Titanium-doped high-density spherical nano sodium ferric pyrophosphate sodium ion battery positive electrode material and preparation method thereof

Abstract

The invention relates to the technical field of sodium-ion battery positive electrode materials, in particular to a titanium-doped high-density spherical nano sodium ferric pyrophosphate sodium-ion battery positive electrode material and a preparation method of the titanium-doped high-density spherical nano sodium ferric pyrophosphate sodium-ion battery positive electrode material. The chemical general formula of the titanium-doped high-density spherical nano sodium ferric pyrophosphate sodium ion battery positive electrode material is NaFe3-xTix (PO) PO), x is more than or equal to 0.05 and less than or equal to 0.2, the particle size of the positive electrode material is 20-300 nm, and the surface of the positive electrode material is coated with a layer of carbon. Through multi-dimensional innovation of material-process-structure, the invention provides a high-performance positive electrode solution for commercialization of the sodium ion battery, and has both technical advancement and engineering potential.

Description

Technical Field

[0001] The present application relates to the technical field of cathode materials for sodium-ion batteries, and particularly relates to a sodium-ion battery cathode material of titanium-doped high-density spherical nano-sodium iron pyrophosphate phosphate and a preparation method thereof. Background Art

[0002] Lithium-ion batteries are limited in their large-scale applications due to the uneven distribution of lithium resources and high costs. Sodium-ion batteries have become an alternative technology because of the abundant sodium resources and low costs. Sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, abbreviated as NFPP) has advantages such as high theoretical capacity, high working voltage (3.1 V), and low volume expansion (<4%), but has problems such as low intrinsic conductivity and poor cycling performance. Existing technologies modify it by carbon coating or introducing crystal defects, but the effects are limited, and it is difficult to balance capacity, rate, and cycling stability. Therefore, a new modification method is urgently needed to improve the comprehensive performance of NFPP. Summary of the Invention

[0003] In order to solve the problems that impurities are easily generated during the synthesis of NFPP and the rate performance of intrinsic NFPP is poor, the present application provides a sodium-ion battery cathode material of titanium-doped high-density spherical nano-sodium iron pyrophosphate phosphate and a preparation method thereof. By doping titanium metal in NFPP and through a series of processes of "wet mixing - rough grinding - pre-sintering - ultrafine grinding - spray drying - sintering", doping titanium reduces the iron ratio in NFPP, inhibits the generation of impurities, and at the same time, titanium doping optimizes the lattice structure and improves the electron / ion transport efficiency. The prepared sodium-ion battery cathode material of titanium-doped high-density spherical nano-sodium iron pyrophosphate phosphate has high conductivity, high cycling stability, and excellent rate performance. The discharge specific capacity is ≥107 mAh / g at a 1C rate, and the capacity retention rate is ≥90% after 500 cycles.

[0004] In the first aspect, the present application provides a sodium-ion battery cathode material of titanium-doped high-density spherical nano-sodium iron pyrophosphate phosphate, adopting the following technical solution: A sodium-ion battery cathode material of titanium-doped high-density spherical nano-sodium iron pyrophosphate phosphate has a chemical general formula of Na4Fe3-xTix(PO4)2P2O7, where 0.05 ≤ x ≤ 0.2, and the particle size of the cathode material is 20 - 300 nm, and a layer of carbon is coated on the surface.

[0005] By adopting the above technical solution, by using Ti 4+ to partially replace Fe 2+ (0.05 ≤ x ≤ 0.2), the concentration of Fe 2+ is reduced; the oxidation of Fe 2+ to Fe 3+The formation of resulting impurities (such as FePO4) stabilizes the lattice structure while ensuring the crystalline purity of the material. The doping of Ti adjusts the connection mode of FeO6 octahedra and PO4 tetrahedra, broadens the sodium ion diffusion channels, and reduces the Na + diffusion energy barrier. Through two-step wet milling (final particle size 20 - 300 nm) to form nanoscale particles, the Na + solid-phase diffusion path is shortened to the nanoscale, significantly improving the rate performance (1C discharge capacity ≥ 107 mAh / g). The Ti-O bond strength is higher than that of the Fe-O bond, alleviating the lattice volume change caused by Na + insertion / extraction during charge and discharge and reducing particle breakage (capacity retention rate ≥ 90% after 500 cycles). In summary, through the synergistic optimization of chemical doping and nanostructure, this cathode material solves the key bottlenecks of traditional NFPP materials, providing a feasible technical path for high-power and long-life sodium-ion batteries.

[0006] Preferably, the specific surface area of the cathode material is 14 - 20 m 2 / g, and the tap density ≥ 2.2 g / cm 3 .

[0007] In a second aspect, the present application provides a method for preparing a sodium-ion battery cathode material of titanium-doped high-density spherical sodium iron pyrophosphate phosphate, adopting the following technical solution: As a general technical concept, the present application also provides the method for preparing the above-mentioned sodium-ion battery cathode material of titanium-doped high-density spherical sodium iron pyrophosphate phosphate, including the following steps: S31. Put the sodium source compound, iron source compound, phosphorus source compound, titanium source compound, and carbon source compound into a solvent, mix evenly to obtain mixture A; S32. Wet-mill mixture A with a sand mill to a particle size of 200 - 600 nm to obtain mixture B; S33. Pre-burn mixture B in an argon atmosphere at 300 - 450 °C for 4 - 6 h to obtain mixture C, preferably pre-burn at 350 °C in an argon atmosphere for 5 h; S34. Put mixture C into a sand mill, and add a solvent 1 - 2 times the mass of mixture C for wet milling to a particle size of 20 - 300 nm to obtain mixture D; preferably wet mill to a particle size of 150 nm; S35. Spray-dry mixture D to form a precursor; S36. Sinter the precursor in a mixed argon-hydrogen gas at 450 - 600 °C for 8 - 12 h to obtain the sodium-ion battery cathode material of titanium-doped high-density spherical sodium iron pyrophosphate phosphate, preferably sinter at 550 °C in a mixed argon-hydrogen gas for 10 h.

[0008] By adopting the above technical solutions, the raw materials in S31 are mixed, and the molecular-level dispersion of a sodium source (such as sodium bicarbonate), an iron source (such as iron nitrate), a titanium source (such as titanium tetrachloride), a phosphorus source (such as ammonium dihydrogen phosphate), and a carbon source (such as glucose) is achieved through a solvent (such as water / ethanol). The addition of ethanol reduces the surface tension and promotes the dispersion of nanoparticles; the pre-embedding of the carbon source (such as glucose) lays the foundation for the subsequent construction of the conductive network. S32 Coarse grinding treatment, the mixture is ground to 200 - 600 nm by a sand mill, and the high-energy mechanical force breaks the raw material agglomerates, increasing the reaction contact area and enabling Ti 4+ to be mixed with Fe 2+ at the atomic level. S33 Pre-sintering treatment to eliminate organic residues and form primary crystal nuclei. S34 Ultra-fine grinding, secondary sand grinding to 20 - 300 nm (preferably 150 nm), adopting the post-pre-sintering grinding strategy to avoid surface defects caused by direct nano-grinding, increasing the specific surface area to 14 - 20 m 2 / g and shortening the Li+ diffusion path. S35 Spray drying, for example, with an inlet temperature of 200 - 350 °C, an atomization pressure of 0.4 - 0.6 MPa, and an outlet temperature of 60 - 120 °C, forming spherical particles induced by surface tension and increasing the tapped density to 2.2 - 2.5 g / cm 3 . S36 High-temperature sintering, Ti 4+ replaces Fe 2+ , an Ar / H2 (95:5) reducing atmosphere maintains the valence state of Fe 2+ , and carbothermal reduction forms a 3D conductive network, and in-situ carbon coating improves the electronic conductivity. In summary, this process chain realizes the coordinated optimization of the intrinsic properties and macroscopic morphology of the material through triple regulation of "particle size gradient control - stepwise crystallization - defect engineering", and finally enables the material to have a discharge specific capacity ≥ 107 mAh / g at a 1C rate and a capacity retention rate ≥ 90% after 500 cycles.

[0009] Preferably, the titanium source compound is selected from one or more of titanium hydroxide, titanium sulfide, titanium halide, titanium tetrachloride, titanium dichloride, titanium oxychloride, titanium fluoride, titanium bromide, titanium iodide, titanium silicide, titanium oxide, titanium oxychloride, and titanium carbide.

[0010] Preferably, the sodium source compound is selected from one or more of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium nitrate, sodium fluoride, sodium fluorophosphate, sodium trifluoroacetate, sodium oxalate, sodium acetate, sodium persulfate, sodium hydroxide, sodium formate, sodium citrate, sodium pyrophosphate, sodium dihydrogen pyrophosphate, and sodium alginate.

[0011] Preferably, the iron source compound is selected from one or more of reduced iron powder, magnetite, iron oxide, ferrous oxide, iron nitrate, iron chloride, iron sulfate, iron phosphate, iron oxalate, ferrous oxalate, iron acetate, iron citrate, and iron fluoride.

[0012] Preferably, the phosphorus source compound is selected from one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium pyrophosphate, sodium dihydrogen pyrophosphate, phosphoric acid, ammonium dihydrogen phosphate, and ammonium phosphate; the carbon source compound is selected from one or more of glucose, sucrose, citric acid, malic acid, polyaniline, polyethylene glycol, adipic acid, phenolic resin, polypyrrole, ascorbic acid, chitosan, and soluble starch.

[0013] Preferably, the solvent is selected from at least one of water and ethanol, preferably a solvent composed of water and ethanol in a molar ratio of 1:1.

[0014] Preferably, the spray drying is carried out by one of centrifugal spraying and two-fluid spraying. The inlet temperature is 200 - 350 °C, the outlet temperature is 60 - 120 °C, and the atomization pressure of the two-fluid spraying is 0.4 - 0.6 MPa.

[0015] By adopting the above technical solutions, the centrifugal spray disperses the liquid mixture into fine droplets through a centrifugal disk rotating at high speed. Advantages: The droplet size distribution is relatively narrow, which is beneficial to the formation of uniform spherical particles; it is suitable for high-solid-content slurries. Combined with the subsequent sintering process, it can improve the material density and structural consistency. The two-fluid spray uses compressed gas (such as air or inert gas) to mix with the liquid at the nozzle to achieve droplet atomization. By adjusting the gas-liquid ratio, the droplet size is optimized, and it cooperates with the ultrafine grinding step (S34) to control the final nanoparticle morphology. The inlet temperature (200 - 350 °C) rapidly evaporates the solvent (water / ethanol), preventing the aggregation of precursor particles; the high temperature promotes the removal of the solvent, avoiding the influence of residual organic matter on the subsequent sintering. Connecting with the pre-sintering step (S33), it avoids solvent residue caused by low temperature or pre-decomposition caused by high temperature. The atomization pressure (0.4 - 0.6 MPa) controls the droplet size and distribution, directly affecting the particle size of the precursor particles (in the target range of 20 - 300 nm). Combined with the nanoscale slurry after wet grinding (S34), it ensures that the particles still maintain ultrafine characteristics after spray drying. The outlet temperature (60 - 120 °C) balances the drying efficiency and the protection of heat-sensitive components, avoiding oxidation of the precursor or structural damage caused by too high temperature. Cooperating with the sintering in an argon-hydrogen mixed atmosphere (S36), it maintains a reducing environment to prevent Fe 2+ from being oxidized to Fe 3+Spraying method + atomization pressure: By combining centrifugal or two-fluid spraying with medium-high pressure atomization, dual regulation of droplet size and component uniformity is achieved, ensuring the sphericity and high density of precursor particles. Temperature gradient control: A gradient design of high temperature at the inlet (rapid drying) → low temperature at the outlet (gentle termination of drying) prevents particle cracking or agglomeration caused by thermal stress. The porous spherical precursor formed by spray drying (step S35) and subsequent sintering (step S36) cooperate to promote grain densification and optimize the electron / ion transport path. Through the fine regulation of the above spray drying process, the cathode material is significantly optimized in terms of microstructure, component uniformity, and electrochemical performance, meeting the requirements of high energy density and long cycle life for sodium-ion batteries.

[0016] Preferably, the volume ratio of argon to hydrogen in the argon-hydrogen mixed gas is 19:1.

[0017] In summary, the beneficial technical effects of this application are as follows: I. Optimization of material properties 1. High electron / ion conductivity: Doping with titanium elements optimizes the lattice structure by replacing some Fe sites, expands the Li + / Na + diffusion channels, reduces the ion diffusion energy barrier, introduces the high electronegativity of Ti 4+ , enhances the intrinsic electron conductivity of the material, and combines with nanoscale particles (20 - 300 nm) to shorten the ion transport path, so that the discharge specific capacity at 1C rate ≥ 107 mAh / g.

[0018] 2. Significantly improved cycle stability: Titanium doping inhibits lattice distortion during charge and discharge, reduces volume expansion (capacity retention rate ≥ 90% @ 500 cycles), and two-stage heat treatment of pre-sintering (300 - 450 °C) and sintering (450 - 600 °C) forms a stable crystal framework. 3. Breakthrough in rate performance: Ultra-fine grinding (to 150 nm) combined with spherical morphology optimization improves the electrode-electrolyte interface contact. The carbon source compound generates a conductive carbon network during sintering, strengthening the charge transport kinetics.

[0019] II. Innovation in preparation process 1. Heterophase suppression mechanism: Wet grinding (200 - 600 nm → 20 - 300 nm) realizes atomic-level uniform mixing, reduces local composition segregation, and sintering in an Ar / H2 (19:1) reducing atmosphere inhibits the generation of heterophases (such as FePO4) caused by Fe 2+ oxidation.

[0020] 2. Morphology and structure control: The sand mill gradient grinding technology (coarse grinding + ultra-fine grinding) precisely regulates the particle size distribution to the nanoscale, and the spray drying process constructs high-density spherical particles, improving the tap density.

[0021] 3. Process Synergy: The pre-sintering stage (e.g., 350°C × 5 h) eliminates organic residues and forms primary crystal nuclei; the sintering stage (550°C × 10 h) promotes the oriented growth of grains and the ordering of Ti doping sites.

[0022] III. Technical and Economic Advantages 1. Cost Control: Using a water / ethanol mixed solvent (1:1) reduces raw material costs and environmental pressure, with a low Ti doping amount (0.05 ≤ x ≤ 0.2), avoiding excessive consumption of precious metal resources.

[0023] 2. Mass Production Feasibility: The wet grinding + spray drying process is compatible with the existing lithium-ion battery material production line equipment. The argon-hydrogen mixture ratio (19:1) balances safety and reduction effects, making it suitable for industrial production. Detailed Implementation Modes

[0024] The following will describe the implementation schemes of the present application in detail in conjunction with embodiments. However, those skilled in the art will understand that the following embodiments are only used to illustrate the present application and should not be regarded as limiting the scope of the present application. For those not specified in the embodiments, the conventional conditions or the conditions recommended by the manufacturer are used. For reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0025] Example 1 A preparation method of a sodium-ion battery cathode material of titanium-doped high-density spherical nano-sodium iron pyrophosphate phosphate, comprising the following steps: S31. Weigh 0.4 mol of sodium bicarbonate, 0.285 mol of iron nitrate, 0.4 mol of ammonium dihydrogen phosphate, 0.005 mol of titanium fluoride, and 0.02 mol of glucose in proportion, and add them to 500 mL of a water / ethanol (the molar ratio of ethanol to water is 1:1) solution, and mix evenly to obtain mixture A; S32. Grind mixture A with a sand mill at a speed of 2000 r / min to a particle size of 600 nm to obtain mixture B; S33. Pre-sinter mixture B in an argon atmosphere at 300°C for 6 h to obtain mixture C; S34. Put mixture C into a sand mill, and add a solvent (the molar ratio of ethanol to water is 1:1) equal to 1 times the mass of mixture C for wet grinding to a particle size of 20 nm to obtain mixture D; S35. Spray-dry mixture D to form a precursor. The spray drying uses centrifugal spraying, with an inlet temperature of 200°C, an outlet temperature of 60°C, and a spray atomization pressure of 0.6 MPa; S36. Put the precursor into a tubular furnace under atmosphere, and sinter it in an argon-hydrogen mixture (the volume ratio of argon to hydrogen is 19:1) at 450°C for 12 h to obtain a sodium-ion battery cathode material of titanium-doped high-density spherical nano-sodium iron pyrophosphate phosphate, marked as Na4Fe2.95 Ti 0.05 (PO4)2P2O7。

[0026] Example 2 A preparation method of a sodium-ion battery cathode material of titanium-doped high-density spherical nano-sodium iron pyrophosphate phosphate, comprising the following steps: S31. Weigh 0.4 mol of sodium bicarbonate, 0.285 mol of iron nitrate, 0.4 mol of ammonium dihydrogen phosphate, 0.015 mol of titanium oxide, 0.02 mol of glucose in proportion, and add them to 500 mL of a water / ethanol (the molar ratio of ethanol to water is 1:1) solution, mix evenly to obtain mixture A; S32. Grind mixture A with a sand mill at a speed of 2000 r / min to a particle size of 200 nm to obtain mixture B; S33. Pre-burn mixture B in an argon atmosphere at 450 °C for 4 h to obtain mixture C; S34. Put mixture C into a sand mill, and add a solvent (the molar ratio of ethanol to water is 1:1) twice the mass of mixture C for wet grinding to a particle size of 300 nm to obtain mixture D; S35. Spray-dry mixture D to form a precursor. The spray drying uses two-fluid spraying (the gas introduced is argon and hydrogen, and the volume ratio of argon to hydrogen is 19:1), its inlet temperature is 350 °C, the outlet temperature is 120 °C, and the atomization pressure of the two-fluid spraying is 0.4 MPa; S36. Put the precursor into an atmosphere tube furnace and sinter it in an argon-hydrogen mixture (the volume ratio of argon to hydrogen is 19:1) at 600 °C for 8 h to obtain a sodium-ion battery cathode material of titanium-doped high-density spherical nano-sodium iron pyrophosphate phosphate, marked as Na4Fe 2.85 Ti 0.15 (PO4)2P2O7。

[0027] Example 3 A preparation method of a sodium-ion battery cathode material of titanium-doped high-density spherical nano-sodium iron pyrophosphate phosphate, comprising the following steps: S31. Weigh 0.4 mol of sodium bicarbonate, 0.285 mol of iron nitrate, 0.4 mol of ammonium dihydrogen phosphate, 0.020 mol of titanium tetrachloride, 0.02 mol of glucose in proportion, and add them to 500 mL of a water / ethanol (the molar ratio of ethanol to water is 1:1) solution, mix evenly to obtain mixture A; S32. Grind mixture A with a sand mill at a speed of 2000 r / min to a particle size of 400 nm to obtain mixture B; S33. Pre-burn mixture B in an argon atmosphere at 350 °C for 5 h to obtain mixture C; S34. Put the mixture C into a sand mill, and add a solvent that is 1.5 times the mass of the mixture C (the molar ratio of ethanol to water is 1:1) for wet grinding until the particle size reaches 150 nm to obtain mixture D; S35. Spray-dry the mixture D to form a precursor. The spray-drying uses centrifugal spraying, with an inlet temperature of 300 °C, an outlet temperature of 80 °C, and a spraying atomization pressure of 0.5 MPa; S36. Put the precursor into an atmosphere tube furnace and sinter it in an argon-hydrogen mixture (the volume ratio of argon to hydrogen is 19:1) at 550 °C for 10 h to obtain a sodium-ion battery cathode material of titanium-doped high-density spherical sodium iron pyrophosphate phosphate, labeled as Na4Fe 2.8 Ti 0.2 (PO4)2P2O7.

[0028] Comparative Example 1 Same as Example 3, except that titanium tetrachloride was not added, and the addition amount of iron nitrate was 0.3 mol to obtain Na4Fe3(PO4)2P2O7.

[0029] Comparative Example 2 Same as Example 3, except that 0.27 mol of iron nitrate and 0.030 mol of titanium tetrachloride were used.

[0030] Comparative Example 3 A preparation method of a sodium-ion battery cathode material of titanium-doped high-density spherical sodium iron pyrophosphate phosphate, comprising the following steps: S31. Weigh 0.4 mol of sodium bicarbonate, 0.285 mol of iron nitrate, 0.4 mol of ammonium dihydrogen phosphate, 0.020 mol of titanium tetrachloride, and 0.02 mol of glucose in proportion, and add them to 500 mL of a water / ethanol (the molar ratio of ethanol to water is 1:1) solution, mix evenly to obtain mixture A; S32. Grind the mixture A with a sand mill at a speed of 2000 r / min until the particle size reaches 150 nm to obtain mixture B; S35. Spray-dry the mixture B to form a precursor. The spray-drying uses centrifugal spraying, with an inlet temperature of 300 °C, an outlet temperature of 80 °C, and a spraying atomization pressure of 0.5 MPa; S36. Put the precursor into an atmosphere tube furnace and sinter it in an argon-hydrogen mixture (the volume ratio of argon to hydrogen is 19:1) at 550 °C for 10 h to obtain a sodium-ion battery cathode material of titanium-doped high-density spherical sodium iron pyrophosphate phosphate, labeled as Na4Fe 2.8 Ti 0.2 (PO4)2P2O7.

[0031] Performance Test The sodium-ion battery cathode materials of titanium-doped high-density spherical nano-sodium iron pyrophosphate prepared in Examples 1-3 and Comparative Examples 1-3 were sampled and tested as follows. The test results are shown in Table 1.

[0032] The particle size of the above cathode materials was tested using a laser particle size analyzer (DLS); the specific surface area was tested using a fully automatic nitrogen adsorption BET specific surface area tester BSD-BET400 from Beishide Instruments; the tapped density was tested according to the GB / T 5162-2021 standard; Battery performance test: The sodium-ion battery cathode materials of titanium-doped high-density spherical nano-sodium iron pyrophosphate prepared in Examples 1-3 and Comparative Examples 1-3 were respectively made into the following sodium-ion batteries: The cathode material, conductive agent Super P, and carboxymethyl cellulose were mixed into a slurry with water in a ratio of 8:1:1, stirred for 24 h, coated on carbon-coated aluminum foil, and dried in an oven at 80 °C for 24 h. The dried electrode sheet was cut into 12-mm circular pieces with a loading of 1-1.4 mg / cm 2 , and a sodium foil was used as the counter electrode to press into a sodium-ion battery. Charge-discharge test conditions: at a 1C rate, constant current charge-discharge, voltage range 1.5 V to 4.2 V.

[0033] Electronic conductivity test: An AC impedance test was performed on it using a Solartron 1260A impedance / gain-phase analyzer in the frequency range of 1 mHz - 0.1 Hz. The cathode material was pressed into a 10-mm diameter circular piece, and carbon-coated aluminum foil was attached to both sides, placed in a PEEK sleeve of a die battery, and then stainless steel column blocking electrodes were inserted on both sides, and the test was performed under a pressure of 360 MPa, and the electronic conductivity was calculated.

[0034] Table 1 Performance test Analyzing the data in Table 1, it can be seen that: 1) The performance test results of the sodium-ion battery cathode materials of titanium-doped high-density spherical nano-sodium iron pyrophosphate prepared in Examples 1-3 show that in this application, by doping titanium metal in NFPP and through a series of processes of "wet mixing - coarse grinding - pre-burning - ultrafine grinding - spray drying - sintering", doping titanium reduces the iron ratio in NFPP, inhibits the formation of impurity phases, and at the same time, titanium doping optimizes the lattice structure, improves the electron / ion transport efficiency. The prepared sodium-ion battery cathode materials of titanium-doped high-density spherical nano-sodium iron pyrophosphate have high conductivity, high cycle stability, and excellent rate performance. The discharge specific capacity is ≥107 mAh / g at a 1C rate, and the capacity retention rate is ≥90% after 500 cycles.

[0035] 2) Performance comparative analysis of the sodium-ion battery cathode material of titanium-doped high-density spherical sodium iron pyrophosphate phosphate prepared in Example 3 and Comparative Examples 1-2 shows that by using Ti 4+ to partially replace Fe 2+ (0.05 ≤ x ≤ 0.2), the concentration of Fe 2+ is reduced, and the oxidation of Fe 2+ to Fe 3+ during high-temperature sintering, which leads to the formation of impurity phases (such as FePO4), is inhibited. While ensuring the crystal purity of the material, the lattice structure is stabilized. The doping of Ti adjusts the connection mode of FeO6 octahedra and PO4 tetrahedra, broadens the sodium-ion diffusion channels, and reduces the Na + diffusion barrier. However, when titanium doping is not used or the amount of titanium doping is excessive, the performance of the obtained cathode material significantly deteriorates.

[0036] 3) Performance comparative analysis of the sodium-ion battery cathode material of titanium-doped high-density spherical sodium iron pyrophosphate phosphate prepared in Example 3 and Comparative Example 3 shows that through the series of processes of "wet mixing - rough grinding - pre-sintering - ultrafine grinding - spray drying - sintering" in this application, doping with titanium reduces the iron ratio in NFPP, inhibits the formation of impurity phases, and at the same time, titanium doping optimizes the lattice structure and improves the electron / ion transport efficiency. The prepared sodium-ion battery cathode material of titanium-doped high-density spherical sodium iron pyrophosphate phosphate has high electrical conductivity, high cycle stability, and excellent rate performance. The rough grinding and pre-sintering processes play an important role in the preparation process flow of this application.

[0037] The above embodiments are only used to explain the technical solutions of this application rather than limit them. Although the above embodiments have specifically described this application, those skilled in the art should understand that the specific implementation manners of the present invention can still be modified or equivalently replaced. Any modification and equivalent replacement without departing from the spirit and scope of this application shall be covered by the protection scope of this application.

Claims

1. A titanium-doped high-density spherical nano-sodium iron pyrophosphate sodium ion battery positive electrode material, characterized in that: The general chemical formula is Na4Fe 3-x Ti x (PO4)2P2O7, where 0.05≤x≤0.2, and the particle size of the positive electrode material is 20-300 nm, and the surface is coated with a layer of carbon.

2. According to claim 1, a titanium-doped high-density spherical nano-sodium iron pyrophosphate sodium ion battery positive electrode material, characterized in that: The specific surface area of ​​the positive electrode material is 14-20 m² / g, and the tap density is ≥2.2 g / cm³.

3. A method for preparing the titanium-doped high-density spherical nano-sodium iron pyrophosphate sodium ion battery positive electrode material according to any one of claims 1-2, characterized in that: The following steps are involved: S31, placing a sodium source compound, an iron source compound, a phosphorus source compound, a titanium source compound and a carbon source compound in a solvent, and mixing them uniformly to obtain a mixture A; S32, wet-grinding the mixture A with a sand mill to a particle size of 200-600 nm to obtain a mixture B; S33, pre-calcining the mixture B at 300-450° C. in an argon atmosphere for 4-6 hours to obtain a mixture C; S34, placing mixture C in a sand mill, and adding a solvent 1-2 times the mass of mixture C to perform wet grinding to a particle size of 20-300 nm, to obtain mixture D; S35, spray drying the mixture D to form a precursor; S36. Sinter the precursor in an argon-hydrogen mixed gas at 450-600°C for 8-12 hours to obtain a titanium-doped high-density spherical nano-sodium iron pyrophosphate sodium ion battery positive electrode material.

4. The method for preparing a titanium-doped high-density spherical nano-sodium iron pyrophosphate sodium ion battery positive electrode material according to claim 3, characterized in that: The titanium source compound is selected from one or more of titanium hydroxide, titanium sulfide, titanium halide, titanium tetrachloride, titanium dichloride, titanium oxychloride, titanium fluoride, titanium bromide, titanium iodide, titanium silicide, titanium oxide, titanium oxychloride and titanium carbide.

5. The method for preparing a titanium-doped high-density spherical nano-sodium iron pyrophosphate sodium ion battery positive electrode material according to claim 3, characterized in that: The sodium source compound is selected from one or more of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium nitrate, sodium fluoride, sodium fluorophosphate, sodium trifluoroacetate, sodium oxalate, sodium acetate, sodium persulfate, sodium hydroxide, sodium formate, sodium citrate, sodium pyrophosphate, sodium dihydrogen pyrophosphate and sodium alginate.

6. The method for preparing a titanium-doped high-density spherical nano-sodium iron pyrophosphate sodium ion battery positive electrode material according to claim 3, characterized in that: The iron source compound is selected from one or more of reduced iron powder, ferroferric oxide, ferric oxide, ferrous oxide, ferric nitrate, ferric chloride, ferric sulfate, ferric phosphate, ferric oxalate, ferrous oxalate, ferrous acetate, ferric citrate and ferric fluoride.

7. The method for preparing a titanium-doped high-density spherical nano-sodium iron pyrophosphate sodium ion battery positive electrode material according to claim 3, characterized in that: The phosphorus source compound is selected from one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium pyrophosphate, sodium dihydrogen pyrophosphate, phosphoric acid, ammonium dihydrogen phosphate and triammonium phosphate; the carbon source compound is selected from one or more of glucose, sucrose, citric acid, malic acid, polyaniline, polyethylene glycol, adipic acid, phenolic resin, polypyrrole, ascorbic acid, chitosan and soluble starch.

8. The method for preparing a titanium-doped high-density spherical nano-sodium iron pyrophosphate sodium ion battery positive electrode material according to claim 3, characterized in that: The solvent is selected from at least one of water and ethanol, preferably a solvent consisting of water and ethanol in a molar ratio of 1:

1.

9. The method for preparing a titanium-doped high-density spherical nano-sodium iron pyrophosphate sodium ion battery positive electrode material according to claim 3, characterized in that: The spray drying adopts one of centrifugal spray and two-fluid spray, with an inlet temperature of 200-350° C., an outlet temperature of 60-120° C., and an atomization pressure of the two-fluid spray of 0.4-0.6 MPa.

10. The method for preparing a titanium-doped high-density spherical nano-sodium iron pyrophosphate sodium ion battery positive electrode material according to claim 3, characterized in that: The volume ratio of argon to hydrogen in the argon-hydrogen mixed gas is 19:1.

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