Diaphragm and lithium battery using same

Abstract

The utility model provides a diaphragm and a lithium battery using the same. The diaphragm comprises a base material layer, wherein the base material layer comprises a first polypropylene film, a polyethylene film and a second polypropylene film which are sequentially compounded; micropores are formed in the first polypropylene film, the polyethylene film and the second polypropylene film; the porosity A of the polyethylene film, the porosity B of the first polypropylene film and the porosity C of the second polypropylene film meet the conditions that C is greater than A and C is greater than B. When the diaphragm provided by the utility model is applied to the lithium battery, the problem that a large number of lithium ions are gathered at a negative electrode to cause lithium precipitation of the negative electrode can be improved, the risk that the diaphragm is punctured by lithium dendrites formed by the large number of lithium precipitation of the negative electrode is reduced, the safety performance of the lithium battery is improved, the condition that lithium ion transmission is blocked is improved, and the internal resistance of the lithium battery is reduced; and the rate capability of the lithium battery is improved.

Description

Technical Field

[0001] The utility model relates to the technical field of batteries, in particular to a diaphragm and a lithium battery using the same. Background Art

[0002] Lithium batteries have the advantages of high specific energy, high specific power, high voltage platform, low self-discharge, long cycle life, low environmental pollution and no memory effect. They have been widely used in mobile phones, computers, electric vehicles, military, space technology and other related fields.

[0003] The separator is a crucial component of lithium-ion batteries. Its primary function is to effectively isolate the positive and negative electrodes, preventing electron conduction between them and preventing internal short circuits. It also ensures the efficient and rapid passage of lithium ions. Furthermore, the separator is a crucial component in ensuring lithium-ion battery safety.

[0004] During the charge and discharge process of lithium batteries, lithium dendrites are easily generated on the surface of the negative electrode. Lithium dendrites can easily pierce the diaphragm, causing internal short circuits in the lithium battery and even the risk of fire, combustion, and explosion, seriously affecting the safety performance of the lithium battery. Utility Model Content

[0005] In order to solve the problem that lithium dendrites generated at the negative electrode of the existing lithium battery pierce the diaphragm during the charge and discharge process, resulting in poor safety performance of the lithium battery, and to improve the safety performance of the diaphragm and the lithium battery, the utility model provides a diaphragm and a lithium battery using the same.

[0006] According to a first aspect of the present invention, a diaphragm is provided, which includes a substrate layer, which includes a first polypropylene film, a polyethylene film, and a second polypropylene film that are laminated in sequence; the first polypropylene film, the polyethylene film, and the second polypropylene film are all provided with micropores; the porosity of the polyethylene film is A, the porosity of the first polypropylene film is B, and the porosity of the second polypropylene film is C, A, B, and C satisfy C>A, and C>B.

[0007] The diaphragm provided by the utility model uses a first polypropylene film, a polyethylene film, and a second polypropylene film that are compounded in sequence as a substrate layer. The first polypropylene film, the polyethylene film, and the second polypropylene film in the substrate layer are compounded in sequence. The diaphragm provided by the utility model is applied to a lithium battery. On the one hand, the porosity B of the first polypropylene film on the side close to the positive electrode sheet is lower than the porosity C of the second polypropylene film on the side close to the negative electrode sheet. During the transfer process, lithium ions are blocked by the first polypropylene film with lower porosity, which slows down the migration speed of lithium ions to the negative electrode. The lithium ions released from the positive electrode are enriched at the positive electrode. The invention relates to a lithium battery which has a large amount of lithium ions and a large amount of lithium ions deposited on the negative electrode. The invention relates to a lithium battery which has a large amount of lithium ions and a large amount of lithium ions deposited on the negative electrode. The invention relates to a lithium battery which has a large amount of lithium ions and a large amount of lithium ions deposited on the negative electrode. The invention relates to a lithium battery which has a large amount of lithium ions and a large amount of lithium ions deposited on the negative electrode. The invention relates to a lithium battery which has a large amount of lithium ions and a large amount of lithium ions deposited on the negative electrode

[0008] Preferably, C>A>B.

[0009] In the direction from the positive electrode sheet to the negative electrode sheet, the porosity of the first polypropylene film, the polyethylene film, and the second polypropylene film in the substrate layer of the diaphragm is increased successively, which can further reduce the risk of lithium ions released from the positive electrode accumulating on the surface of the negative electrode in large quantities, improve the problem of lithium plating at the negative electrode, reduce the formation of lithium dendrites at the negative electrode, and improve the safety performance of the lithium battery using the diaphragm. It can also further improve the situation where lithium ion transmission is easily blocked when lithium ions are transmitted from the first polypropylene film of the diaphragm to the second polypropylene film, reduce the internal resistance of the lithium battery, and thus improve the rate performance of the lithium battery.

[0010] Preferably, A=40-60%, B=30-50%, and C=55-70%.

[0011] Preferably, the micropore diameter of the polyethylene film is 0.1-1.0 μm, and / or the micropore diameter of the first polypropylene film is 0.2-0.6 μm, and / or the micropore diameter of the second polypropylene film is 0.5-1.0 μm.

[0012] The porosity and micropore diameter of the polyethylene film, the first polypropylene film and the second polypropylene film in the substrate layer of the diaphragm are adjusted. Specifically, the porosity A of the polyethylene film is controlled between 40 and 60%, and the micropore diameter is controlled between 0.1 and 1.0 μm. The porosity B of the first polypropylene film is controlled between 30 and 50, and the micropore diameter is controlled between 0.2 and 0.6 μm. The porosity C of the second polypropylene film is controlled between 55 and 70%, and the micropore diameter is controlled between 0.5 and 1.0 μm. The substrate layer of the diaphragm is provided with a microporous structure of three-layer composite polyolefin membranes and the porosity and micropore diameter of the polyolefin membrane meet the above design, which can give the diaphragm the characteristics of high strength and low closed pores. When thermal runaway occurs in the lithium battery, the diaphragm shrinkage area can be effectively reduced and the polyolefin pores can be closed in time to play a circuit-breaking role, thereby improving the safety performance of the lithium battery.

[0013] Preferably, the polyethylene film has a thickness of 1 to 8 μm, and / or the first polypropylene film has a thickness of 2 to 7 μm, and / or the second polypropylene film has a thickness of 2 to 7 μm.

[0014] Preferably, the diaphragm further includes a first alumina ceramic layer and / or a second alumina ceramic layer, the first alumina ceramic layer is composited with the first polypropylene film, and the second alumina ceramic layer is composited with the second polypropylene film.

[0015] Preferably, the first alumina ceramic layer and the second alumina ceramic layer are both provided with micropores; the porosity of the first alumina ceramic layer is greater than the porosity of the second alumina ceramic layer, and the porosity of the first alumina ceramic layer is greater than the porosity of the first polypropylene film, and the porosity of the second alumina ceramic layer is less than the porosity of the second polypropylene film.

[0016] By compounding an inorganic ceramic material on the surface of the first polypropylene film and / or the second polypropylene film with a microporous structure, the organic material polypropylene film can give the diaphragm sufficient flexibility to meet the requirements of the lithium battery assembly process. When the lithium battery is continuously squeezed and thermal runaway occurs, under high temperature conditions, the organic components in the polypropylene film will melt and block the micropores, giving the diaphragm the characteristics of low closed pores and the function of sealing the micropores, which can prevent the lithium battery from short circuiting to a certain extent; the inorganic ceramic material is distributed in the three-dimensional structure of the diaphragm, forming a special rigid skeleton, which has excellent heat resistance and can effectively prevent shrinkage and melting during thermal runaway; in addition, since the thermal conductivity of the inorganic ceramic material decreases with increasing temperature, during the continuous squeezing of the lithium battery, the temperature of the lithium battery will increase, causing the thermal conductivity of the inorganic ceramic material to decrease, avoiding the expansion of certain thermal runaway points in the lithium battery and the formation of overall thermal runaway of the lithium battery, thereby improving the safety performance of the lithium battery. In addition, the porosity of the first alumina ceramic layer is greater than that of the second alumina ceramic layer, the porosity of the first alumina ceramic layer is greater than that of the first polypropylene film, and the porosity of the second alumina ceramic layer is less than that of the second polypropylene film. In the process of lithium ions being transmitted from the positive electrode to the negative electrode, the porosity of the first alumina ceramic layer is greater than that of the first polypropylene film, which can increase the transmission speed of lithium ions from the first alumina ceramic layer to the first polypropylene film and improve the situation of excessive accumulation of lithium ions near the positive electrode. At the same time, the porosity of the first alumina ceramic layer is greater than that of the first polypropylene film, and the porosity of the second alumina ceramic layer is less than that of the second polypropylene film. The first polypropylene film and the second alumina ceramic layer with lower porosity can both slow down the migration speed of lithium ions to the negative electrode. Through the above design, the safety performance of the lithium battery can be further improved.

[0017] Preferably, the porosity of the first alumina ceramic layer is 60-80%, and the porosity of the second alumina ceramic layer is 40-55%.

[0018] Preferably, the thickness of the first alumina ceramic layer is 2 to 4 μm, and the thickness of the second alumina ceramic layer is 2 to 4 μm.

[0019] Preferably, the puncture strength of the diaphragm is ≥ 200 gf.

[0020] Preferably, the tensile strength of the separator in the MD direction is ≥1300 kgf / cm 2 , the tensile strength of the diaphragm in the TD direction is ≥100kgf / cm 2 .

[0021] Preferably, the elongation of the separator in the MD direction is ≥40%, and the elongation of the separator in the TD direction is ≥50%.

[0022] The diaphragm whose tensile strength and elongation in the MD direction and TD direction meet the above range has excellent tensile and elongation properties. Applying this diaphragm to lithium batteries can reduce the risk of lithium batteries breaking due to continuous extrusion, improve the pass rate of the plane extrusion test of lithium batteries, and thus improve the safety performance of lithium batteries.

[0023] Preferably, the thermal shrinkage rate of the separator in the MD direction is ≤1%, and the thermal shrinkage rate of the separator in the TD direction is ≤0.5%.

[0024] The diaphragm has an extremely low thermal shrinkage rate of ≤1% in the MD direction and ≤0.5% in the TD direction. When used in lithium batteries, it can further reduce the shrinkage of the diaphragm during the continuous extrusion and heating process of the lithium battery, thereby further improving the safety performance of the lithium battery.

[0025] According to a second aspect of the present invention, a lithium battery is provided, comprising the above-mentioned separator.

[0026] Preferably, the lithium battery further comprises a positive electrode sheet and a negative electrode sheet, the separator is arranged between the positive electrode sheet and the negative electrode sheet, the first polypropylene film is arranged on the side of the separator close to the positive electrode sheet, and the second polypropylene film is arranged on the side of the separator close to the negative electrode sheet.

[0027] The diaphragm provided by the present invention is applied to a lithium battery. On the one hand, by making the porosity of the first polypropylene film on the side close to the positive electrode sheet lower than the porosity of the second polypropylene film on the side close to the negative electrode sheet, lithium ions will be blocked by the first polypropylene film with lower porosity during the transfer process, which slows down the speed of lithium ion migration to the negative electrode. The lithium ions released from the positive electrode are enriched near the positive electrode, so that the lithium ions cannot accumulate in large quantities on the surface of the negative electrode, thereby improving the problem of lithium plating at the negative electrode caused by the accumulation of a large number of lithium ions at the negative electrode, reducing the risk of lithium dendrites piercing the diaphragm due to a large amount of lithium plating at the negative electrode, and thus improving the safety performance of the lithium battery. On the other hand, in the direction from the positive electrode sheet to the negative electrode sheet, the porosity of the first polypropylene film, the polyethylene film, and the second polypropylene film in the substrate layer of the diaphragm increases successively, which improves the situation where lithium ion transmission is easily blocked when lithium ions are transmitted from the first polypropylene film of the diaphragm to the second polypropylene film, reduces the internal resistance of the lithium battery, and thus improves the rate performance of the lithium battery.

[0028] Preferably, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active coating disposed on at least one surface of the negative electrode current collector; the thickness of the negative electrode current collector is 8 to 9 μm.

[0029] Preferably, the tensile strength of the negative electrode current collector is ≥60kg / mm 2 ; The elongation of the negative electrode current collector is ≥9.5%.

[0030] Preferably, the negative electrode current collector is copper foil.

[0031] By combining a separator that meets the above conditions with a thickness of 8 to 9 μm and a tensile strength of ≥ 60 kg / mm 2 , the negative electrode current collector copper foil with an elongation ≥9.5% is used in lithium batteries. During the plane extrusion test of the lithium battery, it can protect the relative integrity of the diaphragm to a certain extent, reduce the risk of diaphragm rupture and short circuit in the lithium battery. The pass rate of the plane extrusion test of the lithium battery is as high as 100%, giving the lithium battery excellent safety performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic structural diagram of the diaphragm in the lithium battery provided by the utility model.

[0033] Description of the drawings: 1 polyethylene film, 2 first polypropylene film, 3 second polypropylene film, 4 first alumina ceramic layer, 5 second alumina ceramic layer. DETAILED DESCRIPTION

[0034] The following is a further clear and complete description of the technical features of the technical solution provided by the present invention in conjunction with specific embodiments. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making any creative efforts shall fall within the scope of protection of the present invention.

[0035] The lithium battery provided by the utility model includes a positive electrode sheet and a negative electrode sheet, and a diaphragm is provided between the positive electrode sheet and the negative electrode sheet. The structural diagram of the diaphragm is as shown in FIG. Figure 1 As shown. Figure 1 The diaphragm includes a substrate layer, which includes a first polypropylene film 2, a polyethylene film 1, and a second polypropylene film 3 that are compounded in sequence. The first polypropylene film 2 is arranged on the side of the diaphragm close to the positive electrode sheet, and the second polypropylene film 3 is arranged on the side of the diaphragm close to the negative electrode sheet; the first polypropylene film 2, the polyethylene film 1, and the second polypropylene film 3 are all provided with micropores; the porosity of the polyethylene film 1 is A, the porosity of the first polypropylene film 2 is B, and the porosity of the second polypropylene film 3 is C, A, B, and C satisfy, C>A, and, C>B.

[0036] The separator in the lithium battery is composed of a first polypropylene film 2, a polyethylene film 1, and a second polypropylene film 3 that are compounded in sequence as the base material layer. The first polypropylene film 2, the polyethylene film 1, and the second polypropylene film 3 in the base material layer are compounded in sequence. On the one hand, by making the porosity B of the first polypropylene film 2 close to the positive electrode lower than the porosity C of the second polypropylene film 3 close to the negative electrode, lithium ions will be blocked by the first polypropylene film 2 with lower porosity during the transfer process, slowing down the speed of lithium ion migration to the negative electrode, and the lithium ions released from the positive electrode are enriched near the positive electrode, making the lithium ions non-reactive. The second polypropylene film 3 has a large number of lithium ions, which are concentrated on the surface of the negative electrode, thereby improving the problem of lithium plating at the negative electrode caused by the accumulation of a large number of lithium ions at the negative electrode, reducing the risk of lithium dendrites piercing the diaphragm due to a large amount of lithium plating at the negative electrode, and thus improving the safety performance of the lithium battery. On the other hand, the porosity C of the second polypropylene film 3 close to the negative electrode side is greater than the porosity A of the polyethylene film 1 located in the middle of the substrate layer in the diaphragm, which is beneficial to improving the situation where lithium ions are blocked when they are transmitted from the polyethylene film 1 of the diaphragm to the second polypropylene film 3, reducing the internal resistance of the lithium battery, and thus improving the rate performance of the lithium battery.

[0037] Furthermore, C>A>B.

[0038] In the direction from the positive electrode sheet to the negative electrode sheet, the porosity of the first polypropylene film 2, the polyethylene film 1, and the second polypropylene film 3 in the substrate layer of the diaphragm is increased successively, which can not only further reduce the risk of lithium ions released from the positive electrode accumulating on the surface of the negative electrode in large quantities, improve the problem of lithium plating at the negative electrode, reduce the formation of lithium dendrites at the negative electrode, and improve the safety performance of the lithium battery using the diaphragm, but also further improve the situation where lithium ion transmission is easily blocked when lithium ions are transmitted from the first polypropylene film 2 of the diaphragm to the second polypropylene film 3, reduce the internal resistance of the lithium battery, and thus improve the rate performance of the lithium battery.

[0039] If the porosity A of the polyethylene film 1, the porosity B of the first polypropylene film 2, and the porosity C of the second polypropylene film 3 in the substrate layer of the diaphragm do not meet the requirements of C>A>B, lithium deposition is likely to occur near the negative electrode during the charge and discharge process of the lithium battery. A large amount of deposited lithium ions will form lithium dendrites, which will pierce the diaphragm, thereby causing the safety performance of the lithium battery to decline. The transmission of lithium ions from the positive electrode to the negative electrode will be hindered, the internal resistance of the lithium battery will increase, and the rate performance of the lithium battery will also decline.

[0040] Furthermore, A=40-60%, B=30-50%, and C=55-70%. Any value within the above ranges for A, B, and C can achieve the desired technical effect of the present invention. For example, A can be 40%, 45%, 50%, 55%, or 60%, B can be 30%, 35%, 40%, 45%, or 50%, and C can be 55%, 60%, 65%, or 70%.

[0041] Furthermore, the diameter of the micropores of the polyethylene film 1 is 0.1 to 1.0 μm, the diameter of the micropores of the first polypropylene film 2 is 0.2 to 0.6 μm, and the diameter of the micropores of the second polypropylene film 3 is 0.5 to 1.0 μm. The micropore diameters of the polyethylene film 1, the first polypropylene film 2, and the second polypropylene film 3 can take any values within the above range to achieve the technical effect that the present invention intends to achieve. For example, the micropore diameters of the polyethylene film 1 can be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, or 1.0 μm; the micropore diameters of the first polypropylene film 2 can be 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, or 0.6 μm; and the micropore diameters of the second polypropylene film 3 can be 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, or 1.0 μm.

[0042] The porosity and micropore diameter of the polyethylene film 1, the first polypropylene film 2 and the second polypropylene film 3 in the substrate layer of the diaphragm are adjusted. Specifically, the porosity A of the polyethylene film 1 is controlled between 40 and 60%, and the micropore diameter is controlled between 0.1 and 1.0 μm. The porosity B of the first polypropylene film 2 is controlled between 30 and 50, and the micropore diameter is controlled between 0.2 and 0.6 μm. The porosity C of the second polypropylene film 3 is controlled between 55 and 70%, and the micropore diameter is controlled between 0.5 and 1.0 μm. The substrate layer of the diaphragm is provided with a microporous structure of three-layer composite polyolefin membranes and the porosity and micropore diameter of the polyolefin membrane meet the above design, which can give the diaphragm the characteristics of high strength and low closed pores. When thermal runaway occurs in the lithium battery, the diaphragm shrinkage area can be effectively reduced and the polyolefin pores can be closed in time to play a circuit-breaking role, thereby improving the safety performance of the lithium battery.

[0043] Furthermore, the thickness of the polyethylene film 1 is 1 to 8 μm, the thickness of the first polypropylene film 2 is 2 to 7 μm, and the thickness of the second polypropylene film 3 is 2 to 7 μm.

[0044] The thickness of the polyethylene film 1, the first polypropylene film 2, and the second polypropylene film 3 can take any value within the above range to achieve the technical effect that the present invention wants to achieve. For example, the thickness of the polyethylene film 1 can be 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, and 8μm, the thickness of the first polypropylene film 2 can be 2μm, 3μm, 4μm, 5μm, 6μm, and 7μm, and the thickness of the second polypropylene film 3 can be 2μm, 3μm, 4μm, 5μm, 6μm, and 7μm.

[0045] Furthermore, the diaphragm further includes a first alumina ceramic layer 4 and / or a second alumina ceramic layer 5 . The first alumina ceramic layer 4 is composited with the first polypropylene film 5 , and the second alumina ceramic layer 5 is composited with the second polypropylene film 3 .

[0046] Furthermore, the first alumina ceramic layer 4 and the second alumina ceramic layer 4 are both provided with micropores; the porosity of the first alumina ceramic layer 4 is greater than the porosity of the second alumina ceramic layer 5, and the porosity of the first alumina ceramic layer 4 is greater than the porosity of the first polypropylene film 2, and the porosity of the second alumina ceramic layer 5 is less than the porosity of the second polypropylene film 3.

[0047] By compounding an inorganic ceramic material on the surface of the first polypropylene film and / or the second polypropylene film with a microporous structure, the organic material polypropylene film can give the diaphragm sufficient flexibility to meet the requirements of the lithium battery assembly process. When the lithium battery is continuously squeezed and thermal runaway occurs, under high temperature conditions, the organic components in the polypropylene film will melt and block the micropores, giving the diaphragm the characteristics of low closed pores and the function of sealing the micropores, which can prevent the battery from short circuiting to a certain extent; the inorganic ceramic material is distributed in the three-dimensional structure of the diaphragm, forming a special rigid skeleton, which has excellent heat resistance and can effectively prevent shrinkage and melting during thermal runaway; in addition, since the thermal conductivity of the inorganic ceramic material decreases with increasing temperature, during the continuous squeezing of the lithium battery, the temperature of the lithium battery will increase, causing the thermal conductivity of the inorganic ceramic material to decrease, avoiding the expansion of certain thermal runaway points in the lithium battery and the formation of overall thermal runaway of the lithium battery, thereby improving the safety performance of the lithium battery. In addition, the porosity of the first alumina ceramic layer is greater than that of the second alumina ceramic layer, the porosity of the first alumina ceramic layer is greater than that of the first polypropylene film, and the porosity of the second alumina ceramic layer is less than that of the second polypropylene film. In the process of lithium ions being transmitted from the positive electrode to the negative electrode, the porosity of the first alumina ceramic layer is greater than that of the first polypropylene film, which can increase the transmission speed of lithium ions from the first alumina ceramic layer to the first polypropylene film and improve the situation of excessive accumulation of lithium ions near the positive electrode. At the same time, the porosity of the first alumina ceramic layer is greater than that of the first polypropylene film, and the porosity of the second alumina ceramic layer is less than that of the second polypropylene film. The first polypropylene film and the second alumina ceramic layer with lower porosity can both slow down the migration speed of lithium ions to the negative electrode. Through the above design, the safety performance of the lithium battery can be further improved.

[0048] Furthermore, the porosity of the first alumina ceramic layer 4 is 60-80%, and the porosity of the second alumina ceramic layer 5 is 40-55%. The porosity of the first alumina ceramic layer 4 and the porosity of the second alumina ceramic layer 5 can be any value within the above range to achieve the technical effect intended by the present invention. For example, the porosity of the first alumina ceramic layer 4 can be 60%, 65%, 70%, 75%, or 80%, and the porosity of the second alumina ceramic layer 5 can be 40%, 45%, 50%, or 55%.

[0049] Furthermore, the thickness of the first alumina ceramic layer 4 is 2 to 4 μm, and the thickness of the second alumina ceramic layer 5 is 2 to 4 μm. The thickness of the first alumina ceramic layer 4 and the second alumina ceramic layer 5 can be any value within the above range to achieve the technical effect of the utility model. For example, the thickness of the first alumina ceramic layer 4 can be 2 μm, 3 μm, or 4 μm, and the thickness of the second alumina ceramic layer 5 can be 2 μm, 3 μm, or 4 μm.

[0050] Furthermore, the puncture strength of the diaphragm is ≥ 200 gf. Any value within the above range can achieve the technical effect of the present invention. For example, the puncture strength of the diaphragm can be 200 gf, 210 gf, 220 gf, 230 gf, 240 gf, 250 gf, 260 gf, 270 gf, 280 gf, 290 gf, or 300 gf.

[0051] Furthermore, the tensile strength of the separator in the MD direction is ≥1300 kgf / cm 2 , the tensile strength of the diaphragm in the TD direction is ≥100kgf / cm 2 The tensile strength of the diaphragm in the MD direction and the tensile strength in the TD direction can take any value within the above range to achieve the technical effect that the present invention intends to achieve. For example, the tensile strength of the diaphragm in the MD direction can be 1300kgf / cm 2 、1320kgf / cm 2 、1340kgf / cm 2 、1360kgf / cm 2 、1380kgf / cm 2 、1400kgf / cm 2 、1420kgf / cm 2 、1440kgf / cm 2 、1460kgf / cm 2 、1480kgf / cm 2 、1500kgf / cm 2 , the tensile strength of the diaphragm in the TD direction can be 100kgf / cm 2、120kgf / cm 2 、140kgf / cm 2 、160kgf / cm 2 、180kgf / cm 2 , 200kgf / cm 2 、220kgf / cm 2 、240kgf / cm 2 、260kgf / cm 2 、280kgf / cm 2 、300kgf / cm 2 .

[0052] Furthermore, the elongation of the separator in the MD direction is ≥40%, and the elongation of the separator in the TD direction is ≥50%. Any value within the above range for the elongation of the separator in the MD direction and the elongation of the separator in the TD direction can achieve the technical effect intended by the utility model. For example, the elongation of the separator in the MD direction can be 40%, 45%, 50%, 55%, or 60%, and the elongation of the separator in the TD direction can be 50%, 55%, 60%, 65%, or 70%.

[0053] The diaphragm whose tensile strength and elongation in the MD direction and TD direction meet the above range has excellent tensile and elongation properties. Applying this diaphragm to lithium batteries can reduce the risk of lithium batteries breaking due to continuous extrusion, improve the pass rate of the plane extrusion test of lithium batteries, and thus improve the safety performance of lithium batteries.

[0054] Furthermore, the thermal shrinkage rate of the separator in the MD direction is ≤1%, and the thermal shrinkage rate of the separator in the TD direction is ≤0.5%. The thermal shrinkage rate of the separator in the MD direction and the thermal shrinkage rate in the TD direction can be any value within the above ranges to achieve the technical effect intended by the utility model. For example, the thermal shrinkage rate of the separator in the MD direction can be 1%, 0.9%, 0.8%, 0.7%, 0.6%, or 0.5%, and the thermal shrinkage rate of the separator in the TD direction can be 0.5%, 0.4%, 0.3%, 0.2%, or 0.1%.

[0055] The diaphragm has an extremely low thermal shrinkage rate of ≤1% in the MD direction and ≤0.5% in the TD direction. When used in lithium batteries, it can further reduce the shrinkage of the diaphragm during the continuous extrusion and heating process of the lithium battery, thereby further improving the safety performance of the lithium battery.

[0056] Furthermore, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active coating provided on at least one surface of the negative electrode current collector; the thickness of the negative electrode current collector is 8 to 9 μm; the tensile strength of the negative electrode current collector is ≥ 60 kg / mm2 The elongation of the negative electrode current collector is ≥9.5%. Any value within the above ranges for the thickness, tensile strength, and elongation of the negative electrode current collector can achieve the desired technical effect of the present invention. For example, the thickness of the negative electrode current collector can be 8μm, 8.1μm, 8.2μm, 8.3μm, 8.4μm, 8.5μm, 8.6μm, 8.7μm, 8.8μm, 8.9μm, or 9.0μm, and the tensile strength of the negative electrode current collector can be 60kg / mm 2 、65kg / mm 2 , 70kg / mm 2 , 75kg / mm 2 、80kg / mm 2 , the elongation of the negative electrode current collector can be 9.5%, 10.0%, 10.5%, 11.0%, 11.5%, and 12.0%.

[0057] Furthermore, the negative electrode current collector is copper foil.

[0058] By combining a separator that meets the above conditions with a thickness of 8 to 9 μm and a tensile strength of ≥ 60 kg / mm 2 , the negative electrode current collector copper foil with an elongation ≥9.5% is used in lithium batteries. During the plane extrusion test of the lithium battery, it can protect the relative integrity of the diaphragm to a certain extent, reduce the risk of diaphragm rupture and short circuit in the lithium battery. The pass rate of the plane extrusion test of the lithium battery is as high as 100%, giving the lithium battery excellent safety performance.

[0059] 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, ordinary technicians in the relevant field should understand that the technical solutions of the present invention can be modified or replaced by equivalents, but these modifications or replacements are all within the scope of protection of the present invention.

Claims

1. A diaphragm, characterized in that: The diaphragm comprises a substrate layer, wherein the substrate layer comprises a first polypropylene film (2), a polyethylene film (1), and a second polypropylene film (3) which are compounded in sequence; The first polypropylene film (2), the polyethylene film (1), and the second polypropylene film (3) are all provided with micropores; The porosity of the polyethylene film (1) is A, the porosity of the first polypropylene film (2) is B, and the porosity of the second polypropylene film (3) is C. A, B, and C satisfy C>A, and C>B.

2. The diaphragm according to claim 1, wherein: C>A>B.

3. The diaphragm according to claim 2, wherein: A=40~60%, B=30~50%, C=55~70%.

4. The diaphragm according to claim 1, wherein: The micropore diameter of the polyethylene film (1) is 0.1 to 1.0 μm, and / or the micropore diameter of the first polypropylene film (2) is 0.2 to 0.6 μm, and / or the micropore diameter of the second polypropylene film (3) is 0.5 to 1.0 μm.

5. The diaphragm according to claim 1, wherein: The thickness of the polyethylene film (1) is 1 to 8 μm, and / or the thickness of the first polypropylene film (2) is 2 to 7 μm, and / or the thickness of the second polypropylene film (3) is 2 to 7 μm.

6. The diaphragm according to claim 1, wherein: The diaphragm further comprises a first alumina ceramic layer (4) and / or a second alumina ceramic layer (5), wherein the first alumina ceramic layer (4) is composited with the first polypropylene film (2), and the second alumina ceramic layer (5) is composited with the second polypropylene film (3).

7. The diaphragm according to claim 6, wherein: The first alumina ceramic layer (4) and the second alumina ceramic layer (5) are both provided with micropores; The porosity of the first alumina ceramic layer (4) is greater than the porosity of the second alumina ceramic layer (5), and the porosity of the first alumina ceramic layer (4) is greater than the porosity of the first polypropylene film (2), and the porosity of the second alumina ceramic layer (5) is less than the porosity of the second polypropylene film (3).

8. The diaphragm according to claim 7, wherein: The porosity of the first alumina ceramic layer (4) is 60-80%, and the porosity of the second alumina ceramic layer (5) is 40-55%; and / or, The thickness of the first alumina ceramic layer (4) is 2 to 4 μm, and the thickness of the second alumina ceramic layer (5) is 2 to 4 μm.

9. The diaphragm according to claim 1, wherein: The puncture strength of the diaphragm is ≥200 gf.

10. The diaphragm according to claim 1, wherein: The tensile strength of the diaphragm in the MD direction is ≥1300 kgf / cm 2 The tensile strength of the diaphragm in the TD direction is ≥100kgf / cm 2 .

11. The diaphragm according to claim 1, wherein: The elongation of the separator in the MD direction is ≥40%, and the elongation of the separator in the TD direction is ≥50%.

12. The diaphragm according to claim 1, wherein: The thermal shrinkage rate of the separator in the MD direction is ≤1%, and the thermal shrinkage rate of the separator in the TD direction is ≤0.5%.

13. A lithium battery, characterized in that: The lithium battery comprises the separator according to any one of claims 1 to 12.

14. The lithium battery according to claim 13, wherein: The lithium battery further comprises a positive electrode sheet and a negative electrode sheet, the diaphragm is arranged between the positive electrode sheet and the negative electrode sheet, the first polypropylene film (2) is arranged on a side of the diaphragm close to the positive electrode sheet, and the second polypropylene film (3) is arranged on a side of the diaphragm close to the negative electrode sheet.

15. The lithium battery according to claim 14, wherein: The negative electrode sheet includes a negative electrode current collector and a negative electrode active coating disposed on at least one surface of the negative electrode current collector; The thickness of the negative electrode current collector is 8 to 9 μm.

16. The lithium battery according to claim 15, wherein: The tensile strength of the negative electrode current collector is ≥60kg / mm 2 , and / or, the elongation of the negative electrode current collector is ≥9.5%.

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