Preparation method and application of positive electrode material

Abstract

The invention discloses a preparation method and application of a positive electrode material. The invention provides a preparation method of a positive electrode material Na4Fe3 (PO4) 2P2O7. The preparation method comprises the following steps: adding an iron source, a surfactant, water, a resin monomer, a catalyst, a sodium source and a phosphorus source in stages to obtain a mixed system; defoaming, freezing and drying the obtained mixed system to obtain powder; and sintering the obtained powder to obtain the carbon-coated iron-based polyanion positive electrode material. According to the preparation method, an ionic surfactant is adsorbed on FePO4 to reduce the surface energy of FePO4, conformal polymerization coating of phenolic resin monomers on the surface of FePO4 is induced, and finally, a uniform carbon coating layer is formed on the surface of Na4Fe3 (PO4) 2P2O7 through high-temperature sintering. The prepared positive electrode material has better carbon coating and conductivity, and can further improve the rate capability and cycle performance of the battery when being applied to the sodium ion battery. The preparation method is simple and efficient, conditions are easy to control, and the preparation method has a good industrialization prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion battery materials, and in particular to a method for preparing sodium ion battery positive electrode material Na4Fe3(PO4)2P2O7 by using interfacial polymer in-situ coating technology and its application. Background Art

[0002] In the sodium ion battery system, the cathode material is the key factor that determines the battery's energy density, safety, cycle life and other performance. Polyanion cathode materials have the advantages of low cost, good thermal stability and cycle stability, and environmental friendliness. Among them, iron-based composite phosphate has a theoretical specific capacity of 129mAh / g and has good application prospects in the fields of low-speed power and energy storage.

[0003] NFPP (sodium iron pyrophosphate) has a typical NASICON (sodium superfast conductor) structure, and its electron conduction is much weaker than its ion transmission, which seriously restricts its performance and is the main shortcoming. Therefore, the effective construction of the conductive network is crucial, which can be regulated by screening organic and inorganic carbon sources.

[0004] Carbon coating formed by conversion of organic precursors can effectively improve the conductivity of the material, but commonly used carbon sources such as glucose, sucrose, PEG, etc. are difficult to achieve uniform coating, and the thickness and distribution cannot be precisely controlled.

[0005] In view of this, the present invention is proposed. Summary of the invention

[0006] The purpose of the present invention is to propose a method for in-situ polymerization of phenolic resin to coat iron-based polyanion positive electrode material Na4Fe3(PO4)2P2O7 using surfactants. This method improves the uniformity of the carbon coating layer, regulates the thickness of the carbon coating layer, constructs an effective conductive network, and thus improves the electrochemical performance of sodium ion batteries, providing technical reserves for the large-scale production of a new generation of novel sodium ion battery positive electrode materials.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides a method for preparing a positive electrode material Na4Fe3(PO4)2P2O7, comprising the following steps: S1, adding an iron source, a surfactant and water, a resin monomer, a catalyst, a sodium source, and a phosphorus source in stages to obtain a mixed system; S2, degassing, freezing, and drying the mixed system obtained in step S1 to obtain a powder; S3. Sinter the powder obtained in step S3 to obtain a carbon-coated iron-based polyanion positive electrode material Na4Fe3(PO4)2P2O7.

[0008] The present invention uses an ionic surfactant to adsorb on FePO4 to reduce its surface energy, induces conformal polymerization and coating of phenolic resin monomer on the surface of FePO4, and finally forms a uniform carbon coating layer on the surface of Na4Fe3(PO4)2P2O7 through high-temperature sintering. The thickness of the carbon coating layer can be regulated by the amount of phenolic resin monomer added. The prepared positive electrode material has better carbon coating and conductivity, and can be applied to sodium ion batteries to further improve the rate performance and cycle performance of the battery. This preparation method is simple and efficient, the conditions are easy to control, and it has good industrialization prospects.

[0009] In the above preparation method, in step S1, the surfactant is selected from at least one of hexadecyltrimethylammonium bromide, dodecyltrimethylammonium bromide, sodium dodecyl sulfate and sodium dodecylbenzene sulfonate.

[0010] In the above preparation method, in step S1, the resin monomer includes a phenolic compound and an aldehyde compound.

[0011] In the above preparation method, in step S1, the phenolic compound is selected from one or more of phenol, resorcinol and phloroglucinol, preferably resorcinol.

[0012] In the above preparation method, in step S1, the aldehyde compound is selected from one or more of formaldehyde, paraformaldehyde and acetaldehyde, preferably formaldehyde.

[0013] In the above preparation method, in step S1, the catalyst is selected from one or more of formic acid, acetic acid, oxalic acid, tartaric acid and hydrochloric acid, preferably acetic acid.

[0014] In the above preparation method, in step S1, the mass ratio of the iron source, surfactant, resin monomer and catalyst is 1: (0.005-0.01): (0.01-0.1): (0.005-0.01), preferably 1: 0.005: 0.05: 0.005.

[0015] In the above preparation method, in step S1, in the mixed system, the molar ratio of sodium: iron: phosphorus is (3.9~4.1): (2.90~3.0):4.

[0016] In the above preparation method, in step S1, the iron source is selected from at least one of ferric phosphate, ferrous oxalate and ferrosoferric oxide.

[0017] In the above preparation method, in step S1, the sodium source is selected from at least one of sodium carbonate, sodium pyrophosphate decahydrate, sodium acetate, sodium oxalate, sodium citrate and sodium phosphate.

[0018] In the above preparation method, in step S1, the phosphorus source is selected from at least one of sodium dihydrogen phosphate, sodium phosphate and phosphoric acid.

[0019] In the above preparation method, in step S1, the staged addition is performed according to the following operation: mixing the iron source, surfactant and water to obtain a dispersion; continuing to add the resin monomer and the catalyst, mixing; then adding the sodium source and the phosphorus source, mixing, to obtain a mixed system. By rationally designing the order of feeding each material, the uniformity of material mixing can be improved, which is conducive to the full progress of the subsequent reaction, thereby obtaining a uniform carbon coating layer and good electrochemical performance.

[0020] In the above preparation method, in step S1, the mixing is performed using a planetary ball mill with a rotation speed of 300-600 rpm and a time of 1-5 hours.

[0021] In the above preparation method, in step S2, the degassing conditions are: vacuum degassing, stirring speed of 100-1000 rpm, and time of 1-5 hours.

[0022] In the above preparation method, in step S2, the freezing condition is: temperature ≤-50°C.

[0023] In the above preparation method, in step S2, the drying conditions are: using a freeze dryer, the vacuum degree is <100Pa, and the cold trap temperature is ≤-50°C.

[0024] In the above preparation method, in step S3, the sintering conditions are: under an inert atmosphere, the sintering temperature is 450-550°C, the time is 5-10 hours, and the heating rate is 0.5-5°C / min.

[0025] In the above preparation method, in step S3, the gas used in the inert atmosphere is nitrogen, argon, nitrogen-hydrogen mixture, and argon-hydrogen mixture.

[0026] In a second aspect, the present invention also provides a carbon-coated positive electrode material Na4Fe3(PO4)2P2O7 obtained by the above preparation method. The material has better carbon coating and conductivity, and can be applied to sodium ion batteries to further improve the rate performance and cycle performance of the battery.

[0027] The positive electrode material comprises a substrate and a carbon coating layer; the thickness of the carbon coating layer ranges from 1 to 10 nm, preferably from 3.5 to 7.0 nm.

[0028] In a third aspect, the present invention further provides a sodium ion battery, comprising a positive electrode and a negative electrode; the material of the positive electrode comprises the above-mentioned carbon-coated positive electrode material Na4Fe3(PO4)2P2O7. The sodium ion battery has better rate performance and cycle performance.

[0029] Compared with the prior art, the beneficial effects of the present invention are embodied in the following two aspects: (1) The present invention uses an ionic surfactant to adsorb on FePO4, thereby reducing its surface energy, inducing phenolic compounds and aldehyde compounds to settle on the surface of FePO4, promoting conformal polymerization and coating of phenolic resin monomer on the surface of FePO4, and finally forming a uniform carbon coating layer on the surface of Na4Fe3(PO4)2P2O7 after high-temperature sintering. The thickness of the carbon coating layer can be controlled by the amount of phenolic resin monomer added.

[0030] (2) The positive electrode material prepared in the present invention has better carbon coating and conductivity, and can be applied to sodium ion batteries to further improve the rate performance and cycle performance of the battery. The preparation method is simple and efficient, the conditions are easy to control, and it has good industrialization prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic structural diagram of the positive electrode material precursor provided by the present invention.

[0032] Figure 2 This is the TEM photo of Na4Fe3(PO4)2P2O7 prepared in Comparative Example 1.

[0033] Figure 3 This is the TEM photo of Na4Fe3(PO4)2P2O7 prepared in Comparative Example 2.

[0034] Figure 4 This is the TEM photo of Na4Fe3(PO4)2P2O7 prepared in Example 1.

[0035] Figure 5 This is the TEM photo of Na4Fe3(PO4)2P2O7 prepared in Example 3.

[0036] Figure 6 This is the Raman spectrum of Na4Fe3(PO4)2P2O7 prepared in Comparative Example 2 and Example 1. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited to the following embodiments.

[0038] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0039] Unless otherwise specified, the reagents, materials, instruments, etc. used in the following examples can be obtained from commercial sources.

[0040] Example 1 13.58 g of ferric phosphate, 67.9 mg of hexadecyltrimethylammonium bromide and 80 mL of deionized water were added to a 250 mL zirconium oxide ball mill and ball milled at 500 rpm for 2 h to obtain the first mixed system. 130 mg of resorcinol, 150 mL of formaldehyde (mass fraction 37%) and 100 mL of glacial acetic acid were added to the first mixed system and mechanically stirred at 500 rpm for 2 h to obtain the second mixed system. 6.36 g of sodium carbonate and 3.46 g of phosphoric acid were added to the second mixed system and mechanically stirred at 500 rpm for 2 h to obtain the third mixed system.

[0041] The third mixed system was transferred to a vacuum degassing machine to remove bubbles in the slurry, and then transferred to a -80°C freezer for rapid freezing. The frozen block was transferred to a freeze dryer, the vacuum degree was set to 10Pa, the cold trap temperature was set to -50°C, and dried for 48 hours to obtain a precursor powder.

[0042] The precursor powder was then placed in a nitrogen atmosphere, heated to 500°C at 2°C / min, and maintained for 10 h to obtain Na4Fe3(PO4)2(P2O7). The button cell was assembled in a glove box with water and oxygen below 0.01 ppm.

[0043] Example 2 The phenolic resin nanofiber aerogel material was prepared according to the method of Example 1, except that 260 mg of resorcinol and 300 mL of formaldehyde (mass fraction 37%) were added to the first mixed system. The rest was the same.

[0044] Example 3 The phenolic resin nanofiber aerogel material was prepared according to the method of Example 1, except that 520 mg of resorcinol and 600 mL of formaldehyde (mass fraction 37%) were added to the first mixed system. The rest was the same.

[0045] Example 4 The phenolic resin nanofiber aerogel material was prepared according to the method of Example 1, except that 150 mg of phenol and 235 mL of formaldehyde (mass fraction 37%) were added to the first mixed system. The rest was the same.

[0046] Comparative Example 1 13.58 g of iron phosphate, 6.36 g of sodium carbonate, 3.46 g of phosphoric acid, 3.14 g of glucose, and 80 mL of deionized water were added to a 250 mL zirconium oxide ball mill, ball milled at 500 rpm for 6 h, and then quickly frozen in a -80 ° C freezer. The frozen block was transferred to a freeze dryer, the vacuum degree was set to 10 Pa, the cold trap temperature was set to -50 ° C, and dried for 48 h to obtain a precursor powder; then the precursor powder was placed in a nitrogen atmosphere, heated to 500 ° C at 2 ° C / min, and maintained for 10 h to obtain Na4Fe3(PO4)2(P2O7). The button battery was assembled in a glove box with water oxygen below 0.01 ppm.

[0047] Comparative Example 2 13.58 g of iron phosphate and 80 mL of deionized water were added to a 250 mL zirconium oxide ball mill, and the mixture was ball milled at 500 rpm for 2 h to obtain the first mixed system. 130 mg of resorcinol, 150 mL of formaldehyde (mass fraction 37%), and 100 mL of glacial acetic acid were added to the first mixed system, and the mixture was mechanically stirred at 500 rpm for 2 h to obtain the second mixed system. 6.36 g of sodium carbonate and 3.46 g of phosphoric acid were added to the second mixed system, and the mixture was mechanically stirred at 500 rpm for 2 h to obtain the third mixed system. The third mixed system was quickly frozen in a -80 ° C freezer. The frozen block was transferred to a freeze dryer, the vacuum degree was set to 10 Pa, the cold trap temperature was set to -50 ° C, and the precursor powder was dried for 48 h; then the precursor powder was placed in a nitrogen atmosphere, heated to 500 ° C at 2 ° C / min, and maintained for 10 h to obtain Na4Fe3(PO4)2(P2O7). The button cells were assembled in a glove box with water and oxygen concentration below 0.01 ppm.

[0048] Testing Trials 1. Structural characterization Figure 1 This is a schematic diagram of the structure of the iron phosphate positive electrode material precursor provided by the present invention. Figure 1 It can be seen that the surfactant combines with the surface of iron phosphate to reduce its surface energy, inducing the phenolic resin monomer to adsorb on the particle surface and polymerize in situ, thereby forming a uniform polymer layer. This layer can then be sintered at high temperature to form a uniform carbon coating layer.

[0049] like Figures 2~5 As shown in Figure 2, after high-temperature sintering, the phenolic resin forms a carbon coating in situ, which is conformally coated on the FePO4 surface with a thickness of about 3.6 nm. It can be calculated from the Raman spectrum ( Figure 6), the sp3 / sp2 of the carbon layer of Comparative Example 2 and Example 1 are 1.05 and 1.03 respectively, which proves that the surfactant does not change the chemical structure of the phenolic resin derived carbon. With the increase of the amount of phenolic compounds and aldehyde compounds added, the uniformity of the carbon coating layer does not change significantly, but only causes the thickness to gradually increase from 3.6nm to 13.3nm. This shows that the carbon coating effect can be effectively improved by using a surfactant.

[0050] 2. Physical parameters and battery performance Table 1 Physical properties of materials and battery performance

[0051] As shown in Table 1, the comprehensive performance of the positive electrode materials obtained in Comparative Examples 1 and 2 is much lower than that of the positive electrode materials obtained in Examples 1 to 4, especially in terms of rate and cycle performance, indicating that the uniformity and thickness of carbon coating have an important influence on the electrochemical performance.

[0052] It is worth noting that when the FePO4 surface is not treated with a surfactant, the carbon layer derived from the phenolic resin presents an irregular "island-like" non-uniform distribution morphology, which makes it difficult to form an effective conductive network, increases the ohmic polarization and electrochemical polarization, and results in lower rate and voltage performance for Comparative Examples 1 and 2.

[0053] Another thing worth noting is that as the thickness of the carbon layer increases, the rate and voltage performance show the characteristics of first increasing and then decreasing. This is because the appropriate thickening of the carbon layer helps to build a conductive network, but an overly thick carbon layer will hinder the transmission of sodium ions at the interface, thereby inhibiting the kinetics of the electrochemical reaction and reducing the rate and voltage performance. In addition, the capacity gradually decreases with the increase of the carbon layer thickness. This is because the increase in carbon reduces the proportion of active substances, thereby limiting the capacity.

[0054] Although the present invention has been described in detail above with general descriptions and specific embodiments, it is obvious to those skilled in the art that some modifications or improvements may be made thereto based on the present invention. Therefore, these modifications or improvements made without departing from the spirit of the present invention all fall within the scope of protection claimed by the present invention.

Claims

1. A method for preparing a positive electrode material, comprising the following steps: S1, adding an iron source, a surfactant and water, a resin monomer, a catalyst, a sodium source, and a phosphorus source in stages to obtain a mixed system; S2, degassing, freezing, and drying the mixed system obtained in step S1 to obtain a powder; S3. Sinter the powder obtained in step S3 to obtain a carbon-coated iron-based polyanion positive electrode material Na4Fe3(PO4)2P2O7.

2. The preparation method according to claim 1, characterized in that: In step S1, the surfactant is selected from at least one of hexadecyltrimethylammonium bromide, dodecyltrimethylammonium bromide, sodium dodecyl sulfate and sodium dodecylbenzene sulfonate; The resin monomers include phenolic compounds and aldehyde compounds; The phenolic compound is selected from one or more of phenol, resorcinol and phloroglucinol; The aldehyde compound is selected from one or more of formaldehyde, paraformaldehyde and acetaldehyde; The catalyst is selected from one or more of formic acid, acetic acid, oxalic acid, tartaric acid and hydrochloric acid.

3. The preparation method according to claim 1 or 2, characterized in that: In step S1, the mass ratio of the iron source, surfactant, resin monomer and catalyst is 1:(0.005-0.01):(0.01-0.1):(0.005-0.01); In the mixed system, the molar ratio of sodium: iron: phosphorus is (3.9-4.1): (2.90-3.0):

4.

4. The preparation method according to any one of claims 1 to 3, characterized in that In step S1, the iron source is selected from at least one of ferric phosphate, ferrous oxalate and ferrosoferric oxide; The sodium source is selected from at least one of sodium carbonate, sodium pyrophosphate decahydrate, sodium acetate, sodium oxalate, sodium citrate and sodium phosphate; The phosphorus source is selected from at least one of sodium dihydrogen phosphate, sodium phosphate and phosphoric acid.

5. The preparation method according to any one of claims 1 to 4, characterized in that: In step S1, the stepwise addition is performed according to the following operations: mixing the iron source, the surfactant and water to obtain a dispersion; continuing to add the resin monomer and the catalyst and mixing; Then, a sodium source and a phosphorus source are added and mixed to obtain a mixed system; The mixing is carried out using a planetary ball mill at a rotation speed of 300-600 rpm for a time of 1-5 hours.

6. The preparation method according to any one of claims 1 to 5, characterized in that: In step S2, the degassing conditions are: vacuum degassing, stirring speed of 100-1000 rpm, and time of 1-5 hours; The freezing conditions are: temperature ≤ -50°C; The drying conditions are: using a freeze dryer, vacuum degree <100Pa, cold trap temperature ≤-50°C.

7. The preparation method according to any one of claims 1 to 6, characterized in that: In step S3, the sintering conditions are: under an inert atmosphere, the sintering temperature is 450-550°C, the time is 5-10 hours, and the heating rate is 0.5-5°C / min; The gas used in the inert atmosphere is nitrogen, argon, nitrogen-hydrogen mixture, and argon-hydrogen mixture.

8. The carbon-coated positive electrode material Na4Fe3(PO4)2P2O7 obtained by the preparation method according to any one of claims 1 to 7.

9. The cathode material Na4Fe3(PO4)2P2O7 according to claim 8, characterized in that The positive electrode material comprises a substrate and a carbon coating layer; The thickness of the carbon coating layer is in the range of 1-10 nm, preferably 3.5-7.0 nm.

10. A sodium ion battery comprising a positive electrode and a negative electrode; the material of the positive electrode comprises the carbon-coated positive electrode material Na4Fe3(PO4)2P2O7 according to claim 8 or 9.

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