Fluorine-free positive electrode binder, positive electrode plate and lithium ion secondary battery

Abstract

The invention discloses a fluoride-free positive electrode binder, a positive electrode plate and a lithium ion secondary battery. The fluoride-free positive electrode binder comprises a copolymer, the copolymer comprises a first structural unit and a second structural unit, the first structural unit comprises an acrylonitrile structural unit, and the second structural unit comprises a cyano group, a functional structural unit and a polyether structural unit. The fluoride-free positive electrode binder disclosed by the invention has relatively low electrolyte swelling degree, a positive electrode plate prepared from the binder has good flexibility, and a lithium ion secondary battery prepared from the positive electrode plate has relatively low impedance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of battery materials, and particularly relates to a fluorine-free positive electrode binder, a positive electrode tab and a lithium ion secondary battery. BACKGROUND

[0002] A lithium ion secondary battery is generally composed of a positive electrode, a negative electrode, a separator, an electrolyte and a battery shell. The positive electrode is usually made of metal as a substrate, and the positive electrode active material is attached to the surface of the metal through a binder. The currently commercialized positive electrode binder is polyvinylidene fluoride (PVDF), which is used in an organic oily system with N-methyl pyrrolidone (NMP). PVDF has excellent performance in terms of electrochemical stability, adhesion, lithium ion migration ability and thermal stability, and can basically meet the demand of the positive electrode binder of the lithium ion battery.

[0003] However, as the industry requires higher energy density of the lithium ion battery, high-nickel ternary positive electrode material has become a development trend. The higher the nickel content in lithium nickel cobalt manganese oxide is, the stronger the alkalinity of the material is. Since the molecular chain of PVDF will lose HF molecules in a strong alkaline environment, and a continuous double bond will be formed on the molecular chain, and cross-linking between the double bond and other molecular chains may occur, which eventually leads to gelation of the slurry, affecting normal batching, coating and subsequent processes. Secondly, PVDF is prone to swelling in an organic electrolyte, and the adhesion of the soaked electrode tab decreases and the conductive network is damaged, and more seriously, the tab may be detached, resulting in significant degradation of the long-term cycle performance of the battery. Thirdly, the recycling of PVDF is difficult to achieve in the battery recycling process. Fourthly, PVDF is a fluorinated chemical material with high production cost and serious environmental pollution. In addition, the global resources for producing PVDF are limited, causing a shortage of PVDF supply and high sales price, which increases the production cost of the lithium ion battery.

[0004] Based on the above-mentioned shortcomings of PVDF, attempts have been made to replace PVDF with fluorine-free positive electrode binders. At present, the common fluorine-free positive electrode binders mainly include polyacrylonitrile, hydrogenated butyl nitrile rubber and polyimide. The polyacrylonitrile is very polar and has a high glass transition temperature, so the positive electrode tab made of polyacrylonitrile is usually hard and brittle, and is prone to powdering or even breaking during the processing of the positive electrode tab. Hydrogenated butyl nitrile rubber is prepared by copolymerization of butadiene and acrylonitrile and hydrogenation, but the electrolyte mass swelling rate of the hydrogenated butyl nitrile rubber material is large, and the use of the hydrogenated butyl nitrile rubber material as a positive electrode binder usually has the weaknesses of large internal resistance of the battery and poor kinetics. The use of polyimide material may cause large internal resistance of the battery due to its good film forming property. Therefore, it is necessary to develop a fluorine-free positive electrode binder with small electrolyte swelling degree, good softness and low internal resistance. SUMMARY

[0005] In order to solve the problems existing in the prior art positive electrode binder, the application provides a fluorine-free positive electrode binder, a positive electrode tab and a lithium ion secondary battery.

[0006] The purpose of the application is achieved by the following technical solutions.

[0007] In a first aspect, the application provides a fluorine-free positive electrode binder, which comprises a copolymer, and the copolymer comprises a first structural unit and a second structural unit, the first structural unit comprises an acrylonitrile structural unit, and the second structural unit comprises a cyano group, a functional structural unit and a polyether structural unit.

[0008] Further, the electrolyte mass swelling rate of the fluorine-free positive electrode binder is 10% to 100%.

[0009] Further, the electrolyte dissolution rate of the fluorine-free positive electrode binder is ≤5%.

[0010] Further, the mass ratio of the first structural unit to the second structural unit is (60-95):(5-40).

[0011] Further, the acrylonitrile structural unit is an acrylonitrile structural unit or a methacrylonitrile structural unit.

[0012] Further, the polyether structural unit comprises at least one of a polyethylene glycol ether structural unit, a polypropylene glycol ether structural unit and an ethylene glycol / propylene glycol copolyether structural unit; and the functional structural unit is selected from an acrylate structural unit, a methacrylate structural unit, an allyl ether structural unit or a methallyl ether structural unit.

[0013] Further, the second structural unit is selected from a cyano-containing polyether acrylate structural unit, a cyano-containing polyether methacrylate structural unit, a cyano-containing polyether allyl ether structural unit or a cyano-containing polyether methallyl ether structural unit.

[0014] Further, the total number of repeating units of ethoxy and / or propoxy in the polyether structural unit is 2 to 20.

[0015] In a second aspect, the application provides a positive electrode tab, 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, and the positive electrode active material layer comprises a conductive agent, a positive electrode active material and the fluorine-free positive electrode binder as described in the first aspect. The positive electrode active material comprises at least one of lithium cobaltate, lithium iron phosphate, lithium nickel cobalt aluminum oxide or lithium nickel cobalt manganese oxide.

[0016] In a third aspect, the application provides a lithium ion secondary battery, which comprises a negative electrode tab, an electrolyte and the positive electrode tab as described in the second aspect.

[0017] Compared with the prior art, the present application has the following beneficial effects.

[0018] The introduction of the acrylonitrile structure unit in the fluorine-free positive electrode binder polymer can improve the electrolyte resistance, mechanical strength and battery performance of the binder. The acrylonitrile structure unit has a large polarity, which can improve the peel strength of the binder and reduce the electrolyte swelling degree and dissolution rate of the binder. The cyano group has good affinity with lithium ions, so the battery internal resistance can be reduced and the battery performance can be improved. The polyether chain has very excellent softness, which can improve the softness of the binder, so that the prepared positive electrode plate has better softness. In addition, the polyether structure unit can complex lithium ions to improve the ionic conductivity of the binder and further reduce the battery internal resistance. The cyano group in the second structure unit can also improve the adhesion of the binder. The first structure unit and the second structure unit both contain a large number of cyano groups, and the cyano groups have very strong polar interactions. The combination of the two can limit the movement ability of the polyether chain, further reducing the swelling rate and dissolution rate of the binder. DETAILED DESCRIPTION

[0019] 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. They 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.

[0020] It should be noted that in the present application, as known by those skilled in the art in the field of chemical synthesis, each structure unit represents the corresponding structure part of the monomer in the obtained polymer after the monomer participates in the polymerization reaction. The mass ratio of each structure unit is the mass ratio of the monomer providing each structure unit.

[0021] In a first aspect, the present application provides a fluorine-free positive electrode binder, which comprises a copolymer, and the copolymer comprises a first structure unit and a second structure unit, the first structure unit comprises an acrylonitrile structure unit, and the second structure unit comprises a cyano group, a functional structure unit and a polyether structure unit.

[0022] The introduction of the acrylonitrile structure unit in the fluorine-free positive electrode binder polymer can improve the electrolyte resistance, mechanical strength and battery performance of the binder. The acrylonitrile structure unit has a large polarity, which can improve the peel strength of the binder, and can also reduce the electrolyte swelling degree and dissolution rate of the binder. The cyano group has good affinity with lithium ions, so the battery internal resistance can be reduced and the battery performance can be improved. The polyether chain has very excellent softness, which can improve the softness of the binder, so that the prepared positive electrode plate has better softness. In addition, the polyether structure unit can complex lithium ions, improve the ionic conductivity of the binder, and further reduce the battery internal resistance. The cyano group in the second structure unit can also improve the adhesion of the binder. The first structure unit and the second structure unit both contain a large number of cyano groups, and the cyano groups have a very strong polar interaction. The combination of the two can limit the movement ability of the polyether chain, further reducing the swelling rate and dissolution rate of the binder.

[0023] In some embodiments, the electrolyte mass swelling rate of the fluorine-free positive electrode binder is 10% to 100%.

[0024] In some embodiments, the electrolyte dissolution rate of the fluorine-free positive electrode binder is ≤5%.

[0025] In some embodiments, the mass ratio of the first structure unit to the second structure unit is (60-95):(5-40). Specifically, the mass ratio of the first structure unit to the second structure unit can be 60:40, 65:35, 70:30, 75:25, 80:20, 85:15, 90:10, 95:5, etc.

[0026] Based on the total mass of the copolymer of the fluorine-free positive electrode binder being 100%, when the mass fraction of the first structure unit is less than 60% and the mass fraction of the second structure unit is more than 40%, the electrolyte mass swelling rate of the fluorine-free binder is large, resulting in an increase in battery internal resistance and a decrease in battery performance. When the mass fraction of the first structure unit is more than 95% and the mass fraction of the second structure unit is less than 5%, the positive electrode plate prepared by using the fluorine-free binder has great brittleness and poor softness.

[0027] In some embodiments, the acrylonitrile structure unit is an acrylonitrile structure unit or a methacrylonitrile structure unit.

[0028] In some embodiments, the polyether structure unit includes at least one of a polyethylene glycol ether structure unit, a polypropylene glycol ether structure unit, and an ethylene glycol / propylene glycol copolyether structure unit; and the functional structure unit is selected from an acrylate structure unit, a methacrylate structure unit, an allyl ether structure unit, or a methallyl ether structure unit.

[0029] In some embodiments, the second structural unit is selected from a cyano-containing polyether acrylate structural unit, a cyano-containing polyether methacrylate structural unit, a cyano-containing polyether allyl ether structural unit, or a cyano-containing polyether methallyl ether structural unit.

[0030] The second structural unit can be obtained by polymerization of a second structural unit monomer, which is a terminal cyano-containing and functional structure unit-containing polyether monomer. The terminal cyano-containing and functional structure unit-containing polyether monomer includes but is not limited to a terminal cyano-containing polyether acrylate monomer, a terminal cyano-containing polyether methacrylate monomer, a terminal cyano-containing polyether allyl ether monomer, and a terminal cyano-containing polyether methallyl ether monomer.

[0031] The terminal cyano-containing and functional structure unit-containing polyether monomer can be obtained by esterification of a terminal hydroxyl-containing and functional structure unit-containing ether monomer and cyanoacetic acid. The terminal cyano-containing and functional structure unit-containing polyether monomer includes but is not limited to a terminal cyano-containing polyethylene glycol acrylate, a terminal cyano-containing polyethylene glycol methacrylate, a terminal cyano-containing polypropylene glycol acrylate, a terminal cyano-containing polypropylene glycol methacrylate, a terminal cyano-containing allyl polyethylene glycol, a terminal cyano-containing allyl polypropylene glycol, a terminal cyano-containing methallyl polyethylene glycol, a terminal cyano-containing methallyl polypropylene glycol, a terminal cyano-containing polyethylene glycol-polypropylene glycol acrylate, a terminal cyano-containing polyethylene glycol-polypropylene glycol methacrylate, a terminal cyano-containing allyl polyethylene glycol-polypropylene glycol, and a terminal cyano-containing methallyl polyethylene glycol-polypropylene glycol.

[0032] The terminal hydroxyl-containing and functional structure unit-containing ether monomer has a hydroxyl group at one end and a functional structure unit at the other end, and includes but is not limited to polyethylene glycol (2) monoacrylate, polyethylene glycol (10) monoacrylate, polyethylene glycol (20) monoacrylate, polypropylene glycol (10) monoacrylate, polypropylene glycol (5) methacrylate, polyethylene glycol (5) polypropylene glycol (5) monoacrylate, polyethylene glycol (10) monoacrylate, polyethylene glycol (10) allyl ether, and polyethylene glycol (10) methallyl ether. These monomers can be purchased or customized, such as polypropylene glycol (5) methacrylate and polyethylene glycol (10) allyl ether, which can be purchased from Liaoning Kelong Fine Chemical Co., Ltd.

[0033] The esterification reaction method is not limited, for example, a cyanacetic acid and an ether monomer with a hydroxyl group at the end and a functional structure unit can be used to carry out esterification reaction under the condition of a catalyst to obtain a polyether monomer with a cyan group at the end and a functional structure unit, xylene solvent is used as a water-carrying agent, a polymerization inhibitor is added, 100 parts of the ether monomer with a hydroxyl group at the end and a functional structure unit and 150 parts of the cyanacetic acid are added, 2 parts of p-toluenesulfonic acid is added as a catalyst, esterification reaction is carried out under high-temperature reflux, a water trap is used to remove water generated in the reaction process, after the esterification reaction is completed, the solvent and excess cyanacetic acid are removed by distillation under reduced pressure, and the polyether monomer with a cyan group at the end and a functional structure unit is obtained after the catalyst is removed by alkali washing.

[0034] The polymerization inhibitor is a conventional polymerization inhibitor, including but not limited to hydroquinone, p-tert-butyl hydroquinone, 6-tert-butyl-2,4-dimethylphenol, 2,6-di-tert-butyl-p-methylphenol, 4,4'-diphenyl and bisphenol A.

[0035] In some specific embodiments, the total number of repeating units of ethoxy and / or propoxy in the polyether structure unit is 2-20; preferably, the total number of repeating units thereof is 4-15. When the total number of repeating units is 2, it means that the polyether structure unit contains two ethoxy groups, or contains two propoxy groups, or contains one ethoxy group and one propoxy group.

[0036] In some specific embodiments, the ratio of the total number of repeating units of the cyan group, the total number of repeating units of the functional structure unit and the total number of repeating units of the polyether structure unit is 1:1:2-20.

[0037] The fluorine-free positive electrode binder of the present application can be obtained by a conventional polymerization method of monomers containing a first structure unit and monomers of a second structure unit, including emulsion polymerization, solution polymerization, precipitation polymerization and suspension polymerization. Taking solution polymerization as an example, the monomers containing the first structure unit and the monomers of the second structure unit are added to dimethyl sulfoxide together with an initiator (such as azobisisobutyronitrile), uniformly mixed, heated and stirred, and the fluorine-free positive electrode binder solution obtained after polymerization can be obtained after precipitation and drying.

[0038] In the fluorine-free positive electrode binder, the acrylonitrile structure unit has a large polarity, and the polarity is much larger than that of the electrolyte, so when the content of the acrylonitrile structure unit is high, the fluorine-free binder has a low electrolyte mass swelling rate.

[0039] In a second aspect, the present 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, wherein the positive electrode active material layer comprises a conductive agent, a positive electrode active material and the fluorine-free positive electrode binder as described in the first aspect. The conductive agent includes, but is not limited to, at least one of conductive carbon black, conductive graphite, ketjen black, acetylene black, carbon nanotube, carbon fiber, graphene and conductive polymer.

[0040] The positive electrode active material is at least one of lithium cobaltate, lithium iron phosphate, lithium nickel cobalt aluminum oxide or lithium nickel cobalt manganese oxide. By selecting the above positive electrode active material in combination with the fluorine-free positive electrode binder, the cyano group in the fluorine-free positive electrode binder and the polar group and metal elements (such as hydroxyl group, cobalt element, iron element, aluminum element and manganese element on the surface of the active material) in the positive electrode active material produce polar interaction, thereby generating better adhesion. The polyether structural unit has good lithium ion conductivity and good molecular chain softness, so that the positive electrode sheet prepared by using the fluorine-free binder has good softness and is not easy to fall off or break during processing.

[0041] In a third aspect, the present application provides a lithium ion secondary battery, which comprises a negative electrode sheet, an electrolyte and the positive electrode sheet as described in the second aspect. The polyether structural unit has good lithium ion conductivity, so that the lithium ion secondary battery prepared by using the positive electrode sheet containing the fluorine-free binder has lower impedance.

[0042] The specific embodiments of the present application will be further explained by examples and comparative examples.

[0043] 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 are conventional methods in the art, unless otherwise specified. The monomers meeting the present application can be commercially available.

[0044] Example 1

[0045] 1) Preparation of fluorine-free positive electrode binder:

[0046] The xylene solvent is used as a water-carrying agent, 100 parts of polyethylene glycol (10) monoacrylate and 150 parts of cyanoacetic acid are added, 2 parts of p-toluenesulfonic acid is added as a catalyst, and esterification reaction is carried out under high temperature reflux. The water generated during the reaction is removed by a water trap. After the esterification reaction is completed, the solvent and excess cyanoacetic acid are removed by distillation under reduced pressure. After the catalyst is removed by alkaline washing, the polyether acrylate monomer with cyano group at the end, i.e. the second structural unit monomer, is obtained, wherein the polyether segment is polyethylene glycol with a repeat unit number of 10.

[0047] The 70 parts of acrylonitrile, 30 parts of the prepared second structural unit monomer, and 0.1 parts of azobisisobutyronitrile are added into 100 parts of dimethyl sulfoxide, mixed uniformly, heated and stirred, and then polymerized for 6 hours to obtain a fluorine-free positive electrode binder solution. After precipitation and drying, a fluorine-free positive electrode binder powder is obtained.

[0048] 2) Preparation of the positive electrode sheet:

[0049] The 97.8 parts of lithium cobaltate, 1 part of conductive carbon black, and 1.2 parts of the prepared fluorine-free positive electrode binder are added into N-methyl pyrrolidone and stirred to prepare a positive electrode slurry. Then, the positive electrode slurry is coated on both surfaces of the positive electrode current collector aluminum foil, and after drying, cold pressing, and slitting, a positive electrode sheet is obtained.

[0050] 3) Preparation of the lithium ion secondary battery:

[0051] The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator between the positive electrode and the negative electrode to play a role of isolation, to prepare an electrode assembly. The electrode assembly is placed in an outer package, injected with a commercially available electrolyte and packaged. After injection, formation, and exhaust processes, a lithium ion secondary battery is obtained.

[0052] Example 2

[0053] In this example, a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium ion secondary battery are prepared by using most of the operation steps and monomer types in Example 1. The difference is that in the fluorine-free positive electrode binder, the monomer usage is 60 parts of acrylonitrile and 40 parts of the second structural unit monomer.

[0054] Example 3

[0055] In this example, a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium ion secondary battery are prepared by using most of the operation steps and monomer types in Example 1. The difference is that in the fluorine-free positive electrode binder, the monomer usage is 95 parts of acrylonitrile and 5 parts of the second structural unit monomer.

[0056] Example 4

[0057] In this example, a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium ion secondary battery are prepared by using most of the operation steps and monomer types in Example 1. The difference is that in the fluorine-free positive electrode binder, the monomer usage is 85 parts of acrylonitrile and 15 parts of the second structural unit monomer.

[0058] Example 5

[0059] This example employs most of the procedure steps included in Example 1 to prepare a fluorine-free cathode binder, cathode electrode sheet, and lithium ion secondary battery, except that in the fluorine-free cathode binder, the monomer composition is 75 parts of methacrylonitrile, 25 parts of a second structural unit monomer, and the second structural unit monomer is a polyether methacrylate-based monomer having a cyano group at the terminal end, in which the polyether segment is polyethylene glycol having a repeat unit number of 10.

[0060] Example 6

[0061] This example employs most of the procedure steps included in Example 1 to prepare a fluorine-free cathode binder, cathode electrode sheet, and lithium ion secondary battery, except that in the fluorine-free cathode binder, the monomer composition is 75 parts of methacrylonitrile, 25 parts of a second structural unit monomer, and the second structural unit monomer is a polyether methacrylate-based monomer having a cyano group at the terminal end, in which the polyether segment is polyethylene glycol having a repeat unit number of 10.

[0062] Example 7

[0063] This example employs most of the procedure steps included in Example 1 to prepare a fluorine-free cathode binder, cathode electrode sheet, and lithium ion secondary battery, except that in the fluorine-free cathode binder, the monomer composition is 75 parts of methacrylonitrile, 25 parts of a second structural unit monomer, and the second structural unit monomer is a polyether methacrylate-based monomer having a cyano group at the terminal end, in which the polyether segment is polyethylene glycol having a repeat unit number of 10.

[0064] Example 8

[0065] This example employs most of the procedure steps included in Example 1 to prepare a fluorine-free cathode binder, cathode electrode sheet, and lithium ion secondary battery, except that in the fluorine-free cathode binder, the polyether segment is polyethylene glycol having a repeat unit number of 2.

[0066] Example 9

[0067] This example employs most of the procedure steps included in Example 1 to prepare a fluorine-free cathode binder, cathode electrode sheet, and lithium ion secondary battery, except that in the fluorine-free cathode binder, the polyether segment is polyethylene glycol having a repeat unit number of 20.

[0068] Example 10

[0069] This example employs most of the procedure steps included in Example 1 to prepare a fluorine-free cathode binder, cathode electrode sheet, and lithium ion secondary battery, except that in the fluorine-free cathode binder, the polyether segment is polyethylene glycol having a repeat unit number of 20.

[0070] Example 11

[0071] This example uses most of the operation steps of Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium ion secondary battery, except that in the fluorine-free positive electrode binder, the polyether segment is polyethylene glycol-propylene glycol, and the number of repeating units of the ethylene glycol segment and the propylene glycol segment is each 5.

[0072] Example 12

[0073] This example uses most of the operation steps of Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium ion secondary battery, except that in the positive electrode sheet, the positive electrode active material is lithium nickel cobalt aluminate.

[0074] Example 13

[0075] This example uses most of the operation steps of Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium ion secondary battery, except that in the positive electrode sheet, the positive electrode active material is lithium nickel cobalt manganese oxide.

[0076] Comparative Example 1

[0077] This comparative example uses a fluorine-free positive electrode binder that is a polyacrylonitrile type. Other than that, it is the same as Example 1.

[0078] Comparative Example 2

[0079] This comparative example uses a fluorine-free positive electrode binder that is a PVDF type. Other than that, it is the same as Example 1.

[0080] Comparative Example 3

[0081] This comparative example uses most of the operation steps and monomer types of Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium ion secondary battery, except that in the fluorine-free positive electrode binder, the monomer amount is 50 parts of acrylonitrile, 50 parts of a second structural unit monomer.

[0082] Comparative Example 4

[0083] This comparative example uses most of the operation steps of Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium ion secondary battery, except that in the fluorine-free positive electrode binder, the monomer composition is 70 parts of acrylonitrile, 30 parts of cyanoethyl acrylate.

[0084] Comparative Example 5

[0085] This comparative example uses most of the operation steps of Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium ion secondary battery, except that in the fluorine-free positive electrode binder, the monomer composition is 70 parts of acrylonitrile, 30 parts of polyoxyethylene ether acrylate, and the number of repeating units of the polyoxyethylene ether segment is 10.

[0086] Performance test:

[0087] In order to better understand the present application, the positive electrode binder, the positive electrode tab and the lithium ion secondary battery prepared in the above examples and comparative examples were tested as follows, and the test results are shown in Table 1.

[0088]

Test of electrolyte mass swelling rate

[0089]

Test of electrolyte leaching rate

[0090]

Test of softness

[0091]

Test of positive electrode tab peeling strength

[0092]

Test of internal resistance

[0093] Table 1 Test results

[0094]

[0095] From the test results in Table 1, it can be seen that:

[0096] From the test results of examples 1-13 and comparative examples 1-5, it can be seen that the electrolyte swelling degree of the fluorine-free positive electrode binder of the application is small, the prepared positive electrode sheet has good softness, and the battery has low internal resistance, and the electrode sheet peeling strength (≥17 N / m) is better than PVDF, and the softness of the electrode sheet and the internal resistance of the battery can reach the level close to PVDF.

[0097] From the test results of examples 1-5 and comparative examples 1, 3, it can be seen that as the proportion of the first structural unit decreases, the electrolyte mass swelling rate and / or dissolution rate of the positive electrode binder will also increase; as the proportion of the first structural unit increases, the peeling strength of the electrode sheet is relatively strong and / or the softness is relatively good; however, when the proportion of the first structural unit is too low and the proportion of the second structural unit is too high, the electrolyte mass swelling rate and dissolution rate of the positive electrode binder are both large, resulting in an increase in the internal resistance of the battery and a decrease in the performance of the battery; when the proportion of the first structural unit is too high and the proportion of the second structural unit is too low, the positive electrode sheet prepared by using the positive electrode binder is extremely brittle and has extremely poor softness, and the active material layer is easily damaged during electrode sheet rolling, thereby also resulting in poor peeling strength. When the mass ratio of the first structural unit to the second structural unit is (60-95):(5-40), the comprehensive performance of the positive electrode binder, the positive electrode sheet and the battery is better.

[0098] From the test results of example 1 and comparative examples 4, 5, it can be seen that if the second structural unit of the positive electrode binder lacks a polyether structural unit (comparative example 4), the positive electrode sheet prepared by using the positive electrode binder is extremely brittle and has extremely poor softness, and the active material layer is easily damaged during electrode sheet rolling, thereby resulting in poor peeling strength, and the battery internal resistance increases due to the lack of polyether structural units. If the second structural unit of the positive electrode binder lacks a cyano group (comparative example 5), the interaction between the first structural unit and the second structural unit is relatively weak, resulting in a relatively high swelling rate and dissolution rate of the binder, and the adhesion between the positive electrode binder and the positive active material is relatively weak due to the relatively small amount of cyano groups in the positive electrode binder, resulting in poor peeling strength of the electrode sheet and an increase in the internal resistance of the battery. This also shows that when the second structural unit contains a cyano group, a functional structural unit and a polyether structural unit at the same time, the comprehensive performance of the positive electrode binder, the positive electrode sheet and the battery is better.

[0099] The above describes the application by means of specific examples, but it should be understood that the specific description herein should not be understood as limiting the essence and scope of the application, and various modifications made by those skilled in the art to the above examples after reading the specification are within the scope of the application.

Claims

1. A fluorine-free positive electrode binder, characterized in that, The fluorine-free positive electrode binder comprises a copolymer, and the copolymer comprises a first structural unit and a second structural unit. The first structural unit comprises an acrylonitrile structural unit, and the second structural unit comprises a cyano group, a functional structural unit, and a polyether structural unit.

2. The fluorine-free positive electrode binder according to claim 1, characterized in that, The electrolyte mass swelling rate of the fluorine-free positive electrode binder is 10%~100%.

3. The fluorine-free positive electrode binder according to claim 1, characterized in that, The electrolyte dissolution rate of the fluorine-free positive electrode binder is ≤5%.

4. The fluorine-free positive electrode binder according to any one of claims 1 to 3, characterized in that, The mass ratio of the first structural unit to the second structural unit is (60~95):(5~40).

5. The fluorine-free positive electrode binder according to claim 4, characterized in that, The acrylonitrile structural unit is an acrylonitrile structural unit or a methacrylonitrile structural unit.

6. The fluorine-free positive electrode binder according to claim 4, characterized in that, The polyether structural unit includes at least one of polyethylene glycol ether structural unit, polypropylene glycol ether structural unit, and ethylene glycol / propylene glycol copolyether structural unit; the functional structural unit is selected from acrylate structural unit, methacrylate structural unit, allyl ether structural unit, or methyl allyl ether structural unit.

7. The fluorine-free positive electrode binder according to claim 6, characterized in that, The second structural unit is selected from cyano-containing polyether acrylate structural units, cyano-containing polyether methacrylate structural units, cyano-containing polyether allyl ether structural units, or cyano-containing polyether methallyl ether structural units.

8. The fluorine-free positive electrode binder according to claim 7, characterized in that, The total number of repeating ethoxy and / or propoxy units in the polyether structural unit is 2 to 20.

9. 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, wherein the positive active material layer includes a conductive agent, a positive active material, and a fluorine-free positive electrode binder as described in any one of claims 1 to 8.

10. A lithium-ion secondary battery, characterized in that, It includes a negative electrode, an electrolyte, and a positive electrode as described in claim 9.

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