High-conductivity thin carbon layer coated ferric sodium pyrophosphate positive electrode material and preparation method thereof

Abstract

The invention discloses a high-conductivity thin carbon layer coated sodium ferric pyrophosphate positive electrode material and a preparation method thereof, the molecular formula of the positive electrode material is Na4Fe3-aMaP4-bXbO15-cFc / C, M is a metal element, X is a non-metal element, and the numerical values of a, b and c are all 0.1-0.5; the positive electrode material is based on ferric sodium pyrophosphate, iron site-phosphorus site-oxygen site is co-doped in the positive electrode material to construct and form a surface region electric field, and a carbon coating layer on the surface is formed; the preparation method comprises the following steps: mixing an iron source, a sodium source, a phosphorus source, a metal compound, a fluorine source and a carbon source in water according to a stoichiometric ratio, and carrying out ball milling and spray drying to obtain a spray material; and uniformly mixing the spray material with a nonmetal simple substance, calcining, and cooling. The sodium ferric phosphate pyrophosphate positive electrode material disclosed by the invention can be charged to over 114 mAh / g under the current density of 0.1 C, discharged to over 102 mAh / g and over 94 mAh / g under the multiplying power of 1C, and meanwhile, the capacity retention rate can be over 96% after 100 cycles, so that the optimal electrochemical performance and the stability are synchronously improved.

Description

Technical Field

[0001] This invention belongs to the technical field of sodium iron pyrophosphate cathode materials, and particularly relates to a highly conductive thin carbon layer coated sodium iron pyrophosphate cathode material and its preparation method. Background Technology

[0002] Sodium-ion batteries, due to sodium's high abundance in the Earth's crust (2.74%, approximately 420 times that of lithium) and its combination of low cost and wide operating temperature range, are considered an ideal alternative to lithium-ion batteries. However, the current scarcity of high-performance cathode materials is a key bottleneck restricting the commercialization of sodium-ion batteries. Among various cathode materials, polyanionic compounds, especially Na4Fe3(PO4)2P2O7 (NFPP), are recognized as one of the most promising technologies for industrialization due to their unique three-dimensional framework structure, moderate operating voltage (~3.1 V), high theoretical specific capacity (129 mAh / g), and small charge-discharge volumetric strain. However, their low intrinsic electronic conductivity and unsatisfactory electrochemical performance caused by slow ion diffusion kinetics hinder their industrial application.

[0003] Currently, carbon coating is a commonly used commercial method to improve the electronic conductivity and ion diffusion kinetics of NFPP, thereby enhancing the electrochemical performance of the material. However, the introduction of high carbon content can lead to two problems: firstly, it generates a large amount of gas during sintering, increasing morphological porosity and reducing compaction density; secondly, it introduces inert substances, resulting in a decrease in the specific capacity of the composite material. Furthermore, elemental doping is currently the most common strategy to improve the conductivity of materials. However, existing elemental doping methods employ single iron-site doping or Na / O dual-site doping. These methods have limited effectiveness in improving the electronic conductivity of cathode materials. Moreover, while dual-site doping can improve conductivity to some extent, it also leads to a decrease in the stability of the prepared cathode material. Summary of the Invention

[0004] Objective of the invention: This invention provides a highly conductive thin carbon layer coated sodium iron pyrophosphate cathode material and its preparation method, which can significantly improve the electronic conductivity of the cathode material while improving its stability, and avoid the negative effect of decreased stability caused by the increase in conductivity.

[0005] Technical solution: The present invention provides a highly conductive thin carbon layer coated sodium iron pyrophosphate cathode material, which is based on sodium iron pyrophosphate, and is co-doped with iron sites, phosphorus sites, and oxygen sites to form a surface electric field and form a carbon coating layer on the surface.

[0006] The molecular formula of the positive electrode material is Na₄Fe₂ 3-a M a P 4-bX b O 15-c F c / C, M is a metallic element, X is a non-metallic element, and the values ​​of a, b, and c are all between 0.1 and 0.5.

[0007] Furthermore, the M element used in this cathode material can be Ti, Cr, Mn, Co, Ni, Cu or Zn, and the corresponding raw material is a metal compound selected from titanium sulfate, chromium sulfate, manganese sulfate, cobalt sulfate, nickel sulfate, copper sulfate or zinc sulfate.

[0008] Furthermore, the X element used in this cathode material can be S, Se, or B, and the corresponding raw material is a non-metallic element selected from selenium powder, sulfur powder, or boric acid.

[0009] Furthermore, the fluorine source corresponding to the F element used in this cathode material is selected from ammonium fluoride, ammonium hydrogen fluoride, sodium fluoride, or iron fluoride.

[0010] The method for preparing the above-mentioned highly conductive thin carbon layer coated sodium iron pyrophosphate cathode material according to the present invention includes the following steps:

[0011] (1) Iron source, sodium source, phosphorus source, metal compound and fluorine source are mixed with carbon source in water according to stoichiometry, and the mixture is ball-milled and spray-dried to obtain spray material;

[0012] (2) After the aerosol material is mixed evenly with the non-metallic element, it is sintered in an inert atmosphere at 300℃-400℃ for 5-7 hours, and then sintered at 500℃-600℃ for 10-20 hours. The cathode material is obtained by cooling.

[0013] Furthermore, the iron source used in the preparation of this cathode material can be selected from one or more of the following: iron oxide, ferric phosphate, ferrous oxalate, ferrous sulfate, ferric sulfate, ferric oxalate, metallic iron powder, and ferric nitrate. The sodium source can be selected from one or more of the following: sodium carbonate, sodium hydroxide, trisodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium bicarbonate, sodium sulfate, sodium fluoride, sodium citrate, sodium nitrate, and sodium oxalate. The phosphorus source can be selected from one or more of the following: phosphoric acid, ferric phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate.

[0014] Furthermore, the amount of carbon source added in the preparation of the cathode material accounts for 5-10% of the total mass of the sodium source, iron source, phosphorus source and carbon source; it is selected from one or more of glucose, sucrose, citric acid, starch, ascorbic acid, cyclodextrin and polyethylene glycol.

[0015] Beneficial effects: Compared with the prior art, the significant advantages of the present invention are: the sodium iron pyrophosphate cathode material can reach more than 114 mAh / g when charged at a current density of 0.1C, more than 102 mAh / g when discharged, and more than 94 mAh / g at a 1C rate, while maintaining a capacity retention rate of more than 96% after 100 cycles, thus achieving the best simultaneous improvement in electrochemical performance and stability. Attached Figure Description

[0016] Figure 1 Na4Fe 2.7 Cu 0.3 P 3.8 Se 0.2 O 14.6 F 0.4 SEM image of / C spray material;

[0017] Figure 2 Na4Fe 2.7 Cu 0.3 P 3.8 Se 0.2 O 14.6 F 0.4 SEM image of / C sintered sample;

[0018] Figure 3 Na4Fe 2.7 Cu 0.3 P 3.8 Se 0.2 O 14.6 F 0.4 XRD pattern of / C sintered sample;

[0019] Figure 4 Na4Fe 2.7 Cu 0.3 P 3.8 Se 0.2 O 14.6 F 0.4 The first charge-discharge curve of the / C sintered sample. Detailed Implementation

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the amount of carbon source added in the thin carbon layer of the present invention accounts for 5-10% of the total mass of the sodium source, iron source, phosphorus source and carbon source.

[0021] Example 1

[0022] The sodium iron pyrophosphate with a highly conductive thin carbon layer coated in Example 1 has the chemical formula Na₄Fe₂O₃. 2.7 Cu 0.3 P 3.8 Se 0.2 O14.6 F 0.4 / C, the preparation method includes the following steps:

[0023] (1) Weigh ferric phosphate, sodium carbonate, ammonium dihydrogen phosphate, copper sulfate, ammonium fluoride and selenium powder according to stoichiometry;

[0024] (2) Iron phosphate, sodium carbonate, ammonium dihydrogen phosphate, copper sulfate and ammonium fluoride are mixed with glucose in water, stirred evenly and then ground in a sand mill until the particle size is 0.24 μm. The water is then evaporated in a spray tower with an inlet air temperature of 220 °C and an outlet air temperature of 100 °C to obtain spray material. The amount of glucose added accounts for 8% of the total mass of sodium carbonate, iron phosphate, ammonium dihydrogen phosphate and glucose.

[0025] (3) After the spray material and selenium powder are physically mixed evenly, the mixture is sintered at 300℃ for 6 hours in a nitrogen atmosphere, and then sintered at 550℃ for 10 hours. The heating rate is set to 5℃ min. -1 The target product is obtained by naturally cooling to room temperature.

[0026] Structural characterization

[0027] The morphology and structure of the cathode material in Example 1 were characterized using scanning electron microscopy (SEM), such as... Figure 1 and Figure 2 As shown, the sprayed sample exhibits a spherical morphology (result of spray granulation). Furthermore, the sintered sample displays a similar intact spherical morphology to the sprayed sample, indicating that the material's structure is not destroyed during sintering, thus facilitating electron conduction and ion transport. XRD testing confirms the successful synthesis of this material. Figure 3 The absence of a distinct inert sodium iron phosphate phase indicates the synthesis of its pure phase. Furthermore, the high intensity and sharpness of all diffraction peaks in NFPP reveal its higher degree of crystallinity and more ordered crystal arrangement, which significantly enhances the material's electronic conductivity, structural stability, and electrochemical performance.

[0028] Electrochemical performance testing 1

[0029] The positive electrode material, super-P, and PVDF from Example 1 were coated onto aluminum foil in a 9:1:1 ratio. A glass fiber separator was used, and sodium perchlorate was employed as the electrolyte. The resulting coin cell was tested on a blue battery testing system with a voltage window of 2.0-4.0 V and a 1C = 120 mAh / g. The results are as follows: Figure 4 As shown, this material exhibits excellent charge-discharge performance, with a capacity of 118.60 mAh / g at 0.1C charging.

[0030] Comparative Example 1-1

[0031] Comparative Example 1-1 is essentially the same as Example 1, except that no fluorine source or metal compound is added during the ball milling process. That is, only Se is used to replace the phosphorus sites in NFPP, named Na4Fe3P. 3.8 Se 0.2 O 15 / C.

[0032] Comparative Examples 1-2

[0033] Comparative Examples 1-2 are essentially the same as Example 1, except that the spray-dried material is sintered separately without mixing with non-metallic elements. Specifically, only Cu and F are used to replace the iron and oxygen sites in NFPP, and the resulting material is named Na4Fe. 2.7 Cu 0.3 P4O 14.6 F 0.4 / C.

[0034] Comparative Examples 1-3

[0035] Comparative Examples 1-3 are essentially the same as Example 1, except that no metal compounds are added during the ball milling process; only a fluorine source is added. That is, only F and Se are used to replace the oxygen and phosphorus sites in NFPP, and the resulting product is named Na4Fe3P. 3.8 Se 0.2 O 14.6 F 0.4 / C.

[0036] Comparative Examples 1-4

[0037] Comparative Examples 1-4 are essentially the same as Example 1, except that no fluorine source is added during the ball milling process; only a metal compound is added. Specifically, Cu and Se are used to replace the iron and phosphorus sites in NFPP, and the compound is named Na4Fe. 2.7 Cu 0.3 P 3.8 Se 0.2 O 15 / C.

[0038] Comparative Examples 1-5

[0039] Comparative Examples 1-5 are basically the same as Comparative Examples 1-4, except that Zn is used to replace the iron sites of NFPP, and the result is named Na4Fe. 2.7 Zn 0.3 P 3.8 Se 0.2 O 15 / C.

[0040] Example 2

[0041] The highly conductive thin carbon layer-coated sodium iron pyrophosphate cathode material of Example 2 is named Na4Fe 2.7 Cu 0.3 P3.8 S 0.2 O 14.6 F 0.4 / C. This is basically the same as Example 1, except that Cu, S, and F are used to replace the iron, phosphorus, and oxygen sites in the original NFPP, and the corresponding raw materials are copper sulfate, sulfur powder, and ammonium fluoride.

[0042] Example 3

[0043] The highly conductive thin carbon layer-coated sodium iron pyrophosphate cathode material of Example 3 is named Na4Fe 2.7 Mn 0.3 P 3.8 Se 0.2 O 14.6 F 0.4 / C. The basic steps are basically the same as in Example 1, except that Mn, Se, and F are used to replace the iron, phosphorus, and oxygen sites in the original NFPP, and the corresponding raw materials are manganese sulfate, selenium powder, and ammonium fluoride.

[0044] Comparative Example 3-1

[0045] Comparative Example 3-1 is essentially the same as Example 3, except that the spray-dried material is sintered separately without mixing with non-metallic elements. Specifically, only Mn and F are used to replace the iron and oxygen sites in NFPP, and it is named Na4Fe. 2.7 Mn 0.3 P4O 14.6 F 0.4 / C.

[0046] Comparative Example 3-2

[0047] Comparative Example 3-2 is essentially the same as Example 3, except that no fluorine source is added during the ball milling process; only a metal compound is added. Specifically, Mn and Se are used to replace the iron and phosphorus sites in NFPP, and the compound is named Na4Fe. 2.7 Mn 0.3 P 3.8 Se 0.2 O 15 / C.

[0048] Example 4

[0049] The highly conductive thin carbon layer-coated sodium iron pyrophosphate cathode material of Example 4 is named Na4Fe 2.7 Zn 0.3 P 3.8 S 0.2 O 14.6 F 0.4 / C. The basic steps are basically the same as in Example 1, except that Zn, S, and F are used to replace the iron, phosphorus, and oxygen sites in the original NFPP, and the corresponding raw materials are zinc sulfate, sulfur powder, and ammonium fluoride.

[0050] Comparative Example 4-1

[0051] Comparative Example 4-1 is essentially the same as Example 4, except that the spray-dried material is sintered separately without mixing with non-metallic elements. Specifically, only Zn and F are used to replace the iron and oxygen sites in NFPP, and it is named Na4Fe. 2.7 Zn 0.3 P4O 14.6 F 0.4 / C.

[0052] Blank comparison example

[0053] This blank control example is basically the same as Example 1, except that it does not simultaneously perform doping at all three sites: iron, phosphorus, and oxygen, i.e., its chemical formula is Na4Fe3P4O. 15 / C, the preparation method includes the following steps:

[0054] (1) Weigh ferric phosphate, sodium carbonate, and ammonium dihydrogen phosphate according to stoichiometry;

[0055] (2) Mix ferric phosphate, sodium carbonate, ammonium dihydrogen phosphate and glucose in water, stir evenly, grind in a sand mill until the particle size is 0.24 μm, and then evaporate the water in a spray tower with an inlet air temperature of 220 ℃ and an outlet air temperature of 100 ℃ to obtain spray material; the amount of glucose added accounts for 8% of the total mass of sodium carbonate, ferric phosphate, ammonium dihydrogen phosphate and glucose.

[0056] (3) The spray material was placed in a nitrogen atmosphere and sintered at 300°C for 6 hours, and then sintered at 550°C for 10 hours. The heating rate was set to 5°C / min. After natural cooling to room temperature, the target product was obtained.

[0057] Electrochemical performance testing 2

[0058] The cathode materials prepared in the examples, comparative examples, and blank control examples were subjected to electrochemical performance testing using the same testing method as in Example 1. The results are shown in Table 1 below. The error range of the performance data described below is ±1%-2%.

[0059] Table 1. Electrochemical performance test results for the examples and comparative examples.

[0060] Example Doping elements 0.1 C charging specific capacity (mAh / g) 0.1C discharge specific capacity (mAh / g) 1C discharge specific capacity (mAh / g) 1C cycle 100 cycles capacity retention (%) Blank comparison example none 99.83 90.40 85.46 93.67 Example 1 Cu, Se, F 118.60 105.50 98.75 99.24 Comparative Example 1-1 Se 103.38 91.79 86.32 88.21 Comparative Examples 1-2 Cu, F 111.34 100.21 93.45 94.21 Comparative Examples 1-3 Se、F 108.93 98.47 89.35 89.32 Comparative Examples 1-4 Cu, Se 110.45 98.28 94.89 91.65 Comparative Examples 1-5 Zn, Se 112.38 98.79 95.32 91.21 Example 2 Cu, S, F 115.45 103.37 94.45 97.34 Example 3 Mn, Se, F 116.83 104.21 96.21 98.22 Comparative Example 3-1 Mn, F 111.25 99.12 94.23 92.12 Comparative Example 3-2 Mn, Se 109.78 97.32 94.15 90.72 Example 4 Zn, S, F 114.67 102.76 95.67 96.45 Comparative Example 4-1 Zn, F 112.78 98.15 92.45 93.23

[0061] As can be seen from the comparison of the examples in Table 1, the simultaneous doping of the iron-phosphorus-oxygen sites using the three sites of the present invention not only improves the electrochemical performance of the prepared cathode material, but also enhances the structural stability of the prepared cathode material through simultaneous doping of the three sites. Compared with Examples 2 to 4, the simultaneous doping of Cu, Se, and F in Example 1 achieves the best simultaneous improvement in electrochemical performance and stability, achieving a charging current density of over 118.6 mAh / g at 0.1C, a discharging current density of 105.5 mAh / g, and a capacity retention of 98.75 mAh / g at 1C rate, while maintaining a capacity retention of 99.24% after 100 cycles.

[0062] As can be seen from the examples and comparative data in Table 1, three-dimensional doping with iron-phosphorus-oxygen sites, compared to single-site or dual-site doping, simultaneously improves both electrochemical performance and structural stability of the cathode material under the same limiting conditions. As can be seen from Example 1 and its corresponding comparative examples, Example 3 and its corresponding comparative examples, and Examples 2 and 4 and their corresponding comparative examples, conductivity and structural stability can be significantly improved based on different combinations of doping elements.

[0063] Based on the above data characterization, further mechanistic analysis reveals that this invention promotes the rearrangement of the electronic structure in the surface region by precisely doping internal iron, phosphorus, and oxygen sites, thereby constructing a large-area electric field in the multiple surfaces inside the cathode material. This not only greatly improves the electronic conductivity of the prepared cathode material, but also enhances the stability of the prepared cathode material by combining surface doping with these three specific sites (iron, phosphorus, and oxygen), avoiding the negative impact of reduced stability caused by doping.

[0064] Furthermore, the large-area electric field inside the cathode material, combined with the surface carbon layer, forms an internal and external electric field, which greatly accelerates the intrinsic and interfacial electron-ion transport dynamics of the composite material, further enhancing the material's rate performance, charge-discharge capacity, and cycle performance.

[0065] In addition to the above embodiments, the technical effects claimed by the present invention can be achieved by using the processes and parameters defined in the present invention, so there is no need to conduct individual tests and verifications.

Claims

1. A highly conductive thin carbon layer coated sodium iron pyrophosphate cathode material, characterized in that, The cathode material is based on sodium iron pyrophosphate, with iron-phosphorus-oxygen sites co-doped inside to form a surface electric field and a carbon coating layer on the surface. The molecular formula of the positive electrode material is Na₄Fe₂ 3-a M a P 4-b X b O 15-c F c / C, M is a metallic element, X is a non-metallic element, and the values ​​of a, b, and c are all between 0.1 and 0.

5.

2. The highly conductive thin carbon layer coated sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The element M is Ti, Cr, Mn, Co, Ni, Cu or Zn, and the corresponding raw material is a metal compound selected from titanium sulfate, chromium sulfate, manganese sulfate, cobalt sulfate, nickel sulfate, copper sulfate or zinc sulfate.

3. The highly conductive thin carbon layer coated sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The element X is S, Se, or B, and the corresponding raw material is a non-metallic element selected from selenium powder, sulfur powder, or boric acid.

4. The highly conductive thin carbon layer coated sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The fluorine source corresponding to the F element is selected from ammonium fluoride, ammonium hydrogen fluoride, sodium fluoride, or iron fluoride.

5. A method for preparing the highly conductive thin carbon layer coated sodium iron pyrophosphate cathode material as described in claim 1, characterized in that, Includes the following steps: (1) Iron source, sodium source, phosphorus source, metal compound and fluorine source are mixed with carbon source in water according to stoichiometry, and the mixture is ball-milled and spray-dried to obtain spray material; (2) After the aerosol material is mixed evenly with the non-metallic element, it is sintered in an inert atmosphere at 300℃-400℃ for 5-7 hours, and then sintered at 500℃-600℃ for 10-20 hours. The cathode material is obtained by cooling.

6. The method for preparing sodium iron pyrophosphate cathode material according to claim 5, characterized in that, The iron source is selected from one or more of the following: iron oxide, ferric phosphate, ferrous oxalate, ferrous sulfate, ferric sulfate, ferric oxalate, metallic iron powder, and ferric nitrate.

7. The method for preparing sodium iron pyrophosphate cathode material according to claim 5, characterized in that, The sodium source is selected from one or more of sodium carbonate, sodium hydroxide, trisodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium bicarbonate, sodium sulfate, sodium fluoride, sodium citrate, sodium nitrate, and sodium oxalate.

8. The method for preparing sodium iron pyrophosphate cathode material according to claim 5, characterized in that, The phosphorus source is selected from one or more of phosphoric acid, iron phosphate, diammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate.

9. The method for preparing sodium iron pyrophosphate cathode material according to claim 5, characterized in that, The amount of carbon source added accounts for 5-10% of the total mass of the sodium source, iron source, phosphorus source and carbon source; it is selected from one or more of glucose, sucrose, citric acid, starch, ascorbic acid, cyclodextrin and polyethylene glycol.

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