A composite binder, its preparation method and use

Abstract

The application belongs to the technical field of binders, and particularly relates to a composite binder, a preparation method and application thereof. The composite binder has a core-shell structure, the core material is modified or unmodified carbon nanotubes, and the shell material is PTFE. The core-shell structure not only makes the carbon nanotubes avoid agglomeration and curling into a ball in the mixing process under the coating of PTFE, but also enables the carbon nanotubes to exert excellent electrical conductivity and mechanical properties, and makes the composite binder have high stability and uniformity. When the composite binder is applied to the preparation of a dry electrode sheet, the mechanical properties of the electrode sheet, especially the increase of the tensile strength, can improve the operability of the continuous film preparation process. In addition, the electrode conductivity is also significantly improved, thereby reducing the electrode polarization, which is conducive to the development and application of thick electrode high-capacity batteries and high-power batteries.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of adhesives, and particularly relates to a composite adhesive as well as a preparation method and application thereof. BACKGROUND

[0002] In recent years, with the rapid development of electric vehicles and energy storage industries, the application of secondary batteries is also becoming more and more widespread. The electrode sheets in existing secondary batteries are generally prepared by wet method, and a large amount of deionized water and N-methyl pyrrolidone (NMP) solvent is needed in the pulping process, and the energy consumption in the drying process is huge, which is difficult to meet the requirements of energy saving and environmental protection. The dry electrode preparation technology only needs to mix the active material with the adhesive and then perform hot roller pressing to complete the preparation of the electrode sheet. In the preparation process, no NMP solvent needs to be added and no drying and recovery process is needed. The preparation process is greatly simplified, and the dry electrode prepared is not easy to fall off and can guarantee the stability and long cycle life of the device. In addition, the dry film electrode preparation technology can improve the electrode density and help to improve the battery capacity, so it is expected to become the mainstream electrode preparation technology of the next generation.

[0003] At present, in the process of preparing dry electrodes, polytetrafluoroethylene (PTFE) adhesive is a commonly used adhesive. For example, patent CN115579458A discloses a method for preparing a lithium ion battery electrode film by dry method, which uses modified ultra-high molecular weight PTFE as an adhesive. By modifying and adjusting the particle size and molecular weight of the PTFE adhesive, the mechanical properties of the PTFE adhesive are optimized, and the mode, speed, temperature and time of fiberization of the adhesive, as well as the precision of the hot pressing roller, are controlled to produce electrode film sheets with good quality, which is beneficial to improve the compaction density and energy density of the battery. However, there are still some problems in the preparation of dry batteries using PTFE adhesive at present, such as the distribution of adhesive fiber network in the fibrillation process, which increases the contact resistance between active particles, thereby causing the overall impedance of the electrode sheet to be too high. The current solution includes introducing some conductive carbon materials to improve the conductivity between particles, such as carbon nanotubes which have been maturely applied in wet preparation. Carbon nanotubes are a one-dimensional material with seamless and hollow single-walled or multi-walled tubular structure formed by curling graphite, which has excellent mechanical properties and electrical conductivity, good lithium intercalation performance and extremely high aspect ratio.

[0004] Carbon nanotubes are an ideal additive for dry process technology, mainly due to its excellent electrical conductivity and extremely high axial tensile strength, which is crucial for film forming and mechanical strength of dry film forming. Therefore, it is an important issue to make good use of this key additive. However, in the process practice, it is often found that due to the high specific surface area and adsorption capacity of carbon nanotubes, carbon nanotubes will agglomerate in the wet homogenate coating process or in the dry powder mixing process, which is manifested in that under the microscope, carbon nanotubes sometimes gather locally, and do not appear ideal uniform distribution, and the axial direction is not unfolded into a line but curled into a mass. Therefore, solving the agglomeration problem of carbon nanotubes is very crucial for dry process technology. SUMMARY

[0005] The first technical purpose of the present application is to provide a composite binder and a preparation method thereof, the composite binder having a core-shell structure, good uniformity, high adhesion, and being applicable to the preparation of dry electrode sheets, and capable of improving the mechanical properties and conductivity of the electrode sheets, reducing the polarization of the electrode sheets, and being beneficial to the development and application of thick electrode high-capacity batteries and high-power batteries.

[0006] The second technical purpose of the present application is to provide a dry electrode sheet comprising the composite binder.

[0007] The third technical purpose of the present application is to provide a secondary battery.

[0008] In order to achieve the above purposes, the technical solutions adopted by the present application are as follows.

[0009] In one aspect, the present application provides a composite binder, which has a core-shell structure, the material of the core is modified or unmodified carbon nanotubes, and the material of the shell is polytetrafluoroethylene, and the mass ratio of the core to the shell in the core-shell structure is 0.2-0.8:1.

[0010] Further, the mass ratio of the core to the shell in the core-shell structure is 0.33-0.5:1.

[0011] Further, the mass ratio of the core to the shell in the core-shell structure is 0.33:1.

[0012] Further, the particle size of the composite binder is D50=10-50μm.

[0013] Further, the diameter of the carbon nanotubes is 2-30nm, and the length is 10-50μm; preferably, the diameter of the carbon nanotubes is 21nm, and the length is 14μm.

[0014] Further, the shell structure in the composite binder is obtained by generating polytetrafluoroethylene fiber clusters in-situ on the surface of the carbon nanotubes; the polytetrafluoroethylene fiber clusters and the carbon nanotubes are linked by conductive fibers between particles, and the polytetrafluoroethylene fiber clusters and the carbon nanotubes are one-to-one corresponding.

[0015] The present application uses carbon nanotubes obtained after oxidation and fluorination treatment as a core and PTFE as a shell to prepare a composite binder with a core-shell structure. The coating of PTFE reduces the surface energy of the carbon nanotubes, avoids the agglomeration and curling of the carbon nanotubes, greatly improves the dispersing performance, and can play the excellent conductive performance and mechanical performance of the carbon nanotubes in actual use, and reduces the use amount and cost. This composite method forms PTFE fiber clusters and the carbon nanotubes of the core part to constitute conductive fiber links between particles when the PTFE on the surface of the carbon nanotubes is fibrillated and deformed. The PTFE fiber clusters and the carbon nanotubes are one-to-one corresponding, that is, the PTFE fiber clusters and the carbon nanotubes exist simultaneously at any fibrillation position, and the uniformity of the conductive fiber network distribution in the whole space is ensured. The composite binder is used to prepare a dry electrode sheet, which can improve the mechanical performance and conductivity of the electrode sheet, reduce the polarization of the electrode sheet, and is beneficial to the development and application of thick electrode high-capacity batteries and high-power batteries.

[0016] In another aspect, the present application also provides a preparation method of the composite binder, comprising the following steps:

[0017] (1) The carbon nanotubes are subjected to oxidation treatment and fluorination treatment to obtain surface-modified carbon nanotubes;

[0018] (2) Tetrafluoroethylene is polymerized in-situ on the surface of the surface-modified carbon nanotubes obtained in step (1) to obtain a carbon nanotube / polytetrafluoroethylene homopolymer emulsion;

[0019] (3) The carbon nanotube / polytetrafluoroethylene homopolymer emulsion obtained in step (2) is coagulated and dried to obtain a composite binder.

[0020] Through the above method, the carbon nanotubes are first subjected to oxidation and fluorination surface modification, and then PTFE is coated on the surface of the carbon nanotubes by in-situ polymerization to form a composite binder with a core-shell structure. The obtained composite binder has good uniformity and strong adhesion, and can improve the performance when used to prepare a dry electrode sheet.

[0021] Further, the preparation method of the composite binder comprises the following steps:

[0022] (1) The carbon nanotubes are ultrasonically dispersed in a concentrated nitric acid solution, stirred and refluxed at 60-90℃ for 4-6h, filtered to obtain surface-hydrophilic carbon nanotubes after oxidation treatment, and then dispersed in a hydrofluoric acid solution, reacted at 180-200℃ for 4-8h to obtain surface-modified carbon nanotubes;

[0023] (2) mixing water, the surface modified carbon nanotubes obtained in step (1), a dispersant, and a stabilizer to form a liquid phase; under inert gas condition, introducing tetrafluoroethylene monomer into the gas phase, adding an initiator, and performing a polymerization reaction at a pressure of 1.8-2.5 MPa and a temperature of 60-80℃ to obtain a carbon nanotube / polytetrafluoroethylene homopolymer emulsion;

[0024] (3) performing condensation, washing, and drying on the carbon nanotube / polytetrafluoroethylene homopolymer emulsion obtained in step (2) to obtain the composite adhesive.

[0025] Further, in step (1), the mass fraction of the concentrated nitric acid solution is 50-70%, preferably 68%; and the mass ratio of the carbon nanotube powder to the concentrated nitric acid solution is 0.2-0.3:1, preferably 0.25:1.

[0026] Further, in step (1), the mass fraction of the hydrofluoric acid solution is 10-30%, preferably 20%; and the mass ratio of the carbon nanotube powder to the hydrofluoric acid solution is 0.1-0.2:1, preferably 0.125:1.

[0027] Further, in step (1), the ultrasonic power is 300-500 W, and the stirring rate is 30-70 r / min; preferably, the ultrasonic power is 350 W, and the stirring rate is 50 r / min.

[0028] The carbon nanotubes have a relatively low specific gravity and a relatively large specific surface area, and are prone to agglomeration. The present application first performs oxidation treatment on the carbon nanotubes using concentrated nitric acid to make the carbon nanotubes hydrophilic, improve the dispersibility of the carbon nanotubes, and provide surface functional groups for subsequent fluorination treatment, which is conducive to the formation of C-F covalent bonds. Then, fluorination treatment is performed on the carbon nanotubes using hydrofluoric acid to form fluorinated functional groups on the surface of the carbon nanotubes, which provides active sites for the polymerization reaction of tetrafluoroethylene and is conducive to the formation of a core-shell structure.

[0029] Further, in step (2), the dispersant includes ammonium perfluorooctanoate and ammonium lauryl sulfate / ammonium dodecylbenzenesulfonate, and the mass ratio of the ammonium perfluorooctanoate to the ammonium lauryl sulfate / ammonium dodecylbenzenesulfonate is 1-5:1, preferably 1:1; and the mass fraction of the dispersant in the liquid phase is 0.05-0.2 wt%, preferably 0.186%. The dispersant plays a role in emulsifying and dispersing polytetrafluoroethylene and in water-based dispersion of carbon nanotubes, effectively making the two uniformly dispersed in water, so that the in-situ polymerization reaction is more uniform and effective.

[0030] Further, in step (2), the stabilizer is any one of paraffin, montmorillonite, rare earth, methyl phosphite, ethyl phosphite, epoxy soybean oil, carboxymethyl cellulose or carboxymethyl cellulose modifier, preferably any one of paraffin, montmorillonite, rare earth, more preferably paraffin; the mass fraction of the stabilizer in the liquid phase is 3-6wt%, preferably 4wt%.

[0031] Further, in step (2), the initiator is any one of peroxide, azo, redox initiator, preferably one or more of benzoyl peroxide, lauroyl peroxide, potassium persulfate, sodium persulfate, ammonium persulfate, azobisisobutyronitrile, more preferably ammonium persulfate; the amount of the initiator added is 0.0003-0.0006% of the mass of the liquid phase, preferably the amount of the initiator added is 0.0005% of the mass of the liquid phase.

[0032] Further, in step (2), the mass fraction of the surface-modified carbon nanotube in the liquid phase is 0.5-5%, preferably 2.8-3.6%, more preferably 2.8%.

[0033] Further, in step (2), the mass ratio of the amount of the tetrafluoroethylene monomer to the surface-modified carbon nanotube is 2-5:1, preferably 2-3:1, more preferably 2:1.

[0034] Further, in step S2, in step (2), stirring and ultrasonic dispersion are maintained during the polymerization reaction, wherein the power of the ultrasonic is 300-500w, and the stirring rate is 30-50r / min; preferably the power of the ultrasonic is 350w, and the stirring rate is 50r / min. The carbon nanotube composite PTFE is prepared by the emulsion polymerization method, and the ultrasonic is used to assist the reaction in the process, so as to ensure the dispersion of the carbon nanotube, thereby avoiding the agglomeration of the reaction product binder in the electrode sheet.

[0035] In another aspect, the application further discloses a dry electrode sheet, comprising an electrode active material, a conductive agent and the aforementioned composite binder.

[0036] Further, the electrode active material includes but is not limited to any one of active carbon, graphite, silicon, hard carbon, soft carbon, lithium iron phosphate, lithium iron manganese phosphate, prussian white, lithium titanate, tin or silicon lithium alloy system material, etc.

[0037] Further, the conductive agent includes but is not limited to any one of acetylene black, Super-P (i.e. small particle conductive carbon black), carbon nanotube, carbon fiber, Ketjen black, graphite conductive agent, graphene or a mixture of several thereof.

[0038] Further, the mixing mass ratio of the components is: polytetrafluoroethylene in the composite binder: carbon nanotubes and conductive agent in the composite binder: electrode active material = 2.5-3.5: 1.5-2.5: 95.

[0039] Further, the mixing mass ratio of the components is: polytetrafluoroethylene in the composite binder: carbon nanotubes and conductive agent in the composite binder: electrode active material = 3:2:95.

[0040] In another aspect, the application also discloses a preparation method of the dry electrode sheet, comprising the following steps:

[0041] After the electrode active material, the composite binder and the conductive agent are uniformly mixed, the fibrillation treatment is performed by the airflow mill, the film is formed by the screw extrusion, the film is thinned by the roller, and the dry electrode sheet is obtained after the hot pressing and compounding.

[0042] Further, the airflow pressure in the airflow mill process is 0.2-0.8 Mpa, and preferably the airflow pressure is 0.4-0.6 Mpa.

[0043] Further, the screw temperature in the screw extrusion film forming process is 100-200℃, and preferably the screw temperature is 120-200℃.

[0044] Further, the pressure of the roller is 0-70 t and the temperature is 40-80℃ in the roller thinning process; preferably the pressure is 10-70 t and the temperature is 50-60℃.

[0045] Further, the roller is thinned to 80-200 um, and preferably the roller is thinned to 100 um.

[0046] Further, the temperature of the hot pressing and compounding is 100-180℃ and the pressure is 5-50 t; preferably the temperature is 120-180℃ and the pressure is 10-40 t.

[0047] In another aspect, the application also discloses a secondary battery comprising a separator, an electrolyte and the dry electrode sheet.

[0048] Further, the separator comprises but is not limited to a polyolefin porous membrane, and preferably is a copolymer of one or more of polyethylene, polypropylene, polybutylene and poly-4-methylpentene.

[0049] Further, the electrolyte comprises but is not limited to any one of LiPF6, LiBF4 and LiClO4.

[0050] Finally, the application also discloses a preparation method of the secondary battery, comprising: assembling the dry electrode sheet, the separator and the electrolyte into a secondary battery.

[0051] Compared with the prior art, the present application has the following beneficial effects:

[0052] 1. The present application provides a composite binder with carbon nanotube / PTFE core-shell structure, which avoids the agglomeration and curling phenomenon in the mixing process, and can play the excellent conductive performance and mechanical performance of carbon nanotubes in practical application, and reduce the dosage and cost.

[0053] 2. In this in-situ composite mode, when the PTFE on the surface of the carbon nanotube is fibrillated and deformed, the PTFE fiber clusters and the carbon nanotubes in the core part form a conductive fiber link between the particles. The PTFE fiber clusters and the carbon nanotubes are one-to-one corresponding, that is, the PTFE fiber clusters and the carbon nanotubes exist simultaneously at any fibrillation position, which ensures the uniformity of the conductive fiber network distribution in the whole space.

[0054] 3. The in-situ formed composite structure has high stability and uniformity. Compared with mechanical compounding, it also solves the practical engineering problem from the perspective of adhesive raw material production, and has realizability and economy.

[0055] 4. The composite binder is applied to the preparation of dry electrode sheet, has good adhesion, can improve the mechanical performance of the electrode sheet, especially the increase of tensile strength improves the operability of the continuous manufacturing process, and also significantly improves the electrode conductivity, thereby reducing the electrode polarization, which is beneficial to the development and application of thick electrode high-capacity batteries and high-power batteries. BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1 is a structural schematic diagram of the conductive fiber network in the dry electrode sheet;

[0057] Figure 2 is a scanning electron microscope image of the positive electrode sheet prepared in Example 1.

[0058] Figure 3 is a scanning electron microscope image of the positive electrode sheet prepared in Comparative Example 1. DETAILED DESCRIPTION

[0059] The following non-limiting examples can make those skilled in the art more fully understand the present application, but do not limit the present application in any way. The following content is only an exemplary description of the scope of the present application, and those skilled in the art can make various changes and modifications to the present application according to the disclosed content, which should also belong to the scope of the present application.

[0060] In the present application, "negative electrode" or "positive electrode" is collectively referred to as "electrode", i.e., "electrode sheet" refers to at least one of "negative electrode sheet" and "positive electrode sheet". "D50" refers to the particle size corresponding to the cumulative particle size distribution percentage of 50% of a sample. During the preparation of the secondary battery, the separator, electrolyte, and assembly method are not subject to conventional limitations.

[0061] The present application is further described below in the manner of specific examples. The various chemical reagents used in the examples of the present application are obtained through conventional commercial channels unless otherwise specified.

[0062] Example 1

[0063] A method for preparing a composite binder, comprising:

[0064] (1) 250 g of carbon nanotube powder (average diameter 21 nm, average length 14 um) is ultrasonically dispersed in 1 L of 68% concentrated nitric acid solution, the ultrasonic power is 400 W, heating and refluxing in a reflux device for 4 h, the heating temperature is 90°C, after cooling, filtering and washing, then ultrasonically dispersing in 2 L of 20% hydrofluoric acid solution, transferring to a Teflon liner, and then placing it in a stainless steel pressure tank, and keeping it at 180°C for 5 h, cooling, filtering and washing to obtain surface-modified carbon nanotubes;

[0065] (2) In a 10 L high-pressure explosion-proof polymerization reactor, 7 L of deionized water, 210 g of surface-modified carbon nanotubes (dry weight), 7 g of ammonium perfluorooctanoate, 7 g of ammonium lauryl sulfate, and 300 g of paraffin wax are added and uniformly dispersed, wherein the mass fraction of carbon nanotubes is about 2.8%; vacuum and nitrogen replacement are performed until the oxygen content is less than 50 ppm, the liquid phase temperature in the kettle is heated to 75°C, stirring and ultrasonic are started, the stirring rate is 50 r / min, and the ultrasonic power is set to 350 w; slowly introduce tetrafluoroethylene monomer, add 0.0375 g of ammonium persulfate, maintain the reaction pressure at 2.0 MPa, and stop the reaction when the consumption of tetrafluoroethylene is 1.05 kg, to obtain carbon nanotube / polytetrafluoroethylene homopolymer emulsion;

[0066] (3) The carbon nanotube / polytetrafluoroethylene homopolymer emulsion is washed by coagulation and then spray dried to obtain a composite binder dry powder.

[0067] This embodiment also provides a dry-process positive electrode sheet, comprising preparing a positive electrode dry powder by combining a obtained composite binder, carbon black conductive agent, and lithium iron phosphate, wherein the composite binder is formulated according to a mass ratio of polytetrafluoroethylene / (carbon nanotubes and carbon black conductive agent in the composite binder) / lithium iron phosphate = 3:2:95. After stirring, mixing and dispersing, the powder is subjected to fibrillation treatment by an air jet mill at an air pressure of 0.6 MPa. The resulting powder is then extruded into a film by a screw extruder at a screw temperature of 120-200°C. After thinning by roller pressing, a 100 μm positive electrode dry-process film is obtained at a roller pressing pressure of 25 t and a temperature of 50°C. Finally, the film is hot-pressed onto an aluminum foil to obtain a positive electrode sheet at a hot-pressing temperature of 130°C and a pressure of 30 t. A schematic diagram of the conductive fiber network in the positive electrode sheet is shown below. Figure 1 As shown.

[0068] This embodiment also provides a secondary battery, which is assembled from the above-mentioned dry-process positive electrode, separator, graphite negative electrode and electrolyte.

[0069] Example 2

[0070] A method for preparing a composite adhesive, the steps of which are basically the same as those in Example 1, except that the amount of tetrafluoroethylene monomer consumed in step (2) is 0.63 kg.

[0071] The preparation methods for the dry-process positive electrode and the secondary battery are the same as in Example 1.

[0072] Example 3

[0073] A method for preparing a composite adhesive, the steps of which are basically the same as those in Example 2, except that the amount of tetrafluoroethylene monomer consumed in step (2) is 0.42 kg.

[0074] The preparation methods for the dry-process positive electrode and the secondary battery are the same as in Example 2.

[0075] Example 4

[0076] A method for preparing a composite adhesive is basically the same as that in Example 2, except that the amount of surface-modified carbon nanotubes added in step (2) is 270g, and the amount of tetrafluoroethylene monomer consumed is 0.81kg.

[0077] The preparation methods for the dry-process positive electrode and the secondary battery are the same as in Example 2.

[0078] Example 5

[0079] A method for preparing a composite adhesive is basically the same as that in Example 2, except that the amount of surface-modified carbon nanotubes added in step (2) is 380g, and the amount of tetrafluoroethylene monomer consumed is 1.14kg.

[0080] The dry method cathode sheet and the secondary battery were prepared in the same manner as in Example 2.

[0081] Comparative Example 1

[0082] A method for preparing a composite binder was substantially the same as in Example 2, except that the amount of tetrafluoroethylene monomer consumed in step (2) was 2.1 kg.

[0083] The dry method cathode sheet and the secondary battery were prepared in the same manner as in Example 2.

[0084] Comparative Example 2

[0085] A method for preparing a composite binder was substantially the same as in Example 2, except that the amount of surface-modified carbon nanotubes added in step (2) was 790 g, and the amount of tetrafluoroethylene monomer consumed was 2.37 kg.

[0086] The dry method cathode sheet and the secondary battery were prepared in the same manner as in Example 2.

[0087] Comparative Example 3

[0088] A method for preparing a composite binder was substantially the same as in Example 2, except that the carbon nanotubes were not surface-modified.

[0089] The dry method cathode sheet and the secondary battery were prepared in the same manner as in Example 2.

[0090] Comparative Example 4

[0091] A method for preparing a composite binder was substantially the same as in Example 2, except that the ultrasonic device was not turned on in step (2).

[0092] The dry method cathode sheet and the secondary battery were prepared in the same manner as in Example 2.

[0093] Comparative Example 5

[0094] A dry method cathode sheet includes a commercial carbon nanotube having an average tube diameter of 21 nm and an average length of 14 um, and a polytetrafluoroethylene binder, a carbon black conductive agent, and a lithium iron phosphate dry powder mixed, wherein the mass ratio of polytetrafluoroethylene / carbon nanotube / carbon black conductive agent / lithium iron phosphate is 3:1:1:95, and after stirring, mixing, and dispersing, fibrillation treatment is performed by an air flow mill at an air flow pressure of 0.6 MPa, the obtained powder is extruded into a film by a screw at a screw temperature of 120-200°C, and a 100 um cathode dry film is obtained after roll thinning at a roll pressure of 25 t and a roll temperature of 50°C, and a cathode sheet is obtained by hot pressing the cathode dry film on an aluminum foil at a hot pressing temperature of 130°C and a pressure of 30 t.

[0095] The comparative example also provides a secondary battery, which is assembled by the dry positive electrode sheet, the separator, the graphite negative electrode sheet and the electrolyte.

[0096] Experimental example

[0097] The composite binder, the electrode sheet and the battery obtained in Examples 1-5 and Comparative Examples 1-5 are tested as follows: the median particle size of the composite binder is measured by the particle size distribution laser diffraction method, the agglomeration of the carbon nanotubes in the electrode sheet is analyzed by the scanning electron microscope; the mechanical strength of the electrode sheet is tested by the tensile strength testing device; the membrane layer resistance of the electrode sheet is tested by the electrode resistance meter; and the cycle capacity retention rate of the battery is tested by the electrochemical workstation. The measured results are shown in Table 1.

[0098] Table 1

[0099]

[0100] From the test results of Examples 1-5 and Comparative Example 5 in Table 1, it can be seen that the resistance of the electrode sheet obtained in Examples 1-5 is obviously lower than that of Comparative Example 5, the longitudinal tensile strength is higher than that of Comparative Example 5, and the capacity retention rate after 500 cycles is higher than that of Comparative Example 5. Therefore, within the given ratio of carbon nanotubes / polytetrafluoroethylene (0.2-0.8:1) in the application, the dry positive electrode sheet prepared by the binder has higher conductivity and mechanical strength, and good stability; in the microscopic analysis, it can be seen that the agglomeration of the carbon nanotubes is greatly improved (as shown in Figure 2 ), the carbon nanotubes are uniformly distributed between the particles in the membrane layer, and the fibrillated polytetrafluoroethylene fibers form a conductive fiber network (as shown in Figure 1 ), which reduces the polarization of the electrode sheet, and the cycle performance of the prepared battery is also improved, which proves the superiority of the in-situ synthesized carbon nanotube / polytetrafluoroethylene core-shell structure.

[0101] In addition, the composite ratio of carbon nanotubes / polytetrafluoroethylene is also critical. From the test results of Examples 1-3 and Comparative Examples 1 and 2, it can be seen that the higher the amount of the PTFE coating layer, the larger the particle size of the obtained composite binder, and the worse the distribution of the carbon nanotubes after the fibrillation of the polytetrafluoroethylene. In Comparative Example 1, the agglomeration of the carbon nanotubes is caused by the too large amount of polytetrafluoroethylene (as shown in Figure 3 ), which indicates that the carbon nanotubes and polytetrafluoroethylene are not uniformly coated in the in-situ polymerization reaction.

[0102] In addition, in Examples 2, 4, 5 and Comparative Example 2, the composite ratio of carbon nanotube / polytetrafluoroethylene is the same, but the amount of carbon nanotube and polytetrafluoroethylene is increased, i.e. the dispersion concentration of carbon nanotube in the aqueous phase is increased. As can be seen from the test results of Examples 2, 4 and 5 and Comparative Example 2, the greater the dispersion concentration of carbon nanotube in the aqueous phase polymerization, the worse the in-situ polymerization effect. In Comparative Example 2, the too high dispersion concentration of carbon nanotube leads to the inhibition of the formation of the core-shell structure, and the phenomenon of carbon nanotube agglomeration still occurs after the electrode sheet is prepared.

[0103] Comparative Examples 3 and 4 are control experiments of carbon nanotube surface modification and ultrasonic effect in polymerization, respectively. The test results show that the electrode sheet has a large resistance and poor mechanical tensile property, and the microanalysis shows that the agglomeration of carbon nanotube is more serious. Therefore, it is shown that the surface modification of carbon nanotube provides active sites for polymerization, which is beneficial to the formation of the core-shell structure; and the ultrasonic effect in the polymerization process ensures the dispersibility of carbon nanotube as a nucleating agent and promotes the order and uniformity of the polymerization.

[0104] Example 6

[0105] A preparation method of a composite binder comprises:

[0106] (1) 250 g of carbon nanotube powder (average tube diameter 11 nm, average length 35 um) is ultrasonically dispersed in 1 L of 68% concentrated nitric acid solution, the ultrasonic power is 400 W, heating and refluxing in a reflux device for 4 h, the heating temperature is 80°C, after cooling, filtering and washing, then ultrasonically dispersed in 2 L of 20% hydrofluoric acid solution, transferred to a Teflon liner, and then placed in a stainless steel pressure tank, and then kept at 200°C for 6 h, and then cooled, filtered and washed to obtain surface-modified carbon nanotube;

[0107] (2) In a 10 L high-pressure explosion-proof polymerization reactor, 7 L of deionized water, 210 g of surface-modified carbon nanotube, 9 g of ammonium perfluorooctanoate, 3 g of ammonium lauryl sulfate and 300 g of paraffin wax are added and uniformly dispersed; the mass fraction of carbon nanotube is about 2.8%; vacuum is applied and replaced with nitrogen to an oxygen content of less than 30 ppm, the liquid phase temperature in the heating kettle is heated to 60°C, stirring and ultrasonic are started, the stirring rate is 50 r / min, and the ultrasonic power is set to 400 W; four fluorine ethylene monomer is slowly introduced, 0.0375 g of ammonium persulfate is added, the reaction pressure is kept at 2.0 MPa, and the reaction is stopped when the consumption of four fluorine ethylene monomer is 0.63 kg, and a carbon nanotube / polytetrafluoroethylene homopolymer emulsion is obtained;

[0108] (3) The carbon nanotube / polytetrafluoroethylene homopolymer emulsion is washed by coagulation and then spray dried to obtain a composite binder dry powder.

[0109] This embodiment also provides a dry-process positive electrode sheet, comprising preparing a positive electrode dry powder by combining a obtained composite binder, a Super-P conductive agent, and lithium manganese iron phosphate, wherein the composite binder is formulated according to a mass ratio of polytetrafluoroethylene / (carbon nanotubes and Super-P conductive agent in the composite binder) / lithium manganese iron phosphate = 3:4:93. After stirring, mixing and dispersing, the powder is subjected to fibrillation treatment by an air jet mill at an air jet pressure of 0.6 MPa. The resulting powder is then extruded into a film by a screw extruder at a screw temperature of 120-200°C. After thinning by roller pressing, a 100 μm positive electrode dry film is obtained at a roller pressing pressure of 25 t and a temperature of 50°C. Finally, the film is hot-pressed onto an aluminum foil to obtain a positive electrode sheet at a hot-pressing temperature of 130°C and a pressure of 30 t.

[0110] This embodiment also provides a lithium-ion battery, with the same preparation steps as in Embodiment 2.

[0111] Example 7

[0112] A method for preparing a composite adhesive, comprising:

[0113] (1) 250g of carbon nanotube powder (average diameter 10nm, average length 50um) was ultrasonically dispersed in 1L of 68% concentrated nitric acid solution with an ultrasonic power of 400W. The mixture was heated and refluxed in a reflux device for 4h at a heating temperature of 90℃. After cooling, it was filtered and cleaned. Then, it was ultrasonically dispersed in 2L of 20% hydrofluoric acid solution, transferred to a Teflon liner, and placed in a stainless steel pressure tank. It was kept at 180℃ for 5h, cooled, filtered and cleaned to obtain surface-modified carbon nanotubes.

[0114] (2) In a 10L high-pressure explosion-proof polymerization reactor, add 7L of deionized water, 210g of surface-modified carbon nanotubes (dry weight), 6g of ammonium perfluorooctanoate, 3g of ammonium dodecyl sulfate, and 300g of paraffin wax, and disperse them evenly. The mass fraction of carbon nanotubes is about 2.8%. Evacuate the reactor and replace it with nitrogen until the oxygen content is below 50ppm. Heat the liquid phase temperature in the reactor to 75℃, and turn on the stirring and sonication. The stirring rate is 50r / min, and the sonication power is set to 500w. Slowly introduce tetrafluoroethylene monomer, add 0.0375g of ammonium persulfate, and maintain the reaction pressure at 2.0MPa until the consumption of tetrafluoroethylene is 0.42kg. Stop the reaction to obtain a carbon nanotube / polytetrafluoroethylene homopolymer emulsion.

[0115] (3) After coagulation and washing, carbon nanotube / polytetrafluoroethylene homopolymer emulsion is spray-dried to obtain composite binder powder.

[0116] The embodiment also provides a dry-method positive electrode sheet, which is prepared by mixing the obtained composite binder, Super-P conductive agent and Prussian white in a mass ratio of polytetrafluoroethylene / (carbon nanotubes in the composite binder and Super-P conductive agent) / Prussian white = 3:4:93, stirring, mixing and dispersing, performing fibrillation treatment by using an air flow mill with an air flow pressure of 0.6 MPa, extruding the obtained powder into a film by using a screw with a screw temperature of 120-200 DEG C, reducing the film by rolling to obtain a 100 um dry-method positive electrode film, and hot-pressing the film on an aluminum foil to obtain the positive electrode sheet, wherein the rolling pressure is 25 t, the rolling temperature is 50 DEG C, the hot-pressing temperature is 130 DEG C, and the hot-pressing pressure is 30 t.

[0117] The embodiment also provides a secondary battery, which is prepared by assembling the dry-method positive electrode sheet, a diaphragm, a commercial hard carbon negative electrode sheet and an electrolyte.

[0118] Embodiment 8

[0119] (1) 250 g of carbon nanotube powder (average tube diameter 11 nm, average length 35 um) is ultrasonically dispersed in 1 L of 68% concentrated nitric acid solution, ultrasonic power is 400 W, heating and refluxing in a reflux device for 4 h, heating temperature is 90 DEG C, after cooling, filtering and washing, then ultrasonically dispersing in 2 L of 20% hydrofluoric acid solution, transferring to a Teflon liner, and then placing in a stainless steel pressure tank, and then heating at 180 DEG C for 5 h, cooling, filtering and washing to obtain surface-modified carbon nanotubes;

[0120] (2) In a 10 L high-pressure explosion-proof polymerization reactor, 7 L of deionized water, 380 g of surface-modified carbon nanotubes (dry weight), 7 g of ammonium perfluorooctanoate, 7 g of ammonium lauryl sulfate and 300 g of paraffin are added and uniformly dispersed, wherein the mass fraction of the carbon nanotubes is about 4.9%; vacuumizing and nitrogen replacing to less than 50 ppm of oxygen content, heating the liquid phase in the kettle to 75 DEG C, and then starting stirring and ultrasonic, the stirring rate is 50 r / min, and the ultrasonic power is set to 400 W; slowly introducing tetrafluoroethylene monomer, adding 0.0375 g of ammonium persulfate, maintaining the reaction pressure at 2.5 MPa, and stopping the reaction until the consumption of tetrafluoroethylene is 0.76 kg, to obtain carbon nanotube / polytetrafluoroethylene homopolymer emulsion;

[0121] (3) The carbon nanotube / polytetrafluoroethylene homopolymer emulsion is washed by condensation and then spray-dried to obtain a composite binder dry powder.

[0122] The embodiment also provides a dry-method negative sheet, which comprises preparing a negative dry powder by mixing the obtained composite binder, carbon black conductive agent and graphite, wherein the mass ratio of polytetrafluoroethylene / (carbon nanotubes and carbon black conductive agent in the composite binder) / graphite in the composite binder is 1.5:1.5:97, the obtained powder is subjected to fibrillation treatment by an air flow mill with an air flow pressure of 0.6 MPa, the obtained powder is extruded into a film by a screw with a screw temperature of 120-200 DEG C, and the film is thinned by rolling to obtain a positive dry film with a thickness of 100 um, the film is rolled with a pressure of 25 t and a temperature of 50 DEG C, and the positive dry film is hot-pressed on an aluminum foil to obtain the positive sheet, wherein the hot-pressing temperature is 130 DEG C and the pressure is 30 t.

[0123] Example 9

[0124] (1) 250 g of carbon nanotube powder (average diameter 21 nm, average length 14 um) is ultrasonically dispersed in 1 L of 68% concentrated nitric acid solution, ultrasonic power is 400 W, heating and refluxing in a reflux device for 4 h, heating temperature is 90 DEG C, after cooling, filtering and washing, then ultrasonically dispersed in 2 L of 20% hydrofluoric acid solution, transferred to a Teflon liner, and then placed in a stainless steel pressure tank, and heated at 180 DEG C for 5 h, cooled, filtered and washed to obtain surface-modified carbon nanotubes;

[0125] (2) In a 10 L high-pressure explosion-proof polymerization reactor, 7 L of deionized water, 210 g of surface-modified carbon nanotubes (dry weight), 7 g of ammonium perfluorooctanoate, 7 g of ammonium lauryl sulfate, and 300 g of paraffin are added and uniformly dispersed, wherein the mass fraction of the carbon nanotubes is about 2.8%; vacuum is applied and replaced with nitrogen to an oxygen content of less than 50 ppm, the liquid phase temperature in the reactor is maintained at 20 DEG C, stirring and ultrasonic are started, the stirring rate is 50 r / min, and the ultrasonic power is set to 350 W; four fluorine ethylene monomer is slowly introduced, 0.0375 g of ammonium persulfate is added, the reaction pressure is maintained at 2.0 MPa, and the reaction is stopped when the consumption of four fluorine ethylene is 0.63 kg, to obtain carbon nanotube / polytetrafluoroethylene homopolymer emulsion;

[0126] (3) The carbon nanotube / polytetrafluoroethylene homopolymer emulsion is washed by condensation and spray dried to obtain a composite binder dry powder.

[0127] The embodiment also provides a dry-method positive electrode sheet, which comprises: preparing a positive dry powder by using the obtained composite binder, carbon black conductive agent and lithium iron phosphate, wherein the mass ratio of polytetrafluoroethylene / (carbon nanotube and carbon black conductive agent in the composite binder) / lithium iron phosphate in the composite binder is 3:2:95, the obtained powder is subjected to fibrillation treatment by using an air flow mill, the air flow pressure is 0.6 MPa, the obtained powder is extruded into a film by using a screw, the screw temperature is 120-200 DEG C, the film is thinned by rolling, the rolling pressure is 25 t, the rolling temperature is 50 DEG C, the positive electrode dry film with a thickness of 100 um is obtained, the positive electrode sheet is obtained by hot-pressing the positive electrode dry film on an aluminum foil, the hot-pressing temperature is 130 DEG C, and the pressure is 30 t. A schematic diagram of the conductive fiber network in the positive electrode sheet is shown in Figure 1

[0128] The embodiment also provides a secondary battery, which is assembled by using the dry-method positive electrode sheet, a separator, a graphite negative electrode sheet and an electrolyte.

[0129] The embodiment also provides a secondary battery, which is assembled by using a dry-method negative electrode sheet, a separator, a dry-method lithium iron phosphate positive electrode sheet and an electrolyte.

[0130] The embodiments 6-9 also verify the effective application of the composite binder to various lithium / sodium ion battery positive and negative electrode materials.

[0131] The above only describes the preferred embodiments of the present application and is not used to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.​

Claims

1. A composite binder, characterized by, The composite binder is a core-shell structure, the material of the core is modified or unmodified carbon nanotube, the material of the shell is polytetrafluoroethylene, and the mass ratio of the core to the shell in the core-shell structure is 0.2-0.8:

1.

2. The composite binder of claim 1, wherein, The particle size of the composite binder is D50=10-50 μm; the diameter of the carbon nanotube is 2-30 nm, and the length is 10-50 μm.

3. The composite binder of claim 1, wherein, The shell structure in the composite binder is obtained by generating polytetrafluoroethylene fiber clusters in situ on the surface of the carbon nanotube; the polytetrafluoroethylene fiber clusters and the carbon nanotube are linked by conductive fibers between particles, and the polytetrafluoroethylene fiber clusters and the carbon nanotube are one-to-one corresponding.

4. A method of producing the composite binder as claimed in any one of claims 1 to 3, characterized in that, The method comprises the following steps: (1) the carbon nanotube is subjected to oxidation treatment and fluorination treatment to obtain surface-modified carbon nanotube; (2) tetrafluoroethylene is polymerized in situ on the surface of the surface-modified carbon nanotube obtained in step (1) to obtain carbon nanotube / polytetrafluoroethylene homopolymer emulsion; (3) the carbon nanotube / polytetrafluoroethylene homopolymer emulsion obtained in step (2) is coagulated and dried to obtain the composite binder.

5. The preparation method according to claim 4, characterized in that, The method comprises the following steps: (1) the carbon nanotube is dispersed in concentrated nitric acid solution by ultrasonic dispersion, stirred and refluxed at 60-90 ℃ for 4-6 h, filtered to obtain surface-hydrophilic carbon nanotube wet material after oxidation treatment, and then the wet material is dispersed in hydrofluoric acid, and reacted at 180-200 ℃ for 4-8 h to obtain surface-modified carbon nanotube; (2) water, the surface-modified carbon nanotube obtained in step (1), a dispersant and a stabilizer are uniformly mixed to form a liquid phase; under the condition of inert gas, gaseous tetrafluoroethylene monomer is introduced, and an initiator is added to perform polymerization reaction, the reaction pressure is 1.8-2.5 MPa, and the temperature is 60-80 ℃ to obtain carbon nanotube / polytetrafluoroethylene homopolymer emulsion; (3) the carbon nanotube / polytetrafluoroethylene homopolymer emulsion obtained in step (2) is coagulated, washed and dried to obtain the composite binder.

6. The production method according to claim 5, wherein In step (2), the mass fraction of the surface-modified carbon nanotube in the liquid phase is 0.5-5%.

7. The preparation method according to claim 5, characterized in that, In step (2), the dispersant comprises ammonium perfluorooctanoate and ammonium lauryl sulfate / ammonium dodecylbenzenesulfonate, the mass ratio of the ammonium perfluorooctanoate to the ammonium lauryl sulfate / ammonium dodecylbenzenesulfonate is 1-5:1, the mass fraction of the dispersant in the liquid phase is 0.05-0.2 wt%, the mass fraction of the stabilizer in the liquid phase is 3-6 wt%, and the addition amount of the initiator is 0.0003-0.0006% of the mass of the liquid phase.

8. The preparation method according to claim 5, characterized in that, In step (2), stirring and ultrasonic dispersion are maintained during the polymerization reaction, wherein the stirring rate is 30-50 r / min, and the ultrasonic power is 300-500 W.

9. A dry electrode sheet, characterized by The composite binder prepared by the method according to any one of claims 5-8, an electrode active material and a conductive agent.

10. A secondary battery characterized by comprising: The dry electrode sheet according to claim 9, a separator and an electrolyte.

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