A Binder Pretreatment Method for Dry Electrodes and Its Application

Abstract

The present invention relates to the technical field of electrode materials, and discloses a binder pretreatment method for dry electrodes and its application. The binder pretreatment method includes the following steps: mixing polytetrafluoroethylene with a pretreatment agent and aging to obtain a mixed material; the pretreatment agent is an organic liquid with a surface tension not higher than 40 mN / m at 20°C; extracting the pretreatment agent in the mixed material by subcritical carbon dioxide extraction, and performing freeze-vacuum drying to obtain pretreated polytetrafluoroethylene. By using the pretreatment method of the present invention, it is possible to make PTFE have a high degree of fibrillation while reducing the damage to the electrode active material caused by shear force during the preparation process of the dry electrode, thereby enabling the dry electrode to have both high mechanical strength, conductivity and specific capacity.

Description

Technical Field

[0001] The present invention relates to the technical field of electrode materials, and particularly to a binder pretreatment method for dry electrodes and its application. Background Art

[0002] The dry electrode is a new type of electrode preparation process, and the difference from the traditional wet process mainly lies in the front-end electrode production link. The dry electrode process can form an electrode by dry-mixing a binder, an active material, and a conductive agent without using a solvent and then pressing it on a current collector. Compared with the wet process, the dry electrode process has significant advantages in terms of cost efficiency, performance improvement, structural stability, and production adaptability, and has strong compatibility with sulfide solid-state batteries, which is an important boost to the mass production process of sulfide all-solid-state batteries.

[0003] Since no solvent is used in the preparation process of the dry electrode, it is difficult for the binder to form a uniform fiber network, resulting in poor mechanical properties of the dry electrode. The existing solution is to mix the active material powder with the conductive agent and then add the binder polytetrafluoroethylene (PTFE), and then apply an external high shear force to the dry mixture to fibrillate PTFE and bond the electrode film powder, and finally extrude the mixture to form a self-supporting film (for example, in patent CN119725367A, PTFE fibrillation is achieved by performing two air flow grinding shears at a speed of 8000 - 12000 rpm). This method requires applying an external high shear force to ensure a high degree of PTFE fibrillation, but the high shear force will greatly damage the structure of the electrode active particles, resulting in a decrease in the conductivity and specific capacity of the electrode. Summary of the Invention

[0004] In order to solve the technical problem that the existing method of fibrillating PTFE will cause low conductivity and specific capacity of dry electrodes, the present invention provides a binder pretreatment method for dry electrodes and its application. By using the pretreatment method of the present invention, while enabling PTFE to have a high degree of fibrillation, the damage to the electrode active material caused by the shear force during the preparation process of the dry electrode can be reduced, so that the dry electrode has both high mechanical strength, conductivity, and specific capacity.

[0005] The specific technical solution of the present invention is as follows:

[0006] In the first aspect, the present invention provides a binder pretreatment method for dry electrodes, including the following steps:

[0007] S1: Mix and age polytetrafluoroethylene (PTFE) with a pretreatment agent to obtain a mixture; the pretreatment agent is an organic liquid with a surface tension not higher than 40 mN / m at 20°C;

[0008] S2: Extract the pretreatment agent from the mixture using subcritical carbon dioxide, and perform freeze-vacuum drying to obtain pretreated polytetrafluoroethylene.

[0009] In step S1, during the mixing and aging of the pretreatment agent and the binder PTFE, the pretreatment agent with low surface tension can penetrate into the interior of the PTFE particles, achieving wetting inside the PTFE particles, thereby weakening the van der Waals force between the PTFE molecular chains. In step S2, both subcritical carbon dioxide extraction and freeze-vacuum drying are methods for removing the pretreatment agent. The present invention combines these two methods in a specific order, which can produce the following effects: Subcritical carbon dioxide can cause a certain degree of swelling of PTFE, reducing the coiled state of the PTFE fibers, and then removing the residual pretreatment agent through freeze-drying can maintain the loose structure inside the PTFE particles and reduce the possibility of entanglement and aggregation between the molecular chains; When only using subcritical carbon dioxide extraction or freeze-vacuum drying, or changing the order between the two, the above effects will be poor.

[0010] By the above method, pretreating the binder PTFE according to steps S1 and S2 can improve the degree of PTFE fibrillation, reduce the curling, entanglement and aggregation degree of PTFE fibers. When the pretreated PTFE is used in a dry electrode, without applying high shear force to the dry mixture of the electrode active material, conductive agent and binder, the binder can have a good adhesion effect. Therefore, high shear force can be avoided from damaging the electrode active material, so that the finally prepared electrode has a high conductivity and specific capacity.

[0011] Preferably, in step S2, the subcritical carbon dioxide extraction is carried out until the content of the pretreatment agent is 0.1 - 1 wt%.

[0012] After the present invention removes a part of the pretreatment agent by subcritical carbon dioxide extraction and then removes the residual pretreatment agent by freeze-vacuum drying. During this process, by controlling the degree of subcritical carbon dioxide extraction within the scope of the present invention (extracting until the content of the pretreatment agent is 0.1 - 1 wt%), the cooperation effect between subcritical carbon dioxide extraction and freeze-vacuum drying can be improved, thereby further improving the PTFE fibrillation effect and endowing the dry electrode with higher mechanical strength.

[0013] Preferably, in step S2, the temperature of the subcritical carbon dioxide is 25 - 30 °C and the pressure is 5 - 7 MPa.

[0014] Preferably, in step S2, the process of the freeze-vacuum drying includes: freezing at a temperature below -70 °C and then drying under a vacuum degree of 0.1 - 50 Pa until the content of the pretreatment agent is lower than 0.001 wt%.

[0015] Further, the freezing time is 200 - 400 min.

[0016] Preferably, in step S1, the pretreatment agent is one or more of aviation kerosene, petroleum ether, paraffin oil, and isoparaffin.

[0017] Preferably, in step S1, the mass ratio between the polytetrafluoroethylene and the pretreatment agent is 4 - 19:1.

[0018] Preferably, in step S1, the conditions for mixing are as follows: the temperature is lower than 19°C, the rotation speed is 15 - 20 r / min, and the time is 30 - 40 min.

[0019] Preferably, in step S1, the conditions for aging are as follows: the temperature is 25 - 30°C, and the time is 8 - 10 h.

[0020] In a second aspect, the present invention provides a method for preparing a dry electrode, comprising the following steps:

[0021] 1) According to the binder pretreatment method described above, obtain the pretreated polytetrafluoroethylene;

[0022] 2) After mixing the electrode active material and the pretreated polytetrafluoroethylene, press them on the current collector to obtain a dry electrode.

[0023] Preferably, the dry electrode is a positive electrode or a negative electrode.

[0024] Preferably, in step 2), the mixing process is carried out at a temperature lower than 19°C, the rotation speed is 2000 - 3000 r / min, and the time is 3 - 5 min.

[0025] Preferably, in step 2), the electrode active material is a positive electrode active material or a negative electrode active material; the positive electrode active material is one or more of lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium manganate; the negative electrode active material is one or more of graphite, hard carbon, and soft carbon.

[0026] Preferably, in step 2), a conductive agent is mixed with the electrode active material and the pretreated polytetrafluoroethylene together.

[0027] Further, the conductive agent is one or more of acetylene black, Ketjen black, and conductive graphite.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] (1) By subjecting the binder PTFE to low surface tension pretreatment agent treatment, subcritical carbon dioxide extraction, and freeze-drying in sequence, the present invention can effectively improve the degree of fibrillation of PTFE, reduce the curling, winding, and agglomeration of PTFE fibers. When used in dry electrodes, it can reduce the damage of shear force to the electrode active material, and thus the dry electrode has both high mechanical strength, conductivity, and specific capacity.

[0030] (2) By controlling the degree of subcritical carbon dioxide extraction within a specific range, the present invention can improve the cooperation effect with freeze-vacuum drying, thereby improving the PTFE fibrillation effect to a greater extent and endowing the dry electrode with higher mechanical strength. Detailed implementation mode

[0031] The present invention will be further described below in conjunction with embodiments.

[0032] First, the present invention relates to a binder pretreatment method for dry electrodes, including the following steps:

[0033] S1: Mix polytetrafluoroethylene with a pretreatment agent and age to obtain a mixed material; the pretreatment agent is an organic liquid with a surface tension not higher than 40 mN / m at 20°C;

[0034] S2: Use subcritical carbon dioxide to extract the pretreatment agent in the mixed material, and perform freeze-vacuum drying to obtain pretreated polytetrafluoroethylene.

[0035] In the above pretreatment process, when mixing and aging the pretreatment agent with the binder PTFE, the pretreatment agent with low surface tension can penetrate into the interior of PTFE particles to achieve internal wetting of PTFE particles, thereby weakening the van der Waals force between PTFE molecular chains; subcritical carbon dioxide can cause a certain degree of swelling of PTFE, reduce the curled state of PTFE fibers, and then remove the residual pretreatment agent by freeze-drying, which can maintain the loose structure inside PTFE particles and reduce the possibility of entanglement and agglomeration between molecular chains. After the above pretreatment, the degree of fibrillation of PTFE can be effectively improved, and the curling, winding, and agglomeration of PTFE fibers can be reduced. When the pretreated PTFE is used in dry electrodes, it is not necessary to apply high shear force to the dry mixture of electrode active material, conductive agent, and binder, and the binder can have a good adhesion effect, so the high shear force can be avoided from damaging the electrode active material, and thus the finally prepared electrode has high mechanical strength, conductivity, and specific capacity.

[0036] In some specific implementation modes, in step S1, the pretreatment agent is one or more of aviation kerosene, petroleum ether, paraffin oil, and isoparaffin.

[0037] In some specific embodiments, in step S1, the mass ratio between the polytetrafluoroethylene and the pretreatment agent is 4 to 19:1.

[0038] In some specific embodiments, in step S1, the conditions for mixing are as follows: the temperature is lower than 19 °C, the rotation speed is 15 to 20 r / min, and the time is 30 to 40 min.

[0039] In some specific embodiments, in step S1, the conditions for aging are as follows: the temperature is 25 to 30 °C, and the time is 8 to 10 h.

[0040] In some specific embodiments, in step S2, the temperature of the subcritical carbon dioxide is 25 to 30 °C, the pressure is 5 to 7 MPa, and the extraction is carried out until the content of the pretreatment agent is 0.1 to 1 wt%. By controlling the degree of subcritical carbon dioxide extraction within a specific range, that is, extracting until the content of the pretreatment agent is 0.1 to 1 wt%, the cooperation effect between subcritical carbon dioxide extraction and freeze-vacuum drying can be improved, thereby further improving the PTFE fibrillation effect and endowing the dry electrode with higher mechanical strength.

[0041] In some specific embodiments, in step S2, the process of freeze-vacuum drying includes: freezing at a temperature lower than -70 °C for 200 to 400 min, and then drying under a vacuum of 0.1 to 50 Pa until the content of the pretreatment agent is lower than 0.001 wt%.

[0042] Second, the present invention relates to a method for preparing a dry electrode, comprising the following steps:

[0043] 1) According to the binder pretreatment method described above, obtain pretreated polytetrafluoroethylene;

[0044] 2) After mixing the electrode active material and the pretreated polytetrafluoroethylene, press them on the current collector to obtain a dry electrode.

[0045] In some specific embodiments, the dry electrode is a positive electrode or a negative electrode.

[0046] In some specific embodiments, in step 2), the mixing process is carried out at a temperature lower than 19 °C, the rotation speed is 2000 to 3000 r / min, and the time is 3 to 5 min.

[0047] In some specific embodiments, in step 2), the electrode active material is a positive electrode active material or a negative electrode active material; the positive electrode active material can be selected from one or more of lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium manganese oxide; the negative electrode active material can be selected from one or more of graphite, hard carbon, and soft carbon.

[0048] In some specific embodiments, in step 2), the conductive agent is mixed with the electrode active material and the pretreated polytetrafluoroethylene. The conductive agent can be selected from one or more of acetylene black, Ketjen black, and conductive graphite.

[0049] The present invention will be described below through specific examples. It should be understood that these examples are only used to illustrate the present invention and not to limit the scope of the present invention. Without departing from the spirit and scope of the inventive concept, the changes and advantages that can be conceived by those skilled in the art are included in the present invention, and the scope of protection of the present invention is the appended claims and any equivalents thereof.

[0050] Example 1

[0051] According to the following steps, the binder PTFE is pretreated, and a dry electrode (negative electrode) is prepared using the pretreated PTFE:

[0052] S1: Mix PTFE and petroleum ether with a mass ratio of 4:1 at a temperature of 15 °C and a rotation speed of 15 r / min for 30 min, and then let it stand and age at 25 °C for 8 h to obtain a mixed material.

[0053] S2: Under the conditions of 25 °C and 5 MPa, perform subcritical carbon dioxide extraction on the mixed material until the content of the pretreatment agent is 0.1 wt% to obtain a preliminarily dried material.

[0054] S3: Freeze the preliminarily dried material at a temperature of -80 °C for 300 min, and then dry it under a vacuum of 50 Pa until the content of the pretreatment agent is 0.0005 wt% to obtain the pretreated PTFE.

[0055] S4: Mix graphite, acetylene black, and the pretreated PTFE with a mass ratio of 90:5:5 at a temperature of 15 °C and a rotation speed of 2000 r / min for 3 min, and then press it onto a carbon-coated copper foil current collector to obtain a dry electrode.

[0056] Example 2

[0057] According to the following steps, the binder PTFE is pretreated, and a dry electrode (negative electrode) is prepared using the pretreated PTFE:

[0058] S1: Mix PTFE and paraffin oil with a mass ratio of 19:1 at a temperature of 15 °C and a rotation speed of 20 r / min for 40 min, and then let it stand and age at 30 °C for 10 h to obtain a mixed material.

[0059] S2: Under the conditions of 30 °C and 7 MPa, perform subcritical carbon dioxide extraction on the mixed material until the content of the pretreatment agent is 1 wt% to obtain a preliminarily dried material.

[0060] S3: After freezing the preliminarily dried material at -80 °C for 300 min, it is dried under a vacuum of 0.1 Pa until the content of the pretreatment agent is 0.0001 wt% to obtain pretreated PTFE.

[0061] S4: Graphite, acetylene black, and pretreated PTFE with a mass ratio of 90:5:5 are mixed at a temperature of 15 °C at a rotation speed of 2000 r / min for 3 min, and then pressed onto a carbon-coated copper foil current collector to obtain a dry process electrode.

[0062] Example 3

[0063] The difference between this example and Example 1 is only that: in step S2, the degree of subcritical carbon dioxide extraction is changed to extract until the content of the pretreatment agent is 0.08 wt%. The remaining steps are the same as those in Example 1. Specifically, in this example, the binder PTFE is pretreated according to the following steps, and a dry process electrode (negative electrode) is prepared using the pretreated PTFE:

[0064] S1: PTFE and petroleum ether with a mass ratio of 4:1 are mixed at a temperature of 15 °C at a rotation speed of 15 r / min for 30 min, and then left to age at 25 °C for 8 h to obtain a mixed material.

[0065] S2: The mixed material is subjected to subcritical carbon dioxide extraction at 25 °C and 5 MPa until the content of the pretreatment agent is 0.08 wt% to obtain a preliminarily dried material.

[0066] S3: After freezing the preliminarily dried material at -80 °C for 300 min, it is dried under a vacuum of 50 Pa until the content of the pretreatment agent is 0.0005 wt% to obtain pretreated PTFE.

[0067] S4: Graphite, acetylene black, and pretreated PTFE with a mass ratio of 90:5:5 are mixed at a temperature of 15 °C at a rotation speed of 2000 r / min for 3 min, and then pressed onto a carbon-coated copper foil current collector to obtain a dry process electrode.

[0068] Example 4

[0069] The difference between this example and Example 2 is only that: in step S2, the degree of subcritical carbon dioxide extraction is changed to extract until the content of the pretreatment agent is 2 wt%. The remaining steps are the same as those in Example 2. Specifically, in this example, the binder PTFE is pretreated according to the following steps, and a dry process electrode (negative electrode) is prepared using the pretreated PTFE:

[0070] S1: Mix PTFE and paraffin oil with a mass ratio of 19:1 at a temperature of 15°C and a rotation speed of 20 r / min for 40 min, and then let it stand and age at 30°C for 10 h to obtain a mixed material.

[0071] S2: Carry out subcritical carbon dioxide extraction on the mixed material under the conditions of 30°C and 7 MPa until the content of the pretreatment agent is 2 wt% to obtain a preliminarily dried material.

[0072] S3: Freeze the preliminarily dried material at a temperature of -80°C for 300 min, and then dry it under a vacuum degree of 0.1 Pa until the content of the pretreatment agent is 0.0001 wt% to obtain pretreated PTFE.

[0073] S4: Mix graphite, acetylene black and pretreated PTFE with a mass ratio of 90:5:5 at a temperature of 15°C and a rotation speed of 2000 r / min for 3 min, and then press it onto a carbon-coated copper foil current collector to obtain a dry electrode.

[0074] Comparative Example 1

[0075] The difference between this comparative example and Example 1 is only that: PTFE is not pretreated, and a conventional method of applying high shear force during the preparation of the dry electrode is used to fibrillate PTFE. Specifically, this comparative example prepares a dry electrode (negative electrode) according to the following steps:

[0076] Mix graphite, acetylene black and pretreated PTFE at a temperature of 15°C and a rotation speed of 20000 r / min for 10 min, and then press it onto a carbon-coated copper foil current collector to obtain a dry electrode.

[0077] Comparative Example 2

[0078] The difference between this comparative example and Example 1 is only that: Step S3 is not carried out, and only subcritical carbon dioxide extraction is used to remove the pretreatment agent; during the mixing of graphite, acetylene black and pretreated PTFE, the rotation speed and mixing duration are increased. The remaining steps are the same as those in Example 1. Specifically, this comparative example pretreats the binder PTFE according to the following steps and prepares a dry electrode (negative electrode) using the pretreated PTFE:

[0079] S1: Mix PTFE and petroleum ether with a mass ratio of 4:1 at a temperature of 15°C and a rotation speed of 15 r / min for 30 min, and then let it stand and age at 25°C for 8 h to obtain a mixed material.

[0080] S2: Carry out subcritical carbon dioxide extraction on the mixed material under the conditions of 25°C and 5 MPa until the content of the pretreatment agent is 0.0005 wt% to obtain pretreated PTFE.

[0081] S3: Mix graphite, acetylene black, and pretreated PTFE with a mass ratio of 90:5:5 at a temperature of 15°C and a rotation speed of 10,000 r / min for 5 min, and then press them onto a carbon-coated copper foil current collector to obtain a dry electrode.

[0082] Comparative Example 3

[0083] The difference between this comparative example and Example 1 is only that: Step S2 is not carried out, and the pretreatment agent is removed only by freeze-vacuum drying; during the mixing process of graphite, acetylene black, and pretreated PTFE, the rotation speed and mixing duration are increased. The remaining steps are the same as those in Example 1. Specifically, in this comparative example, the binder PTFE is pretreated according to the following steps, and a dry electrode (negative electrode) is prepared using the pretreated PTFE:

[0084] S1: Mix PTFE and petroleum ether with a mass ratio of 4:1 at a temperature of 15°C and a rotation speed of 15 r / min for 30 min, and then let it stand and age at 25°C for 8 h to obtain a mixed material.

[0085] S2: Freeze the mixed material at a temperature of -80°C for 300 min, and then dry it under a vacuum of 50 Pa until the content of the pretreatment agent is 0.0005 wt% to obtain pretreated PTFE.

[0086] S3: Mix graphite, acetylene black, and pretreated PTFE with a mass ratio of 90:5:5 at a temperature of 15°C and a rotation speed of 15,000 r / min for 5 min, and then press them onto a carbon-coated copper foil current collector to obtain a dry electrode.

[0087] Comparative Example 4

[0088] The difference between this comparative example and Example 1 is only that: The order between Step S2 and Step S3 is swapped, and the pretreatment agent is removed by first freeze-vacuum drying and then subcritical carbon dioxide extraction; in Step S4, during the mixing process of graphite, acetylene black, and pretreated PTFE, the rotation speed and mixing duration are increased. The remaining steps are the same as those in Example 1. Specifically, in this comparative example, the binder PTFE is pretreated according to the following steps, and a dry electrode (negative electrode) is prepared using the pretreated PTFE:

[0089] S1: Mix PTFE and a pretreatment agent (one or more of aviation kerosene, petroleum ether, paraffin oil, and isoparaffin) with a mass ratio of 4:1 at a temperature of 15°C and a rotation speed of 15 r / min for 30 min, and then let it stand and age at 25 - 30°C for 8 h to obtain a mixed material.

[0090] S3: After freezing the mixture at -80 °C for 300 min, dry it under a vacuum of 50 Pa until the content of the pretreatment agent is 0.15 wt% to obtain the preliminarily dried material.

[0091] S2: Under the conditions of 25 °C and 5 MPa, perform subcritical carbon dioxide extraction on the preliminarily dried material until the content of the pretreatment agent is 0.0008 wt% to obtain the pretreated PTFE.

[0092] S4: Mix graphite, acetylene black, and the pretreated PTFE with a mass ratio of 90:5:5 at a temperature of 15 °C and a rotation speed of 12,000 r / min for 5 min, and then press it onto the current collector to obtain the dry electrode.

[0093] Test Example

[0094] Take the dry electrodes prepared in each example and comparative example, and conduct tensile strength, ohmic impedance, and specific capacity tests. The results are shown in Table 1.

[0095] Table 1 Performance test results of dry electrodes

[0096]

[0097] Analyzing the performance test results in Table 1, it can be seen that:

[0098] (1) The dry electrodes prepared in Examples 1 - 4 have higher mechanical strength than Comparative Example 1, lower ohmic impedance than Comparative Example 1, and higher specific capacity than Comparative Example 1. This is because: compared with the prior art (Comparative Example 1) that makes PTFE fibrillate by applying high shear force during the preparation of dry electrodes, Examples 1 - 4 use the method of the present invention to pretreat PTFE, which can effectively improve the degree of PTFE fibrillation, reduce the curling, winding, and agglomeration of PTFE fibers without applying high shear force, thereby better maintaining the integrity of the electrode active material.

[0099] (2) Compared with Examples 1 to 4, although the mixing speed and duration of graphite, acetylene black, and binder were increased during the preparation of the dry electrode in Comparative Examples 2 to 4, the mechanical strength of the prepared dry electrode was still lower than that of Examples 1 to 4. The reason is as follows: In Examples 1 to 4, the pretreatment agent was removed by first performing subcritical carbon dioxide extraction and then freeze-vacuum drying. During this process, subcritical carbon dioxide can cause a certain degree of swelling of PTFE, reducing the curled state of PTFE fibers. During the subsequent freeze-vacuum drying process, PTFE is in a low-temperature state, which can maintain the loose structure inside the PTFE particles and reduce the possibility of entanglement and aggregation between molecular chains. In Comparative Example 2, only subcritical carbon dioxide extraction was used, and the effect of maintaining the loose structure inside the PTFE particles was not good. In Comparative Example 3, only freeze-vacuum drying was used, and PTFE particles could not be swollen, so the PTFE fibrillation effect was relatively poor. In Comparative Example 4, subcritical carbon dioxide was carried out after freeze-vacuum drying. Since the temperature during the subcritical carbon dioxide process is higher than that during freeze-vacuum drying, the possibility of molecular weight entanglement and aggregation will increase.

[0100] (3) Compared with Examples 1 and 2, the mechanical strength of the dry electrodes in Examples 3 and 4 was relatively low. This indicates that the degree of subcritical carbon dioxide extraction will affect its cooperation effect with subcritical carbon dioxide extraction. On the basis of combining these two methods for removing the pretreatment agent, by controlling the degree of subcritical carbon dioxide extraction within a certain range (extracting to a pretreatment agent content of 0.1 to 1 wt%), the PTFE fibrillation effect can be improved to a greater extent.

[0101] Unless otherwise defined, all technical terms and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the art to which the present disclosure belongs. The raw materials and equipment used in the present invention are conventional raw materials and equipment in the art and can be obtained from conventional commercial channels without special instructions; the methods used in the present invention are conventional methods in the art without special instructions.

[0102] The above are only the preferred embodiments of the present invention and do not impose any limitations on the present invention. Any simple modifications, changes, and equivalent transformations made to the above embodiments according to the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A binder pretreatment method for dry electrodes, characterized in that, It includes the following steps: S1: Mix polytetrafluoroethylene with a pretreatment agent and age them to obtain a mixed material; the pretreatment agent is an organic liquid with a surface tension not higher than 40 mN / m at 20°C; S2: Extract the pretreatment agent from the mixed material using subcritical carbon dioxide, and perform freeze-vacuum drying to obtain pretreated polytetrafluoroethylene.

2. The binder pretreatment method according to claim 1, wherein, In step S2, the subcritical carbon dioxide extraction is carried out until the content of the pretreatment agent is 0.1 - 1 wt%.

3. The binder pretreatment method according to claim 1 or 2, characterized in that, In step S2, the temperature of the subcritical carbon dioxide is 25 - 30°C, and the pressure is 5 - 7 MPa.

4. The binder pretreatment method according to claim 1 or 2, characterized in that, In step S2, the process of freeze-vacuum drying includes: freezing at a temperature below -70°C, and then drying under a vacuum degree of 0.1 - 50 Pa until the content of the pretreatment agent is lower than 0.001 wt%.

5. The binder pretreatment method according to claim 1, wherein, In step S1, the pretreatment agent is one or more of aviation kerosene, petroleum ether, paraffin oil, and isoparaffin.

6. The binder pretreatment method according to claim 1 or 5, characterized in that, In step S1, the mass ratio between the polytetrafluoroethylene and the pretreatment agent is 4 - 19:

1.

7. The binder pretreatment method according to claim 1, characterized in that, In step S1, the conditions for mixing are as follows: the temperature is lower than 19°C, the rotation speed is 15 - 20 r / min, and the time is 30 - 40 min.

8. The binder pretreatment method according to claim 1, wherein In step S1, the conditions for aging are as follows: the temperature is 25 - 30°C, and the time is 8 - 10 h.

9. A method for preparing a dry electrode, characterized in that, It includes the following steps: 1) Obtain pretreated polytetrafluoroethylene according to the binder pretreatment method described in any one of claims 1 - 8; 2) Mix the electrode active material and the pretreated polytetrafluoroethylene, and press them on the current collector to obtain a dry electrode.

10. The preparation method according to claim 9, characterized in that, In step 2), the mixing process is carried out at a temperature lower than 19°C, the rotation speed is 2000 - 3000 r / min, and the time is 3 - 5 min.

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