A dual-gradient thick electrode structure and a preparation method and application thereof

Abstract

The present application relates to a kind of double gradient thick electrode structure and its preparation method and application, double gradient thick electrode structure includes current collector along the thickness direction, inner layer close to current collector and outer layer away from current collector;Inner layer includes large particle size positive active particle and first pore, outer layer includes small particle size positive active particle and second pore, and the porosity of outer layer is higher than the porosity of inner layer, form the double gradient of particle size and pore.The double gradient thick electrode structure is applied in liquid battery, can reduce the ion transport tortuosity in the electrode inside in liquid battery, reduce concentration polarization, improve rate performance and cycle stability;The structure is applied in solid battery, can optimize solid-solid interface contact, reduce interface impedance.The present application uses double-layer continuous wet coating method to combine low-temperature slow baking method to prepare double gradient thick electrode structure, with good industrial application prospect with existing wet coating production line height compatibility.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and in particular to a dual-gradient thick electrode structure, its preparation method, and its application. Background Technology

[0002] Secondary batteries, as core components of electrochemical energy storage systems, have been widely used in portable electronic devices, electric vehicles, and large-scale energy storage. Improving battery energy density and power density is currently a key research focus. Using thick electrodes is an effective way to increase battery areal capacity, reduce the proportion of inactive materials, and thus improve volumetric energy density. However, with increasing electrode thickness, the internal ion transport path is significantly lengthened and its tortuosity increases, leading to limited ion diffusion kinetics. Especially under medium-to-high rate operating conditions, electrode polarization intensifies, reversible capacity decreases, and capacity utilization is insufficient.

[0003] To alleviate these problems, researchers have tried various improvement strategies, including: increasing the proportion of conductive agents to improve electronic conductivity, increasing overall porosity to promote electrolyte wetting, using small-particle-size active materials to shorten the solid-phase diffusion distance, and using laser drilling to construct artificial ion channels. However, each of these methods has significant drawbacks, as follows.

[0004] (1) Increasing the proportion of conductive agent will reduce the content of active material and sacrifice volumetric energy density.

[0005] (2) Increasing the overall porosity will reduce the compaction density and mechanical stability of the electrode, affecting the cycle life.

[0006] (3) Although using small-diameter particles is beneficial to reaction kinetics, it increases the specific surface area, exacerbates interfacial side reactions, and still cannot solve the problem of non-uniform reaction distribution from the diaphragm to the current collector in thick electrodes.

[0007] (4) Physical hole-making methods such as laser drilling are complex, have high equipment costs, and are not compatible with existing wet coating production lines, making it difficult to achieve large-scale application.

[0008] The aforementioned problems are even more pronounced in solid-state batteries. Solid-state batteries use solid electrolytes instead of liquid electrolytes, fundamentally improving safety and the upper limit of energy density. Among them, polyethylene oxide (PEG) solid electrolytes have become one of the most promising solid electrolyte systems for industrialization due to their good lithium / sodium salt solubility, excellent film-forming properties, and interfacial compatibility with electrodes. However, PEG solid-state batteries suffer from severe solid-solid interface contact problems within the cathode composite structure: the solid electrolyte lacks adequate wetting from the liquid electrolyte between the solid electrolyte and the cathode active material particles, ion transport occurs only at a limited number of contact points, and the interfacial impedance is significantly high. At the same time, the problem of long and tortuous ion transport paths within thick electrodes is further amplified under solid-solid interface conditions, resulting in rate performance and cycle stability that are far below theoretical expectations.

[0009] Therefore, there is an urgent need for a novel cathode structure that can simultaneously improve the ion transport dynamics inside thick electrodes, optimize the solid-solid interface contact state, and be compatible with existing wet coating processes. This structure should be applicable to both liquid and solid-state batteries to achieve a technological leap from liquid to solid state. Summary of the Invention

[0010] This invention addresses the problems of high ion transport tortuosity and poor rate performance of existing thick electrodes in liquid batteries, and proposes a solution for solid-state batteries. To address the technical shortcomings of existing modification methods (such as laser drilling and complex multilayer step-by-step coating), including poor interfacial contact, high interfacial impedance, complexity, incompatibility with mainstream wet coating production lines, and difficulty in achieving large-scale production, this paper proposes a dual-gradient thick electrode structure suitable for both liquid and solid-state batteries, along with its preparation method and applications. Through the synergistic design of particle size gradient and pore size gradient, this method improves ion diffusion kinetics, reduces interfacial polarization and impedance, and is compatible with existing production equipment, thus solving the process bottleneck for industrial scale-up.

[0011] To achieve the above objectives, in a first aspect, the present invention provides a dual-gradient thick electrode structure, wherein the dual-gradient thick electrode structure comprises, along the thickness direction: a current collector, an inner layer close to the current collector, and an outer layer away from the current collector; The inner layer includes: first positive electrode active material particles and first pores formed by a first pore-forming agent; The outer layer includes: second positive electrode active material particles and second pores formed by a second pore-forming agent; The particle size of the first positive electrode active material particles is larger than that of the second positive electrode active material particles; The porosity of the outer layer is greater than that of the inner layer.

[0012] Preferably, the thickness ratio of the inner layer to the outer layer is 0.8:1 to 1.2:1.

[0013] Preferably, the first positive electrode active material particles include one or more of the following: lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, layered transition metal oxides, Prussian blue analogs, or polyanionic compounds; the volume median particle size Dv50 of the first positive electrode active material particles is 4.5 μm to 8.0 μm. The second positive electrode active material particles include one or more of the following: lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, layered transition metal oxides, Prussian blue analogs, or polyanionic compounds; the volume median particle size Dv50 of the second positive electrode active material particles is 1.0 μm to 3.0 μm.

[0014] Preferably, the pore size of the first pore is 0.05 μm to 0.5 μm; The pore size of the second pore is 1.0 μm to 8.0 μm; The porosity of the outer layer is 35%–55%; The porosity of the inner layer is 20% to 35%.

[0015] In a second aspect, the present invention provides a method for fabricating the dual-gradient thick electrode structure described in the first aspect, the method comprising: Step S1: Disperse the first positive electrode active material particles, the first binder, the first conductive agent and the first pore-forming agent in the first solvent, mix them evenly to obtain the inner layer slurry; Step S2: Disperse the second positive electrode active material particles, the second binder, the second conductive agent, and the second pore-forming agent in the second solvent and mix them evenly to obtain an outer layer slurry; wherein, the percentage of the second pore-forming agent in the total solid mass of the outer layer slurry is higher than the percentage of the first pore-forming agent in the total solid mass of the inner layer slurry; the particle size of the first positive electrode active material particles is larger than the particle size of the second positive electrode active material particles; Step S3: The inner layer slurry is coated onto the surface of the current collector, and then the outer layer slurry is coated onto the surface of the inner layer slurry to obtain a thick electrode intermediate. Step S4: Bake the thick electrode intermediate to decompose the first pore-forming agent and the second pore-forming agent and generate gas to be expelled, so that the inner and outer layers formed on the surface of the current collector have a porosity gradient, and obtain the electrode precursor. Step S5: Vacuum dry the electrode precursor to obtain a dual-gradient thick electrode structure.

[0016] Preferably, the first positive electrode active material particles include one or more of the following: lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, layered transition metal oxides, Prussian blue analogs, or polyanionic compounds; The first adhesive comprises one or more of the following: polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyacrylic acid, polyimide, or sodium alginate; The first conductive agent includes one or more of the following: multi-arm carbon nanotubes, single-walled carbon nanotubes, conductive carbon black, acetylene black, Ketjen black, carbon fiber, graphite, or graphene. The first pore-forming agent includes one or more of ammonium bicarbonate and ammonium carbonate; The first solvent includes one or more of N-methylpyrrolidone, water, ethanol, or acetone; The mass ratio of the first positive electrode active material particles, the first binder and the first conductive agent is 87-92:5-8:3-5; The mass of the first pore-forming agent is 20% to 30% of the total mass of the first positive electrode active material particles, the first binder, and the first conductive agent.

[0017] Preferably, the second active material particles include one or more of the following: lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, layered transition metal oxides, Prussian blue analogs, or polyanionic compounds. The second adhesive includes one or more of the following: polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyacrylic acid, polyimide, or sodium alginate; The second conductive agent includes one or more of the following: multi-arm carbon nanotubes, single-walled carbon nanotubes, conductive carbon black, acetylene black, Ketjen black, carbon fiber, graphite, or graphene. The second pore-forming agent includes one or more of ammonium bicarbonate and ammonium carbonate; The second solvent includes one or more of N-methylpyrrolidone, water, ethanol, or acetone; The mass ratio of the second positive electrode active material particles, the second binder, and the second conductive agent is 87-92:5-8:3-5; The mass of the second pore-forming agent is 40% to 45% of the total mass of the second positive electrode active material particles, the second binder, and the second conductive agent.

[0018] Preferably, the current collector comprises: aluminum foil or carbon-coated aluminum foil; The thickness of the inner layer slurry applied by scraping is 100μm to 1000μm; The thickness of the outer layer slurry applied by scraping is 100μm to 1000μm.

[0019] Preferably, the baking conditions are: baking at 50℃~70℃ for 2 to 4 hours; The vacuum drying temperature is 80℃~120℃, and the time is 12 hours~24 hours.

[0020] Thirdly, the present invention provides a secondary battery, the secondary battery comprising the dual gradient thick electrode structure described in the first aspect, or the dual gradient thick electrode structure prepared by the preparation method described in the second aspect.

[0021] The present invention provides a dual-gradient thick electrode structure, its preparation method and application, which has the following technical effects.

[0022] (1) The dual-gradient thick electrode structure provided by this invention significantly improves the ion transport dynamics inside the thick electrode: This invention constructs a dual-gradient structure of "large particle size + low porosity in the inner layer and small particle size + high porosity in the outer layer," forming a stable framework support and a low specific surface area in the inner layer (near the current collector side), reducing deep side reactions; and forming a high-porosity network in the outer layer (away from the current collector side), promoting effective electrolyte wetting and rapid ion entry. The synergistic matching of the particle size gradient and the porosity gradient in the thickness direction constructs a low-torsion ion transport channel, significantly reducing the ion diffusion resistance inside the thick electrode. Experimental data show that when the dual-gradient thick electrode structure of this invention is applied to a liquid battery, at a 5C rate and 7 mg / cm³, it significantly improves the ion transport dynamics inside the thick electrode. 2 ~13mg / cm 2 Under a certain load, the capacity retention rate can reach over 98% after 100 cycles, which is significantly better than the control electrode with only a single gradient or uniform structure.

[0023] (2) The dual-gradient thick electrode structure provided by this invention effectively reduces polarization at medium and high rates and improves capacity utilization: Due to the optimization of the ion transport path, the utilization rate of reactive materials in the thickness direction tends to be uniform, and concentration polarization is significantly reduced. The dual-gradient thick electrode structure of this invention is applied to polyoxyethylene solid-state batteries, improving the discharge specific capacity and ion diffusion coefficient, and effectively solving the problem of poor rate performance of solid-state batteries due to high interface impedance.

[0024] (3) When the dual-gradient thick electrode structure provided by this invention is applied in solid-state batteries, it can optimize the solid-solid interface contact and reduce the interface impedance. In this invention, the high porosity and small particle size of the outer layer work synergistically to increase the effective contact area between the positive electrode active material and the solid electrolyte, constructing a continuous three-phase interface, namely, active material-conductive agent-solid electrolyte. Simultaneously, the stable framework formed by the large-diameter particles in the inner layer provides structural support for the entire electrode, avoiding interface delamination caused by volume changes during charge-discharge cycles.

[0025] (4) The dual gradient thick electrode structure provided by the present invention can improve the cycle stability of the battery and has the ability to suppress structural decay: the dual gradient thick electrode structure makes the lithium / sodium ion flux uniformly distributed in the electrode thickness direction during the charging and discharging process, avoiding local overcharging / over-discharging and local volume expansion concentration.

[0026] (5) The method for preparing the dual-gradient thick electrode structure provided by this invention is highly compatible with existing wet coating production lines and has strong feasibility for process scaling: This invention adopts a double-layer continuous wet coating process (the upper layer is coated immediately while the lower layer is not dry), combined with low-temperature slow baking to induce the decomposition of the pore-forming agent to form a pore gradient. This process does not require the addition of complex equipment and can be directly implemented on existing coating machines by adjusting the slurry formula and coating head configuration, and has good prospects for industrial application. Attached Figure Description

[0027] Figure 1 This is a cross-sectional schematic diagram of the dual-gradient thick electrode structure provided in an embodiment of the present invention.

[0028] Figure 2 This is a method for preparing a dual-gradient thick electrode structure provided in the embodiments of the present invention.

[0029] Figure 3 These are scanning electron microscope (SEM) images of the surfaces of the dual gradient thick electrode structure provided in Embodiment 1 of the present invention and the comparative electrode provided in Comparative Example 1.

[0030] Figure 4 This is a comparison chart of the coulombic efficiency and cycle capacity of the liquid lithium-ion batteries assembled in Example 1 and Comparative Example 1 of the present invention.

[0031] Figure 5 This is a comparison chart of the coulombic efficiency and cycle capacity of the liquid lithium-ion batteries assembled in Example 2 and Comparative Example 2 of the present invention.

[0032] Figure 6 This is a comparison chart of the rate performance of the liquid lithium-ion batteries assembled in Example 3 and Comparative Example 3 of the present invention.

[0033] Figure 7 This is a comparison chart of the coulombic efficiency and cycle capacity of the solid-state batteries assembled in Example 4 and Comparative Example 4 of the present invention.

[0034] Figure 8 This is a comparison chart of the coulombic efficiency and cycle capacity of the solid-state batteries assembled in Example 5 and Comparative Example 5 of the present invention. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0036] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0037] This invention provides a dual-gradient thick electrode structure, comprising: a current collector, an inner layer close to the current collector, and an outer layer away from the current collector.

[0038] The inner layer includes: first positive electrode active material particles and first pores formed by a first pore-forming agent.

[0039] The outer layer includes: second positive electrode active material particles and second pores formed by a second pore-forming agent.

[0040] The first positive electrode active material particles and the second positive electrode active material particles may be of the same or different types, specifically including one or more of the following: lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, layered transition metal oxides, Prussian blue analogs, or polyanionic compounds.

[0041] Among them, nickel-cobalt-manganese ternary materials include NCM111 (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2), NCM523 (LiNi) 0.5 Co 0.2 Mn 0.3 O2), NCM622 (LiNi) 0.6 Co 0.2 Mn 0.2 O2), NCM811 (LiNi) 0.8 Co 0.1 Mn 0.1 One or more of the following: O2; the general chemical formula of layered transition metal oxides is Na. x TMO2, wherein TM is one or more of Ni, Mn, Fe, Co, Cu, and Cr, and 0.5 ≤ x ≤ 1; Prussian blue analogues include Na. 1.88 Fe[Fe(CN)6] 0.98• One or more of 1,8H2O, Na2Mn[Fe(CN)6], and Na2Ni[Fe(CN)6]; polyanionic compounds include one or more of sodium vanadium phosphate, sodium iron pyrophosphate, sodium iron sulfate, and sodium vanadium fluorophosphate.

[0042] The particle size of the first positive electrode active material is larger than that of the second positive electrode active material.

[0043] The volume median particle size Dv50 of the first positive electrode active material particles is 4.5μm to 8.0μm, and can be any value within this range, such as 4.5μm, 5.0μm, 5.5μm, 6.0μm, 6.5μm, 7.0μm, 7.5μm, 8.0μm, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0044] The volume median particle size Dv50 of the second positive electrode active material particles is 1.0μm to 3.0μm, and can be any value within this range, such as 1.0μm, 1.5μm, 2.0μm, 2.5μm, 3.0μm, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0045] In this invention, the volume median particle size (Dv50) refers to the volume median particle size of a material, representing the particle size corresponding to 50% of the material's volume distribution, a meaning known in the art. The Dv50 is tested using conventional methods, such as laser diffraction. According to the standard ISO 13320:2020 Particle size analysis—Laser diffraction method, the Dv50 is determined using a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0046] The porosity of the outer layer is greater than that of the inner layer.

[0047] The porosity of the outer layer is 35% to 55%, and can be any value within this range, such as 35%, 40%, 45%, 50%, 55%, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0048] The porosity of the inner layer is 20% to 35%, and can be any value within this range, such as 20%, 25%, 30%, 35%, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0049] This invention employs the Archimedes principle immersion weighing method to determine the open porosity of electrode coatings. For the porosity of the inner and outer layers of a double-layer gradient structure, separate samples of the inner layer (coated only with the lower slurry) and the outer layer (coated with the upper slurry on a peelable substrate and then peeled off) are prepared and tested using the same method. The testing equipment includes: an electronic analytical balance (accuracy 0.1 mg), an electronic spring balance, vernier calipers, and a vacuum drying oven.

[0050] The pore size of the first pore is 0.05μm to 0.5μm, and can be any value within this range, such as 0.05μm, 0.1μm, 0.2μm, 0.3μm, 0.4μm, 0.5μm, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0051] The pore size of the second pore is 1.0μm to 8.0μm, and can be any value within this range, such as 1.0μm, 2.0μm, 3.0μm, 4.0μm, 5.0μm, 6.0μm, 7.0μm, 8.0μm, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0052] This invention employs mercury porosimetry to determine the pore size distribution and average pore size of electrode coatings. The electrode sheet to be tested (or individual samples of the inner / outer layer) is weighed and placed into the sample cell of the mercury porosimeter. Pressure is gradually applied (0.1–60000 psi), and the volume of mercury injected under different pressures is recorded. The instrument automatically calculates the pore size distribution curve and the average pore size (volume median pore size). The testing equipment is a mercury porosimeter.

[0053] The thickness ratio of the inner layer to the outer layer is 0.8:1 to 1.2:1, and can be any ratio within this range, such as 0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, etc., but is not limited to the listed ratios. Other unlisted ratios within this range are also applicable.

[0054] A schematic diagram of the cross-sectional structure of the dual-gradient thick electrode structure provided in this embodiment of the invention is shown below. Figure 1 As shown, the outer layer consists of the second positive electrode active material particles and their internal second pores, the inner layer consists of the first positive electrode active material particles and their internal first pores, and the bottom layer is the current collector; wherein, the outer layer corresponds to the high porosity small particle size layer far away from the current collector, and the inner layer corresponds to the low porosity large particle size layer close to the current collector. The two layers work together to construct a low tortuosity ion transport channel through the gradient distribution of particle size and porosity.

[0055] The dual-gradient thick electrode structure provided in this invention uses large-diameter particles (Dv50 of 4.5μm to 8.0μm) in the inner layer (near the current collector) and small-diameter particles (Dv50 of 1.0μm to 3.0μm) in the outer layer (away from the current collector), combined with high porosity (35% to 55%) in the outer layer and low porosity (20% to 35%) in the inner layer, to construct a dual gradient of particle size and porosity, synergistically reducing tortuosity and improving ion transport rate.

[0056] This invention utilizes the small particle size of the outer layer to shorten the solid-phase diffusion distance, the high porosity of the outer layer to promote the rapid entry of ions, and the large particle size of the inner layer to stabilize the framework and suppress side reactions, resulting in a more uniform reaction distribution.

[0057] When the dual-gradient thick electrode structure provided in this invention is applied to a solid-state battery, the small-diameter active particles in the outer layer can increase the specific surface area and the number of contact points with the solid electrolyte. The high porosity provides more ion channels and wetting space, thus solving the problem of solid-state electrolyte membrane adhesion between the cathode and the solid electrolyte membrane. Addressing the issue of poor interfacial contact by reducing interfacial impedance can improve several aspects. Firstly, it increases the migration rate between the positive electrode active material and the solid electrolyte, enhancing rate performance and reducing the polarization potential difference during charge and discharge, thus improving energy efficiency. Secondly, good interfacial contact and lower interfacial impedance help homogenize the reaction distribution along the electrode thickness, suppressing localized current concentration and interfacial stripping, improving cycle performance, and extending battery life. Furthermore, reduced interfacial impedance also decreases Joule heating, improving low-temperature performance and battery safety. Therefore, this invention effectively reduces interfacial impedance through a dual-gradient structure, simultaneously achieving high rate capability, long cycle life, and high safety.

[0058] This invention provides a method for fabricating the above-mentioned dual-gradient thick electrode structure, such as... Figure 2 As shown, the specific steps include:

[0059] Step S1: Disperse the first positive electrode active material particles, the first binder, the first conductive agent and the first pore-forming agent in the first solvent and mix them evenly to obtain the inner layer slurry.

[0060] The first positive electrode active material particles include one or more of the following: lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, layered transition metal oxides, Prussian blue analogs, or polyanionic compounds.

[0061] The first adhesive includes one or more of the following: polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyacrylic acid, polyimide, or sodium alginate.

[0062] The first conductive agent includes one or more of the following: multi-arm carbon nanotubes, single-walled carbon nanotubes, conductive carbon black, acetylene black, Ketjen black, carbon fiber, graphite, or graphene.

[0063] The first pore-forming agent includes one or more of ammonium bicarbonate and ammonium carbonate.

[0064] The first solvent includes one or more of N-methylpyrrolidone, water, ethanol, or acetone.

[0065] The mass ratio of the first positive electrode active material particles, the first binder, and the first conductive agent is 87-92:5-8:3-5.

[0066] The mass of the first pore-forming agent is 20% to 30% of the total mass of the first positive electrode active material particles, the first binder, and the first conductive agent.

[0067] Step S2: Disperse the second positive electrode active material particles, the second binder, the second conductive agent, and the second pore-forming agent in the second solvent and mix them evenly to obtain an outer layer slurry; wherein, the percentage of the second pore-forming agent in the total solid mass of the outer layer slurry is higher than the percentage of the first pore-forming agent in the total solid mass of the inner layer slurry; the particle size of the first positive electrode active material particles is larger than the particle size of the second positive electrode active material particles.

[0068] The second active material particles include one or more of the following: lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, layered transition metal oxides, Prussian blue analogs, or polyanionic compounds. The second active material particles may be the same as or different from the first active material particles.

[0069] The second adhesive includes one or more of the following: polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyacrylic acid, polyimide, or sodium alginate. The second adhesive may be the same as or different from the first adhesive.

[0070] The second conductive agent includes one or more of the following: multi-arm carbon nanotubes, single-walled carbon nanotubes, conductive carbon black, acetylene black, Ketjen black, carbon fiber, graphite, or graphene. The second conductive agent may be the same as or different from the first conductive agent.

[0071] The second pore-forming agent includes one or more of ammonium bicarbonate and ammonium carbonate. The second pore-forming agent may be the same as or different from the first pore-forming agent.

[0072] The second solvent includes one or more of N-methylpyrrolidone (NMP), water, ethanol, or acetone.

[0073] The mass ratio of the second positive electrode active material particles, the second binder, and the second conductive agent is 87-92:5-8:3-5.

[0074] The mass of the second pore-forming agent is 40% to 45% of the total mass of the second positive electrode active material particles, the second binder, and the second conductive agent.

[0075] Step S3: Apply an inner layer slurry to the surface of the current collector, and then apply an outer layer slurry to the surface of the inner layer slurry to obtain a thick electrode intermediate.

[0076] This step involves single-sided coating; the current collector includes aluminum foil or carbon-coated aluminum foil.

[0077] The thickness of the inner layer slurry applied by the blade is 100μm to 1000μm, and can be any value within this range, such as 100μm, 200μm, 300μm, 400μm, 500μm, 500μm, 700μm, 800μm, 900μm, 1000μm, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. The preferred thickness of the inner layer slurry is 350μm to 1000μm.

[0078] The thickness of the outer layer of slurry applied by the blade is 100μm to 1000μm, and can be any value within this range, such as 100μm, 200μm, 300μm, 400μm, 500μm, 500μm, 700μm, 800μm, 900μm, 1000μm, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. The preferred thickness of the outer layer of slurry applied by the blade is 350μm to 1000μm.

[0079] Step S4: The thick electrode intermediate is baked to decompose the first pore-forming agent and the second pore-forming agent and generate gas to be expelled, so that the inner and outer layers formed on the surface of the current collector have a porosity gradient, and the electrode precursor is obtained.

[0080] The baking conditions are 50℃~70℃ for 2 to 4 hours. Step S5: Vacuum dry the electrode precursor to obtain a double-gradient thick electrode structure.

[0081] Vacuum drying is used to remove residual solvent. The vacuum drying temperature is 80℃~120℃, and the time is 12 hours~24 hours.

[0082] The preparation method provided in this invention uses a double-layer continuous wet coating (the upper layer is coated before the lower layer is dry) combined with slow drying at low temperature (50℃~70℃) to decompose pore-forming agents such as ammonium bicarbonate, which is directly compatible with existing coating production lines and improves process compatibility.

[0083] The dual-gradient thick electrode structure prepared by the above-described preparation method provided in this invention is used as a positive electrode in a secondary battery, which includes a liquid battery or a solid battery. The liquid battery includes a liquid lithium-ion battery or a liquid sodium-ion battery; the solid battery includes a solid lithium-ion battery or a solid sodium-ion battery.

[0084] The secondary batteries provided in this embodiment of the invention can be applied to electric vehicles, hybrid vehicles, electric bicycles, energy storage power stations, portable electronic devices, mobile power supplies, drones, power tools, wearable devices, medical electronic devices, aerospace vehicles, marine power systems, or grid frequency regulation energy storage systems.

[0085] To better understand the technical solution provided by the present invention, the following uses several specific examples to illustrate the fabrication process and characteristics of the dual-gradient thick electrode structure of the present invention.

[0086] Example 1 This embodiment provides a fabrication process for a dual-gradient thick electrode structure, the specific process of which is as follows.

[0087] (1) Lithium iron phosphate (LFP) with a particle size Dv50 of 6.5 μm is used as the first positive electrode active material particle. 0.89 g of lithium iron phosphate (LFP), 0.08 g of polyvinylidene fluoride (PVDF) as the first binder, and 0.03 g of carbon nanotubes (CNT) as the first conductive agent are weighed in a mass ratio of 89:8:3. 0.25 g of ammonium bicarbonate as the first pore-forming agent is also weighed. The total solid mass is 1.25 g. The mass of the first pore-forming agent is 25% of the total mass of the first positive electrode active material particle, the first binder, and the first conductive agent. Lithium iron phosphate, polyvinylidene fluoride, carbon nanotubes, and ammonium bicarbonate are added to N-methylpyrrolidone (NMP) and stirred to disperse evenly. The solid content of the slurry is controlled by the amount of NMP added to obtain an inner layer slurry with a solid content of about 35 wt%.

[0088] (2) Lithium iron phosphate (LFP) with a particle size Dv50 of 1.5 μm is used as the second positive electrode active material particles. 0.89 g of LFP, 0.08 g of PVDF, 0.03 g of CNT and 0.40 g of ammonium bicarbonate are weighed in a mass ratio of 89:8:3. LFP, PVDF, CNT and ammonium bicarbonate are added to NMP and stirred and dispersed evenly. The solid content of the slurry is controlled by the amount of NMP added to obtain an outer layer slurry with a solid content of about 35 wt%.

[0089] (3) Coating and drying: The inner layer slurry is coated on the surface of the aluminum foil current collector, with a wet film thickness of 350 μm; while the inner layer slurry is not dry, the outer layer slurry is immediately coated on its surface, with a wet film thickness of 350 μm, and the total wet film thickness is 700 μm, to obtain a thick electrode intermediate.

[0090] (4) The obtained thick electrode intermediate is placed in a 60°C forced-air oven and baked for 2 hours to decompose ammonium bicarbonate and generate gas, thereby forming a pore gradient and obtaining the electrode precursor.

[0091] (5) The electrode precursor was transferred to a vacuum oven and dried under vacuum at 120°C for 12 hours to obtain a double gradient thick electrode structure with an inner layer porosity of about 25% and an outer layer porosity of about 40%.

[0092] The lithium iron phosphate loading of the dual-gradient thick electrode structure (hereinafter referred to as the thick electrode positive electrode) prepared in this embodiment is 7.5 mg / cm³. 2 .

[0093] The loading test method is: (mass of electrode sheet - mass of current collector) × active material ratio / area of ​​thick electrode positive electrode. Since the pore-forming agent will eventually volatilize, the active material ratio is calculated as 89%.

[0094] SEM image of the outer surface of the dual-gradient thick electrode structure prepared in this embodiment, as shown below. Figure 3 As shown in b.

[0095] The dual-gradient thick electrode structure prepared in this embodiment was used to assemble a liquid battery and tested.

[0096] The liquid battery assembly process is as follows: The dual-gradient thick electrode structure prepared in this embodiment is cut into 10mm diameter discs as positive electrodes and placed in an argon glove box (water and oxygen content <0.1ppm) for later use. A lithium metal sheet is used as the negative electrode, and a double-sided ceramic-coated separator is used (each side of the ceramic coating is 2mm thick, and the polyethylene separator is 7mm thick). A solution containing 1mol / L LiPF6 in dimethyl carbonate / ethylene carbonate / ethyl methyl carbonate (DMC:EC:EMC volume ratio 1:1:1) is used as the electrolyte. 150μL is added dropwise and then the mixture is placed into a coin cell casing and sealed to obtain a liquid lithium-ion battery. This battery has a low tortuosity and is denoted as LT.

[0097] The testing process for the liquid battery was as follows: a constant current charge-discharge test was conducted using the Xinwei Battery Testing System (CT-4008Q-5V50mA-HWX) at a test temperature of 30℃. First, the liquid battery underwent formation: constant current charging to 3.8V at 0.1C, discharging to 2.5V at 0.1C, cycling for 5 cycles, followed by cycle performance testing and ion diffusion rate testing.

[0098] Cyclic performance test: Constant current charge-discharge cycle test was performed at 0.5C rate, and the discharge capacity of the 1st and 100th cycles was recorded. The capacity retention rate was calculated as (capacity of the 100th cycle / capacity of the 1st cycle × 100%).

[0099] Ion diffusion coefficient test (GITT test): The GITT test was performed on the NEWARE battery test system, with a voltage range of 2.5-3.8V. The test consisted of a series of "constant current pulse-relaxation" steps. First, the battery was charged (or discharged) at a constant current of 0.1C for 30 minutes; then, the battery was allowed to rest for 2 hours to allow its voltage to relax to a quasi-equilibrium state. The above process was repeated within a set voltage window until the cutoff voltage was reached. Test data are detailed in Table 1.

[0100] To better illustrate the effects of the embodiments of the present invention, Comparative Example 1 is compared with Embodiment 1 above.

[0101] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 only has a particle size gradient and no pore size gradient.

[0102] In step (1), no pore-forming agent is added during the preparation of the inner layer slurry. 0.89g of lithium iron phosphate (LFP) with a particle size Dv50 of 6.5μm, 0.08g of polyvinylidene fluoride (PVDF) as the first binder, and 0.03g of carbon nanotubes (CNT) as the first conductive agent are added to NMP and stirred to disperse evenly. The solid content of the slurry is controlled by the amount of NMP added, and an inner layer slurry with a solid content of about 30wt% is obtained.

[0103] In step (2), no pore-forming agent is added during the preparation of the outer layer slurry. Weigh 0.89g of LFP with a particle size Dv50 of 1.5μm, 0.08g of PVDF, and 0.03g of CNT and add them to NMP. Stir and disperse evenly. Adjust the solid content of the slurry by adding the amount of NMP to obtain an outer layer slurry with a solid content of about 30wt%.

[0104] Steps (3) and (4) are the same as in Example 1, resulting in a comparative electrode with only a particle size gradient and no pore size gradient.

[0105] The lithium iron phosphate loading of the comparative electrode prepared in this example was 7.5 mg / cm³. 2 .

[0106] SEM image of the outer surface of the comparative electrode prepared in this example, as shown below. Figure 3 As shown, Figure 3 a is a SEM image of the surface of the control electrode prepared in this comparative example. Figure 3 b is a SEM image of the outer surface of the dual-gradient thick electrode structure in Example 1. Figure 3 The display shows that the overall surface of the contrast electrode is relatively dense, the particles are highly coated with binder, the porosity is not obvious, and the particles are tightly packed together; while Figure 3b shows that the dual-gradient thick electrode structure prepared in Example 1 has abundant and uniformly distributed pores, clear particle boundaries, and obvious interparticle gaps. The overall structure is a loose and porous stacked structure. In the electrode structure of Example 1, the abundant pores provide wetting channels for the electrolyte, shorten the lithium ion diffusion distance, and significantly improve the rate performance and fast charging capability. The gradient structure can buffer the volume expansion during charging and discharging, and improve cycle stability.

[0107] Liquid lithium-ion batteries were assembled and tested using the comparative electrode prepared in this comparative example. The battery assembly and testing process was the same as in Example 1. The battery assembled in Comparative Example 1 has a higher degree of tortuosity than that in Example 1, denoted as HT. Detailed test data are shown in Table 1.

[0108] The coulombic efficiency and cycle capacity curves of the batteries assembled in Comparative Example 1 and Example 1 are shown in the figure. Figure 4 As shown in the figure, the horizontal axis represents the number of cycles, the left vertical axis represents the discharge specific capacity (mAh / g), and the right vertical axis represents the coulombic efficiency (%). LT-Charge capacity represents the charging capacity curve of the liquid battery assembled in Example 1, LT-Discharge capacity represents the discharging capacity curve of the liquid battery assembled in Example 1, HT-Charge capacity represents the charging capacity curve of the liquid battery assembled in Comparative Example 1, and HT-Discharge capacity represents the discharging capacity curve of the liquid battery assembled in Comparative Example 1. The 1C cycle capacity and coulombic efficiency curves of Example 1 (LT) and Comparative Example 1 (HT) under low load conditions (approximately 7.5 mg / cm²) are presented. Figure 4 As can be seen, the initial capacity of Example 1 was approximately 164 mAh / g, and after 100 cycles, it remained at approximately 163 mAh / g, with a retention rate of 99.21%. The initial capacity of Comparative Example 1 was approximately 152 mAh / g, and after 100 cycles, it remained at approximately 141 mAh / g, with a retention rate of 92.8%. Both examples exhibited coulombic efficiencies close to 100%. The test results demonstrate that the battery assembled with the dual-gradient (LT) electrode in Example 1 exhibits more efficient ion transport under low load, and its capacity is significantly higher than that of the battery assembled with only the particle size gradient (HT) electrode in Comparative Example 1.

[0109] Example 2 The difference between this embodiment and Embodiment 1 is that the wet film thickness is different, and the lithium iron phosphate loading of the prepared dual-gradient thick electrode structure is 13 mg / cm³. 2 .

[0110] Wet film thickness: The inner layer slurry coating thickness is 600 μm, the outer layer slurry coating thickness is 600 μm, and the total wet film thickness is 1200 μm. The rest of the preparation process is exactly the same as in Example 1.

[0111] The dual-gradient thick electrode structure prepared in this embodiment was used to assemble a liquid lithium-ion battery and tested. The battery assembly and testing process was the same as in Example 1. Test data are detailed in Table 1.

[0112] To better illustrate the effects of the embodiments of the present invention, Comparative Example 2 is compared with Example 2 above.

[0113] Comparative Example 2 The difference between Comparative Example 2 and Example 2 is that Comparative Example 2 only has a particle size gradient and no pore size gradient. The wet film thickness and loading are the same as in Example 2.

[0114] The comparative structure prepared using this comparative example was assembled into a liquid lithium-ion battery and tested. The battery assembly and testing process was the same as in Example 1. Detailed test data are shown in Table 1.

[0115] The coulombic efficiency and cycle capacity curves of the batteries assembled in Comparative Example 2 and Example 2 are shown in the figure. Figure 5 As shown in the figure, LT-Charge capacity represents the charging capacity curve of the liquid battery assembled in Example 2, LT-Dischargecapacity represents the discharging capacity curve of the liquid battery assembled in Example 2, HT-Charge capacity represents the charging capacity curve of the liquid battery assembled in Comparative Example 2, and HT-Discharge capacity represents the discharging capacity curve of the liquid battery assembled in Comparative Example 2. The liquid battery has a moderate load (approximately 13 mg / cm³). 2 Under the conditions described, the 1C cycle capacity and coulombic efficiency curves of Example 2 (LT) and Comparative Example 2 (HT) are shown. Figure 5 As can be seen, Example 2 initially had a capacity of approximately 158 mAh / g, and after 100 cycles, it reached approximately 155 mAh / g (retention rate); Comparative Example 2 initially had a capacity of approximately 146 mAh / g, and after 100 cycles, it reached approximately 133 mAh / g (retention rate of 91.1%). Both examples exhibited coulombic efficiencies close to 100%. The test results demonstrate that as the load increases, the capacity advantage of the LT electrode expands by 22 mAh / g, indicating that the dual-gradient structure of Example 2 effectively reduces concentration polarization under high areal capacity.

[0116] Table 1 summarizes the test data of the liquid batteries assembled in Examples 1-2 and Comparative Examples 1-2.

[0117] Table 1 As can be seen from the test data in Table 1, the dual-gradient thick electrode (LT) of Examples 1-2 of the present invention, when applied in liquid batteries, exhibits better cycle capacity retention and ion diffusion coefficient than the particle size gradient electrode (HT) of Comparative Examples 1-2: The ion diffusion coefficient of LT (3.5 × 10⁻⁶) is significantly higher than that of the HT electrode with only particle size gradient.-9 ~2.9×10 -9 cm 2 / s) is HT (0.9×10 -9 ~0.7×10 -9 cm 2 The ion diffusion coefficient (LT) is approximately 3 to 4 times that of the particle size gradient electrode (HT), corresponding to an initial capacity increase of approximately 12 mAh / g and a cycle capacity retention rate increase of 6 to 7 percentage points (LT≥98.1%, HT≤92.8%). Therefore, in liquid batteries, the dual gradient thick electrode (LT) provided in this embodiment of the invention can significantly improve the ion diffusion coefficient, initial capacity, and cycle capacity retention rate, exhibiting superior electrochemical performance compared to the particle size gradient electrode (HT).

[0118] Example 3 The difference between this embodiment and Embodiment 1 is that the wet film thickness is different, and the lithium iron phosphate loading of the prepared dual-gradient thick electrode structure is 12 mg / cm³. 2 .

[0119] Wet film thickness: The inner layer slurry coating thickness is 500 μm, the outer layer slurry coating thickness is 500 μm, and the total wet film thickness is 1000 μm. The rest of the preparation process is exactly the same as in Example 1.

[0120] The dual-gradient thick electrode structure prepared in this embodiment was used to assemble a liquid lithium-ion battery and the rate performance and ion diffusion coefficient were tested. The battery assembly and initialization process was the same as in Example 1.

[0121] Rate performance testing of the assembly in this embodiment: constant current charging (upper limit voltage 3.8V) and constant current discharging (lower limit voltage 2.5V) were performed at rates of 0.2C, 0.5C, 1C, and 2C, respectively, with 5 cycles at each rate. The discharge specific capacity at each rate was recorded, and the capacity retention rate was calculated as (2C capacity / 0.1C capacity × 100%).

[0122] The method for testing the ion diffusion rate was the same as in Example 1, and the GITT diffusion coefficient was measured to be 3.1 × 10⁻⁶. - 9 cm 2 / s.

[0123] To better illustrate the effects of the embodiments of the present invention, Comparative Example 3 is compared with Example 3 above.

[0124] Comparative Example 3 The difference between Comparative Example 3 and Example 3 is that Comparative Example 3 only has a particle size gradient and no pore size gradient. The wet film thickness and loading are the same as in Example 3.

[0125] The comparative structure prepared in this comparative example was assembled into a liquid lithium-ion battery and its rate performance and ion diffusion coefficient were tested. The battery assembly and testing process was the same as in Example 3.

[0126] The GITT diffusion coefficient of Comparative Example 3 is 0.8 × 10⁻⁶. -9 cm 2 / s, much smaller than 3.1 × 10 in Example 3. -9 cm 2 / s, which indicates that the dual gradient structure of "large inner particle size + low porosity and small outer particle size + high porosity" constructed in the embodiments of the present invention effectively reduces the tortuosity of ion transport inside the electrode, forms a smoother ion diffusion channel, thereby reducing concentration polarization, and enabling lithium ions to be inserted / extracted more quickly during charging and discharging.

[0127] The rate performance curves of the batteries assembled in Comparative Example 3 and Example 3 are shown below. Figure 6 As shown in the figure, the horizontal axis represents the number of cycles, and the vertical axis represents the discharge specific capacity (mAh / g). The discharge specific capacity was tested at different rates (0.1C, 0.2C, 0.5C, 1C, 2C). LT-Charge capacity represents the charging capacity curve of the liquid battery assembled in Example 3, LT-Dischargecapacity represents the discharging capacity curve of the liquid battery assembled in Example 3, HT-Charge capacity represents the charging capacity curve of the liquid battery assembled in Comparative Example 3, and HT-Discharge capacity represents the discharging capacity curve of the liquid battery assembled in Comparative Example 3. Figure 6 As can be seen, the discharge capacity of Example 3 is significantly higher than that of Comparative Example 3 at all rates, indicating that the low-torsional ion channel constructed by the dual-gradient structure in the thick electrode of Example 3 significantly improves the rate performance.

[0128] Example 4 The difference between the dual-gradient thick electrode structure prepared in this embodiment and that in Example 1 is that the lithium iron phosphate loading is 5.5 mg / cm³. 2 The thickness of the inner and outer slurries is controlled by adjusting their solid content. The thickness of the inner and outer layers, as well as the total thickness of the wet film, are the same as in Example 1, and all other preparation processes are the same as in Example 1.

[0129] The dual-gradient thick electrode structure prepared in this embodiment was used to assemble a solid-state battery and tested. The solid-state battery assembly and testing process is as follows.

[0130] The solid-state battery assembly process is as follows: The dual-gradient thick electrode structure prepared in this embodiment is cut into 10mm diameter circular pieces as positive electrodes and placed in an argon glove box (water and oxygen content <0.1ppm) for later use. Using a lithium metal sheet as the negative electrode, a polyethylene oxide (PEO) based solid electrolyte membrane (PEO-LiTFSI, EO:Li molar ratio of 18:1, membrane thickness controlled at 200μm by hot pressing) is placed between the positive electrode and the lithium sheet. The membrane is then hot-pressed at 60°C for 10 minutes to improve contact. Finally, it is installed in a button cell casing and sealed to assemble the solid-state battery.

[0131] The solid-state battery testing process is as follows: Constant current charge-discharge testing is performed using the Xinwei Battery Testing System (CT-4008Q-5V50mA-HWX) at a temperature of 60℃. First, the solid-state battery undergoes formation: constant current charging to 3.8V at 0.05C, discharging to 2.5V at 0.1C, cycling 5 times, followed by rate performance testing and cycle performance testing.

[0132] Cyclic performance test: Constant current charge-discharge cycle test was performed at 0.1C rate, and the discharge specific capacity on the 100th cycle was recorded. The test data are detailed in Table 2.

[0133] Ion diffusion coefficient test (GITT test): The GITT test was performed on the NEWARE battery testing system, with a voltage range of 2.5-3.8V. The test consisted of a series of "constant current pulse-relaxation" steps. First, the battery was charged (or discharged) at a constant current of 0.1C for 30 minutes; then, the battery was allowed to rest for 2 hours to allow its voltage to relax to a quasi-equilibrium state. The above process was repeated within a set voltage window until the cutoff voltage was reached. Test data are detailed in Table 2.

[0134] To better illustrate the effects of the embodiments of the present invention, Comparative Example 4 is compared with Example 4 above.

[0135] Comparative Example 4 The difference between Comparative Example 4 and Example 4 is that Comparative Example 4 only has a particle size gradient and no pore size gradient. The wet film thickness and loading are the same as in Example 4.

[0136] The comparative structure prepared using this comparative example was assembled into a liquid lithium-ion battery and tested. The battery assembly and testing process was the same as in Example 4. Detailed test data are shown in Table 1.

[0137] The coulombic efficiency and cycle capacity curves of the solid-state batteries assembled in Comparative Example 4 and Example 4 are shown in the figure. Figure 7As shown in the figure, LT-Charge capacity represents the charging capacity curve of the solid-state battery assembled in Example 4, LT-Dischargecapacity represents the discharging capacity curve of the solid-state battery assembled in Example 4, HT-Charge capacity represents the charging capacity curve of the solid-state battery assembled in Comparative Example 4, and HT-Discharge capacity represents the discharging capacity curve of the solid-state battery assembled in Comparative Example 4.

[0138] pass Figure 7 As can be seen, in the polyoxyethylene solid-state battery, under medium loading conditions (approximately 5.5 mg / cm²), the discharge specific capacity of Example 4 (LT) after 100 cycles at 0.1C rate is 109 mAh / g, while that of Comparative Example 4 (HT) is only 82 mAh / g. This indicates that the dual-gradient structure effectively increases the contact area between the positive electrode active material and the PEO-based solid electrolyte through the synergistic effect of the small particle size and high porosity of the outer layer, optimizes the solid-solid interface contact state, reduces the interface impedance, and thus improves the electrochemical performance of the solid-state battery.

[0139] Example 5 The difference between the dual-gradient thick electrode structure prepared in this embodiment and that in Example 1 is that the lithium iron phosphate loading is 5 mg / cm³. 2 The thickness of the inner and outer slurries is controlled by adjusting their solid content. The thickness of the inner and outer layers, as well as the total thickness of the wet film, are the same as in Example 1, and all other preparation processes are the same as in Example 1.

[0140] The dual-gradient thick electrode structure prepared in this embodiment was used to assemble a solid-state battery and tested. The solid-state battery assembly and testing process is as follows.

[0141] The test data for the solid-state battery assembled in Example 5 are detailed in Table 1.

[0142] To better illustrate the effects of the embodiments of the present invention, Comparative Example 5 is compared with Example 5 above.

[0143] Comparative Example 5 The difference between Comparative Example 5 and Example 5 is that Comparative Example 5 only has a particle size gradient and no pore size gradient. The wet film thickness and loading are the same as in Example 5.

[0144] The comparative structure prepared using this comparative example was assembled into a liquid lithium-ion battery and tested. The battery assembly and testing process was the same as in Example 5. Detailed test data are shown in Table 1.

[0145] The coulombic efficiency and cycle capacity curves of the batteries assembled in Comparative Example 5 and Example 5 are shown in the figure. Figure 8 As shown in the figure, LT-Charge capacity represents the charging capacity curve of the battery assembled in Example 5, LT-Discharge capacity represents the discharging capacity curve of the solid-state battery assembled in Example 5, HT-Charge capacity represents the charging capacity curve of the solid-state battery assembled in Comparative Example 5, and HT-Discharge capacity represents the discharging capacity curve of the solid-state battery assembled in Comparative Example 5. The discharge specific capacity of Example 5 (LT) after 100 cycles at 0.1C is 130 mAh / g, while the discharge specific capacity of Comparative Example 5 (HT) after 100 cycles at 0.1C is only 88 mAh / g. This further illustrates that the dual gradient structure effectively increases the contact area between the positive electrode active material and the PEO-based solid electrolyte through the synergistic effect of the small particle size and high porosity of the outer layer, optimizes the solid-solid interface contact state, reduces the interface impedance, and thus improves the electrochemical performance of the solid-state battery.

[0146] Table 2 summarizes the test data of the solid-state batteries assembled in Examples 4-5 and Comparative Examples 4-5.

[0147] Table 2 As can be seen from the test data in Table 2, in solid-state batteries, the diffusion coefficient of the dual-gradient thick electrode structure (LT) in Examples 4-5 is 3.2 × 10⁻⁶. -10 ~3.4×10 -10 cm 2 / s) is the diffusion coefficient of the control electrode (HT) in Comparative Example 4-5 (0.8 × 10⁻⁶). -10 ~0.9×10 -10 cm 2 Four times that of ( / s); when the loading is 5.5 mg / cm³ 2 At that time, the discharge specific capacity of Example 4 (LT) after 100 cycles was 109 mAh / g, while that of Comparative Example 4 (HT) was only 82 mAh / g, which was 27 mAh / g higher (an improvement of about 33%), when the loading was 5.0 mg / cm³. 2At that time, Example 5 (LT) had a specific capacity of 130 mAh / g, while Comparative Example 5 (HT) had 88 mAh / g, a difference of 42 mAh / g (an improvement of approximately 48%). The test results show that the dual-gradient thick electrode (LT) provided by this invention, in solid-state batteries, exhibits significantly better discharge specific capacity and lithium-ion diffusion coefficient after 100 cycles than the particle size gradient electrode (HT) regardless of the loading level. Higher specific capacity at lower loading levels is normal and does not negate the technical advantages of the dual-gradient structure. The dual-gradient thick electrode structure provided by this invention optimizes solid-state... Solid-solid interface and ion transport kinetics provide a reliable solution for improving the cycle stability of PEO-based solid-state batteries.

[0148] In summary, the dual-gradient thick electrode structure provided by this invention, through the synergistic design of "large particle size + low porosity in the inner layer and small particle size + high porosity in the outer layer," constructs a stable framework in the inner layer and reduces side reactions, while forming a high-porosity network in the outer layer to promote electrolyte wetting and rapid ion entry, thereby significantly reducing ion diffusion resistance. Simultaneously, it optimizes reaction uniformity in the thickness direction, resulting in a smaller polarization potential difference at medium to high rates and improved reversible capacity. When applied to polyoxyethylene solid-state batteries, the small particle size and high porosity of the outer layer increase the effective contact area between the active material and the solid electrolyte, constructing a continuous three-phase interface. The large particle size framework in the inner layer provides structural support, reduces interfacial impedance, improves cycle performance, and effectively suppresses local volume expansion and interfacial delamination. Furthermore, this structure employs a double-layer continuous wet coating combined with low-temperature slow baking for pore formation, making it fully compatible with existing coating production lines without requiring additional complex equipment, and possessing excellent prospects for industrial scale-up.

[0149] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A dual-gradient thick electrode structure, characterized in that, The dual-gradient thick electrode structure includes, along the thickness direction: a current collector, an inner layer close to the current collector, and an outer layer away from the current collector; The inner layer includes: first positive electrode active material particles and first pores formed by a first pore-forming agent; The outer layer includes: second positive electrode active material particles and second pores formed by a second pore-forming agent; The particle size of the first positive electrode active material particles is larger than that of the second positive electrode active material particles; The porosity of the outer layer is greater than that of the inner layer.

2. The dual-gradient thick electrode structure according to claim 1, characterized in that, The thickness ratio of the inner layer to the outer layer is 0.8:1 to 1.2:

1.

3. The dual-gradient thick electrode structure according to claim 1, characterized in that, The first positive electrode active material particles include one or more of the following: lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, layered transition metal oxides, Prussian blue analogs, or polyanionic compounds; the volume median particle size Dv50 of the first positive electrode active material particles is 4.5 μm to 8.0 μm. The second positive electrode active material particles include one or more of the following: lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, layered transition metal oxides, Prussian blue analogs, or polyanionic compounds; the volume median particle size Dv50 of the second positive electrode active material particles is 1.0 μm to 3.0 μm.

4. The dual-gradient thick electrode structure according to claim 1, characterized in that, The pore size of the first pore is 0.05 μm to 0.5 μm; The pore size of the second pore is 1.0 μm to 8.0 μm; The porosity of the outer layer is 35%–55%; The porosity of the inner layer is 20% to 35%.

5. A method for fabricating a dual-gradient thick electrode structure according to any one of claims 1-4, characterized in that, The preparation method includes: Step S1: Disperse the first positive electrode active material particles, the first binder, the first conductive agent and the first pore-forming agent in the first solvent, mix them evenly to obtain the inner layer slurry; Step S2: Disperse the second positive electrode active material particles, the second binder, the second conductive agent, and the second pore-forming agent in the second solvent and mix them evenly to obtain an outer layer slurry; wherein, the percentage of the second pore-forming agent in the total solid mass of the outer layer slurry is higher than the percentage of the first pore-forming agent in the total solid mass of the inner layer slurry; the particle size of the first positive electrode active material particles is larger than the particle size of the second positive electrode active material particles; Step S3: The inner layer slurry is coated onto the surface of the current collector, and then the outer layer slurry is coated onto the surface of the inner layer slurry to obtain a thick electrode intermediate. Step S4: Bake the thick electrode intermediate to decompose the first pore-forming agent and the second pore-forming agent and generate gas to be expelled, so that the inner and outer layers formed on the surface of the current collector have a porosity gradient, and obtain the electrode precursor. Step S5: Vacuum dry the electrode precursor to obtain a dual-gradient thick electrode structure.

6. The preparation method according to claim 5, characterized in that, The first positive electrode active material particles include one or more of the following: lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, layered transition metal oxides, Prussian blue analogs, or polyanionic compounds. The first adhesive comprises one or more of the following: polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyacrylic acid, polyimide, or sodium alginate; The first conductive agent includes one or more of the following: multi-arm carbon nanotubes, single-walled carbon nanotubes, conductive carbon black, acetylene black, Ketjen black, carbon fiber, graphite, or graphene. The first pore-forming agent includes one or more of ammonium bicarbonate and ammonium carbonate; The first solvent includes one or more of N-methylpyrrolidone, water, ethanol, or acetone; The mass ratio of the first positive electrode active material particles, the first binder and the first conductive agent is 87-92:5-8:3-5; The mass of the first pore-forming agent is 20% to 30% of the total mass of the first positive electrode active material particles, the first binder, and the first conductive agent.

7. The preparation method according to claim 5, characterized in that, The second active material particles include one or more of the following: lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, layered transition metal oxides, Prussian blue analogs, or polyanionic compounds; The second adhesive includes one or more of the following: polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyacrylic acid, polyimide, or sodium alginate; The second conductive agent includes one or more of the following: multi-arm carbon nanotubes, single-walled carbon nanotubes, conductive carbon black, acetylene black, Ketjen black, carbon fiber, graphite, or graphene. The second pore-forming agent includes one or more of ammonium bicarbonate and ammonium carbonate; The second solvent includes one or more of N-methylpyrrolidone, water, ethanol, or acetone; The mass ratio of the second positive electrode active material particles, the second binder, and the second conductive agent is 87-92:5-8:3-5; The mass of the second pore-forming agent is 40% to 45% of the total mass of the second positive electrode active material particles, the second binder, and the second conductive agent.

8. The preparation method according to claim 5, characterized in that, The current collector includes: aluminum foil or carbon-coated aluminum foil; The thickness of the inner layer slurry applied by scraping is 100μm to 1000μm; The thickness of the outer layer slurry applied by scraping is 100μm to 1000μm.

9. The preparation method according to claim 5, characterized in that, The baking conditions are: baking at 50℃~70℃ for 2 to 4 hours; The vacuum drying temperature is 80℃~120℃, and the time is 12 hours~24 hours.

10. A secondary battery, characterized in that, The secondary battery includes the dual gradient thick electrode structure according to any one of claims 1 to 4, or the dual gradient thick electrode structure prepared by the preparation method according to any one of claims 5 to 8.

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