Negative plate with directional gradient pore structure, composite pore-forming agent for preparing negative plate, preparation method of composite pore-forming agent and lithium ion battery

Abstract

The invention discloses a negative plate with a directional gradient pore structure, a composite pore-forming agent for preparing the negative plate, a preparation method and a lithium ion battery, the composite pore-forming agent has a core-shell coating structure and a surface functionalization modification layer, the core-shell coating structure comprises an inner core and an outer shell coating the outer surface of the inner core; wherein the inner core is a thermal decomposition type pore-forming material, and the thermal decomposition type pore-forming material can be subjected to thermal decomposition in a temperature range of 80-120 DEG C and release gas; the shell is a thermal dissolution type polymer coating layer, and the surface functionalization modification layer is a charged polymer layer and uniformly coats the outer surface of the shell, so that the composite pore-forming agent is endowed with surface charge characteristics. According to the invention, the composite pore-forming agent combining core-shell coating and surface functional modification is designed and is matched with an electric field-assisted gradient sedimentation and segmented heating pore-forming process, so that directional gradient regulation and control of a pore structure in the thickness direction of the pole piece are realized in a single pore-forming agent system.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, specifically to a negative electrode sheet with a directional gradient pore structure, a composite pore-forming agent for preparing the negative electrode sheet and a preparation method thereof, and a lithium-ion battery. Background Technology

[0002] With the widespread adoption of mobile electronic devices and electric vehicles, lithium-ion batteries have become the most important energy storage devices due to their high energy density and long cycle life. To further improve battery energy density, thick electrode design is a crucial technological direction. However, traditional thick electrodes, due to the increased coating thickness, bring a series of kinetic performance problems: the ion diffusion path is significantly lengthened and its tortuosity increased, making electrolyte wetting in the thick coating more difficult. Especially under high-current charge and discharge conditions, the electrode polarization is exacerbated due to untimely lithium-ion transport, and uneven local lithium-ion concentration distribution causes a sudden voltage surge, severely limiting the battery's rate performance and cycle life. Furthermore, at low temperatures, the electrolyte viscosity increases and the lithium-ion diffusion coefficient decreases significantly, further exacerbating the aforementioned kinetic problems.

[0003] Introducing a controllable pore structure into the negative electrode coating is one of the effective ways to improve the above-mentioned problems. A reasonable pore structure can shorten the diffusion path of lithium ions, reduce tortuosity, promote electrolyte wetting, and alleviate the mechanical stress caused by the volume expansion of the active material during charging and discharging to a certain extent. Currently reported pore-forming techniques mainly include: using sacrificial templates (such as polymer microspheres, carbonate particles, etc., which are eliminated by dissolution or decomposition during post-processing to leave pores), freeze-drying, and laser drilling.

[0004] However, existing pore-forming technologies generally have the following shortcomings: First, the process is complex, difficult to prepare, and costly, making it difficult to be compatible with existing roll-to-roll coating production lines; Second, the pore size is difficult to control precisely. If the pore size is too large, it will directly reduce the volume ratio of active material in the electrode, thus losing energy density, while if the pore size is too small, it will have limited effect on improving ion transport; Third, the pore structure formed inside the electrode by traditional pore-forming methods is usually uniformly distributed, failing to consider the differences in pore structure requirements for different thickness layers of the electrode. Ideally, the surface layer of the electrode should have a high porosity to facilitate rapid electrolyte wetting and lithium ion introduction, while the bottom layer should maintain a low porosity to ensure sufficient active material volume ratio and energy density.

[0005] To achieve the aforementioned gradient pore structure, an intuitive approach is to use a mixture of multiple pore-forming agents with different decomposition temperatures. However, in practice, due to differences in density, particle size, surface properties, and solubility, different types of pore-forming agents are prone to unstable phenomena such as stratification, sedimentation, and agglomeration during slurry mixing and coating, making it impossible to form a predictable directional pore gradient. The distribution of traditional single pore-forming agents (such as ammonium bicarbonate and polymethyl methacrylate microspheres) in the slurry is essentially random, and it is even more impossible to achieve an ordered gradient arrangement along the electrode thickness direction. Therefore, how to design a simple, controllable technical solution that can directionally construct a gradient pore structure in the negative electrode is a key problem that urgently needs to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the aforementioned background technology. This invention provides a negative electrode sheet with a directional gradient pore structure, a composite pore-forming agent for preparing the negative electrode sheet, a method for preparing the same, and a lithium-ion battery comprising the negative electrode sheet. This invention achieves directional gradient control of the pore structure along the electrode sheet thickness direction within a single pore-forming agent system by designing a composite pore-forming agent that combines core-shell coating with surface functionalization modification, and by employing an electric field-assisted gradient sedimentation and segmented heating pore-forming process.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A first aspect of the present invention provides a composite pore-forming agent having a core-shell coating structure and a surface functionalized modification layer. The core-shell coating structure includes a core and a shell covering the outer surface of the core. The core is a thermally decomposable pore-forming material capable of thermal decomposition and gas release within a temperature range of 80–120°C. The shell is a thermally soluble polymer coating layer capable of dissolving or softening and being removed within a temperature range of 50–70°C. The dissolution temperature of the shell is lower than the decomposition temperature of the core, allowing the shell to be eliminated before the core during segmented heating, thereby sequentially forming pores of different sizes at different thicknesses of the negative electrode. The surface functionalized modification layer is a charged polymer layer uniformly coating the outer surface of the shell, imparting surface charge characteristics to the composite pore-forming agent, enabling it to maintain dispersion stability in the slurry system and achieve directional migration under an applied electric field.

[0008] In some preferred embodiments, the thermally decomposable pore-forming material is selected from at least one of ammonium carbonate, ammonium bicarbonate, and azodicarbonamide.

[0009] Furthermore, the material of the heat-dissolving polymer coating layer is polymethyl methacrylate (PMMA).

[0010] Furthermore, the surface functionalized modification layer is a polydopamine layer (PDA), which carries a negative charge under neutral and weakly alkaline conditions.

[0011] Furthermore, the polymethyl methacrylate (PMMA) shell has a melting temperature window of 55–65°C, and the ammonium carbonate core has a decomposition temperature window of 90–110°C. The temperature difference between the two is not less than 25°C, to ensure that the elimination behavior of the shell and the core is fully separated in time and space during the segmented heating process.

[0012] Furthermore, the composite pore-forming agent has a particle size of 1-50 μm, and the polydopamine modification layer ensures that the absolute value of the surface potential of the pore-forming agent is not less than 20 mV. Through electrostatic repulsion, it maintains colloidal stability in the aqueous slurry system and effectively inhibits the aggregation between pore-forming agent particles.

[0013] A second aspect of the present invention provides a method for preparing the composite pore-forming agent, comprising the following steps: (1) Synthesis of core-shell structure: The thermally decomposable pore-forming material powder is dispersed in an organic solvent, and a dispersant of 0.05-0.2 wt% of the pore-forming material mass is added and stirred to form a uniform suspension; then, a polymeric monomer is added dropwise to the suspension, and a free radical polymerization reaction is initiated at 60-80°C, so that the polymeric monomer is polymerized in situ on the surface of the pore-forming material particles to form a polymer coating layer. After the reaction is completed, the precursor with a core-shell structure is obtained by filtration and washing.

[0014] (2) Surface functionalization modification: The core-shell structure precursor obtained in step (1) is dispersed in a Tris buffer solution containing dopamine hydrochloride, the pH value of the system is adjusted to 8.0-9.0, and the dopamine oxidation self-polymerization reaction is carried out continuously under room temperature for 8-36 hours, so that dopamine is deposited on the surface of the polymer coating layer to form a polydopamine modified layer. After filtration, washing and drying, the composite pore-forming agent is obtained.

[0015] In some preferred embodiments, the pore-forming material in step (1) is ammonium carbonate, the organic solvent is anhydrous ethanol, the dispersant is sodium dodecyl sulfonate (SDS), the monomer is methyl methacrylate (MMA), the polymerization temperature is 70°C, and the polymerization time is 2–6 hours. In step (2), the pH value of the dopamine self-polymerization reaction is 8.5, the reaction time is 20–28 hours, and the concentration of dopamine hydrochloride is 1–5 mg / mL.

[0016] A third aspect of the present invention provides a method for preparing a negative electrode sheet with a directional gradient pore structure, using the above-mentioned composite pore-forming agent, comprising the following steps: (a) Slurry preparation: The negative electrode active material, conductive agent, binder and the composite pore-forming agent are dispersed in a solvent in a preset ratio, and a uniform negative electrode slurry is obtained after vacuum stirring and degassing treatment.

[0017] (b) Electric field assisted coating: The negative electrode slurry is coated on the surface of the current collector. During the coating process or in the wet film state after coating, a DC electric field is applied in a direction perpendicular to the plane of the current collector. The direction of the electric field is from the electrode surface to the current collector side, so that the composite pore-forming agent with surface charge migrates in a direction closer to the current collector under the drive of the electric field force, thereby forming a gradient distribution in the wet film in which the concentration of the pore-forming agent gradually increases from the surface layer to the bottom layer along the thickness direction of the electrode.

[0018] (c) Segmented drying and pore formation: The electrode sheet treated in step (b) undergoes segmented heating treatment. The first stage involves drying at 50–70°C, which preferentially dissolves or softens and removes the polymer shell of the composite pore-forming agent. Since the concentration of the pore-forming agent is lower on the surface and higher in the bottom layer, the removal of the shell results in relatively large pore channels on the electrode surface. The second stage involves heating to 80–120°C, causing the thermally decomposable core of the composite pore-forming agent to decompose and release gas. This results in a dense microporous structure in the bottom layer region near the current collector due to the enrichment of the pore-forming agent. Through this segmented heating process, a gradient pore distribution with decreasing porosity and smaller pore size is established within the electrode sheet from the surface to the current collector side.

[0019] (d) Cold pressing: The electrode sheet after the pore-forming process is cold pressed, and the gradient pore structure is locked during the compaction process to obtain a negative electrode sheet with a directional gradient pore structure.

[0020] In some preferred embodiments, the negative electrode active material is artificial graphite and / or natural graphite, the conductive agent is conductive carbon black, the binder is a composite binder system of sodium carboxymethyl cellulose and styrene-butadiene rubber, the solvent is deionized water, and the amount of composite pore-forming agent added is 1-10 wt% of the mass of the negative electrode active material. The DC electric field strength is 50-100 V / cm, and the current collector is copper foil. The first stage drying temperature is preferably 60°C, so that the porosity of the large pore channels formed on the surface of the electrode is 30-40%; the second stage heating temperature is preferably 100°C, so that the porosity of the microporous structure formed near the bottom layer of the current collector is 8-15%.

[0021] A fourth aspect of the present invention provides a negative electrode sheet having an oriented gradient pore structure prepared by the above method. The negative electrode sheet exhibits a continuous gradient distribution of porosity decreasing from the surface layer away from the current collector to the bottom layer near the current collector along its thickness direction, wherein the porosity of the surface layer region is 30-40% and the porosity of the bottom layer region is 8-15%.

[0022] A fifth aspect of the present invention provides a lithium-ion battery comprising the above-described negative electrode sheet having a directional gradient pore structure.

[0023] Compared with the prior art, the present invention has the following advantages: Firstly, this invention combines the segmented elimination mechanism of core-shell pore-forming agents with electric field-assisted directional sedimentation technology, enabling the construction of gradient pores along the thickness direction of the negative electrode sheet within a single pore-forming agent system. In the core-shell structure, the PMMA outer shell and the thermally decomposable core have different elimination temperature windows, ensuring the orderly generation of macropores and micropores at different layers of the electrode sheet during segmented heating. Polydopamine surface modification imparts surface charge to the pore-forming agent particles, which, combined with the applied electric field, achieves directional gradient migration of the pore-forming agent in the wet film. This synergistic effect of multiple mechanisms overcomes the inherent defect of random pore distribution in traditional pore-forming technologies.

[0024] Secondly, the gradient pore structure obtained in this invention, with its decreasing pore size from the surface to the interior, creates preferential channels for electrolyte wetting and lithium-ion transport, significantly reducing ion diffusion resistance. Under high-current charge-discharge conditions, delayed lithium-ion transport can lead to localized concentration unevenness and a sudden voltage surge. The directional gradient pore structure, by optimizing the uniformity of lithium-ion distribution across different layers of the electrode, effectively alleviates electrode polarization and improves reversible capacity under high current. At low temperatures, ion transport kinetics slow down significantly. The preferential transport paths provided by the directional gradient pore structure can partially offset the kinetic degradation caused by low temperatures, allowing the battery to maintain good discharge capability even at low temperatures.

[0025] Thirdly, the gradient pore structure of this invention, acting as a reserved buffer space, can accommodate the volume expansion of the graphite anode during lithium intercalation, significantly reducing the concentration of mechanical stress inside the electrode, suppressing the pulverization of active material particles and the damage to the electrode structure, and thus helping to maintain the stability of the solid electrolyte interface film. Furthermore, the gradient pore structure optimizes the uniformity of lithium-ion distribution, reducing side reactions caused by localized concentration anomalies, thereby reducing the irreversible consumption of active lithium and electrolyte during cycling and extending the battery's lifespan.

[0026] Fourth, the gradient pore design of this invention achieves a differentiated configuration of "high porosity on the surface and low porosity on the bottom": the higher porosity on the surface enhances the introduction and transport efficiency of lithium ions, providing kinetic assurance for fast charging; the lower porosity on the bottom maintains a high volume ratio of active material, ensuring that energy density is not excessively lost. This differentiated design allows the electrode to possess both high energy density and excellent fast charging capability, rather than requiring a compromise between the two as is necessary with traditional uniform pore-forming technology.

[0027] Fifth, the large pores in the surface layer of the gradient pore structure of this invention reduce the initial wetting resistance between the electrolyte and the electrode surface. Simultaneously, the gradually decreasing pore structure creates a "capillary pump" effect, guiding the electrolyte to continuously penetrate from the electrode surface along the thickness direction towards the current collector side, significantly shortening the electrolyte wetting time. For thick electrode designs, electrolyte wetting is one of the key process bottlenecks restricting its practical application. The gradient pore structure of this invention provides an effective way to solve this bottleneck.

[0028] Sixth, the negative electrode preparation method of this invention is highly compatible with existing coating production lines. It only requires the addition of an electric field generating device at the coating station, without fundamentally modifying the slurry system, the coating equipment itself, or the subsequent cold pressing process. The composite pore-forming agent is prepared using conventional emulsion polymerization and dopamine self-polymerization processes. The raw materials are readily available, the process conditions are mild, and large-scale production is feasible. These process characteristics enable the technical solution of this invention to promote the commercial application of thick-electrode lithium-ion batteries at a relatively low modification cost. Attached Figure Description

[0029] Figure 1 This is a schematic flowchart of the preparation method of the composite pore-forming agent according to an embodiment of the present invention; Figure 2 This is a schematic flowchart illustrating the preparation method of a negative electrode sheet with a directional gradient pore structure according to an embodiment of the present invention. Detailed Implementation

[0030] The following details the negative electrode sheet with a directional gradient pore structure, the composite pore-forming agent for preparing the negative electrode sheet, the preparation method, and embodiments of the lithium-ion battery of the present invention. This description is provided to enable those skilled in the art to fully understand the invention and is not intended to limit the subject matter of the claims.

[0031] The "scope" disclosed in this invention is defined in the form of a lower limit and an upper limit. A given scope is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific scope. The scope defined in this way can include or exclude end values, and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a scope.

[0032] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined to form new technical solutions. All technical features and optional technical features of the present invention can be combined to form new technical solutions. All steps of the present invention can be performed sequentially or randomly.

[0033] I. Implementation Examples Example 1 This embodiment provides a negative electrode sheet with a directional gradient pore structure and a method for preparing the composite pore-forming agent used therein. The specific steps are as follows: (I) Preparation of composite pore-forming agent (1) Synthesis of core-shell structure: Weigh an appropriate amount of ammonium carbonate powder and disperse it in anhydrous ethanol to form a suspension. Add sodium dodecyl sulfonate (SDS) at 0.1 wt% of the mass of ammonium carbonate as a dispersant to the suspension and stir until the ammonium carbonate particles are fully dispersed. Then, under continuous stirring, slowly add methyl methacrylate (MMA) monomer to the suspension, controlling the mass ratio of MMA to ammonium carbonate to be 0.3:1. Heat to 70°C and add azobisisobutyronitrile (AIBN) as an initiator to initiate a free radical polymerization reaction. Maintain the reaction for 4 hours to allow MMA to polymerize in situ on the surface of ammonium carbonate particles to form a polymethyl methacrylate (PMMA) coating layer. After the reaction is completed, filter, wash with ethanol and dry under vacuum at room temperature to obtain a precursor with a core-shell structure.

[0034] (2) Surface functionalization modification: The above core-shell structure precursor was dispersed in a 10 mM Tris-HCl buffer solution, and dopamine hydrochloride was added to make its concentration 3 mg / mL. The pH of the system was adjusted to 8.5 with 0.1 M NaOH solution, and the dopamine oxidative self-polymerization reaction was carried out at room temperature with continuous stirring for 24 hours. Dopamine underwent oxidative self-polymerization under weakly alkaline conditions, depositing a uniformly thick polydopamine (PDA) modification layer on the surface of the PMMA shell. After the reaction was completed, the mixture was filtered, washed with deionized water, and vacuum dried to obtain a composite pore-forming agent with a core-shell coating structure and a polydopamine surface functionalization modification layer.

[0035] Testing revealed that the average particle size of the obtained composite pore-forming agent was approximately 15 μm, and the Zeta potential was -35 mV, indicating good dispersion stability in aqueous systems. The PMMA shell began to show significant weight loss at approximately 60°C, while the ammonium carbonate core underwent rapid decomposition at approximately 100°C. The temperature difference between the two weight loss steps was approximately 40°C, meeting the temperature window requirements for segmented pore formation.

[0036] (II) Preparation of negative electrode sheets with directional gradient pore structure (a) Slurry preparation: Artificial graphite (as the negative electrode active material), conductive carbon black (Super P, as a conductive agent), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were weighed in a mass ratio of 95:2:1.5:1.5; the above-mentioned composite pore-forming agent was also taken, with an addition amount of 5 wt% of the mass of artificial graphite. CMC was first dissolved in deionized water to prepare a glue solution, and then artificial graphite, conductive carbon black, and composite pore-forming agent were added in sequence. The mixture was stirred at 1500 rpm for 2 hours to ensure that the components were fully dispersed and uniform. Finally, SBR emulsion was added and stirred at 800 rpm for 30 minutes to mix evenly. After vacuum degassing, a uniform negative electrode slurry was obtained with a solid content of approximately 48%.

[0037] (b) Electric field-assisted coating: The above-mentioned negative electrode slurry was uniformly coated onto the copper foil current collector using a doctor blade coating method, controlling the wet film thickness to be approximately 200 μm. Immediately after coating, flat plate electrodes were placed on the upper and lower sides of the wet film, and a DC electric field with an intensity of 75 V / cm was applied in a direction perpendicular to the copper foil plane, with the electric field direction pointing from the electrode surface to the copper foil current collector side, for 10 minutes. Driven by the electric field, the negatively charged composite pore-forming agent particles migrated directionally towards the bottom layer near the current collector along the electric field direction, thereby forming a gradient distribution in the wet film where the concentration of the pore-forming agent gradually increases from the surface to the bottom layer along the electrode thickness direction.

[0038] (c) Segmented drying and pore formation: The wet film electrode treated with an electric field is placed in a forced-air drying oven for segmented heating treatment. In the first stage, the temperature is controlled at 60°C and the drying time is 1 hour. At this temperature, the PMMA outer shell is preferentially dissolved and removed. Since the concentration of the pore-forming agent on the electrode surface is low, dispersed macroporous channels are formed on the surface after the outer shell is removed. In the second stage, the temperature is raised to 100°C and held for 1.5 hours. At this temperature, the ammonium carbonate core decomposes into ammonia and carbon dioxide and escapes, generating a dense microporous structure in the bottom layer near the current collector where the pore-forming agent is concentrated. Through the above segmented heating process, a gradient pore distribution with decreasing porosity and decreasing pore size is established inside the electrode from the surface to the current collector side.

[0039] (d) Cold pressing: The electrode sheet after the hole-making process is cold pressed on a roller press at a pressure of 30MPa to compact the electrode sheet to the target thickness. During the compaction process, the gradient pore structure is locked to obtain a negative electrode sheet with a directional gradient pore structure.

[0040] Example 2 This embodiment provides a negative electrode sheet with a directional gradient pore structure and a method for preparing the composite pore-forming agent used therein. The difference from Embodiment 1 lies in the following specific steps: (I) Preparation of composite pore-forming agent (1) Synthesis of core-shell structure: Ammonium carbonate powder was dispersed in anhydrous ethanol, and SDS (0.05 wt% of ammonium carbonate) was added as a dispersant. The mixture was stirred to form a uniform suspension. MMA monomer was added dropwise to the suspension, and the mass ratio of MMA to ammonium carbonate was controlled at 0.2:1. The temperature was raised to 60°C and AIBN was added to initiate a free radical polymerization reaction. The reaction was maintained for 6 hours. The lower polymerization temperature (60°C) resulted in a slower reaction rate, and the reaction time was extended to 6 hours to ensure the integrity of the coating. After the reaction, the product was filtered, washed, and vacuum dried at room temperature to obtain a core-shell structure precursor with a relatively thin PMMA coating layer.

[0041] (2) Surface functionalization modification: The core-shell structure precursor was dispersed in Tris-HCl buffer solution, and dopamine hydrochloride was added to a concentration of 1 mg / mL. The pH was adjusted to 8.0, and the reaction was carried out with stirring at room temperature for 36 hours. The lower dopamine concentration (1 mg / mL) resulted in a slower deposition rate of polydopamine, requiring a longer reaction time (36 hours) to ensure sufficient coverage of the modified layer on the PMMA shell surface. After filtration, washing, and vacuum drying, the composite pore-forming agent was obtained.

[0042] The Zeta potential of the obtained composite pore-forming agent was tested to be -22mV. Although this Zeta potential is lower than that of Example 1 (-35mV), its absolute value still exceeds 20mV, which meets the requirements for maintaining stable colloidal dispersion in an aqueous slurry system.

[0043] (II) Preparation of negative electrode sheet The negative electrode material formulation in the slurry preparation was the same as in Example 1, and the amount of composite pore-forming agent added was 1 wt% of the mass of artificial graphite. The electric field strength applied during the electric field-assisted coating stage was 50 V / cm, and the application time was 15 minutes. In the segmented drying pore-forming stage, the first stage temperature was 50℃ and the drying time was 2 hours, and the second stage temperature was 80℃ and the heating time was 3 hours. The cold pressing molding process was the same as in Example 1.

[0044] Example 3 This embodiment provides a negative electrode sheet with a directional gradient pore structure and a method for preparing the composite pore-forming agent used therein. The difference from Embodiment 1 lies in the following specific steps: (I) Preparation of composite pore-forming agent (1) Core-shell structure synthesis: Ammonium carbonate powder was dispersed in anhydrous ethanol, and SDS (0.2 wt% of ammonium carbonate) was added as a dispersant. The mixture was stirred thoroughly. MMA monomer was added dropwise to the suspension, controlling the mass ratio of MMA to ammonium carbonate to be 0.5:1. The temperature was raised to 80°C, and AIBN was added to initiate a free radical polymerization reaction. The reaction was maintained for 2 hours. The higher polymerization temperature (80°C) resulted in a faster rate of free radical generation and chain growth, allowing for the formation of a thicker and denser PMMA coating layer within a shorter reaction time (2 hours). The higher proportion of SDS dispersant (0.2 wt%) effectively prevented the agglomeration of ammonium carbonate particles during the polymerization process. The core-shell structure precursor was obtained after post-treatment.

[0045] (2) Surface functionalization modification: The core-shell precursor was dispersed in Tris-HCl buffer solution, and dopamine hydrochloride was added to a concentration of 5 mg / mL. The pH was adjusted to 9.0, and the reaction was carried out with stirring at room temperature for 8 hours. The higher dopamine concentration (5 mg / mL) and stronger alkaline conditions (pH=9.0) significantly accelerated the oxidative self-polymerization rate of dopamine, and a sufficiently thick polydopamine modification layer could be formed on the surface of the PMMA shell within 8 hours. The composite pore-forming agent was obtained after post-treatment.

[0046] The obtained composite pore-forming agent was found to have an average particle size of approximately 20 μm and a Zeta potential of -42 mV, exhibiting excellent dispersion stability and strong electrophoretic response.

[0047] (II) Preparation of negative electrode sheet The amount of composite pore-forming agent added in the slurry preparation is 10 wt% of the mass of artificial graphite. The electric field strength applied during the electric field-assisted coating stage is 100 V / cm, and the application time is 5 minutes. In the segmented drying pore-forming stage, the first stage temperature is 70℃ and the drying time is 0.5 hours; the second stage temperature is 120℃ and the heating time is 0.5 hours. The remaining steps are the same as in Example 1.

[0048] The higher amount of pore-forming agent (10wt%) makes the gradient pore structure inside the electrode most significant, the stronger electric field (100V / cm) promotes the gradient migration of the pore-forming agent in a shorter time, and the higher drying temperature accelerates the dissolution of the outer shell and the decomposition of the core.

[0049] Example 4 This embodiment provides a composite pore-forming agent with ammonium bicarbonate as the core material and a negative electrode sheet with a directional gradient pore structure prepared using this pore-forming agent, to verify the applicability of different thermally decomposable pore-forming materials in the technical solution of this invention. The specific steps are as follows: (I) Preparation of composite pore-forming agent (1) Core-shell structure synthesis: Ammonium bicarbonate was used instead of ammonium carbonate as the core material. Ammonium bicarbonate powder was dispersed in anhydrous ethanol, and SDS (0.1 wt% of the mass of ammonium bicarbonate) was added. After stirring to form a uniform suspension, MMA monomer was added dropwise. The mass ratio of MMA to ammonium bicarbonate was 0.3:1. AIBN was added at 70°C to initiate the polymerization reaction, and the reaction was maintained for 4 hours. The thermal decomposition initiation temperature of ammonium bicarbonate is about 60°C, which is lower than that of ammonium carbonate. Therefore, it is necessary to pay attention to whether the core decomposes prematurely under the polymerization condition of 70°C. In actual operation, the ammonium bicarbonate particles were dispersed in the ethanol solvent system. The PMMA coating layer that was rapidly formed on the particle surface played a certain protective role. At the same time, the evaporation of ethanol carried away some heat, so that the decomposition rate of ammonium bicarbonate during the polymerization stage was controlled at a low level. After post-processing, the ammonium bicarbonate@PMMA core-shell structure precursor was obtained.

[0050] (2) Surface functionalization modification: Same as step (2) in Example 1, that is, react for 24 hours at pH=8.5 and dopamine hydrochloride concentration of 3 mg / mL. The composite pore-forming agent is obtained after post-treatment.

[0051] The obtained composite pore-forming agent was tested and found to have a Zeta potential of -33mV. TGA analysis showed that the ammonium bicarbonate core completely decomposed at approximately 90°C, and there was still a sufficient temperature difference between it and the dissolution temperature window of the PMMA shell (approximately 60°C) to meet the requirements for segmented pore formation.

[0052] (II) Preparation of negative electrode sheet The negative electrode preparation process parameters are the same as in Example 1, namely, 5wt% pore-forming agent, 75V / cm electric field strength, drying at 60℃ for 1 hour in the first stage, and heating at 100℃ for 1.5 hours in the second stage. The ammonium bicarbonate core can be completely decomposed at 100℃, forming a microporous structure in the bottom layer region. After cold pressing, a negative electrode with a directional gradient pore structure is obtained.

[0053] Example 5 This embodiment provides a composite pore-forming agent with azodicarbonamide as the core material and a negative electrode sheet with a directional gradient pore structure prepared using the pore-forming agent. The specific steps are as follows: (I) Preparation of composite pore-forming agent (1) Core-shell structure synthesis: Azodicarbonamide (ADCA) was used instead of ammonium carbonate as the core material. Azodicarbonamide is a commonly used organic chemical foaming agent. Its pure product usually decomposes in the range of 130-200℃, but the decomposition temperature can be reduced to the target range by adding a small amount of activator. In this embodiment, azodicarbonamide powder was mixed and ground evenly with zinc oxide (as a decomposition activator) accounting for 5% of the mass of ADCA, dispersed in anhydrous ethanol, 0.1wt% SDS was added, and MMA monomer was added dropwise. The polymerization was carried out at 70℃ for 4 hours to form a PMMA coating layer. Azodicarbonamide has good thermal stability at the polymerization temperature of 70℃ and will not decompose prematurely. Therefore, the coating process is smoother, and the coating uniformity of the obtained core-shell structure precursor is better than that of the ammonium carbonate system. After post-processing, the ADCA@PMMA core-shell precursor was obtained.

[0054] (2) Surface functionalization modification: Same as step (2) in Example 1. The composite pore-forming agent is obtained after post-treatment.

[0055] The obtained composite pore-forming agent was tested and found to have a Zeta potential of -36mV. TGA analysis showed that the PMMA shell began to lose weight at about 60°C, and the ADCA core (containing zinc oxide activator) began to decompose and produce gas at about 110°C. The temperature difference between the two weight loss stages was about 50°C, and the temperature window separation effect of segmented pore formation was good.

[0056] (II) Preparation of negative electrode sheet The negative electrode preparation process parameters are basically the same as in Example 1, except that the heating temperature in the second stage is adjusted to 110℃ to match the decomposition temperature characteristics of ADCA. The main gaseous products generated during ADCA decomposition include nitrogen, carbon monoxide, carbon dioxide, and a small amount of ammonia. The gas production is greater than that of ammonium carbonate, and under the same addition amount, a slightly higher micropore porosity can be formed in the bottom layer region. After cold pressing, a negative electrode with a directional gradient pore structure is obtained.

[0057] Example 6 The only difference between this embodiment and Example 1 is that the amount of composite pore-forming agent added is 3wt% of the mass of artificial graphite. The rest of the composite pore-forming agent preparation method and negative electrode preparation process parameters (including electric field strength of 75V / cm, drying at 60℃ for 1 hour in the first stage, and heating at 100℃ for 1.5 hours in the second stage) are the same as in Example 1.

[0058] Compared to Example 1 (5wt%), the gradient pore structure formed within the electrode under this addition amount is more moderate, with reduced surface porosity and bottom micropore density, but a higher volumetric proportion of active material, which is beneficial for improving the volumetric energy density of the electrode. The parameter selection in this example is suitable for application scenarios with high energy density requirements but moderate fast charging rate requirements.

[0059] Example 7 The only difference between this embodiment and Embodiment 1 is that the amount of composite pore-forming agent added is 8 wt% of the mass of artificial graphite. The other preparation methods of the composite pore-forming agent and the process parameters for the preparation of the negative electrode sheet are the same as those in Embodiment 1.

[0060] Compared to Example 1 (5wt%), the gradient pore structure formed within the electrode under this addition amount is more significant, with improved porosity at each layer and a larger porosity difference between the surface and bottom layers. This addition amount sacrifices the discharge specific capacity at low rates to some extent, but significantly improves the electrolyte wetting rate and lithium-ion transport performance, making it suitable for thick electrode designs or applications requiring high fast-charging capabilities.

[0061] Example 8 The difference between this embodiment and Example 1 is as follows: In the preparation of the composite pore-forming agent, the amount of SDS dispersant is 0.15 wt% of the mass of ammonium carbonate, the free radical polymerization temperature is 75°C, the polymerization time is 3 hours, the dopamine hydrochloride concentration is 2 mg / mL, and the self-polymerization reaction time is 20 hours; in the preparation of the negative electrode, the electric field strength is 60 V / cm, the application time is 12 minutes, the first stage drying temperature is 55°C, the time is 1.5 hours, and the second stage heating temperature is 95°C, the time is 2 hours. The remaining material formulations and operating steps are the same as in Example 1. The Zeta potential of the obtained composite pore-forming agent was measured to be -30 mV, and a clear gradient distribution of decreasing porosity from the surface to the bottom layer was observed.

[0062] Example 9 This embodiment provides a lithium-ion battery comprising a negative electrode sheet with a directional gradient pore structure, and the specific steps are as follows: (I) Preparation of positive electrode sheet Lithium nickel cobalt manganese oxide (NCM523, LiNi0.5Co0.2Mn0.3O2), conductive carbon black (SuperP), and polyvinylidene fluoride (PVDF) were weighed in a mass ratio of 96:2:2. PVDF was dissolved in N-methylpyrrolidone (NMP) to prepare a gel solution, which was then added sequentially to the positive electrode active material and conductive carbon black. The mixture was stirred at 1200 rpm for 3 hours to ensure thorough and uniform dispersion of all components. After vacuum degassing, a uniform positive electrode slurry was obtained. The positive electrode slurry was uniformly coated onto an aluminum foil current collector using a doctor blade coating method. After drying in a 120℃ forced-air drying oven for 2 hours, the slurry was rolled to obtain the positive electrode sheet. The areal density of the positive electrode sheet was approximately 18 mg / cm³. 2 .

[0063] (II) Preparation of negative electrode sheet A negative electrode sheet with a directional gradient pore structure was prepared using the method described in Example 1, and the preparation process parameters were exactly the same as in Example 1.

[0064] (III) Assembly of Lithium-ion Batteries The positive and negative electrode sheets were cut to suitable sizes. A Celgard 2400 polypropylene microporous membrane was used as the separator, and a 1M LiPF6 solution dissolved in ethylene carbonate (EC) / dimethyl carbonate (DMC) (volume ratio 1:1) was used as the electrolyte. The cells were assembled into CR2032 coin cells in an argon-filled glove box (water and oxygen content both below 0.1 ppm) in the order of negative electrode sheet / separator / positive electrode sheet. The N / P ratio of the positive and negative electrodes was controlled to be 1.05–1.10 to ensure battery safety. After assembly, the cells were allowed to stand for 12 hours to allow the electrolyte to fully impregnate them.

[0065] II. Comparative Example Comparative Example 1 The difference between this comparative example and Example 1 is that the PMMA shell coating step is omitted in the preparation of the pore-forming agent; instead, the ammonium carbonate powder is directly modified with polydopamine. Specifically, in a Tris-HCl buffer solution, ammonium carbonate powder and dopamine hydrochloride (concentration 3 mg / mL) are reacted at pH 8.5 for 24 hours, allowing polydopamine to be directly deposited on the surface of the ammonium carbonate particles. The resulting pore-forming agent has only a two-layer structure of an ammonium carbonate core and a polydopamine surface layer (without the PMMA intermediate shell layer), while the remaining negative electrode preparation process parameters are the same as in Example 1.

[0066] Due to the lack of PMMA shell for isolation and protection, ammonium carbonate partially dissolved and decomposed during the stirring process of the aqueous slurry due to contact with water and the heat generated by stirring, resulting in slurry stability significantly lower than in Example 1. In the segmented drying stage, because the "pre-dissolution" step of the PMMA shell was absent, ammonium carbonate had already begun to partially decompose during the first stage of low-temperature drying at 60°C, making it impossible to achieve segmented elimination of the shell and core in time and space. The resulting electrode exhibited relatively small differences in porosity across its three layers, with indistinct gradient characteristics.

[0067] Comparative Example 2 The difference between this comparative example and Example 1 is that the polydopamine surface functionalization modification step is omitted when preparing the composite pore-forming agent. That is, only the synthesis of the ammonium carbonate@PMMA core-shell structure is completed, and the dopamine self-polymerization reaction is no longer carried out. The resulting pore-forming agent is an ammonium carbonate@PMMA two-layer core-shell structure (without the polydopamine modification layer). The other negative electrode preparation process parameters are the same as in Example 1.

[0068] Because the pore-forming agent surface lacks the charge properties imparted by polydopamine (PMMA itself is an electrically neutral polymer), it cannot generate an effective electrophoretic driving force when an electric field is applied. Therefore, the distribution of the pore-forming agent in the coating slurry is essentially random, failing to form the expected gradient distribution along the electrode thickness direction. Furthermore, the lack of electrostatic repulsion stabilization from the polydopamine modification layer leads to significant agglomeration of the pore-forming agent particles during slurry stirring and settling, affecting the uniformity of the pores.

[0069] Comparative Example 3 The difference between this comparative example and Example 1 is that no electric field is applied during the preparation of the negative electrode sheet; that is, the slurry is directly applied to the copper foil and then proceeds directly to the segmented drying stage without undergoing the electric field-assisted gradient sedimentation step. The preparation method of the composite pore-forming agent and the remaining process parameters such as segmented drying and cold pressing are consistent with those of Example 1.

[0070] Without an applied electric field, although the composite pore-forming agent has a complete core-shell coating structure and polydopamine surface modification, it only undergoes weak natural sedimentation in the slurry coating due to gravity, and cannot establish an effective concentration gradient along the thickness direction in a short time. After segmented drying, the porosity of each layer inside the electrode is relatively similar. Although segmented heating can still eliminate the outer shell and core at different temperatures (generating pores of two different scales, macropores and micropores), the distribution of these pores of different scales in each layer of the electrode is nearly uniform, rather than showing a gradient arrangement from macropores on the surface to micropores on the bottom.

[0071] Comparative Example 4 The difference between this comparative example and Example 1 is that the drying process was changed from segmented heating to single-stage heating. That is, after coating and applying an electric field, the electrode was directly placed in a 100°C forced-air drying oven for 2.5 hours, without a low-temperature pre-drying stage. The preparation method of the composite pore-forming agent and the electric field-assisted coating parameters are the same as in Example 1.

[0072] Under direct heating at 100°C, the dissolution of the PMMA shell and the thermal decomposition of the ammonium carbonate core highly overlap in time. While the PMMA rapidly dissolves during heating, the ammonium carbonate also rapidly decomposes and produces gas. This means that the two-step pore-forming process, which should have sequentially formed macropores and micropores at different temperature windows and in different layers of the electrode, is compressed into a nearly synchronous single-step process. Although the electric field still enriches the pore-forming agent in the bottom layer of the electrode, creating a certain concentration gradient, the lack of a temporal separation and elimination mechanism between the shell and the core significantly weakens the difference in pore size distribution between the bottom and surface layers, and the gradient characteristics are not as pronounced as in Example 1.

[0073] Comparative Example 5 This comparative example uses traditional single PMMA microspheres as the pore-forming agent, without core-shell structure or surface functionalization modification, and without the application of an electric field. Specifically, commercially available PMMA microspheres with an average particle size of approximately 15 μm are directly added to the negative electrode slurry at a ratio of 5 wt% of artificial graphite. After the slurry is coated onto copper foil, it is dried at 60°C for 2 hours to dissolve the PMMA microspheres and create pores. The remaining negative electrode material formulation (artificial graphite, conductive carbon black, CMC / SBR binder) and cold pressing process are the same as in Example 1.

[0074] PMMA microspheres are randomly distributed in the slurry, without electric field driving or concentration gradient. After drying, they form uniform pores of similar size at each layer of the electrode, lacking the characteristics of a gradient pore structure. Furthermore, due to the pore-forming mechanism of only PMMA (dissolution and elimination), it is impossible to generate pores of different sizes at different temperatures. This comparative example represents the most common PMMA pore-forming scheme in the prior art, used to compare with the gradient pore technology of this invention.

[0075] Comparative Example 6 Using the negative electrode sheet prepared in Comparative Example 1, a full cell was assembled according to the same method as in Example 9.

[0076] Comparative Example 7 Using the negative electrode sheet prepared in Comparative Example 5, a full cell was assembled according to the same method as in Example 9.

[0077] III. Test Methods and Performance Evaluation (I) Characterization of pore structure Pore ​​size distribution measurement: The pore size distribution curves of each negative electrode were determined using a mercury porosimeter (MIP). The mercury contact angle was set to 130° and the test pressure range was 0.1 to 60,000 psi. Based on this, the volume ratio and distribution characteristics of macropores (pore size > 1 μm) and micropores (pore size < 1 μm) in the electrode were analyzed.

[0078] (ii) Electrolyte wettability test The negative electrode sheets prepared in each embodiment and comparative example were cut into circular pieces with a diameter of 14 mm. In an argon-filled glove box, 2 μL of standard electrolyte (1 M LiPF6 dissolved in a 1:1 vol% EC / DMC mixture) was dropped onto the center of the electrode surface using a microsyringe. The change in the contact angle of the electrolyte droplet on the electrode surface over time was recorded using a contact angle meter. The time required for the droplet to completely penetrate the electrode (contact angle dropping to 0°) was defined as the wetting time. Three samples of each type were tested, and the average value was taken. A shorter wetting time indicates better wettability of the electrode to the electrolyte.

[0079] (III) Electrochemical performance testing The negative electrode sheets prepared in each example and comparative example were assembled with lithium metal sheets to form CR2032 coin cells. The electrolyte was a 1M LiPF6 solution dissolved in EC / DMC (volume ratio 1:1), and the separator was a Celgard 2400 polypropylene membrane. Battery assembly was performed in an argon-filled glove box (water and oxygen content both below 0.1 ppm). All electrochemical tests were conducted at a constant temperature of 25°C (except for low-temperature tests). Three cells were tested in parallel under each condition, and the average value was taken.

[0080] (1) Rate performance test: After the battery is activated for 3 cycles at 0.1C, it is charged and discharged for 5 cycles each at 0.2C, 0.5C, 1C, 2C and 3C. The discharge specific capacity of the stable cycle at each rate is recorded. The charge and discharge voltage range is 0.01 to 2.0V.

[0081] (2) Cycle life test: After the battery is activated 3 times at 0.1C, a long cycle test is conducted at a charge / discharge rate of 1C / 1C. The discharge specific capacity of each cycle is recorded, and the capacity retention rate of the 500th cycle relative to the 1st cycle is calculated.

[0082] (3) Low temperature performance test: After placing the battery in a -20℃ constant temperature chamber for 4 hours to allow the temperature to fully equalize, charge and discharge test is performed at a rate of 0.5C. The discharge specific capacity at -20℃ is recorded and compared with the discharge specific capacity at the same rate (0.5C) at 25℃. The low temperature capacity retention rate is calculated (low temperature capacity retention rate = -20℃ discharge specific capacity / 25℃ discharge specific capacity × 100%).

[0083] (iv) Full battery performance test Electrochemical performance was tested using the full cell assembled in Example 9. The charge / discharge voltage range was 2.8–4.2 V. After three activation cycles at 0.1 C, five charge / discharge cycles were performed sequentially at 0.5 C, 1 C, and 2 C, and the discharge specific capacity at each stable cycle was recorded. Cycling performance was tested at 1 C / 1 C charge / discharge rates for 300 cycles, and the capacity retention was recorded. Comparative Examples 6 and 7 were tested using the same method. Three cells were tested in parallel under each condition, and the average value was taken.

[0084] Test Results and Analysis The test results are shown in Tables 1-5: Table 1. Pore structure characterization and electrolyte wettability test results Table 2 Rate performance test results (discharge specific capacity, unit: mAh / g) Table 3 Cycle life test results (1C / 1C, 25℃) Table 4. Low-temperature performance test results (0.5°C, -20°C) Table 5. Full cell performance test results (NCM523 / graphite system) Test Result Analysis (I) Analysis of pore structure and electrolyte wettability As can be seen from the test results in Table 1, the negative electrode sheets prepared in Examples 1 to 8 all exhibit a gradient distribution characteristic of decreasing porosity from the surface layer to the bottom layer, indicating that the composite pore-forming agent of the present invention, combined with electric field assistance and segmented heating process, can effectively construct a directional gradient porosity structure. Among them, Example 1 is the preferred scheme, with a surface porosity of 36.2%, a middle layer of 22.5%, and a bottom layer of 11.8%, showing a continuous decreasing gradient transition between the three layers, with obvious gradient characteristics. In Example 3, due to the highest amount of pore-forming agent and the largest electric field strength, the porosity difference between the surface layer and the bottom layer reaches 25.2%, and the gradient effect is the most significant among all examples.

[0085] The pore structure characterization results of Examples 4 and 5 are similar to those of Example 1. The surface porosities of the three examples are 35.5%, 35.8%, and 36.2%, respectively, and the bottom layer porosities are 11.2%, 12.5%, and 11.8%, respectively, with differences within a reasonable experimental error range. This result shows that different types of thermally decomposable pore-forming materials can effectively achieve the construction of gradient pore structures through the core-shell coating and surface modification strategy of this invention, verifying the versatility of the technical solution. The comparison between Examples 6 and 7 further shows that as the amount of pore-forming agent added increases, the porosity of each layer shows an upward trend, and the gradient characteristics gradually strengthen.

[0086] In contrast, the porosities of the three layers in Comparative Example 1 were 28.2%, 25.5%, and 22.8%, respectively. Although there seemed to be a downward trend in the numerical values, the difference between the layers was only 5.4%, far less than the 24.4% in Example 1. Furthermore, the overall porosity was low, which is because the premature decomposition of ammonium carbonate during slurry preparation and low-temperature drying led to a loss of some pore-forming effect. The porosities of the electrodes in Comparative Examples 2 and 3 were very close, exhibiting a near-uniform distribution, strongly demonstrating that the synergistic effect of surface charge characteristics and the applied electric field is indispensable for achieving the directional gradient migration of the pore-forming agent. Comparative Example 4 showed some gradient characteristics, but significantly weaker than Example 1, indicating that the segmented heating strategy played an irreplaceable role in enhancing the gradient effect. The electric field assisted in solving the spatial distribution problem of "where the pore-forming agent is," while segmented heating solved the temporal and scale problems of "when and how large the pores are formed." The porosity of the electrode in Comparative Example 5 was the most uniform, completely lacking gradient characteristics.

[0087] Regarding electrolyte wettability, the wetting time of Example 1 was only 18 seconds, 60% shorter than that of Comparative Example 5. The large pores on the surface of the gradient pore structure significantly reduced the initial resistance to electrolyte penetration. Simultaneously, the decreasing gradient pore structure generated a continuous driving effect similar to a capillary pump, guiding the electrolyte to continuously penetrate from the electrode surface to the deeper layers of the current collector. The comparative examples, lacking effective surface large pores or gradient guiding effects, all had wetting times ranging from 28 to 45 seconds.

[0088] (II) Ratio Performance Analysis As shown in Table 2, the discharge specific capacity differences between the examples and the comparative examples are small at low rates (0.2C) (all within the range of 345–359 mAh / g). This is because, under low current density conditions, the kinetic resistance of lithium-ion transport is not the main factor limiting capacity. However, as the charge / discharge rate increases, the performance gap between the examples and the comparative examples widens rapidly. Taking 3C high-rate discharge as an example, the discharge specific capacity of Example 1 is 295.6 mAh / g, while that of Comparative Example 5 is only 242.5 mAh / g, a difference of 53.1 mAh / g. The capacity retention rates (3C / 0.2C) are 83.2% and 68.9%, respectively.

[0089] This advantage stems from the systematic optimization of the lithium-ion transport path by the gradient pore structure: the high porosity region on the surface provides a wide introduction channel for lithium-ion diffusion from the electrolyte to the active material interface, significantly reducing liquid-phase mass transfer resistance and initial concentration polarization of ions entering the electrode; the transitional porosity of the intermediate layer maintains the continuity of ion transport; although the bottom layer has lower porosity, its densely distributed micropore structure still provides an efficient short-range diffusion path for lithium-ions, and the higher volume fraction of active material in the bottom layer ensures the full utilization of lithium intercalation capacity. This decreasing pore distribution from the surface to the bottom layer perfectly matches the mass transfer requirements faced by lithium-ions at different thicknesses on the electrode: the surface layer needs to rapidly introduce a large number of lithium-ions, the intermediate layer needs efficient transport, and the bottom layer needs rapid short-range intercalation into the active material.

[0090] The rate performance of Comparative Example 4 is between that of the Examples and the other Comparative Examples (270.5 mAh / g at 3C), indicating that the rate improvement effect is limited due to the lack of pore size gradient caused by segmented pore formation, which is only generated by the concentration gradient assisted by the electric field. The 3C discharge specific capacity of Comparative Examples 1-3 and Comparative Example 5 are all in the range of 238-256 mAh / g, which is much lower than that of the Examples.

[0091] (III) Cycle life analysis As shown in Table 3, Example 1 exhibited the best capacity retention rate of 90.6% after 500 cycles at 1C / 1C conditions, outperforming all other samples. Examples 2–8 showed capacity retention rates ranging from 88.0% to 90.4% after 500 cycles, significantly higher than the comparative examples (81.0%–86.3%). The improvement in cycle life due to the gradient pore structure is mainly reflected in two aspects: Firstly, the gradient porosity, acting as a buffer space, effectively accommodates approximately 10% volume expansion of the graphite anode during lithium intercalation. In a uniform and dense electrode, the repeated expansion and contraction of the active material induces cumulative mechanical stress within the electrode, leading to loosening of interparticle contacts, particle pulverization, and repeated rupture and regeneration of the SEI film. The gradient porosity structure effectively releases mechanical stress and mitigates the aforementioned degradation process by providing deformation buffer space around the active material particles.

[0092] Secondly, the gradient pore structure optimizes the uniformity of lithium ion concentration distribution in the electrode, reduces the degree of electrode polarization, and reduces side reactions caused by excessively high local lithium ion concentration (risk of oversaturation and lithium plating) or excessively low local lithium ion concentration (insufficient utilization of active materials), thereby reducing the irreversible consumption rate of active lithium and electrolyte during cycling.

[0093] Comparative Example 5 showed the lowest capacity retention rate of 81.0% after 500 cycles, indicating that the uniformly distributed pores could not provide a directional deformation buffering effect. Comparative Example 1 had a retention rate of 82.0%, indicating that the irregular and disordered pore structure generated by the premature decomposition of ammonium carbonate had poor stability during cycling, with some pores collapsing during cold pressing and cyclic expansion.

[0094] (iv) Low-temperature performance analysis As shown in Table 4, Example 1 exhibited a low-temperature capacity retention rate of 75.4% at -20°C and 0.5°C, significantly higher than the comparative examples (61.0%–69.0%). Under low-temperature conditions, the increased viscosity of the electrolyte, the decreased diffusion coefficient of lithium ions in the electrolyte, and the increased charge transfer resistance make ion transport kinetic bottlenecks a key factor restricting the low-temperature performance of the battery. The gradient pore structure partially offset the kinetic degradation caused by low temperature through the following mechanisms: the large pores on the surface maintain a relatively unobstructed ion introduction path even under conditions of increased electrolyte viscosity; the gradient decreasing pore distribution shortens the effective diffusion distance of lithium ions within the electrode and reduces tortuosity; and the micropores in the bottom layer provide a short-range, rapid diffusion channel between lithium ions and the active material. Among the comparative examples, Comparative Example 5 had the lowest low-temperature capacity retention rate (61.0%), indicating that uniform pores could not provide a preferential channel for ion transport at low temperatures.

[0095] Based on the above test results, the test results of the above embodiments and comparative examples show that the four technical elements of core-shell coating structure, polydopamine surface functionalization modification, electric field-assisted gradient sedimentation, and segmented heating pore formation work together to significantly enhance the battery's rate performance, cycle life, and low-temperature performance.

[0096] (V) Full Battery Performance Analysis As shown in Table 5, the NCM523 / graphite full cell assembled in Example 9 (using the gradient porosity anode sheet of Example 1) exhibited superior discharge specific capacity compared to the comparative full cell at all rates. At 0.5C, the discharge specific capacity of Example 9 was 152.6 mAh / g, an improvement of 7.6% compared to Comparative Example 7. At a high rate of 2C, the discharge specific capacity of Example 9 was 132.5 mAh / g, while that of Comparative Example 7 was only 100.5 mAh / g, representing an improvement of 31.8%, indicating that the gradient porosity structure also significantly improved the rate performance of the full cell system. Regarding cycle life, Example 9 retained 91.2% of its capacity after 300 cycles, while Comparative Examples 6 and 7 retained 83.5% and 80.8%, respectively. These full cell test results further validate the application value of the directional gradient porosity anode sheet of the present invention in practical battery systems, confirming its effectiveness in improving the overall electrochemical performance of lithium-ion batteries.

[0097] The above are merely specific embodiments of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the protection scope of the present invention. All other details not described in detail belong to the prior art.

Claims

1. A composite pore-forming agent for preparing negative electrode sheets with oriented gradient pore structures, characterized in that, The composite pore-forming agent has a core-shell coating structure and a surface functionalized modification layer. The core-shell coating structure includes a core and a shell covering the outer surface of the core. The core is a thermally decomposable pore-forming material that can undergo thermal decomposition and release gas in a temperature range of 80–120°C. The outer shell is a thermosoluble polymer coating layer that can dissolve or soften and be removed within a temperature range of 50–70°C. The dissolution temperature of the outer shell is lower than the decomposition temperature of the core, so that the outer shell is eliminated before the core during the segmented heating process, thereby forming pores of different sizes at different thicknesses of the negative electrode sheet. The surface functionalized modification layer is a charged polymer layer that uniformly coats the outer surface of the outer shell, giving the composite pore-forming agent surface charge characteristics.

2. The composite pore-forming agent for preparing a negative electrode sheet with a directional gradient pore structure according to claim 1, characterized in that, The thermally decomposable pore-forming material is selected from at least one of ammonium carbonate, ammonium bicarbonate, and azodicarbonamide; The material of the heat-dissolving polymer coating layer is polymethyl methacrylate; the surface functionalized modification layer is a polydopamine layer, which carries a negative charge under neutral and weakly alkaline conditions.

3. The composite pore-forming agent for preparing a negative electrode sheet with a directional gradient pore structure according to claim 2, characterized in that, The thickness of the polymethyl methacrylate shell allows for a dissolution temperature window of 55–65°C, while the decomposition temperature window of the ammonium carbonate core is 90–110°C, with a temperature difference of no less than 25°C between the two. The particle size of the composite pore-forming agent is 1–50 μm. The absolute value of the surface potential of the polydopamine-modified layer is no less than 20 mV.

4. A method for preparing a composite pore-forming agent for preparing a negative electrode sheet with a directional gradient pore structure as described in any one of claims 1 to 3, characterized in that, Includes the following steps: (1) Disperse the thermally decomposable pore-forming material powder in an organic solvent, add a dispersant of 0.05-0.2 wt% of the pore-forming material mass and stir to form a uniform suspension; then add a polymeric monomer dropwise to the suspension and initiate a free radical polymerization reaction at 60-80°C, so that the polymeric monomer polymerizes in situ on the surface of the pore-forming material particles to form a polymer coating layer. After the reaction is completed, filter and wash to obtain a precursor with a core-shell structure. (2) The core-shell structure precursor obtained in step (1) is dispersed in a Tris buffer solution containing dopamine hydrochloride, the pH of the system is adjusted to 8.0-9.0, and the dopamine oxidation self-polymerization reaction is carried out continuously at room temperature for 8-36 hours, so that dopamine is deposited on the surface of the polymer coating layer to form a polydopamine modified layer. After filtration, washing and drying, the composite pore-forming agent is obtained.

5. The preparation method according to claim 4, characterized in that, In step (1), the thermally decomposable pore-forming material is ammonium carbonate; the organic solvent is anhydrous ethanol; the dispersant is sodium dodecyl sulfonate, and its addition amount is 0.1 wt% of the pore-forming material; the polymer monomer is methyl methacrylate; the temperature of the free radical polymerization reaction is 70°C, and the reaction time is 2 to 6 hours; In step (2), the pH value of the dopamine oxidative self-polymerization reaction is 8.5, and the reaction time is 20 to 28 hours; the concentration of the dopamine hydrochloride is 1 to 5 mg / mL.

6. A method for preparing a negative electrode sheet with a directional gradient pore structure, characterized in that, The composite pore-forming agent according to any one of claims 1 to 3 is used, comprising the following steps: (a) The negative electrode active material, conductive agent, binder and the composite pore-forming agent are dispersed in a solvent in a preset ratio, and a uniform negative electrode slurry is obtained after vacuum stirring and degassing treatment; (b) The negative electrode slurry is coated onto the surface of the current collector. During the coating process or in the wet film state after coating, a DC electric field is applied in a direction perpendicular to the plane of the current collector. The direction of the electric field is from the surface of the electrode to the side of the current collector, so that the composite pore-forming agent with surface charge migrates in a direction closer to the current collector under the drive of the electric field force, thereby forming a gradient distribution in the wet film in which the concentration of the pore-forming agent gradually increases from the surface to the bottom layer along the thickness direction of the electrode. (c) The electrode sheet after step (b) is subjected to segmented heating treatment. In the first stage, it is dried at 50-70°C to dissolve or soften and remove the polymer shell of the composite pore-forming agent. After the shell is removed, a relatively large pore channel is formed on the surface of the electrode sheet. In the second stage, the temperature is raised to 80-120°C to decompose the thermal decomposition core of the composite pore-forming agent and release gas. A dense microporous structure is generated in the bottom layer area near the current collector due to the enrichment of the pore-forming agent. A gradient pore distribution with decreasing porosity and decreasing pore size is established inside the electrode sheet from the surface to the current collector side. (d) After the pore-forming process is completed, the electrode sheet is cold-pressed to lock the gradient pore structure during the compaction process, thereby obtaining a negative electrode sheet with an oriented gradient pore structure.

7. The preparation method according to claim 6, characterized in that, In step (a), the negative electrode active material is artificial graphite and / or natural graphite, the conductive agent is conductive carbon black, the binder is a composite binder system of sodium carboxymethyl cellulose and styrene-butadiene rubber, and the solvent is deionized water; the amount of the composite pore-forming agent added is 1-10 wt% of the mass of the negative electrode active material. In step (b), the electric field strength of the DC electric field is 50-100V / cm; the current collector is copper foil; and the electric field is applied for 1-30 minutes after coating is completed.

8. The preparation method according to claim 6 or 7, characterized in that, In step (c), the drying temperature of the first stage is 60°C and the drying time is 0.5 to 2 hours, so that the porosity of the large pore channels formed on the surface of the electrode is 30 to 40%; the heating temperature of the second stage is 100°C and the heating time is 0.5 to 3 hours, so that the porosity of the microporous structure formed near the bottom layer of the current collector is 8 to 15%.

9. A negative electrode sheet with a directional gradient pore structure, characterized in that, The negative electrode sheet is obtained by the preparation method according to any one of claims 6 to 8; the negative electrode sheet exhibits a continuous gradient distribution of decreasing porosity along its thickness direction from the surface layer away from the current collector to the bottom layer near the current collector, wherein the porosity of the surface layer region is 30-40% and the porosity of the bottom layer region is 8-15%.

10. A lithium-ion battery, characterized in that, The lithium-ion battery comprises the negative electrode sheet with a directional gradient pore structure as described in claim 9.

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