A preparation method of a fluorine ion and manganese ion co-doped non-stoichiometric sodium iron pyrophosphate positive electrode material

Abstract

This invention discloses a method for preparing a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluoride and manganese ions, relating to the field of sodium-ion battery cathode material preparation technology. The method is characterized by: doping non-stoichiometric sodium iron pyrophosphate with fluoride and manganese ions. The introduction of manganese ions expands the lattice volume, widening the lattice channels, while the introduction of highly electronegative fluoride ions affects the electron cloud density of surrounding atoms. Both synergistically improve the kinetic performance of the cathode material, resulting in a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluoride and manganese ions exhibiting superior electrochemical performance. The preparation method of this invention has advantages such as simple preparation, environmental friendliness, and low cost, and has good prospects for commercial application.

Description

Technical Field

[0001] This invention relates to sodium-ion battery cathode materials and preparation methods, and particularly to a method for preparing a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions. Background Technology

[0002] For a long time, fossil fuels have met the primary energy demand. However, their overconsumption and the resulting environmental pollution have made the search for green energy sources such as solar, wind, and tidal power a pressing issue. However, green energy is intermittent, as it is highly dependent on environmental factors such as weather, geographical location, and season. Currently, energy storage systems are widely considered the best candidate for concentrating these intermittent energy sources into a continuous, stable, and controllable power grid. Among these, secondary batteries in electrochemical energy storage are one of the most promising energy storage methods. Of the many secondary batteries, lithium-ion batteries are currently one of the most commercially successful, but with the rapid development of new energy vehicles, the demand for lithium-ion batteries has increased dramatically, leading to a year-on-year rise in their cost. Sodium-ion batteries, benefiting from the abundance and availability of sodium resources and the use of aluminum foil as both positive and negative electrode current collectors, further reduce costs, making them one of the most promising energy storage media.

[0003] In recent years, various types of cathode materials have been reported, including layered / tunnel transition metal oxides, Prussian blue analogs, organic compounds, and polyanionic compounds. Among these four categories, polyanionic compounds possess a unique three-dimensional framework structure, which can significantly reduce Na+. + The structural changes caused by the insertion / extraction process also provide open ion transport channels for ion migration. This is a typical material currently under study.

[0004] Currently, the main polyanionic cathode materials under research include phosphates, pyrophosphates, fluorophosphates, and mixed phosphates. Pyrophosphates have the advantages of low cost, excellent thermal stability, and environmental friendliness, but their disadvantages are low conductivity and relatively low theoretical capacity. The emergence of non-stoichiometric sodium iron pyrophosphate cathode materials has compensated for the shortcoming of low theoretical capacity, but due to the presence of sodium... + The larger ionic radius of Na leads to + The process of extraction and insertion causes material structural fatigue, leading to shortened cycle life and lower Na content. + The diffusion rate is crucial for improving the performance of non-stoichiometric sodium iron pyrophosphate cathode materials; therefore, promoting the diffusion rate of Na+ is key. + Minimize Na while maintaining diffusion capacity +The impact of diffusion processes on structure is a core research direction driving the commercial application of sodium-ion battery cathode materials. Among various modification strategies, doping modification is generally considered an effective means to improve the intrinsic properties of materials. Currently, most of the non-stoichiometric solid solution modification studies involve single cation or single anion doping, with few reports on co-doping strategies involving both anions and cations. Summary of the Invention

[0005] The first objective of this invention is to provide a method for preparing a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions, which has high electronic conductivity and excellent cycle stability.

[0006] The technical solution adopted in this invention is as follows: A method for preparing a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions includes the following steps: (1) Add sodium source, iron source, manganese source, fluorine source and phosphorus source to deionized water and stir to obtain a solution; (2) The solution obtained in step (1) is heated in an oil bath and stirred to obtain a wet gel; (3) The wet gel obtained in step (2) is dried in a vacuum environment to obtain a dry gel. The dry gel is then ground to obtain a powder. (4) The powder obtained in step (3) is pre-calcined in an inert gas atmosphere to obtain the precursor; (5) Grind the precursor obtained in step (4) and then calcine it at high temperature in an inert gas atmosphere to obtain a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine ions and manganese ions.

[0007] Further settings include: In step (1): The amounts of sodium, iron, manganese, fluorine, and phosphorus sources are as follows: the molar ratio of sodium, iron, manganese, fluorine, and phosphorus, calculated by element, is (2.57-2.66):(2.57-2.66):(0.01-0.1):(0.01-0.1):4; preferably, the molar ratio of sodium, iron, manganese, fluorine, and phosphorus is (2.62-2.64):(2.62-2.64):(0.03-0.05):(0.03-0.05):4.

[0008] The iron source is one or more of ferrous citrate, ferric sulfate, ferric acetate, ferrous oxalate, ferrous sulfate, ferric chloride, or ferric nitrate nonahydrate.

[0009] The sodium source is one or more of sodium acetate, sodium nitrate, sodium citrate, or sodium oxalate.

[0010] The phosphorus source is one or more of hydroxyethylidene diphosphonic acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate.

[0011] In step (2): The oil bath heating temperature is 60-90℃.

[0012] The stirring speed is 200-300 rpm, and the stirring time is 6-12 hours.

[0013] In step (3): The temperature conditions for vacuum drying are 60-90℃.

[0014] In step (4): The inert gas is an argon-hydrogen mixture, the pre-burning temperature is 200-300℃, and the pre-burning time is 1-5h.

[0015] In step (5): The inert gas is an argon-hydrogen mixture, the high-temperature calcination temperature is 500-800℃, and the calcination time is 6-24h.

[0016] A second objective of this invention is to provide a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions prepared by the aforementioned method.

[0017] A third objective of this invention is to provide an application of a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluoride and manganese ions in a sodium-ion battery. This cathode material, modified by co-doping with fluoride and manganese ions, exhibits a capacity of 121.2 mAh g / L in a sodium-ion battery at a rate of 0.1C. -1 Specific capacity.

[0018] The beneficial effects of this invention are as follows: (1) The present invention prepares a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluoride ions and manganese ions. By doping non-stoichiometric sodium iron pyrophosphate with fluoride ions and manganese ions, the introduction of manganese ions can expand the lattice volume to broaden the lattice channels, while the introduction of strongly electronegative fluoride ions can affect the electron cloud density of surrounding atoms. The two work together to improve the kinetic performance of the cathode material, so that the non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluoride ions and manganese ions has better electrochemical performance.

[0019] (2) The preparation method of the present invention has the advantages of simple preparation, environmental friendliness and low cost, and has the prospect of commercial application.

[0020] (3) When the non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions prepared by the present invention is applied to sodium-ion batteries, it improves the reversible capacity release and cycle stability of sodium-ion batteries.

[0021] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0022] Figure 1 The X-ray diffraction (XRD) pattern of the non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions prepared in Example 1 is shown.

[0023] Figure 2 The image shown is a scanning electron microscope (SEM) image of the non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions prepared in Example 1.

[0024] Figure 3 The X-ray diffraction pattern (XRD pattern) of the original non-stoichiometric sodium iron pyrophosphate cathode material prepared for Comparative Example 1.

[0025] Figure 4 The charge-discharge curves of a coin cell assembled from the non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions prepared in Example 1 at a rate of 0.1C are shown.

[0026] Figure 5 The charge-discharge curves of a coin cell assembled from the original non-stoichiometric sodium iron pyrophosphate cathode material prepared for Comparative Example 1 at a rate of 0.1C are shown.

[0027] Figure 6 A comparison of the long-cycle performance of coin cells assembled from the cathode material prepared in Example 1 and the cathode material prepared in Comparative Example 1 at a 1C rate. Detailed Implementation

[0028] The embodiments of the present invention are described in detail below. These embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments. Unless otherwise specified, the raw materials and reagents used in the embodiments are all existing technologies in the art or commercially available products.

[0029] Example 1

[0030] This embodiment provides a method for preparing a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine ions and manganese ions, including the following steps: (1) Preparation of precursor solution: In a beaker containing 30 ml of deionized water, according to the stoichiometric ratio of sodium, iron, manganese, fluorine, and phosphorus of 2.64:2.64:0.03:0.03:4, add 7.83 mmol of sodium nitrate, 7.92 mmol of ferric nitrate nonahydrate, 0.09 mmol of manganese acetate tetrahydrate, 0.09 mmol of sodium fluoride, and 6 mmol of hydroxyethylidene diphosphonic acid. After stirring, a pale yellow transparent solution is obtained.

[0031] (2) The pale yellow transparent solution obtained in step (1) was heated in an oil bath at 80°C and stirred at a speed of 300 rpm for 8 hours to obtain a yellowish-brown wet gel.

[0032] (3) The wet gel obtained in step (2) is vacuum dried at 80°C until the residual solvent and water are completely removed to obtain a loose and porous yellow-brown dry gel. Then it is thoroughly ground to obtain a yellow-brown powder. (4) The yellowish-brown powder from step (3) is loaded into a ceramic boat and placed in a tube furnace. First, inert gas is purged for 30 minutes. The inert gas is an argon-hydrogen mixture (95% Ar2 / 5% H2). Then, it is heated to 300°C in the argon-hydrogen mixture at a heating rate of 2°C / min and held for 2 hours for pre-firing to form a preliminary phase. (5) The dark brown powder pre-calcined in step (4) is further ground, then placed in a ceramic boat and placed in a tube furnace for 30 minutes of inert gas purging. The inert gas is an argon-hydrogen mixture (95% Ar2 / 5% H2). Then, it is heated to 600°C in the argon-hydrogen mixture at a heating rate of 2°C / min and held at that temperature for 10 hours for high-temperature calcination. Finally, the non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine ions and manganese ions, Na, is obtained. 2.64 Fe 2.64 Mn 0.03 (P2O 6.985 )2F 0.03 .

[0033] Product confirmation: Figure 1 The image shows the X-ray diffraction (XRD) pattern of the non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions prepared in Example 1. Figure 1 As shown: The non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions prepared in this embodiment is Na 2.64 Fe 2.64 Mn 0.03 (P2O 6.985 )2F 0.03 The fact that it is a pure phase indicates that small amounts of fluorine and manganese doping will not affect its crystal structure.

[0034] Figure 2The image shown is a scanning electron microscope (SEM) image of the non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions prepared in Example 1. Figure 2 As shown: The non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions prepared in this embodiment is Na 2.64 Fe 2.64 Mn 0.03 (P2O 6.985 )2F 0.03 The particles are irregular nanoparticles with small particle size, which is more conducive to Na + The transmission.

[0035] Comparison 1

[0036] This comparative example provides a method for preparing the original non-stoichiometric sodium iron pyrophosphate cathode material. The preparation method is the same as in Example 1, except that fluorine and manganese ion doping modification is not performed. The specific method is as follows: (1) Preparation of precursor solution: According to the stoichiometric ratio, 7.92 mmol of sodium nitrate, 7.92 mmol of ferric nitrate nonahydrate, and 6 mmol of hydroxyethylidene diphosphonic acid were added to a beaker containing 30 ml of deionized water, and the mixture was stirred to obtain a pale yellow transparent solution.

[0037] (2) The pale yellow transparent solution obtained in step (1) was heated in an oil bath at 80°C and stirred for 8 hours to obtain a yellowish-brown wet gel.

[0038] (3) The wet gel in step (2) is vacuum dried at 80°C until the residual solvent and water are completely removed, resulting in a loose and porous yellow-brown dry gel, which is then thoroughly ground to obtain a yellow-brown powder.

[0039] (4) The yellowish-brown powder from step (3) is loaded into a ceramic boat and placed in a tube furnace. First, inert gas is purged for 30 minutes. The inert gas is an argon-hydrogen mixture (95% Ar2 / 5% H2). Then, it is heated to 300°C in the argon-hydrogen mixture at a heating rate of 2°C / min and held for 2 hours for pre-firing to form a preliminary phase. (5) The dark brown powder pre-calcined in step (4) is further ground, then placed in a ceramic boat and placed in a tube furnace for 30 minutes of inert gas purging. The inert gas is an argon-hydrogen mixture (95% Ar2 / 5% H2). Then, it is heated to 600°C in the argon-hydrogen mixture at a heating rate of 2°C / min and held at that temperature for 10 hours for high-temperature calcination. Finally, the original non-stoichiometric sodium iron pyrophosphate cathode material Na is obtained. 2.67 Fe 2.67 (P2O7)2.

[0040] Product characterization: Figure 3 The image shows the X-ray diffraction (XRD) pattern of the original non-stoichiometric sodium iron pyrophosphate cathode material prepared for Comparative Example 1. Figure 3 As shown: Comparative Example 1 prepared the original non-stoichiometric sodium iron pyrophosphate cathode material Na 2.67 Fe 2.67 (P2O7)2 is the pure phase.

[0041] Example 2

[0042] This embodiment mainly examines the effects of different doping amounts of fluorine and manganese ions on the performance of the cathode material.

[0043] The preparation method is the same as in Example 1, except that in step (1), the molar ratio of sodium, iron, manganese, fluorine and phosphorus is adjusted to 2.63:2.63:0.04:0.04:4, and the amounts are adjusted to 7.77 mmol sodium nitrate, 7.89 mmol ferric nitrate nonahydrate, 0.12 mmol manganese acetate tetrahydrate, 0.12 mmol sodium fluoride and 6 mmol hydroxyethylidene diphosphonic acid; Na is then prepared. 2.63 Fe 2.63 Mn 0.04 (P2O 6.98 )2F 0.04 .

[0044] Example 3

[0045] This embodiment mainly examines the effects of different doping amounts of fluorine and manganese ions on the performance of the cathode material.

[0046] The preparation method is the same as in Example 1, except that in step (1), the molar ratio of sodium, iron, manganese, fluorine and phosphorus is adjusted to 2.62:2.62:0.05:0.05:4, and the amounts are adjusted to 7.71 mmol sodium nitrate, 7.86 mmol ferric nitrate nonahydrate, 0.15 mmol manganese acetate tetrahydrate, 0.15 mmol sodium fluoride and 6 mmol hydroxyethylidene diphosphonic acid; Na is then prepared. 2.62 Fe 2.62 Mn 0.05 (P2O 6.975 )2F 0.05 .

[0047] Performance testing The cathode materials prepared in Examples 1-3 and Comparative Example 1 were applied to sodium-ion batteries, and their electrochemical performance was tested.

[0048] Battery assembly: The positive electrode materials prepared in Examples 1-3 and Comparative Example 1 were used to prepare positive electrode sheets, and sodium sheets were used as negative electrodes. The electrolyte was a mixed solution of NaClO4 dissolved in ethylene carbonate (EC) and propylene carbonate (PC) (volume ratio 1:1) and containing 5% fluoroethylene carbonate (FEC). The separator was made of glass fiber (GF / D). The assembled sodium-ion batteries were then assembled into coin cells. Various electrochemical tests were performed on the assembled sodium-ion batteries. The voltage window for electrochemical performance testing was set to 1.5-4.2V.

[0049] The test results are shown in Table 1.

[0050] Table 1 .

[0051] As shown in Table 1: (1) Based on Examples 1-3, the effect of fluoride and manganese ion doping on sodium iron pyrophosphate cathode material is shown. The molar ratio of sodium, iron, manganese, fluorine and phosphorus is (2.62-2.64):(2.62-2.64):(0.03-0.05):(0.03-0.05):4, which is considered optimal. The best performance is achieved by the Na prepared in Example 1. 2.64 Fe 2.64 Mn 0.03 (P2O 6.985 )2F 0.03 The charge / discharge capacity at a 0.1C rate can reach 121.2 mAh·g. -1 The capacity retention rate is 97% after 100 cycles at 1C.

[0052] (2) The non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions prepared in Example 1 was compared with the original non-stoichiometric sodium iron pyrophosphate cathode material prepared in Comparative Example 1. Figures 4-6 As shown: Figure 4 The charge-discharge curves of a coin cell assembled from the non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions prepared in Example 1 at a rate of 0.1C are shown. Figure 5 The charge-discharge curves of a coin cell assembled from the original non-stoichiometric sodium iron pyrophosphate cathode material prepared for Comparative Example 1 at a 0.1C rate are shown below. Figures 4-5 As shown: The non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions prepared in Example 1 of this invention, compared with the original non-stoichiometric sodium iron pyrophosphate cathode material prepared in Comparative Example 1, exhibits a capacity release of 106.6 mAh·g at 0.1C rate. -1 Increased to 121.2 mAh·g -1 The reversible capacity release of the cathode material is significantly improved.

[0053] Figure 6 This is a comparison chart of the long-cycle performance of coin cells assembled from the cathode materials prepared in Example 1 and Comparative Example 1 at a 1C rate; Figure 6 As shown, under higher charge and discharge rates, the non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions prepared in Example 1 of this invention outperforms the non-stoichiometric sodium iron pyrophosphate cathode material prepared in Comparative Example 1 in terms of both capacity release and capacity retention. After 100 cycles at 1C, Example 1 still has a capacity retention of 97%, which is higher than the 84.7% of Comparative Example 1, demonstrating the effectiveness of fluorine and manganese ion co-doping in improving electrochemical performance.

[0054] The foregoing has only described the embodiments of the present invention in detail; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combining the various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine ions and manganese ions, characterized in that, Includes the following steps: (1) Add sodium source, iron source, manganese source, fluorine source and phosphorus source to deionized water and stir to obtain a solution; (2) The solution obtained in step (1) is heated in an oil bath and stirred to obtain a wet gel; (3) The wet gel obtained in step (2) is dried in a vacuum environment to obtain a dry gel. The dry gel is then ground to obtain a powder. (4) The powder obtained in step (3) is pre-calcined in an inert gas atmosphere to obtain a precursor; (5) Grind the precursor obtained in step (4) and then calcine it at high temperature in an inert gas atmosphere to obtain a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine ions and manganese ions.

2. The method for preparing a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions according to claim 1, characterized in that: In step (1): the molar ratio of sodium, iron, manganese, fluorine and phosphorus, calculated by elements, is (2.57-2.66): (2.57-2.66): (0.01-0.1): (0.01-0.1):

4.

3. The method for preparing a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions according to claim 2, characterized in that: In step (1): the molar ratio of sodium, iron, manganese, fluorine and phosphorus, calculated by elements, is (2.62-2.64): (2.62-2.64): (0.03-0.05): (0.03-0.05):

4.

4. The method for preparing a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions according to claim 1, characterized in that: In step (1): the iron source is one or more of ferrous citrate, ferric sulfate, ferric acetate, ferrous oxalate, ferrous sulfate, ferric chloride, or ferric nitrate nonahydrate; the sodium source is one or more of sodium acetate, sodium nitrate, sodium citrate, or sodium oxalate; and the phosphorus source is one or more of hydroxyethylidene diphosphonic acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate.

5. The method for preparing a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions according to claim 1, characterized in that: In step (2): the temperature of the oil bath heating is 60-90℃.

6. The method for preparing a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions according to claim 1, characterized in that: In step (2): the stirring speed is 200-300 rpm and the stirring time is 6-12 hours.

7. The method for preparing a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions according to claim 1, characterized in that: In step (3): the temperature conditions for vacuum drying are 60-90℃.

8. The method for preparing a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions according to claim 1, characterized in that: In step (4): the inert gas is an argon-hydrogen mixture, the pre-burning temperature is 200-300℃, and the pre-burning time is 1-5h.

9. The method for preparing a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine ions and manganese ions according to claim 1, characterized in that: In step (5): the inert gas is an argon-hydrogen mixture, the high-temperature calcination temperature is 500-800℃, and the calcination time is 6-24h.

10. The application of a non-stoichiometric sodium iron pyrophosphate cathode material co-doped with fluorine and manganese ions prepared by any one of claims 1-9 in a sodium-ion battery.

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