Sodium ferric phosphate pyrophosphate positive electrode material, preparation method thereof, positive electrode plate, electrochemical energy storage device and electric equipment

Abstract

The invention discloses a ferric sodium pyrophosphate positive electrode material, a preparation method thereof, a positive electrode plate, an electrochemical energy storage device and electric equipment, and belongs to the technical field of iron-based phosphate positive electrode materials. The ferric sodium phosphate pyrophosphate positive electrode material comprises ferric sodium phosphate pyrophosphate compounded with carbon, the chemical formula of the ferric sodium phosphate pyrophosphate is Na < 4 + n > Fe < 3-m < x > / 2M < x > (PO4) < 2 + > n P < 2 > O < 7 >, and M is a metal element; m is the valence state of M, and m > = 2; 0 lt; x is smaller than or equal to 0.5; 0 lt; n < = 0.1. The sodium ferric phosphate pyrophosphate positive electrode material effectively inhibits formation of an inactive impurity phase and optimizes a crystal phase and a crystal structure at the same time, so that the sodium ferric phosphate pyrophosphate positive electrode material has high crystal phase purity, and a multi-site defect is constructed and formed; and finally, the first-circle specific capacity, the electronic conductivity, the sodium ion transmission rate and the electrochemical performance of the ferric sodium pyrophosphate positive electrode material are comprehensively improved synergistically.

Description

Technical Field

[0001] This application belongs to the technical field of electrode materials, and particularly relates to sodium iron pyrophosphate phosphate cathode materials, their preparation methods, cathode sheets, electrochemical energy storage devices, and electricity-related devices. Background Art

[0002] The cathode material determines the energy density of the sodium-ion battery, making the preparation of high-performance sodium-ion cathode materials an effective solution for optimizing the battery. Sodium iron pyrophosphate phosphate [Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 has advantages such as low volume strain (<4%), high voltage (3.1V), and high theoretical capacity (129 mAh·g -1 ), and has become a research hotspot among iron-based phosphate cathode materials for sodium-ion batteries. However, the impurity phases formed during the preparation of sodium iron pyrophosphate phosphate limit the performance of the specific capacity, making its actual capacity far lower than the theoretical capacity. Therefore, how to improve the actual capacity of sodium iron pyrophosphate phosphate has become the research focus.

[0003] Currently, it is disclosed in the related art that the synergistic improvement of metal doping and carbon coating can improve the electrochemical performance of sodium iron pyrophosphate phosphate. For example, a sodium iron pyrophosphate phosphate cathode material with co-doping of anions and cations composed of a sodium iron pyrophosphate phosphate core with the chemical formula Na 4- 2y Fe 2.91-x Q x (PO 4 ) 2-y P 2 O 7 F y disclosed in the patent application with the publication number CN 119230796A. Its improved design of introducing Q cations and F anions for co-doping into sodium iron pyrophosphate phosphate and combining carbon coating effectively improves the actual capacity of the cathode material.

[0004] However, the existing synergistic improvement scheme of metal doping and carbon coating still has deficiencies. For example, although introducing Q cations and F anions simultaneously can improve the specific capacity of the cathode material through the synergistic effect of anions and cations, the excessive raw material components introduced also exacerbate the formation of impurity phases, making the improvement effect of the actual specific capacity relatively limited. At the same time, it also increases the raw material cost and limits industrial production. Summary of the Invention

[0005] This application discloses a sodium iron pyrophosphate phosphate cathode material, its preparation method, a cathode electrode sheet, an electrochemical energy storage device, and an electricity-related device, effectively solving the technical problems of poor specific capacity improvement effect and high cost faced by the above-mentioned metal doping and carbon coating synergistic improvement solutions.

[0006] To achieve the above object, the technical solution provided by this application is:

[0007] In the first aspect, this application provides a sodium iron pyrophosphate phosphate cathode material, which is sodium iron pyrophosphate phosphate compounded with carbon;

[0008] The sodium iron pyrophosphate phosphate has the following chemical formula (1):

[0009] Na 4+n Fe 3-mx / 2 M x (PO4) 2+n P 2 O 7 (1)

[0010] Among them, M is a metal element; m is the valence state of M, m≥2; 0<x≤0.5; 0<n≤0.1.

[0011] In a preferred embodiment, M is selected from one of Zr, Sc, Hf, Ta, W, Nb, Al, V, Ti, Cr, Co, Ni, Mn, Cu.

[0012] In a preferred embodiment, the compounding amount of carbon is 0.1-8 wt% of the total amount of the sodium iron pyrophosphate phosphate cathode material.

[0013] In the second aspect, this application also provides a preparation method of the above-mentioned sodium iron pyrophosphate phosphate cathode material, and the steps include:

[0014] Providing a precursor powder containing a sodium source compound, an iron source compound, an M metal source compound, a phosphorus source compound, and a carbon source compound;

[0015] In a non-oxidizing atmosphere, sinter the precursor powder at 400-650°C for 6-20 h to obtain the sodium iron pyrophosphate phosphate cathode material, where the non-oxidizing atmosphere here refers to an inert gas atmosphere or a weakly reducing gas atmosphere.

[0016] In a preferred embodiment, the molar ratio of sodium, iron, phosphorus, and M metal contained in the precursor powder is (4-4.1):(2.5-3):(2-2.1):(0-0.5).

[0017] In a preferred embodiment, the molar ratio of the iron source compound and the carbon source compound contained in the precursor powder is 1:(0.5-5).

[0018] In a preferred embodiment, the sodium source compound is selected from at least one of the following compounds containing sodium in its composition:

[0019] Sodium hydrogen phosphate, sodium phosphate, sodium carbonate, sodium nitrate, sodium oxalate, sodium acetate, sodium sulfate, sodium hydroxide, sodium alginate, sodium formate, sodium citrate, sodium pyrophosphate, sodium dihydrogen pyrophosphate.

[0020] In a preferred embodiment, the iron source compound is selected from at least one of the following compounds containing iron in its composition:

[0021] Ferric citrate, ferrous citrate, ferric nitrate, ferrous nitrate, ferric sulfate, ferrous sulfate, ammonium ferrous sulfate, ferric chloride, ferrous chloride, magnetite, ferric oxide, ferrous oxide, ferric oxalate, ferrous oxalate, ferric acetate, ferric phosphate, ferric pyrophosphate.

[0022] In a preferred embodiment, the phosphorus source compound is selected from at least one of the following compounds containing phosphorus in its composition:

[0023] Sodium monohydrogen phosphate, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, sodium phosphate, phosphoric acid, pyrophosphoric acid, sodium pyrophosphate, sodium dihydrogen pyrophosphate.

[0024] In a preferred embodiment, the carbon source compound is selected from at least one of the following compounds containing carbon in its composition:

[0025] Oxalic acid, ascorbic acid, lactic acid, citric acid, oxalic acid, adipic acid, citric acid, cyclodextrin, soluble starch, sucrose and glucose.

[0026] In a preferred embodiment, the method for preparing the precursor powder includes:

[0027] After mixing the sodium source compound, iron source compound, M metal source compound, phosphorus source compound and carbon source compound, spray granulation is carried out;

[0028] And / or, the mixing method includes ball milling, sand milling and aqueous solution dispersion.

[0029] In a third aspect, the present application also provides a positive electrode sheet, which contains the sodium iron phosphate pyrophosphate positive electrode material described in the above application.

[0030] In a fourth aspect, the present application provides an electrochemical energy storage device, which contains the positive electrode sheet described in the above application.

[0031] In a fifth aspect, the present application also provides an electricity-related device, which contains the electrochemical energy storage device described in the above application.

[0032] Compared with the prior art, the advantages or beneficial effects of the embodiments of the present application at least include:

[0033] The sodium iron zirconium pyrophosphate positive electrode material provided by the present application can perform multi-site dynamic regulation on the sodium iron zirconium pyrophosphate component. On the one hand, while enabling metal elements to be doped into the iron lattice sites to form iron vacancy defects, it can reduce the local Fe / P ratio and form sodium vacancy defects, making the sodium iron zirconium pyrophosphate positive electrode material have multi-site defects. On the other hand, it effectively inhibits the formation of inactive impurity phases and further optimizes the crystal phase and crystal structure, effectively improving the crystal phase purity of the prepared sodium iron zirconium pyrophosphate positive electrode material, and finally synergistically achieving a comprehensive improvement in the first-cycle specific capacity, electronic conductivity, sodium ion transport rate, and electrochemical performance of the sodium iron zirconium pyrophosphate positive electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings described below are only some embodiments recorded in the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0035] Figure 1 SEM diagram of Na 4.01 Fe 2.86 Zr 0.07 (PO 4 ) 2.01 P 2 O 7 / C;

[0036] Figure 2 SEM diagrams of Na 4.01 Fe 2.86 Zr 0.07 (PO 4 ) 2.01 P 2 O 7 / C, Na 4.02 Fe 2.96 Zr 0.03 (PO 4 ) 2.02 P 2 O 7 / C, Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 / C and Na 4 Fe 2.96 Zr 0.03 (PO 4) 2 P 2 O 7 XRD pattern of P / O / C;

[0037] Figure 3 It is an enlarged view of the XRD patterns of the positive electrode materials obtained in Examples 1 to 3 and Comparative Example 1 of this application in the range of 31° to 35°;

[0038] Figure 4 It is the rate test chart of the positive electrode materials obtained in Examples 1 to 3 and Comparative Example 1 of this application in the voltage range of 1.7 V to 4.3 V. Specific embodiments

[0039] Next, the technical solutions in the embodiments of this application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments described in this application without creative efforts shall fall within the scope of protection of this application.

[0040] In the following description of this specification, the term "and / or" is used to describe the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may indicate: A exists alone, B exists alone, and both A and B exist simultaneously. Among them, A and B may be singular or plural; the symbol " / " means "or".

[0041] In the following description of this specification, the term "at least one" means one or more, and "a plurality" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single items or plural items. For example, "at least one of A, B, or C", or, "at least one of A, B, and C", may represent any one of A, B, C, or A + B, or A + C, or B + C, or A + B + C, where A, B, and C may be single or multiple respectively.

[0042] In the following description of this specification, the sequence number does not mean the order of execution. Some or all steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and does not constitute any limitation to the execution process of this embodiment.

[0043] In the following description of this specification, the numerical range should be understood as specifically disclosing each intermediate value between the upper and lower limits of the range. Any stated value or intermediate value within the stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in this embodiment, and the upper and lower limits of the smaller range can be independently included or excluded from the range.

[0044] Unless otherwise specified, the technical / scientific terms used in this specification have the meanings commonly understood by those of ordinary skill in the art. Although this specification only describes preferred materials and methods, any similar or equivalent methods and materials may also be used in specific examples or test cases. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0045] Example 1

[0046] This example provides a preparation method for the sodium iron zirconium pyrophosphate cathode material Na 4.01 Fe 2.86 Zr 0.07 (PO4) 2.01 P 2 O 7 / C, and the specific steps include:

[0047] S1: Dissolve 0.0401 mol of sodium dihydrogen phosphate, 0.0286 mol of iron(III) nitrate nonahydrate, 0.0007 mol of zirconyl nitrate, and citric acid equivalent to 20 wt% of their total weight in deionized water and stir evenly. Then, spray-dry the obtained dispersion at 170 °C through a spray dryer to obtain the precursor powder.

[0048] S2: Place the precursor powder in an argon / hydrogen mixed gas (volume ratio of 95:5), and heat it to 600 °C at a heating rate of 2 °C / min, hold for sintering for 10 h, and cool it to room temperature with the furnace after sintering to obtain the sodium iron zirconium pyrophosphate cathode material Na 4.01 Fe 2.86 Zr 0.07 (PO 4 ) 2.01 P 2 O 7 / C.

[0049] Among them, Figure 1 is the SEM image of Na 4.01 Fe 2.86 Zr 0.07 (PO 4 ) 2.01 P 2 O 7 / C.

[0050] According to Figure 1 it can be known that the Na 4.01 Fe 2.86 Zr 0.07 (PO 4 ) 2.01 P 2 O7 / C is spherical particles with a particle size in the range of 2 - 4 μm and has high uniformity.

[0051] Example 2

[0052] This example provides a preparation method of sodium iron zirconium pyrophosphate cathode material Na 4.01 Fe 2.96 Zr 0.03 (PO4) 2.01 P 2 O 7 / C, and the specific steps include:

[0053] S1: Dissolve 0.0401 mol of sodium dihydrogen phosphate, 0.0296 mol of iron(III) nitrate nonahydrate, 0.0003 mol of zirconyl nitrate, and citric acid equivalent to 20 wt% of their total weight in deionized water and stir evenly. Then, spray-dry the obtained dispersion at 170 °C through a spray dryer to obtain a precursor powder;

[0054] S2: Place the precursor powder in an argon / hydrogen mixed gas (volume ratio of 95:5), and heat it to 600 °C at a heating rate of 2 °C / min, hold for sintering for 10 h, and cool to room temperature with the furnace after sintering to obtain the sodium iron zirconium pyrophosphate cathode material Na 4.01 Fe 2.96 Zr 0.03 (PO 4 ) 2.01 P 2 O 7 / C.

[0055] Example 3

[0056] This example provides a preparation method of sodium iron zirconium pyrophosphate cathode material Na 4.02 Fe 2.96 Zr 0.03 (PO4) 2.02 P 2 O 7 / C, and the specific steps include:

[0057] S1: Dissolve 0.0402 mol of sodium dihydrogen phosphate, 0.0296 mol of iron(III) nitrate nonahydrate, 0.0003 mol of zirconyl nitrate, and citric acid equivalent to 20 wt% of their total weight in deionized water and stir evenly. Then, spray-dry the obtained dispersion at 170 °C through a spray dryer to obtain a precursor powder;

[0058] S2: Place the precursor powder in an argon / hydrogen mixed gas (volume ratio 95:5), heat it at a heating rate of 2 °C / min to 600 °C, hold for sintering for 10 h, and then cool it to room temperature with the furnace, thus obtaining the sodium iron zirconium pyrophosphate phosphate cathode material Na 4.02 Fe 2.96 Zr 0.03 (PO 4 ) 2.02 P 2 O 7 / C.

[0059] Example 4

[0060] This example provides a preparation method for the sodium iron zirconium pyrophosphate phosphate cathode material Na 4.03 Fe 2.96 Zr 0.03 (PO4) 2.03 P 2 O 7 / C, and the specific steps include:

[0061] S1: Dissolve 0.0403 mol of sodium dihydrogen phosphate, 0.0296 mol of iron(III) nitrate nonahydrate, 0.0003 mol of zirconyl nitrate, and citric acid equivalent to 20 wt% of their total weight in deionized water and stir evenly. Then spray-dry the obtained dispersion at 170 °C through a spray dryer to obtain the precursor powder;

[0062] S2: Place the precursor powder in an argon / hydrogen mixed gas (volume ratio 95:5), heat it at a heating rate of 2 °C / min to 600 °C, hold for sintering for 10 h, and then cool it to room temperature with the furnace, thus obtaining the sodium iron zirconium pyrophosphate phosphate cathode material Na 4.03 Fe 2.96 Zr 0.03 (PO 4 ) 2.03 P 2 O 7 / C.

[0063] Example 5

[0064] This example provides a preparation method for the sodium iron titanium pyrophosphate phosphate cathode material Na 4.02 Fe 2.90 Ti 0.05 (PO4) 2.02 P 2 O 7 / C, and the specific steps include:

[0065] S1: Dissolve 0.0402 mol of sodium dihydrogen phosphate, 0.029 mol of iron(III) nitrate nonahydrate, 0.0005 mol of titanium nitrate, and citric acid equivalent to 20 wt% of their total weight in deionized water and stir evenly. Subsequently, spray-dry the obtained dispersion at 170 °C through a spray dryer to obtain the precursor powder;

[0066] S2: Place the precursor powder in an argon / hydrogen mixed gas (volume ratio 95:5), and heat it to 600 °C at a heating rate of 2 °C / min, hold for sintering for 10 h, and cool to room temperature with the furnace after sintering to obtain the sodium iron titanium pyrophosphate phosphate cathode material Na 4.02 Fe 2.90 Ti 0.05 (PO 4 ) 2.02 P 2 O 7 / C.

[0067] Comparative Example 1

[0068] This comparative example provides a preparation method of the sodium iron titanium pyrophosphate phosphate cathode material Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 / C, and the specific steps include:

[0069] S1: Dissolve 0.04 mol of sodium dihydrogen phosphate, 0.03 mol of iron(III) nitrate nonahydrate, and citric acid equivalent to 20 wt% of their total weight in deionized water and stir evenly. Subsequently, spray-dry the obtained dispersion at 170 °C through a spray dryer to obtain the precursor powder;

[0070] S2: Place the precursor powder in an argon / hydrogen mixed gas (volume ratio 95:5), and heat it to 600 °C at a heating rate of 2 °C / min, hold for sintering for 10 h, and cool to room temperature with the furnace after sintering to obtain the sodium iron titanium pyrophosphate phosphate cathode material Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 / C.

[0071] Comparative Example 2

[0072] This comparative example provides the sodium iron zirconium pyrophosphate phosphate cathode material Na 4 Fe 2 Zr 0.5 (PO 4 ) 2 P 2 O7 Preparation method of NaFeZr(PO4)2P2O7 / C, the specific steps include:

[0073] S1: Dissolve 0.04 mol of sodium dihydrogen phosphate, 0.02 mol of iron(III) nitrate nonahydrate, 0.005 mol of zirconyl nitrate, and citric acid equivalent to 20 wt% of their total weight in deionized water and stir evenly. Then spray-dry the obtained dispersion at 170 °C through a spray dryer to obtain a precursor powder.

[0074] S2: Place the precursor powder in an argon / hydrogen mixed gas (volume ratio 95:5), and heat it to 600 °C at a heating rate of 2 °C / min, hold for sintering for 10 h, and cool to room temperature with the furnace after sintering to obtain the sodium iron zirconium pyrophosphate phosphate cathode material NaFeZr(PO4)2P2O7 / C. 4 Fe 2 Zr 0.5 (PO4) 4 ) 2 P 2 O 7 / C.

[0075] Comparative Example 3

[0076] This comparative example provides a preparation method of sodium iron zirconium pyrophosphate phosphate cathode material NaFeZr(PO4)2P2O7 / C, the specific steps include: 4 Fe 2.96 Zr 0.03 (PO4) 2 P 2 O 7 / C.

[0077] S1: Dissolve 0.04 mol of sodium dihydrogen phosphate, 0.0296 mol of iron(III) nitrate nonahydrate, 0.003 mol of zirconyl nitrate, and citric acid equivalent to 20 wt% of their total weight in deionized water and stir evenly. Then spray-dry the obtained dispersion at 170 °C through a spray dryer to obtain a precursor powder.

[0078] S2: Place the precursor powder in an argon / hydrogen mixed gas (volume ratio 95:5), and heat it to 600 °C at a heating rate of 2 °C / min, hold for sintering for 10 h, and cool to room temperature with the furnace after sintering to obtain the sodium iron zirconium pyrophosphate phosphate cathode material NaFeZr(PO4)2P2O7 / C. 4 Fe 2.96 Zr 0.03 (PO4) 2 P 2 O 7 / C.

[0079] Comparative Example 4

[0080] This example provides a sodium iron zirconium pyrophosphate phosphate cathode material NaFeZr(PO4)2P2O7 / C 4.01 Fe3 (PO4) 2.01 P 2 O 7 Preparation method of Na

[0081] S1: Dissolve 0.0401 mol of sodium dihydrogen phosphate, 0.03 mol of ferric nitrate nonahydrate, 0.001 mol of ammonium dihydrogen phosphate, and citric acid equivalent to 20 wt% of their total weight in deionized water and stir evenly. Then spray-dry the obtained dispersion through a spray dryer at 170 °C to obtain a precursor powder;

[0082] S2: Place the precursor powder in an argon / hydrogen mixed gas (volume ratio of 95:5), and heat it to 600 °C at a heating rate of 2 °C / min, keep it sintered for 10 h, and then cool it to room temperature with the furnace after sintering to obtain the sodium iron zirconium pyrophosphate cathode material Na 4.01 Fe 2.86 Zr 0.07 (PO4) 2.01 P 2 O 7 / C.

[0083] Comparative Example 5

[0084] This example provides a preparation method of sodium iron zirconium pyrophosphate cathode material Na 4.03 Fe 3 (PO 4 ) 2.03 P 2 O 7 / C, and the specific steps include:

[0085] S1: Dissolve 0.0403 mol of sodium dihydrogen phosphate, 0.03 mol of ferric nitrate nonahydrate, and citric acid equivalent to 20 wt% of their total weight in deionized water and stir evenly. Then spray-dry the obtained dispersion through a spray dryer at 170 °C to obtain a precursor powder;

[0086] S2: Place the precursor powder in an argon / hydrogen mixed gas (volume ratio of 95:5), and heat it to 600 °C at a heating rate of 2 °C / min, keep it sintered for 10 h, and then cool it to room temperature with the furnace after sintering to obtain the sodium iron zirconium pyrophosphate cathode material Na 4.03 Fe 3 (PO4) 2.03 P 2 O 7 / C.

[0087] To clarify the electrochemical performance of the sodium iron pyrophosphate phosphate cathode material prepared in this application, the cathode materials obtained in Examples 1-5 and Comparative Examples 1-5 were made into sodium-ion battery cathode sheets and assembled into coin cells for battery performance testing, specifically including:

[0088] After mixing the sodium iron pyrophosphate phosphate cathode materials prepared in each example and comparative example with conductive carbon black and binder in a ratio of 8:1:1, N-methylpyrrolidone was added and mixed evenly to form a slurry. Then, the slurry was evenly coated on aluminum foil with a 100-μm scraper, dried and cut into circular pieces with a diameter of 12 mm to obtain the sodium-ion battery cathode;

[0089] Using the sodium-ion battery cathode as the electrode sheet, sodium sheet as the counter electrode, the electrolyte was 1 M sodium perchlorate dissolved in a solvent of propylene carbonate (EC) and ethylene carbonate (PC) at a ratio of 1:1, and 5% fluoroethylene carbonate (FEC) was added. Glass fiber was used as the separator to assemble a sodium-ion coin cell, and the obtained coin cell was subjected to electrochemical testing, and the results are shown in Table 1.

[0090] Table 1: Electrochemical performance test results

[0091]

[0092]

[0093] As can be seen from Table 1, the sodium iron pyrophosphate phosphate cathode materials prepared in Examples 1-5 have better specific capacity and higher cycle stability than Comparative Examples 1-5 at the rates of 0.1C and 10C, indicating that the synchronous metal element doping and regulation of the iron-phosphorus ratio in this application can effectively improve the electrochemical performance of the prepared sodium iron pyrophosphate phosphate cathode material. Specifically, in Comparative Example 1, compared with Examples 1-5, neither element doping nor regulation of the phosphorus element content to reduce the iron-phosphorus ratio was carried out, resulting in a decrease in the conductivity and phase purity of the sodium iron pyrophosphate phosphate cathode material, affecting the specific capacity and cycle stability; in Comparative Example 2, compared with Examples 1-5, excessive Zr element doping modification of the iron site was carried out. Since the Zr element does not have electrochemical activity and the electrochemical redox activity of the Fe element, the number of electrons available for redox of the obtained material decreases, and the electrochemical performance deteriorates; in Comparative Example 3, compared with Examples 2-4, only Zr element doping modification was carried out, and the phosphorus element content was not regulated, resulting in trace impurity phases still remaining in the bulk phase, affecting the material purity, and no sodium vacancies were formed to promote the sodium ion transport kinetics, resulting in a decrease in the actual specific capacity; in Comparative Examples 4 and 5, compared with Examples 2 and 4, due to the improvement of the phase purity and material conductivity of the material by Zr element doping modification on the basis of regulating the phosphate content, the electrochemical performance of the material is improved. This shows the beneficial effects of the dual-site dynamic regulation strategy proposed in this application.

[0094] To prove that the sodium iron zirconium pyrophosphate positive electrode material prepared in this application has a high phase purity, XRD tests were carried out on the sodium iron zirconium pyrophosphate positive electrode materials prepared in Examples 1 and 3 and Comparative Examples 1 and 3, and the results are as follows Figures 2 to 3 as shown. Among them, Figure 2 for Na 4.01 Fe 2.86 Zr 0.07 (PO 4 ) 2.01 P 2 O 7 / C, Na 4.02 Fe 2.96 Zr 0.03 (PO 4 ) 2.02 P 2 O 7 / C, Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 / C and Na 4 Fe 2.96 Zr 0.03 (PO 4 ) 2 P 2 O 7 / C XRD patterns; Figure 3 for Na 4.01 Fe 2.86 Zr 0.07 (PO 4 ) 2.01 P 2 O 7 / C, Na 4.02 Fe 2.96 Zr 0.03 (PO 4 ) 2.02 P 2 O 7 / C, Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 / C and Na 4 Fe 2.96 Zr 0.03 (PO 4 ) 2 P 2 O 7 / C XRD pattern enlarged view in the range of 31 - 35°.

[0095] According toFigures 2 - 3 It can be seen that, compared with Comparative Example 1, the positive composite material obtained in this application shows no diffraction peaks at the positions corresponding to 32.8° and 33.1° for NaFePO 4 The impurity phase, indicating that the product has a high purity and good crystallinity.

[0096] To demonstrate that the positive composite material prepared in this application has better rate performance, the rate tests of Examples 1 and 3 and Comparative Examples 1 and 3 are carried out in the voltage range of 1.7 - 4.2V below.

[0097] Figure 4 The constant current charge-discharge cycling test diagrams of the batteries assembled with the positive electrode materials prepared in Examples 1 and 3 and Comparative Examples 1 and 3 at different current densities are shown to illustrate that the positive electrode materials obtained in this application have more excellent rate performance and increased specific capacity compared with the comparative examples. At 0.1C, the initial discharge specific capacities of Examples 1 and 3 are 117.2 mAh·g -1 and 113.8 mAh·g -1 respectively, both higher than 89.3 mAh·g -1 and 105.7 mAh·g -1 of Comparative Examples 1 and 3. It should be noted that, compared with the direct introduction of Zr element doping, the decrease in the iron-phosphorus ratio caused by the increase in phosphate groups also has a greater impact on the specific capacity. This is because the proportion regulation of the Fe site and (PO 4 ) double sites in the examples leads to a reduction in the content of inactive impurity phases, thereby increasing the proportion of active substances. At the same time, the introduction of sodium vacancies improves the sodium ion diffusion rate.

[0098] The various embodiments in this specification are described in a progressive manner. The same or similar parts between the various embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments.

[0099] The above embodiments are only used to illustrate the technical solutions of this application, rather than to limit this application; although this application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of this application.

Claims

1. A sodium iron phosphate pyrophosphate positive electrode material, characterized in that: comprising sodium ferric pyrophosphate complexed with carbon; The sodium ferric phosphate pyrophosphate has the following chemical formula (1): So 4+n Feb 3-mx / 2 M x (PO4) 2+n P2O7(1) Where M is a metal element; m is the valence state of M, m≥2; 0 <x≤0.5;0<n≤0.1。 2. The sodium iron phosphate pyrophosphate positive electrode material according to claim 1, characterized in that M is selected from one of Zr, Sc, Hf, Ta, W, Nb, Al, V, Ti, Cr, Co, Ni, Mn and Cu.

3. The sodium iron phosphate pyrophosphate positive electrode material according to claim 1, characterized in that The composite amount of the carbon is 0.1-8 wt % of the total amount of the sodium iron phosphate pyrophosphate positive electrode material.

4. A method for preparing the sodium iron phosphate pyrophosphate positive electrode material according to any one of claims 1 to 3, characterized in that: Includes steps: Providing a precursor powder containing a sodium source compound, an iron source compound, an M metal source compound, a phosphorus source compound and a carbon source compound; The precursor powder is sintered at 400-650° C. for 6-20 hours in a non-oxidizing atmosphere to obtain the product.

5. The preparation method according to claim 4, characterized in that: The molar ratio of sodium, iron, phosphorus and M metal contained in the precursor powder is (4-4.1):(2.5-3):(2-2.1):(0-0.5); The molar ratio of the iron source compound to the carbon source compound contained in the precursor powder is 1:(0.5-5).

6. The preparation method according to claim 4, characterized in that: The sodium source compound is selected from at least one of the following compounds containing sodium: Sodium hydrogen phosphate, sodium phosphate, sodium carbonate, sodium nitrate, sodium oxalate, sodium acetate, sodium sulfate, sodium hydroxide, sodium alginate, sodium formate, sodium citrate, sodium pyrophosphate, sodium dihydrogen pyrophosphate; And / or, the iron source compound is selected from at least one of the following compounds containing iron: Ferric citrate, ferrous citrate, ferric nitrate, ferrous nitrate, ferric sulfate, ferrous sulfate, ammonium ferrous sulfate, ferric chloride, ferrous chloride, ferrous tetroxide, ferrous oxide, ferrous oxide, ferrous oxalate, ferrous oxalate, ferric acetate, ferric phosphate, ferric pyrophosphate; And / or, the phosphorus source compound is selected from at least one of the following compounds containing phosphorus: Sodium monohydrogen phosphate, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, triammonium phosphate, sodium phosphate, phosphoric acid, pyrophosphoric acid, sodium pyrophosphate, sodium dihydrogen pyrophosphate; And / or, the carbon source compound is selected from at least one of the following compounds containing carbon: Oxalic acid, ascorbic acid, lactic acid, citric acid, oxalic acid, adipic acid, citric acid, cyclodextrin, soluble starch, sucrose and glucose.

7. The preparation method according to claim 4, characterized in that: The preparation method of the precursor powder comprises: After mixing the sodium source compound, the iron source compound, the M metal source compound, the phosphorus source compound and the carbon source compound, spraying and granulating; And / or, the mixing method includes ball milling, sand milling and aqueous solution dispersion.

8. A positive electrode sheet, characterized in that: The composition contains the sodium iron phosphate pyrophosphate positive electrode material according to any one of claims 1 to 3 or the sodium iron phosphate pyrophosphate positive electrode material prepared by the preparation method according to any one of claims 4 to 7.

9. An electrochemical energy storage device, characterized in that: A positive electrode sheet comprising the positive electrode sheet according to claim 8.

10. An electrical equipment, characterized in that: A sodium ion battery comprising the sodium ion battery according to claim 9.

Images