High-compaction ferric sodium pyrophosphate positive electrode material, pole piece and preparation method and application of high-compaction ferric sodium pyrophosphate positive electrode material and pole piece

Abstract

The invention relates to the technical field of sodium ion battery positive electrode materials, and discloses a high-compaction ferric sodium phosphate pyrophosphate positive electrode material, a pole piece and a preparation method and application of the high-compaction ferric sodium phosphate pyrophosphate positive electrode material and the pole piece. And calculating the mass ratio of the sodium ferric phosphate pyrophosphate A and the sodium ferric phosphate pyrophosphate B according to the true density and the compaction density of the sodium ferric phosphate pyrophosphate A and the sodium ferric phosphate pyrophosphate B, and mixing to obtain the high-compaction sodium ferric phosphate pyrophosphate positive electrode material. The compaction density of the prepared ferric sodium pyrophosphate positive electrode material is remarkably improved, and the compaction density can reach 2.36 g / cm < 3 > or above. And the electrode plate and the battery prepared by adopting the composite material also show better processing performance and higher cycling stability. The method is suitable for being applied to preparation of the sodium-ion battery positive pole piece.

Description

Technical Field

[0001] The present invention relates to the technical field of cathode materials for sodium-ion batteries, and particularly to a high-compactness sodium iron pyrophosphate phosphate cathode material, a pole piece, and a preparation method and application thereof. More particularly, the present invention relates to a preparation method of a high-compactness sodium iron pyrophosphate phosphate cathode material, and other sodium iron pyrophosphate phosphate cathode materials and cathode pole pieces prepared by using this method. Background Art

[0002] The statements in this section only provide background information related to the disclosure of the present application and may not constitute prior art.

[0003] Due to their high safety and long life characteristics, and also because of their low cost, easy availability of materials, environmental friendliness and sustainability, sodium-ion batteries have developed rapidly in recent years. Poly-anion compounds are commonly used cathode materials for sodium-ion batteries. Sodium iron pyrophosphate phosphate is a typical poly-anion structure material, but its tap density is low, and it is not easy to form a dense packing between powder particles. At present, the tap density of most of the prepared electrode pole pieces is difficult to exceed 2.2 g / cm 3 , resulting in a low energy density of the final battery and hindering the application of this material.

[0004] In order to improve the energy density of the battery, it is almost a consensus in the industry to develop a high-compactness sodium iron pyrophosphate phosphate cathode material.

[0005] However, while increasing the tap density of the material, it often leads to a sharp decline in other properties of the material or difficult processing.

[0006] A prior art preparation method and application of a cathode material for a sodium-ion battery provide a method of dispersing a sodium source compound, an iron source mixture, and a carbon source compound in water in a certain proportion and stirring and mixing them to obtain a dispersion liquid, controlling the proportion of the sodium source compound and the iron source mixture, and preparing a cathode material for a sodium-ion battery with a higher tap density and energy density. When the tap density of the sodium iron pyrophosphate phosphate prepared by this prior art reaches 2.25, for the prepared battery, at a current density of 0.1C and a voltage range of 2.0 - 4.0V, its discharge capacity reaches 106 mAh / g; when the tap density reaches 2.29, for the prepared battery, at a current density of 0.1C and a voltage range of 2.0 - 4.0V, its discharge capacity reaches 105 mAh / g. This prior art improves from the perspective of the preparation raw materials of sodium iron pyrophosphate phosphate and prepares sodium iron pyrophosphate phosphate with a higher tap density. However, improving from the raw material level has problems such as a long preparation cycle, complex technology, and difficulty in rapid application, and the technical flexibility is poor. Summary of the Invention

[0007] The object of the present invention is to provide a high-compactness sodium iron pyrophosphate cathode material, a pole piece, a preparation method and an application thereof, aiming at the problems of low tap density and long production cycle of the current sodium iron pyrophosphate cathode material. The tap density of the sodium iron pyrophosphate cathode material is increased through particle size matching, and the tap density can reach 2.36 g / cm 3 or more. At the same time, while ensuring the high tap density of the material, it also has excellent electrochemical performance and processing performance.

[0008] The technical solution of the present invention is as follows: On the one hand, the present invention provides a preparation method of a high-compactness sodium iron pyrophosphate cathode material, comprising the following steps: Step S1: Screen sodium iron pyrophosphate A and sodium iron pyrophosphate B according to the following conditions: The particle size of the material satisfies the following formula: ≤0.3, ≤0.3, and ≤35 μm; In the formula, , and are the particle sizes of the sodium iron pyrophosphate A material corresponding to the cumulative volume distribution percentages of 50%, 90%, and 100% of the material, respectively; , and are the particle sizes of the sodium iron pyrophosphate B material corresponding to the cumulative volume distribution percentages of 50%, 90%, and 100% of the material, respectively.

[0009] Step S2: Compound the sodium iron pyrophosphate A and sodium iron pyrophosphate B screened in step S1 according to the following mass ratio: ; In the formula, in the formula, is the mass of the sodium iron pyrophosphate A material; is the mass of the sodium iron pyrophosphate B material; is the true density of the sodium iron pyrophosphate B material; and are the powder tap densities of the sodium iron pyrophosphate A material and the sodium iron pyrophosphate B material, respectively; γ is an empirical parameter, 0 < γ <0.15.

[0010] Preferably, the particle size distribution of the sodium iron pyrophosphate cathode material has two peaks.

[0011] On the other hand, the present invention provides the application of the high tap density sodium iron pyrophosphate phosphate cathode material as described above in the preparation of a cathode electrode sheet for a sodium ion battery.

[0012] On the other hand, the present invention provides the application of the high tap density sodium iron pyrophosphate phosphate cathode material as described above in the preparation of a sodium ion battery.

[0013] On the other hand, the present invention provides a cathode electrode sheet for a sodium ion battery, comprising a cathode current collector and a cathode active material layer coated on the surface of the cathode current collector, wherein the cathode active material layer comprises the high tap density sodium iron pyrophosphate phosphate cathode material as described above, a conductive agent, and a binder.

[0014] The cathode active material layer is formed by coating a slurry prepared from the high tap density sodium iron pyrophosphate phosphate cathode material, a conductive agent, a binder, and a solvent as described above on the surface of the cathode current collector; wherein the solvent can be one or more of N-methylpyrrolidone (NMP), acetone, and dimethylacetamide (DMAC). The cathode current collector includes any one of aluminum foil, carbon-coated aluminum foil, and perforated aluminum foil. The conductive agent includes, but is not limited to, one or more of carbon nanotubes, graphene, carbon black, and carbon fibers. The binder includes, but is not limited to, one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyacrylonitrile (PAN).

[0015] Preferably, the high tap density sodium iron pyrophosphate phosphate cathode material, the conductive agent, and the binder are configured according to conventional ratios in the art.

[0016] According to a preferred embodiment, the maximum tap density of the cathode electrode sheet for a sodium ion battery is not less than 2.35 g / cm 3 . In some embodiments, the maximum tap density of the electrode sheet can reach 2.46 g / cm 3 or more.

[0017] On the other hand, the present invention provides a sodium ion battery, comprising a cathode electrode sheet, wherein the cathode electrode sheet comprises a cathode current collector and a cathode active material layer coated on the surface of the cathode current collector, and the cathode active material layer comprises the high tap density sodium iron pyrophosphate phosphate cathode material as described above, a conductive agent, and a binder.

[0018] Compared with the existing technology, the beneficial effects of the present invention are as follows: 1. The high tap density sodium iron pyrophosphate phosphate cathode material, the prepared material not only has a high tap density, but also can maintain a large energy density. When the current density is 0.1C and the voltage range is 2.0 - 4.0V, the tap density increases from 2.285 g / cm 3 to 2.327 g / cm 3The discharge capacity is also increased; it combines the two major elements of tap density and performance, which helps to comprehensively enhance the competitiveness of sodium iron pyrophosphate phosphate products; 2. Provide a preparation method of a high-tap-density sodium iron pyrophosphate phosphate cathode material. By grading two materials with different particle sizes, its tap density is improved, and the data of the true density and powder tap density of the material are innovatively used to confirm the weight ratio of the material. The method is novel, effective and universal, and is suitable for popularization and use. Description of the Drawings

[0019] Figure 1 SEM images of the positive electrode sheets prepared in Example 1, Example 3, Comparative Example 1 and Comparative Example 3; Figure 2 Particle size distribution curves of the high-tap-density sodium iron pyrophosphate phosphate cathode materials prepared in Example 1 and Example 3; Figure 3 Charge and discharge curves of the cathode materials prepared in Example 1 and Example 3 at a current of 0.1 C; Figure 4 Rate performance and long cycle performance of the cathode materials prepared in Example 2 and Example 5. Detailed Description of the Invention

[0020] The specific examples listed in the present invention are only examples of the present invention, and the present invention is not limited to the specific examples described below. For those skilled in the art, any equivalent modifications and substitutions to the examples described below are also within the scope of the present invention. Therefore, equivalent transformations and modifications made without departing from the spirit and scope of the present invention should be covered within the scope of the present invention. Conditions not specified in the examples are carried out according to conventional conditions or conditions recommended by the manufacturer. All reagents or instruments not specified by the manufacturer can be purchased through the market as conventional products. In order to better illustrate the present invention, numerous specific details are given in the following detailed description. Those skilled in the art should understand that the present invention can also be implemented without some specific details. In other embodiments, methods, means, equipment and steps well known to those skilled in the art are not described in detail in order to highlight the gist of the present invention.

[0021] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art. Unless otherwise specified, the units used in this specification are all international standard units, and the numerical values and numerical ranges appearing in the present invention should be understood to include the inevitable systematic errors in industrial production.

[0022] The features and performance of the present invention will be further described in detail below in conjunction with the examples.

[0023] Example 1 A preparation method of a high-compact sodium iron pyrophosphate phosphate cathode material includes the following steps: Step S1: Sodium iron pyrophosphate A and sodium iron pyrophosphate B are screened according to the following conditions: Select sodium iron pyrophosphate A, and measure its particle sizes as = 1.939 μm, = 4.443 μm, = 9.907 μm; select sodium iron pyrophosphate B, and measure its particle sizes as = 10.510 μm, = 21.064 μm, = 33.535 μm. After calculation, = 0.184, = 0.219, and or ≤ 35 μm, satisfying the foregoing relationship.

[0024] Step S2: Calculate the grading ratio of materials A and B according to the density parameters of sodium iron pyrophosphate A and sodium iron pyrophosphate B. After measurement, the powder compact densities of materials A and B are respectively = 2.203 g / cm 3 , = 2.185 g / cm 3 , and the true density of material B is = 2.998 g / cm 3 . After calculation, = 0.273. Therefore, the optimized mass ratio of the composite of materials A and B is 0.273 ≤ ≤ 0.423.

[0025] Step S3: Weigh sodium iron pyrophosphate A and sodium iron pyrophosphate B according to the mass ratios of 0.28:1, 0.38:1, and 0.42:1 respectively, and use a horizontal tank mill to mix them evenly to obtain high-compact sodium iron pyrophosphate phosphate cathode materials C1, C2, and C3. The powder compact densities of the high-compact sodium iron pyrophosphate phosphate cathode materials C1, C2, and C3 are measured to be 2.278 g / cm 3 , 2.327 g / cm 3 , 2.330 g / cm 3 .

[0026] Step S4: Prepare the above positive electrode active material into a positive electrode sheet. Weigh a certain mass of the high-compaction sodium iron pyrophosphate phosphate positive electrode material C, Super-P, and PVDF powder according to a mass ratio of 95:2.5:2.5. Add an appropriate amount of N-methylpyrrolidone (NMP) to it, control the solid content of the slurry to be about 50 wt.%, and use a centrifuge to mix evenly to obtain a positive electrode slurry; coat both sides of the aluminum foil with the positive electrode slurry, and obtain a positive electrode sheet after drying. Test the maximum compaction density of the positive electrode sheet without particle breakage. The maximum compaction densities of the positive electrode sheets obtained with C1, C2, and C3 as the positive electrode materials are 2.375 g / cm 3 、2.415 g / cm 3 、2.417 g / cm 3 respectively.

[0027] Example 2 A preparation method of a high-compaction sodium iron pyrophosphate phosphate positive electrode material, comprising the following steps: Step S1: Screen sodium iron pyrophosphate phosphate A and sodium iron pyrophosphate phosphate B according to the following conditions: Select sodium iron pyrophosphate phosphate A, and measure its particle sizes to be = 1.476 μm, = 3.896 μm, = 9.076 μm; select sodium iron pyrophosphate phosphate B, and measure its particle sizes to be = 10.510 μm, = 21.064 μm, = 33.535 μm. After calculation, = 0.140; 0.184. The particle sizes of sodium iron pyrophosphate phosphate A and sodium iron pyrophosphate phosphate B satisfy the foregoing relationship.

[0028] Step S2: Calculate the optimized ratio of the grading of materials A and B according to the density parameters of sodium iron pyrophosphate phosphate A and sodium iron pyrophosphate phosphate B. After measurement, the powder compaction densities of materials A and B are respectively = 2.163 g / cm 3 , = 2.185 g / cm 3 , and the true density of material B is = 2.998 g / cm 3 .

[0029] According to the foregoing relationship, the optimized mass ratio of the composite of materials A and B is 0.268 ≤ ≤ 0.418.

[0030] Step S3: Weigh materials A and B according to the mass ratios of 0.27:1, 0.35:1, and 0.41:1 respectively, and use a horizontal tank mill to mix them evenly to obtain high-compactness sodium pyrophosphate iron phosphate cathode materials C1, C2, and C3. The powder compaction densities of the high-compactness sodium pyrophosphate iron phosphate cathode materials C1, C2, and C3 are measured to be 2.313 g / cm 3 , 2.361 g / cm 3 , and 2.353 g / cm 3 .

[0031] Step S4: The same as in Example 1. After measurement, the compaction densities of the electrode sheets made of C1, C2, and C3 are 2.392 g / cm 3 , 2.457 g / cm 3 , and 2.435 g / cm 3 .

[0032] Example 3 A preparation method of a high-compactness sodium pyrophosphate iron phosphate cathode material, comprising the following steps: Step S1: Select sodium pyrophosphate iron phosphate A, and measure its particle sizes to be = 1.079 μm, = 2.976 μm, = 7.093 μm; select sodium pyrophosphate iron phosphate B, and measure its particle sizes to be = 5.490 μm, 11.779 μm, 20.534 μm. = 0.196; = 0.252; After calculation, the particle sizes of materials A and B satisfy the foregoing relationship.

[0033] Step S2: Calculate the optimized ratio of the grading of materials A and B according to the density parameters of materials A and B. After measurement, the powder compaction densities of materials A and B are = 2.208 g / cm 3 , = 1.987 g / cm 3 , and the true density of material B is = 3.085 g / cm 3 .

[0034] According to the foregoing relationship, the optimized mass ratio of the composite of materials A and B is 0.396 ≤ ≤ 0.546.

[0035] Step S3: Weigh material A and material B according to the mass ratio of 0.45:1, and use a horizontal tank mill to mix them evenly to obtain the high-compactness sodium pyrophosphate iron phosphate cathode material C. The powder tap density of the high-compactness sodium pyrophosphate iron phosphate cathode material C is measured to be 2.285 g / cm 3 .

[0036] Step S4: The same as Example 1. After measurement, the tap density of the electrode is 2.353 g / cm 3 .

[0037] Example 4 A preparation method of a high-compactness sodium pyrophosphate iron phosphate cathode material, comprising the following steps: Step S1: Select sodium pyrophosphate iron phosphate A, and measure its particle sizes to be = 1.079μm, = 2.976μm, = 7.093μm; Select sodium pyrophosphate iron phosphate B, and measure its particle sizes to be = 9.781μm, = 18.064μm, = 25.535μm. After calculation, = 0.110, = 0.164, and or ≤ 35μm, meeting the foregoing relationship.

[0038] Step S2: Calculate the optimized ratio of the grading of materials A and B according to the density parameters of materials A and B. After measurement, the powder tap densities of materials A and B are respectively = 2.208 g / cm 3 , = 2.215 g / cm 3 , and the true density of material B is = 2.960g / cm 3 .

[0039] According to the foregoing relationship, the optimized mass ratio of the composite of materials A and B is 0.251 ≤ ≤ 0.401.

[0040] Step S3: Weigh material A and material B according to the mass ratio of 0.40:1, and use a horizontal tank mill to mix them evenly to obtain the high-compactness sodium pyrophosphate iron phosphate cathode material C. The powder tap density of the high-compactness sodium pyrophosphate iron phosphate cathode material C is measured to be 2.304 g / cm 3 .

[0041] Step S4: The same as Example 1. After measurement, the tap density of the electrode is 2.363 g / cm3 。

[0042] Example 5 A preparation method of a high-compaction sodium iron pyrophosphate phosphate cathode material, comprising the following steps: Step S1: Select sodium iron pyrophosphate phosphate A, and measure its particle sizes to be = 1.177 μm, = 3.076 μm, = 6.093 μm; select sodium iron pyrophosphate phosphate B, and measure its particle sizes to be = 7.512 μm, = 12.113 μm, = 18.815 μm. After calculation, = 0.157, = 0.253, and or ≤ 35 μm, satisfying the foregoing relationship.

[0043] Step S2: Calculate the optimized ratio of the grading of materials A and B according to the density parameters of materials A and B. After measurement, the powder compaction densities of materials A and B are respectively = 2.130 g / cm 3 , = 2.105 g / cm 3 , and the true density of material B is = 2.973 g / cm 3 . According to the foregoing relationship, the optimized mass ratio of the composite of materials A and B is 0.295 ≤ ≤ 0.445.

[0044] Step S3: Weigh materials A and B according to a mass ratio of 0.38:1, and use a horizontal tank mill to mix them evenly to obtain a high-compaction sodium iron pyrophosphate phosphate cathode material C. The powder compaction density of the high-compaction sodium iron pyrophosphate phosphate cathode material C is measured to be 2.286 g / cm 3 .

[0045] Step S4: The same as Example 1. After measurement, the compaction density of the electrode sheet is 2.369 g / cm 3 .

[0046] Comparative Example 1 A preparation method of a high-compaction sodium iron pyrophosphate phosphate cathode material, comprising the following steps: Basically similar to Example 1, the difference between the two is that: sodium iron pyrophosphate phosphate A and sodium iron pyrophosphate phosphate B are mixed according to a mass ratio of 0.18:1. After measurement, the compaction density of the prepared high-compaction sodium iron pyrophosphate phosphate cathode material is 2.197 g / cm3 , the tap density of the electrode is 2.255 g / cm 3 .

[0047] Comparative Example 2 A preparation method of a high-tap-density sodium pyrophosphate iron phosphate cathode material, comprising the following steps: It is basically similar to Example 2, and the difference between the two is that: sodium pyrophosphate iron phosphate A and sodium pyrophosphate iron phosphate B are mixed according to a mass ratio of 0.11:1. After measurement, the tap density of the prepared high-tap-density sodium pyrophosphate iron phosphate cathode material is 2.119 g / cm 3 , and the tap density of the electrode is 2.207 g / cm 3 .

[0048] Comparative Example 3 A preparation method of a high-tap-density sodium pyrophosphate iron phosphate cathode material, comprising the following steps: It is basically similar to Example 3, and the difference between the two is that: sodium pyrophosphate iron phosphate A and sodium pyrophosphate iron phosphate B are mixed according to a mass ratio of 4:1. After measurement, the tap density of the material is 2.035 g / cm 3 , and the tap density of the electrode is 2.113 g / cm 3 .

[0049] Comparative Example 4 A preparation method of a high-tap-density sodium pyrophosphate iron phosphate cathode material, comprising the following steps: Step S1: Select sodium pyrophosphate iron phosphate A, and measure its particle sizes as = 5.490μm, = 11.779μm, = 20.534μm. Select sodium pyrophosphate iron phosphate B, and measure its particle sizes as = 10.510μm, = 21.064μm, = 33.535μm. After calculation, = 0.522, 0.558, not meeting the aforementioned relationship.

[0050] Step S2: The tap density of material A is = 1.978 g / cm 3 , = 2.203 g / cm 3 and the true density is: = 3.023 g / cm 3 .

[0051] According to the aforementioned relationship, the optimized mass ratio of the composite of materials A and B is 0.245 ≤ ≤0.394。

[0052] Step S3: Weigh material A and material B according to the mass ratio of 0.3:1, and use a horizontal tank mill to mix them evenly to obtain the high-compactness sodium pyrophosphate iron phosphate cathode material C. The powder compact density of the high-compactness sodium pyrophosphate iron phosphate cathode material C is measured to be 2.142 g / cm 3 。

[0053] Step S4: The same as Example 1. After measurement, the compact density of the electrode sheet is 2.204 g / cm 3 。

[0054] Comparative Example 5 A preparation method of a high-compactness sodium pyrophosphate iron phosphate cathode material, comprising the following steps: Step S1: Select sodium pyrophosphate iron phosphate A, and measure its particle sizes to be =4.030μm, =10.479μm, =20.534μm. Select sodium pyrophosphate iron phosphate B, and measure its particle sizes to be =5.990μm, 12.779μm, =26.944μm. After calculation, =0.734, =0.890, which does not satisfy the foregoing relationship.

[0055] Step S2: The compact density of material A is =1.815g / cm 3 , =2.211g / cm 3 and the true density is: =3.015g / cm 3 。

[0056] According to the foregoing relationship, the optimized mass ratio of the composite of materials A and B is 0.219 ≤ ≤0.370.

[0057] Step S3: Weigh material A and material B according to the mass ratio of 0.3:1, and use a horizontal tank mill to mix them evenly to obtain the high-compactness sodium pyrophosphate iron phosphate cathode material C. The powder compact density of the high-compactness sodium pyrophosphate iron phosphate cathode material C is measured to be 1.937 g / cm 3 。

[0058] Step S4: The same as Example 1. After measurement, the compact density of the electrode sheet is 2.077 g / cm 3 。

[0059] The tap density of the high-tap-density sodium iron pyrophosphate phosphate cathode materials prepared in Examples 1-5 and Comparative Examples 1-5 was detected, and the results are shown in Table 1 below: Table 1: Tap density of the high-tap-density sodium iron pyrophosphate phosphate cathode materials and electrode sheets prepared in Examples 1-5 and Comparative Examples 1-5

[0060] As Figure 1 shown in the comparison between Example 1, Example 3 and Comparative Example 1 and Comparative Example 3, it can be seen with the naked eye that Examples 1 and 3 have fewer pores and higher tap density.

[0061] Figure 2 Shown is the particle size comparison between Example 1 and Example 3. It can be seen from the figure that Example 1 with a higher tap density has a particle size distribution with two peaks.

[0062] Figure 3 Shown are the charge-discharge curves of the cathode materials prepared in Example 1 and Example 3 at a current of 0.1 C. It can be seen from the figure that the discharge capacity of the cathode material in Example 1 at a current of 0.1 C is 105.3 mAh / g, and the discharge capacity of the cathode material in Example 3 at a current of 0.1 C is 101.5 mAh / g. Comparing the data in Table 2, it is not difficult to find that after compound modification, compared with the original material, the capacity and first Coulomb efficiency of the material hardly decay.

[0063] Figure 4 Shown are the rate performance and long-cycle performance of the cathode materials prepared in Examples 2 and 5. It can be seen from the figure that Examples 2 and 5 exhibit excellent rate performance. The retention rates of both at a current of 1 C are greater than 90%, and the capacity retention rates at a large current of 2 C are greater than 80%, showing good rate performance. After cycling 50 times at a current of 1 C, the capacity retention rates of both are greater than 99%, showing good cycle characteristics.

[0064] The states of the cathode electrode sheet prepared in Example 1 of this application under different bending conditions show that under the condition of double-sided high tap density, the electrode sheet does not show fracture behavior after being folded in half twice, showing strong practical performance.

[0065] Sodium ion batteries were assembled using the positive electrode sheets prepared in Examples 1-5 and Comparative Examples 1-5. The specific preparation method was as follows: in a glove box filled with nitrogen, the back sides of the positive electrode sheets prepared in Examples 1-5 and Comparative Examples 1-5 of the present application and the positive electrode sheets prepared from the raw materials sodium iron pyrophosphate A and sodium iron pyrophosphate B were closely attached to the center of the CR2016 positive electrode case. 50 μL of electrolyte was dropped to moisten the front side of the positive electrode sheet. Subsequently, a glass fiber separator was placed on the positive electrode sheet, and 100 μL of electrolyte was dropped again to moisten the separator. A commercial sodium sheet was placed on the separator, and the CR2016 negative electrode case was buckled. Finally, the battery was encapsulated with an automatic sealing machine to obtain a sodium ion battery. The performance parameters of the sodium ion batteries prepared from different positive electrode sheets are shown in Table 2 below: Table 2: Performance parameters of sodium ion batteries assembled from different positive electrode sheets prepared in Examples 1-5 and Comparative Examples 1-5

[0066] The above-described embodiments merely represent the specific implementation manners of the present application. The description is relatively specific and detailed, but it should not be construed as a limitation on the protection scope of the present application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the technical solution of the present application, several modifications and improvements can still be made, and these all fall within the protection scope of the present application.

Claims

1. A method for preparing a high-density sodium iron pyrophosphate positive electrode material, characterized in that: The steps include: Step S1: Sodium ferric pyrophosphate A and sodium ferric pyrophosphate B are obtained by screening according to the following conditions: The particle size of the material satisfies the following formula: ≤0.3, ≤0.3, and ≤35μm; In the formula, , and They are the particle sizes corresponding to the material when the volume cumulative distribution percentage of sodium ferric pyrophosphate A material reaches 50%, 90% and 100% respectively; , and They are the particle sizes corresponding to the material when the volume cumulative distribution percentage of sodium iron pyrophosphate B material reaches 50%, 90% and 100% respectively; Step S2: Compounding the sodium ferric pyrophosphate A and the sodium ferric pyrophosphate B screened in step S1 according to the following mass ratio: ； In the formula, is the mass of sodium ferric acid pyrophosphate A material; is the mass of sodium ferric acid pyrophosphate B material; is the true density of sodium iron acid pyrophosphate B material; and are the powder compaction densities of sodium acid ferric pyrophosphate A material and sodium acid ferric pyrophosphate B material respectively; γ is an empirical parameter, 0< γ <0.

15.

2. A high-density sodium iron phosphate pyrophosphate positive electrode material, characterized in that: The preparation is obtained by the preparation method as claimed in claim 1.

3. A high-density sodium iron phosphate pyrophosphate positive electrode material according to claim 2, characterized in that: The maximum compacted density of the material is greater than or equal to 2.25 g / cm 3 .

4. A high-density sodium iron phosphate pyrophosphate positive electrode material according to claim 3, characterized in that: The maximum compacted density of the material is greater than or equal to 2.36 g / cm 3 .

5. Application of a high-density sodium iron phosphate pyrophosphate positive electrode material in the preparation of a positive electrode sheet for a sodium ion battery, characterized in that: The high-density sodium iron phosphate pyrophosphate positive electrode material is the high-density sodium iron phosphate pyrophosphate positive electrode material as described in any one of claims 2 to 4.

6. Application of a high-density sodium iron pyrophosphate positive electrode material in the preparation of a sodium ion battery, characterized in that: The high-density sodium iron phosphate pyrophosphate positive electrode material is the high-density sodium iron phosphate pyrophosphate positive electrode material as described in any one of claims 2 to 4.

7. A sodium ion battery positive electrode plate, characterized in that: The invention comprises a positive electrode current collector and a positive electrode active material layer coated on the surface of the positive electrode current collector, wherein the positive electrode active material layer comprises the high-density sodium iron phosphate pyrophosphate positive electrode material as claimed in any one of claims 2 to 4, a conductive agent and a binder.

8. The sodium ion battery positive electrode sheet according to claim 7, characterized in that: Maximum compacted density greater than or equal to 2.35 g / cm 3 .

9. The sodium ion battery positive electrode sheet according to claim 7, characterized in that: The positive electrode current collector is any one of aluminum foil, carbon-coated aluminum foil and perforated aluminum foil; the conductive agent is one or more of carbon nanotubes, graphene, carbon black and carbon fibers; the binder is one or more of polyvinylidene fluoride, polytetrafluoroethylene and polyacrylonitrile.

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