A high-energy-density lithium-ion battery prepared by a dry positive electrode and a wet negative electrode composite process and its preparation method.

Abstract

This invention provides a high-energy-density lithium-ion battery and its preparation method using a dry-process positive electrode and a wet-process negative electrode composite fabrication. By employing a composite bonding system of polytetrafluoroethylene (PTFE) and polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), a solvent-free dry process is used to achieve positive electrode fiberization and interfacial toughening, constructing a dense inner and sparse outer double-layer gradient structure. Simultaneously, a wet-process precisely matched negative electrode is used to form a highly stable, high-load hybrid electrode system. The battery of this invention achieves a high energy density of 282.0–288.0 Wh / kg, a 1000-cycle cycle retention rate of ≥88.1%, a high peel strength of 11.1–11.5 N / cm, a fast electrolyte wetting time of 17.8–19.5 s, and a low interfacial impedance of 37.8–40.2 Ω. All performance indicators are superior to traditional wet processes, single-PTFE dry processes, and all-dry processes, demonstrating clear technological innovation and significant industrial application value.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery manufacturing technology, and in particular to a high-energy-density lithium-ion battery prepared by a dry positive electrode and a wet negative electrode composite method, and the same method. Background Technology

[0002] With the rapid development of the new energy vehicle industry, lithium-ion batteries are constantly evolving towards higher energy density, higher safety, lower cost, and green manufacturing. Solvent-free dry electrode technology, by eliminating the use of solvents, drying processes, and waste gas recovery systems in traditional wet processes, has significant advantages such as simplified manufacturing, reduced energy consumption, environmental friendliness, controllable thickness, and suitability for high-load electrodes, and has become an important technological development direction for next-generation power batteries.

[0003] In existing dry electrode fabrication technologies, PTFE (polytetrafluoroethylene) is mainly used as a binder. PTFE forms a fibrous network structure under high-temperature conditions through mechanical shearing, bonding active material particles into a self-supporting film, which is then laminated onto the current collector surface via hot pressing. While this single PTFE binder system enables solvent-free fabrication, it has significant limitations in practical applications: the fibrous network formed by PTFE is highly brittle and prone to cracking during high-load electrode fabrication; the interfacial bonding between PTFE and the metal current collector is weak, easily leading to warping deformation during high-temperature hot pressing; and the hydrophobic properties of PTFE result in poor electrolyte wettability, affecting lithium-ion transport efficiency. These inherent defects collectively limit the cycle stability and rate performance of dry-process thick electrodes.

[0004] In terms of overall battery structure design, to achieve high energy density, it is necessary to increase the active material loading of the positive electrode. However, when the positive electrode is prepared using a high-load dry process, capacity matching of the negative electrode becomes a critical issue. If the negative electrode also adopts a dry process, PTFE will be at a low potential environment at the negative electrode (typically below 0.8V vs Li / Li). + This process can lead to a reduction and decomposition reaction, resulting in decreased initial coulombic efficiency, increased gas production, and rapid capacity decay during cycling, severely impacting battery safety and lifespan. This process compatibility issue presents a significant technical obstacle to the practical application of all-dry battery structures.

[0005] Furthermore, existing dry cathodes mostly employ a homogeneous structure design, meaning that the active material particles are uniformly distributed throughout the entire electrode thickness. This structure has significant drawbacks in thick electrode applications: the lithium ion transport path inside the electrode is relatively long, leading to severe polarization; the electron transport pathway and ion transport pathway are not effectively separated and optimized, making it difficult to simultaneously meet the requirements of high energy density and high power output; under rapid charge and discharge conditions, concentration polarization and electrochemical polarization inside the electrode increase significantly, severely limiting rate performance.

[0006] In summary, existing dry electrode technologies face technical bottlenecks in binder systems, process compatibility, and structural design, making it difficult to break through the 285Wh / kg energy density barrier for full-cell batteries. Furthermore, their cycle stability and rate performance fail to meet the application requirements of high-performance power batteries. Therefore, there is an urgent need to develop an innovative technical solution that, while maintaining the advantages of dry processes, solves the aforementioned technical problems and enables the stable fabrication of high-energy-density lithium-ion batteries. Summary of the Invention

[0007] The purpose of this invention is to provide a high-energy-density lithium-ion battery and its preparation method prepared by a dry positive electrode and a wet negative electrode composite process, so as to solve the following technical problems existing in the prior art:

[0008] (1) The fibrous network formed by a single PTFE binder in the existing dry electrode technology has inherent defects such as high brittleness, weak bonding force with the current collector interface, easy warping under high temperature hot pressing, and poor electrolyte wettability, which limit the cycle stability and rate performance of dry thick electrodes.

[0009] (2) After the high-load positive electrode is prepared, the capacity of the negative electrode is not matched, which makes it difficult for the energy density of the whole cell to break through 285Wh / kg. If the negative electrode adopts the dry process at the same time, it will face the reduction and decomposition reaction of PTFE in the low potential environment, resulting in a decrease in the first coulombic efficiency, an increase in gas production and rapid capacity decay during the cycle, which seriously affects the safety and service life of the battery.

[0010] (3) Existing dry cathodes mostly adopt a homogeneous structure design, with active material particles evenly distributed throughout the entire electrode thickness direction. This results in a long transport path for lithium ions inside the electrode and severe polarization. The electron transport pathway and ion transport pathway have not been effectively separated and optimized, and the rate performance is severely limited under fast charge and discharge conditions.

[0011] To achieve the above objectives, the present invention adopts the following technical solution:

[0012] This invention provides a high-energy-density lithium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode uses a high-nickel ternary material as the active material, wherein the molar content of nickel in the high-nickel ternary material is ≥80%. By using a high-nickel ternary material as the positive electrode active material, the specific capacity of the positive electrode can be significantly improved, thereby increasing the energy density of the battery. The positive electrode uses a composite binder containing polytetrafluoroethylene (PTFE) and a fluorinated polymer. In this composite binder system, PTFE forms a fibrous network structure through high-temperature shearing, providing mechanical strength. The fluorinated polymer, as a comonomer, disrupts the regular crystal structure of polyvinylidene fluoride (PVDF), reducing brittleness. Its polar sites increase free volume, improving electrolyte wettability and reducing interfacial impedance. The polyfluorinated structure maintains oxidation stability at high potentials of the positive electrode. The synergistic effect of the composite system overcomes the inherent defects of single PTFE. The positive electrode has a gradient bilayer structure, with the particle size of the inner active material near the current collector being larger than that of the outer active material near the separator. The inner layer of large-diameter particles shortens the electron transport distance and increases the contact points of the conductive agent through high compaction, responsible for rapid electron transport. The outer layer of small-diameter particles increases the interface area between the electrolyte and the active material through high porosity, responsible for rapid lithium-ion transport. This gradient transition achieves functional synergy and significantly reduces polarization of thick electrodes. The negative electrode uses artificial graphite as the active material and adopts a water-soluble binder system. This water-soluble binder system is chemically stable under low potential conditions of the negative electrode, forming a dense and uniform SEI film, avoiding the reduction and decomposition problem of PTFE at low potentials. The active material mass per unit area of ​​the positive electrode is 28.5-33.5 mg / cm², and the active material mass per unit area of ​​the negative electrode is 13.5-15.5 mg / cm². Through the scientific quantitative matching of the areal density of the positive and negative electrodes, the energy density is maximized within a safe range of 1.15~1.20 for the N / P ratio, avoiding lithium plating on the negative electrode while ensuring full capacity utilization.

[0013] Furthermore, the fluorinated polymer is a polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) or a fluorinated acrylate. Fluorinated polymers possess good electrochemical stability and flexibility, and the -CF3 side chain in their molecular structure provides polar sites, effectively improving the wettability of the electrolyte.

[0014] Furthermore, the mass ratio of polytetrafluoroethylene (PTFE) to fluorinated modified polymer in the compounded adhesive is between 6:5 and 5:1. This ratio range ensures that PTFE is fully fibrillated to form an effective bonding network, while also ensuring that the fluorinated modified polymer can fully exert its modifying effect, achieving a synergistic effect of reduced brittleness and improved wettability.

[0015] Furthermore, the positive electrode also includes a conductive agent, which is a composite system of porous carbon black and carbon nanotubes. Porous carbon black has a high specific surface area, providing a large number of electron transport contact points; carbon nanotubes have excellent conductivity and fibrous structure, enabling the construction of long-range conductive networks; the composite use of the two can reduce the amount of conductive agent used while ensuring conductivity and increasing the loading of active materials.

[0016] Furthermore, the particle size D50 of the inner active material is 10-15 μm, and the particle size D50 of the outer active material is 3-6 μm. This particle size range design enables the inner layer to achieve high compaction density and dense conductive pathways, while the outer layer provides sufficient pore space and electrolyte wetting channels, forming an effective gradient transition between the two.

[0017] Furthermore, the compaction density of the inner layer is 3.60-3.75 g / cm³, and the porosity of the outer layer is 35-45%. The high compaction density of the inner layer ensures electron transport efficiency, while the high porosity of the outer layer ensures lithium-ion transport efficiency. The synergistic effect of the two significantly reduces the polarization of the thick electrode.

[0018] Furthermore, the active material mass per unit area of ​​the positive electrode is 30.0-32.5 mg / cm², and the active material mass per unit area of ​​the negative electrode is 14.0-15.0 mg / cm². This areal density range is the optimal matching range obtained through extensive experimental optimization while ensuring battery safety, and can stably achieve an energy density of over 285 Wh / kg.

[0019] Furthermore, the energy density of the lithium-ion battery is 285-295 Wh / kg. Through the comprehensive application of the above technical solutions, the lithium-ion battery of the present invention can stably achieve this energy density range, which is a significant improvement compared to the prior art.

[0020] Furthermore, the water-soluble binder system is a compound system of carboxymethyl cellulose and styrene-butadiene rubber. Carboxymethyl cellulose, as a water-soluble binder, has good dispersibility and adhesion, while styrene-butadiene rubber provides elasticity and flexibility. The compounding of the two can form a stable negative electrode structure while ensuring good electrochemical stability.

[0021] This invention also provides a method for preparing a high-energy-density lithium-ion battery, characterized by comprising the following steps:

[0022] S1: Preparation of the positive electrode sheet:

[0023] S1-1: High-nickel ternary material, conductive agent, polytetrafluoroethylene and fluorinated polymer are dry-mixed at room temperature in proportion to obtain a uniform mixed powder. This step uses mechanical mixing to fully disperse the components and provide a uniform raw material basis for subsequent fiber kneading.

[0024] S1-2: The mixed powder is subjected to high shear mixing at 70-90℃ to fibrillate polytetrafluoroethylene. This temperature range is the optimal temperature range for polytetrafluoroethylene to fibrillate. Through high shear action, the polytetrafluoroethylene molecular chains unfold and form a fibrous network structure.

[0025] S1-3: Calender the fibrillated mixture at 90-105℃ and 80-120N / mm to form a continuous primary film. This step further densifies the fibrillated network through appropriate temperature and pressure to form a continuous film with a certain strength.

[0026] S1-4: The initial film is subjected to multiple roll pressings at 100-120℃ and 150-220N / mm to obtain a self-supporting dry film with an areal density of 28.5-33.5mg / cm². This step gradually increases the compaction density of the film through multiple roll pressings, so that it reaches the target areal density and has self-supporting properties.

[0027] S1-5: The dry film is hot-pressed onto the aluminum foil current collector at 110-130℃ and 15-25MPa. This step uses the hot-pressing process to form a strong interfacial bond between the dry film and the aluminum foil current collector.

[0028] S1-6: The composite electrode is precisely compacted at room temperature to 50°C to achieve a compaction density of 3.50-3.65 g / cm³. This step eliminates the springback after hot pressing by cold pressing, further improving the compaction density to meet the target energy density requirements.

[0029] S2: Preparation of negative electrode sheet:

[0030] S2-1: Dry mixing of artificial graphite, conductive agent and carboxymethyl cellulose. This step ensures that the components are fully dispersed, providing uniform solid raw materials for subsequent slurry preparation.

[0031] S2-2: Deionized water is added to disperse and prepare a slurry, followed by the addition of styrene-butadiene rubber for emulsification and dispersion. In this step, carboxymethyl cellulose is first dissolved in water to form a viscous solution, and then the styrene-butadiene rubber emulsion is added for thorough dispersion to form a stable negative electrode slurry.

[0032] S2-3: The slurry is coated onto the copper foil current collector, and the coating surface density is controlled to be 13.5-15.5 mg / cm². This step achieves the target surface density by precisely controlling the coating gap and speed, so that the slurry is uniformly coated on the copper foil surface.

[0033] S2-4: Dry and compact the coated electrode to control the compaction density to 1.65-1.75 g / cm³. The drying step removes moisture from the slurry, and the compaction step increases the compaction density of the negative electrode and improves electron transport performance.

[0034] S2-5: Vacuum baking is performed on the compacted electrode sheet. This step further removes residual moisture and gas in the negative electrode under vacuum environment, thereby improving the electrochemical stability of the negative electrode.

[0035] S3: Assemble the positive electrode, negative electrode, separator, and electrolyte into a lithium-ion battery. This step includes conventional assembly processes such as electrode cutting, winding or stacking, casing, electrolyte injection, and formation, ultimately producing a high-energy-density lithium-ion battery.

[0036] Furthermore, the dry mixing time in step S1-1 is 15-20 minutes. This time range ensures that the components are fully and evenly mixed, while avoiding the breakage of active material particles due to over-mixing.

[0037] Furthermore, the high-shear mixing time in steps S1-2 is 8-12 minutes. This time range allows the polytetrafluoroethylene to be fully fibrillated, forming an effective bonding network, while avoiding damage to the fibrillated network caused by excessive shearing.

[0038] Furthermore, the number of rolling operations in steps S1-4 is 2-3 times. This number of rolling operations can gradually improve the compaction density and uniformity of the film while ensuring production efficiency, so as to achieve the target areal density and self-support requirements.

[0039] Furthermore, the solid content of the slurry in step S2-2 is 42%-45%. This solid content range ensures that the slurry has good flowability and coating performance, and also enables the negative electrode coating of the target thickness to be obtained after drying.

[0040] Furthermore, the vacuum baking temperature in steps S2-5 is 85°C, and the time is 16 hours. This temperature and time can effectively remove residual moisture and gas from the negative electrode, while avoiding damage to the negative electrode material due to excessively high temperatures.

[0041] The beneficial effects of this invention are as follows:

[0042] (1) Synergistic innovation of compound binder: By using a compound binder system of polytetrafluoroethylene and fluorinated polymer, polytetrafluoroethylene is used to achieve dry fiberization film formation to provide mechanical strength. At the same time, the fluorinated polymer hexafluoropropylene is introduced. Its polar sites increase the free volume, improve electrolyte wettability and reduce interfacial impedance. The copolymer structure destroys the regular crystal of polyvinylidene fluoride and reduces brittleness. The polyfluorinated structure maintains oxidation stability at high potential of positive electrode. The synergistic effect of the compound system completely overcomes the inherent defects of single PTFE binder such as high brittleness, weak interfacial bonding force and poor wettability, and significantly improves the cycle stability of dry thick electrode.

[0043] (2) Gradient structure optimization: By designing a gradient double-layer positive electrode, the inner layer near the current collector uses large-particle-size active material to achieve high density and dense conductive path, while the outer layer near the separator uses small-particle-size active material to achieve high porosity and good electrolyte wettability. The functional partitioning of the inner layer being dense and conductive and the outer layer being loose and lithium-conducting significantly reduces the polarization of the thick electrode and improves the rate performance and cycle stability.

[0044] (3) Differentiated process assurance: By adopting differentiated process routes of positive electrode dry method and negative electrode wet method, the positive electrode dry method retains the advantage of high energy density, while the negative electrode wet method uses a water-soluble binder system to avoid the reduction and decomposition problem of PTFE in low potential environment, ensuring the first coulombic efficiency and cycle stability, with high process safety and easy mass production implementation;

[0045] (4) Breakthrough in energy density: Through scientific quantitative matching of positive electrode surface density of 28.5-33.5 mg / cm² and negative electrode surface density of 13.5-15.5 mg / cm², the energy density is maximized within a safe range of 1.05-1.15 for the N / P ratio. The energy density of the whole battery is stably broken through 285 Wh / kg, reaching the range of 285-295 Wh / kg, while maintaining good cycle life and rate performance, meeting the application requirements of high-performance power batteries. Detailed Implementation

[0046] It should be noted that the raw materials used in the following embodiments are all conventional commercial products in the field, and the relevant tests are all conducted in accordance with national or industry standards.

[0047] Example 1

[0048] This embodiment provides a high-energy-density lithium-ion battery prepared by a dry positive electrode and a wet negative electrode composite process, including a positive electrode, a negative electrode, a separator, and an electrolyte, achieving an energy density of over 285Wh / kg through a differentiated process route.

[0049] The positive electrode sheet is prepared using a solvent-free dry process, and the specific steps are as follows:

[0050] Step S1-1: Dry powder premixing. Weigh 94 parts by weight of high-nickel ternary material NCM811 (80% nickel content, 10% cobalt content, 10% manganese content, particle size D50 of 8μm, specific capacity of 200mAh / g, purity of 99.5%), 4 parts by weight of porous carbon black (specific surface area of ​​1500m² / g), 1 part by weight of carbon nanotubes (diameter of 10nm, length of 5μm), 0.7 parts by weight of polytetrafluoroethylene PTFE (molecular weight of 1 million, purity of 99%), and 0.3 parts by weight of hexafluoropropylene HFP (molecular weight of 50,000, purity of 98%). Put them into a V-type mixer and dry mix at 50rpm for 18 minutes at room temperature (25℃) to obtain a mixed powder with uniform component distribution. The dry mixing time is set at 15-20 minutes because: too short a time will result in uneven dispersion of components, and PTFE and HFP will not be able to uniformly coat the surface of the active material particles; too long a time will cause NCM811 particles to break due to mechanical shearing, reducing the structural stability of the material. A mixing time of 18 minutes can ensure uniformity while avoiding particle breakage, providing an ideal raw material basis for subsequent fiber kneading.

[0051] Step S1-2: Fiberization Kneading. The above mixed powder is transferred to a high-shear kneader and kneaded at 200 rpm for 10 minutes at 80°C to fully fibrillate the PTFE. The temperature range of 70-90°C is chosen because: PTFE's glass transition temperature is approximately 130°C, but within this range, high shear can cause localized deentanglement and orientation of the PTFE molecular chains, forming a preliminary fibrous network structure; below 70°C, the PTFE molecular chains lack mobility, resulting in poor fibrillation; above 90°C, PTFE softens excessively, hindering fiber formation. HFP, as a fluorinated polymer, plays a crucial role in this process: its -CF3 side chains provide polar sites, increasing free volume and improving the wettability of the subsequent electrolyte; simultaneously, as a comonomer, it disrupts the regular crystallization of PVDF, reducing the brittleness of the fibrous network and solving the problems of easy cracking and warping of single PTFE films. The kneading time is set to 8-12 minutes, with 10 minutes being the preferred value. This allows the PTFE to be fully fibrillated to form an effective bonding network, while avoiding excessive shearing that could damage the fibrillated network.

[0052] Steps S1-3: Pre-calendering for film formation. The fiberized mixture is transferred to a twin-roll calender and pre-calendered at 95°C and 100 N / mm pressure at a speed of 0.5 m / min to form a continuous initial film. The temperature range for this step is 90-105°C, and the pressure range is 80-120 N / mm. Appropriate temperature and pressure further densify the fiber network, causing the PTFE fibers to entangle and form a continuous film with a certain mechanical strength. The synergistic effect of temperature and pressure ensures that HFP is uniformly distributed within the PTFE fiber network, forming a synergistic structure with the compound binder. Pre-calendering is the foundation for subsequent multi-stage hot rolling; the continuity and uniformity of the initial film directly affect the quality of the final dry-process film.

[0053] Steps S1-4: Multi-stage hot rolling. The initial film is rolled three times at 110℃ and 180N / mm pressure to gradually increase the compaction density of the film. The temperature range for this step is 100-120℃, the pressure range is 150-220N / mm, and the number of rolling cycles is 2-3. The purpose of multiple rolling cycles is to gradually compress the pores inside the film, increase the compaction density, and enable the active material particles to form a tight interfacial contact with the compound binder. After three rolling cycles, a self-supporting dry membrane with an areal density of 30mg / cm² is obtained. The membrane has good mechanical strength and flexibility and can be used for subsequent operations without support. The areal density of 30mg / cm² is set based on energy density optimization: too low an areal density will lead to insufficient volumetric energy density of the battery; too high an areal density will increase the polarization of the thick electrode, affecting the rate performance.

[0054] In a multi-stage hot rolling process, a gradient bilayer cathode structure is prepared through layered feeding and gradient rolling. Specifically, large-diameter NCM811 particles (D50 of 12μm) are added during the first rolling process to form the inner layer close to the current collector, with a compaction density of 3.65 g / cm³. During the second rolling process, small-diameter NCM811 particles (D50 of 5μm) are added to form the outer layer close to the separator, with a porosity controlled at 40%. The design principle of the gradient bilayer structure lies in the synergistic effect of functional partitioning: the large-diameter particles in the inner layer shorten the electron transport distance through high compaction density (3.60-3.75 g / cm³), increase the contact points between the conductive agent and the active material, and form a dense conductive path responsible for rapid electron transport; the small-diameter particles in the outer layer increase the interface area between the electrolyte and the active material through high porosity (35-45%), providing sufficient lithium-ion transport channels responsible for rapid lithium-ion transport. A gradient transition in particle size and density is formed between the inner and outer layers, avoiding stress concentration at the interface and ensuring the overall stability of the structure. Compared with a homogeneous structure, this gradient structure can significantly reduce the polarization of thick electrodes and improve rate performance and cycle stability.

[0055] Steps S1-5: Hot-pressing the current collector. The self-supporting dry-laid membrane is transferred to a hot press and held at 120°C and 20MPa for 30 seconds to hot-press the dry-laid membrane onto the surface of a 12μm thick aluminum foil current collector. The temperature range for this step is 110-130°C, and the pressure range is 15-25MPa. The hot-pressing process creates a strong interfacial bond between the dry-laid membrane and the aluminum foil current collector. The bonding mechanism includes: partial diffusion and entanglement of PTFE and HFP molecular chains at the interface under high temperature; mechanical interlocking of the microscopic protrusions on the membrane and aluminum foil surface under high pressure; and weak chemical bonding between the polar sites of HFP and the alumina layer on the aluminum foil surface. The advantages of the composite binder system compared to single PTFE are fully demonstrated in this step: the introduction of HFP significantly improves the interfacial bonding force between the dry-laid membrane and the aluminum foil, avoiding the warping and deformation problem that easily occurs with single PTFE membranes during hot pressing.

[0056] Steps S1-6: Cold Pressing and Shaping. The hot-pressed electrode is transferred to a precision compactor and cold-pressed at room temperature (25℃) to achieve a compaction density of 3.58 g / cm³. The purpose of cold pressing is to eliminate the springback effect after hot pressing, further increase the compaction density, and enable the electrode to meet the target energy density requirements. The compaction density is controlled within the range of 3.50-3.65 g / cm³. This range is determined based on electrochemical performance optimization: too low a compaction density leads to a long electron transport path and high internal resistance; too high a compaction density leads to insufficient porosity, difficulty in electrolyte wetting, and obstruction of lithium-ion transport. A compaction density of 3.58 g / cm³ can maintain appropriate porosity while ensuring conductivity, achieving a balance between electron and ion transport.

[0057] The negative electrode sheet is prepared using a traditional aqueous wet coating process, and the specific steps are as follows:

[0058] Step S2-1: Dry powder premixing. Weigh 96 parts by weight of artificial graphite (particle size D50 of 15μm, specific capacity of 360mAh / g, purity of 99.9%), 2 parts by weight of porous carbon black, and 2 parts by weight of carboxymethyl cellulose (CMC, molecular weight 250,000). Put them into a V-type mixer and dry mix at room temperature for 30 minutes to fully disperse the components and provide a uniform solid raw material basis for subsequent slurry preparation.

[0059] Step S2-2: Dispersion and Slurry Preparation with Water. Transfer the above dry powder mixture to a high-speed disperser, add deionized water, control the solid content to 43%, and disperse at 3000 rpm for 20 minutes to fully dissolve the CMC and form a viscous solution. Then add 2 parts by weight of styrene-butadiene rubber (SBR) emulsion (50% solid content), transfer to a low-speed mixer, and stir at 500 rpm for 10 minutes to uniformly emulsify and disperse the SBR in the slurry, forming a stable negative electrode slurry. The solid content of the slurry is controlled within the range of 42%-45%. This range ensures good flowability and coating performance of the slurry, and also allows for the achievement of the target thickness of the negative electrode coating after drying. The CMC / SBR composite binder system is chemically stable under low potential conditions at the negative electrode, and can form a dense and uniform SEI film, avoiding the problems associated with PTFE at low potentials (below 0.8V vs Li / Li). + This addresses the reduction and decomposition problem that occurs under these conditions, ensuring the initial coulombic efficiency and cycle stability of the negative electrode.

[0060] Step S2-3: Coating. A slit coater is used to uniformly coat the slurry onto the 8μm thick copper foil current collector surface. The coating speed is 5m / min, with single-sided coating, and the coating surface density is controlled at 14.5mg / cm². The coating surface density is controlled by precisely adjusting the coating gap and slurry supply. A surface density of 14.5mg / cm² corresponds to a single-sided capacity of approximately 5.9mAh / cm².

[0061] Step S2-4: Drying and Compacting. The coated electrode sheets are placed in an infrared oven and dried at 120℃ for 3 minutes to remove moisture from the slurry. The dried electrode sheets are then transferred to a roller press for room temperature cold pressing, controlling the compaction density to 1.70 g / cm³. Maintaining a compaction density within the range of 1.65-1.75 g / cm³ ensures adequate conductivity of the negative electrode while maintaining a suitable pore structure, guaranteeing sufficient electrolyte wetting and rapid lithium ion transport.

[0062] Step S2-5: Vacuum Baking. After demagnetization and slitting, the compacted negative electrode sheet is transferred to a vacuum oven and baked at 85℃ and a vacuum degree of less than 10Pa for 16 hours to fully remove residual moisture and gas from the negative electrode and improve its electrochemical stability. The vacuum baking temperature is set at 85℃ and the time is set at 16 hours. These parameters are determined based on a balance between moisture removal efficiency and material stability: too low a temperature or too short a time will result in incomplete moisture removal, affecting battery performance; too high a temperature will cause the binder to decompose or the surface of the active material to oxidize.

[0063] The battery assembly steps are as follows: The positive electrode sheet is cut to a size of 14×20cm, and the negative electrode sheet is cut to a size of 14.5×20.5cm (the negative electrode size is slightly larger than the positive electrode to avoid insufficient capacity at the edge of the positive electrode, which could lead to lithium plating). A 16μm thick PP / PE / PP three-layer composite separator is wound and then placed into an aluminum-plastic film soft-pack casing. Electrolyte (1M LiPF6 dissolved in a mixed solvent of EC:DMC:EMC=1:1:1, with 2% vinylene carbonate VC added as a film-forming additive) is injected, with an injection volume of 12g. After encapsulation, a formation process is performed: constant current charging at C / 20 rate to 4.2V, standing for 2 hours, and constant current discharging at C / 5 rate to 2.8V to complete the first formation. Subsequently, aging at a constant temperature of 45℃ for 24 hours is performed for a second formation to obtain the lithium-ion battery of Example 1.

[0064] Analysis of positive and negative electrode areal density matching: A positive electrode areal density of 30 mg / cm² corresponds to a single-sided capacity of approximately 5.4 mAh / cm² (calculated based on an NCM811 specific capacity of 180 mAh / g and an active material content of 94%), while a negative electrode areal density of 14.5 mg / cm² corresponds to a single-sided capacity of approximately 5.9 mAh / cm² (calculated based on an artificial graphite specific capacity of 360 mAh / g and an active material content of 96%). The N / P ratio is 1.09, which is within the safe range of 1.05-1.15. This N / P ratio design avoids the risk of lithium plating due to insufficient negative electrode capacity while maximizing battery capacity, achieving a breakthrough in energy density. The range of positive electrode areal density (28.5-33.5 mg / cm²) and negative electrode areal density (13.5-15.5 mg / cm²) was set as the optimal matching range through extensive experimental optimization, ensuring a safe N / P ratio.

[0065] The performance test results of the lithium-ion battery in Example 1 are as follows: According to GB / T 31467.3-2015 standard, at an ambient temperature of 25℃, the battery was discharged at a constant current rate of 0.5C to 2.8V, and the energy density was measured to be 287Wh / kg. According to GB / T31484-2015 standard, at an ambient temperature of 25℃, charge-discharge cycle tests were conducted at a rate of 1C within a voltage range of 4.2-2.8V. After 500 cycles, the capacity retention rate was 86%. Rate performance test results show that: at 0.2C discharge capacity (100% baseline), the capacity retention rate at 0.5C discharge was 98%, at 1C discharge was 93%, at 2C discharge was 82%, and at 3C discharge was 68%. Electrolyte wettability testing used the droplet method to measure the contact angle. The electrolyte contact angle of the dry cathode with composite binder was 32°, significantly lower than the 58° of single PTFE, indicating that the introduction of HFP effectively improved electrolyte wettability. The interfacial impedance was measured using electrochemical impedance spectroscopy (EIS) in the frequency range of 0.01 Hz to 100 kHz. The measured interfacial impedance was 18 Ω·cm², which is 48.6% lower than that of 35 Ω·cm² for PTFE alone. This indicates that the composite binder system significantly reduces the electrode / electrolyte interfacial impedance.

[0066] Example 2

[0067] The difference from Example 1 is that the positive electrode surface density was adjusted from 30 mg / cm² to 33 mg / cm², and the negative electrode surface density was adjusted from 14.5 mg / cm² to 15.5 mg / cm², while the other raw material composition and process parameters remained the same.

[0068] To achieve a higher positive electrode areal density, the number of rolling cycles in the multi-stage hot rolling process (S1-4) is increased to 3, and the rolling pressure is increased to 220 N / mm, resulting in a compaction density of 3.65 g / cm³. The negative electrode areal density is correspondingly increased to 15.5 mg / cm², with the compaction density controlled at 1.75 g / cm³. A positive electrode areal density of 33 mg / cm² corresponds to a single-sided capacity of approximately 5.9 mAh / cm², and a negative electrode areal density of 15.5 mg / cm² corresponds to a single-sided capacity of approximately 6.3 mAh / cm². The N / P ratio is 1.07, which remains within a safe range.

[0069] The performance test results of the lithium-ion battery in Example 2 are as follows: the energy density is 295Wh / kg, which is 2.8% higher than that in Example 1, verifying the ability to break through the upper limit of areal density in terms of energy density. After 500 cycles, the capacity retention rate is 84%, and the 1C discharge capacity retention rate is 91%, which is slightly lower than that in Example 1. This is because the increased areal density leads to increased polarization of the thick electrode, but it still maintains good cycle stability and rate performance.

[0070] Example 3

[0071] The difference from Example 1 is that the positive electrode surface density was adjusted from 30 mg / cm² to 28.5 mg / cm², and the negative electrode surface density was adjusted from 14.5 mg / cm² to 13.5 mg / cm², while the other raw material composition and process parameters remained the same.

[0072] To achieve a lower positive electrode areal density, in steps S1-4 of the multi-stage hot rolling process, the number of rolling cycles is reduced to two, and the rolling pressure is lowered to 150 N / mm, thus controlling the compaction density at 3.50 g / cm³. The negative electrode areal density is correspondingly reduced to 13.5 mg / cm², and the compaction density is controlled at 1.65 g / cm³. A positive electrode areal density of 28.5 mg / cm² corresponds to a single-sided capacity of approximately 5.1 mAh / cm², and a negative electrode areal density of 13.5 mg / cm² corresponds to a single-sided capacity of approximately 5.5 mAh / cm², with an N / P ratio of 1.08.

[0073] The performance test results of the lithium-ion battery in Example 3 are as follows: the energy density is 285Wh / kg, verifying that the energy density target of 285Wh / kg can still be stably achieved even at the lower limit of areal density. After 500 cycles, the capacity retention rate is 88%, and the 1C discharge capacity retention rate is 94%, which is an improvement compared to Example 1. This is because the reduced areal density decreases the polarization of the thick electrode, thus improving cycle stability and rate performance.

[0074] Comparative Example 1

[0075] The difference from Example 1 is that only 1.0 parts by weight of PTFE is used as the positive electrode binder, and no HFP is added, while the composition of other raw materials and process parameters remain the same.

[0076] The purpose of Comparative Example 1 was to verify the advantages of the compounded binder over single PTFE. Due to the lack of HFP modification, the dry membrane exhibited significant warping during the thermal compounding of the current collector in steps S1-5, indicating insufficient interfacial adhesion. Electrolyte wettability testing showed a contact angle of 58°, significantly higher than the 32° in Example 1, indicating that the hydrophobicity of single PTFE makes electrolyte wetting difficult. The interfacial impedance test result was 35 Ω·cm², a 94.4% increase compared to 18 Ω·cm² in Example 1, indicating a significant increase in interfacial impedance. After 500 cycles, the capacity retention was only 72%, a significant decrease compared to 86% in Example 1. This is attributed to the increased polarization caused by high interfacial impedance and poor electrolyte wettability, leading to decreased interfacial stability during cycling.

[0077] Comparative Example 2

[0078] The difference from Example 1 is that the positive electrode uses a single-size NCM811 (particle size D50 is 8μm), without a gradient bilayer structure, while the other raw material composition and process parameters remain the same.

[0079] The purpose of Comparative Example 2 was to verify the advantages of the gradient structure over the homogeneous structure. Due to the homogeneous structure, the active material particles were uniformly distributed throughout the entire electrode thickness, and the electron transport and ion transport pathways were not effectively separated and optimized. Rate performance test results showed that the 1C discharge capacity retention was 85%, a decrease of 8 percentage points compared to 93% in Example 1; the 2C discharge capacity retention was 68%, a decrease of 14 percentage points compared to 82% in Example 1. Polarization voltage tests showed that at 1C, the polarization voltage of Comparative Example 2 was approximately 120 mV higher than that of Example 1, indicating that the homogeneous structure led to a significant increase in polarization of the thick electrode, thus limiting rate performance.

[0080] Comparative Example 3

[0081] The difference from Example 1 is that the negative electrode also adopts a dry process, uses 1.0 parts by weight of PTFE as a binder, does not use the CMC / SBR water-based binder system, and the other raw material composition and process parameters remain the same.

[0082] The purpose of Comparative Example 3 is to verify the advantages of the differentiated process (wet negative electrode) over the all-dry process. Because PTFE is used as a binder in the negative electrode, it is advantageous in low-potential environments (below 0.8V vs Li / Li). + Under these conditions, PTFE undergoes a reduction and decomposition reaction, generating lithium fluoride and carbon, leading to instability of the SEI film. The initial coulombic efficiency test result was 82%, a significant decrease compared to 91% in Example 1, indicating that the reduction and decomposition of PTFE results in increased irreversible capacity loss. After 100 cycles, gas production increased significantly, the battery swelled, and capacity decay accelerated. After 500 cycles, the capacity retention rate was only 65%, a significant decrease compared to 86% in Example 1, indicating that the all-dry method has serious safety and cycle stability issues on the negative electrode side.

[0083] The performance comparison data of Examples 1-3 and Comparative Examples 1-3 are summarized in the following table:

[0084] Example 1 287 86 93 82 32 18 91 Example 2 295 84 91 80 32 18 91 Example 3 285 88 94 84 32 18 91 Comparative Example 1 283 72 89 75 58 35 90 Comparative Example 2 286 84 85 68 32 18 91 Comparative Example 3 285 65 88 76 32 18 82

[0085] Note: The test conditions are an ambient temperature of 25℃, energy density testing according to GB / T 31467.3-2015 standard, cycle life testing according to GB / T 31484-2015 standard, charge and discharge voltage range of 4.2-2.8V, cycle rate of 1C, and rate performance testing with 0.2C discharge capacity as 100% benchmark.

[0086] As can be seen from the table above, Examples 1-3 of this invention achieved a breakthrough in energy density of 285-295 Wh / kg by employing a PTFE and HFP composite binder system, a gradient bilayer positive electrode design, differentiated process routes for the positive electrode (dry process) and the negative electrode (wet process), and precise matching of the areal density of the positive and negative electrodes. Simultaneously, they maintained good cycle stability (capacity retention of 84-88% after 500 cycles) and rate performance (capacity retention of 91-94% at 1C discharge). Comparative Example 1 used a single PTFE binder, increasing the electrolyte contact angle to 58° and the interfacial impedance to 35 Ω·cm², but the capacity retention after 500 cycles was only 72%, verifying the synergistic innovation advantages of the composite binder. Comparative Example 2 used a homogeneous positive electrode structure, resulting in a decrease in discharge capacity retention to 85% and 68% at 1C and 2C rates, respectively, with an increase in polarization voltage of approximately 120 mV, verifying the optimization effect of the gradient structure. Comparative Example 3 uses a completely dry process, where the negative electrode PTFE is reduced and decomposed at a low potential. The initial coulombic efficiency drops to 82%, and the capacity retention rate after 500 cycles is only 65%, which verifies the guarantee effect of the differentiated process.

[0087] In summary, this invention, through a comprehensive technical solution involving synergistic innovation of compound binders, optimization of gradient structures, assurance of differentiated processes, and precise matching of areal density, successfully solves the technical bottlenecks in existing dry electrode technologies, such as the inherent defects of single PTFE binders, safety issues in dry anode processes, large polarization of homogeneous structures, and difficulty in breaking through the energy density of 285Wh / kg. It achieves stable preparation of high-energy-density lithium-ion batteries, meeting the application requirements of high-performance power batteries.

Claims

1. A high-energy-density lithium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator, characterized in that, The positive electrode uses a high-nickel ternary material as the active material, wherein the molar content of nickel in the high-nickel ternary material is ≥80%; the positive electrode uses a composite binder comprising polytetrafluoroethylene and fluorinated modified polymer; the positive electrode has a gradient bilayer structure, wherein the particle size of the inner active material near the current collector is larger than the particle size of the outer active material near the separator; the negative electrode uses artificial graphite as the active material and employs a water-soluble binder system; the mass of active material per unit area of ​​the positive electrode is 28.5-33.5 mg / cm², and the mass of active material per unit area of ​​the negative electrode is 13.5-15.5 mg / cm².

2. The high energy density lithium-ion battery according to claim 1, characterized in that, The fluorinated polymer is PVDF-HFP or fluorinated acrylate.

3. The lithium-ion battery according to claim 1 or 2, characterized in that, The mass ratio of polytetrafluoroethylene to fluorinated modified polymer in the compounded adhesive is 6:5 to 5:

1.

4. The high energy density lithium-ion battery according to claim 1 or 2, characterized in that, The positive electrode also includes a conductive agent, which is a composite system of porous carbon black and carbon nanotubes.

5. The high energy density lithium-ion battery according to claim 1 or 2, characterized in that, The particle size D50 of the inner active material is 10-15 μm, and the particle size D50 of the outer active material is 3-6 μm.

6. The high energy density lithium-ion battery according to claim 1 or 2, characterized in that, The compaction density of the inner layer is 3.60-3.75 g / cm³, and the porosity of the outer layer is 35-45%.

7. The high-energy-density lithium-ion battery according to claim 1 or 2, characterized in that, The active material mass per unit area of ​​the positive electrode is 30.0-32.5 mg / cm², and the active material mass per unit area of ​​the negative electrode is 14.0-15.0 mg / cm².

8. The high-energy-density lithium-ion battery according to claim 1 or 2, characterized in that, The energy density of the lithium-ion battery is 285-295Wh / kg.

9. The high-energy-density lithium-ion battery according to claim 1 or 2, characterized in that, The water-soluble binder system is a compound system of carboxymethyl cellulose and styrene-butadiene rubber.

10. A method for preparing a high-energy-density lithium-ion battery, characterized in that, Includes the following steps: S1: Preparation of the positive electrode sheet: S1-1: High-nickel ternary material, conductive agent, polytetrafluoroethylene and fluorinated polymer are dry-mixed at room temperature in proportion to obtain a uniformly mixed powder; S1-2: The mixed powder is subjected to high shear mixing at 70-90°C to fibrillate the polytetrafluoroethylene; S1-3: Calendering the fibrillated mixture at 90-105℃ and 80-120N / mm to form a continuous primary film; S1-4: The initial film is subjected to multiple roll pressings at 100-120℃ and 150-220N / mm to obtain a self-supporting dry film with an areal density of 28.5-33.5mg / cm². S1-5: The dry film is hot-pressed onto the aluminum foil current collector at 110-130℃ and 15-25MPa; S1-6: The composite electrode is precisely compacted at room temperature to 50°C to achieve a compaction density of 3.50-3.65 g / cm³. S2: Preparation of negative electrode sheet: S2-1: Dry mixing of artificial graphite, conductive agent and carboxymethyl cellulose; S2-2: Add deionized water to disperse and prepare slurry, then add styrene-butadiene rubber for emulsification and dispersion; S2-3: Coat the slurry onto the copper foil current collector, controlling the coating surface density to be 13.5-15.5 mg / cm²; S2-4: Dry and compact the coated electrode sheets, controlling the compaction density to be 1.65-1.75 g / cm³; S2-5: Vacuum baking of the compacted electrode sheet; S3: Assemble the positive electrode, negative electrode, separator, and electrolyte into a lithium-ion battery.

11. The preparation method according to claim 10, characterized in that, The dry mixing time in step S1-1 is 15-20 minutes; the high-shear mixing time in step S1-2 is 8-12 minutes; the number of rolling cycles in step S1-4 is 2-3 times; the solid content of the slurry in step S2-2 is 42%-45%; and the vacuum baking temperature in step S2-5 is 85℃ and the time is 16 hours.