Na / P double-site doped phosphoric acid ferric sodium pyrophosphate / carbon positive electrode active material as well as preparation method and application of Na / P double-site doped phosphoric acid ferric sodium pyrophosphate / carbon positive electrode active material

Abstract

The invention provides a Na / P double-site doped phosphoric acid ferric sodium pyrophosphate / carbon positive electrode active material and a preparation method and application of the Na / P double-site doped phosphoric acid ferric sodium pyrophosphate / carbon positive electrode active material. The material comprises a Na / P double-site doped ferric sodium pyrophosphate base material and a carbon coating layer coating the surface of the base material, the general formula of the base material is Na (4-x) MxFe3 (PO4) 2-y (QO4) y (P2O7), 0 lt, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 X is less than or equal to 0.4, 0lt; y < = 0.2; m is at least one of K, Li, Mg, Ca and Sr, and Q is at least one of Si, Ge, B and S; specific elements are introduced to the Na site and the P site to form a synergistic doping structure, so that the crystal structure and charge distribution are regulated and controlled, and the electrochemical performance of the material is further improved; the material disclosed by the invention is simple and convenient to prepare, wide in raw material source, suitable for electrochemical energy storage systems such as sodium ion batteries and the like, and excellent in rate capability and long in cycle life.

Description

Technical Field

[0001] The present invention relates to the technical field of electrochemical energy storage materials, and more specifically, to a Na / P dual-site doped sodium iron phosphate pyrophosphate / carbon positive electrode active material and a preparation method thereof, as well as an application thereof in a sodium ion electrochemical energy storage device. Background Art

[0002] As energy storage systems develop towards large-scale, low-cost, and resource-friendly development, sodium-ion batteries (SIBs) are being considered as a highly promising next-generation energy storage technology due to the abundance of sodium resources and chemical properties similar to lithium. Among them, sodium ferric pyrophosphate (Na4Fe3(PO4)2(P2O7), abbreviated as NFPP), has become an important candidate for SIB cathode materials due to its high structural stability, stable voltage platform, and environmental friendliness.

[0003] NFPP has a NASICON structure and a theoretical specific capacity of about 120 mAh g -1 , has reversible sodium insertion and extraction reactions in the 2.5-4.3V range and exhibits excellent thermodynamic stability, making it suitable for use in high-safety scenarios. However, during cycling, it is prone to microcrack evolution and interface polarization induced by bulk structural expansion and contraction, further accelerating capacity fading.

[0004] To address these issues, previous studies have attempted to improve their transport properties and structural stability through methods such as surface carbon coating, particle size reduction, and heterogeneous interface construction. However, such modifications primarily focus on external material regulation and struggle to achieve synergistic improvements within the crystal. Recent reports have shown that cation or anion doping is an effective strategy for improving the comprehensive performance of NASICON-type materials. Doping can introduce lattice distortion, modify electronic structure, or induce beneficial defect states, thereby improving their kinetic response and long-term stability.

[0005] Although some doping work has shown some success, most current research focuses on modifying a single lattice site, or primarily on the Fe site through transition metal doping. For example, patent application CN202380066770.3 discloses a cathode active material dual-doped at both the Na and Fe sites. This dual-site doping improves the energy density and rate performance of the cathode active material. However, existing research lacks a systematic exploration of the synergistic regulation mechanism of multiple sites, such as the Na and P sites.

[0006] Therefore, constructing a doping system with synchronous regulation of Na and P sites and improving the conductivity and cycle stability of NFPP by means of a dual action mechanism is a key technical problem that needs to be urgently solved in the field of sodium-ion battery positive electrode materials. Summary of the Invention

[0007] Aiming at the limitations of sodium iron pyrophosphate (Na4Fe3(PO4)2(P2O7)) cathode active material in terms of conductivity and structural stability, the present invention proposes a strategy based on co-doping of Na site and P site. Different cations and anions are introduced into its crystal structure respectively, and the electron transport, ion diffusion and crystal structure stability are synchronously optimized through a two-site regulation mechanism, so as to comprehensively improve the electrochemical performance.

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

[0009] A Na / P dual-site doped sodium iron pyrophosphate / carbon cathode active material, the cathode active material includes a Na / P dual-site doped sodium iron pyrophosphate matrix material and a carbon coating layer coated on the surface of the matrix material, and the general formula of the matrix material is Na 4-x M x Fe3(PO4) 2-y (QO4) y (P2O7), where 0 < x ≤ 0.4, 0 < y ≤ 0.2; M is at least one of K, Li, Mg, Ca, Sr, and Q is at least one of Si, Ge, B, S.

[0010] First, based on the NASICON-type NFPP structure, the present invention selects to introduce specific monovalent or divalent metal cations at the sodium site for doping to regulate the lattice size and local structure. Such cations have a relatively large ionic radius and can, while partially replacing Na + , cause slight expansion of the crystal and symmetry perturbation, expand the movement space of Na + in the three-dimensional migration channel, and significantly reduce the ion diffusion resistance; at the same time, the release of lattice stress between the doped ions and the surrounding Fe-O coordination environment is also beneficial to promoting the formation of electron local states, thereby improving the overall electronic conductivity of the material.

[0011] Furthermore, introducing tetravalent and / or pentavalent non-metal cations with a higher oxidation state (such as Si 4+ , B 5 + , Ge 4+ , S 4+ etc.) at the phosphorus site can achieve partial replacement of the PO4 3- structural unit, thereby forming a positive charge reduction region in the local crystal region. To maintain overall electrical neutrality, the crystal structure spontaneously forms a charge compensation mechanism, such as inducing the adjustment of the variable valence Fe 2+ / Fe 3+ ratio or triggering microstructural defects such as oxygen vacancies. Such ordered defect structures not only help to improve Na +transmission efficiency, while suppressing phase change and structural collapse in long cycles, thereby enhancing cycle stability and service life.

[0012] In some embodiments, 0.01≤x≤0.3, 0.01≤y≤0.2.

[0013] In some embodiments, the crystal structure of the matrix material obtained after doping is a uniform solid solution structure, and the positive electrode active material is in the form of particles with a particle size of 0.5-5 μm.

[0014] The present invention also provides a method for preparing the Na / P dual-site doped sodium iron phosphate pyrophosphate positive electrode active material of any of the above embodiments, the method comprising the following steps:

[0015] S1. Weigh a sodium source, an iron source, a phosphorus source, a carbon source, and an M element doping source according to stoichiometric amounts, add the mixture to deionized water, and mix well to obtain a mixed slurry.

[0016] S2. Place the mixed slurry in a spray dryer for spray drying, then ball-mill the dried product with the Q element doping source, and then heat it to 300-400°C in an inert atmosphere for sintering, and then heat it to 500-600°C for secondary sintering to obtain a Na / P dual-site doped sodium iron pyrophosphate / carbon positive electrode active material.

[0017] 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.

[0018] 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.

[0019] In some embodiments, the phosphorus source is at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid.

[0020] In some embodiments, the carbon source is at least one of soluble starch, cellulose, sucrose, glucose, and ascorbic acid.

[0021] In some embodiments, the carbon coating layer is further doped with at least one of S, N, and F.

[0022] In some embodiments, the carbon source further comprises at least one carbon layer doping source selected from the group consisting of thiourea, elemental sulfur, polytetrafluoroethylene, urea, and lignin sulfonic acid.

[0023] In some embodiments, the M element doping source is at least one of a soluble acetate, nitrate, chloride, oxide, hydroxide, and sulfate of at least one of K, Li, Mg, Ca, and Sr; specifically, including but not limited to potassium acetate, potassium nitrate, potassium oxide, potassium chloride, potassium hydroxide, potassium sulfate, potassium phosphate, calcium acetate, calcium nitrate, calcium chloride, magnesium nitrate, magnesium chloride, magnesium acetate, strontium acetate, strontium nitrate, strontium oxide, strontium chloride, strontium hydroxide, strontium sulfate, strontium phosphate, etc.

[0024] In some embodiments, the Q element doping source is at least one of the salts and oxides of at least one of Si, Ge, B, and S; specifically, including but not limited to sodium orthosilicate, silicon dioxide, sodium tetraborate, elemental boron, germanium dioxide, potassium sulfite, sodium sulfite, magnesium sulfite, etc.

[0025] In some embodiments, in step S2, the inert atmosphere is one of nitrogen atmosphere, argon atmosphere, and helium atmosphere.

[0026] In some embodiments, in step S2, the ball milling rate is 400-800 r / min; and the ball milling time is 3-12 h.

[0027] The present invention also provides a positive electrode material, which includes the positive electrode active material described in any of the above embodiments or the positive electrode active material obtained by the preparation method described in any of the above embodiments.

[0028] The present invention also provides a positive electrode, comprising the positive electrode material.

[0029] The present invention also provides an electrochemical energy storage device comprising the above-mentioned positive electrode. Specifically, the electrochemical energy storage device includes but is not limited to a sodium ion battery and a sodium ion capacitor.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The present invention achieves multi-dimensional coordinated regulation of lattice structure, charge distribution, and conductivity properties by simultaneously introducing Na-site cation doping and P-site cation doping into the main crystal structure of sodium iron pyrophosphate. The introduction of heterogeneous cations at the Na site can induce local lattice expansion and microstructural distortion, effectively widening the sodium ion migration channel, improving ion diffusion kinetics, and helping to establish a continuous electron migration path. The introduction of non-metallic high-valent cations at the P site induces charge imbalance in the crystal, forming a charge compensation mechanism such as oxygen vacancies or iron ion valence regulation while maintaining electrical neutrality, further enhancing bulk conductivity and structural stability. Through the dual-site doping design of the present invention, the rate performance and cycle life of the material are intrinsically improved at the crystal level. This doping strategy is highly versatile and can be adapted to a variety of ion combinations. The preparation path is simple, the doping concentration and structural response are easily regulated, and it has good scalability and industrialization potential. The prepared positive electrode active materials exhibit excellent cycling performance under extreme electrochemical conditions, providing a practical and effective new path for the high performance of phosphate-based sodium ion battery positive electrode active materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 The SEM morphology and EDS element distribution map of the Na-site Ca and P-site Si-doped sodium iron pyrophosphate / carbon positive electrode active material obtained in Example 1. DETAILED DESCRIPTION

[0033] 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.

[0034] 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.

[0035] Example 1

[0036] A method for preparing a Na / P dual-site doped sodium iron phosphate pyrophosphate / carbon positive electrode active material comprises the following steps:

[0037] S1. Add 490 g of sodium dihydrogen phosphate, 460 g of ferrous oxalate dihydrate, 100 g of citric acid, 20 g of thiourea, and 6 g of calcium acetate to 3 L of deionized water, and stir in a ball mill at a speed of 600 rpm for 5 h to obtain a slurry;

[0038] S2. The slurry was transported to a spray dryer via a peristaltic pump at a feed rate of 2500 ml / h, an air inlet temperature of 200°C, and an air outlet temperature of 130°C to obtain dry particles; the dry particles were ball-milled with 5 g of sodium orthosilicate for 3 h, and then heated to 350°C at a rate of 5°C / min in a nitrogen atmosphere for sintering for 4 h, and then heated to 550°C at a rate of 5°C / min for secondary sintering for 5 h to obtain Na / P dual-site doped sodium iron phosphate pyrophosphate / carbon material (Na 3.96 Ca 0.04 Fe3(PO4) 1.97 (SiO4) 0.03 (P2O7) / C);

[0039] The material obtained in this example was subjected to scanning electron microscopy test, and the test results are as follows: Figure 1 As shown in the SEM morphology and EDS element distribution. Figure 1 The dual-site doped sodium iron phosphate pyrophosphate / C material prepared in this embodiment shows the phenomenon of Ca / Si uniform doping.

[0040] After testing, as shown in Table 1, the material obtained in this example has a discharge specific capacity of 92 mAh / g at a voltage of 1.7-4.3 V and a current density of 0.5 C (1 C = 129 mAh / g).

[0041] The material obtained in this embodiment has a discharge capacity retention rate of 96.7% after 100 cycles at a voltage of 1.7-4.3 V and a current density of 0.5 C.

[0042] Comparative Example 1

[0043] A method for preparing a Na-position Ca-doped sodium iron phosphate pyrophosphate / carbon positive electrode active material comprises the following steps:

[0044] S1. Add 490 g of sodium dihydrogen phosphate, 460 g of ferrous oxalate dihydrate, 100 g of citric acid, 20 g of thiourea, and 6 g of calcium acetate to 3 L of deionized water, and stir in a ball mill at a speed of 600 rpm for 5 h to obtain a slurry;

[0045] S2. The slurry was transported to a spray dryer by a peristaltic pump at a feed rate of 2500 ml / h, an air inlet temperature of 200°C, and an air outlet temperature of 130°C to obtain dry particles; in a nitrogen atmosphere, the temperature was raised to 350°C at a rate of 5°C / min for sintering for 4 h, and then raised to 550°C at a rate of 5°C / min for secondary sintering for 5 h to obtain Na-position Ca-doped sodium iron phosphate / carbon material (Na 3.96 Ca 0.04 Fe3(PO4)2(P2O7) / C);

[0046] After testing, as shown in Table 1, the discharge capacity of the material obtained in this comparative example is 86 mAh / g at a voltage of 1.7-4.3 V and a current density of 0.5 C.

[0047] The material obtained in this comparative example has a discharge capacity retention rate of 94.2% after 100 cycles at a voltage of 1.7-4.3 V and a current density of 0.5 C.

[0048] Comparative Example 2

[0049] A method for preparing a P-site Si-doped sodium iron phosphate pyrophosphate positive electrode active material comprises the following steps:

[0050] 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 600 rpm for 5 h to obtain a slurry;

[0051] S2. The slurry was transported to a spray dryer via a peristaltic pump at a feed rate of 2500 ml / h, an air inlet temperature of 200°C, and an air outlet temperature of 130°C to obtain dry particles; the dry particles were ball-milled with 5 g of sodium orthosilicate for 3 h, and then heated to 350°C at a rate of 5°C / min in a nitrogen atmosphere for sintering for 4 h, and then heated to 550°C at a rate of 5°C / min for secondary sintering for 5 h to obtain P-site Si-doped sodium iron phosphate pyrophosphate / carbon material (Na4Fe3(PO4) 1.97 (SiO4) 0.03 (P2O7) / C);

[0052] After testing, as shown in Table 1, the discharge capacity of the material obtained in this comparative example is 84 mAh / g at a voltage of 1.7-4.3 V and a current density of 0.5 C.

[0053] The material obtained in this comparative example has a discharge capacity retention rate of 95.2% after 100 cycles at a voltage of 1.7-4.3 V and a current density of 0.5 C.

[0054] Comparative Example 3

[0055] A method for preparing an undoped sodium iron phosphate pyrophosphate positive electrode active material comprises the following steps:

[0056] 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 600 rpm for 5 h to obtain a slurry;

[0057] S2. The slurry was transported to a spray dryer via a peristaltic pump at a feed rate of 2500 ml / h, an air inlet temperature of 200°C, and an air outlet temperature of 130°C to obtain dry particles; in a nitrogen atmosphere, the temperature was raised to 350°C at a rate of 5°C / min for sintering for 4 h, and then the temperature was raised to 550°C at a rate of 5°C / min for secondary sintering for 5 h to obtain undoped sodium iron pyrophosphate / carbon material (Na4Fe3(PO4)2(P2O7) / C);

[0058] After testing, as shown in Table 1, the material obtained in this comparative example has a discharge capacity of 81 mAh / g at a voltage of 1.7-4.3 V and a current density of 0.5 C.

[0059] The material obtained in this comparative example has a discharge capacity retention rate of 85.2% after 100 cycles at a voltage of 1.7-4.3 V and a current density of 0.5 C.

[0060] Example 2

[0061] A method for preparing a Na / P dual-site doped sodium iron phosphate pyrophosphate / carbon positive electrode active material comprises the following steps:

[0062] S1. Add 490 g of sodium dihydrogen phosphate, 460 g of ferrous oxalate dihydrate, 100 g of citric acid, 20 g of thiourea, and 6 g of magnesium nitrate to 3 L of deionized water, and stir in a ball mill at a speed of 600 rpm for 5 h to obtain a slurry;

[0063] S2. The slurry was transported to a spray dryer via a peristaltic pump at a feed rate of 2500 ml / h, an air inlet temperature of 200°C, and an air outlet temperature of 130°C to obtain dry particles; the dry particles were ball-milled with 8 g of sodium tetraborate for 3 h, and then heated to 350°C at a rate of 5°C / min in a nitrogen atmosphere for sintering for 4 h, and then heated to 550°C at a rate of 5°C / min for secondary sintering for 5 h to obtain Na / P dual-site doped sodium iron phosphate pyrophosphate / carbon material (Na 3.96 Mg 0.04 Fe3(PO4) 1.84 (BO4) 0.16 (P2O7) / C);

[0064] After testing, as shown in Table 1, the material obtained in this example has a discharge capacity of 88 mAh / g at a voltage of 1.7-4.3 V and a current density of 0.5 C.

[0065] The material obtained in this example has a discharge capacity retention rate of 93.2% after 100 cycles at a voltage of 1.7-4.3 V and a current density of 0.5 C.

[0066] Table 1 Discharge capacity and capacity retention rate of different samples after first cycle

[0067]

[0068] As can be seen from Table 1, the present invention can effectively improve the capacity density and cycle stability of the sodium iron pyrophosphate material by specifically modifying the Na / P site, thereby improving the electrochemical performance of the material.

[0069] 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.

[0070] The above-mentioned embodiments only express several implementation methods of the present invention, and their description is relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent.

[0071] It is obvious that for those skilled in the art, without departing from the concept of the present invention,

[0072] Several modifications and improvements can be made, which all fall within the scope of protection of the present invention.

[0073] Therefore, the protection scope of the patent for this invention shall be based on the attached claims.

Claims

1. A Na / P dual-site doped sodium iron phosphate pyrophosphate / carbon positive electrode active material, characterized in that: The positive electrode active material includes a sodium / potassium dual-site doped sodium iron pyrophosphate matrix material and a carbon coating layer coated on the surface of the matrix material. The general formula of the matrix material is Na 4-x M x Fe3(PO4) 2-y (QO4) y (P2O7), where 0 < x ≤ 0.4 and 0 < y ≤ 0.2; M is at least one of K, Li, Mg, Ca, and Sr, and Q is at least one of Si, Ge, B, and S.

2. The Na / P dual-site doped sodium iron phosphate pyrophosphate / carbon positive electrode active material according to claim 1, characterized in that: 0.01≤x≤0.3, 0.01≤y≤0.

2.

3. The Na / P dual-site doped sodium iron phosphate pyrophosphate / carbon positive electrode active material according to any one of claims 1 to 2, characterized in that: The crystal structure of the matrix material obtained after doping is a uniform solid solution structure, and the positive electrode active material is in the form of particles with a particle size of 0.5-5 μm.

4. The method for preparing a Na / P dual-site doped sodium iron phosphate pyrophosphate positive electrode active material / carbon according to claims 1-3, characterized in that: The following steps are involved: S1. Weigh a soluble sodium source, an iron source, a phosphorus source, a carbon source, and an M element doping source according to stoichiometric amounts, add the mixture to deionized water, and mix well to obtain a mixed slurry; S2. Place the mixed slurry in a spray dryer for spray drying, then ball-mill the dried product with the Q element doping source, and then heat it to 300-400°C in an inert atmosphere for sintering, and then heat it to 500-600°C for secondary sintering to obtain a Na / P dual-site doped sodium iron pyrophosphate / carbon positive electrode active material.

5. The method for preparing the Na / P dual-site doped sodium iron phosphate pyrophosphate / carbon positive electrode active material according to claim 4, 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, ferrous ammonium 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 the M element doping source is at least one of a soluble acetate, nitrate, chloride, oxide, hydroxide, and sulfate of at least one of K, Li, Mg, Ca, and Sr; and / or the Q element doping source is at least one of a salt, oxide, or element of at least one of Si, Ge, B, and S.

6. The method for preparing the Na / P dual-site doped sodium iron phosphate pyrophosphate / carbon positive electrode active material according to claim 4, characterized in that: In step S2, the inert atmosphere is one of nitrogen atmosphere, argon atmosphere, and helium atmosphere.

7. The method for preparing the Na / P dual-site doped sodium iron phosphate pyrophosphate / carbon positive electrode active material according to claim 4, characterized in that: In step S2, the ball milling rate is 400-800 r / min; and / or the ball milling time is 3-12 h.

8. A positive electrode material, characterized in that The invention comprises the positive electrode active material according to any one of claims 1 to 3 or the positive electrode active material obtained by the preparation method according to any one of claims 4 to 7.

9. A positive electrode, characterized in that Comprising the positive electrode material according to claim 8.

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

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