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

Abstract

The application discloses a fluorine-free positive electrode binder, a positive electrode tab and a lithium ion secondary battery. The fluorine-free positive electrode binder comprises a copolymer, and the copolymer comprises first structural units and second structural units, the first structural units comprise acrylonitrile structural units, and the second structural units comprise cyano groups, functional structural units and polyether structural units. The fluorine-free positive electrode binder has low electrolyte swelling degree, the positive electrode tab prepared by using the binder has good softness, and the lithium ion secondary battery prepared by using the positive electrode tab has low impedance.

Description

Technical Field

[0001] This invention belongs to the field of battery material technology, specifically relating to a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion secondary battery. Background Technology

[0002] Lithium-ion rechargeable batteries typically consist of a positive electrode, a negative electrode, a separator, an electrolyte, and a battery casing. The positive electrode usually uses a metal substrate, with the active material attached to the metal surface using a binder. Currently, commercially available positive electrode binders are polyvinylidene fluoride (PVDF), which is used in combination with N-methylpyrrolidone (NMP) to form an organic oil-based system. PVDF exhibits excellent performance in terms of electrochemical stability, adhesion, lithium-ion migration ability, and thermal stability, and can basically meet the requirements for positive electrode binders in lithium-ion batteries.

[0003] However, as the industry demands increasingly higher energy density for lithium-ion batteries, high-nickel ternary cathode materials have become a development trend. The higher the nickel content in lithium nickel cobalt manganese oxide (LiNO3) batteries, the stronger the alkalinity of the material. In a strongly alkaline environment, PVDF molecular chains lose HF molecules and form continuous double bonds on the chain. Furthermore, double bond breakage and cross-linking with other molecular chains can occur, ultimately leading to gelation of the slurry, affecting normal batching, coating, and subsequent processes. Secondly, PVDF easily swells in organic electrolytes, reducing the adhesion of the impregnated electrode sheets and damaging the conductive network. More seriously, this can lead to electrode detachment, resulting in significant long-term degradation of battery cycle performance. Thirdly, PVDF recycling is difficult to achieve during battery recycling. Fourthly, PVDF is a fluorochemical material with high production costs and severe environmental pollution. In addition, the global resources of raw materials for PVDF production are limited, resulting in a tight supply and high price, further increasing the production cost of lithium-ion batteries.

[0004] Based on the aforementioned drawbacks of PVDF, attempts are being made to replace it with fluorine-free cathode binders. Currently, common fluorine-free cathode binders mainly include polyacrylonitrile (PAC) compounds, hydrogenated nitrile butadiene rubber (HNBR) compounds, and polyimides. Due to its strong polarity and high glass transition temperature, PAC-based cathode sheets are typically hard and brittle, prone to powdering and even breakage during processing. HNBR is produced by copolymerizing butadiene and acrylonitrile and then hydrogenating it; however, due to its large electrolyte swelling ratio, its use as a cathode binder often results in high internal resistance and poor kinetic performance. Polyimides, due to their good film-forming properties, tend to cause high internal resistance in batteries. Therefore, there is a need to develop fluorine-free cathode binders with low electrolyte swelling, good flexibility, and low internal resistance. Summary of the Invention

[0005] To address the problems existing in current positive electrode binders, this invention provides a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion secondary battery.

[0006] The objective of this invention is achieved through the following technical solution.

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

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

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

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

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

[0012] Furthermore, 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 methallyl ether structural unit.

[0013] Furthermore, 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.

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

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

[0016] Thirdly, the present invention provides a lithium-ion secondary battery, comprising a negative electrode, an electrolyte, and a positive electrode as described in the second aspect.

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

[0018] The introduction of acrylonitrile structural units into the fluorine-free positive electrode binder polymer of this invention can improve the binder's electrolyte resistance, mechanical strength, and battery performance. Due to the high polarity of the acrylonitrile structural units, the peel strength of the binder can be improved, and the electrolyte swelling and dissolution rate of the binder can be reduced. The cyano groups have a good affinity for lithium ions, thus reducing the battery's internal resistance and improving battery performance. The polyether chains have excellent flexibility, which can improve the flexibility of the binder, thereby making the prepared positive electrode sheet more flexible. In addition, the polyether structural units can complex lithium ions, improving the ionic conductivity of the binder and further reducing the battery's internal resistance. The cyano groups in the second structural unit can also improve the binder's adhesion. Furthermore, both the first and second structural units contain a large number of cyano groups, which have very strong polar interactions. Together, they can restrict the movement of the polyether chains, further reducing the swelling and dissolution rate of the binder. Detailed Implementation

[0019] 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.

[0020] It should be noted that, 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.

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

[0022] The introduction of acrylonitrile structural units into the fluorine-free positive electrode binder polymer of this invention can improve the binder's electrolyte resistance, mechanical strength, and battery performance. Due to the high polarity of the acrylonitrile structural units, the peel strength of the binder can be improved, and the electrolyte swelling and dissolution rate of the binder can be reduced. The cyano groups have a good affinity for lithium ions, thus reducing the battery's internal resistance and improving battery performance. The polyether chains have excellent flexibility, which can improve the flexibility of the binder, thereby making the prepared positive electrode sheet more flexible. In addition, the polyether structural units can complex lithium ions, improving the ionic conductivity of the binder and further reducing the battery's internal resistance. The cyano groups in the second structural unit can also improve the binder's adhesion. Furthermore, both the first and second structural units contain a large number of cyano groups, which have very strong polar interactions. Together, they can restrict the movement of the polyether chains, further reducing the swelling and dissolution rate of the binder.

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

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

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

[0026] Taking the total mass of the copolymer of the fluorine-free positive electrode binder as 100%, when the mass fraction of the first structural unit is less than 60% and the mass fraction of the second structural unit is greater than 40%, the mass swelling rate of the electrolyte of the fluorine-free binder is too large, which leads to an increase in the internal resistance of the battery and a decrease in battery performance; when the mass fraction of the first structural unit is greater than 95% and the mass fraction of the second structural unit is less than 5%, the positive electrode sheet prepared using the fluorine-free binder is extremely brittle and has extremely poor flexibility.

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

[0028] In some specific embodiments, 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.

[0029] In some specific embodiments, 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.

[0030] The second structural unit can be obtained by polymerization of a second structural unit monomer, wherein the second structural unit monomer is a polyether monomer that contains a cyano group at the end and a functional structural unit. The polyether monomer that contains a cyano group at the end and a functional structural unit includes, but is not limited to, polyether acrylate monomers, polyether methacrylate monomers, polyether allyl ether monomers, and polyether methallyl ether monomers.

[0031] The polyether monomers containing cyano groups at the end and functional structural units can be obtained by esterification of ether monomers with hydroxyl groups at the end and functional structural units with cyanoacetic acid. The polyether monomers containing cyano groups at the end and functional structural units include, but are not limited to, polyethylene glycol acrylates, polyethylene glycol methacrylates, polypropylene glycol acrylates, polypropylene glycol methacrylates, allyl polyethylene glycol, allyl polyethylene glycol, methallyl polyethylene glycol, methallyl polyethylene glycol, polyethylene glycol-propylene glycol acrylate, polyethylene glycol-propylene glycol methacrylate, polyethylene glycol-propylene glycol methacrylate, polyethylene glycol-propylene glycol acrylate, polyethylene glycol-propylene glycol methacrylate, polyethylene glycol-propylene glycol, and methallyl polyethylene glycol-propylene glycol.

[0032] The ether monomers with hydroxyl groups at the end and functional structural units at the other end include, but are 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) monomethacrylate, polyethylene glycol (10) allyl ether, and polyethylene glycol (10) methyl allyl ether. All of the above monomers can be obtained by purchase or customization. For example, polypropylene glycol (5) methacrylate and polyethylene glycol (10) allyl ether can be purchased from Liaoning Kelong Fine Chemical Co., Ltd.

[0033] The esterification reaction method is not limited. For example, cyanoacetic acid can be used to esterify ether monomers with hydroxyl groups at the end and functional structural units under catalytic conditions to obtain the following: using xylene solvent as a dehydrating agent, adding a polymerization inhibitor, 100 parts of ether monomers with hydroxyl groups at the end and functional structural units, and 150 parts of cyanoacetic acid, and adding 2 parts of p-toluenesulfonic acid as a catalyst, the esterification reaction is carried out under high temperature reflux, the water generated during the reaction is removed by a water separator, the solvent and excess cyanoacetic acid are removed by vacuum distillation after the esterification reaction is completed, and the catalyst is removed by alkaline washing to obtain a polyether monomer with cyano groups at the end and functional structural units.

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

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

[0036] In some specific embodiments, the ratio of the total number of repeating cyano groups, the total number of repeating functional structural units, and the total number of repeating polyether structural units is 1:1:2~20.

[0037] The fluorine-free positive electrode binder of this invention can be obtained by conventional polymerization methods, including emulsion polymerization, solution polymerization, precipitation polymerization, and suspension polymerization, using monomers containing a first structural unit and a second structural unit. Taking solution polymerization as an example, monomers containing the first and second structural units, together with an initiator (such as azobisisobutyronitrile), are added to dimethyl sulfoxide. After mixing evenly, the mixture is heated and stirred. The resulting fluorine-free positive electrode binder solution is precipitated and dried to obtain fluorine-free positive electrode binder powder.

[0038] In fluorine-free positive electrode binders, because the acrylonitrile structural unit has a large polarity, which is much greater than that of the electrolyte, when the content of acrylonitrile structural unit is high, the fluorine-free binder can have a lower electrolyte mass swelling rate.

[0039] 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 comprising a conductive agent, a positive active material, and a fluorine-free positive electrode binder as described in the first aspect. The conductive agent includes, but is not limited to, at least one selected from conductive carbon black, conductive graphite, Ketjen black, acetylene black, carbon nanotubes, carbon fibers, graphene, and conductive polymers.

[0040] The positive electrode active material is at least one of lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt aluminum oxide, or lithium nickel cobalt manganese oxide. By using the above-mentioned positive electrode active material in conjunction with a fluorine-free positive electrode binder, the cyano groups in the fluorine-free binder interact polarly with the polar groups and metal elements (such as hydroxyl groups, cobalt, iron, aluminum, and manganese on the surface of the active material), resulting in good adhesion. The polyether structural unit has good lithium-ion conductivity and molecular chain flexibility, enabling the positive electrode sheet prepared with the fluorine-free binder to have good flexibility and be less prone to powdering or breakage during processing.

[0041] Thirdly, the present invention provides a lithium-ion secondary battery, comprising a negative electrode, an electrolyte, and a positive electrode as described in the second aspect. Because the polyether structural unit has good lithium-ion conductivity, the lithium-ion secondary battery prepared from the positive electrode containing a fluorine-free binder has low impedance.

[0042] The specific embodiments of the present invention will be further explained and described below through examples and comparative examples.

[0043] Unless otherwise specified, all reagents, materials, and instruments used in the following description are conventional reagents, materials, and instruments, all of which are commercially available. The reagents involved can also be synthesized using conventional synthetic methods. Unless otherwise specified, the methods in the examples are conventional methods in the art. Monomers conforming to this invention are commercially available.

[0044] Example 1

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

[0046] Using xylene solvent as a dehydrating agent, hydroquinone, 100 parts of polyethylene glycol (10) monoacrylate, and 150 parts of cyanoacetic acid were added. Two parts of p-toluenesulfonic acid were added as a catalyst. The esterification reaction was carried out under high temperature reflux. Water generated during the reaction was removed using a water separator. After the esterification reaction was completed, the solvent and excess cyanoacetic acid were removed by vacuum distillation. After removing the catalyst by alkaline washing, polyether acrylate monomers with cyano groups at the end were obtained, which are the second structural unit monomers. The polyether segment is polyethylene glycol with 10 repeating units.

[0047] 70 parts of acrylonitrile, 30 parts of the prepared second structural unit monomer, and 0.1 parts of azobisisobutyronitrile were added to 100 parts of dimethyl sulfoxide. After mixing evenly, the mixture was heated and stirred. After polymerization for 6 hours, a fluorine-free positive electrode binder solution was obtained. After precipitation and drying, a fluorine-free positive electrode binder powder was obtained.

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

[0049] 97.8 parts of lithium cobalt oxide, 1 part of conductive carbon black, and 1.2 parts of the prepared fluorine-free positive electrode binder were added to N-methylpyrrolidone and stirred to prepare a positive electrode slurry. Then, the positive electrode slurry was coated on both surfaces of the positive electrode current collector aluminum foil, and after drying, cold pressing, and slitting, the positive electrode sheet was obtained.

[0050] 3) Preparation of lithium-ion secondary batteries:

[0051] The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator positioned between the positive and negative electrodes to act as an separator, thus forming an electrode assembly. The electrode assembly is placed in an outer package, injected with commercially available electrolyte, and sealed. After processes such as electrolyte injection, formation, and venting, a lithium-ion secondary battery is obtained.

[0052] Example 2

[0053] This embodiment uses most of the operating steps and monomer types from Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion secondary battery. The difference is that in the fluorine-free positive electrode binder, the monomer dosage is 60 parts acrylonitrile and 40 parts second structural unit monomer.

[0054] Example 3

[0055] This embodiment uses most of the operating steps and monomer types from Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion secondary battery. The difference is that in the fluorine-free positive electrode binder, the monomer dosage is 95 parts acrylonitrile and 5 parts second structural unit monomer.

[0056] Example 4

[0057] This embodiment uses most of the operating steps and monomer types from Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion secondary battery. The difference is that in the fluorine-free positive electrode binder, the monomer dosage is 85 parts acrylonitrile and 15 parts second structural unit monomer.

[0058] Example 5

[0059] This embodiment uses most of the operating steps in Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion secondary battery. The difference is that in the fluorine-free positive electrode binder, the monomer composition is 75 parts methacrylonitrile and 25 parts second structural unit monomer. The second structural unit monomer is a polyether methacrylate monomer with a cyano group at the end, wherein the polyether segment is polyethylene glycol with 10 repeating units.

[0060] Example 6

[0061] This embodiment uses most of the operating steps in Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion secondary battery. The difference is that in the fluorine-free positive electrode binder, the monomer composition is 75 parts methacrylonitrile and 25 parts second structural unit monomer. The second structural unit monomer is a polyether allyl ether monomer with a cyano group at the end, wherein the polyether segment is polyethylene glycol with 10 repeating units.

[0062] Example 7

[0063] This embodiment uses most of the operating steps in Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion secondary battery. The difference is that in the fluorine-free positive electrode binder, the monomer composition is 75 parts methacrylonitrile and 25 parts second structural unit monomer. The second structural unit monomer is a polyether methyl allyl ether monomer with a cyano group at the end, wherein the polyether segment is polyethylene glycol with 10 repeating units.

[0064] Example 8

[0065] This embodiment uses most of the operating steps in Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion secondary battery. The difference is that in the fluorine-free positive electrode binder, the polyether segment is polyethylene glycol with a repeating unit number of 2.

[0066] Example 9

[0067] This embodiment uses most of the operating steps in Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion secondary battery. The difference is that in the fluorine-free positive electrode binder, the polyether segment is polyethylene glycol with 20 repeating units.

[0068] Example 10

[0069] This embodiment uses most of the operating steps in Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion secondary battery. The difference is that in the fluorine-free positive electrode binder, the polyether segment is polypropylene glycol with 10 repeating units.

[0070] Example 11

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

[0072] Example 12

[0073] This embodiment uses most of the operating steps in Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion secondary battery. The difference is that the positive electrode active material in the positive electrode sheet is lithium nickel cobalt aluminum oxide.

[0074] Example 13

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

[0076] Comparative Example 1

[0077] This comparative example uses a fluorine-free positive electrode binder of polyacrylonitrile. Everything else is the same as in Example 1.

[0078] Comparative Example 2

[0079] This comparative example uses a fluorine-free positive electrode binder of the PVDF type. Everything else is the same as in Example 1.

[0080] Comparative Example 3

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

[0082] Comparative Example 4

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

[0084] Comparative Example 5

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

[0086] Performance testing:

[0087] To better understand the present invention, the positive electrode binder, positive electrode sheet and lithium-ion secondary battery prepared in the above embodiments and comparative examples were tested as follows, and the test results are shown in Table 1.

[0088] [Electrolyte Mass Swelling Rate Test]: Under standard atmospheric pressure and 25°C, 1M lithium hexafluorophosphate was added to a 1:1 mass ratio mixture of ethylene carbonate and methyl ethyl carbonate as the electrolyte. Two containers were taken and the electrolyte was added. The positive electrode binder was formed into a thin film, dried, and weighed to obtain W. 初始 The film is a circular sheet with a diameter of 20 mm and a thickness of 20 μm. The positive electrode binder film is immersed in the electrolyte and left for 72 hours. After being removed and the electrolyte on the surface is wiped dry, the film is weighed to obtain W. 平衡 Mass swelling ratio = (W 平衡 -W 初始 ) / W 初始 *100%.

[0089] [Electrolyte Dissolution Rate Test]: The positive electrode binder film was dried and weighed to obtain m1; after immersion in a 1:1 (EC:EMC=1:1) mixed solvent of ethylene carbonate and methyl ethyl carbonate at 60℃ for 72 hours, it was dried and weighed to obtain m2. The electrolyte dissolution rate is [(m1-m2) / m1]*100%.

[0090] [Flexibility Test]: The flexibility of the positive electrode sheet is tested by winding it with a steel needle. The electrode sheet is wound around a shaft of a certain diameter (mm), and the diameter at which cracks, powdering, or peeling occur is observed; the smaller the value, the better the flexibility.

[0091] [Positive Electrode Sheet Peel Strength Test]: After the positive electrode sheet is compacted, the peel strength of the positive electrode sheet is measured using a tensile testing machine. The test method refers to GB2792-2014. The positive electrode coating is fixed on a stainless steel plate, and the Al foil is peeled off with tape at 180° to obtain the peel strength. The unit is N / m.

[0092] [Internal Resistance Test]: The prepared positive electrode, negative electrode, and separator are assembled into a 503040-550mAh battery. After the cells are formed and tested, the AC internal resistance is tested using an AC low resistance tester, with the unit being mΩ.

[0093] Table 1 Test Results

[0094]

[0095] The test results in Table 1 show that:

[0096] The test results of Examples 1-13 and Comparative Examples 1-5 show that the electrolyte swelling degree of the fluorine-free positive electrode binder of the present invention is small, the prepared positive electrode sheet has good flexibility, the battery internal resistance is low, the electrode sheet peel strength (≥17N / m) is better than PVDF, and the electrode sheet flexibility and battery internal resistance can reach a level close to that of PVDF.

[0097] The test results from Examples 1-5 and Comparative Examples 1 and 3 show that: as the proportion of the first structural unit decreases, the electrolyte mass swelling rate and / or dissolution rate of the positive electrode binder also increase; as the proportion of the first structural unit increases, the peel strength and / or flexibility of the electrode sheet are relatively stronger; 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 too high, resulting in increased battery internal resistance and decreased battery performance; 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 using this positive electrode binder is extremely brittle and has extremely poor flexibility, and the active material layer is easily damaged during electrode rolling, which also leads to poor peel strength. When the mass ratio of the first structural unit to the second structural unit is (60~95):(5~40), the overall performance of the positive electrode binder, the positive electrode sheet, and the battery is better.

[0098] The test results from Examples 1 and Comparative Examples 4 and 5 show that: if the second structural unit of the positive electrode binder lacks a polyether structural unit (Comparative Example 4), the positive electrode sheet prepared with the positive electrode binder is extremely brittle and has very poor flexibility. The active material layer is easily damaged during electrode rolling, resulting in poor peel strength. The lack of a polyether structural unit also leads to increased battery internal resistance. If the second structural unit of the positive electrode binder lacks a cyano group (Comparative Example 5), the interaction between the first and second structural units is relatively weak, resulting in a relatively high swelling and dissolution rate of the binder. Because there are relatively few cyano groups in the positive electrode binder, its adhesion to the positive electrode active material is relatively weak, leading to poor peel strength of the electrode sheet and increased battery internal resistance. This also demonstrates that when the second structural unit simultaneously contains cyano groups, functional structural units, and polyether structural units, the overall performance of the positive electrode binder, positive electrode sheet, and battery is better.

[0099] 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 fluorine-free positive electrode binder, characterized in that, The fluorine-free positive electrode binder is obtained by polymerization of a monomer containing a first structural unit and a monomer containing a second structural unit, wherein the mass ratio of the first structural unit to the second structural unit is (60~95):(5~40); the first structural unit includes an acrylonitrile structural unit, and 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.

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 claim 1, characterized in that, The acrylonitrile structural unit is an acrylonitrile structural unit or a methacrylonitrile structural unit.

5. 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 4.

6. A lithium-ion secondary battery, characterized in that, It includes a negative electrode, an electrolyte, and a positive electrode as described in claim 5.

Images