Modified polyvinyl alcohol binder and preparation method therefor, and modified composite current collector

Abstract

Disclosed in the present disclosure are a modified polyvinyl alcohol binder and a preparation method therefor, and a modified composite current collector. The preparation method comprises the following steps: alternately freezing and unfreezing an aqueous solution of polyvinyl alcohol, so as to obtain pre-crosslinked polyvinyl alcohol; and placing the pre-crosslinked polyvinyl alcohol in a lithium salt solution to treat same, so as to obtain the modified polyvinyl alcohol binder. The modified polyvinyl alcohol binder has a dual-crosslinked network and abundant branched chain structures, and has high extensibility and good bonding performance and lithium-replenishing effect. A modified composite current collector containing the modified polyvinyl alcohol binder can implement self-repairing during rolling extension to ensure the connection of a conductive network, thereby having good conductivity, and can perform lithium replenishment on an electrode material to counteract irreversible lithium loss caused by formation of an SEI film, thereby improving the total capacity, energy density and first-cycle coulombic efficiency of a lithium-ion battery and enabling the lithium-ion battery to have better electrochemical performance.

Description

A modified polyvinyl alcohol binder and its preparation method, and a modified composite current collector Technical Field

[0001] The embodiments of the present application relate to the field of lithium-ion battery technology, such as a modified polyvinyl alcohol binder and a preparation method thereof, and a modified composite current collector. Background Art

[0002] Conventional new energy batteries use metal foil as current collectors, generally aluminum foil for the positive electrode and copper foil for the negative electrode. However, with the popularity of electric vehicles and consumer electronics, people have an increasing demand for battery products with high energy density and long cycle life, and composite current collectors have emerged. Composite current collectors are multi-layer composite materials, usually including two metal layers and a substrate sandwiched between the two metal layers. Compared with traditional copper foil, composite current collectors can save 65% of resources, and compared with traditional aluminum foil, composite current collectors can save 85% of resources. Therefore, composite current collectors have the characteristics of saving resources, 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, and the prospects are very broad.

[0003] The composite current collector is made by setting a metal layer on both surfaces of the substrate. Currently known methods for setting the metal layer include electroplating, evaporation (physical vapor deposition), chemical deposition, magnetron sputtering, bonding, coating composite, etc. A more commonly used method is to first form a metal layer of a certain thickness on the upper and lower surfaces of the substrate by evaporation or magnetron sputtering to achieve a certain square resistance, and then use electroplating or chemical plating to thicken the metal layer so that the square resistance of the material reaches the standard required by the secondary battery. For another example, CN112234211A discloses a method for preparing a current collector, which first uses an adhesive to composite a first foil and a second foil that have been calendered on both sides of the substrate, and then uses an alkaline solution to thin the first foil and the second foil to the required thickness, so that the substrate and the foil are tightly composited, and the density of the foil and the safety of the composite current collector are improved by calendering. CN116247220A discloses a method for preparing a composite current collector. First, a thin metal layer is plated onto a strippable medium by evaporation and electroplating and rolled up. The obtained strippable copper-plated film is rolled up. Then, a coating and compounding method is adopted, that is, a molten polymer material is coated to bond two thin metal coatings together. After coating and bonding, the overall thickness is controlled according to a certain pressure to form a structure of metal coating-polymer layer-metal coating.

[0004] At present, the elongation of the composite current collector is 5-80%. The substrate used to prepare the composite current collector is a polymer material, such as polyethylene terephthalate (PET), biaxially oriented polypropylene film (BOPP), polyimide (PI), polyethylene (PE), polypropylene (PP), polyamide (PA), polystyrene (PS), polyvinyl chloride (PVC), etc., which has a high elongation, can improve the safety of the battery and easily pass the battery's safety tests such as needle puncture and extrusion. However, the high elongation has an adverse effect on the processing of the current collector. Specifically, in the preparation of the electrode sheet, the positive / negative electrode active material is first coated on the current collector, and then roller pressing is performed to compact the active material and increase the volume energy density of the battery. For the composite current collector, extension will occur during the rolling process, and cracks will appear on the surface metal layer. As the extension increases, the cracks become more obvious, which in turn leads to a decrease in the conductivity of the current collector and a degradation of the battery performance. In addition, during the first charging process of lithium-ion batteries, the organic electrolyte will be reduced and decomposed on the surface of the negative electrode such as graphite to form a solid electrolyte interface (SEI) film, which permanently consumes a large amount of lithium from the positive electrode, resulting in a low coulombic efficiency (ICE) of the first cycle, reducing the capacity and energy density of the lithium-ion battery.

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

[0006] Summary of the Invention

[0007] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0008] The embodiments of the present application provide a modified polyvinyl alcohol binder, a preparation method thereof, and a modified composite current collector. Through the design and mutual combination of raw materials such as polyvinyl alcohol and lithium salt and preparation processes, the modified polyvinyl alcohol binder has high ductility and excellent bonding properties. It is used to modify the composite current collector, which can give the modified composite current collector self-healing properties and excellent conductivity, and replenish lithium to the electrode material, thereby improving the electrochemical performance of the lithium-ion battery.

[0009] In a first aspect, an embodiment of the present application provides a method for preparing a modified polyvinyl alcohol binder, the preparation method comprising:

[0010] Alternately freezing and thawing the polyvinyl alcohol aqueous solution to obtain pre-crosslinked polyvinyl alcohol;

[0011] The pre-crosslinked polyvinyl alcohol is placed in a lithium salt solution for treatment to obtain the modified polyvinyl alcohol binder.

[0012] In the preparation method of the modified polyvinyl alcohol binder provided by the present application, the polyvinyl alcohol (PVA) aqueous solution is first alternately frozen and thawed. This process effectively enhances the hydrogen bonding force between the PVA chains, promotes the formation of PVA crystals, and forms a pre-crosslinked polyvinyl alcohol with a single crosslink; the pre-crosslinked polyvinyl alcohol is then placed in a lithium salt solution, and the salting-out effect further enhances the intermolecular interactions within the PVA chain, causing internal chain aggregation and chain entanglement to form a double crosslink, ultimately obtaining a structure of "metal lithium ion complexation with hydroxyl groups + microcrystal crosslinking, chain entanglement crosslinking". The present application uses the raw material design of PVA and lithium salt, the design of a specific preparation process, and their mutual coordination to provide a modified polyvinyl alcohol binder with a double crosslinked network and a rich branched structure, a highly elongated and shrinkable structure, and excellent bonding properties.

[0013] The modified polyvinyl alcohol binder is used in the modified composite current collector. Based on its high extensibility, it can make the conductive coating on the modified composite current collector have self-repairing properties. When the modified composite current collector is rolled and stretched, it can achieve self-repair, ensuring the connection of the conductive network, so that the modified composite current collector has excellent conductive properties. At the same time, due to the complexation between lithium ions and hydroxyl groups in PVA, lithium is introduced into the modified polyvinyl alcohol binder, so that the modified composite current collector can replenish lithium for the electrode material, offsetting the irreversible lithium loss caused by the formation of the SEI film, thereby improving the total capacity, energy density and first cycle coulombic efficiency (ICE) of the lithium-ion battery, and giving the lithium-ion battery more excellent electrochemical performance.

[0014] In the present application, the polyvinyl alcohol aqueous solution includes polyvinyl alcohol and water, with water being the main solvent. Of course, the polyvinyl alcohol aqueous solution may also include other optional solvents, such as alcohol solvents; the alcohol solvents illustratively include but are not limited to: any one of methanol, ethanol, n-propanol, isopropanol, or a combination of at least two thereof.

[0015] 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 point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0016] Preferably, the mass percentage of polyvinyl alcohol in the polyvinyl alcohol aqueous solution is 3-50%, for example, it can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40% or 45%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0017] Preferably, the method for preparing the polyvinyl alcohol aqueous solution comprises: mixing polyvinyl alcohol with water to obtain the polyvinyl alcohol aqueous solution.

[0018] Preferably, the water is hot water with a temperature of 90-100°C, for example, it can be 91°C, 92°C, 93°C, 94°C, 95°C, 96°C, 97°C, 98°C or 99°C, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0019] Preferably, the mixing is carried out under stirring conditions so that the PVA is quickly and fully dissolved in water.

[0020] Preferably, the freezing temperature is ≤-10°C, for example, it can be -12°C, -15°C, -18°C, -20°C, -22°C, -25°C, -28°C, -30°C, -32°C, -35°C, -38°C, -40°C, -42°C, -45°C, -48°C, -50°C, -55°C, -60°C, -65°C or -70°C, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of brevity, this application no longer exhaustively lists the specific point values ​​included in the range, and -10°C to -70°C is further preferred.

[0021] 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 point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range, and 10-50h is further preferred.

[0022] As a preferred technical solution of this application, the freezing and thawing are performed alternately and repeatedly to enhance 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 reorganize. After multiple freeze-thaw cycles, the PVA polymer segments are fully cross-linked. The freezing temperature is ≤ -10°C and the freezing time is ≥ 10 hours, which is the duration of a single freezing cycle.

[0023] Preferably, the thawing temperature 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 point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range, and room temperature or normal temperature is further preferred.

[0024] 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, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of brevity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0025] Preferably, 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, and lithium hexafluorophosphate is more preferred.

[0026] Preferably, the mass percentage of lithium salt in the lithium salt solution is 30-60%, for example, it can be 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 52%, 55% or 58%, as well as specific values ​​between the above points. Due to space limitations and for the sake of brevity, this application no longer exhaustively lists the specific points included in the range.

[0027] Preferably, the solvent of the lithium salt solution includes water.

[0028] Preferably, the method for treating the pre-cross-linked polyvinyl alcohol in a lithium salt solution is immersion, so that the lithium salt fully enters the internal structure of the pre-cross-linked polyvinyl alcohol, enhances the intermolecular interaction within the PVA chain through salting out, forms more abundant interchain aggregation and chain entanglement inside, realizes double cross-linking, and obtains a structure of "lithium ion and hydroxyl complexation + microcrystal cross-linking, chain entanglement cross-linking".

[0029] Preferably, the treatment time of the pre-cross-linked polyvinyl alcohol in the lithium salt solution is 4-12 hours, for example, it can be 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours or 11 hours, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0030] Preferably, the treatment temperature of the pre-cross-linked 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 point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range, and room temperature is further preferred.

[0031] Preferably, the preparation method specifically comprises the following steps:

[0032] (1) alternately freezing and thawing a polyvinyl alcohol aqueous solution for at least three 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 each time is independently ≤-10°C, and the freezing time each time is independently ≥10 hours;

[0033] (2) placing the pre-crosslinked polyvinyl alcohol obtained in step (1) in a lithium salt solution and soaking it at 15-40° C. for 4-12 hours to obtain the modified polyvinyl alcohol binder; the mass percentage of lithium salt in the lithium salt solution is 30-60%.

[0034] In a second aspect, an embodiment of the present application provides a modified polyvinyl alcohol binder, which is prepared by the preparation method described in the first aspect.

[0035] Preferably, the elongation of the modified polyvinyl alcohol binder is 80-130%, for example, it can be 85%, 90%, 95%, 100%, 105%, 110%, 115%, 120% or 125%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range, and 90-125% is further preferred.

[0036] In a third aspect, an embodiment of the present application provides a conductive material composition, which includes a combination of the modified polyvinyl alcohol binder and a conductive agent as described in the second aspect.

[0037] Preferably, the conductive agent includes any one of conductive carbon black, graphite, graphene, carbon nanotubes, and carbon fibers, or a combination of at least two thereof.

[0038] Preferably, the carbon fibers include carbon nanofibers (VGCF).

[0039] Based on its morphological characteristics, the conductive agent has different contact modes, including point contact (such as conductive carbon black), line contact (such as carbon nanotubes, carbon fibers), and surface contact (such as graphene, graphite); among them, the use of sheet-like conductive agents (such as graphene, graphite) and linear conductive agents (such as carbon nanotubes, carbon fibers) is conducive to forming more conductive bridges and improving conductive performance.

[0040] 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.

[0041] Preferably, the conductive material composition further includes an auxiliary agent.

[0042] Preferably, the auxiliary agent includes any one of a pH regulator, a wetting agent, a dispersant, and a coupling agent, or a combination of at least two thereof.

[0043] Preferably, the conductive material composition further comprises a pH regulator and / or a wetting agent.

[0044] Preferably, the pH adjuster comprises an alkaline compound, more preferably sodium hydroxide.

[0045] Preferably, the wetting agent includes any one of isopropyl alcohol, n-propyl alcohol, propylene glycol, n-octanol, polyethylene glycol, and polyoxyethylene ether, or a combination of at least two thereof.

[0046] As a preferred technical solution of the present application, the conductive material composition includes a wetting agent, which can reduce the surface energy of the conductive paste and facilitate subsequent processing and coating.

[0047] On the other hand, the present application further provides a conductive paste, which includes the conductive material composition as described in the third aspect and a solvent.

[0048] Preferably, the conductive paste is obtained by dissolving and / or dispersing the conductive material composition in a solvent.

[0049] Preferably, the solvent includes an aqueous solvent or an oily solvent, and more preferably an aqueous solvent.

[0050] The aqueous solvent includes water and / or an organic solvent miscible with water; the organic solvent miscible with water includes an alcohol solvent, illustratively including but not limited to: any one of methanol, ethanol, n-propanol, isopropanol or a combination of at least two thereof.

[0051] The oily solvent includes acetone, N-methylpyrrolidone (NMP) and the like.

[0052] In the present application, the amount of the solvent is not particularly limited, as long as the amount can make the conductive paste meet the requirements of processing and coating.

[0053] 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%, as well as specific point values ​​between the above point values. Due to limited space and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0054] In a fourth aspect, an embodiment of the present application provides a modified composite current collector, comprising a composite current collector and a conductive coating disposed on the surface of the composite current collector, wherein the conductive coating comprises the conductive material composition as described in the third aspect.

[0055] In the modified composite current collector provided in the present application, the surface of the composite current collector is modified with a conductive coating, and the conductive coating contains the aforementioned modified polyvinyl alcohol binder and a conductive agent, wherein the modified polyvinyl alcohol binder has high extensibility and excellent adhesion, and the conductive agent can build a conductive network on the surface of the composite current collector to enhance conductivity; the conductive coating containing specific components, on the one hand, can achieve self-repair when the composite current collector is rolled and stretched based on its high extensibility, ensure the connection of the conductive network, and give the modified composite current collector excellent conductive properties; on the other hand, the conductive coating helps to increase the battery electrode's absorption and retention of electrolyte. At the same time, the conductive coating (modified polyvinyl alcohol binder) contains lithium, which can replenish lithium to the electrode material and offset the irreversible lithium loss caused by the formation of the SEI film, thereby improving the total capacity, energy density and first cycle coulomb efficiency (ICE) of the lithium-ion battery, so that the lithium-ion battery has better electrochemical performance.

[0056] Preferably, the thickness of the conductive coating is 0.5-2 μm, for example, it can be 0.6 μm, 0.8 μm, 1 μm, 1.1 μm, 1.3 μm, 1.5 μm, 1.7 μm or 1.9 μm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0057] In the present application, 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. The present application does not specifically limit the type of composite current collector, and any composite current collector known in the related art for use in secondary batteries is applicable to the present application.

[0058] Exemplarily, the composite current collector includes a composite copper current collector or a composite aluminum current collector.

[0059] Illustratively, 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; applying the conductive slurry on both surfaces of the composite current collector, and drying to obtain the modified composite current collector.

[0060] Preferably, the coating method includes gravure roller coating, dip coating, narrow slot extrusion or spray coating; further preferably, a double-sided coating one-step forming machine is used to improve production efficiency.

[0061] 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, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0062] Preferably, the drying temperature is 60-100°C, for example, it can be 65°C, 70°C, 75°C, 80°C, 85°C, 90°C or 95°C, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0063] On the other hand, the present application provides a battery pole piece, which includes the modified composite current collector described in the fourth aspect and an active material layer arranged on the surface of the modified composite current collector.

[0064] The battery electrode sheet may be a positive electrode sheet or a negative electrode sheet, and correspondingly, the active material layer may be a positive electrode active material layer or a negative electrode active material layer.

[0065] In this application, the active material layer includes a combination of an active material (positive electrode active material or negative electrode active material), a binder, and a conductive material. This application does not specifically limit the specific types of active materials, binders, and conductive materials. Active materials, binders, and conductive materials known in the relevant art for use in lithium-ion batteries are applicable to this application.

[0066] In a fifth aspect, the present application provides a lithium-ion battery, comprising the modified composite current collector as described in the fourth aspect.

[0067] In the present application, the lithium-ion battery comprises a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator; wherein at least one of the positive electrode sheet and the negative electrode sheet is the battery electrode sheet described above. That is, the current collector of at least one of the positive electrode sheet and the negative electrode sheet is the modified composite current collector described in the third aspect.

[0068] Compared with the related art, the embodiments of the present application have the following beneficial effects:

[0069] (1) In the preparation method of the modified polyvinyl alcohol binder provided in the embodiment of the present application, through the raw material design of PVA and lithium salt, the design of a specific preparation process and their mutual cooperation, the obtained modified polyvinyl alcohol binder has a double cross-linked network and a rich branched structure, a high elongation and shrinkable structure, an elongation of 90-125%, and at the same time has excellent bonding properties and can also achieve a lithium supplement effect.

[0070] (2) In the modified composite current collector provided in the embodiment of the present application, 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 extensibility and can achieve self-repair when the composite current collector is rolled and stretched, ensuring the connection of the conductive network, giving the modified composite current collector excellent conductive properties, and still maintaining high conductivity in the stretched state, with a 3% square resistance change rate of ≤20%, and a 5% square resistance change rate of <38%; at the same time, based on the high bonding performance of the modified polyvinyl alcohol binder, the conductive coating has a high bonding force with the composite current collector, and the modified The peeling force of the composite current collector is ≥498N / 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 described in the present application is increased by 10-15%, and the ICE increase ratio is 1.0-1.4%, which gives the lithium-ion battery better electrochemical performance.

[0071] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] The accompanying drawings are used to provide a further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of this application, they are used to explain the technical solution of this article and do not constitute a limitation to the technical solution of this article.

[0073] FIG1 is a scanning electron microscope image of the modified polyvinyl alcohol binder provided in Preparation Example 1. DETAILED DESCRIPTION

[0074] The technical solution of the present application is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only used to help understand the present application and should not be regarded as specific limitations of the present application.

[0075] As used herein, the terms "comprises," "including," "having," "containing" or any other variations thereof, are intended to cover a non-exclusive inclusion. For example, a composition, process, method, article, or apparatus that comprises a listed element is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, process, method, article, or apparatus.

[0076] "Optionally" or "either" means that the subsequently described event or circumstance can or cannot occur, and that the description includes instances where the event occurs and instances where it does not.

[0077] The indefinite articles "a" and "an" before an element or component of the present application do not limit the quantity requirement (i.e., the number of times the element or component appears). Therefore, "a" or "an" should be interpreted as including one or at least one, and elements or components in the singular also include plural forms, unless the quantity clearly refers to only the singular form.

[0078] The raw materials involved in the following specific embodiments of this application are all commercially available products; among them, polyvinyl alcohol (PVA, degree of polymerization is 2000), brand SG-181, purchased from Anhui Wanwei High-tech Materials Co., Ltd.; the composite current collector is a composite aluminum current collector, brand NP060AA, purchased from Yangzhou Nali New Materials Co., Ltd.

[0079] Preparation Example 1

[0080] A modified polyvinyl alcohol binder and a preparation method thereof, wherein the specific steps of the preparation method are as follows:

[0081] (1) In a stirring tank, 17 kg of PVA powder was added to 100 kg of 100°C hot water and stirred at 1000 rpm for 6 h to form a PVA aqueous solution;

[0082] (2) Cooling the PVA aqueous solution to room temperature, and alternately freezing and thawing, each freezing temperature is -12 ° C., and the time is 14 hours; each thawing is performed at room temperature and thawed to room temperature; the cycle is repeated 6 times to obtain pre-crosslinked polyvinyl alcohol;

[0083] (3) The pre-crosslinked polyvinyl alcohol obtained in step (2) was placed in a lithium hexafluorophosphate aqueous solution (the mass percentage of lithium hexafluorophosphate was 40%) and soaked at room temperature for 6 hours to obtain the modified polyvinyl alcohol binder, hereinafter abbreviated as mPVA-1.

[0084] The modified polyvinyl alcohol binder (mPVA-1) provided in this preparation example was characterized by the following properties:

[0085] 1. Microscopic morphology

[0086] The microscopic morphology of the modified polyvinyl alcohol binder provided in this preparation example was tested using a scanning electron microscope. The resulting scanning electron microscope image is shown in FIG1 . As can be seen from FIG1 , 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 extensibility.

[0087] 2. Elongation

[0088] The modified polyvinyl alcohol adhesive provided in this preparation example was placed in a rectangular mold and tested in its gel state according to the method specified in GB / T 1040.3-2006. Specific parameters included: specimen width of 15 mm, tensile speed of 10 mm / min, and initial clamp distance of 50 mm. Five specimens were taken, 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, the test result would be invalidated if the edge of the sample had notched. Using this method, the elongation of the adhesive mPVA-1 was 120%.

[0089] Preparation Example 2

[0090] A modified polyvinyl alcohol binder and a preparation method thereof, wherein the specific steps of the preparation method are as follows:

[0091] (1) In a stirring tank, add 8 kg of PVA powder to 100 kg of 98°C hot water and stir at 1000 rpm for 5 h to fully dissolve the PVA to form a PVA aqueous solution;

[0092] (2) Cooling the PVA aqueous solution to room temperature, and alternately freezing and thawing, each freezing temperature is -35 ° C, and the time is 12 hours; each thawing is performed at room temperature and thawed to room temperature; the cycle is repeated 5 times to obtain pre-crosslinked polyvinyl alcohol;

[0093] (3) The pre-crosslinked polyvinyl alcohol obtained in step (2) was placed in a lithium hexafluorophosphate aqueous solution (the mass percentage of lithium hexafluorophosphate was 30%) and soaked at room temperature for 4 hours to obtain the modified polyvinyl alcohol binder, hereinafter abbreviated as mPVA-2.

[0094] The performance of mPVA-2 was tested using the same method as in Preparation Example 1, and the elongation was found to be 125%.

[0095] Preparation Example 3

[0096] A modified polyvinyl alcohol binder and a preparation method thereof, wherein the specific steps of the preparation method are as follows:

[0097] (1) In a stirring tank, add 40 kg of PVA powder to 100 kg of 98°C hot water and stir at 1000 rpm for 8 h to fully dissolve the PVA to form a PVA aqueous solution;

[0098] (2) cooling the PVA aqueous solution to room temperature, and alternately freezing and thawing, each freezing temperature is -15°C, and the time is 20 hours; each thawing is performed at room temperature, and thawing is performed to room temperature; the cycle is repeated 8 times to obtain pre-crosslinked polyvinyl alcohol;

[0099] (3) The pre-crosslinked polyvinyl alcohol obtained in step (2) is placed in a lithium hexafluorophosphate aqueous solution (the mass percentage of lithium hexafluorophosphate is 60%) and soaked at room temperature for 8 hours to obtain the modified polyvinyl alcohol binder, hereinafter abbreviated as mPVA-3.

[0100] The performance of mPVA-3 was tested using the same method as in Preparation Example 1, and the elongation was found to be 90%.

[0101] Comparative Preparation Example 1

[0102] A modified polyvinyl alcohol binder and a preparation method thereof, wherein the specific steps of the preparation method are as follows:

[0103] (1) In a stirring tank, 17 kg of PVA powder was added to 100 kg of 100°C hot water and stirred at 1000 rpm for 6 h to form a PVA aqueous solution;

[0104] (2) The PVA aqueous solution was cooled to room temperature and alternately frozen and thawed, with each freezing temperature being -12°C and the time being 14 hours; each thawing was performed at room temperature until it was thawed to room temperature; the cycle was repeated 6 times to obtain the modified polyvinyl alcohol binder, hereinafter abbreviated as mPVA-D1.

[0105] The performance of mPVA-D1 was tested using the same method as in Preparation Example 1, and the elongation was found to be 10%.

[0106] Comparative Preparation Example 2

[0107] A modified polyvinyl alcohol binder and a preparation method thereof, wherein the specific steps of the preparation method are as follows:

[0108] (1) In a stirring 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 h to form a PVA lithium salt aqueous solution;

[0109] (2) The PVA lithium salt aqueous solution was cooled to room temperature, and then alternately frozen and thawed, with each freezing temperature being -12°C and the time being 14 hours; each thawing was performed at room temperature until it was thawed to room temperature; the cycle was repeated 6 times to obtain the modified polyvinyl alcohol binder, hereinafter abbreviated as mPVA-D2.

[0110] The same method as in Preparation Example 1 was used to perform a performance test on mPVA-D2, and the elongation was measured to be 13%.

[0111] Example 1

[0112] A modified composite current collector comprises a composite aluminum current collector and a conductive coating disposed on the composite aluminum current collector, wherein the conductive coating is made of a conductive material composition comprising, by mass, 4 parts of conductive carbon black, 0.7 parts of graphite, 13 parts of a binder mPVA-1 (Preparation Example 1), 0.9 parts of sodium hydroxide, and 15 parts of isopropyl alcohol.

[0113] The preparation method of the modified composite current collector is as follows:

[0114] (1) Mixing the formulated amount of conductive carbon black, graphite, binder and 66 parts of water to form a slurry, then adding sodium hydroxide to adjust the pH of the slurry, and finally adding a wetting agent, mixing and dispersing uniformly to obtain a conductive slurry;

[0115] (2) The conductive slurry obtained in step (1) is coated on both sides of the composite aluminum current collector by gravure roller coating, using a double-sided coating one-time forming machine, and a coating speed of 100 m / min; after coating, it is placed in an oven at 75°C for drying to obtain the modified composite current collector, wherein the thickness of the conductive coating is 1 μm.

[0116] Examples 2-5, Comparative Examples 1-2

[0117] A modified composite current collector includes a composite aluminum current collector and a conductive coating disposed thereon. The conductive coating is made of a conductive material composition. The modified composite current collector differs from Example 1 only in the components and / or amounts of the conductive material composition, as shown in Table 1. The amounts of each component in Table 1 are all in parts, with "--" indicating that the component was not added. The preparation methods for each modified composite current collector are the same as those for Example 1.

[0118] Table 1

[0119] Comparative Example 3

[0120] A modified composite current collector comprises a composite aluminum current collector and a conductive coating disposed on the composite aluminum current collector, wherein the material of the conductive coating is a conductive material composition. The only difference between the modified composite current collector 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., the PVA powder in Preparation Example 1). Other materials, amounts, and preparation methods are the same as those in Example 1.

[0121] Comparative Example 4

[0122] A modified composite current collector comprises a composite aluminum current collector and a conductive coating disposed on the composite aluminum current collector, wherein the material of the conductive coating is a conductive material composition. The only difference between the modified composite current collector and Example 1 is that the binder mPVA-1 in the conductive material composition is replaced with a commercially available polyacrylic acid-type binder (Source Leaf S59106) of equal mass; other materials, amounts, and preparation methods are the same as those in Example 1.

[0123] Comparative Example 5

[0124] A composite aluminum current collector without a conductive coating is used as Comparative Example 5.

[0125] The performance tests were conducted on the modified composite current collectors provided in Examples 1-5 and Comparative Examples 1-4, and the composite aluminum current collector of Comparative Example 5. The specific methods are as follows:

[0126] 1. Conductive properties

[0127] The square resistance (in mΩ) of each modified composite current collector was tested to characterize its conductive performance. The smaller the square resistance test value, the better the conductive performance.

[0128] The test method for the initial square resistance is as follows: the test instrument uses a four-probe resistivity tester, and the specific test parameter requirements are: point mode, probe HPS58006, spacing 2mm+2mm+2mm, to obtain the initial square resistance;

[0129] The 3% square resistance test method is as follows: the modified composite current collector to be tested is stretched using a tensile tester with a stretching ratio (elongation) of 3%, and then the square resistance of the stretched sample is measured using a four-probe resistivity tester according to the test method for the initial square resistance;

[0130] The test method for 5% square 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%, and then the square resistance of the stretched sample is tested using a four-probe resistivity tester according to the test method for initial square resistance.

[0131] 2. Adhesive properties

[0132] The peel force (unit: N / m) of each modified composite current collector was tested to characterize the bonding performance. The greater the peel force, the better the bonding performance.

[0133] The test method for peeling force is as follows: unfold 3-4 circles and take a sample once, the sample size is 15mm×100mm, stick the 3M-9080A-15mm tape on the stainless steel plate, and then evenly stick the modified composite current collector to be tested on the double-sided tape, use a 2kg standard small pressure roller to squeeze back and forth once, then stick the 3M-9080A-14mm tape on the surface of the foil, use a 2kg standard small pressure roller to squeeze back and forth once, then take the pressed sample to the tensile testing machine and stretch it 180°, the speed is 100mm / min, the width is set to 14mm, and the maximum value is taken.

[0134] 3. Electrochemical Performance

[0135] The modified composite current collector to be tested is used to prepare the positive electrode sheet, and then assembled into a lithium-ion battery to test the first cycle coulombic efficiency of the lithium-ion battery. The specific method is as follows:

[0136] (1) Preparation of positive electrode sheets: The positive electrode active material (lithium iron phosphate), conductive agent (conductive carbon black) and binder (polyvinylidene fluoride, PVDF) were mixed in a mass ratio of 96:2:2, N-methylpyrrolidone (NMP) was added at a ratio of 50% of the system solid content, and the mixture was stirred thoroughly to prepare a uniform positive electrode slurry; the positive electrode slurry was coated on each modified composite current collector to be tested, dried and then rolled to obtain a positive electrode sheet.

[0137] (2) Preparation of negative electrode sheets: negative electrode active material (silicon carbon), conductive agent (SP) and thickener (carboxymethyl cellulose CMC), adhesive (styrene-butadiene latex SBR) and water are mixed at a pure material mass ratio of 100:0.5:1.2:2.6:78, water is added according to a system solid content of 45%, and the mixture is fully stirred. A uniform negative electrode slurry is prepared by narrow slit extrusion coating; the negative electrode slurry is coated on copper foil, dried, and then rolled to obtain a negative electrode sheet.

[0138] (3) Assembly of lithium-ion batteries: Assemble the positive electrode, separator, and negative electrode into a battery cell; package the battery cell, bake it to remove water, and then inject the electrolyte. After packaging and formation, a lithium-ion battery is obtained.

[0139] The following performance tests are performed on lithium-ion batteries:

[0140] (A) First Cycle Coulombic Efficiency (ICE): Connect the lithium-ion battery to be tested to a coulomb meter. Charge the battery and record the charge coulomb value. Then discharge the battery and record the discharge coulomb value. Based on the recorded coulomb values, calculate the first cycle coulombic efficiency of each battery.

[0141] Taking the ICE test value of the lithium ion battery using the composite aluminum current collector in Comparative Example 5 as 100.0%, the ratio of the ICE test values ​​of the lithium ion batteries using other modified composite current collectors to the ICE test value of Comparative Example 5 was calculated.

[0142] (B) Energy density: Connect the lithium-ion battery to a constant current load and calculate the total energy released by the battery by recording the battery voltage and discharge time. Measure the battery mass and calculate the battery's energy density based on the measured energy and the battery mass. The formula for calculating energy density is energy / mass.

[0143] Taking the energy density test value of the lithium ion battery using the composite aluminum current collector in Comparative Example 5 as 100.0%, the ratio of the energy density test values ​​of the lithium ion batteries using other modified composite current collectors to the energy density test value of Comparative Example 5 was calculated.

[0144] The specific results are shown in Table 2.

[0145] Table 2

[0146] Combined with the data in Table 2, it can be seen that the modified composite current collector provided in this application is provided with a conductive coating containing a specific modified polyvinyl alcohol binder. The conductive coating has high extensibility and can achieve self-repair when the composite current collector is rolled and stretched, ensuring the connection of the conductive network and maintaining high conductivity in the stretched state. The initial square resistance is 35mΩ, the 3% square resistance and the 5% square resistance are 40-42mΩ and 44.5-48mΩ respectively, the change rate of the 3% square resistance is ≤20%, which is 14.2-20%, and the change rate of the 5% square resistance is <38%, which is 27.1-37.1%. At the same time, due to the excellent bonding properties of the modified polyvinyl alcohol binder, the peel force of the modified composite current collector is high, reaching 498-720N / m. Moreover, the modified polyvinyl alcohol binder contains lithium, which enables 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 of the lithium-ion battery. Due to the high elongation, excellent adhesion, and lithium replenishment properties of the modified polyvinyl alcohol binder, lithium-ion batteries using the modified composite current collectors described in Examples 1-5 achieved ICE improvements of 1.0-1.4% and energy density increases of 10-15%, compared to lithium-ion batteries using the composite aluminum current collector in Comparative Example 5. Furthermore, a comparison of Examples 1 and 2 demonstrates that the combined use of graphite and conductive carbon black in the conductive coating further improves the conductivity of the modified composite current collector.

[0147] In this application, a PVA aqueous solution is first subjected to multiple freeze-thaw cycles to form a pre-crosslinked PVA with a single crosslink. The pre-crosslinked PVA is then treated in a lithium salt solution to achieve a double crosslink. The resulting modified polyvinyl alcohol binder contains a structure of "metal lithium ion complexation with hydroxyl groups + microcrystalline crosslinking and chain entanglement crosslinking," exhibiting high elongation, excellent adhesion, and lithium replenishment. The binder in Comparative Example 1 is a modified PVA with only a single crosslink. Although the binder in Comparative Example 2 uses PVA and lithium salt as raw materials, it does not adopt the preparation process of this application, resulting in insufficient elongation of the modified PVA and its inability to perform self-repairing functions during the extension of the composite current collector, resulting in a break in the conductive network, an increase in square resistance, and a deterioration in conductive performance. The modified composite aluminum current collector of Comparative Example 3 uses unmodified PVA as the binder for the conductive coating. The modified composite aluminum current collector of Comparative Example 4 uses a conventional binder. Due to the low elongation of the binder and the conductive coating, the conductive network of the composite current collector breaks during roller extension, resulting in a decrease in conductive performance.

[0148] The applicant declares that while the above-mentioned embodiments are used to illustrate the modified polyvinyl alcohol binder and its preparation method, as well as the modified composite current collector, the present application is not limited to the above-mentioned process steps, which does not mean that the present application must rely on the above-mentioned process steps for implementation. Those skilled in the art should understand that any improvements to the present application, equivalent replacements for the raw materials used in the present application, addition of auxiliary components, and selection of specific methods, etc., fall within the scope of protection and disclosure of the present application.

Claims

1. A method for preparing a modified polyvinyl alcohol binder, comprising the following steps: The polyvinyl alcohol aqueous solution is alternately frozen and thawed to obtain pre-crosslinked polyvinyl alcohol; The pre-crosslinked polyvinyl alcohol is placed in a lithium salt solution for treatment to obtain the modified polyvinyl alcohol binder.

2. The preparation method according to claim 1, wherein The degree of polymerization of the polyvinyl alcohol is 1000-2000.

3. The preparation method according to claim 1 or 2, wherein The mass percentage of polyvinyl alcohol in the polyvinyl alcohol aqueous solution is 3-50%.

4. The preparation method according to any one of claims 1 to 3, wherein: The freezing temperature is ≤-10°C; Preferably, the freezing time is ≥ 10 h; Preferably, the number of cycles of alternating freezing and thawing is ≥3.

5. The preparation method according to any one of claims 1 to 4, wherein: The lithium salt includes any one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium carbonate, lithium nitrate, lithium oxalate, lithium chloride, and lithium sulfate, or a combination of at least two thereof, preferably lithium hexafluorophosphate; Preferably, the mass percentage of lithium salt in the lithium salt solution is 30-60%.

6. The preparation method according to any one of claims 1 to 5, wherein: The treatment time of the pre-crosslinked polyvinyl alcohol in the lithium salt solution is 4-12 hours; Preferably, the treatment temperature of the pre-crosslinked polyvinyl alcohol in the lithium salt solution is 15-40°C.

7. The preparation method according to any one of claims 1 to 6, comprising the steps of: (1) freezing and thawing a polyvinyl alcohol aqueous solution alternately 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 each time is independently ≤-10°C, and the freezing time each time is independently ≥10h; (2) placing the pre-crosslinked polyvinyl alcohol obtained in step (1) in a lithium salt solution and soaking it at 15-40° C. for 4-12 hours to obtain the modified polyvinyl alcohol binder; the mass percentage of lithium salt in the lithium salt solution is 30-60%.

8. A modified polyvinyl alcohol binder, wherein: The modified polyvinyl alcohol binder is prepared by the preparation method according to any one of claims 1 to 7.

9. A conductive material composition, wherein: The conductive material composition comprises a combination of the modified polyvinyl alcohol binder as claimed in claim 8 and a conductive agent.

10. The conductive material composition according to claim 9, wherein: The conductive agent includes any one of conductive carbon black, graphite, graphene, carbon nanotubes, and carbon fibers, or a combination of at least two of them.

11. The conductive material composition according to claim 9 or 10, wherein: The mass ratio of the modified polyvinyl alcohol binder to the conductive agent is (1-6):

1.

12. The conductive material composition according to any one of claims 9 to 11, wherein: The conductive material composition further includes a pH adjuster and / or a wetting agent.

13. A modified composite current collector, wherein: The modified composite current collector comprises a composite current collector and a conductive coating disposed on the surface of the composite current collector, wherein the conductive coating comprises the conductive material composition according to any one of claims 9 to 12.

14. The modified composite current collector according to claim 13, wherein: The thickness of the conductive coating is 0.5-2 μm.

15. A lithium ion battery, wherein: The lithium ion battery comprises the modified composite current collector as claimed in claim 14.

Images