Carbon-coated ferric sodium pyrophosphate composite material as well as preparation method and application thereof

Abstract

The invention provides a carbon-coated ferric sodium phosphate pyrophosphate composite material as well as a preparation method and application thereof. The composite material comprises a ferric sodium phosphate pyrophosphate matrix material and a composite carbon layer coated on the surface of ferric sodium phosphate pyrophosphate, the structural formula of the ferric sodium pyrophosphate is Na < 4 > Fe < 3 > (PO4) < 2 > P < 2 > O < 7 >; the composite carbon layer is composed of carbon quantum dots and amorphous carbon; according to the invention, the sodium ferric phosphate pyrophosphate material is coated with the composite carbon layer, so that the electrochemical performance of the sodium ferric phosphate pyrophosphate material, the compaction density of a pole piece and the energy density of a sodium ion battery cell prepared from the material can be effectively improved.

Description

Technical Field

[0001] The invention belongs to the technical field of electrode materials, and in particular relates to a carbon-coated sodium iron phosphate pyrophosphate composite material and a preparation method and application thereof. Background Art

[0002] As the world's reliance on renewable energy increases, battery technology is increasingly used in electric vehicles, energy storage systems, and portable electronic devices. Lithium-ion batteries are widely used due to their high energy density and good cycle stability, but the limited lithium resources and cost issues restrict their further development. As a potential alternative, sodium-ion batteries have attracted attention due to their abundant sodium resources and less impact on the environment. However, existing sodium-ion cells still have shortcomings in energy density, cycle life, and safety.

[0003] Among the numerous sodium-ion battery cathode materials, polyanionic compounds are considered to be the most promising class of electrode materials due to their excellent structural stability, safety, and suitable voltage platform. Among them, composite sodium iron phosphate with low cost, high safety, and cyclic stability is gaining more and more favor. However, poor electronic conductivity, slow ion diffusion rate, and difficulty in controlling high-temperature sintering structure are the problems that need to be solved. Summary of the invention

[0004] Based on the above technical problems existing in the prior art, the present invention provides a carbon-coated sodium iron phosphate pyrophosphate composite material, which can effectively improve the electrochemical properties of the sodium iron phosphate pyrophosphate material by coating a composite carbon layer composed of carbon quantum dots and amorphous carbon on the surface of the sodium iron phosphate pyrophosphate material.

[0005] In order to achieve the above object, the technical solution proposed by the present invention is:

[0006] A carbon-coated sodium iron phosphate pyrophosphate composite material, the positive electrode material comprises a sodium iron phosphate pyrophosphate matrix material and a composite carbon layer coated on the surface of the sodium iron phosphate pyrophosphate; the structural formula of the sodium iron phosphate pyrophosphate is: Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 ; The composite carbon layer consists of carbon quantum dots and amorphous carbon.

[0007] In some embodiments, the mass ratio of the carbon quantum dots to amorphous carbon is 1:(1-5).

[0008] The present invention also provides a method for preparing the carbon-coated sodium iron phosphate pyrophosphate composite material of any of the above embodiments, comprising the following steps:

[0009] S1. Add a sodium source, an iron source, a phosphorus source, carbon quantum dots and a common carbon source into water according to a stoichiometric ratio, stir evenly to obtain a slurry, and grind and spray-dry the slurry to obtain a precursor;

[0010] S2, sintering the precursor in a protective gas atmosphere, and crushing it after cooling to obtain the carbon-coated sodium iron phosphate pyrophosphate composite material;

[0011] Wherein, in step S2, pre-sintering is first performed at a temperature of 300-400°C, and then secondary sintering is performed at a temperature of 500-600°C.

[0012] In some embodiments, in step S1, the sodium source includes at least one of sodium carbonate, sodium pyrophosphate, sodium dihydrogen phosphate, sodium oxalate, sodium acetate, and sodium formate.

[0013] In some embodiments, in step S1, the iron source includes at least one of ferric chloride, ferric sulfate, ferric nitrate, and ferrous oxalate.

[0014] In some embodiments, in step S1, the phosphorus source includes at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, and phosphoric acid.

[0015] In some embodiments, in step S1, the carbon quantum dots include at least one of aldol condensation carbon quantum dots, S-doped carbon quantum dots, N-doped carbon quantum dots, and F-doped carbon quantum dots.

[0016] In some embodiments, in step S1, the common carbon source includes at least one of glucose, sucrose, starch, and cellulose.

[0017] In some embodiments, in step S1, the slurry has a solid content of 20%-40%; the slurry is stirred for 2-8 hours; and the slurry is sand-milled for 4-10 hours so that the D50 of the slurry particles after sand-milling is ≤60nm.

[0018] In some embodiments, in step S1, during the spray drying process, the inlet air temperature is 180°C-240°C, the outlet air temperature is 90°C-125°C, and the feed rate is 2.2-5 L / min.

[0019] In some embodiments, in step S2, the atmosphere during the sintering process is Ar or N 2 ; The sintering heating rate is 1-5℃ / min; The QLM series air flow mill is used in the crushing process, and D50 is ≤10μm after crushing.

[0020] The present invention also provides a positive electrode material, which includes the composite material of any of the above embodiments or the composite material obtained by the preparation method of any of the above embodiments.

[0021] The present invention also provides a positive electrode, which comprises the positive electrode material mentioned above.

[0022] The present invention also provides a sodium ion battery cell, which comprises the above-mentioned positive electrode.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The present invention uses carbon quantum dots and amorphous carbon as a composite carbon layer to carbon-coat the sodium iron pyrophosphate material. The carbon quantum dots are ultrafine spherical structures that can be evenly coated on the surface of the composite sodium iron phosphate, significantly improving the compaction density of the positive electrode sheet, thereby improving the energy density of the battery core; the surface of the carbon quantum dots contains abundant functional groups such as hydroxyl and carboxyl, which is conducive to electronic conduction. In addition, as a new type of carbon-based nanomaterial, carbon quantum dots have the advantages of low cost, eco-friendliness, high stability and electron mobility, and can effectively improve the conductivity and stability of the composite sodium iron phosphate.

[0025] In addition, the present invention also has the following advantages:

[0026] (1) The present invention directly mixes various raw materials and then stirs, sand-mills, sprays, and calcines to obtain sodium iron phosphate pyrophosphate coated with carbon quantum dots and amorphous carbon composite carbon layer. The operation is simple, and the obtained sodium iron pyrophosphate material not only has improved conductivity, but also greatly improves its stability in the air under the coating of the composite carbon layer.

[0027] (2) The present invention uses carbon quantum dots with rich functional groups on the surface and amorphous carbon composite carbon materials for coating, which effectively enhances the chemical composition and structural characteristics of the active materials in the battery, thereby improving the electrochemical activity and stability.

[0028] Furthermore, the preparation method of the present invention uses a QLM series air flow mill to crush the composite sodium iron phosphate, and the product has fine material size, regular particles, smooth surface, stable equipment performance, simple operation, high energy utilization rate, and easy industrialization.

[0029] The method for preparing the sodium ferric phosphate pyrophosphate composite-coated with carbon quantum dots and amorphous carbon of the present invention is simple to operate and can be realized under existing lithium battery manufacturing conditions, which can greatly reduce the production cost and is suitable for industrialization. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the XRD pattern of the composite material obtained in Example 1;

[0031] Figure 2 This is a SEM image of the composite material obtained in Example 1;

[0032] Figure 3 is a charge and discharge curve diagram of the composite material obtained in Example 1;

[0033] Figure 4 The charge and discharge curve of the composite material prepared in Comparative Example 1;

[0034] Figure 5 The cycle diagram of the button cell assembled with the composite material prepared in Example 1 under 1C;

[0035] Figure 6 The cycle diagram of the button cell assembled with the composite material prepared in Comparative Example 3 under 1C;

[0036] Figure 7 This is a cycle diagram of a sodium ion battery cell assembled with the composite material prepared in Example 1 under 1C. DETAILED DESCRIPTION

[0037] Many specific details are set forth in the following description to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar modifications without violating the connotation of the present invention, so the present invention is not limited to the specific implementation disclosed below.

[0038] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0039] Example 1

[0040] A preparation method of a carbon-coated sodium iron phosphate pyrophosphate composite material comprises the following steps:

[0041] (1) 0.02 mol sodium carbonate, 0.015 mol ferrous oxalate, 0.04 mol sodium dihydrogen phosphate, glucose and carbon quantum dots were weighed and added into water according to the stoichiometric ratio, stirred for 6 h and then sand-milled for 8 h to obtain a slurry with a D50 of ≤ 60 nm, wherein the amount of carbon quantum dots and glucose added was 5% of the total mass of the raw materials, the mass ratio of the two was 1:1, and the solid content of the slurry was 35%;

[0042] (2) spray drying the slurry at a feed rate of 3.5 L / min to obtain a precursor; during the drying process, the inlet air temperature is 220° C. and the outlet air temperature is 110° C.;

[0043] (3) The precursor is heated to 350° C. at a rate of 3° C. / min in an Ar atmosphere for pre-sintering for 1 h, and then heated to 500° C. at a rate of 3° C. / min for high-temperature sintering for 6 h. After cooling, the precursor is crushed to obtain a carbon-coated sodium iron phosphate pyrophosphate composite material.

[0044] The obtained composite material was subjected to XRD test, and the results were as follows: Figure 1 As shown, it can be seen that the obtained sodium iron phosphate pyrophosphate has a good crystal form and a high degree of crystallinity; its electron microscope image is shown in Figure 2 As shown, it can be seen that the surface of the particles is nanoscale carbon-coated material.

[0045] Example 2

[0046] Compared with Example 1, the only difference is that the mass ratio of carbon quantum dots to glucose is 1:3, and other conditions are the same as Example 1.

[0047] Example 3

[0048] Compared with Example 1, the only difference is that the mass ratio of carbon quantum dots to glucose is 1:5, and other conditions are the same as Example 1.

[0049] Example 4

[0050] Compared with Example 1, the only difference is that the added amount of carbon quantum dots and glucose is 8%, and the other conditions are the same as Example 1.

[0051] Example 5

[0052] Compared with Example 1, the only difference is that the carbon quantum dots used are F-doped carbon quantum dots, and other conditions are the same as Example 1.

[0053] Comparative Example 1

[0054] Compared with Example 1, the only difference is that the carbon source is only glucose (the amount added is 5% of the total mass of the raw materials), and other conditions are the same as Example 1.

[0055] Comparative Example 2

[0056] Compared with Example 1, the only difference is that the carbon source is only carbon quantum dots (the added amount is 5% of the total mass of the raw materials), and other conditions are the same as Example 1.

[0057] Comparative Example 3

[0058] Compared with Example 1, the only difference is that the carbon source is citric acid and carbon quantum dots (the mass ratio of the two is 1:1), and the other conditions are the same as Example 1.

[0059] Comparative Example 4

[0060] Compared with Example 1, the only difference is that the temperature is directly raised to 500° C. and sintered for 6 hours without pre-firing. Other conditions are the same as Example 1.

[0061] The composite materials obtained in Examples 1-5 and Comparative Examples 1-4 were subjected to electrochemical performance tests, as follows:

[0062] The carbon-coated carbon-coated sodium iron phosphate pyrophosphate composite material (94wt%), Super P (2.8wt%) and PVDF (3.2wt%) are uniformly mixed, and then slurried, coated, dried and rolled to obtain a positive electrode sheet;

[0063] After hard carbon (92wt%), Super P (3wt%), CMC (1.5wt%) and SBR (3.5wt%) are uniformly mixed, the negative electrode sheet is obtained by slurrying, coating, drying and rolling;

[0064] The N / P ratio is controlled to be 1.1-1.2, the positive and negative electrode sheets are assembled into a soft-pack battery by lamination, and the sodium ion battery cell is obtained by liquid injection and formation treatment; wherein the electrolyte is NP-007; the injection amount is 5-10g / Ah; the conditions of the formation treatment are shown in Table 1 below:

[0065] Table 1

[0066] Starting voltage Cut-off voltage Activation conditions First stage charging OCV (Open Circuit Voltage) 3.6 Constant current charging (0.1C) Second stage charging 3.6 Constant voltage charging (0.02C)

[0067] The test results are shown in Table 2 and Figure 3-Figure 6 shown.

[0068] Table 2 Electrochemical properties of the composite materials of Examples 1-5 and Comparative Examples 1-3

[0069]

[0070] 2. Pole compaction data:

[0071] The compaction density of the positive electrode sheets prepared in Examples 1-5 and Comparative Examples 1-3 was tested after rolling, and the results are shown in Table 3:

[0072] Table 3 Compacted density of positive electrode sheets and energy density of cells of Examples 1-5 and Comparative Examples 1-3

[0073] <![CDATA[Compaction density (g cm -3 )]]> <![CDATA[Cell energy density (Wh kg -1 )]]> Example 1 2.24 81.5 Example 2 2.15 78.8 Example 3 2.1 78.3 Example 4 2.16 79.4 Example 5 2.22 81.7 Comparative Example 1 2.01 75.4 Comparative Example 2 1.96 74.2 Comparative Example 3 1.94 73.8

[0074] Note: The mass in the calculation of battery cell energy density is calculated based on the mass of the entire battery cell (including positive and negative electrodes, separator, electrolyte, and aluminum-plastic film).

[0075] It can be seen from the embodiments and comparative examples that the use of the novel carbon material carbon quantum dots and glucose as carbon sources to coat the composite sodium iron phosphate in the technical solution of the present invention can effectively improve the electrochemical performance and the compaction density of the electrode.

[0076] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0077] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A carbon-coated sodium iron phosphate pyrophosphate composite material, characterized in that: The positive electrode material includes a sodium iron phosphate pyrophosphate matrix material and a composite carbon layer coated on the surface of the sodium iron phosphate pyrophosphate; the structural formula of the sodium iron phosphate pyrophosphate is: Na4Fe3(PO4)2P2O7; the composite carbon layer is composed of carbon quantum dots and amorphous carbon.

2. The carbon-coated sodium iron phosphate pyrophosphate composite material according to claim 1, characterized in that The mass ratio of the carbon quantum dots to the amorphous carbon is 1:(1-5).

3. The method for preparing the carbon-coated sodium iron phosphate pyrophosphate composite material according to claim 1 or 2, characterized in that: The following steps are involved: S1. Add a sodium source, an iron source, a phosphorus source, carbon quantum dots and a common carbon source into water according to a stoichiometric ratio, stir evenly to obtain a slurry, and grind and spray-dry the slurry to obtain a precursor; S2, sintering the precursor in a protective gas atmosphere, and crushing it after cooling to obtain the carbon-coated sodium iron phosphate pyrophosphate composite material; Wherein, in step S2, pre-sintering is first performed at a temperature of 300-400°C, and then secondary sintering is performed at a temperature of 500-600°C.

4. The method for preparing the carbon-coated sodium iron phosphate pyrophosphate composite material according to claim 3, characterized in that In step S1, the sodium source includes at least one of sodium carbonate, sodium pyrophosphate, sodium dihydrogen phosphate, sodium oxalate, sodium acetate, and sodium formate; and / or, the iron source includes at least one of ferric chloride, ferric sulfate, ferric nitrate, and ferrous oxalate; and / or, the phosphorus source includes at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, and phosphoric acid; and / or, the carbon quantum dots include at least one of aldol condensation carbon quantum dots, S-doped carbon quantum dots, N-doped carbon quantum dots, and F-doped carbon quantum dots; and / or, the common carbon source includes at least one of glucose, sucrose, starch, and cellulose.

5. The method for preparing the carbon-coated sodium iron phosphate pyrophosphate composite material according to claim 3, characterized in that In step S1, the solid content of the slurry is 20%-40%; stirring is performed for 2-8 hours; sand milling is performed for 4-10 hours so that the D50 of the slurry particles after sand milling is ≤60nm.

6. The method for preparing the carbon-coated sodium iron phosphate pyrophosphate composite material according to claim 3, characterized in that: In step S1, during the spray drying process, the inlet air temperature is 180°C-240°C, the outlet air temperature is 90°C-125°C, and the feed rate is 2.2-5L / min.

7. The method for preparing the carbon-coated sodium iron phosphate pyrophosphate composite material according to claim 3, characterized in that: In step S2, the atmosphere during the sintering process is Ar or N2; the sintering heating rate is 1-5°C / min; the pulverizing process uses a QLM series air flow pulverizer, and the D50 after pulverization is ≤10μm.

8. A positive electrode material, characterized in that The composite material according to claim 1 or 2 or the composite material obtained by the preparation method according to any one of claims 3 to 7.

9. A positive electrode, characterized in that Comprising the positive electrode material as claimed in claim 8.

10. A sodium ion battery cell, characterized in that: Comprising the positive electrode as claimed in claim 9.

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