High-magnification ferric sodium pyrophosphate composite material as well as preparation method and application thereof

Abstract

The invention discloses a high-magnification ferric sodium pyrophosphate composite material as well as a preparation method and application thereof, and belongs to the technical field of sodium ion battery positive electrode materials. According to the positive electrode material, phosphoric acid and ferric pyrophosphate sodium nano-microspheres grow on multi-walled carbon nanotubes (MCNTs), so that the MCNTs are coated with the phosphoric acid and ferric pyrophosphate sodium nano-microspheres, and a composite material composed of a matrix MCNTS and a phosphoric acid and ferric pyrophosphate sodium microsphere coating layer is formed. The chemical formula of the material is Na4Fe3 (PO4) P2O7 (at) MCNTS. The ultrahigh-rate ferric sodium phosphate pyrophosphate composite material comprises an MCNTS carrier and a coating layer coated on the surface of the MCNTS. According to the pyrophosphate and ferric phosphate sodium nanosphere coated multi-walled carbon nanotube composite material provided by the invention, due to a cross-linked three-dimensional network structure, a sodion deintercalation path is greatly shortened, electrons are transferred among nanospheres at an ultra-fast speed, and aggregation of the nanospheres is also inhibited. Due to ultrahigh electronic and ionic conductivity, the nanospheres show remarkable high capacity, ultrahigh rate capability and super-long cycle performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of cathode materials for sodium-ion batteries, and particularly relates to a high-rate sodium iron pyrophosphate phosphate composite material, a preparation method thereof and an application thereof. Background Art

[0002] At present, large-scale energy storage and conversion (EESC) of sustainable resources (solar and wind energy) or smart grids (peak shaving and valley filling) has become a key issue for replacing fossil fuels. Although lithium-ion batteries (LIBs) have been widely used in small portable electronics, electric vehicles (EVs) or plug-in hybrid electric vehicles (PHEVs), their high cost and limited lithium reserves still limit their use in EESC. As an ideal EESC device, the price, safety and element reserves of electrode materials are key issues. It is worth noting that the "host" structural iron-based phosphate cathode materials containing abundant sodium and iron elements and relatively high stability in the earth may be an ideal choice. Regarding a limited number of iron-based phosphate cathode materials in sodium-ion batteries (SIBs) such as NaFePO4, Na2FePO4F, Na 3.32 Fe 2.34 (P2O7)2, Na3Fe2(PO4)3 and Na2FeP2O7, there have been many reports.

[0003] Recently, researchers have focused on a new mixed iron-based phosphate, Na4Fe3(PO4)2P2O7 (abbreviated as NFPP). This compound contains phosphate groups and pyrophosphate groups in its structure and takes advantage of the advantages of phosphate and pyrophosphate cathodes. Therefore, it has many excellent properties, including high stability, a good theoretical capacity of 129 mAhg -1 −1, low cost, environmental friendliness and easy synthesis. In particular, previous studies have shown that the sodium storage mechanism of NFPP electrodes involves a single-phase reaction, and its volume change is less than 4%. Therefore, NFPP electrodes should provide a long cycle life.

[0004] Although NFPP shows great potential, its performance is still far from expectations. The main problem of NFPP in SIBs is its low electronic and ionic conductivity, which limits its rate capability and cycling ability. To solve the above problems, researchers have carried out various modifications to improve its conductivity and electrochemical performance. These include: surface carbon coating, material particle nanosizing, transition metal ion doping, and material morphology regulation. For example, in the patent No. CN 117352700 A, a dual-atom doped carbon material-coated composite phosphate cathode material and its preparation method, a carbon nanocomposite modified by a polyviologen organic film is used to prepare a dual-atom doped sodium iron pyrophosphate composite material. The three-dimensional carbon skeleton constructed by this carbon nanocomposite effectively adapts to the volume change of the material during the insertion and extraction of sodium ions in the sodium iron pyrophosphate material, and further improves the surface electronic conductivity of the composite material, enhancing the rate and capacity performance. In the patent No. CN 116002650 A, a preparation method of a composite sodium iron pyrophosphate polyanionic cathode material and its application in sodium ion batteries, the composite polyanionic cathode material prepared by the method coupling co-precipitation, spray drying and pyrolysis reaction exhibits excellent comprehensive electrochemical performance. However, these methods generally have problems such as high production cost, low efficiency, and difficulty in large-scale production. Therefore, there is an urgent need for a simple and efficient method to synthesize a sodium iron pyrophosphate cathode material with excellent electrochemical performance. Summary of the Invention

[0005] Aiming at the current technical problems, the present invention aims to provide a high-rate sodium iron pyrophosphate composite material, its preparation method and application. The precursor is prepared by a one-step spray drying method to improve the uniformity of the material, forming a composite material composed of a matrix MCNT S (multi-walled carbon nanotube) and a sodium iron pyrophosphate microsphere coating layer, realizing the synchronous carbon coating of sodium iron pyrophosphate particles. While simplifying the process steps, effective control is achieved in terms of environmental protection, safety, cost, etc.; the obtained sodium iron pyrophosphate cathode material has the characteristics of good conductivity, large specific capacity, high rate performance and long cycle life.

[0006] The present invention is achieved through the following technical solutions

[0007] S1: First, add MCNTs to a beaker containing strong acid for acidification and dispersion to obtain an MCNTs acidic suspension;

[0008] S2: Concentrate and reflux the acidic suspension to obtain a concentrated solution of acidified MCNTs;

[0009] S3: Filter and wash the concentrated solution with deionized water until the filtrate is clear and the pH value is neutral, and then vacuum dry the filtered MCNTs to obtain acidified MCNTs;

[0010] S4: Dissolve the acidified MCNT obtained in S3 S in deionized water, dissolve the iron source in the deionized water containing the acidified MCNT S , ultrasonically disperse it, then slowly add the sodium source, phosphorus source and carbon source, stir magnetically at high speed, and finally spray-dry the homogeneous solution at an inlet and outlet temperature of 180 - 240 °C and 80 - 120 °C to obtain the precursor powder of the sodium iron phosphate composite material;

[0011] S5: Place the precursor powder in a muffle furnace and perform in-situ reduction reaction treatment under a reducing atmosphere. The reducing atmosphere is hydrogen or a hydrogen-nitrogen mixture. The heat treatment procedure is as follows: heat up to 100 - 400 °C at a rate of 2 - 5 °C / min and hold for 2 - 10 h, then heat up to 450 - 550 °C at a rate of 5 - 10 °C / min and hold for 4 - 16 h, and cool to room temperature to obtain the sodium iron pyrophosphate composite material.

[0012] Further, the strong acid used in step S1 is at least one of H2SO4, HNO3, HCIO4, HCI, and HMnO4.

[0013] Further, the MCNTs added in step S1 account for 5 - 20% of the total mass of the sodium iron pyrophosphate composite material.

[0014] Further, the MCNT in step S1 S has a particle size D50 of 1 - 200 μm.

[0015] Further, the mass ratio of the strong acid to the MCNTs in step S1 is (1 - 20):1.

[0016] Further, the dispersion in step S1 is mechanical stirring and ultrasonic treatment. The rotation speed of the mechanical stirring is 200 - 800 rmp, and the treatment time is 30 - 60 min.

[0017] Further, the suspension in step S1 is black.

[0018] Further, the temperature of the concentration and reflux in step S2 is 50 - 120 °C, and the time is 1 - 5 h.

[0019] Further, the temperature of the vacuum drying in step S3 is 60 - 90 °C, and the drying time is 10 - 24 h.

[0020] Further, the iron source used in step S4 is one or more of ferrous oxalate, ferrous nitrate, ferrous sulfate, iron hydroxide, ferric oxalate, ferric sulfate, and ferric nitrate; the phosphorus source is one or more of sodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, and sodium monohydrogen phosphate; the sodium source is one or more of sodium carbonate, sodium sulfate, sodium hydroxide, sodium bicarbonate, sodium pyrophosphate, sodium dihydrogen phosphate, and sodium citrate; and the carbon source is one or more of glucose, citric acid, sucrose, fructose, starch, glycine, carbon black, tartaric acid, and oxalic acid.

[0021] Further, in step S4, the inlet temperature of spray drying is 120 - 260 °C, the outlet temperature is 80 - 120 °C, the gas source is compressed air, and the spray feed flow rate is 100 - 3000 mL / h.

[0022] Further, in step S4, the molar ratio of sodium salt, phosphate salt, and iron salt is 3.9 - 4.1: 2.8 - 3.2: 3.95 - 4.1, and the mass of the carbon source accounts for 5 - 20% of the total mass of the sodium source, iron source, phosphate source, and carbon source.

[0023] Further, in step S5, the sintering process is as follows: at a heating rate of 2 - 5 °C / min, heat up to 300 °C and hold for 6 - 10 h, then at a heating rate of 2 - 10 °C / min, heat up to 450 - 550 °C and hold for 8 - 12 h. After the sintering is completed, cool the material in the furnace, and the cooling rate is 2 °C / min.

[0024] The present invention has the following beneficial effects compared with the prior art:

[0025] By treating the original MCNTs with strong acid, on the one hand, the impurities inside the MCNTs can be removed, and on the other hand, a large number of -COOH functional groups are added to its surface. The iron ions provided by the previously added iron source can combine with COOH to form nucleation on the surface of MCNTs. After rapid spray drying, the uniform precursor will react and nucleate along the surface of MCNTs during the annealing process; finally, NFPP particle morphology similar to corn cob shape is obtained.

[0026] The present invention obtains a composite material of sodium iron pyrophosphate nanospheres coated on MCNTs through a simple spray drying method. This composite material with MCNTs as the matrix and sodium iron pyrophosphate as the coating layer greatly improves the electron and ion conductivity, and at the same time inhibits the aggregation of nanospheres. Due to its ultra-high conductivity, these nanospheres exhibit remarkable high capacity, ultra-high rate capability, and long cycle performance. Description of the Drawings

[0027] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only exemplary. For those of ordinary skill in the art, without creative efforts, other implementation drawings can also be obtained based on the provided drawings.

[0028] Figure 1 is the technical roadmap of the high-rate sodium iron pyrophosphate composite material;

[0029] Figure 2 is the XRD pattern of the high-rate sodium iron pyrophosphate composite material;

[0030] Figure 3 is the scanning electron microscope image of the high-rate sodium iron pyrophosphate composite material;

[0031] Figure 4 is the structural schematic diagram of the high-rate sodium iron pyrophosphate composite material;

[0032] Figure 5 is the first charge-discharge curve of the coin cell of the high-rate sodium iron pyrophosphate composite material. Specific embodiments

[0033] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0034] The following first explains the concepts involved in this application in conjunction with the drawings. It should be noted here that the following explanations of each concept are only for making the content of this application easier to understand, and do not represent a limitation on the protection scope of this application; at the same time, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The following will refer to the drawings and combine the embodiments to detail this application.

[0035] Embodiment 1

[0036] Preparation of the high-rate sodium iron pyrophosphate composite material: Using MCNTs as the matrix material, ferrous oxalate as the iron source, glucose as the carbon source, sodium carbonate and ammonium dihydrogen phosphate as the sodium source and the phosphorus source respectively, the steps are as follows:

[0037] First, according to the proportion that the mass of MCNTs accounts for 10% of the total mass of sodium iron pyrophosphate, 11.7 g of MCNTs with a D50 of 100 nm were added to a beaker containing 50 g of concentrated nitric acid. During the process, continuous stirring and ultrasonic treatment were carried out. After dispersion, concentration was carried out at 80 °C for 2 h. The concentrated MCNTs were repeatedly washed and pressure-filtered until the pH showed neutrality, and finally dried to obtain acidified MCNTs.

[0038] Dissolve 200 g of ferrous oxalate in deionized water containing acidified MCNTs, disperse it ultrasonically, and successively add 78.4 g of sodium carbonate, 170.5 g of ammonium dihydrogen phosphate, and 10 g of glucose. Stir magnetically quickly, and spray-dry under the conditions that the inlet and outlet temperatures are 200 °C and 100 °C respectively to obtain the precursor powder of sodium iron pyrophosphate composite material. Finally, under a nitrogen atmosphere, heat it at a heating rate of 2 °C / min to 300 °C, keep it warm for 4 h, then heat it to 500 °C, keep it warm for 10 h, and then cool it down naturally to obtain the high-rate sodium iron pyrophosphate composite material.

[0039] Example 2

[0040] Compared with Example 1, the difference in this example is that the added MCNTs account for 5% of the mass of sodium iron pyrophosphate, and other conditions and parameters are the same as those in Example 1.

[0041] Example 3

[0042] Compared with Example 1, the difference in this example is that the added MCNTs account for 15% of the mass of sodium iron pyrophosphate, and other conditions and parameters are the same as those in Example 1.

[0043] Example 4

[0044] Compared with Example 1, the difference in this example is that the added MCNTs account for 20% of the mass of sodium iron pyrophosphate, and other conditions and parameters are the same as those in Example 1.

[0045] Comparative Example 1

[0046] Compared with Example 1, the difference in this comparative example is that MCNTs are replaced with activated carbon, and other conditions and parameters are the same as those in Example 1.

[0047] Comparative Example 2

[0048] Compared with Example 1, the difference in this comparative example is that MCNTs are replaced with acetylene black, and other conditions and parameters are the same as those in Example 1.

[0049] Comparative Example 3

[0050] Compared with Example 1, the difference in this comparative example is that MCNTs are replaced with graphene, and other conditions and parameters are the same as those in Example 1.

[0051] Comparative Example 4

[0052] This comparative example is different from Example 1 in that no conductive material is used as the matrix material, and other conditions and parameters are the same as those in Example 1.

[0053] Select the materials prepared in Examples 1 to 4 and Comparative Examples 1 to 4 as electrode materials, and prepare button cells for charge and discharge tests respectively according to the following steps:

[0054] All the tests in this example were carried out with CR2032 button cells as the reference. A metal sodium sheet was used as the counter electrode, glass fiber was used as the separator, and NaPF6 was used as the electrolyte; the working electrode was composed of 80 wt% active material, 10 wt% conductive agent and 10 wt% polyvinylidene fluoride (PVDF), and N-methyl-2-pyrrolidone (NMP) was used as the solvent; it was uniformly coated on aluminum foil by a coater and dried in a vacuum furnace at 120 °C for 12 h. The thickness achieved by rolling the electrode sheet was calculated according to the compaction density and rolling treatment was carried out. The diameter of the electrode was 12 mm, and a button half-cell was assembled in a vacuum glove box for constant current charge and discharge and low temperature rate performance tests as shown in Table 1 below:

[0055] Table 1

[0056] Discharge specific capacity at 0.1C at 30℃ Discharge specific capacity at 10C at 30℃ Discharge specific capacity at 20C at 30℃ Example 1 118.2 mAh / g 98.5 mAh / g 84.4 mAh / g Example 2 106.7 mAh / g 70.6 mAh / g 52.9 mAh / g Example 3 113.5 mAh / g 83.5 mAh / g 68.2 mAh / g Example 4 108.9 mAh / g 80.4 mAh / g 62.3 mAh / g Comparative Example 1 103.1 mAh / g 67.6 mAh / g 58.7 mAh / g Comparative Example 2 100.8 mAh / g 58.2 mAh / g 53.4 mAh / g Comparative Example 3 104.4 mAh / g 73.6 mAh / g 65.3 mAh / g Comparative Example 4 93.2 mAh / g 51.7 mAh / g 32.1 mAh / g

[0057] The process for preparing the high-rate sodium iron pyrophosphate sodium ion cathode composite material of the present invention is as Figure 1 shown. According to the experimental data in Table 1, the prepared high-rate sodium iron pyrophosphate composite material has high initial capacity and rate performance. From Figure 2 the XRD pattern in, it can be seen that the prepared high-rate sodium iron pyrophosphate composite material belongs to the orthorhombic system (PDF standard card number: PDF#01-089-0579). This result indicates that the prepared sodium iron pyrophosphate composite material is a pure phase, and the added MCNTs will not change the crystal structure of sodium iron pyrophosphate. In order to more vividly and intuitively understand the structure of a high-rate sodium iron pyrophosphate composite material prepared by the present invention, as Figure 3 and Figure 4The SEM and structural schematic diagrams are shown. It can be seen from the figures that the sodium iron pyrophosphate phosphate nanospheres tightly wrap on the surface of MCNTs. This three-dimensional network structure similar to a corncob greatly shortens the path of sodium ion insertion and extraction, not only enabling ultra-fast electron transfer between the nanospheres, but also inhibiting the aggregation of the nanospheres. Due to their ultra-high electronic and ionic conductivities, these nanospheres exhibit remarkable high capacity, ultra-high rate capability, and long cycling performance. The reason for the relatively low electrical performance in Example 2 may be that less MCNTs were added, and part of the sodium iron pyrophosphate phosphate could not adhere to its surface, resulting in agglomeration, leading to slightly worse rate performance and lower electrical performance. In Examples 3 and 4, excessive MCNTs may cause the sodium iron pyrophosphate phosphate nanospheres on their surfaces to be too dispersed, lengthening the ion transport path and resulting in poor rate performance. From Figure 5 It was found by comparison that Comparative Example 1 showed poor electrical performance through activated carbon composite, which may be related to the adsorption of activated carbon. The activated carbon with strong adsorption caused serious agglomeration of sodium iron pyrophosphate phosphate. In Comparative Examples 2 and 3, due to their spherical and flaky structures, different degrees of agglomeration of sodium iron pyrophosphate phosphate nanospheres occurred on their surfaces, showing lower electrical performance.

[0058] Finally, it should be noted that the specific embodiments described herein are merely illustrative of the spirit of the present invention and not a limitation on the implementation manners of the present invention. Those skilled in the art to which the present invention pertains can make various modifications or supplements to the described embodiments or use similar methods for substitution. It is not necessary and impossible to list all the implementation manners here. And these obvious changes or variations derived from the essence of the present invention still fall within the protection scope of the present invention. Interpreting them as any additional limitation is contrary to the spirit of the present invention.

Claims

1. A high-rate sodium iron pyrophosphate phosphate composite, characterized in that: Including MCNT S A carrier and a coating layer coated on the surface of MCNT S The coating layer is a mixed coating layer formed by sodium iron pyrophosphate phosphate microspheres and a carbon source.

2. A preparation method of a high-rate sodium iron pyrophosphate phosphate composite material as defined in claim 1, comprising the following steps: S1: First, take MCNT S and perform acidification treatment in a strong acid solution: Take MCNT S and add it to a beaker containing strong acid for acidification and dispersion to obtain an MCNT S acidic suspension; S2: Subject the MCNT obtained in S1 S acidic suspension to concentration and reflux to obtain acidified MCNT S concentrate; S3: Filter and wash the concentrated solution of the acidified MCNT obtained in S2 with deionized water until the pH value of the filtrate is neutral, and finally vacuum-dry the filtered MCNT S to obtain the acidified MCNT S ; S ; S4: The acidified MCNT obtained in S3 S is dissolved in deionized water, and the iron source is dissolved in the deionized water containing the acidified MCNT S and ultrasonically dispersed. Then, the sodium source, phosphorus source, and carbon source are slowly added, and rapid magnetic stirring is performed. Finally, the homogeneous solution is spray-dried at an inlet and outlet temperature of 180 - 240 °C and 80 - 120 °C to obtain the sodium iron phosphate composite precursor powder; S5: Place the precursor powder in a muffle furnace and perform in-situ reduction reaction treatment under a reducing atmosphere, where the reducing atmosphere is hydrogen or a hydrogen-nitrogen mixture. The heat treatment procedure is as follows: Heat up to 100 - 400 °C at a rate of 2 - 5 °C / min and hold for 2 - 10 h, then heat up to 450 - 550 °C at a rate of 5 - 10 °C / min and hold for 4 - 16 h, and cool to room temperature to obtain the sodium iron pyrophosphate phosphate composite material.

3. The preparation method according to claim 2, wherein: The strong acid used in step S1 is at least one of H2SO4, HNO3, HCIO4, HCI, and HMnO4; The MCNT added in step S1 S accounts for 5 to 20% of the total mass percentage of the sodium iron pyrophosphate phosphate composite material; The MCNT described in step S1 S has a particle size D50 of 1 - 200 um; The strong acid and MCNT described in step S1 S have a mass ratio of (1 - 20):1; The dispersion in step S1 refers to mechanical stirring and ultrasonic treatment. The rotation speed of the mechanical stirring is 200 - 800 rmp, and the time is 30 - 60 min; The suspension in step S1 is black.

4. The preparation method according to claim 2, characterized in that: The temperature of the concentration reflux in step S2 is 50 - 120 °C, and the time is 1 - 5 h.

5. The preparation method according to claim 2, characterized in that: The temperature of the vacuum drying in step S3 is 60 - 90 °C, and the drying time is 10 - 24 h.

6. The preparation method according to claim 2, characterized in that: The iron source in step S4 is one or more of ferrous oxalate, ferrous nitrate, ferrous sulfate, iron oxide, magnetite, iron hydroxide, ferric oxalate, ferric sulfate, and ferric nitrate; The phosphorus source in step S4 is one or more of sodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, and sodium hydrogen phosphate; The sodium source in step S4 is one or more of sodium carbonate, sodium sulfate, sodium hydroxide, sodium bicarbonate, sodium pyrophosphate, sodium dihydrogen phosphate, and sodium citrate; The carbon source in step S4 is one or more of glucose, citric acid, sucrose, fructose, starch, glycine, carbon black, tartaric acid, and oxalic acid.

7. The preparation method according to claim 2, characterized in that: In step S4, the inlet temperature of the spray drying is 120 - 260 °C, the outlet temperature is 80 - 120 °C, the gas source is compressed air, and the spray feeding flow rate is 100 - 3000 mL / h.

8. The preparation method according to claim 2, characterized in that: In step S4, the molar ratio of the sodium salt, phosphorus salt, and iron salt is 3.9 - 4.1: 2.8 - 3.2: 3.95 - 4.1, and the mass of the carbon source accounts for 5 - 20% of the total mass of the sodium source, iron source, phosphorus source, and carbon source.

9. The preparation method according to claim 2, wherein: The sintering process in step S5 is as follows: Heat up at a rate of 2 - 5 °C / min to 300 °C and hold for 6 - 10 h, then heat up at a rate of 2 - 10 °C / min to 450 - 550 °C and hold for 8 - 12 h. After the sintering is completed, cool the material in the furnace, and the cooling temperature is 2 °C / min.

10. Application of a high-rate sodium iron pyrophosphate phosphate composite material in a battery, characterized in that: The high-rate sodium iron pyrophosphate phosphate composite material prepared by the preparation method according to any one of claims 2 - 9 is used as a cathode material for a battery.

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