Softening agent for binder, positive pole piece and electrochemical device

Abstract

The invention provides a softening agent for a binder, a positive pole piece and an electrochemical device. The softening agent 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 parts of the two ends of the copolymer comprise polar groups. The positive pole piece prepared by adopting the softening agent for the binder has good flexibility, can effectively restrain the problems of coating cracking, rolling belt breaking, winding light transmission and the like of the positive pole piece, and also can improve the compaction density of the positive pole piece, so that the energy density of a lithium ion battery is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of battery materials, and particularly relates to a softener for a binder, a positive electrode sheet and an electrochemical device. BACKGROUND

[0002] The positive electrode is an important component of a lithium ion battery. In order to improve the energy density of the positive electrode, there are currently three development directions: firstly, improving the specific capacity and working voltage platform of the active material; secondly, improving the mass fraction of the active material in the electrode sheet and reducing the amount of auxiliary materials such as the binder and the conductive agent; and thirdly, improving the compaction density or coating thickness of the electrode sheet. Among them, the improvement of the specific capacity and working voltage platform of the active material has almost reached a bottleneck; the application of new binders and conductive agents has gradually improved the mass fraction of the active material in the electrode sheet, but the current limit has also been reached. Therefore, at present, the electrode sheet manufacturers focus on improving the compaction density and coating thickness of the electrode sheet as the key research direction for improving the energy density of the positive electrode.

[0003] However, with the increase of the compaction density and coating thickness, the brittleness of the positive electrode sheet is significantly increased, and the process is faced with problems such as coating cracking, roll pressing belt breaking and winding light transmission, which are difficult to solve.

[0004] The positive electrode sheet is mainly composed of an active material, a conductive agent, a binder and a dispersing agent. Among them, the binder commonly used is polyvinylidene fluoride (PVDF), and the mass fraction in the electrode sheet is 1% to 3%. The positive electrode binder is usually a semi-crystalline polymer with high modulus, and therefore poor flexibility, which is the main source of brittleness of the electrode sheet.

[0005] In order to reduce the brittleness of the positive electrode binder and increase its flexibility, one solution is to modify the binder by copolymerization, grafting, blending and the like, but the cost is high, the process is complex, and other adverse effects may be brought, such as increased electrolyte swelling and poor thermal stability. Another solution is to add a softener to the electrode sheet, and the role of the softener is to embed between the high molecular chains of the binder, weaken the interaction between the chains and reduce the crystallinity of the binder. The second solution is simple in process and good in compatibility.

[0006] However, although the existing disclosed softeners can improve the flexibility of the positive electrode sheet and increase the compaction density or coating thickness of the electrode sheet, some materials are triphenylbenzene derivatives with strong rigidity of the main structure, and the effect of reducing the brittleness of the binder is limited; and the materials have fewer groups interacting with the binder and weaker interaction force with the binder, and the effect of reducing the crystallinity of the binder is general. SUMMARY

[0007] In order to solve the problems existing in the existing softeners, the application provides a softener for a binder, a positive electrode sheet and an electrochemical device.

[0008] The object of the application is achieved by the following technical solutions.

[0009] In a first aspect, the present application provides a flexibilizer for a binder, the flexibilizer comprising a copolymer, the copolymer having a number average molecular weight of 200-100000, the copolymer having a glass transition temperature Tg of -50-10℃, the copolymer having a main chain comprising a first structural unit from a polyol monomer and a second structural unit from a carbonate monomer, and the copolymer having end groups at both ends comprising a polar group.

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

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

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

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

[0014] Further, the structure of the polyol monomer further comprises at least one of 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] Further, the number average molecular weight of the copolymer of the flexibilizer is 1000-12000, the glass transition temperature Tg of the copolymer of the flexibilizer is -50--20℃, and the boiling point of the flexibilizer is 250-450℃.

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

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

[0018] In a third aspect, the present application provides an electrochemical device comprising the positive electrode tab as described in the second aspect.

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

[0020] The copolymer of the flexibilizing agent for the binder has a low Tg and a moderate molecular weight, has good fluidity, is easy to embed between the high molecular chains of the binder, weakens the interaction between the chains, and reduces the crystallinity. Meanwhile, the main chain of the flexibilizing agent for the binder is a linear saturated alkyl chain, and the main structure is also very flexible, which can effectively reduce the brittleness of the binder; the interaction between the flexibilizing agent and the binder is stronger through the hydrogen bond or dipole interaction formed by the polar groups at the ends of the flexibilizing agent structure, the flexibilizing agent can embed between the high molecular chains of the binder, and hinder the crystallization of the binder; and the crystallization of the high molecular chains of the binder is hindered by the steric hindrance of the carbonate groups in the main chain, and the flexibility of the binder is improved.

[0021] The flexibilizing agent for the binder has a high boiling point, and is retained in the positive electrode tab during the coating and drying of the positive electrode tab, and can continuously soften the binder.

[0022] The positive electrode tab prepared by using the flexibilizing agent for the binder has good flexibility, can effectively inhibit the problems of cracking during coating, belt breaking during rolling, and light transmission during winding of the positive electrode tab, and can also improve the compaction density of the positive electrode tab, thereby improving the energy density of the lithium ion battery. DETAILED DESCRIPTION

[0023] In order to make the technical problems, technical solutions and beneficial effects of the present application clearer, the present application will be further described in detail below in combination with specific embodiments. It should be understood that the embodiments described herein are part of the embodiments of the present application, rather than all the embodiments, and are used to explain the present application, but not to limit the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the present application.

[0024] In the present application, each structural unit represents the corresponding structure in the obtained polymer after the monomer participates in the polymerization reaction, as known by those skilled in the art in the field of chemical synthesis. The mass ratio of each structural unit is the mass ratio of the monomers providing each structural unit.

[0025] In a first aspect, the present application provides a flexibilizer for adhesives, the flexibilizer comprising a copolymer, the copolymer having a number average molecular weight of 200-100000, the copolymer having a glass transition temperature Tg of -50-10°C, the copolymer having a main chain comprising a first structural unit from a polyol monomer and a second structural unit from a carbonate monomer, the copolymer having end groups at both ends comprising polar groups.

[0026] In some embodiments, the polar groups are selected from at least one of -OH, -COOH or salts formed after neutralization, -PO3H or salts formed after neutralization, -SO3H or salts formed after neutralization, -NH2, -NHCH3, -NH4 + , -CN, -CONH2; the polar groups are capable of forming hydrogen bonds or dipole interactions with the adhesives.

[0027] In some embodiments, the polar groups are from phosphorous pentoxide, thionyl chloride, maleic anhydride, acrylonitrile, butanediol, tetrafluorobutanediol, bisphenol A, diethanolamine, 1,4-cyclohexanediol, 2,2'-dithiodiethanol.

[0028] In the present application, the polar groups at both ends of the copolymer can be the same or different. In some embodiments, the polar groups at both ends of the copolymer are different for different adhesives. For polyvinylidene fluoride, polyacrylic acid and polyacrylate adhesives, the polar groups are selected from at least one of -OH, -COOH, -NH2; for polyacrylonitrile adhesives, the polar groups are selected from -CN; for polyimide adhesives, the polar groups are selected from at least one of -COOH, -CONH2. In this way, the polar groups at the ends of the copolymer interact more strongly with the molecular chains of the adhesives. For example, the end groups are -OH, which can form hydrogen bonds with the C-F bonds of polyvinylidene fluoride; the end groups are -CN, which can form dipole interactions with the -CN of polyacrylonitrile adhesives.

[0029] In some embodiments, the mass ratio of the polyol monomer to the carbonate monomer is (40-85):(15-60).

[0030] In some embodiments, the polyol monomer comprises dihydric alcohol and oligomer containing terminal hydroxyl groups, the mass ratio of the dihydric alcohol to the oligomer is (2-20):(20-83). The oligomer is selected from at least one of polyether polyol, polyester polyol or polyolefin polyol. The carbonate monomer is selected from at least one of dimethyl carbonate, dibutyl carbonate, diphenyl carbonate.

[0031] In some embodiments, the structure of the polyol monomer further comprises at least one of a five-membered alicyclic group, a six-membered alicyclic group, a fluorine substituent, a sulfide group, a disulfide bond, a secondary amine group, and a siloxane group. By adjusting the structure of the polyol monomer, these groups can be introduced into the molecular structure of the flexibilizer to exert different functionalities. The bond length and bond angle of heteroatoms are larger, and the internal rotation barrier is lower. By introducing heteroatoms into the chain, the flexibility of the molecular chain can be further improved, and the flow effect can be improved. The alicyclic group has a large steric hindrance, which can more effectively destroy the crystallinity of the PVDF binder and improve the flexibilization effect. By adjusting the molecular structure, the needs of different formulations can be met.

[0032] More specifically, the oligomer can be preferably polyethylene glycol 400, diethylene glycol, polyethylene glycol 1000, polyester polyol 1000, or hydroxyl silicone oil Mn-550, etc. The diol can be preferably butanediol, tetrafluorobutanediol, bisphenol A, diethanolamine, 1,4-cyclohexanediol, or 2,2'-dithiodiethanol, etc.

[0033] In the present application, the number average molecular weight of the copolymer is 200-100000. If the number average molecular weight is too large, the flowability is poor, it is difficult to embed between the binder chains, and it cannot play a role in inhibiting the crystallization of the binder. In the case of a copolymer meeting the corresponding structure of the present application, the number average molecular weight is as small as possible, preferably 1000-12000.

[0034] In the present application, the glass transition temperature Tg of the copolymer is -50-10℃; more specifically, Tg can be -50℃, -46℃, -41℃, -35℃, -30℃, -27℃, -22℃, -18℃, -14℃, -10℃, -5℃, 0℃, or 10℃, etc. If Tg is too high, the flexibility is poor, and the flexibilization effect on the binder and the pole piece is limited. Therefore, the Tg of the copolymer is preferably -50 to -20℃.

[0035] The present application adopts a copolymer with an optimized structure, a low Tg, and a moderate molecular weight, so that the flexibilizer for the binder has a high boiling point. The boiling point of the flexibilizer is 250-450℃; more specifically, the boiling point can be 250℃, 280℃, 300℃, 315℃, 329℃, 336℃, 342℃, 350℃, 360℃, 370℃, 383℃, 396℃, 412℃, 423℃, 431℃, 440℃, or 450℃, etc. If the boiling point is too low, it is easy to volatilize in the pole piece coating and drying section, and it is difficult to remain inside the pole piece, so as to continuously exert the flexibilization effect on the binder. If the boiling point is too high, the molecular weight is too large, the flowability is poor, and it is difficult to embed between the binder chains, so as to play a role in inhibiting the crystallization of the binder. Therefore, the boiling point of the flexibilizer is preferably 280-350℃.

[0036] The softener for the binder comprises a copolymer, and a repeating unit structure of the copolymer comprises an ether, an ester and a carbonate structural unit, and is obtained by transesterification of a polyol monomer and a carbonate monomer under the action of an alkaline catalyst. The alkaline catalyst can be selected from sodium methoxide, sodium ethoxide, butyl titanate, lithium acetylacetonate, cesium carbonate, lithium acetylacetonate and the like. The amount of the alkaline catalyst is 0.02-0.65 wt% of the total mass of the polyol monomer and the carbonate monomer.

[0037] The softener for the binder can be prepared by the following method: according to the mass ratio of the monomers and the amount of the alkaline catalyst, the polyol monomer and the alkaline catalyst are first added to a reaction container, and then, after nitrogen blowing for 10-20 min, the temperature is raised to 90-105 DEG C under sufficient stirring, the carbonate monomer is added dropwise into the reaction container under a nitrogen atmosphere within 1-3 h, after the dropwise addition is completed, the temperature is raised to 105-120 DEG C, and the reaction is carried out for 4-6 h to obtain a prepolymer, then the temperature is slowly raised to 140-160 DEG C, and the excess monomer and most of the alcohol by-product are distilled off. In the polycondensation stage, the pressure in the reactor is reduced by a vacuum pump, and the temperature is raised to 190-210 DEG C within 4-5 h, and then, after being kept for 2-4 h, the temperature is lowered to discharge the product. The product is dissolved in dichloromethane and separated in methanol, and after 2-4 cycles, the product is vacuum dried to obtain the softener for the binder.

[0038] In the second aspect, the application provides a positive electrode sheet, which comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one side surface of the positive electrode current collector; the positive electrode active material layer comprises a positive electrode active material, a binder, a conductive agent and the softener for the binder 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, and the softener for the binder has better flexibility and peeling strength.

[0039] In some embodiments, the positive electrode active material is one or more of lithium cobaltate, lithium nickel cobalt manganese phosphate, lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt aluminum phosphate, lithium manganese phosphate and lithium nickel cobalt phosphate. The conductive agent is a commonly used conductive material in the art, such as conductive carbon black SP and carbon nanotube CNT.

[0040] In some embodiments, the mass ratio of the positive electrode active material, the binder, the conductive agent and the softener for the binder is (95-97):(1-2):(1-2):(0.05-1). Correspondingly, the mass of the softener for the binder is 2.5-50% of the mass of the binder. If the amount of the softener for the binder is too small, the softening effect is poor, and if the amount of the softener for the binder is too large, the peeling force of the positive electrode sheet decreases, which cannot meet the use requirements. Preferably, the amount of the softener for the binder is 10-40% of the mass of the binder.

[0041] More specifically, the mass ratio of the positive active material, the binder, the conductive agent, and the flexibilizer 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 flexibilizer can be 2.5%, 6%, 9%, 13%, 17%, 22%, 35%, 40%, 43%, 47%, or 50% of the added mass of the binder, etc.

[0042] The method for preparing the positive electrode sheet in the present application at least includes the following steps: dispersing the positive active material, the binder, the conductive agent, and the flexibilizer in a non-aqueous solvent (such as NMP) in a certain proportion, stirring and mixing them uniformly, to obtain a positive electrode slurry with a solid content of 60-76 wt% and a viscosity of 3000-8000 mPa·s. Then, the prepared positive electrode slurry is uniformly coated on one surface of the positive electrode current collector, dried, to obtain a positive electrode sheet coated with positive active material on one side, and then the above steps are repeated on the other surface of the positive electrode current collector, to obtain a positive electrode sheet coated with positive active material on both sides.

[0043] In a third aspect, the present application provides an electrochemical device comprising the positive electrode sheet according to the second aspect.

[0044] In addition, the electrochemical device also comprises a negative electrode sheet, an electrolyte, and a separator. The electrolyte comprises an organic solvent, an electrolyte lithium salt, and an additive. The electrochemical device of the present application can include any device that undergoes an electrochemical reaction, and specific examples thereof include all kinds of primary batteries or secondary batteries. In particular, the electrochemical device is a lithium secondary battery, including a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0045] The specific embodiments of the present application will be further explained by examples and comparative examples.

[0046] In the following description, the reagents, materials, and instruments used are conventional reagents, conventional materials, and conventional instruments, which are commercially available, and the reagents involved can also be synthesized by conventional synthesis methods. The methods in the examples and comparative examples are conventional methods in the art, unless otherwise specified. The parts, percentages, and ratios listed below are based on weight, and the raw materials used are commercially available or synthesized by conventional methods, unless otherwise specified.

[0047] Example 1

[0048] 1) Preparation of the flexibilizer for the binder: 65.40 parts by mass of polyethylene glycol 400, 4.91 parts by mass of butanediol, and 0.23 parts by mass of lithium acetylacetonate were added to a reaction vessel, and after purging with nitrogen for 15 min, the temperature was raised to 100°C under sufficient stirring, and 29.46 parts by mass of dimethyl carbonate was added dropwise to the reaction vessel under a nitrogen atmosphere within 2 h, after which the temperature was raised to 110°C, and the reaction was carried out for 5 h, to obtain a prepolymer; then the temperature was slowly raised to 150°C, and the excess monomer and most of the alcohol byproduct were distilled off. In the polycondensation stage, the pressure in the reactor was reduced using a vacuum pump, and the temperature was raised to 200°C within 4-5 h, and after being maintained for 3 h, the temperature was lowered to discharge the product. The product was dissolved in dichloromethane and separated in methanol. After 3 cycles, the product was vacuum dried to obtain the flexibilizer.

[0049] 2) Preparation of the positive electrode sheet: lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP, and the flexibilizer prepared above were dispersed in a solvent NMP in a mass ratio of 95.95:2:2:0.05, and stirred and mixed uniformly to obtain a positive electrode slurry with a solid content of 61wt% and a viscosity of 6000 mPa·s. Then, the prepared positive electrode slurry was uniformly coated on one surface of a positive current collector aluminum foil with a thickness of 9μm, and dried at 120°C to obtain a single-sided coated positive electrode active material positive electrode sheet with a positive electrode active material layer thickness of 80μm. Then, the above steps were repeated on the other surface of the positive current collector aluminum foil to obtain a double-sided coated positive electrode active material positive electrode sheet. The coated positive electrode sheet was cold-pressed and then cut into sheets for use.

[0050] 3) Preparation of the electrochemical device (lithium ion battery)

[0051] The positive electrode sheet prepared above, the separator, and the negative electrode sheet were sequentially stacked in order, and then wound to obtain an electrode assembly. After the tab was welded, the electrode assembly was placed in an aluminum plastic film, dried to remove water, and then injected with an electrolyte, and after processes such as vacuum packaging, standing, formation, capacity testing, and shaping, a lithium ion battery was obtained. In the present application, unless otherwise specified, the preparation can be carried out by referring to the conventional technical means in the art.

[0052] Examples 2-12

[0053] The preparation method of the flexibilizer for the binder in Examples 2-12 was the same as that in Example 1, and the preparation processes of the positive electrode sheet and the lithium ion battery were also the same as those in Example 1; the difference was that the formula composition of the flexibilizer was different, and the monomers and catalysts used and their mass parts were different, as shown in Table 1.

[0054] Table 1

[0055]

[0056] Examples 13-20

[0057] The formulation composition of the softener for the binder and its preparation method in Examples 13-20 are the same as those in Example 1, and the preparation process of the positive electrode sheet and the lithium ion battery is also the same as that in Example 1; the difference lies in that in the preparation of the positive electrode sheet, the mass ratio of the positive electrode active material or lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP to the softener.

[0058] Example 13

[0059] In the preparation of the positive electrode sheet, the positive electrode active material is lithium cobaltate.

[0060] Example 14

[0061] In the preparation of the positive electrode sheet, the positive electrode active material is lithium nickel cobalt manganese oxide (NCM622).

[0062] Example 15

[0063] In the preparation of the positive electrode sheet, the mass ratio of lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP to the softener is 95.9:2:2:0.1.

[0064] Example 16

[0065] In the preparation of the positive electrode sheet, the mass ratio of lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP to the softener is 95.8:2:2:0.2.

[0066] Example 17

[0067] In the preparation of the positive electrode sheet, the mass ratio of lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP to the softener is 95.6:2:2:0.4.

[0068] Example 18

[0069] In the preparation of the positive electrode sheet, the mass ratio of lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP to the softener is 95.4:2:2:0.6.

[0070] Example 19

[0071] In the preparation of the positive electrode sheet, the mass ratio of lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP to the softener is 95.2:2:2:0.8.

[0072] Example 20

[0073] In the preparation of the positive electrode sheet, the mass ratio of lithium iron phosphate, polyvinylidene fluoride, conductive carbon black SP to the softener is 96:2:2:1.

[0074] Comparative Example 1

[0075] Without adding the softener, the mass ratio of lithium iron phosphate, polyvinylidene fluoride and conductive agent in the preparation of the positive electrode sheet was 96:2:2, and the preparation process of the positive electrode sheet was the same as that of Example 1; the preparation process of the lithium ion battery was also the same as that of Example 1.

[0076] Comparative Example 2

[0077] Without adding the softener, the mass ratio of lithium cobalt oxide, polyvinylidene fluoride and conductive agent in the preparation of the positive electrode sheet was 96:2:2, and the preparation process of the positive electrode sheet was the same as that of Example 1; the preparation process of the lithium ion battery was also the same as that of Example 1.

[0078] Comparative Example 3

[0079] Without adding the softener, the mass ratio of lithium nickel cobalt manganese oxide (NCM622), polyvinylidene fluoride and conductive agent in the preparation of the positive electrode sheet was 96:2:2, and the preparation process of the positive electrode sheet was the same as that of Example 1; the preparation process of the lithium ion battery was also the same as that of Example 1.

[0080] Comparative Examples 4-7

[0081] The preparation method of the softener for the binder in Comparative Examples 4-7 was the same as that of Example 1, and the preparation processes of the positive electrode sheet and the lithium ion battery were also the same as those of Example 1; the difference was that the formula composition for preparing the softener was different, and the monomers and catalysts used and their mass fractions were as shown in Table 2.

[0082] Table 2

[0083]

[0084] Performance test:

[0085] The softeners, positive electrode sheets and lithium ion batteries prepared in the above examples and comparative examples were subjected to the following performance tests.

[0086]

Number average molecular weight

[0087]

Glass transition temperature Tg

[0088]

Boiling point

[0089]

Flexibility

[0090]

Adhesion

[0091]

Compaction density

[0092] The performance test results of each example and comparative example are shown in Table 3.

[0093] Table 3

[0094]

[0095] As can be seen from the test results of examples 1-12, 15-20 and comparative example 1, the test results of examples 13 and comparative example 2, and the test results of examples 14 and comparative example 3, after adding the flexibilizer for the binder, the flexibility of the lithium iron phosphate, lithium cobaltate or lithium nickel cobalt manganese ternary positive electrode sheet is improved, and the compaction density and adhesion are also improved.

[0096] As can be seen from the test results of examples 1-12 and comparative examples 4-7, when the composition of the flexibilizer is outside the preferred range, the boiling point of the flexibilizer is too low (comparative example 4) or too high (comparative example 5), and the glass transition temperature is too high (comparative examples 5 and 6), which has adverse effects on the flexibility, compaction density and adhesion of the prepared positive electrode sheet. When the mass ratio of the polyol monomer to the carbonate monomer is (40-85):(15-60) and the mass ratio of the diol to the oligomer is (2-20):(20-83), the performance of the electrode sheet is better.

[0097] As can be seen from the test results of examples 15-20, as the amount of flexibilizer increases, the flexibility and compaction density of the positive electrode sheet are improved, and the adhesion first increases and then decreases.

[0098] The application is further described in the detailed description that follows, by reference to various embodiments, and with the aid of the accompanying drawings. It should be apparent, however, that the description herein and the drawings are illustrative only and should not be considered to narrow the scope of the application in any way. Changes and modifications within the scope of the application can be made by those of ordinary skill, and it is the intent that the application not be limited as described herein but only by the claims and that they be interpreted as liberally as possible.

Claims

1. A flexibilizer for a binder, characterized by, The softening agent 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℃, 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.

2. The flexibilizer for a binder according to claim 1, characterized by said polar group is selected from at least one of -OH, -COOH or a salt formed after neutralization, -PO3H or a salt formed after neutralization, -SO3H or a salt formed after neutralization, -NH2, -NHCH3, -NH4 + , -CN, -CONH2.

3. The flexibilizer for a binder according to claim 2, characterized by The polar group is selected from at least one of -OH, -COOH, -NH2 for a binder for polyvinylidene fluoride, polyacrylic acid and polyacrylate; the polar group is selected from -CN for a binder for polyacrylonitrile; and the polar group is selected from at least one of -COOH, -CONH2 for a binder for polyimide.

4. The flexibilizer for a binder according to claim 1, characterized by The mass ratio of the polyol monomer to the carbonate monomer is (40-85):(15-60).

5. The flexibilizer for a binder according to claim 4, characterized by The polyol monomer comprises a dihydric alcohol and an oligomer containing a terminal hydroxyl group, the mass ratio of the dihydric alcohol to the oligomer is (2-20):(20-83); and the carbonate monomer is selected from at least one of dimethyl carbonate, dibutyl carbonate and diphenyl carbonate.

6. The flexibilizer for a binder according to claim 5, characterized by The structure of the polyol monomer further comprises at least one of 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.

7. The flexibilizer for a binder according to claim 1, characterized by The number average molecular weight of the copolymer of the softening agent is 1000-12000, the glass transition temperature Tg of the copolymer of the softening agent is -50--20℃, and the boiling point of the softening agent is 250-450℃.

8. A positive electrode sheet characterized by comprising: The positive electrode tab comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one side surface of the positive electrode current collector; the positive electrode active material layer comprises a positive electrode active material, a binder, a conductive agent and the softening agent according to any one of claims 1-7; and the binder is selected from at least one of polyvinylidene fluoride, polyacrylonitrile, polyimide, polyacrylic acid and polyacrylate.

9. The cathode electrode plate of claim 8, wherein, The mass of the softening agent is 2.5-50% of the mass of the binder.

10. An electrochemical device, characterized by, The positive electrode tab comprises the positive electrode tab according to claim 8 or 9.

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