Lithium ion battery negative electrode assembly and preparation method thereof

Abstract

The invention relates to a lithium ion battery negative electrode assembly and a preparation method thereof. The lithium ion battery negative electrode assembly comprises a current collector and an active substance layer arranged on the surface of the current collector, the active substance layer comprises a main body region and a thinned region which are different in thickness, and the thickness of the thinned region is smaller than that of the main body region; a functional layer is arranged on the surface of the thinned area and completely covers the thinned area, and the functional layer is flush with the thickness of the main body area in the thickness direction; and in the direction from the current collector to the active substance layer, the functional layer comprises a water-based bonding layer and an electrolyte layer which are sequentially stacked on the surface of the thinned region. According to the lithium ion battery negative electrode assembly, the gradient functional layer formed by sequentially overlapping the water-based bonding layer and the electrolyte layer is constructed on the surface of the thinned area, and the two layers form an integrated stable structure through molecular interpenetration in the co-curing process, so that the lithium ion battery negative electrode assembly is environment-friendly, and has stable ion conduction and long-acting interface stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium battery, in particular to a lithium ion battery negative electrode assembly and a preparation method thereof. BACKGROUND

[0002] Lithium ion batteries are widely used in electric vehicles and energy storage fields. The negative electrode sheet often has a thinned area to adapt to the welding and winding process. However, the thinned area is prone to insufficient electrolyte infiltration and ion transmission obstruction due to the sudden change in thickness and pore structure, which affects the performance and safety of the battery.

[0003] To improve the performance of the thinned area, a functional layer is usually coated at the thinned area, and a solvent-based system based on PVDF-HFP and other materials is often used, which relies on organic solvents such as NMP. There are problems such as high emission, high recycling cost, and production safety hazards. Solvent residues can weaken the interface bonding, affecting the stability of the structure and the continuity of ion transmission in long-term cycling. Water-based slurry has been widely used in the coating of the main body area of the electrode sheet, but there is still a lack of ion conduction design for the pore structure and infiltration dynamics of the thinned area, making it difficult to achieve strong interface bonding and efficient conduction.

[0004] Therefore, there is an urgent need in the art to establish a structure that can take into account environmental protection, stable ion conduction, and long-term interface stability, to improve the performance of the negative electrode thinned area and prolong the cycle life of the battery. SUMMARY

[0005] Therefore, there is an urgent need in the art to establish a structure that can take into account environmental protection, stable ion conduction, and long-term interface stability, to improve the performance of the negative electrode thinned area and prolong the cycle life of the battery.

[0006] A lithium ion battery negative electrode assembly, comprising: a current collector and an active material layer arranged on the surface of the current collector, the active material layer comprising a main body area and a thinned area with different thicknesses, the thickness of the thinned area being less than the thickness of the main body area. The surface of the thinned area is provided with a functional layer, the functional layer completely covers the thinned area, and the thickness of the functional layer is level with the thickness of the main body area in the thickness direction. From the direction of the current collector to the active material layer, the functional layer comprises a water-based adhesive layer and an electrolyte layer arranged in sequence on the surface of the thinned area.

[0007] The lithium ion battery negative electrode assembly of the technical scheme of the present application has a gradient functional layer formed by sequentially stacking a water-based adhesive layer and an electrolyte layer on the surface of the thinning area. The functional layer realizes close wetting and firm combination of the active material surface by using the water-based adhesive layer, and the electrolyte layer forms a continuous, stable and rapid ion transmission channel, optimizes the current distribution of the thinning area and compensates for the ion deficiency. The two layers form an integrated stable structure through molecular interpenetration during co-curing. In addition, both layers are full water-based systems, completely eliminating the use of organic solvents and the hazards they bring. The above-mentioned whole enables the lithium ion battery negative electrode assembly of the present application to have environmental protection, stable ion conduction and long-term interface stability.

[0008] In some embodiments, the water-based adhesive layer comprises a water-based adhesive and a nano-enhancing material dispersed in the water-based adhesive, and the mass ratio of the water-based adhesive to the nano-enhancing material is (20-40):(2-10).

[0009] In some embodiments, the nano-enhancing material comprises one or more of nanocellulose, nanocrystalline cellulose, carbon nanotubes, nanosilica and graphene oxide.

[0010] In some embodiments, the electrolyte layer comprises a polymer matrix, a lithium salt and a plasticizer, and the mass ratio of the polymer matrix, the lithium salt and the plasticizer is (15-25):(8-15):(3-10).

[0011] In some embodiments, the polymer matrix comprises one or more of polyvinyl alcohol, polyethylene oxide and its derivatives, water-based dispersion of fluoropolymer, polyacrylonitrile and its copolymer, natural polymer and its derivatives, and synthetic water-soluble polymer.

[0012] In some embodiments, the lithium salt comprises one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium bis(oxalate)borate, lithium difluoro(oxalato)borate and lithium tetrafluoroborate.

[0013] In some embodiments, the thickness of the functional layer is 5-60 μm, and the dry thickness ratio of the water-based adhesive layer to the electrolyte layer is 1:2-1:4.

[0014] A manufacturing method of the lithium ion battery negative electrode assembly according to any one of the above, comprising the following steps: providing a negative electrode substrate having a main area and a thinning area; preparing a first slurry for forming a water-based adhesive layer and a second slurry for forming an electrolyte layer; coating the first slurry on the surface of the thinning area to form a first wet film, and then coating the second slurry on the surface of the first wet film to form a second wet film; and The pole piece coated with the first wet film and the second wet film is subjected to heat treatment to remove the solvent and form a functional layer.

[0015] The preparation method of the lithium ion battery negative electrode assembly has simple process steps and is easy to implement on an existing electrode production line. The key "wet film superimposed coating" process allows the two layers of slurry to interpenetrate before solidification, and after co-solidification, an integrated gradient structure with no clear interface is formed, and the bonding force is stronger.

[0016] In some embodiments, the first slurry includes, in terms of mass fraction: 20-40% of the aqueous binder, 2-10% of the nano-enhancing material, and 50-70% of the first aqueous solvent; and the second slurry includes: 15-25% of the polymer matrix, 8-15% of the lithium salt, 3-10% of the plasticizer, and 50-65% of the second aqueous solvent. The first aqueous solvent and the second aqueous solvent independently include one or more of deionized water, distilled water, ultrapure water, and a mixed solvent of water and a water-soluble organic solvent; when the mixed solvent of water and a water-soluble organic solvent is selected, the mass fraction of water in the mixed solvent is not less than 70%; The water-soluble organic solvent includes one or more of ethanol, isopropanol, ethylene glycol, and propylene glycol.

[0017] In some embodiments, the heat treatment is hot air circulation solidification at 60-80℃, and the solidification time is 10-15 minutes. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 FIG. 1 is a schematic diagram of a lithium ion battery negative electrode assembly according to an embodiment of the present application; Figure 2 FIG. 2 is a flowchart of a preparation method of a lithium ion battery negative electrode assembly according to an embodiment of the present application.

[0019] Marked in the figure: 100-lithium ion battery negative electrode assembly, 110-current collector, 120-active material layer, 121-main body area, 122-thinned area, 130-functional layer, 131-aqueous adhesive layer, 132-electrolyte layer. DETAILED DESCRIPTION

[0020] In order to make the above objectives, features and advantages of the present application more clear and comprehensible, specific embodiments of the present application will be described in detail below with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. However, the present application can be practiced in many different ways from what is described herein, and skilled artisans will be able to make similar modifications to the embodiments disclosed without departing from the scope of the present application, and therefore the present application is not limited to the specific embodiments disclosed below.

[0021] 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 in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0022] As used herein, "dry thickness" refers to the final thickness of each layer after the slurry is coated and heat treated to remove solvent.

[0023] See Figure 1 In one embodiment of the present application, the negative electrode assembly 100 of a lithium ion battery comprises a current collector 110 and an active material layer 120 disposed on the surface of the current collector 110. The current collector 110 is a conductive substrate commonly known in the art for use in the negative electrode of a lithium ion battery, such as a copper foil, a copper alloy foil, a three-dimensional porous copper current collector, or a copper composite current collector, and the present application is not limited in this regard.

[0024] The active material layer 120 comprises a main body region 121 and a thinned region 122 having different thicknesses, and the thickness of the thinned region 122 is less than that of the main body region 121.

[0025] In some embodiments, the active material layer 120 comprises a negative electrode active material, which comprises one or more of graphite, soft carbon, hard carbon, silicon-based materials (such as elemental silicon, silicon oxides, silicon-carbon composites), tin-based materials, lithium titanate, and composites thereof, and the present application is not limited in this regard.

[0026] In some embodiments, the active material layer 120 further comprises a conductive agent and a binder. The conductive agent is used to provide an electronic conduction network, and can comprise one or more of conductive carbon black, carbon nanotubes, graphene, carbon fibers; the binder is used to adhere the active material and the conductive agent to the current collector 110, and can comprise an aqueous binder or a solvent-type binder. In the present application, the active material layer 120 in the main body region 121 and the thinned region 122 can have the same or different compositions and ratios.

[0027] It should be noted that the thinning area 122 is a thickness-reduced area formed along the length direction of the pole piece and at one or both edges of the width of the pole piece. The thinning area 122 can exist only at one edge of the pole piece or at both edges symmetrically, and the present application does not limit this. In addition, in the specific embodiments of the present application, the cross-sectional shape of the thinning area 122 is not limited and can be flat, arcuate, or stepped, etc. The formation of the thinning area 122 can be achieved by laser etching, die cutting, mask coating, etc. The width and depth of the thinning area 122 can be designed according to the requirements of tab welding, optimization of internal space of the battery, or uniformity of ion distribution, etc. For example, the width thereof can be 2 mm to 20 mm. The shape shown in the drawing is only one example and is not a limitation of the present application.

[0028] It should be noted that the thinning area 122 can be completely removing the active material layer in this area to partially expose the underlying current collector 110, or can still retain a layer of thinned active material layer, and the present application does not limit this.

[0029] Further, in the present embodiment, the surface of the thinning area 122 is provided with a functional layer 130, the functional layer 130 completely covers the thinning area 122, and the functional layer 130 is flush with the thickness of the main area 121 in the thickness direction. The functional layer 130 fills the thinning area 122 and eliminates the physical step, which is beneficial to the flattening of the pole piece and the assembly of the battery.

[0030] Further, in the present embodiment, from the current collector 110 to the active material layer 120, the functional layer 130 includes a water-based adhesive layer 131 and an electrolyte layer 132 which are sequentially stacked on the surface of the thinning area 122. Among them, the electrolyte layer 132 is a solid-state electrolyte layer.

[0031] The lithium ion battery negative electrode assembly 100 of the present embodiment constructs a gradient functional layer 130 composed of a water-based adhesive layer 131 and an electrolyte layer 132 sequentially superimposed on the surface of the thinning area 122. The bottom layer water-based adhesive layer 131 cooperates with the top layer high ion conduction to solve the problems of weak interface bonding and poor ion transmission existing in the thinning area 122 while reducing pollution with water-based solvent, and improve the cycle performance of the battery.

[0032] In some embodiments, the water-based adhesive layer 131 includes a water-based adhesive and a nano-enhancing material dispersed in the water-based adhesive, and the mass ratio of the water-based adhesive to the nano-enhancing material is (20-40):(2-10). Among them, the water-based adhesive is used to provide the basic adhesion, rheological property and good bonding with the surface of the active material of the slurry. Among them, the nano-enhancing material is used to achieve strong interface adhesion and structural enhancement, and the nano-enhancing material is exemplarily selected from nano materials with high aspect ratio or high specific surface area.

[0033] In some embodiments, the aqueous binder includes one or more of cellulose-based binders, rubber-based elastic binders, polyacrylic acid (PAA) and its salts, polyvinyl alcohol (PVA), sodium alginate (SA), polyurethane (PU) emulsion, and acrylate emulsion. The cellulose-based binders include one or more of sodium carboxymethylcellulose (CMC), potassium carboxymethylcellulose, hydroxyethyl cellulose (HEC), hydroxypropyl methylcellulose (HPMC), methylcellulose (MC), microcrystalline cellulose, and nanocellulose. The rubber-based elastic binders include one or more of styrene-butadiene rubber (SBR) emulsion, carboxylated SBR emulsion, nitrile rubber (NBR) emulsion, acrylate rubber (ACM) emulsion, styrene-butadiene-styrene block copolymer (SBS) latex, natural rubber latex, and modified versions thereof. These mature aqueous binders provide reliable adhesion, good slurry rheology, and compatibility with active material surfaces. For example, the combination of CMC and SBR provides the advantages of rigid adhesion and elastic cushioning, and better accommodates cyclic stress.

[0034] In some embodiments, the nano-enhancing material includes one or more of nanocellulose (CNF), nanocrystalline cellulose (CNC), carbon nanotube (CNT), nanosilica, and graphene oxide (GO). The introduction of these high aspect ratio or high specific surface area nanomaterials forms a three-dimensional reinforcing network in the aqueous binder layer, significantly improving the mechanical strength, modulus, and toughness of the coating.

[0035] In some embodiments, the nano-enhancing material is nanocellulose (CNF). Compared to other nanomaterials, nanocellulose has a very high aspect ratio and abundant surface hydroxyl groups, and can self-assemble into a three-dimensional nanofiber network in the aqueous slurry through hydrogen bonding. This network runs through the binder system, improving the mechanical strength, modulus, and toughness of the coating after drying. During coating and curing, the nanocellulose partially embeds into the micropores and gaps of the underlying porous active material layer, physically locking the electrolyte layer and the active material together, improving the interfacial peeling strength, and solving the problem of interface weakening caused by residual organic solvent systems.

[0036] In some embodiments, the electrolyte layer 132 comprises a polymer matrix, a lithium salt, and a plasticizer, and the mass ratio of the polymer matrix, the lithium salt, and the plasticizer is (15-25):(8-15):(3-10). The polymer matrix is a high molecular material that can be dissolved or dispersed in an aqueous solvent and can form a gel film, which plays a dual role of a film-forming skeleton and an adhesive. On the one hand, it builds a three-dimensional network to fix and transport lithium ions; on the other hand, it is firmly combined with the lower aqueous adhesive layer through intermolecular forces during the solidification process. The lithium salt is a lithium salt stable to water, which does not undergo significant hydrolysis reaction in the aqueous slurry and subsequent solidification process. The plasticizer is used to insert between the molecular chains of the polymer matrix, weaken its crystallinity or intermolecular forces, and improve the flexibility and ionic conductivity of the electrolyte layer after solidification.

[0037] In some embodiments, the polymer matrix comprises one or more of polyvinyl alcohol (PVA), polyethylene oxide (PEO) and its derivatives, aqueous dispersion of fluorine-containing polymer (such as polyvinylidene fluoride-hexafluoropropylene copolymer, PVDF-HFP), polyacrylonitrile (PAN) and its copolymer, natural polymer and its derivatives, and synthetic water-soluble polymer. The natural polymer and its derivatives include one or more of chitosan (CS) and sodium alginate (SA). The synthetic water-soluble polymer includes one or more of polyvinylpyrrolidone (PVP), polyacrylic acid (PAA) and its salt, polyacrylamide (PAM), and polyhydroxyethyl methacrylate (PHEMA). These polymers can form a film in an aqueous environment, build a three-dimensional network to accommodate and transport lithium ions. Preferably, polyvinyl alcohol (PVA) has excellent film-forming property, hydrophilicity, and good compatibility with lithium salt, and the hydroxyl groups on its molecular chain can form strong hydrogen bonding with the lower aqueous adhesive layer to promote interface fusion.

[0038] In some embodiments, the lithium salt comprises one or more of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), and lithium tetrafluoroborate (LiBF4). The use of these lithium salts with high water stability can ensure the retention of active lithium content in the entire aqueous processing system, avoid corrosion of equipment, damage to the polymer matrix, or damage to the electrode interface due to hydrolysis of acidic substances. It ensures that the electrolyte layer after solidification has a high and stable effective lithium ion carrier concentration, thereby obtaining excellent and stable ionic conductivity. The scope of protection of the present application is not limited to the above-mentioned lithium salts, and any lithium salt that is stable to water and can achieve the purpose of the present application belongs to the scope of protection of the present application.

[0039] In some embodiments, the plasticizer is a water-soluble polyol, an oligomeric ether compound, or a sugar alcohol. Exemplarily, the plasticizer includes one or more of glycerol, ethylene glycol, propylene glycol, polyethylene glycol, sorbitol, and xylitol. The addition of the plasticizer can insert between the polymer molecular chains, weaken the inter-chain force and crystallinity, increase the chain segment movement ability, thereby significantly improve the flexibility and ionic conductivity of the electrolyte layer after solidification.

[0040] In some embodiments, the thickness h2 of the functional layer 130 is 5 μm to 60 μm, and the thickness h1 of the active material layer 120 of the main body area 121 can be 30 μm to 60 μm. It should be noted that the thickness h1 and h2 in the present application, unless otherwise specified, refer to the final dry thickness after the completion of the electrode tab preparation. Further, the thickness h1 of the active material layer 120 of the main body area 121 can be, but is not limited to, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, or 60 μm, and any point value therebetween; the thickness h2 of the functional layer 130 can be, but is not limited to, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, or 60 μm, and any point value therebetween.

[0041] In some embodiments, the dry thickness ratio (h3:h4) of the aqueous adhesive layer 131 and the electrolyte layer 132 is 1:2 to 1:4. By controlling the thickness ratio of the aqueous adhesive layer 131 and the electrolyte layer 132 within the range of 1:2 to 1:4, sufficient interface anchoring depth is ensured while avoiding overall hindrance to ion transmission, thereby achieving the goals of long-term interface stability and efficient ion conduction, and avoiding the problems of volatile organic emissions, production costs, and safety hazards in traditional organic solvent processes.

[0042] When the thickness of the aqueous adhesive layer 131 and the electrolyte layer 132 is close to 1:2, the aqueous adhesive layer 131 can provide more sufficient interface anchoring material to ensure firm bonding between the functional layer and the active material layer 120, and the interface stability is outstanding. It is more suitable for application scenarios with high requirements for cycle life and safety.

[0043] When the thickness of the aqueous adhesive layer 131 and the electrolyte layer 132 is close to 1:4, the ion conduction layer is thicker, providing a more sufficient and low-resistance bulk phase transmission path for lithium ions, optimizing the ion transmission efficiency and reducing the interface impedance. It is more suitable for application scenarios that pursue high rate and fast charging performance.

[0044] Deviation from the above preferred range can cause imbalance in performance. If the aqueous binder layer 131 is too thick, the electrolyte layer 132 is relatively insufficient (less than 1:2), the interface is firmly bonded, but the ion conduction capacity may become a bottleneck, the overall impedance of the battery increases, and the rate performance is limited; if the aqueous binder layer 131 is too thin, the electrolyte layer 132 is too thick (more than 1:4), the interface anchoring effect may be insufficient, there is a risk of interface peeling under long-term cycling, and at the same time, the over-thick gel layer may affect the mechanical stability due to increased internal stress.

[0045] In some embodiments, the dry-state thickness ratio of the aqueous binder layer 131 to the electrolyte layer 132 can be, but is not limited to, 1:2, 1:3, or 1:4, and specific point values between the above point values.

[0046] In some embodiments, both surfaces of the current collector 110 are provided with active material layers 120, and both surfaces of the active material layers 120 include thinning regions 122, and both surfaces of the double-sided thinning regions 122 are provided with functional layers 130.

[0047] The lithium ion battery negative electrode assembly of the technical scheme of the present application constructs a gradient functional layer on the surface of the thinning region, which is sequentially stacked by an aqueous binder layer and an electrolyte layer, and controls the dry-state thickness ratio of the aqueous binder layer and the electrolyte layer to be between 1:2 and 1:4. The functional layer uses the aqueous binder layer to achieve close wetting and firm bonding of the active material surface; the introduction of nano-enhanced materials forms a physical anchoring network, combined with the elasticity of the binder system, to ensure the integrity of the interface under long-term cycling and mechanical stress. The electrolyte layer constructs a continuous and stable fast ion transport channel, optimizes the current distribution of the thinning region and compensates for the ion deficiency. The two layers form an integrated stable structure through molecular interpenetration during co-curing. In addition, both layers are full aqueous systems, completely eliminating the use of organic solvents and the hazards they bring. By optimizing the thickness ratio of the aqueous binder layer and the electrolyte layer, the present application achieves a balance between interface anchoring strength and ion transport efficiency, and provides a lithium ion battery negative electrode assembly with environmental friendliness, long-term interface stability, and high-efficiency ion conduction capacity.

[0048] See Figure 2 The preparation method of the lithium ion battery negative electrode assembly of an embodiment of the present application includes the following steps: S10, providing a negative electrode substrate having a main region and a thinning region.

[0049] This step aims to prepare a negative electrode substrate with specific structural characteristics. Specifically, it includes coating an active material layer on the surface of the current collector, and forming a thinning region with a thickness less than that of the main region on the edge or designated area of the electrode sheet by laser etching, mechanical polishing, or printing, etc.

[0050] Preferably, the surface of the thinned area is pretreated before the functional layer is coated, such as ultrasonic cleaning, to remove surface dust, improve wettability, and enhance interface bonding activity, to provide a clean and activated substrate for the subsequent coating of the functional layer. Alternatively, the pretreatment is performed by ultrasonic cleaning at a power of 200-400 W for 15-25 seconds.

[0051] S20, a first slurry for forming a water-based adhesive layer and a second slurry for forming an electrolyte layer are prepared.

[0052] This step involves the preparation of two types of functional slurries. The first slurry is prepared by mixing a water-based adhesive, a nano-enhancing material, and a first water-based solvent in a specific mass ratio, and then dispersing at a speed of 2000-4000 r / min for 30-60 minutes to form a uniform and stable system, ensuring good coating properties and interface penetration ability. The second slurry is composed of a polymer matrix, a lithium salt, a plasticizer, and a second water-based solvent, which are mixed by stirring to form a uniform solution or dispersion, aiming to build a gel precursor with ion conduction properties. Both types of slurries are fully water-based systems, environmentally friendly and good process compatibility.

[0053] In some embodiments, the first slurry includes 20-40% water-based adhesive, 2-10% nano-enhancing material, and 50-70% first water-based solvent by mass fraction; the second slurry includes 15-25% polymer matrix, 8-15% lithium salt, 3-10% plasticizer, and 50-65% second water-based solvent.

[0054] wherein the first water-based solvent and the second water-based solvent are solvent systems with water as the main component.

[0055] In some embodiments, the first water-based solvent and the second water-based solvent independently include one or more of deionized water, distilled water, ultrapure water, and a mixed solvent of water and water-soluble organic solvent; when the mixed solvent of water and water-soluble organic solvent is included, the mass fraction of water in the mixed solvent is not less than 70%.

[0056] wherein the water-soluble organic solvent includes one or more of ethanol, isopropyl alcohol, ethylene glycol, and propylene glycol.

[0057] Preferably, the first water-based solvent and the second water-based solvent are the same, which can ensure the formation of an integrated gradient structure during the curing process.

[0058] S30, the first slurry is coated on the surface of the thinned area to form a first wet film, and then the second slurry is coated on the surface of the first wet film to form a second wet film.

[0059] The step adopts a sequential wet film superimposition coating process, first precisely coating the first slurry on the surface of the thinning area to form a uniform bottom layer wet film; then immediately superimposing the second slurry in a state that has not yet solidified to form an upper layer wet film. The coating method is selected from one of doctor blade coating, slot coating, and spraying, to ensure uniform coating thickness. This "sequential wet film superimposition" coating method facilitates mutual penetration and fusion of the two layers of slurry at the interface, promotes the formation of an integrated gradient structure during subsequent co-solidification, and enhances the interlayer bonding force and structural continuity.

[0060] S40, heat treating the pole piece coated with the first wet film and the second wet film to remove the solvent and form a functional layer.

[0061] This step co-solidifies the two layers of wet film through mild heat treatment. Under the set temperature and time conditions, the moisture and other volatile components are gradually removed, while the polymer components are gelled or crosslinked, the nano-enhanced materials interact with the bonding system, the polymer matrix, and the lithium salt, and finally a stable two-layer composite functional layer with strong adhesion and high ionic conductivity is formed on the surface of the thinning area, and the total thickness is flat with the main body area.

[0062] In some embodiments, the operation of heat treatment is hot air circulation curing at 60-80℃, and the curing time is 10-15 minutes. Further, the heat treatment temperature can be, but is not limited to, 60℃, 65℃, 70℃, 75℃, or 80℃, and specific point values between the above point values; the curing time can be, but is not limited to, 10 minutes, 11 minutes, 12 minutes, 13 minutes, 14 minutes, or 15 minutes, and specific point values between the above point values.

[0063] After step S40, the operation of cutting the solidified pole piece can be further included.

[0064] This step is to cut the continuous pole piece after the functional layer coating and solidification to obtain a single negative electrode assembly that meets the size requirements of battery assembly. During the cutting process, attention should be paid to protect the functional layer area from damage to ensure the structural integrity of the thinning area and the functional layer, thereby ensuring the performance consistency of the negative electrode assembly in subsequent battery manufacturing and use.

[0065] The preparation method of the lithium ion battery negative electrode assembly of the technical scheme of the present application has simple process steps and is easy to implement on existing electrode production lines. The key "wet film superimposition coating" process allows the two layers of slurry to penetrate each other at the interface before solidification, and forms an integrated gradient structure without a clear interface after co-solidification, with stronger bonding force.

[0066] The battery of one embodiment includes the lithium ion battery negative electrode assembly of any of the above.

[0067] Further, the battery is a lithium ion battery. Specifically, the lithium ion battery usually further includes a positive electrode, a separator, an electrolyte, and a battery case.

[0068] The battery comprising the negative electrode assembly of the present application has excellent interface stability and ion transmission efficiency in the thinned area, thereby significantly improving the overall cycle life, rate performance and safety of the battery, especially under harsh conditions such as high temperature and long cycle.

[0069] With reference to the above implementation, in order to make the technical solutions of the present application more specific, clear, and easy to understand, the present application will be exemplified. However, it should be noted that the content to be protected by the present application is not limited to the following examples.

[0070] Preparation of the negative electrode substrate: Graphite + 3% silicon carbon negative electrode active material (96% by mass), conductive agent conductive carbon black SP (1%), and binder CMC (3%) were added to deionized water, and after uniform stirring, a negative electrode slurry was obtained. The above negative electrode slurry was coated on a 6 μm copper foil, dried, and the thickness of the dry active material layer was 45 μm. Laser etching was performed at the edges of the copper foil in the width direction to form a thinned area with a width of 5 mm. The thinned area was ultrasonically cleaned for 20 seconds at a power of 300 W.

[0071] Both the examples and the comparative examples used the above negative electrode substrate.

[0072] Example 1 A first slurry for forming a water-based adhesive layer and a second slurry for forming an electrolyte layer were prepared, respectively. The first slurry included, by mass percentage, a water-based adhesive 30% (sodium carboxymethyl cellulose 25% + butadiene rubber 5%), nanocellulose 5%, and deionized water 65%, which were mixed and then subjected to a dispersion treatment at a rotation speed of 3000 r / min for 20 min to obtain a uniform slurry.

[0073] The second slurry included, by mass percentage, polyvinyl alcohol 20%, lithium bis(trifluoromethanesulfonyl)imide 12%, and glycerol 5%, and deionized water 63%, which were mixed and then stirred at a low speed of 1000 r / min for 10 min to obtain a uniform slurry.

[0074] The first slurry was coated on the surface of the thinned area to form a first wet film, and then the second slurry was superimposed and coated on the surface of the first wet film to form a second wet film. By controlling the coating amount of the first slurry and the second slurry, the functional layer formed after drying completely covers and fills the thinned area, and the surface is leveled with the thickness of the adjacent non-thinned area.

[0075] The coated pole piece is placed in a hot air circulation oven at 65°C for 12 minutes to remove the solvent and gel the components, forming an integrated functional layer consisting of a water-based adhesive layer and an electrolyte layer on the surface of the thinning area. After curing, the water-based adhesive layer is 1.5 μm thick, and the electrolyte layer is 4.5 μm thick, with a thickness ratio of 1:3.

[0076] Example 2 A first slurry for forming a water-based adhesive layer and a second slurry for forming an electrolyte layer are prepared respectively; The first slurry includes, by mass percentage, a water-based adhesive 30% (sodium carboxymethyl cellulose 25% + butadiene-styrene rubber 5%), cellulose nanocrystals (CNC) 5%, and deionized water 65%. After mixing, a dispersion treatment is performed at a speed of 3000 r / min for 20 min to obtain a uniform slurry.

[0077] The second slurry includes, by mass percentage, polyvinyl alcohol 20%, lithium bis(trifluoromethanesulfonyl)imide 12%, and glycerol 5%, and deionized water 63%. After mixing, the mixture is stirred at a low speed of 1000 r / min for 10 min to obtain a uniform slurry.

[0078] The first slurry is coated on the surface of the thinning area to form a first wet film, and then the second slurry is superimposed on the surface of the first wet film to form a second wet film. By controlling the coating amount of the first slurry and the second slurry, the functional layer formed after drying completely covers and fills the thinning area, making the surface level with the thickness of the adjacent non-thinning area.

[0079] The coated pole piece is placed in a hot air circulation oven at 65°C for 12 minutes to remove the solvent and gel the components, forming an integrated functional layer consisting of a water-based adhesive layer and an electrolyte layer on the surface of the thinning area. After curing, the water-based adhesive layer is 1.5 μm thick, and the electrolyte layer is 4.5 μm thick, with a thickness ratio of 1:3.

[0080] Example 3 A first slurry for forming a water-based adhesive layer and a second slurry for forming an electrolyte layer are prepared respectively; The first slurry includes, by mass percentage, a water-based adhesive 30% (sodium carboxymethyl cellulose 25% + butadiene-styrene rubber 5%), nanocellulose 5%, and deionized water 62%. After mixing, a dispersion treatment is performed at a speed of 3000 r / min for 20 min to obtain a uniform slurry.

[0081] The second slurry includes, by mass percentage, polyethylene oxide (PEO) 20%, lithium bis(trifluoromethanesulfonyl)imide 12%, and glycerol 5%, and deionized water 63%. After mixing, the mixture is stirred at a low speed of 1000 r / min for 10 min to obtain a uniform slurry.

[0082] The first slurry is coated on the surface of the thinning area to form a first wet film; then the second slurry is superimposedly coated on the surface of the first wet film to form a second wet film. By controlling the coating amount of the first slurry and the second slurry, the functional layer formed after drying completely covers and fills the thinning area, so that the surface is flat with the thickness of the adjacent non-thinning area.

[0083] The coated pole piece is placed in a hot air circulation oven at 65°C for 12 minutes to remove the solvent and gel the components, forming an integrated functional layer composed of a water-based adhesive layer and an electrolyte layer on the surface of the thinning area. After curing, the thickness of the water-based adhesive layer is 1.5μm, and the thickness of the electrolyte layer is 4.5μm, with a thickness ratio of 1:3.

[0084] Example 4 The first slurry for forming the water-based adhesive layer and the second slurry for forming the electrolyte layer are prepared respectively; The first slurry includes, by mass percentage, water-based adhesive 30% (sodium carboxymethyl cellulose 25% + butadiene-styrene rubber 5%), nanocellulose 5%, and deionized water 65%. After mixing, a uniform slurry is obtained by dispersing treatment at a speed of 3000r / min for 20min.

[0085] The second slurry includes, by mass percentage, polyvinyl alcohol 20%, lithium bis(trifluoromethanesulfonyl)imide 12%, and glycerol 5%. After mixing, a uniform slurry is obtained by stirring at a low speed of 1000r / min for 10min.

[0086] The first slurry is coated on the surface of the thinning area to form a first wet film; then the second slurry is superimposedly coated on the surface of the first wet film to form a second wet film. By controlling the coating amount of the first slurry and the second slurry, the functional layer formed after drying completely covers and fills the thinning area, so that the surface is flat with the thickness of the adjacent non-thinning area.

[0087] The coated pole piece is placed in a hot air circulation oven at 65°C for 12 minutes to remove the solvent and gel the components, forming an integrated functional layer composed of a water-based adhesive layer and an electrolyte layer on the surface of the thinning area. After curing, the thickness of the water-based adhesive layer is 2μm, and the thickness of the electrolyte layer is 4μm, with a thickness ratio of 1:2.

[0088] Example 5 The first slurry for forming the water-based adhesive layer and the second slurry for forming the electrolyte layer are prepared respectively; The first slurry includes, by mass percentage, water-based adhesive 30% (sodium carboxymethyl cellulose 25% + butadiene-styrene rubber 5%), nanocellulose 5%, and deionized water 65%. After mixing, a uniform slurry is obtained by dispersing treatment at a speed of 3000r / min for 20min.

[0089] The second slurry comprises, by mass percentage, polyvinyl alcohol 20%, lithium bis(trifluoromethanesulfonyl)imide 12%, glycerol 5%, and deionized water 63%. After mixing, the slurry is stirred at a low speed of 1000 r / min for 10 min to obtain a uniform slurry.

[0090] The first slurry is coated on the surface of the thinning area to form a first wet film, and then the second slurry is coated on the surface of the first wet film to form a second wet film. By controlling the coating amount of the first slurry and the second slurry, the functional layer formed after drying completely covers and fills the thinning area, and the surface of the thinning area is leveled with the thickness of the adjacent non-thinning area.

[0091] The coated pole piece is placed in a hot air circulation oven at 65°C for 12 minutes to remove the solvent and gel the components, forming an integrated functional layer composed of a water-based adhesive layer and an electrolyte layer on the surface of the thinning area. After curing, the thickness of the water-based adhesive layer is 1.2 μm, and the thickness of the electrolyte layer is 4.8 μm, with a thickness ratio of 1:4.

[0092] Example 6 The first slurry for forming the water-based adhesive layer and the second slurry for forming the electrolyte layer are prepared respectively. The first slurry comprises, by mass percentage, water-based adhesive 30% (sodium carboxymethyl cellulose 25% + styrene-butadiene rubber 5%), nanocellulose 5%, and deionized water 65%. After mixing, the slurry is dispersed at a speed of 3000 r / min for 20 min to obtain a uniform slurry.

[0093] The second slurry comprises, by mass percentage, polyvinyl alcohol 20%, lithium bis(trifluoromethanesulfonyl)imide 12%, glycerol 5%, and deionized water 63%. After mixing, the slurry is stirred at a low speed of 1000 r / min for 10 min to obtain a uniform slurry.

[0094] The first slurry is coated on the surface of the thinning area to form a first wet film, and then the second slurry is coated on the surface of the first wet film to form a second wet film. By controlling the coating amount of the first slurry and the second slurry, the functional layer formed after drying completely covers and fills the thinning area, and the surface of the thinning area is leveled with the thickness of the adjacent non-thinning area.

[0095] The coated pole piece is placed in a hot air circulation oven at 65°C for 12 minutes to remove the solvent and gel the components, forming an integrated functional layer composed of a water-based adhesive layer and an electrolyte layer on the surface of the thinning area. After curing, the thickness of the water-based adhesive layer is 3 μm, and the thickness of the electrolyte layer is 3 μm, with a thickness ratio of 1:1.

[0096] Comparative Example 1 The negative electrode thinning area is not coated with any functional layer.

[0097] Comparative Example 2 The thinned area was coated only with the water-based adhesive layer of the formulation of Example 1, and after drying, the water-based adhesive layer formed completely covered and filled the thinned area, with its surface level with the thickness of the adjacent main body area.

[0098] Comparative Example 3 The thinned area was coated only with the electrolyte layer of the formulation of Example 1, and after drying, the water-based adhesive layer formed completely covered and filled the thinned area, with its surface level with the thickness of the adjacent main body area.

[0099] Comparative Example 4 10 wt% PVDF-HFP, 10 wt% LiTFSI were added into 80 wt% NMP solvent, after mixing, a uniform slurry was obtained by dispersion treatment at a rotation speed of 3000 r / min for 20 min. It was coated on the thinned area of the negative electrode substrate, and the dry thickness was controlled to be 6 μm. Then it was dried at 90°C under vacuum conditions for 12 hours to remove the solvent.

[0100] Preparation of the battery: Preparation of the positive electrode: NCM622, conductive carbon black (SP), and binder PVDF were added into the solvent NMP according to a mass ratio of 97%:1.8%:1.2%, after mixing, they were coated on an aluminum foil by a coating machine, dried, and rolled to obtain the positive electrode.

[0101] Separator: PE separator was selected.

[0102] The positive electrode, separator, and negative electrode sheet of Examples 1-6 and Comparative Examples 1-4 prepared in the above steps were wound, sealed after liquid injection (1M LiPF6 in EC:DEC=1:1 v / v electrolyte), and assembled into a battery for testing.

[0103] Performance test: The batteries assembled from the negative electrode sheets of Examples 1-6 and Comparative Examples 1-4 were tested for performance, and the test method is as follows, and the test results are shown in Table 1.

[0104] (1) Test method of peel strength Cut the prepared negative electrode sheet (including the thinned area and the functional layer) into a strip-shaped sample with a width of 25.0 ± 0.2 mm and a length of not less than 150 mm. Cut a high-strength double-sided adhesive tape with a length of about 100 mm, tear off the protective film on one side, and paste it flat on the center of the standard steel plate. Roll it with a pressure roller at a speed of about 300 mm / min for 3 times in one direction to ensure that there are no bubbles between the tape and the steel plate and that the tape is firmly attached. Tear off the protective film on the other side of the double-sided adhesive tape. Align the active material layer (including the functional layer) of the negative electrode sheet sample accurately and paste it flat on the exposed surface of the double-sided adhesive tape with the active material layer facing down. Ensure that the electrode sheet is in full contact with the tape, and that enough length (about 50 mm) is reserved at one end of the current collector for clamping. Roll back and forth on the tape with a standard 2 kg roller for 3 times to provide constant and sufficient pressing force. Fix the sample with the tape on the rigid plate with the free end of the tape facing up and the end to be peeled (about 20 mm) hanging freely. Fix the prepared sample plate on the base of the testing machine. Carefully clamp the free end of the tape into the upper clamp of the testing machine to ensure firm and centered clamping. Start the test program and move the upper clamp upward at a constant speed (300 mm / min) to peel the tape together with the functional layer adhered thereto from the negative electrode substrate. Continue peeling for a length of at least 100 mm and record the force value (F) in real time during peeling. From the force-displacement curve, discard the data of the initial and final 20 mm, and take the average peeling force F_avg (unit: N) of the middle stable peeling section (about 60 mm).

[0105] Calculate the peeling strength (σ, unit: N / m): σ = F_avg / w Where: F_avg is the average peeling force (N), and w is the sample width (0.025 m).

[0106] (2) Battery cycle test method: Test temperature: 45℃ ± 2℃ ① Charge at 1C to the cut-off voltage (4.4V), with a cut-off current of 0.05C, and stand for 30 min; ② Discharge at 1C to the discharge cut-off voltage (2.8V), stand for 30 min; Cycle ①~②, record the discharge capacity of the 1st to 5th cycle, and calculate the arithmetic mean as the initial discharge capacity; Continue to cycle to the 500th cycle, and record the discharge capacity of the 500th cycle; Capacity retention rate after 500 cycles of battery cycling = discharge capacity of the 500th cycle / initial discharge capacity * 100%.

[0107] (3) Interface peeling strength test after battery cycling: After the cycle, the battery was disassembled in a glove box (filled with argon, water and oxygen content <0.1 ppm). The negative electrode sheet was carefully taken out and immersed in a sufficient amount of anhydrous dimethyl carbonate (DMC) solvent, gently shaken and washed for 30 seconds to remove residual electrolyte and soluble by-products. The washed electrode sheet was naturally dried at room temperature in the glove box. According to the method described in the cycle before peeling strength test, a strip-shaped sample with a width of 25.0 mm and a length of at least 150 mm was cut at the corresponding position of the thinned area of the electrode sheet for testing.

[0108] Table 1 Performance test results of examples and comparative examples The experimental data in Table 1 show that: (1) The cycle capacity retention rate of all examples (double layer) is significantly higher than that of Comparative Example 2 (only adhesive layer) and Comparative Example 3 (only electrolyte layer). The interface combination of Comparative Example 3 is poor, and the ion conduction of Comparative Example 2 is insufficient, which proves the indispensability of the double layer cooperative design of the application.

[0109] (2) Example 1 is superior to Comparative Example 4 (organic solvent system) in cycle retention rate and interface strength after cycling, which verifies the core technical advantages of the all-aqueous system in solving interface residual weakening and achieving long-term stability.

[0110] (3) Examples 1, 4 and 5 (thickness ratios of 1:3, 1:2 and 1:4, respectively) all show excellent comprehensive performance. Although Example 6 (thickness ratio 1:1) has the highest initial and post-cycling peeling strength, the capacity retention rate is relatively low, indicating that when the aqueous adhesive layer is too thick, although the interface combination is stronger, the relatively thin electrolyte layer limits the efficiency of ion transport, resulting in a decrease in long-term capacity retention. This proves the importance of controlling the thickness ratio to 1:2~1:4 for balancing performance.

[0111] The application eliminates the use of organic solvents through an all-aqueous system, avoiding the environmental, safety and interface residue hazards brought by organic solvents. The aqueous adhesive layer containing nano-enhancing materials achieves strong interface, solving the problem of weak interface combination in traditional systems; the electrolyte layer builds a continuous ion channel and cooperates with the aqueous adhesive layer to compensate for the insufficient ion transport caused by the sudden change in thickness in the thinned area.

[0112] The technical features of the above-described examples can be combined in any way. To make the description concise, not all possible combinations of the technical features in the above-described examples are described, but as long as the combinations of the technical features do not contradict, they should be considered within the scope of the present disclosure.

[0113] The above embodiments only express several implementation manners of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the patent. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, several modifications and improvements can be made, which are within the scope of protection of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A lithium-ion battery negative electrode assembly, characterized in that, include: A current collector and an active material layer disposed on the surface of the current collector, the active material layer comprising a main region and a thinned region of different thicknesses, wherein the thickness of the thinned region is less than the thickness of the main region; A functional layer is provided on the surface of the thinning area, the functional layer completely covers the thinning area, and the thickness of the functional layer is flush with the thickness of the main body area in the thickness direction; In the direction from the current collector to the active material layer, the functional layer includes an aqueous adhesive layer and an electrolyte layer sequentially stacked on the surface of the thinned area.

2. The lithium-ion battery negative electrode assembly according to claim 1, characterized in that, The water-based adhesive layer includes a water-based adhesive and a nano-reinforcing material dispersed in the water-based adhesive, wherein the mass ratio of the water-based adhesive to the nano-reinforcing material is (20~40):(2~10).

3. The lithium-ion battery negative electrode assembly according to claim 2, characterized in that, The nano-reinforcing materials include one or more of nanocellulose, nanocrystalline cellulose, carbon nanotubes, nanosilica, and graphene oxide.

4. The lithium-ion battery negative electrode assembly according to claim 1, characterized in that, The electrolyte layer comprises a polymer matrix, a lithium salt, and a plasticizer, wherein the mass ratio of the polymer matrix, the lithium salt, and the plasticizer is (15~25):(8~15):(3~10).

5. The lithium-ion battery negative electrode assembly according to claim 4, characterized in that, The polymer matrix includes one or more of polyvinyl alcohol, polyethylene oxide and its derivatives, aqueous dispersions of fluorinated polymers, polyacrylonitrile and its copolymers, natural polymers and their derivatives, and synthetic water-soluble polymers.

6. The lithium-ion battery negative electrode assembly according to claim 4, characterized in that, The lithium salt includes one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium bis(oxalate)borate, lithium difluorooxalateborate, and lithium tetrafluoroborate.

7. The lithium-ion battery negative electrode assembly according to claim 1, characterized in that, The thickness of the functional layer is 5μm~60μm; The dry thickness ratio of the aqueous adhesive layer to the electrolyte layer is 1:2 to 1:

4.

8. A method for manufacturing a lithium-ion battery negative electrode assembly as described in any one of claims 1 to 7, characterized in that, Includes the following steps: Provide a negative electrode substrate with a main region and a thinned region; Prepare a first slurry for forming an aqueous binder layer and a second slurry for forming an electrolyte layer; The first slurry is coated onto the surface of the thinned area to form a first wet film, and then the second slurry is coated onto the surface of the first wet film to form a second wet film; as well as The electrode coated with the first wet film and the second wet film is heat-treated to remove the solvent and form a functional layer.

9. The method for manufacturing a lithium-ion battery negative electrode assembly according to claim 8, characterized in that, The first slurry comprises, by mass fraction: 20% to 40% aqueous binder, 2% to 10% nano-reinforcing material and 50% to 70% first aqueous solvent; the second slurry comprises: 15% to 25% polymer matrix, 8% to 15% lithium salt, 3% to 10% plasticizer and 50% to 65% second aqueous solvent. Wherein, the first aqueous solvent and the second aqueous solvent independently include one or more of the following: deionized water, distilled water, ultrapure water, and a mixture of water and a water-soluble organic solvent; when a mixture of water and a water-soluble organic solvent is selected, the mass percentage of water in the mixture is not less than 70%; The water-soluble organic solvent includes one or more of ethanol, isopropanol, ethylene glycol, and propylene glycol.

10. The method for manufacturing a lithium-ion battery negative electrode assembly according to claim 8, characterized in that, The heat treatment process involves hot air circulation curing at 60℃~80℃ for 10 to 15 minutes.

Images