阅读说明：本技术 一种锂离子电池 (Lithium ion battery ) 是由 赵君义 邹浒 于 2020-12-10 设计创作，主要内容包括：一种锂离子电池,包括卷芯和极耳,所述卷芯由内层隔膜、第一极片、外层隔膜、第二极片叠加卷绕形成；内层隔膜位于卷芯的最内层,内、外层隔膜均具有夹持段、第一平直段、尾部贴合段,第一平直段位于第一极片之前,尾部贴合段为隔膜的末端,内层隔膜具有第一内层贴合段,内层隔膜的夹持段、第一平直段、尾部贴合段分别和外层隔膜的夹持段、第一平直段、尾部贴合段贴合在一起,内层隔膜的第一内层贴合段为内层隔膜和内层隔膜贴合；内层隔膜的第一内层贴合段的胶面的湿法剥离力≥2N/m且陶瓷面的湿法剥离力≥1N/m。本发明基于湿法剥离力来选择粘接力较好的隔膜制备电芯,达到粘接力最优化输出,进而输出硬度较好和性能较好的电池。(A lithium ion battery comprises a winding core and a tab, wherein the winding core is formed by winding an inner-layer diaphragm, a first pole piece, an outer-layer diaphragm and a second pole piece in an overlapping manner; the inner-layer diaphragm is positioned at the innermost layer of the winding core, the inner-layer diaphragm and the outer-layer diaphragm are respectively provided with a clamping section, a first flat section and a tail attaching section, the first flat section is positioned in front of the first pole piece, the tail attaching section is the tail end of the diaphragm, the inner-layer diaphragm is provided with the first inner-layer attaching section, the clamping section, the first flat section and the tail attaching section of the inner-layer diaphragm are respectively attached with the clamping section, the first flat section and the tail attaching section of the outer-layer diaphragm, and the first inner-layer attaching section of the inner-layer diaphragm is attached with the inner-layer diaphragm; the wet stripping force of the rubber surface of the first inner layer attaching section of the inner layer diaphragm is more than or equal to 2N/m, and the wet stripping force of the ceramic surface is more than or equal to 1N/m. The invention selects the diaphragm with better bonding force to prepare the battery cell based on the wet stripping force, thereby achieving the optimal output of the bonding force and further outputting the battery with better hardness and performance.)

1. The utility model provides a lithium ion battery, includes core and utmost point ear, its characterized in that: the winding core is formed by winding an inner diaphragm, a first pole piece, an outer diaphragm and a second pole piece in an overlapping mode, and the polarities of the first pole piece and the second pole piece are opposite;

the inner diaphragm is positioned at the innermost layer of the winding core, the inner diaphragm and the outer diaphragm are respectively provided with a clamping section, a first straight section which is connected with the clamping section and is positioned behind the clamping section, and a tail laminating section which exceeds the tail end of the first pole piece, the first straight section is positioned in front of the first pole piece, the tail laminating section is the tail end of the diaphragm, the inner diaphragm is provided with a first inner laminating section, the clamping section, the first straight section and the tail laminating section of the inner diaphragm are respectively laminated with the clamping section, the first straight section and the tail laminating section of the outer diaphragm, and the first inner laminating section of the inner diaphragm is laminated with the inner diaphragm and the inner diaphragm;

the wet stripping force of the rubber surface of the first inner layer attaching section of the inner layer diaphragm is not less than 2N/m, and the wet stripping force of the ceramic surface of the first inner layer attaching section of the inner layer diaphragm is not less than 1N/m.

2. The lithium ion battery of claim 1, wherein: the wet peel force is determined by the following method:

s1, cutting the diaphragm to be tested into a sample with a proper size, and aligning and stacking two pieces of diaphragm samples to be tested;

s2, placing the two stacked tested diaphragm samples into an aluminum plastic film for packaging, injecting electrolyte, and vacuumizing and sealing;

s3, carrying out hot pressing on the sealed aluminum plastic film by using a formation machine, wherein the hot pressing temperature is 80 ℃, the pressure is 0.8Mpa, and the hot pressing time is 2 hours;

s4, after hot pressing is completed, taking out the diaphragm sample to be tested from the aluminum plastic film, wiping the electrolyte, putting the diaphragm sample to be tested into a hard sealing tape, and carrying out hot pressing at 100 ℃ under the pressure of 0.2Mpa for 10 seconds;

and S5, after hot pressing is completed, separating the two pieces of tested diaphragm samples which are pressed together from one end, peeling at 90 degrees, and recording the peeling force when the tested diaphragm samples are separated, wherein the peeling force is the wet peeling force.

3. The lithium ion battery of claim 1, wherein: the length of the first inner layer attaching section of the inner layer diaphragm is 30-60% of the width of the winding core.

4. The lithium ion battery of claim 1, wherein: the inlayer diaphragm with outer diaphragm all includes base film, ceramic layer and glue film, a side surface of base film sets up ceramic layer, opposite side surface set up the glue film, the outside surface setting of ceramic layer the surface that has ceramic layer and glue film when the glue film, diaphragm is the ceramic surface, and the surface of diaphragm only the glue film is for gluing the face.

5. The lithium ion battery of claim 4, wherein: the surfaces of the inner-layer diaphragm and the outer-layer diaphragm, which are opposite to the clamping section, the first flat section and the tail part attaching section, are ceramic surfaces, and the surface of the inner-layer diaphragm, which is opposite to the inner-layer diaphragm, is a rubber surface at the first inner-layer attaching section.

6. The lithium ion battery of claim 4, wherein: the surfaces of the inner-layer diaphragm and the outer-layer diaphragm, which are opposite to the clamping section, the first flat section and the tail part attaching section, are rubber surfaces, and the surface of the inner-layer diaphragm, which is opposite to the inner-layer diaphragm, in the first inner-layer attaching section is a ceramic surface.

7. The lithium ion battery of claim 1, wherein: the length of the clamping section of the inner-layer diaphragm and the length of the clamping section of the outer-layer diaphragm are both 1-15% of the width of the winding core; and/or the length of the first flat section of the inner-layer membrane and the length of the first flat section of the outer-layer membrane are 40-50% of the width of the winding core.

8. The lithium ion battery of claim 1, wherein: the length of the tail attaching section of the inner-layer diaphragm and the length of the tail attaching section of the outer-layer diaphragm are both larger than or equal to 5mm, and/or the length of the tail attaching section of the inner-layer diaphragm and the length of the tail attaching section of the outer-layer diaphragm are both 0.1-10% of the width of the winding core.

9. The lithium ion battery of claim 1, wherein: the overall hardness of the lithium ion battery is more than or equal to 200N.

10. The lithium ion battery of claim 1, wherein: the dry peel force of the first straight section of the inner membrane and the first straight section of the outer membrane is less than 8N/m, the dry peel force being determined by:

s1, cutting the diaphragm to be tested into a sample with a proper size, and aligning and stacking two pieces of diaphragm samples to be tested;

s2, hot-pressing the stacked diaphragm samples to be tested together, wherein the hot-pressing temperature is 100 ℃, the pressure is 0.2Mpa, and the hot-pressing time is 10 seconds;

and S3, after the hot pressing is finished, separating the tested diaphragm sample which is pressed together from one end, peeling at 90 degrees, and recording the peeling force when the tested diaphragm sample is separated, wherein the peeling force is the dry-method peeling force.

Technical Field

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a lithium ion battery.

Background

The lithium battery comprises a battery core and a tab arranged on the battery core, wherein the battery core comprises a diaphragm, a positive plate and a negative plate, the diaphragm is one of key materials of the lithium battery, is arranged between the positive plate and the negative plate of the battery and is used for isolating the positive plate and the negative plate and preventing the short circuit of the battery. At present, the diaphragm used in the lithium battery is generally a polyolefin product with a pore structure, such as a PE diaphragm, a PP/PE/PP three-layer diaphragm and the like. Coating inorganic particles such as alumina, boehmite, magnesium oxide and the like on one side surface or two side surfaces of a base material diaphragm, then carrying out double-sided pure glue coating or glue and ceramic particle mixed coating on the diaphragm, and finally obtaining a diaphragm product, wherein the glue can be single PVDF or multiple PVDF mixed, and the coating mode can be water system coating or oil system coating. The water system diaphragm is a finished product obtained by dispersing and grinding a single or a plurality of PVDF, a dispersing agent and glue in water to form turbid liquid, filtering and coating, and the coating mode of the water system diaphragm can be micro gravure roll transfer coating or high-speed spray head coating; the oil-based separator is a product obtained by dissolving a single PVDF or a plurality of PVDF s in an organic solvent (for example, NMP, DMAC, or the like) at a specific ratio to form a solution and then coating the solution, and the coating method of the oil-based separator may be gravure roll transfer coating or dip coating. In the lithium battery, the diaphragm and the pole piece are bonded together, the bonding force of the surface coating of the diaphragm directly influences the quality of a finished product of the battery core, and the bonding force of the surface coating of the diaphragm is unqualified, so that the hardness of the battery is possibly insufficient, and even the cycle performance, the capacity retention rate and the like of the battery are influenced.

Disclosure of Invention

The invention aims to provide a lithium ion battery with good hardness and cycle performance.

In order to achieve the purpose, the invention adopts the following technical solutions:

the lithium ion battery comprises a winding core and a tab, wherein the winding core is formed by winding an inner-layer diaphragm, a first pole piece, an outer-layer diaphragm and a second pole piece in an overlapping manner, and the polarities of the first pole piece and the second pole piece are opposite; the inner diaphragm is positioned at the innermost layer of the winding core, the inner diaphragm and the outer diaphragm are respectively provided with a clamping section, a first straight section which is connected with the clamping section and is positioned behind the clamping section, and a tail attaching section which exceeds the tail end of the first pole piece, the first straight section is positioned in front of the first pole piece, the tail attaching section is the tail end of the diaphragm, the inner diaphragm is provided with a first inner attaching section which is overlapped and contacted together after a needle is drawn, the clamping section, the first straight section and the tail attaching section of the inner diaphragm are respectively attached with the clamping section, the first straight section and the tail attaching section of the outer diaphragm, and the first inner attaching section of the inner diaphragm is attached with the inner diaphragm; the wet stripping force of the rubber surface of the first inner layer attaching section of the inner layer diaphragm is not less than 2N/m, and the wet stripping force of the ceramic surface of the first inner layer attaching section of the inner layer diaphragm is not less than 1N/m.

More specifically, the wet peel force is determined by:

s1, cutting the diaphragm to be tested into a sample with a proper size, and aligning and stacking two pieces of diaphragm samples to be tested;

s2, placing the two stacked tested diaphragm samples into an aluminum plastic film for packaging, injecting electrolyte, and vacuumizing and sealing;

s3, carrying out hot pressing on the sealed aluminum plastic film by using a formation machine, wherein the hot pressing temperature is 80 ℃, the pressure is 0.8Mpa, and the hot pressing time is 2 hours;

s4, after hot pressing is completed, taking out the diaphragm sample to be tested from the aluminum plastic film, wiping the electrolyte, putting the diaphragm sample to be tested into a hard sealing tape, and carrying out hot pressing at 100 ℃ under the pressure of 0.2Mpa for 10 seconds;

and S5, after hot pressing is completed, separating the two pieces of tested diaphragm samples which are pressed together from one end, peeling at 90 degrees, and recording the peeling force when the tested diaphragm samples are separated, wherein the peeling force is the wet peeling force.

More specifically, the length of the first inner layer attaching section of the inner layer diaphragm is 30-60% of the width of the winding core.

More specifically, the inlayer diaphragm with outer diaphragm all includes base film, ceramic layer and glue film, a side surface of base film sets up ceramic layer, opposite side surface set up the glue film, the outside surface of ceramic layer sets up the glue film, the surface that has ceramic layer and glue film when the diaphragm is the ceramic face, and the surface of diaphragm only the glue film is for gluing the face.

More specifically, the surfaces of the inner-layer diaphragm and the outer-layer diaphragm, which are opposite to the clamping section, the first flat section and the tail attaching section, are ceramic surfaces, and the surface of the inner-layer diaphragm, which is opposite to the inner-layer diaphragm, in the first inner-layer attaching section is a rubber surface.

More specifically, the surfaces of the inner-layer diaphragm and the outer-layer diaphragm, which are opposite to the clamping section, the first flat section and the tail attaching section, are adhesive surfaces, and the surface of the inner-layer diaphragm, which is opposite to the inner-layer diaphragm, in the first inner-layer attaching section is a ceramic surface.

More specifically, the length of the clamping section of the inner-layer diaphragm and the length of the clamping section of the outer-layer diaphragm are both 1-15% of the width of the winding core.

More specifically, the length of the first straight section of the inner-layer diaphragm and the length of the first straight section of the outer-layer diaphragm are both 40-50% of the width of the winding core.

More specifically, the length of the tail attaching section of the inner-layer diaphragm and the length of the tail attaching section of the outer-layer diaphragm are both larger than or equal to 5mm, and/or the length of the tail attaching section of the inner-layer diaphragm and the length of the tail attaching section of the outer-layer diaphragm are both 0.1-10% of the width of the winding core.

More specifically, the overall hardness of the lithium ion battery is more than or equal to 200N.

According to the technical scheme, the inner-layer diaphragm with the specific wet stripping force at the first inner-layer joint section is used for preparing the winding core, the diaphragm has a good bonding effect, can be well bonded with the positive electrode sheet and the negative electrode sheet, and forms the battery with good hardness. The wet stripping force of the diaphragm disclosed by the invention is characterized in that the diaphragm and the diaphragm are swelled under certain temperature and pressure in an electrolyte environment, then a sample is formed by high-temperature and low-speed thermal compounding, an electronic universal tester is used for stripping the adhesive layer or the ceramic layer of the diaphragm to test, the hardness of the battery in the subsequent process can be distinguished in advance before formation through the interlayer wet stripping force, the adhesion force among main materials of the battery core can be identified in advance, and therefore, the diaphragm with the wet stripping force meeting the requirement is selected, the optimal output of the adhesion force is achieved, and the battery with better hardness and better performance is obtained.

Further, the dry peel force of the first straight section of the inner membrane and the first straight section of the outer membrane is less than 8N/m, which is determined by the following method:

s1, cutting the diaphragm to be tested into a sample with a proper size, and aligning and stacking two pieces of diaphragm samples to be tested;

s2, hot-pressing the stacked diaphragm samples to be tested together, wherein the hot-pressing temperature is 100 ℃, the pressure is 0.2Mpa, and the hot-pressing time is 10 seconds;

and S3, after the hot pressing is finished, separating the tested diaphragm sample which is pressed together from one end, peeling at 90 degrees, and recording the peeling force when the tested diaphragm sample is separated, wherein the peeling force is the dry-method peeling force.

Further, the dry peel force is a dry peel force of the ceramic face of the separator.

Furthermore, the transfer area ratio of the diaphragm is 20-40%, the transfer area is transfer mass/diaphragm area, and the transfer mass is the mass of the diaphragm before dry stripping-the mass of the diaphragm after dry stripping.

And a proper diaphragm material is selected based on the dry stripping force of the diaphragm, so that the diaphragm is well matched with the winding needle, and the phenomena of winding, unloading, packaging and baking of the winding core and folding of the pole piece are reduced, thereby improving the yield of products and the quality of finished products. More preferably, the dry peel force is the dry peel force of the ceramic face of the separator.

More specifically, the ceramic layer comprises ceramic particles and a binding polymer, the ceramic particles are one or more of alumina, boehmite and magnesia, and the binding polymer is at least one of polyvinylidene fluoride, polyvinylpyrrolidone, vinylidene fluoride-hexafluoropropylene polymer, polyacrylonitrile, sodium carboxymethylcellulose, sodium polyacrylate, polyacrylic acid, polyacrylate, styrene-butadiene copolymer, butadiene-acrylonitrile polymer, polyvinyl alcohol, polymethyl acrylate, polymethyl methacrylate, polyethyl acrylate and polyacrylic acid-styrene polymer.

More specifically, the ceramic particles have a particle size distribution of: the particle size of D10 is 0.15-0.3 μm, the particle size of D50 is 0.35-0.45 μm, the particle size of D90 is 0.6-0.8 μm, and the particle size of D100 is less than 4.5 μm.

More specifically, the adhesive layer contains an adhesive polymer, and the adhesive polymer is at least one of polyvinylidene fluoride, polyvinylpyrrolidone, vinylidene fluoride-hexafluoropropylene polymer, polyacrylonitrile, sodium carboxymethylcellulose, sodium polyacrylate, polyacrylic acid, polyacrylate, styrene-butadiene copolymer, butadiene-acrylonitrile polymer, polyvinyl alcohol, polymethyl acrylate, polymethyl methacrylate, polyethyl acrylate and polyacrylic acid-styrene polymer.

More specifically, the thickness of the adhesive layer is 0.5-3 μm, and the filling density of the adhesive layer is 0.6g/m²～3.0g/m²。

More specifically, the diaphragm is a water-system diaphragm, and the glue layer comprises an adhesive polymer, an adhesive and a dispersing agent, wherein the content of the adhesive polymer is 92-96%, the content of the adhesive is 2.5-5.5%, and the content of the dispersing agent is 1.5-2.5%; or the diaphragm is an oil system mixed coating diaphragm, and the glue layer comprises adhesive polymer and ceramic particles, wherein the content of the adhesive polymer is 30-50%, and the content of the ceramic particles is 50-70%; or the diaphragm is a pure oil diaphragm, the adhesive layer comprises an adhesive polymer, and the molecular weight of the adhesive polymer is 30-100 ten thousand.

Drawings

In order to illustrate the embodiments of the present invention more clearly, the drawings that are needed in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and that other drawings can be obtained by those skilled in the art without inventive effort.

FIG. 1 is a schematic structural view of a lithium battery separator;

FIGS. 2a and 2b are SEM images of a water-based membrane substrate side and a ceramic side, respectively;

FIG. 3 is an SEM image of the surface of an oil-based hybrid coated separator;

FIGS. 4a and 4b are SEM images of a base material side and a ceramic side of an oil-based separator, respectively;

FIG. 5 is a schematic structural diagram of a positive plate of a lithium battery;

FIG. 6 is a schematic structural diagram of a negative electrode sheet of a lithium battery;

fig. 7a is a schematic view when a winding needle winds the separator and the positive and negative electrode sheets;

FIG. 7 is a schematic view of the core after withdrawal of the needle;

FIG. 8 is a cross-sectional view of the winding core;

FIG. 9 is a schematic view of a cut sample

FIG. 10 is a schematic view of a diaphragm being placed in an aluminum plastic film for liquid injection and sealing;

FIG. 11 is a schematic view of hot pressing the sealed aluminum plastic film;

FIG. 12 is a schematic view of a membrane being hot-pressed;

FIG. 13 is a schematic view of a 90 ° peel performed by an electronic universal tester;

FIG. 14 is a graph of cycle testing;

FIG. 15 is an SEM image of the ceramic surface before hot press lamination of the EJ oil-based separator 5;

fig. 16 is an SEM image of the base material surface before the hot press lamination of the EJ oil-based separator 1;

fig. 17 is an SEM image of the ceramic surface after the EJ oil-based separator 5 is peeled off;

fig. 18 is an SEM image of the ceramic surface after the EJ oil-based separator 6 is peeled;

fig. 19 is an SEM image of the base material surface after the EJ oil-based separator 1 is peeled off;

fig. 20 is an SEM image of the base material surface after the EJ oil-based separator 2 is peeled off;

FIG. 21 is a three-point hardness test.

The following describes embodiments of the present invention in further detail with reference to the accompanying drawings.

Detailed Description

The invention will be described in detail below with reference to the accompanying drawings, wherein for the purpose of illustrating embodiments of the invention, the drawings showing the structure of the device are not to scale but are partly enlarged, and the schematic drawings are only examples, and should not be construed as limiting the scope of the invention. It is to be noted, however, that the drawings are designed in a simplified form and are not to scale, but rather are to be construed in an attempt to more clearly and concisely illustrate embodiments of the present invention.

The lithium battery comprises a winding core and a pole lug, wherein the winding core is formed by stacking a positive plate (a first pole piece), a negative plate (a second pole piece) and a diaphragm together and then winding the positive plate, the negative plate and the diaphragm, and the diaphragm is positioned between the positive plate and the negative plate. As shown in fig. 1, the separator for a lithium battery includes a base film 11, a ceramic layer 12 disposed on one or both surfaces of the base film 11, and a glue layer 13 disposed on the outermost layer of the separator, the separator shown in fig. 1 has the ceramic layer 12 disposed on only one surface of the base film 11, the glue layer 13 disposed on the other surface of the base film 11, and the glue layer 13 disposed on the outer surface of the ceramic layer 12, and the separator of the present embodiment has a structure of the base film + a single-layer ceramic layer + a double-sided glue layer. The surface of the diaphragm, which is provided with the ceramic layer and the adhesive layer, is defined as a ceramic surface, and the surface of the diaphragm, which is only provided with the adhesive layer, is defined as an adhesive surface or a substrate surface.

The basement membrane can be a single-layer PE (polyethylene) or a single-layer PP (polypropylene) or a PP-PE-PP three-layer structure, and the thickness of the basement membrane can be 3-20 mu m. The thickness of the ceramic layer may be 0.5 to 3 μm when the separator has only a single-sided ceramic layer, and 0.5 to 5 μm when the separator has both sides of the ceramic layer. The ceramic layer comprises ceramic particles and a bonding polymer, wherein the ceramic particles can be alumina or boehmite or magnesia or a combination thereof, and the bonding polymer is at least one of polyvinylidene fluoride, polyvinylpyrrolidone, vinylidene fluoride-hexafluoropropylene polymer, polyacrylonitrile, sodium carboxymethylcellulose, sodium polyacrylate, polyacrylic acid, polyacrylate, styrene-butadiene copolymer, butadiene-acrylonitrile polymer, polyvinyl alcohol, polymethyl acrylate, polymethyl methacrylate, polyethylacrylate and polyacrylic acid-styrene polymer. The ceramic layer comprises 85-92% of ceramic particles (in percentage by mass), and the balance of adhesive polymers, wherein the ceramic particles have the following particle size distribution: the particle size of D10 is 0.15-0.3 μm, the particle size of D50 is 0.35-0.45 μm, the particle size of D90 is 0.6-0.8 μm, and the particle size of D100 is less than 4.5 μm.

The thickness of the adhesive layer is 0.5-3 μm, and the filling density of the adhesive layer is 0.6g/m²～3.0g/m²The adhesive layer contains an adhesive polymer, and the adhesive polymer is at least one of polyvinylidene fluoride, polyvinylpyrrolidone, vinylidene fluoride-hexafluoropropylene polymer, polyacrylonitrile, sodium carboxymethylcellulose, sodium polyacrylate, polyacrylic acid, polyacrylate, styrene-butadiene copolymer, butadiene-acrylonitrile polymer, polyvinyl alcohol, polymethyl acrylate, polymethyl methacrylate, polyethyl acrylate and polyacrylic acid-styrene polymer. When the diaphragm is a water-based diaphragm, the adhesive layer comprises an adhesive polymer, an adhesive and a dispersing agent, wherein the adhesive polymer accounts for 92-96 wt%, the adhesive accounts for 2.5-5.5 wt%, and the dispersing agent accounts for 1.5-2.5 wt%. Fig. 2a and 2b are SEM images of a water-based separator substrate surface and a ceramic surface, respectively. When the diaphragm is an oil-based mixed coating diaphragm, the adhesive layer comprises adhesive polymer and ceramic particles, wherein the content (mass percentage) of the adhesive polymer is 30-50%, and the content of the ceramic particles is 50-70%. Fig. 3 is an SEM image of an oil-based hybrid coated membrane surface. When the diaphragm is a pure oil diaphragm, the content of the adhesive polymer in the adhesive layer is 100%, and the molecular weight of the adhesive polymer is 30 to 100 ten thousand. Fig. 4a and 4b are SEM images of an oil-based separator substrate surface and a ceramic surface, respectively.

As shown in fig. 5, the positive electrode sheet for a lithium battery includes a positive electrode foil 14 and positive electrode active material layers 15 coated on both side surfaces of the positive electrode foil 14, and the positive electrode foil 14 may be an aluminum foil having a thickness of 8 to 14 μm. The positive active material layer comprises positive material, conductive agent and binder, and the positive material can be LiCoO₂、LiNiO₂、LiFePO₄、LiMn₂O₄、LiNixCoyMn_1-x-yO₂The conductive agent may be conductive carbonThe positive electrode active material layer comprises one or more of black, carbon nano tubes, conductive graphite and graphene, wherein the binder can be one or more of polyvinylidene fluoride, vinylidene fluoride-fluorinated olefin copolymer, polytetrafluoroethylene, sodium carboxymethylcellulose, styrene butadiene rubber, polyurethane, fluorinated rubber and polyvinyl alcohol, the content (mass percentage) of a positive electrode material in the positive electrode active material layer is 96-98.5%, the content of a conductive agent is 0.5-2.5%, and the content of the binder is 1-1.5%.

As shown in fig. 6, the negative electrode sheet for a lithium battery includes a negative electrode foil 16 and negative electrode active material layers 17 coated on both side surfaces of the negative electrode foil 16. The negative foil 16 may be a copper foil having a thickness of 5 μm to 10 μm. The negative electrode active material layer includes a negative electrode material, a conductive agent, a binder, and a dispersant. The negative electrode material can be mesocarbon microbeads, artificial graphite, natural graphite, hard carbon, soft carbon, lithium titanate, silicon-based materials, tin-based materials and lithium metal, the conductive agent can be one or more of conductive carbon black, carbon nanotubes, conductive graphite and graphene, the binder can be one or more of polyvinylidene fluoride, vinylidene fluoride-fluorinated olefin copolymer, polytetrafluoroethylene, sodium carboxymethylcellulose, styrene butadiene rubber, polyurethane, fluorinated rubber and polyvinyl alcohol, and the dispersant can be sodium carboxymethylcellulose or potassium carboxymethylcellulose. In the negative electrode active material layer, the content (mass percentage) of the negative electrode material is 95-97%, the content of the conductive agent is 1-2%, the content of the bonding agent is 1-1.5%, and the content of the dispersing agent is 0-1.5%.

As shown in fig. 7a, when preparing a winding core, a winding needle a clamps the heads of two layers of diaphragms to drive the diaphragms to rotate, wherein a thick dotted line in fig. 7a represents a diaphragm positioned at the inner side in the two layers of diaphragms, namely an inner layer diaphragm B1, an inner layer diaphragm B1 is in contact with the winding needle a, a thin dotted line represents a diaphragm positioned at the outer side in the two layers of diaphragms, namely an outer layer diaphragm B2, and an outer layer diaphragm B2 is stacked at the outer side of the inner layer diaphragm B1 and is not in direct contact with the winding needle a; the winding needle A clamps the heads of the two layers of diaphragms during winding, the part of the diaphragm clamped by the winding needle is defined as a clamping section, and the clamping section of the inner diaphragm B1 and the clamping section of the outer diaphragm B2 are overlapped together, such as the part positioned in a line frame in FIG. 7 a; defining the part of the diaphragm before the first bending during winding as a first straight section, wherein the first straight section is positioned behind the clamping section, the first straight section of the inner diaphragm B1 and the first straight section of the outer diaphragm B2 are overlapped, such as the part positioned in a B-line frame in FIG. 7a, after the diaphragm is driven by the winding needle A to rotate for a half circle, a negative pressure wheel (not shown) presses a negative plate C (a thick solid line in FIG. 7a represents the negative plate), the negative plate C is brought by the diaphragm, the negative plate C is positioned between the inner diaphragm B1 and the outer diaphragm B2, after the half circle is rotated, a positive pressure wheel (not shown) presses a positive plate D (a thin solid line in FIG. 7a represents the positive plate), the positive plate D enters along the diaphragm, so that the positive plate D and the negative plate C are separated by the diaphragm, the winding precursor action is finished, the positive pressure wheel and the negative pressure wheel are retracted, and the positive plate and the negative plate rotate along with, forming a winding structure of a layer of diaphragm, a layer of negative plate, a layer of diaphragm and a layer of positive plate. In addition, another winding process can be adopted, after the clamping section of the diaphragm is clamped by a winding needle to rotate for half a circle, the positive plate and the negative plate are simultaneously fed, and a winding structure of a layer of diaphragm, a layer of positive plate, a layer of diaphragm and a layer of negative plate is formed.

The two-layer diaphragm afterbody all exceeds positive plate tail end in the book core of this embodiment, and the afterbody of two-layer diaphragm has the part that overlaps the laminating together, and the part that the laminating was in the same place behind exceeding positive plate with two-layer diaphragm end is defined as the afterbody laminating section, like the part that arrow Q pointed in figure 7. When the winding needle a is drawn out, the part of the inner layer membrane B1 directly contacting with the winding needle is overlapped and contacted together due to the drawing out of the winding needle, the part of the inner layer membrane B1 overlapped and contacted together after the winding needle a is drawn out is defined as a first inner layer attaching section, as the part pointed by an arrow P in fig. 7, and the length of the first inner layer attaching section of the inner layer membrane B1 is 30-60% of the width of the winding core.

There are two configurations for the winding core shown in fig. 7, one configuration being: the opposite surfaces of the diaphragm and the winding needle are ceramic surfaces, the opposite surfaces between two adjacent layers of diaphragms are rubber surfaces (the part in a line frame a, the part in a line frame B and the part pointed by an arrow Q in fig. 7 a), and after the winding needle is drawn out, the clamping section of the inner-layer diaphragm B1 is opposite to the first straight section of the inner-layer diaphragm B1, namely the ceramic surface is opposite to the ceramic surface; the other structure is as follows: the opposite surfaces of the diaphragm and the winding needle are rubber surfaces, the opposite surfaces between two adjacent layers of diaphragms are ceramic surfaces, and after the winding needle is drawn out, the clamping section of the inner-layer diaphragm B1 is opposite to the first flat section of the inner-layer diaphragm B1, namely the rubber surfaces are opposite to the rubber surfaces. In other words, in the two structures, the opposite surfaces between the two adjacent membranes are the surfaces with the same material, and the surfaces of the inner membrane which are overlapped and opposite to the inner membrane are also the surfaces with the same material. More specifically, the length of the clamping section of the diaphragm (inner diaphragm and outer diaphragm) is 1-15% of the width of the winding core, the length of the first flat section of the diaphragm (inner diaphragm and outer diaphragm) is 40-50% of the width of the winding core, the length of the tail attaching section of the diaphragm (inner diaphragm and outer diaphragm) is at least 5mm, and the length of the tail attaching section of the diaphragm (inner diaphragm and outer diaphragm) can be 0.1-10% of the width of the winding core. The thick short lines in fig. 7a and 7 represent the tabs M. The surface of the inner-layer diaphragm in the winding core has swelling and bonding effects under the electrolyte environment, so that bonding force is formed between diaphragm layers, and the bonding force is mainly expressed as van der Waals force.

The inventor finds that the diaphragm based on the composition structure (including selection of diaphragm materials, setting of material components and arrangement of structures of each layer of the diaphragm) has a specific wet-method stripping force, the bonding effect of the diaphragm between winding core layers can be identified by detecting the wet-method stripping force (swelling bonding force of a coating on the surface of the diaphragm) of the diaphragm, so that the adhesion force between the diaphragm and between the diaphragm and a positive electrode and a negative electrode can be identified in advance according to the wet-method stripping force, whether the diaphragm can meet the bonding requirement after the cell is formed by hot pressing is judged in advance, the adhesion force between main materials of the cell is identified, and the diaphragm with the specific wet-method stripping force is selected in the production process to prepare the cell winding core, so that the cell with better hardness and better performance is obtained. The greater the wet peel force of the separator, the better the bonding effect between the separators and the greater the tear force. As shown in fig. 8, when the adhesion of the separator in the electrolyte environment is good, in the cell formation process, the adhesion phenomenon generated by swelling can occur in the adjacent separator layers, so as to achieve the separator adhesion effect, and the larger the separator adhesion force is, the better the adhesion effect is, the smaller the probability of short circuit caused by internal contact between the positive and negative electrode plates when the cell falls is, thereby increasing the cell safety performance. The wet stripping force (swelling adhesive force, namely the adhesive force of the surface glue coating of the diaphragm after the diaphragm glue swells in the electrolyte environment) of the diaphragm plays a role in fixing a roll core structure.

The wet stripping force is to place the diaphragm in the environment of electrolyte for one-time hot pressing, then carry out second hot pressing compounding on the diaphragm in the environment without electrolyte, and then carry out 90-degree stripping, wherein the stripping force is the wet stripping force and can be used for reflecting the adhesive force performance of the surface coating of the diaphragm after the diaphragm is dissolved and swelled. The following describes a method for testing the wet peeling force with reference to fig. 9 to 12:

s1, cutting the membrane to be tested into a sample with proper size, such as cutting the membrane to be tested into a long strip with a certain width, for example, a small strip with a width of 15mm, aligning and stacking the two cut membrane samples to be tested, adding paper at one end of the membrane samples, separating the two membrane samples to be tested by using the paper (figure 9), and cutting the membrane adhered at the first inner layer adhering section into samples and aligning and stacking the samples when testing the membrane at the first inner layer adhering section;

s2, placing the two stacked tested diaphragm samples into an aluminum plastic film for packaging, then injecting electrolyte, such as 10g of electrolyte, and then vacuumizing and sealing (figure 10);

s3, carrying out hot pressing on the sealed aluminum plastic film by using a formation machine (figure 11), wherein the hot pressing temperature can be 68-90 ℃, the pressure is 0.7-1.1 MPa, the hot pressing temperature of the embodiment is 80 ℃, the pressure is 0.8MPa, and the hot pressing time is 2 hours;

s4, after hot pressing, taking out the diaphragm sample to be tested from the aluminum plastic film, wiping the electrolyte, putting the diaphragm sample to be tested into a hard sealing tape, and carrying out high-temperature hot-pressing compounding (figure 12) again, wherein the model of the hot-pressing machine used in the embodiment is SKY-325R6, the hot-pressing temperature is 100 ℃, the surface pressure is 0.2Mpa, and the speed is 1 mm/S;

s5, after hot pressing is completed, drawing out the paper clamped between the tested diaphragm samples, separating the two tested diaphragm samples pressed together from one end, peeling at 90 degrees (figure 13), and recording the wet peeling force when the two tested diaphragm samples are separated; in the embodiment, an electronic universal testing machine is used for carrying out 90-degree peeling test on a tested diaphragm sample, one end of one tested diaphragm sample is fixed to the movable end of the electronic universal testing machine, one end of the other tested diaphragm sample is fixed to the fixed end of the electronic universal testing machine, the preloading speed and the testing speed are set to be 100mm/min, the two tested diaphragm samples are separated, and the peeling force when the tested diaphragm samples are separated is recorded.

The following table shows the results of the wet stripping test performed on 3 different types of separators.

Selecting 6 diaphragms from the diaphragms subjected to the wet stripping force test, winding the 6 diaphragms together with the positive and negative plates into a battery core, packaging the battery core, injecting electrolyte, and forming to prepare the battery, wherein the width of the diaphragm is 83.8mm, the width of the positive plate is 79.5mm, the width of the negative plate is 81.5mm, and the positive plate and the negative plate are both the positive plate and the negative plate of the conventional lithium battery. Meanwhile, two existing diaphragms and the same positive and negative plates are selected to prepare a battery as a comparative example. The prepared battery is tested for hardness, thickness expansion rate and capacity retention rate, and the hardness test can be carried out by adopting a three-point hardness test method. The three-point hardness test method is as follows: as shown in fig. 21, the bottom surface of the battery is supported by two steel pipes, the two steel pipes are positioned at two ends of the battery, the other steel pipe is pressed at the middle position of the top surface of the battery, the steel pipes apply pressure to the surface of the battery during testing, and the testing parameters are as follows: the pressing deformation limit displacement is-3 mm, the pressing speed is-6 mm/min, and the pressure when the battery deforms is the hardness test result.

The test results of hardness, thickness expansion rate and capacity retention rate are shown in the following table:

from the above results, it can be seen that the adhesive force (wet peeling force) of the hot-pressed composite adhesive surface and the ceramic surface of the aqueous separator after swelling in the electrolyte environment is smaller than the adhesive force (wet peeling force) of the gravure oil-based separator and the gravure oil-based mixed coating separator, the adhesive force effect between the separators can be indirectly expressed by the hardness data after the battery is put down, and the current cycle characteristics of the battery can be expressed (fig. 14). The adhesive force of the oil-based mixed coating diaphragm is superior to that of the water-based diaphragm, the adhesive force effect of the diaphragm after the diaphragm glue swells in the electrolyte environment is indirectly proved, when the wet stripping force is increased, the subsequent cycle performance of the battery is excellent, the hardness of the battery is hard, and the quantitative possibility and the execution performance are realized. Based on the above data for testing battery hardness, the following conclusions can be drawn:

when the wet stripping force of the rubber surface of the diaphragm is less than 2N/m and the wet stripping force of the ceramic surface is less than 1N/m, the whole battery is soft, and the whole hardness of the battery is less than 200N;

when the wet stripping force of the rubber surface of the diaphragm is 2N/m-10N/m and the wet stripping force of the ceramic surface is 1N/m-2N/m, the overall hardness of the battery is 200N-400N;

when the wet stripping force of the rubber surface of the diaphragm is more than 10N/m and the wet stripping force of the ceramic surface is 2N/m-10N/m, the overall hardness of the battery is 400N-550N;

when the wet stripping force of the rubber surface of the diaphragm is larger than 10N/m and the wet stripping force of the ceramic surface is larger than 10N/m, the overall hardness of the battery is 550N-800N.

According to the test results of the hardness test, the thickness expansion rate and the capacity retention rate, when the wet stripping force of the rubber surface of the diaphragm is not less than 2N/m and the wet stripping force of the ceramic surface of the diaphragm is not less than 1N/m, the hardness of the prepared battery and the battery cycle can meet 500cycles, the greater the wet stripping force of the diaphragm is, the harder the battery is, the better the bonding effect between the pole piece and the diaphragm is, the greater the battery hardness is, the better the long-term cycle capacity of the battery is, the better the capacity retention rate is and the smaller the thickness expansion rate is. Therefore, when the winding core is prepared, the diaphragm can be selected according to the wet stripping force of the surface of the diaphragm, the wet stripping force of the glue surface of the first inner layer attaching section of the inner diaphragm of the lithium battery winding core is not less than 2N/m, and the wet stripping force of the ceramic surface of the first inner layer attaching section of the inner diaphragm is not less than 1N/m.

According to the invention, according to the relation between the wet stripping force of the diaphragm and the hardness of the battery and the cycle performance of the battery, before the winding process, the diaphragm with the wet stripping force meeting certain requirements is selected, so that the diaphragm is easy to be bonded, and van der Waals force bonding effect is generated between the diaphragm and the negative plate and the positive plate, so that a good bonding effect is formed, the hardness of the obtained battery is relatively hard, the cycle failure probability of the battery caused by over standard expansion is reduced at the later stage of the cycle, and the cycle life of the battery with good bonding can be prolonged. According to the invention, the adhesive force exhibited in the formation swelling process is controlled by controlling the wet stripping force of the diaphragm, and the hardness degree of the battery after formation can be effectively controlled. In addition, the bonding effect of the diaphragm coating between the winding core layers is identified through the wet stripping force, the quantitative index of the diaphragm coating can be macroscopically provided, the management and control of the diaphragm coating are facilitated, the method can be used as a supplied material management means of the diaphragm, whether the diaphragm can meet the bonding requirement after the hot-pressing formation of the battery cell is judged in advance through identifying the macroscopic index of the supplied material of the diaphragm, and therefore a proper diaphragm material is selected to prepare the battery cell.

The inventor further finds that the adhesive force of the surface (coating) of the diaphragm also influences the production process of the winding core and the quality of a finished product, and the adhesive force of the coating on the surface of the diaphragm is unqualified, so that the poor production phenomena such as folding of the pole piece can be caused. The dry stripping force is to perform 90-degree stripping after hot-pressing compounding on the diaphragm in an electrolyte-free environment, select a proper diaphragm material through the stripping force, avoid the problems of winding the winding core on the next stage and folding the pole piece after packaging and baking by controlling the dry stripping force of the diaphragm, and improve the production yield of the winding core. The dry peel force test comprises the following specific steps:

s1, cutting the tested diaphragm into a sample with a proper size, for example, cutting the tested diaphragm into a long strip with a certain width, for example, a small strip with a width of 15mm, aligning and stacking the two cut tested diaphragm samples, adding paper at one end of the two cut tested diaphragm samples, and separating the two tested diaphragm samples by the paper;

s2, performing heat sealing treatment on the binding surfaces of the two stacked diaphragm samples to be detected by using a thermoplastic machine, wherein the type of the thermoplastic machine used in the embodiment is SKY-325R6, the hot pressing temperature is 100 ℃, the surface pressure is 0.2Mpa, and the hot pressing time is 10 seconds;

s3, after hot pressing is completed, drawing out the paper clamped between the tested diaphragm samples, separating the two tested diaphragm samples pressed together from the end parts, peeling at 90 degrees, and recording the dry peeling force when the two tested diaphragm samples are separated; in the embodiment, an electronic universal testing machine is used for carrying out 90-degree peeling test on a tested diaphragm sample, one end of one tested diaphragm sample is fixed to the movable end of the electronic universal testing machine, one end of the other tested diaphragm sample is fixed to the fixed end of the electronic universal testing machine, the preloading speed and the testing speed are set to be 100mm/min, the two tested diaphragm samples are separated, and the peeling force when the tested diaphragm samples are separated is recorded.

The invention reflects the adhesive force performance between the surface coatings of the diaphragms through the dry stripping force, so that when the dry stripping force of the diaphragms is tested, the diaphragms which are pressed together and made of the same material are separated, the two diaphragms have the same coating material, and auxiliary materials with different surface materials, such as quick-drying adhesive, double-sided adhesive and the like, are not used, and more accurate data of the adhesive force of the surface coatings of the diaphragms can be obtained. In addition, the sample to be tested is pressed together in a hot-pressing compounding mode, the operation is simple and rapid, and the 90-degree stripping test is more convenient compared with the 180-degree stripping test by means of other auxiliary materials.

The following table shows the results of dry peel force testing of 3 different types of oil-based separators.

5 kinds of diaphragms are selected from the diaphragms subjected to the dry stripping force test, and the diaphragms are wound together with the positive and negative plates into a battery cell, wherein the width of the diaphragm is 83.8mm, the width of the positive plate is 79.5mm, the width of the negative plate is 81.5mm, and the positive plate and the negative plate are both conventional positive and negative plates of the lithium battery. Fig. 15 and 16 are SEM images of the base material surface before hot press-compounding of the EJ oil-based separator 5 and the EJ oil-based separator 1, respectively, the SEM images of the base material surface before hot press-compounding of the EJ oil-based separators 6 to 8 are similar to fig. 15, and the SEM images of the base material surface before hot press-compounding of the EJ oil-based separators 2 to 4 are similar to fig. 16. And (3) taking a part of the battery cell obtained by winding for quality inspection, and inspecting the phenomena of folding of the diaphragm and peeling and transferring of the coating, wherein the inspection results are shown in the following table, and the distance of the diaphragm covering pole piece in the following table refers to the size of the diaphragm covering the negative pole piece, and is used for preventing the internal short circuit of the battery cell. The invention further uses the transfer area to reflect the glue transfer performance of the dry stripping of the diaphragm, and selects a proper diaphragm as another performance parameter. Transfer area is the mass transferred per membrane area, transfer mass is the mass of the membrane before dry stripping (measured) -the mass of the membrane after dry stripping (measured). Taking a separator having a width of 15mm and a length of 150mm as an example, when the mass of the separator before dry peeling is 0.18g and the mass of the separator after dry peeling is 0.12g, the transfer area is 0.06/0.00225 to 26.67 and the transfer ratio of the separator paste is 0.06/0.18 × 100% to 33% (mass of the separator before dry peeling (measured) — mass of the separator after dry peeling (measured) — mass of the separator before dry peeling (measured) × 100%). The larger the transfer area ratio is, the larger the adhesiveness is, the larger the adhesive force between the battery core pole piece diaphragms is, the larger the winding difficulty is, the larger the bad ratio of the core pulling to the space is, the higher the breaking possibility is, and the higher the battery hardness is.

According to the sampling inspection result, for the gravure oil-based diaphragm, when the dry stripping force of the ceramic surface of the diaphragm is more than 10N/m, the pole piece and the empty foil of the winding core are easy to fold when the electric core is wound at the next stage, the empty foil or the pole piece is easy to fold when the winding core is hot-pressed or the winding core is baked, the folding percentage is more than 80%, SEM images of the ceramic surface (ceramic layer and adhesive layer) and the base material surface (adhesive layer) of the diaphragm after being stripped are shown in figures 17 to 20, the peeling transfer phenomenon is easy to occur on the ceramic surface or the adhesive layer of the base material surface of the diaphragm, the transfer area percentage reaches 40-80%, and the proportion of the adhesive transfer after being stripped is increased along with the increase of the dry stripping force, and the transfer area is also increased; for the same diaphragm, the stripping force of the rubber surface (the substrate surface) is larger than that of the ceramic surface, so that the dry-process stripping force of the ceramic surface can be used for selecting the diaphragm material;

when the dry stripping force of the ceramic surface is between 5N/m and 8N/m, the proportion of folding of the pole piece and the empty foil of the winding core during the next winding process of the battery core is obviously reduced, the proportion of folding of the empty foil and the pole piece of the winding core during hot pressing or baking of the winding core is obviously reduced to 30 percent, the ceramic surface and the base material surface of the diaphragm after dry pressing have SEM effects similar to those of figures 17 to 20, the ceramic surface or the adhesive layer of the base material surface of the diaphragm is stripped and transferred, and the transfer area ratio is reduced to 20-40 percent;

when the dry stripping force of the ceramic surface is less than 5N/m, the rolled core is not easy to have folding of pole pieces and empty foils when the cell winding process is off-line, the rolled core is not easy to have folding of empty foils and pole pieces when the rolled core is hot-pressed or the rolled core is baked, and the ceramic surface and the base material surface of the diaphragm after stripping do not have SEM effects similar to those of figures 17 to 20.

In summary, the separator with a large dry peeling force (surface adhesion force) cannot be well applied in the winding process if the dry peeling force is larger than 8N/m, the folding phenomenon of the pole piece is improved if the dry peeling force is smaller than 8N/m, and the separator is well applied in the winding process if the dry peeling force is smaller than 5N/m, the manufacturing process of the winding core is stable, and the folding proportion of the pole piece is low and can even be as low as 0. However, the diaphragm with larger bonding force shows better bonding performance when the thermal compression molding is carried out in the post process, the bonding between the diaphragm and the positive and negative plates can be realized, the hardness of the battery is better, the contact bonding performance between the core head part of the battery core and the diaphragm at the bottom of the battery core becomes better when the thermal compression molding is carried out, when the battery core is tested at the furnace temperature in the over-safety performance, the internal short circuit caused by the contact of the positive and negative electrodes can be avoided, but if the folding phenomenon of the pole piece or the diaphragm occurs, the safety performance of the battery core can be reduced.

Therefore, in order to avoid the problem that the pole piece after the winding core is wound on the next stage and packaged and baked is folded, the diaphragm can be further selected according to the dry stripping force on the surface of the diaphragm, and the dry stripping force of the first straight section of the inner diaphragm and the first straight section of the outer diaphragm in the lithium battery winding core is smaller than 8N/m so as to improve the folding phenomenon of the pole piece. The folding phenomenon of the pole piece is improved by using the first flat section of the diaphragm with small dry stripping force, and the adhesion of other parts of the diaphragm is not influenced, so that the hardness of the battery is ensured. More preferably, the dry peel force is the dry peel force of the ceramic face of the separator.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.