A negative electrode binder, a negative electrode sheet, and a secondary battery.

Abstract

To address the problem that existing secondary battery negative electrode binders cannot effectively suppress the volume change of hard carbon negative electrodes during cycling, this invention provides a negative electrode binder, a negative electrode sheet, and a secondary battery. The negative electrode binder comprises a polymer, which includes structural units containing acidic functional groups, rigid structural units, functional structural units, and random copolymer structural units. The functional structural units contain polyoxyethylene segments, and the random copolymer structural units contain polyether segments. The rigid structural units are structural units formed by the polymerization of rigid monomers, and the glass transition temperature of the homopolymer corresponding to the rigid monomers is 90-100°C. The negative electrode binder provided by this invention effectively solves the problem in existing sodium-ion battery negative electrode binders of difficulty in simultaneously achieving good mechanical properties, volume change adaptability, and high ionic conductivity through the synergistic effect of specific structural units.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery technology, specifically relating to a negative electrode binder, a negative electrode sheet, and a secondary battery. Background Technology

[0002] Sodium-ion batteries, due to their abundant sodium resources, low cost, and similar working principle to lithium-ion batteries, are considered a powerful supplement and potential alternative to lithium-ion batteries in large-scale energy storage. However, the commercial application of sodium-ion batteries still faces many challenges, especially the volume change of the anode material during cycling. Among sodium-ion battery anode materials, hard carbon is considered one of the most promising due to its good sodium storage performance, suitable interlayer spacing, and high initial efficiency. However, hard carbon undergoes significant volume changes during sodium ion insertion / extraction, leading to electrode structural instability and consequently affecting battery performance.

[0003] As a key component in maintaining the structural integrity of the negative electrode in sodium-ion batteries, binders stabilize active materials and conductive agents in the electrolyte environment, which is crucial for alleviating the aforementioned problems and improving battery performance. Currently, traditional polyvinylidene fluoride (PVDF) binders are widely used in lithium-ion batteries due to their excellent bonding properties. However, they are unsuitable for sodium-ion battery applications because they are insoluble in water, rely on the highly toxic N-methyl-2-pyrrolidone (NMP) solvent, have poor electronic and ionic conductivity, and do not adapt well to volume changes in the negative electrode of sodium-ion batteries.

[0004] Therefore, there is an urgent need for a new type of anode binder for sodium-ion batteries to suppress volume changes in hard carbon anodes during cycling and improve electrode structural stability. Summary of the Invention

[0005] This invention addresses the problem that existing secondary battery negative electrode binders cannot effectively suppress the volume change of hard carbon negative electrodes during cycling. This invention provides a negative electrode binder, a negative electrode sheet, and a secondary battery.

[0006] To solve the above-mentioned technical problems, the present invention provides a negative electrode binder, the negative electrode binder comprising a polymer, the polymer comprising structural units containing acidic functional groups, rigid structural units, functional structural units and random copolymer structural units, the functional structural units containing polyoxyethylene segments, and the random copolymer structural units containing polyether segments; The rigid structural unit is a structural unit formed by the polymerization of rigid monomers, and the glass transition temperature of the rigid monomers is 90-100℃.

[0007] Preferably, the mass ratio of the acid-containing functional group structural unit, the rigid structural unit, the functional structural unit and the random copolymer structural unit is (55-75):(11-30):(6-15):(2-8).

[0008] Preferably, the functional structural unit is a structural unit formed by the polymerization of functional monomers, and the functional monomers include at least one of methoxy polyethylene glycol acrylate, polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, 2-[2-(2-methoxyethoxy)ethoxy]ethyl acrylate, and polyoxyethylene methacrylate.

[0009] Preferably, the random copolymer structural unit comprises an acrylic-co-methoxy polyethylene glycol methacrylate structural unit.

[0010] Preferably, the acrylic acid-co-methoxy polyethylene glycol methacrylate structural unit is a structural unit formed by the polymerization of poly(acrylic acid-co-methoxy polyethylene glycol methacrylate), and the weight-average molecular weight of the poly(acrylic acid-co-methoxy polyethylene glycol methacrylate) is 400-1000 g / mol. And / or, in the poly(acrylic acid-co-methoxy polyethylene glycol methacrylate), the number-average degree of polymerization of the methoxy polyethylene glycol structural unit is 9-23.

[0011] Preferably, the rigid monomer includes at least one of acrylonitrile, isobornyl acrylate, pentaerythritol triacrylate, trimethylolpropane triacrylate, and divinylbenzene.

[0012] Preferably, the acidic functional group includes at least one selected from carboxylic acids, carboxylates, sulfonic acids, sulfonates, phosphoric acid, and phosphates.

[0013] Preferably, the polymer further includes crosslinking structural units, wherein the mass ratio of the structural units containing acidic functional groups to the crosslinking structural units is (55-75):(2-8).

[0014] Secondly, this application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector, wherein the negative electrode active material layer includes the negative electrode binder described in any of the above claims.

[0015] Thirdly, this application provides a secondary battery, including the negative electrode sheet as described above.

[0016] In this application, the polymer in the negative electrode binder effectively solves the problem in the prior art that sodium-ion battery negative electrode binders struggle to simultaneously achieve good mechanical properties, adaptability to volume changes, and high ionic conductivity through the synergistic effect of specific structural units. The structural units containing acidic functional groups enhance the interfacial adhesion between the binder and the active material by forming hydrogen bonds with the hard carbon surface through these acidic functional groups.

[0017] When rigid monomers copolymerize to form polymers, their high glass transition temperature (Tg) significantly improves the polymer's modulus and dimensional stability, effectively suppressing volume changes in hard carbon particles during charge and discharge, preventing microcrack formation, and thus improving the integrity and cycle life of the electrode structure. The copolymerization of functional structural units with rigid structural units and structural units containing acidic functional groups constructs a rigid framework network with both high Tg and low shrinkage, endowing the film binder with high modulus and dimensional stability. This further effectively suppresses the initiation and propagation of microcracks in hard carbon particles and SEI under repeated stress, improving the structural integrity and cycle life of the electrode.

[0018] The random copolymer structural units contain polyether segments. The functional structural units and the random copolymer structural units construct continuous hydrophilic / ion transport channels, significantly improving electrolyte wettability and Na+. + It improves solvation / migration efficiency, reduces interfacial impedance and concentration polarization, enhances rate and low-temperature performance, and suppresses the "solvent co-intercalation-swelling" chain reaction triggered by local ion accumulation and side reactions. It not only enhances the hydrophilicity of the polymer but also improves the elasticity and stress relaxation ability of the binder through its dynamically reversible chemical bonds, adapting to the volume changes of the hard carbon anode, while simultaneously reducing interfacial impedance and increasing ionic conductivity.

[0019] Controlling the mass ratio of the above structural units within the range specified in this application is more conducive to forming a four-fold synergistic network of "rigid skeleton-flexible bridging-ion channel-carboxyl adhesion". This not only suppresses local stress concentration, but also reduces the pulverization of active materials, the breakage of conductive networks and the peeling of interfaces through uniform ion flux, significantly improving the long-cycle stability and capacity retention of sodium-ion batteries, while maintaining the environmentally friendly, safe and easy-to-mass-produce characteristics of the binder. Detailed Implementation

[0020] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0021] One embodiment of this application provides a negative electrode binder, the negative electrode binder comprising a polymer, the polymer comprising structural units containing acidic functional groups, rigid structural units, functional structural units, and random copolymer structural units, the functional structural units containing polyoxyethylene segments, and the random copolymer structural units containing polyether segments; The rigid structural unit is a structural unit formed by the polymerization of rigid monomers, and the glass transition temperature of the homopolymer corresponding to the rigid monomer is 90-100℃.

[0022] In this embodiment, the polymer in the negative electrode binder effectively solves the problem in the prior art that it is difficult to simultaneously achieve good mechanical properties, adaptability to volume changes, and high ionic conductivity in secondary battery negative electrode binders through the synergistic effect of specific structural units. The structural units containing acidic functional groups enhance the interfacial adhesion between the binder and the active material by forming hydrogen bonds with the hard carbon surface through the acidic functional groups.

[0023] When rigid monomers copolymerize to form polymers, their high glass transition temperature (Tg) significantly improves the polymer's modulus and dimensional stability, effectively suppressing volume changes in hard carbon particles during charge and discharge, preventing microcrack formation, and thus improving the integrity and cycle life of the electrode structure. The copolymerization of functional structural units with rigid structural units and structural units containing acidic functional groups constructs a rigid framework network with both high Tg and low shrinkage, endowing the film binder with high modulus and dimensional stability. This further effectively suppresses the initiation and propagation of microcracks in hard carbon particles and SEI under repeated stress, improving the structural integrity and cycle life of the electrode.

[0024] The random copolymer structural units contain polyether segments. The functional structural units and the random copolymer structural units construct continuous hydrophilic / ion transport channels, significantly improving electrolyte wettability and Na+. + This improves solvation / migration efficiency, reduces interfacial impedance and concentration polarization, enhances rate and low-temperature performance, and suppresses the "solvent co-intercalation-swelling" chain reaction triggered by local ion accumulation and side reactions. It not only enhances the polymer's hydrophilicity but also improves the binder's elasticity and stress relaxation ability through its dynamically reversible chemical bonds, adapting to the volume changes of hard carbon anodes. Simultaneously, it reduces interfacial impedance and increases ionic conductivity, significantly improving the long-cycle stability and capacity retention of sodium-ion batteries, while maintaining the binder's environmental friendliness, safety, and ease of large-scale production.

[0025] In some embodiments, the mass ratio of the acidic functional group-containing structural unit, rigid structural unit, functional structural unit, and random copolymer structural unit is (55-75):(11-30):(6-15):(2-8). Controlling the mass ratio of the above structural units within the range defined in this application is more conducive to forming a quadruple synergistic network of "rigid skeleton-flexible bridging-ion channel-carboxyl adhesion". This not only suppresses local stress concentration, but also reduces active material pulverization, conductive network breakage, and interface peeling through uniform ion flux, significantly improving the long-cycle stability and capacity retention of the secondary battery, while maintaining the environmentally friendly, safe, and easily mass-producible characteristics of the binder.

[0026] Specifically, the mass ratios of structural units containing acidic functional groups, rigid structural units, functional structural units, and random copolymer structural units include, but are not limited to, 55:11:6:2, 65:11:6:2, 75:11:6:2, 55:20:6:2, 55:30:6:2, 55:11:10:2, 55:11:15:2, 55:11:6:5, 55:11:6:8, 75:30:15:8, or 65:20:10:5.

[0027] In some embodiments, the functional structural unit is a structural unit formed by the polymerization of functional monomers, and the functional monomers include at least one of methoxy polyethylene glycol acrylate (MPEG-AC), polyethylene glycol diacrylate (PEGDA), polyethylene glycol dimethacrylate (PEGDMA), 2-[2-(2-methoxyethoxy)ethoxy]ethyl acrylate (MEO3EA), and polyoxyethylene methacrylate (POEM).

[0028] In this embodiment, the functional monomers contain polyoxyethylene segments. The functional structural units formed by introducing these functional monomers aim to introduce flexibility, enhance hydrophilicity, and form efficient ion conduction pathways. The selection of functional monomers in this preferred embodiment, working in conjunction with rigid monomers, provides the polymer with segmental mobility and low internal stress, which helps to construct a "flexible bridge" within the electrode material. This mitigates the volume changes of the hard carbon anode during charging and discharging, reduces the thickness rebound and pulverization of the active material layer in the electrode, and balances high elasticity and crack resistance. Simultaneously, the presence of polyoxyethylene segments or ether bonds optimizes the wettability of the electrolyte, enhances the solubilization and transport efficiency of sodium ions, thereby achieving a significant improvement in electrochemical cycling performance within the basic framework.

[0029] In some embodiments, the random copolymer structural unit includes an acrylic-co-methoxy polyethylene glycol methacrylate (AA-co-MPEGMA) structural unit, which is a structural unit formed by the polymerization of poly(acrylic-co-methoxy polyethylene glycol methacrylate). The polyether segments contained therein impart excellent hydrophilicity and ionic conductivity to the binder, effectively improving the electrochemical performance of the sodium-ion battery. The polyether segments act as ion channels in the electrolyte, not only enhancing the wettability of the electrolyte to the hard carbon anode, but also significantly improving the sodium ion transfer rate between electrode materials, reducing charge transfer impedance, optimizing battery rate performance and low-temperature operation characteristics, and in particular achieving a significant improvement in sodium ion conductivity.

[0030] Specifically, the introduction of the acrylic-co-methoxy polyethylene glycol methacrylate structural unit, in conjunction with the aforementioned functional monomers, constructs a continuous polyoxyethylene ion channel for the overall binder system, significantly improving the wettability of the electrolyte and Na+. + The solvation / migration efficiency is crucial for optimizing the electrochemical cycling performance of the battery. Furthermore, by precisely controlling the ratio of the random copolymer to other monomers, the binder can not only effectively suppress the volume change of the hard carbon anode during cycling, but also significantly reduce interfacial impedance, improving cycle stability and capacity retention.

[0031] In some embodiments, the preparation method of poly(acrylic acid-co-methoxy polyethylene glycol methacrylate) can be found in DOI: 10.3724 / SP.J.1105.2010.00160.

[0032] In some embodiments, the number-average degree of polymerization (NUP) of the methoxy polyethylene glycol structural units of the poly(acrylic acid-co-methoxy polyethylene glycol methacrylate) is 9-23. This ensures that the length and distribution of polyether segments within the random copolymer structural units are just right, providing sufficient hydrophilicity and ion channels to improve sodium ion transport efficiency, while avoiding excessive swelling of the binder or damage to the internal structure that may be caused by excessively long segments. The methoxy polyethylene glycol structural units are structural units formed by the polymerization of methoxy polyethylene glycol monomers, and the NUP of the methoxy polyethylene glycol structural units is the same as the NUP of the methoxy polyethylene glycol monomers.

[0033] In some embodiments, the weight-average molecular weight of the poly(acrylic acid-co-methoxy polyethylene glycol methacrylate) is 400–1000 g / mol. The weight-average molecular weight can be controlled by techniques commonly used in the art, such as controlling the proportion of reactants and the reaction conditions (e.g., reaction temperature) during synthesis.

[0034] Specifically, the appropriate polyether segment length and molecular weight distribution promote uniform wetting of the electrolyte between electrode materials, ensuring smooth migration of sodium ions at different rates, reducing concentration polarization, and thus significantly enhancing the cycle stability and rate performance of the battery. Simultaneously, it strengthens the cohesive force of the polymer, enabling the binder to more effectively maintain the integrity of the electrode structure when faced with volume changes in the hard carbon anode material, inhibiting the pulverization and shedding of active materials, and extending the battery's lifespan. Within this overall framework, this optimized solution not only achieves a perfect match between the electrode materials and the binder but also improves the overall performance of the sodium-ion battery.

[0035] In addition, functional monomers, such as MEO3EA and AA-co-MPEGMA, work synergistically to form continuous EO ion channels and carboxyl adhesion sites, which not only enhance electrolyte wettability and sodium ion migration efficiency, but also reduce charge transfer resistance, improve rate and low temperature performance, and suppress the "solvent co-intercalation-expansion" chain reaction triggered by local ion accumulation and side reactions.

[0036] In some embodiments, the rigid monomer includes at least one selected from acrylonitrile (AN), isobornyl acrylate (IBOA), pentaerythritol triacrylate (PETA), trimethylolpropane triacrylate (TMPTA), and divinylbenzene (DVB). The rigid framework of the binder is reinforced by the aforementioned rigid monomers, improving its resistance to deformation and solvent resistance, effectively suppressing volume changes in the hard carbon anode material during charge and discharge, and maintaining the integrity and stability of the electrode structure.

[0037] In some embodiments, the rigid structural unit includes an acrylonitrile structural unit and an isobornyl acrylate structural unit, and the mass ratio of the structural unit containing acidic functional groups to the acrylonitrile structural unit is (55-70):(10-20). The mass ratio of the structural unit containing acidic functional groups to the acrylonitrile structural unit is set within a specific range in order to coordinate mechanical properties and ion adhesion forces, and to ensure the structural stability of the hard carbon anode material during cycling.

[0038] The mass ratio of isobornyl acrylate, the functional structural unit, and the random copolymer structural unit is (8-18):(6-12):(4-8). This optimized mass ratio aims to introduce appropriate flexibility and improve hydrophilicity, creating a dynamic cross-linked network and polyether ion channels, thereby reducing internal stress and improving electrolyte wettability and Na+. + The migration efficiency.

[0039] The strongly polar backbone formed by the monomer containing acidic functional groups and AN provides the basic mechanical and solvent resistance of the binder. The introduction of sterically hindered alicyclic ester side groups of IBOA can significantly improve the Tg and hardness of the polymerization and reduce shrinkage, giving the binder high modulus and dimensional stability. This rigid continuous phase can effectively inhibit the initiation and propagation of microcracks in hard carbon particles and SEI under repeated stress, thereby improving the structural integrity and cycle life of the electrode.

[0040] IBOA and functional monomers together provide the polymer with chain segment mobility and low internal stress relaxation, forming a flexible "bridging phase" that runs through the binder network. Under rapid charging and discharging and high load conditions, it achieves strain homogenization and reversible energy dissipation, significantly alleviates the thickness rebound and pulverization of the active material layer, and takes into account both high elasticity and crack resistance.

[0041] The dual design of rigid skeleton network and flexible bridging allows the binder to absorb and release stress during cycling without damage, maintaining the structural integrity and interface stability of the electrode.

[0042] In some embodiments, the polymer further includes crosslinking structural units, which are structural units in which crosslinking monomers participate in polymerization. The crosslinking monomers include at least one of N,N'-methylenebisacrylamide (MBAA) and N,N'-methylenebisacrylamide. By introducing crosslinking structural units, the binder forms a crosslinked network, enabling the binder to absorb and release stress during cycling without damage, thus maintaining the structural integrity and interfacial stability of the electrode.

[0043] In some embodiments, the mass ratio of the structural unit containing acidic functional groups to the cross-linked structural unit is (55-75):(2-8).

[0044] In some embodiments, the acidic functional group includes at least one of carboxylic acid, carboxylate, sulfonic acid, sulfonate, phosphoric acid, and phosphate.

[0045] The structural units containing acidic functional groups in adhesives are mainly carboxylates, sulfonates, or phosphates. Taking carboxylates as an example, -COO - / Na + The full formation of the internal salt network enhances the formation of multi-point hydrogen bonds / ion-dipole anchoring with the conductive agent / current collector, strengthens interfacial adhesion and cohesion, and works synergistically with the ion channels formed by the polyether segments to reduce the interfacial energy barrier and stabilize the SEI, thereby improving the continuity and durability of the electronic / ion pathway.

[0046] In some embodiments, the structural unit containing acidic functional groups is a structural unit formed by the polymerization of acidic monomers, wherein the acidic monomers include at least one of acrylic acid (AA), methacrylic acid (MAA), itaconic acid (IA), fumaric acid (FA), sodium methacrylate sulfonate, sodium vinyl sulfonate, 2-acrylamide-2-methylpropanesulfonic acid (AMPS), sodium allyl sulfonate (SAS), and vinylphosphonic acid (VPA).

[0047] Specifically, acidic monomers can be neutralized before participating in polymerization, with the pH adjusted to 7.0-8.0. Alternatively, the corresponding salts can be used directly.

[0048] In some embodiments, the solid content of the negative electrode binder is 8%-15%, and the pH of the negative electrode binder is 6.5-8.0.

[0049] The solid content of the negative electrode binder is set between 8% and 15%. This range ensures sufficient dispersion and adhesion of the binder in the electrode slurry, while avoiding the increased processing difficulty and electrode internal resistance caused by excessively high solid content. The pH of the binder is controlled between 6.5 and 8.0 to form a stable internal salt network, enhancing the interfacial bonding with the conductive agent / current collector, while avoiding water absorption and swelling and abnormal processing viscosity caused by excessively high pH.

[0050] Furthermore, one embodiment of this application provides a method for preparing a negative electrode binder polymer, comprising the following steps: Neutralize the pH of the acidic monomer to 7.0-8.0 using NaOH or NH3·H2O. Under N2 protection, thoroughly mix the acidic monomer, rigid monomer, functional monomer, and random copolymer with water at 60-80℃. Add 0.8-1.2% initiator semi-continuously, react for 6-10 hours, and then add NaOH or NH3·H2O to control the target aqueous binder dispersion within the range of 6.5-8.0.

[0051] The initiator includes at least one of ammonium persulfate (APS), potassium persulfate (KPS), hydrogen peroxide, and azobisisobutyronitrile (AIBN).

[0052] In this invention, each of the aforementioned structural units 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 based on the mass content of the corresponding monomer in the total amount of monomers participating in the polymerization.

[0053] As those skilled in the art know, the reactions in the above steps are conventional free radical polymerization, and the specific methods and reaction conditions are common free radical polymerization methods in the prior art, which will not be described in detail in this invention.

[0054] An embodiment of the present invention also provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector, wherein the negative electrode active material layer comprises the negative electrode binder described in any of the preceding claims. By using the aforementioned negative electrode binder, a network structure combining a rigid framework and flexible bridging is constructed in the negative electrode active material layer, effectively suppressing the volume change of the hard carbon negative electrode during charging and discharging, while providing chain segment mobility and low internal stress buffering, enabling the electrode structure to remain stable under rapid charging and discharging and high load conditions, reducing the pulverization and shedding of active materials, and improving the structural integrity of the electrode and the cycle life of the battery.

[0055] Furthermore, the negative electrode active material layer also includes a negative electrode active material and a negative electrode conductive agent.

[0056] Specifically, the mass ratio of the negative electrode active material, the negative electrode conductive agent and the negative electrode binder is (75-96): (2-15): (2-10).

[0057] Negative electrode active materials include hard carbon.

[0058] The areal density of the negative electrode active material layer is 1.0-2.0 mg / cm³. 2 .

[0059] Furthermore, the negative electrode active material layer is obtained by coating with a negative electrode active slurry, the solid content of which is 45%-55% and the pH is 7.0-8.0.

[0060] An embodiment of the present invention also provides a secondary battery, including the negative electrode sheet as described above. The above-mentioned high-performance negative electrode sheet, when applied to sodium-ion batteries, can solve the problems of poor cycle performance, poor rate performance, and short service life caused by the unstable negative electrode structure in existing sodium-ion batteries, thereby improving the overall performance of sodium-ion batteries. The present invention will be further illustrated by the following examples.

[0061] Specifically, this invention discloses the negative electrode binder, negative electrode, and secondary battery.

[0062] Example 1 Negative electrode binder: 65 parts of acrylic acid were neutralized with NaOH and the pH was neutralized to 5.0-6.0 to obtain pre-neutralized acrylate.

[0063] The above-mentioned acrylate, 13 parts acrylonitrile (AN), 5 parts isobornyl acrylate (IBOA), 10 parts 2-[2-(2-methoxyethoxy)ethoxy]ethyl acrylate (MEO3EA), and 4 parts polyacrylate-co-methoxy polyethylene glycol methacrylate (AA-co-MPEGMA) were thoroughly mixed with 590 parts water. Ammonium persulfate (APS) was added as an initiator in a semi-continuous manner at 1% of the monomer mass. The reaction was carried out for 8 hours, followed by neutralization with NaOH to obtain a negative electrode binder dispersion with pH=7. The weight-average molecular weight of AA-co-MPEGMA was 700 g / mol, and the number-average degree of polymerization of the methoxy polyethylene glycol structural units was 16.

[0064] The negative electrode sheet is prepared as follows: hard carbon, negative electrode binder, and conductive agent Super P are blended in a mass ratio of 96:2:2, and then mixed with deionized water to obtain a slurry.

[0065] The slurry was coated onto copper foil, dried, and then cut into negative electrode sheets of a fixed size. The areal density of the negative electrode active material layer was 1.5 mg / cm³. 2 .

[0066] Sodium-ion battery preparation: After stacking negative electrode and Na3V2(PO4)3 positive electrode to form a dry cell, the electrolyte is injected to obtain a lithium battery. The electrolyte is made by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 mass ratio, and then adding NaPF6 to a molar concentration of 1.0 mol / L.

[0067] Examples 2-19 Examples 2-19 are largely the same as Example 1, except that they use the formulations in Table 1.

[0068] Example 20 Example 20 uses the same monomer type as Example 1, except that 0.5 parts of the cross-linking monomer N,N'-methylenebisacrylamide (MBAA) are added in Example 20.

[0069] Example 21 Example 21 uses the same monomer type as Example 1, except that 2 parts of the cross-linking monomer N,N'-methylenebisacrylamide (MBAA) are added in Example 21.

[0070] Example 22 Example 22 uses the same monomer type as Example 1, except that 5 parts of the cross-linking monomer N,N'-methylenebisacrylamide (MBAA) are added in Example 22.

[0071] Example 23 Example 23 uses the same monomer type as Example 1, except that 8 parts of the cross-linking monomer N,N'-methylenebisacrylamide (MBAA) are added in Example 23.

[0072] Example 24 Example 24 uses the same monomer type as Example 1, except that 10 parts of the cross-linking monomer N,N'-methylenebisacrylamide (MBAA) are added in Example 24.

[0073] Example 25 Example 25 is similar to Example 1 in most steps, except that the weight-average molecular weight of AA-co-MPEGMA is 400 g / mol and the number-average degree of polymerization of the methoxy polyethylene glycol structural units is 9.

[0074] Example 26 Example 26 is similar to Example 1 in most steps, except that the weight-average molecular weight of AA-co-MPEGMA is 1000 g / mol and the number-average degree of polymerization of the methoxy polyethylene glycol structural units is 23.

[0075] Comparative Examples 1-13 Most of the steps in Comparative Examples 1-13 are the same as those in Example 1, except that the formulations in Table 1 are used; wherein the acrylic-co-methoxy polyethylene glycol methacrylate used in Comparative Example 13 has a weight-average molecular weight of 1200 g / mol and a number-average degree of polymerization of 30.

[0076] Comparative Example 14 Comparative Example 14 and Example 1 follow most of the same steps, except that the negative electrode binder is a 1:1 mass ratio of CMC and SBR, with SBR being an emulsion and having a solid content of 48%.

[0077] Table 1 The negative electrode binder, negative electrode sheet, and sodium-ion battery prepared in the above embodiments and comparative examples were tested as follows.

[0078] 1. Film strength and elongation at break: Sample preparation method: The negative electrode adhesive liquid of the examples and comparative examples is spread evenly in the mold. The mold size is: length * width * depth = 8cm * 1.5cm * 0.5cm; put it in an oven and dry at 80℃ for 2 hours.

[0079] The dried film was tested using a universal tensile testing machine at a tensile rate of 1 mm / min.

[0080] 2. Ionic conductivity test: The negative electrode binder solutions from the examples and comparative examples were uniformly coated onto a flat substrate (such as glass, stainless steel sheet, etc.) to form a 25 μm film. After drying, the film was peeled off from the substrate to prepare the binder film. The binder film was then sandwiched between two stainless steel sheets to assemble a symmetrical battery. Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation. The resistance R was obtained by fitting the impedance spectrum, and the ionic conductivity was finally calculated using the formula: σ = R / AL, where L is the sample thickness (cm), R is the sample resistance (Ω), and A is the electrode contact area (cm²). 2 ).

[0081] 3. Cyclic performance: In a constant temperature test chamber at 25℃±2℃, the batteries of the examples and comparative examples were left to stand for 1 hour. After 3 cycles of charge and discharge at 0.5C, the initial capacity of the cycle was measured. Then, the batteries were charged to 3.65V at 1C constant current and constant voltage, and the cutoff current was 0.05C. The batteries were then discharged to 2.5V at 1C constant current, and the discharge capacity was recorded. The above steps were repeated 200 times, and the capacity retention rate was calculated.

[0082] 4. Ratio performance: The discharge capacity was tested at different rates of 0.1C, 0.5C, 1C, 2C, 3C, 4C, and 0.5C in a constant temperature test chamber at 25℃±2℃.

[0083] The test results are shown in Tables 2 and 3.

[0084] Table 2 Table 3 As shown in Tables 2 and 3, the test results of Examples 1-19 and Comparative Examples 1-14 indicate that when the negative electrode binder includes structural units containing acidic functional groups, rigid structural units, functional structural units, and random copolymer structural units, the resulting film exhibits good mechanical properties and ionic conductivity. Batteries using this negative electrode binder demonstrate good capacity retention and rate performance. When the mass ratio of the structural units containing acidic functional groups, rigid structural units, functional structural units, and random copolymer structural units is within the range of (55-75):(11-30):(6-15):(2-8), the mechanical properties and ionic conductivity of the resulting film are further improved. The capacity retention and rate performance of batteries using this negative electrode binder are also further enhanced.

[0085] As can be seen from the test results of Examples 1 and 16-19, when the negative electrode binder contains both acrylonitrile structural units and isobornyl acrylate structural units, and the mass ratio of isobornyl acrylate, functional structural units and random copolymer structural units is in the range of (8-18):(6-12):(4-8), the mechanical properties and ionic conductivity of the negative electrode binder are further improved, and the capacity retention rate and rate performance of the battery are further improved.

[0086] The test results of Examples 1 and 20-24 show that when the mass ratio of the structural unit containing acidic functional groups to the cross-linked structural unit is in the range of (55-75): (2-8), the mechanical properties of the film formed by the negative electrode binder are further improved, ensuring the integrity of the electrode structure and further improving the cycle performance of the battery.

[0087] The test results of Examples 1, 25, 26 and Comparative Example 13 show that when the weight-average molecular weight of poly(acrylic acid-co-methoxy polyethylene glycol methacrylate) is 400-1000 g / mol and the number-average degree of polymerization of its methoxy polyethylene glycol structural units is 9-23, the binder has high ionic conductivity, which ensures the smooth migration of sodium ions at different rates, reduces concentration polarization, and thus significantly enhances the cycle stability and rate performance of the battery.

[0088] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode binder, characterized in that, The negative electrode binder comprises a polymer, which includes structural units containing acidic functional groups, rigid structural units, functional structural units, and random copolymer structural units. The functional structural units contain polyoxyethylene segments, and the random copolymer structural units contain polyether segments. The rigid structural unit is a structural unit formed by the polymerization of rigid monomers, and the glass transition temperature of the rigid monomers is 90-100℃.

2. The negative electrode binder according to claim 1, characterized in that, The mass ratio of the acid-containing functional group structural unit, rigid structural unit, functional structural unit and random copolymer structural unit is (55-75): (11-30): (6-15): (2-8).

3. The negative electrode binder according to claim 2, characterized in that, The functional structural unit is a structural unit formed by the polymerization of functional monomers, and the functional monomers include at least one of methoxy polyethylene glycol acrylate, polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, 2-[2-(2-methoxyethoxy)ethoxy]ethyl acrylate, and polyoxyethylene methacrylate.

4. The negative electrode binder according to claim 1, characterized in that, The random copolymer structural unit includes acrylic-co-methoxy polyethylene glycol methacrylate structural unit.

5. The negative electrode binder according to claim 4, characterized in that, The acrylic-co-methoxy polyethylene glycol methacrylate structural unit is a structural unit formed by the polymerization of poly(acrylic-co-methoxy polyethylene glycol methacrylate), and the weight-average molecular weight of poly(acrylic-co-methoxy polyethylene glycol methacrylate) is 400-1000 g / mol. And / or, in the poly(acrylic acid-co-methoxy polyethylene glycol methacrylate), the number-average degree of polymerization of the methoxy polyethylene glycol structural unit is 9-23.

6. The negative electrode binder according to claim 1, characterized in that, The rigid monomer includes at least one of acrylonitrile, isobornyl acrylate, pentaerythritol triacrylate, trimethylolpropane triacrylate, and divinylbenzene.

7. The negative electrode binder according to claim 1, characterized in that, The acidic functional group includes at least one of carboxylic acid, carboxylate, sulfonic acid, sulfonate, phosphoric acid, and phosphate.

8. The negative electrode binder according to any one of claims 1-7, characterized in that, The polymer further includes cross-linked structural units, wherein the mass ratio of the structural units containing acidic functional groups to the cross-linked structural units is (55-75):(2-8).

9. A negative electrode sheet, characterized in that, It includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector, wherein the negative electrode active material layer includes the negative electrode binder according to any one of claims 1 to 8.

10. A secondary battery, characterized in that, Includes the negative electrode sheet as described in claim 9.

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