A negative electrode sheet, a preparation method thereof, and a lithium ion battery

Abstract

In view of the problem that the material homogenization design of the single coating of the existing lithium ion battery negative electrode sheet is difficult to balance the energy density and the rate performance of the battery, the application provides a negative electrode sheet, a preparation method thereof and a lithium ion battery. The negative electrode sheet comprises a negative electrode current collector and a multilayer negative electrode active layer, the multilayer negative electrode active layer comprises a first negative electrode active layer, a second negative electrode active layer, …, and an n-th negative electrode active layer in sequence, and n is greater than or equal to 2; the i-th and the i-1-th negative electrode active layers comprise i-th and i-1-th negative electrode active materials respectively, and satisfy the relationship formula ① or ②: ① C i > C i‑1 ; ② C i = C i‑1 , and SOC i > SOC i‑1 ; C i and C i‑1 are minimum lithium precipitation rates of the i-th and the i-1-th negative electrode active layers respectively, and SOC i and SOC i‑1 are critical lithium precipitation states of the i-th and the i-1-th negative electrode active layers respectively.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery technology, specifically relating to a negative electrode sheet and its preparation method, and a lithium-ion battery. Background Technology

[0002] With the rapid development of new energy technologies, lithium-ion batteries, as core components of energy storage systems, face increasingly stringent requirements in terms of energy density, power performance, and cycle life. Especially in the fields of electric vehicles, portable electronic devices, and renewable energy storage, battery performance directly impacts device efficiency and user experience. However, while existing traditional thick electrode designs improve energy density, they also expose some technical challenges, particularly in terms of battery energy density and cycle performance.

[0003] The main advantage of traditional thick electrodes lies in their ability to increase battery energy density by increasing the amount of active material. However, this design also presents several challenges. First, the high areal density of thick electrodes leads to longer ion transport paths, limiting the migration speed of ions within the battery. The diffusion rate of lithium ions within the electrode decreases, further increasing resistance to ion transport. This not only reduces the battery's charge and discharge efficiency but also exacerbates polarization during high-rate charge and discharge, causing voltage drops and limiting high-power output and rapid charge / discharge capabilities. Especially under high-load conditions, battery performance deteriorates significantly, and polarization shortens cycle life and accelerates capacity decay.

[0004] Furthermore, the homogenization design of a single coating often struggles to simultaneously meet the multiple requirements of a battery, such as high conductivity, strong adhesion, and rapid ion migration. In traditional thick electrodes, due to the relatively simple material structure, achieving a balance between conductivity and adhesion is difficult, resulting in the electrode material not functioning fully during high-rate charge and discharge. More importantly, thick electrodes have lower porosity, leading to insufficient electrolyte wetting and preventing the electrolyte from uniformly penetrating deep into the electrode region. This results in the inefficient utilization of active materials within the battery, thereby reducing the battery's energy density and cycle performance. Summary of the Invention

[0005] To address the issue that the homogenization design of existing lithium-ion battery negative electrode sheets with a single coating makes it difficult to simultaneously meet multiple requirements such as high conductivity, high bonding strength, and rapid ion migration, especially in thick electrodes where uneven electrolyte wetting and low utilization of active materials are prone to occur, thus affecting the energy density and cycle performance of the battery, this invention provides a negative electrode sheet, its preparation method, and a lithium-ion battery.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0007] In a first aspect, the present invention provides a negative electrode sheet, comprising a negative electrode current collector and a multilayer negative electrode active layer disposed on at least one side of the negative electrode current collector, wherein the multilayer negative electrode active layers are sequentially a 1st, a 2nd, ..., an nth negative electrode active layer, n≥2, and the direction from the 1st negative electrode active layer to the nth negative electrode active layer is a direction away from the negative electrode current collector; the i-th negative electrode active layer is any negative electrode active layer from the 2nd negative electrode active layer to the nth negative electrode active layer, the i-th negative electrode active layer includes an i-th negative electrode active material, and the (i-1)th negative electrode active layer includes an (i-1)th negative electrode active material;

[0008] The i-th negative electrode active layer and the (i-1)-th negative electrode active layer satisfy the following relationship ① or ②:

[0009] ①C i >C i-1 ;

[0010] ②C i =C i-1 And SOC i >SOC i-1 ;

[0011] Among them, C i C represents the minimum lithium plating rate of the i-th negative electrode active layer. i-1 The minimum lithium plating rate of the (i-1)th negative electrode active layer; SOC i The critical lithium plating state of the i-th negative electrode active layer, SOC i-1 This represents the critical lithium plating state of the (i-1)th negative electrode active layer.

[0012] Furthermore, C i and SOC i The test calculation steps are as follows:

[0013] S1. Prepare the i-th negative electrode active layer using the i-th negative electrode active material alone, and fabricate a coin cell;

[0014] S2. The button cell battery is charged at a rate of 0.1C to obtain its charging capacity, denoted as Q. ci ;

[0015] S3. The button cell is discharged at different rates until the discharge curve of the button cell is below 0V; the different rates are a*C, where a is an integer multiple of 0.1 and a≥1;

[0016] When the discharge curve of the coin cell is below 0V, the corresponding rate is recorded as the C of the i-th negative electrode active material. i Meanwhile, the discharge capacity of the coin cell at this rate, up to 0V, is denoted as Q of the i-th negative electrode active material. di ;

[0017] S4. According to the relation SOC i =Q di / Q ci The SOC of the i-th negative electrode active layer was calculated. i ;

[0018] Repeat test steps S1~S4 to obtain the C of the (i-1)th negative electrode active layer. i-1 and SOC i-1 .

[0019] Furthermore, the average carbon coating amount of the i-th particle is ω. i The average carbon coating amount of the (i-1)th particle is ω i-1 ω i ≥ω i-1 .

[0020] Furthermore, the ω i and ω i-1 The range is 0% to 3.0%.

[0021] Furthermore, the i-th particle and the (i-1)-th particle are primary particles, and the average particle size of the i-th particle is D. i The average particle size of the (i-1)th particle is D i-1 D i <D i-1 ;

[0022] Alternatively, the i-th particle and the (i-1)-th particle are secondary particles, which are formed by granulation of primary particles, and the average particle size of the primary particles of the i-th particle is D. i The average particle size of the (i-1)th particle is D. i-1 D i <D i-1 ;

[0023] Alternatively, the i-th particle and the (i-1)-th particle comprise primary particles and secondary particles, with the secondary particles formed by granulation of the primary particles. The average particle size of the primary particles in the i-th particle is D. i The average particle size of the (i-1)th particle is D. i-1 D i <D i-1 .

[0024] Furthermore, the D i and D i-1 The range is 3~25μm.

[0025] Furthermore, the average interlayer spacing t of the i-th particle i The average interlayer spacing t of the (i-1)th particle i-1 , t i ≥ti-1 .

[0026] Furthermore, the t i and t i-1 The range is 0.30~0.40nm.

[0027] Furthermore, the total thickness L of the multilayer negative electrode active layer ranges from 30 to 150 μm.

[0028] Furthermore, the total compaction density ρ of the multilayer negative electrode active layer ranges from 1.2 to 2.2 g / cm³. 3 .

[0029] Furthermore, the orientation degrees of the i-th negative electrode active layer and the (i-1)-th negative electrode active layer are OI, respectively. i and OI i-1 The OI i and OI i-1 The range is 2~30, and OI i <OI i-1 .

[0030] Furthermore, the i-th negative electrode active material and / or the (i-1)-th negative electrode active material includes one or more of the following: graphite material, amorphous carbon material, graphene material, carbon fiber material, silicon-carbon material, silicon-oxygen material, lithium titanate material, tin-based material, and alloy material.

[0031] Furthermore, the ratio of the mass of the binder in the i-th negative electrode active layer to the total mass of the i-th negative electrode active layer is b. i The ratio of the mass of the binder in the (i-1)th negative electrode active layer to the total mass of the (i-1)th negative electrode active layer is b. i-1 b i ≤b i-1 .

[0032] Furthermore, the b i and b i-1 The range is 1.0% to 10.0%.

[0033] The adhesive comprises one or more of carboxymethyl cellulose (CMC), polytetrafluoroethylene (PTFE) and its modified polymers, rubber, styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polyimide (PI), modified polyimide, polyamide (PAI), polyethyleneimine (PEI), polyurethane (PU), polymethyl methacrylate (PMMA), polypropylene, modified polypropylene, polyacrylic acid (PAA), modified polyacrylic acid, polyvinyl alcohol (PVA), modified polyvinyl alcohol, polyvinyl butyral, modified polyvinyl butyral, polyacrylonitrile (PAN), and modified polyacrylonitrile.

[0034] Furthermore, the ratio of the mass of the conductive agent in the i-th negative electrode active layer to the total mass of the i-th negative electrode active layer is e. i The ratio of the mass of the conductive agent in the (i-1)th negative electrode active layer to the total mass of the (i-1)th negative electrode active layer is e. i-1 e i ≥e i-1 .

[0035] Furthermore, the e i and e i-1 The range is 0.1% to 4.0%.

[0036] Furthermore, the conductive agent includes one or more of the following: conductive nanofibers, conductive nanotubes, carbon nanofibers, graphene, sheet-like graphite flakes, carbon black, graphene microspheres, three-dimensional conductive metal-organic frameworks, and porous spherical carbon.

[0037] It should be understood that the thick electrode in this invention is: the total thickness of the positive electrode active layer is ≥60μm, and the total thickness of the negative electrode active layer is ≥30μm.

[0038] Secondly, the present invention provides a method for preparing a negative electrode sheet as described in any one of the above claims, comprising the following steps:

[0039] The negative electrode active material, binder and conductive agent of each negative electrode active layer are mixed and dispersed in water to obtain the first negative electrode slurry, the second negative electrode slurry, ..., the nth negative electrode slurry;

[0040] The first negative electrode slurry, the second negative electrode slurry, ..., the nth negative electrode slurry are coated sequentially on at least one side of the negative electrode current collector, and after drying and rolling, a negative electrode sheet is obtained.

[0041] Thirdly, the present invention provides a lithium-ion battery, including a negative electrode sheet, wherein the negative electrode sheet is any of the negative electrode sheets described above, or the negative electrode sheet is prepared by the method for preparing a negative electrode sheet as described above. Detailed Implementation

[0042] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0043] An embodiment of the present invention provides a negative electrode sheet, comprising a negative electrode current collector and a multilayer negative electrode active layer disposed on at least one side of the negative electrode current collector, wherein the multilayer negative electrode active layers are sequentially designated as a 1st, 2nd, ..., nth negative electrode active layer, n≥2, and the direction from the 1st negative electrode active layer to the nth negative electrode active layer is away from the negative electrode current collector; the i-th negative electrode active layer is any negative electrode active layer from the 2nd negative electrode active layer to the nth negative electrode active layer, the i-th negative electrode active layer includes an i-th negative electrode active material, and the (i-1)th negative electrode active layer includes an (i-1)th negative electrode active material;

[0044] The i-th negative electrode active layer and the (i-1)-th negative electrode active layer satisfy the following relationship ① or ②:

[0045] ①C i >C i-1 ;

[0046] ②C i =C i-1 And SOC i >SOC i-1 ;

[0047] Among them, C i C represents the minimum lithium plating rate of the i-th negative electrode active layer. i-1 The minimum lithium plating rate of the (i-1)th negative electrode active layer; SOC i The critical lithium plating state of the i-th negative electrode active layer, SOC i-1 This represents the critical lithium plating state of the (i-1)th negative electrode active layer.

[0048] Among them, C i and SOC i The test calculation steps are as follows:

[0049] S1. Prepare the i-th negative electrode active layer using the i-th negative electrode active material alone, and fabricate a coin cell;

[0050] S2. The button cell battery is charged at a rate of 0.1C to obtain its charging capacity, denoted as Q. ci ;

[0051] S3. The button cell is discharged at different rates until the discharge curve of the button cell is below 0V; the different rates are a*C, where a is an integer multiple of 0.1 and a≥1;

[0052] When the discharge curve of the coin cell is below 0V, the corresponding rate is recorded as the C of the i-th negative electrode active material. i Meanwhile, the discharge capacity of the coin cell at this rate, up to 0V, is denoted as Q of the i-th negative electrode active material. di ;

[0053] S4. According to the relation SOC i =Q di / Q ci The SOC of the i-th negative electrode active layer was calculated. i ;

[0054] Repeat test steps S1~S4 to obtain the C of the (i-1)th negative electrode active layer. i-1 and SOC i-1 .

[0055] This invention introduces a multi-layer structure into the negative electrode sheet and designs the negative electrode active material between adjacent layers to possess increasing physicochemical properties. i >C i-1 (or C) i =C i-1 And SOC i >SOC i-1 This technology achieves significant improvements in electrochemical performance. On one hand, the outer negative electrode material possesses superior kinetic characteristics, enabling faster lithium-ion insertion and extraction, thereby accelerating the overall battery charge and discharge rate. On the other hand, the rapidly reacting outer layer helps lithium ions form a uniform distribution within the electrode, preventing localized overcharging or over-discharging and improving cycle life and battery safety. Simultaneously, through synergistic optimization of the performance of different layer materials, the utilization rate of active materials is effectively improved, polarization and internal resistance are reduced, further enhancing the battery's energy density and efficiency.

[0056] In some embodiments, for the i-th particle and / or the (i-1)-th particle that are carbon-coated using the same coating method, the same process, and the same raw materials, the average carbon coating amount of the i-th particle is ω. i The average carbon coating amount of the (i-1)th particle is ω i-1 ω i ≥ω i-1 .

[0057] Furthermore, the methods for carbon coating on the i-th particle and / or the (i-1)-th particle in this invention include, but are not limited to, solid phase coating, liquid phase coating and chemical vapor deposition (CVD).

[0058] In this invention, the average carbon coating amount ω of the particles contained in the i-th negative electrode active material is... i The carbon coating amount ω of the (i-1)th negative electrode active material is greater than or equal to i-1 (i.e. ω) i ≥ω i-1 This invention significantly enhances the lithium-ion diffusion efficiency of the outer negative electrode active material. The higher carbon coating effectively reduces interfacial resistance and increases charge transfer rate, thereby accelerating the lithium-ion insertion and extraction process and enhancing overall reaction kinetics.

[0059] In some embodiments, the ω i and ω i-1 The range is 0% to 3.0%.

[0060] Limiting the average carbon coating content of the i-th and (i-1)-th particles to within the range of 0% to 3.0% helps to achieve an effective balance between improving conductivity and ion diffusion performance. An appropriate amount of carbon coating can reduce interfacial resistance and enhance electron transport capability; however, excessively high coating content may introduce additional interfacial defects, increasing the risk of side reactions, such as electrolyte decomposition. Furthermore, an excessively thick carbon layer can lengthen the lithium-ion diffusion path, limiting its rapid migration and thus hindering the improvement of kinetic performance. Therefore, limiting the carbon coating content within this range helps ensure material surface stability, optimize lithium-ion transport efficiency, and improve overall electrochemical performance.

[0061] In some embodiments, the i-th particle and the (i-1)-th particle are primary particles, and the average particle size of the i-th particle is D. i The average particle size of the (i-1)th particle is D i-1 D i <D i-1 ;

[0062] Alternatively, the i-th particle and the (i-1)-th particle are secondary particles, which are formed by granulation of primary particles, and the average particle size of the primary particles of the i-th particle is D. i The average particle size of the (i-1)th particle is D. i-1 D i <D i-1 ;

[0063] Alternatively, the i-th particle and the (i-1)-th particle comprise primary particles and secondary particles, with the secondary particles formed by granulation of the primary particles, and the average particle size of the primary particles in the i-th particle being D. i The average particle size of the (i-1)th particle is D. i-1 D i <D i-1 .

[0064] Furthermore, the average particle size of the i-th particle is the same as that of the primary particles. It should be understood that the negative electrode active material includes primary particles and / or secondary particles. Primary particles are the most basic and smallest units, while secondary particles are larger structural units formed by granulation of primary particles.

[0065] By setting the average particle size D of the i-th layer of negative electrode particles (primary particles) i The particle size D smaller than that of the (i-1)th layer i-1This approach enables particle size gradient control, facilitating synergistic function of different layers within the electrode structure. The smaller outer particles provide shorter lithium-ion diffusion paths, significantly enhancing lithium-ion insertion and extraction rates, thus improving the kinetic performance of the outer active layer. Conversely, the larger inner particles increase compaction density and the mass of active material per unit volume, enhancing overall energy density. Through this optimal combination of small outer and large inner particles, the electrode maintains high rate performance while achieving high energy density, thus optimizing and balancing the battery's electrochemical performance.

[0066] In some embodiments, the D i and D i-1 The range is 3~25μm.

[0067] Limiting the average particle size of the i-th and (i-1)-th particles to the range of 3–25 μm helps achieve an optimal balance between processability, electrochemical performance, and structural stability of the anode active material. While excessively large particle sizes improve compaction density, they also prolong lithium-ion transport distance, thus limiting the lithium-ion diffusion rate and negatively impacting rate performance and cycle life. Conversely, excessively small particle sizes significantly increase the material surface area, easily triggering side reactions such as electrolyte decomposition, reducing initial coulombic efficiency, and potentially increasing electrode internal resistance and deteriorating slurry dispersion stability during preparation, thus increasing processing difficulty. Therefore, controlling the particle size within the range of 3–25 μm satisfies the requirements for structural compaction and high energy density while also ensuring good rate performance and cycle stability, making it suitable for multilayer gradient anode systems.

[0068] In some embodiments, the average interlayer spacing t of the i-th particle i The average interlayer spacing t of the (i-1)th particle i-1 , t i ≥t i-1 .

[0069] By setting the average interlayer spacing t of the i-th particle i The average interlayer spacing t of the (i-1)th particle is greater than or equal to that of the average interlayer spacing t. i-1 (i.e. t) i ≥t i-1 This invention achieves gradient control of interlayer spacing along the electrode thickness direction, effectively improving lithium-ion diffusion efficiency. A larger interlayer spacing provides more spacious migration channels for lithium-ion intercalation and delamination, thereby significantly reducing ion diffusion resistance and improving the material's reaction kinetics. This design enables the outer negative electrode active layer near the electrolyte side to possess superior ion migration rate and rate response capability, better meeting its rapid reaction requirements during charge and discharge, further strengthening the synergistic effect of each functional layer in the multilayer structure, and optimizing the overall electrochemical performance of the battery.

[0070] In some embodiments, the average interlayer spacing t of the i-th particle i and the average interlayer spacing t of the (i-1)th particle i-1 The range is 0.33~0.40nm.

[0071] In this invention, the average interlayer spacing between the i-th particle and the (i-1)-th particle is limited to the range of 0.33~0.40 nm, mainly based on the inherent structural characteristics of various carbon-based anode materials. Different types of carbon materials have relatively fixed interlayer spacing distributions. For example, the interlayer spacing of artificial graphite and natural graphite is generally between 0.33~0.34 nm, coke is 0.34~0.37 nm, activated carbon is 0.34~0.35 nm, hard carbon is about 0.38 nm, soft carbon is 0.36~0.37 nm, and carbon fiber and pyrolytic graphite are usually 0.34 nm. Considering the physical limitations of the various carbon materials used in this invention (including graphite, coke, hard carbon, soft carbon, etc.) in terms of interlayer spacing, setting the interlayer spacing in the range of 0.33~0.40 nm not only covers the average interlayer spacing characteristics of most common carbon materials, but also helps to ensure the compatibility and hierarchical design requirements between materials.

[0072] In some embodiments, the total thickness L of the multilayer negative electrode active layer ranges from 30 to 150 μm.

[0073] In this invention, the total thickness L of the multi-layer negative electrode active layer is controlled within the range of 30-150 μm, achieving a balance between energy density and rate performance. If the total thickness L is too small, insufficient negative electrode active material content will limit the battery's lithium loading capacity, leading to a decrease in energy density. Simultaneously, due to the particle size limitation of the negative electrode active material itself, the total thickness L must be larger than the particle size of the negative electrode active material. Conversely, if the thickness L is too large, it will lengthen the lithium ion diffusion path, resulting in a decrease in rate performance. Furthermore, the significant gradient in the lithium intercalation reaction from the surface to the bottom of the negative electrode active material can cause lithium plating on the surface, preventing full utilization of the bottom layer's negative electrode active material. By controlling the total thickness L of the multi-layer negative electrode active layer to 30-150 μm, this invention achieves sufficient lithium loading and energy density while maintaining a reasonable ion transport distance, avoiding the problems of increased polarization and decreased rate performance caused by excessively thick electrodes.

[0074] In some embodiments, the total compaction density ρ of the multilayer negative electrode active layer ranges from 1.2 to 2.2 g / cm³. 3 .

[0075] In this invention, the total compaction density ρ of the multilayer negative electrode active layer is controlled at 1.2~2.2 g / cm³. 3This invention achieves an optimized balance between electrode pore structure, electron conduction network, and energy density. If the total compaction density is too high, the reduced pore size within the electrode sheet leads to insufficient electrolyte wetting of the negative electrode active layer, obstructing ion diffusion paths and thus reducing rate performance and fast-charging capability. Conversely, if the total compaction density is too low, it reduces the contact points between negative electrode active material particles and between particles and the conductive agent, resulting in discontinuous electron transport channels, decreased kinetic performance, and reduced battery energy density. In this invention, the total compaction density ρ of the multilayer negative electrode active layer is controlled at 1.2~2.2 g / cm³. 3 This can ensure that the negative electrode active material has sufficient interparticle contact and electron conduction path, while avoiding problems such as reduced porosity, insufficient electrolyte wetting and limited ion diffusion caused by excessive pressure compaction.

[0076] In this invention, the total compaction density ρ of the multilayer negative electrode active layer is controlled at 1.2~2.2 g / cm³. 3 This can ensure that the negative electrode active material has sufficient interparticle contact and electron conduction path, while avoiding problems such as reduced porosity, insufficient electrolyte wetting and limited ion diffusion caused by excessive pressure compaction.

[0077] In some embodiments, the orientation degrees of the i-th negative electrode active layer and the (i-1)-th negative electrode active layer are respectively OI i and OI i-1 The OI i and OI i-1 The range is 2~30, and OI i <OI i-1 In the negative electrode active layer with lower orientation, the graphite sheets are more randomly arranged, and there are a large number of edge sites with different orientations within the graphite particles. Lithium ions can only be inserted into the interlayer from the edges of the graphite sheets and cannot directly penetrate the graphite sheets. Therefore, the aforementioned low-orientation structure can provide more and shorter insertion channels for lithium ions. As a result, the negative electrode active material near the electrolyte side has a lower orientation, which is beneficial to improving the lithium ion intercalation-deintercalation kinetics and increasing rate performance. At the same time, in the negative electrode active layer with higher orientation, most of the graphite sheets are arranged parallel to the current collector surface, and the degree of exposure of the sheet edges is lower. This results in a higher orientation of the negative electrode active material near the current collector side, thereby reducing the number of highly active edge sites. This is beneficial to suppressing side reactions with the electrolyte, reducing excessive growth of the SEI film, and reducing excessive consumption of active lithium and electrolyte.

[0078] In some embodiments, the i-th negative electrode active material and / or the (i-1)-th negative electrode active material includes one or more of graphite materials, amorphous carbon materials, graphene materials, carbon fiber materials, silicon-carbon materials, silicon-oxygen materials, lithium titanate materials, tin-based materials, and alloy materials.

[0079] It is important to understand that graphene materials include single-layer graphene and multilayer graphene. Single-layer graphene does not have a defined average interlayer spacing. Therefore, when using single-layer graphene in actual production processes, there is no need to restrict its average interlayer spacing.

[0080] In some embodiments, the ratio of the mass of the binder in the i-th negative electrode active layer to the total mass of the i-th negative electrode active layer is b. i The ratio of the mass of the binder in the (i-1)th negative electrode active layer to the total mass of the (i-1)th negative electrode active layer is b. i-1 b i ≤b i-1 .

[0081] By designing the mass ratio b of the binder in the i-th negative electrode active layer i The proportion of binder mass in the (i-1)th negative electrode active layer is less than or equal to b. i-1 (i.e. b) i ≤b i-1 This invention achieves a binder gradient distribution along the electrode thickness direction, effectively coordinating the battery's kinetic performance and structural stability. A lower binder content in the outer layer helps reduce its obstruction of lithium-ion migration channels, lowers interfacial resistance, and improves ion diffusion rate and electrode reaction kinetics. A relatively higher binder proportion in the inner layer enhances the bonding force between the active material and the current collector, improving the overall bonding strength and mechanical structural stability of the negative electrode. Simultaneously, due to the binder "floating" phenomenon, a higher proportion of inner layer binder is necessary to ensure a uniform final binder distribution.

[0082] In some embodiments, the b i and b i-1 The range is 1.0% to 10.0%; and / or, the adhesive includes one or more of carboxymethyl cellulose (CMC), polytetrafluoroethylene (PTFE) and its modified polymers, rubber, styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polyimide (PI), modified polyimide, polyamide (PAI), polyethyleneimine (PEI), polyurethane (PU), polymethyl methacrylate (PMMA), polypropylene, modified polypropylene, polyacrylic acid (PAA), modified polyacrylic acid, polyvinyl alcohol (PVA), modified polyvinyl alcohol, polyvinyl butyral, modified polyvinyl butyral, polyacrylonitrile (PAN), and modified polyacrylonitrile.

[0083] Limiting the mass percentage of binder in the i-th and (i-1)-th anode active layers to between 1.0% and 10.0% helps optimize the battery's electrochemical performance while ensuring structural stability. Excessive binder content not only hinders electron conduction but may also block lithium-ion migration channels, leading to increased internal resistance and decreased diffusion rate, thus weakening rate performance. Furthermore, an excessively high binder ratio can increase electrode brittleness and reduce mechanical flexibility. Conversely, an insufficient binder ratio makes it difficult to provide sufficient bonding strength, easily causing electrode pulverization or active material detachment, thereby affecting cycle stability and battery life. Therefore, controlling the binder content within the range of 1.0% to 10.0% ensures both the structural integrity of the anode active layer and efficient lithium-ion and electron transport, thus improving overall battery performance.

[0084] In some embodiments, the ratio of the mass of the conductive agent in the i-th negative electrode active layer to the total mass of the i-th negative electrode active layer is e. i The ratio of the mass of the conductive agent in the (i-1)th negative electrode active layer to the total mass of the (i-1)th negative electrode active layer is e. i-1 e i ≥e i-1 .

[0085] By setting the mass percentage e of the conductive agent in the i-th negative electrode active layer i The mass percentage e of the conductive agent in the (i-1)th negative electrode active layer is greater than or equal to that of the conductive agent. i-1 (i.e. e) i ≥e i-1 This invention achieves a gradient distribution of conductive agent content along the thickness of the negative electrode sheet, which helps to improve the electron transport capability of the outer active layer. A higher proportion of conductive agent allows for the construction of a more continuous and efficient electron conduction network within the electrode, significantly reducing resistance in electron migration paths, thereby lowering the overall battery internal resistance and improving the electrode's kinetic response performance. This structural design enables the outer layer near the electrolyte to have stronger electron transport efficiency, meeting its high-rate operation requirements under rapid charge and discharge conditions. Simultaneously, together with the relatively lower proportion of conductive agent in the inner layer, it forms a reasonable conduction structure, balancing energy density and electrochemical reaction rate, achieving synergistic optimization of battery performance.

[0086] In some embodiments, the e i and e i-1 The range is 0.1% to 4.0%; and / or, the conductive agent includes one or more of conductive nanofibers, conductive nanotubes, carbon nanofibers, graphene, sheet-like graphite flakes, carbon black, graphene microspheres, three-dimensional conductive metal-organic frameworks, and porous spherical carbon.

[0087] Limiting the mass percentage of conductive agent in the i-th and (i-1)-th negative electrode active layers to within the range of 0.1% to 4.0% helps to improve electron conductivity while simultaneously controlling energy density and cost. An appropriate amount of conductive agent can construct an effective electron transport network in the negative electrode, reducing internal resistance and enhancing kinetic performance. However, if its content is too high, it will not only dilute the proportion of active material, reducing the battery's volumetric or gravimetric energy density, but may also lead to decreased material utilization and increased cost. Conversely, if the conductive agent content is too low, it will result in insufficient conductive pathways in the battery, hindering electron transport and thus affecting the battery's rate performance and cycle stability. Therefore, controlling the conductive agent content within the conventional and reasonable range of 0.1% to 4.0% can effectively balance conductivity, electrochemical performance, and material utilization efficiency.

[0088] On the other hand, one embodiment of the present invention also provides a method for preparing a negative electrode sheet as described in any of the above embodiments, comprising the following steps:

[0089] The negative electrode active material, binder and conductive agent of each negative electrode active layer are mixed and dispersed in water to obtain the first negative electrode slurry, the second negative electrode slurry, ..., the nth negative electrode slurry;

[0090] The first negative electrode slurry, the second negative electrode slurry, ..., the nth negative electrode slurry are coated sequentially on at least one side of the negative electrode current collector, and after drying and rolling, a negative electrode sheet is obtained.

[0091] On the other hand, one embodiment of the present invention provides a lithium-ion battery, including a negative electrode sheet, wherein the negative electrode sheet is the negative electrode sheet described in any of the above embodiments, or the negative electrode sheet is prepared by the preparation method of the negative electrode sheet described in the above embodiments.

[0092] The present invention will be further illustrated by the following examples.

[0093] Example 1

[0094] In this embodiment, n=2, meaning there are two negative electrode active layers: a first negative electrode active layer and a second negative electrode active layer. The first negative electrode active material in the first negative electrode active layer comprises first particles, and the second negative electrode active material in the second negative electrode active layer comprises second particles.

[0095] This embodiment illustrates the negative electrode sheet and lithium-ion battery disclosed in this invention, and includes the following operational steps:

[0096] (1) C of the negative electrode active layer i and SOC i test:

[0097] ① First negative electrode active layer:

[0098] The first particle of the first negative electrode active material is selected as petroleum coke coated secondary particle graphite. The average carbon coating amount ω1 of the first particle is 1.88%, the average particle size D1 is 8.2μm, the average interlayer spacing t1 is 0.33nm, and the orientation degree OI1 is 12.9.

[0099] Dissolve 2.0 parts of CMC as a thickener in water, then add 2.0 parts of carbon black as a conductive agent and disperse evenly to obtain a conductive slurry. Add 100 parts of the first particle to the conductive slurry and disperse evenly to obtain a slurry. Then add 1.6 parts of SBR and 0.6 parts of PAA as binders to the above slurry and disperse evenly to obtain the first active layer slurry.

[0100] The first active layer slurry is uniformly coated onto the negative electrode current collector copper foil, and then baked, dried, rolled, and cut to obtain the electrode sheet for assembling coin cells.

[0101] A coin cell was fabricated using the aforementioned electrode as the working electrode, a lithium metal sheet as the counter electrode, and a polyethylene microporous membrane as the separator. The electrolyte consisted of LiPF6 dissolved in a mixture of ethyl carbonate (EC) and diethyl carbonate (DEC) (EC to DEC volume ratio of 1:1), with a LiPF6 concentration of 1 mol / L.

[0102] After the button cell is left to stand for 4 hours, it is discharged at 0.1C to 0.005V. After standing for 10 minutes, it is charged at 0.1C to 1.5V. The charging capacity at this time is Qc1, and the charging capacity Qc1 is the reversible capacity.

[0103] Multiple coin cells were discharged at rates of 0.1C, 0.2C, 0.3C, and so on, with each coin cell discharged until its discharge capacity equaled the reversible capacity Qc1. When the discharge curve of a coin cell drops below 0V, the corresponding rate is recorded as C1 of the first negative electrode active material, and the discharge capacity of the coin cell at that rate up to 0V is recorded as Qc1 of the first negative electrode active material. d1 According to the relation SOC1=Q d1 / Q c1 The SOC1 of the first negative electrode active material was calculated.

[0104] ② Second negative electrode active layer:

[0105] The second particle of the second negative electrode active material is a mixture of needle-shaped coke-coated single-particle graphite and petroleum coke hard carbon, with a mass ratio of needle-shaped coke-coated single-particle graphite to petroleum coke hard carbon of 9:1. The average carbon coating amount ω2 of the second particle is 2.47%, the average particle size D2 is 7.1 μm, the average interlayer spacing t2 is 0.35 nm, and the orientation degree OI2 is 5.3.

[0106] Dissolve 1.8 parts of CMC as a thickener in water, then add 2.1 parts of carbon black as a conductive agent and disperse evenly to obtain a conductive slurry. Add 100 parts of the second particle to the conductive slurry and disperse evenly to obtain a slurry. Then add 1.0 parts of SBR and 1.0 parts of PAA as binders to the above slurry and disperse evenly to obtain a second active layer slurry.

[0107] The second active layer slurry is uniformly coated onto the negative electrode current collector copper foil, and then baked, dried, rolled, and cut to obtain an electrode sheet that can be assembled.

[0108] A coin cell was fabricated using the aforementioned electrode as the working electrode, a lithium metal sheet as the counter electrode, and a polyethylene microporous membrane as the separator. The electrolyte consisted of LiPF6 dissolved in a mixture of ethyl carbonate (EC) and diethyl carbonate (DEC) (EC to DEC volume ratio of 1:1), with a LiPF6 concentration of 1 mol / L.

[0109] After letting the button cell battery stand for 4 hours, discharge it to 0.005V at 0.1C, let it stand for 10 minutes, and then charge it to 1.5V at 0.1C. At this point, the charging capacity... Quantity The value is Qc2, and this charging capacity Qc2 is a reversible capacity.

[0110] Multiple coin cells were discharged at rates of 0.1C, 0.2C, 0.3C, and so on, with each coin cell discharged until its discharge capacity equaled the reversible capacity Qc2. When the discharge curve of a coin cell dropped below 0V, the corresponding rate was recorded as C2 of the second negative electrode active material, and the discharge capacity at that rate, ending at 0V, was recorded as Q of the second negative electrode active material. d2 According to the relation SOC2=Q d2 / Q c2 The SOC2 of the second negative electrode active material was calculated.

[0111] (2) Preparation of lithium-ion batteries:

[0112] ① Preparation of the negative electrode sheet:

[0113] The first particle of the first negative electrode active material is selected as petroleum coke coated secondary particle graphite. The average carbon coating amount ω1 of the first particle is 1.88%, the average particle size D1 is 8.2μm, the average interlayer spacing t1 is 0.33nm, and the orientation degree OI1 is 12.9.

[0114] Dissolve 2.0 parts of CMC in water, then add 2.0 parts of carbon black as a conductive agent and disperse evenly to obtain a conductive slurry. Add 100 parts of the first particle to the conductive slurry and disperse evenly to obtain a slurry. Then add 1.6 parts of SBR and 0.6 parts of PAA as binders to the above slurry and disperse evenly to obtain the first active layer slurry. It should be understood that in this operation, CMC, SBR, and PAA are all binders.

[0115] The second particle of the second negative electrode active material is a mixture of needle-shaped coke-coated single-particle graphite and petroleum coke hard carbon, with a mass ratio of needle-shaped coke-coated single-particle graphite to petroleum coke hard carbon of 9:1. The average carbon coating amount ω2 of the second particle is 2.47%, the average particle size D2 is 7.1 μm, the average interlayer spacing t2 is 0.35 nm, and the orientation degree OI2 is 5.3.

[0116] Dissolve 1.8 parts of CMC in water, then add 2.1 parts of carbon black as a conductive agent and disperse evenly to obtain a conductive slurry. Add 100 parts of the second particle to the conductive slurry and disperse evenly to obtain a slurry. Then add 1.0 part of SBR and 1.0 part of PAA as binders to the above slurry and disperse evenly to obtain a second active layer slurry. It should be understood that in this operation, CMC, SBR, and PAA are all binders.

[0117] The first and second active layer slurries are uniformly coated onto the negative electrode current collector copper foil, and then baked, dried, rolled, slit and cut to obtain a negative electrode sheet that can be assembled.

[0118] ② Preparation of the positive electrode sheet:

[0119] Lithium iron phosphate (LiFePO4), CNT (CNT), and PVDF (PVDF) were thoroughly mixed in NMP solvent at a weight ratio of 96.5:1.5:2.0 to obtain a positive electrode slurry. The positive electrode slurry was then uniformly coated onto aluminum foil, and the resulting positive electrode sheet was manufactured through processes including drying, cold pressing, slitting, sheet forming, welding tabs, and adhesive bonding.

[0120] ③ Cell fabrication:

[0121] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes and the positive electrode in the center of the negative electrode. This stacking process yields the battery cell.

[0122] ④ Assembly of lithium-ion batteries:

[0123] The above-mentioned cells are processed through hot pressing, welding of tabs, packaging, baking and drying, electrolyte injection and full immersion, vacuum sealing, formation and final sealing to obtain lithium-ion batteries.

[0124] Examples 2-13

[0125] The examples are used to illustrate the negative electrode sheet and lithium-ion battery disclosed in this invention, including most of the operating steps in Example 1, except that the formulations in Tables 1 and 2 are used.

[0126] Comparative Examples 1-6

[0127] The comparative examples are used to illustrate the negative electrode sheet and lithium-ion battery disclosed in this invention, including most of the operating steps in Example 1, except that the formulations in Tables 1 and 2 are used.

[0128] Table 1

[0129]

[0130] Table 2

[0131]

[0132] Performance testing

[0133] The lithium-ion batteries obtained in the above embodiments and comparative examples were subjected to the following performance tests:

[0134] 1. Rate test: The lithium-ion battery prepared above was charged at 0.5C in the range of 2.5-3.65V and 25±2℃, left to stand for 10 minutes, and then discharged at 4C. The discharge capacity at 4C rate was recorded.

[0135] 2. DC internal resistance test:

[0136] Adjusting to 50% SOC: Discharge the lithium-ion battery to 50% SOC, place it at 25±2℃ for 60 minutes, record the ending voltage V1, then discharge it at a 2C current for 30 seconds, and record the ending voltage V2; calculate the measured DC internal resistance value using the formula: (V1-V2) / (2C current)*1000, where V1 and V2 are in V, (2C current) is in A, and the DC internal resistance value is in mΩ.

[0137] 3. High-rate discharge cycle test: After being placed at 25±2℃ for 2 hours, the battery is charged at 1C to 3.65V and placed for 10 minutes. Then it is discharged at 6C to 2.5V and placed for 10 minutes. The battery is continuously cycled in the order of charging-placement-discharging-placement, and the capacity retention rate of the battery after the cycle is calculated.

[0138] The calculation formula is as follows:

[0139] Capacity retention rate after nth cycle (%) = (Discharge capacity after nth cycle) / (Discharge capacity after first cycle) * 100%.

[0140] 4. Energy density test: Charge at 0.5C within the range of 2.5-3.65V and at 25±2℃, let stand for 10 minutes, then discharge at 0.5C and record the discharge energy at the 0.5C rate. Calculate the energy density using the formula: discharge energy / battery mass, where the unit of discharge energy is Wh, the unit of battery mass is kg, and the unit of energy density is Wh / kg.

[0141] The test results are shown in Table 3.

[0142] Table 3

[0143]

[0144] The test results of Examples 2 and 3 show that increasing the carbon coating of the negative electrode active material enhances lithium-ion diffusion efficiency, which is beneficial to the battery's kinetic performance and thus achieves higher Cg. i The test results from Examples 4 and 5 show that when the average particle size (average particle size of primary particles) of the negative electrode active material decreases, the reduced ion transport path is beneficial to improving the kinetic performance of the battery. i Higher; as shown by the test results of Examples 1 and 6, when the interlayer spacing of the negative electrode active material increases, the migration channels for lithium ion intercalation and delamination become more open, significantly reducing ion diffusion resistance and improving the reaction kinetics performance of the negative electrode active material, thus exhibiting higher C. i The test results of Examples 7 and 8 show that when the OI value of the negative electrode active material decreases, the hindering effect of the graphite sheets on lithium ions is reduced, which is beneficial to improving the kinetic performance of the battery and exhibiting higher C. i The test results from Examples 9 and 10 show that reducing the amount of binder in the negative electrode active material layer helps to reduce its obstruction of lithium-ion migration channels, lowers interfacial resistance, improves ion diffusion rate and electrode reaction kinetics, and exhibits higher Cg. i The test results of Examples 11 and 12 show that increasing the amount of conductive agent in the negative electrode active material layer enhances the conductivity of the negative electrode, improves its kinetic performance, and exhibits higher Cg. i C i At higher rates, the battery's reaction kinetics are faster, which means higher specific capacity in rate tests, lower internal resistance, and higher capacity retention during high-rate cycling.

[0145] C i It can directly reflect the intrinsic kinetic properties of the negative electrode active material layer, C iThe larger the negative electrode active material layer, the better its kinetic performance, and the better the battery's rate performance. In high-power applications, the negative electrode surface needs to receive a large number of lithium ions from the electrolyte. If the kinetic performance of the negative electrode surface is insufficient to receive such a large number of lithium ions, lithium plating can easily occur, leading to irreversible loss of active lithium and the risk of lithium dendrites piercing the separator and causing a short circuit. Therefore, improving the kinetic performance of the negative electrode is crucial. However, if the kinetic performance of the entire negative electrode is uniformly improved (as in Comparative Example 6), the cycle performance often decreases. This is because high-rate operating conditions accelerate all the side reactions that lead to aging and failure within the battery. At the same time, the huge difference in lithium concentration between the electrode surface and the interior causes the surface layer of negative electrode active material to expand / contract violently, while the change in the interior negative electrode active material lags behind. This generates huge internal stress, leading to problems such as the breakage of negative electrode active material particles, collapse of the negative electrode structure, and rupture of the SEI film, thereby deteriorating the battery's cycle performance.

[0146] When the mass percentage of binder in the first or second active layer is less than 1.0% (b1 < 1.0% or b2 < 1.0%), insufficient adhesion will cause the negative electrode active material to detach, making it difficult to form a complete negative electrode. When the proportion of binder in the first or second active layer is greater than 10.0% (as in Comparative Example 3), although it can effectively improve the adhesion of the negative electrode, excessive binder significantly increases the obstruction effect on lithium-ion migration channels, drastically increases the internal resistance of the electrode, greatly restricts the reaction kinetics of the electrode, resulting in a decrease in the specific capacity of the battery in rate testing and a decrease in the capacity retention rate during high-rate cycling. At the same time, excessive binder will seriously reduce the energy density of the battery.

[0147] When the conductive agent in the negative electrode active layer is greater than 4% (as in Comparative Example 4), although it has no negative impact on the electrochemical performance of the battery, it may even increase C. i (As in Example 12 and Comparative Example 6), but because the proportion of active material is diluted, the energy density of the battery is greatly reduced, and the cost also increases. At the same time, when the conductive agent in the first active layer or the second active layer is less than 0.1% (as in Comparative Example 5), the conductivity of the negative electrode sheet will be greatly reduced, which will seriously affect the dynamic performance of the battery. Therefore, the specific capacity in the rate test and the capacity retention rate in high-rate cycling are both lower.

[0148] To achieve faster reaction kinetics, higher carbon coating, smaller particle size, larger interlayer spacing, lower OI value, less binder, and more conductive agent are often required. These measures usually reduce the energy density of the battery. Therefore, the kinetic performance of the outer negative electrode active layer (i.e., higher C) can be improved through artificially designed multilayer structures. iThis ensures the battery's high-rate performance, while the inner negative electrode active layer is filled with negative electrode active material to maximize energy density.

[0149] The test results of Example 13 show that even when C1=C2, the magnitudes of SOC1 and SOC2 can reflect the kinetic performance of the first and second negative electrode active materials. Only when the kinetic performance of the second active material layer is better than that of the first active material layer (SOC2>SOC1) can the specific capacity in the rate test and the capacity retention rate in the high-rate cycle be better. However, when SOC2≤SOC1 (as in Comparative Example 2), or even C2<C1 (as in Comparative Example 1), due to the lower ion concentration and more tortuous transport path in the inner layer, the superior kinetic performance of the material cannot be fully utilized in the inner layer. Instead, because the kinetic performance of the outer layer is poor, the rate performance of the battery decreases, and the specific capacity in the rate test and the capacity retention rate in the high-rate cycle are both poor.

[0150] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The device includes a negative electrode current collector and multiple negative electrode active layers disposed on at least one side of the negative electrode current collector. The multiple negative electrode active layers are sequentially designated as the 1st, 2nd, ..., nth negative electrode active layers, where n ≥ 2. The direction from the 1st negative electrode active layer to the nth negative electrode active layer is away from the negative electrode current collector. The i-th negative electrode active layer is any negative electrode active layer from the 2nd negative electrode active layer to the nth negative electrode active layer. The i-th negative electrode active layer includes the i-th negative electrode active material, and the (i-1)th negative electrode active layer includes the (i-1)th negative electrode active material. The i-th negative electrode active material includes the i-th particle, and the (i-1)th negative electrode active material includes the (i-1)th particle. The i-th negative electrode active layer and the (i-1)-th negative electrode active layer satisfy the following relationship ① or ②: ①C i >C i-1 ; ②C i =C i-1 And SOC i >SOC i-1 ; Among them, C i C represents the minimum lithium plating rate of the i-th negative electrode active layer. i-1 The minimum lithium plating rate of the (i-1)th negative electrode active layer; SOC i The critical lithium plating state of the i-th negative electrode active layer, SOC i-1 This represents the critical lithium plating state of the (i-1)th negative electrode active layer; C i and SOC i The test calculation steps are as follows: S1. Prepare the i-th negative electrode active layer using the i-th negative electrode active material alone, and fabricate a coin cell; S2. The button cell battery is charged at a rate of 0.1C to obtain its charging capacity, denoted as Q. ci ; S3. The button cell is discharged at different rates until the discharge curve of the button cell is below 0V; the different rates are a*C, where a is an integer multiple of 0.1 and a≥1; When the discharge curve of the coin cell is below 0V, the corresponding rate is recorded as the C of the i-th negative electrode active material. i Meanwhile, the discharge capacity of the coin cell at this rate, up to 0V, is denoted as Q of the i-th negative electrode active material. di ; S4. According to the relation SOC i =Q di / Q ci The SOC of the i-th negative electrode active layer was calculated. i ; Repeat test steps S1~S4 to obtain the C of the (i-1)th negative electrode active layer. i-1 and SOC i-1 ; The i-th particle and the (i-1)-th particle are primary particles, and the average particle size of the i-th particle is D. i The average particle size of the (i-1)th particle is D i-1 D i <D i-1 ; Alternatively, the i-th particle and the (i-1)-th particle are secondary particles, which are formed by granulation of primary particles, and the average particle size of the primary particles of the i-th particle is D. i The average particle size of the (i-1)th particle is D. i-1 D i <D i-1 ; Alternatively, the i-th particle and the (i-1)-th particle comprise primary particles and secondary particles, with the secondary particles formed by granulation of the primary particles. The average particle size of the primary particles in the i-th particle is D. i The average particle size of the (i-1)th particle is D. i-1 D i <D i-1 .

2. The negative electrode sheet according to claim 1, characterized in that, The average carbon coating amount of the i-th particle is ω i The average carbon coating amount of the (i-1)th particle is ω i-1 ω i ≥ω i-1 .

3. The negative electrode sheet according to claim 2, characterized in that, The ω i and ω i-1 The range is 0% to 3.0%.

4. The negative electrode sheet according to claim 1, characterized in that, The D i and D i-1 The range is 3~25μm.

5. The negative electrode sheet according to claim 1, characterized in that, The average interlayer spacing t of the i-th particle i The average interlayer spacing t of the (i-1)th particle i-1 , t i ≥t i-1 .

6. The negative electrode sheet according to claim 5, characterized in that, The t i and t i-1 The range is 0.30~0.40nm.

7. The negative electrode sheet according to claim 1, characterized in that, The total thickness L of the multilayer negative electrode active layer ranges from 30 to 150 μm.

8. The negative electrode sheet according to claim 1, characterized in that, The total compaction density ρ of the multilayer negative electrode active layer ranges from 1.2 to 2.2 g / cm³. 3 .

9. The negative electrode sheet according to claim 1, characterized in that, The orientation degrees of the i-th negative electrode active layer and the (i-1)-th negative electrode active layer are OI, respectively. i and OI i-1 The OI i and OI i-1 The range is 2~30, and OI i <OI i-1 .

10. The negative electrode sheet according to claim 1, characterized in that, The i-th and / or i-1-th negative electrode active materials include one or more of the following: graphite materials, amorphous carbon materials, graphene materials, carbon fiber materials, silicon-carbon materials, silicon-oxygen materials, lithium titanate materials, tin-based materials, and alloy materials.

11. The negative electrode sheet according to claim 1, characterized in that, The ratio of the mass of the binder in the i-th negative electrode active layer to the total mass of the i-th negative electrode active layer is b. i The ratio of the mass of the binder in the (i-1)th negative electrode active layer to the total mass of the (i-1)th negative electrode active layer is b. i-1 b i ≤b i-1 .

12. The negative electrode sheet according to claim 11, characterized in that, The b i and b i-1 The range is 1.0% to 10.0%; and / or, The adhesive comprises one or more of carboxymethyl cellulose (CMC), polytetrafluoroethylene (PTFE) and its modified polymers, rubber, styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polyimide (PI), modified polyimide, polyamide (PAI), polyethyleneimine (PEI), polyurethane (PU), polymethyl methacrylate (PMMA), polypropylene, modified polypropylene, polyacrylic acid (PAA), modified polyacrylic acid, polyvinyl alcohol (PVA), modified polyvinyl alcohol, polyvinyl butyral, modified polyvinyl butyral, polyacrylonitrile (PAN), and modified polyacrylonitrile.

13. The negative electrode sheet according to claim 1, characterized in that, The ratio of the mass of the conductive agent in the i-th negative electrode active layer to the total mass of the i-th negative electrode active layer is e. i The ratio of the mass of the conductive agent in the (i-1)th negative electrode active layer to the total mass of the (i-1)th negative electrode active layer is e. i-1 e i ≥e i-1 .

14. The negative electrode sheet according to claim 13, characterized in that, The e i and e i-1 The range is 0.1% to 4.0%; and / or, The conductive agent includes one or more of the following: conductive nanofibers, conductive nanotubes, carbon nanofibers, graphene, sheet-like graphite flakes, carbon black, graphene microspheres, three-dimensional conductive metal-organic frameworks, and porous spherical carbon.

15. A method for preparing a negative electrode sheet as described in any one of claims 1 to 14, characterized in that, Includes the following steps: The negative electrode active material, binder and conductive agent of each negative electrode active layer are mixed and dispersed in water to obtain the first negative electrode slurry, the second negative electrode slurry, ..., the nth negative electrode slurry; The first negative electrode slurry, the second negative electrode slurry, ..., the nth negative electrode slurry are coated sequentially on at least one side of the negative electrode current collector, and after drying and rolling, a negative electrode sheet is obtained.

16. A lithium-ion battery, comprising a negative electrode, characterized in that, The negative electrode sheet is the negative electrode sheet according to any one of claims 1 to 14, or the negative electrode sheet is prepared by the method for preparing the negative electrode sheet according to claim 15.

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