Safe functional current collector, preparation method thereof and battery

Abstract

The invention belongs to the technical field of battery manufacturing, and relates to a safe functional current collector, a preparation method thereof and a battery, and the ratio of the elongation at break of the safe functional current collector in any two mutually perpendicular directions is (0.86-1.18): 1. The preparation method comprises the following steps: (1) sequentially performing crystallization, drying, melt extrusion, sheet casting, asynchronous stretching and heat treatment on a polymer raw material to obtain a base film layer; (2) metal layers are deposited on the surfaces of the two sides of the base film layer obtained in the step (1) respectively, and the safe functional current collector is obtained; wherein the asynchronous stretching in the step (1) comprises longitudinal stretching and transverse stretching which are sequentially carried out, and the ratio of the transverse stretching multiplying power to the longitudinal stretching multiplying power is (1.1-3): 1. According to the safe functional current collector provided by the invention, when the battery is impacted or extruded, the heat spreading risk of the current collector is reduced, and the overall safety of the battery is fully improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery manufacturing, and relates to a functional current collector, and in particular to a safe functional current collector and a preparation method thereof, and a battery. Background Art

[0002] As we all know, when a battery experiences impact or extrusion, due to deformation inside the battery, lithium dendrites or tiny foreign matter inside the battery can easily amplify their adverse effects under the action of extrusion, and even pierce the diaphragm or current collector, causing a short circuit between the positive and negative electrodes.

[0003] The functional current collector that has appeared in recent years is a "sandwich" structure, with a middle layer of polymer (such as PET, PP or PI) and metal conductive layers (such as Al or Cu) on both sides. The functional current collector has the characteristics of both lightness and high safety. Since the polymer layer in the middle layer can provide effective resistance to puncture and impact short circuit, it improves the battery's resistance to puncture and impact, and effectively reduces the risk of thermal runaway caused by external impact on the battery.

[0004] Compared with traditional pure metal current collectors, existing functional current collectors can increase impact resistance to more than 80%. However, with the impact of the current trend of cost reduction for new energy vehicles, the overall stiffness of the external tempered parts of the battery pack has decreased. When a collision occurs, the battery cell needs to withstand greater impact force. Therefore, it is urgent to develop a safe functional current collector to further optimize the impact resistance and improve the overall safety of the battery pack. Summary of the invention

[0005] In view of the deficiencies in the prior art, the object of the present invention is to provide a safe functional current collector and a preparation method and a battery, wherein the safe functional current collector reduces the risk of heat spread of the current collector when the battery is impacted or squeezed, thereby fully improving the overall safety of the battery.

[0006] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a safe functional current collector, comprising a base film layer and a metal layer arranged on the two side surfaces of the base film layer, wherein the ratio of the elongation at break of the safe functional current collector in any two mutually perpendicular directions is (0.86-1.18):1.

[0008] The present invention ensures the uniformity of the ductility of the functional current collector in different directions by strictly limiting the ratio of the elongation at break of the functional current collector in any two mutually perpendicular directions. When the battery is subjected to impact or extrusion, the risk of heat spread of the current collector is significantly reduced, thereby fully improving the overall safety of the battery.

[0009] Preferably, the safety-type functional current collector has a breaking elongation in any direction of 8%-50%.

[0010] Preferably, the base film layer is prepared by an asynchronous stretching process, including longitudinal stretching and transverse stretching performed sequentially.

[0011] Preferably, the material of the base film layer includes polyethylene terephthalate or polypropylene.

[0012] Preferably, the metal layer includes a copper layer or an aluminum layer.

[0013] In a second aspect, the present invention provides a method for preparing the safe functional current collector as described in the first aspect, the preparation method comprising the following steps:

[0014] (1) subjecting the polymer raw material to crystallization, drying, melt extrusion, sheet casting, asynchronous stretching and heat treatment in sequence to obtain a base film layer;

[0015] (2) Metal layers are deposited on both side surfaces of the base film layer obtained in step (1) to obtain a safe functional current collector.

[0016] Wherein, the polymer raw material in step (1) includes polyethylene terephthalate or polypropylene; the asynchronous stretching includes longitudinal stretching and transverse stretching performed sequentially, and the ratio of the transverse stretching ratio to the longitudinal stretching ratio is (1.1-3):1.

[0017] Preferably, when the polymer raw material is polyethylene terephthalate, the longitudinal stretching ratio is 1.5-6 times, and the transverse stretching ratio is 1.6-10 times.

[0018] Preferably, the ratio of the transverse stretching ratio to the longitudinal stretching ratio is (1.33-1.67):1.

[0019] Preferably, when the polymer raw material is polypropylene, the longitudinal stretching ratio is 4-5 times, and the transverse stretching ratio is 10-12 times.

[0020] Preferably, the ratio of the transverse stretching ratio to the longitudinal stretching ratio is (2.5-3):1.

[0021] Preferably, the longitudinal stretching and transverse stretching respectively include preheating, stretching and cooling performed sequentially.

[0022] Preferably, the preheating temperatures for the longitudinal stretching and the transverse stretching are 40-80°C, respectively.

[0023] Preferably, the stretching temperature of the transverse stretching is higher than the stretching temperature of the longitudinal stretching.

[0024] Preferably, the cooling temperature of the longitudinal stretching is ≥0.6Tg, and Tg is the glass transition temperature of the polymer raw material.

[0025] Preferably, when the polymer raw material is polyethylene terephthalate, the stretching temperature of the longitudinal stretching is 90-110°C.

[0026] Preferably, the stretching temperature of the transverse stretching is 100-120°C.

[0027] Preferably, when the polymer raw material is polypropylene, the stretching temperature of the longitudinal stretching is 125-145°C.

[0028] Preferably, the stretching temperature of the transverse stretching is 160-170°C.

[0029] Preferably, the deposition method in step (2) includes any one of vacuum evaporation, magnetron sputtering or water electroplating, or a combination of at least two of them.

[0030] In a third aspect, the present invention provides a battery, comprising the safety functional current collector as described in the first aspect.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] The present invention ensures the uniformity of the ductility of the functional current collector in different directions by strictly limiting the ratio of the elongation at break of the functional current collector in any two mutually perpendicular directions. When the battery is subjected to impact or extrusion, the risk of heat spread of the current collector is significantly reduced, thereby fully improving the overall safety of the battery. DETAILED DESCRIPTION

[0033] The technical solution of the present invention is further described below by specific implementation methods. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0034] An embodiment of the present invention provides a safe functional current collector, comprising a base film layer and a metal layer arranged on the surfaces of both sides of the base film layer, wherein the ratio of the elongation at break of the safe functional current collector in any two mutually perpendicular directions is (0.86-1.18):1, for example, it can be 0.86:1, 0.88:1, 0.9:1, 0.92:1, 0.94:1, 0.96:1, 0.98:1, 1:1, 1.02:1, 1.04:1, 1.06:1, 1.08:1, 1.1:1, 1.12:1, 1.14:1, 1.16:1 or 1.18:1, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0035] The present invention ensures the uniformity of the ductility of the functional current collector in different directions by strictly limiting the ratio of the elongation at break of the functional current collector in any two mutually perpendicular directions. When the battery is subjected to impact or extrusion, the risk of heat spread of the current collector is significantly reduced, thereby fully improving the overall safety of the battery.

[0036] In some embodiments, the breaking elongation of the safety functional current collector in any direction is 8%-50%, for example, it can be 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% or 50%, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0037] In some embodiments, the base film layer is prepared by an asynchronous stretching process, including sequential longitudinal stretching and transverse stretching.

[0038] In some embodiments, the base film layer is made of polyethylene terephthalate or polypropylene.

[0039] In some embodiments, the metal layer includes a copper layer or an aluminum layer.

[0040] An embodiment of the present invention further provides a method for preparing the safe functional current collector as described in any of the above embodiments, the preparation method comprising the following steps:

[0041] (1) subjecting the polymer raw material to crystallization, drying, melt extrusion, sheet casting, asynchronous stretching and heat treatment in sequence to obtain a base film layer;

[0042] (2) Metal layers are deposited on both side surfaces of the base film layer obtained in step (1) to obtain a safe functional current collector.

[0043] Wherein, the polymer raw material in step (1) includes polyethylene terephthalate or polypropylene; the asynchronous stretching includes longitudinal stretching and transverse stretching performed sequentially, and the ratio of the transverse stretching ratio to the longitudinal stretching ratio is (1.1-3):1, for example, it can be 1.1:1, 1.2:1, 1.4:1, 1.6:1, 1.8:1, 2:1, 2.2:1, 2.4:1, 2.6:1, 2.8:1 or 3:1, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0044] After research analysis and experimental verification, the applicant found that the molding process of the base film layer in the functional current collector will affect its elongation at break in different directions. The polymer base films currently on the market are obtained by melt extrusion and then longitudinal and transverse stretching. In theory, the transverse and longitudinal stretching can be carried out simultaneously in the biaxial stretching process of the base film, but the equipment investment cost of the synchronous stretching process is high, the loss of trimming waste is large, and high-speed production is difficult, making it difficult to achieve mass production. Therefore, asynchronous stretching is mostly used in actual mass production. The main problem with asynchronous stretching is that the obtained base film is very likely to be anisotropic.

[0045] Specifically, in order to avoid the inefficiency problem caused by increasing the number of changes in the direction of the tape run on the production line, technicians first perform longitudinal stretching and then transverse stretching on the polymer raw materials after melt extrusion. Since the longitudinally oriented macromolecular structure has been partially fixed during the longitudinal stretching process, the final product has a larger elongation at break in the longitudinal stretching direction and a smaller elongation at break in the transverse stretching direction. Therefore, when the functional current collector undergoes extrusion, the transverse stretching direction will become a "short board" of force, which is more likely to be pierced by lithium dendrites or tiny foreign objects inside the battery or cause cracking due to extrusion.

[0046] In this regard, the present invention adjusts the molding process of the base film layer in the functional current collector so that the ratio of the elongation at break of the functional current collector in any two mutually perpendicular directions is appropriately reduced, so that the lower limit of the elongation at break in each direction when it is squeezed is increased, thereby ensuring the uniformity of the ductility of the functional current collector in different directions, and ultimately improving the overall impact resistance and extrusion resistance of the battery.

[0047] In some embodiments, when the polymer raw material is polyethylene terephthalate, the longitudinal stretching ratio is 1.5-6 times, for example, it can be 1.5 times, 2 times, 2.5 times, 3 times, 3.5 times, 4 times, 4.5 times, 5 times, 5.5 times or 6 times, and the transverse stretching ratio is 1.6-10 times, for example, it can be 1.6 times, 2 times, 3 times, 4 times, 5 times, 6 times, 7 times, 8 times, 9 times or 10 times, but it is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0048] In some embodiments, when the polymer raw material is polyethylene terephthalate, the ratio of the transverse stretch ratio to the longitudinal stretch ratio is (1.33-1.67):1, for example, it can be 1.33:1, 1.35:1, 1.4:1, 1.45:1, 1.5:1, 1.55:1, 1.6:1, 1.65:1 or 1.67:1, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0049] In some embodiments, when the polymer raw material is polypropylene, the longitudinal stretching ratio is 4-5 times, for example, it can be 4 times, 4.1 times, 4.2 times, 4.3 times, 4.4 times, 4.5 times, 4.6 times, 4.7 times, 4.8 times, 4.9 times or 5 times, and the transverse stretching ratio is 10-12 times, for example, it can be 10 times, 10.2 times, 10.4 times, 10.6 times, 10.8 times, 11 times, 11.2 times, 11.4 times, 11.6 times, 11.8 times or 12 times, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0050] In some embodiments, when the polymer raw material is polypropylene, the ratio of the transverse stretch ratio to the longitudinal stretch ratio is (2.5-3):1, for example, it can be 2.5:1, 2.55:1, 2.6:1, 2.65:1, 2.7:1, 2.75:1, 2.8:1, 2.85:1, 2.9:1, 2.95:1 or 3:1, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0051] For different types of polymer raw materials, the present invention limits the longitudinal stretching ratio to be lower than the transverse stretching ratio, that is, appropriately reduces the longitudinal stretching ratio, thereby improving the stretching elasticity during the transverse stretching process and improving the isotropic uniformity of the base film layer.

[0052] In certain embodiments, the longitudinal stretching and the transverse stretching respectively include preheating, stretching and cooling performed sequentially.

[0053] In some embodiments, the preheating temperatures for the longitudinal stretching and the transverse stretching are 40-80°C, for example, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C or 80°C, but are not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0054] In certain embodiments, the stretching temperature of the transverse stretching is higher than the stretching temperature of the longitudinal stretching.

[0055] The present invention appropriately increases the preheating temperature and stretching temperature of the transverse stretching, and specifically limits the transverse stretching temperature to be higher than the longitudinal stretching temperature, so that the transversely stretched macromolecular chains are easier to form, thereby significantly improving the aggregation structure of the transverse macromolecular chains, avoiding the transverse stretching direction from becoming a force-bearing "short board", and further improving the isotropic uniformity of the base film layer.

[0056] In certain embodiments, the cooling temperature of the longitudinal stretching is ≥0.6Tg, and Tg is the glass transition temperature of the polymer raw material, for example, it can be 0.6Tg, 0.65Tg, 0.7Tg, 0.75Tg, 0.8Tg, 0.85Tg, 0.9Tg, 0.95Tg or 1Tg, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0057] The present invention limits the cooling temperature of the longitudinal stretching to not less than 0.6 times of the glass transition temperature, thereby preventing the longitudinally oriented macromolecular structure from being partially fixed before the transverse stretching, which causes difficulty in transverse stretching, thereby reducing the difficulty of subsequent transverse stretching.

[0058] In some embodiments, when the polymer raw material is polyethylene terephthalate, the stretching temperature of the longitudinal stretching is 90-110°C, for example, it can be 90°C, 92°C, 94°C, 96°C, 98°C, 100°C, 102°C, 104°C, 106°C, 108°C or 110°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0059] In some embodiments, when the polymer raw material is polyethylene terephthalate, the stretching temperature of the transverse stretching is 100-120°C, for example, it can be 100°C, 102°C, 104°C, 106°C, 108°C, 110°C, 112°C, 114°C, 116°C, 118°C or 120°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0060] In some embodiments, when the polymer raw material is polypropylene, the stretching temperature of the longitudinal stretching is 125-145°C, for example, it can be 125°C, 126°C, 128°C, 130°C, 132°C, 134°C, 136°C, 138°C, 140°C, 142°C, 144°C or 145°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0061] In some embodiments, when the polymer raw material is polypropylene, the stretching temperature of the transverse stretching is 160-170°C, for example, it can be 160°C, 161°C, 162°C, 163°C, 164°C, 165°C, 166°C, 167°C, 168°C, 169°C or 170°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0062] In some embodiments, the deposition method in step (2) includes any one of vacuum evaporation, magnetron sputtering or water electroplating, or a combination of at least two of them. Typical but non-limiting combinations include a combination of vacuum evaporation and magnetron sputtering, a combination of magnetron sputtering and water electroplating, a combination of vacuum evaporation and water electroplating, or a combination of vacuum evaporation, magnetron sputtering and water electroplating.

[0063] A certain embodiment of the present invention further provides a battery, wherein the battery comprises the safe functional current collector as described in any of the above embodiments.

[0064] The numerical range described in the present invention not only includes the point values ​​listed above, but also includes any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0065] Example 1

[0066] This embodiment provides a safe functional current collector and a preparation method thereof, the preparation method comprising the following steps:

[0067] (1) Polyethylene terephthalate (molecular weight 43000 g / mol) with a glass transition temperature Tg of 75±2°C is used as a polymer raw material to prepare a base film layer, specifically:

[0068] (1.1) Crystallization and drying: First, the polymer raw material is crystallized at 155°C for 50 minutes, and then dried at 160°C for 180 minutes;

[0069] (1.2) Melt extrusion: The crystallized and dried polymer raw material is heated to melt at 260°C and then extruded through a die;

[0070] (1.3) Casting: The molten extruded polymer raw material is cast onto a casting roll, cooled to 40°C and cast into a thick sheet;

[0071] (1.4) Longitudinal stretching: preheat the polymer raw material to 70°C, then stretch it longitudinally at 110°C, control the longitudinal stretching ratio to 3 times, and cool it at 70°C after stretching;

[0072] (1.5) Transverse stretching: Preheat the polymer raw material to 80°C, then stretch it transversely at 110°C, control the transverse stretching ratio to 4 times, and cool it at 40°C after stretching;

[0073] (1.6) Heat treatment: The film obtained by asynchronous stretching is placed at 80°C for heat treatment;

[0074] (1.7) Winding: The film is naturally cooled and then rolled up to obtain a base film layer with a thickness of 6 μm.

[0075] (2) depositing metal layers on both sides of the base film layer obtained in step (1), specifically:

[0076] The base film layer is placed in a vacuum evaporation chamber, and the high-purity aluminum wire (purity greater than 99.99%) in the metal evaporation chamber is melted and evaporated at a high temperature of 800°C. The evaporated aluminum vapor is deposited on the two sides of the base film layer. Argon gas is introduced for protection, and the vacuum degree is controlled at 5×10 -2 Pa, the winding speed is controlled at 280m / min, the wire feeding speed is 320mm / min, and finally a single-layer aluminum metal layer with a thickness of 1μm is formed, thereby obtaining a safe aluminum current collector with a total thickness of 8μm.

[0077] Example 2

[0078] This embodiment provides a safe functional current collector and a preparation method thereof. Except that the transverse stretching ratio is changed to 3.3 times, the remaining steps and conditions are the same as those in Example 1 and are not described in detail here.

[0079] Example 3

[0080] This embodiment provides a safe functional current collector and a preparation method thereof. Except that the transverse stretching ratio is changed to 5 times, the remaining steps and conditions are the same as those in Example 1 and are not described in detail here.

[0081] Example 4

[0082] This embodiment provides a safe functional current collector and a preparation method thereof. Except that the longitudinal stretching ratio is changed to 2 times and the transverse stretching ratio is changed to 6 times, the remaining steps and conditions are the same as those in Example 1 and are not described in detail here.

[0083] Example 5

[0084] This embodiment provides a safe functional current collector and a preparation method thereof. Except that the stretching temperature of the transverse stretching is changed to 120° C., the remaining steps and conditions are the same as those in Embodiment 1 and are not described in detail here.

[0085] Example 6

[0086] This embodiment provides a safe functional current collector and a preparation method thereof. Except that the stretching temperature of the transverse stretching is changed to 80° C., the remaining steps and conditions are the same as those in Embodiment 1 and are not described in detail here.

[0087] Example 7

[0088] This embodiment provides a safe functional current collector and a preparation method thereof. Except that the cooling temperature of the longitudinal stretching is changed to 40° C., the remaining steps and conditions are the same as those in Embodiment 1 and are not described in detail here.

[0089] Example 8

[0090] The present embodiment provides a safe functional current collector and a preparation method thereof, except that the polymer raw material is changed to polypropylene (Tg=0°C, molecular weight of 100000 g / mol), the longitudinal stretching ratio is changed to 4 times, the longitudinal stretching temperature is changed to 135°C, the longitudinal stretching cooling temperature is changed to 10°C, and the transverse stretching ratio is changed to 11 times, and the transverse stretching temperature is changed to 165°C. The remaining steps and conditions are the same as those in Example 1 and are not repeated here.

[0091] Example 9

[0092] This embodiment provides a safe functional current collector and a preparation method thereof, which is different from Embodiment 1 only in step (2), which is specifically:

[0093] (2.1) The base film layer is placed in a magnetron sputtering chamber, and a pure copper target (purity of 99.99%) is sputtered on both sides of the base film layer by high vacuum magnetron sputtering to form a single-layer copper seed layer with a thickness of 100 nm to obtain a semi-finished product; the sputtering process parameters are: vacuum degree 6×10 -3 Pa, working gas Ar, gas flow rate 150sccm, sputtering power 7kW, sputtering main roller cooling temperature -20℃, target power 13kW, winding speed 6m / min;

[0094] (2.2) Placing the semi-finished product obtained in step (2.1) on a DC temperature-controlled winding electroplating line for water electroplating, electroplating on both sides of the semi-finished product until the copper layer is thickened to 1 μm, thereby obtaining a safe copper current collector with a total thickness of 8 μm; wherein the electroplating solution comprises the following concentration components: CuSO4 60 g / L, H2SO4 130 g / L, SPS 0.02 g / L, and the solvent is deionized water; the process parameters of the water electroplating treatment are: electroplating temperature ≤ 25°C, winding speed 12 m / min, square resistance ≤ 20 μΩ, and the current density of one electroplating tank is 1 A / dm 2 The current density increases by 1A / dm from electroplating tank 1 to electroplating tank 8. 2 Until the current density of 8 electroplating tanks is 8A / dm 2 .

[0095] Comparative Example 1

[0096] This comparative example provides a functional current collector and a preparation method thereof. Except that the cooling temperature of the longitudinal stretching is changed to 40°C and the transverse stretching ratio is changed to 2 times, the remaining steps and conditions are the same as those in Example 1 and are not described in detail here.

[0097] Comparative Example 2

[0098] This comparative example provides a functional current collector and a preparation method thereof. Except that the stretching temperature of the transverse stretching is changed to 120° C. and the transverse stretching ratio is changed to 10 times, the remaining steps and conditions are the same as those in Example 1 and are not described in detail here.

[0099] Comparative Example 3

[0100] This comparative example provides a functional current collector and a preparation method thereof. In addition to performing transverse and longitudinal stretching simultaneously, and controlling the longitudinal and transverse stretching ratios to be 3 times, the preheating temperature is 70°C, the stretching temperature is 110°C, and the cooling temperature is 40°C. The remaining steps and conditions are the same as those in Example 1 and are not repeated here.

[0101] Performance Testing

[0102] (1) Elongation at break test: The functional current collectors obtained in Examples 1-8 and Comparative Examples 1-3 were selected and cut into 5 samples with a length greater than 175 mm and a width greater than 150 mm, respectively. The samples were then cut in a T-shape to obtain 5 samples with the longitudinal direction being the TD direction and the transverse direction being the MD direction, and the size of each sample was 25 mm × 150 mm. The samples cut from the same sample were numbered TD1, TD2, TD3, TD4, TD5 and MD1, MD2, MD3, MD4, MD5, respectively. Each sample was tested in accordance with the national standard GB / T1040.3-2006. It was required that the longitudinal direction of the sample was parallel to the axis of the fixture during the test, and the sample was kept in a straight line. The tensile speed of the tensile machine was 50 mm / min. The ratio of the elongation at break of each TDn sample to the elongation at break of the MDn sample was the arithmetic average of the 5 samples. The relevant test results are shown in Table 1 below.

[0103] Table 1

[0104]

[0105] (2) Battery preparation:

[0106] (2.1) Preparation of positive electrode sheets: The ternary active material NCM811, conductive carbon black and binder PVDF are fully stirred in NMP at a weight ratio of 92:4:4 to obtain a positive electrode slurry; the positive electrode slurry is then applied to the surface of the above-mentioned aluminum current collector (Examples 1-8 and Comparative Examples 1-3), dried, rolled and then die-cut to obtain a positive electrode sheet, and the compaction density of the positive electrode active material layer is controlled to be 3.3 g / cm3 .

[0107] (2.2) Preparation of negative electrode sheet: The negative electrode active material artificial graphite, binder SBR, dispersant CMC and conductive carbon black are fully stirred in deionized water at a weight ratio of 96:2:1.2:0.8 to obtain a negative electrode slurry; then the negative electrode slurry is coated on the surface of the above copper current collector (Example 9), dried, rolled and die-cut to obtain a negative electrode sheet, and the compaction density of the negative electrode active material layer is controlled to be 1.55 g / cm 3 .

[0108] (2.3) PE film is selected as the diaphragm.

[0109] (2.4) Preparation of electrolyte: Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 4:6 as the solvent, and lithium salt LiPF6 was selected as the solute with a concentration of 1 mol / L.

[0110] (2.5) Assembling batteries: stack the positive electrode, separator, and negative electrode in order, wind them up to obtain a battery cell, put them into a battery case, and then add electrolyte. After packaging, standing, formation, and capacity division, a sample lithium battery is obtained.

[0111] (3) Anti-stamping test:

[0112] (3.1) Charge the battery. The charging steps are as follows: at 25°C, first charge to 4.2V at a constant current of C / 3, then switch to constant voltage charging to 0.05C cutoff current.

[0113] (3.2) Place the battery in a 2°C incubator for 1 hour.

[0114] (3.3) Place the fully charged batteries to be tested on the same plane and squeeze the batteries through the roller plane. The diameter of the roller is 150 mm and the feed speed is 2 mm / s.

[0115] (3.4) When the deformation of the battery in the deformation direction reaches 15%, or the extrusion pressure reaches 100kN, stop the cylinder feed, observe whether the battery catches fire or explodes at this time, and read the extrusion pressure; if no fire or explosion occurs, continue to roll until the deformation reaches 25% and maintain for 10 minutes, and read the extrusion pressure; if no fire or explosion occurs, continue to roll until the deformation reaches 35% and maintain for 10 minutes, and read the extrusion pressure; if no fire or explosion occurs, continue to roll until the battery catches fire or explodes or the maximum force of the equipment is reached, and read the deformation and extrusion pressure under extreme conditions.

[0116] According to the above method, the batteries corresponding to Examples 1-8 and Comparative Examples 1-3 were subjected to X-direction impact resistance tests (X-direction refers to the test in which the extrusion force is perpendicular to the large surface of the battery, that is, the battery is placed flat). The relevant test results are shown in Table 2 below.

[0117] Table 2

[0118]

[0119] In Table 2 above, state 1 refers to the state when the deformation reaches 15%; extrusion pressure 1 refers to the extrusion pressure when the deformation reaches 15%; state 2 refers to the state when the deformation reaches 25%; extrusion pressure 2 refers to the extrusion pressure when the deformation reaches 25%; state 3 refers to the state when the deformation reaches 35%; extrusion pressure 3 refers to the extrusion pressure when the deformation reaches 35%.

[0120] It can be seen that the present invention ensures the uniformity of the ductility of the functional current collector in different directions by strictly limiting the ratio of the elongation at break of the functional current collector in any two mutually perpendicular directions. When the battery is subjected to impact or extrusion, the risk of heat spread of the current collector is significantly reduced, thereby fully improving the overall safety of the battery.

[0121] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention are within the protection scope and disclosure scope of the present invention.

Claims

1. A safe functional current collector, comprising a base film layer and a metal layer disposed on both sides of the base film layer, characterized in that: The ratio of the elongation at break of the safety functional current collector in any two mutually perpendicular directions is (0.86-1.18):1; The base film layer is prepared by an asynchronous stretching process, including longitudinal stretching and transverse stretching performed sequentially, and the ratio of the transverse stretching ratio to the longitudinal stretching ratio is (1.1-3):

1.

2. The safety functional current collector according to claim 1, characterized in that: The breaking elongation of the safety functional current collector in any direction is 8%-50%; And / or, the material of the base film layer includes polyethylene terephthalate or polypropylene; And / or, the metal layer includes a copper layer or an aluminum layer.

3. A method for preparing the safe functional current collector according to claim 1 or 2, characterized in that: The preparation method comprises the following steps: (1) subjecting the polymer raw material to crystallization, drying, melt extrusion, sheet casting, asynchronous stretching and heat treatment in sequence to obtain a base film layer; (2) depositing metal layers on both sides of the base film layer obtained in step (1) to obtain a safe functional current collector; Wherein, the polymer raw material in step (1) includes polyethylene terephthalate or polypropylene.

4. The preparation method according to claim 3, characterized in that: When the polymer raw material is polyethylene terephthalate, the longitudinal stretching ratio is 1.5-6 times, and the transverse stretching ratio is 1.6-10 times; And / or, the ratio of the transverse stretching ratio to the longitudinal stretching ratio is (1.33-1.67):

1.

5. The preparation method according to claim 3, characterized in that: When the polymer raw material is polypropylene, the longitudinal stretching ratio is 4-5 times, and the transverse stretching ratio is 10-12 times; And / or, the ratio of the transverse stretching ratio to the longitudinal stretching ratio is (2.5-3):

1.

6. The preparation method according to any one of claims 3 to 5, characterized in that: The longitudinal stretching and transverse stretching respectively include preheating, stretching and cooling performed sequentially; Wherein, the preheating temperature of the longitudinal stretching and the transverse stretching is 40-80°C respectively; and / or, the stretching temperature of the transverse stretching is higher than the stretching temperature of the longitudinal stretching; And / or, the cooling temperature of the longitudinal stretching is ≥0.6Tg, and Tg is the glass transition temperature of the polymer raw material.

7. The preparation method according to claim 6, characterized in that: When the polymer raw material is polyethylene terephthalate, the stretching temperature of the longitudinal stretching is 90-110°C; And / or, the stretching temperature of the transverse stretching is 100-120°C.

8. The preparation method according to claim 6, characterized in that: When the polymer raw material is polypropylene, the stretching temperature of the longitudinal stretching is 125-145°C; And / or, the stretching temperature of the transverse stretching is 160-170°C.

9. The preparation method according to claim 3, characterized in that: The deposition method in step (2) includes any one of vacuum evaporation, magnetron sputtering or water electroplating, or a combination of at least two of them.

10. A battery, characterized in that: The battery comprises the safe functional current collector according to claim 1 or 2.

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