was produced, matching the dimensions of the negative electrode sheet described above. 2 The positive electrode sheet.

[0134] 4) Preparation of electrolyte

[0135] Lithium salt LiPF6 was dissolved in an organic solvent consisting of ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 volume ratio to prepare an electrolyte with a concentration of 1 mol / L for lithium salt LiPF6.

[0136] 5) Battery assembly

[0137] The positive electrode, separator, and negative electrode prepared above are stacked alternately in sequence to form a battery cell. The battery cell is placed in an aluminum shell, and a sufficient amount of the above electrolyte is injected. After encapsulation, settling, formation, and aging processes, a lithium-ion battery with a designed capacity of 180 Ah is assembled.

[0138] Example 2

[0139] This embodiment is basically the same as Embodiment 1, except that:

[0140] The opening width (h1) of the first mold head area is 1.22 mm, the opening width (h2) of the second mold head area is 1.32 mm, and the opening width (h3) of the third mold head area is 1.42 mm; other conditions remain unchanged.

[0141] Example 3

[0142] This embodiment is basically the same as Embodiment 1, except that:

[0143] The opening width (h1) of the first mold head area is 1.22 mm, the opening width (h2) of the second mold head area is 1.42 mm, and the opening width (h3) of the third mold head area is 1.62 mm; other conditions remain unchanged.

[0144] Example 4

[0145] This embodiment is basically the same as Embodiment 1, except that:

[0146] The opening width (h1) of the first mold head area is 1.25mm, the opening width (h2) of the second mold head area is 1.55mm, and the opening width (h3) of the third mold head area is 1.85mm; other conditions remain unchanged.

[0147] Example 5

[0148] This embodiment is basically the same as Embodiment 1, except that:

[0149] The opening width (h1) of the first mold head area is 1.35mm, the opening width (h2) of the second mold head area is 1.65mm, and the opening width (h3) of the third mold head area is 2.00mm; other conditions remain unchanged.

[0150] Example 6

[0151] This embodiment is basically the same as embodiment 2, except that:

[0152] The base film raw materials include a first raw material, a second raw material, and a third raw material. The first raw material includes polypropylene (weight-average molecular weight of 250,000), the second raw material includes polypropylene (weight-average molecular weight of 170,000), and the third raw material includes polypropylene (weight-average molecular weight of 250,000).

[0153] Molten first, second, and third raw materials are extruded and cast through the first, second, and third dies of a composite die head, respectively. The extrusion temperature of the first and third raw materials is 240℃, and the extrusion temperature of the second raw material is 220℃. The temperature of the first, second, and third dies is also 240℃. The composite die head is divided into three sections along its length: a first die head section, a second die head section, and a third die head section. The opening width (h1) of the first die head section is 1.22mm, the opening width (h2) of the second die head section is 1.32mm, and the opening width (h3) of the third die head section is 1.42mm. Figure 2 As shown, the base film precursor formed by extrusion using this composite die has a gradient thickness distribution characteristic. Specifically, the base film precursor has a first section corresponding to the first die section of the composite die, a second section corresponding to the second die section of the composite die, and a third section corresponding to the third die section of the composite die. The first, second, and third sections are distributed sequentially along the width direction of the base film precursor. The widths of the first, second, and third sections are d1 = 15 cm, d2 = 15 cm, and d3 = 15 cm. The length direction of the opening of the composite die is parallel to the width direction of the base film precursor formed by extrusion using this composite die; other conditions remain unchanged.

[0154] Example 7

[0155] This embodiment is basically the same as embodiment 2, except that:

[0156] On the base membrane prepared in Example 2, an adhesive layer with uneven thickness is coated. The adhesive layer is a slurry with a solid content of 40% prepared by mixing polyvinylidene fluoride (PVDF) and polyacrylonitrile with water in a mass ratio of 9:1. The slurry is then coated on the base membrane prepared in Example 1 by a scraping method, so that the overall thickness of the diaphragm in this example is uniform.

[0157] Example 8

[0158] This embodiment is basically the same as embodiment 2, except that:

[0159] On the base membrane prepared in Example 2, a uniformly thick adhesive layer is coated. The adhesive layer is a slurry with a solid content of 40% prepared by mixing polyvinylidene fluoride (PVDF) and polyacrylonitrile with water in a mass ratio of 9:1. The slurry is coated on the base membrane prepared in Example 1 by roller coating, so that the thickness of the diaphragm in this example shows a trend corresponding to the thickness trend of the base membrane in Example 1.

[0160] Comparative Example 1

[0161] This comparative example is basically the same as Example 2, except that:

[0162] The opening width (i.e., the width of the opening) of the first, second, and third mold head areas is 1.20 mm; other conditions remain unchanged.

[0163] Comparative Example 2

[0164] This comparative example is basically the same as Example 2, except that:

[0165] The opening width (h1) of the first mold head area is 1.62 mm, the opening width (h2) of the second mold head area is 1.42 mm, and the opening width (h3) of the third mold head area is 1.22 mm; other conditions remain unchanged.

[0166] Comparative Example 3

[0167] This comparative example is basically the same as Comparative Example 1, except that:

[0168] On the base membrane prepared in Comparative Example 1, an adhesive layer with uneven thickness was coated. The adhesive layer was a slurry with a solid content of 40% prepared by mixing polyvinylidene fluoride (PVDF) and polyacrylonitrile with water in a mass ratio of 9:1. The slurry was then sprayed onto the base membrane prepared in Comparative Example 1, forming an uneven adhesive layer on the surface, so that the thickness of the diaphragm tends to be thicker on both sides and thinner in the middle.

[0169] Comparative Example 4

[0170] This comparative example is basically the same as Example 2, except that:

[0171] The opening width (h1) of the first die head area is 1.20 mm, the opening width (h2) of the second die head area is 1.22 mm, and the opening width (h3) of the third die head area is 1.25 mm; the temperature of the first stretching treatment is 140℃, the stretching ratio of the first stretching treatment is 1.20, the temperature of the second stretching treatment is 175℃, the stretching ratio of the second stretching treatment is 3.75, and the product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment is 4.5; other conditions remain unchanged.

[0172] Comparative Example 5

[0173] This comparative example is basically the same as Example 2, except that:

[0174] The opening width (h1) of the first die head area is 1.80 mm, the opening width (h2) of the second die head area is 3.80 mm, and the opening width (h3) of the third die head area is 6.00 mm; the temperature of the first stretching treatment is 30℃, the stretching ratio of the second stretching treatment is 1.08, the temperature of the second stretching treatment is 110℃, the stretching ratio of the second stretching treatment is 1.20, and the product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment is 1.3; other conditions remain unchanged.

[0175] Comparative Example 6

[0176] This comparative example is basically the same as Example 2, except that:

[0177] The opening width (h1) of the first die head area is 1.20 mm, the opening width (h2) of the second die head area is 1.21 mm, and the opening width (h3) of the third die head area is 1.22 mm; the stretching ratio of the first stretching process is 1.02, the stretching ratio of the second stretching process is 1.08, and the product of the stretching ratio of the first stretching process and the stretching ratio of the second stretching process is 1.1; other conditions remain unchanged.

[0178] Table 1 summarizes some parameters of Examples 1 to 8 and Comparative Examples 1 to 6.

[0179] Table 1

[0180]

[0181] Test case

[0182] The following parameters of each embodiment and comparative example were tested.

[0183] 1) Thickness H of the base film x and width Lx , β.

[0184] Along the width direction of the base film, the location of the centerline in the width direction is defined as the center, and the average thickness of the base film at this center is measured as γ (unit: μm). Along the width direction of the base film, a point is taken between the center and the edge of the base film as position X, and the average thickness of the base film on the strip perpendicular to the width direction at position X is measured and denoted as H. x (Unit: μm); Simultaneously, the straight-line distance from measurement position X, parallel to the width direction, to the center of the base film is denoted as L. x (Unit: cm). According to β=(H x -γ) / L x 2 β is calculated. Where position X satisfies: the straight-line distance L from position X along the width direction to the center of the base film. x For L / 6, L / 3, L / 2, that is, through β1=(H L / 6 -γ) / L L / 6 2 , β2=(H L / 3 -γ) / L L / 3 2 ,β3=(H L / 2 -γ) / L L / 2 2 .

[0185] 2) Ionic conductivity of the membrane

[0186] The diaphragm, in its width direction, comprises three sub-regions along its center to edge, corresponding one-to-one with the first, second, and third sections of the base membrane precursor. First, within each of these three sub-regions, five equally spaced positions are selected along the longitudinal direction perpendicular to the diaphragm's width. A square sample of the specified size (5cm × 5cm) is punched out using a standard die in a single pass. Immediately after cutting, the thickness of each sample at its center and periphery is measured in a dry state, and the average value is taken as its thickness d. The sample is then thoroughly immersed in a standard electrolyte for at least one hour until completely wetted. Next, 1 to 5 layers of wetted diaphragms are sequentially assembled in a symmetrical electrode fixture, and the bulk resistance at different numbers of layers is measured using an electrochemical workstation. A linear fit is performed with the number of diaphragm layers as the abscissa and the measured bulk resistance as the ordinate. When the fit (R0) is... 2 When the slope is greater than 0.999, the slope of the fitted straight line is the average surface resistance per unit area of ​​the membrane. The final ionic conductivity σ is calculated using the formula σ = d / (slope × effective test area), where the test area should use the actual effective contact area during assembly. Along the direction from the center to the edge of the base membrane, the ionic conductivity of the three sub-regions are σ1, σ2, and σ3, respectively, as shown in Table 4.

[0187] 3) Battery's DC internal resistance at 50% SOC

[0188] The battery was adjusted to 50% SOC at 25°C, and then pulse-charged at 1.5C for 30 seconds. The voltage change (ΔV) during the pulse was recorded, and the DC internal resistance (DCIR) was calculated as ΔV / I, where ΔV is the voltage change during the pulse in V and I is the magnitude of the pulse current in A.

[0189] 4) Long cycle life of the battery

[0190] The test temperature was 25℃;

[0191] Charging stages: Step 1 (constant current charging): Charge the battery with a constant current of 0.5C until the voltage reaches the upper limit of 3.8V; Step 2 (constant voltage charging): Maintain the voltage at 3.8V and continue charging until the current decays to a set cutoff value (e.g., 0.05C or below). At this point, the battery can be considered fully charged (≈100% SOC).

[0192] Discharge phase: The battery is discharged at a constant current of 0.5C until the voltage drops to the cutoff voltage of 2.0V. The amount of charge remaining at the end of this process is the discharge capacity of this cycle.

[0193] Repeat: Repeat the above "charging phase" and "discharging phase" process continuously.

[0194] The test is stopped when the capacity released by the battery during discharge (i.e., the discharge capacity) drops to 80% of its initial discharge capacity. The corresponding number of cycles represents the long cycle life of the battery.

[0195] The test results are shown in Tables 2, 3, and 4.

[0196] Table 2

[0197]

[0198] Table 3

[0199]

[0200] Table 4

[0201]

[0202] Examples 1-8 and Comparative Examples 1-6 show that the thickness trend of the base film satisfies 1×10⁻⁶. -4 ≤β≤2×10 -2 Its DC internal resistance and cycling performance are improved.

[0203] Compared to Example 2, the diaphragm of Example 7 includes a non-uniform coating, which further improves the DC internal resistance and cycling performance of the diaphragm.

[0204] Compared to Example 2, the diaphragm of Example 8 includes a uniform coating, which further improves the DC internal resistance and cycling performance of the diaphragm.

[0205] Compared with Example 2, the thickness of the base film in Comparative Example 1 did not show a significant trend, and its DC internal resistance and cycle performance were significantly worse; while the diaphragm of Comparative Example 3 had a coating added to the base film of the comparative example, which slightly improved the DC internal resistance and cycle performance of Comparative Example 3, but it was still worse than Example 2.

[0206] Compared with Example 2, the thickness of the base film in Comparative Examples 2, 4 and 5 does not conform to the trend of base film thickness variation defined in this invention, and their DC internal resistance and cycle performance are significantly worse.

[0207] Compared with Example 2, the product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment in Comparative Example 6 is too small, resulting in the thickness trend of the base film not conforming to the limited range of the present invention, and its DC internal resistance and cycle performance are relatively poor. Similarly, compared with Example 13, the product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment in Comparative Example 7 is too large, resulting in the thickness trend of the base film not conforming to the limited range of the present invention, and its DC internal resistance and cycle performance are relatively poor.

[0208] Since different sized batteries use different sized separators, this invention further investigated the performance of separators with different widths through Examples 9, 10, 11, and 14, and Comparative Examples 8 to 10. Example 12 and Comparative Example 12 investigated the performance of separators with different thicknesses. All investigations found that the separator thickness trend satisfies 1×10⁻⁶. -4 ≤β≤2×10 -2 Its DC internal resistance and cycling performance were both improved. Furthermore, the performance of membranes made from different raw materials was studied through Examples 13, 7, and 11, and it was found that the membrane thickness trend met the requirement of 1×10⁻⁶. -4 ≤β≤2×10 -2 Its DC internal resistance and cycling performance have been improved.

[0209] Example 9

[0210] This embodiment is basically the same as embodiment 2, except that:

[0211] The width of the first section is d1=1cm, the width of the second section is d2=1cm, and the width of the third section is d3=1cm; the opening (i.e., the width of the opening) h1 of the first die head area is 1.20mm, the opening (i.e., the width of the opening) h2 of the second die head area is 1.21mm, and the opening (i.e., the width of the opening) h3 of the third die head area is 1.22mm; the temperature of the first stretching treatment is 40℃, the stretching ratio of the first stretching treatment is 1.05, the temperature of the second stretching treatment is 120℃, the stretching ratio of the second stretching treatment is 1.14, and the product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment is 1.2; other conditions remain unchanged.

[0212] Example 10

[0213] This embodiment is basically the same as embodiment 2, except that:

[0214] The width of the first section is d1=35cm, the width of the second section is d2=35cm, and the width of the third section is d3=35cm; the temperature of the first stretching treatment is 110℃, and the stretching ratio of the first stretching treatment is 1.20; the temperature of the second stretching treatment is 170℃, and the stretching ratio of the second stretching treatment is 2.92; the product of the stretching ratios of the first and second stretching treatments is 3.5; other conditions remain unchanged.

[0215] Example 11

[0216] This embodiment is basically the same as embodiment 2, except that:

[0217] The width of the first section is d1=2cm, the width of the second section is d2=2cm, and the width of the third section is d3=2cm; the opening (i.e., the width of the opening) h1 of the first die head area is 0.20mm, the opening (i.e., the width of the opening) h2 of the second die head area is 0.25mm, and the opening (i.e., the width of the opening) h3 of the third die head area is 0.30mm; the stretching ratio of the first stretching treatment is 1.20, the stretching ratio of the second stretching treatment is 1.25, and the product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment is 1.5; other conditions remain unchanged.

[0218] Example 12

[0219] This embodiment is basically the same as embodiment 2, except that:

[0220] The opening width (h1) of the first die head area is 3.00 mm, the opening width (h2) of the second die head area is 3.10 mm, and the opening width (h3) of the third die head area is 3.18 mm; the stretching ratio of the first stretching process is 1.40, the stretching ratio of the second stretching process is 4.00, and the product of the stretching ratio of the first stretching process and the stretching ratio of the second stretching process is 5.6; other conditions remain unchanged.

[0221] Example 13

[0222] This embodiment is basically the same as embodiment 2, except that:

[0223] Polyethylene (weight average molecular weight of 1.5 million) was used as the base film material; the opening width (h1) of the first die area was 1.22 mm, the opening width (h2) of the second die area was 1.27 mm, and the opening width (h3) of the third die area was 1.35 mm; the temperature of the first stretching treatment was 130℃, the stretching ratio of the first stretching treatment was 4.50, the temperature of the second stretching treatment was 170℃, the stretching ratio of the second stretching treatment was 8.89, and the product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment was 40; other conditions remained unchanged.

[0224] Example 14

[0225] This embodiment is basically the same as Embodiment 1, except that:

[0226] The opening width (h1) of the first mold head area is 1.45mm, the opening width (h2) of the second mold head area is 2.28mm, and the opening width (h3) of the third mold head area is 3.70mm.

[0227] The width of the first section is d1=167cm, the width of the second section is d2=166cm, and the width of the third section is d3=167cm.

[0228] The temperature of the first stretching treatment is 110℃, and the stretching ratio of the first stretching treatment is 1.60. The temperature of the second stretching treatment is 170℃, and the stretching ratio of the second stretching treatment is 6.25. The product of the stretching ratios of the first and second stretching treatments is 10. Other conditions remain unchanged.

[0229] Comparative Example 7

[0230] This comparative example is basically the same as Example 13, except that:

[0231] The opening width (h1) of the first die head area is 1.20 mm, the opening width (h2) of the second die head area is 1.21 mm, and the opening width (h3) of the third die head area is 1.22 mm; the stretching ratio of the second stretching process is 9.11, and the product of the stretching ratio of the first stretching process and the stretching ratio of the second stretching process is 41.0; other conditions remain unchanged.

[0232] Comparative Example 8

[0233] This comparative example is basically the same as Example 9, except that:

[0234] The opening width (h1) of the first mold head area is 1.20 mm, the opening width (h2) of the second mold head area is 1.20 mm, and the opening width (h3) of the third mold head area is 1.20 mm; other conditions remain unchanged.

[0235] Comparative Example 9

[0236] This comparative example is basically the same as Example 10, except that:

[0237] The opening width (h1) of the first mold head area is 1.20 mm, the opening width (h2) of the second mold head area is 1.20 mm, and the opening width (h3) of the third mold head area is 1.20 mm; other conditions remain unchanged.

[0238] Comparative Example 10

[0239] This comparative example is basically the same as Example 11, except that:

[0240] The opening width (h1) of the first mold head area is 1.20 mm, the opening width (h2) of the second mold head area is 1.20 mm, and the opening width (h3) of the third mold head area is 1.20 mm; other conditions remain unchanged.

[0241] Comparative Example 11

[0242] This comparative example is basically the same as Example 13, except that:

[0243] The opening width (h1) of the first mold head area is 1.20 mm, the opening width (h2) of the second mold head area is 1.20 mm, and the opening width (h3) of the third mold head area is 1.20 mm; other conditions remain unchanged.

[0244] Comparative Example 12

[0245] This comparative example is basically the same as Example 12, except that:

[0246] The opening width (h1) of the first mold head area is 3.00 mm, the opening width (h2) of the second mold head area is 3.00 mm, and the opening width (h3) of the third mold head area is 3.00 mm; other conditions remain unchanged.

[0247] The parameters of some examples of Examples 9 to 14 and Comparative Examples 7 to 12 are summarized in Table 5.

[0248] Table 5

[0249]

[0250] The test results of Examples 9 to 14 and Comparative Examples 7 to 12 are explained below.

[0251] 1. Test results of Example 9 and Comparative Example 8, Example 10 and Comparative Example 9, Example 11 and Comparative Example 10, and Example 14 are shown in Tables 6, 7, and 8.

[0252] Table 6

[0253]

[0254] Table 7

[0255]

[0256] Table 8

[0257]

[0258] Compared with Example 9, the thickness of the base film in Comparative Example 8 did not show a significant trend, and its DC internal resistance and cycle performance were significantly worse.

[0259] Compared with Example 10, the thickness of the base film in Comparative Example 9 did not show a significant trend, and its DC internal resistance and cycle performance were significantly worse.

[0260] Compared with Example 11, the thickness of the base film in Comparative Example 10 did not show a significant trend, and its DC internal resistance and cycle performance were significantly worse.

[0261] Compared with Example 13, the thickness of the base film in Comparative Example 11 did not show a significant trend, and its DC internal resistance and cycle performance were significantly worse.

[0262] Compared with Example 2, the base film of Examples 9 and 11 has a relatively small width, and the corresponding single cell area is smaller. Therefore, its DC internal resistance is relatively large and its cycle performance is relatively poor. On the other hand, the base film of Examples 10 and 14 has a relatively large width, and the corresponding single cell area is larger. Therefore, its DC internal resistance is relatively small and its cycle performance is relatively good.

[0263] 2. Test results of Example 12 and Comparative Example 12 are shown in Tables 9, 10 and 11.

[0264] Table 9

[0265]

[0266] Table 10

[0267]

[0268] Table 11

[0269]

[0270] Compared with Example 12, the thickness of the base film in Comparative Example 12 did not show a significant trend, and its DC internal resistance and cycle performance were significantly worse.

[0271] 3. Test results of Example 13, Comparative Example 7, and Comparative Example 11 are shown in Tables 12, 13, and 14.

[0272] Table 12

[0273]

[0274] Table 13

[0275]

[0276] Table 14

[0277]

[0278] In summary, the base membrane of the present invention satisfies the requirement that the ionic conductivity decreases along the direction from the center to the edge of the base membrane, and that the DC internal resistance of the diaphragm is reduced and the cycle performance is improved, effectively solving the problem of uniform current distribution.

[0279] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A secondary battery, characterized in that, It includes a separator, a positive electrode plate, and a negative electrode plate, wherein the separator is located between the positive electrode plate and the negative electrode plate; The diaphragm includes a base membrane, the thickness of which varies along the direction from the center to the edge of the base membrane, satisfying the following trend: 1×10⁻⁶. -4 ≤β≤2×10 -2 , β=(H x -γ) / L x 2 ; Among them, H x The thickness of the base film at any location X between the center and the edge of the base film, expressed in μm, L x The distance from position X to the center of the base film is expressed in cm, and γ is the thickness of the base film at its center in μm. The position X includes the L / 6 position, the L / 3 position, and the L / 2 position, where L is the total length of the base film in the direction from the center of the base film to the edge of the base film; The positive electrode plate is provided with a positive electrode tab, and the positive electrode tab is disposed at a position corresponding to the edge of the positive electrode plate and the edge of the separator along the direction from the center of the separator to the edge of the separator; and / or, The negative electrode sheet is provided with a negative electrode tab, which is disposed at a position corresponding to the edge of the negative electrode sheet in the direction from the center of the diaphragm to the edge of the diaphragm.

2. The secondary battery according to claim 1, characterized in that, 1μm≤γ≤50μm.

3. The secondary battery according to claim 2, characterized in that, 2μm≤γ≤30μm.

4. The secondary battery according to any one of claims 1-3, characterized in that, In the direction from the center of the base film to its edge, the distance between the center of the base film and its edge is 3cm to 500cm.

5. The secondary battery according to claim 4, characterized in that, The distance between the center of the base film and the edge of the base film is 3cm to 105cm.

6. The secondary battery according to any one of claims 1-3 and 5, characterized in that, In the width direction of the base film, the thickness of the base film satisfies the aforementioned variation trend.

7. The secondary battery according to any one of claims 1-3 and 5, characterized in that, The diaphragm is a single base membrane or may include multiple sub-base membranes.

8. The secondary battery according to any one of claims 1-3 and 5, characterized in that, The base film comprises a polymer, which includes one or more of polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, polyurethane, polymethylpentene, polyethylene terephthalate, polycarbonate, polyester, polyimide, polyvinyl alcohol, polyacrylonitrile, polyoxymethylene, polymethyl methacrylate, polyoxyethylene, and cellulose.

9. The secondary battery according to any one of claims 1-3 and 5, characterized in that, The diaphragm also includes a coating disposed on at least one surface of the base membrane.

10. The secondary battery according to claim 9, characterized in that, The coating has a uniform thickness; or the coating has an uneven thickness.

11. The secondary battery according to any one of claims 1-3, 5, and 10, characterized in that, The method for preparing the diaphragm includes: The raw materials used to form the base film are processed to obtain a base film precursor; wherein the thickness of the base film precursor tends to increase in the direction from the center of the base film precursor to the edge of the base film precursor; The base film precursor is subjected to stretching and heat setting treatment in sequence. The stretching treatment includes a first stretching treatment and a second stretching treatment in sequence. The temperature of the first stretching treatment is 40℃~130℃, the temperature of the second stretching treatment is 120℃~170℃, and the product of the stretching ratio of the first stretching treatment and the stretching ratio of the second stretching treatment is 1.2~40.0, so that the base film precursor forms the base film and the diaphragm is obtained.

12. The secondary battery according to claim 11, characterized in that, The thickness of the base film precursor increases in a gradient direction from the center of the base film precursor to the edge of the base film precursor; And / or, the direction from the center of the base film precursor to the edge of the base film precursor is parallel to the width direction of the base film precursor.

13. The secondary battery according to claim 12, characterized in that, The gradient increasing trend satisfies the following: the base film precursor includes at least three partitions distributed sequentially along the direction from the center of the base film precursor to the edge of the base film precursor, and in any two adjacent partitions, the thickness of the partition closer to the edge of the base film precursor is greater than the thickness of the partition closer to the center of the base film precursor.

14. The secondary battery according to any one of claims 12-13, characterized in that, The molding process includes: melting and extruding the raw material used to form the base film, and casting it into a film to obtain the base film precursor.

15. The secondary battery according to claim 14, characterized in that, The process of melting and extruding the raw material used to form the base film includes: melting at least a portion of the raw material and extruding it through a first die, wherein the opening of the first die increases along the direction from the preset center of the base film precursor to the preset edge of the base film precursor.

16. The secondary battery according to claim 15, characterized in that, The first mold head includes at least three mold head areas that are sequentially distributed along a predetermined direction from the predetermined center of the base film precursor to the predetermined edge of the base film precursor. In any two adjacent mold head areas, the opening of the mold head area closer to the predetermined edge of the base film precursor is greater than the opening of the mold head area closer to the predetermined center of the base film precursor.

17. The secondary battery according to claim 15 or 16, characterized in that, The raw materials are melt-extruded using a co-extrusion method to obtain the base film precursor comprising a multilayer sub-base film precursor; wherein, of the raw materials, the raw material used to form at least one of the multilayer sub-base films is extruded through the first die.

18. The secondary battery according to any one of claims 12-13 and 15-16, characterized in that, The direction of the stretching process intersects with the direction from the center of the base film precursor to the edge of the base film precursor; And / or, the stretching ratio of the first stretching treatment is less than the stretching ratio of the second stretching treatment.

19. The secondary battery according to any one of claims 1-3, 5, and 10, characterized in that, The positive electrode tab is disposed on at least one side of the positive electrode sheet in the width direction.

20. The secondary battery according to any one of claims 1-3, 5, and 10, characterized in that, The negative electrode tab is disposed on at least one side of the negative electrode sheet in the width direction.

21. An electrical appliance, characterized in that, The device includes a secondary battery as described in any one of claims 1-20, wherein the secondary battery serves as the power supply for the electrical device.

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