Preparation method of negative pole piece, secondary battery and electronic equipment

Abstract

The invention provides a preparation method of a negative pole piece, a secondary battery and electronic equipment, and particularly relates to the technical field of battery manufacturing. The preparation method of the negative electrode plate comprises the following steps: coating at least one side surface of a negative electrode current collector with negative electrode slurry so as to form a negative electrode active layer on the negative electrode current collector; carrying out primary rolling on the negative electrode active layer according to a first direction; carrying out secondary rolling on the negative electrode active layer subjected to the primary rolling according to a second direction to obtain a negative electrode plate; wherein the first direction and the second direction are both along the length direction of the negative electrode current collector, and the second direction is opposite to the first direction. According to the preparation method disclosed by the invention, the crushing and pulverization of the negative electrode particles and the binder can be reduced, so that the pole piece gaps and the upper and lower pore size distribution are more uniform, the pole piece standing rebound is smaller, the thickness consistency is better, and the electrolyte infiltration effect is better, thereby improving the performance of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of battery manufacturing, and in particular to a method for preparing a negative electrode sheet, a secondary battery and an electronic device. Background Art

[0002] Silicon-based negative electrode materials are considered to be the most competitive candidate materials for the next generation of anodes for lithium-ion batteries due to their high specific energy density, natural abundance, and attractive operating voltage. However, silicon-based negative electrode materials usually undergo significant volume changes during the cycle, resulting in large rebound and extension of the negative electrode plates during both static and cycling processes. In addition, the current negative electrode plates are mostly prepared using a single rolling process, which not only easily causes the rupture and pulverization of the negative electrode particles, but also leads to uneven distribution of voids in the negative electrode plates (the pore size above the plate is larger than the pore size below the plate), resulting in uneven electrolyte infiltration and large plate resistance.

[0003] Therefore, it is necessary to provide a method for preparing a negative electrode plate, a secondary battery and an electronic device to solve the above-mentioned problems of silicon-based negative electrode materials. Summary of the invention

[0004] In view of the above shortcomings of the prior art, the present invention provides a method for preparing a negative electrode plate to improve the problems of cracking and pulverization of negative electrode particles and uneven distribution of gaps in the negative electrode plate, resulting in uneven electrolyte infiltration and large plate resistance.

[0005] To achieve the above-mentioned object and other related objects, the present invention provides a method for preparing a negative electrode sheet, the preparation method comprising: coating a negative electrode slurry on at least one side surface of a negative electrode current collector to form a negative electrode active layer on the negative electrode current collector; rolling the negative electrode active layer once in a first direction; rolling the negative electrode active layer after the first rolling for a second time in a second direction to obtain a negative electrode sheet; wherein the first direction and the second direction are both along the length direction of the negative electrode current collector, and the second direction is opposite to the first direction.

[0006] In one embodiment of the present invention, the first rolling is performed by hot roller pressing, and the temperature of the hot roller pressing is 60°C to 90°C.

[0007] In one embodiment of the present invention, the secondary rolling is performed by hot roller pressing or cold roller pressing, the temperature of the hot roller pressing is 60°C to 90°C, and the temperature of the cold roller pressing is 20°C to 30°C.

[0008] In one embodiment of the present invention, the thickness of the pole piece after the first rolling is D1, and the thickness of the pole piece after the second rolling is D2, and the second rolling coefficient is defined as P=D1 / D2, P>1.

[0009] In one embodiment of the present invention, the secondary rolling coefficient P has a value of 1.3 to 2.8.

[0010] In one embodiment of the present invention, the negative electrode current collector has a first surface and a second surface arranged opposite to each other in the thickness direction, and the slurry coating step includes: coating the negative electrode slurry on the first surface of the negative electrode current collector, and forming a first active layer after drying; turning over the negative electrode current collector, coating the negative electrode slurry on the second surface of the negative electrode current collector, and forming a second active layer after drying.

[0011] In an embodiment of the present invention, the one rolling comprises: synchronously rolling the first active layer and the second active layer in the first direction.

[0012] In one embodiment of the present invention, the slurry coating further includes slurry preparation before the slurry coating, and the slurry preparation includes: mixing the negative electrode active material, the negative electrode conductive agent, the negative electrode binder and the negative electrode thickener, injecting a solvent and stirring evenly to obtain the negative electrode slurry.

[0013] In one embodiment of the present invention, the negative electrode active material includes a silicon-based negative electrode material and / or a graphite negative electrode material.

[0014] In one embodiment of the present invention, the mass ratio of the negative electrode active material, the negative electrode conductive agent, the negative electrode binder and the negative electrode thickener is (93-98): (0.2-1.5): (1-3): (0.01-1.5).

[0015] Another aspect of the present invention provides a secondary battery, the secondary battery comprising a positive electrode sheet and a negative electrode sheet, the negative electrode sheet being prepared by the preparation method described above in the present invention.

[0016] The present invention further provides an electronic device, wherein the electronic device comprises the secondary battery provided by the present invention.

[0017] The present invention adopts a secondary rolling technology to prepare negative electrode sheets. By controlling the rolling directions of the two rolling processes to be opposite, the crushing and pulverization of negative electrode particles and binders can be reduced, so that the gaps and upper and lower pore size distributions of the negative electrode sheets are more uniform, the static rebound of the negative electrode sheets is smaller, the thickness consistency is better, the electrolyte infiltration effect is better, the resistance of the negative electrode sheets is reduced, and the electrical performance of the battery is improved. In addition, the use of hot rollers for the first rolling can overcome the inconsistent electrode thickness caused by the friction temperature rise of the cold roller. The electrode can be pressed to the required thickness and surface density using a lower rolling force. Therefore, the electrode gaps and the upper and lower pore size distribution are more uniform. Combined with the secondary rolling direction, the deformation of active particles can be effectively reduced, the microcracks of the electrode binder can be reduced, the binder performance can be improved, and the battery cycle life can be improved. The ratio P value of the electrode thickness after the first rolling and the electrode thickness after the second rolling is controlled to be between 1.3 and 2.8, which can ensure the uniformity of the pore size of the upper and lower layers of the negative electrode, reduce the breakage and shedding of the negative electrode particles, improve the electrolyte wettability and the bonding force of the negative electrode, and thus improve the battery cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0019] Figure 1 The flowchart of the method for preparing the negative electrode sheet of the present invention is as follows;

[0020] Figure 2 It is a structural schematic diagram of a method for preparing a negative electrode sheet of the present invention in one embodiment;

[0021] Figure 3 The figure is a relationship curve between the secondary rolling coefficient P value and the cycle capacity and the expansion rate of the negative electrode sheet in the preparation method of the negative electrode sheet of the present invention. DETAILED DESCRIPTION

[0022] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict.

[0023] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0024] Unless otherwise specified or incompatible herewith, the terms and phrases used herein shall have the following meanings:

[0025] As used herein, "plurality", "multiple", "multiple times", etc., unless otherwise specified, refer to a quantity greater than or equal to 2. For example, "one or more" means one or greater than or equal to two.

[0026] Herein, "preferred", "better" and "more preferred" are only used to describe implementation methods or examples with better effects, and it should be understood that they do not constitute a limitation on the scope of protection of the present invention. If multiple "preferred" items appear in a technical solution, unless otherwise specified and there is no contradiction or mutual restriction, each "preferred" item is independent.

[0027] Herein, “further”, “furthermore”, “particularly”, etc. are used for descriptive purposes to indicate differences in content, but should not be construed as limiting the scope of protection of the present invention.

[0028] In this article, when it comes to numerical ranges, unless otherwise specified, the distribution of optional values ​​within the numerical range is considered continuous and includes the two numerical endpoints (i.e., the minimum value and the maximum value) of the numerical range, as well as every value between the two numerical endpoints. When multiple numerical ranges are provided to describe features or characteristics, these numerical ranges can be combined.

[0029] The present invention provides a method for preparing a negative electrode plate, a secondary battery and an electronic device comprising the secondary battery, adopting a secondary rolling process and controlling the rolling directions of the two rolling processes to improve the problems of negative electrode particle breakage and pulverization and uneven distribution of negative electrode plate gaps in the prior art.

[0030] See also Figure 1 The method for preparing the negative electrode sheet provided by the present invention comprises at least the following steps:

[0031] S1, coating the negative electrode slurry on at least one side surface of the negative electrode current collector to form a negative electrode active layer on the negative electrode current collector;

[0032] S2, rolling the negative electrode active layer once in a first direction;

[0033] S3, rolling the negative electrode active layer after the first rolling for a second time in the second direction to obtain a negative electrode sheet.

[0034] Specifically, step S1 is a slurry coating step, and the negative electrode slurry in this step includes a negative electrode active material, a negative electrode conductive agent, a negative electrode binder, a negative electrode dispersant and a solvent. Among them, the negative electrode active material includes a silicon-based negative electrode material and / or a graphite negative electrode material, the silicon-based negative electrode material includes any one or more of elemental silicon, silicon oxide compounds, and silicon carbon compounds, and the graphite negative electrode material includes any one or more of natural graphite, artificial graphite, soft carbon, and hard carbon. The negative electrode conductive agent is selected from one of conductive carbon black, nano silver powder, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc., or a combination of two or more mixed in any proportion. The negative electrode binder is selected from any one of polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), or a combination of several mixed in any proportion; the negative electrode thickener can be sodium carboxymethyl cellulose (CMC), etc., and the solvent is usually selected from deionized water or N-methylpyrrolidone (NMP), etc.

[0035] Before executing step S1, the above materials need to be configured into negative electrode slurry, and the configuration process of negative electrode slurry can be carried out according to conventional methods in the art. As an example, the negative electrode active material, negative electrode conductive agent, negative electrode binder and negative electrode thickener are mixed and stirred evenly in a mixer according to a certain proportion, and deionized water solvent is injected and stirred evenly to obtain negative electrode slurry. Among them, the mass ratio between the negative electrode active material, the negative electrode conductive agent, the negative electrode binder and the negative electrode thickener is (93-98): (0.2-1.5): (1-3): (0.01-1.5), and the solvent is added at a solid content of 45% to 70%.

[0036] The negative electrode current collector in step S1 can be selected from conventional current collector types in the art, such as copper foil, carbon-coated copper foil, etc. When coating the slurry, the negative electrode slurry is coated on at least one side surface of the negative electrode current collector to form a negative electrode active layer on the negative electrode current collector. That is, the negative electrode current collector has two surfaces opposite to each other in its own thickness direction, which are respectively recorded as the first surface and the second surface. The negative electrode slurry can be coated on any one of the first surface and the second surface of the negative electrode current collector, or on both the first surface and the second surface. When both surfaces of the negative electrode current collector are coated with the negative electrode slurry, a coating machine can be used to first coat the negative electrode slurry on the first surface, and after drying, a first negative electrode active layer is formed on the first surface of the negative electrode current collector; then the negative electrode current collector is turned over, and the negative electrode slurry is coated on the second surface, and after drying, a second negative electrode active layer is formed on the second surface of the negative electrode current collector. It should be noted that the single-sided coating surface density and coating amount of the negative electrode slurry can be set according to actual needs, and are not limited here.

[0037] See also Figure 1 and Figure 2 , step S2 is to roll the negative electrode active layer formed in step S1. Specifically, the negative electrode active layer is rolled once in a first direction using a rolling device. The specific type of the rolling device is not limited here, and it can be any device in the art that can meet the requirements of electrode sheet rolling, such as a roller press. The above-mentioned first direction (rolling direction of the first rolling) is along the length direction of the negative electrode current collector. Since the length direction of the negative electrode current collector includes two opposite directions, here, we record one direction as direction A, and the direction opposite to direction A as direction B (see Figure 2 ), the first direction can be direction A or direction B.

[0038] Furthermore, the first rolling process uses hot roller rolling, and the hot roller rolling process uses a relatively low rolling force to press the pole piece to the required thickness and surface density, so the pole piece gap and the pore size distribution of the upper and lower layers are uniform. Furthermore, the temperature of the hot roller rolling process is 60°C to 90°C, that is, any temperature within the above temperature range can be selected for the first rolling process, for example, 60°C, 70°C, 80°C or 90°C, etc. Rolling within this temperature range can not only meet the rolling requirements, but also save energy consumption.

[0039] It should be noted that, when the first surface and the second surface of the negative electrode current collector are both coated with negative electrode slurry, during one rolling process, the first active material layer formed on the first surface and the second active material layer formed on the second surface are rolled synchronously in a first direction, that is, the front and back sides of the negative electrode sheet are rolled simultaneously.

[0040] See also Figure 1 and Figure 2 , step S3 is to perform a secondary rolling on the negative electrode active layer after the primary rolling. That is, the negative electrode active material layer after the primary rolling is rolled a second time in the second direction using a rolling device. Among them, the second direction (the rolling direction of the secondary rolling) is along the length direction of the negative electrode current collector, and the second direction is opposite to the first direction. That is to say, if the first direction is direction A, the second direction is direction B; if the first direction is direction B, the second direction is direction A. The present application adopts a secondary rolling process, and the rolling directions of the primary rolling and the secondary rolling are opposite, which can reduce the crushing and pulverization of the negative electrode material particles and the binder, and the gaps and the upper and lower pore sizes of the negative electrode sheets are more uniform, so that the static rebound of the negative electrode sheets is smaller, the thickness consistency is better, the electrolyte infiltration effect is better, the resistance of the negative electrode sheets is reduced, and the performance of the battery is improved.

[0041] Furthermore, the secondary rolling can be performed by hot roller pressing or cold roller pressing. When hot roller pressing is performed, the rolling temperature is 60°C to 90°C, such as 60°C, 75°C or 90°C; when cold roller pressing is performed, the rolling temperature is 20°C to 30°C, such as 20°C, 25°C or 30°C. However, considering the economic efficiency, cold roller pressing is preferred for the secondary rolling.

[0042] In order to further clarify the influence of primary and secondary rolling on the negative electrode sheet, the inventors defined a secondary rolling coefficient P, and recorded the thickness of the electrode sheet after primary rolling as D1, and the thickness of the electrode sheet after secondary rolling as D2. The secondary rolling coefficient P = D1 / D2. Since the secondary rolling is to roll the negative electrode active layer again based on the primary rolling, D1>D2, that is, P>1. Further, 1.3≤P≤2.8, as an example, P can be 1.3, 1.5, 2.0 or 2.8, etc. The inventors have found through a lot of research that when the P value is between 1.3 and 2.8, the effect of secondary rolling is better, the gap and upper and lower pore size distribution uniformity of the negative electrode sheet is good, the cycle stability is good, and the expansion rate of the negative electrode sheet is low; when P≤1.2, the thickness after the first rolling is approximately equal to the thickness after the second rolling, the secondary rolling technology fails to play a role, the gap and upper and lower pore size distribution uniformity of the negative electrode sheet is relatively poor, the capacity is only slightly improved, and the expansion of the negative electrode sheet is still serious (>20%); when P≥3, the thickness of the electrode sheet after the first rolling is much greater than the thickness after the second rolling, the gap and upper and lower pore size distribution uniformity of the negative electrode sheet is relatively poor, the capacity is slightly improved compared to the first rolling technology, and the expansion of the negative electrode sheet is still serious (>20%). Therefore, the secondary rolling coefficient P is preferably 1.3 to 2.8.

[0043] After the second rolling, the electrode sheets usually need to be cut to obtain negative electrode sheets that meet the requirements.

[0044] The present invention also provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, a diaphragm and an electrolyte, wherein the diaphragm is arranged between the positive electrode sheet and the negative electrode sheet to play an isolating role; the electrolyte plays a role of conducting lithium ions between the positive electrode sheet and the negative electrode sheet. During the battery charging and discharging process, lithium ions are embedded and released back and forth between the positive electrode sheet and the negative electrode sheet. The negative electrode sheet is prepared by the preparation method described above in the present invention. The gap and upper and lower pore size distribution of the negative electrode sheet are relatively uniform, the sheet has smaller static rebound, better thickness consistency, better electrolyte infiltration effect, and reduced resistance of the negative electrode sheet, thereby improving the electrochemical performance of the secondary battery.

[0045] Specifically, the positive electrode plate includes a positive electrode current collector and a positive electrode active layer disposed on at least one surface of the positive electrode current collector. The positive electrode current collector can be made of a material with good electrical conductivity and mechanical strength, such as aluminum foil or carbon-coated aluminum foil. The positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode active layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector. The positive electrode active layer includes a positive electrode material, a positive electrode conductive agent, and a positive electrode binder, wherein the positive electrode material, the positive electrode conductive agent, and the positive electrode binder can be selected from conventional material types in the art, and no specific restrictions are made here.

[0046] Taking lithium batteries as an example, the positive electrode material can be selected from any one of lithium iron phosphate, lithium iron manganese phosphate, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide or lithium nickel cobalt aluminum oxide, or a combination of at least two. The positive electrode conductor is selected from one of conductive carbon black (SP), acetylene black, nano metal powder, graphene, carbon nanotubes, carbon nanofibers, or a combination of two or more mixed in any proportion. The positive electrode binder is selected from one or more mixtures of polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene, sodium carboxymethyl cellulose, and styrene-butadiene rubber. Preferably, the positive electrode active layer also includes a lithium supplement, and the lithium supplement includes but is not limited to Li 6 CoO 4 , Li 2 NiO 2 , Li 5 FeO 4 .

[0047] The separator is selected from conventional types in the art, for example, a 12 μm polypropylene (PP) or polyethylene (PE) porous membrane is selected as the separator.

[0048] The electrolyte includes an organic solvent and a lithium salt (selected according to the type of battery), wherein the solvent can be selected from conventional organic solvents in the art. As an example, the organic solvent is selected from one of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), etc., or a plurality of them mixed in any proportion. The lithium salt is selected from LiPF 6 (Lithium hexafluorophosphate), LiBF 4 (Lithium Tetrafluoroborate)LiClO 4 (Lithium Perchlorate), LiAsF 6 (lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl imide), LiTFSI (lithium bis(trifluoromethanesulfonyl imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalatoborate), LiBOB (lithium dioxalatoborate), LiPO2 F 2 (lithium difluorophosphate), LiDFOP (lithium difluorobis(oxalate phosphate), LiTFOP (lithium tetrafluorooxalate phosphate) or a combination thereof in any proportion. The mass content of lithium salt in the electrolyte is 5% to 20%. Additives may also be added to the electrolyte, including film-forming additives and functional additives that can improve battery performance, such as vinyl carbonate (VC), 1,3-propane sultone (PS), diethylene sulfate (DTD), etc., and those skilled in the art may select according to actual needs. The conventional dosage of additives in the electrolyte is 1% to 4% of the electrolyte, such as 2%.

[0049] The secondary battery can be assembled together with the above-mentioned positive electrode sheet, negative electrode sheet, electrolyte and separator by the commonly used method in the field, wherein the positive electrode sheet, separator, negative electrode sheet, etc. are wound or stacked in sequence to form a bare battery cell, and then encapsulated in, for example, an aluminum-plastic film, and the electrolyte is injected. After formation, packaging and testing, a secondary battery is obtained. The secondary batteries prepared in this application are all lithium-ion secondary batteries.

[0050] The preparation method of lithium ion secondary battery is exemplified as follows:

[0051] (1) Preparation of positive electrode sheet: The positive electrode material, lithium supplement agent, positive electrode conductive agent and positive electrode binder are mixed evenly, and a solvent (such as N-methylpyrrolidone, referred to as NMP) is added, and the mixture is stirred under the action of a vacuum stirrer until the system is stable and uniform to obtain a positive electrode slurry; wherein the mass ratio of the positive electrode material, lithium supplement agent, positive electrode conductive agent and positive electrode binder in the positive electrode slurry is (88-97):(1-10):1:1, and the solvent NMP is added at a solid content ratio of 45% to 70%.

[0052] The positive electrode slurry is then coated on the positive electrode current collector, and the positive electrode sheet is obtained through processes such as room temperature drying, oven drying, cold pressing, and slitting.

[0053] (2) Preparation of negative electrode sheet: The negative electrode sheet is prepared according to the preparation method described above in the present invention, which will not be described in detail here.

[0054] (3) Preparation of electrolyte: In an argon atmosphere glove box with a water content of <10 ppm, fully dried lithium salt (LiPF 6 ) is dissolved in an organic solvent and mixed evenly to obtain an electrolyte, wherein LiPF 6 The concentration is 1 mol / L.

[0055] (4) Preparation of the diaphragm: A 12 μm thick polyethylene (PE) porous polymer film is selected, wherein the thickness of the diaphragm is 9 μm to 18 μm; the air permeability is 180 s / 100 mL to 380 s / 100 mL; and the porosity is 30% to 50%.

[0056] (5) Preparation of battery: The positive electrode sheet, separator, and negative electrode sheet prepared above are stacked or wound in sequence to obtain an electrode assembly, with the separator being placed between the positive and negative electrode sheets to play a role of isolation. The electrode assembly is placed in a packaging shell, fully baked to reduce the water content to below 450 ppm, and then injected with electrolyte. The secondary battery is obtained through processes such as formation, sealing, and inspection.

[0057] Those skilled in the art will understand that the above-described method for preparing a secondary battery is only an example, and other commonly used methods in the art may be used without departing from the contents disclosed in the present application.

[0058] The present invention also provides an electronic device, which comprises the secondary battery described above in the present invention. The secondary battery can be used in the electronic device in the form of a single cell, a battery module or a battery pack.

[0059] The electronic devices of the present invention include, but are not limited to, mobile phones, tablets, laptop computers, electric toys, battery cars, new energy vehicles, ships, spacecraft, etc. Among them, electric toys can include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys and electric airplane toys, etc., and spacecraft can include airplanes, rockets, space shuttles and spacecraft, etc. New energy vehicles can be pure electric vehicles, hybrid vehicles or extended-range vehicles, etc.

[0060] The technical scheme of the present invention is described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are all commercially available products, or can be prepared by conventional methods in the art, and the instruments used in the examples are all commercially available.

[0061] 1. The influence of rolling temperature and rolling direction on the negative electrode sheet

[0062] Example 1

[0063] This embodiment provides a method for preparing a negative electrode sheet, comprising the following steps:

[0064] (1) Coating slurry: artificial graphite of negative electrode materials, Si, conductive agent acetylene black, thickener sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber latex are mixed in a mass ratio of 89:8:1:1:1, deionized water is added, and the mixture is stirred in a vacuum mixer until it is stable and uniform to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on the first surface and the second surface of a copper foil with a thickness of 8 μm, and after drying, a first negative electrode active layer and a second negative electrode active layer are formed on the copper foil; it should be noted that after each surface is coated, the copper foil is first dried at room temperature and then transferred to a 120°C forced air oven for drying for 1 hour;

[0065] (2) Primary rolling: The current collector coated with the negative electrode slurry is rolled once using a roller press, wherein the primary rolling is performed using a hot roller, the temperature of the hot roller is 75°C, and the rolling direction of the primary rolling is along Figure 2 In the A direction, the thickness of the pole piece after one rolling is 155μm;

[0066] (3) Secondary rolling: The electrode sheet after the first rolling is subjected to secondary rolling. The secondary rolling is performed using a cold roller. The temperature of the cold roller is 25°C. The rolling direction of the secondary rolling is the B direction opposite to the A direction. The thickness of the electrode sheet after the secondary rolling is 85 μm. After the rolling is completed, the negative electrode sheet is obtained by slitting.

[0067] Example 2

[0068] The preparation process of this embodiment is consistent with that of embodiment 1, except that the second rolling is performed by hot rolling, and the rolling temperature is the same as that of the first rolling.

[0069] Example 3

[0070] The preparation process of this embodiment is consistent with that of embodiment 1, except that the rolling temperature of the first rolling is 60° C., and the rolling temperature of the second rolling is 20° C.

[0071] Example 4

[0072] The preparation process of this embodiment is consistent with that of embodiment 1, except that the rolling temperature of the first rolling is 90° C., and the rolling temperature of the second rolling is 30° C.

[0073] Comparative Example 1

[0074] The preparation process of this comparative example is consistent with that of Example 1, except that the rolling temperature of the first rolling is 25° C., the rolling temperature of the second rolling is 25° C., and the rolling directions are all along the A direction.

[0075] Comparative Example 2

[0076] The preparation process of this comparative example is consistent with that of Example 1, except that the rolling temperature of the first rolling is 25°C, and the rolling temperature of the second rolling is 25°C.

[0077] Comparative Example 3

[0078] The preparation process of this comparative example is consistent with that of Example 1, except that the rolling temperature of the first rolling is 25° C., the rolling temperature of the second rolling is 75° C., and the rolling directions are all along the A direction.

[0079] Comparative Example 4

[0080] The preparation process of this comparative example is consistent with that of Example 1, except that the rolling temperature of the first rolling is 25° C., and the rolling temperature of the second rolling is 75° C.

[0081] Comparative Example 5

[0082] The preparation process of this comparative example is consistent with that of Example 1, except that the rolling temperature of the first rolling is 75° C., the rolling temperature of the second rolling is 25° C., and the rolling directions are all along the A direction.

[0083] Comparative Example 6

[0084] The preparation process of this comparative example is consistent with that of Example 1, except that the rolling temperature of the first rolling is 75° C., the rolling temperature of the second rolling is 75° C., and the rolling directions are all along the A direction.

[0085] The rebound rate and thickness deviation of the negative electrode sheets prepared in Examples 1 to 4 and Comparative Examples 1 to 6 were tested after 24 hours. The test results are shown in Table 1. The test method is as follows:

[0086] 1. Rebound rate test: Test the thickness of the electrode pieces of each embodiment and comparative example before standing, stand at room temperature for 24 hours, and test the thickness of the electrode pieces after standing. Rebound rate = (thickness after standing - thickness before standing) / thickness before standing.

[0087] 2. Thickness deviation test: The thickness of the pole pieces prepared in each embodiment and comparative example was tested respectively. During the test, 10 test points were randomly selected on each sample to test the thickness of the pole piece, where the difference between the maximum thickness and the minimum thickness was the thickness deviation of the pole piece.

[0088] 2. Effect of the secondary rolling coefficient P value on the negative electrode sheet and battery performance

[0089] Example 5

[0090] (1) Preparation of negative electrode sheet: artificial graphite of negative electrode material, Si, acetylene black as a conductive agent, sodium carboxymethyl cellulose (CMC) as a thickener, and styrene-butadiene rubber latex as a binder are mixed in a mass ratio of 89:8:1:1:1, deionized water is added, and the mixture is stirred in a vacuum mixer until it is stable and uniform to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on the first surface and the second surface of a copper foil having a thickness of 8 μm, and after drying, a first negative electrode active layer and a second negative electrode active layer are formed on the copper foil; it should be noted that after coating each surface, the copper foil is first dried at room temperature and then transferred to a 120°C forced air oven for drying for 1 hour;

[0091] The current collector coated with the negative electrode slurry is rolled once using a roller press, wherein the first rolling is performed by hot roller rolling, the temperature of the hot roller rolling is 75°C, and the rolling direction of the first rolling is along Figure 2In the A direction, the thickness of the electrode sheet after the first rolling is 235μm; the electrode sheet after the first rolling is rolled for the second time, and the second rolling is carried out by cold rolling. The temperature of the cold rolling is 25°C, and the rolling direction of the second rolling is the B direction opposite to the A direction. The thickness of the electrode sheet after the second rolling is 85μm; the rolling coefficient P = 2.8; after the rolling is completed, it is cut to obtain the negative electrode sheet.

[0092] (2) Preparation of positive electrode sheet:

[0093] The positive electrode active material is lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O 2 ), lithium supplement (Li 6 CoO 4 ), binder polyvinylidene fluoride, and conductive agent acetylene black are mixed in a mass ratio of 89:9:1:1, N-methylpyrrolidone is added, and the mixture is stirred until stable and uniform under the action of a vacuum stirrer to obtain a positive electrode slurry; the positive electrode slurry is uniformly coated on an aluminum foil with a thickness of 12 μm; the aluminum foil is dried at room temperature and then transferred to a 120°C forced air oven for drying for 1 hour, and then cold pressed and cut to obtain a positive electrode sheet.

[0094] (3) Preparation of electrolyte

[0095] In an argon atmosphere glove box with a water content of <10 ppm, EC, EMC and DEC were mixed in a mass ratio of 1:1:1 to obtain an organic solvent, and then the fully dried lithium salt LiPF 6 Dissolve in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / l.

[0096] (4) Preparation of diaphragm

[0097] A polyethylene (PE) porous polymer film is used as the separator.

[0098] (5) Preparation of secondary batteries

[0099] The positive electrode sheet, separator and negative electrode sheet prepared above are stacked or wound in sequence to obtain an electrode assembly, the electrode assembly is placed in a packaging shell, and after being fully baked to reduce the water content to below 450ppm, an electrolyte is injected, and a secondary battery is prepared through processes such as sealing and inspection.

[0100] Example 6

[0101] The preparation process of this embodiment is consistent with that of embodiment 5, except that: when preparing the negative electrode sheet, the thickness of the sheet after the first rolling is 185 μm; the thickness of the sheet after the second rolling is 85 μm; and the rolling coefficient P=2.2.

[0102] Example 7

[0103] The preparation process of this embodiment is consistent with that of embodiment 5, except that: when preparing the negative electrode sheet, the thickness of the sheet after the first rolling is 155 μm; the thickness of the sheet after the second rolling is 85 μm; and the rolling coefficient P=1.8.

[0104] Example 8

[0105] The preparation process of this embodiment is consistent with that of embodiment 5, except that: when preparing the negative electrode sheet, the thickness of the sheet after the first rolling is 110 μm; the thickness of the sheet after the second rolling is 85 μm; and the rolling coefficient P=1.3.

[0106] Comparative Example 7

[0107] The preparation process of this comparative example is consistent with that of Example 5, except that: when preparing the negative electrode sheet, the thickness of the sheet after the first rolling is 305 μm; the thickness of the sheet after the second rolling is 85 μm; and the rolling coefficient P=3.6.

[0108] Comparative Example 8

[0109] The preparation process of this comparative example is consistent with that of Example 5, except that: when preparing the negative electrode sheet, the thickness of the sheet after the first rolling is 255 μm; the thickness of the sheet after the second rolling is 85 μm; and the rolling coefficient P=3.0.

[0110] Comparative Example 9

[0111] The preparation process of this comparative example is consistent with that of Example 5, except that: when preparing the negative electrode sheet, the thickness of the single-layer negative electrode active layer after the first rolling is 105 μm; the thickness of the sheet after the second rolling is 85 μm; and the rolling coefficient P=1.2.

[0112] Comparative Example 10

[0113] The preparation process of this comparative example is consistent with that of Example 5, except that: when preparing the negative electrode sheet, the thickness of the single-layer negative electrode active layer after the first rolling is 95 μm; the thickness of the sheet after the second rolling is 85 μm; and the rolling coefficient P=1.1.

[0114] Comparative Example 11

[0115] The preparation process of this comparative example is consistent with that of Example 5, except that: when preparing the negative electrode pole piece, the thickness of the pole piece after the first rolling is 85 μm; the thickness of the pole piece after the second rolling is 85 μm; and the rolling coefficient P=1.0.

[0116] The secondary batteries prepared in Examples 5 to 8 and Comparative Examples 7 to 11 were subjected to cycle performance tests, and the secondary batteries after the cycles were disassembled to test the expansion rate of the negative electrode plates. The test results are shown in Table 2, and the test method is as follows:

[0117] 1. Cycle performance test: first adjust the temperature of the thermostat to 25℃, discharge at 1C constant current to 2.5V, rest for 30min; then charge at 0.5C constant current to 4.25V, rest for 30min. Repeat this cycle for 800 times, collect the charge and discharge capacity, and calculate the capacity retention rate.

[0118] Cycle capacity retention rate=(discharge capacity at the 800th cycle / discharge capacity at the first cycle)×100%.

[0119] 2. Expansion rate of negative electrode sheet: disassemble the battery after the cycle, test the thickness of the negative electrode sheet after the cycle, and calculate the expansion rate using the thickness of the negative electrode sheet before the cycle and the negative electrode sheet after the cycle.

[0120] Expansion rate = (thickness of negative electrode sheet after cycle - thickness of negative electrode sheet before cycle) / thickness of negative electrode sheet before cycle.

[0121] Table 1: Rebound rate and thickness deviation of negative electrode sheets prepared in Examples 1 to 4 and Comparative Examples 1 to 6

[0122]

[0123] It can be seen from Table 1 that in Examples 1 to 4, a hot roller (60°C to 90°C) is used for the first rolling, and the two rolling directions are opposite. A hot roller or a cold roller is used for the second rolling, which can reduce the static rebound rate of the electrode to about 2.9%; the thickness deviation is reduced to about 1.0 μm; considering the economy, the process of first hot rolling and then cold rolling can be adopted, combined with opposite rolling directions, which can effectively reduce the deformation of the active particles, reduce the microcracks of the electrode adhesive, and improve the adhesive performance.

[0124] Comparative Examples 1 to 4: Cold rollers (20-30°C) were used for rolling at one time. A higher rolling force was required to press the pole piece to the thickness and surface density required by the process and reduce damage to the foil and active particles due to excessive pressure. Therefore, the pole piece gaps and the upper and lower pore size distribution uniformity were poor, and the pole piece had a large rebound rate (>4%) after standing for 24 hours; at the same time, the thickness deviation was large (>2.0μm).

[0125] Comparative Examples 5 and 6: The first rolling uses a hot roller (60-90°C), which can overcome the inconsistent electrode thickness caused by the friction temperature rise of the cold roller. The electrode can be pressed to the required thickness and surface density using a lower rolling force. Therefore, the electrode gap and the upper and lower pore size distribution are more uniform, and the rebound rate of the electrode after standing for 24 hours is reduced to ~3.5%; however, since the secondary rolling and the primary rolling have the same rolling direction, the negative electrode active particles are deformed more in one direction, so the consistency of the electrode thickness is not reduced and the thickness deviation is still large (>2.0μm).

[0126] Table 2: Performance of Examples 5 to 8 and Comparative Examples 7 to 11

[0127]

[0128] It can be seen from Table 2 that in Examples 5 to 8, the secondary rolling coefficient P is between 1.3 and 2.8. At this time, the secondary rolling effect is better, the gaps and upper and lower pore size distribution of the negative electrode plate are uniform, the capacity is greatly improved after 800 cycles, the cycle stability is good, and the expansion rate of the negative electrode plate is low (all <16%).

[0129] Comparative Examples 7 and 8: The secondary rolling coefficient P≥3. At this time, the thickness of the primary rolling is much greater than the thickness of the secondary rolling. The gaps and upper and lower pore size distribution uniformity of the pole piece are poor. The capacity is slightly improved compared to the primary rolling technology, and the expansion of the negative pole piece is still serious (>20%).

[0130] Comparative Examples 9 and 10: The secondary rolling coefficient P≤1.2. At this time, the thickness of the first rolling is approximately equal to the thickness of the second rolling. The secondary rolling technology fails to work. The gaps of the pole piece and the upper and lower pore size distribution uniformity are poor, and the capacity is only slightly improved. The expansion of the negative pole piece is still serious (>20%).

[0131] Comparative Example 11: Rolling coefficient P = 1 (i.e., one-time rolling technology is used). At this time, the gap and upper and lower pore size distribution uniformity of the electrode sheet are the worst, and the capacity is the lowest; the negative electrode sheet expands most seriously (36%).

[0132] The relationship curve between the secondary rolling coefficient P value, the cycle capacity and the expansion rate of the negative electrode sheet is plotted according to the test results of Examples 5 to 8 and Comparative Examples 7 to 11. Figure 3 , where the solid line represents the relationship curve between the cycle capacity and the P value, and the dotted line represents the relationship curve between the expansion rate and the P value.

[0133] The present invention adopts a secondary rolling technology to prepare negative electrode sheets. By controlling the rolling directions of the two rollings to be opposite, the crushing and pulverization of negative electrode particles and binders can be reduced, so that the gaps of the sheets and the upper and lower pore sizes are more uniform, the static rebound of the sheets is smaller, the thickness consistency is better, the electrolyte infiltration effect is better, the resistance of the sheets is reduced, and the performance of the battery is improved. In addition, the first rolling uses a hot roller, which can overcome the inconsistent thickness of the sheets caused by the friction temperature rise of the cold roller. The sheets can be pressed to the required thickness and surface density using a lower rolling force, so the gaps of the sheets and the upper and lower pore sizes are more uniform. Combined with the secondary rolling direction, the deformation of active particles can be effectively reduced, the microcracks of the sheet binder can be reduced, the binder performance can be improved, and the battery cycle life can be improved; the ratio P of the sheet thickness after the first rolling to the sheet thickness after the second rolling is controlled to be between 1.3 and 2.8, which can ensure the uniformity of the upper and lower pore sizes of the negative electrode sheets, reduce the crushing and shedding of the negative electrode particles, improve the electrolyte wettability and the bonding force of the negative electrode sheets, and thus improve the battery cycle performance. Therefore, the present invention effectively overcomes some practical problems in the prior art and thus has high utilization value and use significance.

[0134] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A method for preparing a negative electrode sheet, It is characterized in that The following steps are involved: Applying slurry, coating the negative electrode slurry on at least one side of the negative electrode current collector to form a negative electrode active layer on the negative electrode current collector; Rolling once, rolling the negative electrode active layer once in a first direction; Secondary rolling, rolling the negative electrode active layer after the primary rolling for a second time in a second direction to obtain a negative electrode sheet; The first direction and the second direction are both along the length direction of the negative electrode current collector, and the second direction is opposite to the first direction.

2. The preparation method according to claim 1, It is characterized in that The first rolling process uses hot roller pressing, and the temperature of the hot roller pressing is 60°C to 90°C.

3. The preparation method according to claim 2, It is characterized in that The secondary rolling is performed by hot roller pressing or cold roller pressing. The temperature of the hot roller pressing is 60°C to 90°C, and the temperature of the cold roller pressing is 20°C to 30°C.

4. The preparation method according to claim 1, It is characterized in that The thickness of the pole piece after the first rolling is D1, and the thickness of the pole piece after the second rolling is D2. The second rolling coefficient is defined as P=D1 / D2, and P>1.

5. The preparation method according to claim 4, It is characterized in that The secondary rolling coefficient P is between 1.3 and 2.

8.

6. The preparation method according to claim 1, It is characterized in that The negative electrode current collector has a first surface and a second surface arranged opposite to each other in a thickness direction, and the slurry coating step comprises: coating the negative electrode slurry on the first surface of the negative electrode current collector, and forming a first negative electrode active layer after the negative electrode slurry is dried; turning over the negative electrode current collector, coating the negative electrode slurry on the second surface of the negative electrode current collector, and forming a second negative electrode active layer after the negative electrode current collector is dried.

7. The preparation method according to claim 6, It is characterized in that The one rolling includes: synchronously rolling the first active layer and the second active layer once along the first direction.

8. The preparation method according to claim 1, It is characterized in that The coating slurry also includes slurry configuration before the coating slurry, and the slurry configuration includes: mixing the negative electrode active material, the negative electrode conductive agent, the negative electrode binder and the negative electrode thickener, injecting the solvent and stirring evenly to obtain the negative electrode slurry; wherein the negative electrode active material includes a silicon-based negative electrode material and / or a graphite negative electrode material.

9. A secondary battery, It is characterized in that It comprises a positive electrode sheet and a negative electrode sheet, wherein the negative electrode sheet is prepared by the preparation method according to any one of claims 1 to 8.

10. An electronic device, It is characterized in that The electronic device comprises the secondary battery according to claim 9.

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