Diaphragm for lithium battery and lithium battery applying diaphragm

Abstract

The invention provides a diaphragm for a lithium battery and the lithium battery applying the diaphragm. The lithium battery comprises a winding battery cell, wherein the winding battery cell comprises a positive plate and a diaphragm; the area, exceeding the width direction of the positive plate, of the diaphragm base material is used as a remaining area, and other areas are used as normal coating areas; the remaining area and the normal coating area are provided with a first coating layer and a second coating layer respectively; the thickness a of the first coating layer and the thickness b of the second coating layer satisfy 1.1 < = a / b < = 2. Through differential coating design, the thickness of the coating layer in the margin area of the diaphragm base material is increased in a targeted manner, and the thickness a of the coating layer in the margin area and the thickness b of the coating layer in the normal coating area meet the condition that a / b is greater than or equal to 1.1 and less than or equal to 2, so that the thickness of the diaphragm base material can be increased on the premise of not increasing the diameter or thickness of a winding battery cell and not reducing the cycling stability of the lithium battery. The safety risk of positive and negative electrode contact short circuit caused by easy shrinkage of the diaphragm when the lithium battery is short-circuited is reduced, and the safety performance of the lithium battery is improved.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and more specifically to a separator for lithium batteries and a lithium battery using the same. Background Technology

[0002] Lithium-ion batteries have advantages such as high energy density, long cycle life, and low environmental pollution, and have been widely used in consumer electronics, new energy vehicles, energy storage, and other fields. The separator, as one of the core components of a lithium-ion battery, mainly functions to isolate the positive and negative electrodes, prevent short circuits, and allow lithium ions to pass through. Its performance directly affects the safety and cycle stability of the lithium-ion battery.

[0003] Existing lithium-ion battery separators typically improve their thermal stability and mechanical properties by uniformly coating a substrate with materials such as adhesives, ceramics, or aramid fibers. However, during the assembly of wound lithium-ion battery cells, the separator width needs to be slightly larger than the positive electrode width to create a margin area to ensure complete isolation between the positive and negative electrodes. In traditional uniform coating processes, the margin area often maintains the same coating thickness as the main separator area, which can easily lead to insufficient thermal stability in this margin area. When a short circuit or other abnormal situation occurs in a lithium-ion battery, the temperature rises sharply, and the separator in the margin area is prone to shrinkage, leading to direct contact between the positive and negative electrodes and potentially causing safety accidents such as fires and explosions. Summary of the Invention

[0004] To address the problems of insufficient thermal stability in the margin area of ​​existing lithium battery separators and their tendency to shrink under abnormal operating conditions such as short circuits, leading to direct contact between the positive and negative electrodes and causing safety accidents such as fires and explosions, this invention provides a separator for lithium batteries and a lithium battery using the same.

[0005] According to a first aspect of the present invention, a lithium battery is provided, the lithium battery including a wound cell, the wound cell including a positive electrode sheet and a separator; the separator including a substrate, the area of ​​the substrate extending beyond the width direction of the positive electrode sheet is defined as a margin area, and other areas are defined as normal coating areas; the margin area is provided with a first coating layer, and the normal coating area is provided with a second coating layer; the thickness of the first coating layer is a, and the thickness of the second coating layer is b, a and b satisfy 1.1≤a / b≤2.

[0006] The existing lithium battery separator has insufficient thermal stability in the margin area, and it is prone to shrinkage under abnormal operating conditions such as short circuits, which can lead to direct contact between the positive and negative electrodes and cause safety accidents such as fire and explosion. While overall thickening of the coating can improve thermal stability, it will increase the diameter or thickness of the lithium battery winding cell, reduce energy density and assembly compatibility, which is not in line with the development trend of lithium battery miniaturization and high energy density.

[0007] In the lithium battery provided by this invention, through differentiated coating design, the coating thickness in the excess area of ​​the separator substrate is specifically increased, and the thickness 'a' of the first coating layer formed on the surface of the excess area and the thickness 'b' of the second coating layer formed on the surface of the normal coating area satisfy the relationship 1.1 ≤ a / b ≤ 2. This significantly improves the thermal stability of the excess area of ​​the separator without increasing the diameter or thickness of the lithium battery winding cell or reducing the cycle stability of the lithium battery. It effectively reduces the safety risk of short circuits caused by separator shrinkage during lithium battery short circuits, thereby improving the safety performance of the lithium battery. Furthermore, the lithium battery provided by this invention significantly improves safety while maintaining the original energy density and assembly compatibility, meeting the high safety and high reliability requirements of lithium batteries in consumer electronics, new energy vehicles, energy storage, and other fields.

[0008] When applying slurry to the excess area and normal coating area of ​​the separator substrate to form a coating layer, single-sided or double-sided coating can be flexibly selected according to the performance requirements of the lithium battery. Double-sided coating can further improve the overall stability of the separator.

[0009] Preferably, a and b satisfy 1.4≤a / b≤1.8.

[0010] By adjusting the thickness 'a' of the first coating layer formed on the surface of the margin region and the thickness 'b' of the second coating layer formed on the surface of the normal coating region to satisfy 1.4 ≤ a / b ≤ 1.8, the thermal stability of the margin region of the separator can be further improved, thereby further enhancing the safety performance of the lithium battery.

[0011] Preferably, a = 1.1 to 10 μm.

[0012] By controlling the thickness 'a' of the first coating layer formed on the surface of the allowance region within the aforementioned range, thermal shrinkage of the allowance region can be effectively suppressed, and the energy density of the lithium battery will not be reduced due to an increase in the diameter or thickness of the wound cell caused by excessive thickness.

[0013] Preferably, b = 1 to 5 μm.

[0014] By controlling the thickness b of the second coating layer formed on the surface of the normally coated area within the above-mentioned range, both the ion conduction performance of the diaphragm and the basic thermal stability of the diaphragm can be guaranteed.

[0015] Preferably, the substrate material is selected from at least one of polyethylene (PE), polypropylene (PP), and polyolefin (POE).

[0016] Preferably, the thickness of the substrate is 10~15 μm.

[0017] By using a diaphragm substrate containing the above-mentioned materials and controlling the thickness of the substrate within the above-mentioned range, the diaphragm substrate can have good ion permeability and mechanical strength, and the diaphragm substrate has good compatibility with subsequent coating processes, thereby improving the adhesion of the coating layer on the substrate surface.

[0018] Preferably, the materials of the first coating layer and the second coating layer are independently selected from at least one of adhesive materials, ceramic materials, and aramid fibers.

[0019] Preferably, the adhesive material is selected from at least one of polyvinylidene fluoride, polyacrylate, and styrene-butadiene rubber.

[0020] Preferably, the ceramic material is selected from at least one of alumina, zirconium oxide, and silicon dioxide.

[0021] Using the aforementioned adhesive materials in the first and second coating layers can improve the compatibility between the first and second coating layers and the substrate, thereby improving the adhesion of the coating layers to the substrate surface. Using the aforementioned ceramic materials in the first and second coating layers can endow the coating layers with excellent high-temperature resistance, thereby significantly improving the thermal stability of the diaphragm. Using aramid materials in the first and second coating layers can enable the coating layers to have both high strength and high-temperature resistance, which can further optimize the mechanical properties and thermal stability of the diaphragm.

[0022] Preferably, the total width of the allowance area is 2 to 8 mm.

[0023] Preferably, the allowance area includes a first area and a second area, which are symmetrically arranged along the central axis of the positive electrode sheet along its length; the width of the first area and the second area is 1~4 mm.

[0024] By controlling the size of the allowance area within the above range, we can ensure a good isolation effect while avoiding material waste caused by an excessively large allowance area.

[0025] Preferably, the diaphragm is prepared by the following steps: coating a first slurry in the normal coating area to form a first coating layer, and coating a second slurry in the remaining area to form a second coating layer; the coating speed of the first slurry and the second slurry is 5 to 30 m / min.

[0026] During the preparation of the separator, the coating speed of the normal coating area and the reserve area can be controlled by the partition flow adjustment module of the coating head, so as to accurately match the thickness requirements of the normal coating area and the reserve area, ensure the uniformity and consistency of the coating layer, and avoid performance fluctuations of the separator and lithium battery caused by thickness deviation.

[0027] Furthermore, the above-mentioned diaphragm can be prepared using an intermittent, step-by-step coating process, which is simple and feasible. It can be achieved by modifying existing diaphragm coating production lines without the need for large-scale new equipment, resulting in lower production costs and easy industrialization.

[0028] Preferably, the coating speed of both the first slurry and the second slurry is 15-20 m / min. Attached Figure Description

[0029] Figure 1 A schematic diagram of the planar structure of the separator in the lithium battery provided in Example 1. Figure 2 This is a schematic diagram of the cross-sectional structure of the separator in the lithium battery provided in Example 1.

[0030] Figure reference numerals: 1. Substrate, 2. Normal coating area, 3. Remaining area. Detailed Implementation

[0031] The technical features of the technical solution provided by the present invention will be further clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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] Example 1 A lithium battery is prepared by the following steps: (1) Preparation of positive electrode LiNi, the positive electrode active material 0.6 Co 0.2 Mn 0.2 O2, conductive carbon black (Super P), carbon nanotubes, and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 96:1.8:0.5:1.7 and then added to N-methylpyrrolidone (NMP) to prepare a positive electrode slurry with a solid content of 70%. The above positive electrode slurry is coated on both surfaces of the positive electrode current collector aluminum foil to form a positive electrode active material layer. After drying, a positive electrode sheet is obtained. (2) Preparation of negative electrode Graphite, conductive carbon black (Super P), carbon nanotubes, and carboxymethyl cellulose were mixed in a mass ratio of 96:1.0:0.5:2.5 and added to deionized water to prepare a negative electrode slurry with a solid content of 70%. The negative electrode slurry was coated on both surfaces of the copper foil of the negative electrode current collector to form a negative electrode active material layer. After drying, a negative electrode sheet was obtained. (3) Preparation of the diaphragm A 12 μm thick polyethylene (PE) film is used as substrate 1. Substrate 1 includes a margin region 3 (the region extending beyond the width of the positive electrode) and a normal coating region 2 corresponding to and covering the positive electrode. A first region and a second region with a margin width of 2 mm are respectively set above and below the margin region 3 of substrate 1 (the total width of the margin region is 4 mm). An intermittent coating process and a zoned controlled-volume coating process are used to coat a water-based ceramic coating liquid containing alumina with a particle size of 1 μm onto substrate 1 on one side at a coating speed of 15 m / min. After drying, a second coating layer with a thickness of 3 μm is formed on the surface of the normal coating region 2 of substrate 1, and a first coating layer with a thickness of 4.5 μm (1.5 times the thickness of the second coating layer formed on the surface of the normal coating region 2) is formed on the surfaces of the first and second regions of margin region 3. A separator is thus obtained, and its planar structure is shown in the figure below. Figure 1 As shown in the schematic diagram of the cross-sectional structure. Figure 2 As shown; exist Figure 1 , Figure 2 In the illustrated diaphragm structure, substrate 1 is a long strip-shaped carrier, the middle region is the normally coated region 2 corresponding to the electrode, and the two side regions along the width direction are the allowance regions 3. Figure 1 In order to better show the normal coating area 2 and the excess area 3, the surface of the substrate 1 is not completely covered by the coating layer, but in the actual preparation process, the entire surface of the substrate 1 is covered by the coating layer. (4) Preparation of electrolyte Propylene carbonate (PC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) were mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Lithium salt LiPF6 was added to the organic solvent to prepare an electrolyte with a LiPF6 concentration of 1 mol / L. (5) Assembly of lithium batteries The separator, positive electrode, and negative electrode are wound together in the order of "positive electrode - separator - negative electrode" to form a wound cell. The excess area of ​​the separator completely covers both sides of the positive electrode in the width direction to ensure isolation between the positive and negative electrodes. Electrolyte is injected into the wound cell and packaged into a 10 Ah soft-pack lithium battery. After standing for 12 hours to ensure that the electrolyte fully wets the separator and electrodes, the lithium battery of this embodiment is obtained.

[0033] Example 2 This embodiment provides a lithium battery, which differs from Embodiment 1 in that the separator used is different; The separator used in this embodiment is prepared through the following steps: A 10 μm thick polypropylene (PP) film is used as the substrate. The substrate includes a margin area (the area extending beyond the width of the positive electrode) and a normal coating area corresponding to the positive electrode. A first area and a second area with a margin width of 1.5 mm are respectively set above and below the margin area of ​​the substrate (the total width of the margin area is 3 mm). A coating liquid containing polyvinylidene fluoride (PVDF) is applied to the substrate on one side at a coating speed of 20 m / min using an intermittent coating process and a zoned controlled coating process. After drying, a second coating layer with a thickness of 2 μm is formed on the surface of the normal coating area of ​​the substrate, and a first coating layer with a thickness of 2.4 μm is formed on the surfaces of the first and second areas of the margin area (1.2 times the thickness of the second coating layer formed on the surface of the normal coating area), thus obtaining the separator.

[0034] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0035] Example 3 This embodiment provides a lithium battery, which differs from Embodiment 1 in that the separator used is different; The separator used in this embodiment is prepared through the following steps: A 15 μm thick polyolefin composite separator (PE / PP / PE) film is used as the substrate. The substrate includes a margin area (the area extending beyond the width of the positive electrode) and a normal coating area corresponding to the positive electrode. A first area and a second area with a margin width of 3 mm are respectively set above and below the margin area of ​​the substrate (the total width of the margin area is 6 mm). A composite coating liquid containing 5 μm long para-aramid short-cut fibers and 1 μm particle size alumina ceramics is coated onto the substrate on one side at a coating speed of 10 m / min using an intermittent coating process and a zoned controlled coating process. After drying, a second coating layer with a thickness of 5 μm is formed on the surface of the normal coating area of ​​the substrate, and a first coating layer with a thickness of 10 μm is formed on the surfaces of the first and second areas of the margin area (which is 2.0 times the thickness of the second coating layer formed on the surface of the normal coating area), thus obtaining the separator.

[0036] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0037] Example 4 This embodiment provides a lithium battery, which differs from Embodiment 1 in that the separator used is different; The separator used in this embodiment is prepared through the following steps: A 12 μm thick polyolefin composite separator (PE / PP / PE) film is used as the substrate. The substrate includes a margin area (the area extending beyond the width of the positive electrode) and a normal coating area corresponding to the positive electrode. A first area and a second area with a margin width of 2.5 mm are respectively set above and below the margin area of ​​the substrate (the total width of the margin area is 5 mm). A composite coating liquid containing alumina ceramic with a particle size of 1 μm and PVDF (mass ratio 7:3) is coated on both sides of the substrate at a coating speed of 25 m / min using an intermittent coating process and a zoned controlled coating process. After drying, a second coating layer with a thickness of 3 μm is formed on the surface of the normal coating area of ​​the substrate. A first coating layer with a thickness of 4.2 μm is formed on the surfaces of the first and second areas of the margin area (1.4 times the thickness of the second coating layer formed on the surface of the normal coating area), thus obtaining the separator.

[0038] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0039] Example 5 This embodiment provides a lithium battery, which differs from Embodiment 1 in that the separator used is different; The separator used in this embodiment is prepared through the following steps: a 10 μm thick polypropylene (PP) film is used as the substrate. The substrate includes a margin area (the area extending beyond the width of the positive electrode) and a normal coating area corresponding to the positive electrode. A first area and a second area with a margin width of 1 mm are respectively set above and below the margin area of ​​the substrate (the total width of the margin area is 2 mm). Styrene-butadiene rubber (SBR) adhesive is applied to the substrate on one side at a coating speed of 30 m / min using an intermittent coating process and a zoned controlled coating process. After drying, a second coating layer with a thickness of 1 μm is formed on the surface of the normal coating area of ​​the substrate, and a first coating layer with a thickness of 1.1 μm is formed on the surfaces of the first and second areas of the margin area (1.1 times the thickness of the second coating layer formed on the surface of the normal coating area), thus obtaining the separator.

[0040] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0041] Example 6 This embodiment provides a lithium battery. Compared with embodiment 1, the difference in the structure is: (1) In the preparation process of the separator, the thickness of the second coating layer formed on the surface of the normal coating area of ​​the substrate is 2 μm, and the thickness of the first coating layer formed on the surface of the first area and the second area respectively set above and below the remaining area of ​​the substrate is 3.5 μm (1.5 times the thickness of the second coating layer formed on the surface of the normal coating area); (2) In the preparation process of the separator, the coating speed of the aqueous ceramic coating liquid is 15 m / min.

[0042] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0043] Example 7 This embodiment provides a lithium battery. Compared with embodiment 1, the difference in the structure is: (1) In the preparation process of the separator, the thickness of the second coating layer formed on the surface of the normal coating area of ​​the substrate is 2 μm, and the thickness of the first coating layer formed on the surface of the first area and the second area respectively set above and below the remaining area of ​​the substrate is 3.2 μm (1.6 times the thickness of the second coating layer formed on the surface of the normal coating area); (2) In the preparation process of the separator, the coating speed of the aqueous ceramic coating liquid is 15 m / min.

[0044] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0045] Example 8 This embodiment provides a lithium battery. Compared with embodiment 1, the difference in the structure is: during the preparation of the separator, the thickness of the second coating layer formed on the surface of the normal coating area of ​​the substrate is 3 μm, and the thickness of the first coating layer formed on the surface of the first area and the second area respectively set above and below the residual area of ​​the substrate is 5.4 μm (1.8 times the thickness of the second coating layer formed on the surface of the normal coating area); (2) during the preparation of the separator, the coating speed of the aqueous ceramic coating liquid is 15 m / min.

[0046] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0047] Example 9 This embodiment provides a lithium battery. Compared with embodiment 1, the difference in the structure is: (1) In the preparation process of the separator, the thickness of the second coating layer formed on the surface of the normal coating area of ​​the substrate is 1 μm, and the thickness of the first coating layer formed on the surface of the first area and the second area respectively set above and below the remaining area of ​​the substrate is 1.9 μm (1.9 times the thickness of the second coating layer formed on the surface of the normal coating area); (2) In the preparation process of the separator, the coating speed of the aqueous ceramic coating liquid is 15 m / min.

[0048] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0049] Example 10 This embodiment provides a lithium battery. Compared with embodiment 1, the difference in the structure is: (1) In the preparation process of the separator, the thickness of the second coating layer formed on the surface of the normal coating area of ​​the substrate is 5.5 μm, and the thickness of the first coating layer formed on the surface of the first area and the second area respectively set above and below the excess area of ​​the substrate is 8.25 μm (1.5 times the thickness of the second coating layer formed on the surface of the normal coating area); (2) In the preparation process of the separator, the coating speed of the aqueous ceramic coating liquid is 15 m / min.

[0050] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0051] Example 11 This embodiment provides a lithium battery. Compared with embodiment 1, the difference in the structure is: (1) In the preparation process of the separator, the thickness of the second coating layer formed on the surface of the normal coating area of ​​the substrate is 0.8 μm, and the thickness of the first coating layer formed on the surface of the first area and the second area respectively set above and below the remaining area of ​​the substrate is 1.2 μm (1.5 times the thickness of the second coating layer formed on the surface of the normal coating area); (2) In the preparation process of the separator, the coating speed of the aqueous ceramic coating liquid is 15 m / min.

[0052] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0053] Example 12 This embodiment provides a lithium battery. Compared with embodiment 1, the difference in the structure is: (1) In the preparation process of the separator, the thickness of the second coating layer formed on the surface of the normal coating area of ​​the substrate is 0.8 μm, and the thickness of the first coating layer formed on the surface of the first area and the second area respectively set above and below the remaining area of ​​the substrate is 1 μm (1.25 times the thickness of the second coating layer formed on the surface of the normal coating area); (2) In the preparation process of the separator, the coating speed of the aqueous ceramic coating liquid is 15 m / min.

[0054] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0055] Example 13 This embodiment provides a lithium battery. Compared with Embodiment 1, the difference in configuration is that the coating speed of the aqueous ceramic coating liquid is 3 m / min during the preparation of the separator.

[0056] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0057] Example 14 This embodiment provides a lithium battery. Compared with Embodiment 1, the difference in configuration is that the coating speed of the aqueous ceramic coating liquid is 32 m / min during the preparation of the separator.

[0058] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0059] Example 15 This embodiment provides a lithium battery. Compared with Embodiment 1, the difference in configuration is that the coating speed of the aqueous ceramic coating liquid is 10 m / min during the preparation of the separator.

[0060] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0061] Example 16 This embodiment provides a lithium battery. Compared with Embodiment 1, the difference in configuration is that the coating speed of the aqueous ceramic coating liquid is 20 m / min during the preparation of the separator.

[0062] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0063] Example 17 This embodiment provides a lithium battery. Compared with Embodiment 1, the difference in configuration is that the coating speed of the aqueous ceramic coating liquid is 25 m / min during the preparation of the separator.

[0064] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0065] Comparative Example 1 This comparative example provides a lithium battery, which differs from Example 1 in that the thickness of the second coating layer formed on the surface of the normal coating area of ​​the substrate and the first coating layer formed on the surface of the first and second areas of the reserve area are both 3 μm.

[0066] Apart from the differences mentioned above, the materials, formulation ratios, and preparation procedures used in this comparative example are strictly consistent with those in Example 1.

[0067] Comparative Example 2 This comparative example provides a lithium battery, which differs from Example 1 in that the thickness of the second coating layer formed on the surface of the normal coating area of ​​the substrate and the first coating layer formed on the surface of the first and second areas in the remaining area are both 4.5 μm during the preparation of the separator.

[0068] Apart from the differences mentioned above, the materials, formulation ratios, and preparation procedures used in this comparative example are strictly consistent with those in Example 1.

[0069] Comparative Example 3 This comparative example provides a lithium battery. Compared with Example 1, the difference in configuration is that, during the preparation of the separator, the thickness of the second coating layer formed on the surface of the normal coating area of ​​the substrate is 3 μm, and the thickness of the first coating layer formed on the surface of the first and second areas in the remaining area is 3.15 μm (1.05 times the thickness of the second coating layer formed on the surface of the normal coating area).

[0070] Apart from the differences mentioned above, the materials, formulation ratios, and preparation procedures used in this comparative example are strictly consistent with those in Example 1.

[0071] Comparative Example 4 This comparative example provides a lithium battery. Compared with Example 1, the difference in configuration is that, during the preparation of the separator, the thickness of the second coating layer formed on the surface of the normal coating area of ​​the substrate is 3 μm, and the thickness of the first coating layer formed on the surface of the first and second areas in the remaining area is 7.5 μm (which is 2.5 times the thickness of the second coating layer formed on the surface of the normal coating area).

[0072] Apart from the differences mentioned above, the materials, formulation ratios, and preparation procedures used in this comparative example are strictly consistent with those in Example 1.

[0073] Test case 1. Participants This test example uses the separators and lithium batteries prepared in Examples 1-17 and Comparative Examples 1-4 as test objects to conduct relevant performance tests.

[0074] 2. Test Content (1) Heat shrinkage rate test The diaphragms prepared in Examples 1-17 and Comparative Examples 1-4 were cut into test samples with a size of 100 mm × 100 mm. The test samples were placed in an oven at 150°C and kept at that temperature for 1 h. After cooling to room temperature, the dimensional change in the width direction was measured, with a focus on collecting the remaining area. Each group of samples was tested 5 times, and the average value was taken to ensure the accuracy of the data. The thermal shrinkage rate of the diaphragm was calculated according to the following formula: Thermal shrinkage rate (%) = (Initial width - Width after heat preservation) / Initial width × 100%.

[0075] (2) Short-circuit safety test The needle penetration test was used. A steel needle with a diameter of 3 mm was used to pierce the center of the lithium batteries (fully charged, voltage 4.2 V) prepared in Examples 1-17 and Comparative Examples 1-4 at a speed of 10 mm / s. The occurrence of fire or explosion of the lithium batteries was observed and recorded. 100 batteries were tested in each group. The safety pass rate was calculated according to the following formula: Safety pass rate (%) = Number of lithium batteries that did not catch fire or explode / Total number of lithium batteries tested × 100%.

[0076] (3) Thickness test of lithium battery wound cells The thickness of the wound cells of the lithium batteries assembled in Examples 1-17 and Comparative Examples 1-4 was measured using a vernier caliper with an accuracy of 0.01 mm. Ten lithium batteries were tested in each group, and the average value was taken. The thickness difference of the wound cells was compared to verify whether there was a problem of increased thickness of the wound cells.

[0077] (4) Cyclic stability test Under a constant temperature environment of 25℃, the lithium batteries prepared in Examples 1-17 and Comparative Examples 1-4 were charged to 4.2 V at a charging rate of 1C (1.5A), and kept constant until the current ≤0.05A. Then, they were discharged to 3.0 V at a discharging rate of 1C (1.5A). This constituted one charge-discharge cycle. The lithium batteries were subjected to 1000 charge-discharge cycles, and the discharge capacity of the lithium batteries was recorded. Three lithium batteries were tested in each group, and the average value was taken. The capacity retention rate of the lithium batteries was calculated according to the following formula: Capacity retention rate (%) = Discharge capacity at 1000th cycle / Discharge capacity at first cycle × 100%; 3. Experimental Results Table 1. Relevant parameters involved in the preparation of the diaphragm.

[0078] Table 2. Performance test results of the separator and lithium battery

[0079] For ease of comparison, the parameters involved in the preparation of the diaphragm in Examples 1-17 and Comparative Examples 1-4 are summarized in Table 1.

[0080] The performance test results of the separators and lithium batteries prepared in Examples 1-17 and Comparative Examples 1-4 are shown in Table 2.

[0081] From Table 1 and Table 2, we can see that: In Comparative Example 1, the thickness 'a' of the first coating layer formed on the surface of the excess area of ​​the substrate in the separator of the lithium-ion battery and the thickness 'b' of the second coating layer formed on the surface of the normally coated area are both 3 μm (a / b=1.0). In Comparative Example 2, the slurry coating thickness 'a' of the first coating layer formed on the surface of the excess area of ​​the substrate in the separator of the lithium-ion battery and the coating thickness 'b' of the second coating layer formed on the surface of the normally coated area are both 4.5 μm. μm (a / b=1.0), while the thickness 'a' of the first coating layer formed on the surface of the excess area of ​​the substrate in the separator of the lithium-ion battery provided in Examples 1-17 and the thickness 'b' of the first coating layer formed on the surface of the normal coating area both satisfy the relationship 1.1≤a / b≤2. The test results show that, in Comparative Example 1, due to the use of a traditional uniform coating method, the coating layer thickness in the excess area and the normal coating area is consistent, resulting in insufficient thermal stability. The thermal shrinkage rate of the separator (15.3%) is significantly higher than that in Example 1, and the short-circuit safety pass rate of the lithium battery is only 63%, lower than that in Examples 1-17, which easily leads to fire and explosion accidents. Although Comparative Example 2 can reduce the thermal shrinkage rate of the separator by increasing the overall thickness of the coating layer in the excess area and the normal coating area, the thickness of the wound cell will increase, which seriously affects the assembly compatibility and energy density of the battery, violating the miniaturization and high energy density of lithium batteries. The trend of density development, and because the overall coating is too thick, it will increase the ion transport resistance and reduce the cycle stability of lithium battery; the ratio of the thickness a of the first coating layer in the excess area of ​​the substrate and the thickness b of the second coating layer in the normal coating area of ​​the lithium battery separator provided by Comparative Examples 3 and 4 is 1.05 and 2.5 respectively, which do not satisfy the relationship 1.1≤a / b≤2. The test results show that although the thickness of the wound cell in the lithium battery of Comparative Example 3 is lower than that of Example 1, the thermal shrinkage rate of the separator prepared by Comparative Example 3 is higher than that of Example 1, and the short circuit safety pass rate of the lithium battery is lower than that of Examples 1 to 17. The thickness of the wound cell, the thermal shrinkage rate of the prepared separator and the short circuit safety pass rate of the lithium battery in Comparative Example 4 are comparable to those of Examples 1 to 17, but the capacity retention rate of the lithium battery after 1000 cycles of charge and discharge is lower than that of Examples 1 to 17. The above results demonstrate that the lithium battery provided by this invention, through differentiated coating design, specifically thickens the coating layer in the excess area of ​​the separator substrate, and ensures that the thickness 'a' of the first coating layer formed on the surface of the excess area and the thickness 'b' of the second coating layer formed on the surface of the normal coating area satisfy the relationship 1.1≤a / b≤2. This significantly improves the thermal stability of the excess area of ​​the separator without increasing the thickness of the lithium battery winding cell or reducing the cycle stability of the lithium battery. It effectively solves the technical problem of easy separator shrinkage during lithium battery short circuits, fundamentally avoiding positive and negative electrode contact caused by separator shrinkage during short circuits, significantly reducing safety hazards, and thus improving the safety performance of the lithium battery.

[0082] In the lithium battery separators provided in Examples 1, 4, 6, 7, and 8, the ratio a / b of the thickness 'a' of the first coating layer in the excess area of ​​the substrate and the thickness 'b' of the second coating layer in the normal coating area satisfies the relationship 1.4 ≤ a / b ≤ 1.8. However, in the lithium battery separators provided in Examples 5 and 9, the ratio a / b of the thickness 'a' of the first coating layer in the excess area of ​​the substrate and the thickness 'b' of the second coating layer in the normal coating area does not satisfy this relationship. Test results show that the thermal shrinkage rate of the separators prepared in Examples 1, 4, 6, 7, and 8 is lower than that in Examples 5 and 9, and the short-circuit safety pass rate of the lithium batteries is higher than that in Examples 5 and 9. These results indicate that by adjusting the thickness 'a' of the first coating layer formed on the surface of the excess area and the thickness 'b' of the second coating layer formed on the surface of the normal coating area to further satisfy 1.4 ≤ a / b ≤ 1.8, the thermal stability of the excess area of ​​the separator can be further improved, thereby further enhancing the safety performance of the lithium battery.

[0083] In Example 1, the ratio of the thickness 'a' of the first coating layer in the excess area of ​​the substrate and the thickness 'b' of the second coating layer in the normal coating area of ​​the separator of the lithium battery simultaneously satisfies the three relationships: 1.4 ≤ a / b ≤ 1.8, a = 1.1–10 μm, and b = 1–5 μm. Examples 10 and 11 satisfy 1.4 ≤ a / b ≤ 1.8 and a = 1.1–10 μm but not b = 1–5 μm. Example 12 satisfies 1.4 ≤ a / b ≤ 1.8 but not a = 1.1–10 μm and b = 1–5 μm. Test results show that although the thickness of the wound cell in the lithium batteries of Examples 11 and 12 has decreased, the thermal shrinkage rate of the separators prepared in Examples 10 and 12 is higher than that of Example 1, and the short-circuit safety pass rate and capacity retention rate of the lithium batteries after 1000 charge-discharge cycles are lower than those of Example 1.

[0084] By comparing the data of Examples 1, 13, and 14, it can be seen that controlling the coating speed of the slurry between 5 and 30 m / min during the preparation of the separator can improve the safety performance of the separator and the cycle stability of the lithium battery to a certain extent. Specifically, the thermal shrinkage rate of the separator in Example 1 is lower than that in Examples 13 and 14, and the short-circuit safety pass rate of the lithium battery and the capacity retention rate of the lithium battery after 1000 cycles of charge and discharge are higher than those in Examples 13 and 14.

[0085] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention, but such modifications or substitutions are all within the scope of protection of the present invention.

Claims

1. A lithium battery, characterized in that: The lithium battery includes a wound cell, and the wound cell includes a positive electrode and a separator. The separator includes a substrate, and the area of ​​the substrate that extends beyond the width direction of the positive electrode sheet is defined as the allowance area, while the other areas are defined as the normal coating area. The excess area is provided with a first coating layer, and the normal coating area is provided with a second coating layer; Let the thickness of the first coating layer be a, and the thickness of the second coating layer be b, wherein a and b satisfy 1.1≤a / b≤2.

2. The lithium battery as described in claim 1, characterized in that: The condition a and b satisfy that 1.4 ≤ a / b ≤ 1.

8.

3. The lithium battery as described in claim 1, characterized in that: a = 1.1–10 μm, and / or b = 1–5 μm.

4. The lithium battery as described in claim 1, characterized in that: The material of the substrate is selected from at least one of polyethylene, polypropylene, and polyolefin, and / or the thickness of the substrate is 10~15 μm.

5. The lithium battery as described in claim 1, characterized in that: The materials of the first coating layer and the second coating layer are independently selected from at least one of adhesive materials, ceramic materials, and aramid fibers.

6. The lithium battery as described in claim 5, characterized in that: The adhesive material is selected from at least one of polyvinylidene fluoride, polyacrylate, and styrene-butadiene rubber, and / or the ceramic material is selected from at least one of alumina, zirconium oxide, and silicon dioxide.

7. The lithium battery as described in claim 1, characterized in that: The total width of the allowance area is 2~8 mm.

8. The lithium battery as described in claim 7, characterized in that: The remaining area includes a first area and a second area, which are symmetrically arranged about the central axis of the positive electrode sheet along its length. The width of both the first region and the second region is 1 to 4 mm.

9. The lithium battery as described in claim 1, characterized in that, The diaphragm is prepared by the following steps: coating the normal coating area with a first slurry to form the first coating layer, and coating the remaining area with a second slurry to form the second coating layer; The coating speed of both the first slurry and the second slurry is 5 to 30 m / min.

10. The lithium battery as described in claim 9, characterized in that: The coating speed of both the first slurry and the second slurry is 15-20 m / min.

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