Composite negative electrode, method for preparing the same, and lithium ion battery

Abstract

The application discloses a composite negative electrode and a preparation method thereof and a lithium ion battery. The composite negative electrode comprises a current collector, a negative active material layer and a gradient composite structure arranged on at least one side of the current collector. The negative active material layer comprises an active material area and a thinned area at an end portion, and the gradient composite structure covers and adheres to the thinned area. The gradient composite structure comprises a rigid bonding layer, an elastic buffer layer and an ion conductive layer which are sequentially combined along the thickness direction of the negative active material layer and gradually away from the current collector. The ion conductive layer overlaps with the edge of the active material area. The gradient composite structure provided by the application is suitable for 200%-300% volume expansion of the high-silicon thinned area, effectively inhibits problems such as cracking of the thinned area and lithium precipitation caused by volume expansion of the high-silicon thinned area, and improves the cycle life and safety of the battery.

Description

Composite anode and its preparation method, lithium-ion battery Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a composite negative electrode and its preparation method, and a lithium-ion battery. Background Technology

[0002] Battery electrode sheets typically consist of a current collector and an active material layer, which is formed by coating a slurry onto the current collector. Since the slurry is a Newtonian fluid, material buildup is prone to occur at the edges during coating, affecting the thickness of the electrode edge area and subsequent rolling. To avoid defects such as thick edges or bulging edges during coating, the edge thickness is generally controlled during the coating process, reducing the thickness of the active material layer edge area (referred to in the industry as the thinning zone).

[0003] High silicon content Silicon-based anodes, due to their high theoretical capacity density (>1500mAh / g), are a core technology for improving the energy density of lithium batteries. However, the thinned region suffers from problems such as accelerated expansion and deterioration of wetting. Compared to the traditional graphite anode with a volume expansion rate of only 10%, the volume expansion rate of silicon-based materials after lithium intercalation reaches 200%~300%, causing cracking of the active material layer in the thinned region. The cracked active material loses electrical contact with the current collector, forming an electrochemical "dead zone," which leads to a rapid drop in battery capacity. Furthermore, the expansion causes the pore structure to collapse, preventing electrolyte penetration. This drastically increases local impedance, forcing lithium ions to deposit as lithium dendrites on the anode surface during charging, significantly increasing the risk of internal short circuits and thermal runaway. At the same time, uneven current distribution accelerates the overall aging of the battery, resulting in a significant reduction in the battery's cycle performance. Summary of the Invention

[0004] This invention aims to at least solve one of the technical problems existing in the prior art. To this end, this invention proposes a composite anode and its preparation method, as well as a lithium-ion battery, to solve the problem that the thinned area at the edge of the current silicon-based anode material cracks and the pore structure collapses due to excessive expansion rate, leading to internal short circuits, thermal runaway, and low battery cycle performance.

[0005] In a first aspect, the present invention provides a composite negative electrode, comprising: a current collector, a negative electrode active material layer disposed on at least one side of the current collector, and a gradient composite structure;

[0006] The negative electrode active material layer includes an active material region and a thinned region located at the end, and the gradient composite structure covers and adheres to the thinned region;

[0007] The gradient composite structure includes a rigid bonding layer, an elastic buffer layer, and an ion-conducting layer sequentially composited along the thickness direction of the negative electrode active material layer and gradually moving away from the current collector. The ion-conducting layer overlaps with the edge of the active material region; the overlap between the ion-conducting layer and the edge of the active material region in the negative electrode active material layer is 1mm to 3mm; the thickness of the rigid bonding layer is 1.5μm to 5μm; the thickness of the elastic buffer layer is 2μm to 10μm; and the thickness of the ion-conducting layer is 1.5μm to 5μm.

[0008] The rigid adhesive layer includes an adhesive and a conductive agent. The adhesive includes at least one of polyimide, polyamide-imide, polybenzimidazole, and epoxy resin. The conductive agent includes at least one of carbon nanotubes, graphene, acetylene black, Super P, and Ketjen black.

[0009] The elastic buffer layer comprises an elastomer, a first lithium salt, and a toughening agent. The elastomer comprises at least one of polyurethane, polysiloxane, hydrogenated nitrile rubber, and polyolefin elastomer. The first lithium salt comprises at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, and lithium dioxaborate. The toughening agent comprises at least one of calcium carbonate, silica, alumina, montmorillonite, and carbon fiber powder.

[0010] The ion-conducting layer comprises a polymer, a second lithium salt, and an organic solvent. The polymer comprises at least one of polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene glycol, polyethylene oxide, polyacrylonitrile, and polymethyl methacrylate. The second lithium salt comprises at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium difluorooxalateborate, lithium trifluoromethanesulfonylimide, and lithium bis(pentafluoroethylsulfonyl)imino.

[0011] In some embodiments, the thickness of the gradient composite structure is 5 μm to 20 μm.

[0012] In some embodiments, the mass ratio of the conductive agent to the adhesive is 1:(5~10).

[0013] The mass ratio of the elastomer to the toughening agent is (7~15):1, and the mass percentage of the first lithium salt in the elastic buffer layer is 8%~15%.

[0014] The mass ratio of the polymer to the organic solvent is (1~10):1, and the mass of the second lithium salt accounts for 12%~20% of the total mass of the ion-conducting layer.

[0015] In some embodiments, the mass ratio of the negative electrode active material in the thinned region to the negative electrode active material in the active material region is (3-4):5.

[0016] In a second aspect, the present invention also provides a method for preparing a composite negative electrode, the method being used to prepare a composite negative electrode as described in any one of the first aspects, the method comprising:

[0017] A negative electrode sheet is provided, having an active material region and a thinned region at its end;

[0018] Slurries for a rigid bonding layer, an elastic buffer layer, and an ion-conducting layer were prepared separately.

[0019] On the surface of the thinned area, the rigid adhesive layer, the elastic buffer layer, and the ion-conducting layer are sequentially coated in a direction away from the current collector.

[0020] The coated electrode is subjected to gradient curing treatment to form a gradient composite structure on the surface of the thinned area;

[0021] The gradient curing process includes: pre-baking at 70℃~90℃ and then curing under vacuum conditions at 100℃~130℃.

[0022] In some embodiments, after the step of providing a negative electrode sheet having an active material region and a thinned region at its end, and before the step of sequentially coating the surface of the thinned region in a direction away from the current collector to form the rigid adhesive layer, the elastic buffer layer, and the ion conduction layer, the step of surface roughening the thinned region is further included.

[0023] Thirdly, the present invention also provides a lithium-ion battery comprising the composite negative electrode described in the first aspect.

[0024] The above-described embodiments of the present invention have at least one or more of the following beneficial effects:

[0025] This invention provides a composite negative electrode and its preparation method, as well as a lithium-ion battery. The negative electrode active material layer has a thinned region at its end, and the thinned region is connected to a gradient composite structure. The gradient composite structure includes a rigid bonding layer, an elastic buffer layer, and an ion conducting layer that are sequentially composited in the thickness direction of the negative electrode active material layer as they move away from the current collector. This structure is adapted to the 200%~300% volume expansion of the high-silicon thinned region. The interfacial bonding force between the gradient composite structure and the expanded active material is improved, thereby effectively suppressing problems such as cracking and lithium plating in the thinned region caused by the high-silicon volume expansion.

[0026] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0027] The disclosure of this invention will become more readily understood with reference to the accompanying drawings. It will be readily understood by those skilled in the art that these drawings are for illustrative purposes only and are not intended to limit the scope of protection of this invention. Furthermore, similar numbers in the drawings are used to denote similar components, wherein:

[0028] Figure 1 is a schematic diagram of the composite negative electrode provided in one embodiment of the present invention.

[0029] Among them: 1. Negative electrode active material layer; 11. Thinning area; 12. Active material area; 2. Gradient composite structure; 21. Rigid bonding layer; 22. Elastic buffer layer; 23. Ion conduction layer; A. Overlapping area; 3. Current collector. Detailed Implementation

[0030] Some embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0031] As described in the background section, silicon content In the thinned region formed by the electrode coating process, the high silicon content of the high-silicon anode material is high. The volume expansion of the material causes cracks in the active material layer of the thinned area, forming an electrochemical "dead zone" that leads to rapid capacity decay. The expansion also causes the pore structure to collapse, preventing the electrolyte from effectively wetting and penetrating, drastically increasing the local ion transport impedance. This increased local impedance significantly increases the risk of internal short circuits and thermal runaway. Furthermore, the uneven current distribution caused by the thinned area accelerates the aging of the entire battery, resulting in a significant reduction in cycle performance.

[0032] To address the aforementioned problems, this invention creatively proposes a composite negative electrode and its preparation method, as well as a lithium-ion battery. A gradient composite structure is overlapped in the thinned region at the end of the negative electrode active material layer. The gradient composite structure includes a rigid bonding layer, an elastic buffer layer, and an ion-conducting layer sequentially laminated along the thickness direction of the negative electrode active material layer and gradually moving away from the current collector, adapting to the high-silicon thinned region. The volume expansion of silicon enhances the interfacial bonding between the gradient composite structure and the expanded active material, thereby effectively suppressing problems such as cracking in the thinned area and lithium plating caused by the volume expansion of high silicon.

[0033] The present invention will be specifically described below through specific embodiments.

[0034] Specifically, the present invention provides a composite negative electrode, as shown in FIG1, including a current collector 3, a negative electrode active material layer 1 disposed on at least one side of the current collector 3, and a gradient composite structure 2. The negative electrode active material layer 1 includes an active material region 12 and a thinned region 11 located at the end. The gradient composite structure 2 covers and adheres to the thinned region 11.

[0035] Thinning region 11 refers to the gently sloping region with gradually decreasing thickness formed at the end edge of the negative electrode active material layer 1. The gradient composite structure 2 covering and adhering to the thinning region 11 means that the gradient composite structure 2 is disposed on the sloping region at the end of the negative electrode active material layer 1. For ease of understanding and description, the region with uniform thickness outside the sloping region in the negative electrode active material layer 1 is named active material region 12 in this application.

[0036] The gradient composite structure 2 includes a rigid bonding layer 21, an elastic buffer layer 22, and an ion conduction layer 23 that are sequentially composited along the thickness direction of the negative electrode active material layer 1 and gradually move away from the current collector 3. The ion conduction layer 23 overlaps with the edge of the active material region 12.

[0037] In this application, the rigid adhesive layer 21, the elastic buffer layer 22, and the ion conduction layer 23 are flush with the outer edges of the negative electrode active material layer 1 in the length and width directions in the horizontal direction.

[0038] In some embodiments, the negative current collector 3 may be selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrate coated with conductive metal, and combinations thereof.

[0039] In some embodiments, the ion-conducting layer 23 overlaps with the edge of the active material region 12 in the negative electrode active material layer 1 by 1 mm to 3 mm, forming an overlap region A. Specifically, one end of the ion-conducting layer 23 near the active material region 12 extends out of the thinned region 11 and overlaps with the active material region 12 to avoid the appearance of an interface gap between the negative electrode active material layer 1 and the gradient composite structure 2 after expansion, and at the same time eliminate the interface transport breakpoint, thereby suppressing the formation of lithium dendrites and ensuring battery cycle safety.

[0040] Optionally, the overlap length between the ion-conducting layer 23 and the edge of the active material region 12 in the negative electrode active material layer can be 1 mm, 1.3 mm, 1.5 mm, 1.8 mm, 2 mm, 2.2 mm, 2.5 mm, 2.7 mm, 3 mm, or any value within the above overlap length range.

[0041] In some embodiments, the thickness of the rigid adhesive layer 21 is 1.5 μm to 5 μm. Optionally, the thickness of the rigid adhesive layer 21 can be 1.5 μm, 1.7 μm, 2 μm, 2.3 μm, 2.5 μm, 2.8 μm, 3.0 μm, 3.4 μm, 4.0 μm, 4.5 μm, 4.6 μm, 5.0 μm, or any value within the above thickness range. The rigid adhesive layer 21 provides mechanical strength and strong adhesion for the gradient composite structure 2. If the thickness of the rigid adhesive layer 21 is less than 1.5 μm, the thickness will be too thin, resulting in insufficient adhesion and mechanical strength, making it impossible to anchor the subsequent elastic buffer layer 22 and ion-conducting layer 23. If the thickness of the rigid adhesive layer 21 is greater than 5 μm, the thickness will be too thick, limiting the deformation space of the elastic buffer layer 22 and increasing the interfacial impedance.

[0042] In some preferred embodiments, the bonding strength of the rigid adhesive layer 21 can reach ≥2.5N / mm, which can ensure that the rigid adhesive layer 21 firmly anchors the thinned area 11 and the elastic buffer layer 22, resists the peeling stress generated by high silicon expansion, and prevents the gradient composite structure 2 from detaching from the thinned area 11; the tensile strength ≥100MPa can provide sufficient mechanical toughness to prevent itself from cracking and failing when high borosilicate expands, while providing a stable support substrate for the elastic buffer layer 22 and the ion conduction layer 23.

[0043] In some embodiments, the thickness of the elastic buffer layer 22 is 2 μm to 10 μm. Optionally, the thickness of the elastic buffer layer 22 can be 2 μm, 3.2 μm, 4.5 μm, 5.1 μm, 6.0 μm, 7.3 μm, 8.3 μm, 9.5 μm, or 10 μm, or any value within the above thickness range. If the thickness of the elastic buffer layer 22 is less than 2 μm, it cannot adequately absorb expansion stress and is difficult to mitigate the impact caused by volume changes in the high-silicon negative electrode active material layer 1. If it is greater than 10 μm, it will increase the ion transport path length, leading to an increase in interface impedance.

[0044] Preferably, the elastic buffer layer 22 has an elongation at break ≥300%, possessing extremely strong deformation adaptability, effectively absorbing expansion stress, preventing the gradient composite structure 2 from cracking due to stress concentration, protecting the structural integrity of the ion conduction layer 23, and preventing ion channel breakage; the ion conductivity is ≥5×10⁻⁶. -4 With a density of s / cm, it can fill the conduction gradient between the rigid bonding layer 21 and the ion conduction layer 23, avoid the formation of ion transport breakpoints, and ensure that lithium ions can quickly pass through the elastic buffer layer 22 during silicon expansion. In conjunction with the ion conduction layer 23, it reduces local ion accumulation and suppresses the formation of lithium dendrites.

[0045] In some embodiments, the thickness of the ion-conducting layer 23 is 1.5 μm to 5 μm. Optionally, the thickness of the ion-conducting layer 23 can be 1.5 μm, 2.0 μm, 2.3 μm, 2.5 μm, 3.0 μm, 3.5 μm, 3.7 μm, 4.0 μm, 4.5 μm, 5.0 μm, or any value within the above thickness range. If the thickness of the ion-conducting layer 23 is less than 1.5 μm, the ion conduction path will be incomplete and susceptible to fracture due to expansion stress; if the thickness is greater than 5.0 μm, the thickness will be too large, increasing the ion transport distance and raising the interface impedance.

[0046] More preferably, the ion conductivity of the ion-conducting layer 23 is ≥1.2×10⁻⁶. -3 With a conductivity of s / cm, the high conductivity can quickly conduct lithium ions. Combined with the medium-high conductivity of the elastic buffer layer 22, it forms a gradient transport system to avoid the accumulation of lithium ions caused by the collapse of pores after silicon expansion.

[0047] In some embodiments, the thickness of the gradient composite structure 2 is 5 μm to 20 μm. Optionally, the thickness of the gradient composite structure 2 can be 5 μm, 7.5 μm, 10 μm, 13.4 μm, 15 μm, 17.6 μm, 19.5 μm, 20 μm, or any value within the above thickness range.

[0048] In some embodiments, the thickness ratio of the rigid adhesive layer 21, the elastic buffer layer 22, and the ion-conducting layer 23 is (1.5~5):(2~10):(1.5~5). Optionally, the thickness ratio of the rigid adhesive layer 21, the elastic buffer layer 22, and the ion-conducting layer 23 can be 1.5:2:1.5, 2.1:2.5:1.8, 3:5:4, 3.5:2:1.5, 3.2:8:3.5, 4:4:3, 5:2:1.5, 5:10:1.5, 4.1:7.2:3.1, or any value within the above range.

[0049] The thickness ratio of the rigid bonding layer 21, the elastic buffer layer 22, and the ion conduction layer 23 is (1.5~5):(2~10):(1.5~5), ensuring that the elastic buffer layer 22 has sufficient space to absorb expansion stress. The rigid bonding layer 21 has strong anchoring and the ion conduction layer 23 has high conduction efficiency without encroaching on the deformation space of the elastic buffer layer 22. The three work together to form a gradient fit of stability-buffering-efficient conduction, avoiding synergistic failure caused by excessive thickness or thinness of a single functional layer.

[0050] In some embodiments, the rigid adhesive layer 21 includes an adhesive and a conductive agent. The adhesive provides strong adhesion to stably bond the gradient composite structure 2 to the thinned region 11, and the conductive agent improves electron transport efficiency.

[0051] The elastic buffer layer 22 includes an elastomer, a first lithium salt, and a toughening agent. The elastomer has high elasticity and absorbs silicon expansion stress through stretching and rebound. The first lithium salt and the ion conduction layer 23 form an ion gradient supply to ensure the continuity of ion transport at the negative electrode during the silicon expansion process. The toughening agent inhibits excessive deformation of the elastomer and enhances the interlayer mechanical strength to prevent permanent failure of the elastic buffer layer 22.

[0052] The ion-conducting layer 23 comprises a polymer, a second lithium salt, and an organic solvent. The polymer, organic solvent, and second lithium salt are mixed to construct the ion-conducting channel. Even if the active material expands and the pore structure collapses, making it difficult for the liquid electrolyte to re-wet, it ensures efficient and uniform lithium ion transport, avoiding lithium plating caused by localized ion depletion. Furthermore, the polymer itself possesses a certain degree of flexibility, allowing it to deform in coordination with the underlying elastic buffer layer 22 without breaking during significant elastic deformation, thus maintaining the physical integrity of the ion-conducting channel.

[0053] The ion-conducting layer 23 is in direct contact with the active material region 12 in the non-thinned area, which rapidly conducts ions and reduces the interface impedance. The first lithium salt in the elastic buffer layer 22 and the second lithium salt in the ion-conducting layer 23 form an ion gradient supply. Combined with the electronic conduction efficiency improved by the conductive agent in the rigid adhesive layer 21, the overall interface impedance of the battery is kept at a low level and can be cycled stably.

[0054] In some embodiments, the mass ratio of the conductive agent to the adhesive is 1:(5~10). Optionally, the mass ratio of the conductive agent to the adhesive may be 1:5, 1:6.5, 1:7.3, 1:8, 1:9.2, 1:10, or any ratio within the above range.

[0055] The mass ratio of the elastomer to the toughening agent is (7~15):1. Optionally, the mass ratio of the elastomer to the toughening agent can be 7:1, 8.2:1, 9.5:1, 10:1, 11.4:1, 12.6:1, 13.5:1, 14.3:1, 15:1, or any ratio within the above range. The mass percentage of the first lithium salt in the elastic buffer layer 22 is 8%~15%. Optionally, the mass percentage of the first lithium salt in the elastic buffer layer 22 can be 8%, 9.1%, 10%, 11.3%, 12.5%, 13.8%, 14.7%, 15%, or any value within the above range.

[0056] The mass ratio of the polymer to the organic solvent is (1~10):1. Optionally, the mass ratio of the polymer to the organic solvent can be 1:1, 2:1, 3.5:1, 4.7:1, 5.4:1, 6.3:1, 7.2:1, 8.6:1, 9.1:1, 10:1, or any value within the above range. The mass percentage of the second lithium salt in the ion-conducting layer 23 is 12%~20%. Optionally, the mass percentage of the second lithium salt in the ion-conducting layer 23 can be 12%, 13.2%, 14.5%, 15%, 16.7%, 17.3%, 18.6%, 19.4%, 20%, or any value within the above range.

[0057] In some embodiments, the binder includes at least one of polyimide, polyamide-imide, polybenzimidazole, and epoxy resin; the conductive agent includes at least one of carbon nanotubes, graphene, acetylene black, Super P, and Ketjen black; the elastomer includes at least one of polyurethane, polysiloxane, hydrogenated nitrile rubber, and polyolefin elastomer; the first lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, and lithium dioxalate borate; the toughening agent includes at least one of calcium carbonate, silica, alumina, montmorillonite, and carbon fiber powder; the polymer includes at least one of polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene glycol, polyethylene oxide, polyacrylonitrile, and polymethyl methacrylate; the second lithium salt includes at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium dioxalate borate, lithium trifluoromethanesulfonylimide, and lithium bis(pentafluoroethylsulfonyl)imino; and the organic solvent includes at least one of carbonates, ethers, nitrile solvents, and ester solvents. For illustrative purposes only and not as a limitation: carbonates include ethylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, etc.; ethers include 1,3-dioxolane, tetrahydrofuran, ethylene glycol dimethyl ether, etc.; nitriles include acetonitrile, adiponitrile, succinic anhydride, etc.; and ester solvents include propyl propionate, ethyl acetate, fluorocarboxylic acid esters, γ-butyrolactone, etc.

[0058] In some embodiments, in the negative electrode active material layer 1, the mass ratio of the negative electrode active material in the thinned region 11 to that in other regions (active material region 12) is (3-4):5. Optionally, the mass ratio of the negative electrode active material in the thinned region 11 to that in the active material region 12 can be 3:5, 3.3:5, 3.5:5, 3.7:5, 4:5, or any ratio within the above range.

[0059] This invention also provides a method for preparing a composite negative electrode, the method being used to prepare a composite negative electrode as described in any of the above embodiments, the method comprising:

[0060] S110, providing a negative electrode sheet having an active material region and a thinned region located at its end.

[0061] The negative electrode sheet includes a current collector and a negative electrode active material layer disposed on at least one side of the current collector. The negative electrode active material layer includes an active material region and a thinned region located at the end of the active material region.

[0062] S120. Prepare slurries for the rigid bonding layer, elastic buffer layer and ion-conducting layer respectively.

[0063] S130. On the surface of the thinned area, the rigid adhesive layer, the elastic buffer layer and the ion-conducting layer are sequentially coated in a direction away from the current collector to form the rigid adhesive layer, the elastic buffer layer and the ion-conducting layer.

[0064] S140. The coated electrode is subjected to gradient curing treatment to form the gradient composite structure on the surface of the thinned area.

[0065] The gradient curing process includes: pre-baking at 70℃~90℃, followed by curing under vacuum conditions at 100℃~130℃. In some embodiments, S120 for preparing the rigid adhesive layer slurry includes:

[0066] First, the binder matrix is ​​dissolved in the first solvent, and then the conductive agent is added. The ratio of the conductive agent to the binder is (1:5) to (1:10). The mixture is dispersed at a high speed of 1500 r / min to 4500 r / min for 30 min to 60 min. The first solvent can be any organic solvent that can dissolve the conductive agent and the binder and completely evaporate during the subsequent curing process, such as N-methylpyrrolidone. This invention does not impose any specific restrictions on this.

[0067] The preparation of the elastic buffer layer slurry includes:

[0068] First, the elastomer and toughening agent are mixed in a mass ratio of (7:1) to (15:1) and stirred for less than 60 minutes at a speed of 1500 r / min to 4500 r / min. Then, the first lithium salt and the second solvent are added and stirred until dissolved. The concentration of the first lithium salt is 0.8 mol / L to 1.5 mol / L and the mass percentage of the first lithium salt is 8% to 15%.

[0069] The preparation of the ion-conducting layer slurry includes: dissolving a polymer in an organic solvent, wherein the mass ratio of the polymer to the organic solvent is (1~10):1, and adding a second lithium salt and stirring until homogeneous, wherein the concentration of the second lithium salt is 1mol / L~2mol / L, and the mass percentage of the second lithium salt in the mixed slurry is 12%~20%.

[0070] In some embodiments, step S130 includes:

[0071] The rigid adhesive layer, elastic buffer layer and ion conduction layer slurry are sequentially coated onto the thinned area at the end of the negative electrode active material layer.

[0072] Specifically, a rigid adhesive layer, an elastic buffer layer, and an ion-conducting layer slurry are applied sequentially using three nozzles. The first nozzle applies the rigid adhesive layer to form a wet film of 5μm to 12μm. After an 8-second interval, the second nozzle applies the elastic buffer layer to form a wet film of 10μm to 25μm. After another 5-second interval, the third nozzle applies the ion-conducting layer to form a wet film of 6μm to 12μm, ensuring interlayer mutual solubility and bonding.

[0073] In some embodiments, step S140 includes:

[0074] After the rigid adhesive layer, elastic buffer layer and ion conduction layer are pre-baked at 70℃~90℃ for 3min~8min, they are vacuum cured at 100℃~130℃ for 10min~30min. After cooling to room temperature, a gradient composite structure is obtained and attached to the active material region to obtain a composite negative electrode.

[0075] Specifically, gradient curing promotes the cross-linking of elastomers in the elastic buffer layer, forming a gradient composite structure after cooling to room temperature. Gradient curing is employed to accommodate the different functional requirements of the three-layer composite structure and avoid interlayer delamination, solvent residue, or structural failure. First, low-temperature pre-baking slowly removes the solvent from each layer to prevent rapid heating from causing solvent boiling and creating pores. Simultaneously, initial interlayer wetting and fusion are followed by high-temperature vacuum curing. This aims to promote the cross-linking reaction of the elastomers (such as polyurethane) in the elastic buffer layer, forming a three-dimensional network structure, thereby enhancing its structural stability and lithium salt fixation while maintaining high elasticity.

[0076] The composite anode prepared in this application gradually transitions in the thickness direction according to the functional and performance gradients of rigidity-elasticity-ion conduction as it moves further away from the current collector. It is not a simple superposition, but a layered design adapted to the needs of the high-silicon thinning region, with each layer having complementary functions and performance.

[0077] In some embodiments, after step S110 of providing the negative electrode sheet and before step S130 of coating, a step of surface roughening treatment of the thinned area is included to improve the mechanical interlocking force between the thinned area and the rigid adhesive layer.

[0078] In some embodiments, roughening the thinned area includes: making the surface roughness of the thinned area Ra=1.0μm-1.5μm by sandblasting, wherein the sand particle size is 30μm~80μm and the pressure is 0.2MPa~0.5MPa.

[0079] The roughening treatment of the thinned region 11 increases the contact area between the gradient composite structure and the surface of the active material in the thinned region 11, providing sufficient mechanical interlocking sites for the rigid adhesive layer 21, allowing its strong adhesion to be fully utilized. The tight bonding between the rigid adhesive layer 21, the surface of the active material in the thinned region 11, and the elastic buffer layer 22 further enhances the interlayer mutual solubility effect between the elastic buffer layer 22 and the ion-conducting layer 23, ultimately achieving high peel strength between the gradient composite structure and the thinned region, preventing detachment during the expansion process.

[0080] In some embodiments, the method further includes: preparing a negative electrode active material layer having a thinned region.

[0081] In some embodiments, preparing a negative electrode active material layer having a thinned region includes:

[0082] Preparation of negative electrode active material membrane;

[0083] By partitioning the negative electrode active material film, a negative electrode active material layer with a thinned region is obtained.

[0084] Specifically, the process of partitioning the negative electrode active material film to obtain a negative electrode active material layer with a thinned region includes: partially removing the edge of the negative electrode active material film to form a negative electrode active material layer with a thinned region.

[0085] In some embodiments, the length of the thinned region in the length direction of the negative electrode active material layer accounts for 3% to 10%.

[0086] In some embodiments, laser etching is used to partially remove the edge of the negative electrode active material film to form a thinned region. The laser wavelength is 355nm~365nm; the laser power is 5W~15W; the number of etching cycles is 1~3; and the etching depth is 5μm~20μm.

[0087] The present application will be further described in detail below with reference to embodiments. It should be noted that the following embodiments are for illustrative purposes only and are not intended to limit the scope of protection claimed in this application.

[0088] Example 1: Preparation of composite negative electrode:

[0089] 1. Preparation of negative electrode active material film: Silicon, graphite, PTFE and SP are mixed evenly in a solvent at a mass ratio of 9.6:86.4:2:2 to form a negative electrode slurry, which is then coated on the surface of a copper foil with a thickness of 15μm to form a negative electrode active material film.

[0090] 2. Partitioning of the negative electrode active material film: Laser etching is used to form thinned areas, with the surface density of the high-silicon active material area reaching 140 g / m². 2 The surface density of the thinned area is 98 g / m² (70% coating amount), and the length of the thinned area at the edge along the first direction of the negative electrode active material layer accounts for 6% of the active material layer.

[0091] 3. Roughening treatment of the thinned area: The surface roughness of the active material Ra=1.2μm is achieved by sandblasting (sand particle size 50μm, pressure 0.3MPa);

[0092] 4. Preparation of each layer of the three-layer composite structure: ① Rigid bonding layer: First, dissolve PI in N-methylpyrrolidone, then add carbon nanotubes. The ratio of carbon nanotubes to PI is 1:9. Disperse at high speed at 3000r / min for 30min.

[0093] ② Elastic buffer layer: After mixing polyurethane and calcium carbonate at a mass ratio of 75:7 at 3000r / min for 30min, add lithium hexafluorophosphate at a mass ratio of 12% (based on the total mass of elastomer, toughening agent and lithium salt being 100%) and a second solvent and stir until dissolved. The concentration of lithium hexafluorophosphate is 1mol / L.

[0094] ③ Ion-conducting layer: Dissolve PVDF-HFP in organic solvents ethylene carbonate and propylene carbonate, with a mass ratio of PVDF-HFP to organic solvent of 65:17. Add 18% (based on a total mass of 100% for the polymer, lithium salt, and organic solvent) of lithium bis(trifluoromethanesulfonylimide) and stir until homogeneous. The concentration of lithium bis(trifluoromethanesulfonylimide) is 1.5 mol / L.

[0095] 5. Place each layer of the three-layer composite structure in the thinning zone: use three nozzles to coat sequentially. The first nozzle coats the rigid adhesive layer to form a 6μm wet film. After an 8s interval, the second nozzle coats the elastic buffer layer to form a 15μm wet film. After another 5s interval, the third nozzle coats the ion-conducting layer to form a 6μm wet film.

[0096] 6. Gradient curing: First, pre-bake at 80℃ for 5 minutes to remove some of the solvent, then vacuum cure at 120℃ for 10 minutes. After cooling to room temperature, a gradient composite structure is formed. The thicknesses of the three layers after drying are 3μm, 10μm, and 3μm, respectively, and the total thickness of the gradient composite structure is 16μm.

[0097] An electrochemical dilatometer was used to simulate volume expansion and detect whether the composite layer of the gradient composite structure was cracked. After passing the test, the composite was cut according to the battery specifications to obtain a high-silicon composite negative electrode sheet.

[0098] Preparation of the positive electrode: Material ratio: LiNi 0.6 Co 0.1 Mn 0.3 O2 (NCM613) 96% + conductive agent Super P 1% + conductive agent CNT 1.5% + binder PVDF 1.5% is mixed and coated onto aluminum foil.

[0099] Battery fabrication: Using the composite negative electrode sheet prepared above as the negative electrode and the positive electrode sheet prepared above as the positive electrode, with a separator (polyethylene / polypropylene composite membrane) sandwiched in between, an electrolyte is injected. The electrolyte contains 1 mol / L lithium hexafluorophosphate and a mixed solution of EC / DMC / EMC in a ratio of 1:1:1 to prepare a soft-pack battery.

[0100] Example 2: The difference between this example and Example 1 is that the adhesive matrix in the rigid adhesive layer is selected from polyimide; the elastomer in the elastic buffer layer is selected from polyurethane, and the toughening agent is selected from Al2O3; the polymer in the ion conduction layer is selected from polyethylene oxide (PEO).

[0101] Example 3: The difference between this example and Example 1 is that the thickness of the rigid adhesive layer is 5 μm, the thickness of the elastic buffer layer is 6 μm, the thickness of the ion conduction layer is 5 μm, and the total thickness is 16 μm.

[0102] Example 4: The difference between this example and Example 1 is that the thickness of the rigid adhesive layer is 1.5 μm, the thickness of the elastic buffer layer is 2 μm, the thickness of the ion conduction layer is 1.5 μm, and the total thickness is 5 μm.

[0103] Example 5: The difference between this example and Example 1 is that the thickness of the rigid adhesive layer is 4 μm, the thickness of the elastic buffer layer is 9 μm, the thickness of the ion conduction layer is 7 μm, and the total thickness is 20 μm.

[0104] Comparative Example 1: The difference from Example 1 is that a high-silicon anode is used and an active material region and a thinning region are set, but a gradient composite structure is not set.

[0105] Comparative Example 2: The difference from Example 1 is that no rigid adhesive layer is provided, only an elastic buffer layer and an ion conduction layer are provided, with thicknesses of 10 μm and 6 μm, respectively.

[0106] Comparative Example 3: The difference from Example 1 is that no elastic buffer layer is provided, only a rigid adhesive layer and an ion-conducting layer are provided, with thicknesses of 10 μm and 6 μm, respectively.

[0107] Comparative Example 4: The difference from Example 1 is that no ion-conducting layer is provided, only a rigid adhesive layer and an elastic buffer layer are provided, with thicknesses of 10 μm and 6 μm, respectively.

[0108] The performance of the batteries prepared in the above embodiments and comparative examples was tested:

[0109] 1. Peel strength test: Cut a 10mm section perpendicular to the coating direction. Using a 100mm sample strip, adjust the tensile testing machine, setting the peel angle to 180°, the tensile speed to 50mm / min, and the sensor range to 0N~50N. Calibrate the instrument's zero point. Fix the copper foil end of the sample to the lower clamp of the testing machine, and the free end of the polyester film to the upper clamp, ensuring the sample is tension-free and wrinkle-free. Align the starting point of the composite layer peel with the center line of the clamp. Start the equipment and record the peel force-displacement curve in real time, ensuring the peel length is not less than 50mm. Discard the initial 5mm fluctuation data, and take the average peel force of the stable 20mm~40mm segment as the peel strength (unit: N / mm). Calculate the average value of three parallel samples, with an allowable error ≤5%.

[0110] 2. Lithium Deposition Area Test: After cycling, the cell interface is disassembled, and the proportion of lithium dendrite deposition area on the negative electrode surface is quantified through visual observation and image analysis. Image analysis software is used to automatically identify and calculate the lithium deposition area by setting the grayscale threshold of the lithium dendrite deposition area; Lithium deposition area proportion = (Lithium deposition area / Effective electrode area) 100%.

[0111] 3. Capacity retention test: Test the discharge capacity / constant capacity after 100 cycles.

[0112] 4. Interface impedance test: The test method is electrochemical impedance spectroscopy (EIS), and the test frequency range is 10mHz. With an AC signal amplitude of 5mV and a frequency of 100kHz, connect the positive and negative terminals of the battery to the corresponding interfaces of the electrochemical workstation to confirm that the circuit connection is stable. Start the test program and wait for the impedance spectrum (Nyquist plot) to be acquired. Save the test data and then fit the Nyquist plot using the electrochemical workstation software or ZView software.

[0113] 5. Cracking rate test after 100 cycles: After 100 cycles, the cell interface is disassembled, and the cracking rate of the sample is quantified through visual observation and image analysis.

[0114] The performance test results are shown in the table below:

[0115]

[0116] As shown in the table above, the battery embodiments of this application exhibit significantly increased peel strength, a substantial reduction in lithium plating area, improved capacity retention, and a significant decrease in interface impedance, with a low or even zero cracking rate. Furthermore, only by employing a three-layer composite structure consisting of a rigid adhesive layer, an elastic buffer layer, and an ion-conducting layer can excellent peel strength and capacity retention be simultaneously maintained while significantly reducing the lithium plating area, interface impedance, and cracking rate.

[0117] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0118] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0119] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A composite negative electrode, characterized in that, include: The present invention comprises a current collector, a negative electrode active material layer disposed on at least one side of the current collector, and a gradient composite structure; the negative electrode active material layer includes an active material region and a thinned region located at the end, and the gradient composite structure covers and adheres to the thinned region; the gradient composite structure includes a rigid bonding layer, an elastic buffer layer, and an ion conducting layer sequentially composited along the thickness direction of the negative electrode active material layer and gradually moving away from the current collector, the ion conducting layer overlapping the edge of the active material region; the ion conducting layer overlapping the edge of the active material region in the negative electrode active material layer by 1mm to 3mm; the thickness of the rigid bonding layer is 1.5μm to 5μm; the thickness of the elastic buffer layer is 2μm to 10μm; the thickness of the ion conducting layer is 1.5μm to 5μm; the rigid bonding layer includes an adhesive and a conductive agent, the adhesive including at least one of polyimide, polyamide-imide, polybenzimidazole, and epoxy resin, and the conductive agent including carbon nanotubes, graphene, acetylene black, and Super At least one of P and Ketjen black; the elastic buffer layer comprises an elastomer, a first lithium salt, and a toughening agent, wherein the elastomer comprises at least one of polyurethane, polysiloxane, hydrogenated nitrile rubber, and polyolefin elastomer, and the first lithium salt comprises at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, and lithium dioxalate borate; the toughening agent comprises at least one of calcium carbonate, silica, alumina, montmorillonite, and carbon fiber powder; the ion-conducting layer comprises a polymer, a second lithium salt, and an organic solvent, wherein the polymer comprises at least one of polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene glycol, polyethylene oxide, polyacrylonitrile, and polymethyl methacrylate; the second lithium salt comprises at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium dioxalate borate, lithium trifluoromethanesulfonylimide, and lithium bis(pentafluoroethylsulfonyl)imino.

2. The composite negative electrode according to claim 1, characterized in that, The thickness of the gradient composite structure is 5μm~20μm.

3. The composite negative electrode according to claim 1, characterized in that, The mass ratio of the conductive agent to the binder is 1:(5~10); the mass ratio of the elastomer to the toughening agent is (7~15):1; the mass of the first lithium salt accounts for 8%~15% of the total mass of the elastic buffer layer; the mass ratio of the polymer to the organic solvent is (1~10):1; and the mass of the second lithium salt accounts for 12%~20% of the total mass of the ion-conducting layer.

4. The composite negative electrode according to claim 1, characterized in that, The mass ratio of the negative electrode active material in the thinning zone to the negative electrode active material in the active material zone is (3-4):

5.

5. A method for preparing a composite negative electrode, characterized in that, The method is used to prepare a composite negative electrode as described in any one of claims 1-4. The method includes: providing a negative electrode sheet having an active material region and a thinned region located at its end; preparing a rigid bonding layer, an elastic buffer layer, and an ion-conducting layer slurry respectively; sequentially coating the rigid bonding layer, the elastic buffer layer, and the ion-conducting layer on the surface of the thinned region in a direction away from the current collector; subjecting the coated electrode sheet to a gradient curing treatment to form a gradient composite structure on the surface of the thinned region; wherein the gradient curing treatment includes: pre-baking at 70℃~90℃ and then curing under vacuum conditions at 100℃~130℃.

6. The preparation method according to claim 5, characterized in that, After the step of providing a negative electrode sheet having an active material region and a thinned region at its end, and before the step of sequentially coating the surface of the thinned region with the rigid adhesive layer, the elastic buffer layer and the ion conduction layer in a direction away from the current collector, the method further includes a step of surface roughening the thinned region.

7. A lithium-ion battery, characterized in that, Includes the composite negative electrode as described in any one of claims 1-4.

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