Lithium-ion secondary battery and electric device

Abstract

The present application relates to a lithium-ion secondary battery and an electric device. The lithium-ion secondary battery comprises a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode active material layer; the positive electrode active material layer comprises a positive electrode active material and an acrylate polymer; the glass transition temperature of the acrylate polymer is denoted as Tg, where -50ºC≤Tg≤25ºC; and the mass proportion of the acrylate polymer in the positive electrode active material layer is 0.5-2%. By introducing the acrylate polymer into the positive electrode active material layer, the flexibility of the positive electrode sheet can be improved; moreover, the cycle performance of a lithium-ion secondary battery comprising the positive electrode sheet is also improved.

Description

Lithium-ion secondary batteries and electrical devices

[0001] Related applications

[0002] This application claims priority to Chinese patent application filed on October 16, 2024, application number 2024114433276, entitled "Lithium-ion Secondary Battery and Power Consumption Device", the entire contents of which are incorporated herein by reference. Technical Field

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

[0004] In recent years, with the increasingly widespread application of secondary batteries such as lithium-ion batteries, 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 power tools, electric bicycles, electric motorcycles, electric cars, military equipment, aerospace, and many other fields. Due to the significant development of lithium-ion batteries, higher requirements have been placed on their capacity, energy density, and other performance characteristics.

[0005] During the winding of the electrodes in a lithium-ion secondary battery, the positive electrode is prone to partial cracking or even complete breakage, resulting in the shedding of some positive electrode active material and severely affecting the cycle performance of the lithium-ion secondary battery. Summary of the Invention

[0006] This application provides a lithium-ion secondary battery and an electrical device to improve the flexibility of the positive electrode and enhance the cycle performance of the lithium-ion secondary battery containing the positive electrode.

[0007] A first aspect of this application provides a lithium-ion secondary battery, comprising a positive electrode sheet, the positive electrode sheet including a positive electrode active material layer, the positive electrode active material layer comprising a positive electrode active material and an acrylate polymer, the glass transition temperature of the acrylate polymer being denoted as T. g -50℃≤T g At ≤25℃, the acrylate polymer accounts for 0.5% to 2% of the mass of the positive electrode active material layer.

[0008] Acrylic polymers exhibit good adhesion to current collectors and are resistant to the erosion of organic solvents in the electrolyte, making them a relatively ideal non-fluorinated binder. However, when acrylic polymers are used as binders for the positive electrode sheet, the positive electrode active material layer is subjected to significant stress during bending when the positive electrode sheet is wound, leading to partial cracking or even breakage at the bending points. Therefore, this application introduces a -50℃≤T... gAcrylic polymers with a temperature ≤25℃ can improve the flexibility of the positive electrode sheet while ensuring its adhesion performance. By controlling the mass ratio of acrylic polymers in the positive electrode active material layer, the adhesion performance of acrylic polymers to the positive electrode active material can be improved, while ensuring more uniform dispersion of the positive electrode active material, which is beneficial to improving the adhesion and cohesion of the positive electrode sheet. The glass transition temperature (Tg) of acrylic polymers refers to the temperature at which the acrylic polymer transitions from the glassy state to the elastic state. When the temperature is below the glass transition temperature, the acrylic polymer is in the glassy state, and the molecular chains and segments cannot move; only the atoms or groups constituting the molecules vibrate in their equilibrium positions, resulting in high rigidity. When the temperature is above or equal to the glass transition temperature, although the molecular chains cannot move, the chain segments begin to move, exhibiting high elasticity. The positive electrode sheet of lithium-ion secondary batteries is usually wound at a temperature above 25℃. By introducing a temperature of -50℃ ≤ Tg into the positive electrode active material layer of the positive electrode sheet... g Acrylic polymers with a temperature of ≤25℃ exhibit high elasticity at the aforementioned processing temperature, possessing a certain degree of elasticity and flexibility. This can improve the flexibility of the positive electrode sheet, reduce problems such as cracking or even breakage of the positive electrode sheet during winding, and buffer the stress caused by the volume expansion of the positive electrode sheet during repeated charging and discharging, reducing the risk of shedding of the positive electrode active material layer, thereby improving the cycle performance of lithium-ion secondary batteries.

[0009] In some implementations, -40℃≤T g ≤25℃.

[0010] In some embodiments, the acrylate polymer comprises a first structural unit, the first structural unit having the following structural formula:

[0011] R1 to R3 are each independently selected from any one of hydrogen, substituted or unsubstituted C1-C8 saturated alkyl groups, substituted or unsubstituted C3-C8 cycloalkyl groups, substituted or unsubstituted heterocyclic alkylene groups, acid anhydrides, hydroxyl groups, carboxyl groups, and carboxyl salts. R4 is selected from any one of hydrogen, substituted or unsubstituted C1-C8 saturated alkyl groups, substituted or unsubstituted C3-C8 cycloalkyl groups, substituted or unsubstituted heterocyclic alkylene groups, and metal cations. * represents the bonding position. Acrylic polymers containing the first structural unit can improve the dispersibility of the positive electrode slurry, the adhesion and stability of the positive electrode sheet, which is beneficial to improving the cycle performance of lithium-ion secondary batteries.

[0012] In some embodiments, the substituent groups in R1-R3 and R4, including the substituted C1-C8 saturated alkyl groups, the substituted C3-C8 cycloalkyl groups, and the substituted heterocyclic alkylene groups, are each independently selected from at least one of ester groups, carbonyl groups, amide groups, and acid anhydrides. Selecting the above-mentioned types of substituent groups can increase the polarity of the first structural unit, which is beneficial for improving the adhesion of acrylate polymers to the positive electrode active material, thereby enhancing the adhesion and cohesion of the positive electrode sheet.

[0013] In some embodiments, the metal cation includes at least one of sodium ions and potassium ions.

[0014] In some embodiments, the mass percentage of the first structural unit in the acrylate polymer is denoted as M11, wherein M11 satisfies: 0 < M11 ≤ 100%.

[0015] In some embodiments, the acrylate polymer further comprises a second structural unit, the second structural unit having the following structural formula:

[0016] R5 to R8 are each independently selected from hydrogen, substituted or unsubstituted C1 to C8 saturated alkyl groups, and * represents the bonding position. Introducing a weaker polar second structural unit into acrylate polymers can regulate the polarity of the acrylate polymers, weaken the intermolecular forces between the polymer and the positive electrode active material, improve the dispersibility of the positive electrode slurry, and reduce the film resistance of the positive electrode sheet.

[0017] In some embodiments, the acrylate polymer further comprises a third structural unit, the structural formula of which includes... * Represents a bonding site. Introducing the aforementioned third structural unit into acrylate polymers can enhance the adhesion of acrylate polymers to the positive electrode active material, which is beneficial for further improving the adhesion and cohesion of the positive electrode sheet.

[0018] In some embodiments, the mass percentage of the second structural unit in the acrylate polymer is denoted as M12, wherein M12 satisfies: 65% ≤ M12 < 100%. When M12 is within this range, the polarity of the acrylate polymer can be reduced, improving the dispersibility of the positive electrode active material, which is beneficial to the processing performance of the electrode and the cycle performance of the lithium-ion secondary battery.

[0019] In some embodiments, 70% ≤ M12 ≤ 90%. This reduces the polarity of the acrylate polymer, improves the dispersibility of the positive electrode active material, and benefits the processing performance of the electrode and the cycle performance of the lithium-ion secondary battery.

[0020] In some embodiments, the mass percentage of the first structural unit in the acrylate polymer is denoted as M11, and the mass percentage of the third structural unit in the acrylate polymer is denoted as M13. M11 and M13 satisfy the following conditions: 0 < M11 + M13 ≤ 35%, 0 ≤ M13 ≤ 5%. Meeting these conditions improves the adhesion of the acrylate polymer to the positive electrode active material, which is beneficial for further enhancing the adhesion and cohesion of the positive electrode sheet.

[0021] In some embodiments, 10% ≤ M11 + M13 ≤ 30%. This can improve the adhesion of acrylate polymers to the positive electrode active material, which is beneficial to further improving the adhesion and cohesion of the positive electrode sheet.

[0022] In some embodiments, the acrylate polymer includes at least one of the following: a copolymer of a first acrylate monomer and a second acrylate monomer; an olefin-acrylate copolymer; a vinyl acetate-acrylate copolymer; an olefin-acrylate-maleic anhydride terpolymer; an olefin-acrylate-glycidyl methacrylate terpolymer; a maleic anhydride-grafted olefin-acrylate copolymer; a methacrylate-glycidyl methacrylate-grafted olefin-acrylate copolymer; an olefin-acrylate-2-octeneylsuccinic anhydride terpolymer; and an olefin-acrylate-(2-methyl-2-propene)succinic anhydride terpolymer.

[0023] In some embodiments, the first acrylate monomer includes at least one of n-butyl acrylate, isobutyl acrylate, ethyl acrylate, n-octyl acrylate, isooctyl acrylate, and isooctyl methacrylate.

[0024] In some embodiments, the second acrylate monomer includes at least one of methyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, isobornyl methacrylate, and cyclohexyl methacrylate.

[0025] In some embodiments, the olefin includes at least one of ethylene, propylene, and 1-butene.

[0026] In some embodiments, the acrylate includes at least one selected from methyl acrylate, ethyl acrylate, methyl methacrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, n-hexyl acrylate, and isooctyl acrylate.

[0027] Choosing the above-mentioned types of acrylate polymers can further improve the dispersion uniformity of the positive electrode active material.

[0028] In some embodiments, the weight-average molecular weight of the acrylate polymer is 50,000 to 500,000. This design can improve the dispersion effect of the acrylate polymer on the positive electrode active material, while also taking into account the adhesion of the acrylate polymer to the positive electrode active material. This helps to reduce the film resistance of the positive electrode sheet, buffer the stress caused by the volume expansion of the positive electrode sheet during repeated charge and discharge, and further improve the cycle performance of the lithium-ion secondary battery.

[0029] In some embodiments, the acrylate polymer comprises a blend of a first acrylate polymer and a second acrylate polymer, wherein the weight-average molecular weight of the first acrylate polymer is 50,000 to 200,000, and the weight-average molecular weight of the second acrylate polymer is 300,000 to 500,000. The first acrylate polymer can enhance its dispersion effect on the positive electrode active material, while the second acrylate polymer can act as a binder. The combined use of the first and second acrylate polymers helps to reduce the film resistance of the positive electrode sheet, buffer the stress caused by the volume expansion of the positive electrode sheet during repeated charge and discharge, and further improve the cycle performance of the lithium-ion secondary battery.

[0030] In some embodiments, the acrylate polymer accounts for 1% to 2% of the mass of the positive electrode active material layer. This further improves the adhesion performance of the acrylate polymer to the positive electrode active material, while also ensuring more uniform dispersion of the positive electrode active material, which is beneficial for enhancing the adhesion and cohesion of the positive electrode sheet.

[0031] In some embodiments, the positive electrode active material includes lithium phosphate.

[0032] In some embodiments, the lithium-containing phosphate includes the chemical formula Li m A a Fe x D d P y E e O z G g The material, wherein A includes at least one element selected from Al, Na, K, or Mg; D includes at least one element selected from Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, Ti, or V; E includes at least one element selected from B, S, Si, or N; and G includes at least one element selected from S, F, Cl, or Br; wherein 0.5 ≤ m ≤ 1.5; 0 ≤ a ≤ 0.1; 0.5 ≤ x ≤ 1; 0 ≤ d ≤ 0.5; 0.5 ≤ y ≤ 1; 0 ≤ e ≤ 0.5; 3.5 ≤ z ≤ 4; and 0 ≤ g ≤ 0.5.

[0033] In some embodiments, the compaction density of the positive electrode sheet is ≥2.6 g / cm³.3 .

[0034] In some embodiments, the compaction density of the positive electrode sheet is 2.6 g / cm³. 3 ~2.8g / cm 3 .

[0035] In some embodiments, the lithium-ion secondary battery is a wound battery.

[0036] A second aspect of this application provides an electrical device including the lithium-ion secondary battery described in the first aspect of this application.

[0037] The electrical device of this application includes the aforementioned lithium-ion secondary battery, and therefore the electrical device has at least the same advantages as the lithium-ion secondary battery.

[0038] 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

[0039] 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:

[0040] Figure 1 is a schematic diagram of a battery cell according to an embodiment of this application.

[0041] Figure 2 is an exploded view of a battery cell according to an embodiment of this application shown in Figure 1.

[0042] Figure 3 is a schematic diagram of a battery module according to one embodiment of this application.

[0043] Figure 4 is a schematic diagram of a battery pack according to one embodiment of this application.

[0044] Figure 5 is an exploded view of the battery pack of one embodiment of this application shown in Figure 4.

[0045] Figure 6 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.

[0046] Explanation of reference numerals in the attached drawings: 1 Battery pack; 2 Upper casing; 3 Lower casing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Cover plate; 6 Electrical device. Detailed Implementation

[0047] Hereinafter, some embodiments of the lithium-ion secondary battery and power-consuming device of this application are described in detail 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 practically 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.

[0048] 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 a particular range. Ranges defined in this way can include or exclude endpoints; any endpoint can be independently included or excluded, and they can be combined arbitrarily, meaning 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 ​​1 and 2 are listed, and maximum range values ​​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 "ab" 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 describing a parameter as an integer ≥ 2, it is equivalent to listing integers such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12 for that parameter. For instance, when describing a parameter as an integer selected from "2-10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0049] 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.

[0050] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] Unless otherwise stated or in case of contradiction, the terms or phrases used herein shall have the following meanings:

[0055] The term "alkyl" refers to a saturated hydrocarbon containing a primary (normal) carbon atom, or a secondary carbon atom, or a tertiary carbon atom, or a quaternary carbon atom, or a combination thereof. Phrases containing this term, such as "C1-C8 alkyl," refer to alkyl groups containing 1 to 8 carbon atoms, and each time it appears, it can independently be C1 alkyl, C2 alkyl, C3 alkyl, C4 alkyl, C5 alkyl, C6 alkyl, C7 alkyl, or C8 alkyl. Suitable examples include, but are not limited to: methyl (Me, -CH3), ethyl (Et, -CH2CH3), 1-propyl (n-Pr, n-propyl, -CH2CH2CH3), 2-propyl (i-Pr, i-propyl, -CH(CH3)2), 1-butyl (n-Bu, n-butyl, -CH2CH2CH2CH3), 2-methyl-1-propyl (i-Bu, i-butyl, -CH2CH(CH3)2), 2-butyl (s-Bu) ,s-butyl,-CH(CH3)CH2CH3), 2-methyl-2-propyl (t-Bu, t-butyl,-C(CH3)3), 1-pentyl (n-pentyl,-CH2CH2CH2CH2CH3), 2-pentyl (-CH(CH3)CH2CH2CH3), 3-pentyl (-CH(CH2CH3)2), 2-methyl-2-butyl (-C(CH3)2CH2CH3), 3-methyl-2-butyl (-CH(CH3)2CH2CH3) )CH(CH3)2), 3-methyl-1-butyl(-CH2CH2CH(CH3)2), 2-methyl-1-butyl(-CH2CH(CH3)CH2CH3), 1-hexyl(-CH2CH2CH2CH2CH2CH3), 2-hexyl(-CH(CH3)CH2CH2CH2CH3), 3-hexyl(-CH(CH2CH3)(CH2CH2CH3)), 2-methyl-2-pentyl(-C(CH3)) 2CH2CH2CH3), 3-methyl-2-pentyl (-CH(CH3)CH(CH3)CH2CH3), 4-methyl-2-pentyl (-CH(CH3)CH2CH(CH3)2), 3-methyl-3-pentyl (-C(CH3)(CH2CH3)2), 2-methyl-3-pentyl (-CH(CH2CH3)CH(CH3)2), 2,3-dimethyl-2-butyl (-C(CH3)2CH(CH3)2).

[0056] The term "alkylene" refers to a hydrocarbon group derived from an alkyl group by removing one hydrogen atom, forming a group with two monovalent centers. This group can be a saturated branched alkyl group or a saturated straight-chain alkyl group. For example, "C1-C8 alkylene" means that the alkyl moiety contains 1-8 carbon atoms, and each occurrence can be independently C1, C4, C5, C6, C7, C8, or C9 alkylene. Suitable examples include, but are not limited to: methylene (-CH2-), 1,1-ethyl (-CH(CH3)-), 1,2-ethyl (-CH2CH2-), 1,1-propyl (-CH(CH2CH3)-), 1,2-propyl (-CH2CH(CH3)-), 1,3-propyl (-CH2CH2CH2-), and 1,4-butyl (-CH2CH2CH2CH2-).

[0057] The term "cycloalkyl" refers to a non-aromatic hydrocarbon containing a ring of carbon atoms, which can be monocycloalkyl, spirocycloalkyl, or bridged cycloalkyl. Phrases containing this term, such as "C3-C8 cycloalkyl," refer to cycloalkyl compounds containing 3 to 8 carbon atoms, and each occurrence can independently be C3, C4, C5, C6, C7, or C8 cycloalkyl. Suitable examples include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and cycloheptyl. Additionally, "cycloalkyl" may also contain one or more double bonds.

[0058] The term "heterocyclic group" refers to a non-aromatic monocyclic or bicyclic cycloalkyl group containing 3 to 8 atoms, wherein 1 to 6 carbon atoms in the ring are replaced by heteroatoms selected from O, S or N.

[0059] The term “heterocyclic alkylene” refers to an alkylene-heterocyclic group linked by an alkylene moiety, wherein the definitions of alkylene and heterocyclic group are as described above.

[0060] Polyvinylidene fluoride (PVDF) is commonly used as a binder for the positive electrode in lithium-ion secondary batteries. However, due to its simple molecular structure, regular molecular chain arrangement, and high crystallinity, PVDF has poor flexibility, resulting in significant brittleness of the positive electrode. During the winding of the positive electrode, the active material layer is subjected to considerable stress during bending, leading to partial cracking or even complete fracture at the bending point. Part of the active material layer detaches from the positive electrode, affecting the cycle performance of the lithium-ion secondary battery. Furthermore, PVDF contains fluorine, which may have some environmental impact. Therefore, there is an urgent need to develop a non-fluorinated binder that can improve the flexibility of the positive electrode and the cycle performance of lithium-ion secondary batteries.

[0061] Based on this, one embodiment of this application provides a lithium-ion secondary battery, including a positive electrode sheet, the positive electrode sheet including a positive electrode active material layer, the positive electrode active material layer containing a positive electrode active material and an acrylate polymer, the glass transition temperature of the acrylate polymer being denoted as T. g -50 degrees Celsius (°C) ≤ T g At ≤25℃, the acrylate polymer accounts for 0.5% to 2% of the mass of the positive electrode active material layer.

[0062] Acrylic polymers exhibit good adhesion to current collectors and are resistant to the erosion of organic solvents in the electrolyte, making them a relatively ideal non-fluorinated binder. However, when acrylic polymers are used as binders for the positive electrode sheet, the positive electrode active material layer is subjected to significant stress during bending when the positive electrode sheet is wound, leading to partial cracking or even breakage at the bending points. Therefore, this application introduces a -50℃≤T... g Acrylic polymers with a temperature ≤25℃ can improve the flexibility of the positive electrode sheet while ensuring its adhesion performance. By controlling the mass ratio of acrylic polymers in the positive electrode active material layer, the adhesion performance of acrylic polymers to the positive electrode active material can be improved, while ensuring more uniform dispersion of the positive electrode active material, which is beneficial to improving the adhesion and cohesion of the positive electrode sheet. The glass transition temperature (Tg) of acrylic polymers refers to the temperature at which the acrylic polymer transitions from the glassy state to the elastic state. When the temperature is below the glass transition temperature, the acrylic polymer is in the glassy state, and the molecular chains and segments cannot move; only the atoms or groups constituting the molecules vibrate in their equilibrium positions, resulting in high rigidity. When the temperature is above or equal to the glass transition temperature, although the molecular chains cannot move, the chain segments begin to move, exhibiting high elasticity. The positive electrode sheet of lithium-ion secondary batteries is usually wound at a temperature above 25℃. By introducing a temperature of -50℃ ≤ Tg into the positive electrode active material layer of the positive electrode sheet... g Acrylic polymers with a temperature of ≤25℃ exhibit high elasticity at the aforementioned processing temperature, possessing a certain degree of elasticity and flexibility. This can improve the flexibility of the positive electrode sheet, reduce problems such as cracking or even breakage of the positive electrode sheet during winding, and buffer the stress caused by the volume expansion of the positive electrode sheet during repeated charging and discharging, reducing the risk of shedding of the positive electrode active material layer, thereby improving the cycle performance of lithium-ion secondary batteries.

[0063] When the T of acrylate polymers g When the temperature is too high, its rigidity is relatively large, and the flexibility of the positive electrode sheet containing the positive electrode active material and acrylate polymer is relatively poor, leading to a deterioration in the cycle performance of the lithium-ion secondary battery; when the T of the acrylate polymer is too high... gWhen the temperature is too low, acrylate polymers exhibit a viscous flow state at the processing temperature of the positive electrode or the operating temperature of the lithium-ion secondary battery. This causes adhesion between positive electrode sheets or between the positive electrode sheet and the separator, affecting the cycle performance of the lithium-ion secondary battery. Furthermore, the positive electrode sheet may stick to the roller during the coating and cold pressing stage, resulting in production and processing problems and a relatively low yield of positive electrode sheets.

[0064] Specifically, T g Including but not limited to: -50℃, -40℃, -30℃, -20℃, -10℃, 0℃, 10℃, 20℃, 25℃, or any range between the foregoing. Optionally, -40℃ ≤ T g ≤25℃. The mass percentage of acrylate polymers in the positive electrode active material layer includes, but is not limited to: 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, or any two of the foregoing.

[0065] In some embodiments, the acrylate polymer comprises a first structural unit, the structural formula of which includes...

[0066] R1 to R3 are each independently selected from any one of hydrogen, substituted or unsubstituted C1 to C8 saturated alkyl groups, substituted or unsubstituted C3 to C8 cycloalkyl groups, substituted or unsubstituted heterocyclic alkylene groups, acid anhydrides, hydroxyl groups, carboxyl groups, and carboxyl salts. R4 is selected from any one of hydrogen, substituted or unsubstituted C1 to C8 saturated alkyl groups, substituted or unsubstituted C3 to C8 cycloalkyl groups, substituted or unsubstituted heterocyclic alkylene groups, and metal cations. * represents the bonding position.

[0067] In the above embodiments, the acrylate polymer containing the first structural unit can improve the dispersibility of the positive electrode slurry, the adhesion and stability of the positive electrode sheet, which is beneficial to improving the cycle performance of the lithium-ion secondary battery.

[0068] In some embodiments, the substituent groups in R1-R3 and R4, including the substituted C1-C8 saturated alkyl groups, the substituted C3-C8 cycloalkyl groups, and the substituted heterocyclic alkylene groups, are each independently selected from at least one of ester groups, carbonyl groups, amide groups, and acid anhydrides. Selecting the above-mentioned types of substituent groups can increase the polarity of the first structural unit, which is beneficial for improving the adhesion of acrylate polymers to the positive electrode active material, thereby enhancing the adhesion and cohesion of the positive electrode sheet.

[0069] In some embodiments, the metal cation includes at least one of sodium ions and potassium ions.

[0070] In some embodiments, the mass percentage of the first structural unit in the acrylate polymer is denoted as M11, where M11 satisfies: 0 < M11 ≤ 100%. It is understood that the acrylate polymer may contain only the first structural unit, or it may contain other structural units in addition to the first structural unit.

[0071] In some embodiments, the acrylate polymer further comprises a second structural unit, the structural formula of which includes...

[0072] R5 to R8 are each independently selected from hydrogen, substituted or unsubstituted C1 to C8 saturated alkyl groups, and * represents the bonding position.

[0073] In the above embodiments, introducing a weaker polarity second structural unit into the acrylate polymer can regulate the polarity of the acrylate polymer, weaken the intermolecular forces between the polymer and the positive electrode active material, improve the dispersibility of the positive electrode slurry, and reduce the film resistance of the positive electrode sheet.

[0074] In some embodiments, the acrylate polymer further comprises a third structural unit, the structural formula of which includes... * indicates the bonding location.

[0075] In the above embodiments, the third structural unit introduced into the acrylate polymer can enhance the adhesion of the acrylate polymer to the positive electrode active material, which is beneficial to further improve the adhesion and cohesion of the positive electrode sheet.

[0076] In some embodiments, the mass percentage of the second structural unit in the acrylate polymer is denoted as M12, where M12 satisfies the condition: 65% ≤ M12 < 100%. Within this range, M12 can reduce the polarity of the acrylate polymer, improve the dispersibility of the positive electrode active material, and benefit the processing performance of the electrode and the cycle performance of the lithium-ion secondary battery. Specifically, M12 includes, but is not limited to, 65%, 68%, 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, 92%, 95%, or any combination thereof. Further, 70% ≤ M12 ≤ 90%.

[0077] In some embodiments, the mass percentage of the first structural unit in the acrylate polymer is denoted as M11, and the mass percentage of the third structural unit in the acrylate polymer is denoted as M13. M11 and M13 satisfy the following conditions: 0 < M11 + M13 ≤ 35%, 0 ≤ M13 ≤ 5%. Meeting these conditions improves the adhesion of the acrylate polymer to the positive electrode active material, which is beneficial for further enhancing the adhesion and cohesion of the positive electrode sheet. It can be understood that when M13 = 0, M11 satisfies the condition: 0 < M11 ≤ 35%. Specifically, M11 includes, but is not limited to, 5%, 10%, 15%, 20%, 25%, 30%, 35%, or any two of the aforementioned values, and M13 includes, but is not limited to, 0%, 1%, 2%, 3%, 4%, 5%, or any two of the aforementioned values. Further, 10% ≤ M11 + M13 ≤ 30%.

[0078] In some embodiments, the acrylate polymer includes at least one of the following: a copolymer of a first acrylate monomer and a second acrylate monomer; an olefin-acrylate copolymer; a vinyl acetate-acrylate copolymer; an olefin-acrylate-maleic anhydride terpolymer; an olefin-acrylate-glycidyl methacrylate terpolymer; a maleic anhydride-grafted olefin-acrylate copolymer; a methacrylate-glycidyl methacrylate-grafted olefin-acrylate copolymer; an olefin-acrylate-2-octeneylsuccinic anhydride terpolymer; and an olefin-acrylate-(2-methyl-2-propene)succinic anhydride terpolymer.

[0079] In some embodiments, the first acrylate monomer includes at least one of n-butyl acrylate, isobutyl acrylate, ethyl acrylate, n-octyl acrylate, isooctyl acrylate, and isooctyl methacrylate.

[0080] In some embodiments, the second acrylate monomer includes at least one of methyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, isobornyl methacrylate, and cyclohexyl methacrylate.

[0081] As a non-limiting example, acrylate polymers may include, but are not limited to, at least one of the following: n-butyl acrylate-methyl acrylate copolymer, isobutyl acrylate-methyl methacrylate copolymer, n-octyl acrylate-ethyl methacrylate copolymer, isooctyl methacrylate-methyl methacrylate copolymer, and ethyl acrylate-cyclohexyl methacrylate copolymer.

[0082] In some embodiments, the olefin includes at least one of ethylene, propylene, and 1-butene.

[0083] In some embodiments, the acrylate includes at least one of methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl 2-methacrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, n-hexyl acrylate, and isooctyl acrylate.

[0084] Choosing the above-mentioned types of acrylate polymers can further improve the dispersion uniformity of the positive electrode active material.

[0085] In some embodiments, the weight-average molecular weight of the acrylate polymer is 50,000 to 500,000. A weight-average molecular weight within this range can enhance the dispersion effect of the acrylate polymer on the positive electrode active material while maintaining its adhesion to the material. This helps reduce the film resistance of the positive electrode sheet, buffers the stress caused by the volume expansion of the positive electrode sheet during repeated charge and discharge, and further improves the cycle performance of the lithium-ion secondary battery. Specifically, the weight-average molecular weight of the polymer includes, but is not limited to: 50,000, 100,000, 150,000, 200,000, 250,000, 300,000, 350,000, 400,000, 450,000, 500,000, or any combination thereof.

[0086] Non-limitingly, the weight-average molecular weight of acrylate polymers can be determined using gel permeation chromatography (GPC). For example, using a 0.1% (w / w) polystyrene solution as a reference, a matched column can be selected. A 5% analyte solution is prepared using purified N-methylpyrrolidone (NMP) solvent and allowed to stand for one day. During testing, tetrahydrofuran is first drawn into a syringe and used to flush the solution, repeating this several times. Then, 5 mL of the test solution is drawn, air is expelled from the syringe, and the needle tip is dried. Finally, the sample solution is slowly injected into the injection port. After the reading stabilizes, the data is acquired, and the weight-average molecular weight is recorded.

[0087] In some embodiments, the acrylate polymer includes a blend of a first acrylate polymer and a second acrylate polymer. The first acrylate polymer has a weight-average molecular weight of 50,000 to 200,000, and the second acrylate polymer has a weight-average molecular weight of 300,000 to 500,000. The first acrylate polymer can enhance its dispersion effect on the positive electrode active material, while the second acrylate polymer can act as a binder. The combined use of the first and second acrylate polymers helps to reduce the film resistance of the positive electrode sheet, buffer the stress caused by the volume expansion of the positive electrode sheet during repeated charge and discharge, and further improve the cycle performance of the lithium-ion secondary battery.

[0088] In some embodiments, the first acrylate polymer and the second acrylate polymer are each independently selected from at least one of the following: copolymers of the first acrylate monomer and the second acrylate monomer; olefin-acrylate copolymers; vinyl acetate-acrylate copolymers; olefin-acrylate-maleic anhydride terpolymers; olefin-acrylate-glycidyl methacrylate terpolymers; maleic anhydride-grafted olefin-acrylate copolymers; glycidyl methacrylate-grafted olefin-acrylate copolymers; olefin-acrylate-2-octenyl succinic anhydride terpolymers; and olefin-acrylate-(2-methyl-2-propene)succinic anhydride terpolymers.

[0089] In the above embodiments, the first acrylate polymer and the second acrylate polymer can be the same type of acrylate polymer with different weight average molecular weights, or they can be different types of acrylate polymers. As a non-limiting example, the first acrylate polymer can be an olefin-acrylate-maleic anhydride terpolymer with a weight average molecular weight of 50,000, and the second acrylate polymer can be an olefin-acrylate-maleic anhydride terpolymer with a weight average molecular weight of 300,000; or the first acrylate polymer can be an olefin-acrylate-glycidyl methacrylate terpolymer with a weight average molecular weight of 200,000, and the second acrylate polymer can be a copolymer of a first acrylate monomer and a second acrylate monomer with a weight average molecular weight of 500,000. For example, the first acrylate polymer can be an ethylene-methyl acrylate-maleic anhydride terpolymer with a weight average molecular weight of 50,000, and the second acrylate polymer can be an ethylene-methyl acrylate-maleic anhydride terpolymer with a weight average molecular weight of 300,000; or the first acrylate polymer can be an ethylene-methyl acrylate-glycidyl methacrylate terpolymer with a weight average molecular weight of 200,000, and the second acrylate polymer can be a n-butyl acrylate-methyl acrylate copolymer with a weight average molecular weight of 500,000.

[0090] In other embodiments, the acrylate polymers include only third acrylate polymers with a weight-average molecular weight of 200,000 to 500,000. This improves the dispersion effect of the third acrylate polymer on the positive electrode active material while maintaining its adhesion to the material, which helps reduce the film resistance of the positive electrode and buffers the stress caused by the volume expansion of the positive electrode during repeated charge and discharge, further improving the cycle performance of the lithium-ion secondary battery. Further, the weight-average molecular weight of the third acrylate polymer is 300,000 to 400,000.

[0091] In some embodiments, the third acrylate polymer is selected from at least one of the following: copolymers of the first and second acrylate monomers, olefin-acrylate copolymers, vinyl acetate-acrylate copolymers, olefin-acrylate-maleic anhydride terpolymers, olefin-acrylate-glycidyl methacrylate terpolymers, maleic anhydride-grafted olefin-acrylate copolymers, glycidyl methacrylate-grafted olefin-acrylate copolymers, olefin-acrylate-2-octenyl succinic anhydride terpolymers, and olefin-acrylate-(2-methyl-2-propene)succinic anhydride terpolymers.

[0092] In some embodiments, the acrylate polymer accounts for 1% to 2% of the mass of the positive electrode active material layer. This can further improve the adhesion performance of the acrylate polymer to the positive electrode active material, while also making the positive electrode active material more uniformly dispersed, which is beneficial to improving the adhesion and cohesion of the positive electrode sheet.

[0093] In some implementations, the positive electrode active material includes lithium phosphate.

[0094] In some embodiments, the lithium phosphate includes the chemical formula Li m A a Fe x D d P y E e O z G g The material, A includes at least one element selected from Al, Na, K, or Mg; D includes at least one element selected from Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, Ti, or V; E includes at least one element selected from B, S, Si, or N; G includes at least one element selected from S, F, Cl, or Br; wherein 0.5 ≤ m ≤ 1.5; 0 ≤ a ≤ 0.1; 0.5 ≤ x ≤ 1; 0 ≤ d ≤ 0.5; 0.5 ≤ y ≤ 1; 0 ≤ e ≤ 0.5; 3.5 ≤ z ≤ 4; 0 ≤ g ≤ 0.5. As an example, lithium-containing phosphates include lithium iron phosphate (LiFePO4) (LFP).

[0095] 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. In the examples of positive electrode active materials listed in this application, unless otherwise specified, the Li content refers to the initial state of the material. When positive electrode active materials are applied to the positive electrode in a battery system, the Li content in the positive electrode active 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 active materials are also within the scope of positive electrode active materials. The aforementioned appropriate modification refers to acceptable modification methods for positive electrode active materials, and non-limiting examples include coating modification.

[0096] In the examples of positive electrode active 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.

[0097] As a non-limiting example, lithium phosphates may include those with the chemical formula Li. m A a Fe x D d P y E e O z G g A composite material of materials and carbon. Considering the chemical formula Li m A a Fe x D d P y E e O z G g The material has weak electrical conductivity, and is usually produced by using the chemical formula Li. m A a Fe x D d P y E e O z G g The surface of the material is coated with carbon to improve its conductivity.

[0098] In some embodiments, the compaction density of the positive electrode sheet is ≥2.6 g / cm³. 3 Furthermore, the compaction density of the positive electrode sheet is 2.6 g / cm³. 3 ~2.8g / cm 3Specifically, the compaction density of the positive electrode sheet includes, but is not limited to, 2.6 g / cm³. 3 2.65g / cm 3 2.7g / cm 3 2.75g / cm 3 2.8g / cm 3 Or the range between any two of the aforementioned.

[0099] In some embodiments, the positive electrode sheet further includes a positive current collector, and the positive active material layer is disposed on at least one surface of the positive current collector.

[0100] As a non-limiting example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0101] 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).

[0102] In some embodiments, the positive electrode active material layer 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.

[0103] In some embodiments, the positive electrode sheet can be prepared by dispersing the components used to prepare the positive electrode sheet, such as the positive electrode active material, conductive agent, the acrylate polymer described in this application, and any other components, in a solvent to form a positive electrode slurry; coating the positive electrode slurry onto at least one surface of the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing, and other processes. The surface of the positive electrode current collector coated with the positive electrode slurry can be a single surface of the positive electrode current collector or both surfaces of the positive electrode current collector.

[0104] In some embodiments, the lithium-ion secondary battery further includes a negative electrode sheet, which includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, the negative active material layer including a negative active material.

[0105] 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.

[0106] 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 polymeric material substrate and a metal layer formed on at least one surface of the polymeric material substrate. The composite current collector can be obtained by forming a metal material on the polymeric 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 polymeric 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).

[0107] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As a non-limiting example, the negative electrode active material 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.

[0108] 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).

[0109] 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.

[0110] In some embodiments, the negative electrode active material layer may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0111] 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 the negative electrode current collector, and then 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 (mass percentage) of the negative electrode slurry can be 40wt%-60wt%. The viscosity of the negative electrode slurry at room temperature can be adjusted to 2000 mPa·s-10000 mPa·s.

[0112] In some embodiments, the lithium-ion secondary battery further includes an electrolyte that facilitates ion conduction between the positive and negative electrodes. This application does not impose any particular limitation on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid.

[0113] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0114] In some embodiments, the electrolyte salt 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 difluorophosphate (LiPO2F2), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0115] In some embodiments, the solvent may include 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), butene carbonate Fluoroethylene carbonate (FEC), methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] 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.

[0120] In some embodiments, the thickness of the isolation membrane is 6 micrometers (μm) to 40 μm, and optionally 12 μm to 20 μm.

[0121] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0122] In some embodiments, the lithium-ion secondary battery is a wound battery.

[0123] 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.

[0124] 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.

[0125] A lithium-ion secondary battery includes at least one battery cell. A lithium-ion secondary battery may include one or more battery cells.

[0126] 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.

[0127] 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.

[0128] 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. The positive electrode sheet, negative electrode sheet, and separator may 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 the battery cell 5 may be one or more, which can be selected by those skilled in the art according to actual needs.

[0129] The lithium-ion secondary battery can be either battery module 4 or battery pack 1.

[0130] A battery module includes at least one battery cell. The number of battery cells in a battery module can be one or more, and those skilled in the art can select an appropriate number based on the application and capacity of the battery module.

[0131] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple battery cells 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple battery cells 5 can be fixed in place using fasteners.

[0132] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0133] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the battery pack may contain one or more battery modules. Those skilled in the art can select an appropriate number based on the application and capacity of the battery pack.

[0134] Figures 4 and 5 show a battery pack 1 as an example. Referring to Figures 4 and 5, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box 2 and a lower box 3, with the upper box 2 covering the lower box 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0135] Another embodiment of this application provides a positive electrode slurry, comprising a positive electrode active material, an acrylate polymer, and a solvent, wherein the glass transition temperature of the acrylate polymer is denoted as T. g -50℃≤T g ≤25℃.

[0136] By introducing -50℃≤T into the positive electrode slurry g Acrylic polymers with a temperature of ≤25℃ exhibit high elasticity at the aforementioned processing temperature, possessing a certain degree of elasticity and flexibility. This can improve the flexibility of the positive electrode sheet, reduce problems such as cracking or even breakage of the positive electrode sheet during winding, and buffer the stress caused by the volume expansion of the positive electrode sheet during repeated charging and discharging, reducing the risk of shedding of the positive electrode active material layer, thereby improving the cycle performance of lithium-ion secondary batteries.

[0137] In some embodiments, the acrylate polymer comprises a first structural unit, the structural formula of which includes...

[0138] R1 to R3 are each independently selected from any one of hydrogen, substituted or unsubstituted C1-C8 saturated alkyl groups, substituted or unsubstituted C3-C8 cycloalkyl groups, substituted or unsubstituted heterocyclic alkylene groups, acid anhydrides, hydroxyl groups, carboxyl groups, and carboxyl salts. R4 is selected from any one of hydrogen, substituted or unsubstituted C1-C8 saturated alkyl groups, substituted or unsubstituted C3-C8 cycloalkyl groups, substituted or unsubstituted heterocyclic alkylene groups, and metal cations. * represents the bonding position. Acrylic polymers containing the first structural unit can improve the dispersibility of the positive electrode slurry, the adhesion and stability of the positive electrode sheet, which is beneficial to improving the cycle performance of lithium-ion secondary batteries.

[0139] In some embodiments, the substituent groups in R1-R3 and R4, including the substituted C1-C8 saturated alkyl groups, the substituted C3-C8 cycloalkyl groups, and the substituted heterocyclic alkylene groups, are each independently selected from at least one of ester groups, carbonyl groups, amide groups, and acid anhydrides. Selecting the above-mentioned types of substituent groups can increase the polarity of the first structural unit, which is beneficial for improving the adhesion of acrylate polymers to the positive electrode active material, thereby enhancing the adhesion and cohesion of the positive electrode sheet.

[0140] In some embodiments, the metal cation includes at least one of sodium ions and potassium ions.

[0141] In some embodiments, the acrylate polymer further comprises a second structural unit, the structural formula of which includes...

[0142] R5 to R8 are each independently selected from hydrogen, substituted or unsubstituted C1 to C8 saturated alkyl groups, and * represents the bonding position. Introducing a weaker polar second structural unit into acrylate polymers can regulate the polarity of the acrylate polymers, weaken the intermolecular forces between the polymer and the positive electrode active material, improve the dispersibility of the positive electrode slurry, and reduce the film resistance of the positive electrode sheet.

[0143] In some embodiments, the acrylate polymer further comprises a third structural unit, the structural formula of which includes... * Represents a bonding site. The introduction of a third structural unit into acrylate polymers can enhance the adhesion of acrylate polymers to the positive electrode active material, which is beneficial for further improving the adhesion and cohesion of the positive electrode sheet.

[0144] In some embodiments, the mass percentage of the second structural unit in the acrylate polymer is denoted as M12, and the solvent, M12, and the acrylate polymer satisfy at least one of the following conditions (1) to (2):

[0145] (1) M12 < 80% or the acrylate polymer contains a first structural unit but does not contain a second structural unit, and the solvent in the positive electrode slurry includes at least one of N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide and acetone;

[0146] (2) 80% ≤ M12 < 100%, the solvent in the positive electrode slurry includes at least one of solvent oil, pentylbenzene and petroleum ether.

[0147] By varying the M12 and the type of acrylate polymer, different combinations of acrylate polymers and solvents can be designed to improve the polymer's solubility in the corresponding solvents, further promoting the uniform dispersion of the cathode slurry. Solvent oils include, but are not limited to, one or more of the following: D20, D40, D60, D80, D100, D120, and D200 solvent oils.

[0148] Another embodiment of this application provides an electrical device including the lithium-ion secondary battery described above.

[0149] Lithium-ion secondary batteries can be used as a power source for electrical devices or as an energy storage unit for electrical devices. Electrical devices can include, but are not limited to, mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc. Mobile devices can include, for example, mobile phones and laptops; electric vehicles can include, for example, pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc., but are not limited to these.

[0150] As an electrical device, lithium-ion rechargeable batteries can be selected according to its usage requirements.

[0151] Figure 6 shows an example of an electrical device 6. This electrical device 6 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.

[0152] 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.

[0153] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where the technology or conditions are not specified in the embodiments, they are performed according to the technology or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0154] Example 1

[0155] (1) Preparation of the positive electrode sheet:

[0156] (1.1) Acrylate polymers were prepared according to the following method:

[0157] (1.1.1) Add the emulsifier sodium dodecyl sulfate to a beaker containing deionized water and stir at 500 rpm to dissolve it. Then add methyl acrylate monomer and stir at 800 rpm for 2 hours at 50°C to obtain a mixed solution.

[0158] (1.1.2) Add the mixed solution to the reactor that has been purged with ethylene gas, add the initiator benzoyl peroxide (total initiator accounts for 0.5% of the total mass of the reactants), introduce ethylene gas (mass ratio of ethylene monomer a to methyl acrylate monomer b a:b = 80:20), heat and stir, and keep the pressure of the system constant. When the temperature rises to 70°C, maintain the temperature and pressure for 4 hours.

[0159] (1.1.3) After the reaction is complete, the reaction liquid is poured into a methanol solution containing a trace amount of hydroquinone to demulsify. When the precipitate no longer increases, it is filtered and the upper filter cake is dried to obtain ethylene-methyl acrylate copolymer (acrylate polymer).

[0160] The glass transition temperature T of the obtained acrylate polymer g The temperature is 0.6℃. The second structural unit of the acrylate polymer is derived from ethylene, and the first structural unit is derived from methyl acrylate. The mass percentages of the second structural unit in the acrylate polymer (M12) and the first structural unit in the acrylate polymer (M11) are approximately 80% and 20%, respectively. The weight-average molecular weight of the acrylate polymer is 400,000.

[0161] The first structural unit derived from methyl acrylate is shown in Formula I, and the second structural unit derived from ethylene is shown in Formula II:

[0162] (1.2) Ethylene-methyl acrylate copolymer (acrylate polymer) was dissolved in solvent D60 to obtain a binder solution. Then, lithium iron phosphate (LiFePO4) (LFP), conductive carbon black (Super P), and the binder solution were mixed to obtain a positive electrode slurry. The mass ratio of LFP, Super P, and acrylate polymer in the positive electrode slurry was 97.8:0.4:1.8. The positive electrode slurry was coated on both sides of the positive electrode current collector aluminum foil. After drying and cold pressing, a positive electrode sheet was obtained. The compacted density of the positive electrode sheet was approximately 2.6 g / cm³. 3 .

[0163] (2) Preparation of negative electrode sheet:

[0164] The negative electrode active material graphite, thickener sodium carboxymethyl cellulose, binder styrene-butadiene rubber and conductive agent acetylene black were mixed in a mass ratio of 97:1:1:1, deionized water was added, and a negative electrode slurry was obtained under the action of a vacuum stirrer. The negative electrode slurry was uniformly coated on both sides of a copper foil. After the copper foil was dried at room temperature, it was transferred to a 120°C oven to dry for 1 hour. Then, it was cold-pressed and slit to obtain the negative electrode sheet.

[0165] (3) Separating membrane:

[0166] A 12μm thick polypropylene separator membrane was selected.

[0167] (4) Preparation of electrolyte:

[0168] The organic solvent was a mixture containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), with a volume ratio of EC:20:20:60. Thoroughly dried lithium salt LiPF6 was dissolved in the organic solvent and mixed thoroughly in an argon-atmosphere glove box with a water content of <10 ppm to obtain the electrolyte. The concentration of the lithium salt was 1 mol / L.

[0169] (5) Battery fabrication:

[0170] The positive electrode, negative electrode, and separator are made into an electrode assembly by a winding process. The separator is placed between the positive and negative electrode sheets to isolate them. The assembly is then filled with an aluminum-plastic film, baked at 80°C to remove water, injected with electrolyte, sealed, and subjected to processes such as standing, hot and cold pressing, formation, clamping, and capacity testing to obtain a lithium-ion secondary battery.

[0171] Examples 2-5

[0172] The preparation of lithium-ion secondary batteries is basically the same as in Example 1, except that the types and mass ratios of monomers in steps (1.1.1) and (1.1.2) are changed, thereby changing the types of acrylate polymers prepared in step (1.1), as detailed in Table 1.

[0173] In Example 2, the methyl acrylate monomer in steps (1.1.1) and (1.1.2) is replaced with n-butyl acrylate monomer;

[0174] In Example 3, the methyl acrylate monomer in steps (1.1.1) and (1.1.2) is replaced with ethyl acrylate monomer;

[0175] In Example 4, the methyl acrylate monomer in steps (1.1.1) and (1.1.2) is replaced with methyl acrylate monomer and glycidyl methacrylate monomer. In step (1.1.2), the mass ratio of ethylene monomer a: methyl acrylate monomer b: glycidyl methacrylate monomer c is a:b:c = 70:25:5.

[0176] Example 5

[0177] The preparation of lithium-ion secondary batteries is basically the same as in Example 1, except that the type of acrylate polymer prepared in step (1.1) is changed, as detailed in Table 1.

[0178] In Example 5, acrylate polymers were prepared according to the following method:

[0179] (1.1.1) Add the emulsifier sodium dodecyl sulfate to a beaker containing deionized water and stir at 500 rpm to dissolve it. Then add vinyl acetate monomer and isobutyl acrylate monomer and stir at 800 rpm for 2 h at 50 °C to obtain a mixed solution.

[0180] (1.1.2) Add the mixed solution to a nitrogen-protected reactor, add the initiator benzoyl peroxide (total initiator accounts for 0.5% of the total mass of the reactants), and the mass ratio of vinyl acetate monomer a to isobutyl acrylate monomer b is a:b = 80:20. Heat and stir. When the temperature rises to 70℃, maintain the temperature and pressure for 4 hours.

[0181] (1.1.3) After the reaction is complete, the reaction liquid is poured into a methanol solution containing a trace amount of hydroquinone to demulsify. When the precipitate no longer increases, the mixture is filtered, and the upper filter cake is dried to obtain vinyl acetate-isobutyl acrylate copolymer (acrylate polymer).

[0182] Examples 6-7

[0183] The preparation of lithium-ion secondary batteries is basically the same as in Example 5, except that the types and mass ratios of monomers in steps (1.1.1) and (1.1.2) are changed, thereby changing the types of acrylate polymers prepared in step (1.1), as detailed in Table 1.

[0184] In Example 6, the vinyl acetate monomer and isobutyl acrylate monomer in steps (1.1.1) and (1.1.2) are replaced with ethyl acrylate monomer and methyl methacrylate monomer, respectively.

[0185] In Example 7, the vinyl acetate monomer and isobutyl acrylate monomer in steps (1.1.1) and (1.1.2) were replaced with ethyl acrylate monomer and methyl acrylate monomer, respectively.

[0186] Example 8

[0187] The preparation of lithium-ion secondary batteries is basically the same as in Example 1, except that the ethylene-methyl acrylate copolymer used in step (1.2) is replaced with ethylene-methyl acrylate copolymer (first acrylate polymer) and ethylene-methyl acrylate-maleic anhydride terpolymer (second acrylate polymer), with a mass ratio of 0.5:1.

[0188] The preparation method of the first acrylate polymer is similar to that of the acrylate polymer in Example 1, except that the heat and pressure holding reaction time in step (1.1.2) is changed, thereby changing the weight-average molecular weight of the first acrylate polymer to 53,000, as detailed in Table 1.

[0189] The preparation method of the second acrylate polymer is as follows:

[0190] Sodium dodecyl sulfate emulsifier was added to a beaker containing deionized water and stirred at 500 rpm to dissolve it. Then, methyl acrylate monomer and maleic anhydride monomer were added and stirred at 800 rpm for 2 hours at 50°C to obtain a mixed solution.

[0191] The mixed solution was added to a reactor that had been purged with ethylene gas. Benzoyl peroxide (the total amount of initiator accounted for 0.5% of the total mass of the reactants) was added as an initiator. Ethylene gas was introduced (the mass ratio of ethylene monomer a, methyl acrylate monomer b, and maleic anhydride monomer c was a:b:c = 73:25.5:1.5). The mixture was heated and stirred, and the pressure of the system was kept constant. When the temperature reached 70°C, the reaction was carried out under the same temperature and pressure for 4.5 hours.

[0192] After the reaction was completed, the reaction liquid was poured into a methanol solution containing a trace amount of hydroquinone to demulsify. When the precipitate no longer increased, it was filtered, and the upper filter cake was dried to obtain ethylene-methyl acrylate-maleic anhydride terpolymer (second acrylate polymer).

[0193] The obtained ethylene-methyl acrylate-maleic anhydride terpolymer has a weight-average molecular weight of 482,000 and a Tg of -20℃. The first structural unit in the ethylene-methyl acrylate-maleic anhydride terpolymer is derived from methyl acrylate, the second structural unit is derived from ethylene, and the third structural unit is derived from maleic anhydride. The mass percentage of the first structural unit M11 is approximately 25.5%, the mass percentage of the second structural unit M12 is approximately 73%, and the mass percentage of the third structural unit M13 is approximately 1.5%.

[0194] Example 9

[0195] The preparation of lithium-ion secondary batteries is basically the same as in Example 1, except that the ethylene-methyl acrylate copolymer used in step (1.2) is replaced with ethylene-methyl acrylate copolymer (first acrylate polymer) and ethylene-methyl acrylate-maleic anhydride terpolymer (second acrylate polymer), with a mass ratio of 0.5:1.

[0196] The preparation method of the first acrylate polymer is similar to that of the acrylate polymer in Example 1, except that the heat and pressure holding reaction time in step (1.1.2) is changed, thereby changing the weight-average molecular weight of the first acrylate polymer to 203,000, as detailed in Table 1.

[0197] The preparation method of the second acrylate polymer is as follows:

[0198] Sodium dodecyl sulfate emulsifier was added to a beaker containing deionized water and stirred at 500 rpm to dissolve it. Then, methyl acrylate monomer and maleic anhydride monomer were added and stirred at 800 rpm for 2 hours at 50°C to obtain a mixed solution.

[0199] The mixed solution was added to a reactor that had been purged with ethylene gas. Benzoyl peroxide (the total amount of initiator accounted for 0.5% of the total mass of the reactants) was added as an initiator. Ethylene gas was introduced (the mass ratio of ethylene monomer a, methyl acrylate monomer b, and maleic anhydride monomer c was a:b:c = 73:25.5:1.5). The mixture was heated and stirred, and the pressure of the system was kept constant. When the temperature reached 70°C, the reaction was carried out under the same temperature and pressure for 3.5 hours.

[0200] After the reaction was completed, the reaction liquid was poured into a methanol solution containing a trace amount of hydroquinone to demulsify. When the precipitate no longer increased, it was filtered, and the upper filter cake was dried to obtain ethylene-methyl acrylate-maleic anhydride terpolymer (second acrylate polymer).

[0201] The obtained ethylene-methyl acrylate-maleic anhydride terpolymer has a weight-average molecular weight of 308,000 and a Tg of -30℃. The first structural unit in the ethylene-methyl acrylate-maleic anhydride terpolymer is derived from methyl acrylate, the second structural unit is derived from ethylene, and the third structural unit is derived from maleic anhydride. The mass percentage of the first structural unit M11 is approximately 25.5%, the mass percentage of the second structural unit M12 is approximately 73%, and the mass percentage of the third structural unit M13 is approximately 1.5%.

[0202] Examples 10-12

[0203] The preparation of lithium-ion secondary batteries is basically the same as in Example 8, except that the mass ratio of positive electrode active material LFP, conductive agent carbon black (Super P) and acrylate polymer in the positive electrode slurry in step (1.2) is changed, that is, the mass ratio of acrylate polymer in the positive electrode active material layer is changed, as shown in Table 1.

[0204] In Example 10, the mass ratio of the positive electrode active material LFP, the conductive agent carbon black (Super P), and the acrylate polymer in the positive electrode slurry was 99.1:0.4:0.5.

[0205] In Example 11, the mass ratio of the positive electrode active material LFP, the conductive agent carbon black (Super P), and the acrylate polymer in the positive electrode slurry was 98.6:0.4:1;

[0206] In Example 12, the mass ratio of positive electrode active material LFP, conductive agent carbon black (Super P), and acrylate polymer in the positive electrode slurry was 97.6:0.4:2.

[0207] Examples 13-16

[0208] The preparation of lithium-ion secondary batteries is basically the same as in Example 8, except that the mass ratio of ethylene monomer a to methyl acrylate monomer b in step (1.1.2) is changed to a:b, thereby changing the mass percentage of the second structural unit in the ethylene-methyl acrylate copolymer (the first acrylate polymer), as shown in Table 1.

[0209] In Example 13, a:b = 65:35;

[0210] In Example 14, a:b = 70:30;

[0211] In Example 15, a:b = 90:10;

[0212] In Example 16, a:b = 95:5.

[0213] Example 17

[0214] The preparation of the lithium-ion secondary battery is basically the same as in Example 1, except that the methyl acrylate monomer in steps (1.1.1) and (1.1.2) is replaced with methyl methacrylate monomer, and the mass ratio of ethylene monomer a: methyl methacrylate monomer b in step (1.1.2) is a:b = 67:33, as shown in Table 1.

[0215] Example 18

[0216] The preparation of lithium-ion secondary batteries is basically the same as in Example 1, except that the type of acrylate polymer prepared in step (1.1) is changed, as detailed in Table 1.

[0217] In Example 18, acrylate polymers were prepared according to the following method:

[0218] (1.1.1) Add the emulsifier sodium dodecyl sulfate to a beaker containing deionized water and stir at 500 rpm to dissolve it. Then add ethyl acrylate monomer and n-butyl acrylate monomer and stir at 800 rpm for 2 hours at 50°C to obtain a mixed solution.

[0219] (1.1.2) Add the mixed solution to a nitrogen-protected reactor, add the initiator benzoyl peroxide (total initiator accounts for 0.5% of the total mass of the reactants), and the mass ratio of ethyl acrylate monomer a to n-butyl acrylate monomer b is a:b = 98.5:1.5. Heat and stir, and when the temperature rises to 70℃, keep the temperature and pressure for 4 hours.

[0220] (1.1.3) After the reaction is complete, the reaction liquid is poured into a methanol solution containing a trace amount of hydroquinone to demulsify. When the precipitate no longer increases, the mixture is filtered, and the upper filter cake is dried to obtain ethyl acrylate-n-butyl acrylate copolymer (acrylate polymer).

[0221] Comparative Example 1

[0222] The preparation of lithium-ion secondary batteries is basically the same as in Example 1, except that the type of acrylate polymer prepared in step (1.1) is changed, as detailed in Table 1.

[0223] In Comparative Example 1, acrylate polymers were prepared according to the following method:

[0224] (1.1.1) Add the emulsifier sodium dodecyl sulfate to a beaker containing deionized water and stir at 500 rpm to dissolve it. Then add methyl methacrylate monomer and n-butyl acrylate monomer and stir at 800 rpm for 2 h at 50 °C to obtain a mixed solution.

[0225] (1.1.2) Add the mixed solution to a nitrogen-protected reactor, add the initiator benzoyl peroxide (total initiator accounts for 0.5% of the total mass of the reactants), and the mass ratio of methyl methacrylate monomer a to n-butyl acrylate monomer b is a:b = 71.5:28.5. Heat and stir, and when the temperature rises to 70℃, maintain the temperature and pressure for 4 hours.

[0226] (1.1.3) After the reaction is complete, the reaction liquid is poured into a methanol solution containing a trace amount of hydroquinone to demulsify. When the precipitate no longer increases, it is filtered and the upper filter cake is dried to obtain methyl methacrylate-n-butyl acrylate copolymer (acrylate polymer).

[0227] Comparative Example 2

[0228] The preparation of the lithium-ion secondary battery is basically the same as in Example 18, except that the heat and pressure holding time in step (1.1.2) is changed, thereby changing the weight average molecular weight of the prepared ethyl acrylate-n-butyl acrylate copolymer (acrylate polymer) to 400,000, as shown in Table 1.

[0229] Comparative Example 3

[0230] The preparation of the lithium-ion secondary battery is basically the same as in Example 1, except that in step (1.2), PVDF (the manufacturer and model of the PVDF used in this comparative example is Solvay 5130) is used instead of the ethylene-methyl acrylate copolymer in Example 1 as a binder and dissolved in the solvent NMP to obtain a binder solution. Then, the positive electrode active material lithium iron phosphate LiFePO4 (LFP), the conductive agent carbon black (Super P) and the binder solution are mixed to obtain a positive electrode slurry. The mass ratio of positive electrode active material LFP, conductive agent carbon black (Super P) and PVDF in the positive electrode slurry is 97.8:0.4:1.8.

[0231] Test case

[0232] (1) Glass transition temperature test:

[0233] The glass transition temperature of acrylate polymers was tested using differential scanning calorimetry (DSC) under a nitrogen atmosphere at a heating rate of 10 °C / min, with a temperature range of -80 °C to 100 °C.

[0234] (2) Adhesion test of positive electrode sheet:

[0235] The prepared positive electrode sheet was cut into test specimens of 20mm × 100mm size for later use. One side of the double-sided tape was attached to the surface of the steel plate, and the other side was attached to the side of 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 entire positive electrode active material layer was peeled off from the surface of the current collector. The displacement and force during the process were recorded. The force at which the forces were balanced was taken as the bonding force of the electrode sheet. The bonding strength, i.e., the bonding force of the positive electrode sheet, was obtained by dividing this force by the adhesion length of the specimen.

[0236] (3) Cohesive force test of the positive electrode sheet:

[0237] The prepared positive electrode sheet was cut into test specimens of 20mm × 100mm size for later use. One side of the double-sided adhesive was pasted onto the surface of the steel plate, and the other side was pasted onto the side of the electrode sheet to be tested. The adhesive was then pressed with a pressure roller to ensure complete adhesion to the electrode sheet. Cohesion test tape was pasted onto the other side of the electrode sheet and pressed with a pressure roller. One end of the cohesion test tape 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 current collector. The displacement and force during the process were recorded. The force when the force was balanced was taken as the bonding force of the electrode sheet. The bonding strength, i.e., the cohesion of the positive electrode sheet, was obtained by dividing this force by the adhesion length of the specimen.

[0238] (4) Flexibility test of the positive electrode sheet:

[0239] Take a 20mm × 100mm (longitudinal) sample of the prepared positive electrode sheet and sample it along the rolling direction. Place the pre-folded experimental electrode sheet on the experimental table and roll it with a 2kg cylindrical roller. After each rolling, observe whether the electrode sheet is translucent. Record the corresponding number of rolling cycles when the electrode sheet is translucent. The number of rolling cycles represents the flexibility of the electrode sheet. The more rolling cycles, the better the flexibility of the electrode sheet.

[0240] (5) Battery cycle performance test:

[0241] Place the battery in a 25℃ environment and let it stand for 2 hours. After the battery temperature reaches 25℃, charge the battery at a constant current of 1C and constant voltage to 3.65V, then charge at constant voltage to the cutoff current of 0.05C, let it rest for 5 minutes, and then discharge it at 1C to 2.5V. Record the initial capacity Q0. The discharge capacity after 400 cycles is recorded as the battery capacity Q2. The capacity retention rate (%) of the battery after 400 cycles is calculated as Q2 / Q0 × 100%. The higher the capacity retention rate, the better the battery's cycle performance.

[0242] The product parameters and test results of each embodiment and comparative example are shown in Tables 1 and 2.

[0243] Table 1

[0244] In Table 1, " / " indicates that the substance or parameter does not exist; M11 represents the mass percentage of the first structural unit in the acrylate polymer, and M12 represents the mass percentage of the second structural unit in the acrylate polymer.

[0245] Table 2

[0246] As shown in Table 2, compared with Comparative Examples 1 and 3, the positive electrode sheets of Examples 1-18 have higher rolling cycles and higher capacity retention after 400 battery cycles. Compared with Comparative Example 2, the batteries of Examples 1-18 have higher capacity retention after 400 battery cycles. This indicates that the positive electrode sheets of the batteries in Examples 1-18 use materials with a glass transition temperature of -50℃≤T. g Acrylic polymers with a temperature ≤25℃ can effectively improve the flexibility of the positive electrode and the cycle performance of the battery. While the positive electrodes of Comparative Examples 1 and 3 possess suitable adhesion and cohesion, their poor flexibility leads to deterioration in battery cycle performance. Although the positive electrode of Comparative Example 2 has suitable adhesion and flexibility, its glass transition temperature is too low. This causes the electrode to stick together at room temperature or battery operating temperature, affecting the battery's cycle performance. Furthermore, the electrode exhibits roller sticking during the coating and cold pressing stage, indicating manufacturing problems and resulting in a low yield rate.

[0247] 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.

[0248] 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, the positive electrode sheet comprising a positive electrode active material layer, the positive electrode active material layer comprising a positive electrode active material and an acrylate polymer, wherein the glass transition temperature of the acrylate polymer is denoted as T. g -50℃≤T g At ≤25℃, the acrylate polymer accounts for 0.5% to 2% of the mass of the positive electrode active material layer.

2. The lithium-ion secondary battery according to claim 1, wherein -40℃≤T g ≤25℃。 3. The lithium-ion secondary battery according to claim 1 or 2, wherein The acrylate-based polymer comprises a first structural unit, the structural formula of which includes R1 to R3 are each independently selected from any one of hydrogen, substituted or unsubstituted C1 to C8 saturated alkyl groups, substituted or unsubstituted C3 to C8 cycloalkyl groups, substituted or unsubstituted heterocyclic alkylene groups, acid anhydrides, hydroxyl groups, carboxyl groups, and carboxyl salts. R4 is selected from any one of hydrogen, substituted or unsubstituted C1 to C8 saturated alkyl groups, substituted or unsubstituted C3 to C8 cycloalkyl groups, substituted or unsubstituted heterocyclic alkylene groups, and metal cations. * represents the bonding position.

4. The lithium-ion secondary battery according to claim 3, wherein The substituted C1-C8 saturated alkyl groups, substituted C3-C8 cycloalkyl groups, and substituted heterocyclic alkylene groups in R1-R3 and R4 are each independently selected from at least one of ester, carbonyl, amide, and acid anhydride groups; and / or, The metal cation includes at least one of sodium ions and potassium ions.

5. The lithium-ion secondary battery according to claim 3 or 4, wherein The mass percentage of the first structural unit in the acrylate polymer is denoted as M11, where M11 satisfies: 0 < M11 ≤ 100%.

6. The lithium-ion secondary battery according to any one of claims 3 to 5, wherein The acrylate-based polymer further comprises a second structural unit, the structural formula of which comprises R5 to R8 are each independently selected from hydrogen, substituted or unsubstituted C1 to C8 saturated alkyl groups, and * represents the bonding position.

7. The lithium-ion secondary battery according to claim 6, wherein The acrylate-based polymer further comprises a third structural unit, the structural formula of which includes * represents a bonding position.

8. The lithium-ion secondary battery according to claim 6 or 7, wherein The mass percentage of the second structural unit in the acrylate polymer is denoted as M12, and M12 satisfies: 65% ≤ M12 < 100%.

9. The lithium-ion secondary battery according to claim 8, wherein 70％≤M12≤90％。 10. The lithium-ion secondary battery according to any one of claims 7 to 9, wherein The mass percentage of the first structural unit in the acrylate polymer is denoted as M11, and the mass percentage of the third structural unit in the acrylate polymer is denoted as M13. M11 and M13 satisfy the following conditions: 0 < M11 + M13 ≤ 35%, 0 ≤ M13 ≤ 5%.

11. The lithium-ion secondary battery according to claim 10, wherein 10% ≤ M11 + M13 ≤ 30%.

12. The lithium-ion secondary battery according to any one of claims 1 to 11, wherein The acrylate polymers include at least one of the following: copolymers of a first acrylate monomer and a second acrylate monomer; olefin-acrylate copolymers; vinyl acetate-acrylate copolymers; olefin-acrylate-maleic anhydride terpolymers; olefin-acrylate-glycidyl methacrylate terpolymers; maleic anhydride-grafted olefin-acrylate copolymers; methacrylic acid glycidyl acrylate-grafted olefin-acrylate copolymers; olefin-acrylate-2-octenyl succinic anhydride terpolymers; and olefin-acrylate-(2-methyl-2-propene)succinic anhydride terpolymers.

13. The lithium-ion secondary battery according to claim 12, wherein The first acrylate monomer includes at least one selected from n-butyl acrylate, isobutyl acrylate, ethyl acrylate, n-octyl acrylate, isooctyl acrylate, and isooctyl methacrylate; and / or, The second acrylate monomer includes at least one selected from methyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, isobornyl methacrylate, and cyclohexyl methacrylate; and / or, The olefin includes at least one selected from ethylene, propylene, and 1-butene; and / or, The acrylates include at least one of methyl acrylate, ethyl acrylate, methyl methacrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, n-hexyl acrylate, and isooctyl acrylate.

14. The lithium-ion secondary battery according to any one of claims 1 to 13, wherein The weight-average molecular weight of the acrylate polymer is 50,000 to 500,000.

15. The lithium-ion secondary battery according to claim 14, wherein The acrylate polymers include blends of a first acrylate polymer and a second acrylate polymer, wherein the weight-average molecular weight of the first acrylate polymer is 50,000 to 200,000, and the weight-average molecular weight of the second acrylate polymer is 300,000 to 500,000.

16. The lithium-ion secondary battery according to any one of claims 1 to 15, wherein The acrylate polymer accounts for 1% to 2% of the mass of the positive electrode active material layer.

17. The lithium-ion secondary battery according to any one of claims 1 to 16, wherein The positive electrode active material includes lithium phosphate.

18. The lithium-ion secondary battery according to claim 17, wherein The lithium-containing phosphate includes those with the chemical formula Li m A a Fe x D d P y E e O z G g The material, wherein A includes at least one element selected from Al, Na, K, or Mg; D includes at least one element selected from Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, Ti, or V; E includes at least one element selected from B, S, Si, or N; and G includes at least one element selected from S, F, Cl, or Br; wherein 0.5 ≤ m ≤ 1.5; 0 ≤ a ≤ 0.1; 0.5 ≤ x ≤ 1; 0 ≤ d ≤ 0.5; 0.5 ≤ y ≤ 1; 0 ≤ e ≤ 0.5; 3.5 ≤ z ≤ 4; and 0 ≤ g ≤ 0.

5.

19. The lithium-ion secondary battery according to any one of claims 1 to 18, wherein The compacted density of the positive electrode plate is ≥ 2.6 g / cm 3 .

20. The lithium-ion secondary battery according to claim 19, wherein, The compacted density of the positive electrode plate is 2.6 g / cm 3 ~ 2.8 g / cm 3 .

21. The lithium-ion secondary battery according to any one of claims 1 to 20, wherein The lithium-ion secondary battery is a wound battery.

22. An electrical device comprising a lithium-ion secondary battery as described in any one of claims 1 to 21.

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