Preparation method of iron-based polyanionic composite positive electrode material, positive electrode material, pole piece and battery

Abstract

The invention relates to the field of sodium ion batteries, and particularly provides a preparation method of an iron-based polyanionic composite positive electrode material, the positive electrode material, a pole piece and a battery. The preparation method of the iron-based polyanionic composite positive electrode material comprises the following steps: mixing precursor powder of M element doped Na4Fe3-3x M3x (PO4) 2 (P2O7) with a carbon source to form a mixture, where X is greater than 0 and less than or equal to 0.2, and the Dv50 particle size of the precursor powder is less than or equal to 20 microns; the mixture is sintered under the oxygen-free condition, and Na4Fe3-3x M3x (PO4) 2 (P2O7) (at) C is obtained. The gram volume and the compaction density of the positive electrode material are improved, so that the energy density of the material is improved.

Description

Technical Field

[0001] The present invention relates to the field of sodium-ion batteries, and specifically provides a preparation method of an iron-based polyanionic composite cathode material, a cathode material, an electrode sheet, and a battery. Background Art

[0002] Under the background of high raw material costs of lithium-ion batteries and limited lithium resources, sodium-ion batteries have received extensive attention due to their low cost and wide sources. The polyanionic composite sodium iron phosphate material has a relatively stable structure and low cost, and has become a candidate with great potential in the future energy storage field. However, its specific capacity and tap density are relatively low, resulting in a large gap in its energy density compared with lithium iron phosphate and sodium layered oxides, thus limiting its industrialization process.

[0003] Sodium-ion battery cathodes can be broadly classified into three categories: Prussian blue, layered oxides, and polyanions. Among them, polyanions (such as Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 )) have a relatively stable structure and strong safety performance, and have great application potential in the energy storage field. However, the tap density and specific capacity of polyanions are much lower than those of layered oxides, resulting in an energy density that cannot meet the actual use requirements. As the composite sodium iron phosphate cathode, which has the best comprehensive performance among polyanions, also faces this problem in terms of energy density. To achieve large-scale industrialization, improving the energy density of this material has become an issue that people must face. To increase the energy density, people consider doping to improve the specific capacity of the material. However, the calcination temperature of this material is relatively low (<600 °C), and it is difficult for doped elements to enter the crystal lattice. Summary of the Invention

[0004] To overcome the above defects, the present invention provides a preparation method of an iron-based polyanionic composite cathode material, a cathode material, an electrode sheet, and a battery, which can improve the specific capacity and tap density of the cathode material, thereby improving the energy density of the material.

[0005] In a first aspect, the present invention provides a preparation method of an iron-based polyanionic composite cathode material, including:

[0006] Mixing Na 4 Fe 3-3x M 3x (PO 4 ) 2 (P 2 O 7) The precursor powder and a carbon source are mixed to form a mixture, where 0 < X ≤ 0.2, and the Dv50 particle size of the precursor powder ≤ 20 μm;

[0007] The mixture is sintered under an oxygen-free condition to obtain Na 4 Fe 3-3x M 3x (PO 4 ) 2 (P 2 O 7 )@C.

[0008] Furthermore, the preparation method of the precursor powder of M element-doped Na 4 Fe 3-3x M 3x (PO 4 ) 2 (P 2 O 7 ) includes:

[0009] A coprecipitate is obtained from a first mixture formed by mixing an iron source, a first phosphorus source, and an M element doping source;

[0010] The product after sintering the coprecipitate is mixed with a second phosphorus source, a sodium source, and water to obtain a precursor slurry;

[0011] After the precursor slurry is dried and pulverized, a precursor powder with a Dv50 particle size ≤ 20 μm is obtained.

[0012] Furthermore, the pH value of the first mixture is adjusted to a preset value to obtain the coprecipitate.

[0013] Furthermore, the mixing ratio of the iron source, the first phosphorus source, and the M element doping source is:

[0014] The molar ratio of Fe in the iron source: P in the first phosphorus source: M element in the M element doping source is (1 - x): 1: x, where 0 < X ≤ 0.2.

[0015] Furthermore, during the mixing of the product after sintering the coprecipitate with the second phosphorus source, the sodium source, and water, the mixing ratio is:

[0016] The molar ratio of the sintered product, P in the second phosphorus source, and Na in the sodium source is 3:1:4;

[0017] In the precursor slurry formed by mixing the product after sintering the coprecipitate with the second phosphorus source, the sodium source, and water, the amount of water added is such that the solid content of the precursor slurry is greater than 20% and less than 50%.

[0018] Furthermore, the mixing of the precursor powder and the carbon source includes:

[0019] The precursor slurry is dried and then pulverized to a Dv50 particle size ≤ 20 μm to obtain a second mixture, and the second mixture is the precursor powder;

[0020] The second mixture is mixed with a carbon source.

[0021] Furthermore, the carbon source is a carbon source solution.

[0022] Furthermore, the mass ratio of the carbon source in the second mixture to the carbon source in the carbon source solution is (85 - 95):(5 - 15).

[0023] Furthermore, the sintering temperature of the coprecipitate is a first preset temperature.

[0024] Furthermore, the sintering of the mixture under an oxygen-free condition includes:

[0025] Under an oxygen-free condition, the mixture is sintered at a second preset temperature.

[0026] Furthermore, the first preset temperature is greater than the second preset temperature.

[0027] Furthermore, the M element includes one or more of the elements B, Ce, Co, Cu, Ga, Ti, V, Zr, Ca, La, Lu, Y, Zn, Al, Mg, and Mn.

[0028] In a second aspect, the present invention provides an iron-based polyanionic composite cathode material, and the structural general formula of the iron-based polyanionic composite cathode material is Na 4 Fe 3-3x M 3x (PO 4 ) 2 (P 2 O 7 )@C, where 0 < X ≤ 0.2.

[0029] Furthermore, the M includes one or more of the elements B, Ce, Co, Cu, Ga, Ti, V, Zr, Ca, La, Lu, Y, Zn, Al, Mg, and Mn.

[0030] In a third aspect, the present invention provides a positive electrode sheet, including: a current collector and a positive electrode material layer coated on the current collector, and the positive electrode material layer includes the iron-based polyanionic composite cathode material described in the second aspect or the iron-based polyanionic composite cathode material prepared by using the preparation method described in the first aspect.

[0031] In a fourth aspect, the present invention provides a sodium-ion battery, including the positive electrode sheet described in the third aspect.

[0032] One or more of the above technical solutions of the present invention have at least one or more of the following beneficial effects:

[0033] In implementing the technical solution of the present invention, through the doping treatment of elements, the specific capacity and average voltage performance of sodium iron phosphate composite can be effectively improved.

[0034] On the basis of element doping, carbon coating is carried out. Specifically, the precursor slurry is soaked with a carbon source in a powder state, which can further improve the tap density of the material.

[0035] Combining the above two methods can significantly improve the energy density of sodium iron phosphate composite. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Referring to the accompanying drawings, the disclosure of the present invention will become more readily understood. It is easy for those skilled in the art to understand that these drawings are only for illustrative purposes and are not intended to limit the scope of protection of the present invention. In addition, similar numbers in the drawings are used to represent similar components, where:

[0037] Figure 1 is a schematic flow chart of the main steps of a method for preparing an iron-based polyanionic composite cathode material according to an embodiment of the present invention;

[0038] Figure 2 is a scanning electron micrograph of the cathode material obtained according to Embodiment 1 of the present invention;

[0039] Figure 3 is a scanning electron micrograph of the cathode material obtained according to Comparative Example 1 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0040] The following describes some embodiments of the present invention with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are only used to explain the technical principle of the present invention and are not intended to limit the scope of protection of the present invention.

[0041] The present invention provides a method for preparing an iron-based polyanionic composite cathode material, referring to Figure 1 , including:

[0042] S1, mixing the precursor powder of Na 4 Fe 3-3x M 3x (PO 4 ) 2 (P 2 O 7 ) doped with M element and a carbon source to form a mixture, where 0 < X ≤ 0.2, and the Dv50 particle size of the precursor powder ≤ 20 μm;

[0043] S2, the mixture is sintered under anaerobic conditions to obtain Na 4 Fe 3-3x M 3x (PO 4 ) 2 (P 2 O 7 )@C.

[0044] In one embodiment, the M element includes one or more of the elements B, Ce, Co, Cu, Ga, Ti, V, Zr, Ca, La, Lu, Y, Zn, Al, Mg, and Mn.

[0045] The present invention provides a high-energy-density composite sodium iron phosphate cathode and a preparation method thereof. First, an iron source, a phosphorus source, and a sodium source are doped with the M element. Through doping, the doped elements can be evenly distributed in the lattice. The above-mentioned doped elements can improve the conductivity of the composite sodium iron phosphate, increase the specific capacity and discharge average voltage. Next, the product after element doping in the present invention is carbon-coated in a pulverized state. First, it is dried to obtain a dry material, then the dry material is pulverized to reduce its particle size and then mixed with a carbon source, and finally sintered. In this way, the carbon source can penetrate into the gaps of the pulverized precursor dry powder. During sintering pyrolysis, in-situ carbonization occurs on the particle surface and in the gaps, avoiding the influence of the generated gas on its structure. At the same time, it can also fill a certain amount of the original pores, improve the compaction density of the material, and make the carbon coating continuous, uniform, and effective.

[0046] Conventional composite sodium iron phosphate often mixes the sodium source, phosphorus source, iron source, and carbon source together to make a precursor slurry, and then dries, high-temperature sinters, and then air-grinds. When the carbon source and the precursor are mixed and pyrolyzed, a lot of gas is often generated due to the decomposition of the carbon source. These gases will cause the internal structure to be loose and porous, reducing the compaction density of the material. In addition, carbon layer peeling is likely to occur during air-grinding after carbon coating, which is not conducive to improving the conductivity.

[0047] The present invention conducts metal doping and carbon coating on the iron-based polyanionic cathode material Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) to improve the specific capacity, discharge average voltage, and compaction density of the material, thereby jointly improving the energy density of the cathode material.

[0048] The present invention is based on the iron-based polyanionic cathode material Na 4 Fe 3 (PO 4 ) 2 P 2 O7 Metal doping and carbon coating treatments have significantly improved the specific capacity, average discharge voltage, and tap density of the material. Specifically, during the doping process, by introducing other elements, the electrochemical performance of the material is improved, thereby increasing its specific capacity, that is, the charge storage capacity per unit mass. At the same time, using the carbon coating technology, a conductive carbon layer is formed on the surface of the material, which not only enhances the conductivity of the material but also improves its structural stability. The increase in the average discharge voltage means that the battery can output a higher voltage more stably during the discharge process, which is directly related to the energy density and charge storage capacity of the battery. For sodium-ion batteries, this is an important sign of technological progress and helps to enhance its application value in the fields of energy storage and power. The increase in tap density means that more active materials can be stored in the same volume, thus further increasing the energy density of the positive electrode material.

[0049] The present invention has significantly improved the overall performance of the positive electrode material, making it have a higher energy density in practical applications and meeting the requirements of modern battery technology for high-performance materials.

[0050] In one embodiment, in step S1, the precursor powder of Na 4 Fe 3-3x M 3x (PO 4 ) 2 (P 2 O 7 ) is prepared by the following method:

[0051] (1) A coprecipitate is obtained from a first mixture formed by mixing an iron source, a first phosphorus source, and an M element doping source.

[0052] In one embodiment, the pH value of the first mixture is adjusted to a preset value to obtain the coprecipitate.

[0053] In one embodiment, the mixing ratio of the iron source, the first phosphorus source, and the M element doping source is as follows:

[0054] The molar ratio of Fe in the iron source: P in the first phosphorus source: M element in the M element doping source is (1 - x): 1: x, where 0 < X ≤ 0.2.

[0055] In one embodiment, the pH value of the first mixture is adjusted to pH 2 - 5 to obtain the coprecipitate.

[0056] (2) The product after sintering the coprecipitate is mixed with a second phosphorus source, a sodium source, and water to obtain a precursor slurry.

[0057] In one embodiment, during the mixing process of the product after sintering the coprecipitate with the second phosphorus source, the sodium source, and water, the mixing ratio is as follows:

[0058] The molar ratio of P in the sintered product and the second phosphorus source to Na in the sodium source is 3:1:4;

[0059] In the precursor slurry formed by mixing the product obtained by sintering the coprecipitate with the second phosphorus source, the sodium source and water, the amount of water added is such that the solid content of the precursor slurry is greater than 20% and less than 50%.

[0060] The specific preparation process of the precursor slurry is as follows:

[0061] S11, mixing the iron source, the first phosphorus source and the M element doping source uniformly to form a first mixture;

[0062] S12, adjusting the pH of the first mixture to a preset value to obtain a coprecipitate;

[0063] S13, sintering the coprecipitate at a first preset temperature;

[0064] S14, mixing the sintered product with the second phosphorus source, the sodium source and water to obtain a precursor slurry.

[0065] In one embodiment, the temperature for sintering the coprecipitate is the first preset temperature.

[0066] In one embodiment, the sintering of the coprecipitate at the first preset temperature includes:

[0067] Sintering the coprecipitate at 650 - 900 °C for 6 - 12 h.

[0068] (3) After the precursor slurry is dried and pulverized, a precursor powder with a Dv50 particle size ≤ 20 μm is obtained.

[0069] In one embodiment, the mixing process of the precursor powder and the carbon source includes:

[0070] The precursor slurry is dried and pulverized to a Dv50 particle size ≤ 20 μm to obtain a second mixture, which is a precursor powder of M - element - doped Na 4 Fe 3-3x M 3x (PO 4 ) 2 (P 2 O 7 ).

[0071] Mix the second mixture and the carbon source.

[0072] In one embodiment, after the precursor slurry is spray - dried, it is subjected to jet milling to obtain a second mixture.

[0073] The mixture in step S2 includes a second mixture and a carbon source.

[0074] In the present invention, the first mixture, the second mixture, and the mixture respectively represent three different mixed contents.

[0075] In step S2, the sintering of the mixture under an oxygen-free condition includes:

[0076] After mixing the second mixture and the carbon source, sintering is carried out under an oxygen-free condition.

[0077] The second mixture is a precursor powder of Na 4 Fe 3-3x M 3x (PO 4 ) 2 (P 2 O 7 ) with a Dv50 particle size of 0.2 μm - 20 μm. Within this particle size range, it is beneficial for Na + to diffuse inward, which is beneficial for fully forming the desired product.

[0078] In one embodiment, the carbon source is a carbon source solution.

[0079] In one embodiment, the mass ratio of the carbon source in the precursor powder and the carbon source solution is (85 - 95):(5 - 15).

[0080] The mixing method of the precursor powder and the carbon source is:

[0081] Soak the precursor powder into the carbon source solution.

[0082] In one embodiment, the sintering of the mixture under an oxygen-free condition includes:

[0083] Under an oxygen-free condition, the mixture is sintered at a second preset temperature.

[0084] In one embodiment, the sintering at the second preset temperature includes:

[0085] Sintering is carried out under a protective atmosphere. During sintering, first keep the temperature at 200 - 300 °C for 1 - 3 h, and then keep the temperature at 500 - 600 °C for 10 - 16 h.

[0086] In one embodiment, the oxygen-free condition is under a nitrogen protective atmosphere.

[0087] In one embodiment, the first preset temperature is greater than the second preset temperature.

[0088] The present invention uses a relatively high temperature for element doping to effectively dope elements into the lattice structure. However, for the synthesis of iron-based polyanionic cathode materials, if the temperature is too high, the material will lose its original properties due to decomposition. Therefore, during the carbon coating process, the temperature used will be lower than the process temperature of element doping to ensure the stability and performance of the material.

[0089] In one embodiment, the iron source includes one or more of iron nitrate, iron sulfate, and ferrous oxalate;

[0090] The first phosphorus source includes one or more of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid, or sodium phosphate;

[0091] The second phosphorus source includes one or more of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid, or sodium phosphate;

[0092] The first phosphorus source and the second phosphorus source can be the same or different.

[0093] The M element doping source is a nitrate doped with the M element;

[0094] The sodium source includes one or more of sodium carbonate, sodium phosphate, and sodium oxalate.

[0095] The present invention relates to a new element doping method aimed at improving the conductivity and ionic conductivity of the material, thereby enhancing the specific capacity of the material. The specific steps are as follows: First, a soluble iron source, such as iron nitrate, iron sulfate, or ferrous oxalate, is mixed with a phosphorus source such as diammonium hydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid, or sodium phosphate. Then, a nitrate of a soluble doping element is added, and these doping elements include, but are not limited to, boron (B), cerium (Ce), cobalt (Co), copper (Cu), gallium (Ga), titanium (Ti), vanadium (V), zirconium (Zr), calcium (Ca), lanthanum (La), lutetium (Lu), yttrium (Y), zinc (Zn), aluminum (Al), magnesium (Mg), and manganese (Mn). Then, ammonia water is used to adjust the pH of the mixture to a value between 2 and 5 to obtain a coprecipitate.

[0096] Next, the obtained coprecipitate is sintered in air. The sintering temperature T1 is set between 650 °C and 900 °C to ensure the structural stability of the material and the uniform distribution of the doping elements. If the temperature is too high, the material will lose its original properties due to decomposition. If the calcination temperature (<600 °C) is too low, the doping cannot be evenly distributed into the lattice. Through this process, sodium iron phosphate doped with metal Fe 1-x M x PO 4, where M represents one or more of the above-mentioned doping elements (B, Ce, Co, Cu, Ga, Ti, V, Zr, Ca, La, Lu, Y, Zn, Al, Mg, Mn), and the value of x ranges from 0 to 0.2.

[0097] By doping specific elements (such as boron, cerium, cobalt, copper, gallium, titanium, vanadium, zirconium, calcium, lanthanum, lutetium, yttrium, zinc, aluminum, magnesium, and manganese, etc.) in a specific ratio, the present invention can significantly improve the problems of poor conductivity and low ionic conductivity of the sodium iron phosphate body. This doping method not only increases the specific capacity of the material but also ensures the uniform distribution of the doping elements in the material through the element doping method of coprecipitation followed by sintering, avoiding the formation of segregation and impurity phases, and thus not having a negative impact on the performance of the material.

[0098] In one embodiment, the mixing of the sintered product with the second phosphorus source, sodium source, and water includes:

[0099] Mix Fe 1-x M x PO 4 (0 < x ≤ 0.2) with a phosphorus source (such as diammonium hydrogen phosphate, phosphoric acid, sodium phosphate, ammonium dihydrogen phosphate, etc.), a sodium source (such as sodium carbonate, sodium oxalate, sodium phosphate, etc.), and water, and after sand grinding, a precursor slurry with a solid content of g is formed, where 20% < g < 50%.

[0100] In one embodiment, during the spray drying process, the inlet air temperature is controlled at 230 - 280°C, and the outlet air temperature is controlled at 70 - 90°C.

[0101] In one embodiment, the precursor slurry is spray-dried through a spray dryer, with the inlet air temperature controlled at 250°C and the outlet air temperature controlled at 80°C; a precursor dry material F1 is obtained.

[0102] The F1 is subjected to airflow pulverization to obtain a precursor powder F2 with a Dv50 particle size of L μm, where (0.2 < L < 20 μm).

[0103] In one embodiment, the carbon source includes: one or more of sucrose, glucose, citric acid, and ethylenediaminetetraacetic acid.

[0104] The process of carbon coating is: soaking the precursor powder F2 in a liquid carbon source (such as sucrose, glucose, citric acid, ethylenediaminetetraacetic acid, etc., one or more), where the mass ratio of F2 to the carbon source (pure carbon source) is f (85:15 < f < 95:5); soaking for 0.5 h and passing N 2 Sintering, the sintering temperature is first heated at a rate of 5 - 10 ° / min, held at 200 - 300°C for 2 h, and then held at 500 - 600°C for 10 - 16 h to finally obtain the required sample.

[0105] The present invention proposes a new method for carbon coating. First, the precursor is subjected to air jet milling after spray drying treatment, and then the milled precursor powder is immersed in a liquid-phase carbon source. In this way, the tiny gaps between the milled precursor powders can be fully filled with the carbon source. Such treatment can effectively avoid the generation of pores by gas inside the precursor powder during the carbonization process, thereby significantly improving the tap density. In addition, carbon source coating after air jet milling can effectively prevent the carbon layer from peeling off during the milling process, thereby significantly improving the coating efficiency.

[0106] By this method, not only the specific capacity and compaction effect are improved, but also the energy density of the material is further enhanced.

[0107] The present invention also provides an iron-based polyanionic composite cathode material, and the structural general formula of the iron-based polyanionic composite cathode material is Na 4 Fe 3-3x M 3x (PO 4 ) 2 (P 2 O 7 )@C, where M is one or more of B, Ce, Co, Cu, Ga, Ti, V, Zr, Ca, La, Lu, Y, Zn, Al, Mg, and Mn, and 0 < X ≤ 0.2.

[0108] @C means that a carbon source is mixed into Na 4 Fe 3-3x M 3x (PO 4 ) 2 (P 2 O 7 ) For example, a carbon source material is coated on the surface of Na 4 Fe 3-3x M 3x (PO 4 ) 2 (P 2 O 7 )

[0109] The present invention also provides a positive electrode plate, including: a current collector and a positive electrode material layer coated on the current collector, and the positive electrode material layer includes the iron-based polyanionic composite cathode material or the iron-based polyanionic composite cathode material prepared by using the preparation method.

[0110] The present invention also provides a sodium ion battery, including the positive electrode plate described above.

[0111] Next, the iron-based polyanionic composite cathode material was prepared by the preparation method of the present invention, and the cathode electrode sheet and battery were made from this cathode material to test its performance.

[0112] Example 1

[0113] 1: M element doping was carried out based on iron source, phosphorus source and sodium source to obtain a precursor slurry; specifically, the following steps were included:

[0114] 1-1, Ferric nitrate, diammonium hydrogen phosphate and aluminum nitrate were mixed in a ratio of Fe:P:Al molar ratio of 0.9:1:0.1 to form a first mixture.

[0115] 1-2, The pH of the first mixture was adjusted to about 3 with ammonia water to obtain coprecipitate 1.

[0116] 1-3, The coprecipitate 1 was sintered in air at a sintering temperature of 700 °C for 10 h to obtain Al-doped Fe 0.9 Al 0.1 PO 4 .

[0117] 1-4, Fe 0.9 Al 0.1 PO 4 was mixed with NH 4 H 2 PO 4 and Na 2 CO 3 in a ratio of 3:1:2 and mixed with H 2 O to form a precursor slurry 1 with a solid content of 40%, and this precursor slurry 1 was a kind of mixture.

[0118] 2, The precursor slurry was dried and then pulverized to obtain a precursor powder; specifically as follows:

[0119] The precursor slurry 1 was spray-dried by a spray dryer, the inlet air temperature was controlled at 250 °C, and the outlet air temperature was controlled at 80 °C; the precursor dry material 1 was obtained; the dry material 1 was subjected to airflow pulverization to obtain a precursor powder with a Dv50 particle size of 5 μm, namely the second mixture 1, specifically the precursor powder of Fe 0.9 Al 0.1 PO 4 .

[0120] 3, The precursor powder and the carbon source were mixed and then sintered under an oxygen-free condition to obtain (Na 4 Fe 2.7 Al 0.3 P 2 O 7 (PO 4 ) 2) @C (carbon content 1.8 wt%); specifically as follows:

[0121] Soak the precursor powder obtained in step 2 in a saturated aqueous citric acid solution, where the mass ratio of the precursor powder to citric acid in the citric acid aqueous solution is 90:10. After soaking for 0.5 h, pass N 2 Perform sintering; the sintering temperature is first heated at a rate of 10 ° / min, held at 300 °C for 2 h, and then held at 500 °C for 10 h to finally obtain the sample of Example 1 (Na 4 Fe 2.7 Al 0.3 P 2 O 7 (PO 4 ) 2 ) @C (carbon content 1.8 wt%).

[0122] Example 2

[0123] 1: Based on iron source, phosphorus source and sodium source, perform M element doping to obtain a precursor slurry; specifically including the following steps:

[0124] 1-1, Mix ferric nitrate, diammonium hydrogen phosphate and calcium nitrate in a ratio of Fe:P:Ca molar ratio of 0.85:1:0.15 to form a first mixture.

[0125] 1-2, Adjust the pH of the first mixture to about 3 with ammonia water to obtain a coprecipitate 2; 1-3, Sinter the coprecipitate 2 in air at a sintering temperature of 700 °C for 10 h to obtain Ca-doped Fe 0.85 Ca 0.15 PO 4 .

[0126] 1-4, Mix Fe 0.85 Ca 0.15 PO 4 with NH 4 H 2 PO 4 and Na 2 CO 3 in a ratio of 3:1:2 and mix with H 2 O to form a precursor slurry 2 with a solid content of 40%.

[0127] Steps 2 and 3 are the same as those in Example 1.

[0128] Finally, obtain the sample of Example 2 (Na 4 Fe 2.55 Ca 0.45 P 2 O 7 (PO 4 ) 2) @C (carbon content 1.8 wt%).

[0129] Example 3

[0130] 1: Dope with element M based on iron source, phosphorus source and sodium source to obtain a precursor slurry; specifically including the following steps:

[0131] 1-1: Mix ferric nitrate, diammonium hydrogen phosphate and magnesium nitrate in a molar ratio of Fe:P:Mg of 0.85:1:0.15 to form a first mixture.

[0132] 1-2: Adjust the pH of the first mixture to about 3 with ammonia water to obtain a coprecipitate 3; 1-3: Sinter the coprecipitate 3 in air at a sintering temperature of 700 °C for 10 h to obtain Mg-doped Fe 0.85 Mg 0.15 PO 4 .

[0133] 1-4: Mix Fe 0.85 Mg 0.15 PO 4 with NH 4 H 2 PO 4 and Na 2 CO 3 in a molar ratio of 3:1:2 and mix with H 2 O to form a precursor slurry 3 with a solid content of 40%.

[0134] Steps 2 and 3 are the same as those in Example 1.

[0135] Finally, obtain the sample of Example 3 (Na 4 Fe 2.55 Mg 0.45 P 2 O 7 (PO 4 ) 2 ) @C (carbon content 1.8 wt%).

[0136] Example 4

[0137] 1: Dope with element M based on iron source, phosphorus source and sodium source to obtain a precursor slurry; specifically including the following steps:

[0138] 1-1: Mix ferric nitrate, diammonium hydrogen phosphate and manganese nitrate in a molar ratio of Fe:P:Mn of 0.85:1:0.15 to form a first mixture.

[0139] 1-2: Adjust the pH of the first mixture to about 3 with ammonia water to obtain a coprecipitate 4;

[0140] 1-3, sinter the coprecipitate 4 in air at a sintering temperature of 700 °C for 10 h to obtain Mn-doped Fe 0.85 Mn 0.15 PO 4 。

[0141] 1-4, mix Fe 0.85 Mn 0.15 PO 4 with NH 4 H 2 PO 4 and Na 2 CO 3 in a molar ratio of 3:1:2 and mix with H 2 O to form a precursor slurry 4 with a solid content of 40%.

[0142] Steps 2 and 3 are the same as in Example 1.

[0143] Finally, obtain the sample of Example 4 (Na 4 Fe 2.55 Mn 0.45 P 2 O 7 (PO 4 ) 2 )@C (carbon content 1.8 wt%).

[0144] Example 5

[0145] The difference between Example 5 and Example 1 is only that the sintering temperature in Steps 1-3 is 800 °C.

[0146] Example 6

[0147] The difference between Example 6 and Example 1 is only that the solid content of the precursor slurry in Steps 1-4 is 30%.

[0148] Example 7

[0149] The difference between Example 7 and Example 1 is only that in Step 2, the precursor powder with a Dv50 particle size of 10 μm is obtained after air jet milling.

[0150] Example 8

[0151] The difference between Example 8 and Example 1 is only that in Step 3, the precursor powder obtained in Step 2, i.e., the second mixture, is immersed in a saturated sucrose aqueous solution, and finally obtain the sample of Example 8 (Na 4 Fe 2.7 Al 0.3 P 2 O 7 (PO 4 ) 2 )@C (carbon content 2.1 wt%).

[0152] Example 9

[0153] The difference between Example 9 and Example 1 is only that in step 3, the mass ratio of the precursor powder to citric acid in the aqueous citric acid solution is 93:7, and the sample of Example 9 (Na 4 Fe 2.7 Al 0.3 P 2 O 7 (PO 4 ) 2 )@C (carbon content 1.6 wt%) is obtained.

[0154] Example 10

[0155] The difference between Example 10 and Example 1 is only that in step 3, the sintering temperature is that the heating rate is 10 ° / min first, keep the temperature at 250 °C for 2 h, and then keep the temperature at 550 °C for 10 h.

[0156] Example 11

[0157] 1: Doping with M element is carried out based on iron source, phosphorus source and sodium source to obtain a precursor slurry; specifically, the following steps are included:

[0158] 1-1, Mix ferric sulfate, sodium phosphate and calcium nitrate in a molar ratio of Fe:P:Ca of 0.82:1:0.18 to form a first mixture.

[0159] 1-2, Adjust the pH of the first mixture to pH 2 with ammonia water to obtain a coprecipitate 11; 1-3, Sinter the coprecipitate 11 in air, the sintering temperature is 650 °C, and sinter for 12 h to obtain Ca-doped Fe 0.82 Ca 0.18 PO 4 .

[0160] 1-4, Mix Fe 0.82 Ca 0.18 PO 4 with diammonium hydrogen phosphate and sodium oxalate in a molar ratio of 3:1:2 and mix with H 2 O to form a precursor slurry 11 with a solid content of 20%.

[0161] 2, Dry the precursor slurry and then pulverize it to obtain a mixture; specifically as follows:

[0162] Spray-dry the precursor slurry 11 through a spray dryer, control the inlet air temperature at 230 °C and the outlet air temperature at 70 °C; obtain a precursor dry material 11; carry out air flow pulverization on the precursor dry material 11 to obtain a precursor powder with a Dv50 particle size of 20 μm, that is, a second mixture 11.

[0163] 3. Mix the precursor powder and the carbon source and then sinter them to obtain (Na 4 Fe 2.46 Ca 0.54 P 2 O 7 (PO 4 ) 2 )@C; specifically as follows:

[0164] Soak the precursor dry material 11 (i.e., the second mixture 11) obtained in step 2 into a saturated glucose aqueous solution. Among them, the mass ratio of the precursor dry material 11 to glucose is 85:5. After soaking for 0.5 h, pass N 2 and conduct sintering; the sintering temperature is that the heating rate is 10 ° / min first, keep it at 200 °C for 1 h, then keep it at 530 °C for 13 h, and finally obtain the sample of Example 11 (Na 4 Fe 2.46 Ca 0.54 P 2 O 7 (PO 4 ) 2 )@C (carbon content 1.95 wt%).

[0165] Example 12

[0166] 1. Perform M element doping based on an iron source, a phosphorus source, and a sodium source to obtain a precursor slurry; specifically including the following steps:

[0167] 1-1. Mix ferric sulfate, phosphoric acid, and aluminum nitrate in a molar ratio of Fe:P:Al of 0.9:1:0.1 to form a first mixture.

[0168] 1-2. Adjust the pH of the first mixture to 5 with ammonia water to obtain a coprecipitate 12.

[0169] 1-3. Sinter the coprecipitate 12 in air at a sintering temperature of 900 °C for 6 h to obtain Al-doped Fe 0.9 Al 0.1 PO 4 .

[0170] 1-4. Mix Fe 0.9 Al 0.1 PO 4 with Na 3 PO 4 and Na 2 C 2 O 4 in a molar ratio of 3:1:2 and mix them with H 2 O to form a precursor slurry 12 with a solid content of 50%.

[0171] 2. Dry the precursor slurry and then crush it to obtain precursor powder 12. Specifically as follows:

[0172] Perform spray drying on the precursor slurry 12 through a spray dryer, control the inlet air temperature at 280 °C, and the outlet air temperature at 90 °C; obtain the precursor dry material 12; perform air flow crushing on the precursor dry material 12 to obtain precursor powder 12 with a Dv50 particle size of 0.2 μm, that is, the second mixture 12.

[0173] 3. Mix the precursor powder 12 and the carbon source and then sinter them to obtain (Na 4 Fe 2.7 Al 0.3 P 2 O 7 (PO 4 ) 2 )@C. Specifically as follows:

[0174] Soak the precursor powder 12, that is, the second mixture obtained in step 2, in a saturated glucose aqueous solution. Among them, the mass ratio of the precursor powder 12 to glucose is 95:15. After soaking for 0.5 h, pass N 2 Perform sintering; the sintering temperature is to first increase the temperature at a rate of 10 ° / min, keep the temperature at 210 °C for 3 h, and then keep the temperature at 600 °C for 16 h. Finally, obtain the sample of Example 12 (Na 4 Fe 2.7 Al 0.3 P 2 O 7 (PO 4 ) 2 )@C (carbon content 4.7 wt%).

[0175] Example 13

[0176] The difference between Example 13 and Example 1 is only that the molar ratio of Fe:P:Al is changed to 0.8:1:0.2. Finally, obtain Na 4 Fe 2.4 Al 0.6 (PO 4 ) 2 (P 2 O 7 )@C (carbon content 1.8 wt%).

[0177] Comparative Example 1

[0178] (1) Mix ferric nitrate, diammonium hydrogen phosphate, and sodium carbonate in a molar ratio of Fe:P:Na of 3:4:4 to form a mixture.

[0179] (2) Then mix the mixture with citric acid (citric acid as the carbon source) in a mass ratio of 90:10, and then add water to form a precursor slurry with a solid content of 40%.

[0180] (3) Spray-dry the precursor slurry, control the inlet air temperature at 250 °C and the outlet air temperature at 80 °C to obtain the precursor dry powder.

[0181] (4) Pass the precursor dry powder obtained in step (2) through N 2 for sintering. The sintering temperature is to first increase the temperature at a rate of 10 ° / min, hold for 2 h at 300 °C, and then hold for 10 h at 500 °C to finally obtain the sample of Comparative Example 1, with the chemical formula of Na 4 Fe 3 P 2 O 7 (PO 4 ) 2 @C (carbon content 1.8 wt%).

[0182] In Comparative Example 1, only carbon coating was carried out, and no doping with metal M was performed.

[0183] Comparative Example 2

[0184] (1) The same as in Example 1.

[0185] (2) Mix Fe 0.9 Al 0.1 PO 4 with NH 4 H 2 PO 4 and Na 2 CO 3 in a molar ratio of 3:1:2, then mix with citric acid, and then mix with H 2 O to form a precursor slurry with a solid content of 40%.

[0186] (3) Spray-dry the precursor slurry through a spray dryer, control the inlet air temperature at 250 °C and the outlet air temperature at 80 °C; obtain the precursor dry powder.

[0187] (4) Pass the precursor dry powder obtained in step (3) through N 2 for sintering; the sintering temperature is to first increase the temperature at a rate of 10 ° / min, hold for 2 h at 300 °C, and then hold for 10 h at 500 °C to finally obtain the sample of Comparative Example 2, with the chemical formula of (Na 4 Fe 2.7 Al 0.3 P 2 O 7 (PO 4 ) 2) @C (carbon content 1.8 wt%).

[0188] In Comparative Example 2, during the metal doping process, carbon coating was carried out simultaneously, and finally sintering was performed to obtain a product with the same component content as that in Example 1.

[0189] Comparative Example 3

[0190] (1) Mix FePO 4 with NH 4 H 2 PO 4 and Na 2 CO 3 in a molar ratio of 3:1:2 and mix with H 2 O to form a precursor slurry with a solid content of 40%.

[0191] Steps (2) and (3) are the same as Steps 2 and 3 in Example 1 to obtain a sample without precursor doping, that is, the sample of Comparative Example 3 (Na 4 Fe 3 P 2 O 7 (PO 4 ) 2 ) @C (carbon content 1.8 wt%).

[0192] In Comparative Example 3, no element M doping was carried out, and carbon coating was directly performed.

[0193] Comparative Example 4

[0194] (1) Mix ferric nitrate, diammonium hydrogen phosphate, aluminum nitrate and sodium carbonate in a molar ratio of Fe:P:Al:Na of 2.7:4:0.3:4;

[0195] (2) Then mix the mixture with citric acid (citric acid as the carbon source) in a mass ratio of 90:10, and then add water to form a precursor slurry with a solid content of 40%.

[0196] (3) Spray-dry the precursor slurry, control the inlet air temperature at 250 °C and the outlet air temperature at 80 °C to obtain the precursor dry powder.

[0197] (4) Pass the precursor dry powder obtained in (3) through N 2 for sintering. The sintering temperature is first heated at a rate of 10 ° / min, held at 300 °C for 2 h, and then held at 500 °C for 10 h to finally obtain the sample of Comparative Example 4, with the chemical formula (Na 4 Fe 2.7 Al 0.3 P 2 O 7 (PO 4 ) 2)@C (carbon content 1.8wt%).

[0198] Comparative Example 4 performed metal doping and carbon coating in sequence, but in the synthesis of Na 4 Fe 2.7 P 2 O 7 (PO 4 ) 2 When the phosphorus doping is performed in two separate steps, the phosphorus element required for the synthesis is added at one time. A product with the same content and components as in Example 1 is obtained.

[0199] Material characterization

[0200] The (Na 4 Fe 2.7 Al 0.3 P 2 O 7 (PO 4 ) 2 )@C and the (Na 4 Fe 3 P 2 O 7 (PO 4 ) 2 )@C were used for material characterization, see Figures 2 - 3 .

[0201] from Figure 2 It can be observed that the positive electrode material obtained by implementing Example 1 of the present invention has a very dense particle structure. This dense particle structure significantly improves the compaction density of the material. The increase in compaction density directly leads to an increase in energy density, which is of great significance for improving battery performance. Therefore, the positive electrode material prepared by the method of the present invention performs well in terms of energy density and can meet the needs of higher performance batteries.

[0202] from Figure 3 It can be clearly seen that the positive electrode material prepared by the method of Comparative Example 1 presents a porous structure with more pores. This porous structure leads to a relatively low compaction density of the material. As the compaction density decreases, the energy density of the material also decreases. The low energy density makes this positive electrode material unable to meet the performance requirements of industrial applications, limiting its application prospects in the field of high-performance batteries.

[0203] Electrochemical performance test

[0204] Using the positive electrode materials prepared in Examples 1-10 and Comparative Examples 1-4, 1.5Ah sodium ion soft-pack batteries were prepared in the same manner. The obtained batteries were tested respectively, and the test results are shown in Table 1.

[0205] Table 1 Electrochemical performance test results of the cells in Examples 1-10 and Comparative Examples 1-4

[0206]

[0207] In Table 1, the method for capacity testing is as follows:

[0208] For the 1.5 Ah soft-pack sodium-ion cells prepared in the examples and comparative examples, in the voltage range of 1.5-3.65 V, the first charge test was carried out at a rate of 0.05C. When charging to 3.65 V, constant voltage charging was carried out, and the cut-off current was 0.05C. Then, 0.2C discharge was carried out until discharging to 1.5 V to cut off, and the first charge and discharge capacities were tested; the average discharge voltage was the discharge energy / discharge capacity.

[0209] Table 1 details the electrochemical test data of the above examples and comparative examples. Through comparison, it can be clearly seen that the present invention is significantly superior to the comparative examples in terms of performance, and this significant advantage is mainly attributed to the unique preparation process of the present invention. Specifically, the present invention proposes a method for preparing a composite sodium iron phosphate cathode material with high energy density and its corresponding preparation method. First, by doping specific elements in the composite sodium iron phosphate, its ionic conductivity and electronic conductivity are significantly improved, thereby effectively enhancing the specific capacity and average voltage performance of the material. Then, the precursor after spray drying is subjected to air flow pulverization, and then it is soaked in a liquid carbon source. This process can not only reduce the particle size but also significantly reduce the pores between and inside the particles, thereby further enhancing the tap density of the material. In this way, the overall energy density of the material is significantly improved. In addition, during the synthesis of Na 4 Fe 3-3x (PO 4 ) 2 (P 2 O 7 ) of the present invention adopts a two-step separate addition method of phosphorus element, and this innovative process further improves the synthesis quality and performance of the product.

[0210] It can be seen from the performance comparison data in Table 1 that by doping elements under specific conditions, the elements can be effectively doped into the crystal lattice of the cathode material, thereby enhancing the specific capacity and average voltage performance of the composite sodium iron phosphate. On the basis of element doping, carbon coating is carried out. Specifically, the precursor slurry is soaked with the carbon source in a pulverized state, which can further improve the tap density of the material. Combining these two methods can significantly improve the energy density of the composite sodium iron phosphate, making it show more excellent performance in practical applications.

[0211] It should be noted that although the above embodiments describe the various steps in a specific order, those skilled in the art can understand that in order to achieve the effects of the present invention, it is not necessary for different steps to be executed in such an order. They can be executed simultaneously (in parallel) or in other orders, and these variations are all within the protection scope of the present invention.

[0212] So far, the technical solutions of the present invention have been described in conjunction with the preferred embodiments shown in the accompanying drawings. However, it is easy for those skilled in the art to understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the protection scope of the present invention.

Claims

1. A method for preparing an iron-based polyanion composite positive electrode material, characterized in that: include: M-doped Na4Fe 3-3x M 3x A precursor powder of (PO4)2(P2O7) and a carbon source are mixed to form a mixture, wherein 0<X≤0.2, and the Dv50 particle size of the precursor powder is ≤20 μm; The mixture is sintered in the absence of oxygen to obtain Na4Fe 3-3x M 3x (PO4)2(P2O7)@C.

2. The method according to claim 1, characterized in that: The M element doped Na4Fe 3-3x M 3x The preparation method of the precursor powder of (PO4)2(P2O7) comprises: Obtaining a coprecipitate by mixing a first mixture formed by an iron source, a first phosphorus source, and an M element doping source; The sintered product of the coprecipitate is mixed with a second phosphorus source, a sodium source and water to obtain a precursor slurry; After the precursor slurry is dried and crushed, a precursor powder with a Dv50 particle size of ≤20 μm is obtained.

3. The method according to claim 2, characterized in that The pH value of the first mixture is adjusted to a preset value to obtain the coprecipitate.

4. The method according to claim 2, characterized in that: The mixing ratio of the iron source, the first phosphorus source and the M element doping source is: The molar ratio of Fe in the iron source: P in the first phosphorus source: M element in the M element doping source is (1-x): 1: x, where 0 < X ​​≤ 0.2; and / or During the mixing of the sintered product of the coprecipitate with the second phosphorus source, the sodium source and water, the mixing ratio is: The molar ratio of the sintered product, P in the second phosphorus source, and Na in the sodium source is 3:1:4; In the precursor slurry formed by mixing the sintered product of the coprecipitate with a second phosphorus source, a sodium source and water, the amount of water added is such that the solid content of the precursor slurry is greater than 20% and less than 50%.

5. The method according to claim 1, characterized in that The carbon source is a carbon source solution; The mass ratio of the precursor powder to the carbon source in the carbon source solution is (85-95):(5-15).

6. The preparation method according to claim 2, characterized in that: The sintering temperature of the coprecipitate is a first preset temperature; The mixture is sintered under oxygen-free conditions comprising: The mixture is sintered at a second preset temperature in the absence of oxygen; The first preset temperature is greater than the second preset temperature.

7. The method according to claim 1, characterized in that The M element includes one or more of B, Ce, Co, Cu, Ga, Ti, V, Zr, Ca, La, Lu, Y, Zn, Al, Mg and Mn elements.

8. An iron-based polyanion composite positive electrode material, characterized in that: The general structural formula of the iron-based polyanion composite positive electrode material is Na4Fe 3-3x M 3x (PO4)2(P2O7)@C, where 0<X≤0.

2.

9. The material according to claim 8, characterized in that The M includes one or more of B, Ce, Co, Cu, Ga, Ti, V, Zr, Ca, La, Lu, Y, Zn, Al, Mg and Mn elements.

10. A positive electrode sheet, characterized in that: include: A current collector and a positive electrode material layer coated on the current collector, wherein the positive electrode material layer comprises the iron-based polyanion composite positive electrode material according to claim 8 or 9 or the iron-based polyanion composite positive electrode material prepared by the preparation method according to any one of claims 1-7.

11. A sodium ion battery, characterized in that: Including the positive electrode sheet as described in claim 10.

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