Lithium iron phosphate dry-method electrode film and preparation method thereof

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a lithium iron phosphate dry-method electrode membrane and a preparation method thereof.The preparation method comprises the steps that S1, lithium iron phosphate and a low-tap-density conductive agent are premixed, and a first mixture which is evenly mixed is obtained; s2, adding a binder into the first mixture to obtain a uniformly mixed second mixture; s3, converting the second mixture into an electrode group, and repeatedly folding and rolling the electrode group to obtain a positive electrode membrane with no abnormal appearance; a low-tap-density conductive agent is added into lithium iron phosphate with a small particle size (D50 < 1 mu m), the input proportion of the lithium iron phosphate, the conductive agent and PTFE particles is optimized, and the continuous self-supporting lithium iron phosphate electrode membrane with no abnormal appearance can be obtained by using a dry preparation process.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium - ion batteries, and particularly to a dry - process lithium iron phosphate electrode film and a preparation method thereof. Background Art

[0002] Currently, the preparation process of lithium - ion battery electrode membranes is the traditional wet process. The traditional wet electrodes are prepared by mixing powder materials with solvents to prepare slurries, and then through processes such as coating, drying, and solvent recovery. However, this preparation process has problems such as high energy consumption, high cost, environmental pollution, electrode delamination, and limited electrode thickness. Therefore, the dry - process electrode process has become a better choice because it does not require solvents and has advantages such as low production cost, low energy consumption, and environmental friendliness.

[0003] The dry - process electrode process mainly includes steps such as dry - powder mixing, fibrillation treatment, and dry - film formation. Among them, the dry - powder mixing step mainly mixes active materials, conductive agents, and binders in dry - powder form in proportion, and realizes uniform mixing through mechanical stirring, high - speed shearing, or air - flow dispersion and other methods to avoid particle agglomeration and ensure that the conductive agent and binder uniformly coat the active material.

[0004] Existing dry processes can easily prepare negative - electrode membranes, but there are still great difficulties in the preparation of positive - electrode membranes. Especially for lithium iron phosphate particles (active materials) with a small particle size (D50 < 1μm), it is difficult to form a self - supporting lithium iron phosphate electrode film with a continuous appearance and no abnormalities. Summary of the Invention

[0005] The purpose of the present invention is to provide a dry - process lithium iron phosphate electrode film and a preparation method thereof to solve the problems raised in the above - mentioned background art.

[0006] The technical solution adopted by the present invention is as follows:

[0007] A preparation method of a dry - process lithium iron phosphate electrode film, the preparation method comprising the following steps:

[0008] S1. Premix lithium iron phosphate and a low - tap - density conductive agent to obtain a uniformly mixed first mixture;

[0009] S2. Add a binder to the first mixture to obtain a uniformly mixed second mixture;

[0010] S3. Convert the second mixture into electrode agglomerates, and repeatedly fold and roll the electrode agglomerates to obtain a positive - electrode film with a normal appearance.

[0011] Optionally, the mass ratio of the lithium iron phosphate, the low - tap - density conductive agent, and the binder is (88 - 93):(10 - 5):2.

[0012] Optionally, the mass ratio of the lithium iron phosphate, the low tap density conductive agent, and the binder is preferably 93:5:2.

[0013] Optionally,

[0014] The low tap density conductive agent is one of CNT and conductive carbon black;

[0015] The lithium iron phosphate is small particle size particles with D50 < 1 μm.

[0016] Optionally, the lower the tap density of the low tap density conductive agent, the less the content of the low tap density conductive agent required to form a self-supporting positive electrode film with a continuous appearance and no abnormality.

[0017] Optionally, in the steps S1 and S2, mixing is carried out through a stirring tank, and the temperature of the stirring tank is controlled between 0 and 15 °C.

[0018] Optionally, in the step S1, the stirring of the stirring tank includes:

[0019] The first stirring stage, and the stirring time of the first stirring stage is controlled between 60 and 180 S;

[0020] The second stirring stage, and the stirring time of the second stirring stage is controlled between 300 and 900 S.

[0021] Optionally, in the step S2, the stirring of the stirring tank includes:

[0022] Slow stirring, and the slow stirring time is controlled between 60 and 180 S;

[0023] Fast stirring, and the fast stirring time is controlled between 60 and 180 S.

[0024] Optionally, in the step S2, the binder needs to be cooled and sieved before being put into the first mixture.

[0025] Optionally, the dry lithium iron phosphate electrode film is prepared by the method according to any one of claims 1-9.

[0026] Compared with the prior art, the beneficial effects of the present invention are:

[0027] By adding a low tap density conductive agent to small particle size (D50 < 1 μm) lithium iron phosphate, optimizing the input ratio of lithium iron phosphate, conductive agent, and PTFE particles, a self-supporting lithium iron phosphate electrode film with a continuous appearance and no abnormality can be obtained by using a dry preparation process. Detailed implementation mode

[0028] In view of the problem in the current existing technologies that it is difficult to form a self-supporting lithium iron phosphate electrode film with a continuous appearance and no abnormality for lithium iron phosphate particles (active materials) with a small particle size (D50 < 1 μm).

[0029] Example 1

[0030] This example provides a preparation method for a dry lithium iron phosphate electrode film. In this example, the active material is lithium iron phosphate with a small particle size (D50 < 1 μm), the low tap density conductive agent is CNT with a tap density of 0.025 g / cm3, and the binder is PTFE. The preparation method includes the following steps:

[0031] 1. First, add lithium iron phosphate particles and CNT into a stirring tank, turn on the chilled water, control the temperature between 0 and 15 °C, reverse the stirring shaft with a speed of 150 rpm, and at the same time rotate the stirring barrel forward with a speed of 45 rpm. Control the stirring time between 60 and 180 s, preferably 120 s, to premix the lithium iron phosphate particles and CNT, and at the same time prevent CNT from generating dust. Preventing CNT from generating dust is an existing technology and will not be elaborated here;

[0032] 2. Keep the chilled water on, control the temperature between 0 and 15 °C, reverse the stirring shaft with a speed of 2250 rpm, and at the same time rotate the stirring barrel forward with a speed of 45 rpm. Control the stirring time between 300 and 900 s, preferably 600 s, to coat the lithium iron phosphate particles with CNT and obtain a uniformly mixed first mixture;

[0033] 3. First, put PTFE particles into the refrigerator and refrigerate them for about 24 h, and screen them through a 2 mm sieve before feeding to prevent PTFE particles from agglomerating. Keep the chilled water on, control the temperature between 0 and 15 °C, add PTFE particles to the first mixture, first stir slowly, reverse the stirring shaft with a speed of 150 rpm, and at the same time rotate the stirring barrel forward with a speed of 45 rpm. Control the stirring time between 60 and 180 s, preferably 120 s; then stir quickly, reverse the stirring shaft with a speed of 2250 rpm, and at the same time rotate the stirring barrel forward with a speed of 45 rpm. Control the stirring time between 60 and 180 s, preferably 120 s, to obtain a uniformly mixed second mixture of lithium iron phosphate particles, CNT, and PTFE particles. At the same time, the particle size of PTFE particles is reduced from 425 - 525 μm to about 10 μm without fibrillation;

[0034] 4. Turn off the chilled water, reverse the stirring shaft at a speed of 4500 rpm, and at the same time rotate the stirring barrel forward at a speed of 45 rpm. Raise the temperature to between 60 and 90 °C. Preferably, end the stirring when the temperature reaches 70 °C. Transform the uniformly mixed powder into electrode agglomerates by inducing fibrillation of PTFE particles. The induction of fibrillation of PTFE particles is a prior art and will not be elaborated here.

[0035] 5. Place the electrode agglomerates in an oven at 100 °C and bake for 2 h to further promote fibrillation of PTFE particles. Control the environmental humidity within 10% to prevent PTFE particles from absorbing water and sticking to the rollers. Then place the baked electrode agglomerates between the two rollers of an open mill at a temperature of 120 °C, a gap of 200 μm, and a differential speed ratio of 3 to obtain a low-strength film. Fold the film and place it between the two rollers of the open mill again with the same parameters to obtain a film with improved strength. Repeat this process 2 - 4 times to obtain a lithium iron phosphate positive electrode film with a smooth surface without wrinkles and cracks. By adjusting the gap, a self-supporting film with a thickness of 100 μm can be obtained.

[0036] Among them, the mass ratio of lithium iron phosphate, CNT, and PTFE is 93:5:2.

[0037] Example 2

[0038] This example provides a method for preparing a dry electrode film of lithium iron phosphate. In this example, the active material is lithium iron phosphate with a small particle size (D50 < 1 μm), the low tap density conductive agent is CNT with a tap density of 0.025 g / cm3, and the binder is PTFE. The preparation method includes the following steps:

[0039] 1. First, add lithium iron phosphate particles and CNT to a stirring tank, turn on the chilled water, control the temperature between 0 and 15 °C, reverse the stirring shaft at a speed of 150 rpm, and at the same time rotate the stirring barrel forward at a speed of 45 rpm. Control the stirring time between 60 and 180 s. Preferably, control the time at 120 s to premix the lithium iron phosphate particles and CNT, and at the same time prevent CNT from generating dust. Preventing CNT from generating dust is a prior art and will not be elaborated here.

[0040] 2. Keep the chilled water on, control the temperature between 0 and 15 °C, reverse the stirring shaft at a speed of 2250 rpm, and at the same time rotate the stirring barrel forward at a speed of 45 rpm. Control the stirring time between 300 and 900 s. Preferably, control it at 600 s to coat the lithium iron phosphate particles with CNT to obtain a uniformly mixed first mixture.

[0041] 3. First, place the PTFE particles in the refrigerator for refrigeration and static settlement for about 24 hours. Before feeding, sieve them through a 2-mm sieve to prevent the PTFE particles from agglomerating. Keep the chilled water open, control the temperature between 0 and 15 °C, add the PTFE particles to the first mixture, first perform slow stirring, with the stirring shaft rotating in reverse at a speed of 150 rpm, and at the same time the stirring barrel rotating forward at a speed of 45 rpm. Control the stirring time between 60 and 180 s, preferably 120 s. Then perform fast stirring, with the stirring shaft rotating in reverse at a speed of 2250 rpm, and at the same time the stirring barrel rotating forward at a speed of 45 rpm. Control the stirring time between 60 and 180 s, preferably 120 s, to obtain a second mixture of uniformly mixed lithium iron phosphate particles, CNT, and PTFE particles. At the same time, the particle size of the PTFE particles is reduced from 425 - 525 μm to about 10 μm without fibrillation.

[0042] 4. Close the chilled water, with the stirring shaft rotating in reverse at a speed of 4500 rpm, and at the same time the stirring barrel rotating forward at a speed of 45 rpm. Raise the temperature to between 60 and 90 °C, preferably end stirring when the temperature rises to 70 °C. Transform the uniformly mixed powder into electrode agglomerates by inducing the fibrillation of the PTFE particles. The induction of the fibrillation of the PTFE particles is a prior art and will not be elaborated here.

[0043] 5. Place the electrode agglomerates in an oven at 100 °C for baking for 2 hours to promote further fibrillation of the PTFE particles. Control the environmental humidity within 10% to prevent the PTFE particles from absorbing water and sticking to the rollers. Then place the baked electrode agglomerates between the two rollers of a two-roll mill at a temperature of 120 °C, a gap of 200 μm, and a differential speed ratio of 3 to obtain a low-strength film. After folding it in half, place it between the two rollers of the two-roll mill again with the same parameters to obtain a film with improved strength. Repeat this 2 - 4 times to obtain a lithium iron phosphate positive electrode film with a smooth surface and no wrinkles or cracks. By adjusting the gap, a self-supporting film with a thickness of 100 μm can be obtained.

[0044] Among them, the mass ratio of lithium iron phosphate, CNT, and PTFE is 88:10:2.

[0045] The difference between this example and Example 1 is that in this example, the mass ratio of lithium iron phosphate, CNT, and PTFE is 88:10:2.

[0046] Example 3

[0047] This example provides a preparation method of a dry electrode film of lithium iron phosphate. In this example, the active material is lithium iron phosphate with a small particle size (D50 < 1 μm), the low tap density conductive agent is CNT with a tap density of 0.08 g / cm3, and the binder is PTFE. The preparation method includes the following steps:

[0048] 1. First, add lithium iron phosphate particles and CNT into the stirring tank. Turn on the chilled water and control the temperature between 0 and 15°C. The stirring shaft rotates in reverse at a speed of 150 rpm, while the stirring barrel rotates forward at a speed of 45 rpm. Control the stirring time between 60 and 180 s. Preferably, control the time at 120 s to premix the lithium iron phosphate particles and CNT, and at the same time prevent CNT from raising dust. Preventing CNT from raising dust is an existing technology and will not be elaborated here.

[0049] 2. Keep the chilled water on and control the temperature between 0 and 15°C. The stirring shaft rotates in reverse at a speed of 2250 rpm, while the stirring barrel rotates forward at a speed of 45 rpm. Control the stirring time between 300 and 900 s. Preferably, control it at 600 s to coat the CNT on the lithium iron phosphate particles and obtain a uniformly mixed first mixture.

[0050] 3. First, put the PTFE particles into the refrigerator and refrigerate them for about 24 h. Sieve them through a 2 - mm sieve before feeding to prevent the PTFE particles from agglomerating. Keep the chilled water on and control the temperature between 0 and 15°C. Add the PTFE particles to the first mixture. First, conduct slow stirring. The stirring shaft rotates in reverse at a speed of 150 rpm, while the stirring barrel rotates forward at a speed of 45 rpm. Control the stirring time between 60 and 180 s. Preferably, control it at 120 s. Then conduct fast stirring. The stirring shaft rotates in reverse at a speed of 2250 rpm, while the stirring barrel rotates forward at a speed of 45 rpm. Control the stirring time between 60 and 180 s. Preferably, control it at 120 s to obtain a uniformly mixed second mixture of lithium iron phosphate particles, CNT, and PTFE particles. At the same time, reduce the particle size of the PTFE particles from 425 - 525 μm to about 10 μm without fibrillation.

[0051] 4. Turn off the chilled water. The stirring shaft rotates in reverse at a speed of 4500 rpm, while the stirring barrel rotates forward at a speed of 45 rpm. Raise the temperature to between 60 and 90°C. Preferably, end the stirring when the temperature rises to 70°C. Transform the uniformly mixed powder into electrode agglomerates by inducing the fibrillation of the PTFE particles. Inducing the fibrillation of the PTFE particles is an existing technology and will not be elaborated here.

[0052] 5. Put the electrode agglomerates into an oven at 100°C and bake for 2 h to promote further fibrillation of the PTFE particles. Control the environmental humidity within 10% to prevent the PTFE particles from absorbing water and sticking to the rollers. Then put the baked electrode agglomerates between the two rollers of an open mill at a temperature of 120°C, a gap of 200 μm, and a differential speed ratio of 3 to obtain a low - strength film. Fold it in half and put it between the two rollers of the open mill again with the same parameters to obtain a film with improved strength. Repeat this 2 - 4 times to obtain a lithium iron phosphate positive electrode film with a smooth surface, no wrinkles or cracks. By adjusting the gap, a self - supporting film with a thickness of 100 μm can be obtained.

[0053] Among them, the mass ratio of lithium iron phosphate, CNT, and PTFE in this embodiment is 88:10:2. The difference between this embodiment and the second embodiment is that the tap density of CNT in this embodiment is 0.08 g / cm3.

[0054] Example 4

[0055] This embodiment provides a method for preparing a dry electrode film of lithium iron phosphate. In this embodiment, the active material is lithium iron phosphate with a small particle size (D50 < 1 μm), the low-tap-density conductive agent is conductive carbon black with a tap density of 0.16 g / cm3, and the binder is PTFE. The preparation method includes the following steps:

[0056] 1. First, add lithium iron phosphate particles and conductive carbon black to a stirring tank, turn on the chilled water, control the temperature between 0 and 15 °C, reverse the stirring shaft at a speed of 150 rpm, and at the same time rotate the stirring barrel forward at a speed of 45 rpm. Control the stirring time between 60 and 180 s, preferably 120 s, to premix the lithium iron phosphate particles and conductive carbon black;

[0057] 2. Keep the chilled water on, control the temperature between 0 and 15 °C, reverse the stirring shaft at a speed of 2250 rpm, and at the same time rotate the stirring barrel forward at a speed of 45 rpm. Control the stirring time between 300 and 900 s, preferably 600 s, to coat the lithium iron phosphate particles with conductive carbon black to obtain a uniformly mixed first mixture;

[0058] 3. First, place the PTFE particles in the refrigerator and let them stand still for about 24 h, and screen them through a 2 mm sieve before feeding to prevent the PTFE particles from agglomerating. Keep the chilled water on, control the temperature between 0 and 15 °C, add the PTFE particles to the first mixture, first stir slowly, reverse the stirring shaft at a speed of 150 rpm, and at the same time rotate the stirring barrel forward at a speed of 45 rpm. Control the stirring time between 60 and 180 s, preferably 120 s; then stir quickly, reverse the stirring shaft at a speed of 2250 rpm, and at the same time rotate the stirring barrel forward at a speed of 45 rpm. Control the stirring time between 60 and 180 s, preferably 120 s, to obtain a uniformly mixed second mixture of lithium iron phosphate particles, conductive carbon black, and PTFE particles. At the same time, the particle size of the PTFE particles is reduced from 425 - 525 μm to about 10 μm without fibrillation;

[0059] 4. Turn off the chilled water, reverse the stirring shaft at a speed of 4500 rpm, and at the same time rotate the stirring barrel forward at a speed of 45 rpm. Raise the temperature to between 60 and 90 °C, preferably end the stirring when the temperature rises to 70 °C, and transform the uniformly mixed powder into electrode agglomerates by inducing the fibrillation of the PTFE particles. The induction of the fibrillation of the PTFE particles is a prior art and will not be elaborated here;

[0060] 5. Place the electrode mass in an oven at 100 °C and bake for 2 h to further fibrillate the PTFE particles; control the environmental humidity within 10% to prevent the PTFE particles from absorbing water and sticking to the rollers; then place the baked electrode mass between the two rollers of an open mill at a temperature of 120 °C, a gap of 200 μm, and a differential speed ratio of 3 to obtain a low-strength film. Fold the film and place it between the two rollers of the open mill again with the same parameters to obtain a film with improved strength. Repeat this process 2 - 4 times to obtain a lithium iron phosphate positive electrode film with a smooth surface and no wrinkles or cracks. By adjusting the gap, a self-supporting film with a thickness of 100 μm can be obtained.

[0061] Among them, the mass ratio of lithium iron phosphate, conductive carbon black, and PTFE is 58:40:2.

[0062] The difference between this example and Example 1 is that in this example, the low tap density conductive agent is conductive carbon black with a tap density of 0.16 g / cm3, and the mass ratio of lithium iron phosphate, conductive carbon black, and PTFE is 58:40:2.

[0063] Furthermore, by comparing Examples 1 to 4, it can be seen that on the premise of ensuring that the positive electrode film has no abnormal appearance, there is a certain law between the tap density of the low tap density conductive agent and the amount of the low tap density conductive agent required for film formation.

[0064] As shown in the following table:

[0065]

[0066] As can be seen from the above table, the lower the tap density of the low tap density conductive agent, the less the content of the low tap density conductive agent required to form a continuous self-supporting lithium iron phosphate electrode film with no abnormal appearance.

[0067] As can be seen from the above table, only 5% of CNT with an ultra-low tap density (0.025 g / cm3) can be added to form a continuous self-supporting lithium iron phosphate electrode film with no abnormal appearance.

[0068] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate dry electrode film, characterized in that: The preparation method comprises the following steps: S1, premixing lithium iron phosphate and a low tap density conductive agent to obtain a uniformly mixed first mixture; S2, adding a binder to the first mixture to obtain a uniformly mixed second mixture; S3, converting the second mixture into an electrode mass, and repeatedly folding and rolling the electrode mass to obtain a positive electrode film with normal appearance.

2. The preparation method according to claim 1, characterized in that: The mass ratio of the lithium iron phosphate, the low tap density conductive agent and the binder is (88-93):(10-5):

2.

3. The preparation method according to claim 2, characterized in that: The mass ratio of the lithium iron phosphate, the low tap density conductive agent and the binder is preferably 93:5:

2.

4. The preparation method according to claim 1, characterized in that: The low tap density conductive agent is one of CNT and conductive carbon black; the lithium iron phosphate is a small particle with a D50 of less than 1 μm.

5. The preparation method according to claim 1, characterized in that: The lower the tap density of the low tap density conductive agent, the less the content of the low tap density conductive agent is required to form a continuous self-supporting positive electrode film with no abnormalities in appearance.

6. The preparation method according to claim 1, characterized in that: In step S1 and step S2, mixing is performed in a stirring tank, and the temperature of the stirring tank is controlled between 0 and 15°C.

7. The preparation method according to claim 6, characterized in that: In step S1, the stirring of the stirring tank includes: a first stirring stage, in which the stirring time of the first stirring stage is controlled between 60 and 180 seconds; and a second stirring stage, in which the stirring time of the second stirring stage is controlled between 300 and 900 seconds.

8. The preparation method according to claim 6, characterized in that: In step S2, the stirring of the stirring tank includes: slow stirring, wherein the slow stirring time is controlled between 60 and 180 seconds; and fast stirring, wherein the fast stirring time is controlled between 60 and 180 seconds.

9. The preparation method according to claim 1, characterized in that: In the step S2, the binder needs to be cooled and sieved before being added to the first mixture.

10. A lithium iron phosphate dry electrode membrane, characterized in that: The lithium iron phosphate dry electrode membrane is prepared by the method as described in any one of claims 1-9.

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