Preparation method of doped modified sodium iron pyrophosphate positive electrode material

Abstract

This invention belongs to the field of sodium-ion battery cathode materials, specifically relating to a method for preparing a doped modified sodium iron pyrophosphate cathode material. The method for preparing a doped modified sodium iron pyrophosphate cathode material of this invention includes the following steps: (1) Weighing sodium source, carbon source, phosphorus source, iron source, dopant element M and solvent, first mixing the iron source and dopant element M evenly, then adding sodium source, carbon source, phosphorus source and solvent, ball milling the resulting mixture, drying it, adding a certain amount of sodium supplementer and mixing it evenly to obtain a precursor; the molar ratio of sodium element to iron element is 1.6-2.2:1; (2) Calcining the precursor obtained in step (1) at 400-500℃ for 5-15h under an inert gas atmosphere, and naturally cooling it to room temperature; (3) Crushing and sieving the sintered product obtained in step (2) to obtain the sodium iron pyrophosphate cathode material.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery cathode materials, specifically relating to a method for preparing a doped and modified sodium iron pyrophosphate cathode material. Background Technology

[0002] Sodium-ion batteries have become a hot research topic in battery technology in recent years due to their abundant raw materials, low cost, and high safety. Since sodium ions have a larger radius than lithium ions, the current research focus is on developing electrode materials capable of stable and rapid insertion and extraction of sodium ions. However, the larger radius of sodium ions and lower specific capacity in sodium-ion cathode materials result in poor cycle performance and discharge rate performance of sodium-ion batteries, becoming a major factor restricting the application of sodium-ion battery cathode materials.

[0003] Researchers have made significant efforts to improve the electrochemical properties and structural stability of sodium-ion cathode materials. For example, CN113948697A discloses a doped sodium iron phosphate cathode material, its preparation method, and its applications. The prepared doped sodium iron phosphate cathode material introduces nickel and cobalt to dope sodium iron phosphate. First, a nickel alloy is leached with acid, then a reducing agent and alkali are added to adjust the pH, resulting in a precipitation reaction and obtaining a mixed hydroxide containing ferrous nickel. This ferrous nickel mixed hydroxide is then mixed with a phosphate-containing substance, a sodium source, a cobalt source, a dispersant, and aluminum fluoride, and ball-milled and calcined to obtain the doped sodium iron phosphate cathode material. The phase transition during the sodium ion insertion / extraction process is improved by adjusting the ratio of nickel and cobalt phosphate, and then aluminum fluoride is coated onto the surface of the doped sodium iron phosphate. The resulting material achieves a room-temperature discharge specific capacity of 105.6 mAh / g and an initial charge / discharge efficiency of 72.4%. CN113972364A discloses a method for preparing layered carbon-doped sodium iron phosphate cathode material. The method includes placing carbonate powder under an inert atmosphere, introducing gaseous organic matter, and heating to react, thereby obtaining MCO3 / C layered carbon material. The MCO3 / C layered carbon material, sodium source, ferrous phosphate, and dispersant are mixed under an inert atmosphere, then ground, washed, dried to remove the dispersant, and heated under an inert atmosphere to react, thus obtaining the layered carbon-doped sodium iron phosphate cathode material. The obtained material achieves a room-temperature discharge specific capacity of 116.4 mAh / g and an initial charge-discharge efficiency of 63.6%.

[0004] Although it has improved the electrochemical performance of sodium-ion battery cathode materials to some extent, the operation process is complex and costly, and there is still room for further improvement in its conductivity, specific capacity, and rate performance. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing doped and modified sodium iron pyrophosphate cathode material, which is simple in process and low in cost, and the obtained sodium iron pyrophosphate cathode material has better conductivity, specific capacity, rate performance and longer cycle life.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] A method for preparing a doped and modified sodium iron pyrophosphate cathode material includes the following steps:

[0008] (1) Weigh sodium source, carbon source, phosphorus source, iron source, dopant element M and solvent. First, mix iron source and dopant element M evenly, then add sodium source, carbon source, phosphorus source and solvent. Ball mill the resulting mixture, dry it, add a certain amount of sodium supplement and mix evenly to obtain the precursor; the molar ratio of sodium element to iron element is 1.6-2.2:1.

[0009] Adding elements in this order allows for better in-situ doping at the Fe site. After drying, sodium supplementation is added, allowing the precursor and sodium supplementation to react during sintering, forming a protective layer on the material surface and reducing the formation of sodium dendrites during charging and discharging.

[0010] (2) The precursor obtained in step (1) is heated to 400-500℃ and calcined for 5-15 hours in an inert gas atmosphere, and then naturally cooled to room temperature.

[0011] (3) The sintered product obtained in step (2) is crushed and sieved to obtain the sodium iron pyrophosphate cathode material.

[0012] The sodium source in step (1) is any one or a combination of at least two of sodium hydroxide, sodium carbonate, sodium oxalate, sodium nitrite, disodium hydrogen phosphate, sodium bicarbonate, sodium citrate, anhydrous sodium sulfate, sodium stearate, sodium oleate, sodium tartrate, sodium alginate, sodium carboxymethyl cellulose, or sodium lactate; the carbon source is any one or a combination of at least two of glucose, sucrose, fructose, starch, citric acid, ascorbic acid, tartaric acid, or oxalic acid.

[0013] The phosphorus source mentioned in step (1) is one or a combination of two of the following: phosphoric acid, manganese phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ferric phosphate, and sodium dihydrogen phosphate; the iron source is one or a combination of two of the following: ferric phosphate, ferric oxide, ferrous oxalate, ferric nitrate, ferric oxalate, ferric acetate, ferrous acetate, ferrous sulfate, and ferrous chloride.

[0014] The doping element M mentioned in step (1) is one or a combination of two of Zn, Co, Cu, Mg, Ti, Zr, Al, V, Cr, and Nb.

[0015] The doping element M mentioned in step (1) is a combination of Ti and Cr.

[0016] In step (1), the molar ratio of Na, Ti, Cr, Fe and P is (1.6-2.2):(0.01-0.03):(0.02-0.05):1:1.

[0017] The solvent is one or more of pure water, ethanol, propanol, and acetone.

[0018] The carbon source content in step (1) is 1%-10% of the mass of the cathode material; the solvent content is 20-80% of the mass of the cathode material.

[0019] In step (1), the sodium supplement is at least one of sodium squartzate, sodium oxalate, sodium hydride, sodium nitride, sodium acetylene, and metallic sodium powder; the amount of sodium supplement added is 20%-40% of the mass of the positive electrode material.

[0020] The inert gas mentioned in step (2) is at least one of nitrogen, helium, neon, argon, krypton, xenon, and radon.

[0021] Compared with existing technologies, the method for preparing sodium-ion cathode materials described in this invention has the following advantages:

[0022] (1) By directly controlling the molar ratio of Na:Fe in the starting material, that is, by adjusting the non-stoichiometric ratio within a certain range, defects are induced in the crystal structure of Na2FeP2O7 during the low-temperature sintering process, which can change the basic electrochemical behavior. When a suitable sodium supplement is added, pores are formed on the positive electrode side during decomposition, thereby strengthening the interfacial contact between the active material and the electrolyte. During the charging and discharging process, a large number of sodium ions can be provided while controlling the internal expansion force of the cell, inhibiting the formation of sodium dendrites, alleviating the irreversible capacity loss caused by phase change, increasing storage capacity, improving conductivity and sodium ion mobility.

[0023] (2) By modifying Na2FeP2O7 material through double doping with heterovalent metal cations at the Fe sites, Ti… 4+ and Cr 3+ Doping causes lattice distortion, which leads to lattice shrinkage in the material and increases the intrinsic electronic conductivity. Compared with doped Fe isovalent metal cations, it has better kinetic behavior, allowing sodium ions to diffuse faster and thus improving the bulk conductivity and overall material performance.

[0024] The combined effect of these two factors forms a cathode material with a relatively stable crystal structure, which increases the diffusion rate of sodium ions in the crystal lattice and the doped Ti 4+ and Cr 3+Inducing distortion within the crystal lattice can support the Na2FeP2O7 / C structure, reduce charge transfer resistance and the strain effect of sodium ion insertion / extraction, thereby ensuring the structural stability of the material under high current density and during cycling, resulting in better conductivity, specific capacity, rate performance and longer cycle life.

[0025] This method synthesizes the material through a simple ball milling process, which does not require long-term ball milling or the addition of high-pressure equipment. The preparation process is simple, environmentally friendly, and low-cost, which is conducive to large-scale production and has good application prospects. Attached Figure Description

[0026] Figure 1 These are scanning electron microscope (SEM) images and elemental surface scans of the doped and modified sodium iron pyrophosphate cathode material prepared in Example 1.

[0027] Figure 2 This is a comparison chart of the cycle performance of Examples 1-4 and Comparative Examples 1-2;

[0028] Figure 3 This is a rate performance diagram of the doped and modified sodium iron pyrophosphate cathode material prepared in Example 1;

[0029] Figure 4 These are the AC impedance diagrams for Example 1 and Comparative Examples 1-2. Detailed Implementation

[0030] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. To further understand this invention, the following description, in conjunction with the specification and specific preferred embodiments, will further illustrate the invention. These embodiments should not be construed as limiting the scope of protection claimed in this application.

[0031] Example 1

[0032] This embodiment provides a doped and modified sodium iron pyrophosphate cathode material, and the preparation method of the sodium-ion battery cathode material is as follows:

[0033] Sodium carbonate, nano-chromium trioxide, nano-titanium dioxide, ferrous oxalate, and ammonium dihydrogen phosphate were weighed according to the molar ratio of Na, Cr, Ti, Fe, and P of 2.06:0.02:0.03:1:1. Ferrous oxalate was mixed evenly with nano-chromium trioxide and nano-titanium dioxide, then sodium carbonate and ammonium dihydrogen phosphate were added. Next, 6% glucose and 40% pure water (based on the total weight of the cathode material) were added, and the mixture was ball-milled for 8 hours. After drying, sodium squartz (based on the weight of the cathode material) was added and mixed evenly. The mixture was then placed in a crucible and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was increased to 450℃ at 3℃ / min and calcined for 10 hours. After natural cooling to room temperature, the mixture was pulverized and sieved to obtain the doped and modified sodium iron pyrophosphate cathode material.

[0034] in, Figure 1 These are scanning electron microscope (SEM) images and elemental surface scans of the doped and modified sodium iron pyrophosphate cathode material prepared in Example 1.

[0035] Example 2

[0036] Sodium carbonate, nano-chromium trioxide, nano-titanium dioxide, ferrous oxalate, and diammonium hydrogen phosphate were weighed according to the molar ratio of Na, Cr, Ti, Fe, and P of 2.03:0.03:0.02:1:1. Ferrous oxalate was mixed evenly with nano-chromium trioxide and nano-titanium dioxide. Sodium carbonate and diammonium hydrogen phosphate were then added, followed by 6% glucose and 40% pure water (based on the total weight of the cathode material). The mixture was ball-milled for 8 hours, dried, and then 10% sodium squartz (based on the weight of the cathode material) was added and mixed evenly. The mixture was then placed in a crucible and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was increased to 480℃ at 3℃ / min and calcined for 10 hours. After natural cooling to room temperature, the mixture was pulverized and sieved to obtain the doped and modified sodium iron pyrophosphate cathode material.

[0037] Example 3

[0038] Sodium bicarbonate, nano-zirconia, nano-titanium dioxide, ferrous acetate, and ammonium dihydrogen phosphate were weighed according to a molar ratio of Na, Zr, Ti, Fe, and P of 2.1:0.03:0.02:1:1. Ferrous acetate was mixed evenly with nano-zirconia and nano-titanium dioxide. Sodium bicarbonate and ammonium dihydrogen phosphate were then added, followed by 6% citric acid and 40% pure water (based on the total weight of the cathode material). The mixture was ball-milled for 8 hours, dried, and then 50% sodium squartz (based on the mass of the cathode material) was added and mixed evenly. The mixture was then placed in a crucible and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was increased to 520℃ at 3℃ / min and calcined for 10 hours. After natural cooling to room temperature, the mixture was pulverized and sieved to obtain the doped and modified sodium iron pyrophosphate cathode material.

[0039] Example 4

[0040] Sodium carbonate, nano-alumina, nano-titanium dioxide, ferrous oxalate, and diammonium hydrogen phosphate were weighed according to the molar ratio of Na, Al, Ti, Fe, and P of 2.06:0.02:0.03:1:1. Ferrous oxalate was mixed evenly with nano-alumina and nano-titanium dioxide. Sodium carbonate and diammonium hydrogen phosphate were then added, followed by 8% glucose and 50% pure water (based on the total weight of the cathode material). The mixture was ball-milled for 8 hours, dried, and then 30% sodium nitride (based on the mass of the cathode material) was added and mixed evenly. The mixture was then placed in a crucible and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was increased to 550℃ at 3℃ / min and calcined for 10 hours. After natural cooling to room temperature, the mixture was pulverized and sieved to obtain the doped and modified sodium iron pyrophosphate cathode material.

[0041] Example 5

[0042] Sodium carbonate, nano-chromium trioxide, nano-zinc oxide, ferrous oxalate, and ferric phosphate were weighed according to the molar ratio of Na, Cr, Zn, Fe, and P of 2.3:0.03:0.05:1:1. Ferrous oxalate, nano-chromium trioxide, and nano-zinc oxide were mixed evenly, and sodium carbonate and ferric phosphate were added. Then, 5% glucose and 40% pure water (based on the total weight of the cathode material) were added to the ball mill and milled for 8 hours. After drying, sodium acetylene (based on the weight of the cathode material) was added and mixed evenly. The mixture was then placed in a sagger and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was increased to 350℃ at 3℃ / min and calcined for 10 hours. After natural cooling to room temperature, the mixture was pulverized and sieved to obtain the doped and modified sodium iron pyrophosphate cathode material.

[0043] Example 6

[0044] Sodium carbonate, nano-chromium trioxide, nano-titanium dioxide, ferrous oxalate, and ammonium dihydrogen phosphate were weighed according to the molar ratio of Na, Cr, Ti, Fe, and P as 1.5:0.05:0.03:1:1. Ferrous oxalate was mixed evenly with nano-chromium trioxide and nano-titanium dioxide. Sodium carbonate and ammonium dihydrogen phosphate were then added, followed by 6% glucose and 40% pure water (based on the total weight of the cathode material). The mixture was ball-milled for 8 hours, dried, and then 35% sodium oxalate (based on the weight of the cathode material) was added and mixed evenly. The mixture was then placed in a crucible and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was increased to 450℃ at 3℃ / min and calcined for 12 hours. After natural cooling to room temperature, the mixture was pulverized and sieved to obtain the doped and modified sodium iron pyrophosphate cathode material.

[0045] Example 7

[0046] Sodium sulfate, nano-chromium trioxide, nano-titanium dioxide, ferrous phosphate, and sodium dihydrogen phosphate were weighed according to the molar ratio of Na, Cr, Ti, Fe, and P of 1.95:0.03:0.08:1:1. Ferrous phosphate was mixed evenly with nano-chromium trioxide and nano-titanium dioxide, and then sodium carbonate and sodium dihydrogen phosphate were added. Next, 6% oxalic acid and 50% pure water (based on the total weight of the cathode material) were added to a ball mill and ball-milled for 8 hours. After drying, 40% sodium squartz (based on the weight of the cathode material) was added and mixed evenly. The mixture was then placed in a crucible and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was increased to 500℃ at 3℃ / min and calcined for 10 hours. After natural cooling to room temperature, the mixture was pulverized and sieved to obtain the doped and modified sodium iron pyrophosphate cathode material.

[0047] Example 8

[0048] Sodium nitrate, nano-chromium trioxide, nano-magnesium oxide, ferrous oxalate, and ammonium dihydrogen phosphate were weighed according to the molar ratio of Na, Cr, Mg, Fe, and P as 1:0.02:0.03:1:1. Ferrous oxalate was mixed evenly with nano-chromium trioxide and nano-magnesium oxide. Sodium carbonate and ammonium dihydrogen phosphate were then added. 10% glucose and 40% pure water (based on the total weight of the cathode material) were added to the mixture and ball-milled for 8 hours. After drying, sodium squartz (based on the weight of the cathode material) was added and mixed evenly. The mixture was then placed in a crucible and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was increased to 380℃ at 3℃ / min and calcined for 10 hours. After natural cooling to room temperature, the mixture was pulverized and sieved to obtain the doped and modified sodium iron pyrophosphate cathode material.

[0049] Comparative Example 1

[0050] Sodium carbonate, nano-chromium trioxide, nano-titanium dioxide, ferrous oxalate, and ammonium dihydrogen phosphate were weighed according to the molar ratio of Na, Cr, Ti, Fe, and P as 2.06:0.02:0.03:0.8:1. Ferrous oxalate was mixed evenly with nano-chromium trioxide and nano-titanium dioxide. Sodium carbonate and ammonium dihydrogen phosphate were then added, followed by 6% glucose and 40% pure water (based on the total weight of the cathode material). The mixture was ball-milled for 8 hours, dried, and then 35% sodium squartz (based on the weight of the cathode material) was added and mixed evenly. The mixture was then placed in a crucible and calcined in a tube furnace at 450°C for 10 hours under a nitrogen atmosphere with a heating rate of 3°C / min. After natural cooling to room temperature, the mixture was pulverized and sieved to obtain the doped and modified sodium iron pyrophosphate cathode material.

[0051] Comparative Example 2

[0052] Sodium carbonate, nano-chromium trioxide, nano-titanium dioxide, ferrous oxalate, and ammonium dihydrogen phosphate were weighed according to the molar ratio of Na, Cr, Ti, Fe, and P as 2.06:0.02:0.03:0.75:1. Ferrous oxalate was mixed evenly with nano-chromium trioxide and nano-titanium dioxide. Sodium carbonate and ammonium dihydrogen phosphate were then added, followed by 6% glucose and 40% pure water (based on the total weight of the cathode material). The mixture was ball-milled for 8 hours, dried, and then 35% sodium squartz (based on the weight of the cathode material) was added and mixed evenly. The mixture was then placed in a crucible and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was increased to 450℃ at 3℃ / min and calcined for 10 hours. After natural cooling to room temperature, the mixture was pulverized and sieved to obtain the doped and modified sodium iron pyrophosphate cathode material.

[0053] Comparative Example 3

[0054] Sodium carbonate, nano-chromium trioxide, nano-titanium dioxide, ferrous oxalate, and ammonium dihydrogen phosphate were weighed according to the molar ratio of Na, Cr, Ti, Fe, and P as 2.06:0.02:0.03:1.15:1. Ferrous oxalate was mixed evenly with nano-chromium trioxide and nano-titanium dioxide, then sodium carbonate and ammonium dihydrogen phosphate were added. Next, 6% glucose and 40% pure water (based on the total weight of the cathode material) were added, and the mixture was ball-milled for 8 hours. After drying, sodium squartz (based on the weight of the cathode material) was added and mixed evenly. The mixture was then placed in a crucible and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was increased to 450℃ at 3℃ / min and calcined for 10 hours. After natural cooling to room temperature, the mixture was pulverized and sieved to obtain the doped and modified sodium iron pyrophosphate cathode material.

[0055] Comparative Example 4

[0056] Sodium carbonate, nano-chromium trioxide, nano-titanium dioxide, ferrous oxalate, and ammonium dihydrogen phosphate were weighed according to the molar ratio of Na, Cr, Ti, Fe, and P as 2.06:0.02:0.03:1.05:1. Ferrous oxalate was mixed evenly with nano-chromium trioxide and nano-titanium dioxide, and then sodium carbonate and ammonium dihydrogen phosphate were added. 6% glucose and 40% pure water (based on the total weight of the cathode material) were then added, and the mixture was ball-milled for 8 hours. After drying, sodium squartz (based on the weight of the cathode material) was added and mixed evenly. The mixture was then placed in a crucible and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was increased to 450℃ at 3℃ / min and calcined for 10 hours. After natural cooling to room temperature, the mixture was pulverized and sieved to obtain the doped and modified sodium iron pyrophosphate cathode material.

[0057] Comparative Example 5

[0058] The only difference between this comparative example and Example 1 is that no doping elements are added during the mixing process; all other conditions and parameters are exactly the same as in Example 1.

[0059] Comparative Example 6

[0060] The only difference between this comparative example and Example 1 is that no sodium supplement is added during mixing; all other conditions and parameters are exactly the same as in Example 1.

[0061] Comparative Example 7

[0062] The only difference between this comparative example and Example 1 is that nano-titanium dioxide is not added during mixing; all other conditions and parameters are exactly the same as in Example 1.

[0063] Comparative Example 8

[0064] The only difference between this comparative example and Example 1 is that nano-chromium trioxide is not added during mixing; all other conditions and parameters are exactly the same as in Example 1.

[0065] Table 1

[0066]

[0067]

[0068] As shown in Table 1, Examples 1-8 and Comparative Examples 1-4 modified Na2FeP2O7 material by adjusting the Na:Fe molar ratio in the starting material and by doping Fe sites with heterovalent metal cations, forming a cathode material with a relatively stable crystal structure. The initial charge specific capacity increased from 84.52 mAh / g to 125.48 mAh / g, the discharge specific capacity increased from 66.41 mAh / g to 121.1 mAh / g, and the initial charge-discharge efficiency reached 96.51%.

[0069] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

[0070] Button cell fabrication: The modified sodium iron pyrophosphate composite material obtained above was assembled into button cells. The obtained sodium-ion battery positive electrode material was mixed with conductive carbon black and PVDF binder at a mass ratio of 8:1:1, and N-methylpyrrolidone solution was added and mixed evenly to prepare the battery positive electrode slurry. The slurry was coated on aluminum foil, vacuum dried and rolled to form a positive electrode sheet. Sodium metal sheet was used as the negative electrode, 1 mol / L NaPF6 ethylene carbonate (EC) / dimethyl carbonate (DMC) (volume ratio 1:1) solution was used as the electrolyte, and glass fiber was used as the separator. The button cells were assembled in an argon-filled glove box.

[0071] The button batteries prepared with the cathode materials in Examples 1-8 and Comparative Examples 1-2 were tested using a blue electric field tester. The voltage range was 2.0-4.5V, and one cycle of 0.1C charge-discharge activation was performed to obtain the initial charge-discharge specific capacity and initial coulombic efficiency. The test results are shown in Table 1. Then, constant current and constant voltage charging was performed at 0.5C, with a cutoff current of 0.05C, followed by constant current discharge at 1C for 50 cycles. Data on parameters such as the discharge capacity at the 50th cycle and the capacity retention rate at the 50th cycle were obtained. The test results are as follows: Figure 2 As shown. Button batteries prepared with the positive electrode materials in Examples 1-6 and Comparative Examples 1-2 were tested using a blue electric current tester. The voltage range was 2.0-4.5V. Constant current and constant voltage charging was performed at a current of 0.5C, with a charging cutoff current of 0.05C. Constant current discharging was performed at currents of 0.1C, 0.2C, 0.5C, 1C, 2C, and 0.1C, respectively, with a discharge cutoff voltage of 2V. The test results are as follows. Figure 3 As shown.

[0072] The AC impedance of Na2FeP2O7 / C was measured in the frequency range of 0.01Hz-0.1MHz, and the results are as follows: Figure 4 As shown in the figure, the semicircle in the electrochemical impedance spectroscopy of the material represents the charge transfer impedance (Rct), and the slanted line represents the Warburg impedance. It can be observed from the figure that the slanted line of the Na2FeP2O7 / C sample prepared in Example 1 is larger than that of the comparative example, indicating that the Warburg impedance (Zw) is smaller and the ion diffusion is better.

[0073] The above embodiments are only used to explain the inventive concept of the present invention, and are not intended to limit the protection of the present invention. Any non-substantial modifications made to the present invention using this concept should fall within the protection scope of the present invention.

Claims

1.A method for preparing a doped modified sodium iron pyrophosphate positive electrode material, characterized by comprising the following steps: (1) weighing a sodium source, a carbon source, a phosphorus source, an iron source, a doping element M, and a solvent, mixing the iron source and the doping element M uniformly first, then adding the sodium source, the carbon source, the phosphorus source, and the solvent, ball-milling the obtained mixture, adding a certain amount of a sodium supplementing agent after drying, and mixing uniformly to obtain a precursor; the molar ratio of sodium element to iron element is 1.6-2.2:1; (2) calcining the precursor obtained in step (1) under an inert gas atmosphere to 400-500℃ for 5-15h, and naturally cooling to room temperature; (3) crushing and sieving the sintered product obtained in step (2) to obtain the sodium iron pyrophosphate positive electrode material; in step (1), the doping element M is a combination of Ti and Cr; in step (1), the molar ratio of Na element, Ti element, Cr element, Fe element, and P element is (1.6-2.2):(0.01-0.03):(0.02-0.05):1:1; the solvent is one or two or more of pure water, ethanol, propanol, and acetone; in step (1), the content of the carbon source is 1%-10% of the mass of the positive electrode material; the content of the solvent is 20-80% of the mass of the positive electrode material; in step (1), the sodium supplementing agent is at least one of sodium hypophosphite, sodium oxalate, sodium hydride, sodium nitride, sodium acetylide, and metal sodium powder; the addition amount of the sodium supplementing agent is 20%-40% of the mass of the positive electrode material. In step (1), the sodium source is any one or a combination of at least two of sodium hydroxide, sodium carbonate, sodium oxalate, sodium nitrite, disodium hydrogen phosphate, sodium bicarbonate, sodium citrate, anhydrous sodium sulfate, sodium stearate, sodium oleate, sodium tartrate, sodium alginate, sodium carboxymethyl cellulose, or sodium lactate; the carbon source is any one or a combination of at least two of glucose, sucrose, fructose, starch, citric acid, ascorbic acid, tartaric acid, or oxalic acid. In step (1), the phosphorus source is one or a combination of at least two of phosphoric acid, manganese phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, iron phosphate, and sodium dihydrogen phosphate; the iron source is one or a combination of at least two of iron phosphate, diiron trioxide, ferrous oxalate, ferric nitrate, ferric oxalate, ferric acetate, ferrous acetate, ferrous sulfate, and ferrous chloride. In step (2), the inert gas is at least one of nitrogen, helium, neon, argon, krypton, xenon, and radon. ​ ​ ​ ​ ​ 2. The preparation method of the doped modified sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that: ​ 3. The preparation method of the doped modified sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that: ​ 4. The preparation method of the doped modified sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that: ​

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