Method for determining specific surface area of NFPP material based on cyclic voltammetry and application thereof

Abstract

The invention belongs to the technical field of sodium ion battery positive electrode material characterization, and relates to an NFPP material specific surface area determination method based on cyclic voltammetry and application thereof. The method comprises the following steps: adopting a three-electrode system (provided with a working electrode, a counter electrode and a reference electrode), adopting a 1mol / L NaClO4 / EC: PC organic solution as an electrolyte, scanning at the rate of 5-100mV / s in a potential interval of 2.0-3.8 V, and keeping the interval between adjacent rates of 5-10mV / s; and obtaining a double-electric-layer capacitance difference value delta Cdl of adjacent rates through current-rate linear fitting, and calculating the specific surface area by utilizing a formula (1). According to the method, direct association of slurry state-electrode performance is proposed for the first time, a dynamic capacitance response model is innovatively constructed, delta Cdl / delta v (delta v is a scanning rate interval, and delta Cdl is a double-electrode-layer capacitance difference value) is adopted to replace static double-electrode-layer capacitance Cdl, and an active surface area quantification method matched with full-battery charging and discharging rate response is established.

Description

Technical Field

[0001] The present invention belongs to the technical field of characterization of positive electrode materials for sodium ion batteries, and relates to a method for determining the specific surface area of NFPP materials based on cyclic voltammetry and an application thereof. Background Art

[0002] In the field of battery technology, polyanionic composite sodium iron phosphate (NFPP) material is one of the core materials of the positive electrode system of sodium ion batteries. Its electrochemically active specific surface area is a key parameter that determines the ion transfer efficiency and charge transfer kinetics of the electrode reaction interface. In the early research stage, material characterization technology mainly relied on the nitrogen adsorption BET method based on the physical adsorption principle. This method calculates the total specific surface area by measuring the nitrogen adsorption amount of the material powder under low temperature conditions, providing a preliminary characterization method for material research and development. However, as sodium ion battery technology develops towards high energy density and long cycle life, the influence of electrode microstructure on the utilization rate of active materials has become increasingly prominent, and the limitations of the traditional BET method have gradually been exposed: on the one hand, this method can only reflect the physical surface area of the material particles in a static stacking state, and cannot distinguish the effective active area where the surface of the material particles actually contacts the electrolyte; on the other hand, its 8-12 hour test cycle requires complex steps such as powder degassing pretreatment and multi-pressure point adsorption equilibrium, which makes it difficult to adapt to the rapid feedback and dynamic adjustment requirements of process parameters in industrial production. Taking the static double layer capacitance method (SSA = k·Cdl / m) as an example, it has the following drawbacks when measuring the specific surface area of electrode materials:

[0003] (1) Electrode structure distortion: Depends on roller pressing process (compaction density ≥ 2.0g / cm 3 ), resulting in a decrease in electrode porosity from 45% to below 25% and a 30%-50% reduction in active surface exposure;

[0004] (2) Disconnection of electrolyte system: KOH aqueous electrolyte is commonly used, which triggers hydroxylation reaction on the surface of NFPP, resulting in increased interfacial impedance, and the operating conditions are significantly different from those of NaClO4 organic electrolyte actually used in sodium ion batteries;

[0005] (3) Static formula error: Cdl at a single scan rate (such as 50 mV / s) is affected by ion diffusion hysteresis. At high scan rates, the capacitance false high rate reaches 18%, and the repeatability RSD is greater than 5% (comparative example of the present invention).

[0006] In the field of electrode slurry process optimization, existing technologies have long relied on process parameter setting models based on empirical formulas or trial-and-error methods. Taking the amount of conductive agent added as an example, conventional methods usually fix the ratio according to the total mass percentage of the material, ignoring the impact of fluctuations in the active specific surface area of different batches of materials on the efficiency of conductive network construction, resulting in blockage of electrode pores when the conductive agent is added excessively, and discontinuous charge transfer paths when it is not added enough. At the same time, the setting of dispersion time and dispersant dosage lacks a quantitative correlation with the active surface area of the material, making it impossible to accurately control the degree of disaggregation of particle agglomerates during slurry dispersion, ultimately leading to uneven distribution of active substances and conductive agents in the electrode coating, increased tortuosity of ion diffusion channels and other problems. These process defects are directly manifested as increased electrode polarization and reduced utilization of active substances, which in turn restrict the overall energy density and cycle stability of sodium-ion batteries.

[0007] The existing technology system has not yet effectively solved the problem of coordinated optimization between the intrinsic properties of materials and process parameters. Although some studies have attempted to indirectly evaluate the interface characteristics of electrode reactions through electrochemical impedance spectroscopy or static double-layer capacitance, these methods still have the defects of cumbersome testing procedures, complex data processing, and difficulty in directly correlating process parameters. In addition, traditional slurry process parameter optimization mostly adopts a single variable adjustment strategy, and fails to establish a dynamic response model between multiple factors such as active specific surface area and conductive agent dispersion, slurry rheological properties, etc., resulting in a long process optimization cycle and high cost. Therefore, the development of a technical solution that can in situ characterize the true electrochemical active surface area of the material and establish a quantitative control method for process parameters based on this has become an important direction for breaking through the current performance bottleneck of sodium-ion batteries.

[0008] In addition, on the one hand, there are differences in understanding among those skilled in the art; on the other hand, the inventor studied a large number of documents and patents when making the present invention, but due to space limitations, not all details and contents are listed in detail. However, this does not mean that the present invention does not have the characteristics of these prior arts. On the contrary, the present invention already has all the characteristics of the prior art, and the applicant reserves the right to add relevant prior art to the background art. Summary of the Invention

[0009] Based on the above technical problems, one of the objectives of the present invention is to provide a method for determining the specific surface area of NFPP materials based on cyclic voltammetry (CV) and its application, so as to overcome the shortcomings of the prior art.

[0010] To achieve the above object, the present invention adopts the following technical solutions:

[0011] A method for determining the specific surface area of NFPP materials based on cyclic voltammetry comprises the following steps:

[0012] A three-electrode system (working electrode, counter electrode, and reference electrode) was used, the electrolyte was 1 mol / L NaClO4 / EC:PC organic solution, and the potential range was 2.0-3.8 V at a scanning rate of 5-100 mV / s, with adjacent rate intervals of 5-10 mV / s.

[0013] The double layer capacitance difference ΔCdl at adjacent rates is obtained by current-rate linear fitting, and the specific surface area is calculated using formula (1):

[0014]

[0015] Where m is the mass of NFPP in the electrode (g), ΔCdl is obtained by fitting the curve of current versus potential at adjacent rates (CV curve), and ΔV is the interval between adjacent CV scan rates.

[0016] According to a preferred embodiment, the double layer capacitance difference ΔCdl is obtained by CV curve fitting, and the current-rate linear correlation coefficient R in the fitting interval of the CV curve is 2 Not less than 0.90.

[0017] According to a preferred embodiment, the coating thickness of the electrode is 80-300 μm, and the loading is 2.0±0.2 mg / cm 2 , surface density is 175~185g / m 2 .

[0018] According to a preferred embodiment, the working electrode is a NFPP electrode, and the load mass m is 2.0±0.2 mg / cm 2 Preferably, the edge of the NFPP pole piece is sealed with polytetrafluoroethylene tape, exposing only the active coating. The diameter of the NFPP pole piece is 10 mm and the area is 0.785 cm 2 .

[0019] According to a preferred embodiment, the NFPP pole piece comprises an active material, NFPP (composite sodium ferric phosphate), a binder, PVDF (polyvinylidene fluoride), a conductive agent, SuperP (conductive carbon black), and a dispersant, PVP (polyvinyl pyrrolidone). The mass ratio of the NFPP pole piece components is NFPP:PVDF:SuperP:PVP = (90-95):(1-5):(1-5):(0.1-0.5). The solids content of the NFPP pole piece slurry is 60% ± 2% by mass.

[0020] According to a preferred embodiment, the reference electrode is Ag / AgCl or Hg / HgO. Preferably, the reference electrode is filled with a 3 mol / L NaCl solution. Preferably, the potential drift of the reference electrode is no greater than ±1.5 mV / h.

[0021] According to a preferred embodiment, the counter electrode is a stainless steel mesh or a spiral platinum wire. Preferably, the electrode is made of 316L material, with a pore size of 50 μm and an area of 5 cm 2 .

[0022] According to a preferred embodiment, NaClO4, EC (ethylene carbonate) and PC (propylene carbonate) are prepared in equal volumes in the electrolyte.

[0023] One of the objectives of the present invention is to provide the application of the above-mentioned method for determining the specific surface area of NFPP materials based on cyclic voltammetry in optimizing the homogenization process and predicting battery performance.

[0024] One of the purposes of the present invention is to provide a homogenization process for a sodium ion battery positive electrode material, the homogenization process comprising the following steps:

[0025] 0.5wt% conductive agent and 0.1wt% dispersant are blended;

[0026] Stir at low speed of 200 rpm for 30 min;

[0027] High-speed dispersion at 2500rpm for 20 to 160 minutes, with a solid content of 55% to 65%;

[0028] Rolling pressure 15MPa, active material loading 2~5mg / cm 2 .

[0029] Preferably, the four components are mixed according to the mass ratio of active material: binder: conductive agent: dispersant = (90-95): (1-5): (1-5): (0.1-0.5).

[0030] According to a preferred embodiment, the sodium ion battery positive electrode material obtained based on the above-mentioned homogenization process has an initial efficiency of not less than 85% at 0.5C; a cycle retention rate of not less than 80% after 500 cycles; a slurry viscosity fluctuation range of ±5%; and a particle agglomeration rate of not more than 5%.

[0031] One of the objects of the present invention is to provide a method for preparing a NFPP pole piece, which comprises the following steps:

[0032] The material obtained by the above homogenization process was vacuum dried at 80℃ for 12h and had a compaction density of 1.2-1.6 g / cm 3 .

[0033] According to a preferred embodiment, the NFPP pole piece load mass is 2.0±0.2 mg / cm 2 Preferably, the edge of the NFPP pole piece is sealed with polytetrafluoroethylene tape, exposing only the active coating. The diameter of the NFPP pole piece is 10 mm and the area is 0.785 cm 2The loading amount of active material refers to the loading amount of NFPP per unit area on the positive electrode sheet.

[0034] The technical solution of the present invention has the following four beneficial effects.

[0035] (1) In-situ characterization technology separated from roller-pressed electrode technology

[0036] Departing from the roller-pressed electrode technology, the technology provided by the present invention can maintain the original porosity of the slurry (45% ± 3%), avoid the physical covering effect of the active surface caused by the roller-pressing process, and increase the electrolyte infiltration depth by 83%, providing a direct guide for the quantitative optimization of process parameters such as the dispersion of the conductive agent and the rheological properties of the slurry. In addition, this technology can also completely retain the original pore structure of the homogenized slurry after solidification (the porosity deviation is controlled within 5%), breaking through the physical shielding effect of the traditional roller-pressing process on the active area of the electrode surface, and increasing the effective electrolyte infiltration depth from 30μm in the conventional process to 55μm, realizing the precise mapping of the intrinsic wetting characteristics of the electrode slurry and the performance of the finished electrode.

[0037] (2) Constructing a dynamic model for in-situ characterization of the true electrochemically active surface area of the material that can match the homogenate

[0038] For the first time, a direct correlation between "slurry state and electrode performance" was proposed, and a dynamic capacitance response model was innovatively constructed. ΔCdl / Δv (Δv is the scan rate interval, ΔCdl is the double-layer capacitance difference) was used to replace the static double-layer capacitance Cdl, and a method for quantifying the active surface area that is compatible with the full battery charge and discharge rate response was established. At the same time, an electrolyte correction factor (25μF / cm 2 ), while retaining the industry standard unit (m 2 / g) to achieve compatibility between differential capacitance (μF / mV) and dynamic parameters of scan rate, solving the problem of ion diffusion characterization distortion caused by ignoring rate changes in traditional formulas.

[0039] (3) Electrolyte Adaptation System

[0040] An adaptive electrolyte system was developed based on a 1 mol / L NaClO4 / EC:PC (volume ratio 1:1) organic solution. By adding 0.1% fluoroethylene carbonate (FEC), the hydroxylation side reaction on the surface of the electrode material was effectively suppressed, breaking through the limitations of traditional aqueous electrolytes on the stability of the active interface.

[0041] Example 9 (SSA = 8.1 m 2 / g) full battery internal resistance is 92mΩ·cm 2 , 0.5C capacity 86mAh / g, 200 cycle retention rate 88%, both better than the battery prepared by traditional method (internal resistance 110mΩ·cm 2, capacity 80mAh / g, retention rate 82%); Correlation analysis shows that SSA is negatively correlated with internal resistance (R 2 =0.987), and is positively correlated with capacity and retention rate (R 2 0.991 and 0.993, respectively), demonstrating that SSA can be used as a predictor of battery performance.

[0042] Figures in the specification

[0043] Figure 1 The voltage-current density curves obtained by CV scanning measurements at rates of 10 mV / s, 20 mV / s, 30 mV / s, 40 mV / s, and 50 mV / s are shown in the figure.

[0044] Figure 2 This is the current-rate linear fitting line graph of the present invention. DETAILED DESCRIPTION

[0045] In the description of the present invention, terms are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly indicate the number of technical features indicated. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0046] The meaning of first efficiency (0.5C first efficiency) is the percentage of the first discharge capacity to the first charge capacity, which reflects the utilization efficiency of the active material during the first charge and discharge process of the electrode.

[0047] First-effect (0.5C first-effect) test method: Use a battery charge and discharge tester to charge the battery at 0.5C constant current to 3.8V at 25°C. After standing for 30 minutes, discharge the battery at 0.5C constant current to 2.0V. First-effect = first discharge capacity / first charge capacity × 100%.

[0048] The 500-cycle retention rate means the percentage of the 500th discharge capacity to the initial discharge capacity, which is used to evaluate the battery cycle stability.

[0049] 500-cycle retention rate test method: After completing the initial efficiency test, cycle charge and discharge 500 times at a 1C rate, and calculate the ratio of the 500th discharge capacity to the initial discharge capacity.

[0050] The meaning of slurry viscosity: a quantitative indicator of slurry flow resistance, which affects coating uniformity.

[0051] Slurry viscosity test method: Use a rotational viscometer, measure at 60 rpm at 25°C, and take the average value of 3 times.

[0052] Porosity (SEM porosity) means the ratio of the pore area in the electrode cross section, reflecting the electrolyte infiltration space.

[0053] Porosity (SEM porosity) test method: SEM images of the electrode cross section (5000x magnification) were taken to analyze the pore area ratio.

[0054] Charge transfer resistance (Rct) means: the resistance to electron transfer at the electrode / electrolyte interface, which affects the kinetics of electrochemical reactions.

[0055] Charge transfer resistance (Rct) test method: EIS test was performed on an electrochemical workstation with a frequency range of 10 to 100 kHz and an amplitude of 5 mV, and the charge transfer resistance was obtained by fitting.

[0056] The meaning of electrolyte penetration depth is the penetration depth of the electrolyte inside the electrode, which reflects the accessibility of active sites.

[0057] Electrolyte penetration depth test method: The electrode sheet was immersed in 0.1% methylene blue solution for 30 minutes, and the dye penetration depth was measured using an optical microscope after sectioning along the cross section.

[0058] Zeta potential means: the surface charge density of particles. The larger the absolute value, the better the dispersion stability.

[0059] Zeta potential test method: Use a potentiometer to test, dilute the slurry 10 times with deionized water, balance at 25℃ for 30s, and then measure the value after 3 times.

[0060] Agglomeration rate (SEM agglomeration rate) means the proportion of agglomerated particles with a particle size ≥5μm, reflecting the effectiveness of the dispersant in inhibiting agglomeration.

[0061] Agglomeration rate (SEM agglomeration rate) test method: SEM takes images of the electrode surface and counts the proportion of agglomerated particles.

[0062] Meaning of average particle size: The average size of particles in the slurry, which affects the length of the ion transport path.

[0063] Particle average particle size test method: Laser particle size analyzer is used to measure particle size distribution, and D50 is calculated as the average particle size.

[0064] Dispersion time optimization experiment (SSA=8.2m 2 / g, dispersion time range adjustment)

[0065] Example 1

[0066] The following steps involve a method for determining the specific surface area of NFPP materials based on cyclic voltammetry and the construction of a model for the method. The following steps also involve a validation method to determine the effect of the homogenization parameters defined by the above determination method on the battery positive electrode after the homogenized material is homogenized based on the parameters.

[0067] 1. NFPP pole piece preparation

[0068] (1) Homogenization: The NFPP sample was vacuum-baked at 110°C for 8 h to ensure that its moisture content was less than 500 ppm. Subsequently, NFPP was mixed according to the mass ratio of NFPP (D50 = 5 ± 1 μm): PVDF: SuperP = 93:3:4, and the solvent NMP (N-methylpyrrolidone) was added to adjust the solid content to 60% (mass fraction); the mixture was first stirred at a low speed of 300 rpm for 60 min, and then dispersed at a high speed of 2000 rpm for 120 min to prepare the NFPP slurry.

[0069] (2) Coating: The slurry in step (1) is coated on both sides of a 12 μm aluminum foil (coating is performed by scraping, the gap of the comma scraper is 120 μm, the coating speed is 5 m / min, the roughness of the aluminum foil Ra ≤ 0.2 μm, and the surface density of the electrode after coating is 175-185 g / m 2 , aluminum foil single-sided loading 2.0±0.2mg / cm 2 , coating thickness is 220~240um); then the pole coil is dried (drying process: 80℃ vacuum drying oven (pressure <10Pa) drying for 12h);

[0070] (3) Preparation: After the pole coil in step (2) is naturally cooled to room temperature, the pole coil is die-cut and stripped. The burrs on the die-cut and stripped edges of the obtained pole pieces are controlled within 6 μm, and there are no cracks or particle agglomerations on the surface. Finally, the prepared NFPP pole pieces are punched into pieces with a diameter of 10 mm and an area of 0.785 cm 2 The discs are reserved.

[0071] 2. Construction of three-electrode system and CV test

[0072] 2.1 Electrode configuration

[0073] The working electrode is a circular NFPP electrode (diameter 10 mm, area 0.785 cm 2 ), the edges were sealed with polytetrafluoroethylene tape, exposing only the active coating; the reference electrode was an Ag / AgCl electrode (filled with 3 mol / L NaCl solution, potential drift ≤ ± 1.5 mV / h); the counter electrode was a 316L stainless steel mesh (pore size 50 μm, area 5 cm 2 , electrochemical polishing to surface roughness Ra ≤ 0.1 μm);

[0074] 2.2 Electrolyte preparation

[0075] Solvent: EC:PC (volume ratio 1:1, water <10ppm), solute: 1mol / L NaClO4 (purity ≥99.9%), 0.1% FEC is added to stabilize the interface and avoid the surface hydroxylation problem caused by traditional aqueous electrolytes.

[0076] 2.3CV test parameters

[0077] First, scan the electrode surface for two cycles at a rate of 50 mV / s in the range of 0.01-4.0 V to remove impurities. Then, perform CV scanning at a rate of 5-110 mV / s in the range of 2.0-3.8 V (vs Ag / AgCl, LSV test confirmed the absence of NFPP redox peak) to obtain the voltage-current density image of the NFPP sample.

[0078] 2.4 Data Processing

[0079] A current-rate linear fit (I = av + b) was performed at each voltage point, with the slope a corresponding to 2·Cdl·A (A is the electrode area), and Cdl was calculated. The absolute value of the adjacent rate capacitance difference ΔCdl = Cdl(i+1)-Cdl(i) was taken and inserted into the dynamic formula to calculate SSA.

[0080] 3. Preparation of sodium ion soft pack batteries

[0081] The sodium-supplemented positive electrode sheet in the above embodiment was roller-pressed to a compaction density of 2.1 g / cm 3 ; Assembled with negative electrode sheet, electrolyte, separator, etc. into sodium ion battery, the specific steps include:

[0082] (1) Preparation of negative electrode sheet: Hard carbon, conductive carbon black, SBR, and CMC were mixed with deionized water in a mass ratio of 94:2:2:2 and stirred for 6 hours to obtain negative electrode slurry, which was then coated on aluminum foil, dried, rolled, and cut into pieces to produce negative electrode sheets.

[0083] (2) Electrolyte preparation: EC:PC (volume ratio 1:1, water <10 ppm), solute: 1 mol / L NaClO4 (purity ≥99.9%), 0.1% FEC was added to stabilize the interface and avoid the surface hydroxylation problem caused by traditional aqueous electrolytes;

[0084] (3) Selection of diaphragm: The diaphragm used in the present invention is a commercially available polypropylene diaphragm (PP) with a thickness of 12 μm and a porosity of 31.5%.

[0085] (4) Battery assembly and activation: The positive electrode sheet, negative electrode sheet, and sodium-supplementing diaphragm are stacked into a battery cell, and then the obtained battery cell is placed in a metal shell and injected with electrolyte. Finally, after standing, forming, and capacity separation, a soft-pack sodium-ion battery with a capacity of 20Ah is made.

[0086] 4. Experimental related measurement methods

[0087] (1) Repeatability RSD (Relative Standard Deviation)

[0088] Calculation process: The SSA of NFPP in each embodiment is measured repeatedly n times (n=5) to obtain the data sets x1, x2,…, x n , first calculate the average Then calculate the standard deviation Finally calculate the RSD:

[0089]

[0090] Where s represents the standard deviation and x represents the number of data groups.

[0091] The smaller the RSD, the smaller the standard deviation of this set of data.

[0092] (2) Linear fit R 2 (Coefficient of determination)

[0093] Calculation process: The data (x i ,y i ) to perform linear fitting (x i is the scanning rate, y i is the current density, i = 1 to 5), and the fitting equation y = ax + b is obtained to calculate the following parameters:

[0094] Total sum of squared deviations:

[0095] Residual sum of squares: in is the fitted value,

[0096]

[0097] R 2 The closer it is to 1, the better the fit of this set of data.

[0098] (3) Infiltration depth (unit: μm)

[0099] Calculation process: Take the initial mass of the electrode m0, the mass after infiltration m1, assume the liquid density ρ, and the infiltration area A, then the infiltration depth is:

[0100]

[0101] (4) Electrode resistivity (unit: mΩ·cm) 2 )

[0102] Calculation process: Use the four-probe method to measure the electrode resistance R (unit: Ω), the electrode area A (unit: cm 2 ), then the resistivity R sheet for:

[0103] R sheet =R×A

[0104] Unit: mΩ·cm

[0105] (5) 0.5C discharge capacity (unit: mAh / g)

[0106] Calculation process:

[0107] During the actual measurement, discharge with current I (unit: mA) to the cut-off voltage, record the discharge time t (unit: h), the mass of the active material of the electrode m (unit: g), and the discharge capacity is:

[0108]

[0109] (6) 200-cycle capacity retention rate (unit: %)

[0110] Calculation process: The initial capacity C0 (mAh / g) is obtained by the first charge and discharge;

[0111] The charge and discharge cycle was performed 200 times at a rate of 0.5C, and the 200th discharge capacity C was recorded. 200 ;

[0112] The capacity retention rate B is:

[0113]

[0114] On the one hand, the present invention reflects the dynamic characteristics of the interface double layer through ΔCdl / Δv, which is highly consistent with the rate response of the full battery charge and discharge process, while the traditional static formula cannot capture the impact of rate changes on ion diffusion; on the other hand, the present invention enables the formula to have dynamic characterization capabilities through unit consistency, and through the electrolyte correction factor (25μF / cm 2 ) to compensate for the differential capacitance unit (μF / mV) to ensure that the SSA unit remains in m 2 / g, compatible with industry standards while incorporating the velocity dimension (Δv).

[0115] Furthermore, the proposed model improves SSA testing reliability. Specifically, CV testing is performed on NFPP electrodes that have not been rolled three times, preserving the original NFPP porosity and preventing active surface masking caused by rolling. This increases electrolyte penetration depth and, for the first time, achieves a direct correlation between slurry state and electrode performance. A CV testing method for NaClO4 / EC:PC electrolytes is also established, stabilizing the interface by adding FEC, thus avoiding the surface hydroxylation problem associated with traditional aqueous electrolytes.

[0116] Example 1

[0117] (1) Homogenization: The NFPP sample was vacuum-baked at 110°C for 8 h to ensure that its moisture content was less than 500 ppm. NFPP was then mixed according to a mass ratio of NFPP:PVDF:SuperP = 93:3:4, and the solvent NMP was added to adjust the solid content to 60% (mass fraction). The mixture was first stirred at a low speed of 300 rpm for 60 min, and then dispersed at a high speed of 2000 rpm for 120 min to prepare the NFPP slurry.

[0118] (2) Coating: The slurry in step (1) is coated on both sides of a 12 μm aluminum foil (coating is performed by scraping, the gap of the comma scraper is 120 μm, the coating speed is 5 m / min, the roughness of the aluminum foil Ra ≤ 0.2 μm, and the surface density of the electrode after coating is 175-185 g / m 2 , aluminum foil single-sided loading 2.0±0.2mg / cm 2 , coating thickness is 220-240 μm); then the pole coil is dried (drying process: 80℃ vacuum drying oven (pressure <10Pa) drying for 12 hours);

[0119] (3) Preparation: After the pole coil in step (2) is naturally cooled to room temperature, the pole coil is die-cut and stripped. The burrs on the die-cut and stripped edges of the obtained pole pieces are controlled within 6 μm, and there are no cracks or particle agglomerations on the surface. Finally, the prepared NFPP pole pieces are punched into pieces with a diameter of 10 mm and an area of 0.785 cm 2 The discs are reserved.

[0120] (4) CV test: The working electrode of the CV test is the circular NFPP electrode obtained by the above preparation steps; the reference electrode is an Ag / AgCl electrode; the counter electrode is a 316L stainless steel mesh; the electrolyte is a 1 mol / L NaClO4 / EC:PC (volume ratio 1:1) organic solution. In addition, by adding 0.1% by mass of FEC to stabilize the interface, the surface hydroxylation problem caused by traditional aqueous electrolytes is solved.

[0121] First, scan 2 cycles in the range of 0.01-4.0 V at a rate of 50 mV / s to remove impurities on the electrode surface; then perform CV scans in the range of 2.0-3.8 V (vs Ag / AgCl, LSV test confirmed that there was no NFPP redox peak) at rates of 10 mV / s, 20 mV / s, 30 mV / s, 40 mV / s, and 50 mV / s to obtain the voltage-current density curve, as shown in Figure 2. Figure 1 As shown;

[0122] (5) Data processing: Figure 2As shown, a current-rate linear fit (I = av + b) was performed at each voltage point, and the slope a corresponded to 2·Cdl·A (A is the electrode area), and Cdl was calculated; the adjacent rate capacitance difference ΔCdl = Cdl(i+1)-Cdl(i), and the absolute value was taken and then inserted into the dynamic formula to calculate SSA; then the SSA test repeatability (RSD), linear fit (R2), and electrolyte infiltration depth (μm) of the sample NFPP were tested, and the results are shown in Table 1.

[0123] Example 2

[0124] The only difference between this embodiment and embodiment 1 is that the rate interval Δv of the CV scan is 5 mV / s (rate: 10 mV / s, 15 mV / s, 20 mV / s, 25 mV / s, 30 mV / s).

[0125] Example 3

[0126] The only difference between this embodiment and embodiment 1 is that the rate interval Δv of the CV scan is 15 mV / s (rate: 10 mV / s, 25 mV / s, 40 mV / s, 55 mV / s, 70 mV / s).

[0127] Example 4

[0128] The only difference between this embodiment and embodiment 1 is that 0.1% FEC is not added to the NaClO 4 electrolyte used in the CV test.

[0129] Example 5

[0130] The only difference between this embodiment and embodiment 1 is that the electrolyte used in the CV test is a KOH aqueous solution (1 mol / L).

[0131] Example 6

[0132] The only difference between this embodiment and embodiment 1 is that the reference electrode used in the CV test is an Ag / AgCl reference electrode.

[0133] Example 7

[0134] The only difference between this embodiment and embodiment 1 is that the reference electrode used in the CV test is an SCE reference electrode.

[0135] Example 8

[0136] The homogenization process of NFPP with a test result of SSA of 6.5 was optimized by adding a dispersant with a mass fraction of 15% relative to the main material. The dispersion time during the homogenization process was set to 50 minutes, and NFPP slurry was obtained after the homogenization was completed; the NFPP slurry was coated, rolled, die-cut and striped to obtain NFPP positive electrode sheets, and the electrode resistivity of the positive electrode sheets was tested. Subsequently, it was assembled with the negative electrode sheet, separator and electrolyte, and a sodium ion battery was obtained after formation and capacity separation. The 0.5C capacity and 200-cycle cycle retention rate of the sodium ion battery were tested. The results are shown in Table 2.

[0137] Example 9

[0138] The only difference between this embodiment and embodiment 8 is that the SSA of the NFPP used is 8.1.

[0139] Example 10

[0140] The only difference between this embodiment and embodiment 8 is that the SSA of the NFPP used is 10.5.

[0141] Example 11

[0142] The only difference between this embodiment and embodiment 8 is that the SSA of the NFPP used is 7.5, which is calculated using the traditional SSA calculation formula ((SSA=k·Cdl / m)).

[0143] The summary table of multivariable optimization results is shown in Table 1.

[0144] Table 1

[0145]

[0146] Table 2 shows the experimental results of the correlation between SSA and full battery performance.

[0147] Table 2

[0148]

[0149] Note: The traditional formula group SSA uses Cdl at a single rate of 50mV / s and does not use ΔCdl / Δv.

[0150] Based on Examples 1 to 3, it should be noted that, in terms of physical significance, Δv, as the core parameter of the dynamic test, directly reflects the response sensitivity of the double-layer capacitance to changes in the electrochemical scan rate. Experiments have confirmed that when Δv is set to 10mV / s, the ion diffusion and charge transfer processes in the system reach a quasi-steady-state equilibrium. At this time, the measured ΔCdl (double-layer capacitance difference) can truly reflect the essential differences in the active surface areas of different NFPP materials, avoiding kinetic polarization interference caused by improper rate selection. For the first time, Δv is introduced as a dynamic variable into the active specific surface area calculation formula. At the same time, the present invention verifies that 10mV / s is the optimal interval parameter, which significantly reduces the relative standard deviation (RSD) of the test results from 4.5% at Δv=5mV / s to 2.1%, fundamentally solving the polarization error problem caused by ignoring rate dependence in the traditional single rate test method.

[0151] Based on Examples 4 and 5, it should be noted that, in terms of the interface mechanism, when 1 mol / L NaClO4 / EC:PC electrolyte (Example 4) is used, the Na + With Na in NFPP lattice + The sites are perfectly matched to form a stable interface with low impedance. The electrode infiltration depth is measured to be 55μm, which is 57% higher than that of the traditional KOH aqueous electrolyte (35μm). Synchronous characterization shows that the thickness of the SEI film formed under this system is thinned from 15nm in the aqueous system to 8nm, and the interface impedance is reduced by 42%. Data correlation analysis shows that the first-week efficiency of the electrode group in Example 4 reached 88%, which is significantly higher than the 75% of the aqueous electrolyte system. This result directly confirms the strong correlation between the SSA measurement value of this patent and the actual electrochemical performance of the battery, breaking through the problem of "characterization data-battery performance" disconnection caused by poor ion matching in traditional electrolyte systems.

[0152] Based on Examples 6 and 7, it should be noted that the potential stability test shows that in the organic electrolyte system, the potential drift of the Ag / AgCl reference electrode (Example 6) is only ±1.5 mV per hour, which is 3 times more stable than the ±5.0 mV / h of the saturated calomel electrode (SCE), ensuring the accuracy of the voltage scale and making the linear fit goodness of fit R 2 From 0.985 to 0.998. Adaptability mechanism research shows that the Cl- mobility in organic electrolyte is as low as 0.05cm 2 / V·s, so that the liquid junction potential error of Ag / AgCl electrode is controlled at the level of <1mV; while SCE + with Na + The difference in migration numbers (0.12 vs 0.08) resulted in a liquid junction potential error of up to 5 mV. This series of experiments systematically verified the crucial role of reference electrode compatibility in the accuracy of ΔCdl calculations, establishing a theoretical foundation for standardized testing systems.

[0153] Based on Examples 8 to 11, it should be noted that the internal resistance correlation analysis shows that when SSA=8.1m 2 / g (Example 9), the electrode interface impedance dropped to 92mΩ·cm 2 , which is 16.4% lower than that of the group calculated by the traditional formula (Example 11), which is attributed to the shorter ion transport path and more uniform conductive network distribution in the high SSA electrode. The capacity mechanism study found that for every 1m increase in SSA, the 2 / g, the number of active sites on the material surface increases by about 5%, corresponding to a capacity increase of 3-5mAh / g. This law is highly consistent with the electrochemical double layer surface capacitance theory. Cyclic stability tests show that after SSA optimization of the homogenization process (SSA = 8.1m 2 / g) reduces the volume expansion rate of the electrode during the cycle by 9%, improves the uniformity of SEI film thickness distribution by 35%, and improves the capacity retention rate after 200 cycles by 6% compared with the traditional process group, fully demonstrating the significant improvement effect of this patented method on the stability of the electrode structure.

[0154] Comprehensively consider indicators such as RSD, goodness of fit, and working condition correlation to determine the optimal parameters:

[0155] Scan rate interval: 10 mV / s (Example 1), balancing sensitivity and stability;

[0156] Electrolyte: NaClO4 / EC:PC (Example 4), adapted to the actual working conditions of sodium ion batteries;

[0157] Reference electrode: Ag / AgCl (Example 6), ensuring long-term stability of the potential reference.

[0158] Example 12

[0159] Based on the homogenization method of Example 1, the following ratios of materials were prepared: NFPP:PVDF:SuperP:PVP=(95-x):3:x:0.3 (x=3.0, 3.5, 4.0, 4.5, 5.0). SSA was 8.2 m 2 / g

[0160] Table 3 shows the experimental results after the conductive agent dosage is optimized.

[0161] Table 3

[0162]

[0163] Example 3 (conductive agent added in an amount of 4.0wt%) exhibits the best comprehensive electrochemical performance, with an initial coulombic efficiency of 85.7% and a capacity retention rate of 85.2% after 100 cycles, both of which are the highest values in the experimental group. This excellent performance stems from the optimal ratio of the conductive agent: EIS testing confirmed that the charge transfer resistance (Rct) of the electrode under this ratio dropped to 95Ω, indicating that the conductive agent formed an efficient electron conduction network; at the same time, SEM characterization showed that the electrode had an ideal porosity of 52%, ensuring effective infiltration of the electrolyte (infiltration depth of 45μm), thereby significantly improving the utilization rate of the active material. When the conductive agent is excessive (6.0wt%), the electrode porosity drops to 40%, resulting in obstructed ion transport and a reduced proportion of active materials, ultimately causing the simultaneous attenuation of the first efficiency and cycle performance.

[0164] The slurry viscosity shows a good linear positive correlation with the amount of conductive agent used (6100-7500 mPa·s), both within the ideal processing window (5000-8000 mPa·s). Notably, when the conductive agent addition level is increased to 6.0wt%, the viscosity significantly increases to 8500 mPa·s (exceeding the process standard) due to excessive accumulation of conductive agent particles. This phenomenon verifies the regulatory effect of conductive agent content on the rheological behavior of the slurry.

[0165] Porosity analysis shows that the sample with 4.0wt% conductive agent has the optimal pore structure (porosity of 52%), corresponding to the maximum electrolyte infiltration depth of 45μm. Excessive addition of conductive agent (6.0wt%) triggers pore clogging, reducing the porosity to 40% and the infiltration depth to 35μm. Ultimately, it significantly reduces the contact area between the active material and the electrolyte, affecting the electrochemical reaction kinetics.

[0166] Example 13

[0167] SSA is 8.2m 2 Based on Example 1, NFPP: PVDF: SuperP: PVP = (95-x): 3: x: 0.3 (x = 3.0, 3.5, 4.0, 4.5, 5.0) was prepared. Based on Example 1, Comparative Example 1 was prepared with 6.0 wt% of conductive agent.

[0168] Table 4 shows the results of the dispersant dosage optimization experiment.

[0169] Table 4

[0170]

[0171] Example 3 (0.3wt% dispersant) showed the best dispersion effect, with a Zeta potential of +25mV and a particle agglomeration rate as low as 5%, corresponding to a first efficiency of 88.8%, the highest value in the experimental group. This excellent performance is attributed to the effective adsorption of PVP molecules on the surface of NFPP particles, forming a stable protective layer with a thickness of about 10nm, which significantly inhibits particle agglomeration through the synergistic effect of electrostatic repulsion (Zeta potential) and steric hindrance. In contrast, in the absence of dispersant (0.0wt%), the particles are severely agglomerated due to van der Waals forces (agglomeration rate 18%), resulting in the encapsulation of active sites and a 4.5% reduction in the first efficiency.

[0172] When the dispersant dosage is 0.3wt%, the slurry viscosity (6800mPa·s) is at the center of the target range (5000-8000mPa·s), indicating that the dispersibility and rheological properties are optimally balanced. However, excessive addition of dispersant (0.5wt%) causes entanglement of PVP molecular chains, increasing the viscosity to 7800mPa·s, but the Zeta potential and agglomeration rate are not further optimized. This phenomenon confirms that the optimal adsorption monolayer concentration of the dispersant is 0.3wt%. Beyond this threshold, the dispersion effect is no longer improved, but the slurry processing performance is affected.

[0173] As the amount of dispersant increases, the average particle size gradually decreases. Under the condition of 0.3wt% dispersant, the particle size drops to 5.5μm and is most evenly distributed, corresponding to an electrolyte infiltration depth of 45μm, indicating that a good ion transmission channel is formed inside the electrode. However, the particle size of the sample without dispersion treatment (0.0wt%) is as high as 8.0μm, and it is severely agglomerated, resulting in an electrolyte infiltration depth of only 28μm, a significant increase in interfacial impedance, and affecting the electrochemical reaction kinetics.

[0174] Example 14

[0175] This embodiment is based on the exploration of dispersion time optimization.

[0176] Based on Example 1, a slurry containing 4.0 wt% conductive agent and 0.3 wt% dispersant was prepared. The dispersion time was based on 100 min and distributed in a gradient of 30 min (samples 1-5: 50, 70, 100, 130, 150 min; sample 6: 30 min; sample 7: 170 min).

[0177] Table 5

[0178]

[0179] After 100 minutes of dispersion, the average particle size was 5.5 μm. Sample 3 had a 5C capacity of 85 mAh / g (the highest in the group) and a resistance of 95 mΩ·cm. 2(The lowest in the group). Appropriate dispersion time breaks up large particles (particle size 15μm at 30min), forming a uniform distribution (D50 = 5-6μm), shortening the ion transmission path (electrolyte infiltration depth 45μm). Excessive dispersion (150min, particle size 3.5μm) leads to particle breakage, increasing the specific surface area but destroying the electrode structure, and the resistance increases to 110mΩ·cm 2 .

[0180] The resistance of sample 3 is the lowest, indicating that the contact between particles is the best and the electron transfer efficiency is the highest; the agglomerated particles of sample 6 (particle size 15 μm) increase the interface resistance (Rct = 160 mΩ·cm 2 ), verifying the key influence of dispersion time on electrode kinetic performance.

[0181] A dispersion time that is too short (30 min) will lead to particle agglomeration, while a dispersion time that is too long (170 min) will cause excessive particle breakage, both of which will reduce the stability of the electrode structure. The optimal dispersion time is 100 min to achieve a balance between dispersion effect and structural integrity.

[0182] Through the refined gradient of 0.5wt% conductive agent, 0.1wt% dispersant, and 30min dispersion time (the dispersion time optimization experiment was adjusted to 100min as the benchmark), the mapping relationship of "active specific surface area-process parameters-performance indicators" was established.

[0183] Based on the above process, the present invention achieves:

[0184] Precise control of slurry viscosity within ±5% (e.g., viscosity is 6800mPa·s at 4.0wt% conductive agent, with fluctuation less than 5%) to ensure uniform electrode coating.

[0185] The first efficiency is increased by 6% to 10% (from 80% to 85.7%), and the cycle retention rate is increased by 12% to 18% (500 cycles retention rate increased from 70% to 85.2%), significantly improving the initial performance and cycle stability of the battery;

[0186] High-rate performance optimization (5C capacity increased by 13.3%) breaks through the performance bottleneck caused by blind parameter adjustment in traditional processes by optimizing particle dispersion and conductive network and shortening the ion / electron transmission path.

[0187] In summary, the "CV activity characterization - single-factor gradient experiment - quantitative process optimization" technology system constructed in this invention provides a quantifiable and efficient solution for the preparation of positive electrode materials for sodium-ion batteries. By solving the "uncertain measurement" problem through in situ activity characterization and the "uncertain adjustment" problem through quantitative gradient experimentation, the process transformation from experience-driven to data-driven is ultimately achieved. This significantly improves the scientific nature of electrode microstructure regulation and the stability of battery performance, and has important industrial application value and technological foresight.

[0188] It should be noted that the above-described specific embodiments are illustrative only. Those skilled in the art may devise various solutions based on the disclosure of the present invention, and such solutions fall within the scope of the present invention and are intended to be protected by the present invention. Those skilled in the art should understand that the present description and its accompanying drawings are intended to be illustrative only and are not intended to limit the scope of the claims. The scope of protection of the present invention is defined by the claims and their equivalents.

Claims

1. A method for determining the specific surface area of NFPP materials based on cyclic voltammetry, characterized in that: The following steps are involved: Set up a working electrode, a counter electrode, and a reference electrode. The electrolyte is a 1 mol / L NaClO4 / EC:PC organic solution. Scan the potential range from 2.0 to 3.8 V at a rate of 5 to 100 mV / s, with an interval of 5 to 10 mV / s between adjacent rates. The double layer capacitance difference ΔCdl at adjacent rates is obtained by current-rate linear fitting, and the specific surface area is calculated using formula (1): Where m is the mass of NFPP in the electrode in g, ΔCdl is obtained by fitting the adjacent rate CV curve, and ΔV is the interval between adjacent CV scan rates.

2. The method for measuring the specific surface area of NFPP material according to claim 1, wherein: The reference electrode is Ag / AgCl or Hg / HgO.

3. The method for measuring the specific surface area of NFPP material according to claim 1 or 2, characterized in that: The fitting interval current-rate linear correlation coefficient R of the CV curve 2 Not less than 0.

90.

4. The method for measuring the specific surface area of NFPP material according to any one of claims 1 to 3, characterized in that: The counter electrode is a stainless steel mesh or a spiral platinum wire.

5. The method for measuring the specific surface area of NFPP material according to any one of claims 1 to 4, characterized in that: The working electrode is a NFPP electrode.

6. The method for measuring the specific surface area of NFPP material according to any one of claims 1 to 5, characterized in that: The potential drift of the reference electrode involved is no more than ±1.5 mV / h.

7. The method for measuring the specific surface area of NFPP material according to any one of claims 1 to 6, characterized in that: The electrolyte is prepared with NaClO4 and XX (EC:PC) in equal volumes.

8. Application of the cyclic voltammetry method for determining the specific surface area of NFPP materials in optimizing the homogenization process and predicting battery performance, characterized in that: The method for determining the specific surface area of NFPP materials by cyclic voltammetry is a method for determining the specific surface area of NFPP materials by cyclic voltammetry as described in any one of claims 1 to 7.

9. A homogenization process for a sodium ion battery cathode material, the homogenization process comprising the following steps: 0.5 wt% conductive agent and 0.1 wt% dispersant were mixed and stirred at a low speed of 200 rpm for 30 min; High-speed dispersion at 2500rpm for 20 to 160 minutes, with a solid content of 55% to 65%; Roller pressing pressure 15MPa, active material loading 2~5mg / cm 2 .

10. A method for preparing a NFPP pole piece, characterized in that: The preparation method comprises the following steps: The material obtained by the homogenization process of the sodium ion battery positive electrode material according to claim 9 is vacuum dried at 80°C for 12 hours and has a compaction density of 1.2 to 1.6 g / cm 3 .

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