Secondary battery, method for manufacturing the same, and electric device

Abstract

A secondary battery includes an electrode tab including a current collector and an active material layer, the active material layer is provided on at least one surface of the current collector, the active material layer is divided into a first region and a second region in a thickness direction thereof, the first region is located in a direction close to the surface of the current collector, a thickness of the first region accounts for 1 / 2 of a thickness of the active material layer, a binder content of the first region is denoted as A, the second region is located in a direction away from the surface of the current collector, a thickness of the second region accounts for 1 / 2 of the thickness of the active material layer, a binder content of the second region is denoted as B, and the electrode tab satisfies 1≤B / A≤1.04.

Description

Technical Field

[0001] This application relates to the field of secondary batteries, specifically to a secondary battery and its preparation method and electrical device. Background Technology

[0002] With the increasing application and promotion of power batteries in electric vehicles, the demand for secondary batteries such as lithium-ion batteries continues to rise, placing higher demands on their cycle performance and kinetic performance. The fine structure of the electrode sheets directly affects the wetting effect of the electrolyte, thus influencing important battery performance characteristics such as internal resistance, rate performance, and cycle performance. Electrode sheets produced by traditional processes typically exhibit an uneven gradient material distribution. This uneven material distribution is particularly pronounced when the coating weight of the electrode sheets is increased to improve energy density, negatively impacting both the rate performance and cycle performance of the battery. Summary of the Invention

[0003] To address the aforementioned issues, this application provides a secondary battery, its preparation method, and an electrical device thereof, which can improve the cycle performance and kinetic performance of the secondary battery.

[0004] One aspect of this application provides a secondary battery, including an electrode sheet comprising: a current collector; and an active material layer disposed on at least one surface of the current collector; the active material layer is divided into a first region and a second region in its thickness direction; the first region is located in the direction close to the surface of the current collector, the thickness of the first region is 1 / 2 of the thickness of the active material layer, and the binder content of the first region is denoted as A; the second region is located in the direction away from the surface of the current collector, the thickness of the second region is 1 / 2 of the thickness of the active material layer, and the binder content of the first region is denoted as B; the electrode sheet satisfies: 1 < B / A ≤ 1.04.

[0005] The aforementioned secondary battery, by controlling the binder content of the electrode sheets to ensure that the electrode sheets satisfy: 1 ​​< B / A ≤ 1.04, can effectively improve the problem of binder enrichment on the surface of the active material layer, improve the surface porosity of the electrode sheets, and thus enhance the cycle performance and kinetic performance of the secondary battery.

[0006] In some embodiments, the electrode plates satisfy: 1.01≤B / A≤1.03.

[0007] In some embodiments, the thickness of the active material layer is 40 μm to 180 μm; optionally, the thickness of the active material layer is 60 μm to 100 μm.

[0008] In some embodiments, the porosity of the first region is 25% to 30%.

[0009] In some embodiments, the porosity of the second region is 25% to 30%.

[0010] In some embodiments, the porosity of the first region differs from that of the second region by ≤5%.

[0011] In some embodiments, the porosity of the first region is the same as that of the second region.

[0012] In some embodiments, the active material layer has self-supporting properties.

[0013] In some embodiments, the tensile strength of the active material layer is 0.03 MPa to 1 MPa; optionally, the tensile strength of the active material layer is 0.05 MPa to 0.5 MPa.

[0014] In some embodiments, the electrode sheet is a positive electrode sheet; in the active material layer, the binder includes at least one of polyvinylidene fluoride, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0015] In some embodiments, the electrode sheet is a negative electrode sheet; in the active material layer, the binder comprises a polymer having polar functional groups; optionally, the polar functional groups are selected from at least one of carboxyl, cyano, and amino groups.

[0016] In some embodiments, the polymer has a weight-average molecular weight ≥ 200,000; alternatively, the polymer has a weight-average molecular weight of 300,000 to 800,000.

[0017] In some embodiments, the polymer is selected from at least one of chain polymers and cross-linked network polymers.

[0018] In some embodiments, the polymer is selected from at least one of polyacrylonitrile, polyacrylic acid, polyamide, acrylic acid-acrylonitrile copolymer and its derivatives, acrylic acid-acrylate copolymer and its derivatives, and acrylic acid-acrylamide copolymer and its derivatives.

[0019] In some embodiments, the binder comprises 0.5% to 5% by mass in the active material layer.

[0020] In some embodiments, the active material layer further includes an additive, which includes at least one selected from polytetrafluoroethylene, polyethylene, polypropylene, and polyacrylonitrile; optionally, the additive has a mass percentage of ≤2%.

[0021] Secondly, this application also provides a method for preparing a secondary battery, including the following steps to prepare electrode sheets:

[0022] Prepare electrode slurry with a solid content of ≥65%;

[0023] The electrode slurry is extruded to form an active material layer; and

[0024] The active material layer is combined with a current collector to prepare the electrode sheet;

[0025] The active material layer is disposed on at least one surface of the current collector; the active material layer is divided into a first region and a second region in its thickness direction; the first region is located close to the surface of the current collector, the thickness of the first region is 1 / 2 of the thickness of the active material layer, and the binder content of the first region is denoted as A; the second region is located away from the surface of the current collector, the thickness of the second region is 1 / 2 of the thickness of the active material layer, and the binder content of the first region is denoted as B; the electrode sheet satisfies: 1 < B / A ≤ 1.04.

[0026] In some embodiments, the electrode sheet is a negative electrode sheet, and the solid content of the electrode slurry is 68% to 74%.

[0027] In some embodiments, the electrode sheet is a positive electrode sheet, and the solid content of the electrode slurry is 75% to 80%.

[0028] Thirdly, this application also provides an electrical device, which includes a secondary battery as described above or a secondary battery prepared by the method described above.

[0029] Details of one or more embodiments of this application are set forth in the following drawings and description, and other features, objects and advantages of this application will become apparent from the specification, drawings and claims. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of a secondary battery according to one embodiment of this application;

[0031] Figure 2 for Figure 1 An exploded view of a secondary battery according to an embodiment of this application is shown.

[0032] Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application;

[0033] Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application;

[0034] Figure 5 for Figure 4 An exploded view of a battery pack according to one embodiment of this application is shown;

[0035] Figure 6 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to an embodiment of this application.

[0036] Figure 7 This is a scanning electron microscope (SEM) image of the surface of the negative electrode sheet in Embodiment 1 of this application;

[0037] Figure 8 This is a scanning electron microscope (SEM) image of the surface of the negative electrode of Comparative Example 1 of this application;

[0038] Explanation of reference numerals in the attached figures:

[0039] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Housing; 52 Electrode assembly; 53 Cover plate; 6 Electrical device.

[0040] To better describe and illustrate embodiments and / or examples of the inventions disclosed herein, reference may be made to one or more accompanying drawings. Additional details or examples used to describe the drawings should not be considered as limiting the scope of any of the disclosed inventions, the currently described embodiments and / or examples, or the best mode of these inventions as currently understood. Detailed Implementation

[0041] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings. Preferred embodiments of this application are shown in the drawings. However, this application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of this application.

[0042] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0043] The microstructure of electrode sheets (such as surface porosity and binder distribution) directly affects the electrolyte wetting effect, thus influencing important battery performance characteristics such as internal resistance, rate performance, and cycle performance. In continuous efforts to improve energy density, increasing the weight of active material per unit area of ​​the electrode sheets (by ≥40%) can significantly reduce the amount of current collector and separator substrates used, thereby achieving increased energy density. However, increasing the weight of active material per unit area leads to an uneven gradient distribution of the electrode material in the longitudinal direction.

[0044] Through extensive research, the inventors discovered that during the drying process of the electrode sheet (especially near the transition point (the junction where the upper layer of the electrode sheet is dried while the lower layer is not), the upper solvent of the active material layer evaporates first, while the lower solvent rises rapidly. Therefore, the solvent carries small-molecule binders during its ascent, causing the binders to float due to the following reasons: (1) surface tension; (2) concentration gradient; (3) capillary action; (4) thermodynamic motion of solid particles; and (5) density difference between particles (particle settling causes density differences between the upper and lower layers). Furthermore, as the coating speed increases, the drying temperature needs to be further increased, further reducing the solvent surface tension. This leads to a further increase in the surface tension difference between the upper dried electrode active material layer and the solvent, making it easier for the solvent to migrate and spread upwards, and for the binders to float.

[0045] During the drying process of electrode sheets, the binder floats to the surface, leading to binder enrichment on the electrode sheet surface. After cold pressing, the electrode sheet surface tends to become denser. On the one hand, a dense electrode sheet surface makes electrolyte wetting difficult, severely affecting the production efficiency of secondary batteries. On the other hand, the low porosity of the electrode sheet surface makes it difficult for active ions to shuttle within the electrode sheet, reducing rate performance and low-temperature performance. Furthermore, if the electrode sheet has difficulty wetting, the insufficiently wetting areas cannot achieve electrochemical reactions, leading to abnormal battery performance and the risk of metal deposition of active materials (such as lithium plating) at the interface. Therefore, the microstructure of the electrode sheet directly affects the electrolyte wetting effect, thereby affecting important performance characteristics such as battery internal resistance, rate performance, and lifespan.

[0046] This application provides a secondary battery, and a battery module, battery pack, and electrical device using the secondary battery. This secondary battery is suitable for various battery-powered electrical devices, such as mobile phones, portable devices, laptops, electric vehicles, electric toys, power tools, electric cars, ships, and spacecraft, including aircraft, rockets, space shuttles, and spacecraft.

[0047] One embodiment of this application provides an electrode sheet, comprising: a current collector and an active material layer. The active material layer is disposed on at least one surface of the current collector; the active material layer is divided into a first region and a second region in its thickness direction. The first region is located in the direction close to the surface of the current collector, and the thickness of the first region accounts for 1 / 2 of the thickness of the active material layer, and the binder content of the first region is denoted as A; the second region is located in the direction away from the surface of the current collector, and the thickness of the second region accounts for 1 / 2 of the thickness of the active material layer, and the binder content of the first region is denoted as B; the electrode sheet satisfies: 1 < B / A ≤ 1.04.

[0048] The inventors discovered that in electrode sheets prepared by traditional processes, the binder exhibits a significantly uneven gradient distribution along the thickness direction of the active material layer, resulting in a large accumulation of binder on the electrode sheet surface, with a B / A value much greater than 1.04, which affects the electrochemical performance of the electrode sheet.

[0049] The electrode sheet of this application, by controlling the binder content of the electrode sheet to ensure that the electrode sheet satisfies: 1 < B / A ≤ 1.04, can effectively improve the problem of binder enrichment on the surface of the active material layer, improve the surface porosity of the electrode sheet, avoid the densification of the electrode sheet surface, and thus improve the cycle performance and kinetic performance of the secondary battery.

[0050] The B / A value mentioned above can be selected as 1.01, 1.02, 1.03, or 1.04. Furthermore, the electrode sheet satisfies: 1.01 ≤ B / A ≤ 1.03.

[0051] In some embodiments, the thickness of the single-sided active material layer is 40 μm to 180 μm. By controlling the B / A value of the electrode sheet within the above range, the thickness of the single-sided active material layer in the electrode sheet of the present application embodiment can reach 180 μm, which can avoid the problem of poor electrolyte wettability of the electrode sheet due to the increase in the thickness of the active material layer, thus obtaining a thick electrode sheet with high energy density. Optionally, the thickness of the active material layer is 40 μm to 60 μm, 60 μm to 80 μm, 80 μm to 100 μm, 100 μm to 120 μm, 120 μm to 150 μm, 150 μm to 160 μm, or 160 μm to 180 μm.

[0052] In some embodiments, the porosity of the first region of the active material layer is 25% to 30%. By controlling the B / A value of the electrode sheet within the above range, the higher porosity of the first region of the active material layer, i.e., the surface layer of the electrode sheet, is beneficial for electrolyte wetting and ion transport.

[0053] In some embodiments, the porosity of the second region of the active material layer is 25% to 30%. By controlling the B / A value of the electrode sheet within the above range, the porosity of the second region of the active material layer is closer to that of the first region, which is beneficial for electrolyte wetting and ion transport.

[0054] Porosity, as understood, refers to the percentage of pore volume within a material to its total volume. In this embodiment, the porosity of the first and second regions of the active material layer can be obtained by cross-sectional imaging of the active material layer using a scanning electron microscope (SEM) and analysis using AVIZO software.

[0055] In some embodiments, the porosity gradient decreases in the thickness direction of the active material layer, from the side of the active material layer closer to the current collector to the side of the active material layer farther from the current collector.

[0056] In some embodiments, the porosity of the first region of the active material layer differs from that of the second region of the active material layer by ≤5%. Optionally, the porosity difference between the first region of the active material layer and the second region of the active material layer is ≤5%, ≤4%, ≤3%, ≤2%, or ≤1%.

[0057] In some embodiments, the porosity of the first region of the active material layer is the same as that of the second region of the active material layer. In some embodiments, the porosity of the active material layer is the same along its thickness direction, from the side of the active material layer closest to the current collector to the side of the active material layer furthest from the current collector. In this case, the electrode surface layer has a high porosity, which can improve the electrolyte wettability and ion transport of the electrode, resulting in good kinetic performance of the electrode.

[0058] In some embodiments, the active material layer is self-supporting, which is beneficial for improving the cycle stability of the battery. Self-supporting means that the active material layer can maintain its mechanical integrity even in the absence of a current collector.

[0059] In some embodiments, the tensile strength of the active material layer is 0.03 MPa to 1 MPa. Optionally, the tensile strength of the active material layer is 0.03 MPa, 0.1 MPa, 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa, 0.6 MPa, 0.7 MPa, 0.8 MPa, 0.9 MPa, or 1 MPa. Preferably, the tensile strength of the active material layer is 0.05 MPa to 0.5 MPa. The above-mentioned active material layer has suitable tensile strength, resulting in better mechanical properties of the electrode sheet, reducing the likelihood of pulverization, breakage, or other deterioration of the electrode sheet, and thus improving the cycle stability of the electrode sheet.

[0060] In some embodiments, the electrode sheet is a positive electrode sheet; in the active material layer, the binder includes at least one selected from polyvinylidene fluoride (PVDF), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin. Further, the electrode sheet is a positive electrode sheet; in the active material layer, the binder includes polyvinylidene fluoride (PVDF).

[0061] In some embodiments, the electrode sheet is a negative electrode sheet; in the active material layer, the binder includes a polymer having polar functional groups.

[0062] In some embodiments, the polar functional group is selected from at least one of carboxyl, cyano, and amino groups. Polymers containing the above-mentioned polar functional groups have good adhesion, which is beneficial for the preparation of electrode sheets.

[0063] In some embodiments, the polymer has a weight-average molecular weight ≥ 200,000. Using a polymer with a weight-average molecular weight ≥ 200,000 as a binder can further improve the mechanical strength of the active material layer.

[0064] In some embodiments, the polymer is selected from at least one of chain polymers and cross-linked network polymers. By selecting chain polymers or cross-linked network polymers, the binder can wrap around and fix the active material layer material, further improving the binder flotation ratio of the electrode sheet and improving the cycle performance and kinetic performance of the electrode sheet.

[0065] In some embodiments, the polymer is selected from at least one of polyacrylonitrile (PAN), polyacrylic acid (PAA), polyamide (PA), acrylic acid-acrylonitrile copolymer and its derivatives, acrylic acid-acrylate copolymer and its derivatives, and acrylic acid-acrylamide copolymer and its derivatives. Further, the electrode is a negative electrode; in the active material layer, the binder is a polyacrylic acid-acrylonitrile copolymer.

[0066] In some embodiments, the adhesive content in the active material layer is 0.5% to 5% by mass. Optionally, the adhesive content in the active material layer is 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, or 5% by mass. By controlling the adhesive content within the above range, better bonding and molding effects can be achieved, and stronger adhesion between the active material layer and the current collector can be obtained.

[0067] In some embodiments, the active material layer further includes additives, including at least one selected from polytetrafluoroethylene (PTFE), polyethylene (PE), polypropylene (PP), and polyacrylonitrile (PAN). Optionally, the mass percentage of the additive in the active material layer is ≤2%. By adding the aforementioned specific additives, the uniformity of the component distribution in the active material layer can be further improved, which is beneficial to the preparation of the active material layer. Optionally, the mass percentage of the additive in the active material layer is 0.2% to 2%, for example, the mass percentage of the additive can be 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, or 2%.

[0068] An embodiment of this application also provides a method for preparing the above-mentioned electrode sheet, including the following steps S1 to S3.

[0069] Step S1: Prepare an electrode slurry with a solid content of ≥65%; optionally, the solid content of the electrode slurry is 65%, 70%, 75%, 80%, 85%, 90%, or 95%.

[0070] Step S2: Extrude the electrode slurry to form an active material layer.

[0071] Step S3: Combine the active material layer with the current collector to prepare the electrode sheet.

[0072] The above-mentioned method for preparing electrode sheets, by controlling the solid content of the electrode slurry to be greater than or equal to 65% and adopting a suitable preparation process, can effectively suppress the problem of binder floating during the preparation process. The electrode sheets satisfy 1 < B / A ≤ 1.04, and the prepared electrode sheets have good electrolyte wettability and ion conduction performance, which is beneficial to improving the cycle performance and kinetic performance of secondary batteries.

[0073] In some embodiments, the electrode sheet is a negative electrode sheet, and the solid content of the electrode slurry is 68% to 74%.

[0074] In some embodiments, the electrode sheet is a positive electrode sheet, and the solid content of the electrode slurry is 75% to 80%.

[0075] In some embodiments, the solids content of the electrode slurry, by weight percentage, includes: 0.5% to 5% binder and 0% to 2% additives.

[0076] In some embodiments, the electrode slurry is prepared by a kneading process. Specifically, in step S1, the preparation of the electrode slurry involves: mixing active materials, conductive agents, and additives to prepare a dry powder mixture; preparing a binder into a liquid; mixing the dry powder mixture and the liquid, and preparing a lumpy material with a solid content of 65% to 95% by a kneading process, which is the electrode slurry in the embodiments of this application.

[0077] In some embodiments, step S2 specifically involves: extruding the agglomerated material obtained in step S1 to form a slurry film layer; then rolling the slurry film layer to thin it out, thereby preparing an active material layer with a preset thickness and compaction density.

[0078] In some embodiments, the step of combining the active material layer with the current collector in step S3 can be achieved by rolling the active material layer onto the surface of the current collector, so that the active material layer and the current collector are bonded together to obtain the electrode sheet.

[0079] The secondary battery, battery module, battery pack, and power-consuming device of this application will be described below with appropriate reference to the accompanying drawings.

[0080] In one embodiment of this application, a secondary battery is provided.

[0081] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0082] In the embodiments of this application, the positive and / or negative electrode sheets of the secondary battery are the electrode sheets described above. Optionally, the electrode sheets are prepared by the preparation method described above.

[0083] Positive electrode sheet

[0084] The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer including a positive active material.

[0085] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0086] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy on the polymer substrate. The polymer substrate includes substrates such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0087] In some embodiments, the positive electrode active material may be a known positive electrode active material for batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0088] In some embodiments, the positive electrode active material layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0089] In some embodiments, the positive electrode active material layer may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0090] In some embodiments, the positive electrode uses the electrode sheet described above.

[0091] In some embodiments, the positive electrode can be prepared by the above-described method for preparing electrode sheets.

[0092] Negative electrode sheet

[0093] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, wherein the negative active material layer includes a negative active material.

[0094] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode active material layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0095] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material such as copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy on the polymer substrate. The polymer substrate includes substrates such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0096] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0097] In some embodiments, the negative electrode sheet uses the electrode sheet described above, and the binder comprises a polymer having polar functional groups. Optionally, the polymer has a weight-average molecular weight ≥ 200,000. Optionally, the polymer is selected from at least one of polyacrylonitrile (PAN), polyacrylic acid (PAA), polyamide (PA), acrylic acid-acrylonitrile copolymers and their derivatives, acrylic acid-acrylate copolymers and their derivatives, and acrylic acid-acrylamide copolymers and their derivatives.

[0098] Furthermore, the adhesive may also include at least one of styrene-butadiene rubber (SBR), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0099] In some embodiments, the negative electrode active material layer may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0100] In some embodiments, the negative electrode active material layer may also optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0101] In some embodiments, the negative electrode can be prepared by the above-described method for preparing electrode sheets.

[0102] electrolytes

[0103] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0104] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0105] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0106] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0107] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0108] Separating membrane

[0109] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0110] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0111] In some embodiments, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding process or a stacking process.

[0112] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0113] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The soft pack can be made of plastic, and examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0114] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 This is an example of a square-structured secondary battery 5.

[0115] In some of these embodiments, reference is made to Figure 2The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0116] In some embodiments, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0117] Figure 3 This is battery module 4, used as an example. (See reference...) Figure 3 In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0118] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0119] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0120] Figure 4 and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 4 and Figure 5 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0121] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc. Among them, mobile devices may be, for example, mobile phones, laptops, etc.; electric vehicles may be, for example, pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc., but are not limited to.

[0122] As an electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0123] Figure 6 This is an example of an electrical device 6. This electrical device 6 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery for this electrical device, a battery pack or battery module can be used.

[0124] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0125] Example

[0126] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0127] Example 1:

[0128] Preparation of positive electrode sheet:

[0129] Lithium iron phosphate (LiFePO4), PVDF (polyvinylidene fluoride), and SP (conductive carbon black) were dispersed in an NMP (N-methylpyrrolidone) solvent system at a mass ratio of 95:3:2 and thoroughly mixed to obtain a positive electrode slurry with a solid content of 65%. The positive electrode slurry was then coated onto both surfaces of a current collector aluminum foil using extrusion coating or transfer coating. The positive electrode sheet was obtained through processes such as rolling, drying, and cold pressing. The thickness of the positive electrode active material layer on one side was 106 μm.

[0130] Negative electrode preparation:

[0131] In the negative electrode active material layer, the mass ratio of artificial graphite, acrylic acid-acrylonitrile copolymer and SP (conductive carbon black) is 95:3:2.

[0132] Artificial graphite, the negative electrode active material, is dry-mixed with SP (conductive carbon black) in a ribbon mixer to obtain a well-mixed powder. Acrylic acid-acrylonitrile copolymer and deionized water are mixed to prepare a slurry. The powder and slurry are added to a kneader and kneaded for 30 minutes to obtain a negative electrode slurry with a solid content of 65%. The positive electrode slurry is extruded through an extruder to form a negative electrode active material layer film. The film is then thinned by roller pressing to obtain a negative electrode active material layer of a predetermined thickness. The edges of the film layer are removed using a cutter, and the film is transferred onto both surfaces of a current collector copper foil using pressure. After drying, a negative electrode sheet is obtained. The thickness of the negative electrode active material layer on one side is 79 μm.

[0133] See Figure 7 The figure shows a scanning electron microscope (SEM) image of the surface of the negative electrode sheet of Example 1. As can be seen from the figure, the surface of the negative electrode sheet prepared in Example 1 is rich in pores.

[0134] Separating membrane: PE porous polymer film is used as the separating membrane.

[0135] Electrolyte: In an argon atmosphere glove box with a water content of <10ppm, equal volumes of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a volume ratio of 3:7 to obtain an organic solvent. Then, 1 mol / L of LiPF6 is uniformly dissolved in the organic solvent to obtain the electrolyte.

[0136] Secondary battery fabrication:

[0137] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrode assembly is then wound up and placed in an outer package. After drying, the electrolyte is injected. The battery is then formed and left to stand to obtain a secondary battery.

[0138] The preparation methods of Examples 2 to 5 are similar to those of Example 1, except that the solid content of the negative electrode slurry is adjusted so that the B / A ratio is within the scope of this application, as detailed in Table 1.

[0139] Example 6:

[0140] Preparation of positive electrode sheet:

[0141] In the positive electrode active material layer, lithium iron phosphate, PVDF (polyvinylidene fluoride), and SP (conductive carbon black) are in a mass ratio of 95:3:2.

[0142] Lithium iron phosphate (LiFePO4) and SP (conductive carbon black) were dry-mixed in a ribbon mixer to obtain a well-mixed powder. PVDF (polyvinylidene fluoride) was dissolved in NMP (N-methylpyrrolidone) to prepare a binder solution. The powder and binder solution were added to a kneader and kneaded for 30 minutes to obtain a positive electrode slurry with a solid content of 75%. The positive electrode slurry was extruded into a positive electrode active material layer film using an extruder. The film was then thinned by roller pressing to obtain a positive electrode active material layer of a predetermined thickness. The edges of the film layer were removed using a cutter, and the film was transferred onto both surfaces of a current collector aluminum foil using pressure. After drying, a positive electrode sheet was obtained. The thickness of the positive electrode active material layer on one side was 106 μm.

[0143] Negative electrode preparation:

[0144] Artificial graphite, acrylic-acrylonitrile copolymer, and SP (conductive carbon black) were dispersed in deionized water at a mass ratio of 95:3:2 and thoroughly mixed to obtain a negative electrode slurry with a solid content of 55%. The positive electrode slurry was coated onto both surfaces of the current collector copper foil using extrusion coating or transfer coating. The negative electrode sheet was obtained through processes such as rolling, drying, and cold pressing. The thickness of the negative electrode active material layer on one side was 79 μm.

[0145] The preparation methods of Examples 7 to 12 are similar to those of Example 1, except that the solid content of the positive electrode slurry is adjusted so that the B / A ratio is within the scope of this application, as detailed in Table 1.

[0146] Example 13:

[0147] Example 13 is similar to the method in Example 3, except that:

[0148] Negative electrode preparation: The mass ratio of artificial graphite, acrylic acid-acrylonitrile copolymer, SP (conductive carbon black), and PTFE (polytetrafluoroethylene) is 94.8:3:2:0.2. The negative electrode active material, artificial graphite, is dry-mixed with SP (conductive carbon black) and PTFE (polytetrafluoroethylene) in a ribbon mixer to obtain a well-mixed powder.

[0149] Example 14:

[0150] Example 14 is similar to the method in Example 8, except that:

[0151] Positive electrode preparation: The mass ratio of lithium iron phosphate, PVDF (polyvinylidene fluoride), SP (conductive carbon black), and PTFE (polytetrafluoroethylene) is 94.8:3:2:0.2. The positive electrode active material, lithium iron phosphate, is dry-mixed with SP and PTFE in a ribbon mixer to obtain a well-mixed powder.

[0152] Comparative Example 1:

[0153] The difference between Comparative Example 1 and Example 1 lies in the preparation of the negative electrode sheet.

[0154] Negative electrode sheet: The negative electrode active materials, artificial graphite, acrylic acid-acrylonitrile copolymer, and SP (conductive carbon black) are dispersed in deionized water at a mass ratio of 95:3:2 and thoroughly mixed to obtain a negative electrode slurry with a solid content of 55%. The negative electrode slurry is coated onto the current collector copper foil using extrusion coating or transfer coating, and then processed through rolling, drying, and cold pressing to obtain the negative electrode sheet. The thickness of the negative electrode active material layer on one side is 79 μm.

[0155] See Figure 8 The figure shows a scanning electron microscope (SEM) image of the negative electrode sheet of Comparative Example 1. As can be seen from the figure, the surface of the negative electrode sheet prepared in Comparative Example 1 is dense and has fewer pores.

[0156] Test section:

[0157] (1) Adhesive content test:

[0158] 10–30 mg of powder was scraped from the outer surface (away from the current collector) of the active material layer of the electrode sheet, covering an area of ​​1 / 2 the thickness of the active material layer, as the second region sample; 10–30 mg of powder was scraped from the inner surface (closer to the current collector) of the active material layer of the electrode sheet, covering an area of ​​1 / 2 the thickness of the active material layer, as the first region sample; the samples were placed in an Al2O3 crucible, spread evenly, and the crucible lid was closed. Differential scanning thermogravimetric analysis was performed, with the following parameters: nitrogen atmosphere, purge gas flow rate 60 mL / min, protective gas flow rate 20 mL / min, and a heating rate of 10–30 °C within the temperature range of 35–600 °C. The difference in sample mass before and after heating was evaluated using an electronic balance with an accuracy of 0.01%, and the weight loss rate as a function of temperature was obtained. This was used to determine the binder content of the sample (the first region is denoted as A, and the second region as B), and then the B / A value of the electrode sheet could be calculated.

[0159] (2) Electrode porosity test:

[0160] Electrode sheets prepared in each embodiment and comparative example were used to perform three-dimensional reconstruction of the active material layer by slicing using a dual-beam focused ion beam microscope (FIB-SEM Zeiss Crossbeam 540). The sample was sliced ​​by focused ion beam (FIB), and then the surface morphology data of the slices was collected by scanning electron microscope (SEM). Three-dimensional reconstruction was performed using Avizo software, and the porosity information of the first and second regions was extracted using the Volume Fraction calculation module.

[0161] (3) Tensile strength test: The Instron metal tensile testing machine was used for the test. The specific steps included: cutting the active material layer of the electrode sheet into a test sample with a length of 100 mm, a width of 20 mm, and a thickness of 1 mm, and using the tensile testing machine to test the tensile strength of the sample at a tension of 2 mm / min.

[0162] (4) Cyclic performance test:

[0163] At 25°C, the secondary batteries prepared in the examples and comparative examples were charged at a constant current rate of 1C to the charging cutoff voltage of 3.6V, then charged at a constant voltage until the current ≤0.05C, allowed to stand for 5 minutes, and then discharged at a constant current rate of 0.33C to the discharge cutoff voltage of 2.5V, allowed to stand for 5 minutes. This constitutes one charge-discharge cycle. The batteries were subjected to cyclic charge-discharge tests according to this method until the battery capacity decreased to 80%. The number of cycles at this point is the cycle life of the battery at 25°C.

[0164] (5) Charge / discharge rate performance test:

[0165] 2C charging capacity retention: At 25°C, the secondary batteries of each embodiment and comparative example were charged at a constant current rate of 0.33C to 3.60V, then charged at a constant voltage rate to a current of 0.05C, and allowed to stand for 5 minutes. The charging capacity at this point was recorded as the initial charging capacity. After standing for 5 minutes, the batteries were discharged at a constant current rate of 0.33C to 2.5V and allowed to stand for 30 minutes. Then, the secondary batteries were charged at a constant current rate of 2C to 3.6V, then charged at a constant voltage rate to a current of 0.05C, and allowed to stand for 5 minutes. The charging capacity at this point was recorded.

[0166] Battery capacity retention rate (%) at 2C charging rate = 2C charging capacity / 0.33C charging capacity × 100%.

[0167] 2C discharge capacity retention: At 25°C, the secondary batteries of each embodiment and comparative example were discharged to 2.5V at a rate of 0.33C, then charged to 3.60V with a constant current, and then switched to constant voltage charging. When the charging current decreased to 0.05C, the constant voltage charging ended. After resting for 5 minutes, the batteries were discharged at 0.33C to 2.5V, and the discharge capacity at this point was recorded as the initial discharge capacity. After resting for 30 minutes, the batteries were charged to 3.60V with a constant current of 0.33C, and then switched to constant voltage charging. When the charging current decreased to 0.05C, the constant voltage charging ended. After resting for 5 minutes, the batteries were discharged at 2C to 2.5V, and the discharge capacity at this point was recorded.

[0168] Battery capacity retention rate (%) at 2C discharge rate = 2C discharge capacity / 0.33C discharge capacity × 100%.

[0169] Table 1 shows the slurry solid content, B / A value, cycle performance, and charge / discharge performance of the electrode sheets in Examples 1-14 and Comparative Example 1.

[0170]

[0171] As can be seen from the relevant data in Table 1, in the secondary battery of Comparative Example 1, both the positive and negative electrode sheets were prepared using the traditional wet coating method, and the B / A values ​​of the positive and negative electrode sheets were 1.08 and 1.15, respectively. The secondary battery of Comparative Example 1 had 853 cycle times, a discharge capacity retention rate of 66% at 2C, and a charge capacity retention rate of 54% at 2C. In the secondary batteries of Examples 1 to 14, the positive or negative electrode sheets used were the electrode sheets provided in this application, and the electrode sheets satisfied 1 < B / A ≤ 1.04. The secondary batteries had 873 to 1298 cycle times, a discharge capacity retention rate of 67% to 72% at 2C, and a charge capacity retention rate of 56% to 64% at 2C. The cycle performance and charge / discharge performance of the secondary batteries were improved compared to the secondary battery of Comparative Example 1.

[0172] As can be seen from Examples 1 to 12, increasing the solid content of the electrode slurry within the range of 65% to 95% can reduce the B / A value of the electrode. When the B / A value of the electrode is between 1.01 and 1.03, the cycle performance and charge / discharge performance of the secondary battery are better.

[0173] The negative electrode sheet of Example 13 and the positive electrode sheet of Example 14 contain a small amount of PTFE. The tensile strength of the slurry is good during the preparation process, which is beneficial to the extrusion molding of the slurry. Furthermore, the cycle performance and charge-discharge performance of the secondary batteries of Examples 13 and 14 are both good.

[0174] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0175] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A method for preparing a secondary battery, comprising the following steps: preparing electrode sheets: Prepare electrode slurry with a solid content of ≥65%; The electrode slurry is extruded to form an active material layer; and The active material layer is combined with a current collector to prepare the electrode sheet; The active material layer is disposed on at least one surface of the current collector; the active material layer is divided into a first region and a second region in its thickness direction; The first region is located in the direction close to the surface of the current collector, the thickness of the first region is 1 / 2 of the thickness of the active material layer, and the binder content of the first region is denoted as A; The second region is located away from the surface of the current collector, and the thickness of the second region is 1 / 2 of the thickness of the active material layer. The binder content of the first region is denoted as B. The electrode plates satisfy the following condition: 1.01 ≤ B / A ≤ 1.04; The electrode sheet is a negative electrode sheet.

2. The method for preparing a secondary battery according to claim 1, wherein, The solid content of the electrode slurry is 68%~74%.

3. The method for preparing a secondary battery according to claim 1, wherein, The electrode plates satisfy the following condition: 1.01≤B / A≤1.

03.

4. The method for preparing a secondary battery according to claim 1, wherein, The thickness of the active material layer is 40 μm to 180 μm.

5. The method for preparing a secondary battery according to claim 4, wherein, The thickness of the active material layer is 60 μm to 100 μm.

6. The method for preparing a secondary battery according to claim 1, wherein, The porosity of the first region is 25%~30%; and / or, The porosity of the second region is 25% to 30%.

7. The method for preparing a secondary battery according to claim 6, wherein, The porosity of the first region differs from that of the second region by ≤5%.

8. The method for preparing a secondary battery according to claim 7, wherein, The porosity of the first region is the same as that of the second region.

9. The method for preparing a secondary battery according to claim 1, wherein, The active material layer has self-supporting properties.

10. The method for preparing a secondary battery according to any one of claims 1 to 9, wherein, The tensile strength of the active material layer is 0.03 MPa to 1 MPa.

11. The method for preparing a secondary battery according to claim 10, wherein, The tensile strength of the active material layer is 0.05 MPa to 0.5 MPa.

12. The method for preparing a secondary battery according to any one of claims 1 to 9, wherein, In the active material layer, the binder comprises a polymer having polar functional groups.

13. The method for preparing a secondary battery according to claim 12, wherein, The polar functional group is selected from at least one of carboxyl, cyano, and amino groups.

14. The method for preparing a secondary battery according to claim 12, wherein, The weight-average molecular weight of the polymer is ≥200,000.

15. The method for preparing a secondary battery according to claim 14, wherein, The polymer has a weight-average molecular weight of 300,000 to 800,000.

16. The method for preparing a secondary battery according to claim 12, wherein, The polymer is selected from at least one of chain polymers and cross-linked network polymers.

17. The method for preparing a secondary battery according to any one of claims 13 to 16, wherein, The polymer is selected from at least one of polyacrylonitrile, polyacrylic acid, polyamide, acrylic acid-acrylonitrile copolymer and its derivatives, acrylic acid-acrylate copolymer and its derivatives, and acrylic acid-acrylamide copolymer and its derivatives.

18. The method for preparing a secondary battery according to any one of claims 1 to 9, wherein, In the active material layer, the adhesive has a mass percentage of 0.5% to 5%.

19. The method for preparing a secondary battery according to any one of claims 1 to 9, wherein, The active material layer also includes additives, which include at least one of polytetrafluoroethylene, polyethylene, polypropylene, and polyacrylonitrile.

20. The method for preparing a secondary battery according to claim 19, wherein, The additive has a mass percentage of ≤2%.

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