Se anion doped ferric sodium pyrophosphate / carbon positive electrode active material and preparation method and application thereof

Abstract

The invention provides a Se anion-doped ferric phosphate pyrophosphate / carbon positive electrode active material and a preparation method and application thereof, and relates to the technical field of electrochemical energy storage device positive electrode active materials, the positive electrode active material comprises a ferric phosphate pyrophosphate base material and a carbon coating layer, in the crystal structure of the base material, part of O is partially replaced by Se, and the general formula of the base material is Na4Fe3 (PO4) 2 (P2O7-xSex), x is less than or equal to 1.0; o < 2-> is partially replaced by Se < 2-> in a crystal structure of a matrix material to construct a Se-doped solid solution, so that an electron energy band structure of the material is regulated and controlled; due to introduction of Se, the band gap of the material can be remarkably reduced, the electron conductivity can be improved, local lattice distortion can be caused, a Na < + > diffusion channel can be widened, and the ion migration capability can be improved; the material keeps a complete main phase structure, electron and ion transmission is synergistically optimized, and the rate capability is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion battery materials, and in particular to a Se anion-doped sodium iron phosphate pyrophosphate / carbon positive electrode active material, a preparation method thereof, and an application thereof in energy storage devices. Background Art

[0002] In recent years, sodium-ion batteries have become one of the important energy storage technologies to replace lithium-ion batteries due to their cost advantages and balanced resource distribution. Among the many sodium-ion positive electrode active materials, sodium iron pyrophosphate (Na4Fe3(PO4)2(P2O7, NFPP) has attracted widespread attention due to its moderate theoretical voltage platform, high structural thermal stability, and environmentally friendly and non-toxic raw materials. NFPP has three-dimensional sodium ion diffusion channels, which can achieve reversible sodium ion insertion and extraction reactions over a wide voltage range, and has a theoretical specific capacity of nearly 120mAh·g - 1. Have a good cycle performance foundation.

[0003] However, the practical application of NFPP materials still faces many challenges, the most prominent of which is their extremely low intrinsic electronic conductivity, which is far below the requirements of commercial electrode materials. This deficiency is mainly due to their stable Fe-O coordination network and highly localized electronic state structure, which leads to significant polarization and capacity decay of the electrode under high-rate charge and discharge or high current conditions. Therefore, improving the electronic conductivity of NFPP materials has become one of the core issues to promote their practical application.

[0004] At present, the methods for improving the electronic transport performance of NFPP mainly focus on two categories: one is to construct a conductive network, such as using carbon coating, conductive polymer composite or introducing metal phosphide heterophase interface to improve the interface electron migration channel; the other is through cation doping strategy, such as introducing Ti 4+ 、V 3+ Mg 2+ These methods have improved the electronic structure and dynamic behavior of the material to a certain extent, but are often limited to the regulation of a single lattice site and are difficult to fundamentally change the band gap characteristics of NFPP.

[0005] Compared with cation engineering, anion doping strategy has shown unique advantages in regulating electronic structure in various cathode systems in recent years. 2- 、F - Soft anions such as ions replace O in the crystal 2- , which can effectively lower the valence band top energy level, significantly reduce the band gap of the material, increase the probability of electron transition, and thus enhance the electronic conductivity. 2-There is still a gap in the research on the doping behavior of isoanions. Its stability, substitution mechanism and its regulatory effect on electron transport properties in the phosphate-pyrophosphate dianion structure have not been systematically explored.

[0006] Therefore, a Se 2- As doping source, realize O 2- The new NFPP positive electrode active material system with anion substitution and improving electronic conductivity through band gap regulation is a key research direction to expand its application scenarios and achieve performance leap, which has important scientific significance and engineering application value. Summary of the Invention

[0007] In order to solve the technical bottleneck of low electronic conductivity and poor rate performance of sodium iron pyrophosphate positive electrode active materials in the prior art, the present invention proposes a method based on Se 2- Anion replacement of O 2- A novel structural control strategy is proposed. By introducing Se doping to construct a controlled NFPP solid solution, the band structure and electron distribution are effectively adjusted without destroying the main crystal phase structure and ion channel system, achieving a significant improvement in the intrinsic electronic conductivity.

[0008] In order to achieve the above object, the technical solution of the present invention is as follows:

[0009] A Se anion-doped sodium iron phosphate pyrophosphate / carbon positive electrode active material comprises a sodium iron phosphate pyrophosphate matrix material and a carbon coating layer coated on the matrix surface, wherein part of the O in the crystal structure of the matrix material is replaced by Se, and the general formula is Na4Fe3(PO4)2(P2O 7-x Se x ), where 0 <x≤1.0。

[0010] The material design concept proposed in this invention is based on the fact that Se and O both belong to the VIA group, but their outer electron clouds are more dispersed, their energy levels are higher, and they are more likely to form shallow energy level impurity states in the crystal. 2- Substituting some O in the NFPP crystal structure 2- When the band gap is modulated, it can effectively reduce the material's band gap, increase the energy level at the top of the valence band or the bottom of the conduction band, and change the carrier migration mechanism. This band gap modulation mechanism not only increases the electron migration rate, but also enhances the reversibility of electron gain and loss reactions, providing a material foundation for rapid charge and discharge of batteries under high-rate operating conditions.

[0011] In terms of structure, Se 2- The covalent radius (198pm) is larger than that of O 2-The large (140 pm) doping rate indicates that moderate doping can cause local expansion of the crystal, thereby broadening the diffusion path for sodium ions to a certain extent and lowering the ion diffusion barrier. More importantly, the Fe-Se coordination bond between Se and Fe is more flexible than the Fe-O bond, which facilitates the diffusion of sodium ions.

[0012] The elements in the matrix material formed by the specific structural design of the present invention are evenly distributed, and the doped elements do not form an independent phase or agglomerated structure.

[0013] In some embodiments, 0.005≤x≤0.3.

[0014] In some embodiments, the positive electrode active material is in the form of submicron or micron particles with an average particle size of 0.5-5 μm.

[0015] In some embodiments, the mass of the carbon coating layer is 1-10% of the mass of the positive electrode active material.

[0016] In some embodiments, the carbon coating layer is further doped with at least one of S, N, F, and B.

[0017] In order to achieve the stable formation of the above-mentioned doped structure, the present invention proposes a reasonable and controllable preparation route. First, NFPP is uniformly mixed with a Se source, and then the Se doping and curing is completed in an inert atmosphere through heat treatment. The selected Se source can be Se powder, Na2SeO3, Na2Se, etc., and the active Se is gradually released during the heat treatment process. 2- ions, which react with Fe-O bonds to form a doped solid solution.

[0018] Specifically, the preparation method of the Se anion-doped sodium iron phosphate pyrophosphate / carbon positive electrode active material of any of the above embodiments comprises the following steps:

[0019] S1. Weighing a sodium source, an iron source, a phosphorus source, and a carbon source according to stoichiometric amounts, adding them to deionized water, and stirring evenly to obtain a mixed slurry; placing the mixed slurry in a spray dryer, spray drying it, and then heating it to 300-400° C. in a first inert atmosphere for sintering, and then heating it to 450-600° C. for secondary sintering to obtain a sodium iron pyrophosphate / carbon precursor material;

[0020] S2. Mixing the Se source with the precursor material obtained in step S1, placing the mixture in a second inert atmosphere, heating the mixture to 400-600° C. and sintering the mixture to obtain the Se anion-doped sodium iron pyrophosphate / carbon positive electrode active material.

[0021] In some embodiments, the sodium source is at least one of sodium carbonate, sodium hydroxide, sodium bicarbonate, sodium dihydrogen phosphate, sodium pyrophosphate, disodium dihydrogen pyrophosphate, sodium acetate, and sodium citrate.

[0022] In some embodiments, the iron source is at least one of ferric chloride, ferrous oxalate, ferrous oxalate, ferrous sulfate, ferrous ammonium sulfate, diammonium hydrogen phosphate, and phosphoric acid.

[0023] In some embodiments, the phosphorus source is at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid.

[0024] In some embodiments, the carbon source is at least one of soluble starch, cellulose, sucrose, glucose, and ascorbic acid.

[0025] In some embodiments, the Se source is at least one of Se powder, Na2SeO3, SeO2, and Na2Se.

[0026] In some embodiments, in steps S1 and S2, the first inert atmosphere and the second inert atmosphere are each independently one of a nitrogen atmosphere, an argon atmosphere, and a helium atmosphere.

[0027] In some embodiments, in step S2, the molar ratio of O to Se in the pyrophosphate anion in the precursor material is 1-z:z, where z=0.005-0.3.

[0028] In some embodiments, the raw material in step S1 further includes a doping element source containing at least one of S, N, F, and B.

[0029] In some embodiments, the doping element source includes at least one of thiourea, elemental sulfur, polytetrafluoroethylene, urea, lignin sulfonic acid, and boric acid.

[0030] In some embodiments, the preparation method comprises the following steps:

[0031] S1. Weighing a sodium source, an iron source, a phosphorus source, a carbon source, and a doping element source according to stoichiometric amounts, adding them to deionized water, and stirring evenly to obtain a mixed slurry; placing the mixed slurry in a spray dryer, spray drying it, and then heating it to 300-400° C. in a first inert atmosphere for sintering, and then heating it to 450-600° C. for secondary sintering to obtain a sodium iron pyrophosphate / carbon precursor material;

[0032] S2. Mixing the Se source with the precursor material obtained in step S1, placing the mixture in a second inert atmosphere, heating the mixture to 400-600° C. and sintering the mixture to obtain the Se anion-doped sodium iron pyrophosphate / carbon positive electrode active material.

[0033] The present invention also provides a positive electrode, which includes the positive electrode active material of any of the above embodiments or the positive electrode active material obtained by the preparation method of any of the above embodiments.

[0034] The present invention also provides an electrochemical energy storage device, comprising the above-mentioned positive electrode. Further, the electrochemical energy storage device includes but is not limited to a sodium ion battery, a sodium ion capacitor, and the like.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] Different from the existing NFPP materials which mainly rely on external carbon coating or metal doping, the present invention adopts Se 2- Replace O inside the crystal 2- By effectively controlling the band structure from the perspective of the material's intrinsic electronic structure, Se doping can effectively regulate the material's energy band structure. By appropriately doping with Se, the material's conduction band bottom energy level is raised, narrowing the band gap and facilitating carrier transitions, thereby significantly improving electronic conductivity. Furthermore, the introduction of Se alters the coordination environment of the local Fe-O bond, making the electron distribution more delocalized and improving the electron supply capacity during the electrode reaction. These effects collectively contribute to the electrode's rapid response at high rates.

[0037] In addition, Se 2- Compared to O 2- The larger ion radius causes the crystal to produce a certain degree of lattice expansion and stress adjustment after doping. This change helps to build a wider ion migration channel and reduce Na + Diffusion resistance is reduced, and ion transport performance is improved. Overall, the present invention achieves coordinated regulation of electrons and ions through Se anion doping, and the prepared material shows excellent performance in both rate performance and preparation adaptability.

[0038] The preparation method provided by the present invention has a relatively simple preparation process flow, and the prepared positive electrode active material has an excellent capacity retention rate at a high current density. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 The SEM morphology and EDS element distribution diagram of the sodium iron pyrophosphate / carbon positive electrode active material prepared in Example 1, in which 15% of the oxygen in the pyrophosphate is replaced by Se. DETAILED DESCRIPTION

[0040] The following description sets forth numerous specific details to facilitate a thorough understanding of the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art may make similar modifications without departing from the scope of the present invention. Therefore, the present invention is not limited to the specific implementations disclosed below.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in this specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0042] Example 1

[0043] A method for preparing a sodium iron pyrophosphate / carbon positive electrode active material in which 15% of the oxygen in the pyrophosphate group is replaced by Se, comprising the following steps:

[0044] S1. Add 490 g of sodium dihydrogen phosphate, 460 g of ferrous oxalate dihydrate, 100 g of citric acid, and 20 g of thiourea to 3 L of deionized water, and stir in a ball mill at a speed of 500 rpm for 3 h to obtain a slurry; transport the slurry to a spray dryer through a peristaltic pump at a feed rate of 2500 ml / h, an inlet air temperature of 200°C, and an outlet air temperature of 130°C to obtain dry particles; place the particles in a nitrogen atmosphere, heat the temperature to 350°C at a rate of 5°C / min for pre-sintering for 4 h, and then heat the temperature to 550°C at a rate of 5°C / min for secondary sintering for 5 h to obtain a sodium iron pyrophosphate / carbon precursor material;

[0045] S2. Mix 9.5 g of Se powder with 500 g of the precursor material, stir and ball-mill at 500 rpm for 1 h, place it in a vacuum drying oven at 80 ° C for 5 h, then place it in a nitrogen atmosphere, heat it to 500 ° C at a rate of 5 ° C / min and sinter it for 2 h to obtain a sodium iron pyrophosphate / carbon positive electrode active material in which 15% of the oxygen in the pyrophosphate ion is replaced by Se.

[0046] The positive electrode active material obtained in this example was tested by scanning electron microscopy. The test results are as follows: Figure 1 As shown, the phenomenon of uniform Se doping in the matrix material of the sodium iron pyrophosphate / carbon positive electrode active material can be detected.

[0047] The electrochemical performance of the positive electrode active material prepared in this example was tested as follows:

[0048] The positive electrode active material, conductive agent carbon black: binder PVDF were mixed in a mass ratio of 8:1:1. After thorough grinding, an appropriate amount of NMP was added to obtain a uniform slurry. The slurry was coated on an aluminum foil current collector, placed in a vacuum drying oven, dried at 120°C, then taken out and cut into discs. A metallic sodium sheet was used as the counter electrode; 1M NaClO4 / EC:DMC:EMC (1:1:1) was used as the electrolyte. After assembling button cells, the electrochemical performance was tested.

[0049] After testing, the positive electrode active material obtained in this example has a discharge capacity of 91 mAh / g at a voltage of 1.7-4.3 V and a rate of 0.5 C.

[0050] The positive electrode active material obtained in this example has a discharge specific capacity of 86 mAh / g at a voltage of 1.7-4.3 V and a rate of 5 C, and a corresponding capacity retention rate of 94.5%.

[0051] Comparative Example 1

[0052] A method for preparing a sodium iron pyrophosphate / carbon positive electrode active material comprises the following steps:

[0053] S1. Add 490g of sodium dihydrogen phosphate, 460g of ferrous oxalate dihydrate, 100g of citric acid, and 20g of thiourea to 3L of deionized water, stir in a ball mill, and stir at a speed of 500rpm for 3h to obtain a slurry; transport the slurry to a spray dryer through a peristaltic pump at a feed rate of 2500ml / h, an inlet air temperature of 200℃, and an outlet air temperature of 130℃ to obtain dry particles; in a nitrogen atmosphere, heat to 350℃ at a rate of 5℃ / min and sinter for 4h, then heat to 550℃ at a rate of 5℃ / min and sinter for a second time for 5h to obtain sodium iron pyrophosphate / carbon material.

[0054] The electrochemical performance of the positive electrode active material obtained in this comparative example was tested according to the method of Example 1.

[0055] After testing, the positive electrode active material obtained in this comparative example has a discharge capacity of 86 mAh / g at a voltage of 1.7-4.3 V and a rate of 0.5 C.

[0056] The positive electrode active material obtained in this comparative example has a discharge specific capacity of 65 mAh / g at a voltage of 1.7-4.3 V and a rate of 5 C, and a corresponding capacity retention rate of 75.6%.

[0057] Example 2

[0058] A method for preparing a sodium iron pyrophosphate positive electrode active material in which 30% of the oxygen in the pyrophosphate group is replaced by Se, comprising the following steps:

[0059] S1. Add 490 g of sodium dihydrogen phosphate, 460 g of ferrous oxalate dihydrate, 100 g of citric acid, and 20 g of thiourea to 3 L of deionized water, and stir in a ball mill at a speed of 500 rpm for 3 h to obtain a slurry; transport the slurry to a spray dryer through a peristaltic pump at a feed rate of 2500 ml / h, an inlet air temperature of 200°C, and an outlet air temperature of 130°C to obtain dry particles; in a nitrogen atmosphere, heat the mixture to 350°C at a rate of 5°C / min and sinter for 4 h, and then heat the mixture to 550°C at a rate of 5°C / min and sinter for a second time for 5 h to obtain a sodium iron pyrophosphate / carbon precursor material;

[0060] S2. 19 g of Se powder was mixed with 500 g of sodium iron pyrophosphate / carbon precursor material, and the mixture was stirred at 500 rpm for 1 h in a ball mill, and then placed in a vacuum drying oven at 80 ° C for 5 h. Then, the mixture was placed in a nitrogen atmosphere, heated to 500 ° C at a rate of 5 ° C / min and sintered for 2 h to obtain a sodium iron pyrophosphate / carbon positive electrode active material with a Se doping molar ratio of 5%.

[0061] The electrochemical performance of the positive electrode active material obtained in this example was tested according to the method of Example 1.

[0062] After testing, the positive electrode active material obtained in this example has a discharge capacity of 85 mAh / g at a voltage of 1.7-4.3 V and a rate of 0.5 C.

[0063] The positive electrode active material obtained in this example has a discharge specific capacity of 76 mAh / g at a voltage of 1.7-4.3 V and a rate of 5 C, and a corresponding capacity retention rate of 89.4%.

[0064] Example 3

[0065] A method for preparing a sodium iron pyrophosphate / carbon positive electrode active material in which 15% of the oxygen in the pyrophosphate group is replaced by Se, comprising the following steps:

[0066] S1. Add 490 g of sodium dihydrogen phosphate, 460 g of ferrous oxalate dihydrate, 100 g of citric acid, and 20 g of thiourea to 3 L of deionized water, and stir in a ball mill at a speed of 500 rpm for 3 h to obtain a slurry; transport the slurry to a spray dryer through a peristaltic pump at a feed rate of 2500 ml / h, an inlet air temperature of 200°C, and an outlet air temperature of 130°C to obtain dry particles; in a nitrogen atmosphere, heat the mixture to 350°C at a rate of 5°C / min and sinter for 4 h, and then heat the mixture to 550°C at a rate of 5°C / min and sinter for a second time for 5 h to obtain a sodium iron pyrophosphate / carbon precursor material;

[0067] S2. 13.4 g of SeO2 was mixed with 500 g of sodium iron pyrophosphate / carbon precursor material, and the mixture was stirred and ball-milled at 500 rpm for 1 h in a ball mill, and then dried at 80°C in a vacuum drying oven for 5 h. The mixture was then placed in a nitrogen atmosphere, heated to 500°C at a rate of 5°C / min, and sintered for 2 h to obtain the sodium iron pyrophosphate / carbon positive electrode active material with a Se doping molar ratio of 1% for different Se sources.

[0068] After testing, the positive electrode active material obtained in this example has a discharge capacity of 89 mAh / g at a voltage of 1.7-4.3 V and a rate of 0.5 C.

[0069] The positive electrode active material obtained in this example has a discharge specific capacity of 82 mAh / g at a voltage of 1.7-4.3 V and a rate of 5 C, and a corresponding capacity retention rate of 92.1%.

[0070] The electrochemical performance test results of the positive electrode active materials obtained in Example 1-2 and Comparative Example 1-2 are shown in Table 1.

[0071] Table 1 Discharge capacity and capacity retention rate of different samples after first cycle

[0072]

[0073] As can be seen from Table 1, the present invention can effectively improve the high-rate performance of the sodium iron pyrophosphate material by doping the anion with Se, thereby improving the electrochemical performance of the material.

[0074] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0075] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A Se anion-doped sodium iron phosphate pyrophosphate / carbon positive electrode active material, characterized in that: The invention comprises a sodium iron phosphate pyrophosphate matrix material and a carbon coating layer coated on the surface of the matrix material, wherein part of the O in the crystal structure of the matrix material is replaced by Se, and the general formula is Na4Fe3(PO4)2(P2O 7-x Se x ), where 0 <x≤1.0。 2. The Se anion-doped sodium iron phosphate pyrophosphate / carbon positive electrode active material according to claim 1, characterized in that 0.005≤x≤0.3。 3. The Se anion-doped sodium iron phosphate pyrophosphate / carbon positive electrode active material according to any one of claims 1 to 2, characterized in that: The positive electrode active material is in the form of submicron or micron particles with an average particle size of 0.5-5 μm.

4. The Se anion-doped sodium iron phosphate pyrophosphate / carbon positive electrode active material according to any one of claims 1 to 2, characterized in that: The mass of the carbon coating layer is 1-10% of the mass of the positive electrode active material.

5. The method for preparing the Se anion-doped sodium iron phosphate pyrophosphate positive electrode active material according to any one of claims 1 to 4, characterized in that: The following steps are involved: S1. Weighing a soluble sodium source, an iron source, a phosphorus source, and a carbon source according to stoichiometric amounts, adding them to deionized water and mixing them evenly to obtain a mixed slurry; placing the mixed slurry in a spray dryer and spray drying it, then heating it to 300-400° C. in a first inert atmosphere for sintering, and then heating it to 450-600° C. for secondary sintering to obtain a sodium iron pyrophosphate / carbon precursor material; S2. Mixing the Se source with the precursor material obtained in step S1, placing the mixture in a second inert atmosphere, heating the mixture to 400-600° C. and sintering the mixture to obtain the Se anion-doped sodium iron pyrophosphate / carbon positive electrode active material.

6. The method for preparing Se anion-doped sodium iron phosphate pyrophosphate / carbon positive electrode active material according to claim 5, characterized in that: In step S1, the sodium source is at least one of sodium carbonate, sodium hydroxide, sodium bicarbonate, sodium dihydrogen phosphate, sodium pyrophosphate, disodium dihydrogen pyrophosphate, sodium acetate, and sodium citrate; and / or, the iron source is at least one of ferric chloride, ferrous oxalate, ferrous oxalate, ferrous sulfate, ammonium ferrous sulfate, diammonium hydrogen phosphate, and phosphoric acid; and / or, the phosphorus source is at least one of diammonium hydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid; and / or, the carbon source is at least one of soluble starch, cellulose, sucrose, glucose, and ascorbic acid; and / or, in step S2, the Se source is at least one of Se powder, Na2SeO3, SeO2, and Na2Se.

7. The method for preparing Se anion-doped sodium iron phosphate pyrophosphate / carbon positive electrode active material according to claim 5, characterized in that: In step S2 , the first inert atmosphere and the second inert atmosphere are each independently one of a nitrogen atmosphere, an argon atmosphere, and a helium atmosphere. 8 . The method according to claim 5 , wherein in step S2 , the molar ratio of O to Se in the pyrophosphate anion in the precursor material is 1-z:z, where z=0.005-0.

3.

9. A positive electrode, characterized in that The invention comprises the positive electrode active material according to any one of claims 1 to 4 or the positive electrode active material obtained by the preparation method according to any one of claims 5 to 8.

10. An electrochemical energy storage device, characterized in that: Comprising the positive electrode according to claim 9.

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