Anion-cation co-doped ultra-low temperature type sodium ion battery NFPP positive electrode material and a preparation method thereof

Abstract

This invention relates to the field of battery materials technology, and discloses an anion-cation co-doped ultra-low temperature sodium-ion battery NFPP cathode material and its preparation method. The cathode material is Ni. 2+ / F ‑ Co-doped sodium iron pyrophosphate with anions and cations has the chemical formula Na₄Fe₃(PO₄)₂P₂O₇; in which Ni 2+ The molar percentage of doping is 2% to 5%, F ‑ The molar percentage of doping is 2% to 6%, Ni 2+ Occupying Fe lattice sites, F ‑ Occupying O lattice sites, Ni 2+ With F ‑ The overall electroneutrality of the crystal lattice is maintained through a charge self-compensation mechanism. The preparation method employs an integrated process of stepwise doping, wet classification, high-gradient magnetic separation, spray granulation, and two-stage atmosphere calcination, which can reduce production energy consumption, improve doping uniformity, and control the content of magnetic foreign matter.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, specifically to an anion-cation co-doped ultra-low temperature sodium-ion battery NFPP cathode material and its preparation method. Background Technology

[0002] Sodium-ion batteries are a crucial technology in energy storage and cryogenic power applications, and polyanionic cathode materials have become a research focus due to their structural stability and good cycle performance. To address the issues of slow ion transport and weak conductivity at low temperatures in these materials, existing modification techniques generally employ ion doping strategies. This involves introducing heterogeneous ions with specific valence states and radii, such as potassium, niobium, vanadium, and chromium, into the polyanionic compound lattice to replace some of the original cation sites or embed them into lattice interstices. This doping method can synergistically optimize several key properties of the material. On the one hand, it can broaden sodium ion migration channels, reduce the ion diffusion barrier within the lattice, and enhance intrinsic ion transport kinetics at low temperatures. On the other hand, it can optimize the electronic structure of the material, improve intrinsic electronic conductivity, and alleviate low-temperature electrochemical polarization. Simultaneously, it can stabilize the lattice structure, suppress volume deformation and phase transitions during charging and discharging, especially under cryogenic conditions, ensuring the structural integrity and capacity retention of the electrode during long-term cycling.

[0003] Current doping modification techniques for polyanionic materials still suffer from several insurmountable drawbacks. The doping window is narrow, and the process tolerance is extremely low. In actual production, the control requirements for the type and proportion of dopant elements are extremely stringent. If the proportion exceeds the optimal range, the dopant ions not only fail to achieve the modification effect but also block sodium ion migration channels, causing crystalline distortion and directly leading to a decline in the material's electrochemical performance. Mainstream doping processes rely on high-temperature solid-state reactions, which are energy-intensive, difficult to precisely control, and prone to generating impurity phases or causing uneven distribution of dopant elements within the material, significantly reducing batch consistency and electrochemical performance stability. Furthermore, the introduction of high-valence or large-radius heteroions easily disrupts the original charge balance and lattice matching of the polyanionic framework. Such structural disturbances can induce irreversible phase transitions during long-term cycling, exacerbating side reactions at the electrode interface and becoming a core obstacle to further improving the material's low-temperature performance.

[0004] Sodium iron phosphate pyrophosphate (Na4Fe3(PO4)2P2O7, NFPP) is a promising polyanion cathode material for sodium-ion batteries, but its traditional preparation process has significant drawbacks. The production process requires multiple purification steps, which are complex and energy-intensive, directly increasing production costs. Furthermore, this material has poor intrinsic electronic conductivity, resulting in significant electrochemical polarization in practical applications, short cycle life, and difficulty in adapting to ultra-low temperature and long-cycle operating scenarios. Summary of the Invention

[0005] The purpose of this invention is to provide an anion-cation co-doped ultra-low temperature sodium-ion battery NFPP cathode material, through Ni 2+ With F - The synergistic doping and charge self-compensation mechanism broadens the material doping window, enhances the stability of the crystal structure, reduces the diffusion barrier of sodium ions in the ultra-low temperature environment, significantly improves the ion transport dynamics and low-temperature discharge performance of the material, and solves the problems of low low-temperature capacity, large polarization and poor cycle stability of traditional NFPP materials.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A co-doped cation-anion ultra-low temperature sodium-ion battery NFPP cathode material, wherein the cathode material is Ni. 2+ / F - Co-doped sodium iron pyrophosphate with anions and cations has the chemical formula Na₄Fe₃(PO₄)₂P₂O₇; in which Ni 2+ The molar percentage of doping is 2% to 5%, F - The molar percentage of doping is 2% to 6%, Ni 2+ Occupying Fe lattice sites, F - Occupying O lattice sites, Ni 2+ With F - The overall electroneutrality of the crystal lattice is maintained through a charge self-compensation mechanism.

[0007] Preferably, the positive electrode material has a spherical porous structure, a tap density ≥1.76g / cm³, and a magnetic foreign matter content ≤50μg / kg.

[0008] Preferably, the positive electrode material has a 0.1C discharge capacity ≥ 122.4 mAh / g, a capacity retention rate ≥ 89.5% after 3000 cycles at 1C rate, and a discharge capacity ≥ 86.8 mAh / g at -20℃.

[0009] Another objective of this invention is to provide a method for preparing anion-cation co-doped ultra-low temperature sodium-ion battery NFPP cathode material. The method employs a process combining stepwise doping, graded impurity removal, spray granulation, and two-stage calcination to achieve uniform distribution of doping elements, reduce process energy consumption and production costs, suppress interfacial side reactions, improve batch consistency, and enable the obtained cathode material to possess excellent comprehensive electrochemical performance under ultra-low temperature conditions and long-cycle conditions.

[0010] To achieve another objective of this invention, the following technical solution is adopted: A method for preparing the above-mentioned anion and cation co-doped ultra-low temperature sodium-ion battery NFPP cathode material includes the following steps: S1: Weigh the iron source, phosphate source, sodium source, and Ni according to the stoichiometric ratio. 2+ Doping source and F -Doping source; S2: Iron source, phosphate source, Ni 2+ The dopant source and dispersant are mixed and subjected to high-energy wet ball milling to obtain a mixed slurry; S3: Add F to the slurry - The doping source is used for low-energy mixing to complete stepwise doping. S4: The mixed slurry is sequentially subjected to wet centrifugal classification and high-gradient magnetic separation for impurity removal; S5: Add flocculant and sodium source to the magnetically separated slurry, disperse it by high-speed shearing, and then spray dry it to obtain a spherical porous precursor. S6: The spherical porous precursor is subjected to two-stage high-temperature calcination under an inert atmosphere to obtain the calcined product; S7: The calcined product is crushed, pulverized by air jet mill, and sieved to obtain the cathode material.

[0011] Preferably, the preparation of 1 mol Na4Fe3(PO4)2P2O7 is used as the metric standard, the iron source is ferrous oxalate, and the feed amount is (3-x) mol, where x is Ni. 2+ The molar percentage of Fe substitution is 2%≤x≤5%; the phosphate source is ammonium dihydrogen phosphate, with a feed amount of 3.8~4.2mol; the sodium source is sodium carbonate, with a feed amount of 1.9~2.1mol, wherein sodium carbonate is added in step S5 to avoid sodium volatilization loss during the initial mixing process.

[0012] Preferably, Ni 2+ The doping source is nano-nickel oxide or nickel acetate, requiring a particle size D50 < 1 μm, purity ≥ 99.5%, and a feed rate of (3 × a%) mol, where a% is Ni 2+ Relative to the theoretical doping ratio of Fe, a = 2–5; F - The dopant source is ammonium fluoride or sodium fluoride, with a purity ≥ 99.0%, and the feed amount is (1 × b%) mol, where b% is F. - The doping ratio relative to the total amount of NFPP is b = 2 to 6.

[0013] Ni 2+ The molar percentage of doping is 2% to 5%, F - The molar percentage of doping is 2% to 6%. The doping ratio of both must satisfy a charge balance relationship, i.e., Ni... 2+ The introduced positive charge change is through F - The heterovalent substitution achieves compensation, ensuring that the overall lattice remains electrically neutral, thereby maximizing the synergistic effect.

[0014] Preferably, in step S2, the high-energy wet ball milling is carried out in a high-energy planetary ball mill, using anhydrous ethanol as the ball milling medium and high-density zirconia balls (3-10 mm in diameter) as the grinding medium. The ball-to-material mass ratio is 10:3, that is, the total mass of zirconia balls: the total mass of solid materials = 10:3. The rotation speed is 300-500 rpm, and the ball milling time is 4-6 hours.

[0015] To prevent the agglomeration of ultrafine particles during ball milling, sodium hexametaphosphate (0.5%–1.0% by mass of the total material) is added to the grinding system as a dispersant. Through the high-energy mechanical force of this step, the following technical effects are achieved: (1) Refine the particle size of the mixture to D50=5-10μm to improve the activity of subsequent solid-phase reaction; (2) Utilizing the mechanical activation effect to activate Ni 2+ The source is uniformly dispersed within and on the surface of the iron and phosphate source particles, providing a basis for the subsequent high-temperature calcination of Ni. 2+ Directional occupation of Fe sites lays the foundation for mixing at the atomic scale.

[0016] After one ball milling cycle, a metered amount of anionic F is slowly added to the resulting slurry. - Remove the doping source, switch the ball mill to low-energy mixing mode (100-200 rpm), and continue mixing for 1-2 hours. This step employs a "stepwise doping" strategy, which differs from the traditional single-stage mixing process. Its technical purpose is to avoid F... - with Ni 2+ Premature chemical reactions or localized enrichment (excessive local concentration leading to premature reaction and impurity phase formation or uneven distribution) under high-intensity mechanical forces must be avoided to ensure the synergistic distribution of the two dopant ions in the final product. Low-energy mixing only guarantees F - The source is macroscopically and uniformly distributed in the slurry without destroying its chemical form, thus providing conditions for in-situ doping in the subsequent calcination process.

[0017] The mixed slurry is fed to a horizontal screw centrifuge for wet classification. The particle size distribution of solid particles in the slurry is controlled by adjusting the centrifugal speed and feed flow rate. The target parameters are D50 = 5-15 μm and D90 ≤ 15 μm. Excessively coarse particles are removed to ensure the electrochemical consistency of the final material.

[0018] Preferably, in step S4, the graded slurry enters a wet high-gradient magnetic separator. The magnetic field strength of the high-gradient magnetic separator is 8000–10000 Oersted to remove trace amounts of ferromagnetic impurities (such as wear debris from the grinding media, iron filings in the raw material, etc.) that may be introduced during the grinding process. This step controls the content of magnetic foreign matter in the final product to ≤50 μg / kg, avoiding the adverse effects of magnetic impurities on battery safety and self-discharge performance.

[0019] Preferably, in step S5, anionic polyacrylamide (PAM) is added to the magnetically separated slurry as a flocculant at a concentration of 0.1% to 0.3% of the total solid mass in the slurry. This increases the slurry concentration through flocculation and sedimentation, facilitating subsequent spray drying and granulation. The concentrated slurry is then transported to a spray drying tower. Before feeding, sodium carbonate powder is added according to the NFPP stoichiometric ratio, and the powder is fully dispersed in the slurry using an online high-speed shear emulsifier.

[0020] The inlet temperature of the atomizer in the spray dryer is 200–260℃, the outlet temperature is 70–90℃, and the atomizer rotation speed is 10,000–25,000 rpm (adjusted according to the target particle size D50 = 5–15 μm; higher speed is used when a smaller particle size is required, and lower speed is used when a larger particle size is required). The residence time in the drying tower is 4–5 hours. This step yields spherical precursor powder with a porous structure. This spherical morphology is beneficial for improving the tap density of the material, while the porous structure provides a channel for the smooth discharge of gaseous products (NH3, H2O, CO2, etc.) during subsequent calcination, and also facilitates the uniform diffusion of dopant elements during calcination.

[0021] Preferably, in step S6, the precursor powder obtained by spray drying is placed in an atmosphere furnace and subjected to a high-temperature solid-phase reaction under the protection of high-purity argon gas (purity ≥99.99%, oxygen content <1ppm). A two-stage high-temperature calcination process is employed. The first stage involves heating to 350-450℃ at a rate of 3-5℃ / min and holding for 2-4 hours. This stage mainly achieves the decomposition of ammonium salts (NH4H2PO4, NH4F), the decomposition of oxalate (FeC2O4), and the carbonization of some organic matter, releasing gaseous products such as NH3, H2O, and CO2, and initially forming a porous intermediate with a certain framework structure. The second stage involves heating at 2–4 °C / min to 650–800 °C and holding at that temperature for 8–12 hours. Under these high-temperature conditions, the following physicochemical changes occur: the NFPP main crystalline phase crystallizes and grows, forming a complete polyanionic framework structure; Ni… 2+ Cations, driven by concentration gradient and lattice potential energy, enter the crystal structure and partially occupy Fe sites; F - Anion-substituted O 2- Sites are used to achieve anion doping.

[0022] In step S7, after calcination, the product is naturally cooled to room temperature in the furnace. The sintered block material is first coarsely crushed by a jaw crusher, and then finely pulverized by an air jet mill. The particle size is controlled by adjusting the speed of the separator, with a target particle size D50 of 3-15 μm. The pulverized material is then sieved through a 200-400 mesh vibrating screen to remove a small amount of overly coarse or overly fine particles, yielding the final NFPP cathode material.

[0023] This invention utilizes the resources of fluorine impurities that require multiple steps to remove in traditional processes, by employing a charge self-compensation mechanism to remove Ni 2+ The doping window is broadened to 2%–5%, F - The doping window is widened to 2%–6%, significantly improving process tolerance and batch stability. Ni 2+ It can effectively strengthen the material's crystal lattice framework and suppress structural distortion and volume changes during charging and discharging. - It can significantly suppress side reactions at the electrode interface and reduce electrochemical polarization. The synergistic effect of these two factors achieves multiple benefits, including lattice stability, accelerated ion transport, and improved interface stability. Employing an integrated process of stepwise doping, wet classification, high-gradient magnetic separation, spray granulation, and two-stage atmosphere calcination, it can reduce production energy consumption, improve doping uniformity, and control the content of magnetic foreign matter. This results in NFPP cathode materials with both excellent structural stability and electrochemical performance. The discharge capacity can reach 92.4 mAh / g at -20℃, and the capacity retention rate is as high as 92.0% after 3000 cycles at 1C rate. This effectively solves the technical bottlenecks of traditional NFPP materials, such as poor low-temperature performance, short cycle life, high preparation energy consumption, and narrow doping window, making it suitable for sodium-ion battery applications requiring ultra-low temperature and long-cycle conditions. Detailed Implementation

[0024] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Example 1 Example 1 is an optimal preparation example of an anion and cation co-doped ultra-low temperature sodium-ion battery NFPP cathode material, based on the preparation of 1 kg of sodium iron phosphate pyrophosphate NFPP cathode material.

[0026] S1: Raw material ratio: Ferrous oxalate: 1520g; Ammonium dihydrogen phosphate: 920g; Sodium carbonate: 1060g, added during the spray drying process; Nickel oxide (D50=0.8μm, purity 99.6%): 22.4g, corresponding to Ni 2+ Doping amount 3.5 mol% Ammonium fluoride (purity 99.2%): 11.1g, corresponding to F - Doping amount 3.5 mol% Sodium hexametaphosphate dispersant: 8.5g; Anionic polyacrylamide flocculant: 2.1g; S2: High-energy wet ball milling, ferrous oxalate, ammonium dihydrogen phosphate, nickel oxide and sodium hexametaphosphate dispersant are fed into a high-energy planetary ball mill; anhydrous ethanol is used as the dispersion medium, and zirconia balls of mixed specifications with a diameter of 3-10 mm are used as the grinding medium. The ball-to-material mass ratio is controlled at 10:3 and the grinding speed is 400 rpm. The continuous wet ball milling is carried out for 5 hours to obtain a uniform mixed slurry. S3: Low-energy stepwise doping and mixing. After ball milling, ammonium fluoride dopant is added to the resulting slurry. The equipment is adjusted to low-energy mixing mode, the stirring speed is set to 150 rpm, and the mixture is continuously mixed for 1.5 hours to complete the anion stepwise doping treatment. S4: Centrifugal classification and high-gradient magnetic separation for impurity removal. The doped and modified mixed slurry is fed to a horizontal screw centrifuge for centrifugal classification. The process parameters are adjusted to make the particle size of the solid particles in the slurry D50=6.5μm and D90=13.2μm. Then, the classified slurry is fed into a wet high-gradient magnetic separator, and the magnetic field strength is set to 9000 Oersted for impurity removal and purification. After magnetic separation, the content of magnetic foreign matter in the material is 32μg / kg. S5: Flocculation treatment and spray drying granulation. Anionic polyacrylamide flocculant is added to the magnetically separated slurry. After flocculation and sedimentation, metered sodium carbonate is added simultaneously. The mixture is then fully dispersed and homogenized by a high-speed shear emulsifier. Subsequently, it is conveyed to a spray drying tower for granulation. The spray drying inlet temperature is controlled at 230℃, the outlet temperature at 80℃, the atomizer speed at 18000rpm, and the material residence time in the drying tower is 4.5h. Finally, a spherical porous precursor powder is obtained. S6: Two-stage inert atmosphere calcination: The prepared spherical porous precursor is placed inside an atmosphere furnace, and high-purity argon gas with an oxygen content of <1ppm is introduced as a protective atmosphere throughout the process; a two-stage gradient heating calcination process is adopted. In the first stage, the temperature is increased to 400℃ at a heating rate of 4℃ / min and held at a constant temperature for 3h; in the second stage, the temperature is increased to 720℃ at a heating rate of 3℃ / min and held at a constant temperature for 10h. After calcination, the furnace is naturally cooled to room temperature to obtain the calcined product. S7: The finished product is obtained through post-processing. The cooled calcined product is sequentially subjected to jaw crushing and air jet mill ultrafine grinding. The speed of the air jet mill classifier is adjusted to precisely control the particle size of the finished product D50=5.2μm. Finally, it is sieved through a 300-mesh standard sieve to obtain the anion and cation co-doped modified NFPP cathode material.

[0027] Material performance test results: The cathode material prepared in this embodiment is co-doped modified sodium iron pyrophosphate occupying Fe lattice sites and O lattice sites, which maintains lattice neutrality through a charge self-compensation mechanism. The tap density of the finished product can reach 1.82 g / cm³, and the content of magnetic foreign matter is 32 μg / kg. It has excellent electrochemical performance, with a discharge capacity of 122.4 mAh / g at 0.1C rate, a capacity retention of 92.0% after 3000 cycles at 1C rate, and a discharge capacity of 92.4 mAh / g at an ultra-low temperature of -20℃. It has a stable lattice structure, few interfacial side reactions, and excellent low-temperature kinetic performance.

[0028] Example 2 The types of raw materials, complete processes, and equipment types in this embodiment are the same as in Embodiment 1, except that: Ni 2+ The doping amount is 2.0 mol% (12.8 g of nickel oxide), F - The doping amount was 6.0 mol% (ammonium fluoride 19.0 g), the second stage calcination temperature was 650℃, and the holding time was 12 hours.

[0029] Material performance test results: The tap density is slightly lower than that of Example 1, at 1.76 g / cm³, and the material bulk density has decreased; the magnetic foreign matter content is 35 μg / kg; in terms of electrochemical performance, the 0.1C discharge capacity is 118.7 mAh / g, the cycle stability is slightly inferior to that of Example 1, the capacity retention rate after 3000 cycles at 1C is 89.5%, and the low-temperature discharge capacity at -20℃ is 89.1 mAh / g.

[0030] Example 3 The types of raw materials, complete processes, and equipment types in this embodiment are the same as in Embodiment 1, except that: Ni 2+ The doping amount is 5.0 mol% (32.0 g of nickel oxide), F - The doping amount was 2.0 mol% (6.3 g of ammonium fluoride), the second stage calcination temperature was 800℃, and the holding time was 8 hours.

[0031] Material performance test results: The tap density is slightly higher than that of Example 1, reaching 1.84 g / cm³, indicating that the material is more compact; the content of magnetic foreign matter is 30 μg / kg; in terms of electrochemical performance, the 0.1C discharge capacity is 120.5 mAh / g, slightly lower than that of Example 1, the capacity retention rate after 3000 cycles at 1C is 90.2%, and the low-temperature discharge capacity at -20℃ is 86.8 mAh / g, indicating that the low-temperature performance is lower than that of Example 1.

[0032] Example 4 The types of raw materials, complete processes, and equipment types in this embodiment are the same as in Embodiment 1, except that: Ni 2+The source uses nickel acetate, corresponding to Ni 2+ Doping amount 3.5 mol%, F - The source uses sodium fluoride, corresponding to F - The doping amount was 3.5 mol%, the ball milling time was 4 hours, the second stage calcination temperature was 700℃, and the holding time was 11 hours.

[0033] Material performance test results: tap density is 1.79 g / cm³, slightly lower than that of Example 1; magnetic foreign matter content is slightly higher, at 38 μg / kg; in terms of electrochemical performance, the 0.1C discharge capacity is 121.3 mAh / g, close to the level of Example 1, the capacity retention rate after 3000 cycles at 1C is 91.2%, and the low-temperature discharge capacity at -20℃ is 90.5 mAh / g. All performances are similar to those of Example 1 but slightly different.

[0034] Example 5 The types of raw materials, complete processes, and equipment types in this embodiment are the same as in Embodiment 1, except that: Ni 2+ The doping concentration is 4.0 mol% (25.6 g of nickel oxide), F - The doping amount was 4.0 mol% (ammonium fluoride 12.7 g), the ball milling speed was 300 rpm, the ball milling time was 6 hours, the second stage calcination temperature was 750℃, and the holding time was 9 hours.

[0035] Material performance test results: tap density is 1.81 g / cm³, which is comparable to Example 1, and magnetic foreign matter content is 33 μg / kg; in terms of electrochemical performance, the 0.1C discharge capacity is 121.8 mAh / g, the capacity retention rate after 3000 cycles at 1C is 91.5%, and the low-temperature discharge capacity at -20℃ is 91.2 mAh / g. The overall performance is close to that of Example 1, but the cycle stability and low-temperature performance are slightly inferior.

[0036] Comparative Example 1 (Blank Comparative Example: Undoped NFPP) The raw material formulation, all preparation processes, equipment parameters, calcination curves, and post-treatment methods were completely identical to those in Example 1. No nickel or fluorine source dopants were added to prepare unmodified pure-phase sodium iron pyrophosphate material. Lacking synergistic doping and charge self-compensation mechanisms, the polyanionic framework was not strengthened, and the interface lacked passivation modification, serving as a basic blank control.

[0037] Material performance test results: the tap density is only 1.68 g / cm³, and the material particle morphology is irregular; in terms of electrochemical performance, the 0.1C discharge capacity is 105.3 mAh / g, which is much lower than that of Example 1.

[0038] Comparative Example 2 (Single-cation doping: Single Ni) 2+ Doping) Only nickel oxide (22.4g, corresponding to Ni) was added. 2+ The doping concentration was 3.5 mol%, with no ammonium fluoride added. All other raw materials, the step-by-step doping process, ball milling, magnetic separation, spray drying, and two-stage calcination conditions were completely consistent with Example 1. Single nickel ions can only strengthen the crystal lattice and cannot form a charge self-compensation balance; the lack of F... - The inhibitory effect on interfacial side reactions was studied using a univariate control.

[0039] Material performance test results: The performance is significantly improved compared with the undoped sample, but there is still a gap compared with the co-doped Example 1. The cycle stability and low temperature performance are not as good as Example 1.

[0040] Comparative Example 3 (Single anion doping: F only) - Doping) Only ammonium fluoride (11.1g, corresponding to F) was added. - The doping concentration was 3.5 mol%, without the addition of nickel oxide. The entire preparation process, parameters, and proportions were identical to those in Example 1. Fluoride ions can optimize interfacial stability, but without nickel ions to reinforce the crystal framework, the structural resistance to deformation is insufficient, highlighting the difference in the effect of anion doping alone.

[0041] Material performance test results: The crystallinity of the material has been slightly improved, but the improvement in electrical conductivity is limited, and the capacity decays rapidly during cycling.

[0042] Comparative Example 4 (Process Comparison: Step-by-step doping eliminated, one-step blending) All raw materials, dopant addition amounts, and formulation ratios are consistent with Example 1; the core process of first cation ball milling followed by anion low-energy stepwise doping in this invention is omitted. Instead, the iron, phosphorus, nickel, and fluorine sources are all added to the ball mill at once, and high-energy ball milling (400 rpm, continuous ball milling for 6.5 hours) is performed throughout, without stepwise doping. This is used to compare the effects of stepwise doping on doping uniformity, impurity phase suppression, and ion transport kinetics.

[0043] Material performance test results: All properties were lower than those in Example 1, indicating that the one-time mixing resulted in uneven distribution of doping elements and failed to fully utilize the synergistic effect of Ni-F co-doping.

[0044] Comparative Example 5 (Doping Ratio Imbalance) Ni 2+ The doping concentration is 1.0 mol% (6.4 g of nickel oxide), F - The doping concentration was 7.0 mol% (22.2 g ammonium fluoride), which did not meet the charge balance requirement. The remaining preparation process, temperature regime, and impurity removal and granulation procedures were the same as in Example 1. This demonstrates that limiting the nickel doping range in this invention is a necessary condition for maintaining lattice stability and avoiding structural distortion.

[0045] Material performance test results: 0.1C discharge capacity is 115.6mAh / g, capacity retention rate drops to 82.3% after 2000 cycles at 1C, and low-temperature discharge capacity is 76.4mAh / g at -20℃.

[0046] Comparative Example 6 (Doping Ratio Imbalance) Ni 2+ The doping concentration is 7.0 mol% (44.8 g of nickel oxide), F - The doping amount was 1.0 mol% (3.2 g ammonium fluoride), and the doping ratio did not meet the charge balance relationship. The raw material type, process steps, calcination atmosphere, and molding process were all the same as in Example 1. This verifies that excessive fluorine doping will disrupt the lattice charge balance and induce the formation of impurity phases, thus supporting the rationality of the anion doping range of this invention.

[0047] Material property test results: Excess Ni 2+ The introduction of this substance disrupted the material's structural stability, resulting in a significant decrease in both rate performance and cycle performance, and making it the worst performing of all doped samples.

[0048] Table 1 compares the performance of Examples 1-5 with Comparative Examples 1-6: The comparative results are analyzed as follows: 1. The samples prepared in Examples 1-5 exhibited a stable 0.1C initial discharge specific capacity in the range of 118.7–122.4 mAh / g, significantly higher than the 105.3 mAh / g of Comparative Example 1 (undoped system), and also superior to Comparative Example 2 (single cation doping) and Comparative Example 3 (single anion doping). Experimental data indicate that Ni… 2+ / F - Co-doping of anions and cations is not a simple performance superposition, but rather a technical gain effect of 1+1>2 generated through charge self-compensation mechanism and lattice synergistic regulation, which effectively improves the overall reversible capacity and electrochemical utilization of the material. 2. In Example 1, the sample maintained a 0.2C discharge capacity retention of 78.3% at -20℃, significantly higher than the 52.1% of Comparative Example 1 (undoped), and significantly outperformed the single-doped control groups (63.5% for Comparative Example 2 and 61.2% for Comparative Example 3). This result fully demonstrates that the synergistic doping strategy proposed in this invention can effectively reduce the ion diffusion barrier under ultra-low temperature conditions, suppress interfacial side reactions, and significantly improve the kinetic behavior and capacity utilization of NFPP materials under extreme low-temperature conditions. 3. Comparative Example 5 (Ni) 2+ 1.0 mol% / F - 7 mol%) and Comparative Example 6 (Ni 2+ 7mol% / F -Due to the imbalance in the doping ratio (1.0 mol%), the lattice's electroneutrality was disrupted, leading to crystal structure distortion and increased polarization. Its electrochemical performance was significantly worse than Examples 1-5, even approaching the level of the undoped Comparative Example 1. This comparative result strongly confirms the crucial necessity of charge self-compensation balance in synergistic doping systems; that is, only by strictly controlling Ni... 2+ With F - Only by determining the molar ratio of doping can the synergistic effect of the two be fully realized; 4. Comparative Example 4 uses a traditional "one-step blending" doping process. Although the total amount of doped elements is the same as in Example 1, due to F... - with Ni 2+ Premature local reactions occurred during the high-energy ball milling stage, leading to uneven distribution and local enrichment of dopant elements, and introducing additional interface defects and impurity phases. Test results showed that its 0.1C discharge capacity (121.3 mAh / g) was lower than that of Example 1 (122.4 mAh / g). This difference directly verifies the superiority of the "cation ball milling followed by low-energy stepwise anion doping" process of this invention, which can achieve uniform dispersion of dopant elements, optimize microstructure consistency, and improve electrochemical performance. 5. Electrochemical impedance spectroscopy and GIVS test results show that the charge transfer resistance of Example 1 is only 38.6 Ω, far lower than that of Comparative Example 1 (86.3 Ω); at the same time, the sodium ion diffusion coefficient reaches 6.8 × 10⁻⁶. -12 cm 2 / s, compared to Comparative Example 1 (1.5×10 -12 cm 2 The efficiency of ions transported during charging and discharging was increased by about 4.5 times. The data confirmed that the synergistic doping effect significantly broadened the ion migration channels and optimized the lattice energy barrier structure, which substantially improved the transport efficiency of ions during charging and discharging, laying a kinetic foundation for excellent low-temperature performance. 6. In Example 15, the capacity retention after 500 cycles was between 89.5% and 91.2%, significantly higher than that of Comparative Example 1 (78.6%), single-doped Comparative Examples 2 and 3, and Comparative Examples 5 and 6 with unbalanced doping ratios. This is attributed to the Ni... 2+ Strengthening the lattice framework, suppressing structural distortions during cycling, and F - Suppressing interfacial side reactions and reducing electrolyte decomposition, the two work synergistically on the basis of charge self-compensation to significantly improve the structural stability and long-cycle capacity retention of the material. 7. The initial coulombic efficiency of Example 15 reached 91.8%–92.4%, significantly higher than that of the undoped, single-doped, and unbalanced doping comparative samples. This is mainly due to the charge self-compensation mechanism reducing lattice defects and irreversible capacity loss. -The interface modification effect inhibits the occurrence of side reactions, while the spherical porous morphology of the material shortens the ion transport path, further reducing the irreversible loss during the first charge and discharge and improving the utilization rate of the active material. 8. The 5C / 0.1C rate performance of Example 15 remained at 84.2%–86.5%, significantly higher than the 68.3% of Comparative Example 1 and other comparative examples, indicating that Ni… 2+ Doping can control the lattice spacing and improve the ion diffusion rate, F - Doping can reduce the interfacial charge transfer impedance. Combined with the uniform element distribution achieved by step doping, the material can still quickly complete ion insertion and extraction under high-rate charge and discharge conditions, maintaining excellent capacity performance.

[0049] Finally, it should be noted that in the description of this invention, the terms "vertical," "upper," "lower," "horizontal," etc., indicating orientation or positional relationships are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0050] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0051] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A co-doped cation and anion ultra-low temperature sodium-ion battery NFPP cathode material, characterized in that, The cathode material is Ni 2+ / F - Co-doped sodium iron pyrophosphate with anions and cations has the chemical formula Na₄Fe₃(PO₄)₂P₂O₇; in which Ni 2+ The molar percentage of doping is 2% to 5%, F - The molar percentage of doping is 2% to 6%, Ni 2+ Occupying Fe lattice sites, F - Occupying O lattice sites, Ni 2+ With F - The overall electroneutrality of the crystal lattice is maintained through a charge self-compensation mechanism.

2. The NFPP cathode material for co-doped ultra-low temperature sodium-ion batteries according to claim 1, characterized in that, The positive electrode material has a spherical porous structure with a tap density ≥1.76g / cm³ and a magnetic foreign matter content ≤50μg / kg.

3. The NFPP cathode material for co-doped anion and cation ultra-low temperature sodium-ion batteries according to claim 1 or 2, characterized in that, The cathode material has a 0.1C discharge capacity ≥118.7mAh / g, a capacity retention rate ≥89.5% after 3000 cycles at 1C rate, and a discharge capacity ≥86.8mAh / g at -20℃.

4. A method for preparing the NFPP cathode material for anion and cation co-doped ultra-low temperature sodium-ion batteries as described in any one of claims 1 to 3, characterized in that, Includes the following steps: S1: Weigh the iron source, phosphate source, sodium source, and Ni according to the stoichiometric ratio. 2+ Doping source and F - Doping source; S2: Iron source, phosphate source, Ni 2+ The dopant source and dispersant are mixed and subjected to high-energy wet ball milling to obtain a mixed slurry; S3: Add F to the slurry - The doping source is used for low-energy mixing to complete stepwise doping. S4: The mixed slurry is sequentially subjected to wet centrifugal classification and high-gradient magnetic separation for impurity removal; S5: Add flocculant and sodium source to the magnetically separated slurry, disperse it by high-speed shearing, and then spray dry it to obtain a spherical porous precursor. S6: The spherical porous precursor is subjected to two-stage high-temperature calcination under an inert atmosphere to obtain the calcined product; S7: The calcined product is crushed, pulverized by air jet mill, and sieved to obtain the cathode material.

5. The method for preparing the NFPP cathode material for anion and cation co-doped ultra-low temperature sodium-ion batteries according to claim 4, characterized in that, Using the preparation of 1 mol Na4Fe3(PO4)2P2O7 as the metric standard, ferrous oxalate as the iron source, and a feed amount of (3-x) mol, where x is the amount of Ni... 2+ The molar percentage of Fe substitution is 2%≤x≤5%; the phosphate source is ammonium dihydrogen phosphate, with a feed amount of 3.8~4.2mol; the sodium source is sodium carbonate, with a feed amount of 1.9~2.1mol.

6. The method for preparing the NFPP cathode material for anion and cation co-doped ultra-low temperature sodium-ion batteries according to claim 4 or 5, characterized in that, Ni 2+ The doping source is nano-nickel oxide or nickel acetate, F - The doping source is ammonium fluoride or sodium fluoride.

7. The method for preparing the NFPP cathode material for anion and cation co-doped ultra-low temperature sodium-ion batteries according to claim 4 or 5, characterized in that, In step S2, the high-energy wet ball milling uses anhydrous ethanol as the milling medium and zirconia balls as the grinding medium, with a ball-to-material mass ratio of 10:3, a rotation speed of 300-500 rpm, and a milling time of 4-6 hours.

8. The method for preparing the NFPP cathode material for anion and cation co-doped ultra-low temperature sodium-ion batteries according to claim 4 or 5, characterized in that, In step S4, the magnetic field strength of the high gradient strong magnetic separation is 8000-10000 Oersted.

9. The method for preparing the NFPP cathode material for anion and cation co-doped ultra-low temperature sodium-ion batteries according to claim 4 or 5, characterized in that, In step S5, the inlet temperature of the atomizer for spray drying is 200-260℃, the outlet temperature is 70-90℃, the rotation speed of the atomizer is 10000-25000 rpm, and the residence time in the drying tower is 4-5 hours.

10. The method for preparing the NFPP cathode material for anion and cation co-doped ultra-low temperature sodium-ion batteries according to claim 4 or 5, characterized in that, In step S6, calcination is carried out under a high-purity argon inert atmosphere, and the two-stage high-temperature calcination is as follows: The first stage involves raising the temperature to 350-450℃ at a rate of 3-5℃ / min and holding it at that temperature for 2-4 hours. The second stage involves increasing the temperature at 2–4℃ / min to 650–800℃ and holding it at that temperature for 8–12 hours.

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