Lithium-ion secondary battery and electric apparatus

Abstract

The present application provides a lithium-ion secondary battery and an electric apparatus. The lithium-ion secondary battery comprises a positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector; the single-sided positive electrode film layer has an areal coating density of 20 mg / cm2-30 mg / cm2, the positive electrode film layer comprising a lithium-containing phosphate and a polyacrylate-based binder, and the polyacrylate-based binder having a weight-average molecular weight of 80,000 daltons to 130,000 daltons; and the positive electrode sheet has a compaction density of 2.45 g / cm3, and 2.55 g / cm3.

Description

Lithium-ion secondary batteries and electrical devices

[0001] Cross-references

[0002] This application claims priority to Chinese Patent Application No. 202411625228.X, filed on November 14, 2024, entitled "Lithium-ion Secondary Battery and Power Consumption Device", which is incorporated herein by reference in its entirety. Technical Field

[0003] This application relates to the field of secondary battery technology, and in particular to a lithium-ion secondary battery and an electrical device thereof. Background Technology

[0004] In recent years, as the application scope of secondary batteries has become increasingly wide, they have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles and electric vehicles.

[0005] Rechargeable batteries using lithium iron phosphate (LFP) and other lithium-containing phosphates as positive electrode active materials have advantages such as low cost and good safety, making them a popular area of ​​development. With the rapid development of rechargeable battery technology, higher demands are being placed on their energy density. How to further improve the energy density of rechargeable batteries using lithium-containing phosphates as positive electrode active materials has become one of the important research directions in this field.

[0006] However, a significant problem facing high-energy-density lithium-phosphate secondary batteries is their severe self-discharge, which causes a gradual decrease in capacity during storage, affecting their storage performance. Therefore, how to improve the energy density of lithium-phosphate secondary batteries while reducing self-discharge and enhancing battery performance is a key area of ​​focus for those skilled in the art. Summary of the Invention

[0007] This application is made in view of the above-mentioned issues, and one of its objectives is to provide a lithium-ion secondary battery and an electrical device with low self-discharge.

[0008] To achieve the above objectives, a first aspect of this application provides a lithium-ion secondary battery, including a positive electrode sheet, wherein the positive electrode sheet includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector, and the coating surface density of the positive electrode film layer on one side is 20 mg / cm³. 2 ~30mg / cm 2The positive electrode film layer comprises lithium phosphate and polyacrylate binder, wherein the weight-average molecular weight of the polyacrylate binder is 800,000 to 1,300,000 Daltons; the compaction density of the positive electrode sheet is 2.45 g / cm³. 3 ~2.55g / cm 3 .

[0009] By using a polyacrylate binder with a weight-average molecular weight of 800,000 to 1,300,000 Daltons in the positive electrode film layer, the polyacrylate binder with the above molecular weight has a long polymer molecular chain, more contact points between molecules, stronger interaction forces, and higher cohesive force. Under the condition of high coating density and high compaction density of the positive electrode film layer, it can play a good bonding role for lithium phosphate in the positive electrode film layer, so that the positive electrode sheet has high adhesion and cohesive force, reducing or avoiding the problem of positive electrode film layer powder shedding that is easy to occur during the die-cutting, winding and hot pressing process, thereby reducing battery self-discharge.

[0010] In any embodiment, the weight-average molecular weight of the polyacrylate binder is 1 million to 1.3 million Daltons. This is beneficial for further improving the adhesion and cohesion of the electrode, further improving the problem of electrode powder shedding, and also better reducing battery self-discharge.

[0011] In any embodiment, the polyacrylate adhesive comprises structural units derived from acrylate monomers, including one or more of methyl methacrylate, butyl acrylate, or isooctyl acrylate.

[0012] In any embodiment, the structural units derived from acrylate monomers account for 60% to 80% by mass in the polyacrylate adhesive.

[0013] In any embodiment, the polyacrylate binder further includes structural units derived from a first monomer, which includes one or more of styrene or acrylonitrile. This is beneficial for improving the cohesiveness of the electrode.

[0014] In any embodiment, the structural unit derived from the first monomer accounts for 20% to 40% by mass in the polyacrylate adhesive.

[0015] In any embodiment, the mass fraction of the polyacrylate binder is 1% to 2%, based on the total mass of the positive electrode film layer as 100%. This is beneficial for improving the adhesion and cohesion of the electrode while maintaining the energy density of the battery.

[0016] In any embodiment, the Dv50 particle size of the lithium phosphate is 1.1 μm to 1.8 μm. Using lithium phosphate with the above-mentioned Dv50 particle size is beneficial for shortening the transport distance between lithium ions and electrons, and is beneficial for the energy output of high-energy-density lithium-ion secondary batteries. The polyacrylate binder of this application has good adhesion to this lithium phosphate; combining this lithium phosphate with the binder is beneficial for obtaining high-energy-density lithium-ion secondary batteries with low self-discharge.

[0017] In any embodiment, the positive electrode film layer further includes a dispersant, which includes one or more of phosphate ester compounds, styrene-ethylene / butene-styrene block copolymers, or hydrogenated nitrile rubber. By introducing the above-mentioned dispersant, the problem of gelation that may occur in positive electrode slurries including polyacrylate binders and lithium phosphates can be effectively alleviated, which is beneficial to improving the dispersion uniformity and coating uniformity of the slurry.

[0018] In any embodiment, the mass fraction of the dispersant is 0.3% to 0.5% based on the total mass of the positive electrode film layer as 100%. This facilitates further dispersion of the polyacrylate binder and lithium phosphate, suppressing the gelation phenomenon of the positive electrode slurry.

[0019] In any embodiment, the positive electrode sheet further includes a positive current collector and a base coating layer. The base coating layer is disposed between the positive current collector and the positive electrode film layer. The base coating layer includes a conductive agent and a polar group-grafted modified polymer binder, wherein the polar groups include carboxyl groups and / or ester groups. This helps to further improve the adhesion of the positive electrode sheet and further improve the problem of electrode sheet powder shedding.

[0020] In any embodiment, the polar group grafted modified polymer binder includes structural units derived from a second monomer and structural units derived from a third monomer; the second monomer and the third monomer are of different monomer types;

[0021] The second monomer includes one or more of styrene or acrylonitrile;

[0022] The third monomer includes one or more of ethylene, propylene, butadiene, isoprene, or acrylonitrile.

[0023] In any embodiment, the polar group-grafted modified polymer binder includes one or more of the following: polar group-grafted modified styrene-ethylene / butene-styrene block copolymer, polar group-grafted modified hydrogenated styrene-butadiene rubber, polar group-grafted modified styrene-isoprene-styrene block copolymer, or polar group-grafted modified styrene-ethylene-propylene-styrene block copolymer. This facilitates good adhesion between the undercoat and the positive electrode film, while also giving the positive electrode a certain pressure-sensitive characteristic, which helps improve charge transfer efficiency and better adapts to changes in internal battery pressure.

[0024] In any embodiment, the polar group-grafted modified polymer binder comprises a structural unit derived from a fourth monomer containing a polar group, the fourth monomer comprising one or more of acrylic acid, methyl methacrylate, butyl acrylate, isooctyl acrylate, or maleic anhydride.

[0025] In any embodiment, the mass fraction of the polar group-grafted modified polymer binder is 50% to 70% based on 100% of the total mass of the base coating. This is beneficial for ensuring good adhesion between the base coating and both the positive current collector and the positive electrode film layer.

[0026] In any embodiment, the thickness of the base coating is 1 μm to 2 μm.

[0027] In any embodiment, the mass fraction of lithium phosphate is 90% to 99%, with the total mass of the positive electrode film layer being 100%.

[0028] In any embodiment, the lithium-containing phosphate includes one or more of lithium iron phosphate, lithium manganese phosphate, or lithium manganese iron phosphate.

[0029] A second aspect of this application provides an electrical device including the lithium-ion secondary battery of the first aspect of this application.

[0030] Details of one or more embodiments of this application are set forth in the following drawings and description. Other features, objects, and advantages of this application will become apparent from the specification, drawings, and claims. Attached Figure Description

[0031] To better describe and illustrate the embodiments or examples provided in this application, reference may be made to one or more accompanying drawings. Additional details or examples used to describe the drawings should not be considered as limiting the scope of any of the disclosed applications, the currently described embodiments or examples, or the best mode of conduct of these applications as currently understood. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0032] Figure 1 is a schematic diagram of a battery cell according to an embodiment of this application;

[0033] Figure 2 is an exploded view of a battery cell according to an embodiment of this application shown in Figure 1;

[0034] Figure 3 is a schematic diagram of an electrical device using a lithium-ion secondary battery as a power source according to an embodiment of this application.

[0035] Explanation of reference numerals in the attached drawings: 5. Battery cell; 51. Casing; 52. Electrode assembly; 53. Cover plate; 6. Electrical device. Detailed Implementation

[0036] The embodiments of the lithium-ion secondary battery and power-consuming device of this application are described in detail below with appropriate reference to the accompanying drawings. However, some unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of actually identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0037] The "range" disclosed in this application can be defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific range. Ranges defined in this way can include or exclude endpoints. Any endpoint can be included or excluded independently, and they can be combined arbitrarily; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60–120 and 80–110 are listed for a specific parameter, it is expected that ranges of 60–110 and 80–120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are also listed, then the following ranges are all expected: 1–3, 1–4, 1–5, 2–3, 2–4, and 2–5. In this application, unless otherwise stated, the numerical range "a–b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0" and "5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥ 2, it is equivalent to listing integers such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For instance, when a parameter is described 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 this application, the terms "multiple" or "various" are used unless otherwise specified, referring to a quantity greater than or equal to 2. For example, "one or more" means one or more types.

[0039] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0040] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments. The term "implementation" as used herein has a similar understanding.

[0041] Those skilled in the art will understand that the order in which the steps are written in the methods of various embodiments or examples does not imply a strict execution order and does not constitute any limitation on the implementation process. The detailed execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) can be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0042] In this application, open-ended technical features or solutions described using terms such as "containing," "comprising," or "including" do not exclude additional members beyond those listed unless otherwise specified. They can be considered as providing both closed-ended features or solutions comprised of the listed members and open-ended features or solutions that include additional members beyond the listed members. For example, if A includes a1, a2, and a3, it may also include other members or exclude additional members unless otherwise specified. This can be considered as providing both the feature or solution that "A consists of a1, a2, and a3" and the feature or solution that "A includes not only a1, a2, and a3, but also other members." In this application, unless otherwise specified, A (e.g., B) indicates 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," "optionally," and "optional" mean that something is optional, that is, it means that it is selected from either "with" or "without." If there are multiple "optional" entries in a technical solution, unless otherwise specified and there are no contradictions or mutual constraints, each "optional" entry shall be independent.

[0044] Rechargeable batteries using lithium iron phosphate and other lithium-phosphate compounds as positive electrode active materials have advantages such as low cost and good safety, making them a popular area of ​​development. With the rapid development of rechargeable battery technology, higher demands are being placed on their energy density. How to further improve the energy density of rechargeable batteries using lithium-phosphate compounds as positive electrode active materials has become one of the important research directions in this field. However, a significant problem facing high-energy-density lithium-ion rechargeable batteries containing lithium phosphate compounds is the severe self-discharge phenomenon. The battery capacity gradually decreases during storage, affecting its storage performance. The main reason is that high-energy-density lithium-ion secondary batteries containing lithium phosphate typically require a larger positive electrode film coating density and a larger compaction density. In order to firmly bond the active materials, conductive agents, and other particles together, higher requirements are placed on the adhesion and cohesion of the binder used in the positive electrode film. Conventional binders have insufficient adhesion and cohesion between the positive electrode film and the lithium phosphate positive electrode material under the condition of large coating density, which makes it easy for the positive electrode film to shed powder during processes such as die-cutting, winding, and hot pressing, causing self-discharge of the lithium-ion secondary battery.

[0045] In response, one embodiment of this application provides a lithium-ion secondary battery, which includes a positive electrode sheet, a positive current collector, and a positive electrode film layer disposed on at least one side of the positive current collector, wherein the coating surface density of the positive electrode film layer on one side is 20 mg / cm³. 2 ~30mg / cm 2 The positive electrode film layer includes lithium phosphate and polyacrylate binders; the weight-average molecular weight of the polyacrylate binders is 800,000 to 1,300,000 Daltons; the compaction density of the positive electrode sheet is 2.45 g / cm³. 3 ~2.55g / cm 3 .

[0046] The lithium-ion secondary battery described in this application utilizes a polyacrylate binder with a weight-average molecular weight of 800,000 to 1,300,000 Daltons in the positive electrode film layer. This polyacrylate binder has a long polymer molecular chain, numerous intermolecular contact points, and strong interaction forces, resulting in high adhesion and cohesion for lithium phosphate-containing positive electrode materials. Even with a high coating density and compaction density in the positive electrode film layer, it effectively bonds the lithium phosphate in the positive electrode film layer, giving the positive electrode sheet high adhesion and cohesion. This reduces or avoids the problem of powder shedding from the positive electrode film layer during die-cutting, winding, and hot pressing, thereby reducing battery self-discharge and improving battery storage performance.

[0047] It is understood that the weight-average molecular weight of polyacrylate adhesives can be 800,000 Daltons, 850,000 Daltons, 900,000 Daltons, 950,000 Daltons, 1,000,000 Daltons, 1,050,000 Daltons, 1,100,000 Daltons, 1,150,000 Daltons, 1,200,000 Daltons, 1,250,000 Daltons, 1,300,000 Daltons, or any value within the range formed by any two of the above values. The polyacrylate adhesive can be a commercially available product. Here, a weight-average molecular weight of 10,000 Daltons = 10 kDa.

[0048] It should be noted that compaction density reflects the degree of compaction of the battery electrode sheet after it has been compressed under a certain pressure. The formula for calculating compaction density is: Compaction density = Areal density / (Thickness of the electrode sheet after compaction - Thickness of the current collector). Wherein, areal density refers to the weight of the positive electrode film layer per unit area of ​​the electrode sheet.

[0049] In some embodiments, the weight-average molecular weight of the polyacrylate binder is 1 million to 1.3 million Daltons. Using a polyacrylate binder with the aforementioned weight-average molecular weight in the positive electrode film layer is beneficial for further improving the adhesion and cohesion of the positive electrode sheet using lithium phosphate as the positive electrode active material, further improving the problem of powder shedding from the positive electrode sheet, and further reducing battery self-discharge.

[0050] In some embodiments, the polyacrylate adhesive includes structural units derived from acrylate monomers, including one or more of methyl methacrylate, butyl acrylate, or isooctyl acrylate.

[0051] In some embodiments, the structural units derived from acrylate monomers constitute 60% to 80% of the mass of the polyacrylate adhesive. It is understood that the mass percentage of the structural units derived from acrylate monomers in the polyacrylate adhesive can be 60%, 62%, 65%, 68%, 70%, 72%, 75%, 78%, 80%, or any value within the range formed by any two of the above values.

[0052] In some embodiments, the polyacrylate adhesive further includes structural units derived from a first monomer, which includes one or more of styrene or acrylonitrile. Thus, by employing the aforementioned structural units derived from the first monomer in the polyacrylate adhesive, where styrene contains a benzene ring structure and acrylonitrile contains polar nitrile groups, polymerization with acrylate monomers can increase the interaction sites between polymer molecules, thereby enhancing intermolecular attraction and making the polymer molecules more tightly bound, thus improving cohesive strength.

[0053] In some embodiments, the structural units derived from the first monomer constitute 20% to 40% of the mass of the polyacrylate adhesive. It is understood that the mass percentage of the structural units derived from the first monomer in the polyacrylate adhesive can be 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, or any value within the range formed by any two of the above values.

[0054] Polyacrylate binders can be prepared using existing methods or commercially available products. In some embodiments, polyacrylate binders can be prepared by dissolving styrene (St) and methyl methacrylate (MMA) in a solvent, and then reacting them with the initiator benzoyl peroxide. The aforementioned polyacrylate binders are fluorine-free, which can mitigate the risks of persistent battery contamination and bioaccumulation hazards.

[0055] In some embodiments, the mass fraction of the polyacrylate binder is 1% to 2% based on the total mass of the positive electrode film layer (100%). Controlling the mass fraction of the polyacrylate binder in the positive electrode film layer within the above range is beneficial for improving the adhesion and cohesion of the positive electrode sheet while also considering the energy density of the battery.

[0056] It is understood that the mass fraction of polyacrylate binder in the positive electrode film layer can be 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, or any value within the range formed by any two of the above values.

[0057] In some embodiments, the Dv50 particle size of the lithium phosphate is 1.1 μm to 1.8 μm. Using lithium phosphate with this Dv50 particle size is beneficial for shortening the transport distance between lithium ions and electrons, thus improving the energy output of high-energy-density lithium-ion secondary batteries. The polyacrylate binder of this application has good adhesion to this lithium phosphate; combining this lithium phosphate with the binder is beneficial for obtaining high-energy-density lithium-ion secondary batteries with low self-discharge.

[0058] Among them, Dv50 particle size, also known as volume median particle size, refers to the particle size distribution in which 50% of the particles have a diameter smaller than this value, while 50% of the particles have a diameter larger than this value.

[0059] On the other hand, polyacrylate binders with a weight-average molecular weight of 800,000 to 1,300,000 Daltons are prone to causing gel formation in the cathode slurry; at the same time, cathode slurries using lithium phosphate as the cathode active material are also prone to gelation.

[0060] In some embodiments of this application, the positive electrode film layer further includes a dispersant, which includes one or more of phosphate ester compounds, styrene-ethylene / butene-styrene block copolymers, and hydrogenated nitrile rubber. Using the above-mentioned compounds as dispersants, and with the assistance of these dispersants, a polyacrylate binder with a weight-average molecular weight of 800,000 to 1,300,000 Daltons is employed. This not only alleviates the problem of gelation in the positive electrode slurry and improves its processing performance, but also enhances the adhesion and cohesion of the positive electrode sheet, mitigating the problem of powder shedding during die-cutting, winding, and hot pressing, thus reducing battery self-discharge.

[0061] Among the dispersants mentioned above, phosphate ester compounds are a class of organic compounds containing phosphate ester functional groups (-OPO(OR)2), where R represents an organic group. Styrene-ethylene / Butylene-styrene block copolymer (SEBS) is a linear triblock copolymer, consisting of polystyrene as the end block and ethylene-butene copolymer (obtained by hydrogenating polybutadiene) as the middle elastic block. Hydrogenated nitrile rubber (HNBR) is a highly saturated special elastomer obtained by hydrogenating nitrile rubber (NBR).

[0062] In some embodiments, the mass fraction of the dispersant is 0.3% to 0.5% based on the total mass of the positive electrode film layer (100%). Adding the above-mentioned amount of dispersant to the positive electrode film layer facilitates the dispersion of polyacrylate binders and lithium phosphates, effectively improving the gelation phenomenon of the positive electrode slurry. It is understood that the mass fraction of the dispersant in the positive electrode film layer can be 0.3%, 0.32%, 0.35%, 0.38%, 0.4%, 0.42%, 0.45%, 0.48%, 0.5%, or any value within the range formed by any two of the above values.

[0063] In some embodiments, the positive electrode sheet further includes a positive current collector and a base coating layer. The base coating layer is disposed between the positive current collector and the positive electrode film layer. The base coating layer includes a conductive agent and a polar group grafted modified polymer binder. The polar group grafted modified polymer binder includes polar groups, including carboxyl groups and / or ester groups.

[0064] A base coating layer is formed between the positive electrode current collector and the positive electrode film layer, and a polar group-grafted modified polymer binder is used in the base coating layer. This polar group-grafted modified polymer binder not only has good adhesion to the positive electrode current collector but also to the positive electrode film layer, which is beneficial to further improve the adhesion of the positive electrode sheet and further improve the problem of electrode powder shedding. It is understood that this polar group-grafted modified polymer binder can be a commercially available binder. The positive electrode current collector can be an aluminum foil current collector.

[0065] In some embodiments, the polar group grafted modified polymer binder includes structural units derived from a second monomer and structural units derived from a third monomer; the second monomer and the third monomer are of different monomer types; the second monomer includes one or more of styrene or acrylonitrile; and the third monomer includes one or more of ethylene, propylene, butadiene, isoprene, or acrylonitrile.

[0066] In some embodiments, the polar group-grafted modified polymer binder includes one or more of the following: polar group-grafted modified styrene-ethylene / butene-styrene block copolymer, polar group-grafted modified hydrogenated styrene-butadiene rubber, polar group-grafted modified styrene-isoprene-styrene block copolymer, or polar group-grafted modified styrene-ethylene-propylene-styrene block copolymer. Using the above-mentioned polymer binder in the undercoat layer helps to achieve good adhesion between the undercoat layer and the positive electrode film layer, while also giving the positive electrode a certain pressure-sensitive characteristic, which helps to maintain good contact between the electrode materials, improves charge transfer efficiency, and allows the electrode to better adapt to changes in the internal pressure of the battery.

[0067] Among the aforementioned polymer binders, styrene-ethylene / butylene-styrene block copolymer (SEBS) is a linear triblock copolymer with polystyrene as the end block and ethylene-butene copolymer obtained by hydrogenating polybutadiene as the middle elastic block; hydrogenated styrene-butadiene rubber (HSBR) is a thermoplastic elastomer made by modifying styrene-butadiene rubber using hydrogenation technology; styrene-isoprene-styrene block copolymer (SIS) is an important thermoplastic elastomer material; and styrene-ethylene / propylene-styrene block copolymer (SEPS) is a product obtained by selectively hydrogenating styrene-isoprene polymer.

[0068] In some embodiments, the polar group-grafted modified polymer binder includes a structural unit derived from a fourth monomer containing a polar group, the fourth monomer including one or more of acrylic acid, methyl methacrylate, butyl acrylate, isooctyl acrylate, or maleic anhydride. That is, the polar group-grafted modified polymer binder can be an binder formed by grafting a polymer with one or more raw materials selected from acrylic acid, methyl methacrylate, butyl acrylate, isooctyl acrylate, or maleic anhydride. By introducing polar groups such as carboxyl groups and ester groups into the polymer through the above compounds, the adhesion between the polymer binder and the positive electrode current collector is improved.

[0069] In some embodiments, the mass fraction of the polar group-grafted modified polymer binder is 50% to 70% based on 100% of the total mass of the base coating. Controlling the content of the polar group-grafted modified polymer binder in the base coating within the above range is more conducive to ensuring good adhesion between the base coating and the positive electrode current collector and the positive electrode film layer, resulting in good overall adhesion of the positive electrode sheet.

[0070] It is understood that the mass fraction of the polar group grafted modified polymer binder can be 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, or any value within the range formed by any two of the above values.

[0071] In some embodiments, the thickness of the base coating is 1 μm to 2 μm. It is understood that the thickness of the base coating can be 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, or any value within the range formed by any two of the above values.

[0072] In some embodiments, the mass fraction of lithium phosphate is 90% to 99% based on the total mass of the positive electrode film layer (100%). The lithium phosphate in the positive electrode film layer may be one or more of lithium iron phosphate, lithium manganese phosphate, or lithium manganese iron phosphate.

[0073] One embodiment of this application provides a positive electrode sheet, which includes a positive electrode film layer. The positive electrode film layer includes a lithium phosphate and a polyacrylate binder, wherein the weight-average molecular weight of the polyacrylate binder is 800,000 Daltons to 1,300,000 Daltons. The above-mentioned positive electrode sheet is less prone to powder shedding, thus reducing battery self-discharge.

[0074] One embodiment of this application provides a positive electrode slurry comprising lithium phosphate, a polyacrylate binder, and an organic solvent. The polyacrylate binder has a weight-average molecular weight of 800,000 to 1,300,000 Daltons. Positive electrode sheets prepared using this slurry are less prone to powder shedding, thus reducing battery self-discharge.

[0075] In some embodiments, the organic solvent includes one or more of N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), or triethyl phosphate (TEP). Using the aforementioned organic solvents is more beneficial for improving the dispersion uniformity and coating uniformity of the cathode slurry.

[0076] In some embodiments, the cathode slurry further includes a dispersant, which includes one or more of phosphate ester compounds, styrene-ethylene / butene-styrene block copolymers, or hydrogenated nitrile rubber. This prevents the cathode slurry from gelling, improving the dispersion and coating uniformity of the cathode slurry.

[0077] According to one embodiment of this application, an electrical device is provided, which includes the lithium-ion secondary battery described above.

[0078] The lithium-ion secondary battery and power supply device of this application will be described below with appropriate reference to the accompanying drawings.

[0079] In one embodiment of this application, a lithium-ion secondary battery is provided.

[0080] Typically, a lithium-ion secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0081] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector.

[0082] As a non-limiting example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0083] 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 substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be obtained by forming a metal material on the polymer material substrate. Non-limiting examples of the metal material in the positive electrode current collector may include one or more of aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. Non-limiting examples of the polymer material substrate in the positive electrode current collector may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0084] In some embodiments, the positive electrode active material mainly comprises lithium-containing phosphates, and may also include other positive electrode active materials known in the art for use in lithium-ion secondary batteries. Non-limiting examples of lithium-containing phosphates include, but are not limited to, one or more of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium iron manganese phosphate, and lithium iron manganese phosphate and carbon composites.

[0085] As a non-limiting example, other positive electrode active materials for lithium-ion secondary batteries may include lithium transition metal oxides and their modified compounds. However, this application is not limited to these materials, and other conventional 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. Examples of lithium transition metal oxides 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 their modified compounds. Non-limiting examples of lithium cobalt oxide may include LiCoO2; non-limiting examples of lithium nickel oxide may include LiNiO2; non-limiting examples of lithium manganese oxide may include LiMnO2, LiMn2O4, etc.; non-limiting examples of lithium nickel cobalt manganese oxide may include LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 Examples of lithium nickel cobalt aluminum oxides include LiNi, etc. 0.85 Co 0.1 Al 0.05 O2.

[0086] Understandably, lithium (Li) is intercalated and deintercalated during the charging and discharging process of a battery, and the Li content in the positive electrode varies depending on the state of discharge. Unless otherwise specified, the Li content in the examples of positive electrode materials listed in this application refers to the initial state of the material. When a positive electrode material is applied to a positive electrode in a battery system, the Li content in the positive electrode material typically changes after charge-discharge cycles. The Li content can be measured using molar content, but is not limited to this. Regarding "Li content refers to the initial state of the material," the initial state of the material refers to its state before being added to the positive electrode slurry. It is understood that new materials obtained by appropriately modifying the listed positive electrode materials are also within the scope of positive electrode materials. The aforementioned appropriate modification refers to acceptable modification methods for the positive electrode material; non-limiting examples include coating modification.

[0087] In the examples of cathode materials in this application, the oxygen (O) content is only a theoretical value. Lattice oxygen release will cause changes in the molar content of oxygen, and the actual O content will fluctuate. The O content can be measured in molar content, but is not limited to this.

[0088] The positive electrode active material accounts for more than 80% of the weight of the positive electrode film, based on the total weight of the positive electrode film.

[0089] In some embodiments, the positive electrode film layer also includes a polyacrylate binder and a dispersant.

[0090] In some embodiments, the positive electrode film may optionally 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 conductive agent accounts for 0% to 20% of the positive electrode film by weight, based on the total weight of the positive electrode film.

[0091] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder, dispersant and any other components, in a solvent (e.g. N-methylpyrrolidone) to form a positive electrode slurry, coating the positive electrode slurry on both sides of the positive electrode current collector, drying it and then cold pressing it through a cold rolling mill to form the positive electrode sheet.

[0092] In some embodiments, an undercoating layer is also provided between the positive current collector and the positive electrode film layer.

[0093] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, wherein the negative active material layer includes a negative active material.

[0094] As a non-limiting example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode active material layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0095] 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 substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be obtained by forming a metal material on the polymer material substrate. Non-limiting examples of the metal material in the negative electrode current collector may include one or more of copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. Non-limiting examples of the polymer material substrate in the negative electrode current collector may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0096] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries.

[0097] As a non-limiting example, the negative electrode active material of a lithium-ion secondary battery may include one or more of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. Silicon-based materials may include one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may include one or more of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0098] In some embodiments, the negative electrode active material layer may optionally 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).

[0099] In some embodiments, the negative electrode active material layer may optionally include a conductive agent. 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.

[0100] In some embodiments, the negative electrode active material layer may also optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0101] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder, and any other components, in a solvent (a non-limiting example of a solvent is deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto at least one surface of a negative electrode current collector, and obtaining the negative electrode sheet after drying, cold pressing, and other processes. The surface of the negative electrode current collector coated with the negative electrode slurry can be a single surface of the negative electrode current collector or both surfaces of the negative electrode current collector. The solid content of the negative electrode slurry can be 40wt% to 60wt%. The viscosity of the negative electrode slurry at room temperature can be adjusted to 2000 mPa·s to 10000 mPa·s. When coating the negative electrode slurry, the coating unit areal density (dry weight, minus solvent) can be 75 g / m². 2 ~220g / m 2 The compaction density of the negative electrode sheet can be 1.0 g / cm³. 3 ~1.8g / cm 3 .

[0102] Electrolytes function to conduct ions between the positive and negative electrode plates. This application does not impose any particular restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or completely solid.

[0103] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0104] 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 bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0105] In some embodiments, the solvent may include one or more of the following: fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl 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), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0106] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0107] In some embodiments, the additives in the electrolyte may include, but are not limited to, one or more of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethyl ethylene carbonate (TFPC), etc.

[0108] In some embodiments, the lithium-ion secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0109] In some embodiments, the material of the separator may include one or more of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0110] In some embodiments, the thickness of the separator is 6 μm to 40 μm, and optionally 12 μm to 20 μm.

[0111] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0112] In some embodiments, the lithium-ion secondary battery may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0113] In some embodiments, the outer packaging of the lithium-ion secondary battery can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the lithium-ion secondary battery can also be a soft pack, such as a pouch. The soft pack can be made of plastic; further, non-limiting examples of plastic may include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0114] A lithium-ion secondary battery includes at least one battery cell. A lithium-ion secondary battery may include one or more battery cells.

[0115] In this application, unless otherwise specified, "cell battery" refers to the basic unit capable of converting chemical energy into electrical energy, and generally includes at least a positive electrode, a negative electrode, and an electrolyte. During the charging and discharging process of the battery, active ions move back and forth between the positive and negative electrode plates, inserting and extracting. The electrolyte acts as a conductor for the active ions between the positive and negative electrode plates.

[0116] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square battery cell 5 as an example.

[0117] In some embodiments, referring to FIG2, the outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to close the receiving cavity. A positive electrode sheet, a negative electrode sheet, and a separator can be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in a single battery cell 5 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0118] In some embodiments, the battery cells 5 can be assembled into a battery module, and the number of battery cells 5 contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0119] In the battery module, multiple battery cells 5 can be arranged sequentially along the length of the battery module. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 5 can be secured with fasteners.

[0120] Optionally, the battery module may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0121] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0122] The battery pack may include a battery box and multiple battery modules disposed within the battery box. The battery box includes an upper body and a lower body, with the upper body covering the lower body to form a closed space for accommodating the battery modules. The multiple battery modules can be arranged in any manner within the battery box.

[0123] In addition, this application also provides an electrical device, which includes at least one of the lithium-ion secondary battery, battery module, or battery pack provided in this application. The lithium-ion secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., 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.

[0124] As an electrical device, lithium-ion secondary batteries, battery modules, or battery packs can be selected according to their usage requirements.

[0125] Figure 3 shows an example of an electrical device 6. This electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the lithium-ion secondary battery for this electrical device, a battery pack or battery module can be used.

[0126] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a lithium-ion rechargeable battery as their power source.

[0127] The following are some examples.

[0128] To make the technical problems, technical solutions, and beneficial effects solved by this application clearer, the application will be further described in detail below with reference to embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this application or its applications. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0129] Where specific techniques or conditions are not specified in the examples, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0130] Example 1:

[0131] (1) Preparation of positive electrode sheet

[0132] LiFePO4 (LFP) positive electrode active material, polymethyl methacrylate binder, SEBS dispersant (model 1701, manufactured by Kraton Corporation), and super P conductive agent carbon black were mixed in a mass ratio of 97:1:0.5:1.5. N-methylpyrrolidone solvent was added and mixed thoroughly to obtain a positive electrode slurry with a solid content of 65%. This positive electrode slurry was then coated onto both sides of an aluminum foil current collector, with the coating thickness being the same on both sides. The positive electrode was then obtained through drying, cold pressing, slitting, and cutting processes. The areal density of the positive electrode film layer on one side of the positive electrode sheet was 25 mg / cm³. 2 The compaction density of the positive electrode sheet is 2.5 g / cm³. 3 The Dv50 particle size of the positive electrode active material LiFePO4 is 1.5 μm.

[0133] The preparation method of polymethyl methacrylate (PMMA) adhesive is as follows: 108 mL of distilled water, 0.005 g of polyvinyl alcohol, and 0.009 g of sodium dodecylbenzenesulfonate are added to a 500 mL flask. The water bath is maintained at 81 °C, and the mixture is stirred for 0.5 h. Then, 0.66 g of benzoyl peroxide, 14.14 g of PMMA, and 3.608 g of acrylonitrile are added. The mixture is kept at 81 °C and reacted for 1.5 h. The polymerization product is washed with cold water and filtered. The product, PMMA adhesive, is obtained. The weight-average molecular weight of the PMMA adhesive is 1 million Daltons.

[0134] (2) Preparation of negative electrode sheet

[0135] Artificial graphite (anode active material), sodium carboxymethyl cellulose (thickener), styrene-butadiene rubber (binder), and carbon black (Super P) (conductive agent) were mixed in a mass ratio of 97:1:1:1. Deionized water was added, and a negative electrode slurry with a solid content of 50% was obtained under vacuum stirring. The negative electrode slurry was uniformly coated on both sides of a copper foil current collector. After the copper foil was dried at room temperature, it was transferred to a 120°C oven for 1 hour. Then, it was cold-pressed and slit to obtain the negative electrode sheet.

[0136] (3) Separating membrane

[0137] A 12μm thick polypropylene membrane was selected as the separator.

[0138] (4) Electrolyte

[0139] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a mass ratio of 30:70 to obtain an organic solvent. Fully dried LiPF6 was dissolved in the organic solvent to prepare an electrolyte with a LiPF6 concentration of 1 mol / L.

[0140] (5) Battery assembly

[0141] The positive electrode, separator, and negative electrode are wound together to form an electrode assembly, with the separator positioned between the positive and negative electrodes to provide insulation. The electrode assembly is then placed in an outer package, injected with the electrolyte, and sealed. After standing, hot and cold pressing, formation, clamping, and capacity testing, a battery is formed.

[0142] Example 2:

[0143] This embodiment is basically the same as Embodiment 1, except that: in step (1), the preparation method of polymethyl methacrylate adhesive is as follows: 108 mL of distilled water, 0.005 g of polyvinyl alcohol, and 0.009 g of sodium dodecylbenzenesulfonate are added to a 500 mL flask, and the water bath is kept at 81 °C and stirred for 0.5 h; then 0.99 g of benzoyl peroxide, 14.14 g of methyl methacrylate, and 3.608 g of acrylonitrile are added, and the mixture is kept at 81 °C and reacted for 1.5 h. The polymerization product is washed with cold water and filtered. The product, polymethyl methacrylate adhesive, is obtained. The weight-average molecular weight of the polymethyl methacrylate adhesive is 800,000 Daltons.

[0144] Example 3:

[0145] This embodiment is basically the same as Example 1, except that: in step (1), the preparation method of polymethyl methacrylate adhesive is as follows: 108 mL of distilled water, 0.005 g of polyvinyl alcohol, and 0.009 g of sodium dodecylbenzenesulfonate are added to a 500 mL flask, and the water bath is kept at 81 °C and stirred for 0.5 h; then 0.462 g of benzoyl peroxide, 14.14 g of methyl methacrylate, and 3.608 g of acrylonitrile are added, and the mixture is kept at 81 °C and reacted for 1.5 h. The polymerization product is washed with cold water and filtered. The product, polymethyl methacrylate adhesive, is obtained. The weight-average molecular weight of the polymethyl methacrylate adhesive is 1.3 million Daltons.

[0146] Example 4:

[0147] This embodiment is basically the same as that of embodiment 1, except that: in step (1), the mass ratio of positive electrode active material LFP, polymethyl methacrylate binder, dispersant SEBS and conductive agent carbon black (Super P) is 97:1.8:0.5:0.7; and the amount of polymethyl methacrylate binder is 1.8%.

[0148] Example 5:

[0149] This embodiment is basically the same as that of embodiment 1, except that: in step (1), the mass ratio of positive electrode active material LFP, polymethyl methacrylate binder, dispersant SEBS and conductive agent carbon black (Super P) is 97:2:0.5:0.5; and the amount of polymethyl methacrylate binder is 2%.

[0150] Example 6:

[0151] This embodiment is basically the same as Embodiment 1, except that: in step (1), a phosphate ester dispersant of model BYK-110 produced by BYK Chemie, Germany, is used instead of the dispersant SEBS; the single-sided coating surface density of the positive electrode film in the positive electrode sheet is 30 mg / cm³. 2 The compaction density of the positive electrode sheet is 2.45 g / cm³. 3 The Dv50 particle size of the positive electrode active material LiFePO4 is 1.8 μm.

[0152] Example 7:

[0153] This embodiment is basically the same as Embodiment 1, except that: in step (1), the mass ratio of the positive electrode active material LFP, polymethyl methacrylate binder, dispersant SEBS, and conductive agent carbon black (Super P) is 97:1:0.3:1.7; the amount of dispersant SEBS is 0.3%; and the single-sided coating density of the positive electrode film layer in the positive electrode sheet is 20 mg / cm³. 2 The compaction density of the positive electrode sheet is 2.55 g / cm³. 3 The Dv50 particle size of the positive electrode active material LiFePO4 is 1.1 μm.

[0154] Example 8:

[0155] This embodiment is basically the same as Embodiment 1, except that: the method for preparing the positive electrode sheet in step (1) is as follows:

[0156] (1.1) Preparation of the base coating

[0157] Acrylic grafted modified SEBS binder and conductive carbon black (Super P) were mixed at a mass ratio of 70:30. N-methylpyrrolidone (NMP) was added, and the mixture was stirred for 3 hours in a dual planetary stirrer to prepare a primer slurry with a 20% solids content. This primer slurry was then coated onto both sides of a 13-micron thick aluminum foil current collector and dried to form a primer coating on the current collector. The thickness of the primer coating on each side was 2 μm. The acrylic grafted modified SEBS binder used was product model 1924 manufactured by Kraton Corporation, USA.

[0158] (1.2) Preparation of positive electrode film

[0159] The positive electrode active material LFP, polymethyl methacrylate binder, dispersant SEBS, and conductive agent carbon black (Super P) were mixed in a mass ratio of 97:1:0.5:1.5, and N-methylpyrrolidone solvent was added and mixed evenly to obtain a positive electrode slurry. The positive electrode slurry was then coated onto the base coating of an aluminum foil current collector with a base coating layer to prepare a positive electrode film layer. The coating thickness on both sides was the same. The base coating layer was positioned between the current collector and the positive electrode film layer. The positive electrode sheet was obtained through drying, cold pressing, slitting, and cutting processes. The surface area density of the positive electrode film layer on one side of the positive electrode sheet was 25 mg / cm³. 2 The polymethyl methacrylate adhesive has a weight-average molecular weight of 1 million Daltons and is prepared using the same method as in Example 1.

[0160] Example 9:

[0161] This embodiment is basically the same as embodiment 8, except that the mass ratio of acrylic graft modified SEBS binder to conductive carbon black (Super P) in step (1.1) is 50:50.

[0162] Example 10:

[0163] This embodiment is basically the same as embodiment 8, except that the thickness of the base coating in step (1.1) is 1 μm.

[0164] Example 11:

[0165] This embodiment is basically the same as embodiment 8, except that: lithium manganese iron phosphate (LiMn) is used in step (1.2). 0.6 Fe 0.4 O4 replaces LFP as the positive electrode active material.

[0166] Example 12:

[0167] This embodiment is basically the same as embodiment 3, except that: in step (1), polyisooctyl acrylate with the same weight-average molecular weight is used instead of polymethyl methacrylate as the binder.

[0168] The preparation method of poly(isooctyl acrylate) binder is as follows: Add 108 mL of distilled water, 0.005 g of polyvinyl alcohol, and 0.009 g of sodium dodecylbenzenesulfonate to a 500 mL flask, maintain the water bath temperature at 81 °C, and stir for 0.5 h; then add 0.66 g of benzoyl peroxide, 13.2 g of isooctyl acrylate, and 3.608 g of acrylonitrile, maintain the temperature at 81 °C, and react for 1.5 h. Wash the polymerization product with cold water and filter. The product, poly(isooctyl acrylate) binder, is obtained.

[0169] Comparative Example 1:

[0170] This comparative example is basically the same as Example 2, except that the preparation method of the polymethyl methacrylate adhesive in step (1) is as follows: 108 mL of distilled water, 0.005 g of polyvinyl alcohol, and 0.009 g of sodium dodecylbenzenesulfonate are added to a 500 mL flask, and the water bath is kept at 81 °C and stirred for 0.5 h; then 1.05 g of benzoyl peroxide, 14.14 g of methyl methacrylate, and 3.608 g of acrylonitrile are added, and the mixture is kept at 81 °C and reacted for 1.5 h. The polymerization product is washed with cold water and filtered. The polymethyl methacrylate adhesive is obtained. The weight-average molecular weight of the polymethyl methacrylate adhesive is 600,000 Daltons.

[0171] Comparative Example 2:

[0172] This comparative example is basically the same as Example 1, except that the preparation method of the polymethyl methacrylate binder in step (1) is as follows: 108 mL of distilled water, 0.005 g of polyvinyl alcohol, and 0.009 g of sodium dodecylbenzenesulfonate are added to a 500 mL flask, and the water bath is kept at 81 °C and stirred for 0.5 h; then 0.38 g of benzoyl peroxide, 14.14 g of methyl methacrylate, and 3.608 g of acrylonitrile are added, and the mixture is kept at 81 °C for 1.5 h. The polymerization product is washed with cold water and filtered. The polymethyl methacrylate binder is obtained. The weight-average molecular weight of the polymethyl methacrylate binder is 1.5 million Daltons. Due to the gelation phenomenon in the positive electrode slurry, a uniform slurry could not be made, and the positive electrode sheet and battery were not prepared.

[0173] Test method:

[0174] (1) Electrode brittleness test

[0175] Take a 20mm×100mm (longitudinal) sample of the prepared positive electrode sheet and take the sample along the electrode sheet rolling direction; place the pre-folded experimental electrode sheet on the experimental table plane and roll it with a 2kg cylindrical roller. After each rolling, observe whether the electrode sheet is transparent. When the electrode sheet is transparent, record the corresponding number of rolling times. The number of rolling times represents the brittleness of the electrode sheet.

[0176] (2) Electrode mechanical property testing

[0177] (2.1) Adhesion

[0178] The prepared positive electrode sheet was cut into test specimens with dimensions of 20mm × 100mm and set aside. One side of the double-sided tape was attached to the surface of the steel plate, and the other side was attached to the electrode sheet to be tested. The tape was then pressed with a pressure roller to ensure complete adhesion to the electrode sheet. One end of the current collector was bent in the opposite direction at a bending angle of 180°. The test was conducted using a high-speed rail tensile testing machine. One end of the steel plate was fixed to the lower clamp of the tensile testing machine, and the bent end of the current collector was fixed to the upper clamp. The angle of the current collector was adjusted to ensure that the upper and lower ends were in a vertical position. The specimen was then stretched at a speed of 50mm / min until the current collector was completely peeled off from the surface of the electrode sheet. The displacement and force during the process were recorded, and the force at which the forces were balanced was taken as the adhesion force of the electrode sheet.

[0179] (2.2) Cohesion

[0180] Cut the prepared positive electrode sheet into test specimens of 20mm×100mm size for later use; attach one side of double-sided tape to the surface of the steel plate, and the other side to the electrode sheet to be tested, and press it with a pressure roller to make it completely adhered to the electrode sheet; attach the special tape for cohesion testing to the other side of the electrode sheet and press it with a pressure roller; bend one end of the special tape for cohesion in the opposite direction with a bending angle of 180°; use a high-speed rail tensile testing machine to test, fix one end of the steel plate to the lower clamp of the tensile testing machine, fix the bent end of the current collector to the upper clamp, adjust the angle of the current collector to ensure that the upper and lower ends are in a vertical position, and then stretch the specimen at a speed of 50mm / min until the current collector is completely peeled off from the surface of the electrode sheet, record the displacement and force during the process, and take the force when the force is balanced as the cohesion of the electrode sheet.

[0181] (3) Weight-average molecular weight test

[0182] The binder was dissolved in NMP solvent, and the weight-average molecular weight of the polymer was tested using gel permeation chromatography.

[0183] (4) Battery self-discharge performance test

[0184] The battery cells were left to stand at room temperature for 1 hour. The initial open circuit voltage (OCV) of the cells was measured using a 6.5-digit digital multimeter and recorded as OCV1. After the test, the cells were left to stand at 45°C for 48 hours, then at room temperature for 1 hour. The open circuit voltage was measured again and recorded as OCV2. The self-discharge rate was calculated using the formula: (OCV2 - OCV1) / 48. Ten cells were measured, and the average value was taken.

[0185] (5) Test of surface density of positive electrode film coating

[0186] The coating surface density was tested using the following method: a certain area S (unit: cm²) was punched. 2 Fifteen positive electrode sheets and 15 current collectors (from the same production batch as the positive electrode sheet) were weighed and their average mass was calculated. The average mass of the positive electrode sheets was M1 (mg), and the average mass of the current collectors was M2 (mg). When the positive electrode film layer was applied to both sides of the current collector, the coating surface density was (M1-M2) / 2S.

[0187] (6) Volume average particle size Dv50 test

[0188] Take a clean beaker, add an appropriate amount of the particle sample to be tested, and sonicate at 120W for 5 minutes to ensure complete dispersion. The testing instrument is a Malvern 2000 (USA). After pouring the sample into the injection tower, it circulates with the solution to the test optical path system. Under the irradiation of the laser beam, the particle size distribution characteristics (opause: 8%–12%) can be obtained by receiving and measuring the energy distribution of the scattered light. Based on the test data, a particle size volume distribution map is plotted. From the distribution map, it is found that 50% of the particles in the total volume have a diameter greater than a certain Dv50 value, and another 50% of the particles in the total volume have a diameter less than this Dv50 value. This Dv50 value is the volume average particle size.

[0189] (7) Electrode compaction density test

[0190] Disassemble the battery cell, remove the positive electrode plate, and punch it to 1540.25mm. 2 Take a small circular sheet and measure its weight M and thickness L; take another electrode sheet, remove the film layer from the surface, and cut the remaining empty current collector foil into 1540.25mm pieces. 2 The mass M0 of the air current collector foil is measured using a small circular piece. The compaction density PD = (M-M0) / 1.54025 / (L-L0), where L0 is the thickness of the current collector foil.

[0191] The parameters and performance data of the lithium-ion secondary batteries in the above embodiments and comparative examples are shown in Tables 1, 2, and 3. In the tables, " / " indicates that the battery was not present or was not tested. The molecular weights of the polyacrylate binders and the modified polymer binders are rounded to the nearest integer.

[0192] Table 1

[0193] Table 2

[0194] Table 3

[0195] The data in the table shows that the lithium-ion secondary battery of this application has a relatively low self-discharge. Compared with Examples 1-12, the molecular weight of the polyacrylate binder in Comparative Example 1 is too small, resulting in poor adhesion and a large self-discharge of the battery. In Comparative Example 2, the molecular weight of the polyacrylate binder is too large, causing gelation in the positive electrode slurry, making it impossible to form a uniform slurry and resulting in poor processing performance.

[0196] The description of the various embodiments above tends to emphasize the differences between the various embodiments. The similarities or similarities between them can be referred to, and for the sake of brevity, they will not be repeated here.

[0197] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A lithium-ion secondary battery, comprising a positive electrode sheet, wherein the positive electrode sheet includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector, and the coating areal density of the positive electrode film layer on one side is 20 mg / cm³. 2 ~30mg / cm 2 The positive electrode film layer comprises lithium phosphate and polyacrylate binder, wherein the weight-average molecular weight of the polyacrylate binder is 800,000 to 1,300,000 Daltons; the compaction density of the positive electrode sheet is 2.45 g / cm³. 3 ~2.55g / cm 3 .

2. The lithium-ion secondary battery according to claim 1, wherein, The weight-average molecular weight of the polyacrylate adhesive is 1 million to 1.3 million Daltons.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein, The polyacrylate adhesive comprises structural units derived from acrylate monomers, which include one or more of methyl methacrylate, butyl acrylate, or isooctyl acrylate.

4. The lithium-ion secondary battery according to claim 3, wherein, The structural units derived from acrylate monomers account for 60% to 80% of the mass of the polyacrylate adhesive.

5. The lithium-ion secondary battery according to any one of claims 1 to 4, wherein, The polyacrylate adhesive includes structural units derived from a first monomer, which includes one or more of styrene or acrylonitrile.

6. The lithium-ion secondary battery according to claim 5, wherein, The structural units derived from the first monomer constitute 20% to 40% of the mass of the polyacrylate adhesive.

7. The lithium-ion secondary battery according to any one of claims 1 to 6, wherein, Based on the total mass of the positive electrode film layer being 100%, the mass fraction of the polyacrylate binder is 1% to 2%.

8. The lithium-ion secondary battery according to any one of claims 1 to 7, wherein, The Dv50 particle size of the lithium phosphate is 1.1 μm to 1.8 μm.

9. The lithium-ion secondary battery according to any one of claims 1 to 8, wherein, The positive electrode film layer also includes a dispersant, which includes one or more of phosphate ester compounds, styrene-ethylene / butene-styrene block copolymers, or hydrogenated nitrile rubber.

10. The lithium-ion secondary battery according to claim 9, wherein, Based on the total mass of the positive electrode film layer being 100%, the mass fraction of the dispersant is 0.3% to 0.5%.

11. The lithium-ion secondary battery according to any one of claims 1 to 10, wherein, The positive electrode sheet further includes a positive current collector and a base coating layer. The base coating layer is disposed between the positive current collector and the positive electrode film layer. The base coating layer includes a conductive agent and a polar group grafted modified polymer binder. The polar group grafted modified polymer binder includes polar groups, including carboxyl groups and / or ester groups.

12. The lithium-ion secondary battery according to claim 11, wherein, The polar group grafted modified polymer binder includes structural units derived from a second monomer and structural units derived from a third monomer; the second monomer and the third monomer are of different monomer types; The second monomer includes one or more of styrene or acrylonitrile; The third monomer includes one or more of ethylene, propylene, butadiene, isoprene, or acrylonitrile.

13. The lithium-ion secondary battery according to any one of claims 11 to 12, wherein, The polar group grafted modified polymer adhesive includes one or more of the following: polar group grafted modified styrene-ethylene / butene-styrene block copolymer, polar group grafted modified hydrogenated styrene-butadiene rubber, polar group grafted modified styrene-isoprene-styrene block copolymer, or polar group grafted modified styrene-ethylene-propylene-styrene block copolymer.

14. The lithium-ion secondary battery according to any one of claims 11 to 13, wherein, The polar group grafted modified polymer binder includes a structural unit derived from a fourth monomer containing a polar group, the fourth monomer including one or more of acrylic acid, methyl methacrylate, butyl acrylate, isooctyl acrylate, or maleic anhydride.

15. The lithium-ion secondary battery according to any one of claims 11 to 14, wherein, Based on the total mass of the base coating layer being 100%, the mass fraction of the polar group grafted modified polymer binder is 50% to 70%.

16. The lithium-ion secondary battery according to any one of claims 11 to 15, wherein, The thickness of the base coating is 1μm to 2μm.

17. The lithium-ion secondary battery according to any one of claims 1 to 16, wherein, Based on the total mass of the positive electrode film layer being 100%, the mass fraction of the lithium phosphate is 90% to 99%.

18. The lithium-ion secondary battery according to any one of claims 1 to 17, wherein, The lithium-containing phosphate includes one or more of lithium iron phosphate, lithium manganese phosphate, or lithium manganese iron phosphate.

19. An electrical device comprising a lithium-ion secondary battery as claimed in any one of claims 1 to 18.

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