Preparation method of multiplying power type sodium ion battery pole piece and sodium ion battery

Abstract

The invention discloses a preparation method of a rate type sodium-ion battery pole piece and a sodium-ion battery, the sodium-ion battery pole piece comprises a positive pole piece and a negative pole piece, and the preparation method is characterized by comprising the following steps: S1, preparation of positive bottom layer slurry and positive top layer slurry, S2, preparation of negative bottom layer slurry and negative top layer slurry, and S3, preparation of a rate type sodium-ion battery pole piece. S3, double-chamber linkage coating, S4, three-section type gradient drying and S5, sectional type gradient rolling. The preparation method of the multiplying power type sodium ion battery pole piece and the sodium ion battery have the characteristics of high ion transmission efficiency, good volume stability, strong process applicability and excellent electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a method for preparing a high-rate sodium-ion battery electrode and a sodium-ion battery. Background Technology

[0002] The porosity of sodium-ion battery electrodes is a core parameter affecting electrolyte wetting efficiency, ion diffusion rate, and structural stability. Current technologies generally employ a uniform porosity design for electrodes, which presents the following significant drawbacks:

[0003] For example, the ion transport bottleneck of thick electrodes: when the electrode thickness is ≥100μm, the uniform porosity causes the diffusion path of ions from the separator to the current collector to be as long as 100-200μm. During high-rate charging and discharging (5C and above), an "ion concentration gradient" is easily formed near the current collector, which leads to intensified polarization and a sharp drop in capacity retention (usually <80%).

[0004] For example, stress concentration due to volume change: the volume expansion rate of hard carbon anode during sodium intercalation is 1%-3%, while the volume change of polyanionic cathode (such as Na4Fe3(PO4)2P2O7, abbreviated as NFPP) is relatively small (<3%), but the uniform porosity cannot adapt to the stress distribution in different regions. After long-term cycling, cracks are prone to appear on the surface of the electrode, and the active material shedding rate is >15%.

[0005] Uneven electrolyte wetting: Traditional electrode surfaces have the same porosity as the interior, and it takes more than 120 seconds for the electrolyte to penetrate into the core area, resulting in low initial charge and discharge efficiency (<82%), and the capacity difference between the electrode edge and center is >10%.

[0006] To address the above shortcomings, existing porosity control methods, such as using a single pore-forming agent, step-by-step coating, and fixed process parameters, all have significant technical limitations:

[0007] If the pore-forming agent is of a single type, a gradient cannot be formed:

[0008] Existing technologies mostly employ single-size pore-forming agents (such as nano-sized or micro-sized agents only), controlling the overall porosity by adjusting the dosage, but failing to achieve a gradient distribution along the thickness direction. For example, using only nano-sized pore-forming agents (50-100 nm) results in excessively small pore sizes (<200 nm), leading to high electrolyte wetting resistance; using only micro-sized pore-forming agents (500-800 nm) reduces the continuity of the conductive network, increasing electronic conduction resistance by more than 30%. Furthermore, the standard deviation of pore size distribution with a single pore-forming agent is >8%, resulting in poor batch-to-batch consistency of the electrode.

[0009] For example, step-by-step coating processes are complex and present significant interface problems.

[0010] Some technologies attempt to create gradient porosity through "multiple coatings + layered drying," but there are unavoidable interface defects: weak interlayer bonding (peel strength < 0.5 N / m), easy peeling at the interface after 500 cycles, resulting in impedance growth > 50%; step-by-step coating requires multiple shutdowns to change materials, reducing production efficiency by more than 50% compared to traditional processes, and the cumulative thickness deviation of each layer (total deviation > 5%) cannot meet the needs of large-scale industrialization.

[0011] For example, the lack of material-structure compatibility: existing technologies do not distinguish the characteristic differences between polyanionic cathodes and hard carbon anodes, and adopt a uniform porosity gradient (e.g., both are 20%→40%), resulting in functional mismatch: polyanionic cathodes do not require an excessive porosity difference (excessive porosity will reduce volumetric energy density), but existing designs have not optimized the precise gradient of "high porosity on the surface - low porosity on the bottom" for their low ion diffusion coefficient; hard carbon anodes require a larger porosity difference to buffer volume expansion, but existing designs do not reserve enough surface pore space (usually <35%), resulting in expansion stress concentration in the later stages of cycling.

[0012] For example, the lack of coordinated control of process parameters: In the existing technology, the process parameters such as coating, drying, and rolling are independent of each other and have not formed a gradient coordination mechanism: During coating, the mixing degree of the slurry is not controlled by the flow rate ratio, resulting in a sudden change in porosity in the transition zone (gradient slope deviation > 10%); During drying, a single temperature / wind speed is used, which cannot drive the pore-forming agent to naturally distribute and form a continuous gradient, resulting in a porosity distribution standard deviation > 5%; During rolling, a fixed pressure is used, which easily over-compresses the large pores on the surface or fails to fully compact the small pores in the bottom layer, resulting in the failure of the gradient design.

[0013] The shortcomings of existing technologies are obvious: traditional uniform porosity electrodes have long ion diffusion paths (100-200μm) due to their single porosity, resulting in severe high-rate polarization and an inability to buffer the volume expansion of hard carbon (cracking rate >15%); step-by-step coating technology has interface stress concentration and weak interlayer bonding due to multiple coatings, resulting in a production efficiency of only 50% of that of traditional processes, and the thickness deviation accumulates (total deviation >5%). Summary of the Invention

[0014] The purpose of this invention is to provide a method for preparing a high-rate sodium-ion battery electrode and a sodium-ion battery, which has the characteristics of high ion transport efficiency, good volume stability, strong process applicability and excellent electrochemical performance.

[0015] This invention can be achieved through the following technical solutions:

[0016] The present invention discloses a method for preparing a high-rate sodium-ion battery electrode, the sodium-ion battery electrode comprising a positive electrode and a negative electrode, the preparation method comprising the following steps:

[0017] S1. Preparation of the bottom and top slurries of the positive electrode: Add 3-5% nano-sized pore-forming agent to the positive electrode slurry, adjust the viscosity to 4900-5100 mPa.s, and the solid content to 54-56%, and stir evenly to obtain the bottom slurry of the positive electrode; Add 10-15% micron-sized pore-forming agent to the positive electrode slurry, control the viscosity to 4600-5000 mPa.s, and the solid content to 54-56%, and stir evenly to obtain the top slurry of the positive electrode;

[0018] S2. Preparation of the bottom and top slurries of the negative electrode: Add 2-4% nano-sized pore-forming agent to the negative electrode slurry, control the viscosity to 2700-2900 mPa.s, and the solid content to 49-51%; stir evenly to obtain the bottom slurry of the negative electrode; add 15-20% micron-sized pore-forming agent to the negative electrode slurry, control the viscosity to 2400-2600 mPa.s, and the solid content to 49-51%, and stir evenly to obtain the top slurry of the negative electrode;

[0019] S3. Dual-chamber joint coating: Control the flow rate ratio of the bottom layer slurry to the top layer slurry of the positive electrode to be 1:1.3-1.5, and the wet film thickness to be 245-255μm for positive electrode coating; control the flow rate ratio of the bottom layer slurry to the top layer slurry of the negative electrode to be 1:1.5-2, and the wet film thickness to be 225-235μm for negative electrode coating.

[0020] S4. Three-stage gradient drying: Along the thickness direction of the electrode sheet, the bottom layer curing stage is 80-100℃ and 1-2m / s, the transition adjustment stage is 70-80℃ and 2-3m / s, and the surface layer forming stage is 50-70℃ and 3-4m / s. Gradient drying is carried out on the positive and negative electrode sheets respectively.

[0021] S5. Segmented Gradient Rolling: The positive electrode sheet is rolled in segments at pressures ranging from 225 MPa to 18 MPa to 12 MPa to obtain the positive electrode sheet; the negative electrode sheet is rolled in segments at pressures ranging from 25 MPa to 20 MPa to 10 MPa to obtain the negative electrode sheet; along the thickness direction of the electrode sheet, the void gradient of the positive electrode sheet increases from 20% to 35%, and the void gradient of the negative electrode sheet increases from 15% to 40%.

[0022] Furthermore, in the bottom layer slurry of the positive electrode, the particle size of the nano-sized pore-forming agent is 50-100 nm; in the top layer slurry of the positive electrode, the particle size of the micro-sized pore-forming agent is 500-600 nm.

[0023] Furthermore, in the bottom slurry of the negative electrode, the particle size of the nano-sized pore-forming agent is 50-100 nm; in the top slurry of the negative electrode, the particle size of the micro-sized pore-forming agent is 600-800 nm.

[0024] Furthermore, the nanoscale pore-forming agent is ammonium bicarbonate and / or ammonium oxalate, and the microscale pore-forming agent is starch and / or polyethylene glycol.

[0025] Furthermore, in the dual-chamber linkage coating process, the coating speed of the positive electrode is 6-8 m / min, the coating speed of the negative electrode is 4-6 m / min, and the lateral thickness deviation of the wet film is ≤3%.

[0026] Furthermore, in the three-stage gradient drying, the air dew point of the three-stage drying channel is ≤-40℃, and the moisture content of the electrode after drying is ≤0.05%.

[0027] Furthermore, in the positive electrode, the positive active material is Na4Fe3(PO4)2P2O7; in the negative electrode, the negative active material is hard carbon.

[0028] Furthermore, in segmented gradient rolling, the thickness of the transition zone accounts for 30-50% of the total thickness, the porosity gradient difference of the positive electrode is 14-16%, and the porosity gradient difference of the negative electrode is 24-26%.

[0029] Another aspect of the present invention is to protect a sodium-ion battery, which is formed by stacking or winding the above-mentioned positive and negative electrode sheets.

[0030] This invention discloses a method for preparing a high-rate sodium-ion battery electrode and a sodium-ion battery, which has the following beneficial effects:

[0031] First, it has high ion transport efficiency; the gradient porosity shortens the ion diffusion path by more than 35%, such as... Figure 2 The full cell's 10C rate capacity retention rate increased from 78% to 95%; electrolyte wetting time decreased from 120s to 30s; and the lateral capacity deviation of the electrode was ≤2%.

[0032] Secondly, it has good volume stability. Taking hard carbon anode as an example, after 1000 cycles, the volume expansion rate of hard carbon anode decreased from 2.5% to 0.5%, the electrode cracking rate decreased by 90%, the active material shedding rate was <2%, and the peel strength increased from 0.5N / m to 1.2N / m.

[0033] Third, it has strong process applicability. The dual-chamber coating machine can be implemented by modifying traditional equipment (modification cost < 20% of the original equipment value), and the production efficiency is 40% higher than that of step coating, with a material utilization rate of ≥ 98%.

[0034] Fourth, it exhibits excellent electrochemical performance, with the initial charge-discharge efficiency increasing from 82% to 91%, such as... Figure 3 As shown, the capacity retention rate increased from 87% to 96% after 1000 cycles at 25℃, and the capacity retention rate increased from 78% to 93% at -20℃. Attached Figure Description

[0035] Figure 1 This is a graph showing the relationship between the gradient porosity and electrode thickness of the positive and negative electrode sheets of the present invention.

[0036] Figure 2 This is a comparison chart of the 10C rate discharge of the application embodiment 1 of the present invention and the comparative full cell;

[0037] Figure 3 This is a comparison chart of the application example 1 of the present invention and the comparative example full cell at room temperature 1C cycle. Detailed Implementation

[0038] To enable those skilled in the art to better understand the technical solution of the present invention, the product of the present invention will be further described in detail below with reference to embodiments.

[0039] The present invention discloses a method for preparing a high-rate sodium-ion battery electrode, the sodium-ion battery electrode comprising a positive electrode and a negative electrode, the preparation method comprising the following steps:

[0040] S1. Preparation of the bottom and top slurries of the positive electrode: Add 3-5% nano-sized pore-forming agent to the positive electrode slurry, adjust the viscosity to 4900-5100 mPa.s, and the solid content to 54-56%, and stir evenly to obtain the bottom slurry of the positive electrode; Add 10-15% micron-sized pore-forming agent to the positive electrode slurry, control the viscosity to 4600-5000 mPa.s, and the solid content to 54-56%, and stir evenly to obtain the top slurry of the positive electrode;

[0041] S2. Preparation of the bottom and top slurries of the negative electrode: Add 2-4% nano-sized pore-forming agent to the negative electrode slurry, control the viscosity to 2700-2900 mPa.s, and the solid content to 49-51%; stir evenly to obtain the bottom slurry of the negative electrode; add 15-20% micron-sized pore-forming agent to the negative electrode slurry, control the viscosity to 2400-2600 mPa.s, and the solid content to 49-51%, and stir evenly to obtain the top slurry of the negative electrode;

[0042] S3. Dual-chamber joint coating: Control the flow rate ratio of the bottom layer slurry to the top layer slurry of the positive electrode to be 1:1.3-1.5, and the wet film thickness to be 245-255μm for positive electrode coating; control the flow rate ratio of the bottom layer slurry to the top layer slurry of the negative electrode to be 1:1.5-2, and the wet film thickness to be 225-235μm for negative electrode coating.

[0043] S4. Three-stage gradient drying: Along the thickness direction of the electrode sheet, the bottom layer curing stage is 80-100℃ and 1-2m / s, the transition adjustment stage is 70-80℃ and 2-3m / s, and the surface layer forming stage is 50-70℃ and 3-4m / s. Gradient drying is carried out on the positive and negative electrode sheets respectively.

[0044] S5. Segmented Gradient Rolling: The positive electrode sheet is rolled in segments at pressures ranging from 225 MPa to 18 MPa to 12 MPa to obtain the positive electrode sheet; the negative electrode sheet is rolled in segments at pressures ranging from 25 MPa to 20 MPa to 10 MPa to obtain the negative electrode sheet; along the thickness direction of the electrode sheet, the void gradient of the positive electrode sheet increases from 20% to 35%, and the void gradient of the negative electrode sheet increases from 15% to 40%.

[0045] Furthermore, in the bottom layer slurry of the positive electrode, the particle size of the nano-sized pore-forming agent is 50-100 nm; in the top layer slurry of the positive electrode, the particle size of the micro-sized pore-forming agent is 500-600 nm.

[0046] Furthermore, in the bottom slurry of the negative electrode, the particle size of the nano-sized pore-forming agent is 50-100 nm; in the top slurry of the negative electrode, the particle size of the micro-sized pore-forming agent is 600-800 nm.

[0047] Furthermore, the nanoscale pore-forming agent is ammonium bicarbonate or ammonium oxalate, and the micron-scale pore-forming agent is starch or polyethylene glycol.

[0048] Furthermore, in the dual-chamber linkage coating process, the coating speed of the positive electrode is 6-8 m / min, the coating speed of the negative electrode is 4-6 m / min, and the lateral thickness deviation of the wet film is ≤3%.

[0049] Furthermore, in the three-stage gradient drying, the air dew point of the three-stage drying channel is ≤-40℃, and the moisture content of the electrode after drying is ≤0.05%.

[0050] Furthermore, in the positive electrode, the positive active material is Na4Fe3(PO4)2P2O7; in the negative electrode, the negative active material is hard carbon.

[0051] Furthermore, in segmented gradient rolling, the thickness of the transition zone accounts for 30-50% of the total thickness, the porosity gradient difference of the positive electrode is 14-16%, and the porosity gradient difference of the negative electrode is 24-26%.

[0052] Another aspect of the present invention is to protect a sodium-ion battery, which is formed by stacking or winding the above-mentioned positive and negative electrode sheets.

[0053] In this invention, Figure 1 shows the relationship between electrode thickness (horizontal axis, unit μm) and porosity (vertical axis, unit %), including two fitted curves, which are described in detail below:

[0054] Positive electrode curve: Using "y=0.0101x+0.1909" as the fitting equation, it shows that from the bottom layer (near the current collector, thickness ≈0μm) to the surface layer (near the separator, thickness ≈150μm), the porosity of the positive electrode sheet continuously increases from 20%±1% to 35%±1%, with the gradient difference strictly controlled at 15%±1%.

[0055] Negative electrode curve: Using "y=0.0195x+0.1328" as the fitting equation, it shows that from the bottom layer (thickness≈0μm) to the surface layer (thickness≈132μm), the porosity of the negative electrode sheet continuously increases from 15%±1% to 40%±1%, and the gradient difference is strictly controlled at 25%±1%.

[0056] The above curve analysis shows that:

[0057] 1. Continuity of gradient porosity: Figure 1 The invention demonstrates that through the synergistic process of "composite pore-forming agent - dual-cavity coating - gradient drying - segmented rolling", a continuous porosity gradient without obvious interfaces is formed in the thickness direction of the electrode (the curve is smooth and without abrupt changes). The porosity in the transition zone (thickness ratio of 30%-50%) increases linearly, which solves the interface problem of "abrupt porosity between layers" in the traditional step-by-step coating technology (the gradient difference in Comparative Example 2 is only 9.4% and the curve has an inflection point).

[0058] 2. Differentiated design of positive and negative electrodes: The design clearly reflects the characteristic differences between polyanion positive electrodes (low ion diffusion coefficient) and hard carbon negative electrodes (requiring buffered volume expansion), and different gradient differences (15% for positive electrode and 25% for negative electrode) are customized, which verifies the invention concept of "precise functional adaptation".

[0059] 3. Effectiveness of process control: The curve slope (0.101% / μm for positive electrode and 0.195% / μm for negative electrode) directly corresponds to the process parameters such as "segmented gradient roller pressure gradient" and "dual-cavity coating flow rate ratio" in the examples, proving that multi-process collaboration can achieve precise control of porosity gradient (single batch slope standard deviation < 0.02% / μm).

[0060] Relationship with technical effects:

[0061] The gradient distribution in Figure 1 is the core structural basis for achieving "high ion transport efficiency and good volume stability"—the high porosity of the positive electrode surface shortens the ion diffusion path (increasing the 10C capacity retention rate to 95.3%), and the high porosity of the negative electrode surface buffers volume expansion (reducing the volume expansion rate to 0.87% after 1000 cycles), providing direct structural evidence for the performance data in the specification.

[0062] In this invention, the "segmented gradient rolling" of the positive electrode sheet is achieved through a multi-roller pressure gradient setting during a single continuous rolling process. Specifically, different roller groups of a four-roller press apply pressures of 22 MPa, 18 MPa, and 12 MPa respectively, allowing the electrode sheet to pass through each roller group continuously in one pass, completing the gradient compaction from high pressure to low pressure. The specific implementation method and principle are as follows:

[0063] I. Equipment: Structural Design of a Four-Roll Segmented Pressure Roller Press

[0064] The equipment used for gradient rolling is a four-roll mill (roll diameter 300mm, roll width 400mm), which includes 3 sets of pressure-adjustable rolling units (4 rollers in total), arranged in the following order along the electrode travel direction:

[0065] First roller group (inlet high pressure zone): consists of upper roller 1 and lower roller 1, with a set pressure of 22MPa;

[0066] The second roller group (intermediate medium pressure zone): consists of lower roller 1 and upper roller 2, with a set pressure of 18MPa;

[0067] The third roller group (outlet low-pressure zone): consists of upper roller 2 and lower roller 2, with a set pressure of 12MPa;

[0068] The roller spacing is designed according to the electrode thickness (the thickness of the positive electrode wet film after drying is about 200μm, and the final target thickness is 150μm). The roller speed is synchronized at 5±0.1m / min to ensure that the electrode passes through continuously without stopping.

[0069] II. Gradient Roll Forming Process (Taking Positive Electrode as an Example)

[0070] The electrode sheets pass continuously through the roller press at a speed of 5 m / min, sequentially passing through three pressure zones to complete gradient compaction in one operation.

[0071] 1. First roller group (22MPa, high pressure zone):

[0072] When the electrode enters the first roller group of the roller press, the bottom layer of the electrode (near the current collector) still retains a large amount of undecomposed nano-sized pore-forming agent (ammonium bicarbonate, decomposition temperature 140-160℃). This layer is relatively hard and requires high compaction to ensure the continuity of the conductive network. A high pressure of 22 MPa is applied to the bottom layer, compacting the porosity from 35% after drying to the target 20%, while simultaneously initially fixing the bottom layer structure (nanopores are less prone to collapse). The surface layer (away from the current collector) experiences lower contact pressure at this point (because the electrode has just entered the roller group, the main pressure is concentrated on the bottom layer), and the micron-sized pore-forming agent (starch) is not excessively compressed, retaining some porosity.

[0073] 2. Second roller group (18MPa, medium pressure zone):

[0074] The electrode enters the intermediate roller assembly, where it is applied to the electrode transition zone (a mixed area of ​​the bottom and surface layers, accounting for 30-40% of the total thickness). The pore-forming agent in the transition zone is a mixture of nano- and micro-sized particles (nano-sized particles account for approximately 50%), requiring moderate pressure to achieve a continuous transition in porosity. The moderate pressure of 18 MPa avoids over-compacting the already fixed structure of the bottom layer or prematurely compressing the large pores of the surface layer, allowing the porosity in the transition zone to linearly increase from 20% to 28% (gradient slope 0.1% / μm, deviation <5%). The pressure is uniformly transmitted through the elastic deformation of the rollers, preventing stress concentration in the transition zone (real-time monitoring via online pressure sensors, pressure fluctuation ≤ ±0.5 MPa).

[0075] 3. Third roller group (12MPa, low pressure zone):

[0076] The electrode enters the exit roller assembly, where the pressure is applied to the electrode surface (near the diaphragm side). This surface layer primarily contains micron-sized pore-forming agents (starch, decomposition temperature 200-230℃), requiring high porosity to shorten the ion diffusion path. A low pressure of 12 MPa only lightly compacts the surface layer, fine-tuning the surface porosity from 45% after drying to the target 35%, ensuring that the micron-sized pores (500-600 nm) are not damaged (scanning electron microscopy shows a pore size retention rate >90%). The final electrode thickness is precisely controlled at 150 ± 2 μm, with a continuous porosity gradient between the bottom layer, transition zone, and surface layer (20% → 28% → 35%).

[0077] III. Key Control Parameters:

[0078] Roller parallelism: ≤0.01mm / m (to ensure uniform pressure transmission and no edge overpressure); Pressure feedback frequency: 10Hz (to adjust the roller group pressure in real time and compensate for electrode thickness fluctuations); Electrode tension: 30±5N (to prevent electrode from shifting during rolling, with lateral thickness deviation ≤±2μm).

[0079] In summary, the gradient rolling of the positive electrode is completed in one go through three continuous pressure zones of a four-roll mill. This design ensures the continuity of the porosity gradient and meets the efficiency requirements of industrial production, making it the core process for achieving "low porosity at the bottom layer and high porosity at the surface layer".

[0080] From a technical perspective, this invention addresses the characteristic differences between polyanionic cathodes and hard carbon anodes by designing a differentiated porosity gradient: the cathode increases from 20%±1% to 35%±1% (gradient difference 15%), adapting to its low ion diffusion coefficient to optimize transport efficiency; the anode increases from 15%±1% to 40%±1% (gradient difference 25%), reserving sufficient space to buffer volume expansion. A composite pore-forming agent of nanoscale (50-100nm) and microscale (500-800nm) is used, employing an integrated process of dual-chamber coordinated coating (flow rate ratio 1:1.5-2), three-stage gradient drying (100→85→70℃), and segmented rolling (25→20→10MPa) to form a continuous transition layer (without obvious interfaces) in the electrode thickness direction, with the transition region accounting for 30%-50% of the total thickness.

[0081] Traditional uniform porosity electrodes employ a single porosity design (25%-30%), using a single-particle-size pore-forming agent (e.g., only micron-sized starch). They are prepared through single-cavity coating, single-temperature drying (e.g., 90℃), and fixed-pressure rolling (e.g., 20MPa). While this avoids interface issues, it cannot accommodate the differences in positive and negative electrode characteristics. Step-coated gradient electrodes (e.g., CN115693255A) use a layered design (e.g., 30% bottom layer → 40% top layer), relying on a single pore-forming agent (e.g., ammonium bicarbonate). They are completed through multiple coatings and drying processes, resulting in distinct interfaces between layers and weak adhesion (peel strength <0.8N / m).

[0082] In terms of performance, this invention has significant advantages: such as Figure 2 The full cell shown retains ≥95% capacity at 10C rate, far exceeding traditional uniform porosity electrodes (≤80%) and step-coated electrodes (80%-85%); Figure 3 The full-cell cycle performance is ≥95% after 1000 cycles, which is more than 8% higher than traditional technology (≤88%) and also superior to step-coating technology (85%-91%). The electrolyte wetting time is ≤30s, significantly shorter than traditional electrodes (≥120s) and step-coated electrodes (60-90s). The electrode cracking rate is ≤5%, and the active material shedding rate is ≤2%, while the cracking rate of traditional electrodes is ≥15%, the shedding rate is ≥10%, and the cracking rate of step-coated electrodes is still 8%-12%. In terms of production efficiency, the positive electrode coating speed of this invention is 8m / min and the negative electrode is 6m / min, which is slightly lower than traditional uniform electrodes (10-12m / min), but 40% higher than step-coating technology (4-6m / min), and batch consistency is better (porosity standard deviation <3%).

[0083] The core innovations of this invention are: first, differentiated gradient design, which for the first time customizes a 15% / 25% gradient difference for the positive and negative electrode characteristics to achieve precise functional adaptation; second, integrated preparation process, which forms a continuous transition layer through the synergy of multiple process parameters (flow rate, temperature, pressure gradient) to solve the interface peeling problem of step-by-step coating (peeling strength increased to ≥1.2N / m); and third, multi-scale pore synergy, with nanopores enhancing conductivity (conductivity 10⁻¹S / cm) and micropores accelerating electrolyte wetting (penetration increased by 50%), breaking through the performance limitations of single-scale pores.

[0084] In this invention, by constructing a continuous gradient distribution of "low porosity at the bottom layer and high porosity at the surface layer," the ion diffusion path is shortened by more than 30%, and the 10C rate capacity retention rate is increased to over 95%. Differential gradients are designed for the characteristics of the positive and negative electrodes: a 25% porosity difference is reserved in the hard carbon negative electrode to buffer volume expansion, and a 15% porosity difference is designed in the polyanion positive electrode to optimize ion transport. Continuous gradient preparation is achieved through an integrated process, increasing production efficiency by 40%, and the batch consistency of the electrodes (porosity standard deviation <3%) is superior to traditional processes.

[0085] Taking polyanion cathode materials and hard carbon anode materials as examples, this invention addresses the characteristic differences between polyanion cathodes and hard carbon anodes by achieving a precise porosity gradient distribution along the electrode thickness direction through multi-process synergistic control. The specific technical solution is as follows:

[0086] 1. Core Design of Gradient Porosity Electrodes

[0087] To address the characteristic differences between polyanionic cathodes and hard carbon anodes, a differentiated porosity gradient distribution was designed to achieve functional adaptation of "ion transport-volume stability":

[0088] Polyanion positive electrode: From the current collector side (bottom layer) to the separator side (surface layer), the porosity continuously increases from 20%±1% to 35%±1%, with the gradient difference strictly controlled at 15%±1%; the transition zone thickness accounts for 30%-40% of the total electrode thickness, ensuring no abrupt change in porosity and adapting to its low ion diffusion coefficient (10⁻¹). 0 -10⁻¹²cm² / s) to optimize ion transport.

[0089] Hard carbon negative electrode sheet: from the current collector side (bottom layer) to the separator side (surface layer), the porosity increases continuously from 15%±1% to 40%±1%, and the gradient difference is strictly controlled at 25%±1%; the thickness of the transition zone accounts for 40%-50% of the total thickness of the electrode sheet, and sufficient space is reserved to buffer its 1%-3% volume expansion.

[0090] 2. Raw material system and formulation design

[0091] (1) Positive electrode slurry (polyanionic system)

[0092] Raw material specifications: Active material: Na4Fe3(PO4)2P2O7 (purity ≥99.5%, particle size D50=5-6μm);

[0093] Conductive agent: carbon nanotubes (diameter 8-10nm, length 5-10μm, purity ≥98%), Super P (particle size D50=40-50nm); binder: PVDF (molecular weight 1,000,000±50,000, purity ≥99%).

[0094] Pore-forming agents: nano-sized ammonium bicarbonate (particle size 50-100nm, purity ≥99%), micron-sized starch (particle size 500-600nm, moisture ≤10%); solvent: NMP (purity ≥99.9%, moisture ≤50ppm).

[0095] Formula (parts by weight): Bottom slurry (Group A): 93 parts Na4Fe3(PO4)2P2O7, 2 parts carbon nanotubes, 2 parts Super P, 3 parts PVDF, 5 parts nano-sized ammonium bicarbonate; Top slurry (Group B): 93 parts Na4Fe3(PO4)2P2O7, 2 parts carbon nanotubes, 2 parts Super P, 3 parts PVDF, 15 parts micron-sized starch.

[0096] (2) Negative electrode slurry (hard carbon system)

[0097] Raw material specifications: Active material: Hard carbon (specific surface area 6.5±1m² / g, particle size D50=5-8μm);

[0098] Conductive agent: Ketjen Black (ECP600JD, DBP oil absorption 360±20mL / 100g); Binder: CMC (molecular weight 250000±20000), SBR (solid content 50%±2%, particle size D50=100-200nm);

[0099] Pore-forming agents: nano-sized ammonium oxalate (particle size 50-100nm, purity ≥99%), micron-sized polyethylene glycol (PEG, molecular weight 6000, particle size 600-800nm); solvent: deionized water (resistivity ≥18.2MΩ・cm).

[0100] (3) Formulation (parts by weight): Bottom layer slurry (Group C): 92 parts hard carbon, 3 parts Kejen black, 2 parts CMC, 3 parts SBR, 3 parts nano-sized ammonium oxalate; Top layer slurry (Group D): 92 parts hard carbon, 3 parts Kejen black, 2 parts CMC, 3 parts SBR, 20 parts micron-sized PEG.

[0101] 3. Core Process Collaboration Mechanism

[0102] Through the coordinated process of "composite pore-forming agent - dual-cavity coating - gradient drying - segmented rolling", precise control of gradient porosity is achieved.

[0103] Nanoscale pore-forming agents create small pores of 50-100 nm in the bottom layer to ensure the continuity of the conductive network; micron-scale pore-forming agents create large pores of 500-800 nm in the surface layer to accelerate electrolyte wetting.

[0104] The flow rate ratio of dual-cavity coating (1:1.5-2), the temperature / wind speed gradient of gradient drying (100→85→70℃, 1.5→2.5→3.5m / s), and the pressure gradient of segmented roller pressing (25→20→10MPa) are matched in a coordinated manner to ensure continuous change of porosity in the transition zone and no interface stress concentration.

[0105] The method for fabricating gradient porosity electrodes includes the following steps:

[0106] Step 1: Preparation of functional slurry

[0107] (1) Preparation of positive electrode slurry

[0108] Group A (bottom layer):

[0109] ① Dry mixing: Add 93 parts of Na4Fe3(PO4)2P2O7, 2 parts of carbon nanotubes, and 2 parts of Super P to a 5L double planetary mixer and dry mix at 1000±50r / min for 30±2min (mixing uniformity ≥95%, verified by laser particle size analyzer).

[0110] ② Wet mixing: Add NMP solution containing 3 parts PVDF (PVDF:NMP=1:20, mass ratio), adjust the speed to 2000±100r / min, and stir for 120±5min;

[0111] ③ Addition of pore-forming agent: Add 5 parts of nano-grade ammonium bicarbonate, continue stirring for 120±5 min, and adjust the viscosity to 5000±100 mPa・s using a rotational viscometer (25℃, 60 r / min), with the solid content controlled at 55%±1%.

[0112] Group B (surface layer):

[0113] Follow the procedure in Group A, replacing 5 parts of nano-sized ammonium bicarbonate with 15 parts of micron-sized starch, and adjust the viscosity.

[0114] Up to 4800±100mPa・s, solid content 55%±1%.

[0115] (2) Preparation of negative electrode slurry

[0116] Group C (bottom level):

[0117] ① Dry mixing: Add 92 parts hard carbon and 2 parts Ketjen black to a 5L double planetary mixer and dry mix at 800±50r / min for 40±2min;

[0118] ② Wet mixing: Add deionized water, then add 2 parts CMC and 3 parts SBR in sequence, adjust the speed to 1500±100r / min, and stir for 120±5min;

[0119] ③ Addition of pore-forming agent: Add 3 parts of nano-grade ammonium oxalate, continue stirring for 90±5 min, adjust the viscosity to 2800±100 mPa・s (25℃, 60 r / min), and the solid content is 50%±1%.

[0120] Group D (surface layer):

[0121] Following the procedure in Group C, replace 3 parts of nano-sized ammonium oxalate with 20 parts of micron-sized PEG, and adjust the viscosity.

[0122] Temperature up to 2500±100 mPa・s, solid content 50%±1%.

[0123] Step 2: Dual-chamber coordinated coating

[0124] A custom-designed dual-chamber slit coater (5L / chamber, 300mm nozzle width, equipped with an online laser thickness gauge (accuracy ±1μm) and a viscosity monitor (accuracy ±5mPa·s)) is used:

[0125] Positive electrode coating:

[0126] The flow rate of the bottom layer slurry (Group A) is 10±0.5 mL / min, and the flow rate of the top layer slurry (Group B) is 15±0.5 mL / min (flow rate ratio 1:1.5); the coating speed is 8±0.2 m / min, forming a 250±5 μm wet film on a 15 μm carbon-coated aluminum foil (surface roughness Ra=0.3-0.5 μm), with a transverse thickness deviation ≤3% (adjusted in real time by laser thickness gauge).

[0127] Negative electrode coating:

[0128] The flow rate of the bottom layer slurry (Group C) was 8±0.5 mL / min, and the flow rate of the top layer slurry (Group D) was 16±0.5 mL / min (flow rate ratio 1:2); the coating speed was 6±0.2 m / min, forming a 230±5 μm wet film on a 12 μm aluminum foil (surface roughness Ra=0.2-0.3 μm), with a transverse thickness deviation ≤3%.

[0129] Key control: High-speed camera monitoring of the coating nozzle outlet ensures that the two slurries form a molecular-level mixing transition zone with no obvious interface stratification.

[0130] Step 3: Three-stage gradient drying

[0131] It adopts a 6m long, three-section, independently temperature-controlled drying channel (each section is 2m long and equipped with a wind speed sensor (accuracy ±0.1m / s) and a temperature sensor (accuracy ±0.5℃)):

[0132] Drying parameters:

[0133] Bottom curing section: temperature 100±2℃, wind speed 1.5±0.1m / s, dry air with dew point ≤-40℃ is introduced to quickly evaporate the solvent and lock in nanoscale pores; Transition adjustment section: temperature 85±2℃, wind speed 2.5±0.1m / s, to achieve continuous transition of porosity; Surface forming section: temperature 70±2℃, wind speed 3.5±0.1m / s, to promote the aggregation of micron-sized pore-forming agent to form large pores.

[0134] Drying endpoint:

[0135] The electrode moisture content is ≤0.05% (detected by Karl Fischer moisture analyzer), and the transverse shrinkage rate is ≤1%.

[0136] Step 4: Segmented gradient roller pressing

[0137] A four-roll segmented pressure roller press (roll diameter 300mm, roll width 400mm, equipped with a pressure feedback control system (accuracy ±0.1MPa) and an online thickness monitoring instrument (accuracy ±0.5μm)) is used:

[0138] The rolling pressure gradient (positive electrode 22→12MPa, negative electrode 25→10MPa) is coupled with the degree of pore-forming agent decomposition:

[0139] High-pressure zone (inlet): Compact the undecomposed nanoporous agent in the bottom layer, reducing the porosity from 45% to 20% of the wet film;

[0140] Medium pressure zone (transition): Partially decomposed pore-forming agent forms compressible pores, and the porosity gradient slope is controlled at 0.3% / μm (deviation <5%).

[0141] Low-pressure zone (outlet): The surface layer retains large pores that have been completely decomposed, with a porosity of 35%-40%.

[0142] Positive electrode rolling:

[0143] Pressure gradient: inlet 22±0.5MPa → middle 18±0.5MPa → outlet 12±0.5MPa; roller speed 5±0.1m / min, final thickness 150±2μm (pressure is adjusted in real time through thickness feedback).

[0144] Negative electrode rolling:

[0145] Pressure gradient: inlet 25±0.5MPa → middle 20±0.5MPa → outlet 10±0.5MPa; roller speed 4±0.1m / min, final thickness 132±2μm.

[0146] Porosity verification: A mercury porosimeter (pore size range 0.003-360μm) was used to sample and test every 10μm along the thickness direction. The slope k was calculated as (surface porosity - bottom layer porosity) / total thickness. The required values ​​were k = 0.1% / μm (positive electrode) and 0.19% / μm (negative electrode). The standard deviation of the slope for a single batch was <0.02% / μm. The following conditions were ensured: Positive electrode bottom layer 20%±1%, surface layer 35%±1% (gradient difference 15%±1%); Negative electrode bottom layer 15%±1%, surface layer 40%±1% (gradient difference 25%±1%).

[0147] The key material composition of this invention is described below:

[0148] 1. Cathode material system

[0149] Active material: Na4Fe3(PO4)2P2O7, purity ≥99.5%, particle size D50=5-6μm, specific surface area 10-15m² / g

[0150] Conductive agent: Carbon nanotubes: diameter 8-10nm, length 5-10μm, purity ≥98%, CNT content ≥95%.

[0151] Super P: Particle size D50 = 40-50nm, specific surface area 62±5m² / g

[0152] Adhesive: PVDF (Kynar HSV900), molecular weight 1,000,000 ± 50,000

[0153] Pore-forming agent: Nano-sized ammonium bicarbonate: particle size 50-100nm, decomposition temperature 140-160℃

[0154] Micron-sized starch: particle size 500-600nm, moisture content ≤10%, decomposition temperature 200-230℃

[0155] 2. Anode material system

[0156] Active material: Hard carbon, specific surface area 6.5±0.5m² / g, particle size D50=5-6μm, initial charge / discharge efficiency ≥90%; Conductive agent: Ketjen Black (ECP600JD), DBP oil absorption 360±20mL / 100g, specific surface area 1270±50m² / g; Binder: CMC molecular weight 250000±20000, degree of substitution 0.9-1.2; SBR: solid content 50%±2%, particle size D50=100-200nm

[0157] Pore-forming agent:

[0158] Nano-sized ammonium oxalate: particle size 50-100nm, decomposition temperature 160-180℃; Micron-sized PEG: molecular weight 6000, particle size 600-800nm, melting point 58-62℃

[0159] 3. Current collector

[0160] Positive electrode: 15μm carbon-coated aluminum foil, surface roughness Ra=0.3-0.5μm, tensile strength ≥200MPa

[0161] Negative electrode: 12μm aluminum foil, roughness Ra=0.2-0.3μm, tensile strength ≥150Mpa

[0162] The principle of this invention is as follows:

[0163] By employing gradient porosity design and synergistic control of multiple processes, the ion transport bottleneck and volume change stress issues of sodium-ion battery electrodes are addressed.

[0164] 1. Gradient porosity adapts to material properties

[0165] Polyanionic cathode: The low porosity (20%) of the bottom layer ensures the continuity of the conductive network, while the high porosity (35%) of the surface layer shortens the ion diffusion path, thereby improving the ion transport efficiency by more than 30%.

[0166] Hard carbon anode: The high porosity (40%) of the surface layer allows for volume expansion space, while the low porosity (15%) of the bottom layer enhances structural stability, reducing volumetric stress during cycling by approximately 40%.

[0167] 2. Synergistic effect of composite pore-forming agents

[0168] Nanoscale pore-forming agents (ammonium bicarbonate / ammonium oxalate) create 50-100 nm pores in the bottom layer, optimizing the electron conduction path (conductivity increased to 10⁻¹S / cm); micron-scale pore-forming agents (starch / PEG) create 500-800 nm pores on the surface layer, accelerating electrolyte wetting (permeability increased by 50%); the transition zone achieves a smooth transition of the porosity gradient (gradient slope deviation <5%) through continuous variation of the pore-forming agent ratio.

[0169] 3. Coordinated control of multiple process parameters

[0170] The flow rate ratio of the dual-cavity coating (1:1.5-2) determines the starting point of the gradient; the gradient drying temperature (100→70℃) and wind speed (1.5→3.5m / s) regulate the decomposition rate of the pore-forming agent and the evaporation path of the solvent; the segmented roller pressure (25→10MPa) precisely locks the porosity of different areas to avoid over-compaction or under-compaction.

[0171] The product of this invention has been verified through examples to meet the performance testing requirements of different types of sodium-ion batteries.

[0172] Application Example 1

[0173] This embodiment relates to positive and negative electrode sheets prepared using the method of the present invention, and the preparation and testing process includes the following steps:

[0174] 1. Slurry preparation:

[0175] Positive electrode bottom layer slurry (Group A): Weigh 93 parts Na4Fe3(PO4)2P2O7, 2 parts carbon nanotubes, and 2 parts SuperP by mass, add them to a double planetary mixer and dry mix at 1000 r / min for 30 min; add NMP solution containing 3 parts PVDF and stir at 2000 r / min for 120 min; add 5 parts nano-sized ammonium bicarbonate and continue stirring for 120 min, adjusting the viscosity to 5000 mPa·s.

[0176] Positive electrode surface slurry (Group B): The process is the same as Group A, except that ammonium bicarbonate is replaced with 15 parts of micron-sized starch and the viscosity is adjusted to 4800 mPa·s.

[0177] Negative electrode bottom layer slurry (Group C): Weigh 92 parts hard carbon and 2 parts Ketjen black by weight, add them to a double planetary mixer and dry mix at 800 r / min for 40 min; add deionized water, then add 2 parts CMC and 3 parts SBR in sequence, and stir at 1500 r / min for 120 min; add 3 parts nano-sized ammonium oxalate, continue stirring for 90 min, and adjust the viscosity to 2800 mPa·s.

[0178] Negative electrode surface slurry (Group D): The process is the same as Group C, except that ammonium oxalate is replaced with 20 parts of micron-sized PEG and the viscosity is adjusted to 2500 mPa·s.

[0179] 2. Dual-cavity coating

[0180] The flow rate of the bottom layer of the positive electrode is 10 mL / min, the flow rate of the surface layer is 15 mL / min, and the coating speed is 8 m / min, forming a 250 μm wet film on a 15 μm carbon-coated aluminum foil.

[0181] The flow rate of the negative electrode bottom layer is 8 mL / min, the flow rate of the surface layer is 16 mL / min, and the coating speed is 6 m / min, forming a 230 μm wet film on a 12 μm aluminum foil.

[0182] 3. Gradient drying

[0183] Positive electrode, first stage: temperature 100℃, wind speed 1.5m / s, drying time 1min; second stage: temperature 85℃, wind speed 2.5m / s, drying time 1min; third stage: temperature 70℃, wind speed 3.5m / s, drying time 1min.

[0184] Negative electrode, first stage: temperature 90℃, wind speed 1.5m / s, drying time 1.5min; second stage: temperature 75℃, wind speed 2.5m / s, drying time 1.5min; third stage: temperature 60℃, wind speed 3.5m / s, drying time 2min.

[0185] 4. Segmented roller pressing

[0186] Positive electrode pressure gradient: 22MPa→18MPa→12MPa, roller speed 5m / min, final thickness 150μm.

[0187] Negative electrode pressure gradient: 25MPa→20MPa→10MPa, roller speed 4m / min, final thickness 132μm.

[0188] 5. Performance Testing

[0189] The positive electrode has a bottom layer porosity of 20.3% and a surface layer porosity of 34.8%, with a gradient difference of 14.5%; the negative electrode has a bottom layer porosity of 14.8% and a surface layer porosity of 39.5%, with a gradient difference of 24.7%. Figure 2 The full cell shown retains 95.3% of its capacity at 10C rate. Figure 3 The capacity retention rate after 1000 cycles is 96.75%, the volume expansion rate is 0.87%, and the electrode cracking rate is 2.3%.

[0190] Comparative Example 1

[0191] This embodiment involves conventional uniform porosity positive and negative electrode sheets, and their preparation and testing process includes the following steps:

[0192] 1. Slurry preparation

[0193] The positive electrode is prepared by weighing 93 parts by weight of Na4Fe3(PO4)2P2O7, 2 parts of carbon nanotubes, 2 parts of Super P, and 3 parts of PVDF, and adding 10 parts of ammonium bicarbonate (particle size 200-300nm) to prepare a slurry with a single porosity.

[0194] The negative electrode is prepared by weighing 92 parts hard carbon, 2 parts Ketjen black, 2 parts CMC, and 3 parts SBR by mass, and adding 20 parts micron-sized PEG (600-800nm) to prepare a single porosity material.

[0195] 2. Coating and drying

[0196] The positive electrode was coated in a single cavity with a wet film thickness of 250 μm and dried at 100 °C for 3 min.

[0197] The negative electrode was coated in a single cavity with a wet thickness of 230 μm and dried at 70 °C for 3 min.

[0198] 3. Roller pressing

[0199] The positive electrode is fixed at a pressure of 20 MPa, and the final thickness is 150 μm.

[0200] The negative electrode is fixed at a pressure of 18 MPa, and the final thickness is 132 μm.

[0201] 4. Performance Testing

[0202] Positive electrode porosity 28.5%±1.2%; negative electrode porosity 30.4%±1.2%; full cell 10C rate capacity retention 78.7%; 1000-cycle capacity retention 87.79%; volume expansion 3.69%.

[0203] Comparative Example 2

[0204] This embodiment involves the stepwise coating of gradient positive and negative electrode sheets (refer to CN115693255A), and its preparation and testing process includes the following steps:

[0205] 1. Undercoat

[0206] The positive electrode slurry contains 93 parts Na4Fe3(PO4)2P2O7, 2 parts carbon nanotubes, 2 parts Super P, 3 parts PVDF, and 5 parts ammonium bicarbonate. The coating thickness is 100 μm, and it is dried at 80℃ for 1 min.

[0207] The negative electrode slurry contains 92 parts hard carbon, 2 parts Ketjen black, 2 parts CMC, 3 parts SBR, and 3 parts nano-sized ammonium oxalate. The coating thickness is 105 μm, and it is dried at 70℃ for 1 min.

[0208] 2. Surface coating

[0209] The positive electrode slurry contains 93 parts Na4Fe3(PO4)2P2O7, 2 parts carbon nanotubes, 2 parts Super P, 3 parts PVDF, and 15 parts ammonium bicarbonate. The coating thickness is 150 μm, and it is dried at 80℃ for 2 min.

[0210] The negative electrode slurry contains 92 parts hard carbon, 2 parts Ketjen black, 2 parts CMC, 3 parts SBR, and 20 parts micron-sized PEG. The coating thickness is 125 μm, and it is dried at 70℃ for 2 min.

[0211] 3. Roller pressing

[0212] The positive electrode is fixed at a pressure of 20 MPa, resulting in a final thickness of 150 μm; the negative electrode is fixed at a pressure of 18 MPa, resulting in a final thickness of 132 μm.

[0213] 4. Performance Testing

[0214] The bottom layer porosity of the positive electrode is 29.3%, the surface porosity is 38.7%, and the gradient difference is 9.4%; the bottom layer porosity of the negative electrode is 15.1%, the surface porosity is 39.3%, and the gradient difference is 24.2%; the full cell has a 10C rate capacity retention of 83.5%, a 1000-cycle capacity retention of 90.28%, and a volume expansion of 2.33%.

[0215] The positive and negative electrode sheets of the above embodiments were assembled with separators, electrolytes, and membrane shells under the same process conditions and battery testing conditions. The comparison results of specific embodiments are shown in Table 1 and... Figure 2-3 As shown:

[0216] Table 1 Performance Test Results

[0217] Instance number Electrolyte wetting time (titration method) 10C rate discharge capacity retention 1000-cycle capacity retention @ 1C Battery volume expansion ratio after 1000 cycles Application Example 1 26s 95.3% 96.75% 0.87% Comparative Example 1 133s 78.7% 87.79% 3.69% Comparative Example 2 72s 83.5% 90.28% 2.33%

[0218] From Table 1, Figure 2-3 As can be seen, by controlling the porosity in the thickness direction of the electrode, the overall performance of sodium-ion batteries can be improved.

[0219] The above embodiments are merely specific examples of the present invention, and their descriptions are quite specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these obvious substitutions all fall within the protection scope of the present invention.

Claims

1. A method for preparing a high-rate sodium-ion battery electrode, the sodium-ion battery electrode comprising a positive electrode and a negative electrode, characterized in that... The preparation method includes the following steps: S1. Preparation of the bottom and top slurries of the positive electrode: Add 3-5% nano-sized pore-forming agent to the positive electrode slurry, adjust the viscosity to 4900-5100 mPa.s, and the solid content to 54-56%, and stir evenly to obtain the bottom slurry of the positive electrode; Add 10-15% micron-sized pore-forming agent to the positive electrode slurry, control the viscosity to 4600-5000 mPa.s, and the solid content to 54-56%, and stir evenly to obtain the top slurry of the positive electrode; S2. Preparation of the bottom and top slurries of the negative electrode: Add 2-5% nano-sized pore-forming agent to the negative electrode slurry, control the viscosity to 2700-2900 mPa.s, and the solid content to 49-51%; stir evenly to obtain the bottom slurry of the negative electrode; add 15-20% micron-sized pore-forming agent to the negative electrode slurry, control the viscosity to 2400-2600 mPa.s, and the solid content to 49-51%, and stir evenly to obtain the top slurry of the negative electrode; S3. Dual-chamber joint coating: Control the flow rate ratio of the bottom layer slurry to the top layer slurry of the positive electrode to be 1:1.3-1.5, and the wet film thickness to be 245-255μm for positive electrode coating; control the flow rate ratio of the bottom layer slurry to the top layer slurry of the negative electrode to be 1:1.5-2, and the wet film thickness to be 225-235μm for negative electrode coating. S4. Three-stage gradient drying: Along the thickness direction of the electrode sheet, the bottom layer curing stage is 80-100℃ and 1-2m / s, the transition adjustment stage is 70-80℃ and 2-3m / s, and the surface layer forming stage is 50-70℃ and 3-4m / s. Gradient drying is carried out on the positive and negative electrode sheets respectively. S5. Segmented Gradient Rolling: The positive electrode sheet is rolled in segments at pressures ranging from 22 MPa to 18 MPa to 12 MPa to obtain the positive electrode sheet; the negative electrode sheet is rolled in segments at pressures ranging from 25 MPa to 20 MPa to 10 MPa to obtain the negative electrode sheet; along the thickness direction of the electrode sheet, the void gradient of the positive electrode sheet increases from 20% to 35%, and the void gradient of the negative electrode sheet increases from 15% to 40%.

2. The method for preparing a high-rate sodium-ion battery electrode according to claim 1, characterized in that: In the bottom layer slurry of the positive electrode, the particle size of the nano-sized pore-forming agent is 50-100nm; in the top layer slurry of the positive electrode, the particle size of the micro-sized pore-forming agent is 500-600nm.

3. The method for preparing a high-rate sodium-ion battery electrode according to claim 1, characterized in that: In the bottom slurry of the negative electrode, the particle size of the nano-sized pore-forming agent is 50-100 nm; in the top slurry of the negative electrode, the particle size of the micro-sized pore-forming agent is 600-800 nm.

4. The method for preparing a high-rate sodium-ion battery electrode according to claim 1, characterized in that: The nanoscale pore-forming agent is ammonium bicarbonate or ammonium oxalate, and the micron-scale pore-forming agent is starch or polyethylene glycol.

5. The method for preparing a high-rate sodium-ion battery electrode according to claim 1, characterized in that: In the dual-chamber joint coating process, the coating speed of the positive electrode is 6-8 m / min, the coating speed of the negative electrode is 4-6 m / min, and the lateral thickness deviation of the wet film is ≤3%.

6. The method for preparing a high-rate sodium-ion battery electrode according to claim 1, characterized in that: In the three-stage gradient drying process, the air dew point of the three-stage drying channel is ≤-40℃, and the moisture content of the electrode after drying is ≤0.05%.

7. The method for preparing a high-rate sodium-ion battery electrode according to claim 1, characterized in that: In the positive electrode, the positive active material is Na4Fe3(PO4)2P2O7; in the negative electrode, the negative active material is hard carbon.

8. The method for preparing a high-rate sodium-ion battery electrode according to claim 1, characterized in that: In segmented gradient rolling, the thickness of the transition zone accounts for 30-50% of the total thickness, the porosity gradient difference of the positive electrode is 14-16%, and the porosity gradient difference of the negative electrode is 24-26%.

9. A sodium-ion battery, characterized in that: The positive electrode or negative electrode is formed by stacking or winding any one of the claims 1-8.

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