A modified polyvinyl alcohol binder and a method for preparing the same, a modified composite current collector

Abstract

The application provides a modified polyvinyl alcohol binder and a preparation method thereof, and a modified composite current collector, the preparation method comprising the following steps: alternately freezing and thawing a polyvinyl alcohol aqueous solution to obtain pre-crosslinked polyvinyl alcohol; and treating the pre-crosslinked polyvinyl alcohol in a lithium salt solution to obtain the modified polyvinyl alcohol binder, which has a double-crosslinked network and abundant branched chain structure, and has high elongation, excellent bonding performance and lithium supplement effect. The modified composite current collector containing the modified polyvinyl alcohol binder can realize self-repairing when being rolled and stretched, ensures the connection of the conductive network, has excellent conductive performance, and can supplement lithium for the electrode material, offset the irreversible lithium loss caused by the formation of SEI film, thereby improving the total capacity, energy density and initial cycle coulombic efficiency of the lithium ion battery, and making the lithium ion battery have better electrochemical performance.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a modified polyvinyl alcohol binder and its preparation method, and a modified composite current collector. Background Technology

[0002] Conventional new energy batteries use metal foil as current collectors, typically aluminum foil for the positive electrode and copper foil for the negative electrode. However, with the increasing popularity of electric vehicles and consumer electronics, the demand for batteries with high energy density and long cycle life is growing, leading to the development of composite current collectors. Composite current collectors are multi-layered composite materials, usually consisting of two metal layers and a substrate sandwiched between them. Compared to traditional copper foil, composite current collectors can save 65% of resources, and compared to traditional aluminum foil, they can save 85%. Therefore, composite current collectors offer advantages such as resource conservation, high safety, high energy density, light weight, small thickness, and easier processing. With the continuous development of battery technology, the market share of composite current collectors will gradually increase, indicating a very promising future.

[0003] Composite current collectors are fabricated by depositing metal layers on both surfaces of a substrate. Known methods for depositing these metal layers include electroplating, vapor deposition (physical vapor deposition), chemical deposition, magnetron sputtering, bonding, and coating. A commonly used method is to first deposit metal layers of a certain thickness on the upper and lower surfaces of the substrate using vapor deposition or magnetron sputtering to achieve a specific sheet resistance. Then, electroplating or chemical plating is used to thicken the metal layers, ensuring the sheet resistance meets the standards required for secondary batteries. For example, CN112234211A discloses a method for preparing a current collector. First, a first and second foil, after being rolled, are bonded to both sides of a substrate using an adhesive. Then, an alkaline solution is used to thin the first and second foils to the desired thickness, ensuring a tight bond between the substrate and the foils. Rolling further improves the density of the foils and the safety of the composite current collector. CN116247220A discloses a method for preparing a composite current collector. First, a thin metal layer is deposited onto a peelable medium using vapor deposition and electroplating methods and then wound up. The obtained peelable copper plating film is wound up and then coated and laminated. Molten polymer material is applied to bond the two thin metal plating layers together. After coating and bonding, the overall thickness is controlled by a certain pressure to form a structure of metal plating layer-polymer layer-metal plating layer.

[0004] Currently, the elongation rate of composite current collectors ranges from 5% to 80%. The substrates used to prepare composite current collectors are high-molecular materials, such as polyethylene terephthalate (PET), biaxially oriented polypropylene film (BOPP), polyimide (PI), polyethylene (PE), polypropylene (PP), polyamide (PA), polystyrene (PS), and polyvinyl chloride (PVC). These materials have high elongation rates, which can improve battery safety and facilitate passing safety tests such as needle penetration and compression. However, high elongation rates negatively impact the current collector processing. Specifically, in electrode preparation, positive / negative electrode active materials are first coated onto the current collector, followed by rolling to compact the active materials and increase the battery's volumetric energy density. For composite current collectors, elongation occurs during rolling, leading to cracks in the surface metal layer. These cracks become more pronounced with increasing elongation, resulting in reduced conductivity and decreased battery performance. In addition, during the first charge of a lithium-ion battery, the organic electrolyte will decompose on the surface of the negative electrode, such as graphite, to form a solid electrolyte interphase (SEI) film, which permanently consumes a large amount of lithium from the positive electrode, resulting in a low coulombic efficiency (ICE) for the first cycle and reducing the capacity and energy density of the lithium-ion battery.

[0005] Therefore, developing a composite current collector with excellent conductivity to improve the electrochemical performance of batteries is an urgent problem to be solved in this field. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a modified polyvinyl alcohol binder and its preparation method, as well as a modified composite current collector. Through the design and combination of raw materials such as polyvinyl alcohol and lithium salts, and preparation processes, the modified polyvinyl alcohol binder possesses high ductility and excellent bonding properties. When used in modified composite current collectors, it can endow the modified composite current collectors with self-healing properties and excellent conductivity, and replenish lithium to the electrode materials, thereby improving the electrochemical performance of lithium-ion batteries.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing a modified polyvinyl alcohol adhesive, the method comprising:

[0009] Pre-crosslinked polyvinyl alcohol was obtained by alternating freezing and thawing of an aqueous solution of polyvinyl alcohol.

[0010] The pre-crosslinked polyvinyl alcohol was treated in a lithium salt solution to obtain the modified polyvinyl alcohol binder.

[0011] The method for preparing the modified polyvinyl alcohol (PVA) adhesive provided by this invention involves firstly alternating freezing and thawing of an aqueous PVA solution. This process effectively enhances the hydrogen bonding forces between PVA chains, promotes the formation of PVA crystals, and forms a single-crosslinked pre-crosslinked PVA. Then, the pre-crosslinked PVA is placed in a lithium salt solution. Salting out further enhances the intermolecular interactions within the PVA chains, leading to inter-chain aggregation and chain entanglement, achieving double crosslinking, and ultimately obtaining a structure of "lithium ion-hydroxyl complexation + microcrystalline crosslinking, chain entanglement crosslinking." This invention, through the design of PVA and lithium salt raw materials, the design of a specific preparation process, and their synergistic interaction, results in a modified PVA adhesive with a double crosslinked network and abundant branched structure, exhibiting a highly elongated, shrinkable structure and excellent adhesive properties.

[0012] The modified polyvinyl alcohol binder, used in the modified composite current collector, possesses high elongation, enabling the conductive coating on the modified composite current collector to exhibit self-healing properties. This self-healing occurs during the roll forming process of the modified composite current collector, ensuring the continuity of the conductive network and thus giving the modified composite current collector excellent conductivity. Simultaneously, due to the complexation of lithium ions with the hydroxyl groups in PVA, lithium is introduced into the modified polyvinyl alcohol binder, allowing the modified composite current collector to replenish lithium in the electrode material, offsetting the irreversible lithium loss caused by the formation of the SEI film. This improves the total capacity, energy density, and first-cycle coulombic efficiency (ICE) of the lithium-ion battery, endowing it with superior electrochemical performance.

[0013] In this invention, the polyvinyl alcohol aqueous solution comprises polyvinyl alcohol and water, with water as the main solvent. Of course, the polyvinyl alcohol aqueous solution may also optionally include other solvents, such as alcohol solvents; the alcohol solvents include, but are not limited to, any one or a combination of at least two of methanol, ethanol, n-propanol, and isopropanol.

[0014] Preferably, the degree of polymerization of the polyvinyl alcohol is 1000-2000, for example, it can be 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800 or 1900, as well as specific values ​​between the above points. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

[0015] Preferably, the polyvinyl alcohol content in the aqueous polyvinyl alcohol solution is 3-50% by mass, for example, it can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40% or 45%, and specific values ​​between the above points. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

[0016] Preferably, the method for preparing the polyvinyl alcohol aqueous solution includes: mixing polyvinyl alcohol with water to obtain the polyvinyl alcohol aqueous solution.

[0017] Preferably, the water is hot water with a temperature of 90-100℃, such as 91℃, 92℃, 93℃, 94℃, 95℃, 96℃, 97℃, 98℃ or 99℃, as well as specific values ​​between the above points. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

[0018] Preferably, the mixing is carried out under stirring conditions to allow the PVA to dissolve quickly and completely in the water.

[0019] Preferably, the freezing temperature is ≤-10℃, for example, it can be -12℃, -15℃, -18℃, -20℃, -22℃, -25℃, -28℃, -30℃, -32℃, -35℃, -38℃, -40℃, -42℃, -45℃, -48℃, -50℃, -55℃, -60℃, -65℃ or -70℃, and specific values ​​between the above points. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range, but it is further preferred to be -10℃ to -70℃.

[0020] Preferably, the freezing time is ≥10h, for example, it can be 12h, 15h, 18h, 20h, 22h, 25h, 28h, 30h, 32h, 35h, 38h, 40h, 42h, 45h, 48h or 50h, as well as specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range, but 10-50h is further preferred.

[0021] As a preferred embodiment of the present invention, the freezing and thawing are performed alternately and repeatedly to enhance the hydrogen bonding between PVA segments and between PVA and water, promoting the formation of PVA ice crystals. During the thawing process, the PVA ice crystals melt, and the segments recombine. After multiple freeze-thaw cycles, the polymer segments of PVA undergo sufficient cross-linking. The freezing temperature is ≤-10℃, and the time is ≥10h, where the time is the duration of a single freezing cycle.

[0022] Preferably, the thawing temperature is 15-40℃, for example, it can be 18℃, 20℃, 22℃, 25℃, 28℃, 30℃, 32℃, 35℃ or 38℃, and specific values ​​between the above points. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range, and room temperature or normal temperature is further preferred.

[0023] Preferably, the number of cycles of alternating freezing and thawing is ≥3, for example, it can be 4, 5, 6, 7, 8, 9, 10, 12, 15, 18 or 20, and specific point values ​​between the above point values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific point values ​​included in the range.

[0024] Preferably, the lithium salt comprises any one or a combination of at least two of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium carbonate, lithium nitrate, lithium oxalate, lithium chloride, and lithium sulfate, with lithium hexafluorophosphate being more preferred.

[0025] Preferably, the lithium salt content in the lithium salt solution is 30-60% by mass, for example, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 52%, 55%, or 58%, as well as specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

[0026] Preferably, the solvent of the lithium salt solution includes water.

[0027] Preferably, the method of treating the pre-crosslinked polyvinyl alcohol in a lithium salt solution is immersion, so that the lithium salt can fully enter the internal structure of the pre-crosslinked polyvinyl alcohol, enhance the intermolecular interaction within the PVA chain through salting out, and form richer inter-chain aggregation and chain entanglement inside, thereby achieving double crosslinking and obtaining a structure of "lithium ion and hydroxyl complexation + microcrystalline crosslinking and chain entanglement crosslinking".

[0028] Preferably, the treatment time of the pre-crosslinked polyvinyl alcohol in the lithium salt solution is 4-12 hours, for example, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours or 11 hours, and specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

[0029] Preferably, the treatment temperature of the pre-crosslinked polyvinyl alcohol in the lithium salt solution is 15-40°C, for example, it can be 18°C, 20°C, 22°C, 25°C, 28°C, 30°C, 32°C, 35°C or 38°C, as well as specific values ​​between the above points. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range, and room temperature is further preferred.

[0030] Preferably, the preparation method specifically includes the following steps:

[0031] (1) The polyvinyl alcohol aqueous solution is alternately frozen and thawed for at least 3 cycles to obtain pre-crosslinked polyvinyl alcohol; the mass percentage of polyvinyl alcohol in the polyvinyl alcohol aqueous solution is 3-50%; the freezing temperature of each freezing is independently ≤-10℃ and the freezing time of each freezing is independently ≥10h.

[0032] (2) The pre-crosslinked polyvinyl alcohol obtained in step (1) is placed in a lithium salt solution and soaked at 15-40°C for 4-12 hours to obtain the modified polyvinyl alcohol binder; the lithium salt in the lithium salt solution has a mass percentage content of 30-60%.

[0033] In a second aspect, the present invention provides a modified polyvinyl alcohol adhesive, which is prepared by the preparation method described in the first aspect.

[0034] Preferably, the elongation of the modified polyvinyl alcohol adhesive is 80-130%, for example, it can be 85%, 90%, 95%, 100%, 105%, 110%, 115%, 120% or 125%, and specific values ​​between the above points. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range, but 90-125% is further preferred.

[0035] Thirdly, the present invention provides a conductive material composition comprising a combination of a modified polyvinyl alcohol binder and a conductive agent as described in the second aspect.

[0036] Preferably, the conductive agent includes any one or a combination of at least two of conductive carbon black, graphite, graphene, carbon nanotubes, and carbon fibers.

[0037] Preferably, the carbon fiber comprises carbon nanofibers (VGCF).

[0038] Based on their morphological characteristics, the conductive agents have different contact modes, such as point contact (e.g., conductive carbon black), line contact (e.g., carbon nanotubes, carbon fibers), and surface contact (e.g., graphene, graphite). Among them, the use of sheet-like conductive agents (e.g., graphene, graphite) and line-like conductive agents (e.g., carbon nanotubes, carbon fibers) is beneficial to forming more conductive bridges and improving conductivity.

[0039] Preferably, the mass ratio of the modified polyvinyl alcohol binder to the conductive agent is (1-6):1, for example, it can be 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1 or 5.5:1, etc.

[0040] Preferably, the conductive material composition further includes additives.

[0041] Preferably, the additives include any one or a combination of at least two of the following: pH adjusters, wetting agents, dispersants, and coupling agents.

[0042] Preferably, the conductive material composition further includes a pH adjuster and / or a wetting agent.

[0043] Preferably, the pH adjuster comprises an alkaline compound, and more preferably sodium hydroxide.

[0044] Preferably, the wetting agent includes any one or a combination of at least two of isopropanol, n-propanol, propylene glycol, n-octanol, polyethylene glycol, and polyoxyethylene ether.

[0045] As a preferred embodiment of the present invention, the conductive material composition includes a wetting agent, which can reduce the surface energy of the conductive paste and facilitate subsequent processing and coating.

[0046] On the other hand, the present invention also provides a conductive paste comprising the conductive material composition and solvent as described in the third aspect.

[0047] Preferably, the conductive paste is obtained by dissolving and / or dispersing the conductive material composition in a solvent.

[0048] Preferably, the solvent includes an aqueous solvent or an oil-based solvent, and more preferably an aqueous solvent.

[0049] The aqueous solvent includes water and / or an organic solvent miscible with water; the organic solvent miscible with water includes alcohol solvents, including but not limited to: any one or a combination of at least two of methanol, ethanol, n-propanol, and isopropanol.

[0050] The oily solvents include acetone, N-methylpyrrolidone (NMP), etc.

[0051] In this invention, the amount of solvent used is not particularly limited, as long as it is sufficient to make the conductive paste meet the requirements for processing and coating.

[0052] Preferably, the solid content of the conductive paste is 5-30%, for example, it can be 6%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25% or 28%, and specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

[0053] Fourthly, the present invention provides a modified composite current collector, the modified composite current collector comprising a composite current collector and a conductive coating disposed on the surface of the composite current collector, the conductive coating comprising the conductive material composition as described in the third aspect.

[0054] The modified composite current collector provided by this invention involves modifying the surface of the composite current collector with a conductive coating. This conductive coating includes the aforementioned modified polyvinyl alcohol binder and conductive agent. The modified polyvinyl alcohol binder exhibits high elongation and excellent adhesion, while the conductive agent can build a conductive network on the surface of the composite current collector, enhancing conductivity. The conductive coating, containing specific components, achieves self-repair during the rolling and stretching of the composite current collector due to its high elongation, ensuring the connection of the conductive network and endowing the modified composite current collector with excellent conductivity. Furthermore, the conductive coating helps increase the electrolyte absorption and retention capacity of the battery electrodes. Simultaneously, the conductive coating (modified polyvinyl alcohol binder) contains lithium, which can replenish lithium in the electrode material, offsetting the irreversible lithium loss caused by the formation of the SEI film. This improves the total capacity, energy density, and first-cycle coulombic efficiency (ICE) of the lithium-ion battery, resulting in better electrochemical performance.

[0055] Preferably, the thickness of the conductive coating is 0.5-2μm, for example, it can be 0.6μm, 0.8μm, 1μm, 1.1μm, 1.3μm, 1.5μm, 1.7μm or 1.9μm, as well as specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

[0056] In this invention, the composite current collector includes a first metal layer, a second metal layer, and a polymer substrate disposed between the first metal layer and the second metal layer. This invention does not specifically limit the type of composite current collector; any composite current collector known in the prior art that can be used in secondary batteries is applicable to this invention.

[0057] For example, the composite current collector includes a composite copper current collector or a composite aluminum current collector.

[0058] For example, the preparation method of the modified composite current collector includes: mixing the conductive material composition described in the third aspect with a solvent to obtain a conductive slurry; coating the conductive slurry onto both surfaces of the composite current collector and drying it to obtain the modified composite current collector.

[0059] Preferably, the coating method includes gravure roller coating, dip coating, slot extrusion or spray coating; more preferably, a double-sided coating one-time forming machine is used to improve production efficiency.

[0060] Preferably, the coating speed is 30-120 m / min, for example, it can be 40 m / min, 50 m / min, 70 m / min, 90 m / min, 100 m / min or 110 m / min, and specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

[0061] Preferably, the drying temperature is 60-100℃, for example, it can be 65℃, 70℃, 75℃, 80℃, 85℃, 90℃ or 95℃, and specific values ​​between the above points. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

[0062] On the other hand, the present invention provides a battery electrode, the battery electrode comprising the modified composite current collector as described in the fourth aspect and an active material layer disposed on the surface of the modified composite current collector.

[0063] The battery electrode can be a positive electrode or a negative electrode, and correspondingly, the active material layer can be a positive active material layer or a negative active material layer.

[0064] In this invention, the active material layer comprises a combination of active material (positive electrode active material or negative electrode active material), binder, and conductive material. This invention does not impose any specific limitations on the types of active material, binder, and conductive material; any active material, binder, and conductive material known in the prior art for use in lithium-ion batteries is applicable to this invention.

[0065] Fifthly, the present invention provides a lithium-ion battery, the lithium-ion battery comprising the modified composite current collector as described in the fourth aspect.

[0066] In this invention, the lithium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator; wherein at least one of the positive and negative electrode sheets is the battery electrode described above. That is, the current collector of at least one of the positive and negative electrode sheets is the modified composite current collector described in the third aspect.

[0067] Compared with the prior art, the present invention has the following beneficial effects:

[0068] (1) In the preparation method of the modified polyvinyl alcohol adhesive provided by the present invention, through the design of raw materials such as PVA and lithium salt, the design of specific preparation process and their synergy, the modified polyvinyl alcohol adhesive obtained has a double cross-linking network and rich branched structure, has a high elongation and shrinkable structure, with an elongation rate of 90-125%, and has excellent adhesive performance, and can also achieve lithium replenishment effect.

[0069] (2) In the modified composite current collector provided by the present invention, the surface of the composite current collector is modified with a conductive coating. On the one hand, the conductive coating containing the modified polyvinyl alcohol binder has high elongation and can achieve self-repair during the roll forming and stretching of the composite current collector, ensuring the connection of the conductive network and giving the modified composite current collector excellent conductivity. It maintains high conductivity even under tension, with a 3% sheet resistance change rate ≤20% and a 5% sheet resistance change rate <38%. At the same time, based on the high adhesion performance of the modified polyvinyl alcohol binder, the conductive coating has high bonding force with the composite current collector, and the modified composite... The peel force of the current collector is ≥498 N / m; on the other hand, in the conductive coating of the modified composite current collector, since the modified polyvinyl alcohol binder contains lithium, it can replenish lithium to the electrode material and offset the irreversible lithium loss caused by the formation of the SEI film, thereby improving the energy density and first cycle coulombic efficiency (ICE) of the lithium-ion battery. Compared with lithium-ion batteries using conventional composite current collectors, the energy density of lithium-ion batteries using the modified composite current collector of the present invention is increased by 10-15%, and the ICE is increased by 1.0-1.4%, giving the lithium-ion battery better electrochemical performance. Attached Figure Description

[0070] Figure 1 Scanning electron microscope image of the modified polyvinyl alcohol binder provided in Example 1. Detailed Implementation

[0071] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0072] The terms “comprising,” “including,” “having,” “containing,” or any other variations thereof, as used herein, are intended to cover non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not limited to those elements and may also include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0073] "Optional" or "any one" means that the matter or event described thereafter may or may not occur, and the description includes both the possibility that the event may occur and the possibility that the event may not occur.

[0074] The indefinite articles “a” and “an” preceding an element or component of this invention do not impose any limitation on the quantity (i.e., number of times) of the element or component. Therefore, “an” or “a” should be interpreted as including one or at least one, and the singular form of an element or component also includes the plural form, unless the quantity clearly refers only to the singular form.

[0075] The raw materials involved in the following specific embodiments of the present invention are all commercially available products; among them, polyvinyl alcohol (PVA, degree of polymerization of 2000), grade SG-181, was purchased from Anhui Wanwei High-tech Materials Co., Ltd.; the composite current collector is a composite aluminum current collector, grade NP060AA, purchased from Yangzhou Nali New Materials Co., Ltd.

[0076] Preparation Example 1

[0077] A modified polyvinyl alcohol adhesive and its preparation method, wherein the specific steps of the preparation method are as follows:

[0078] (1) In a mixing tank, 17 kg of PVA powder is added to 100 kg of 100°C hot water and stirred at 1000 rpm for 6 hours to form a PVA aqueous solution.

[0079] (2) Cool the PVA aqueous solution to room temperature and alternate between freezing and thawing. Each freezing temperature is -12℃ and the time is 14h. Each thawing is carried out at room temperature and thawed to room temperature. Repeat the cycle 6 times to obtain pre-crosslinked polyvinyl alcohol.

[0080] (3) The pre-crosslinked polyvinyl alcohol obtained in step (2) is placed in an aqueous solution of lithium hexafluorophosphate (the mass percentage of lithium hexafluorophosphate is 40%) and soaked at room temperature for 6 hours to obtain the modified polyvinyl alcohol adhesive, which is abbreviated as mPVA-1 below.

[0081] The modified polyvinyl alcohol adhesive (mPVA-1) provided in this preparation example was characterized for the following properties:

[0082] I. Microscopic morphology

[0083] The microstructure of the modified polyvinyl alcohol binder provided in this preparation example was tested using scanning electron microscopy. The obtained scanning electron microscopy images are shown below. Figure 1 As shown, from Figure 1 As can be seen from the above, the modified polyvinyl alcohol binder is prepared by double cross-linking and has a rich micron-scale pore structure and branched structure inside. The above structure determines that the modified polyvinyl alcohol binder has high elongation.

[0084] II. Elongation

[0085] The modified polyvinyl alcohol adhesive provided in this preparation example was loaded into a rectangular mold and tested in the gel state according to the method in national standard GB / T 1040.3-2006. Specific parameters included: sample width of 15 mm, tensile speed of 10 mm / min, and initial clamp distance of 50 mm. Five samples were taken for testing, and the average value was used as the test result. During testing, the length direction was parallel to the clamp axis, and the sample was kept straight. If the sample broke within the clamp jaws, or if there were notches at the sample edge, the experimental result was invalid. The elongation of the adhesive mPVA-1 was measured to be 120% using the above method.

[0086] Preparation Example 2

[0087] A modified polyvinyl alcohol adhesive and its preparation method, wherein the specific steps of the preparation method are as follows:

[0088] (1) In a mixing tank, add 8 kg of PVA powder to 100 kg of 98°C hot water and stir at 1000 rpm for 5 hours to fully dissolve the PVA and form a PVA aqueous solution.

[0089] (2) Cool the PVA aqueous solution to room temperature and alternate between freezing and thawing. Each freezing temperature is -35℃ and the time is 12h. Each thawing is carried out at room temperature and thawed to room temperature. Repeat the cycle 5 times to obtain pre-crosslinked polyvinyl alcohol.

[0090] (3) The pre-crosslinked polyvinyl alcohol obtained in step (2) is placed in an aqueous solution of lithium hexafluorophosphate (the mass percentage of lithium hexafluorophosphate is 30%) and soaked at room temperature for 4 hours to obtain the modified polyvinyl alcohol adhesive, which is abbreviated as mPVA-2 below.

[0091] The mPVA-2 was tested using the same method as in Preparation Example 1, and its elongation was found to be 125%.

[0092] Preparation Example 3

[0093] A modified polyvinyl alcohol adhesive and its preparation method, wherein the specific steps of the preparation method are as follows:

[0094] (1) In a mixing tank, add 40 kg of PVA powder to 100 kg of 98°C hot water and stir at 1000 rpm for 8 hours to fully dissolve the PVA and form a PVA aqueous solution.

[0095] (2) Cool the PVA aqueous solution to room temperature and alternate between freezing and thawing. Each freezing temperature is -15℃ and the time is 20h. Each thawing is carried out at room temperature and thawed to room temperature. Repeat the cycle 8 times to obtain pre-crosslinked polyvinyl alcohol.

[0096] (3) The pre-crosslinked polyvinyl alcohol obtained in step (2) is placed in an aqueous solution of lithium hexafluorophosphate (the mass percentage of lithium hexafluorophosphate is 60%) and soaked at room temperature for 8 hours to obtain the modified polyvinyl alcohol adhesive, which is abbreviated as mPVA-3 below.

[0097] The mPVA-3 was tested using the same method as in Preparation Example 1, and its elongation was found to be 90%.

[0098] Comparative Preparation Example 1

[0099] A modified polyvinyl alcohol adhesive and its preparation method, wherein the specific steps of the preparation method are as follows:

[0100] (1) In a mixing tank, 17 kg of PVA powder is added to 100 kg of 100°C hot water and stirred at 1000 rpm for 6 hours to form a PVA aqueous solution.

[0101] (2) Cool the PVA aqueous solution to room temperature and alternate between freezing and thawing. Each freezing temperature is -12℃ and the time is 14h. Each thawing is carried out at room temperature and thawed to room temperature. Repeat the cycle 6 times to obtain the modified polyvinyl alcohol adhesive, which is abbreviated as mPVA-D1 below.

[0102] The mPVA-D1 was tested using the same method as in Preparation Example 1, and its elongation was found to be 10%.

[0103] Comparative Preparation Example 2

[0104] A modified polyvinyl alcohol adhesive and its preparation method, wherein the specific steps of the preparation method are as follows:

[0105] (1) In a mixing tank, 17 kg of PVA powder and 0.5 kg of lithium hexafluorophosphate were added to 100 kg of 100°C hot water and stirred at 1000 rpm for 6 hours to form a PVA lithium salt aqueous solution.

[0106] (2) Cool the PVA lithium salt aqueous solution to room temperature and alternate between freezing and thawing. Each freezing temperature is -12℃ and the time is 14h. Each thawing is carried out at room temperature and thawed to room temperature. Repeat the cycle 6 times to obtain the modified polyvinyl alcohol adhesive, hereinafter abbreviated as mPVA-D2.

[0107] The mPVA-D2 was tested using the same method as in Preparation Example 1, and its elongation was found to be 13%.

[0108] Example 1

[0109] A modified composite current collector includes a composite aluminum current collector and a conductive coating disposed on the composite aluminum current collector. The conductive coating is a conductive material composition comprising, by weight: 4 parts conductive carbon black, 0.7 parts graphite, 13 parts binder mPVA-1 (Preparation Example 1), 0.9 parts sodium hydroxide, and 15 parts isopropanol.

[0110] The modified composite current collector is prepared as follows:

[0111] (1) Mix the conductive carbon black, graphite, binder and 66 parts of water in the formula to make a slurry, then add sodium hydroxide to adjust the pH of the slurry, and finally add wetting agent, mix and disperse evenly to obtain conductive slurry;

[0112] (2) The conductive paste obtained in step (1) is coated on both sides of the composite aluminum current collector by gravure roller coating and a double-sided coating one-time forming machine with a coating speed of 100m / min. After coating, it is placed in an oven at 75°C to dry, and the modified composite current collector is obtained, wherein the thickness of the conductive coating is 1μm.

[0113] Examples 2-5, Comparative Examples 1-2

[0114] A modified composite current collector includes a composite aluminum current collector and a conductive coating disposed on the composite aluminum current collector. The conductive coating is made of a conductive material composition. The only difference between this and Example 1 is the composition and / or amount of the conductive material composition, as shown in Table 1. In Table 1, the amount of each component is expressed in "parts," and "--" indicates that the component was not added. The preparation method of each modified composite current collector is the same as in Example 1.

[0115] Table 1

[0116]

[0117]

[0118] Comparative Example 3

[0119] A modified composite current collector includes a composite aluminum current collector and a conductive coating disposed on the composite aluminum current collector. The material of the conductive coating is a conductive material composition. The only difference between this and Example 1 is that the binder mPVA-1 in the conductive material composition is replaced with an equal mass of PVA (unmodified, i.e., PVA powder in Preparation Example 1). All other materials, amounts, and preparation methods are the same as in Example 1.

[0120] Comparative Example 4

[0121] A modified composite current collector includes a composite aluminum current collector and a conductive coating disposed on the composite aluminum current collector. The material of the conductive coating is a conductive material composition. The only difference between this and Example 1 is that the binder mPVA-1 in the conductive material composition is replaced with an equal mass of commercially available polyacrylic acid type binder (Yuanye S59106); the other materials, amounts and preparation methods are the same as in Example 1.

[0122] Comparative Example 5

[0123] A composite aluminum current collector without a conductive coating was used as a comparative example 5.

[0124] The performance of the modified composite current collectors provided in Examples 1-5 and Comparative Examples 1-4, and the composite aluminum current collector in Comparative Example 5 were tested using the following methods:

[0125] I. Electrical conductivity

[0126] The sheet resistance (in mΩ) of each modified composite current collector is tested to characterize its conductivity. The smaller the sheet resistance value, the better the conductivity.

[0127] The initial sheet resistance is tested as follows: The test instrument is a four-probe resistivity meter. The specific test parameters are as follows: point mode, probe HPS58006, spacing 2mm+2mm+2mm, to obtain the initial sheet resistance.

[0128] The test method for 3% sheet resistance is as follows: The modified composite current collector to be tested is stretched using a tensile testing machine with a stretching ratio (elongation) of 3%. Then, the sheet resistance of the stretched sample is tested using a four-probe resistivity tester according to the test method in the initial sheet resistance.

[0129] The test method for 5% sheet resistance is as follows: The modified composite current collector to be tested is stretched using a tensile testing machine with a stretching ratio of 5%. Then, the sheet resistance of the stretched sample is tested using a four-probe resistivity tester according to the test method in the initial sheet resistance.

[0130] II. Adhesive properties

[0131] The peel force (in N / m) of each modified composite current collector was tested to characterize the adhesive performance. The greater the peel force, the better the adhesive performance.

[0132] The peel strength test method is as follows: Take a sample after unfolding 3-4 turns. The sample size is 15mm×100mm. Attach 3M-9080A-15mm tape to a stainless steel plate, then evenly attach the modified composite current collector to be tested onto the double-sided tape. Use a 2kg standard small pressure roller to press back and forth once. Then attach 3M-9080A-14mm tape to the foil surface and press back and forth once again using a 2kg standard small pressure roller. After that, take the pressed sample to a tensile testing machine and stretch it 180° at a speed of 100mm / min and a width of 14mm. Take the maximum value of the result.

[0133] III. Electrochemical Performance

[0134] The modified composite current collector was used to prepare the positive electrode, which was then assembled into a lithium-ion battery. The first-cycle coulombic efficiency of the lithium-ion battery was tested. The specific method is as follows:

[0135] (1) Preparation of positive electrode sheet: The positive active material (lithium iron phosphate), conductive agent (conductive carbon black) and binder (polyvinylidene fluoride, PVDF) are mixed in a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) is added according to the solid content of the system of 50%. The mixture is stirred thoroughly to prepare a uniform positive electrode slurry. The positive electrode slurry is coated on each modified composite current collector to be tested, dried and then rolled to obtain the positive electrode sheet.

[0136] (2) Preparation of negative electrode sheet: The negative electrode active material (silicon carbon), conductive agent (SP), thickener (carboxymethyl cellulose CMC), binder (styrene-butadiene latex SBR) and water are mixed in a pure material mass ratio of 100:0.5:1.2:2.6:78. Water is added according to the system solid content of 45%, and the mixture is stirred thoroughly. The mixture is then coated by narrow-slit extrusion to form a uniform negative electrode slurry. The negative electrode slurry is coated on copper foil, dried, and then rolled to obtain the negative electrode sheet.

[0137] (3) Assembly of lithium-ion batteries: The positive electrode, separator and negative electrode are assembled into a cell; the cell is packaged, baked to remove water and injected with electrolyte, and after packaging, formation and other processes, a lithium-ion battery is obtained.

[0138] The following performance tests were performed on the lithium-ion battery:

[0139] (A) First Cycle Coulombic Efficiency (ICE): Connect the lithium-ion battery to be tested to the coulombic metering device, charge the battery and record the charging coulombic value, then discharge the battery and record the discharging coulombic value. Calculate the first cycle coulombic efficiency of each battery based on the recorded coulombic values.

[0140] With the ICE test value of the lithium-ion battery using the composite aluminum current collector in Comparative Example 5 as 100.0%, calculate the ratio of the ICE test value of the lithium-ion battery using other modified composite current collectors to the ICE test value of Comparative Example 5.

[0141] (B) Energy Density: The lithium-ion battery under test is connected to a constant current load. By recording the battery voltage and discharge time, the total energy released by the battery is calculated. The mass of the battery is measured, and the energy density is calculated based on the measured energy and the battery mass. The formula for energy density is energy / mass.

[0142] With the energy density test value of the lithium-ion battery using the composite aluminum current collector in Comparative Example 5 as 100.0%, calculate the ratio of the energy density test value of the lithium-ion battery using other modified composite current collectors to the energy density test value of Comparative Example 5.

[0143] The specific results are shown in Table 2.

[0144] Table 2

[0145]

[0146]

[0147] As shown in Table 2, the modified composite current collector provided by this invention has a conductive coating containing a specific modified polyvinyl alcohol binder. This conductive coating exhibits high elongation and can self-repair during the roll forming and stretching of the composite current collector, ensuring the connection of the conductive network. It maintains high conductivity even under tension, with an initial sheet resistance of 35 mΩ, 3% sheet resistance of 40-42 mΩ, and 5% sheet resistance of 44.5-48 mΩ. The change rate of the 3% sheet resistance is ≤20%, ranging from 14.2-20%, while the change rate of the 5% sheet resistance is <38%, ranging from 27.1-37.1%. Simultaneously, due to the excellent adhesive properties of the modified polyvinyl alcohol binder, the modified composite current collector has a high peel strength, reaching 498-720 N / m. Furthermore, the modified polyvinyl alcohol binder contains lithium, enabling the modified composite current collector to replenish lithium to the electrode material, offsetting the irreversible lithium loss caused by the formation of the SEI film, thereby improving the ICE (Inductance, Interval, and Conversion) of the lithium-ion battery. Based on the high elongation, excellent adhesion, and lithium replenishment effect of the modified polyvinyl alcohol binder, the lithium-ion battery using the modified composite current collector described in Examples 1-5 achieves an ICE improvement rate of 1.0-1.4% and an energy density improvement rate of 10-15% compared to the lithium-ion battery using the composite aluminum current collector in Comparative Example 5. Furthermore, a comparison of Examples 1 and 2 shows that the combined use of graphite and conductive carbon black in the conductive coating further improves the conductivity of the modified composite current collector.

[0148] In this invention, PVA aqueous solution is first subjected to multiple freeze-thaw cycles to form pre-crosslinked PVA with single crosslinking. Then, the pre-crosslinked PVA is treated in a lithium salt solution to achieve double crosslinking, resulting in a modified polyvinyl alcohol binder with a structure of "lithium ion-hydroxyl complexation + microcrystalline crosslinking, chain entanglement crosslinking," exhibiting high elongation, excellent adhesion, and lithium replenishment effect. The binder in Comparative Example 1 is modified PVA with only single crosslinking. Although the binder in Comparative Example 2 uses PVA and lithium salt as raw materials, it does not employ the preparation process of this invention, resulting in insufficient elongation of the modified PVA. This prevents it from performing self-repair during the stretching of the composite current collector, leading to conductive network breakage, increased sheet resistance, and deteriorated conductivity. The modified composite aluminum current collector in Comparative Example 3 uses unmodified PVA as the binder for the conductive coating. The modified composite aluminum current collector in Comparative Example 4 uses a conventional binder. Due to the low elongation of both the binder and the conductive coating, the conductive network of the composite current collector breaks during roll stretching, resulting in reduced conductivity.

[0149] The applicant declares that this invention illustrates the modified polyvinyl alcohol adhesive and its preparation method, as well as the modified composite current collector, through the above embodiments. However, this invention is not limited to the above process steps, meaning that this invention does not necessarily rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to this invention, equivalent substitutions of the raw materials used in this invention, additions of auxiliary components, and selection of specific methods all fall within the protection and disclosure scope of this invention.

Claims

1. A modified composite current collector, characterized in that, The modified composite current collector includes a composite current collector and a conductive coating disposed on the surface of the composite current collector, wherein the conductive coating comprises a conductive material composition; the thickness of the conductive coating is 0.5-2 μm. The conductive material composition comprises a combination of a modified polyvinyl alcohol binder and a conductive agent; The preparation method of the modified polyvinyl alcohol adhesive includes the following steps: Pre-crosslinked polyvinyl alcohol was obtained by alternating freezing and thawing of an aqueous solution of polyvinyl alcohol. The pre-crosslinked polyvinyl alcohol was treated in a lithium salt solution to obtain the modified polyvinyl alcohol binder; the elongation of the modified polyvinyl alcohol binder was 90%~125%. The treatment time of the pre-crosslinked polyvinyl alcohol in lithium salt solution is 4-12 h; The pre-crosslinked polyvinyl alcohol is treated in lithium salt solution at a temperature of 15-40℃. The lithium salt solution contains 30-60% lithium salt by mass. The degree of polymerization of the polyvinyl alcohol is 1000-2000.

2. The modified composite current collector according to claim 1, characterized in that, The polyvinyl alcohol aqueous solution contains 3-50% polyvinyl alcohol by mass.

3. The modified composite current collector according to claim 1, characterized in that, The freezing temperature is ≤-10℃.

4. The modified composite current collector according to claim 1, characterized in that, The freezing time is ≥10 h.

5. The modified composite current collector according to claim 1, characterized in that, The number of alternating freezing and thawing cycles is ≥3.

6. The modified composite current collector according to claim 1, characterized in that, The lithium salt includes any one or a combination of at least two of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium carbonate, lithium nitrate, lithium oxalate, lithium chloride, and lithium sulfate.

7. The modified composite current collector according to claim 1, characterized in that, The lithium salt is lithium hexafluorophosphate.

8. The modified composite current collector according to claim 1, characterized in that, The preparation method of the modified polyvinyl alcohol adhesive specifically includes the following steps: (1) The polyvinyl alcohol aqueous solution is alternately frozen and thawed for at least 3 cycles to obtain pre-crosslinked polyvinyl alcohol; the mass percentage of polyvinyl alcohol in the polyvinyl alcohol aqueous solution is 3-50%; the freezing temperature of each freezing is independently ≤-10℃ and the freezing time of each freezing is independently ≥10 h. (2) The pre-crosslinked polyvinyl alcohol obtained in step (1) is placed in a lithium salt solution and soaked at 15-40°C for 4-12 h to obtain the modified polyvinyl alcohol binder; the lithium salt solution contains 30-60% lithium salt by mass.

9. The modified composite current collector according to claim 1, characterized in that, The conductive agent includes any one or a combination of at least two of conductive carbon black, graphite, graphene, carbon nanotubes, and carbon fibers.

10. The modified composite current collector according to claim 1, characterized in that, The mass ratio of modified polyvinyl alcohol binder to conductive agent in the conductive material composition is (1-6):

1.

11. The modified composite current collector according to claim 1, characterized in that, The conductive material composition also includes a pH adjuster and / or a wetting agent.

12. A lithium-ion battery, characterized in that, The lithium-ion battery includes the modified composite current collector as described in any one of claims 1 to 11.

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