Binder for secondary battery, negative electrode slurry, pole piece and lithium ion battery

Abstract

The invention discloses a binder for a secondary battery, negative electrode slurry, a pole piece and a lithium ion battery, the binder for the secondary battery comprises a binder A and a binder B, the binder A comprises a polyacrylic acid polymer, and the binder B comprises styrene butadiene rubber. The two binders are matched, and an effective synergistic effect can be formed between the binders, so that a cross-linked network is formed, cyclic expansion of an active material in an electrode is inhibited, and continuous generation of an SEI film and damage to an electrode structure are avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a binder for secondary batteries and application thereof. Background Art

[0002] The role of the binder in lithium batteries is to bond the active material, conductive agent and current collector together to maintain the integrity of the electrode. At the same time, it has little effect on the transmission of ions and electrons in the electrode and has strong cycle stability.

[0003] Silicon-based anode materials have extremely high theoretical specific capacity (3579mAh / g, Li 15 Si4, low voltage platform and abundant sources, has been widely explored as the next generation of high-performance lithium-ion battery negative electrode materials, and has begun to be used in consumer batteries and power batteries. However, the volume expansion of silicon-based negative electrode materials can reach 300% during the cyclic lithium insertion process. After a long period of circulation, the serious volume effect of silicon particles will lead to the continuous generation of SEI film and the destruction of electrode structure, and ultimately lead to serious battery capacity decay.

[0004] However, current binders perform poorly in suppressing the cyclic expansion of Si-based anode materials. Summary of the invention

[0005] In view of the deficiencies of the prior art, the present invention provides a binder for a secondary battery, a negative electrode material, a pole piece and a lithium ion battery, aiming to solve the problem that the current binder performs poorly in inhibiting the cyclic expansion of silicon-based negative electrode materials.

[0006] In order to achieve the above object, the present invention provides a binder for a secondary battery, wherein the binder comprises a binder A and a binder B, wherein the binder A comprises a polyacrylic acid polymer, and the binder B comprises styrene-butadiene rubber.

[0007] In some embodiments, the solid content ratio of the binder A to the binder B is (4-10):1.

[0008] In some embodiments, the monomer of the binder A includes an acrylic monomer, and the acrylic monomer includes at least one of acrylic acid, methacrylic acid, methylene succinic acid, and maleic acid.

[0009] In some embodiments, the monomers of the binder A include acrylic monomers and vinyl monomers, and the molar ratio of the acrylic monomers in the binder A is greater than or equal to 40%.

[0010] In some embodiments, the monomers of the binder B include a first monomer, a second monomer, and a third monomer.

[0011] The structural formula of the third monomer is shown in formula (I): Wherein, R1=R2=-H; R3=-H or -CH3; R4=-C n H 2n , 1≤n≤4; R5=-OH or

[0012] In some embodiments, the first monomer is a styrene monomer and the second monomer is a butadiene monomer.

[0013] In some embodiments, the molar masses of the first monomer, the second monomer, and the third monomer are a, b, and c, respectively, and a, b, and c follow the following relationship:

[0014] In order to achieve the above-mentioned purpose, the present invention also proposes a negative electrode slurry, which includes a negative electrode active material, a conductive agent and the above-mentioned secondary battery binder, and the mass ratio of the negative electrode active material, the conductive agent and the binder is (60~99):(0.5~20):(0.5~20), and the sum of the three parts is always 100 parts.

[0015] In order to achieve the above object, the present invention further provides a negative electrode plate, wherein the negative electrode plate is a negative electrode plate prepared by using the above negative electrode slurry.

[0016] In some embodiments, the preparation of the negative electrode sheet includes heat treatment, and the heat treatment includes: coating the negative electrode slurry on the negative electrode current collector, and then vacuum baking at a temperature of 60-120° C. for 4-8 hours.

[0017] In order to achieve the above object, the present invention further proposes a lithium-ion battery, wherein the battery comprises the above negative electrode plate, and the battery comprises any one of a battery cell, a battery module, and a battery pack.

[0018] Beneficial effects of the present invention:

[0019] The adhesive provided by the present invention uses two adhesives to be combined, and the functional group with a cross-linking site is modified to the styrene-butadiene rubber instead of the polyacrylic acid, which can avoid a significant change in the viscosity of the polyacrylic acid product and ensure the processing performance of the slurry. In addition, the introduction of acrylic acid ester active monomers into the styrene-butadiene rubber can improve the flexibility of the styrene-butadiene rubber and introduce a flexible segment into the cross-linking network. At the same time, due to the introduction of epoxy functional groups into the styrene-butadiene rubber, through the secondary processing of the electrode sheet, an effective synergistic effect can be formed between the two types of adhesives to form a cross-linking network of alternating soft and hard, with stronger adhesion to maintain good contact between the active material, the conductive additive and the current collector, reduce the material cracking and crushing caused by the volume change of silicon during the charge and discharge cycle, can better limit the expansion of the active material, avoid the destruction of the electrode structure, and thus avoid serious attenuation of the battery capacity. DETAILED DESCRIPTION

[0020] In order to make the technical solutions and advantages of the present invention clearer, the present invention and its beneficial effects will be further described in detail below in conjunction with specific implementation methods. The following description is provided for those skilled in the art to fully understand the present application and is not intended to limit the subject matter recorded in the claims.

[0021] "Scope" disclosed in the present application is limited in the form of lower limit and upper limit, and a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The scope limited in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "ab" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0022] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0023] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0024] If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0025] Silicon-based anode materials have extremely high theoretical specific capacity (3579mAh / g, Li 15 Si4, low voltage platform and abundant sources, has been widely explored as the next generation of high-performance lithium-ion battery negative electrode materials, and has begun to be used in consumer batteries and power batteries. However, the volume expansion of silicon-based negative electrode materials can reach 300% during the cyclic lithium insertion process. After a long period of circulation, the serious volume effect of silicon particles will lead to the continuous generation of SEI film and the destruction of electrode structure, and ultimately lead to serious battery capacity decay.

[0026] However, current binders perform poorly in suppressing the cyclic expansion of Si-based anode materials.

[0027] In view of this, the present invention provides a binder for a secondary battery, the binder comprising a binder A and a binder B, the binder A comprising a polyacrylic acid polymer, and the binder B comprising styrene-butadiene rubber.

[0028] Polyacrylic acid (PAA) polymers can form strong interactions with silicon particles due to their rich carboxyl functional groups, thereby improving the adhesion and electrochemical stability of silicon negative electrodes. Styrene-butadiene rubber (SBR) is often used to improve the mechanical properties of electrodes due to its good flexibility and adhesion. Combining PAA with SBR can form a composite binder with good adhesion and mechanical strength to adapt to the volume changes of silicon-based negative electrodes during charging and discharging.

[0029] In this scheme, two kinds of binders are used in combination, and the functional groups with cross-linking sites are modified to styrene-butadiene rubber instead of polyacrylic acid, which can avoid a significant change in the viscosity of the polyacrylic acid product and ensure the processing performance of the slurry. In addition, the introduction of acrylate active monomers into styrene-butadiene rubber can improve the flexibility of styrene-butadiene rubber and introduce flexible segments into the cross-linking network. At the same time, due to the introduction of epoxy functional groups into styrene-butadiene rubber, through the secondary processing of the electrode sheet, an effective synergistic effect can be formed between the two types of binders to form a cross-linking network of alternating soft and hard layers with stronger adhesion to maintain good contact between the active material, the conductive additive and the current collector, reduce the material cracking and crushing caused by the volume change of silicon during the charge and discharge cycle, better limit the expansion of the active material, avoid damage to the electrode structure, and thus avoid serious attenuation of the battery capacity.

[0030] In some embodiments, the solid content ratio of the binder A to the binder B is (4-10):1. The solid content (solid content) of the binder is the mass percentage of the non-volatile matter in the binder measured under the specified test conditions. The solid content directly affects the mass ratio of the binder. In use, it affects the viscosity of the binder, the thickness of the negative electrode slurry coating, and further affects the construction performance and bonding performance. By mixing the binder A and the binder B according to the solid content ratio, the binder for the secondary battery of this scheme can be obtained.

[0031] In some embodiments, the molecular weight of binder A is between 10w and 100w.

[0032] In some embodiments, the monomer of the binder A includes an acrylic monomer, and the acrylic monomer includes at least one of acrylic acid, methacrylic acid, methylene succinic acid, and maleic acid. Acrylic monomers contain carboxyl groups and can form strong interactions with hydroxyl groups or metal ions on the surface of silicon-based electrode materials by forming hydrogen bonds or ionic bonds, thereby improving the adhesion between the binder and the electrode material. This strong interaction helps to maintain the integrity of the structure during the volume expansion and contraction of the electrode material and improve the cycle stability of the battery. The introduction of acrylic monomers can increase the polarity of the binder, thereby improving the compatibility between the binder and the electrolyte and facilitating the transmission of lithium ions. In some embodiments, the binder A prepared in this scheme can be synthesized by conventional free radical polymerization.

[0033] In some embodiments, the monomers of the binder A include acrylic monomers and vinyl monomers, and the molar ratio of the acrylic monomers in the binder A is greater than or equal to 40%. In some embodiments, the vinyl monomers have pendant functional groups such as carboxylic acid groups, amide groups, sulfonic acid groups, hydroxyl groups, ester groups, nitrile groups, and hydrocarbon groups.

[0034] In addition, the carboxylic acid group of the acrylic monomer can form a complex with lithium ions to further increase the ionic conductivity, thereby improving the rate performance and cycle stability of the battery.

[0035] Preferably, the molar ratio of acrylic monomer in binder A is greater than or equal to 60%. A higher molar ratio of acrylic monomer in the binder helps to form a tighter polymer network structure, which can provide better mechanical support, resist the mechanical stress caused by the volume change of the electrode material during the charge and discharge process, and reduce the breakage and shedding of the electrode material. In addition, a higher content of acrylic monomer helps to improve the fluidity and coating of the slurry during the preparation of the pole piece, so that the active material, conductive agent and binder can be more evenly distributed on the current collector, thereby improving the performance and consistency of the battery.

[0036] In some embodiments, the monomers of the binder B include a first monomer, a second monomer, and a third monomer.

[0037] The structural formula of the third monomer is shown in formula (I): Wherein, R1=R2=-H; R3=-H or -CH3; R4=-C n H 2n , 1≤n≤4; R5=-OH or

[0038] Introducing epoxy functional groups into styrene-butadiene rubber can form a synergistic effect with polyacrylic acid binders. The third monomer with a linear structure will modify the functional group with a cross-linking site to the styrene-butadiene rubber instead of the polyacrylic acid, which can avoid a significant change in the viscosity of the polyacrylic acid product and ensure the processing performance of the slurry.

[0039] In some embodiments, the first monomer is a styrene monomer, and the second monomer is a butadiene monomer. In some embodiments, butadiene, styrene monomer, a third monomer, water and an emulsifier are mixed to form an emulsion with the help of mechanical stirring; the polymerization reaction is started by adding an initiator, and during the polymerization process, the monomer undergoes a polymerization reaction in the emulsion to form a styrene-butadiene rubber polymer. The binder B prepared in this scheme can be synthesized by conventional emulsion polymerization. The polymerization heat of the emulsion polymerization method is easy to diffuse, and the polymerization reaction temperature is easy to control; the polymerization system has a low viscosity even in the later stage of the reaction, which is suitable for preparing highly viscous polymers; a high molecular weight polymer can be obtained; and it can be used directly in the form of an emulsion.

[0040] In some embodiments, the molar masses of the first monomer, the second monomer, and the third monomer are a, b, and c, respectively, and a, b, and c follow the following relationship: The cross-linking principle of binder A and binder B is that the acrylic monomer in binder A and the third monomer in binder B are promoted to undergo esterification reaction between the epoxy group / hydroxyl group of the acrylic monomer and the third monomer through heat treatment, thereby realizing the construction of a cross-linking network.

[0041] when When , the viscosity of binder B is high, resulting in a significant increase in the viscosity of the negative electrode slurry at the same solid content, which is not conducive to subsequent coating processing; when When the concentration of reactive sites in binder B is too low, it is not conducive to the construction of a cross-linked network in the subsequent baking of the electrode. In summary, the three components in binder B must follow the following quantitative relationship:

[0042] In order to solve the above problems, the present invention also proposes a negative electrode slurry, including a negative electrode active material, a conductive agent and the above-mentioned binder, the mass ratio of the negative electrode active material, the conductive agent and the binder is (60-99): (0.5-20): (0.5-20), and the sum of the three parts is always 100 parts.

[0043] In some embodiments, the negative electrode active material includes graphite, silicon-oxygen material, silicon-carbon composite material, etc.

[0044] In some embodiments, the conductive agent includes Super-P Li (SP), Ketjen Black (ECP), conductive graphite KS-6, SFG-6, artificial graphite, carbon fiber, carbon nanotube, graphene, and the like.

[0045] In order to solve the above problem, the present invention further proposes a negative electrode plate, wherein the negative electrode plate is a negative electrode plate prepared by using the above negative electrode slurry.

[0046] In some embodiments, the preparation of the negative electrode sheet includes heat treatment, which includes: after the negative electrode slurry is coated on the negative electrode current collector, vacuum baking at a temperature of 60-120°C for 4-8h, rolling, striping, welding the pole ear and other processes to prepare the negative electrode sheet. In this scheme, the baking treatment forms a non-in-situ cross-linked network, and the alternating soft and hard cross-linked network can better limit the expansion of the active material. If the baking time is less than 4h, the cross-linking reaction cannot be fully carried out; if the baking time is greater than 8h, since the reaction has tended to be fully carried out under the 8h baking time, the baking time greater than 8h does not promote the reaction. The preferred baking time is 8h, and under the baking time of 8h, the cross-linking reaction between the active sites in the pole piece tends to be completed. At the same time, if the baking temperature is less than 60°C, the cross-linking reaction cannot be fully carried out; if the baking temperature is greater than 120°C, it may cause self-cross-linking of the reaction sites in the binder B, which in turn leads to incomplete cross-linking reaction of the pole piece, affecting the inhibitory effect of the cross-linked network on the expansion of the pole piece.

[0047] In order to solve the above problems, the present invention further proposes a lithium-ion battery, wherein the battery comprises the above-mentioned negative electrode plate, and the battery comprises any one of a battery cell, a battery module, and a battery pack.

[0048] Example 1

[0049] 1. Preparation of binder for secondary batteries

[0050] 40 mol% of acrylic acid (acrylic monomer) and 60 mol% of acrylamide (vinyl monomer) were mixed, and 0.5% of ammonium persulfate based on the amount of monomer substance was added as an initiator; deionized water was used as a solvent, and the system concentration was adjusted to 5wt%. Under the conditions of 50°C and normal pressure, protective gas was introduced into the reaction system for 30 minutes for deoxygenation, and the system was kept in a closed state during the subsequent reaction process. The reaction time was 12 hours, and after the preparation, a binder A was obtained;

[0051] SDS is selected as an emulsifier and ammonium persulfate is used as an initiator. 42.85 mol% of butadiene (the second monomer), 42.85 mol% of styrene (the first monomer), 14.3 mol% of the third monomer, 1.5 wt% of the emulsifier accounting for the total mass fraction of the first to third monomers, and water are mixed to form an emulsion with the help of mechanical stirring; an initiator is added according to 0.5 wt% of the monomer to start the polymerization reaction. During the polymerization process, the monomer undergoes a polymerization reaction in the emulsion to form a binder B; wherein the third monomer is Among them, R1=R2=R3=H; R4=C2H8; The molar ratio of a, b, and c is

[0052] The binder A and the binder B were mixed at a weight ratio of 4:1 to obtain a binder for a secondary battery.

[0053] 2. Preparation of lithium-ion batteries

[0054] 1. The porous silicon carbon material (BSO-1), Super-P Li, and the binder prepared above are fully stirred and dispersed at a mass ratio of 95:2.5:2.5 to obtain a negative electrode slurry, and the solid content of the negative electrode slurry is adjusted to 40%. The negative electrode slurry is evenly coated on the negative electrode collector. The current collector coated with the negative electrode slurry is prepared into a negative electrode sheet by vacuum baking at a temperature of 90°C for 8 hours, rolling, striping, welding the pole ears, and other processes.

[0055] 2. Assemble the positive electrode sheet, the negative electrode sheet, the separator, and the aluminum-plastic film, inject the electrolyte after baking, and obtain the lithium-ion battery through the processes of standing, forming, secondary sealing, and testing.

[0056] Example 2

[0057] In Example 2, binder A: binder B are mixed in a weight ratio of 7:1 to obtain a binder for a secondary battery. The rest is the same as in Example 1.

[0058] Embodiment 3:

[0059] In Example 3, binder A: binder B are mixed in a weight ratio of 10:1 to obtain a binder for a secondary battery. The rest is the same as in Example 1.

[0060] Embodiment 4:

[0061] In Example 4, the baking temperature of the electrode is 60° C. and the baking time is 8 h. Other conditions are the same as those in Example 1.

[0062] Embodiment 5:

[0063] In Example 5, the baking temperature of the electrode is 120° C. and the baking time is 4 h. Other conditions are the same as those in Example 1.

[0064] Embodiment 6:

[0065] In Example 6, binder A: binder B are mixed in a weight ratio of 3:1 to obtain a binder for a secondary battery. The rest is the same as in Example 1.

[0066] Embodiment 7:

[0067] In Example 7, binder A:binder B are mixed at a weight ratio of 11:1 to obtain a binder for a secondary battery. The other conditions are the same as those in Example 1.

[0068] Embodiment 8:

[0069] In Example 8, the reaction raw materials of binder A do not contain acrylamide, but only contain acrylic acid, and the amount of acrylic acid used is the same as the weight of "40 mol% acrylic acid and 60 mol% acrylamide" in Example 1, and the rest are the same as Example 1.

[0070] Embodiment 9:

[0071] In Example 9, the reaction raw materials of the binder A include 30 mol % of acrylic acid and 70 mol % of acrylamide, and the rest are the same as in Example 1.

[0072] Embodiment 10:

[0073] In Example 10, the molar ratio of monomers a, b, and c in the binder B is 3:3:2, so that the molar ratio of monomers a, b, and c is The rest are the same as in Example 1.

[0074] Embodiment 11:

[0075] In Example 11, the molar ratio of monomers a, b, and c in the binder B is 9:9:2, so that the molar ratio of monomers a, b, and c is The rest are the same as in Example 1.

[0076] Comparative Example 1:

[0077] In Comparative Example 1, binder B was not added, and binder A the same as in Example 1 was used as a binder for a secondary battery. The porous silicon-carbon material, Super-P Li, and binder A were fully stirred and dispersed in a mass ratio of 95:2.5:2.5 to obtain a negative electrode slurry. The rest was the same as in Example 1.

[0078] Comparative Example 2:

[0079] In Comparative Example 2, the baking temperature of the electrode is 50° C. and the baking time is 8 h. Other conditions are the same as those in Example 1.

[0080] Comparative Example 3:

[0081] In Comparative Example 3, the baking temperature of the electrode is 130° C. and the baking time is 8 h. Other conditions are the same as those in Example 1.

[0082] Comparative Example 4:

[0083] In Comparative Example 4, the binder B was not added to the binder for secondary batteries, and commercially available styrene-butadiene rubber BM-451B was added to obtain a binder for secondary batteries. The other conditions were the same as those in Example 1.

[0084] Performance Testing

[0085] 1. Viscosity test of negative electrode slurry

[0086] After the negative electrode slurry is prepared, a Brookfield DVS viscometer is used with a 63#3 rotor to test for 3 minutes at the minimum range that can measure the viscosity. The final value is recorded as the viscosity of the slurry in mPa·s.

[0087] 2. Pole peeling force test

[0088] The negative electrode prepared according to the above implementation method is cut into strips with a width of 20 mm and a length of 180 mm. Prepare a glass plate with a clean, dust-free and uneven surface, and stick a 3M scotch double-sided tape with a width of 24 mm on the glass plate. Stick one end of the long strip of electrode on the double-sided tape, the length of the adhesive is 100 mm, and use a roller to roll the bonding section of the electrode and the glass plate 3-4 times to make the electrode and the glass plate fit tightly. Use an electronic universal testing machine to perform a pole piece peeling test to measure the adhesion of the pole piece. Make one end of the pole piece fixed on the glass plate face up, and the other end of the glass plate is fixed with a lower clamp. The free end of the pole piece is folded upward 180° and fixed with an upper clamp. At room temperature, keep the lower clamp stationary, and stretch the upper clamp upward at a tensile rate of 100 mm / min. Take the average peeling force of the 25mm-100mm segment as the adhesion obtained by a single group test, in gf.

[0089] 3. Cycle retention rate test

[0090] ① Charge the battery to 4.5V at 0.5C at 25℃, with a cut-off current of 0.05C, and leave it for 5 minutes;

[0091] ②Measure the discharge capacity of the battery at this time, recorded as D before;

[0092] ③Discharge the battery at 0.8C to 3V and leave it for 5 minutes;

[0093] ④ Charge the battery to 4.5V at 1.5C, with a cut-off current of 0.05C, and leave it for 5 minutes; ⑤ Repeat the above steps ③ and ④ for 300 times, and then measure the discharge capacity of the battery at this time, which is recorded as Dafter. Among them, the cycle retention rate = Dafter / Dbefore*100%.

[0094] 4. Cyclic thickness change rate test

[0095] ① Charge the battery to 4.5V at 0.5C at 25℃, with a cut-off current of 0.05C, and leave it for 5 minutes;

[0096] ②Measure the thickness of the battery at this time, recorded as d before;

[0097] ③Discharge the battery at 0.8C to 3V and leave it for 5 minutes;

[0098] ④ Charge the battery to 4.5V at 1.5C, with a cut-off current of 0.05C, and leave it for 5 minutes; ⑤ Repeat the above steps ③ and ④ for 300 times, and then measure the thickness of the battery at this time, which is recorded as d after. Among them, the cycle thickness retention rate = (d after - d before) / d before * 100%.

[0099] The variable statistics of the embodiments and comparative examples and the above performance test results are shown in Table 1 below.

[0100] Table 1. Variable statistics and performance comparison of examples and comparative examples

[0101]

[0102] By comparing Example 1 and Comparative Example 3, it is found that if the system contains only binder A, the viscosity of the negative electrode slurry is relatively high, which is not conducive to subsequent coating processing.

[0103] By comparing Example 1 and Comparative Example 1, it is found that if the binder B is the main component in the system, the viscosity of the negative electrode slurry is low, which is not conducive to the subsequent coating processing.

[0104] By comparing Examples 1 to 3 with Comparative Example 1, it is found that if the ratio of Binder A:Binder B is greater than 10:1, the electrode has fewer cross-linking sites after heat treatment and cannot form an effective cross-linking network, resulting in a slight loss in electrode adhesion and a slight deterioration in cycle performance.

[0105] By comparing Example 1 with Comparative Examples 2 to 3, it is found that if the baking temperature does not reach the range of 60 to 120°C, the bonding strength of the pole piece and the electrical performance of the battery cell will deteriorate. If the baking temperature is lower than 60°C, the secondary baking process of the pole piece cannot allow the cross-linking reaction to proceed completely, resulting in deterioration of the expansion performance; if the baking temperature is greater than 120°C, it may trigger the self-cross-linking of the active sites of the binder B, resulting in deterioration of the bonding strength and expansion performance of the pole piece.

[0106] By comparing Examples 1 to 3 with Comparative Example 1, it is found that when only binder A is used without adding binder B, the viscosity of the negative electrode slurry is high, and due to the absence of a cross-linked network, the adhesion of the electrode and the cycle performance of the battery cell are slightly deteriorated.

[0107] By comparing Examples 1 to 3 and Comparative Example 4, it is found that when the binder B is replaced with conventional styrene-butadiene rubber, the electrode bonding force and cycle performance are slightly deteriorated due to the lack of cross-linking reaction between the binder A and the unmodified styrene-butadiene rubber.

[0108] Among Examples 1 to 11, the preferred solution is Examples 1 to 3, which have better battery cell cycle performance.

[0109] The above performance results indicate that the binder for secondary batteries provided by the present invention uses two binders in combination, and the functional groups with cross-linking sites are modified onto styrene-butadiene rubber instead of polyacrylic acid, which can avoid a significant change in the viscosity of the polyacrylic acid product and ensure the processing performance of the slurry. In addition, the introduction of acrylate active monomers into styrene-butadiene rubber can improve the flexibility of styrene-butadiene rubber and introduce flexible segments into the cross-linking network. At the same time, due to the introduction of epoxy functional groups into styrene-butadiene rubber, through the secondary processing of the electrode sheets, an effective synergistic effect can be formed between the two types of binders to form a cross-linking network of alternating soft and hard layers with stronger adhesion to maintain good contact between the active material, the conductive additive and the current collector, reduce the material cracking and crushing caused by the volume change of silicon during the charge and discharge cycle, better limit the expansion of the active material, avoid damage to the electrode structure, and thus avoid serious attenuation of the battery capacity.

[0110] Those skilled in the art may combine and associate different embodiments or examples and features of different embodiments or examples described in this specification without mutual contradiction.

[0111] The above description is only a partial or preferred embodiment of the present invention, and the scope of protection of the present invention cannot be limited therefrom. All equivalent structural changes made using the contents of the present invention specification under the concept of an overall concept of the present invention, or direct / indirect application in other related technical fields are included in the scope of protection of the present invention.

Claims

1. A binder for a secondary battery, characterized in that: The adhesive comprises an adhesive A and an adhesive B, wherein the adhesive A comprises a polyacrylic acid polymer and the adhesive B comprises styrene-butadiene rubber.

2. The adhesive according to claim 1, characterized in that The solid content ratio of the binder A to the binder B is (4-10):

1.

3. The adhesive according to claim 1, characterized in that The monomer of the binder A includes an acrylic monomer, and the acrylic monomer includes at least one of acrylic acid, methacrylic acid, methylene succinic acid, and maleic acid.

4. The adhesive according to claim 1, characterized in that The monomers of the binder A include acrylic monomers and vinyl monomers, and the molar ratio of the acrylic monomers in the binder A is greater than or equal to 40%.

5. The adhesive according to claim 1, characterized in that The monomers of the binder B include a first monomer, a second monomer and a third monomer. The structural formula of the third monomer is shown in formula (I): Wherein, R1=R2=-H; R3=-H or -CH3; R4=-C n H 2n , 1≤n≤4; R5=-OH or 6. The adhesive according to claim 5, characterized in that The first monomer is a styrene monomer, and the second monomer is a butadiene monomer.

7. The adhesive according to claim 5, characterized in that The molar masses of the first monomer, the second monomer and the third monomer are a, b, and c, respectively, and a, b, and c follow the following relationship:

8. A negative electrode slurry, characterized in that: The negative electrode slurry includes a negative electrode active material, a conductive agent and a binder as described in any one of claims 1 to 7, wherein the mass ratio of the negative electrode active material, the conductive agent and the binder is (60-99):(0.5-20):(0.5-20), and the sum of the three parts is always 100 parts.

9. A negative electrode plate, characterized in that: The negative electrode plate is a negative electrode plate prepared using the negative electrode slurry as claimed in claim 8.

10. The negative electrode sheet according to claim 9, characterized in that: The preparation of the negative electrode plate includes heat treatment, and the heat treatment includes: coating the negative electrode slurry on the negative electrode current collector, and then vacuum baking at a temperature of 60-120° C. for 4-8 hours.

11. A lithium ion battery, characterized in that: The battery comprises the negative electrode sheet as claimed in claim 9 or 10.

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