A borophosphate composite-modified sodium iron phosphate positive electrode material and its preparation method

Abstract

The present invention belongs to the technical field of sodium ion battery materials and discloses a borophosphate-composite modified sodium iron phosphate pyrophosphate cathode material and its preparation method. The present invention prepares the borophosphate-composite sodium iron phosphate pyrophosphate cathode material (Na4Fe3(PO4)2(P2O7)) through a one-step solid-phase method. The method mainly comprises the following steps: high-temperature sintering potassium borate, zinc oxide, dipotassium hydrogen phosphate, and diammonium hydrogen phosphate to produce a K[ZnBP2O8] nanomaterial; solid-phase mixing the nanomaterial with a sodium source, an iron source, and a phosphate; and then high-temperature sintering the nanomaterial to produce a K[ZnBP2O8] composite cathode material. This composite sodium battery cathode material exhibits superior conductivity compared to sodium iron phosphate pyrophosphate. The composite K[ZnBP2O8] layer has a larger lattice spacing, which can promote the transport of sodium ions, providing new ideas for the further application of sodium iron phosphate pyrophosphate.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion battery manufacturing, and in particular relates to a borophosphate composite-modified sodium iron pyrophosphate positive electrode material and a preparation method thereof. Background Art

[0002] Sodium-ion batteries have been a research hotspot in recent years. Due to the abundance of sodium resources, sodium-ion batteries have a significant price advantage over lithium-ion batteries and are more suitable for large-scale energy storage. Among the positive electrode materials for sodium-ion batteries, olivine-type sodium iron phosphate (NaFePO4) has attracted the most attention because it has the consistent structural stability of olivine-type materials and has an excellent electrochemical curve and a high capacity. At present, olivine sodium iron phosphate cannot be directly chemically synthesized. The most commonly used simple preparation method is the ion exchange method based on organic solutions. It is divided into two steps. The first step is to remove lithium from lithium iron phosphate to obtain iron phosphate. The second step is to charge the iron phosphate obtained after deionization with sodium to obtain sodium iron phosphate. However, the electrochemical performance of sodium iron phosphate obtained by this method is not ideal. Its capacity is usually 100-120mA h / g, and significant capacity decay will occur after 100 cycles.

[0003] Based on this, iron-based mixed polyanionic compounds have received widespread attention in recent years. They combine the advantages of iron-based phosphate (NaFePO4) and pyrophosphate (Na2FeP2O7), with high theoretical specific capacity (129mAh / g), high average operating voltage (3.1V), small volume change (less than 4%), and are low-cost, environmentally friendly, and easy to synthesize. However, their electronic conductivity is low and ion transport rate is slow. Further composite layers with high conductivity and large lattice spacing are needed to better meet the requirements of sodium-ion batteries for cathode materials. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the deficiencies and defects mentioned in the above background technology and provide a borophosphate composite modified sodium iron pyrophosphate positive electrode material and a preparation method thereof.

[0005] In order to improve the conductivity and ion diffusion rate of the positive electrode material of sodium ion batteries and enhance its rate capacity, the present invention designs and synthesizes a borophosphate composite modified sodium iron phosphate pyrophosphate positive electrode material. The technical solution proposed is:

[0006] A borophosphate-composite-modified sodium iron phosphate pyrophosphate cathode material, wherein the borophosphate-composite-modified iron-based mixed polyanion sodium cathode material has a molecular formula of K[ZnBP2O8]@Na4Fe3(PO4)2(P2O7).

[0007] A method for preparing a borophosphate-composite-modified sodium iron pyrophosphate positive electrode material comprises the following steps:

[0008] (1) Potassium borate, zinc oxide, dipotassium hydrogen phosphate, and diammonium hydrogen phosphate are sintered at high temperature to prepare K[ZnBP2O8] nanomaterials;

[0009] (2) The K[ZnBP2O8] nanomaterial in step (1) is solid-phase mixed with a sodium source, an iron source, and a phosphate, and then solid-phase sintered to obtain a K[ZnBP2O8]-composite Na4Fe3(PO4)2(P2O7) positive electrode material.

[0010] Preferably, the molar ratio of potassium borate, zinc oxide, dipotassium hydrogen phosphate and diammonium hydrogen phosphate in step (1) is 1:4:1:1.02-1.06.

[0011] Preferably, the sintering atmosphere in step (1) is an oxidizing or air atmosphere, the sintering temperature is 800-1000° C., and the reaction time is 10-36 hours.

[0012] Preferably, the sodium source in step (2) is one or more of sodium oxide, sodium hydroxide, sodium carbonate, and sodium nitrate. The iron source is one or more of ferric nitrate, ferrous acetate, and ferrous oxide. The phosphorus source is one or both of diammonium hydrogen phosphate and ammonium dihydrogen phosphate.

[0013] Preferably, the molar ratio of K[ZnBP2O8], sodium source, iron source and phosphate in step (2) is 0.03-0.08:1.1-1.6:1:1.2-1.7.

[0014] Preferably, the sintering atmosphere in step (2) is nitrogen or argon atmosphere; the sintering temperature is 600-800° C., and the sintering time is 10-30 hours.

[0015] The beneficial effects of the present invention are:

[0016] (1) The present invention prepares a borophosphate-composite sodium iron phosphate pyrophosphate positive electrode material (Na4Fe3(PO4)2(P2O7)) by a one-step solid-phase method, and the material has excellent structural stability.

[0017] (2) The borophosphate composite phase prepared by the present invention has a PO bond similar to that of the sodium iron pyrophosphate compound and a BO bond with high conductivity, and has a large ion size K + , it can provide a larger ion transmission space during the electrochemical process, and the ion transmission of sodium ions in the electrolyte can effectively improve the conductivity and ion diffusion rate of the sodium iron pyrophosphate positive electrode material.

[0018] (3) The preparation process of the present invention is simple and easy to implement, which provides a reference for the further practical application of sodium iron pyrophosphate positive electrode materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is the SEM of the product in Example 1 of the present invention. DETAILED DESCRIPTION

[0020] Example 1

[0021] (1) 0.01 mol potassium borate K2B4O7, 0.04 mol zinc oxide ZnO, 0.01 mol dipotassium hydrogen phosphate K2HPO4, and 0.0102 mol diammonium hydrogen phosphate (NH4)2HPO4 were sintered at 950°C for 20 h in a muffle furnace to prepare K[ZnBP2O8] nanomaterials;

[0022] (2) Take 0.002 mol of the K[ZnBP2O8] nanomaterial in step (1) and mix it with 0.02 mol of sodium oxide Na2O, 0.03 mol of ferrous oxide FeO, and 0.04 mol of diammonium hydrogen phosphate (NH4)2HPO4 in a solid phase, and then solid phase sinter it at 615°C for 16 hours under a nitrogen atmosphere to obtain a K[ZnBP2O8] composite Na4Fe3(PO4)2(P2O7) positive electrode material, the morphology of which is as follows: Figure 1 shown.

[0023] Comparative Example

[0024] Take 0.02 mol of sodium oxide Na2O, 0.03 mol of ferrous oxide FeO, and 0.04 mol of diammonium hydrogen phosphate (NH4)2HPO4, mix them in the solid phase, and then sinter them at 615°C in a nitrogen atmosphere for 16 hours to obtain Na4Fe3(PO4)2(P2O7) positive electrode material.

[0025] Example 2

[0026] (1) 0.01 mol potassium borate K2B4O7, 0.04 mol zinc oxide ZnO, 0.01 mol dipotassium hydrogen phosphate K2HPO4, and 0.0102 mol diammonium hydrogen phosphate (NH4)2HPO4 were sintered in a muffle furnace at 850°C for 30 h to prepare K[ZnBP2O8] nanomaterials;

[0027] (2) 0.002 mol of the K[ZnBP2O8] nanomaterial prepared in step (1) was mixed with 0.02 mol of sodium oxide Na2O, 0.0291 mol of ferrous oxide FeO, and 0.04 mol of diammonium hydrogen phosphate (NH4)2HPO4 in a solid phase, and then solid-phase sintered at 615°C for 16 h under a nitrogen atmosphere to obtain K[ZnBP2O8]-complexed Na4Fe2.91 (PO4)2(P2O7) positive electrode material.

[0028] The battery assembly is completed by the following method:

[0029] The Na4Fe3(PO4)2(P2O7) obtained in Examples 1-2 and the comparative example was used as the positive electrode material, and was mixed with the conductive agent acetylene black (AB) and the binder polyvinylidene fluoride (PVDF) in a mass ratio of 7:2:1. N-methylpyrrolidone (NMP) was used as the solvent, and the mixture was placed in a small beaker and stirred at a speed of 800r / min for 2h to obtain a slurry. The slurry was coated on the current collector aluminum foil using an automatic coating machine, placed flat on tempered glass and dried in a vacuum drying oven at 85°C for 4h. The sheet was punched into a pole piece with a diameter of 12mm and then dried in a vacuum drying oven at 105°C for 4h. It was placed in a glove box filled with argon atmosphere with a water content and oxygen content of less than 0.1ppm for 4h to reduce the moisture adsorbed by the pole piece during the transfer process. It was then assembled into a CR2032 button battery in the glove box. The metallic sodium is rolled into thin sheets and punched into 14mm round sodium sheets to serve as the negative electrode. A 1mol / L NaClO4 solution is used as the electrolyte, and a glass fiber membrane with a diameter of 16mm is used as the diaphragm.

[0030] After aging for 12 hours, the assembled cells were subjected to charge and discharge tests at different potentials. The discharge capacity of the calcined samples was shown in Table 1 after activation for five cycles at a current density of 0.1C at a voltage of 2.2-4V and 100 cycles at 2C.

[0031] Table 1

[0032]

[0033] The above contents are only basic descriptions of the concept of the present invention, and any equivalent transformations made according to the technical solution of the present invention shall fall within the protection scope of the present invention.

Claims

1. A borophosphate-composite-modified sodium iron pyrophosphate positive electrode material, characterized in that: The molecular formula of the borophosphate composite modified sodium iron phosphate pyrophosphate positive electrode material is K[ZnBP2O8]@Na4Fe3(PO4)2(P2O7).

2. A method for preparing a borophosphate-composite-modified sodium iron pyrophosphate positive electrode material, characterized in that: The following steps are involved: (1) Potassium borate, zinc oxide, dipotassium hydrogen phosphate, and diammonium hydrogen phosphate are sintered at high temperature to prepare K[ZnBP2O8] nanomaterials; (2) The K[ZnBP2O8] nanomaterial in step (1) is solid-phase mixed with a sodium source, an iron source, and a phosphate, and then solid-phase sintered to obtain a K[ZnBP2O8]-composite Na4Fe3(PO4)2(P2O7) positive electrode material.

3. The method for preparing a borophosphate composite-modified sodium iron pyrophosphate positive electrode material according to claim 2, characterized in that: The molar ratio of potassium borate, zinc oxide, dipotassium hydrogen phosphate and diammonium hydrogen phosphate in step (1) is 1:4:1:1.02-1.

06.

4. The method for preparing a borophosphate composite modified sodium iron pyrophosphate positive electrode material according to claim 2, characterized in that: The sintering atmosphere in step (1) is an oxidizing or air atmosphere; the sintering temperature is 800-1000° C., and the sintering time is 10-36 hours.

5. The method for preparing a borophosphate composite modified sodium iron pyrophosphate positive electrode material according to claim 2, characterized in that: In step (2), the sodium source is one or more of sodium oxide, sodium hydroxide, sodium carbonate, and sodium nitrate; the iron source is one or more of ferrous nitrate, ferrous acetate, and ferrous oxide; and the phosphate is one or both of diammonium hydrogen phosphate and ammonium dihydrogen phosphate.

6. The method for preparing a borophosphate composite-modified sodium iron pyrophosphate positive electrode material according to claim 2, characterized in that: The molar ratio of K[ZnBP2O8], sodium source, iron source and phosphate in step (2) is 0.03-0.08:1.1-1.6:1:1.2-1.

7.

7. The method for preparing a borophosphate composite modified sodium iron pyrophosphate positive electrode material according to claim 2, characterized in that: The sintering atmosphere in step (2) is nitrogen or argon atmosphere; the sintering temperature is 600-800°C, and the sintering time is 10-30h.

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