Carbon-coated ion-doped ferric sodium pyrophosphate positive electrode material and preparation method thereof

Abstract

The invention discloses a carbon-coated ion-doped ferric sodium pyrophosphate positive electrode material and a preparation method thereof, and belongs to the technical field of batteries. The preparation method comprises the following steps: adding a sodium source, an iron source, an M source, a phosphorus source and a carbon source into water to obtain a clear and transparent mixed solution, then drying the mixed solution to obtain a precursor, and sequentially carrying out pre-sintering, pressing molding and high-temperature sintering on the precursor to obtain the final ferric sodium pyrophosphate positive electrode material. Compared with a traditional solid phase method, a spray drying method and a conventional sol-gel method, the preparation method of the carbon-coated ion-doped ferric sodium pyrophosphate positive electrode material is simpler and more efficient, the equipment cost and the energy consumption are greatly reduced, the stability and the electronic conductivity of a crystal frame can be enhanced, and the service life of the positive electrode material is prolonged. The prepared positive electrode material has very excellent rate capability and long cycle service life, can circulate for 10000 circles at a high rate of 20C, and also has a very good capacity retention ratio.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and more specifically relates to a carbon-coated ion-doped sodium iron phosphate pyrophosphate cathode material and a preparation method thereof. Background Art

[0002] In just a few decades, the development and innovation of lithium-ion batteries (LIBs) has enabled the widespread use of portable devices and electric vehicles, bringing enormous benefits to society. Lithium-ion batteries have been widely used in portable electronic devices and electric vehicles due to their high energy density and long cycle life. While LIBs have performed well in electric vehicles and portable electronic devices in emerging markets, their deployment in large-scale grid storage remains limited by limited resources, high costs, low safety, and sustainability issues. There is an urgent need to find alternatives that meet the comprehensive requirements of low cost, high performance, and safety.

[0003] Sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 (NFPP) has a stable three-dimensional framework structure and belongs to the sodium superion conductor (NASICON) family with a large ion transport channel. NFPP has a higher theoretical specific capacity (129 mAh g -1 ), small battery volume change (< 4%) during sodium ion extraction / insertion, and a higher operating voltage (> 3.1 V). Furthermore, the material offers advantages such as high thermal stability, low cost, high safety, and non-toxicity. These properties make NFPP a potential candidate for high-rate and long-life sodium-ion battery cathode materials, and it is currently a hot research material.

[0004] Previous studies have mostly used solid-phase, spray-drying, and sol-gel methods to synthesize sodium iron phosphate (NFPP) electrode materials. These methods are not only cumbersome and expensive, but also involve the generation of inactive NaFePO4. Furthermore, the NFPP material itself suffers from low electronic conductivity and slow ion diffusion rates, which limits its rate capability and long-cycle stability.

[0005] Therefore, it is necessary to develop a method that can improve the rate performance and cycle stability of sodium iron pyrophosphate positive electrode materials, extend the battery life, and at the same time simplify the synthesis process, reduce costs, and have industrialization potential. Summary of the Invention

[0006] The purpose of the present invention is to provide a carbon-coated ion-doped sodium iron phosphate pyrophosphate cathode material and a preparation method thereof, so as to achieve the characteristics of the cathode material having both excellent rate performance and ultra-high stability.

[0007] A preparation method of a carbon-coated ion-doped sodium iron pyrophosphate cathode material, comprising the following steps: (1) Dissolve the Na source, Fe source, M source, P source and carbon source in water to obtain a clear and transparent mixed solution; (2) Dry the mixed solution to obtain a precursor powder; (3) Perform a low-temperature solid-state reaction on the precursor powder in an inert atmosphere to obtain a pre-sintered powder; (4) Press the pre-sintered powder into a block; (5) Perform a high-temperature solid-state reaction on the block in step (4) in an inert atmosphere to finally obtain a carbon-coated ion-doped sodium iron pyrophosphate cathode material.

[0008] Preferably, the molar ratio of Na + : Fe 2+ : M ions: P source: carbon source in the Na source, Fe source, M source, P source and carbon source is 4:3-x:x:4:4 to 4.5, 0 < x ≤ 0.2; the M source is an inorganic salt containing M; M is at least one of La, Ce, Pr, Nd; the Na source is CH3COONa or NaHCO3; the Fe source is FeSO4·7H2O; the P source is NH4H2PO4; the carbon source is C6H8O7·H2O.

[0009] More preferably, the Na source is CH3COONa. Not all Na sources can be used in the present invention to reduce the formation of NaFePO4. For example, Na2C2O4 or Na2CO3 will increase the formation of NaFePO4, which will lead to a decrease in the rate performance and long cycle life of the cathode material.

[0010] Preferably, in step (2), the drying temperature is 120-160°C and the time is 8-12h.

[0011] More preferably, the drying temperature is 160°C and the time is 10h.

[0012] Preferably, the temperature of the low-temperature solid-state reaction is 250-350°C, the time is 2-5 hours, the heating rate is 2-4°C / min, and the inert atmosphere is one of nitrogen, argon or argon-hydrogen mixture.

[0013] More preferably, the temperature of the low-temperature solid-state reaction is 300°C, the time is 3 hours, and the heating rate is 2°C / min.

[0014] Preferably, the pressure for pressing is 14-18 MPa, and more preferably, the pressure for pressing is 16 MPa.

[0015] Preferably, the temperature of the high-temperature solid-phase reaction is 500-650°C, the time is 8-12 hours, the heating rate is 5-10°C / min, and the inert atmosphere is one of nitrogen, argon or argon-hydrogen mixture.

[0016] More preferably, the temperature of the high-temperature solid-phase reaction is 600°C, the time is 10 hours, and the heating rate is 5°C / min.

[0017] The present invention also claims that the molecular formula of the carbon-coated ion-doped sodium iron pyrophosphate phosphate cathode material is Na4Fe 3-x M x (PO4)2P2O7@C, where 0 < x ≤ 0.2, and M is at least one of La, Ce, Pr, and Nd.

[0018] Compared with the prior art, what are the advantages of the present invention: Compared with the traditional solid-phase method, spray drying method and conventional sol-gel method, the preparation method of the carbon-coated ion-doped sodium iron pyrophosphate phosphate cathode material of the present invention is simpler and more efficient, greatly reducing the equipment cost and energy consumption, and can reduce the generation of inactive NaFePO4. The prepared cathode material has very excellent rate performance and long cycle life, and can cycle 10,000 times at a high rate of 20C and still has a very good capacity retention rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 XRD patterns of the carbon-coated highly stable sodium iron pyrophosphate phosphate cathode materials obtained in Example 1, Comparative Examples 1, 3, and 5.

[0020] Figure 2 FESEM image of the carbon-coated highly stable sodium iron pyrophosphate active material obtained in Example 3.

[0021] Figure 3 Rate performance graph of the carbon-coated highly stable sodium iron pyrophosphate active material obtained in Example 1.

[0022] Figure 4 Charge-discharge curve of the carbon-coated highly stable sodium iron pyrophosphate active material obtained in Example 2.

[0023] Figure 5 Long cycle performance graph of the carbon-coated highly stable sodium iron pyrophosphate active material obtained in Example 2 at a high rate of 20C. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0024] The various exemplary embodiments of the present invention are now described in detail. This detailed description should not be considered as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, characteristics, and embodiments of the present invention.

[0025] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value within any stated value or stated range, and each smaller range between any other stated value or intermediate value within the stated range, is also encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0026] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0027] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be illustrative only.

[0028] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0029] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited to the contents described above. Example

[0030] A method for preparing a carbon-coated high-stability sodium iron phosphate pyrophosphate positive electrode material, the specific steps are as follows: (1) According to the stoichiometric ratio, 4 mmol CH3COONa, 2.94 mmol FeSO4·7H2O, 0.03 mmol La2(SO4)3, 4 mmol NH4H2PO4, and 4.5 mmol C6H8O7·H2O were weighed and added to 10 ml deionized water. The mixture was stirred vigorously for 10 minutes to obtain a clear and transparent mixed solution.

[0031] (2) The mixed solution was transferred to a forced air drying oven and dried at 160° C. for 10 hours to obtain a precursor.

[0032] (3) The precursor was ground into powder and pre-sintered in a tube furnace at 300 °C for 3 hours in an atmosphere of H2 / Ar (7% / 93%).

[0033] (4) The pre-sintered powder was pressed into a coin-shaped block using a powder block pressing mold with a diameter of 15 mm and maintained at a pressure of 16 MPa for 1 minute.

[0034] (5) Finally, the pressed precursor was placed in a tube furnace and sintered at 600 °C for 10 hours in an atmosphere of H2 / Ar (5% / 95%) to obtain Na4Fe 2.94 La 0.06 (PO4)2P2O7 positive electrode material.

[0035] (6) Collect the sintered active materials and assemble them into CR2032 button batteries for electrochemical testing. The specific steps are as follows: 2.94 La 0.06 The (PO4)2P2O7 cathode material, conductive carbon black, and PVDF binder were mixed in a mass ratio of 7:2:1. NMP solvent was then added and mixed in a mixer for half an hour to create a smooth, fine slurry. The slurry was then evenly coated onto aluminum foil to a thickness of 200 µm and dried in a vacuum drying oven at 120°C for 12 hours to completely evaporate the solvent. After drying, the slurry was cut into 13 mm discs for later use. The cut electrodes were assembled into CR2032 button cells in an argon-filled glove box for electrochemical performance testing within a voltage window of 1.7–4.3 V.

[0036] In the XRD spectrum, the peak near 33.1° (shown by the dotted line) represents the characteristic diffraction peak of NaFePO4. Figure 1 It can be seen that the Na4Fe prepared in Example 1 2.94 La 0.06 The characteristic diffraction peak intensity of NaFePO4 in the (PO4)2P2O7 positive electrode material is significantly weaker than that in Comparative Examples 1, 3, and 5, indicating that the synthesis method of the present invention can inhibit the formation of NaFePO4.

[0037] from Figure 3 The rate performance diagram shows that the Na4Fe prepared in Example 1 2.94 La 0.06 The (PO4)2P2O7 positive electrode material exhibits excellent rate performance. As the rate increases, the capacity does not decay rapidly, which also demonstrates its excellent stability. Example

[0038] The preparation method of a carbon-coated high-stability sodium iron phosphate pyrophosphate positive electrode material in this embodiment is different from that in Example 1 in that: in step (1), FeSO4·7H2O is 2.9mmol, La2(SO4)3 is 0.05mmol, CH3COONa is replaced by NaHCO3 of the same amount, and the final synthesized material is Na4Fe 2.9 La 0.1 (PO4)2P2O7.

[0039] Na4Fe prepared in this example 2.9 La 0.1 The steps of assembling and testing the (PO4)2P2O7 battery are the same as those in Example 1.

[0040] Na4Fe prepared in Example 2 2.9 La 0.1 The characteristic diffraction peak intensity of NaFePO4 in the (PO4)2P2O7 positive electrode material is similar to that in Example 1 and is almost invisible.

[0041] from Figure 4 The charge and discharge curves of the sample prepared in Example 2 at a rate of 0.1 C show that the average voltage platform is about 3.2 V (relative to Na+ / Na), which corresponds to Na + Fe during insertion / extraction 3+ / Fe 2+ redox reaction.

[0042] according to Figure 5 The sample was subjected to an ultra-long cycle test at a high current density of 20C, which showed that the capacity retention rate of the sample prepared in Example 2 was still 85.1% after 10,000 cycles. Example

[0043] The method for preparing a carbon-coated high-stability sodium iron phosphate pyrophosphate positive electrode material in this embodiment is different from that in Example 1 in that: in step (1), the M salt is selected as Ce(SO4)2, FeSO4·7H2O is 2.9mmol, and Ce(SO4)2 is 0.1mmol. The final synthesized material is Na4Fe 2.9 Ce 0.1 (PO4)2P2O7.

[0044] Na4Fe prepared in this example 2.9 Ce 0.1 The steps of assembling and testing the (PO4)2P2O7 battery are the same as those in Example 1.

[0045] Na4Fe prepared in Example 3 2.9 Ce 0.1The characteristic diffraction peak intensity of NaFePO4 in the (PO4)2P2O7 positive electrode material is similar to that in Example 1 and is almost invisible.

[0046] from Figure 2 The FESEM image shows that the sample prepared in Example 3 exhibits a typical irregular porous morphology, which is formed by the aggregation of single nanoparticles. This porous structure is conducive to electrolyte penetration and improves the rate performance of the material. Example

[0047] The method for preparing a carbon-coated high-stability sodium iron phosphate pyrophosphate positive electrode material in this embodiment is different from that in Example 1 in that: in step (1), the M salt is selected as Pr2(SO4)3, FeSO4·7H2O is 2.9mmol, and Pr2(SO4)3 is 0.05mmol. The final synthesized material is Na4Fe 2.9 Pr 0.1 (PO4)2P2O7.

[0048] Na4Fe prepared in this example 2.9 Pr 0.1 The steps of assembling and testing the (PO4)2P2O7 battery are the same as those in Example 1.

[0049] Na4Fe prepared in Example 4 2.9 Pr 0.1 The characteristic diffraction peak intensity of NaFePO4 in the (PO4)2P2O7 positive electrode material is similar to that in Example 1 and is almost invisible. Example

[0050] The method for preparing a carbon-coated high-stability sodium iron phosphate pyrophosphate positive electrode material in this embodiment is different from that in Example 1 in that: in step (1), the M salt is selected as Pr2(SO4)3, FeSO4·7H2O is 2.9mmol, and Nd2(SO4)3 is 0.05mmol. The final synthesized material is Na4Fe 2.9 Nd 0.1 (PO4)2P2O7.

[0051] Na4Fe prepared in this example 2.9 Nd 0.1 The steps of assembling and testing the (PO4)2P2O7 battery are the same as those in Example 1.

[0052] Na4Fe prepared in Example 5 2.9 Nd 0.1 The characteristic diffraction peak intensity of NaFePO4 in the (PO4)2P2O7 positive electrode material is similar to that in Example 1 and is almost invisible.

[0053] Comparative Example 1 A method for preparing a carbon-coated high-stability sodium iron phosphate pyrophosphate positive electrode material, the specific steps are as follows: (1) 4 mmol CH3COONa, 3 mmol FeSO4·7H2O, 4 mmol NH4H2PO4, and 4.5 mmol C6H8O7·H2O were weighed in stoichiometric proportions and added to 10 ml deionized water. The mixture was stirred vigorously for 10 min to obtain a clear and transparent mixed solution.

[0054] (2) The mixed solution was transferred to a forced air drying oven and dried at 160° C. for 10 hours to obtain a precursor.

[0055] (3) The precursor was ground into powder and pre-sintered in a tube furnace at 300 °C for 3 hours in an atmosphere of H2 / Ar (7% / 93%).

[0056] (4) The pre-sintered powder was pressed into a coin-shaped block using a powder block pressing mold with a diameter of 15 mm at a pressure of 16 MPa.

[0057] (5) Finally, the pressed precursor was placed in a tube furnace and sintered at 600 °C for 10 h in an atmosphere of H2 / Ar (7% / 93%) to obtain the Na4Fe3(PO4)2P2O7 cathode material, which was recorded as NFPP.

[0058] The steps of preparing the Na4Fe3(PO4)2P2O7 assembled battery and testing in this comparative example are the same as those in Example 1.

[0059] Comparative Example 2 The method for preparing a carbon-coated high-stability sodium iron phosphate pyrophosphate positive electrode material in this embodiment is different from that in embodiment 2 in that: in step (1), C6H8O7·H2O is 3mmol, and the final synthesized material is Na4Fe 2.9 La 0.1 (PO4)2P2O7 is recorded as NFPP2.

[0060] The NFPP2 assembled battery prepared and tested in this comparative example was the same as that in Example 1.

[0061] Comparative Example 3 The method for preparing a carbon-coated high-stability sodium iron phosphate pyrophosphate positive electrode material in this embodiment is different from that in Example 2 in that: the Na source in step (1) is Na2CO3, and the final synthesized material is Na4Fe 2.9 La 0.1 (PO4)2P2O7 is recorded as NFPP3.

[0062] The NFPP2 assembled battery prepared and tested in this comparative example was the same as that in Example 1.

[0063] Comparative Example 4 The method for preparing a carbon-coated high-stability sodium iron phosphate pyrophosphate positive electrode material in this embodiment is different from that in Example 2 in that: The sintering atmosphere in step (5) is nitrogen, the sintering temperature in step (5) is 500° C., and the sintering time is 12 hours. The synthesized sodium iron phosphate pyrophosphate positive electrode material is recorded as NFPP4.

[0064] The NFPP4 assembled battery prepared and tested in this comparative example was the same as that in Example 1.

[0065] Comparative Example 5 A method for preparing a carbon-coated high-stability sodium iron phosphate pyrophosphate positive electrode material, the specific steps are as follows: (1) 40 mmol CH3COONa, 29.4 mmol FeSO4·7H2O, 0.3 mmol La2(SO4)3, 40 mmol NH4H2PO4, and 45 mmol C6H8O7·H2O were weighed into a ball mill in the stoichiometric ratio, and then wet-ground by ball milling. The precursor was dried in a forced air drying oven at 160°C for 10 h to obtain a precursor.

[0066] (2) The precursor was ground into powder and pre-sintered in a tube furnace at 300 °C for 3 hours in an atmosphere of H2 / Ar (7% / 93%).

[0067] (3) The pre-sintered powder is pressed into a coin-shaped block using a powder block pressing mold with a diameter of 15 mm and maintained at a pressure of 16 MPa for 1 minute.

[0068] (4) Finally, the pressed precursor was placed in a tube furnace and sintered at 600 ° C for 10 hours in an atmosphere of H2 / Ar (7% / 93%) to obtain Na4Fe 2.94 La 0.06 (PO4)2P2O7 positive electrode material, denoted as NFPP5.

[0069] The NFPP5 assembled battery prepared and tested in this comparative example was the same as that in Example 1.

[0070] from Figure 1 It can be seen that the characteristic diffraction peak intensity of NaFePO4 in Comparative Example 5 is significantly stronger than that in Example 1, Comparative Examples 1 and 3, indicating that the Na4Fe 2.94 La 0.06 The NaFePO4 content in the (PO4)2P2O7 material is significantly higher than that in the synthesis method adopted in the present invention.

[0071] Effect Examples Electrochemical tests were performed on CR2032 button batteries assembled with the sodium iron phosphate pyrophosphate active materials synthesized in Examples 1-5 and Comparative Examples 1-5 as electrode materials. The test results are shown in Table 1.

[0072] Table 1

[0073] As can be seen from Table 1, the specific capacity and cycle stability of the batteries assembled with the materials prepared in Examples 1-5 of the present invention are better than those in Comparative Examples 1-5, indicating that the electrode materials prepared by the preparation method of the present invention exhibit excellent rate performance and cycle stability, especially the long-range stability of fast charging at a high current of 20C, which can reach more than 80%.

[0074] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one 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 is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a carbon-coated ion-doped sodium iron pyrophosphate positive electrode material, characterized in that: The steps include: (1) dissolving a Na source, an Fe source, an M source, a P source, and a carbon source in water to obtain a clear and transparent mixed solution; (2) drying the mixed solution to obtain a precursor powder; (3) subjecting the precursor powder to a low-temperature solid-phase reaction in an inert atmosphere to obtain a pre-sintered powder; (4) Pressing the pre-sintered powder into a block; (5) subjecting the block of step (4) to a high-temperature solid-phase reaction in an inert atmosphere to finally obtain a carbon-coated ion-doped sodium iron phosphate pyrophosphate positive electrode material.

2. The method for preparing the carbon-coated ion-doped sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that: In the Na source, Fe source, M source, P source and carbon source, the molar ratio of Na + :Fe 2+ :M ions:P source:carbon source is 4:3 - x:x:4:4 to 4.5, where 0 < x ≤ 0.2; the M source is an inorganic salt containing M; M is at least one of La, Ce, Pr, Nd; the Na source is CH3COONa or NaHCO3; the Fe source is FeSO4·7H2O; the P source is NH4H2PO4; the carbon source is C6H8O7·H2O.

3. The method for preparing the carbon-coated ion-doped sodium iron phosphate pyrophosphate positive electrode material according to claim 1, characterized in that: In the step (2), the drying temperature is 120-160° C. and the drying time is 8-12 hours.

4. The method for preparing the carbon-coated ion-doped sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that: The temperature of the low-temperature solid-phase reaction is 250-350°C, the time is 2-5 hours, the heating rate is 2-4°C / min, and the inert atmosphere is one of nitrogen, argon or argon-hydrogen mixed gas.

5. The method for preparing the carbon-coated ion-doped sodium iron phosphate pyrophosphate positive electrode material according to claim 1, characterized in that: The compression pressure is 14-18 MPa.

6. The method for preparing the carbon-coated ion-doped sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that: The temperature of the high-temperature solid-phase reaction is 550-650°C, the time is 8-12 hours, the heating rate is 5-10°C / min, and the inert atmosphere is one of nitrogen, argon or argon-hydrogen mixed gas.

7. A carbon-coated ion-doped sodium iron phosphate positive electrode material, characterized in that: Its molecular formula is Na4Fe 3-x M x (PO4)2P2O7@C, where 0 < x ≤ 0.2 and M is at least one of La, Ce, Pr, and Nd.

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