Binder softener, positive electrode sheet, and electrochemical device

Abstract

The application provides a flexibilizer for a binder, a positive electrode sheet and an electrochemical device. The flexibilizer comprises a copolymer, the number average molecular weight of the copolymer is 200-100000, the glass transition temperature Tg of the copolymer is-50-10 DEG C, the main chain of the copolymer comprises a first structural unit from a polyol monomer and a second structural unit from a carbonate monomer, and the end of the copolymer at both ends comprises a polar group. The positive electrode sheet prepared by using the flexibilizer for the binder has good flexibility, can effectively inhibit the problems of positive electrode sheet coating cracking, roll pressing belt breaking, winding light transmission and the like, and can also improve the compaction density of the positive electrode sheet, thereby improving the energy density of the lithium ion battery.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, specifically relating to a binder flexible agent, a positive electrode sheet, and an electrochemical device. Background Technology

[0002] The cathode is a crucial component of lithium-ion batteries. To improve the energy density of the cathode, there are currently three main development directions: first, increasing the specific capacity and operating voltage platform of the active material; second, increasing the mass proportion of active material in the electrode sheet while reducing the amount of auxiliary materials such as binders and conductive agents; and third, increasing the compaction density or coating thickness of the electrode sheet. Among these, improvements in the specific capacity and operating voltage platform of the active material have almost reached a bottleneck; the application of new binders and conductive agents has gradually increased the mass proportion of active material in the electrode sheet, but this has also approached its limit. Therefore, cell manufacturers are currently focusing on improving the compaction density and coating thickness of the electrode sheet as key areas for enhancing the energy density of the cathode.

[0003] However, with the increase in compaction density and coating thickness, the brittleness of the positive electrode sheet increases significantly, and the process faces difficult problems such as coating cracking, roll breakage, and light transmission during winding.

[0004] The positive electrode sheet is mainly composed of active material, conductive agent, binder, and dispersant. Among them, polyvinylidene fluoride (PVDF) is commonly used as the binder, accounting for 1% to 3% of the electrode sheet by mass. The positive electrode binder is usually a semi-crystalline polymer with a high modulus, resulting in poor flexibility and being the main source of the electrode sheet's brittleness.

[0005] To reduce the brittleness and increase the flexibility of the positive electrode binder, one approach is to modify it through copolymerization, grafting, or blending. However, this method is costly, complex, and may introduce other adverse effects, such as increased electrolyte swelling and decreased thermal stability. Another approach involves adding a flexible agent to the electrode. The flexible agent embeds itself between the polymer chains of the binder, weakening interchain interactions and reducing its crystallinity. This second approach is simpler and has better compatibility.

[0006] However, while existing flexible agents can improve the flexibility of the positive electrode sheet and increase the compaction density or coating thickness, some of these materials are triphenylbenzene derivatives with a strong main structure, which limits their effect on reducing binder brittleness. Furthermore, these materials have fewer groups that interact with the binder, resulting in weaker interaction forces and a generally limited effect on reducing binder crystallinity. Summary of the Invention

[0007] To address the problems existing in current flexible agents, this invention provides a flexible agent for adhesives, a positive electrode sheet, and an electrochemical device.

[0008] The objective of this invention is achieved through the following technical solution.

[0009] In a first aspect, the present invention provides a flexible agent for adhesives, the flexible agent comprising a copolymer having a number-average molecular weight of 200 to 100,000, a glass transition temperature (Tg) of -50 to 10°C, the main chain of the copolymer comprising a first structural unit derived from a polyol monomer and a second structural unit derived from a carbonate monomer, and the ends of the copolymer comprising polar groups.

[0010] Furthermore, the polar group is selected from -OH, -COOH or the salt formed after neutralization, -PO3H or the salt formed after neutralization, -SO3H or the salt formed after neutralization, -NH2, -NHCH3, -NH4 + At least one of -CN and -CONH2.

[0011] Furthermore, in adhesives for polyvinylidene fluoride, polyacrylic acid, and polyacrylate, the polar group is selected from at least one of -OH, -COOH, and -NH2; in adhesives for polyacrylonitrile, the polar group is selected from -CN; and in adhesives for polyimide, the polar group is selected from at least one of -COOH and -CONH2.

[0012] Furthermore, the mass ratio of the polyol monomer to the carbonate monomer is (40~85):(15~60).

[0013] Furthermore, the polyol monomer includes a diol and an oligomer containing a terminal hydroxyl group, wherein the mass ratio of the diol to the oligomer is (2~20):(20~83); the carbonate monomer is selected from at least one of dimethyl carbonate, dibutyl carbonate, and diphenyl carbonate.

[0014] Furthermore, the structure of the polyol monomer also includes at least one of the following: a five-membered alicyclic group, a six-membered alicyclic group, a fluorine substituent, a thioether group, a disulfide bond, a secondary amine group, and a siloxane group.

[0015] Furthermore, the number average molecular weight of the copolymer of the softening agent is 1000~12000, and the glass transition temperature Tg of the copolymer of the softening agent is -50~-20℃.

[0016] In a second aspect, the present invention provides a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector; the positive active material layer comprising a positive active substance, a binder, a conductive agent and a flexible agent as described in the first aspect; the binder is selected from at least one of polyvinylidene fluoride, polyacrylonitrile, polyimide, polyacrylic acid, and polyacrylate.

[0017] Furthermore, the mass ratio of the positive electrode active material, binder, conductive agent, and flexible agent is (95~97):(1~2):(1~2):(0.05~1). Correspondingly, the mass of the flexible agent is 2.5~50% of the mass of the binder.

[0018] Thirdly, the present invention provides an electrochemical device comprising a positive electrode as described in the second aspect.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The copolymer of the binder softener of this invention has a low Tg and moderate molecular weight, exhibiting good flowability and easily embedding between binder polymer chains, weakening inter-chain interactions and reducing its crystallinity. Simultaneously, the main chain of the binder softener structure of this invention is a linear saturated alkyl chain, with excellent structural flexibility, effectively reducing binder brittleness. Through hydrogen bonds or dipole interactions formed between the end polar groups of the softener structure and the binder, the interaction between the softener and the binder is strengthened, allowing the softener to embed between the binder polymer chains and hindering binder crystallization. Furthermore, the steric hindrance of the carbonate groups in the main chain further hinders the crystallization of the binder polymer chains, improving its flexibility.

[0021] The positive electrode sheet prepared by using this binder and flexible agent has good flexibility, which can effectively suppress problems such as coating cracking, roll breakage, and light transmission during winding of the positive electrode sheet. It can also improve the compaction density of the positive electrode sheet, thereby improving the energy density of lithium-ion batteries. Detailed Implementation

[0022] To make the technical problems solved, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the embodiments described herein are only some, not all, of the embodiments of this invention, and are merely illustrative and not intended to limit the invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the protection scope of this invention.

[0023] In this invention, as is known to those skilled in the art of chemical synthesis, each structural unit represents the structural portion of the corresponding monomer present in the resulting polymer after the monomer participates in the polymerization reaction. The mass ratio of each structural unit is the mass ratio of the monomers providing each structural unit.

[0024] In a first aspect, the present invention provides a flexible agent for adhesives, the flexible agent comprising a copolymer having a number-average molecular weight of 200 to 100,000, a glass transition temperature (Tg) of -50 to 10°C, the main chain of the copolymer comprising a first structural unit derived from a polyol monomer and a second structural unit derived from a carbonate monomer, and the ends of the copolymer comprising polar groups.

[0025] In some specific embodiments, the polar group is selected from -OH, -COOH or the salt formed after neutralization, -PO3H or the salt formed after neutralization, -SO3H or the salt formed after neutralization, -NH2, -NHCH3, -NH4 + At least one of -CN, -CONH2; the polar group can form hydrogen bonds or dipole interactions with the binder.

[0026] In some specific embodiments, the polar group is derived from phosphorus pentoxide, thionyl chloride, maleic anhydride, acrylonitrile, butanediol, tetrafluorobutanediol, bisphenol A, diethanolamine, 1,4-cyclohexanediol, and 2,2'-dithiodiethanol.

[0027] In this invention, the polar groups at both ends of the copolymer can be the same or different. In some specific embodiments, the polar groups at both ends of the copolymer are different for different adhesives. For adhesives used with polyvinylidene fluoride, polyacrylic acid, and polyacrylate, the polar group is selected from at least one of -OH, -COOH, and -NH2; for adhesives used with polyacrylonitrile, the polar group is selected from -CN; for adhesives used with polyimide, the polar group is selected from at least one of -COOH and -CONH2. This results in a stronger interaction between the polar groups at the ends of the copolymer and the adhesive molecular chains. For example, a -OH end group can form a hydrogen bond with the CF bond of polyvinylidene fluoride; a -CN end group can form a dipole interaction with the -CN bond of the polyacrylonitrile adhesive.

[0028] In some specific embodiments, the mass ratio of the polyol monomer to the carbonate monomer is (40~85):(15~60).

[0029] In some specific embodiments, the polyol monomer comprises a diol and an oligomer containing terminal hydroxyl groups, wherein the mass ratio of the diol to the oligomer is (2~20):(20~83). The oligomer is selected from at least one of polyether polyols, polyester polyols, or polyolefin polyols. The carbonate monomer is selected from at least one of dimethyl carbonate, dibutyl carbonate, and diphenyl carbonate.

[0030] In some specific embodiments, the structure of the polyol monomer further includes at least one of five-membered alicyclic groups, six-membered alicyclic groups, fluorine substituents, thioether groups, disulfide bonds, secondary amine groups, and siloxane groups. By adjusting the structure of the polyol monomer, these groups can be introduced into the flexible agent molecule structure, thereby exhibiting different functionalities. Heteroatoms have larger bond lengths and bond angles, and possess lower internal rotation barriers. Introducing heteroatoms onto the chain can further improve the flexibility of the molecular chain and enhance flowability. Alicyclic groups have greater steric hindrance, which can more effectively disrupt the crystallinity of PVDF binders and improve the flexible effect. By adjusting the molecular structure, the requirements of different formulations can be met.

[0031] More specifically, the oligomer can preferably be polyethylene glycol 400, diethylene glycol, polyethylene glycol 1000, polyester polyol 1000, or hydroxyl silicone oil Mn-550, etc. The diol can preferably be butanediol, tetrafluorobutylene glycol, bisphenol A, diethanolamine, 1,4-cyclohexanediol, or 2,2'-dithiodiethanol, etc.

[0032] In this invention, the number-average molecular weight of the copolymer is 200 to 100,000. If the number-average molecular weight is too high, the flowability is poor, making it difficult to embed between binder chains and thus failing to inhibit binder crystallization. For copolymers satisfying the corresponding structure of this invention, the number-average molecular weight is as small as possible, preferably 1,000 to 12,000.

[0033] In this invention, the glass transition temperature (Tg) of the copolymer is -50 to 10°C; more specifically, Tg can be -50°C, -46°C, -41°C, -35°C, -30°C, -27°C, -22°C, -18°C, -14°C, -10°C, -5°C, 0°C, or 10°C, etc. If the Tg is too high, the copolymer itself has poor flexibility, and its effect on softening the binder and electrode is limited. Therefore, the preferred Tg of the copolymer is -50 to -20°C.

[0034] The adhesive softener of this invention comprises a copolymer whose repeating unit structure includes ether, ester, and carbonate structural units, obtained through transesterification of a polyol monomer and a carbonate monomer under the action of an alkaline catalyst. The alkaline catalyst may be sodium methoxide, sodium ethoxide, tetrabutyl titanate, lithium acetylacetonate, cesium carbonate, lithium acetylacetonate, etc. The amount of the alkaline catalyst is 0.02~0.65 wt% of the total mass of the polyol monomer and the carbonate monomer.

[0035] The binder softener of this invention can be prepared by the following method: According to the aforementioned monomer mass ratio and the amount of alkaline catalyst, first add the polyol monomer and alkaline catalyst to the reaction vessel. After purging with nitrogen for 10-20 minutes, heat to 90-105°C under thorough stirring. Then, add the carbonate monomer dropwise to the reaction vessel under a nitrogen atmosphere over 1-3 hours. After the addition is complete, heat to 105-120°C and react for 4-6 hours to obtain the prepolymer. Then, slowly heat to 140-160°C to distill off excess monomer and most of the alcohol byproducts. During the polycondensation stage, use a vacuum water pump to reduce the pressure inside the reactor and raise the temperature to 190-210°C over 4-5 hours, maintain this temperature for 2-4 hours, and then cool down to discharge the product. Dissolve the product in dichloromethane and separate it in methanol; after 2-4 cycles, vacuum dry the product to obtain the final product.

[0036] Secondly, the present invention provides a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector; the positive active material layer comprises a positive active substance, a binder, a conductive agent, and a flexibility agent as described in the first aspect. The binder is selected from at least one of polyvinylidene fluoride, polyacrylonitrile, polyimide, polyacrylic acid, and polyacrylate. The binder is preferably polyvinylidene fluoride, which has good flexibility and peel strength.

[0037] In some specific embodiments, the positive electrode active material is one or more of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt aluminum oxide, lithium manganese oxide, and lithium nickel cobalt oxide. The conductive agent is a commonly used conductive material in the art, such as conductive carbon black SP, carbon nanotubes (CNTs), etc.

[0038] In some specific embodiments, the mass ratio of the positive electrode active material, binder, conductive agent, and flexible agent is (95~97):(1~2):(1~2):(0.05~1). Correspondingly, the mass of the flexible agent is 2.5~50% of the mass of the binder. Too little flexible agent results in poor flexibleization, while too much reduces the electrode peel strength, failing to meet usage requirements. Preferably, the amount of flexible agent added is 10~40% of the mass of the binder.

[0039] More specifically, the mass ratio of the positive electrode active material, binder, conductive agent, and flexible agent can be 95:2:2:1, 95:2:2:0.5, 95:1.5:1.5:0.5, 95.5:2:2:0.5, 96:2:1.5:0.5, 96:1.5:2:0.5, 96:2:1.95:0.05, 96.5:1:2:0.5, 96.5:2:1:0.5, 97:1.5:1.45:0.05, 97:1:1.95:0.05, or 97:1.95:1:0.05, etc. The added mass of the flexible agent is 2.5%, 6%, 9%, 13%, 17%, 22%, 35%, 40%, 43%, 47%, or 50% of the added mass of the binder, etc.

[0040] The method for preparing the positive electrode sheet in this invention includes at least the following steps: dispersing positive electrode active material, binder, conductive agent, and flexible agent in a non-aqueous solvent (such as NMP) in a certain proportion, stirring and mixing evenly to obtain a positive electrode slurry with a solid content of 60-76 wt% and a viscosity of 3000-8000 mPa·s. Then, uniformly coating the prepared positive electrode slurry onto one surface of a positive electrode current collector, drying it to obtain a positive electrode sheet with one side coated with positive electrode active material. The above steps are then repeated on the other surface of the same positive electrode current collector to obtain a positive electrode sheet with positive electrode active material coated on both sides.

[0041] Thirdly, the present invention provides an electrochemical device comprising a positive electrode as described in the second aspect.

[0042] In addition, it includes a negative electrode, an electrolyte, and a separator; the electrolyte includes an organic solvent, an electrolyte lithium salt, and additives. The electrochemical device of this invention can include any device in which an electrochemical reaction occurs, and specific examples include all types of primary or secondary batteries. In particular, the electrochemical device is a lithium secondary battery, including lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0043] The specific embodiments of the present invention will be further explained and described below through examples and comparative examples.

[0044] Unless otherwise specified, all reagents, materials, and instruments used in the following description are conventional reagents, materials, and instruments, and are commercially available. The reagents involved can also be synthesized using conventional methods. Unless otherwise specified, the methods in the examples and comparative examples are conventional methods in the art. Unless otherwise stated, all parts, percentages, and ratios listed below are by weight, and all raw materials used are commercially available or synthesized using conventional methods.

[0045] Example 1

[0046] 1) Preparation of the adhesive softener: 65.40 parts by weight of polyethylene glycol 400, 4.91 parts by weight of butanediol, and 0.23 parts by weight of lithium acetylacetonate were added to a reaction vessel. After purging with nitrogen for 15 min, the mixture was heated to 100°C with thorough stirring. 29.46 parts by weight of dimethyl carbonate were added dropwise to the reaction vessel over a nitrogen atmosphere over 2 h. After the addition was complete, the temperature was raised to 110°C, and the reaction was carried out for 5 h to obtain the prepolymer. Then, the temperature was slowly raised to 150°C to distill off excess monomers and most of the alcohol byproducts. During the polycondensation stage, the pressure inside the reactor was reduced using a vacuum water pump, and the temperature was raised to 200°C over 4-5 h and maintained for 3 h before cooling and discharging. The product was dissolved in dichloromethane and separated in methanol. After three cycles, the product was vacuum dried to obtain the final product.

[0047] 2) Preparation of the positive electrode sheet: Lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP, and the softening agent prepared above were dispersed in NMP solvent at a mass ratio of 95.95:2:2:0.05 and stirred until homogeneous, resulting in a positive electrode slurry with a solid content of 61 wt% and a viscosity of 6000 mPa·s. The prepared positive electrode slurry was then uniformly coated onto one surface of a 9 μm thick aluminum foil used as a positive electrode current collector, and dried at 120°C to obtain a single-sided positive electrode sheet with an 80 μm thick positive electrode active material layer. The above steps were then repeated on the other surface of the aluminum foil to obtain a double-sided positive electrode sheet coated with positive electrode active material. The coated positive electrode sheet was then cold-pressed and cut into sheets for later use.

[0048] 3) Preparation of electrochemical devices (lithium-ion batteries)

[0049] The prepared positive electrode sheet, separator, and negative electrode sheet are stacked sequentially and then wound to obtain an electrode assembly. After the tabs are welded, the electrode assembly is placed in an aluminum-plastic film, dried to remove moisture, and then injected with electrolyte. After vacuum sealing, settling, formation, capacity testing, and shaping, a lithium-ion battery is obtained. In this invention, unless otherwise specified, the preparation can be carried out using conventional techniques in the art.

[0050] Examples 2-12

[0051] The preparation methods of the binder flexible agent in Examples 2-12 are the same as those in Example 1, and the preparation processes of the positive electrode sheet and lithium-ion battery are also the same as those in Example 1. The difference is that the formulation composition of the flexible agent is different, and the monomers and catalysts used and their mass fractions are shown in Table 1.

[0052] Table 1

[0053]

[0054] Examples 13-20

[0055] The formulation and preparation method of the binder flexible agent in Examples 13-20 are the same as those in Example 1, and the preparation process of the positive electrode sheet and lithium-ion battery is also the same as that in Example 1. The difference is that in the preparation of the positive electrode sheet, the mass ratio of the positive active material or lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP to the flexible agent is different.

[0056] Example 13

[0057] In the preparation of the positive electrode sheet, the positive active material is lithium cobalt oxide.

[0058] Example 14

[0059] In the preparation of the positive electrode sheet, the positive active material is lithium nickel cobalt manganese oxide (NCM622).

[0060] Example 15

[0061] In the preparation of the positive electrode sheet, the mass ratio of lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP and flexible agent is 95.9:2:2:0.1.

[0062] Example 16

[0063] In the preparation of the positive electrode sheet, the mass ratio of lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP and flexible agent is 95.8:2:2:0.2.

[0064] Example 17

[0065] In the preparation of the positive electrode sheet, the mass ratio of lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP and flexible agent is 95.6:2:2:0.4.

[0066] Example 18

[0067] In the preparation of the positive electrode sheet, the mass ratio of lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP and flexible agent is 95.4:2:2:0.6.

[0068] Example 19

[0069] In the preparation of the positive electrode sheet, the mass ratio of lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP and flexible agent is 95.2:2:2:0.8.

[0070] Example 20

[0071] In the preparation of the positive electrode sheet, the mass ratio of lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP and flexible agent is 96:2:2:1.

[0072] Comparative Example 1

[0073] Without adding a flexibility enhancer, in the preparation of the positive electrode sheet, the mass ratio of lithium iron phosphate, polyvinylidene fluoride, and conductive agent is 96:2:2, and the preparation process of the positive electrode sheet is the same as in Example 1; the preparation process of the lithium-ion battery is also the same as in Example 1.

[0074] Comparative Example 2

[0075] Without adding a flexibility enhancer, in the preparation of the positive electrode sheet, the mass ratio of lithium cobalt oxide, polyvinylidene fluoride, and conductive agent is 96:2:2, and the preparation process of the positive electrode sheet is the same as in Example 1; the preparation process of the lithium-ion battery is also the same as in Example 1.

[0076] Comparative Example 3

[0077] Without adding a flexibility enhancer, in the preparation of the positive electrode sheet, the mass ratio of lithium nickel cobalt manganese oxide (NCM622), polyvinylidene fluoride, and conductive agent is 96:2:2, and the preparation process of the positive electrode sheet is the same as in Example 1; the preparation process of the lithium-ion battery is also the same as in Example 1.

[0078] Comparative Examples 4-7

[0079] The preparation methods of the binder flexible agent in Comparative Examples 4-7 are the same as those in Example 1, and the preparation processes of the positive electrode sheet and lithium-ion battery are also the same as those in Example 1; the difference is that the formulation composition of the flexible agent is different, and the monomers and catalysts used and their mass fractions are shown in Table 2.

[0080] Table 2

[0081]

[0082] Performance testing:

[0083] The flexibility enhancer, positive electrode sheet, and lithium-ion battery prepared in the above examples and comparative examples were subjected to the following performance tests.

[0084] [Number-average molecular weight]: Refer to standard SH / T 1759-2007, and use gel permeation chromatography to test the softening agent.

[0085] [Glass Transition Temperature Tg]: Differential scanning calorimetry (DSC) is used. The softening agent and reference material are placed in the sample and reference pans of the DSC instrument, respectively, and scanned at a certain heating rate. The instrument records the power difference as a function of temperature, i.e., the DSC curve. The glass transition is represented by a baseline shift on the DSC curve, and Tg can be determined by extrapolating the baseline.

[0086] [Flexibility]: Using the needle winding method, the electrode sheet is wound onto the needle at a uniform speed, ensuring the positive electrode sheet adheres tightly to the needle surface. After winding, observe the surface of the positive electrode sheet for cracks, peeling, or loss of active material. Repeat the above steps using needles of different diameters, generally starting with larger diameter needles and gradually decreasing the diameter, to test the flexibility of the positive electrode sheet under different degrees of bending. By comparing the winding behavior of the positive electrode sheet under different needle diameters, the flexibility limit of the electrode sheet can be evaluated. The smaller the needle diameter, the better the flexibility.

[0087] [Adhesion Strength]: Take the cold-pressed positive electrode sheet and punch it using a mold to obtain a test strip with a length of 100mm and a width of 20mm. Clean the surface of the steel plate with alcohol, and attach double-sided tape with a length of 55mm to 70mm and a width of 20mm to the steel plate. Place the test strip in the center on the double-sided tape, with the test side facing down. Use wrinkle adhesive to connect and fix one end of the test strip with a paper strip with a length of 50mm to 75mm and a width equal to that of the test strip. Push a 2kg rubber roller back and forth on the test strip 4 times by hand to obtain the test sample. Test the sample using a tensile testing machine. Fix the test sample on the test table, then fold the paper strip upwards at 90° and fix it with a clamp. Then, slowly pull the paper strip with the tensile testing machine at a speed of 10mm / min until the positive active material layer on the surface of the double-sided tape separates from the positive current collector, and the test ends. The average tensile force in the stable region is taken as the bonding force between the positive electrode active material layer and the positive electrode current collector, and the unit is N / m.

[0088] [Compacted Density]: Take the battery cell electrode sheet after capacity testing, and use a stamping die to cut 12 small round pieces. After zeroing the electronic scale, use tweezers to place the cut small round pieces on the weighing platform and record the weight of each small round piece m1, m2, m3, m4, m5, m6, m7, m8, m9, m10, m11, m12. Use a micrometer to measure the thickness of each small circular piece at four positions: h11, h12, h13, h14; h21, h22, h23, h24; h31, h32, h33, h34; h41, h42, h43, h44; h51, h52, h53, h54; h61, h62, h63, h64; h71, h72, h73, h74; h81, h82, h83, h84; h91, h92, h93, h94; h101, h102, h103, h104; h111, h112, h113, h114; h121, h122, h123, h124. The average values ​​of the four thicknesses of each small disc are h1, h2, h3, h4, h5, h6, h7, h8, h9, h10, h11, and h12. The average compaction density of each small disc is calculated using a formula, and the average value of the compaction density of the 12 small discs is recorded as the compaction density of each embodiment or comparative example.

[0089] The performance test results of each embodiment and comparative example are shown in Table 3.

[0090] Table 3

[0091]

[0092] As can be seen from the test results of Examples 1-12, 15-20 and Comparative Example 1, Example 13 and Comparative Example 2, and Example 14 and Comparative Example 3, after adding the flexible agent for the binder of the present invention, the flexibility of lithium iron phosphate, lithium cobalt oxide, and lithium nickel cobalt manganese oxide ternary cathode sheets is improved, and the compaction density and adhesion are increased.

[0093] As can be seen from the test results of Examples 1-12 and Comparative Examples 4-7, when the composition of the flexible agent is outside the preferred range, the glass transition temperature is too high (Comparative Examples 5 and 6), and the prepared positive electrode sheet has varying degrees of adverse effects on flexibility, compaction density, and adhesion. When the mass ratio of polyol monomer to carbonate monomer is (40-85):(15-60) and the mass ratio of diol to oligomer is (2-20):(20-83), the electrode sheet has better performance.

[0094] As can be seen from the test results of Examples 15-20, as the amount of flexible agent increases, the flexibility and compaction density of the positive electrode sheet increase accordingly, and the adhesion force shows a trend of first increasing and then decreasing.

[0095] The present invention has been further described above with reference to specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the nature and scope of the present invention. Various modifications made to the above embodiments by those skilled in the art after reading this specification are all within the scope of protection of the present invention.

Claims

1. A softening agent for adhesives, characterized in that, The binder is selected from at least one of polyvinylidene fluoride, polyacrylonitrile, polyimide, polyacrylic acid, and polyacrylate; the flexible agent comprises a copolymer, the number average molecular weight of the copolymer is 1000~12000, the glass transition temperature Tg of the copolymer is -50~-20℃, the main chain of the copolymer comprises a first structural unit derived from a polyol monomer and a second structural unit derived from a carbonate monomer, and the ends of the copolymer comprise polar groups. The mass ratio of the polyol monomer to the carbonate monomer is (40~85):(15~60). The polyol monomer comprises a diol and an oligomer containing terminal hydroxyl groups, wherein the mass ratio of the diol to the oligomer is (2~20):(20~83), the oligomer is selected from polyethylene glycol 400, polyethylene glycol 1000, polyester polyol 1000 or hydroxyl silicone oil Mn-550, the diol is selected from butanediol, tetrafluorobutylene glycol, bisphenol A, diethanolamine, 1,4-cyclohexanediol or 2,2'-dithiodiethanol; and the carbonate monomer is selected from at least one of dimethyl carbonate, dibutyl carbonate or diphenyl carbonate.

2. The adhesive softener according to claim 1, characterized in that, The polar group is selected from -OH, -COOH or the salt formed after neutralization, -PO3H or the salt formed after neutralization, -SO3H or the salt formed after neutralization, -NH2, -NHCH3, -NH4 + At least one of -CN and -CONH2.

3. The adhesive softener according to claim 2, characterized in that, For adhesives used with polyvinylidene fluoride, polyacrylic acid, and polyacrylate, the polar group is selected from at least one of -OH, -COOH, and -NH2; for adhesives used with polyacrylonitrile, the polar group is selected from -CN; for adhesives used with polyimide, the polar group is selected from at least one of -COOH and -CONH2.

4. A positive electrode sheet, characterized in that, The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector; the positive active material layer includes a positive active substance, a binder, a conductive agent and a flexible agent as described in any one of claims 1 to 3; the binder is selected from at least one of polyvinylidene fluoride, polyacrylonitrile, polyimide, polyacrylic acid, and polyacrylate.

5. The positive electrode sheet according to claim 4, characterized in that, The mass of the softening agent is 2.5 to 50% of the mass of the adhesive.

6. An electrochemical device, characterized in that, Including the positive electrode sheet as described in claim 4 or 5.

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