A negative electrode binder, a method of preparing the same, a negative electrode, and a secondary battery

Abstract

The application belongs to the field of new energy materials, and discloses a negative electrode binder, which comprises the following monomers with the following weight percentage components: 28wt%-48wt% of soft monomers; 50wt%-70wt% of hard monomers; and 0.5wt%-3wt% of polymerizable monomers containing sulfonic acid groups and / or phosphoric acid groups. The binder is combined with the soft / hard monomers and the polymerizable monomers containing sulfonic acid groups and / or phosphoric acid groups, wherein the soft monomers endow the binder with bonding properties and flexibility, and the hard monomers endow the binder with structural strength; meanwhile, the hard monomers also play a role in constructing current-carrying channels, and the polymerizable monomers containing sulfonic acid groups and / or phosphoric acid groups are used for modifying the current-carrying channels, improving the transmission capacity of the current-carrying channels for Li ions, and improving the electrochemical properties of the binder. Meanwhile, the application also provides a preparation method of the binder, a negative electrode, and a secondary battery.

Description

Technical Field

[0001] This invention belongs to the field of new energy materials, and more specifically, relates to a negative electrode binder and its preparation method, a negative electrode, and a secondary battery. Background Technology

[0002] As the lithium-ion battery industry continues to develop, the performance requirements for binders are also constantly increasing. Due to their high discharge power, power lithium-ion batteries require binders to have good adhesion as well as good electronic and ionic conductivity.

[0003] PAA-type binders are among the most widely used anode binders, primarily used in conjunction with graphite or silicon-based anodes. They effectively control silicon expansion, reduce material shedding, and improve battery cycle performance. However, these binders have low ionic conductivity, affecting the battery's charge / discharge rate and power performance, thus failing to meet the performance requirements of fast-charge / discharge batteries. Summary of the Invention

[0004] The main objective of this invention is to provide a negative electrode binder that employs a combination of soft / hard monomers and polymerizable monomers containing sulfonic acid groups and / or phosphate groups. The soft monomers impart adhesive properties and flexibility to the binder, while the hard monomers impart structural strength. Simultaneously, the hard monomers also play a role in constructing current-carrying channels, and the polymerizable monomers containing sulfonic acid groups and / or phosphate groups are used to modify the current-carrying channels, thereby improving the transport capacity of the current-carrying channels for Li ions and enhancing the electrochemical properties of the binder, such as ionic conductivity and charge transfer impedance.

[0005] In addition, the present invention also provides a method for preparing the binder, a negative electrode, and a secondary battery.

[0006] The specific solution of the present invention is as follows:

[0007] A negative electrode binder comprising monomers in the following weight percentages:

[0008] Soft monomers 28wt%~48wt%;

[0009] Hard monomer 50wt%~70wt%;

[0010] Polymerizable monomers containing sulfonic acid groups and / or phosphate groups, 0.5 wt% to 3 wt%.

[0011] In this field, polymer monomers are generally classified as hard or soft monomers according to their glass transition temperature. Soft monomers have lower glass transition temperatures, such as ethyl acrylate, butyl acrylate, and isooctyl acrylate, with glass transition temperatures ranging from -20°C to -70°C. The use of soft monomers imparts a certain degree of flexibility and extensibility to the resin, enabling it to provide good flexibility and extensibility when participating in resin copolymerization. Hard monomers have higher glass transition temperatures, such as methyl methacrylate, acrylonitrile, and styrene, with glass transition temperatures generally above 0°C. Hard monomers provide cohesion and strength in the copolymer, resulting in copolymers with higher hardness and strength, as well as good wear resistance and heat resistance.

[0012] The mechanism by which the above-mentioned monomers achieve the basic bonding properties and good electrochemical performance of the negative electrode binder in this invention is as follows:

[0013] 1. A proper combination of soft and hard monomers is essential to ensuring the basic bonding performance of the negative electrode binder;

[0014] The use of soft monomers imparts a certain degree of flexibility and extensibility to the resin, enabling it to provide good flexibility, extensibility, and adhesion when participating in copolymerization. Hard monomers provide cohesion and strength in the copolymer, resulting in higher hardness and strength, as well as good abrasion and heat resistance. In other words, increasing the amount of soft monomers can improve adhesion and peel strength, while increasing the amount of hard monomers can improve cohesion; however, not all increases will achieve the above effects, and excessively soft or hard binders will lead to the opposite development of properties.

[0015] Based on the above principles, a suitable ratio of soft and hard monomers can keep the peeling force and cohesion in good condition.

[0016] 2. Controlling the amount of hard monomers and using polymerizable monomers are essential for achieving good electrochemical performance;

[0017] The mechanism by which this invention improves ionic conductivity lies in constructing a stable and efficient ion-carrying channel. Constructing a stable and efficient ion-carrying channel requires the existence of a stable ion-carrying channel, and simultaneously, the current-carrying efficiency of this channel must be sufficiently high. In this invention, a stable ion-carrying channel is constructed by using an appropriate amount of hard monomers, and the conductivity of the current-carrying channel is further improved by polymerizable monomer sulfonic acid groups and / or phosphate groups, thereby enhancing the channel's ability to transport Li ions. Ionic conductivity is determined by the Li ion transport capacity of the current-carrying channel, and the charge transfer impedance and ionic conductivity further reduce the film resistance.

[0018] 3. The dosage of both soft and hard monomers, as well as the dosage of polymerizable monomers, ensured the achievement of the above effects;

[0019] In this invention, if the amount of soft monomer is higher than 48 wt% and the amount of hard monomer is lower than 50 wt%, the negative electrode binder will become too soft, reducing the bonding performance. At the same time, it cannot guarantee the stability of the ion carrying channel, and the ion carrying channel is prone to collapse, which significantly reduces the Li ion transport capacity of the carrying channel, significantly reduces the ion conductivity, and significantly increases the impedance.

[0020] If the amount of soft monomer is less than 28 wt% and the amount of hard monomer is more than 70 wt%, the negative electrode binder will become too hard and brittle, losing its basic application value as an adhesive. At the same time, if too much hard monomer is used, the channel will become too rigid, which will not be able to effectively wet and absorb the electrolyte, significantly affecting the conduction of Li ions.

[0021] If the amount of functional monomer is less than 0.5 wt%, the improvement in the Li ion transport capacity of the current-carrying channel is not significant. If the amount of functional monomer exceeds 3 wt%, the introduction of excessive sulfonic acid groups and / or phosphate groups leads to a significant decrease in ionic conductivity, a significant increase in impedance, and a decrease in peeling force and cohesion, indicating that excessive use of sulfonic acid groups and / or phosphate groups may disrupt the stability of the current-carrying channel.

[0022] Through the above optimizations, the negative electrode binder of the present invention can achieve good bonding performance, good ionic conductivity and low impedance.

[0023] The above-mentioned negative electrode binder includes monomers comprising the following weight percentage components:

[0024] The soft monomer has a weight percentage of 33.33 wt% to 48.25 wt%; the hard monomer has a weight percentage of 50 wt% to 64.91 wt%.

[0025] In the aforementioned negative electrode binder, the soft monomer is any one or a combination of at least two of acrylic acid, alkyl acrylate, ethoxyacrylate, methoxy polyethylene glycol methacrylate, or methethoxyacrylate. The alkyl acrylate includes, but is not limited to, straight-chain alkyl acrylate and branched-chain alkyl acrylate; preferably, the alkyl group in the alkyl acrylate refers to an alkyl group having 1 to 12 carbon atoms; the alkyl acrylate can be methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, isooctyl acrylate, etc.

[0026] In the above-mentioned negative electrode binder, the hard monomer is any one or a combination of at least two of acrylonitrile, methacrylic acid, styrene, isobornyl acrylate, acrylamide, isobornyl methacrylate, methyl methacrylate, or diisopropylbenzene.

[0027] In the above-mentioned negative electrode binder, the polymerizable monomer containing sulfonic acid groups and / or phosphate groups is sodium p-styrene sulfonate or a combination of sodium p-styrene sulfonate with any one of 2-acrylamido-2-methylpropanesulfonic acid, sodium allyl sulfonate, alkyl acrylate phosphate, and ethylene glycol methacrylate phosphate.

[0028] Meanwhile, the present invention also discloses a method for preparing the negative electrode binder as described above, wherein the negative electrode binder is obtained by polymerizing monomers using a free radical emulsion polymerization method.

[0029] In the free radical emulsion polymerization method, the amount of emulsifier used is equivalent to 0.5 wt% to 2.5 wt% of the total monomer; the amount of initiator used is equivalent to 0.1 wt% to 1 wt% of the total monomer.

[0030] In some preferred embodiments of the present invention, the amount of the emulsifier is equivalent to 0.5 wt%, 0.7 wt%, 0.9 wt%, 1.0 wt%, 1.5 wt%, 2.0 wt%, or 2.5 wt% of the total monomers; and the amount of the initiator is equivalent to 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, or 1.0 wt% of the total monomers.

[0031] In the above preparation method, the emulsifier is any one or a combination of at least two of sodium dodecyl sulfonate, sodium dodecyl sulfate, and sodium dodecylbenzene sulfonate.

[0032] The initiator is a redox initiator.

[0033] The method specifically involves adding emulsifier, monomer, and deionized water into a reaction vessel, heating to 50–80°C under inert gas protection, adding an initiator, reacting for 1–5 hours, discharging the product, and adjusting the pH to 6–8. Preferably, the solid content of the product is adjusted to 8 wt%–12 wt%.

[0034] Furthermore, the present invention also discloses the use of the negative electrode binder described above in preparing negative electrode sheets and secondary batteries, and a negative electrode comprising a negative electrode sheet and a negative electrode slurry coated on the negative electrode sheet, wherein the negative electrode slurry contains the negative electrode binder described above.

[0035] In this field, the method for preparing the negative electrode is generally as follows:

[0036] First, the negative electrode active material, such as graphite, is mixed with a conductive agent and a negative electrode binder to form a slurry;

[0037] The slurry is then coated onto the negative electrode sheet, such as an aluminum foil electrode sheet or a copper foil electrode sheet, to form a primary slurry film;

[0038] Finally, the negative electrode sheet and the slurry film are tightly bonded together by drying, rolling and other methods.

[0039] Finally, a secondary battery is also disclosed, including a positive electrode, a negative electrode as described above, an electrolyte, and a separator; the secondary battery is preferably a lithium-ion battery, a sodium-ion battery, etc.; the corresponding positive electrode, separator, and electrolyte are the positive electrode, separator, and electrolyte commonly used for the corresponding secondary battery type.

[0040] One of the above-described technical solutions of the present invention has at least one of the following advantages or beneficial effects:

[0041] 1. A proper combination of soft and hard monomers is essential for achieving the basic bonding performance of the negative electrode binder. A suitable ratio of soft and hard monomers can ensure that the peel strength and cohesion are in good condition.

[0042] 2. Controlling the amount of hard monomers and using polymerizable monomers are essential for achieving good electrochemical performance;

[0043] The mechanism by which this invention improves ionic conductivity lies in the construction of a stable and efficient ion-carrying channel.

[0044] Constructing a stable and efficient ion-carrying channel requires the existence of a stable ion-carrying channel with sufficiently high current-carrying efficiency. In this invention, a stable ion-carrying channel is constructed by using an appropriate amount of hard monomers, and the conductivity of the channel is further improved by polymerizable monomer sulfonic acid groups and / or phosphate groups, thereby enhancing the channel's ability to transport Li ions. The ion conductivity is determined by the Li ion transport capacity of the channel, and the charge transfer impedance and ion conductivity further reduce the membrane resistance.

[0045] 3. The dosage of both soft and hard monomers, as well as the dosage of polymerizable monomers, ensured the achievement of the above effects;

[0046] Through the above optimizations, the negative electrode binder of the present invention can achieve good bonding performance, good ionic conductivity and low impedance. Detailed Implementation

[0047] The embodiments of the present invention are described in detail below. These embodiments are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0048] Examples 1-23

[0049] A lithium-ion battery negative electrode binder, the preparation method of which includes the following steps:

[0050] Add the surfactant, polymerizable monomers containing sulfonic acid groups and / or phosphate groups, hard monomers, soft monomers and 300g of deionized water to a 500mL five-necked flask, start stirring at 25rpm, purge with nitrogen for 30min, and heat to 65℃.

[0051] Dissolve the oxidant in 5g of deionized water and the reducing agent in 5g of deionized water as redox initiators, and add them sequentially to a 1L five-necked flask. After reacting for 120 minutes, raise the temperature to 70℃ and hold for 60 minutes. Cool down and discharge the product, add NaOH solution to neutralize, and adjust the pH to 6-8. Adjust the solid content of the product to 10wt% with deionized water to obtain the binder sample.

[0052] The specific ingredients are shown in Table 1 below;

[0053]

[0054] Comparative Examples 1 to 4

[0055] The preparation method is as described in Example 1; the formulation is as described in Table 2.

[0056] Table 2 Comparative Example Formulations (Unit: g)

[0057]

[0058] Performance testing

[0059] The embodiments and comparative examples of the present invention have undergone electrochemical and mechanical performance tests.

[0060] Electrochemical tests include: ionic conductivity testing, charge transfer impedance testing, and membrane resistance testing; mechanical property tests include: peel force testing and cohesive force testing.

[0061] The testing methods for the above tests are as follows:

[0062] Ionic conductivity test: The adhesive solution was coated onto a film, a button cell was installed, and the test was performed using an electrochemical workstation;

[0063] Charge transfer impedance transfer test: After homogenizing the adhesive solution, a film was coated, a button cell was installed, and the test was performed using an electrochemical workstation.

[0064] Diaphragm resistance test: After homogenizing the adhesive solution, a coating machine is used to coat the film, and a diaphragm resistance meter is used for testing;

[0065] Peel strength test: After homogenizing the adhesive solution, spread it on a sheet using a coating machine and test it using a peel strength tester;

[0066] Cohesive strength test: After homogenizing the adhesive solution, spread it on a sheet using a coating machine and test it using a peel strength tester.

[0067] The test results are shown in Table 3.

[0068] Table 3 Test Results

[0069]

[0070]

[0071] Results analysis:

[0072] 1. As can be seen from Examples 1-22 and Comparative Examples 1-4, when the weight percentage of soft monomers is 28wt% to 48wt%, the weight percentage of hard monomers is 50wt% to 70wt%, and the weight percentage of polymerizable monomers containing sulfonic acid groups and / or phosphate groups is 0.5wt% to 3wt%, the binder has good electrical conductivity and charge transfer resistance; when the weight percentage of hard monomers is 50wt% to 65wt% and the weight percentage of soft monomers is 34wt% to 48wt%, the electrical conductivity and charge transfer resistance of the binder reach the optimal level.

[0073] 2. Examples 1 to 5 show that by fixing the proportion of polymerizable monomers containing sulfonic acid groups and / or phosphate groups (hereinafter referred to as polymerizable monomers), the proportion of soft and hard monomers was increased or decreased accordingly. As can be seen from Examples 1 to 5, when the amount of hard monomer is 50% to 64.91%, its ionic conductivity reaches the maximum and the corresponding impedance decreases to the minimum, indicating that hard monomers are very important for the construction of ion channels. In terms of adhesion performance, the least amount of hard monomer is used and the adhesion performance is the best. The reason is that the glass transition temperature of soft monomers is lower, which gives the negative electrode binder good adhesion performance and flexibility. Therefore, the increase of soft monomers increases the peeling performance accordingly.

[0074] In summary, hard monomers are important for constructing ion channels, but increasing their dosage limits the improvement in exfoliation strength.

[0075] 3. Examples 6, 7, 8, and 9 respectively verified the effect of different ratios of soft and hard monomers on performance under the conditions of maximum and minimum values ​​of polymerizable monomers. The amount of polymerizable monomer used in Example 6 was lower than that in Example 7, but the ionic conductivity of Example 6 was higher than that of Example 7. The amount of polymerizable monomer used in Example 8 was lower than that in Example 9, and conversely, the ionic conductivity of Example 8 was lower than that of Example 9.

[0076] The reason for the above differences is that the amount of hard monomer used in Example 7 is lower than that in Example 6. Although the amount of polymerizable monomer used is more, the stability of its ion channel is not as good as that in Example 6, so it exhibits a lower ionic conductivity than Example 6.

[0077] In summary, ionic conductivity is determined by both hard monomers and polymerizable monomers.

[0078] 4. In order to clarify the effect of polymerizable monomers, in Examples 8 to 11 and Example 1, the amount of polymerizable monomers was adjusted. The amount of increase or decrease in the amount of polymerizable monomers was added to or deducted from n-amyl acrylate. The effect of polymerizable monomers on performance was measured by keeping the total amount of monomers consistent and the amount of hard monomers unchanged.

[0079] The experimental results from Examples 1, 8 to 11 show that as the amount of polymerizable monomer increases, the ionic conductivity gradually increases, while the impedance and resistance decrease. When the amount of polymerizable monomer increases to 2.28%, the corresponding electrochemical performance weakens slightly. Adhesion performance tests show that adjusting the amount of polymerizable monomer has no significant impact on adhesion performance.

[0080] In summary, under the premise of keeping the proportion of hard monomers unchanged, appropriately increasing the amount of polymerizable monomers can increase ionic conductivity, while having little effect on peel strength; this indicates that polymerizable monomers are one of the core factors in achieving increased ionic conductivity.

[0081] 5. As can be seen from Examples 1, 12, and 13, in the scheme of Example 1, the contribution of sodium styrene sulfonate to ionic conductivity is greater than that of 2-acrylamido-2-methylpropanesulfonic acid and sodium allyl sulfonate to ionic conductivity. The possible reason is that the aromatic groups contained in sodium styrene sulfonate have a positive effect on ion conduction.

[0082] Examples 16 to 23 further verified the applicability of the present invention under different formulations and ratios.

[0083] Through Examples 1, 14, 15, 12, and 13, it was found that the scheme using sulfonic acid-based polymerizable monomers significantly improved ionic conductivity compared to the scheme using phosphate-based polymerizable monomers.

[0084] In summary, although different polymerizable monomers exhibit varying performance characteristics in their applications, they all effectively improve ionic conductivity. In practical applications, polymerizable monomers with sulfonic acid groups are preferred.

[0085] 6. Comparative Examples 1, 2, and 1 demonstrate the effect of too much or too little polymerizable monomer on performance;

[0086] Both excessive and insufficient polymerizable monomers lead to a decrease in ionic conductivity, but the reasons for this decrease differ. Comparative Example 1 uses fewer polymerizable monomers, resulting in minimal changes in peel strength and cohesive force, but a significant decrease in ionic conductivity, indicating insufficient sulfonic acid groups in the current-carrying channel. Comparative Example 2 uses excessive sulfonic acid groups, which may reduce the stability of the current-carrying channel, manifesting as decreased peel strength and cohesive force, ultimately leading to a decrease in ionic conductivity.

[0087] Comparative Example 3 demonstrates that excessive soft monomers are detrimental to the stability of the current-carrying channel, manifested as a decrease in peeling force and cohesive force, ultimately leading to a decrease in ionic conductivity.

[0088] Comparative Example 4 demonstrates that while an excessive amount of hard monomers can create stable current-carrying channels and its ionic conductivity is superior to that of Comparative Example 3, the ionic conductivity of Comparative Example 4 is still lower than that of Example 2, which contained 70% hard monomers. This may be because the binder itself is too hard, leading to reduced affinity with the electrolyte and consequently negatively impacting conductivity. As is well known to those skilled in the art, excessive hard monomers also negatively affect peel strength and cohesive strength.

[0089] In summary, the amount of soft and hard monomers and the amount of functional monomers used in this invention should be reasonably controlled.

[0090] In summary, this invention improves the transport capacity of the current-carrying channels for Li ions and enhances the electrochemical properties of the binder, such as ionic conductivity and charge transfer impedance, by using a reasonable ratio of soft and hard monomers and the use of functional monomers, while ensuring basic bonding performance.

[0091] The applicant declares that the present invention is illustrated by the above embodiments, but the present invention is not limited to the above process steps, nor does it mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A negative electrode binder, characterized in that, Monomers comprising the following components by weight percentage: Soft monomers 28wt%~48wt%; Hard monomer 50wt%~70wt%; Polymerizable monomers containing sulfonic acid groups and / or phosphate groups, 0.5 wt% to 3 wt%.

2. The negative electrode binder according to claim 1, characterized in that, The soft monomer has a weight percentage of 33.33 wt% to 48.25 wt%; the hard monomer has a weight percentage of 50 wt% to 64.91 wt%.

3. The negative electrode binder according to claim 1, characterized in that, The soft monomer is any one or a combination of at least two of acrylic acid, alkyl acrylate, ethoxyacrylate, methoxy polyethylene glycol methacrylate, or methethoxyacrylate.

4. The negative electrode binder according to claim 1, characterized in that, The hard monomer is any one or a combination of at least two of acrylonitrile, methacrylic acid, styrene, isobornyl acrylate, acrylamide, isobornyl methacrylate, methyl methacrylate, or diisopropylbenzene.

5. The negative electrode binder according to claim 1, characterized in that, The polymerizable monomer containing sulfonic acid groups and / or phosphate groups is sodium p-styrene sulfonate or a combination of sodium p-styrene sulfonate with any one of 2-acrylamido-2-methylpropanesulfonic acid, sodium allyl sulfonate, alkyl acrylate phosphate, or ethylene glycol methacrylate phosphate.

6. A method for preparing a negative electrode binder as described in any one of claims 1 to 5, characterized in that, Emulsifier, soft monomer, hard monomer, polymerizable monomer containing sulfonic acid group and / or phosphate group, and deionized water are added to the reaction vessel. Under inert gas protection, the temperature is raised to 50-80℃, an initiator is added, and the reaction is carried out for 1-5 hours before discharge. The pH is then adjusted to 6-8.

7. The preparation method according to claim 6, characterized in that, The amount of emulsifier used is equivalent to 0.5 wt% to 2.5 wt% of the total weight of the soft monomers, hard monomers, and polymerizable monomers containing sulfonic acid groups and / or phosphate groups; the amount of initiator used is equivalent to 0.1 wt% to 1 wt% of the total weight of the soft monomers, hard monomers, and polymerizable monomers containing sulfonic acid groups and / or phosphate groups.

8. The preparation method according to claim 7, characterized in that, The emulsifier is any one or a combination of at least two of sodium dodecyl sulfonate, sodium dodecyl sulfate, and sodium dodecylbenzene sulfonate. The initiator is a redox initiator.

9. A negative electrode, characterized in that, It includes a negative electrode sheet and a negative electrode paste coated on the negative electrode sheet, wherein the negative electrode paste contains a negative electrode binder as described in any one of claims 1 to 6.

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

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