Preparation method of negative pole piece and negative pole piece

Abstract

The invention belongs to the technical field of battery manufacturing, and provides a preparation method of a negative pole piece and the negative pole piece. The preparation method comprises the following steps: sequentially coating the surface of a thinned area of a negative electrode substrate with ion conduction layer slurry and bonding layer slurry, and coating the bonding layer slurry when an ion conduction layer wet film is not completely dried; after heat treatment, a functional layer is integrally formed on the surface of the thinned area, the outer surface of the functional layer is flush with the main body area, and the interior of the functional layer comprises an ion conduction layer, a bonding layer and a transition structure interpenetrating at the interface of the ion conduction layer and the bonding layer. The negative pole piece comprises the functional layer. According to the invention, through a wet film superposition process and heat treatment, an integrated functional layer with an efficient ion transmission channel and a strong mechanical bonding interface is constructed in the thinned area, the problems of nonuniform ion transmission and weak interface bonding in the area are solved, local lithium precipitation and interface stripping are inhibited, the cycle life of the battery is effectively prolonged, and the safety of the battery is effectively improved.

Description

Technical Field

[0001] This application belongs to the field of battery technology and relates to a method for preparing a negative electrode sheet and the negative electrode sheet itself. Background Technology

[0002] With the development of new energy vehicles, energy storage systems, and other fields, the demand for high-energy-density and high-safety lithium-ion batteries is becoming increasingly urgent. During lithium battery manufacturing, to adapt to tab welding or winding processes and alleviate the thick edge phenomenon of coating, a thinning zone is usually set in the area of ​​the negative electrode sheet near the tab. Due to the reduced thickness of the active material layer and changes in the pore structure, the thinned zone is prone to problems such as insufficient electrolyte wetting during actual battery operation, leading to localized lithium plating and affecting the overall performance of the battery. Summary of the Invention

[0003] In view of the problems existing in the prior art, the purpose of this application is to provide a method for preparing a negative electrode sheet and a negative electrode sheet, so as to solve the technical contradiction that it is difficult to balance the interface ion transport and mechanical bonding in the thinned region, and effectively improve the interface stability and long cycle life of the battery.

[0004] To achieve this objective, the following technical solution is adopted in this application:

[0005] In a first aspect, some embodiments of this application provide a method for preparing a negative electrode sheet, comprising the following steps:

[0006] A negative electrode substrate is provided, the negative electrode substrate including a current collector, an active material layer disposed on at least one surface of the current collector and a blank area not covered by the active material layer; the active material layer includes a main region and a thinned region with a thickness less than the main region, and the thinned region is disposed between the main region and the blank area;

[0007] Provide ion-conducting layer slurry and adhesive layer slurry;

[0008] The ion-conducting layer slurry is applied to the surface of the active material layer in the thinned area to form a wet film of the ion-conducting layer.

[0009] Before the ion-conducting layer wet film is completely dry, the adhesive layer slurry is applied to its surface to form an adhesive layer wet film;

[0010] Heat treatment is performed on the electrode sheet covered with a wet film;

[0011] By controlling the coating amount of the ion-conducting layer slurry and / or the adhesive layer slurry, and through the heat treatment, a functional layer is formed on the surface of the thinned area; the outer surface of the functional layer is flush with the outer surface of the active material layer of the main body area, and the functional layer includes an ion-conducting layer, an adhesive layer, and a transition structure formed by mutual penetration of the two at the interface.

[0012] In some embodiments, by controlling the coating amount of the ion-conducting layer slurry and / or the adhesive layer slurry, the designed thickness ratio of the ion-conducting layer to the adhesive layer is (1.5~3):1, and the thickness of the transition structure accounts for 1%~8% of the total thickness of the functional layer.

[0013] In some embodiments, the preparation method further includes pre-activating the active material layer of the thinned region before applying the ion-conducting layer slurry; and / or, the activation pre-treatment includes plasma treatment for 30s to 60s.

[0014] In some embodiments, the ion-conducting layer slurry includes a polymer matrix, a first lithium salt, nano-inorganic fillers, and a first solvent; based on the total mass of the ion-conducting layer slurry as 100%, the polymer matrix accounts for 10% to 20% of the mass, the first lithium salt accounts for 5% to 12% of the mass, the nano-inorganic fillers account for 3% to 8% of the mass, and the first solvent accounts for 60% to 80% of the mass.

[0015] In some embodiments, the adhesive layer slurry includes an adhesive, a second lithium salt, and a second solvent; based on the total mass of the adhesive layer slurry being 100%, the adhesive accounts for 8% to 15% of the mass, the second lithium salt accounts for 5% to 10% of the mass, and the second solvent accounts for 75% to 85% of the mass.

[0016] In some embodiments, the first solvent and the second solvent each independently comprise carbonate solvents.

[0017] In some embodiments, the first solvent is the same as the second solvent.

[0018] In some embodiments, both the first solvent and the second solvent are mixed solvents composed of ethylene carbonate, diethyl carbonate and dimethyl carbonate in a volume ratio of 1:(0.7~2.5):(0.3~1.5).

[0019] In some embodiments, the heat treatment method includes first drying at 40°C to 50°C for 3 to 5 minutes; then drying at 60°C to 70°C for 8 to 12 minutes.

[0020] The method for preparing the negative electrode sheet in this application achieves integrated formation of a functional layer on the surface of the thinned area by controlling the amount of slurry coating and employing a process of heat treatment after wet film stacking. This process utilizes the mutual solubility of the wet film interface and the interpenetration of components under heat treatment to naturally form a transitional structure in which the ion-conducting layer and the adhesive layer permeate each other, while ensuring that the surface of the functional layer is flush with the main body area. The process has good controllability and high repeatability.

[0021] Secondly, some embodiments of this application provide a negative electrode sheet.

[0022] The negative electrode sheet includes a current collector; an active material layer and a blank area not covered by the active material layer are disposed on at least one side surface of the current collector; the active material layer includes a main region and a thinned region with a thickness less than the main region; and the thinned region is disposed between the main region and the blank area;

[0023] A functional layer is disposed on the surface of the active material layer in the thinned area away from the current collector, and the outer surface of the functional layer is flush with the outer surface of the active material layer in the main body area.

[0024] The functional layer includes an ion-conducting layer near the active material layer and an adhesive layer disposed on the side of the ion-conducting layer away from the active material layer; the ion-conducting layer and the adhesive layer have a mutually penetrating transition structure, wherein the thickness of the transition structure accounts for 1% to 8% of the total thickness of the functional layer.

[0025] The negative electrode sheet of this application incorporates an integrated functional layer in the thinned region, with an internally interpenetrating transition structure that achieves a strong interfacial bond between the ion-conducting layer and the bonding layer. This structure, on the one hand, creates a continuous and rapid ion transport channel in the thinned region, optimizing the lithium-ion flow distribution; on the other hand, it transforms the traditional rigid interlayer interface into a stable whole with continuously and gradually changing mechanical properties. During battery cycling, this structure effectively absorbs and disperses the stress generated by the volume expansion of the active material, avoiding the risk of interlayer delamination or cracking, thus providing a durable and robust mechanical bonding interface between the electrode sheet and the separator.

[0026] Compared with existing technical solutions, this application has at least the following beneficial effects:

[0027] The negative electrode preparation method provided in this application establishes a specific functional layer in the thinned region. The ion-conducting layer within this functional layer forms a strong interfacial bond with the active material layer, ensuring close contact between the ion-conducting layer and the active material particles. This creates a rapid and uniform ion transport channel in the thinned region, optimizing lithium-ion insertion / extraction behavior, suppressing lithium plating caused by localized lithium-ion aggregation, and preventing interfacial peeling and ion transport path interruption due to active material volume expansion during long-term cycling. Simultaneously, the adhesive layer on the surface of the ion-conducting layer can directly contact the battery separator, eliminating the physical gap between the traditional thinned region and the separator, achieving better initial contact, and shortening the electrolyte wetting time in the thinned region. In the subsequent hot-pressing process of battery assembly, the adhesive layer softens upon heating and forms a tighter bond with the separator, enhancing the interfacial adhesion between the electrode thinning region and the separator, suppressing interfacial peeling caused by volume changes during long-term cycling, thereby stabilizing the ion transport path and effectively extending battery cycle life. Attached Figure Description

[0028] Figure 1 This is a schematic flowchart of the method for preparing the negative electrode sheet in the embodiments of this application.

[0029] Figure 2 This is a partial cross-sectional structural diagram of the negative electrode sheet in an embodiment of this application.

[0030] 1-Current collector, 2-Active material layer, 3-Blank area, 4-Main area, 5-Thinned area, 6-Ion conduction layer, 7-Binder layer, 8-Transition structure. Detailed Implementation

[0031] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application are described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0032] 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 in the specification of this application 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.

[0033] Please see Figure 1 , Figure 1 This is a flowchart illustrating a method for preparing an electrode according to an embodiment of this application. Figure 1 As shown, the preparation method of the electrode includes the following steps:

[0034] S10: Provide a negative electrode substrate, the negative electrode substrate including a current collector, an active material layer disposed on at least one side surface of the current collector and a blank area not covered by the active material layer; the active material layer includes a main region and a thinned region with a thickness less than the main region, and the thinned region is disposed between the main region and the blank area.

[0035] In step S10, the current collector is a conductive substrate known in the art for use as a negative electrode of a lithium-ion battery, such as copper foil, copper alloy foil, three-dimensional porous copper current collector, or copper composite current collector, etc. This application does not limit it.

[0036] In some embodiments of this application, the active material layer may include a negative electrode active material, which may be selected from one or more of graphite, soft carbon, hard carbon, silicon-based materials (such as elemental silicon, silicon oxide, silicon-carbon composites), tin-based materials, lithium titanate, and their composites. In addition, the active material layer typically also includes a conductive agent (such as conductive carbon black, carbon nanotubes, graphene) and a binder (such as CMC, SBR, polyacrylic acid).

[0037] In some embodiments of this application, the active material layers in the main body region and the thinned region may have the same composition and ratio. In this case, because the thickness of the active material layer in the thinned region is reduced, its areal density is less than that in the main body region. The blank area is the region where the current collector is completely exposed, and is typically used for subsequent tab welding.

[0038] It should be noted that the thinning region is typically a thickness reduction area set along the length of the electrode, on one or both edges of the electrode width, to accommodate electrode tab welding, winding processes, or to alleviate coating edge effects. The thinning region can be formed using processes known in the art, such as laser etching, precision die-cutting, and mask coating. In this application, "thickness less than" means that the average thickness of the active material layer in the thinning region is less than that of the main body region. Furthermore, this application does not specifically limit the outline or cross-sectional shape of the thinning region; it can be designed as a straight line, an arc, a stepped shape, or other gradually changing shapes according to actual needs.

[0039] In some embodiments of this application, the preparation method further includes pre-activating the active material layer of the thinned area before applying the ion-conducting layer slurry.

[0040] In some embodiments of this application, the activation pretreatment includes plasma treatment for a duration of 30 to 60 seconds. Exemplarily, the treatment time can be 30 seconds, 35 seconds, 40 seconds, 50 seconds, 55 seconds, or 60 seconds, etc. This treatment effectively cleans the surface, increases surface energy, and introduces active groups, which is beneficial for the subsequent spreading and wetting of the ion-conducting layer slurry and the formation of stronger interfacial bonding.

[0041] S20: Provides ion-conducting layer slurry and adhesive layer slurry;

[0042] In some embodiments of this application, the ion-conducting layer slurry includes a polymer matrix, a first lithium salt, nano-inorganic fillers, and a first solvent. The first lithium salt is dissolved in the polymer matrix, providing freely migrating lithium-ion carriers; the nano-inorganic fillers can construct high-speed lithium-ion transport channels within the layer, promote the dissociation of the lithium salt, and enhance the mechanical strength and thermal stability of the composite layer. Thus, an interface layer with both high intrinsic ionic conductivity and good structural integrity is constructed on the surface of the thinned region before contact with the electrolyte. With the total mass of the ion-conducting layer slurry as 100%, the polymer matrix accounts for 10% to 20% of the mass, the first lithium salt accounts for 5% to 12% of the mass, the nano-inorganic filler accounts for 3% to 8% of the mass, and the first solvent accounts for 60% to 80% of the mass. For example, the mass percentage of the polymer matrix can be 10%, 12%, 15%, 18%, or 20%, etc.; the mass percentage of the first lithium salt can be 5%, 7%, 9%, 11%, or 12%, etc.; the mass percentage of the nano-inorganic filler can be 3%, 4%, 6%, 7%, or 8%, etc.; correspondingly, the mass percentage of the first solvent is the balance, for example, under the above exemplary combination, its percentage can be a corresponding value of 70% to 82%.

[0043] In some embodiments of this application, the polymer matrix includes at least one of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyethylene oxide (PEO) and its derivatives, polyacrylonitrile (PAN)-based polymers, polymethyl methacrylate-ethylene oxide copolymer (PMMA-co-PEO), and polycarbonate-based polymers (such as polypropylene carbonate, PPC); preferably, it includes PVDF-HFP. The polymer matrix constitutes the framework of the ion-conducting layer, mainly providing structural support and serving as a medium for ion transport. PVDF-HFP is preferred because it has good chemical stability, film-forming properties, and compatibility with the electrolyte.

[0044] In some embodiments of this application, the nano-inorganic filler is nanoscale, which facilitates its uniform dispersion in the polymer matrix and the construction of a continuous ion transport network. For example, the average particle size of the filler can be selected in the range of 50 nm to 100 nm. The nano-inorganic filler includes at least one of fast ion conductor fillers, dielectric constant fillers, or layered fillers; the fast ion conductor filler includes at least one of garnet-type, NASICON-type, or sulfide-type lithium-ion conductors; the dielectric constant filler includes at least one of alumina, silica, titanium dioxide, or zirconium oxide; and the layered filler includes montmorillonite and / or boron nitride.

[0045] In some embodiments of this application, the first lithium salt includes at least one of Li2O (lithium oxide), Li2O2 (lithium peroxide), Li2S (lithium sulfide), Li3N (lithium nitride), Li2Se (lithium selenide), Li2CO3 (lithium carbonate), Li5FeO4 (LFO), Li6CoO4 (LCO), Li2NiO2, Li6MnO4, Li6ZnO4 (LZO), Li2C2O4 (lithium oxalate), Li2C4O4 (dilithium cyclobutanetetracarboxylate), LiC2O2, Li2C3O5, Li2C4O6, LiDFOB (lithium difluorooxoborate), LiBOB (lithium dioxoborate), LiDFP (lithium difluorophosphate), LiPO2F2 (lithium difluorophosphate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), or LiPF2O2 (lithium difluorodioxophosphate).

[0046] In some embodiments of this application, the adhesive layer slurry includes an adhesive, a second lithium salt, and a second solvent; based on the total mass of the adhesive layer slurry as 100%, the mass percentage of the adhesive is 8% to 15%, the mass percentage of the second lithium salt is 5% to 10%, and the mass percentage of the second solvent is 75% to 85%; for example, the mass percentage of the adhesive can be 8%, 10%, 12%, 14%, or 15%, etc.; the mass percentage of the second lithium salt can be 5%, 6%, 8%, 9%, or 10%, etc.; correspondingly, the mass percentage of the second solvent is the balance, and under the above exemplary combinations, its percentage can be a corresponding value of 75% to 85%.

[0047] In some embodiments of this application, the adhesive includes at least one of fluoropolymers, acrylic (ester) polymers, rubber elastomers, cellulose derivatives, polyolefin alcohols, or polyether polymers. Exemplarily, the adhesive may include, but is not limited to: polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), polyacrylic acid (PAA) or its salts, polyacrylates, styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), polyethylene oxide (PEO), etc.

[0048] In some embodiments of this application, the second lithium salt includes at least one of Li₂O, Li₂O₂, Li₂S, Li₃N, Li₂Se, Li₂CO₃, Li₅FeO₄, Li₆CoO₄, Li₂NiO₂, Li₆MnO₄, Li₆ZnO₄, Li₂C₂O₄, Li₂C₄O₄, Li₂C₃O₅, Li₂C₄O₆, LiDFOB, LiBOB, LiDFP, LiPO₂F₂, LiFSI, LiTFSI, or LiPF₂O₂. Introducing a second lithium salt helps to form a certain ion pathway in the binder layer, reduces the intrinsic ion impedance of the binder layer, and facilitates more uniform interfacial ion transport with the ion-conducting layer.

[0049] In some embodiments of this application, the second lithium salt may be the same as or different from the first lithium salt. Preferably, the second lithium salt is the same as the first lithium salt. Using the same lithium salt is beneficial for promoting uniform interpenetration of components at the interface between the two layers during heat treatment, forming a more continuous ion transport gradient, simplifying the process, and ensuring electrochemical compatibility and interface stability.

[0050] In some embodiments of this application, the first solvent and the second solvent each independently include carbonate solvents; for example, carbonate solvents include one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), or ethyl methyl carbonate (EMC).

[0051] In some embodiments of this application, the first solvent is the same as the second solvent. Although the first solvent and the second solvent can theoretically be selected from different carbonate solvents, using solvents with the same composition and ratio is beneficial to promote the diffusion and fusion of polymer segments at the interface between the ion conduction layer and the binder layer during subsequent segmented heat treatment, thereby forming an integrated gradient structure with tighter interface bonding and more continuous ion transport path.

[0052] In some embodiments of this application, both the first solvent and the second solvent are mixed solvents composed of ethylene carbonate, diethyl carbonate, and dimethyl carbonate in a volume ratio of 1:(0.7~2.5):(0.3~1.5). Exemplarily, the volume ratio can be 1:0.7:0.3, 1:0.7:0.5, 1:0.7:0.8, 1:0.7:1, 1:0.7:1.2, 1:0.7:1.5, 1:1:0.3, 1:1:0.5, 1:1:0.8, 1:1:1, 1:1:1.2, 1:1:1.5, 1:1.5:0.3, 1:1.5:0.5, 1:2:0.8, 1:2:1, 1:2.5:1, or 1:2.5:1.5, etc.

[0053] S30: The ion-conducting layer slurry is applied to the surface of the active material layer in the thinned area to form a wet film of the ion-conducting layer.

[0054] In some embodiments of this application, the amount and width of the slurry can be controlled by coating methods such as slot coating, spraying, or scraping to ensure that the wet film completely covers the thinned area.

[0055] S40: Before the ion-conducting layer wet film is completely dry, the adhesive layer slurry is applied to its surface to form an adhesive layer wet film.

[0056] It should be noted that "not completely dry" means that the surface of the ion-conducting layer wet film is still in a flowable state. When the adhesive layer wet film is superimposed in this state, the two slurries will physically mix at the contact interface due to solvent miscibility, providing a basis for the mutual diffusion of polymer segments, lithium salts, and other components during subsequent heat treatment.

[0057] S50: Heat treatment of the electrode sheet covered with a wet film.

[0058] In some embodiments of this application, the heat treatment method includes first drying at 40°C to 50°C for 3 to 5 minutes, for example, the drying temperature can be 40°C, 43°C, 45°C, 48°C or 50°C, etc., to initially remove some of the solvent; then drying at 60°C to 70°C for 8 to 12 minutes, for example, the drying temperature can be 60°C, 63°C, 65°C, 68°C or 70°C, etc., to solidify the polymer in the slurry, and to allow the ion-conducting layer and the adhesive layer to dissolve and permeate each other at the interface to form an integrated transition structure.

[0059] S60: By controlling the coating amount of the ion-conducting layer slurry and / or the adhesive layer slurry, and through the heat treatment, a functional layer is formed on the surface of the thinned area; the outer surface of the functional layer is flush with the outer surface of the active material layer of the main body area, and the functional layer includes an ion-conducting layer, an adhesive layer, and a transition structure formed by mutual penetration of the two at the interface.

[0060] In some embodiments of this application, the "controlling coating amount" can be achieved by controlling the coating speed, slit size, or areal density of the slurry, so that after the wet film thickness is dried and cured, the dry film thickness ratio corresponding to the design thickness of the ion-conducting layer and the adhesive layer is (1.5~3):1.

[0061] In some embodiments of this application, by controlling the coating amount of the ion-conducting layer slurry and / or the adhesive layer slurry, the designed thickness ratio of the ion-conducting layer to the adhesive layer is (1.5~3):1. For example, the thickness ratio can be 1.5:1, 1.8:1, 2:1, 2.2:1, 2.4:1, 2.6:1, 2.8:1 or 3:1, etc.

[0062] By controlling the dry-state thickness ratio of the ion-conducting layer to the binder layer within the range of 1.5:1 to 3:1, the relatively thicker ion-conducting layer is more conducive to penetrating and covering the irregular surface and pores of the thinned active material layer, thereby constructing a continuous and uniform ion transport network. This optimizes the lithium-ion insertion / extraction kinetics at the interface and provides a more sufficient electrochemical safety boundary to suppress local lithium-ion aggregation and lithium plating. Simultaneously, the appropriate binder layer thickness within this ratio ensures the presence of sufficient binder material, allowing it to flow fully and embed into the microporous structure of the separator during the subsequent hot-pressing process of battery assembly. This forms strong mechanical interlocking and bonding forces to resist volumetric stress during long-term charge-discharge cycles. If the ion-conducting layer is too thick (e.g., thickness ratio > 3:1), the functional layer as a whole is prone to generating large internal stresses during curing or cycling. Simultaneously, an excessively thin adhesive layer lacks sufficient mechanical interlocking strength during hot pressing, leading to decreased long-term interface durability and a risk of interface delamination. Conversely, if the adhesive layer is too thick (e.g., thickness ratio < 1.5:1), the ion-conducting layer becomes relatively thin, weakening its coverage of the active material layer and its optimization effect on the ion transport network, lowering the safety boundary for suppressing lithium plating. Furthermore, an excessively thick adhesive layer typically has low conductivity, becoming a major obstacle to ion transport, which is detrimental to battery performance. Therefore, controlling the thickness ratio of the ion-conducting layer to the adhesive layer between 1.5:1 and 3:1 is beneficial for achieving a synergistic optimization of rapid ion transport, suppressing lithium plating, and ensuring long-term interface stability.

[0063] Furthermore, the interpenetrating transition structure is the result of the wet film stacking and segmented heat treatment process. Its thickness accounts for 1% to 8% of the total thickness of the functional layer. This range ensures effective interpenetration and fusion of the two layers, forming an integrated functional layer. The thickness of the transition structure can be characterized and measured by scanning electron microscopy (SEM) of the functional layer cross-section. In a clear interface region, the distance from the point where the homogeneous ion-conducting layer image features disappear to the point where the homogeneous bonding layer image features appear, perpendicular to the interface direction, is measured as the thickness of the transition structure at that location. The average of multiple measurement points is taken to obtain the thickness of the transition structure. If the transition region is too thin (e.g., <1%), interpenetration is insufficient, and interlayer bonding is weak; if it is too thick (e.g., >8%), it may lead to blurred internal layering functions within the functional layer. This application optimizes the transition structure within this range by controlling the slurry formulation and heat treatment, ensuring both the continuity of ion transport and improved interfacial toughness.

[0064] It is understood that the functional layer described in this application completely covers all exposed surfaces of the active material layer in the thinned area to compensate for and fill the thickness difference between the thinned area and the main body area, so that the surface of the functional layer in the thinned area is flush with the surface of the active material layer in the main body area.

[0065] In this way, the ion-conducting layer and the thinned active material layer can be integrated at the ion level, while the adhesive layer can be used to achieve a strong mechanical bond with the battery separator. The ion-conducting layer can transform the porous and rough interface of the active material layer into a smooth, dense interface with high ion conductivity, while the accompanying adhesive layer can form a more uniform and robust whole, thereby more effectively enhancing the adhesion between the electrode and the separator. Moreover, during battery cycling, when the active material expands in volume, the flexible ion-conducting layer can absorb some of the stress; the strong bond between the adhesive layer and the separator constrains the displacement of the entire interface. It is evident that the two layers work together to transform the traditionally weak mechanical interface of the thinned region-separator into a robust, integrated interface with stress buffering and uniform load-bearing capacity, effectively suppressing interface delamination. Meanwhile, for the low areal density, abnormal pore structure, and high incidence of lithium plating in the thinned region due to thickness reduction, an ion-conducting layer can be added to improve the bulk ionic conductivity of the thinned region surface. This allows lithium ions to disperse rapidly and uniformly laterally upon reaching the thinned region surface. Furthermore, the free lithium ions in the ion-conducting layer act as a microscopic buffer, rapidly replenishing ions during the initial charging phase and temporarily storing a small amount of ions at the end, smoothing the ion concentration gradient at the active material interface in the thinned region and reducing the risk of instantaneous overpotential. Moreover, the ion-conducting layer can pre-form an interface layer with good ion conductivity, stable mechanical properties, and uniform composition on the active material surface. A stable interface helps reduce side reactions and suppresses the inhomogeneity and lithium plating tendency exacerbated by the repeated rupture and growth of the SEI, which consumes lithium source and electrolyte.

[0066] The negative electrode sheet of this application is finally obtained through the preparation methods described in S10 to S60. Please refer to the diagram for the structure of this negative electrode sheet. Figure 2 The battery comprises a current collector 1 and an active material layer 2 disposed on at least one side of the current collector 1. The active material layer 2 includes a main region 4 and a thinned region 5. A functional layer is laminated on the surface of the active material layer 2 in the thinned region 5. The outer surface of the functional layer is flush with the outer surface of the active material layer in the main region 4. As indicated by the reference numerals, the functional layer includes an ion-conducting layer 6 and a binder layer 7. Characterization techniques such as scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) reveal a transition structure 8 between the ion-conducting layer 6 and the binder layer 7, where components and structures interpenetrate. For example, the thickness of this transition structure 8, measured by cross-sectional SEM images, accounts for 1% to 8% of the total thickness of the functional layer. The negative electrode sheet can be used for subsequent battery assembly.

[0067] The negative electrode obtained by the above preparation method is assembled with the positive electrode, separator, and electrolyte of a conventional battery according to processes known in the art to produce a lithium-ion battery. For example, in one embodiment, NCM622 material is used as the positive electrode, a polyolefin porous membrane as the separator, and a 1M LiPF6 carbonate solution as the electrolyte. After stacking, encapsulation, electrolyte injection, and formation with the negative electrode of this application, a lithium-ion battery can be obtained.

[0068] Referring to the above implementation details, in order to make the technical solution of this application more specific, clear, and easy to understand, examples of the technical solution of this application are given below. However, it should be noted that the content to be protected by this application is not limited to the following embodiments.

[0069] Those skilled in the art should understand that the embodiments described are merely illustrative of this application and should not be construed as specific limitations on this application.

[0070] Example 1

[0071] This embodiment provides a negative electrode sheet. Figure 2 The figure shows a partial cross-sectional view of the negative electrode sheet, with the plane formed by the length and thickness directions of the electrode sheet as the cross-section. The negative electrode sheet includes:

[0072] The current collector 1 is a copper foil with a thickness of 6 μm. Both sides of the current collector 1 are provided with an active material layer 2 and a blank area 3 not covered by the active material layer 2. The active material layer 2 includes a main region 4 with a thickness of 45 μm and a thinned region 5 with a thickness and areal density less than the main region 4. The thinned region 5 is located between the main region 4 and the blank area 3. Specifically, the thinned region is equivalent to removing the apex of the active material layer 2 near the blank area 3. Therefore, the cross-sectional shape of the cut portion is a right-angled triangle. The length of the right-angled side in the electrode length direction is 5 mm, and the thickness of the right-angled side in the electrode thickness direction is 6 μm. Thus, a portion of the active material layer 2 in a right-angled trapezoidal shape remains in the thinned region. It should be noted that... Figure 2 The shape shown is only one example and is not a limitation of this application. The structure, thickness, width, shape and size of the thinned area shall be adjusted according to the actual design and needs.

[0073] A functional layer is disposed on the surface of the active material layer 2 in the thinned region 5 away from the current collector 1. The functional layer includes an ion-conducting layer 6 near the active material layer 2 and an adhesive layer 7 disposed on the side of the ion-conducting layer 6 away from the active material layer 2. The ion-conducting layer 6 and the adhesive layer 7 have a mutually penetrating transition structure 8. Calculated with the mass of the ion-conducting layer 6 as 100%, the ion-conducting layer 6 comprises 53.57% polymer matrix PVDF-HFP, 28.57% first lithium salt LiPF6, and 17.86% nano-inorganic filler nano-silica. Calculated with the mass of the adhesive layer 7 as 100%. The adhesive layer 7 comprises 63.16% PVDF-HFP binder and 36.84% LiPF6 second lithium salt; the thickness ratio of the ion-conducting layer 6 to the adhesive layer 7 is 2:1; and the total thickness of the active material layer 2 in the thinned region 5 and the functional layer is equal to the thickness of the active material layer 2 in the main body region 4; that is, the functional layer completely covers or fills the right-angled triangular portion missing from the active material layer 2 in the thinned region 5, the thickness of the functional layer is equal to the thickness of the thinned region 5 (6 μm), the length of the functional layer is equal to the length of the thinned region 5 (5 mm), and the surface height of the functional layer is level with the surface height of the active material layer 2 in the main body region 4.

[0074] This embodiment also provides a method for preparing the negative electrode sheet, including:

[0075] S1. Preparation of the negative electrode substrate:

[0076] Graphite and 3% (by weight of graphite) of silicon-carbon anode active material were used as the anode active material. The active material, conductive carbon black (anode conductive agent), and CMC (carboxymethyl cellulose) binder were added to deionized water in a mass ratio of 96:1:3 and stirred until homogeneous to obtain an anode slurry. This slurry was then coated onto a 6μm thick copper foil and dried to form a 45μm thick dry active material layer. Laser etching was performed along the edge of the copper foil to create a thinning region 5mm long and 6μm thick, resulting in the anode substrate. The thinned region on the anode substrate was then subjected to plasma treatment for 50 seconds.

[0077] S2. Prepare the ion-conducting layer slurry and the adhesive layer slurry respectively:

[0078] According to the mass percentage, 15% PVDF-HFP, 8% LiPF6, 5% nano silica and 72% mixed solvent (EC:DEC:DMC volume ratio 1:1:1) are mixed and stirred until uniformly dispersed to obtain ion-conducting layer slurry.

[0079] According to the mass percentage, 12% PVDF-HFP, 7% LiPF6, and 81% mixed solvent (EC:DEC:DMC volume ratio 1:1:1) are mixed and stirred until uniformly dispersed to obtain the adhesive layer slurry.

[0080] S3. The ion-conducting layer slurry is coated onto the surface of the active material layer in the thinned area to form a wet film of the ion-conducting layer. Then, within 10 seconds, the adhesive layer slurry is coated onto the surface of the wet film of the ion-conducting layer to form a wet film of the adhesive layer. The final functional layer wet film completely covers the thinned area. The electrode is pre-baked at 45°C for 4 minutes and then vacuum-cured at 65°C for 10 minutes. By controlling and adjusting the coating speed and slit size of the two slurries, the total thickness of the functional layer after drying and curing is 6 μm, and the thickness ratio of the ion-conducting layer to the adhesive layer is 2:1. According to the cross-sectional SEM image, the thickness of the interpenetrating transition structure formed between the ion-conducting layer and the adhesive layer in the functional layer accounts for about 5% of the total thickness of the functional layer.

[0081] Example 2

[0082] The difference from Example 1 is that in step S2, nano-silica is replaced with lithium lanthanum zirconium oxide. All other conditions are exactly the same as in Example 1. Cross-sectional SEM image characterization shows that the interpenetrating transition structure formed between the ion-conducting layer and the adhesive layer in the functional layer accounts for approximately 5% of the total thickness of the functional layer.

[0083] Example 3

[0084] The difference from Example 1 is that in step S2, the PVDF-HFP in the adhesive layer slurry is changed to a crosslinkable acrylate adhesive. Apart from the above, all other conditions are exactly the same as in Example 1. Cross-sectional SEM image characterization shows that the interpenetrating transition structure formed between the ion-conducting layer and the adhesive layer in the functional layer accounts for approximately 4% of the total thickness of the functional layer.

[0085] Example 4

[0086] The difference from Example 1 is that the polymer matrix in the ion-conducting layer slurry is replaced with 18% polyethylene oxide-polyacrylonitrile blend (PEO-PAN), the lithium salt is replaced with 10% lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), the nanofiller is replaced with 5% nano silica, and the solvent is replaced with 67% propylene carbonate (PC).

[0087] In the adhesive layer slurry, the binder was replaced with 15% PEO-PAN, the lithium salt with 8% LiTFSI, and the solvent was adjusted to 77% PC. All other conditions were identical to those in Example 1. Cross-sectional SEM image characterization revealed that the interpenetrating transition structure formed between the ion-conducting layer and the adhesive layer in the functional layer accounted for approximately 5% of the total thickness of the functional layer.

[0088] Example 5

[0089] The difference from Example 1 is that in step S3, the heat treatment regime is adjusted to: first drying at 40°C for 5 minutes, then vacuum drying at 70°C for 10 minutes. Apart from the above, all other conditions are exactly the same as in Example 1. Characterization by cross-sectional SEM images shows that the interpenetrating transition structure formed between the ion-conducting layer and the adhesive layer in the functional layer accounts for approximately 5% of the total thickness of the functional layer.

[0090] Example 6

[0091] The difference from Example 1 is that in step S3, the heat treatment regime is adjusted to: first drying at 45°C for 5 minutes, then vacuum drying at 60°C for 12 minutes. Apart from the above, all other conditions are exactly the same as in Example 1. Characterization by cross-sectional SEM images shows that the interpenetrating transition structure formed between the ion-conducting layer and the adhesive layer in the functional layer accounts for approximately 5% of the total thickness of the functional layer.

[0092] Example 7

[0093] The difference from Example 1 lies in the change of solvent composition in step S2. The solvent system of the ion-conducting layer slurry is the same as in Example 1, a mixed solvent of EC:DEC:DMC in a volume ratio of 1:1:1, accounting for 72% by mass. The solvent system of the binder layer slurry is changed to a mixed solvent of EC:EMC in a volume ratio of 3:7, still accounting for 81% by mass. Except for the above, all other conditions are exactly the same as in Example 1. Characterization by cross-sectional SEM images shows that the interpenetrating transition structure formed between the ion-conducting layer and the binder layer in the functional layer accounts for approximately 4% of the total thickness of the functional layer.

[0094] Example 8

[0095] The difference from Example 1 is that in step S3, the coating amount is controlled to ensure that the designed thickness ratio of the ion-conducting layer to the adhesive layer is 1.5:1. Apart from this, all other conditions are exactly the same as in Example 1. Cross-sectional SEM image characterization shows that the interpenetrating transition structure formed between the ion-conducting layer and the adhesive layer in the functional layer accounts for approximately 6% of the total thickness of the functional layer.

[0096] Example 9

[0097] The difference from Example 1 is that in step S3, the coating amount is controlled to ensure that the designed thickness ratio of the ion-conducting layer to the adhesive layer is 3:1. Apart from this, all other conditions are exactly the same as in Example 1. Cross-sectional SEM image characterization shows that the interpenetrating transition structure formed between the ion-conducting layer and the adhesive layer in the functional layer accounts for approximately 4% of the total thickness of the functional layer.

[0098] Example 10

[0099] The difference from Example 1 is that in step S3, the coating amount is controlled to ensure that the designed thickness ratio of the ion-conducting layer to the adhesive layer is 4:1. Apart from this, all other conditions are exactly the same as in Example 1. Cross-sectional SEM image characterization shows that the interpenetrating transition structure formed between the ion-conducting layer and the adhesive layer in the functional layer accounts for approximately 2% of the total thickness of the functional layer.

[0100] Comparative Example 1

[0101] This comparative example uses the negative electrode substrate obtained in step S1 of Example 1 for subsequent testing.

[0102] Comparative Example 2

[0103] The difference from Example 1 is that only ion-conducting layer slurry is used to form an ion-conducting layer in the thinning zone, and the thickness of the ion-conducting layer is equal to the thickness of the thinning zone, which is 6 μm. The length of the ion-conducting layer is equal to the length of the thinning zone, which is 5 mm. The surface height of the ion-conducting layer is the same as the surface height of the active material layer in the main zone. Apart from the above, the other conditions are exactly the same as in Example 1.

[0104] Comparative Example 3

[0105] The difference from Example 1 is that only the adhesive layer slurry is used to form the adhesive layer in the thinning zone, and the thickness of the adhesive layer is equal to the thickness of the thinning zone, which is 6 μm. The length of the adhesive layer is equal to the length of the thinning zone, which is 5 mm. The height of the surface of the adhesive layer is the same as the height of the surface of the active material layer in the main body zone. Apart from the above, the other conditions are exactly the same as those in Example 1.

[0106] Comparative Example 4

[0107] The difference from Example 1 lies in the coating and drying method of the two slurries in step S3. A traditional dry lamination process is used, specifically: In step S3, the ion-conducting layer slurry is first coated onto the surface of the active material layer in the thinned area, and then dried at 65°C for 10 minutes to completely solidify the wet film of the ion-conducting layer, forming a dry ion-conducting layer. After the electrode cools to room temperature, the adhesive layer slurry is then coated onto the surface of the dried ion-conducting layer to form a wet adhesive layer film. Subsequent processing is the same as in Example 1: the electrode covered with the wet film is pre-baked at 45°C for 4 minutes, and then vacuum-cured at 65°C for 10 minutes. By controlling and adjusting the coating amounts of the two slurries, a laminated functional layer with a total thickness of approximately 6 μm is finally formed on the surface of the thinned area. The thickness ratio of the ion-conducting layer to the adhesive layer in this functional layer is approximately 2:1. Since the bottom layer is completely dry, effective interpenetration between the two layers cannot occur during subsequent heat treatment.

[0108] Characterization and testing:

[0109] I. Test the peel strength between the functional layer and the active material layer in the thinned area:

[0110] To evaluate the interfacial bonding strength of the thinned region, additional negative electrode samples with a thinned region length extended to 25.0 ± 0.2 mm were prepared according to the corresponding embodiments or comparative examples, ensuring that the sample width was consistent with that used in the standard peel test, thus guaranteeing that the test load was fully applied to the target interface. Although the absolute value of the peel strength obtained in this way differs in scale from the interfacial strength at the actual micro-width, the relative strength trends and ranking results of the obtained data are repeatable and of reference value because this method fixes all test conditions except for the target material or process variables. Specific steps include:

[0111] (1) At 25±0.5℃, the prepared negative electrode sheet (including the thinned area and functional layer) is cut into strips with a width of 25.0±0.2 mm and a length of not less than 150 mm;

[0112] (2) Cut a piece of high-strength double-sided tape about 100mm long, peel off the protective film on one side, and stick it flat in the center of the standard steel plate. Use a pressure roller to roll it unidirectionally 3 times at a speed of about 300 mm / min to ensure that there are no air bubbles between the tape and the steel plate and that it is firmly attached.

[0113] (3) Remove the protective film from the other side of the double-sided tape. With the active material layer and functional layer of the negative electrode sample facing down on the side away from the current collector, precisely align and flatly adhere them to the exposed surface of the double-sided tape. Ensure that the electrode is in complete contact with the tape, and leave sufficient length (approximately 50 mm) at the current collector end for clamping;

[0114] (4) Immediately use a standard pressure roller to roll back and forth on the sample 3 times at a speed of 300 mm / min to ensure that the contact between the active material layer (including the functional layer) of the electrode and the double-sided tape is uniform and tight. Then, vertically clamp the steel plate with the sample pasted on it in the lower clamp of the tensile testing machine and fix it securely.

[0115] (5) Clamp the current collector end of the electrode sample in the upper fixture of the testing machine, and adjust the fixture position so that the peeling angle of the sample is 180° (i.e., the electrode and the steel plate are parallel in opposite directions); set the tensile speed of the tensile testing machine to 300 mm / min. Start the testing machine and begin peeling. The effective peeling length should be at least 100 mm (calculated from the functional layer area). The data acquisition system records the force (F)-displacement curve in real time during the peeling process.

[0116] (6) From the force-displacement curve, discard the data for approximately 25 mm at the beginning and end, and take the force value of the stable peeling interval of at least 50 mm in the middle, and calculate its arithmetic mean F. avg (Unit: Newton, N), then the peel strength (σ) is calculated using the following formula: σ = F avg / w; where: σ is the peel strength, in Newtons per meter (N / m); F avg The average peel force is expressed in Newtons (N); w is the sample width in meters (m), which is uniformly set to 0.025m for this test.

[0117] II. Electrochemical performance testing:

[0118] To prepare the positive electrode, NCM622, conductive carbon black, and PVDF were added to the solvent NMP at a mass ratio of 97:1.8:1.2. After mixing, the mixture was coated onto aluminum foil using a coating machine, dried, and rolled to obtain the positive electrode. A separator was provided, using a PE separator. The above positive electrode and separator were wound with the negative electrode sheets obtained in the examples and comparative examples, respectively. After being injected with electrolyte (1M LiPF6 in EC:DEC=1:1 v / v electrolyte), the electrodes were sealed and assembled into a battery, which was then tested as follows.

[0119] Battery cycle test method: Test temperature: 45℃±2℃; First, charge at 1C to the termination voltage (4.4V), cut-off current 0.05C, and let stand for 30 minutes; then discharge at 1C to the final discharge voltage (2.8V), and let stand for 30 minutes, which is one cycle; repeat the cycle and record the discharge capacity of the 500th cycle; then the capacity retention rate of the battery after 500 cycles = 100% × discharge capacity of the 500th cycle / initial discharge capacity;

[0120] The results are recorded in Table 1.

[0121] Table 1

[0122]

[0123] As can be seen from Table 1:

[0124] Comparative Example 1 (without a functional layer) exhibited the lowest cycle retention rate. The performance of Comparative Example 2 (single ion-conducting layer) and Comparative Example 3 (single adhesive layer) was significantly worse than that of Example 1, proving that a single material layer cannot simultaneously meet the dual requirements of rapid ion conduction and strong interfacial adhesion, thus demonstrating the necessity of the ion-conducting layer / adhesive layer composite structure designed in this application.

[0125] Example 1 exhibits higher peel strength and cycle retention than Comparative Example 4. This demonstrates that the interpenetrating transition structure formed by the wet film stacking and heat treatment process of this application is key to achieving strong interfacial bonding and long cycle life, rather than a simple double-layer stacking.

[0126] The peel strength and cycle performance of all embodiments are significantly better than those of the comparative examples. The high peel strength indicates that the functional layer and the active material form a strong interfacial bond, laying the foundation for the construction of uniform ion channels; the high cycle retention rate confirms that this structure can effectively suppress interfacial peeling during long-term cycling, stabilize the ion transport path, and extend battery life.

[0127] Examples 1, 8, and 9 (thickness ratios from 1.5:1 to 3:1) yielded the best performance combinations, while the performance of Example 10 (4:1) declined. This indicates that there is an optimal range for the thickness ratio of the ion-conducting layer to the adhesive layer; a ratio that is too low may weaken ion transport, while a ratio that is too high may damage the integrity of the interfacial bonding. Examples 2 to 7 demonstrate the universality of the dual-layer structural framework of this application for key materials under the preferred parameters.

[0128] In summary, by introducing a composite functional layer of "ion-conducting layer / binder layer" with a specific thickness ratio, the problems of insufficient liquid electrolyte wetting and discontinuous ion conduction in the thinned area, leading to lithium plating and affecting battery cycle life, can be solved. It can also address the issue of poor interfacial bonding and interface delamination during cycling caused by porosity at the interface between the active material layer and the battery separator in the thinned area of ​​the negative electrode. In other words, the technical contradiction of simultaneously achieving ion transport and mechanical bonding at the interface of the thinned area is resolved, effectively improving the battery's interfacial stability and extending its cycle life.

[0129] The preferred embodiments of this application have been described in detail above. However, this application is not limited to the specific details of the above embodiments. Within the scope of the technical concept of this application, various simple modifications can be made to the technical solution of this application, and these simple modifications all fall within the protection scope of this application.

[0130] It should also be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, this application will not describe the various possible combinations separately.

[0131] Furthermore, various different implementations of this application can be combined in any way, as long as they do not violate the spirit of this application, they should also be regarded as the content disclosed in this application.

Claims

1. A method for preparing a negative electrode sheet, characterized in that, Includes the following steps: A negative electrode substrate is provided, the negative electrode substrate including a current collector, an active material layer disposed on at least one surface of the current collector and a blank area not covered by the active material layer; the active material layer includes a main region and a thinned region with a thickness less than the main region, and the thinned region is disposed between the main region and the blank area; Provide ion-conducting layer slurry and adhesive layer slurry; The ion-conducting layer slurry is applied to the surface of the active material layer in the thinned area to form a wet film of the ion-conducting layer. Before the ion-conducting layer wet film is completely dry, the adhesive layer slurry is applied to its surface to form an adhesive layer wet film; Heat treatment is performed on the electrode sheet covered with a wet film; By controlling the coating amount of the ion-conducting layer slurry and / or the adhesive layer slurry, and through the heat treatment, a functional layer is formed on the surface of the thinned area; the outer surface of the functional layer is flush with the outer surface of the active material layer of the main body area, and the functional layer includes an ion-conducting layer, an adhesive layer, and a transition structure formed by mutual penetration of the two at the interface.

2. The method for preparing the negative electrode sheet according to claim 1, characterized in that, By controlling the coating amount of the ion-conducting layer slurry and / or the adhesive layer slurry, the designed thickness ratio of the ion-conducting layer to the adhesive layer is (1.5~3):1, and the thickness of the transition structure accounts for 1%~8% of the total thickness of the functional layer.

3. The method for preparing the negative electrode sheet according to claim 1, characterized in that, The preparation method further includes pre-activating the active material layer in the thinned area before applying the ion-conducting layer slurry. And / or, the activation pretreatment includes plasma treatment for 30s to 60s.

4. The method for preparing the negative electrode sheet according to claim 1, characterized in that, The ion-conducting layer slurry comprises a polymer matrix, a first lithium salt, nano-inorganic fillers, and a first solvent; based on the total mass of the ion-conducting layer slurry as 100%, the polymer matrix accounts for 10% to 20% of the mass, the first lithium salt accounts for 5% to 12% of the mass, the nano-inorganic fillers account for 3% to 8% of the mass, and the first solvent accounts for 60% to 80% of the mass.

5. The method for preparing the negative electrode sheet according to claim 1, characterized in that, The adhesive layer slurry includes an adhesive, a second lithium salt, and a second solvent; based on the total mass of the adhesive layer slurry as 100%, the mass percentage of the adhesive is 8% to 15%, the mass percentage of the second lithium salt is 5% to 10%, and the mass percentage of the second solvent is 75% to 85%.

6. The method for preparing the negative electrode sheet according to claim 4 or 5, characterized in that, The first solvent and the second solvent each independently include carbonate solvents.

7. The method for preparing the negative electrode sheet according to claim 6, characterized in that, The first solvent is the same as the second solvent.

8. The method for preparing the negative electrode sheet according to claim 7, characterized in that, Both the first solvent and the second solvent are mixed solvents composed of ethylene carbonate, diethyl carbonate and dimethyl carbonate in a volume ratio of 1:(0.7~2.5):(0.3~1.5).

9. The method for preparing the negative electrode sheet according to claim 1, characterized in that, The heat treatment method includes first drying at 40℃~50℃ for 3min~5min; then drying at 60℃~70℃ for 8min~12min.

10. A negative electrode sheet, characterized in that, The negative electrode sheet is prepared by the preparation method according to any one of claims 1-9.

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