Modified ferric sodium pyrophosphate positive electrode material, preparation method thereof and electrochemical device

Abstract

The invention relates to the technical field of battery positive electrode materials, in particular to a modified ferric sodium pyrophosphate positive electrode material, a preparation method thereof and an electrochemical device. The preparation method comprises the following steps: (1) uniformly mixing an iron source, a phosphorus source, a sodium source, an organic acid and an organic solvent, and freeze-drying to obtain a modified ferric sodium pyrophosphate positive electrode material precursor; and (2) carrying out carbonization treatment on the modified ferric sodium phosphate pyrophosphate positive electrode material precursor to obtain the modified ferric sodium phosphate pyrophosphate positive electrode material. The high-purity modified ferric sodium pyrophosphate positive electrode material with a strong coating structure is prepared through efficient adjustment of organic acid, the conductivity and the ion / electron transmission rate of the positive electrode material are effectively improved, and electrochemical polarization of the positive electrode material in the energy storage process is effectively relieved; therefore, the electrochemical performance of the positive electrode material is remarkably improved, and an electrochemical device applying the positive electrode material has more excellent cycling stability and reversible specific capacity.

Description

Technical Field

[0001] The invention relates to the technical field of battery positive electrode materials, and in particular to a modified sodium iron phosphate pyrophosphate positive electrode material and a preparation method and an electrochemical device thereof. Background Art

[0002] Electrochemical energy storage systems have the advantages of high energy conversion efficiency and easy assembly, and are considered to be the most promising energy storage devices in the field of new energy. At present, lithium-ion batteries are the most successful commercial energy storage system. However, lithium resources are unevenly distributed and the abundance of lithium in the earth's crust is not high. 80% of my country's lithium resources rely on imports, resulting in high costs, which is not conducive to their application in large-scale energy storage. Among existing energy storage systems, sodium-ion batteries have the advantages of wide sources and low costs, and are the first choice for low-cost, long-life energy storage systems.

[0003] The positive electrode material directly determines the capacity and operating voltage of the entire battery, and often accounts for about 30% of the total battery cost. Sodium iron pyrophosphate positive electrode material has the advantages of low cost and stable structure, and is the most promising positive electrode material in the field of sodium ion batteries. However, sodium iron pyrophosphate positive electrode material has the problem of poor electronic conductivity, which leads to large electrochemical polarization and low cycle life. In addition, there are many impurities and inactive sodium iron phosphate. Therefore, it is very important to develop new methods to improve the electronic conductivity of this material and reduce its impure substance content.

[0004] In order to overcome these problems, carbon-coated structural materials have been proposed. At present, materials such as glucose, fructose, and sucrose are usually used as carbon sources to improve the performance of sodium iron pyrophosphate, but this method often has problems such as many impurities and high carbon content, which makes it difficult to effectively exert its capacity, which seriously limits its application in sodium ion battery storage.

[0005] Therefore, it is necessary to develop a modified sodium iron pyrophosphate positive electrode material with simple preparation process, excellent cycle stability and reversible specific capacity performance. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a modified sodium iron phosphate pyrophosphate positive electrode material and a preparation method and an electrochemical device. The present invention uses organic acid to efficiently adjust and prepare a high-purity modified sodium iron phosphate pyrophosphate positive electrode material with a strong coating structure, effectively improving the conductivity and ion / electron transmission rate of the positive electrode material, and effectively alleviating the electrochemical polarization of the positive electrode material during the energy storage process, thereby significantly improving the electrochemical performance of the positive electrode material, and the electrochemical device using it has better cycle stability and reversible specific capacity.

[0007] In a first aspect, the present invention provides a method for preparing a modified sodium iron phosphate pyrophosphate positive electrode material, the preparation method comprising:

[0008] (1) mixing an iron source, a phosphorus source, a sodium source, an organic acid and an organic solvent, and freeze-drying the mixture to obtain a modified sodium iron pyrophosphate cathode material precursor;

[0009] (2) Carbonizing the modified sodium iron phosphate pyrophosphate positive electrode material precursor to obtain the modified sodium iron phosphate pyrophosphate positive electrode material.

[0010] The preparation method of the modified sodium iron phosphate pyrophosphate positive electrode material provided by the present invention utilizes the chelation effect of organic acid to prepare a high-purity modified sodium iron phosphate pyrophosphate positive electrode material with a strong coating structure, effectively improving the conductivity and ion / electron transmission rate of the positive electrode material, and effectively alleviating the electrochemical polarization of the positive electrode material during the energy storage process, thereby significantly improving the electrochemical performance of the positive electrode material. The electrochemical device using the modified sodium iron phosphate pyrophosphate has better cycle stability and reversible specific capacity. Specifically:

[0011] The preparation method provided by the present invention first uses organic acid and iron source for coordination, and then freeze-drying to obtain a modified sodium ferric phosphate pyrophosphate positive electrode material precursor, which is a molecular-level uniform mixture of iron source, phosphorus source, sodium source and organic acid, etc., and the freeze-drying process can maintain its internal microstructure, that is, the precursor can well maintain the coordination between organic acid and iron source, and the uniformity of the package is good, and the subsequent carbonization process can further improve the coating effect, improve the uniformity and purity of the coating, thereby preparing a high-purity modified sodium ferric phosphate pyrophosphate positive electrode material with a strong coated carbon layer structure, which can effectively improve the conductivity of the positive electrode material, effectively promote the migration rate of electrons and ions, and improve its energy storage performance. The electrochemical device using it has more excellent cycle stability and reversible specific capacity. Moreover, the preparation method provided by the present invention has a simple process and strong scalability, and has great application prospects and research value in the field of battery preparation.

[0012] As a preferred technical solution of the present invention, the organic acid includes any one or more of mellitic acid, trichloroacetic acid, trinitrobenzenesulfonic acid, nitrogen-sulfuric acid, trifluoroacetic acid, 2,4,6-trinitrobenzoic acid and 2,4,6-trinitrophenol.

[0013] The organic acids involved in the preparation method of the present invention are all strong acids, which play three roles in the preparation process: they can be used as a carbon source, a dispersant and a chelating agent, and can coordinate with the iron element in the iron source, thereby effectively improving the coating effect of the precursor. After subsequent carbonization treatment, a modified sodium iron phosphate pyrophosphate positive electrode material with high purity and a strong coated carbon layer structure is obtained.

[0014] As a preferred technical solution of the present invention, the iron source includes any one or more of ferrous oxalate, ferrous chloride and ferrous nitrate.

[0015] As a preferred technical solution of the present invention, the phosphorus source includes any one or more of ammonium dihydrogen phosphate, sodium dihydrogen phosphate and disodium hydrogen phosphate.

[0016] As a preferred technical solution of the present invention, the sodium source includes any one or more of sodium carbonate, sodium oxalate, sodium acetate and sodium nitrate.

[0017] As a preferred technical solution of the present invention, the organic solvent includes any one or more of acetone, ethanol and ethylene glycol.

[0018] As a preferred technical solution of the present invention, the molar ratio of the iron element in the iron source, the phosphorus element in the phosphorus source, the sodium element in the sodium source and the organic acid is 3:4:(3.8-4.5):(0.4-1), for example, it can be 3:4:3.8:0.4, 3:4:4.0:0.5, 3:4:4.1:0.6, 3:4:4.3:0.8, 3:4:4.5:1, etc.

[0019] As a preferred technical solution of the present invention, the mixing method is high-energy ball milling mixing under an inert gas atmosphere, and the inert gas includes nitrogen and / or argon.

[0020] The present invention utilizes high-energy ball milling to fully mix the components and fully coordinate the organic acid and the iron element in the iron source, thereby achieving full mixing and high dispersion of the materials in the precursor, thereby effectively improving the coating effect of the precursor, and obtaining a modified sodium iron phosphate pyrophosphate positive electrode material with high purity and a strong coating carbon layer structure after subsequent carbonization treatment.

[0021] As a preferred technical solution of the present invention, the carbonization temperature is 550-1000°C, for example, 550°C, 600°C, 700°C, 800°C, 900°C, 1000°C, etc.

[0022] As a preferred technical solution of the present invention, the carbonization time is 2-12 hours, for example, 2 hours, 4 hours, 6 hours, 8 hours, 10 hours, 11 hours, 12 hours, etc.

[0023] When the carbonization temperature and time are within the above ranges, the purity of the positive electrode material and the uniformity of the coated carbon layer can be more effectively improved.

[0024] In a second aspect, the present invention provides a modified sodium iron phosphate pyrophosphate positive electrode material prepared by the preparation method described in the first aspect, wherein the chemical composition of the modified sodium iron phosphate pyrophosphate positive electrode material is Na4Fe3(PO4)2P2O7@C, wherein @ represents carbon coating.

[0025] As a preferred technical solution of the present invention, the carbon content in the modified sodium iron phosphate pyrophosphate positive electrode material is 3-10wt%, for example, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt% and the like.

[0026] In a third aspect, the present invention provides an electrochemical device, comprising the modified sodium iron phosphate pyrophosphate positive electrode material described in the second aspect.

[0027] The electrochemical device of the present invention includes any one of a sodium ion battery, a lithium sodium dual ion battery, a hybrid ion battery and a hybrid ion capacitor.

[0028] The technical solution provided by the embodiment of the present invention has the following advantages compared with the prior art:

[0029] 1. The present invention utilizes organic acid to efficiently adjust and prepare a modified sodium iron phosphate pyrophosphate positive electrode material with high purity and a strong coating structure, which effectively improves the conductivity and ion / electron transfer rate of the positive electrode material, and effectively alleviates the electrochemical polarization of the positive electrode material during the energy storage process, thereby significantly improving the electrochemical performance of the positive electrode material. The electrochemical device using the modified sodium iron phosphate has better cycle stability and reversible specific capacity.

[0030] 2. The preparation method provided by the present invention has simple process and strong scalability, and has great application prospects and research value in the field of battery preparation. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0033] Figure 1 This is a SEM image of the modified sodium iron phosphate pyrophosphate positive electrode material prepared in Example 1 of the present invention;

[0034] Figure 2 This is the XRD pattern of the modified sodium iron phosphate pyrophosphate positive electrode material prepared in Example 1 of the present invention;

[0035] Figure 3 This is a SEM image of the modified sodium iron pyrophosphate positive electrode material prepared in Example 2 of the present invention;

[0036] Figure 4This is the XRD pattern of the modified sodium iron phosphate pyrophosphate positive electrode material prepared in Example 2 of the present invention;

[0037] Figure 5 This is the XRD pattern of the modified sodium iron phosphate pyrophosphate positive electrode material prepared in Comparative Example 1 of the present invention;

[0038] Figure 6 This is a charge and discharge curve diagram of a button cell assembled with the modified sodium iron phosphate pyrophosphate positive electrode material prepared in Example 1 of the present invention;

[0039] Figure 7 This is a cycle performance diagram of a button cell assembled with the modified sodium iron phosphate pyrophosphate positive electrode material prepared in Example 1 of the present invention at 0.5C;

[0040] Figure 8 This is a charge and discharge curve diagram of a button battery assembled with the modified sodium iron phosphate pyrophosphate positive electrode material prepared in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0041] In order to more clearly understand the above-mentioned objectives, features and advantages of the present invention, the scheme of the present invention will be further described below. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0042] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein; it is obvious that the embodiments in the specification are only part of the embodiments of the present invention, rather than all of the embodiments.

[0043] Example 1

[0044] This embodiment provides a modified sodium iron phosphate pyrophosphate positive electrode material and a preparation method thereof, the preparation method comprising the following steps:

[0045] (1) 0.4 mmol of ammonium dihydrogen phosphate, 0.3 mmol of ferrous oxalate, 0.2 mmol of sodium oxalate and 0.04 mmol of mellitic acid were dissolved and dispersed in 30 mL of acetone solution, and then transferred to a ball mill, and high-energy ball milled for 2 h under nitrogen protection to allow the organic acid and iron salt to fully coordinate, and then transferred to a freeze dryer and freeze-dried for 10 h to obtain a yellow precursor material;

[0046] (2) The precursor powder prepared in step (1) is heated at 600° C. for 2 h under nitrogen protection to obtain Na4Fe3(PO4)2P2O7@C nanomaterial.

[0047] Example 2

[0048] This embodiment provides a modified sodium iron phosphate pyrophosphate positive electrode material and a preparation method thereof, the preparation method comprising the following steps:

[0049] (1) 0.60 mmol sodium dihydrogen phosphate, 0.45 mmol ferrous nitrate, 0.02 mmol sodium nitrate and 0.10 mmol trifluoroacetic acid were dissolved and dispersed in 50 mL acetone solution, and then transferred to a ball mill, and high-energy ball milled for 2 h under nitrogen protection to allow the organic acid and iron salt to fully coordinate, and then transferred to a freeze dryer and freeze-dried for 10 h to obtain a yellow precursor material;

[0050] (2) The precursor powder prepared in step (1) is heated at 650° C. for 2 h under argon protection to obtain Na4Fe3(PO4)2P2O7@C nanomaterial.

[0051] Example 3

[0052] This embodiment provides a modified sodium iron phosphate pyrophosphate positive electrode material and a preparation method thereof, the preparation method comprising the following steps:

[0053] (1) 0.4 mmol of ammonium dihydrogen phosphate, 0.3 mmol of ferrous oxalate, 0.2 mmol of sodium oxalate and 0.1 mmol of mellitic acid were dissolved and dispersed in 30 mL of acetone solution, and then transferred into a ball mill, and high-energy ball milled for 2 h under nitrogen protection to allow the organic acid and iron salt to fully coordinate, and then transferred into a freeze dryer and freeze-dried for 10 h to obtain a yellow precursor material;

[0054] (2) The precursor powder prepared in step (1) was heated at 1000° C. for 12 h under nitrogen protection to obtain Na4Fe3(PO4)2P2O7@C nanomaterial.

[0055] Embodiment 4-5

[0056] This embodiment provides a modified sodium iron phosphate pyrophosphate positive electrode material and a preparation method thereof. The preparation method is the same as that of Example 1. The difference from Example 1 is that in this embodiment, mellitic acid is replaced by trinitrobenzenesulfonic acid (Example 4) and nitrogen-sulfuric acid (Example 5).

[0057] Example 6

[0058] This embodiment provides a modified sodium iron phosphate pyrophosphate positive electrode material and a preparation method thereof. The preparation method is the same as that of Example 1, and the difference from Example 1 is that in this embodiment, mellitic acid is replaced by acetic acid.

[0059] Comparative Example 1

[0060] This comparative example provides a modified sodium iron phosphate pyrophosphate positive electrode material and a preparation method thereof. The preparation method is the same as that of Example 1, and the difference from Example 1 is that in this comparative example, mellitic acid is replaced by glucose.

[0061] Comparative Example 2

[0062] This comparative example provides a modified sodium iron phosphate pyrophosphate positive electrode material and a preparation method thereof. The preparation method is the same as that of Example 1, and the difference from Example 1 is that in this comparative example, step (1) is directly performed without freeze-drying after high-energy ball milling and step (2) is performed.

[0063] Performance Test 1

[0064] The modified sodium iron phosphate pyrophosphate positive electrode materials prepared in the examples and comparative examples were subjected to SEM and XRD tests.

[0065] The SEM and XRD patterns of the modified sodium iron phosphate pyrophosphate cathode material prepared in Example 1 are as follows: Figure 1 and Figure 2 As shown, the SEM and XRD patterns of the modified sodium iron phosphate pyrophosphate cathode material prepared in Example 2 are shown as follows: Figure 3 and Figure 4 As shown, the XRD pattern of the modified sodium iron phosphate pyrophosphate positive electrode material prepared in Comparative Example 1 is as follows Figure 5 shown.

[0066] Depend on Figure 2 and 5 It can be found from the comparison that the modified sodium iron phosphate pyrophosphate positive electrode material prepared by the present invention has a higher purity, while in Comparative Example 1, after the organic acid is replaced by the same amount of glucose, the crystal composition of the modified sodium iron phosphate pyrophosphate positive electrode material obtained is impure, and there is an obvious impure phase of sodium iron phosphate. In addition, the modified sodium iron phosphate pyrophosphate positive electrode material obtained in Comparative Example 2 also contains a relatively obvious sodium iron impurity phase after detection.

[0067] Application Examples

[0068] The modified sodium iron phosphate pyrophosphate positive electrode material prepared in the embodiment and the comparative example is assembled into a button battery, comprising the following steps:

[0069] (1) Positive electrode sheet: The modified sodium iron phosphate pyrophosphate positive electrode materials prepared in the examples and comparative examples were respectively used as positive electrode active materials, and a binder (PVDF), a conductive agent (Super P), and a positive electrode active material were mixed in a mass ratio of 1:2:7, and slurried to obtain a slurry. The slurry was then coated on a carbon-containing aluminum foil with a scraper to a thickness of 15 μm, and vacuum dried at 120° C. for 10 h to obtain a sheet, which was then cut into discs with a diameter of 14 mm;

[0070] (2) Electrolyte: 1 mol / L NaPF6, solvent: ethylene carbonate (EC) / dimethyl carbonate (DMC) (volume ratio 1:1);

[0071] (3) Assemble in the following order: negative electrode shell - gasket - sodium sheet - glass fiber separator - positive electrode sheet - positive electrode shell.

[0072] Performance Test 2

[0073] The button batteries prepared according to the corresponding use cases are tested:

[0074] The button cells assembled with the modified sodium iron phosphate pyrophosphate cathode materials prepared in the examples and comparative examples were tested for specific capacity and cycle performance at 0.5C. The test method is as follows:

[0075] At 1.7-4.3V (vs.Na + / Na) voltage range, 0.5C (1C = 129mAh g -1 ) current conditions, constant current charge and discharge tests were carried out on the target material.

[0076] The test results are shown in Table 1 and Figure 6-8 shown.

[0077] in, Figure 6 This is a charge and discharge curve of a button cell assembled with the modified sodium iron phosphate pyrophosphate positive electrode material prepared in Example 1. Figure 7 This is a cycle performance diagram of a button cell assembled with the modified sodium iron phosphate pyrophosphate positive electrode material prepared in Example 1 at 0.5C. Figure 8 This is a charge and discharge curve diagram of a button cell assembled with the modified sodium iron phosphate pyrophosphate positive electrode material prepared in Comparative Example 1.

[0078] Table 1

[0079]

[0080] From Table 1 and Figure 6-7 It can be found that the modified sodium iron phosphate pyrophosphate positive electrode material prepared in Example 1 of the present invention has a reversible specific capacity of 97.8 mAh / g at 0.5C, an initial coulombic efficiency of 95.6%, and still has a capacity of 96.2 mAh / g after 100 cycles at 0.5C, indicating that the modified sodium iron phosphate pyrophosphate positive electrode material prepared in the present invention has excellent rate performance and cycle performance.

[0081] From the comparison between Example 1 and Example 6, it can be found that if the strong acid mellitic acid provided by the present invention is replaced by acetic acid, the rate performance and cycle performance of the prepared modified sodium iron phosphate pyrophosphate positive electrode material are both reduced.

[0082] From the comparison between Example 1 and Comparative Examples 1-2, it can be found that in Comparative Example 1, mellitic acid is replaced with glucose, and the reversible specific capacity of the modified sodium iron phosphate pyrophosphate positive electrode material prepared is only 77.1 mAh / g at 0.5C, showing a relatively low reversible specific capacity. In the preparation process of Comparative Example 2, after high-energy ball milling, step (2) is directly performed without freeze drying, and the rate performance and cycle performance of the prepared modified sodium iron phosphate pyrophosphate positive electrode material are both reduced.

[0083] It should be noted that, in this article, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0084] The above description is only a specific embodiment of the present invention, so that those skilled in the art can understand or implement the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments described herein, but should conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a modified sodium iron phosphate pyrophosphate positive electrode material, characterized in that: The preparation method comprises: (1) mixing an iron source, a phosphorus source, a sodium source, an organic acid and an organic solvent, and freeze-drying the mixture to obtain a modified sodium iron pyrophosphate cathode material precursor; (2) Carbonizing the modified sodium iron phosphate pyrophosphate positive electrode material precursor to obtain the modified sodium iron phosphate pyrophosphate positive electrode material.

2. The preparation method according to claim 1, characterized in that: The organic acid includes any one or more of mellitic acid, trichloroacetic acid, trinitrobenzenesulfonic acid, nitrosulfuric acid, trifluoroacetic acid, 2,4,6-trinitrobenzoic acid and 2,4,6-trinitrophenol.

3. The preparation method according to claim 1 or 2, characterized in that: The iron source includes any one or more of ferrous oxalate, ferrous chloride and ferrous nitrate; And / or, the phosphorus source includes any one or more of ammonium dihydrogen phosphate, sodium dihydrogen phosphate and disodium hydrogen phosphate.

4. The preparation method according to any one of claims 1 to 3, characterized in that The sodium source includes any one or more of sodium carbonate, sodium oxalate, sodium acetate and sodium nitrate; And / or, the organic solvent includes any one or more of acetone, ethanol and ethylene glycol.

5. The preparation method according to any one of claims 1 to 4, characterized in that: The molar ratio of the iron element in the iron source, the phosphorus element in the phosphorus source, the sodium element in the sodium source and the organic acid is 3:4:(3.8-4.5):(0.4-1).

6. The preparation method according to any one of claims 1 to 5, characterized in that: The mixing method is high-energy ball milling mixing under an inert gas atmosphere.

7. The preparation method according to any one of claims 1 to 6, characterized in that: The carbonization temperature is 550-1000°C; And / or, the carbonization time is 2-12h.

8. The modified sodium iron phosphate pyrophosphate positive electrode material prepared by the preparation method according to any one of claims 1 to 7.

9. The modified sodium iron phosphate pyrophosphate positive electrode material according to claim 8, characterized in that: The carbon content of the modified sodium iron phosphate pyrophosphate positive electrode material is 3-10 wt %.

10. An electrochemical device, characterized in that: The electrochemical device comprises the modified sodium iron phosphate pyrophosphate positive electrode material according to claim 8 or 9.

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