Lithium ion secondary battery and preparation method thereof, and power-using device

Abstract

The present application provides a lithium-ion secondary battery and a preparation method thereof, and an electric device, wherein the lithium-ion secondary battery comprises a positive electrode plate, the positive electrode plate comprises a positive electrode active material layer, the positive electrode active material layer comprises a lithium-containing phosphate and a thermoplastic elastomer graft-modified polyacrylate binder, the thermoplastic elastomer-modified polyacrylate binder comprises a thermoplastic elastomer polymer and a polyacrylate polymer; the thermoplastic elastomer polymer comprises one or more of styrene-ethylene-butylene-styrene block copolymer, styrene-isoprene-styrene block copolymer, styrene-ethylene / propylene-styrene block copolymer, hydrogenated styrene-butadiene rubber, or hydrogenated nitrile-butadiene rubber. The lithium-ion secondary battery has low internal resistance and good dynamic performance.

Description

Technical Field

[0001] The present application relates to the technical field of secondary batteries, and in particular to a lithium-ion secondary battery and a preparation method and an electrical device thereof. Background Art

[0002] In recent years, as the application scope of secondary batteries becomes wider and wider, secondary batteries are widely used in energy storage power supply systems such as hydropower, thermal power, wind power and solar power stations, as well as in many fields such as electric tools, electric bicycles, electric motorcycles and electric vehicles.

[0003] Secondary batteries using lithium phosphates such as lithium iron phosphate (LFP) as positive electrode active materials have the advantages of low cost and good safety, and have become a hot area for secondary battery development. However, the internal resistance (DCR) of lithium phosphates is usually relatively large, and the dynamic performance of their batteries is poor. Therefore, how to reduce the internal resistance of lithium phosphate batteries and improve their dynamic performance has become one of the key areas of focus for technicians in this field. Summary of the invention

[0004] The present application is made in view of the above-mentioned problems, and one of its purposes is to provide a lithium ion secondary battery with lower internal resistance and better dynamic performance, and correspondingly provide a preparation method and an electrical device of the lithium ion secondary battery.

[0005] In order to achieve the above-mentioned purpose, the first aspect of the present application provides a lithium-ion secondary battery, including a positive electrode plate, wherein the positive electrode plate includes a positive electrode active material layer, wherein the positive electrode active material layer includes a lithium-containing phosphate and a thermoplastic elastomer graft-modified polyacrylate binder, wherein the thermoplastic elastomer graft-modified polyacrylate binder includes a thermoplastic elastomer polymer and a polyacrylate polymer; the thermoplastic elastomer polymer includes one or more of styrene-ethylene-butylene-styrene block copolymer, styrene-isoprene-styrene block copolymer, styrene-ethylene / propylene-styrene block copolymer, hydrogenated styrene-butadiene rubber or hydrogenated nitrile rubber.

[0006] By adding thermoplastic elastomer to the positive electrode active material layer to graft and modify the polyacrylate binder; the polarity of the above thermoplastic elastomer is usually smaller than that of polyacrylate, and the thermoplastic elastomer can wrap or cover the polar groups on the surface of the polyacrylate binder molecule, reducing the contact between the polar groups and the outside world. By grafting and modifying the polyacrylate binder with thermoplastic elastomer, the polarity of the binder can be effectively reduced, the force between the binder and the surface of the lithium-containing phosphate material can be weakened, and the density of the polyacrylate binder film on the surface of the lithium-containing phosphate material can be effectively reduced, which is conducive to the rapid transmission of lithium ions, thereby reducing the internal resistance of the battery and improving the dynamic performance of the battery. In addition, the thermoplastic elastomer part introduced into the binder can significantly reduce the polarity of the polyacrylate binder, making it difficult for the positive electrode slurry to agglomerate and gel, improving the dispersion uniformity of the positive electrode active material in the positive electrode slurry, and then helping to improve the dispersion uniformity of the constituent materials in the positive electrode active material layer, optimizing the uniformity of the ion transmission channel, and helping to improve the dynamic performance of the battery. Furthermore, the glass transition temperature of thermoplastic elastomers is low. They are usually in a highly elastic state at room temperature and have good elasticity. Using thermoplastic elastomers to modify polyacrylate binders can make the polyacrylate binders have better flexibility, so that the positive electrode can better withstand the stress caused by volume changes during the battery cycle, which is beneficial to maintaining the stability of the electrode and improving the dynamic performance of the battery.

[0007] In any embodiment, the thermoplastic elastomer polymer includes structural units derived from a first monomer and a second monomer, respectively; the monomer types of the first monomer and the second monomer are different; the first monomer includes one or more of styrene or acrylonitrile; and the second monomer includes one or more of ethylene, propylene, butadiene, isoprene or acrylonitrile.

[0008] In any embodiment, in the thermoplastic elastomer polymer, a molar ratio of the structural unit derived from the first monomer to the structural unit derived from the second monomer is (2:3) to (3:2).

[0009] In any embodiment, the polyacrylate polymer includes structural units derived from acrylic acid ester monomers, and the acrylic acid ester monomers include one or more of methyl methacrylate, butyl acrylate, or isooctyl acrylate.

[0010] In any embodiment, the mass fraction of the polyacrylate polymer is 40% to 70%, based on the total mass of the thermoplastic elastomer graft-modified polyacrylate binder being 100%. This is beneficial for reducing the internal resistance of the battery, improving the battery dynamics performance, and making the positive electrode sheet have higher bonding force and better mechanical properties.

[0011] In any embodiment, the mass fraction of the polyacrylate polymer is 50% to 60%, based on the total mass of the thermoplastic elastomer graft-modified polyacrylate binder being 100%. This is beneficial for further taking into account the dynamic performance of the secondary battery and the mechanical properties of the positive electrode sheet.

[0012] In any embodiment, the weight average molecular weight of the thermoplastic elastomer graft-modified polyacrylate binder is 150,000 Daltons to 400,000 Daltons, which is conducive to making the binder have good bonding properties, while making the viscosity of the positive electrode slurry moderate and not prone to gelation.

[0013] In any embodiment, the weight average molecular weight of the thermoplastic elastomer graft modified polyacrylate binder is 300,000 Daltons to 400,000 Daltons, which is conducive to making the viscosity of the positive electrode slurry moderate, and is not prone to gelation, while making the binder have better bonding performance.

[0014] In any embodiment, based on the total mass of the positive electrode active material layer being 100%, the mass fraction of the thermoplastic elastomer graft-modified polyacrylate binder is 0.5% to 2%. This is conducive to making the positive electrode slurry have good dispersion performance and the positive electrode sheet have good mechanical properties, while not significantly affecting the energy density of the battery.

[0015] In any embodiment, based on the total mass of the positive electrode active material layer being 100%, the mass fraction of the thermoplastic elastomer graft-modified polyacrylate binder is 1% to 1.8%. This helps to better balance the dispersion performance of the positive electrode slurry, the mechanical properties of the positive electrode sheet, and the energy density of the battery.

[0016] In any embodiment, based on the total mass of the positive electrode active material layer being 100%, the mass fraction of the lithium-containing phosphate is 95% to 99%.

[0017] In any embodiment, the lithium-containing phosphate includes one or more of lithium iron phosphate, lithium manganese phosphate or lithium manganese iron phosphate.

[0018] A second aspect of the present application provides a method for preparing a lithium ion secondary battery, comprising the following steps:

[0019] The lithium-containing phosphate, the thermoplastic elastomer graft-modified polyacrylate binder and the solvent are mixed to obtain a positive electrode slurry;

[0020] Placing the positive electrode slurry on a positive electrode current collector and drying the positive electrode to obtain a positive electrode sheet; and

[0021] The positive electrode sheet, the separator and the negative electrode sheet are stacked in sequence to form an electrode assembly, and the electrode assembly is made into a lithium-ion secondary battery;

[0022] Among them, the thermoplastic elastomer grafted modified polyacrylate adhesive includes a thermoplastic elastomer polymer and a polyacrylate polymer; the thermoplastic elastomer polymer includes one or more of styrene-ethylene-butylene-styrene block copolymer, styrene-isoprene-styrene block copolymer, styrene-ethylene / propylene-styrene block copolymer, hydrogenated styrene-butadiene rubber or hydrogenated nitrile rubber.

[0023] Adding thermoplastic elastomer grafted modified polyacrylate binder to the positive electrode slurry; by using the above-mentioned thermoplastic elastomer polymer to modify the polyacrylate binder, the density of the polyacrylate binder film on the surface of the lithium-containing phosphate material can be effectively reduced, and the transmission of lithium ions can be accelerated, thereby reducing the internal resistance of the secondary battery and improving the dynamic performance of the battery. In addition, the thermoplastic elastomer polymer can significantly reduce the polarity of the polyacrylate binder, making it difficult for the positive electrode slurry to agglomerate and gel. In addition, the glass transition temperature of the thermoplastic elastomer is low, which can make the positive electrode sheet have better flexibility, which is beneficial to improving the dynamic performance of the battery.

[0024] In any embodiment, the preparation method of the thermoplastic elastomer graft-modified polyacrylate adhesive comprises the following steps: mixing a thermoplastic elastomer polymer and an initiator in a solvent, adding an acrylate monomer to carry out a polymerization reaction, and forming the thermoplastic elastomer graft-modified polyacrylate adhesive.

[0025] In any embodiment, the thermoplastic elastomeric polymer includes one or more of styrene-ethylene-butylene-styrene block copolymer, styrene-isoprene-styrene block copolymer, styrene-ethylene / propylene-styrene block copolymer, hydrogenated styrene-butadiene rubber or hydrogenated nitrile-butadiene rubber.

[0026] In any embodiment, the acrylic acid ester monomer includes one or more of methacrylate, butyl acrylate or isooctyl acrylate.

[0027] In any embodiment, the initiator includes one or more of benzoyl peroxide, potassium persulfate or azobisisobutyronitrile.

[0028] The third aspect of the present application provides an electrical device, comprising one or more of the lithium ion secondary battery of the first aspect of the present application and the lithium ion secondary battery prepared by the preparation method of the lithium ion secondary battery of the second aspect of the present application.

[0029] The details of one or more embodiments of the present application are set forth in the following drawings and description. Other features, objects, and advantages of the present application will become apparent from the description, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to better describe and illustrate the embodiments or examples provided by the present application, reference may be made to one or more drawings. The additional details or examples used to describe the drawings should not be considered as limiting the scope of the disclosed application, the embodiments or examples currently described, and any of the best modes of these applications currently understood. Moreover, the same reference numerals are used throughout the drawings to represent the same components. In the drawings:

[0031] Figure 1 A schematic diagram of a battery cell according to an embodiment of the present application;

[0032] Figure 2 for Figure 1 An exploded view of a battery cell according to an embodiment of the present application is shown;

[0033] Figure 3 A schematic diagram of an electrical device using a lithium-ion secondary battery according to an embodiment of the present application as a power source.

[0034] Description of reference numerals:

[0035] 5. Battery cell; 51. Shell; 52. Electrode assembly; 53. Cover plate; 6. Electrical device. DETAILED DESCRIPTION

[0036] Below, the lithium-ion secondary battery and its preparation method, positive electrode sheet, positive electrode slurry and electric device of the present application are described in detail with appropriate reference to the drawings. However, there are cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following descriptions are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0037] The "range" disclosed in the present application can be limited in the form of a lower limit and an upper limit, and a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The range limited in this way can be including or excluding end values, and any end value can be included or not included independently, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if a range of 60 to 120 and 80 to 110 is listed for a specific parameter, it is understood that the range of 60 to 110 and 80 to 120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are also listed, the following ranges can all be expected: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4 and 2 to 5. In the present application, unless otherwise specified, the numerical range "a to b" represents the abbreviation of any real number combination between a and b, wherein a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to listing the parameter as, for example, integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For example, when a parameter is expressed as an integer selected from "2-10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9 and 10.

[0038] In the present application, "plurality", "multiple" and the like, unless otherwise specified, refer to a number greater than or equal to 2. For example, "one or more" means one or greater than or equal to two.

[0039] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0040] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment or implementation of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments. The "implementation methods" mentioned herein have a similar understanding.

[0041] Those skilled in the art will appreciate that, in the methods of each embodiment or example, the order in which each step is written does not mean a strict order of execution and does not constitute any limitation on the implementation process, and the detailed order of execution of each step should be determined by its function and possible internal logic. If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or steps (b) and (a) performed sequentially. For example, the method may also include step (c), indicating that step (c) can be added to the method in any order, for example, the method may include steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0042] In the present application, in the open technical features or technical solutions described by the words "contain", "include", "comprise", etc., unless otherwise specified, additional members other than the listed members are not excluded, and it can be regarded as providing both closed features or solutions consisting of the listed members and open features or solutions including additional members in addition to the listed members. For example, A includes a1, a2 and a3. Unless otherwise specified, it may also include other members or may not include additional members. It can be regarded as providing both the feature or solution of "A consists of a1, a2 and a3" and the feature or solution of "A not only includes a1, a2 and a3, but also includes other members". In the present application, unless otherwise specified, A (such as B) means that B is a non-limiting example of A, and it can be understood that A is not limited to B.

[0043] In this application, "optionally", "optional", and "optional" mean optional, that is, any one of the two parallel schemes of "yes" or "no". If there are multiple "options" in a technical solution, unless otherwise specified and there is no contradiction or mutual restriction, each "option" is independent.

[0044] At present, lithium-ion secondary batteries using lithium phosphates such as lithium iron phosphate as positive electrode active materials have the advantages of low cost and good safety, and have become a hot field for the development of lithium-ion secondary batteries. However, when polyacrylate binders are used in the positive electrode active material layer, the internal resistance of the lithium phosphate secondary battery will be relatively large, affecting the dynamic performance of the battery. In this regard, the present application improves the positive electrode plate of the lithium-ion secondary battery, effectively reducing the internal resistance of the lithium-ion secondary battery using lithium phosphate as the positive electrode active material and using polyacrylate binders, and improving the dynamic performance of the battery.

[0045] In one embodiment of the present application, a lithium-ion secondary battery is provided, which includes a positive electrode plate, the positive electrode plate includes a positive electrode active material layer, the positive electrode active material layer includes a lithium-containing phosphate and a thermoplastic elastomer graft-modified polyacrylate binder, the thermoplastic elastomer graft-modified polyacrylate binder includes a thermoplastic elastomer polymer and a polyacrylate polymer; the thermoplastic elastomer polymer includes one or more of styrene-ethylene-butylene-styrene block copolymer, styrene-isoprene-styrene block copolymer, styrene-ethylene / propylene-styrene block copolymer, hydrogenated styrene-butadiene rubber or hydrogenated nitrile rubber.

[0046] Traditional positive electrode sheets using lithium-containing phosphate as positive electrode active materials usually use PVDF (polyvinylidene fluoride) as a binder. In some batteries using PFAS (per / polyfluoroalkyl substances, perfluorinated and polyfluoroalkyl substances), the use of PVDF needs to be restricted due to the risk of persistent pollution and bioaccumulative hazards. For example, polyacrylate binders with strong bonding force and good stability can be used instead of PVDF. However, the internal resistance (DCR) of lithium-ion secondary batteries containing lithium phosphate using polyacrylate binders is large and the dynamic performance is poor. The main reason is that the polar groups in the polyacrylate binder will form hydrogen bonds and other interactions with the polar sites (such as phosphate) on the surface of the lithium-containing phosphate, thereby forming a relatively dense binder film on the surface of the lithium-containing phosphate, and the polyacrylate binder itself has poor ion conductivity. The dense binder film wrapped on the surface of the lithium-containing phosphate material makes the internal resistance of the lithium-ion secondary battery large and the dynamic performance poor.

[0047] In the above technical solution of the present application, a thermoplastic elastomer is added to the positive electrode active material layer to graft and modify a polyacrylate binder; the polarity of the above thermoplastic elastomer is usually smaller than that of polyacrylate, and the thermoplastic elastomer can wrap or cover the polar groups on the surface of the polyacrylate binder molecules, reducing the contact between the polar groups and the outside world. By grafting and modifying the polyacrylate binder with a thermoplastic elastomer polymer, the polarity of the binder can be effectively reduced, the force between the binder and the polar sites on the surface of the lithium-containing phosphate can be weakened, and the density of the polyacrylate binder film on the surface of the lithium-containing phosphate material can be effectively reduced, and the transmission of lithium ions can be accelerated, thereby reducing the internal resistance of the lithium-ion secondary battery and improving the dynamic performance of the battery. In addition, the thermoplastic elastomer polymer can significantly reduce the polarity of the polyacrylate binder, so that the positive electrode slurry is not easy to agglomerate and gel. The glass transition temperature of thermoplastic elastomers is low. They are usually in a highly elastic state at room temperature and have good elasticity. Using thermoplastic elastomers to modify polyacrylate binders can make the polyacrylate binders have better flexibility, so that the positive electrode can better withstand the stress caused by volume changes during the battery cycle, which is beneficial to maintaining the stability of the electrode and improving the dynamic performance of the battery.

[0048] It should be noted that thermoplastic elastomer (TPE) is a new type of polymer material that has both the high elasticity of rubber and the processability of plastic. It has good elasticity, good impact resistance and fatigue resistance, high tensile strength and a wide range of hardness. Thermoplastic elastomer graft-modified polyacrylate adhesive can be an adhesive obtained by chemically grafting thermoplastic elastomer and polyacrylate adhesive. For example, the thermoplastic elastomer is first mixed with an initiator, and then an acrylate monomer is added, and the chemical grafting reaction between the thermoplastic elastomer and the polyacrylate polymer is initiated by the free radicals generated by the initiator.

[0049] Among the above-mentioned thermoplastic elastomer polymers, Styrene-Ethylene-Butylene-Styrene block copolymer (SEBS) is a linear triblock copolymer, which is a triblock copolymer with polystyrene as the terminal segment and ethylene-butylene copolymer obtained by hydrogenating polybutadiene as the middle elastic block. Styrene-Ethylene / Propylene-Styrene block copolymer (SEPS) is a product obtained by selective hydrogenation of styrene-isoprene polymer, where "ethylene / propylene" refers to the alternating structure of ethylene and propylene. Hydrogenated Styrene Butadiene Rubber (HSBR), also known as saturated styrene butadiene rubber and hydrogenated styrene butadiene rubber, refers to a thermoplastic elastomer made by modifying styrene butadiene rubber using hydrogenation technology. Hydrogenated Nitrile Rubbe (HNBR) is a highly saturated special elastomer obtained by hydrogenating nitrile rubber (NBR).

[0050] In some embodiments, the thermoplastic elastomer polymer includes structural units derived from a first monomer and a second monomer respectively; the first monomer and the second monomer are of different monomer types; wherein the first monomer includes one or more of styrene or acrylonitrile; and the second monomer includes one or more of ethylene, propylene, butadiene, isoprene or acrylonitrile. The above-mentioned thermoplastic elastomer polymer has a relatively lower polarity, which is more conducive to reducing the polarity of the binder, thereby being more conducive to improving the dynamic performance of the battery.

[0051] Furthermore, in some of the embodiments, in the thermoplastic elastomer polymer, the molar ratio of the structural unit derived from the first monomer to the structural unit derived from the second monomer is (2:3) to (3:2).

[0052] In some of these embodiments, the polyacrylate polymer includes structural units derived from acrylic acid ester monomers, and the acrylic acid ester monomers include one or more of methyl methacrylate, butyl acrylate, or isooctyl acrylate.

[0053] In some embodiments, the mass fraction of the polyacrylate polymer is 40% to 70%, based on the total mass of the thermoplastic elastomer graft-modified polyacrylate binder being 100%. Controlling the content of the polyacrylate polymer in the thermoplastic elastomer graft-modified polyacrylate binder within the above range is beneficial for making the lithium-ion secondary battery have lower internal resistance and good dynamic performance, while making the positive electrode sheet have higher bonding force and better mechanical properties.

[0054] It will be appreciated that the mass fraction of the polyacrylate polymer may be 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70% and any value within the range formed by any two of the above values.

[0055] In some embodiments, the mass fraction of the polyacrylate polymer is 50% to 60%, based on the total mass of the thermoplastic elastomer graft-modified polyacrylate binder being 100%. This is conducive to further taking into account the dynamic performance of the lithium-ion secondary battery and the mechanical properties of the positive electrode sheet.

[0056] In some embodiments, the weight average molecular weight of the thermoplastic elastomer graft modified polyacrylate binder is 150,000 Daltons to 400,000 Daltons. Controlling the weight average molecular weight of the thermoplastic elastomer graft modified polyacrylate binder within the above range is conducive to making the binder have good bonding properties, while making the viscosity of the positive electrode slurry moderate and not prone to gelation.

[0057] It is understood that the weight average molecular weight of the thermoplastic elastomer graft modified polyacrylate adhesive can be 150,000 Daltons, 160,000 Daltons, 170,000 Daltons, 180,000 Daltons, 190,000 Daltons, 200,000 Daltons, 210,000 Daltons, 220,000 Daltons, 230,000 Daltons, 240,000 Daltons, 250,000 Daltons, 260,000 Daltons, 270,000 Daltons, 280,000 Daltons, 290,000 Daltons, 300,000 Daltons, 310,000 Daltons, 320,000 Daltons, 330,000 Daltons, 340,000 Daltons, 350,000 Daltons, 360,000 Daltons, 370,000 Daltons, 380,000 Daltons, 390,000 Daltons, 400,000 Daltons and any value within the range formed by any two of the above values. Among them, 10,000 Daltons means that the weight average molecular weight of the polymer is 10,000.

[0058] In some embodiments, the weight average molecular weight of the thermoplastic elastomer grafted modified polyacrylate binder is 300,000 Daltons to 400,000 Daltons. This is conducive to making the viscosity of the positive electrode slurry moderate, and not prone to gelation, while making the binder have better bonding performance.

[0059] In some embodiments, the mass fraction of the thermoplastic elastomer grafted modified polyacrylate binder is 0.5% to 2% based on the total mass of the positive electrode active material layer as 100%. Controlling the mass fraction of the thermoplastic elastomer grafted modified polyacrylate binder within the above range is conducive to making the positive electrode slurry have good dispersibility, making the various raw materials in the positive electrode slurry evenly dispersed, making the positive electrode sheet have good mechanical properties, and at the same time will not significantly affect the energy density of the battery.

[0060] It can be understood that the mass fraction of the thermoplastic elastomer grafted modified polyacrylate binder can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2% and any value within the range formed by any two of the above values.

[0061] In some embodiments, based on the total mass of the positive electrode active material layer being 100%, the mass fraction of the thermoplastic elastomer graft-modified polyacrylate binder is 1% to 1.8%. This helps to better balance the dispersion performance of the positive electrode slurry, the mechanical properties of the positive electrode sheet, and the energy density of the battery.

[0062] In some embodiments, the mass fraction of the lithium-containing phosphate is 95% to 99% based on the total mass of the positive electrode active material layer as 100%. Specifically, the lithium-containing phosphate can be one or more of lithium iron phosphate, lithium manganese phosphate or lithium manganese iron phosphate.

[0063] In one embodiment of the present application, a method for preparing a lithium-ion secondary battery is provided, the method comprising the following steps:

[0064] The lithium-containing phosphate, the thermoplastic elastomer graft-modified polyacrylate binder and the solvent are mixed to obtain a positive electrode slurry;

[0065] Placing the positive electrode slurry on the positive electrode current collector and drying it to obtain a positive electrode sheet;

[0066] The positive electrode sheet, the separator and the negative electrode sheet are stacked in sequence to form an electrode assembly, and the electrode assembly is made into a lithium-ion secondary battery.

[0067] Add thermoplastic elastomer to the positive electrode slurry to graft and modify the polyacrylate binder; by using thermoplastic elastomer to modify the polyacrylate binder, the density of the polyacrylate binder film on the surface of the lithium-containing phosphate material can be effectively reduced, and the transmission of lithium ions can be accelerated, thereby reducing the internal resistance of the lithium-ion secondary battery and improving the dynamic performance of the battery. In addition, the thermoplastic elastomer can significantly reduce the polarity of the polyacrylate binder, making it less likely for the positive electrode slurry to agglomerate and gel. In addition, the thermoplastic elastomer has a low glass transition temperature and good elasticity, which can make the positive electrode sheet have good flexibility.

[0068] In some embodiments, the preparation method of the thermoplastic elastomer graft modified polyacrylate adhesive comprises the following steps: dissolving the thermoplastic elastomer polymer and the initiator in a solvent, adding an acrylic ester monomer to carry out a polymerization reaction, and forming the thermoplastic elastomer graft modified polyacrylate adhesive. It is understood that the thermoplastic elastomer graft modified polyacrylate adhesive can also be prepared by an existing method, or by using an existing commercially available product.

[0069] In some specific examples, the thermoplastic elastomer polymer is first placed in a vacuum oven and dried at 80°C for 6 hours; then the dried thermoplastic elastomer polymer is dissolved in toluene at 60°C; then the initiator benzoyl peroxide is added and stirred at 60°C for 15 minutes; then the acrylic ester monomer is added and stirred at a speed of 600 rpm for 6 hours, the reaction product is precipitated in cold methanol, washed with water three times, filtered and placed in a vacuum oven, and dried at 40°C for 48 hours to obtain a thermoplastic elastomer graft-modified polyacrylate adhesive. Among them, the thermoplastic elastomer polymer can be prepared by existing methods, or by using existing commercially available products.

[0070] It should be noted that after the thermoplastic elastomer polymer and the initiator are dissolved in a solvent, the initiator will decompose to generate free radicals that attack the active sites on the thermoplastic elastomer polymer molecular chain (such as tertiary carbon atoms or unsaturated bonds remaining due to incomplete hydrogenation), forming free radicals of the thermoplastic elastomer polymer; then, the free radicals of the thermoplastic elastomer polymer react with the double bonds or active groups on the polyacrylate molecular chain, thereby achieving the grafting of the thermoplastic elastomer polymer on the polyacrylate polymer, and obtaining a thermoplastic elastomer graft-modified polyacrylate adhesive.

[0071] In some embodiments, the thermoplastic elastomer polymer includes one or more of styrene-ethylene-butylene-styrene block copolymer, styrene-isoprene-styrene block copolymer, styrene-ethylene / propylene-styrene block copolymer, hydrogenated styrene-butadiene rubber or hydrogenated nitrile rubber. The use of the above-mentioned thermoplastic elastomer polymer to modify the polyacrylate binder can effectively reduce the density of the polyacrylate binder film formed on the surface of the lithium-containing phosphate material, accelerate the transmission of lithium ions, reduce the internal resistance of the lithium-ion secondary battery, and improve the dynamic performance of the battery.

[0072] In some embodiments, the acrylate monomer includes one or more of methacrylate, butyl acrylate or isooctyl acrylate; the initiator includes one or more of benzoyl peroxide, potassium persulfate or azobisisobutyronitrile. The mass fraction of the acrylate monomer in the thermoplastic elastomer graft-modified polyacrylate adhesive may be 40% to 70%. The amount of the initiator may be determined according to the reaction requirements.

[0073] In one embodiment of the present application, a positive electrode plate is provided, which includes a positive electrode active material layer, and the positive electrode active material layer includes a lithium-containing phosphate and a thermoplastic elastomer graft-modified polyacrylate binder. By adding a thermoplastic elastomer graft-modified polyacrylate binder to the positive electrode active material layer of the positive electrode plate, the density of the polyacrylate binder film formed on the surface of the lithium-containing phosphate material can be effectively reduced, and the transmission of lithium ions can be accelerated, thereby reducing the internal resistance of the lithium-ion secondary battery and improving the dynamic performance of the battery. In addition, the thermoplastic elastomer can significantly reduce the polarity of the polyacrylate binder, so that the positive electrode slurry is not prone to agglomeration and gelation. In addition, the thermoplastic elastomer has a low glass transition temperature and good elasticity, which can make the positive electrode plate have good flexibility and is also beneficial to improving the dynamic performance of the battery.

[0074] In one embodiment of the present application, a positive electrode slurry is provided, which includes a lithium-containing phosphate, a thermoplastic elastomer-grafted modified polyacrylate binder and an organic solvent, wherein the thermoplastic elastomer-grafted modified polyacrylate binder includes a thermoplastic elastomer polymer and a polyacrylate polymer.

[0075] In some of the embodiments, based on 100% dry weight of the positive electrode slurry, the mass fraction of the thermoplastic elastomer graft-modified polyacrylate binder is 0.5%-2%.

[0076] In some of the embodiments, the solid content of the positive electrode slurry is 55% to 70%.

[0077] Specifically, the organic solvent may be a common solvent for positive electrode slurry. In some specific examples, the organic solvent is N-methylpyrrolidone.

[0078] In one embodiment of the present application, an electrical device is provided, which includes one or more of the lithium ion secondary batteries described above in the present application or lithium ion secondary batteries prepared by the method for preparing lithium ion secondary batteries described above in the present application.

[0079] The lithium ion secondary battery and the electric device of the present application will be described below with reference to the drawings as appropriate.

[0080] In one embodiment of the present application, a lithium ion secondary battery is provided.

[0081] Generally, lithium-ion secondary batteries include a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. During the battery charging and discharging process, active ions are embedded and extracted back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is set between the positive electrode sheet and the negative electrode sheet, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing ions to pass through.

[0082] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector.

[0083] As a non-limiting example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode active material layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.

[0084] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be obtained by forming a metal material on a polymer material substrate. In the positive electrode current collector, non-limiting examples of the metal material may include one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. In the positive electrode current collector, non-limiting examples of the polymer material substrate may include one or more of substrates such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0085] In some embodiments, the positive electrode active material includes a lithium-containing phosphate, and may also include other positive electrode active materials for batteries known in the art. Among them, non-limiting examples of lithium-containing phosphates may include, but are not limited to, one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0086] As a non-limiting example, other positive electrode active materials for batteries may include lithium transition metal oxides and modified compounds thereof. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide and modified compounds thereof. Non-limiting examples of lithium cobalt oxides may include LiCoO2; non-limiting examples of lithium nickel oxides may include LiNiO2; non-limiting examples of lithium manganese oxides may include LiMnO2, LiMn2O4, etc.; non-limiting examples of lithium nickel cobalt manganese oxides may include LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 )、LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 )、LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 )、LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 )、LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ) etc.; Non-limiting examples of lithium nickel cobalt aluminum oxide may include LiNi 0.85 Co 0.1 Al 0.05 O2.

[0087] It is understandable that the battery will be accompanied by lithium (Li) deintercalation and consumption during the charging and discharging process, and the content of Li in the positive electrode plate is different when the battery is discharged to different states. In the list of positive electrode materials in this application, unless otherwise specified, the content of Li is the initial state of the material. The positive electrode material is applied to the positive electrode plate in the battery system, and after the charge and discharge cycle, the content of Li in the positive electrode material contained in the plate usually changes. Among them, the content of Li can be measured by molar content, but is not limited to this. Regarding "the content of Li is the initial state of the material", the initial state of the material refers to the state before the material is added to the positive electrode slurry. It is understandable that the new material obtained by appropriate modification on the basis of the listed positive electrode materials is also within the scope of the positive electrode material. The aforementioned appropriate modification refers to the acceptable modification method for the positive electrode material, and non-limiting examples include coating modification.

[0088] In the list of positive electrode materials in this application, the content of oxygen (O) is only a theoretical state value. The release of oxygen from the lattice will cause the molar content of oxygen to change, and the actual content of O will fluctuate. Among them, the content of O can be measured by molar content, but is not limited to this.

[0089] In some embodiments, the positive electrode active material layer may further include a conductive agent. As a non-limiting example, the conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The weight ratio of the conductive agent in the positive electrode active material layer is 0 wt % to 20 wt %, based on the total weight of the positive electrode active material layer.

[0090] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry, wherein the positive electrode slurry has a solid content of 40wt% to 80wt%, and the viscosity at room temperature is adjusted to 5000mPa·s to 25000mPa·s, the positive electrode slurry is coated on both sides of the positive electrode current collector, and after drying, the positive electrode sheet is formed by cold rolling, and the positive electrode powder coating unit surface density is 150mg / m 2 ~350mg / m 2 The compaction density of the positive electrode is 3.0g / cm 3 ~3.6g / cm 3 , optional 3.3g / cm 3 ~3.5g / cm 3 .

[0091] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material.

[0092] As a non-limiting example, the negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode active material layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.

[0093] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be obtained by forming a metal material on a polymer material substrate. Among them, in the negative electrode current collector, non-limiting examples of the metal material may include one or more of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. In the negative electrode current collector, non-limiting examples of the polymer material substrate may include one or more of substrates such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0094] In some embodiments, the negative electrode active material may be a negative electrode active material for a battery known in the art.

[0095] In some embodiments, the negative electrode active material layer may further include a binder. The binder may include one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0096] In some embodiments, the negative electrode active material layer may further include a conductive agent, which may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0097] In some embodiments, the negative electrode active material layer may further optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0098] In some embodiments, the negative electrode sheet can be prepared in the following manner: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (a non-limiting example of the solvent is deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on at least one side of the surface of the negative electrode collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained. The surface of the negative electrode collector coated with the negative electrode slurry can be on a single surface of the negative electrode collector or on both surfaces of the negative electrode collector. The solid content of the negative electrode slurry can be 40wt%~60wt%. The viscosity of the negative electrode slurry at room temperature can be adjusted to 2000mPa·s~10000mPa·s. When coating the negative electrode slurry, the coating unit surface density on a dry weight basis (excluding the solvent) can be 75g / m 2 ~220g / m 2 The compaction density of the negative electrode sheet can be 1.0g / cm 3 ~1.8g / cm 3 .

[0099] The electrolyte has the function of conducting ions between the positive electrode and the negative electrode. The present application has no particular restrictions on the type of electrolyte, which can be selected according to needs. For example, the electrolyte can be liquid, gel or all-solid.

[0100] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0101] In some embodiments, the electrolyte salt of the lithium ion secondary battery may include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonyl imide (LiFSI), lithium bistrifluoromethanesulfonyl imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobisoxalatophosphate (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP).

[0102] In some embodiments, the solvent may include one or more of fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS), and diethyl sulfone (ESE).

[0103] In some embodiments, the electrolyte may further include additives, such as negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high or low temperature performance, etc.

[0104] In some embodiments, the additive in the electrolyte may include, but is not limited to, one or more of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethylethylene carbonate (TFPC), and the like.

[0105] In some embodiments, the lithium-ion secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical stability and mechanical stability can be selected.

[0106] In some embodiments, the material of the isolation membrane may include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane may be a single-layer film or a multi-layer composite film, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0107] In some embodiments, the thickness of the isolation film is 6 μm to 40 μm, and may be 12 μm to 20 μm.

[0108] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

[0109] In some embodiments, the lithium-ion secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0110] In some embodiments, the outer packaging of the lithium ion secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the lithium ion secondary battery can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, and further, non-limiting examples of plastic can include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0111] A lithium-ion secondary battery includes at least one battery cell. A lithium-ion secondary battery may include one or more battery cells.

[0112] In this application, unless otherwise specified, "battery cell" refers to a basic unit that can realize the mutual conversion of chemical energy and electrical energy, and further, generally speaking, at least includes a positive electrode sheet, a negative electrode sheet and an electrolyte. During the battery charging and discharging process, active ions are embedded and released back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays the role of conducting active ions between the positive electrode sheet and the negative electrode sheet.

[0113] The present application has no particular limitation on the shape of the battery cell, which may be cylindrical, square or any other shape. Figure 1 The battery cell 5 is a square structure as an example.

[0114] In some embodiments, reference Figure 2 , the outer packaging may include a shell 51 and a cover plate 53. Among them, the shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0115] In some embodiments, the battery cells 5 can be assembled into a battery module. The number of battery cells 5 contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0116] In the battery module, the plurality of battery cells 5 may be arranged in sequence along the length direction of the battery module. Of course, they may also be arranged in any other manner. Further, the plurality of battery cells 5 may be fixed by fasteners.

[0117] Optionally, the battery module may further include a housing having a receiving space, and the plurality of battery cells 5 are received in the receiving space.

[0118] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art according to the application and capacity of the battery pack.

[0119] The battery pack may include a battery box and a plurality of battery modules disposed in the battery box. The battery box includes an upper box body and a lower box body, and the upper box body can be covered on the lower box body to form a closed space for accommodating the battery modules. The plurality of battery modules can be arranged in the battery box in any manner.

[0120] In addition, the present application also provides an electrical device, which includes at least one of the lithium-ion secondary battery, battery module, or battery pack provided in the present application. The lithium-ion secondary battery, battery module, or battery pack can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited to these.

[0121] As an electrical device, a lithium-ion secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0122] Figure 3 The power consumption device 6 is taken as an example. The power consumption device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the power consumption device's requirements for high power and high energy density of lithium-ion secondary batteries, a battery pack or a battery module can be used.

[0123] Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a lithium-ion secondary battery may be used as a power source.

[0124] The following are some examples.

[0125] In order to make the technical problems, technical solutions and beneficial effects solved by the present application clearer, the present application will be further described in detail below in conjunction with the embodiments and drawings. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present application and its applications. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present application.

[0126] If no specific technology or conditions are specified in the examples, the technology or conditions described in the literature in the field or the product instructions are used. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be obtained commercially. Among them, SEBS and SEPS in the following examples and comparative examples are products of KRATON, USA.

[0127] Embodiment 1:

[0128] (1) Preparation of positive electrode

[0129] The positive electrode active material LiFePO4, the conductive agent carbon black (Super P) and the HSBR modified polyacrylate binder are mixed evenly in a proper amount of NMP (N-methylpyrrolidone) at a mass ratio of 95:3.5:1.5 to obtain a positive electrode slurry; the positive electrode slurry is coated on both sides of the positive electrode current collector aluminum foil, and the positive electrode sheet is obtained through drying, cold pressing, striping, cutting and other processes. Among them, the solid content of the positive electrode slurry is 60%, and the single-side coating surface density of the positive electrode is 25mg / cm 2 .

[0130] The preparation method of HSBR modified polyacrylate binder is as follows: placing thermoplastic elastomer HSBR in a vacuum oven and drying at 80°C for 6 hours; then dissolving the dried 5g HSBR in toluene at 60°C; then adding 0.2g initiator benzoyl peroxide and stirring at 60°C for 15 minutes; then adding 3.3g butyl acrylate monomer and stirring at a speed of 600 rpm for 6 hours, precipitating the reaction product in cold methanol, washing with water three times, filtering and placing in a vacuum oven, drying at 40°C for 48 hours, and obtaining HSBR modified polyacrylate binder. The mass fraction of polyacrylate polymer in HSBR modified polyacrylate binder is 40%, the weight average molecular weight of HSBR modified polyacrylate binder is 150,000 Daltons, and the molar ratio of styrene monomer unit to butadiene monomer unit in HSBR is 1:1.2.

[0131] (2) Preparation of negative electrode

[0132] The negative electrode active material artificial graphite, thickener sodium carboxymethyl cellulose, binder styrene butadiene rubber, and conductive carbon black (Super P) are mixed in a mass ratio of 97:1:1:1, and deionized water is added to obtain the negative electrode slurry under the action of a vacuum mixer; the negative electrode slurry is evenly coated on the double-sided surface of the negative electrode current collector copper foil; the copper foil is dried at room temperature 25°C and transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the negative electrode sheet. Among them, the solid content of the negative electrode slurry is 50%, and the single-sided coating surface density of the negative electrode is 12mg / cm 2 .

[0133] (3) Isolation film

[0134] A 12 μm thick polypropylene film was selected as the isolation film.

[0135] (4) Electrolyte

[0136] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a mass ratio of 30:70 to obtain an organic solvent, and fully dried LiPF6 is dissolved in the organic solvent to prepare an electrolyte with a LiPF6 concentration of 1 mol / L.

[0137] (5) Battery assembly

[0138] The positive electrode sheet, the isolation film and the negative electrode sheet are made into an electrode assembly through a winding process, the electrode assembly is loaded into the battery outer packaging, and then baked at 80°C to remove water, the electrolyte is injected, and the sealing is carried out. After standing, hot and cold pressing, formation, clamping, capacity division and other processes, a lithium-ion secondary battery is obtained.

[0139] Embodiment 2:

[0140] This embodiment is basically the same as embodiment 1, except that: in step (1), SEBS-modified polyacrylate binder is used instead of HSBR-modified polyacrylate binder; SEBS is used instead of HSBR in the preparation method of the binder, wherein the molar ratio of styrene monomer unit to the sum of ethylene monomer unit and butene monomer unit in SEBS is 1:1.2, and the molar ratio of ethylene monomer unit to butene monomer unit is 1:1.

[0141] Embodiment 3:

[0142] This embodiment is basically the same as embodiment 1, except that: in step (1), SEPS modified polyacrylate binder is used instead of HSBR modified polyacrylate binder; SEPS is used instead of HSBR in the preparation method of the binder, wherein the molar ratio of styrene monomer unit to isoprene monomer unit in SEPS is 1:1.2.

[0143] Embodiment 4:

[0144] This embodiment is basically the same as embodiment 1, except that: the preparation method of the HSBR modified polyacrylate binder in step (1) is as follows: placing the thermoplastic elastomer HSBR in a vacuum oven and drying it at 80°C for 6 hours; then dissolving the dried 5g HSBR in toluene at 60°C; then adding 0.2g initiator benzoyl peroxide and stirring at 60°C for 15 minutes; then adding 5g butyl acrylate monomer and stirring the reaction at a speed of 600 rpm for 6 hours, precipitating the reaction product in cold methanol, washing it with water three times, filtering it, placing it in a vacuum oven, and drying it at 40°C for 48 hours to obtain the HSBR modified polyacrylate binder. The mass fraction of acrylate in the HSBR modified polyacrylate binder is 50%.

[0145] Embodiment 5:

[0146] This embodiment is basically the same as embodiment 1, except that: the preparation method of the HSBR modified polyacrylate binder in step (1) is as follows: placing the thermoplastic elastomer HSBR in a vacuum oven and drying at 80°C for 6 hours; then dissolving the dried 5g HSBR in toluene at 60°C; then adding 0.2g initiator benzoyl peroxide and stirring at 60°C for 15 minutes; then adding 7.5g butyl acrylate monomer and stirring at 600 rpm for 6 hours, precipitating the reaction product in cold methanol, washing with water three times, filtering and placing in a vacuum oven, and drying at 40°C for 48 hours to obtain the HSBR modified polyacrylate binder. The mass fraction of acrylate in the HSBR modified polyacrylate binder is 60%.

[0147] Embodiment 6:

[0148] This embodiment is basically the same as embodiment 1, except that: the preparation method of HSBR modified polyacrylate binder in step (1) is as follows: placing thermoplastic elastomer HSBR in a vacuum oven and drying at 80°C for 6 hours; then dissolving 5g of the dried HSBR in toluene at 60°C; then adding 0.2g of initiator benzoyl peroxide and stirring at 60°C for 15 minutes; then adding 11.7g of butyl acrylate monomer and stirring at 600 rpm for 6 hours, precipitating the reaction product in cold methanol, washing with water three times, filtering and placing in a vacuum oven, and drying at 40°C for 48 hours to obtain HSBR modified polyacrylate binder. The mass fraction of acrylate in the HSBR modified polyacrylate binder is 70%.

[0149] Embodiment 7:

[0150] This embodiment is basically the same as embodiment 1, except that: the preparation method of HSBR modified polyacrylate binder in step (1) is as follows: placing thermoplastic elastomer HSBR in a vacuum oven and drying at 80°C for 6 hours; then dissolving 5g of the dried HSBR in toluene at 60°C; then adding 0.14g of initiator benzoyl peroxide and stirring at 60°C for 15 minutes; then adding 3.3g of butyl acrylate monomer and stirring at 600 rpm for 6 hours, precipitating the reaction product in cold methanol, washing with water three times, filtering and placing in a vacuum oven, and drying at 40°C for 48 hours to obtain HSBR modified polyacrylate binder. The weight average molecular weight of the HSBR modified polyacrylate binder is 300,000.

[0151] Embodiment 8:

[0152] This embodiment is basically the same as embodiment 1, except that: the preparation method of HSBR modified polyacrylate binder in step (1) is as follows: placing thermoplastic elastomer HSBR in a vacuum oven and drying at 80°C for 6 hours; then dissolving 5g of the dried HSBR in toluene at 60°C; then adding 0.1g of initiator benzoyl peroxide and stirring at 60°C for 15 minutes; then adding 3.3g of butyl acrylate monomer and stirring at 600 rpm for 6 hours, precipitating the reaction product in cold methanol, washing with water three times, filtering and placing in a vacuum oven, and drying at 40°C for 48 hours to obtain HSBR modified polyacrylate binder. The weight average molecular weight of the HSBR modified polyacrylate binder is 400,000.

[0153] Embodiment 9:

[0154] This embodiment is basically the same as the embodiment 1, except that in step (1), the mass ratio of the positive electrode active material LiFePO4, the conductive agent carbon black (Super P) and the HSBR modified polyacrylate binder is 95:4.5:0.5.

[0155] Embodiment 10:

[0156] This embodiment is basically the same as the embodiment 1, except that in step (1), the mass ratio of the positive electrode active material LFP, the conductive agent carbon black (Super P) and the HSBR modified polyacrylate binder is 95:4:1.

[0157] Embodiment 11:

[0158] This embodiment is basically the same as the embodiment 1, except that in step (1), the mass ratio of the positive electrode active material LFP, the conductive agent carbon black (Super P) and the HSBR modified polyacrylate binder is 95:3.2:1.8.

[0159] Embodiment 12:

[0160] This embodiment is basically the same as Embodiment 1, except that in step (1), the mass ratio of the positive electrode active material LFP, the conductive agent carbon black (Super P) and the HSBR modified polyacrylate binder is 95:3:2.

[0161] Embodiment 13:

[0162] This embodiment is basically the same as embodiment 1, except that: lithium iron manganese phosphate LiMn is used in step (1) 0.6 Fe 0.4 PO4 replaces lithium iron phosphate as the positive electrode active material.

[0163] Comparative Example 1:

[0164] This comparative example is basically the same as Example 1, except that: in step (1), polybutyl acrylate binder is used instead of thermoplastic elastomer-modified polyacrylate binder; wherein the weight average molecular weight of the polybutyl acrylate binder is 150,000 Daltons.

[0165] Comparative Example 2:

[0166] This comparative example is basically the same as Example 12, except that: in step (1), HSBR modified polyacrylate binder is not used, and HSBR and polybutyl acrylate binder are added separately; that is, the positive electrode active material LiFePO4, the conductive agent carbon black (Super P), HSBR and polybutyl acrylate binder are mixed uniformly in an appropriate amount of NMP in a mass ratio of 95:3:1.2:0.8 to obtain a positive electrode slurry.

[0167] Comparative Example 3:

[0168] This comparative example is basically the same as Example 13, except that: in step (1), polybutyl acrylate binder is used instead of thermoplastic elastomer-modified polyacrylate binder; wherein the weight average molecular weight of the polybutyl acrylate binder is 150,000 Daltons.

[0169] Test method:

[0170] (1) Pole piece adhesion test

[0171] The prepared positive electrode sheet is cut into test specimens of 20mm×100mm size for later use; one side of the double-sided tape is pasted to the surface of the steel plate, and the other side is adhered to the electrode to be tested, and compacted with a roller to make it completely fit with the electrode; one end of the current collector is reversely bent with a bending angle of 180°; a high-speed rail tensile testing machine is used for testing, one end of the steel plate is fixed to the lower fixture of the tensile testing machine, and the bent end of the current collector is fixed to the upper fixture, the angle of the current collector is adjusted to ensure that the upper and lower ends are in a vertical position, and then the sample is stretched at a speed of 50mm / min until the current collector is completely peeled off from the surface of the electrode, the displacement and force in the process are recorded, and the force when the force is balanced is taken as the bonding force of the electrode.

[0172] (2) Pole brittleness test

[0173] Take a 20mm×100mm (longitudinal) sample of the prepared positive electrode sheet and sample along the rolling direction of the sheet; place the pre-folded experimental sheet on the test bench plane and roll it with a 2kg cylindrical roller. After each rolling, observe whether the sheet is light-transmissive. When the sheet is light-transmissive, record the corresponding rolling times. The rolling times represent the flexibility of the sheet. The more rolling times, the better the flexibility of the sheet.

[0174] (3) Diaphragm resistance test

[0175] The prepared positive electrode sheet is cut into a suitable size; it is placed in a membrane resistance tester for testing, the membrane resistance is recorded, and the average value of three measurements is taken as the membrane resistance value of the electrode sheet.

[0176] (4) Battery DCR test

[0177] Let the cell stand for 10 minutes, charge to 3.65V at 1 / 3C constant current, then charge to 0.05C at constant voltage and let stand for 30 minutes; then discharge to 2.5V at 1C to get capacity C1; let stand for 30 minutes, then discharge 0.5C1 at 1 / 3C, and measure the cell voltage V0 after standing for 30 minutes; then discharge at 3C for 30S and measure the cell V1 to calculate DCR, DCR= (V0-V1) / 3C.

[0178] (5) Battery rate performance test

[0179] Let the battery cell stand for 10 minutes, then charge to 3.65V with 1 / 3C constant current, constant voltage to 0.05C, let stand for 30 minutes, and discharge to 2.5V with 1 / 3C to obtain capacity C1; then charge to 3.65V with 1 / 3C constant current, constant voltage to 0.05C, let stand for 30 minutes, and discharge to 2.5V with 1C to obtain capacity C2; then charge to 3.65V with 1 / 3C constant current, constant voltage to 0.05C, let stand for 30 minutes, and discharge to 2.5V with 2C to obtain capacity C3. 1C capacity retention rate = C2 / C1×100%; 2C capacity retention rate = C3 / C1×100%.

[0180] (6) Binder weight average molecular weight test

[0181] The binder was dissolved in NMP solvent, and the weight average molecular weight of the binder was measured using a gel permeation chromatograph.

[0182] (7) Adhesive type test

[0183] The type of binder was determined by using infrared combined with pyrolysis gas chromatography-mass spectrometry.

[0184] The specific testing method is: a trace amount of binder sample is quickly heated in an inert atmosphere to generate cracking products, the cracking products are directly introduced into the gas chromatography system for separation, and then into the mass spectrometer for detection. The characteristic fragment ions after high-temperature cracking are qualitatively and quantitatively analyzed to determine the sample composition and the mass fraction of the monomer unit in the polymer binder.

[0185] The battery parameters of the above embodiments and comparative examples are shown in Table 1, and the battery performance test data are shown in Table 2. Among them, the binder content refers to the mass fraction of the binder in the positive electrode active material layer; the acrylate content in the binder refers to the mass fraction of the acrylate polymer in the binder. The acrylate content in the binder and the weight-average molecular weight of the binder in Table 1 are both rounded values.

[0186] Table 1

[0187]

[0188] Table 2

[0189]

[0190] It can be seen from the data in the table that the lithium-ion secondary batteries of the embodiments of the present application have a lower internal resistance (DCR), and their 1C capacity retention rate and 2C capacity retention rate are both high, indicating that the lithium-ion secondary batteries have good dynamic performance.

[0191] Compared with Examples 1-12, in Comparative Example 1, unmodified polyacrylate binder was used instead of thermoplastic elastomer modified polyacrylate binder, and the internal resistance of the battery increased and the dynamic performance decreased. Compared with Example 12, in Comparative Example 2, HSBR and unmodified polyacrylate binder were added separately, and the internal resistance of the battery increased, the dynamic performance decreased, and the bonding force of the positive electrode sheet decreased significantly. In Comparative Example 3, unmodified polyacrylate binder was used instead of thermoplastic elastomer modified polyacrylate binder on the basis of Example 13, and the internal resistance of the battery increased and the dynamic performance decreased.

[0192] The above description of various embodiments tends to emphasize the differences between the various embodiments. The same or similar aspects can be referenced to each other, and for the sake of brevity, they will not be repeated herein.

[0193] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A lithium ion secondary battery, characterized in that: The invention comprises a positive electrode plate, wherein the positive electrode plate comprises a positive electrode active material layer, wherein the positive electrode active material layer comprises a lithium-containing phosphate and a thermoplastic elastomer graft-modified polyacrylate binder; The thermoplastic elastomer grafted modified polyacrylate adhesive includes a thermoplastic elastomer polymer and a polyacrylate polymer; the thermoplastic elastomer polymer includes one or more of styrene-ethylene-butylene-styrene block copolymer, styrene-isoprene-styrene block copolymer, styrene-ethylene / propylene-styrene block copolymer, hydrogenated styrene-butadiene rubber or hydrogenated nitrile rubber; taking the total mass of the thermoplastic elastomer grafted modified polyacrylate adhesive as 100%, the mass fraction of the polyacrylate polymer is 40% to 70%; wherein ethylene / propylene refers to an alternating structure of ethylene and propylene.

2. The lithium ion secondary battery according to claim 1, characterized in that: The thermoplastic elastomer polymer includes a structural unit derived from a first monomer and a structural unit derived from a second monomer; the first monomer and the second monomer are of different monomer types; The first monomer includes one or more of styrene or acrylonitrile; The second monomer includes one or more of ethylene, propylene, butadiene, isoprene or acrylonitrile.

3. The lithium ion secondary battery according to claim 2, characterized in that: In the thermoplastic elastomer polymer, a molar ratio of the structural unit derived from the first monomer to the structural unit derived from the second monomer is (2:3) to (3:2).

4. The lithium-ion secondary battery according to claim 1, characterized in that: The polyacrylate polymer includes structural units derived from acrylic acid ester monomers, and the acrylic acid ester monomers include one or more of methyl methacrylate, butyl acrylate, or isooctyl acrylate.

5. The lithium ion secondary battery according to claim 1, characterized in that: Taking the total mass of the thermoplastic elastomer graft-modified polyacrylate binder as 100%, the mass fraction of the polyacrylate polymer is 50% to 60%.

6. The lithium ion secondary battery according to any one of claims 1 to 5, characterized in that The weight average molecular weight of the thermoplastic elastomer graft-modified polyacrylate adhesive is 150,000 Daltons to 400,000 Daltons.

7. The lithium ion secondary battery according to claim 6, characterized in that: The weight average molecular weight of the thermoplastic elastomer graft-modified polyacrylate adhesive is 300,000 Daltons to 400,000 Daltons.

8. The lithium ion secondary battery according to any one of claims 1 to 5 and 7, characterized in that: Based on the total mass of the positive electrode active material layer being 100%, the mass fraction of the thermoplastic elastomer graft-modified polyacrylate binder is 0.5% to 2%.

9. The lithium ion secondary battery according to any one of claims 1 to 5 and 7, characterized in that: Based on the total mass of the positive electrode active material layer being 100%, the mass fraction of the thermoplastic elastomer graft-modified polyacrylate binder is 1% to 1.8%.

10. The lithium ion secondary battery according to any one of claims 1 to 5 and 7, characterized in that: Based on the total mass of the positive electrode active material layer being 100%, the mass fraction of the lithium-containing phosphate is 95% to 99%.

11. The lithium ion secondary battery according to any one of claims 1 to 5 and 7, characterized in that: The lithium-containing phosphate includes one or more of lithium iron phosphate, lithium manganese phosphate or lithium manganese iron phosphate.

12. A method for preparing a lithium ion secondary battery, characterized in that: The following steps are involved: The lithium-containing phosphate, the thermoplastic elastomer graft-modified polyacrylate binder and the solvent are mixed to obtain a positive electrode slurry; Placing the positive electrode slurry on a positive electrode current collector and drying the positive electrode to obtain a positive electrode sheet; and The positive electrode sheet, the separator and the negative electrode sheet are stacked in sequence to form an electrode assembly, and the electrode assembly is made into a lithium-ion secondary battery; Wherein, the thermoplastic elastomer grafted modified polyacrylate adhesive includes a thermoplastic elastomer polymer and a polyacrylate polymer; the thermoplastic elastomer polymer includes one or more of styrene-ethylene-butylene-styrene block copolymer, styrene-isoprene-styrene block copolymer, styrene-ethylene / propylene-styrene block copolymer, hydrogenated styrene-butadiene rubber or hydrogenated nitrile rubber; taking the total mass of the thermoplastic elastomer grafted modified polyacrylate adhesive as 100%, the mass fraction of the polyacrylate polymer is 40%~70%; wherein ethylene / propylene refers to an alternating structure of ethylene and propylene.

13. The method for preparing a lithium ion secondary battery according to claim 12, characterized in that: The preparation method of the thermoplastic elastomer graft-modified polyacrylate adhesive comprises the following steps: Thermoplastic elastomer polymer and initiator are mixed in a solvent, and acrylic ester monomer is added to carry out polymerization reaction to form the thermoplastic elastomer graft-modified polyacrylate adhesive.

14. The method for preparing a lithium ion secondary battery according to claim 13, characterized in that: Satisfy one or more of the following (1) to (3): (1) The thermoplastic elastomer polymer includes one or more of styrene-ethylene-butylene-styrene block copolymer, styrene-isoprene-styrene block copolymer, styrene-ethylene / propylene-styrene block copolymer, hydrogenated styrene-butadiene rubber or hydrogenated nitrile-butadiene rubber; (2) The acrylic acid ester monomer includes one or more of methacrylate, butyl acrylate or isooctyl acrylate; (3) The initiator includes one or more of benzoyl peroxide, potassium persulfate or azobisisobutyronitrile.

15. An electrical device, characterized in that: The invention comprises one or more of the lithium ion secondary battery according to any one of claims 1 to 11 or the lithium ion secondary battery prepared by the method for preparing the lithium ion secondary battery according to any one of claims 12 to 14.

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