Isolation film and electrochemical device

Abstract

The application discloses an isolation film and an electrochemical device. The isolation film comprises a first layer, the first layer comprises at least one cellulose layer and at least one polyolefin layer, the cellulose layer and the polyolefin layer are alternately stacked, the cellulose layer has a three-dimensional pore structure, and the polyolefin layer has a unidirectional pore structure. Wherein, Cu ions entering the cellulose layer can diffuse in multiple directions, and the problem that Cu dendrites grow along a single line direction from an anode to a cathode to pierce the isolation film is improved. The cellulose layer and the polyolefin layer are alternately stacked, further hindering the unidirectional growth of copper dendrites, and short circuiting is not easy. In addition, the alternately stacked structure can also improve the mass transfer power, thereby improving the liquid absorption capacity of the isolation film, improving poor wetting, and also increasing the mechanical strength of the prepared isolation film.

Description

Technical Field

[0001] This application relates to the field of chemical apparatus technology, and more particularly to a separating membrane and an electrochemical device. Background Technology

[0002] The separator is one of the core technologies of lithium-ion batteries. Its performance determines the battery's interface structure, internal resistance, and other characteristics, directly affecting capacity, cycle life, and safety. The separator is a porous, electrically insulating film that does not participate in the electrochemical reactions within the battery. Its function is to isolate the positive and negative electrodes, preventing electrons from freely passing through while allowing ions to pass freely between them. Therefore, the separator needs excellent electrical insulation, high ion permeability, good mechanical properties, solvent resistance, high-temperature resistance, and high wettability. When the positive and negative electrodes are thick and large, an unsuitable pore size in the separator will lead to poor electrolyte wetting, lithium plating, and a significant drop in battery cycle performance. For example, while an excessively large pore size can improve electrolyte wetting and rate performance, it also increases the risk of micro-short circuits and copper dendrite growth that can puncture the separator. Summary of the Invention

[0003] To address the aforementioned problems, embodiments of this application provide a separating membrane and an electrochemical device.

[0004] In a first aspect, embodiments of this application provide a separating membrane, the separating membrane comprising a first layer, the first layer comprising a cellulose layer of layer n1 and a polyolefin layer of layer n2, wherein n1 and n2 are both greater than or equal to 1, and the cellulose layer and the polyolefin layer are alternately stacked, the cellulose layer having a three-dimensional porous structure, and the polyolefin layer having a unidirectional porous structure.

[0005] In some exemplary embodiments, the total thickness of the cellulose layer is H1, and the total thickness of the polyolefin layer is H2, wherein H1 and H2 satisfy: 0.2≤H1 / H2≤5.

[0006] In some exemplary embodiments, H1 and H2 satisfy: 1≤H1 / H2≤2.

[0007] In some exemplary embodiments, the separator membrane satisfies at least one of the following conditions:

[0008] Condition a: The total thickness of the cellulose layer is H1, and H1 satisfies: 2μm≤H1≤15μm;

[0009] Condition b: The total thickness of the polyolefin layer is H2, and H2 satisfies: 2μm≤H2≤15μm;

[0010] Condition c: The thickness of the first layer is H0, and H0 satisfies: 4μm≤H0≤30μm.

[0011] In some exemplary embodiments, the polyolefin layer is selected from at least one of polypropylene release films, polyethylene release films, and polyimide release films.

[0012] In some exemplary embodiments, the separator membrane satisfies at least one of the following conditions:

[0013] Condition 1: The average pore size of the three-dimensional pore structure of the cellulose layer is r1, and r1 satisfies: 1μm≤r1≤5μm;

[0014] Condition 2: The average pore size of the unidirectional pore structure of the polyolefin layer is r2, and r2 satisfies: 40nm≤r2≤250nm.

[0015] In some exemplary embodiments, the air permeability of the first layer is W, where W satisfies: 30s / 100cc ≤ W ≤ 625s / 100cc.

[0016] In some exemplary embodiments, the porosity of the three-dimensional pore structure of the cellulose layer is X1, where X1 satisfies: 40% ≤ X1 ≤ 60%; and the porosity of the one-dimensional pore structure of the polyolefin layer is X2, where X2 satisfies: 30% ≤ X2 ≤ 50%.

[0017] In some exemplary embodiments, the liquid absorption capacity of the first layer is m1, where m1 satisfies: 10 mm / min ≤ m1 ≤ 32 mm / min.

[0018] In some exemplary embodiments, the longitudinal tensile strength of the first layer is MD, and the transverse tensile strength of the first layer is TD, satisfying: 1≤MD / TD≤10.

[0019] In some exemplary embodiments, n2 = n1 + 1, the polyolefin layer on one surface of the separator is disposed toward the negative electrode sheet, and the polyolefin layer on the other surface of the separator is disposed toward the positive electrode sheet; or, n2 = n1, the cellulose layer on one surface of the separator is disposed toward the negative electrode sheet, and the polyolefin layer on the other surface of the separator is disposed toward the positive electrode sheet.

[0020] In some exemplary embodiments, the separator further includes a second layer; the second layer is disposed on the surface of the first layer, and the second layer is selected from at least one of alumina, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, or titanium dioxide.

[0021] Secondly, this application provides an electrochemical device including the isolation membrane described above.

[0022] Based on the separator and electrochemical device of this application embodiment, Cu ions can diffuse in multiple directions within the cellulose layer, improving the problem of Cu dendrites growing along a single-line direction from the anode to the cathode and piercing the separator. The alternating layers of cellulose and polyolefin further hinder the unidirectional growth of copper dendrites. Furthermore, this growth requires a larger absolute amount of copper, making short circuits less likely. The pore size of the three-dimensional porous structure of the cellulose layer is larger than that of the unidirectional porous structure of the polyolefin layer, allowing the separator to accommodate a larger amount of copper. The smaller-pore polyolefin layer, located outside the cellulose layer, also hinders the unidirectional growth of copper dendrites, further improving the problem of copper dendrites easily piercing the separator.

[0023] The staggered pore structures of the cellulose and polyolefin layers enhance mass transfer, thereby improving the liquid absorption capacity of the separator and mitigating poor wetting. The three-dimensional pore structure of the cellulose layer provides a larger liquid retention space, contributing to increased liquid retention capacity of the separator. Furthermore, the alternating layers of cellulose and polyolefin increase the mechanical strength of the resulting separator and improve its processability. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a cross-sectional schematic diagram of an embodiment of the separator membrane of this application, comprising a cellulose layer and a polyolefin layer;

[0026] Figure 2 This is a cross-sectional schematic diagram of an embodiment of the separator of this application, comprising a cellulose layer and two polyolefin layers;

[0027] Figure 3 This is a cross-sectional schematic diagram of an embodiment of the separator membrane of this application, comprising multiple cellulose layers and multiple polyolefin layers;

[0028] Figure 4 This is an electron microscope image of a cellulose layer according to an embodiment of this application;

[0029] Figure 5 This is an electron microscope image of the polyolefin layer according to an embodiment of this application.

[0030] 100. Separating membrane; 101. First layer; 102. Second layer; 103. Third layer; 110. Cellulose layer; 120. Polyolefin layer. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0032] This application provides a separator 100 disposed between the positive and negative electrodes of an electrochemical device. The positive electrode includes a positive current collector and a positive active material layer. The positive active material layer is disposed on the surface of the positive current collector and includes positive active material. In this application embodiment, the positive active material is recycled positive active material. The inventors have found that recycled positive active material is prone to mixing with anode copper foil. In related technologies, during the formation (PIEF) process for preparing an electrochemical device, copper is easily oxidized and dissolved, and dendrites are easily deposited at the anode. Copper dendrites can easily pierce the separator 100, leading to problems such as short circuits between the positive and negative electrodes and poor K value.

[0033] This application provides a separating membrane 100, which includes a cellulose layer 110 (n1 layer) and a polyolefin layer 120 (n2 layer), where n1 and n2 are both greater than or equal to 1. Figures 1 to 3 As shown, cellulose layer 110 and polyolefin layer 120 are alternately stacked, as... Figure 4 As shown, the cellulose layer 110 has a three-dimensional porous structure, such as Figure 5 As shown, the polyolefin layer 120 has a unidirectional porous structure. When Cu ions enter the interior of the separator 100, they can more easily pass through the unidirectional pores of the polyolefin layer 120 to reach the cellulose layer 110. Within the cellulose layer 110, Cu ions can diffuse in multiple directions, preventing Cu dendrites from growing along a single line from the anode to the cathode and piercing the separator 100. The alternating layers of the cellulose layer 110 and the polyolefin layer 120 allow the three-dimensional porous structure of the cellulose layer 110 and the unidirectional porous structure of the polyolefin layer 120 to interweave, further hindering the unidirectional growth of copper dendrites. Furthermore, this growth pattern requires a greater absolute amount of copper to penetrate the separator 100, reducing the likelihood of short circuits. The pore size of the three-dimensional pore structure of the cellulose layer 110 is larger than that of the unidirectional pore structure of the polyolefin layer 120, which allows the separator 100 to hold more copper. The smaller pore size of the polyolefin layer 120 on the outside of the cellulose layer 110 can also hinder the unidirectional growth of copper dendrites, further improving the problem that copper dendrites can easily puncture the separator 100.

[0034] The pore sizes of both the three-dimensional porous structure of the cellulose layer 110 and the unidirectional porous structure of the polyolefin layer 120 are in the micrometer or even nanometer range. The alternating arrangement of the pore structures of the cellulose layer 110 and the polyolefin layer 120 can also improve mass transfer dynamics, thereby enhancing the liquid absorption capacity of the separator 100 and improving poor wetting. The three-dimensional porous structure of the cellulose layer 110 provides a larger liquid retention space, which helps to increase the liquid retention capacity of the separator 100. Furthermore, the alternating layering of the cellulose layer 110 and the polyolefin layer 120 can also increase the mechanical strength of the resulting separator 100 and improve its processability.

[0035] In some exemplary embodiments, the total thickness of the cellulose layer 110 is H1, and the total thickness of the polyolefin layer 120 is H2. H1 and H2 satisfy the condition: 0.2 ≤ H1 / H2 ≤ 5. For example, H1 / H2 can be 0.2, 1.0, 1.2, 1.5, 2.0, 3.0, or 5. The thickness of both the cellulose layer 110 and the polyolefin layer 120 satisfying 0.2 ≤ H1 / H2 ≤ 5 helps to obtain a separator membrane 100 with suitable porosity and pore volume, inhibits the growth of Cu dendrites along a single-line direction from the anode to the cathode, and helps to improve the liquid absorption capacity of the separator membrane 100. It also gives the separator membrane 100 good mechanical properties, so that when the separator membrane 100 is sandwiched between the positive electrode and the negative electrode, the separator membrane 100 can provide effective support and prevent short circuit between the positive electrode and the negative electrode. When H1 / H2 < 0.2, the cellulose layer 110 is too thin and insufficient to suppress the unidirectional growth of copper dendrites, making the separator 100 easily punctured. When H1 / H2 > 5, the polyolefin layer 120 is too thin, resulting in poor mechanical strength of the separator 100 and affecting the cycle performance of the electrochemical device. Preferably, H1 and H2 satisfy: 1 ​​≤ H1 / H2 ≤ 2, which has a good inhibitory effect on the unidirectional growth of Cu dendrites from the anode to the cathode.

[0036] In some exemplary embodiments, the separator 100 satisfies condition a: the total thickness of the cellulose layer 110 is H1, where H1 satisfies: 2μm ≤ H1 ≤ 15μm. For example, H1 can be 2μm, 4μm, 6μm, 8μm, 12μm, 15μm, etc. Within the above thickness range, the thickness of the cellulose layer 110 is suitable, providing effective space to accommodate copper dendrite growth, thereby improving the situation where copper dendrites easily puncture the separator 100.

[0037] In some exemplary embodiments, the separator 100 satisfies condition b: the total thickness of the polyolefin layer 120 is H2, and H2 satisfies: 2μm ≤ H2 ≤ 15μm. For example, H2 can be 2μm, 4μm, 6μm, 8μm, 12μm, 15μm, etc. The polyolefin layer 120 is used to provide support for the cellulose layer 110, making the structure of the cellulose layer 110 stable, thereby making the resulting separator 100 structurally stable and with high mechanical strength.

[0038] In some exemplary embodiments, the first layer satisfies condition c: the thickness of the first layer is H0, and H0 satisfies: 4μm ≤ H0 ≤ 30μm. For example, H0 can be 4μm, 8μm, 12μm, 16μm, 20μm, 24μm, 30μm, etc. When the thickness of the first layer satisfies 4μm ≤ H0 ≤ 30μm, the separator 100 of appropriate thickness can provide effective support for the positive and negative electrode plates, prevent short circuits, and prevent the separator 100 from being too thick and occupying too much space, thus reducing the energy density of the electrochemical device.

[0039] In some exemplary embodiments, the polyolefin layer 120 is selected from at least one of polypropylene separator 100, polyethylene separator 100, and polyimide separator 100. The polyolefin layer 120 of the above materials helps to improve the composite stability of the polyolefin layer 120 and the cellulose layer 110, making them less prone to peeling, thereby better preventing copper dendrite growth from piercing the separator 100.

[0040] Optionally, when the polyolefin layer 120 includes multiple layers, each polyolefin layer 120 can be independently selected from one of polypropylene release film 100, polyethylene release film 100, and polyimide release film 100. That is, the materials of the multiple polyolefin layers 120 can be the same or different.

[0041] In some exemplary embodiments, the isolation membrane 100 satisfies condition 1: the pore size of the three-dimensional pore structure of the cellulose layer 110 is r1, and r1 satisfies: 1μm≤r1≤5μm. For example, r1 can be 1μm, 2.0μm, 2.5μm, 3.5μm, 5.0μm, etc.

[0042] In some exemplary embodiments, the separator 100 satisfies condition 2: the pore size of the one-way porous structure of the polyolefin layer 120 is r2, and r2 satisfies: 40nm ≤ r2 ≤ 250nm. For example, r2 can be 40nm, 60nm, 100nm, 180nm, or 250nm. Within the above pore size range, the pore size r2 of the one-way porous structure of the polyolefin layer 120 is suitable, facilitating electrolyte penetration and helping to improve the cycle performance of the electrochemical device in conjunction with the cellulose layer 110. When r2 < 40nm, the pore size of the one-way porous structure of the polyolefin layer 120 is too small, resulting in high impedance; when r2 > 250nm, the pore size of the one-way porous structure of the polyolefin layer 120 is too large, making it prone to pinholes and causing poor K-value in the electrochemical device.

[0043] In some exemplary embodiments, the air permeability of the first layer is W, where W satisfies: 30s / 100cc ≤ W ≤ 625s / 100cc. For example, W can be 30s / 100cc, 110s / 100cc, 230s / 100cc, 430s / 100cc, 550s / 100cc, or 625s / 100cc, etc. Within the above air permeability range, the separator 100 has good liquid absorption capacity, and the separator 100 can accommodate a larger amount of copper, which helps to improve the problem of copper dendrites easily puncturing the separator 100.

[0044] In some exemplary embodiments, the porosity of the three-dimensional pore structure of the cellulose layer 110 is X1, where X1 satisfies: 40% ≤ X1 ≤ 60%, for example, X1 can be 40%, 45%, 50%, 55%, or 60%, etc. Within the above range, the cellulose layer 110 has sufficient pore structure to allow copper dendrites to grow in multiple directions, making the separator 100 less prone to short circuits. When X1 < 40%, the three-dimensional pore structure of the cellulose layer 110 is insufficient, and the space for accommodating copper dendrites is limited, causing the separator 100 to be prone to short circuits; when X1 > 60%, the internal pore volume of the cellulose layer 110 is too large, resulting in low mechanical strength of the cellulose layer 110, thus leading to low mechanical strength of the separator 100.

[0045] In some exemplary embodiments, the porosity of the unidirectional pore structure of the polyolefin layer 120 is X2, where X2 satisfies: 30% ≤ X2 ≤ 50%. For example, X2 can be 30%, 35%, 40%, 45%, or 50%. Within this range, the porosity of the polyolefin layer 120 is suitable, enabling it to provide good support. Simultaneously, the pore size of the unidirectional pore structure ensures a suitable specific surface area, facilitating improved liquid absorption capacity of the separator 100 and mitigating poor wetting. When X2 < 30%, the pore structure of the polyolefin layer 110 is insufficient, resulting in poor electrolyte wetting, high impedance, and negatively impacting the cycle performance of the electrochemical device. When X2 > 50%, the pore structure of the polyolefin layer 120 is excessive, making it difficult to provide adequate barrier protection against short circuits between the positive and negative electrodes. Furthermore, the polyolefin layer 120 is prone to pinholes, leading to poor K-value in the electrochemical device.

[0046] In some exemplary embodiments, the liquid absorption capacity of the first layer is m1, where 10 mm / min ≤ m1 ≤ 32 mm / min. For example, m1 can be 10 mm / min, 15 mm / min, 19 mm / min, 23 mm / min, 28 mm / min, or 32 mm / min, etc. Within the above liquid absorption capacity range, the separator 100 has a fast liquid absorption speed, and the electrolyte can permeate the separator 100 in a short time, which helps to shorten the settling time of the battery formation process and improve efficiency.

[0047] In some exemplary embodiments, the ratio of the longitudinal tensile strength MD to the transverse tensile strength TD of the first layer satisfies: 1 ≤ MD / TD ≤ 10. For example, MD / TD can be 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10. When the MD / TD value of the first layer meets the above range, the separator 100 is applied in an electrochemical device, and the electrochemical device has good drop resistance.

[0048] A separator 100 is disposed between the positive and negative electrodes of the electrochemical device. When n2 = n1, in the thickness direction of the separator 100, a cellulose layer 110 on one surface of the separator 100 is positioned towards the negative electrode, and a polyolefin layer 120 on the other surface of the separator 100 is positioned towards the positive electrode. For example, as... Figure 1 As shown, when the separator 100 includes a cellulose layer 110 and a polyolefin layer 120, the cellulose layer 110 is positioned facing the negative electrode. During the formation process, copper dendrites easily precipitate at the negative electrode. When the cellulose layer 110 is on the surface facing the negative electrode, copper dendrites can penetrate it. The polyolefin layer 120 has a small pore size due to its unidirectional porous structure, which can block copper dendrites and reduce the occurrence of copper dendrites piercing the separator 100. When n2 = n1 + 1, as... Figure 2 and Figure 3 As shown, in the thickness direction of the separator 100, the two opposite surface layers of the separator 100 are both polyolefin layers 120. In this case, this application does not limit the direction of the polyolefin layer 120 on the surface of the separator 100 toward the positive electrode and the negative electrode. Specifically, it can be selected according to actual needs.

[0049] In some exemplary embodiments, the separator 100 further includes a second layer 102 disposed on the surface of the first layer 101. For example, in the thickness direction of the separator 100, the second layer 102 is disposed on two opposite surfaces of the first layer 101. The second layer 102 is used to reduce the thermal shrinkage rate of the separator 100 and improve the high-temperature safety performance of the electrochemical device. The second layer 102 is selected from at least one of alumina, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, or titanium dioxide.

[0050] In some exemplary embodiments, the separator 100 further includes a third layer 103 disposed on the surface of the second layer 102. The third layer 103 is used to adhere to the positive electrode and the negative electrode to fix the position of the separator 100 relative to the positive electrode and the negative electrode. The adhesive layer is selected from at least one of polyvinylidene fluoride and polymethyl methacrylate.

[0051] This application also provides an electrochemical device, including a positive electrode, a negative electrode, and a separator 100 as described above. The positive electrode, separator 100, and negative electrode are sequentially stacked or sequentially stacked and then wound to form an electrode assembly. The positive electrode includes a positive current collector and a positive active material layer disposed on the surface of the positive current collector, and the negative electrode includes a negative current collector and a negative active material layer disposed on the surface of the negative current collector. The secondary battery also includes a positive electrode tab, a negative electrode tab, a packaging bag, and an electrolyte. The electrode assembly is disposed in the internal space of the packaging bag. The positive electrode tab is electrically connected to the positive current collector, and the negative electrode tab is electrically connected to the negative current collector. Both the positive and negative electrode tabs extend from the internal space of the packaging bag for electrical connection to an external circuit. The electrolyte fills the internal space of the packaging bag to wet the positive electrode, negative electrode, and separator 100.

[0052] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0053] The positive electrode active material layer comprises compounds that reversibly insert or remove lithium ions. In some embodiments, the positive electrode active material layer comprises a positive electrode active material with an operating potential of 4.5V or higher relative to metallic lithium, i.e., the positive electrode active material of this application can operate under high voltage. In some embodiments, the positive electrode active material may include at least one of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, or lithium nickel manganese oxide, and the above-mentioned positive electrode active materials may be doped and / or coated. In some embodiments, the coating elements used for the coating layer may include K, Na, Ca, Mg, B, Al, Co, Si, V, Ga, Sn, Zr, or mixtures thereof.

[0054] In some embodiments, the positive electrode active material layer further includes a binder and a conductive agent. In some embodiments, the conductive agent in the positive electrode active material layer may include at least one of conductive carbon black, acetylene black, Ketjen black, sheet graphite, graphene, carbon nanotubes, or carbon fibers. In some embodiments, the binder in the positive electrode active material layer may include at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, styrene-acrylate copolymer, styrene-butadiene copolymer, polyamide, sodium carboxymethyl cellulose, polyvinyl acetate, polyvinylpyrrolidone, polyvinyl ether, polytetrafluoroethylene, polyhexafluoropropylene, or polymethyl methacrylate.

[0055] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, the positive electrode current collector may be aluminum foil. The composite current collector may include a polymeric material substrate and a metal material layer formed on at least one surface of the polymeric material substrate. As an example, the metal material may be selected from one or more of aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. As an example, the polymeric material substrate may be selected from polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.

[0056] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode active material layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0057] The negative electrode active material layer includes a negative electrode active material, which includes a material that reversibly inserts / de-intercalates lithium ions. In some embodiments, the material that reversibly inserts / de-intercalates lithium ions includes at least one of lithium metal, lithium alloy, carbon material, or silicon-based material. In some embodiments, the lithium alloy may include at least one of Li-Ag, Li-Al, Li-B, Li-Mg, Li-Au, Li-Na, Li-Si, Li-Sn, Li-Zn, Li-Ba, Li-Bi, Li-C, Li-Ca, Li-Ge, Li-Cs, Li-Ga, Li-K, Li-Pb, Li-P, Li-Sb, Li-B-Mg, or Li-Mg-Si. The carbon material includes crystalline carbon, amorphous carbon, and combinations thereof. The silicon-based material includes at least one of silicon, silicon oxides, silicon carbide compounds, or silicon alloys.

[0058] In some embodiments, the negative electrode active material layer may further include a conductive agent and / or a binder. The conductive agent in the negative electrode active material layer may include at least one of carbon black, acetylene black, Ketjen black, sheet graphite, graphene, carbon nanotubes, carbon fibers, or carbon nanowires. In some embodiments, the binder in the negative electrode active material layer may include at least one of carboxymethyl cellulose (CMC), polyacrylic acid, polyacrylate, polyacrylate, polyvinylpyrrolidone, polyaniline, polyimide, polyamide-imide, polysiloxane, styrene-butadiene rubber, epoxy resin, polyester resin, polyurethane resin, or polyfluorene.

[0059] In some embodiments, the negative electrode current collector may be at least one of copper foil, nickel foil, or carbon-based current collector.

[0060] The electrolyte acts as a conductor of active ions between the positive and negative electrodes. The electrolyte used in the secondary battery of this application can be any electrolyte known in the prior art. The electrolyte includes a non-aqueous organic solvent and a lithium salt. In some embodiments, the concentration of the lithium salt in the electrolyte is from 0.5 mol / L to 5 mol / L.

[0061] In some embodiments, the non-aqueous organic solvent includes carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, other aprotic solvents, or combinations thereof. Examples of carbonate solvents include diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, and combinations thereof. Examples of carboxylic acid ester solvents include methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerate, mevalonolactone, caprolactone, and combinations thereof. Examples of ether solvents include tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 2-methyl-1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,3-dioxane, 1,4-dioxane, dimethoxypropane, dimethoxymethane, 1,1-dimethoxyethane, 1,2-dimethoxyethane, diethoxymethane, 1,1-diethoxyethane, 1,2-diethoxyethane, ethoxymethoxymethane, 1,1-ethoxymethoxyethane, 1,2-ethoxymethane. Methoxyethane, HCF₂CF₂CH₂OCF₂CF₂H, (CF₃)₂CFCF(CF₂CF₃)(OCH₃), CF₃CHFCF₂CH(CH₃)OCF₂CHFCF₃, HCF₂CF₂CH₂OCF₂CF₂CF₂H, HCF₂CF₂OCH₂CF₂H, HCF₂CF₂OCH₂CH₂CH₂OCF₂CF₂H, and combinations thereof. Examples of sulfone solvents include sulfolane, dimethyl sulfoxide, methyl sulfolane, etc. Examples of other organic solvents include 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, and phosphate esters or combinations thereof.

[0062] In some embodiments, the lithium salt is selected from lithium hexafluorophosphate (LiPF6), lithium bis(oxalato)borate (LiB(C2O4)2, LiBOB), lithium difluorooxalato)borate (LiBF2(C2O4), LiDFOB), lithium tetrafluoroborate (LiBF4), lithium hexafluoroantimonyrate (LiSbF6), lithium hexafluoroarsenate (LiAsF6), lithium perfluorobutyl sulfonate (LiC4F9SO3), lithium perchlorate (LiClO4), lithium aluminate (LiAlO2), lithium tetrachloroaluminate (LiAlCl4), lithium bis(sulfonyl)imide (LiN(C)2), lithium bis(sulfonyl)imide (LiN(C)2), lithium bis(sulfonyl)imide (LiBF2), lithium bis(sulfonyl)imide (LiBF2), lithium bis(sulfonyl)imide (LiBF2), lithium bis(sulfonyl)borate ... x F 2x +1SO2)(C y F 2y +1SO2)(where x and y are natural numbers), at least one of LiFSI (lithium bisfluorosulfonylimide), LiTFSI (lithium bistrifluoromethanesulfonate imide), lithium chloride (LiCl), or lithium fluoride (LiF).

[0063] This application also provides an electronic device, including the secondary battery described above. The electronic device in this application is not particularly limited and can be any electronic device known in the prior art. In some embodiments, the electronic device may include, but is not limited to, laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0064] The rechargeable battery of this application is described below using lithium-ion batteries as an example. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0065] I. Preparation of Lithium-ion Batteries

[0066] 1. Preparation of positive electrode sheet

[0067] Lithium iron phosphate (LFP), lithium manganese oxide (LMO), polyvinylidene fluoride (PVDF) as a positive electrode binder, conductive carbon black (Super P) as a positive electrode conductive agent, and carbon nanotubes (CNT) as a positive electrode conductive agent were mixed evenly in a mass ratio of 19:78:1.5:0.7:0.8. The mixture was then added to N-methylpyrrolidone (NMP) as a solvent and stirred evenly to prepare a positive electrode slurry with a solid content of 70%. The positive electrode slurry was uniformly coated onto one side of a 12 μm thick aluminum foil used as a positive electrode current collector and dried at 90°C to obtain a single-sided coated positive electrode sheet with a coating thickness of 140 μm. The above steps were repeated on the other side of the aluminum foil to obtain a double-sided coated positive electrode sheet. After cold pressing and slitting, a positive electrode sheet with a size of 87 mm × 500 mm was obtained.

[0068] 2. Preparation of negative electrode sheet

[0069] Artificial graphite (anode active material), sodium carboxymethyl cellulose (CMC) (anode binder), styrene-butadiene rubber (SBR) (anode binder), and carbon nanotubes (CNTs) (anode conductive agent) (anode conductive agent) were mixed in a mass ratio of 96:1.5:1.5:1. Deionized water was added, and the mixture was stirred evenly under vacuum to obtain a cathode slurry with a solid content of 45%. The cathode slurry was uniformly coated on one side of a copper foil with a cathode current collector thickness of 6 μm and dried at 90°C to obtain a cathode sheet with a single-sided coating of cathode active material layer with a coating thickness of 100 μm. The above steps were repeated on the other side of the copper foil to obtain a cathode sheet with a double-sided coating of cathode active material layer. After cold pressing and slitting, a cathode sheet with a specification of 78 mm × 875 mm was obtained.

[0070] 3. Preparation of the separating membrane 100

[0071] Cellulose with a weight-average molecular weight of 50,000-1,000,000 was added to deionized water and stirred until homogeneous to obtain a cellulose slurry with a solid content of 70%. The cellulose slurry was coated onto the surface of the polyolefin layer 120 and dried at 100°C to obtain the first layer 101. Alumina of the second layer 102 was coated onto the two opposite surfaces of the first layer 101, with a single layer thickness of 3 μm. PVDF of the third layer 103 was then coated onto the surfaces of the two second layers 102, with a single layer thickness of 4 μm, to obtain the separator membrane 100.

[0072] 4. Preparation of electrolyte

[0073] In a dry argon atmosphere, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed uniformly at a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 was dissolved in the above organic solvent, and then vinylene carbonate was added and mixed uniformly to obtain the electrolyte. Based on the total mass of the electrolyte, the mass percentage of LiPF6 was 12.5%, the mass percentage of vinylene carbonate was 3%, and the mass percentage of the organic solvent was 84.5%.

[0074] 5. Assembly of lithium-ion batteries

[0075] An electrode assembly is formed by sequentially stacking and winding a positive electrode sheet, a separator 100 (as described in each embodiment), and a negative electrode sheet. A positive electrode tab is electrically connected to the positive electrode sheet, and a negative electrode tab is electrically connected to the negative electrode sheet. The electrode assembly is placed inside a packaging bag, with the positive and negative electrode tabs extending out of the bag for electrical connection to an external circuit. Electrolyte is added to the interior space of the packaging bag, and after processes such as encapsulation, settling, formation, and shaping, a lithium-ion battery is obtained.

[0076] II. Performance Testing of Lithium-ion Batteries

[0077] 1. Test method for K-value of lithium-ion batteries

[0078] After one charge-discharge cycle, with 60% remaining charge (SOC), place the lithium-ion battery in a 45℃ oven for 2 days and measure the voltage V1. Then, place it for another 3 days and measure the voltage V2. Calculate the K value: K = (V1 - V2) / 3 / 24. Set the K value to be greater than 0.04 as a defective product.

[0079] 2. Cyclic performance testing methods for lithium-ion batteries

[0080] After measuring the K-value of the lithium-ion battery, it was sent for cycle testing. The lithium-ion battery was placed in a 45℃ oven, charged at a constant current of 0.5C to 4.2V, then charged at a constant voltage of 0.05C, and allowed to stand for 10 minutes. Then it was discharged at 0.5C to 2.8V, allowed to stand for 10 minutes, and the test was repeated. The number of cycles at 45℃ was recorded until the capacity retention rate decreased to 70%.

[0081] 3. 1C discharge temperature rise test method

[0082] The temperature sensor of a multi-channel temperature measuring instrument was attached to the surface of the lithium-ion battery to record the charging, discharging, and overcharging temperatures. The lithium-ion battery was charged at 0.2C to 4.2V, then constant voltage and current was applied to 0.02C, and left to stand for 30 minutes; then it was discharged at 1C to 2.8V, left to stand for 30 minutes, and the test was ended. The temperature rise at 1C discharge was obtained by subtracting the temperature before discharge from the highest temperature at 1C overcharge.

[0083] 4. Average pore size test method

[0084] The lithium-ion battery to be tested was disassembled after discharge, and the separator 100 was removed. The inorganic layer and adhesive layer were washed away with deionized water. The battery was placed in an oven at 45°C for 3 days. The battery was photographed using a scanning tunneling microscope. Twenty areas of 1mm×1mm were randomly selected, and the maximum value of the pore size in each area was measured. The average pore size was obtained by taking the average value.

[0085] 5. Drop resistance test method

[0086] The lithium-ion battery was charged to 4.2V at 0.2C, constant voltage and constant current to 0.02C, and left to stand for 30 minutes. Then, a drop test was performed at a height of 1.5m from the ground. After the drop, the battery was left to stand for 1 hour, and the voltage V1 of the lithium-ion battery was measured. The battery was left to stand for another 3 days, and the voltage V2 was measured. The K value was calculated as K = (V1-V2) / 3 / 24. A K value greater than 0.04 was set as a defective product.

[0087] 6. Test method for the ratio of MD / TD mechanical strength

[0088] The lithium-ion battery to be tested after discharge was disassembled, the separator 100 was removed, the inorganic layer and adhesive layer were washed away with deionized water, and the battery was placed in an oven at 45°C for 3 days. The battery was then cut into 2cm*20cm samples according to the longitudinal (MD) or transverse (TD) direction. The tensile force f1 when the sample in the longitudinal (MD) direction was broken was tested with a tensile testing device, and the tensile force f2 when the sample in the transverse (TD) direction was broken was tested. The mechanical strength ratio of MD / TD was obtained as f1 / f2.

[0089] 7. Air permeability (Gurley) test method

[0090] The discharged lithium-ion battery under test was disassembled, and the separator 100 was removed. The inorganic and adhesive layers were washed away with deionized water, and the battery was left to stand in a 45℃ oven for 3 days. It was then cut into 5cm x 5cm pieces, and the permeability was tested using an air permeability testing device. The area through which 100mL of air passed was found to be 6.25cm². 2 The time required for the separator membrane 100.

[0091] 8. Liquid Absorption Capacity Test Method

[0092] The lithium-ion battery to be tested after discharge is disassembled, the separator 100 is removed, and the separator 100 is cut into 2cm*20cm pieces. The pieces are then placed vertically along the length of the separator 100 into a beaker containing electrolyte at a height of 2cm. The mixture is left to stand for 10 minutes, and the length of the electrolyte that moves through the separator 100 is measured with a ruler to characterize the liquid absorption performance of the separator 100.

[0093] The lithium-ion batteries of the examples and comparative examples were prepared according to the above method and tested. The relevant parameters of the lithium-ion batteries in each example are shown in Tables 1-4.

[0094] Table 1 shows the relevant parameters for Comparative Examples 1-1 to 1-6 and Examples 1-1 to 1-7.

[0095] Table 1

[0096]

[0097] As can be seen from Comparative Examples 1-1 to 1-6, when the first layer 101 of the separator 100 consists solely of the polyethylene layer 120, the K-value defect rate of the lithium-ion battery is high, indicating that an internal short circuit occurs inside the battery, and the polyethylene separator 100 has a poor effect in preventing copper dendrite puncture. When the first layer 101 of the separator 100 consists solely of the cellulose layer 110, the frequency of the separator 100 breaking is high, that is, the separator 100 is easy to break, has poor processing performance, and the strength of the separator 100 is poor, resulting in a low number of cycle cycles.

[0098] As can be seen from Comparative Examples 1-1 to 1-6 and Examples 1-1 to 1-7, when the first layer 101 of the separator 100 simultaneously contains both the cellulose layer 110 and the polyolefin layer 120, the K-value defect rate is 0 compared to when only the polyolefin layer 120 or the cellulose layer 110 is present. No short circuit occurs inside the battery, indicating that the separator 100 with both the cellulose layer 110 and the polyolefin layer 120 present has a better effect in preventing copper dendrite puncture, and also has good electrolyte wettability, good strength, and a higher number of cycles.

[0099] As can be seen from Examples 1-1 to 1-5, when the thickness of the first layer 101 is between 4 μm and 30 μm, the K-value defect rate is 0, and the cycle count of the lithium-ion battery is high, all exceeding 420 cycles. When the thickness of the first layer 101 is less than 4 μm, the separator 100 is too thin, increasing the risk of internal short circuits in the lithium-ion battery. When the thickness of the first layer 101 exceeds 30 μm, the battery thickness is too thick, affecting the volumetric energy density and resulting in a large internal resistance. Preferably, as can be seen from Examples 1-2 to 1-4, when the thickness of the first layer 101 is between 8 μm and 24 μm, the cycle count of the lithium-ion battery is high, and the internal resistance is low.

[0100] Table 2

[0101]

[0102] As can be seen from Examples 1-4 and Examples 2-1 to 2-5, when the separator 100 has both a cellulose layer 110 and a polyolefin layer 120, and there are polyolefin layers 120 on both sides of the cellulose layer 110, the polyolefin layer 120 can provide support for the cellulose layer 110, the separator 100 has higher mechanical strength, the separator 100 is not easily damaged, and the lithium-ion battery has more cycle times.

[0103] As can be seen from Examples 1-4, Examples 2-3 to Examples 2-5, when the total thickness of the separator 100 remains constant, the more layers of cellulose layer 110 and polyolefin layer 120 are alternately arranged, the better the liquid retention of the lithium-ion battery and the more cycle times it can achieve.

[0104] Table 3

[0105]

[0106] As can be seen from Examples 2-2, 3-1 to 3-3, and Comparative Examples 3-1 to 3-2, when the ratio of the total thickness of the cellulose layer 110 to the total thickness of the polyolefin layer 120 is in the range of 0.2 to 5, the lithium-ion battery has a higher cycle count and a lower K-value defect rate. Specifically, when H1 / H2 is greater than 5, the cellulose layer 110 is too thick, resulting in insufficient mechanical strength of the separator 100 and a reduced cycle count for the electrochemical device. When H1 / H2 is less than 0.2, the cellulose layer 110 is too thin, making it easy for copper dendrites to puncture the separator 100, leading to a K-value defect in the lithium-ion battery. Furthermore, when H1 / H2 is in the range of 1 to 2, the lithium-ion battery has a higher cycle count.

[0107] Table 4

[0108]

[0109]

[0110] As can be seen from Examples 2-1, 4-1 to 4-2, and Comparative Examples 4-1 to 4-2, when the average pore size r2 of the polyolefin layer 120 satisfies 40nm ≤ r2 ≤ 250nm, it facilitates electrolyte penetration, helps to improve the cycle performance of the electrochemical device in conjunction with the cellulose layer 110, and no K-value defects occur. When the average pore size of the polyolefin layer 120 is less than 40nm, the electrolyte wettability of the separator 100 is poor, affecting battery performance. The lithium-ion battery has high resistance, a high 1C discharge temperature rise, and the cycle performance is low due to the discharge temperature rise. When the average pore size of the polyolefin layer 120 is greater than 250nm, pinholes are prone to occur, and copper dendrites are prone to unidirectional growth that pierces the separator 100, leading to poor K-value defects in the lithium-ion battery and affecting the number of cycles.

[0111] As can be seen from Examples 2-1, 4-1 to 4-2, and Comparative Examples 4-1 to 4-2, when the Gurley value W of the first layer 101 satisfies 30s / 100cc ≤ W ≤ 625s / 100cc, the separator 100 has good liquid absorption capacity and can hold more copper, which helps to improve the problem of copper dendrites easily piercing the separator 100, and the cycle count is higher. When the Gurley value of the first layer 101 is less than 30s / 100cc, it indicates that the pore size of the polyolefin layer 120 is too large, which easily leads to pinholes, resulting in poor K value and affecting the cycle count. When the Gurley value of the first layer 101 is greater than 625s / 100cc, it indicates that the pore size of the polyolefin layer 120 is too small, resulting in high impedance of the lithium-ion battery, high discharge temperature rise, and thus affecting cycle performance.

[0112] Table 5

[0113]

[0114]

[0115] As can be seen from Examples 2-1 and 5-1 to 5-6, when the ratio of the longitudinal (MD) to the transverse (TD) tensile strength of the first layer 101 (MD / TD) satisfies 1 ≤ MD / TD ≤ 10, the difference between the longitudinal and transverse tensile strengths of the separator 100 is small when applied to the electrochemical device, the corners of the separator 100 are not easily damaged, and the drop resistance of the electrochemical device is good. Furthermore, when the ratio of the longitudinal (MD) to the transverse (TD) tensile strength of the first layer 101 (MD / TD) satisfies 1 ≤ MD / TD ≤ 3, the drop resistance of the electrochemical device is further improved when applied to the separator 100. When the ratio of the longitudinal (MD) to the transverse (TD) tensile strength of the first layer 101 is greater than 10, the difference between the longitudinal and transverse tensile strengths of the separator 100 is large, the corners of the separator 100 are more easily damaged, and the voltage drops significantly after a corner drop, affecting battery performance.

[0116] In the accompanying drawings of this embodiment, the same or similar reference numerals correspond to the same or similar components. In the description of this application, it should be understood that if terms such as "upper," "lower," "left," and "right" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, they are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the accompanying drawings are only for illustrative purposes and should not be construed as limiting this patent. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0117] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A separating membrane, characterized in that, For use in lithium-ion batteries, the separator includes a first layer comprising a cellulose layer (n1) and a polyolefin layer (n2), where n1 and n2 are both greater than or equal to 1, and the cellulose layer and the polyolefin layer are alternately stacked; n2 = n1 + 1, the polyolefin layer on one surface of the separator is positioned towards the negative electrode, and the polyolefin layer on the other surface of the separator is positioned towards the positive electrode; or, n2 = n1, the cellulose layer on one surface of the separator is positioned towards the negative electrode, and the polyolefin layer on the other surface of the separator is positioned towards the positive electrode. The cellulose layer is formed by coating the surface of the polyolefin layer with a cellulose slurry containing cellulose and then drying it. The weight-average molecular weight of the cellulose is 50,000. The cellulose layer has a three-dimensional porous structure with an average pore size of r1, where r1 satisfies: 1μm≤r1≤5μm. The polyolefin layer has a unidirectional porous structure with an average pore size of r2, where r2 satisfies: 40nm≤r2≤250nm.

2. The separator membrane according to claim 1, characterized in that, The total thickness of the cellulose layer is H1, and the total thickness of the polyolefin layer is H2. H1 and H2 satisfy: 0.2≤H1 / H2≤5.

3. The separator membrane according to claim 2, characterized in that, H1 and H2 satisfy: 1≤H1 / H2≤2.

4. The separator membrane according to claim 1, characterized in that, The isolation membrane satisfies at least one of the following conditions: Condition a: The total thickness of the cellulose layer is H1, and H1 satisfies: 2μm≤H1≤15μm; Condition b: The total thickness of the polyolefin layer is H2, and H2 satisfies: 2μm≤H2≤15μm; Condition c: The thickness of the first layer is H0, and H0 satisfies: 4μm≤H0≤30μm.

5. The separator membrane according to claim 1, characterized in that, The polyolefin layer is selected from at least one of polypropylene separator film, polyethylene separator film, and polyimide separator film.

6. The separator according to claim 1, characterized in that, The air permeability of the first layer is W, and W satisfies: 30s / 100cc≤W≤625s / 100cc.

7. The separator membrane according to claim 1, characterized in that, The longitudinal tensile strength of the first layer is MD, and the transverse tensile strength of the first layer is TD, satisfying: 1≤MD / TD≤10.

8. The separator membrane according to claim 1, characterized in that, The isolation membrane further includes a second layer; the second layer is disposed on the surface of the first layer, and the second layer is selected from at least one of alumina, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite or titanium dioxide.

9. An electrochemical device, characterized in that, Includes the separator membrane as described in any one of claims 1-8.

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