Self-supporting electrode diaphragm as well as preparation method and application thereof

Abstract

The invention discloses a self-supporting electrode diaphragm as well as a preparation method and application thereof. The preparation method comprises the following steps: mixing an electrode material and a solvent to obtain a blocky mixture A; wherein the electrode material comprises a small-particle-size active substance and a first adhesive, and the median particle size D50 of the small-particle-size active substance is 0.05 [mu] m-5 [mu] m; drying and crushing the mixture A, and separating to obtain a mixture B; and carrying out dry mixing and shearing on the mixture B and a second adhesive capable of being fiberized to obtain a mixture C, and rolling to obtain the self-supporting electrode diaphragm. The method provided by the invention can improve the problem that the membrane is too hard, easy to crack and difficult to thin when the self-supporting electrode membrane is prepared by using the small-particle-size active substance.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries and relates to a self-supporting electrode membrane and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries are widely used in consumer, energy storage, and electric vehicle fields due to their many advantages. The processing technology of lithium-ion battery pole pieces can be divided into wet electrode process and dry electrode process according to whether solvents are used. The wet electrode process is currently the most common coating process for preparing lithium-ion battery pole pieces. In order to ensure the fluidity of the battery slurry, the solvent content of the battery slurry is generally between 30% and 60%. Therefore, solvent volatilization usually occurs during the wet electrode process, which will cause environmental pollution. Moreover, the energy consumption of solvent drying will increase the manufacturing cost of the battery. Finally, the residual solvent impurities will cause side reactions inside the battery, affecting the battery performance. The dry electrode process is a technology that mixes the electrode active material, conductive agent, and dry binder evenly without adding solvent. After rolling to form a self-supporting membrane, the membrane is hot-pressed with the current collector to prepare the battery pole piece.

[0003] The self-supporting membrane in the dry electrode process relies primarily on the dry binder fiberization to form a mesh structure, thereby fixing the electrode active material and conductive agent within the binder fiber mesh, forming a self-supporting electrode membrane. The wet coating technology for preparing electrode sheets relies on the support of the current collector, and the slurry is coated on the current collector. This is the most direct difference between the dry electrode process and the traditional wet electrode process.

[0004] It can be seen that compared with the wet electrode process, the dry electrode process can simplify the process, reduce costs, and avoid the side effects of solvent impurities on the battery, and has a good application prospect. However, the existing dry electrode process still has the following disadvantages: when the particle size of the active material is small, the compaction density increases rapidly during the dry membrane thinning process, and it tends to become very hard, lack flexibility and easily crack, making it difficult to thin. Dry processing is particularly difficult. This problem limits the mass production application of the dry electrode process and urgently needs to be solved. Summary of the Invention

[0005] In response to the above-mentioned technical problems existing in the prior art, the present invention aims to provide a self-supporting electrode membrane, a method for preparing the membrane, and its application. The method of the present invention can alleviate the problem of using small-particle active materials to prepare self-supporting electrode membranes, which results in the membrane being too hard, prone to cracking, and difficult to thin.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a method for preparing a self-supporting electrode membrane, characterized in that the preparation method comprises the following steps:

[0008] Mixing an electrode material and a solvent to obtain a mixture A in the form of a mass; wherein the electrode material includes a small-particle active material and a first binder, and the median particle size D50 of the small-particle active material is 0.05 μm-5 μm;

[0009] Drying and crushing the mixture A to obtain a mixture B;

[0010] The mixture B is dry-mixed and sheared with a second fiberizable adhesive to obtain a mixture C, which is then rolled to obtain the self-supporting electrode membrane.

[0011] In the present invention, the first adhesive should be soluble in the solvent.

[0012] In the method of the present invention, the median particle size D50 of the small-particle active substance is 0.05 μm-5 μm, for example, it can be 0.05 μm, 0.1 μm, 0.15 μm, 0.2 μm, 0.3 μm, 0.5 μm, 0.7 μm, 1 μm, 1.3 μm, 1.6 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm or 5 μm, etc.

[0013] In the method of the present invention, the electrode material and the solvent are mixed so that the components of the electrode material (such as small-particle active material, the first adhesive and the optional conductive agent) are evenly dispersed and bonded to obtain a mass-like mixture A. After drying, crushing and separating, a granulated mixture B is obtained, which improves the particle size and flexibility of the mixture. Therefore, the dry electrode process (i.e., the process of dry mixing, shearing and rolling) is used to prepare a self-supporting membrane. The second adhesive that can be fiberized forms fiberized adhesive filaments under the action of shear. The adhesive filaments wrap around the particles that bind the second mixture. After rolling, the flexibility of the membrane can be ensured during the thinning process. This solves the problem that the membrane is too hard, easy to crack and difficult to thin when using small-particle active materials to prepare self-supporting electrode membranes, which helps promote the mass production application of dry electrode processes.

[0014] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. Through the following preferred technical solutions, the technical objectives and beneficial effects of the present invention can be better achieved and realized.

[0015] As a preferred technical solution of the method for preparing the self-supporting electrode membrane of the present invention, based on the mass of the electrode material, the mass proportion of the solvent is V, the drying temperature is T, and D50, V and T satisfy the following relationship:

[0016] 20≥D50×V×0.01×16.5+T / 20≥5.

[0017] The unit of D50 is μm, the unit of V is %, the unit of T is °C, and the parameters in the formula do not contain units.

[0018] Generally speaking, the smaller the D50 particle size, the larger the specific surface area of the material, the greater the adsorption capacity of the solvent, and the correspondingly higher the drying temperature required to remove the solvent after subsequent granulation. When selecting parameters to meet the above relationship, better mixing, granulation and drying effects can be achieved.

[0019] Preferably, the mass proportion V of the solvent satisfies: 5%≤V≤35%. For example, V can be 5%, 6%, 7%, 8%, 10%, 12%, 13%, 14%, 15%, 16%, 18%, 20%, 22%, 23%, 25%, 27%, 28%, 30%, 31%, 32%, 33% or 35%, etc.

[0020] Preferably, the drying temperature T is 50°C-250°C, for example, it can be 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 175°C, 185°C, 200°C, 210°C, 220°C, 235°C or 250°C, etc.

[0021] By further limiting the parameter range of V and T, it is beneficial to achieve the best mixing, granulation and drying effects.

[0022] Preferably, the electrode materials and the solvent are mixed by first dry-mixing the electrode materials, and then mixing the dry-mixed mixture with the solvent to obtain a first mixture. This arrangement has the advantage that, through dry mixing, the first binder is pre-coated on the surface of the small-sized active material and conductive agent particles, thereby preventing agglomeration of the small-sized active material and conductive agent particles. Subsequently, mixing with the solvent ensures good mixing uniformity and effectively forms agglomerates.

[0023] Preferably, the electrode material further includes a conductive agent.

[0024] Preferably, based on the total mass of the electrode material, the mass proportions of the small-particle active material, the conductive agent, and the first binder are 90%-99%, 0.5%-5%, and 0.5%-5%, respectively. For example, the mass proportion of the small-particle active material may be 90%, 90.5%, 91%, 91.5%, 92%, 92.5%, 93%, 93.5%, 94%, 94.5%, 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, 98%, or 99%, etc. The mass proportion of the conductive agent may be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, or 5%, etc. The mass proportion of the first adhesive may be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%, etc.

[0025] Preferably, the first adhesive includes at least one of polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC) or polyurethane.

[0026] Preferably, the solvent includes at least one of N-methylpyrrolidone (NMP), water, ethylene carbonate (EC), dimethyl carbonate (DMC) or diethyl carbonate (DEC).

[0027] Preferably, the drying method is baking.

[0028] Preferably, the drying time is 1 h to 3 h, for example, 1 h, 1.5 h, 2 h, 2.5 h or 3 h.

[0029] Preferably, the crushing method is shear crushing.

[0030] Preferably, the process parameters of the shear crushing are: shear speed 3m / s-10m / s, for example, it can be 3m / s, 4m / s, 5m / s, 6m / s, 7m / s, 8m / s, 9m / s or 10m / s, etc.; time 5min-60min, for example, it can be 5min, 10min, 15min, 20min, 25min, 30min, 35min, 40min, 45min, 50min, 55min or 60min, etc.

[0031] Preferably, the separation is carried out by screening.

[0032] Preferably, the particle size of the mixture B is in the range of 50 μm-300 μm, and the particle size can be, for example, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm, 190 μm, 200 μm, 215 μm, 220 μm, 230 μm, 240 μm, 250 μm, 260 μm, 270 μm, 280 μm, 290 μm or 300 μm, etc. The preferred mixture B of the present invention has a larger particle size, which is beneficial to the subsequent dry electrode process and rolling process, and better avoids the problems of poor membrane flexibility and easy cracking caused by direct dry electrode process of small particles, thereby better improving the processing difficulty of dry electrode preparation of small particle size active material.

[0033] Preferably, the mass ratio of the mixture B to the second fiberizable adhesive is 100:(1-3), for example, it can be 100:1, 100:1.2, 100:1.5, 100:1.8, 100:2, 100:2.3, 100:2.5, 100:2.7 or 100:3, etc.

[0034] Preferably, the preparation method further comprises thinning the self-supporting electrode membrane obtained by rolling.

[0035] The present invention does not specifically limit the stirring method, for example, stirring can be performed using a stirring device. Exemplarily, the stirring device can be a double planetary mixer, a powerful mixer, a V-type mixer, or a horizontal mixer.

[0036] In a second aspect, the present invention provides a self-supporting electrode membrane, which is prepared by the preparation method described in the first aspect.

[0037] In the self-supporting electrode membrane of the present invention, the median particle size D50 of the active material is 0.05 μm-5 μm. The small particle size of the active material helps to shorten the transmission path of lithium ions, which is beneficial to improving the battery performance.

[0038] The self-supporting electrode membrane of the present invention can be a positive electrode membrane or a negative electrode membrane. When the self-supporting electrode membrane is a positive electrode membrane, the active material therein is a positive electrode active material; when the self-supporting electrode membrane is a negative electrode membrane, the active material therein is a negative electrode active material.

[0039] The present invention does not impose any particular limitation on the types of the positive electrode active material and the negative electrode active material, and any electrode active material suitable for batteries can be used in the present invention.

[0040] Preferably, the thickness of the self-supporting electrode membrane before thinning is 300μm-400μm, for example, it can be 300μm, 310μm, 320μm, 330μm, 350μm, 360μm, 380μm or 400μm, etc.; the thickness after thinning is 100μm-200μm, for example, it can be 100μm, 110μm, 120μm, 130μm, 140μm, 150μm, 160μm, 180μm or 200μm, etc.

[0041] In a third aspect, the present invention provides an electrode sheet comprising a current collector and an active material layer composited on at least one surface of the current collector, wherein the active material layer is the self-supporting electrode membrane described in the second aspect.

[0042] The electrode sheet in the present invention can be either a positive electrode sheet or a negative electrode sheet. When the electrode sheet is a positive electrode sheet, the current collector is a positive electrode current collector, and the active material layer on its surface is a positive electrode membrane. When the electrode sheet is a negative electrode sheet, the current collector is a negative electrode current collector, and the active material layer on its surface is a negative electrode membrane.

[0043] In a fourth aspect, the present invention provides a battery comprising a positive electrode, a negative electrode and a separator, wherein the positive electrode and / or the negative electrode adopts the electrode sheet described in the third aspect.

[0044] The numerical range described in the present invention includes not only the point values listed above, but also any point values between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values included in the range.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] The method of the present invention obtains a mixture A in the form of a mass by mixing the electrode material and a solvent so that the components in the electrode material are evenly dispersed and bonded. After drying, crushing and separating the mixture, a granulated mixture B is obtained, thereby improving the particle size and flexibility of the mixture. Therefore, the method is used to prepare self-supporting membranes using a dry electrode process, which solves the problem that the membranes are too hard, easy to crack and difficult to thin when using small-particle active materials to prepare self-supporting electrode membranes, and helps promote the mass production application of dry electrode processes.

[0047] (2) The tensile strength of the self-supporting electrode membrane prepared by the present invention is greater than 0.7 MPa, preferably greater than 1.0 MPa, and more preferably greater than 1.3 MPa. The capacity retention rate of the lithium battery prepared using the self-supporting electrode membrane of the present invention is greater than 87.9%, preferably greater than 91.8%, and more preferably greater than 93.7%. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 It is a process flow chart of a method for preparing a self-supporting electrode membrane provided by one embodiment of the present invention.

[0049] Figure 2 This is a photograph of the self-supporting electrode membrane prepared in Example 1.

[0050] Figure 3 This is a photograph of the self-supporting electrode membrane prepared in Comparative Example 1. DETAILED DESCRIPTION

[0051] The technical solution of the present invention will be further described below with reference to the accompanying drawings and through specific implementation methods.

[0052] The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0053] In one embodiment, the present invention provides a method for preparing a self-supporting electrode membrane. Figure 1 , including the following steps:

[0054] (1) Dry-mixing an active material having a median particle size D50 of 0.05 μm to 5 μm, a conductive agent, and a non-fibrillated adhesive in a mixer to obtain a uniformly mixed first mixture. In the first mixture, the active material accounts for 90% to 99% by weight, the conductive agent accounts for 0.5% to 5% by weight, and the first adhesive accounts for 0.5% to 5% by weight; the first adhesive comprises at least one of polyvinyl alcohol, polyacrylic acid, polyvinylidene fluoride, styrene-butadiene rubber, carboxymethyl cellulose, or polyurethane.

[0055] (2) A small amount of solvent is added to the first mixture, and the mixture is mixed and stirred. The active material and the conductive agent are bonded together by a binder to obtain a second mixture in the form of a mass. The solvent accounts for 5% to 35% by weight of the first mixture, and the solvent includes at least one of N-methylpyrrolidone, water, ethylene carbonate, dimethyl carbonate, or diethyl carbonate.

[0056] (3) The second mixture is placed in an oven for baking at a temperature of 50°C to 250°C for 1 hour to 3 hours, and the dried agglomerates are sheared and crushed at a shear speed of 3m / s to 10m / s for 5 minutes to 60 minutes. Finally, the powder obtained by shearing and crushing is sieved to obtain a third mixture with a sieve mesh of 50 to 300 mesh. The particle size of the third mixture particles is controlled to be between 50μm and 300μm.

[0057] (4) Adding a second binder (PTFE) capable of being fiberized to the third mixture particles, mixing and shearing the mixture, so that the second binder is fiberized and dispersed with the third mixture particles, thereby obtaining a fourth mixture.

[0058] (5) Roll-pressing the fourth mixture powder into an independent self-supporting membrane, thinning the membrane to a desired surface density, and then compounding it with a current collector to obtain a desired electrode sheet.

[0059] The following are typical but non-limiting examples:

[0060] Example 1

[0061] This embodiment provides a method for preparing a self-supporting positive electrode membrane, comprising the following steps:

[0062] (1) 8000 g of lithium iron phosphate powder (particle size D50 is 1.12 μm), 125 g of conductive carbon black SP powder, and 83 g of polyvinylidene fluoride (PVDF) powder were added to a double planetary mixer and mixed at a stirring speed of 35 rpm / min and a dispersion speed of 1500 rpm / min for 60 min to obtain a first mixture.

[0063] (2) 1190 g of N-methylpyrrolidone (NMP) was added to the first mixture, and the mixture was mixed at a stirring speed of 20 rpm / min and a dispersion speed of 1000 rpm / min for 30 minutes to obtain a second mixture in the form of agglomerates.

[0064] (3) The second mixture mass was placed in an oven and baked at a temperature of 110° C. for 2 hours. After drying, the mixture was sheared and crushed using a crusher at a shear speed of 8 m / s for 20 minutes. The crushed powder was sieved through a 200-mesh sieve to obtain a third mixture, wherein the mixture particle size D50 was 75 μm.

[0065] (4) 8000 g of the third mixture particles and 160 g of polytetrafluoroethylene (PTFE) were added to a high-pressure shearing machine with a barrel rotation speed of 35 r / min and a dispersion disk speed of 1000 r / min. The mixture was stirred for 20 min to be fully mixed. Then, the barrel rotation speed was increased to 70 r / min and the dispersion disk speed was increased to 5000 r / min. The PTFE was fiberized by high-speed shearing for 15 min to obtain a fourth mixture.

[0066] (5) The fourth mixture was put into a horizontal roller press with a roller gap of 300 μm, a pressure of 5T, a temperature of 120°C, and a rolling speed of 5 m / min to obtain a lithium iron phosphate (LFP) dry film with a thickness of 350 μm. Subsequently, a self-supporting positive electrode film was obtained after two thinning steps with a film thickness of 158 μm and an area density of 33.2 mg / cm 2 , compacted density 2.10g / cm 3 ,like Figure 2 As shown in the figure, the thinned diaphragm has good flexibility and a smooth and complete surface.

[0067] The thinned film prepared above is composited onto a current collector by hot pressing to obtain a positive electrode sheet.

[0068] Example 2

[0069] This embodiment provides a method for preparing a self-supporting positive electrode membrane, comprising the following steps:

[0070] (1) 8000 g of lithium iron phosphate powder (particle size D50 is 0.5 μm), 200 g of conductive carbon black SP powder and 100 g of polyacrylic acid were added to a double planetary mixer and mixed at a stirring speed of 50 rpm / min and a dispersion speed of 1800 rpm / min for 50 min to obtain a first mixture.

[0071] (2) 800 g of N-methylpyrrolidone (NMP) was added to the first mixture, and the mixture was mixed at a stirring speed of 25 rpm / min and a dispersion speed of 900 rpm / min for 35 minutes to obtain a second mixture in the form of agglomerates.

[0072] (3) The second mixture mass was placed in an oven and baked at 200°C for 1 hour. After drying, the mixture was sheared and crushed using a crusher at a shear speed of 7 m / s for 20 minutes. The crushed powder was sieved through a 100-mesh sieve to obtain a third mixture, wherein the particle size D50 of the mixture was 150 μm.

[0073] (4) Add 8000 g of the third mixture particles and 160 g of polytetrafluoroethylene (PTFE) into a high-pressure shearing machine, with the barrel rotating at 40 r / min and the dispersion disk at 700 r / min, and stir for 30 min to fully mix. Then, increase the barrel rotating speed to 80 r / min and the dispersion disk speed to 4000 r / min, and perform high-speed shearing for 15 min to fiberize the PTFE to obtain a fourth mixture.

[0074] (5) The fourth mixture was put into a horizontal roller press with a roller gap of 300 μm, a pressure of 6T, a temperature of 110°C, and a rolling speed of 5 m / min to obtain a lithium iron phosphate (LFP) dry-process membrane with a thickness of 400 μm. Subsequently, the self-supporting positive electrode membrane was obtained after two thinning steps with a membrane thickness of 200 μm and an area density of 40.6 mg / cm 2 , compacted density 2.03g / cm 3 The thinned diaphragm has good flexibility and a smooth and complete surface.

[0075] The thinned film prepared above is composited onto a current collector by hot pressing to obtain a positive electrode sheet.

[0076] Example 3

[0077] This embodiment provides a method for preparing a self-supporting positive electrode membrane, comprising the following steps:

[0078] (1) 8000 g of lithium iron phosphate powder (particle size D50 is 3.5 μm), 300 g of conductive carbon black SP powder and 200 g of polyvinylidene fluoride PVDF powder were added to a double planetary mixer and mixed at a stirring speed of 35 rpm / min and a dispersion speed of 1400 rpm / min for 55 min to obtain a first mixture.

[0079] (2) 1800 g of N-methylpyrrolidone (NMP) was added to the first mixture, and the mixture was mixed at a stirring speed of 20 rpm / min and a dispersion speed of 900 rpm / min for 30 min to obtain a second mixture in the form of agglomerates.

[0080] (3) The second mixture mass was placed in an oven and baked at 70°C for 3 hours. After drying, the mixture was sheared and crushed using a crusher at a shear speed of 10 m / s for 10 minutes. The crushed powder was sieved through a 60-mesh sieve to obtain a third mixture, wherein the particle size D50 of the mixture was 250 μm.

[0081] (4) 8000 g of the third mixture particles and 200 g of polytetrafluoroethylene (PTFE) were added to a high-pressure shearing machine with a barrel rotation speed of 35 r / min and a dispersion disk speed of 750 r / min. The mixture was stirred for 25 min to be fully mixed. Then, the barrel rotation speed was increased to 70 r / min and the dispersion disk speed was increased to 3500 r / min. The PTFE was fiberized by high-speed shearing for 20 min to obtain a fourth mixture.

[0082] (5) The fourth mixture was put into a horizontal roller press with a roller gap of 150 μm, a pressure of 4.5 T, a temperature of 100 °C, and a rolling speed of 4 m / min to obtain a lithium iron phosphate (LFP) dry film with a thickness of 300 μm. Subsequently, a self-supporting positive electrode film was obtained after two thinning steps with a film thickness of 120 μm and an area density of 26.4 mg / cm 2 , compacted density 2.20g / cm 3 The thinned diaphragm has good flexibility and a smooth and complete surface.

[0083] The thinned film prepared above is composited onto a current collector by hot pressing to obtain a positive electrode sheet.

[0084] In the embodiment of the present invention, based on the total weight of the first mixture, the mass proportion of NMP is V, and the baking temperature in step (3) is T (° C.).

[0085] In Examples 1-3, the particle size D50 of the lithium iron phosphate powder, the mass fraction V of NMP, and the baking temperature T are shown in Table 1.

[0086] Table 1

[0087]

[0088] Example 4

[0089] The difference from Example 1 is that the amount of NMP added is adjusted so that V is 4%.

[0090] Example 5

[0091] The difference from Example 1 is that the amount of NMP added is adjusted so that V is 36%.

[0092] Example 6

[0093] The difference from Example 1 is that the baking temperature in step (3) is adjusted to 260°C.

[0094] Example 7

[0095] The difference from Example 3 is that the baking temperature in step (3) is adjusted to 260°C.

[0096] Example 8

[0097] The difference from Example 3 is that the particle size D50 of the lithium iron phosphate powder is adjusted to 0.4 μm.

[0098] In Examples 4-8, the particle size D50 of the lithium iron phosphate powder, the mass fraction V of NMP, and the baking temperature T are shown in Table 2.

[0099] Table 2

[0100]

[0101] Comparative Example 1

[0102] This comparative example provides a method for preparing a self-supporting positive electrode membrane, which adopts a dry electrode process. The specific steps are as follows:

[0103] 8000 g of lithium iron phosphate powder (D50 particle size 1.12 μm), 125 g of conductive carbon black SP powder and 160 g of polytetrafluoroethylene (PTFE) were added to a high-pressure shearing machine. The barrel rotation speed was 35 rpm / min, the speed of the dispersion disk was 1000 rpm / min, and the mixture was stirred for 20 minutes for thorough mixing. Then, the barrel rotation speed was increased to 70 rpm / min, the speed of the dispersion disk was increased to 5000 rpm / min, and high-speed shearing was performed for 15 minutes to fiberize the PTFE to obtain a mixture.

[0104] The mixture particles were sieved, and the sieve was put into a horizontal roller press with a roller gap of 350 μm, a pressure of 5T, a temperature of 120°C, and a speed of 5 m / min to obtain a lithium iron phosphate dry-process membrane with a thickness of 460 μm. The membrane was then thinned twice to 185 μm with an area density of 45.5 mg / cm 2 , compacted density 2.46g / cm 3 ,like Figure 3 As shown, the compaction density of this diaphragm is high and the appearance is severely cracked, and it is impossible to further reduce the thickness.

[0105] Finally, the thinned film is hot-pressed and composited onto the current collector to obtain a dry-process positive electrode sheet.

[0106] Performance testing:

[0107] 1) Diaphragm performance test

[0108] Test method: The surface quality and tensile strength of the membranes prepared in Examples 1-8 and Comparative Example 1 were tested, and the results are summarized in Table 3;

[0109] The tensile strength test steps are as follows: cut each membrane separately to obtain a tensile specimen with a width of 6 mm, then use a tensile testing machine to clamp both sides of each tensile specimen and perform a tensile test on it, wherein the tensile speed is 250 mm / min until the tensile specimen breaks, and record the tensile strength at the time of breakage.

[0110] 2) Preparation of lithium batteries

[0111] Batteries were prepared using the positive electrodes prepared in Examples 1-8 and Comparative Example 1. The specific manufacturing methods are as follows:

[0112] (1) Preparation of negative electrode: Artificial graphite as a negative electrode active material, carbon black as a conductive material, and PAA binder were mixed in a deionized water solvent at a weight ratio of 95:2:3 to prepare a slurry for forming a negative electrode, and the slurry was coated on a copper current collector and dried to prepare a negative electrode.

[0113] (2) Preparation of lithium batteries: A porous polyethylene separator was placed between the positive electrode prepared in Examples 1-8 or Comparative Example 1 and the negative electrode prepared in (1) above to produce an electrode assembly. The electrode assembly was placed in an aluminum-plastic bag, and then an electrolyte was injected into the aluminum-plastic bag to produce a lithium secondary battery. The electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1.0 mol / L in an organic solvent consisting of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate (EC:DMC:EMC volume ratio = 3:3:3).

[0114] 3) Lithium battery cycle capacity retention test:

[0115] The lithium battery manufactured above was subjected to cyclic charge and discharge to evaluate the capacity retention rate. The charging conditions were: first charging at a constant current of 0.5C to 3.65V, and then charging at a constant voltage of 3.65V to a cutoff current of 0.05C; the discharging conditions were: discharging at a constant current of 0.5C to a cutoff voltage of 2.0V; the test environment temperature was 25°C, and the test results after 500 cycles are shown in Table 3.

[0116] Table 3

[0117]

[0118] In summary, the method of the present invention solves the problem of using small-particle active materials to prepare self-supporting electrode membranes, which causes the membranes to be too hard, easy to crack and difficult to thin, and helps promote the mass production application of dry electrode technology.

[0119] Moreover, by comparing Example 1 with Examples 7-8, it can be seen that by preferably satisfying the formula 20≥D50×V×0.01×16.5+T / 20≥5, the quality of the membrane can be further improved, thereby improving the performance of the lithium battery prepared therefrom.

[0120] By comparing Example 1 with Examples 4-6, it can be seen that when the formula 20≥D50×V×0.01×16.5+T / 20≥5 is satisfied, it is further preferred that 5%≤V≤35% and T is 50℃-250℃, which is beneficial to improving the quality of the diaphragm and thus improving the performance of the lithium battery prepared therefrom.

[0121] The applicant states that the present invention is intended to illustrate the detailed methods of the present invention through the above-described embodiments, but the present invention is not limited to the above-described detailed methods, that is, it does not mean that the present invention must rely on the above-described detailed methods in order to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for various raw materials in the products of the present invention, addition of auxiliary ingredients, and selection of specific methods, etc., are all within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing a self-supporting electrode membrane, characterized in that: The preparation method comprises the following steps: Mixing an electrode material and a solvent to obtain a mixture A in the form of a mass; wherein the electrode material includes a small-particle active material and a first binder, and the median particle size D50 of the small-particle active material is 0.05 μm-5 μm; Drying and crushing the mixture A to obtain a mixture B; The mixture B is dry-mixed and sheared with a second fiberizable adhesive to obtain a mixture C, which is then rolled to obtain the self-supporting electrode membrane.

2. The method for preparing a self-supporting electrode membrane according to claim 1, wherein: Based on the mass of the electrode material, the mass proportion of the solvent is V, the drying temperature is T, and D50, V and T satisfy the following relationship: 20≥D50×V×0.01×16.5+T / 20≥5; The unit of D50 is μm, the unit of V is %, the unit of T is °C, and the parameters in the formula do not contain units.

3. The method for preparing a self-supporting electrode membrane according to claim 1 or 2, characterized in that: The mass proportion V of the solvent satisfies: 5%≤V≤35%; Preferably, the drying temperature T is 50°C-250°C; Preferably, the step of mixing the electrode material and the solvent is: first dry-mixing the electrode material, and then mixing the mixture obtained by the dry mixing with the solvent to obtain a mixture A; Preferably, the electrode material further includes a conductive agent; Preferably, based on the total mass of the electrode material, the mass proportions of the small-particle active material, the conductive agent and the first adhesive are 90%-99%, 0.5%-5% and 0.5%-5% respectively; Preferably, the first adhesive includes at least one of polyvinyl alcohol, polyacrylic acid, polyvinylidene fluoride, styrene-butadiene rubber, carboxymethyl cellulose or polyurethane.

4. The method for preparing a self-supporting electrode membrane according to any one of claims 1 to 3, characterized in that: The solvent includes at least one of N-methylpyrrolidone, water, ethylene carbonate, dimethyl carbonate or diethyl carbonate.

5. The method for preparing a self-supporting electrode membrane according to any one of claims 1 to 4, characterized in that: The drying method is baking; Preferably, the drying time is 1 hour to 3 hours.

6. The method for preparing a self-supporting electrode membrane according to any one of claims 1 to 5, characterized in that: The crushing method is shear crushing; Preferably, the process parameters of shear crushing are: shear speed 3m / s-10m / s, time 5min-60min; Preferably, the separation method is screening; Preferably, the particle size of the mixture B is in the range of 50 μm-300 μm; Preferably, the mass ratio of the mixture B to the second fiberizable adhesive is 100:(1-3); Preferably, the preparation method further comprises thinning the self-supporting electrode membrane obtained by rolling.

7. A self-supporting electrode membrane, characterized in that: The self-supporting electrode membrane is prepared by the preparation method according to any one of claims 1 to 6; Preferably, the thickness of the self-supporting electrode membrane before thinning is 300 μm-400 μm, and the thickness after thinning is 100 μm-200 μm.

8. An electrode sheet, characterized in that: The electrode sheet includes a current collector and an active material layer composited on at least one side surface of the current collector, and the active material layer is the self-supporting electrode membrane according to claim 7.

9. A method for preparing an electrode sheet according to claim 8, characterized in that: The method for preparing the electrode sheet comprises the following steps: The self-supporting electrode membrane according to claim 7 is compounded with a current collector to obtain the electrode sheet.

10. A battery comprising a positive electrode, a negative electrode and a separator, characterized in that: The positive electrode and / or negative electrode adopts the electrode sheet according to claim 8.

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