A safe functional current collector, a preparation method thereof, and a battery

Abstract

The present invention belongs to the field of battery manufacturing technology and relates to a safety-type functional current collector, a preparation method thereof, and a battery. The ratio of the elongation at break of the safety-type functional current collector in any two mutually perpendicular directions is (0.86-1.18):1. The preparation method comprises: (1) subjecting a 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 safety-type functional current collector; wherein the asynchronous stretching in step (1) comprises longitudinal stretching and transverse stretching performed in sequence, and the ratio of the transverse stretching ratio to the longitudinal stretching ratio is (1.1-3):1. The safety-type functional current collector provided by the present invention reduces the risk of heat spread of the current collector when the battery is subjected to impact or extrusion, thereby fully improving the overall safety of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery manufacturing and relates to a functional current collector, 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 collectors that have emerged in recent years have a "sandwich" structure, with a middle polymer layer (such as PET, PP, or PI) and two metal conductive layers (such as Al or Cu) on either side. These current collectors combine lightweight design with high safety. The polymer layer in the middle layer provides effective resistance to puncture and short circuits, improving the battery's puncture and impact resistance and effectively reducing the risk of thermal runaway caused by external impacts.

[0004] Compared to traditional pure metal current collectors, existing functional current collectors can improve impact resistance by over 80%. However, with the current trend of cost reduction for new energy vehicles, the overall stiffness of the external tempered components of the battery pack has decreased. In the event of an impact, the battery cell needs to withstand greater impact forces. Therefore, there is an urgent need to develop a safe functional current collector to further optimize impact resistance and enhance the overall safety of the battery pack. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a safe functional current collector and its preparation method and battery. The safe functional current collector reduces the risk of heat spread of the current collector when the battery experiences impact or extrusion, 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 safety-type functional current collector, comprising a base film layer and a metal layer arranged on both side surfaces of the base film layer, wherein the ratio of the elongation at break of the safety-type 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 experiences impact or extrusion, the risk of heat spread in the current collector is significantly reduced, thereby fully improving the overall safety of the battery.

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

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

[0011] Preferably, the base film layer is made of 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) The polymer raw material is sequentially subjected to crystallization, drying, melt extrusion, sheet casting, asynchronous stretching and heat treatment to obtain a base film layer;

[0015] (2) Metal layers are deposited on both sides 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 in sequence.

[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 experiences impact or extrusion, the risk of heat spread in 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 way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely 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 safety functional current collector, comprising a base film layer and a metal layer arranged on both side surfaces of the base film layer, wherein 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, 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 this 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 experiences impact or extrusion, the risk of heat spread in the current collector is significantly reduced, thereby fully improving the overall safety of the battery.

[0036] In some embodiments, the elongation at break 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 this numerical range are also applicable.

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

[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) The polymer raw material is sequentially subjected to crystallization, drying, melt extrusion, sheet casting, asynchronous stretching and heat treatment to obtain a base film layer;

[0042] (2) Metal layers are deposited on both sides 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 all 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, in actual mass production, asynchronous stretching is mostly used. The main problem with asynchronous stretching is that the obtained base film is extremely prone to anisotropy.

[0045] Specifically, in order to avoid the inefficiency problem caused by increasing the number of changes in the direction of the tape 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 fine foreign matter inside the battery or cause cracking due to extrusion.

[0046] To this end, 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 is not limited to the listed values, and other values not listed within this numerical range are also applicable.

[0048] In certain embodiments, when the polymer raw material is polyethylene terephthalate, the ratio of the transverse stretching ratio to the longitudinal stretching 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 this numerical range are also applicable.

[0049] In certain 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 values not listed within this numerical range are also applicable.

[0050] In certain 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 this 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 comprise 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 in the transverse stretching is higher than the stretching temperature in the longitudinal stretching.

[0055] The present invention appropriately increases the preheating temperature and stretching temperature of 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 becoming the "short board" of force, and further improving the isotropic uniformity of the base film layer.

[0056] In certain embodiments, the cooling temperature for 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 longitudinal stretching to no less than 0.6 times the glass transition temperature, thereby preventing the longitudinally oriented macromolecular structure from being partially fixed before 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 this 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 this 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 this 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 this 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] An embodiment of the present invention further provides a battery, comprising the safety functional current collector as described in any of the above embodiments.

[0064] The numerical range described in the present invention includes not only the point values listed above, but also 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 was used as a polymer raw material to prepare the base film layer, specifically:

[0068] (1.1) Crystallization and drying: The polymer raw material was first crystallized at 155°C for 50 min, and then dried at 160°C for 180 min;

[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 to 70°C, then stretch it longitudinally at 110°C with a stretch ratio of 3. After stretching, cool it at 70°C.

[0072] (1.5) Transverse stretching: Preheat the polymer to 80°C, then stretch it transversely at 110°C with a stretch ratio of 4. After stretching, cool it at 40°C.

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

[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 both 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 of 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 for changing the transverse stretching ratio to 3.3 times, the remaining steps and conditions are the same as those in Example 1 and are not described here in detail.

[0079] Example 3

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

[0081] Example 4

[0082] This embodiment provides a safe functional current collector and a preparation method thereof. Except for changing the longitudinal stretching ratio to 2 times and the transverse stretching ratio to 6 times, the remaining steps and conditions are the same as those in Example 1 and are not repeated 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 Example 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 Example 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 for longitudinal stretching is changed to 40° C., the remaining steps and conditions are the same as those in Example 1 and are not described in detail here.

[0089] Example 8

[0090] This 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 100,000 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 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. The only difference between this embodiment and embodiment 1 is step (2), which is:

[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 onto both sides of the base film layer by high vacuum magnetron sputtering to form a single layer of 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 150 sccm, sputtering power 7 kW, sputtering main roller cooling temperature -20 ° C, target power 13 kW, winding speed 6 m / min;

[0094] (2.2) The semi-finished product obtained in step (2.1) is placed on a DC temperature-controlled winding electroplating line for water electroplating treatment, and the copper layer is electroplated 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 includes the following concentration components: CuSO4 60g / L, H2SO4 130g / L, SPS 0.02g / L, and the solvent is deionized water; the process parameters of the water electroplating treatment are: electroplating temperature ≤25°C, winding speed 12m / min, square resistance ≤20μΩ, and current density of 1 electroplating tank is 1A / 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 repeated 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 repeated 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 five samples with a length greater than 175 mm and a width greater than 150 mm. The samples were then cut into a T-shape to obtain five samples with the longitudinal direction being the TD direction and the transverse direction being the MD direction. 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. Each sample was tested in accordance with the national standard GB / T1040.3-2006. During the test, the longitudinal direction of the sample was required to be parallel to the axis of the fixture and the sample was kept in a straight line. The tensile speed of the tensile testing 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 five 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 were thoroughly stirred in NMP at a weight ratio of 92:4:4 to obtain a positive electrode slurry. The positive electrode slurry was 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 positive electrode sheets. The compaction density of the positive electrode active material layer was controlled to be 3.3 g / cm3 .

[0107] (2.2) Preparation of negative electrode sheet: artificial graphite (negative electrode active material), SBR (binder), CMC (dispersant), and conductive carbon black (conductive carbon black) were thoroughly stirred in deionized water at a weight ratio of 96:2:1.2:0.8 to obtain a negative electrode slurry. The negative electrode slurry was then applied to the surface of the copper current collector (Example 9), dried, rolled, and then die-cut to obtain a negative electrode sheet. The compaction density of the negative electrode active material layer was 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 at a concentration of 1 mol / L.

[0110] (2.5) Battery assembly: The positive electrode, separator, and negative electrode are stacked in order, wound to obtain a battery cell, which is placed in a battery case. Then, electrolyte is added. After packaging, standing, formation, and capacity separation, a sample lithium battery is obtained.

[0111] (3) Anti-impact 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 roller has a diameter of 150 mm and a feed speed of 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 crush until the deformation reaches 25%, maintain for 10 minutes, and read the extrusion pressure; if no fire or explosion occurs, continue to crush until the deformation reaches 35%, maintain for 10 minutes, and read the extrusion pressure; if no fire or explosion occurs, continue to crush 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] The batteries corresponding to Examples 1-8 and Comparative Examples 1-3 were subjected to X-direction impact resistance tests according to the above method (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 force 1 refers to the extrusion force when the deformation reaches 15%; state 2 refers to the state when the deformation reaches 25%; extrusion force 2 refers to the extrusion force when the deformation reaches 25%; state 3 refers to the state when the deformation reaches 35%; extrusion force 3 refers to the extrusion force 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 experiences impact or extrusion, the risk of heat spread in the current collector is significantly reduced, thereby fully improving the overall safety of the battery.

[0121] The above description is only a specific embodiment 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 fall within the protection scope and disclosure scope of the present invention.

Claims

1. A safe functional current collector comprising a base film layer and metal layers 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 made of polyethylene terephthalate and is prepared by an asynchronous stretching process, including sequential longitudinal stretching and transverse stretching, and the ratio of the transverse stretching ratio to the longitudinal stretching ratio is (1.33-1.67):1; The longitudinal stretching and transverse stretching respectively include preheating, stretching and cooling performed in sequence; The preheating temperatures for the longitudinal stretching and transverse stretching are 40-80°C respectively; the stretching temperature for the longitudinal stretching is 90-110°C, the stretching temperature for the transverse stretching is 100-120°C, and after the asynchronous stretching, the heat treatment is performed at 80°C; the cooling temperature for the longitudinal stretching is ≥0.6Tg, and Tg is the glass transition temperature of the polymer raw material.

2. The safety functional current collector according to claim 1, characterized in that: The safety functional current collector has a breaking elongation of 8% to 50% in any direction; 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) The polymer raw material is sequentially subjected to crystallization, drying, melt extrusion, sheet casting, asynchronous stretching and heat treatment 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) is polyethylene terephthalate.

4. The preparation method according to claim 3, characterized in that The longitudinal stretching ratio is 1.5-6 times, and the transverse stretching ratio is 1.6-10 times.

5. The preparation method according to claim 3 or 4, characterized in that The stretching temperature of the transverse stretching is higher than the stretching temperature of the longitudinal stretching.

6. 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.

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

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