A double-layer coated negative electrode sheet, a preparation method thereof, and a lithium ion battery

Abstract

This invention discloses a double-layer coated negative electrode sheet and its preparation method, as well as a lithium-ion battery, belonging to the field of lithium-ion battery technology. The double-layer coated negative electrode sheet of this invention includes a negative electrode current collector, a first coating, and a second coating; the first coating contains a first negative electrode active material, and the second coating contains a second negative electrode active material; the performance index θ satisfies the following relationship: [Equation missing]. By defining the above relationship, this invention systematically and synergistically optimizes key parameters of the double-layer coated negative electrode sheet, such as particle size, porosity, compaction density, capacity, elastic modulus, and interfacial bonding strength, achieving a comprehensive improvement in the fast-charging capability, energy density, cycle life, and safety of lithium-ion batteries.

Description

Technical Field

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

[0002] The information disclosed in the background section of this invention is intended only to enhance the understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] As the global energy structure accelerates its transition to clean energy, lithium-ion batteries, as the core energy carrier supporting new energy vehicles, smart terminals, and large-scale energy storage systems, are evolving from the traditional pursuit of single energy density to the synergistic optimization of multiple dimensions, including fast charging capability, cycle stability, safety and reliability, and overall cost control. However, the mainstream lithium-ion battery anode still widely adopts a single-layer coating process, and its homogeneous material system and structural design have become a key bottleneck restricting performance breakthroughs, specifically manifested in multiple irreconcilable technical contradictions.

[0004] In balancing fast-charging performance and energy density, traditional single-layer coated electrodes face a fundamental dilemma in material selection: using small-particle graphite to shorten the lithium-ion diffusion path and improve the fast-charging rate results in a significant decrease in compaction density due to the increased contact area between particles, leading to an energy density loss of over 15%; while using large-particle graphite can improve energy density by increasing packing density, the extended lithium-ion transport distance exacerbates polarization effects, significantly reducing fast-charging performance. This contradiction is particularly prominent in the field of power batteries, directly hindering the simultaneous improvement of fast-charging time and driving range in electric vehicles, becoming a core pain point restricting industry development. The limited functionality of materials further exacerbates the difficulty of performance optimization: for example, although silicon-based materials have an ultra-high theoretical specific capacity of 4200mAh / g, their 300% volume expansion rate during charging and discharging causes electrode coating cracking and current collector peeling, making them unsuitable for direct application in single-layer structures; hard carbon materials exhibit excellent low-temperature charge-discharge performance due to their disordered carbon structure, but their inherent low initial coulombic efficiency cannot be effectively compensated for by single-layer electrode design. In existing technologies, although the above problems can be partially alleviated by processes such as doping modification, surface coating or pre-lithiation, these are often accompanied by a significant increase in the complexity of the production process and a substantial increase in manufacturing costs, making it difficult to achieve large-scale application.

[0005] As the new energy industry continues to upgrade its performance requirements for lithium-ion batteries, the structural limitations of traditional single-layer coated anodes have become a key factor restricting the industry's progress. There is an urgent need to propose breakthrough solutions from the perspectives of material systems and structural design to achieve a synergistic improvement in energy density, fast-charging performance, cycle life, and safety. Summary of the Invention

[0006] In view of this, the present invention provides a double-layer coated negative electrode sheet and its preparation method, and a lithium-ion battery. The present invention adopts double-layer coating technology and gradient physical property parameter synergistic design method, and utilizes the differential matching of physical property parameters of upper and lower layer materials to systematically solve the problem of synergistic optimization of fast charging capability, energy density, cycle life and safety.

[0007] In a first aspect, the present invention provides a double-coated negative electrode sheet, comprising a negative electrode current collector and a first coating and a second coating sequentially stacked on at least one side surface of the negative electrode current collector; the first coating contains a first negative electrode active material and the second coating contains a second negative electrode active material. The performance index θ of the double-layer coated negative electrode sheet satisfies the relationship shown in equation (I): (I); Wherein, D1 and D2 are the D50 particle sizes of the first and second negative electrode active materials, respectively, in μm; ε1 and ε2 are the porosities of the first and second coatings, respectively, in %; ρ1 and ρ2 are the compacted densities of the first and second coatings, respectively, in g / cm³. 3 C1 and C2 are the specific capacities of the first and second coatings, respectively, in mAh / g; E1 and E2 are the elastic moduli of the first and second coatings, respectively, in GPa; A is the interfacial bonding strength in N / m; α, β, and γ are the first, second, and third weighting coefficients, respectively, and α+β+γ=1.

[0008] Preferably, D1 is 15~25μm, D2 is 8~12μm, ε1 is 25~35%, ε2 is 35~45%, and ρ1 is 1.6~1.8g / cm³. 3 ρ2 is 1.3~1.55 g / cm³ 3 C2 is 340~350mAh / g; when the first coating contains silicon carbon, C1 is 450~540mAh / g, and when the first coating does not contain silicon carbon, C1 is 350~370mAh / g; E1 is 2.0~3.0GPa, E2 is 0.5~1.5GPa, A≥2.0, α is 0.45~0.55, β is 0.25~0.35, and γ is 0.15~0.25.

[0009] Furthermore, D2:D1 is 1 : (1.5~3); ε2:ε1 is (1.2~1.8) : 1; ρ1:ρ2 is (1.1~1.3) : 1; E1:E2 is (2~4) : 1, α is 0.5, β is 0.3, and γ is 0.2.

[0010] Furthermore, the first negative electrode active material of the first coating is one or more of graphite, silicon carbide, hard carbon, or soft carbon; the second negative electrode active material of the second coating is one or more of graphite, hard carbon, or soft carbon. When the first coating contains silicon carbon, the ratio of C1 to C2 is (1.2~1.8):1; when the first coating does not contain silicon carbon, the ratio of C1 to C2 is (1.03~1.06):1.

[0011] Preferably, the areal density of the first coating is 6~12 mg / cm³. 2 The areal density of the second coating is 6~12 mg / cm³. 2 The ratio of the areal density of the first coating to that of the second coating is 1:(0.8~1.2).

[0012] Preferably, the first coating further includes a first conductive agent and a first adhesive, and the second coating further includes a second conductive agent and a second adhesive; The mass ratio of the first negative electrode active material, the first conductive agent, and the first binder is (94~98.5): (0.5~3): (1~3); the mass ratio of the second negative electrode active material, the second conductive agent, and the second binder is (94~98.5): (0.5~3): (1~3).

[0013] Furthermore, the first conductive agent and the second conductive agent are each independently selected from one or more of conductive carbon black, carbon nanotubes, graphene, Ketjen black, or carbon nanofibers, and the first binder and the second binder are each independently selected from one or more of styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), or polyimide (PI).

[0014] Preferably, the negative electrode current collector is selected from copper foil or carbon-coated copper foil.

[0015] Secondly, the present invention provides a method for preparing the above-mentioned double-layer coated negative electrode sheet, comprising the following steps: The first negative electrode active material is made into a first slurry, and the second negative electrode active material is made into a second slurry. The two slurries are coated onto the surface of the negative electrode current collector using a double-layer coating method. After drying and rolling, a double-layer coated negative electrode sheet is obtained.

[0016] Thirdly, the present invention provides a lithium-ion battery, comprising the above-described double-coated negative electrode sheet or the double-coated negative electrode sheet prepared by the above-described preparation method.

[0017] Compared with the prior art, the present invention has achieved the following beneficial effects: (1) By introducing a quantitative relationship of performance index θ, this invention systematically and synergistically optimizes key parameters such as particle size, porosity, compaction density, capacity, elastic modulus and interfacial bonding strength of double-layer coated negative electrode sheets, thereby achieving a comprehensive improvement in the fast charging capability, energy density, cycle life and safety of lithium-ion batteries.

[0018] (2) The present invention uses a gradient material design of two-layer materials. The second coating (surface layer) uses small-particle-size active materials and high porosity to shorten the lithium-ion diffusion path and promote electrolyte wetting. The first coating (bottom layer) uses large-particle-size active materials and low porosity to increase compaction density and suppress dendrite penetration. At the same time, by limiting the compaction density ratio, elastic modulus ratio and interfacial bonding strength, the fast charging performance, volumetric energy density and structural stability are taken into account. Attached Figure Description

[0019] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0020] Figure 1 This is a schematic diagram of the structure of the double-layer coated negative electrode sheet in a specific embodiment of the present invention. Detailed Implementation

[0021] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0022] This invention provides a double-layer coated negative electrode sheet, comprising a negative electrode current collector and a first coating and a second coating sequentially stacked on at least one surface of the negative electrode current collector; as a specific embodiment, such as Figure 1 As shown, a first coating and a second coating are sequentially stacked on both sides of the negative electrode current collector.

[0023] The first coating contains a first negative electrode active material, and the second coating contains a second negative electrode active material.

[0024] The performance index θ of the double-layer coated negative electrode sheet satisfies the relationship shown in equation (I): (I); Wherein, D1 and D2 are the D50 particle sizes of the first and second negative electrode active materials, respectively, in μm; ε1 and ε2 are the porosities of the first and second coatings, respectively, in %; ρ1 and ρ2 are the compacted densities of the first and second coatings, respectively, in g / cm³. 3 C1 and C2 are the specific capacities of the first and second coatings, respectively, in mAh / g; E1 and E2 are the elastic moduli of the first and second coatings, respectively, in GPa; A is the interfacial bonding strength in N / m; α, β, and γ are the first, second, and third weighting coefficients, respectively, and α+β+γ=1.

[0025] In the following description, "first coating," "lower layer," and "bottom layer" are used interchangeably, all referring to the coating that is in direct contact with the negative electrode current collector. "Second coating," "upper layer," and "surface layer" are used interchangeably, all referring to the coating on the surface of the first coating. D50 particle size refers to the particle size value corresponding to when the cumulative volume distribution of the sample reaches 50%.

[0026] This invention establishes a quantitative correlation between the microstructure and macroscopic performance of a double-layer coated negative electrode sheet through gradient synergistic design of multi-dimensional physical property parameters, and carries out coupled optimization around three aspects: ion transport kinetics, energy density, and structural stability. Firstly, This study focuses on improving ion transport efficiency. The ratio of D2 (D50 particle size of the upper active material) to D1 (particle size of the lower layer) (D2 / D1) reflects the particle size gradient design, while the ratio of ε2 (porosity of the upper layer) to ε1 (porosity of the lower layer) (ε2 / ε1) constructs the porosity gradient. The product of these two factors, through a weighting coefficient α, enhances their contribution to fast-charging performance, achieving a synergistic effect of "short path + high wetting" in ion transport. Secondly, This project aims to balance energy density and material utilization. The ratio of ρ1 (lower layer compaction density) to ρ2 (upper layer compaction density) (ρ1 / ρ2) reflects the density gradient, while the ratio of C1 (lower layer capacity) to C2 (upper layer capacity) (C1 / C2) optimizes energy output through material proportioning. The product of these two ratios, combined with a weighting coefficient β, achieves a synergistic energy effect of "high density + high capacity." Finally, This approach focuses on ensuring structural stability and cycle life. The ratio of E1 (lower layer elastic modulus) to E2 (upper layer elastic modulus) (E1 / E2) constructs a mechanical gradient; A (interfacial bonding strength) ensures that the bond strength between the upper and lower layers is sufficient to resist the peel stress generated by expansion, preventing interlayer cracking. The product of these two factors, through a weighting coefficient γ, enhances their contribution to structural stability, achieving a synergistic mechanical effect of "hard support - soft buffer - strong interface".

[0027] Overall, the threshold requirement of θ≥1.5 quantifies the three major performance indicators of ion transport, energy density and structural stability into an adjustable mathematical relationship through the weighted allocation of α, β and γ (totaling 1), so that the gradients of each parameter form a synergistic rather than competitive relationship, and ultimately achieve systematic optimization of the fast charging capability, driving range and cycle life of lithium-ion batteries.

[0028] In this invention, D1 is 15~25μm and D2 is 8~12μm. The large-particle active material in the lower layer increases the compaction density through close packing, directly improving the volumetric energy density. The upper layer uses small-particle material to shorten the diffusion path of lithium ions within the particles, reducing concentration polarization at high rates. ε1 is 25~35% and ε2 is 35~45%. The high porosity of the upper layer provides more channels for electrolyte wetting, accelerating the migration rate of ions within the coating, while the low porosity of the lower layer ensures structural compactness to suppress dendrite growth. The difference in porosity between the upper and lower layers forms a transport channel of "dense lower layer suppressing dendrites + loose upper layer promoting electrolyte wetting".

[0029] In this invention, ρ1 is 1.6~1.8 g / cm³. 3 ρ2 is 1.3~1.55 g / cm³ 3 The C2 value is 340~350mAh / g; when the first coating contains silicon-carbon, the C1 value is 450~540mAh / g, and when the first coating does not contain silicon-carbon, the C1 value is 350~370mAh / g. The lower layer uses high-compact and high-specific-capacity materials to increase the volumetric energy density and uniformly disperse silicon expansion stress; the upper layer uses low-compact materials to balance fast charging and structural stability.

[0030] In this invention, E1 is 2.0~3.0GPa, E2 is 0.5~1.5GPa, and A≥2.0. The lower high modulus provides rigid support to suppress the volume expansion stress of the material, while the upper low modulus has flexible buffering capacity to adapt to the coating deformation during the cycle. The interfacial bonding strength (A) ≥2.0N / cm ensures that the adhesion between the upper and lower layers is sufficient to resist the peeling stress generated by expansion and prevent interlayer cracking.

[0031] In this invention, α is 0.45~0.55, more preferably 0.48~0.52, and most preferably 0.5; β is 0.25~0.35, more preferably 0.28~0.32, and most preferably 0.3; γ is 0.15~0.25, more preferably 0.18~0.22, and most preferably 0.2. Ion transport dynamics (fast charging performance) is given priority in the overall optimization, and a comprehensive performance threshold of θ value ≥ 1.5 is ensured through the balance between β (energy density) and γ (structural stability).

[0032] In this invention, D2:D1 is 1:(1.5~3); this ratio ensures that the particle size of the upper layer is significantly smaller than that of the lower layer, maximizing the effect of shortening the diffusion path. ε2:ε1 is (1.2~1.8):1; the ion transport efficiency is improved by the moderately high porosity of the upper layer, while avoiding excessive reduction in volumetric energy density. ρ1:ρ2 is 1:(1.1~1.3); balancing the packing limit of large particle materials and the feasibility of the rolling process. E1:E2 is (2~4):1, so that the lower layer suppresses the expansion of the material through high rigidity, and the upper layer adapts to cyclic deformation through low modulus.

[0033] In this invention, the first negative electrode active material of the first coating is one or more of graphite, silicon carbon, hard carbon or soft carbon, more preferably graphite or a combination of graphite and silicon carbon; the second negative electrode active material of the second coating is one or more of graphite, hard carbon or soft carbon, more preferably graphite.

[0034] When the first coating contains silicon carbon, the C1:C2 ratio is (1.2~1.8):1, which compensates for the loss of active material caused by the volume expansion of silicon carbon due to its high capacity; when the first coating does not contain silicon carbon, the C1:C2 ratio is (1.03~1.06):1, which optimizes the state of charge (SOC) distribution with a small capacity difference and avoids uneven interlayer polarization.

[0035] In this invention, the areal density of the first coating is 6~12 mg / cm³. 2 The areal density of the second coating is 6~12 mg / cm³. 2 The areal density ratio of the first coating to the second coating is 1:(0.8~1.2). If the areal density is too high, it will lead to a longer diffusion path for ions within the coating, exacerbating concentration polarization, especially resulting in significant capacity loss during high-rate charge-discharge cycles; if it is too low, the active material loading will be insufficient, and the energy density will be difficult to meet the target. A similar areal density ratio can avoid excessive differences in the state of charge (SOC) between the two layers, reduce interfacial stress caused by inconsistent expansion / contraction during cycling, and ensure that the slurry leveling and drying shrinkage are matched during double-layer coating, reducing the risk of cracking in the rolling process.

[0036] In this invention, the first coating further includes a first conductive agent and a first binder, and the second coating further includes a second conductive agent and a second binder; the mass ratio of the first negative electrode active material, the first conductive agent and the first binder is (94~98.5): (0.5~3): (1~3); the mass ratio of the second negative electrode active material, the second conductive agent and the second binder is (94~98.5): (0.5~3): (1~3).

[0037] In this invention, the first conductive agent and the second conductive agent are each independently selected from one or more of conductive carbon black, carbon nanotubes, graphene, Ketjen black or carbon nanofibers, and the first binder and the second binder are each independently selected from one or more of styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polyacrylic acid (PAA) or polyimide (PI).

[0038] In this invention, the negative electrode current collector is selected from copper foil or carbon-coated copper foil. Copper foil, as a traditional current collector, has advantages such as high conductivity, chemical stability, and low cost; carbon-coated copper foil further optimizes interface characteristics through a surface carbon layer: the carbon layer can reduce the contact resistance between the current collector and the active material, and at the same time improve the coating adhesion through a physical anchoring effect, reducing the "powder shedding" phenomenon during cycling. For high-capacity silicon-carbon negative electrodes, carbon-coated copper foil can effectively alleviate interface peeling caused by volume expansion and improve battery cycle life.

[0039] The present invention also provides a method for preparing the above-mentioned double-layer coated negative electrode sheet, comprising the following steps: The first negative electrode active material is made into a first slurry, and the second negative electrode active material is made into a second slurry. The two slurries are coated onto the surface of the negative electrode current collector using a double-layer coating method. After drying and rolling, a double-layer coated negative electrode sheet is obtained.

[0040] This invention does not impose any special restrictions on the preparation methods of the first and second slurries; commonly used methods for preparing negative electrode slurries in the art can be used. Preferably, the difference in solid content between the first and second slurries is controlled within ≤5% to avoid uneven thickness caused by interlayer mutual dissolution.

[0041] This invention does not impose any special restrictions on the method of double-layer coating; double-layer coating can be performed using a double-layer coating machine (with a double-layer coating die head) commonly used in the field.

[0042] The present invention also provides a lithium-ion battery, comprising the above-described double-coated negative electrode sheet or the double-coated negative electrode sheet prepared by the above-described preparation method.

[0043] The lithium-ion battery provided by this invention further includes a positive electrode, a separator, and an electrolyte. This invention does not impose any special restrictions on the preparation and selection of the positive electrode, separator, and electrolyte; commonly used methods in the art can be employed. This invention also does not impose any special restrictions on the preparation method of the lithium-ion battery; conventional lithium-ion battery preparation methods in the art can be followed.

[0044] The technical solution of the present invention will be further described below with reference to specific embodiments. The present invention does not impose any special restrictions on the source of reagents used in the following embodiments; commercially available products well known to those skilled in the art can be used.

[0045] In the following embodiments, the porosity test method is GB / T 21650.1 "Determination of Pore Size Distribution of Solid Materials by Mercury Intrusion Porosimetry and Gas Adsorption Method". The interfacial bonding strength test method is: 180° peel test: The specific steps are as follows: First, cut the electrode sheet to the specified size using a die-cutting roller; apply double-sided adhesive to the steel plate and tightly adhere it to the electrode sheet; fix the steel plate vertically to the lower clamp of the tensile testing machine, and fix the other end of the electrode sheet to the upper clamp (at a 180° / 90° angle); set the peel length and speed, and then test; remove the abnormal segments at the beginning and end of the data, and obtain the standard peel curve and average value; the specific standard refers to GB / T 2792 "Test for Peel Strength of Pressure-Sensitive Adhesive Tape". The elastic modulus test method is GB / T 3074.2-2008 "Method for Determination of Elastic Modulus of Graphite Electrode".

[0046] Example 1 This embodiment provides a double-layer coated negative electrode sheet and its preparation method.

[0047] (1) Preparation of the first slurry: Primary artificial graphite with a particle size D50 of 22 μm, conductive agent SP, CMC and SBR are mixed in a mass ratio of 95:1.5:2:3 and deionized water is added to prepare the first slurry.

[0048] (2) Preparation of the second slurry: Secondary artificial graphite with a particle size D50 of 12 μm, conductive agent SP, CMC and SBR are mixed in a mass ratio of 95:1.5:2:3 and deionized water is added to prepare the second slurry.

[0049] (3) A double-layer coating machine is used to coat the copper foil surface. The first slurry and the second slurry are coated simultaneously. After drying, a double-layer coated negative electrode sheet as shown in the attached figure is obtained. The areal density of the first coating and the second coating is controlled to be 8 mg / cm². 2 Roll pressing was used to control the compaction density of the first coating to 1.7 g / cm³. 3 The compaction density of the second coating is 1.36 g / cm³. 3 .

[0050] The specific capacity of the first coating was 360 mAh / g, the porosity was 25%, and the elastic modulus was 2.5 GPa; the specific capacity of the second coating was 340 mAh / g, the porosity was 35%, and the elastic modulus was 1.0 GPa; the interfacial bonding strength between the first and second coatings was 2.2 N / m.

[0051] Example 2 This embodiment provides a double-layer coated negative electrode sheet and its preparation method.

[0052] (1) Preparation of the first slurry: The active material, conductive agent SP, CMC and SBR are mixed in a mass ratio of 95:1.5:2:3. The active material is a composite system of primary particulate artificial graphite (particle size D50 is 20μm) and silicon carbon. The silicon carbon accounts for 7wt% of the active material. Deionized water is added to prepare the first slurry.

[0053] (2) Preparation of the second slurry: Secondary particle artificial graphite with a particle size D50 of 10 μm, conductive agent SP, CMC and SBR are mixed in a mass ratio of 95:1.5:2:3, and deionized water is added to prepare the second slurry.

[0054] (3) A double-layer coating machine is used to coat the copper foil surface. The first slurry and the second slurry are coated simultaneously. After drying, a double-layer coated negative electrode sheet as shown in the attached figure is obtained. The areal density of the first coating and the second coating is controlled to be 8 mg / cm². 2 Roll pressing was used to control the compaction density of the first coating to 1.7 g / cm³. 3 The compaction density of the second coating is 1.5 g / cm³. 3 .

[0055] The specific capacity of the first coating was 450 mAh / g, the porosity was 30%, and the elastic modulus was 3.0 GPa; the specific capacity of the second coating was 350 mAh / g, the porosity was 40%, and the elastic modulus was 1.0 GPa; the interfacial bonding strength between the first and second coatings was 2.5 N / m.

[0056] Example 3 This embodiment provides a double-layer coated negative electrode sheet and its preparation method.

[0057] (1) Preparation of the first slurry: The active material, conductive agent SP, CMC and SBR are mixed in a mass ratio of 95:1.5:2:3. The active material is a composite system of primary particulate artificial graphite and silicon carbon. The silicon carbon accounts for 12wt% of the active material. The particle size D50 of the primary particulate artificial graphite is 16μm. Deionized water is added to prepare the first slurry.

[0058] (2) Preparation of the second slurry: Secondary particle artificial graphite with a particle size D50 of 8 μm, conductive agent SP, CMC and SBR are mixed in a mass ratio of 95:1.5:2:3, and deionized water is added to prepare the second slurry.

[0059] (3) A double-layer coating machine is used to coat the copper foil surface. The first slurry and the second slurry are coated simultaneously. After drying, a double-layer coated negative electrode sheet as shown in the attached figure is obtained. The areal density of the first coating and the second coating is controlled to be 8 mg / cm². 2Roll pressing was used to control the compaction density of the first coating to 1.7 g / cm³. 3 The compaction density of the second coating is 1.5 g / cm³. 3 .

[0060] The specific capacity of the first coating was measured to be 510 mAh / g, the porosity was 25%, and the elastic modulus was 3.0 GPa; the specific capacity of the second coating was 350 mAh / g, the porosity was 45%, and the elastic modulus was 1.0 GPa; the interfacial bonding strength between the first and second coatings was 2.8 N / m.

[0061] Example 4 This embodiment provides a double-layer coated negative electrode sheet and its preparation method.

[0062] (1) Preparation of the first slurry: Primary artificial graphite with a particle size D50 of 18 μm, conductive agent SP, CMC and SBR are mixed in a mass ratio of 95:1.5:2:3 and deionized water is added to prepare the first slurry.

[0063] (2) Preparation of the second slurry: Secondary artificial graphite with a particle size D50 of 12 μm, conductive agent SP, CMC and SBR are mixed in a mass ratio of 95:1.5:2:3 and deionized water is added to prepare the second slurry.

[0064] (3) A double-layer coating machine is used to coat the copper foil surface. The first slurry and the second slurry are coated simultaneously. After drying, a double-layer coated negative electrode sheet as shown in the attached figure is obtained. The areal density of the first coating and the second coating is controlled to be 8 mg / cm². 2 Roll pressing was used to control the compaction density of the first coating to 1.7 g / cm³. 3 The compaction density of the second coating is 1.5 g / cm³. 3 .

[0065] The specific capacity of the first coating was 360 mAh / g, the porosity was 30%, and the elastic modulus was 2.0 GPa; the specific capacity of the second coating was 350 mAh / g, the porosity was 36%, and the elastic modulus was 1.0 GPa; the interfacial bonding strength between the first and second coatings was 2.3 N / m.

[0066] Example 5 This embodiment provides a double-layer coated negative electrode sheet and its preparation method.

[0067] (1) Preparation of the first slurry: The active material, conductive agent SP, CMC and SBR are mixed in a mass ratio of 95:1.5:2:3. The active material is a composite system of primary particulate artificial graphite (particle size D50 is 18μm) and silicon carbon. The silicon carbon accounts for 14wt% of the active material. Deionized water is added to prepare the first slurry.

[0068] (2) Preparation of the second slurry: Secondary artificial graphite with a particle size D50 of 12 μm, conductive agent SP, CMC and SBR are mixed in a mass ratio of 95:1.5:2:3 and deionized water is added to prepare the second slurry.

[0069] (3) A double-layer coating machine is used to coat the copper foil surface. The first slurry and the second slurry are coated simultaneously. After drying, a double-layer coated negative electrode sheet as shown in the attached figure is obtained. The areal density of the first coating layer is controlled to be 12 mg / cm³. 2 The areal density of the second coating is 8 mg / cm³. 2 Roll pressing was used to control the compaction density of the first coating to 1.7 g / cm³. 3 The compaction density of the second coating is 1.5 g / cm³. 3 .

[0070] The specific capacity of the first coating was 540 mAh / g, the porosity was 30%, and the elastic modulus was 2.0 GPa; the specific capacity of the second coating was 350 mAh / g, the porosity was 36%, and the elastic modulus was 1.0 GPa; the interfacial bonding strength between the first and second coatings was 2.3 N / m.

[0071] Comparative Example 1 Compared with Example 3, the difference between this comparative example and Example 3 is that the particle size D50 of the primary artificial graphite in step (1) of this comparative example is 18 μm; the artificial graphite and silicon-carbon composite system (the proportion of silicon-carbon is 12%) is used in step (2), and the particle size of the graphite is 10 μm; the specific capacity of the first coating is 340 mAh / g, the porosity is 30%, and the elastic modulus is 1.8 GPa; the capacity of the second coating is 520 mAh / g, the porosity is 33%, and the interfacial bonding strength is 2.0 N / m.

[0072] Comparative Example 2 Compared with Example 1, the difference between this comparative example and Example 1 is that the particle size D50 of the primary artificial graphite in step (1) of this comparative example is 15 μm; the particle size of the artificial graphite in step (2) is 8 μm; the porosity of the first coating is 30% and the elastic modulus is 2.2 GPa; the porosity of the second coating is 32% and the interfacial bonding strength is 1.8 N / m.

[0073] The parameters of Examples 1-5 and Comparative Examples 1-2 are summarized in Table 1.

[0074] Table 1. Parameters of Examples 1-5 and Comparative Examples 1-2

[0075] In Table 1, D1 and D2 are the D50 particle sizes of artificial graphite in steps (1) and (2), respectively, in μm; ε1 and ε2 are the porosities of the first and second coatings, respectively, in %; ρ1 and ρ2 are the compacted densities of the first and second coatings, respectively, in g / cm³. 3 C1 and C2 are the specific capacities of the first and second coatings, respectively, in mAh / g; E1 and E2 are the elastic moduli of the first and second coatings, respectively, in GPa; A is the interfacial bonding strength in N / m. α, β, and γ are 0.5, 0.3, and 0.2, respectively.

[0076] Test case 1. Lithium-ion battery assembly: A positive electrode slurry was prepared by mixing lithium iron phosphate, conductive agent SP, and binder PVDF in N-methylpyrrolidone at a mass ratio of 97:1:2. The slurry was coated onto aluminum foil, dried, and then rolled to obtain a positive electrode sheet. The negative electrode sheet used was the negative electrode sheet of Examples 1-5 and Comparative Examples 1-2. The separator was a PP / PE composite membrane. The electrodes were stacked in the order of "negative electrode-separator-positive electrode" to assemble a battery cell. An electrolyte (1M LiPF6 dissolved in a mixed solution of ethylene carbonate, methyl ethyl carbonate, and dimethyl carbonate in a volume ratio of 3:5:2) was injected. After formation and capacity testing, a 3Ah lithium-ion battery was obtained.

[0077] 2. Lithium-ion battery performance testing: The energy density of the lithium-ion batteries in the test examples and comparative examples was tested, and the discharge capacity retention rate at 2C rate and the 300-cycle performance at 1C rate were tested in accordance with GB / T 31486-2015 "Requirements and Test Methods for Cycle Life of Power Batteries for Electric Vehicles". The test results are shown in Table 2.

[0078] Table 2. Lithium-ion battery performance of the examples and comparative examples

[0079] As can be seen from Tables 1 and 2, the performance index θ values ​​of the above embodiments of the present invention are all greater than 1.5. This comprehensively considers the synergistic effect between the physical properties of the upper and lower active materials, ensuring a good match between the upper and lower layers, thereby improving the fast-charging capability, energy density, and cycle performance of the lithium-ion battery. Embodiments 2, 3, and 5 introduce silicon-carbon materials into the lower layer, further improving the energy density of the cell while maintaining cycle performance. Embodiment 4 reduces the particle size of the lower layer particles and increases the particle size ratio between the upper and lower layers, thus improving the fast-charging performance of the cell.

[0080] Compared with Example 1, Comparative Example 2 has insufficient interfacial bonding strength, resulting in insufficient bonding strength between the upper and lower layers of the electrode during cycling, thus leading to rapid degradation during cycling.

[0081] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A double-layer coated negative electrode sheet, characterized in that, It includes a negative electrode current collector and a first coating and a second coating sequentially stacked on at least one surface of the negative electrode current collector; the first coating contains a first negative electrode active material and the second coating contains a second negative electrode active material. The performance index θ of the double-layer coated negative electrode sheet satisfies the relationship shown in equation (I): (I)； Wherein, D1 and D2 are the D50 particle sizes of the first and second negative electrode active materials, respectively, in μm; ε1 and ε2 are the porosities of the first and second coatings, respectively, in %; ρ1 and ρ2 are the compacted densities of the first and second coatings, respectively, in g / cm³. 3 C1 and C2 are the specific capacities of the first and second coatings, respectively, in mAh / g; E1 and E2 are the elastic moduli of the first and second coatings, respectively, in GPa; A is the interfacial bonding strength in N / m; α, β, and γ are the first, second, and third weighting coefficients, respectively, and α+β+γ=1; Among them, D1 is 16~22μm, D2 is 8~12μm, ε1 is 25~30%, ε2 is 35~45%, and ρ1 is 1.6~1.8g / cm³. 3 ρ2 is 1.36~1.5 g / cm³ 3 C2 is 340~350mAh / g; when the first coating contains silicon carbon, C1 is 450~540mAh / g, and when the first coating does not contain silicon carbon, C1 is 350~370mAh / g; E1 is 2.0~3.0GPa, E2 is 0.5~1.5GPa, A≥2.0, α is 0.45~0.55, β is 0.25~0.35, and γ is 0.15~0.

25.

2. The double-layer coated negative electrode sheet as described in claim 1, characterized in that, D2: D1 is 1 : (1.5~3); ε2: ε1 is (1.2~1.8) : 1; ρ1: ρ2 is 1 : (1.1~1.3); E1: E2 is (2~4) : 1, α is 0.5, β is 0.3, γ is 0.

2.

3. The double-layer coated negative electrode sheet as described in claim 2, characterized in that, The first negative electrode active material of the first coating is one or more of graphite, silicon carbide, hard carbon, or soft carbon; the second negative electrode active material of the second coating is one or more of graphite, hard carbon, or soft carbon. When the first coating contains silicon carbon, the ratio of C1 to C2 is (1.2~1.8):1; when the first coating does not contain silicon carbon, the ratio of C1 to C2 is (1.03~1.06):

1.

4. The double-layer coated negative electrode sheet as described in claim 1, characterized in that, The areal density of the first coating is 6~12 mg / cm³. 2 The areal density of the second coating is 6~12 mg / cm³. 2 The ratio of the areal density of the first coating to that of the second coating is 1:(0.8~1.2).

5. The double-layer coated negative electrode sheet as described in claim 1, characterized in that, The first coating further includes a first conductive agent and a first adhesive, and the second coating further includes a second conductive agent and a second adhesive; The mass ratio of the first negative electrode active material, the first conductive agent, and the first binder is (94~98.5): (0.5~3): (1~3); the mass ratio of the second negative electrode active material, the second conductive agent, and the second binder is (94~98.5): (0.5~3): (1~3).

6. The double-layer coated negative electrode sheet as described in claim 5, characterized in that, The first conductive agent and the second conductive agent are each independently selected from one or more of conductive carbon black, carbon nanotubes, graphene or carbon nanofibers, and the first binder and the second binder are each independently selected from one or more of styrene-butadiene rubber, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyacrylic acid or polyimide.

7. The double-layer coated negative electrode sheet as described in claim 1, characterized in that, The negative electrode current collector is selected from copper foil or carbon-coated copper foil.

8. The method for preparing a double-layer coated negative electrode sheet according to any one of claims 1 to 7, characterized in that, Includes the following steps: The first negative electrode active material is made into a first slurry, and the second negative electrode active material is made into a second slurry. The two slurries are coated onto the surface of the negative electrode current collector using a double-layer coating method. After drying and rolling, a double-layer coated negative electrode sheet is obtained.

9. A lithium-ion battery, characterized in that, Includes the double-coated negative electrode sheet according to any one of claims 1 to 7 or the double-coated negative electrode sheet prepared by the preparation method according to claim 8.

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