In-situ polymerization coated sodium ferric pyrophosphate composite material, preparation method and application thereof

Abstract

The application relates to the field of sodium ion battery electrode materials, and discloses a sodium pyrophosphate iron phosphate composite material prepared through in-situ polymerization, a preparation method and application. x Fe y (PO4) z P2O7, a preparation process of which is as follows: a precursor is prepared by combining a segmented grinding and drying process, a sodium pyrophosphate iron phosphate positive electrode material is obtained through pre-sintering and sintering under the protection of an inert gas, and then in-situ polymerization coating is carried out to uniformly coat organic monomers on the surface of the material. In the application, the organic monomers are in-situ polymerized on the surface of the prepared sodium pyrophosphate iron phosphate material through electrochemistry, thereby successfully solving the problems of serious side reactions caused by poor structural stability of the material surface and particle breakage caused by internal stress of secondary particles in the charging and discharging process. The prepared material can be applied to the field of sodium ion batteries and can effectively improve the electrochemical performance of the sodium pyrophosphate iron phosphate material.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium ion battery electrode materials, and particularly relates to an in-situ polymerization coated sodium iron pyrophosphate composite material, a preparation method and application thereof. BACKGROUND

[0002] Among many energy storage technologies, electrochemical secondary batteries are the first choice for energy storage systems due to their high conversion efficiency and easy maintenance. In the past 20 years, lithium ion batteries have been widely used as the most mature energy storage devices. However, the uneven distribution and limited storage of lithium resources on earth will seriously restrict the development of lithium ion batteries in the future. Compared with lithium resources, sodium resources are abundant and inexpensive on earth. Sodium ion batteries will surely experience an explosive development and achieve large-scale application.

[0003] In sodium ion batteries, the iron-based mixed polyanion fast ionic conductor sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) material has three Fe occupied sites in the crystal structure, and the typical three-dimensional framework structure ensures that sodium ions can migrate quickly, has high structural stability, and the single electron specific capacity involved in the redox reaction in the material is 43 mAh g -1 , so that the theoretical capacity of the sodium iron pyrophosphate material can reach 129 mAh g -1 .(P2O7) 4− The strong induction of 3.1 V of the average discharge voltage makes the Na4Fe3(PO4)2P2O7 material a promising sodium ion battery anode material. However, the iron-based polyanion Na4Fe3(PO4)2P2O7 material not only has low electronic conductivity, but also generates NaFePO4 and Na2FeP2O7 impurities during the preparation process, which limits the capacity of the material. The obvious voltage decay and poor cycle stability limit its commercial application. SUMMARY

[0004] The problem to be solved by the present application is to solve the defects and deficiencies of the existing sodium iron pyrophosphate anode material, and to provide an in-situ polymerization coated sodium iron pyrophosphate composite material, a preparation method and application thereof. The sodium iron pyrophosphate material is successfully prepared by combining the step-by-step grinding method, drying process and high-temperature solid-phase method, and then the organic monomer is successfully and uniformly coated on the surface of the material by in-situ polymerization, forming a protective layer. The electronic conductivity of the composite material is improved, thereby realizing fast ion transmission, and the coating layer formed after polymerization of the organic monomer further buffers the volume change caused by the Na + extraction process of the electrode. The structural integrity of the material during charging and discharging is ensured, so that the material particles have excellent high-rate performance and cycle stability.

[0005] The application aims to provide a preparation method of sodium pyrophosphate ferric phosphate material with simple synthesis process, environmental friendliness, low cost, good repeatability and practical application potential.

[0006] To solve the technical problems proposed in the application, the application provides a sodium pyrophosphate ferric phosphate composite material coated in situ by polymerization, and the composition of the material is Na x Fe y (PO4) z P2O7(3≤ x ≤4.5、2≤ y ≤3、1≤ z ≤2), the in-situ polymerization coating material is an organic monomer material; the organic monomer includes at least one or more than one mixture of aniline monomer and its derivative and pyrrole monomer and its derivative, and the formed composite material is a conductive polymer (poly-pyrrole, poly-aniline).

[0007] The XRD powder diffraction peak of the sodium pyrophosphate ferric phosphate coated in situ by polymerization mainly includes about 9.7±0.2°, 15.9±0.2°, 16.7±0.2°, 19.4±0.2°, 23.9±0.2°, 25.4±0.2°, 25.8±0.2°, 25.9±0.2°, 27.2±0.2°, 28.8±0.2°, 29.4±0.2°, 29.4±0.2° and 30.2±0.2° in θ°.

[0008] The application further provides a preparation method of the sodium pyrophosphate ferric phosphate composite material coated in situ by polymerization, and the preparation steps are as follows:

[0009] Step one: an electrode sheet containing active material of sodium pyrophosphate ferric phosphate is used as a positive electrode sheet, sodium metal is used as a negative electrode, glass fiber is used as a separator, and a mixed electrolyte formed by mixing sodium battery electrolyte and organic monomer is assembled into a button cell;

[0010] In some specific embodiments, the electrode sheet containing active material of sodium pyrophosphate ferric phosphate is dried in a vacuum drying box for a certain time and then placed in a glove box. The sodium pyrophosphate ferric phosphate electrode sheet is used as a positive electrode, sodium metal is used as a negative electrode, glass fiber is used as a separator, and the button cell is assembled in the order of negative electrode shell, sodium metal, a certain amount of mixed electrolyte (formed by mixing sodium battery electrolyte and organic monomer according to a volume ratio), separator, a certain amount of mixed electrolyte, positive electrode, gasket, spring, and positive electrode shell, and finally sealed by a button sealing machine.

[0011] Step two: after the button cell is cycled for a certain number of times by applying current, the organic monomer in the mixed electrolyte is in-situ polymerized on the surface of the positive electrode sheet of sodium pyrophosphate iron phosphate to form a protective conductive polymer coating layer, thereby obtaining an in-situ polymerization coated sodium pyrophosphate iron phosphate composite material.

[0012] The sodium battery electrolyte in step one is a mixed solution of equal volume ratio of ethylene carbonate / diethyl carbonate / methyl ethyl carbonate containing 0.5-2 M NaPF6; the organic monomer includes at least one of aniline monomer, pyrrole monomer and thiophene monomer or a mixture of one or more thereof.

[0013] Specifically, the sodium battery electrolyte is formed by adding NaPF6 into a mixture formed by mixing ethylene carbonate (EC) / diethyl carbonate (DEC) / methyl ethyl carbonate (EMC) in equal volume ratio.

[0014] The volume ratio of the organic monomer to the sodium battery electrolyte is 1:9~999.

[0015] The current in step two is 0.1~4 C, and the charge and discharge cycle number is 2~10 cycles.

[0016] The preparation steps of the electrode sheet containing sodium pyrophosphate iron phosphate active material are as follows:

[0017] (1) The phosphorus salt, iron source and water are mixed and ground to form slurry A; then the sodium salt, pyrophosphoric acid or pyrophosphate compound, dispersant and carbon source are stirred and mixed with water according to the proportion to form solution B;

[0018] In some embodiments, the grinding method is one of ball milling or sand milling, the grinding time of slurry A is 0.5~6 h, and the stirring time of solution B is 10~60 min.

[0019] (2) Solution B is added to slurry A, and then ground again to obtain precursor solution C, and after drying, a powder-like precursor is obtained;

[0020] In some embodiments, the grinding time of slurry C is 0.5~6 h, and the drying method is one of spray drying, normal pressure drying, freeze drying, infrared drying and microwave drying.

[0021] (3) The obtained powder-like precursor is placed in an atmosphere furnace, and pre-fired at low temperature for a period of time under inert protective atmosphere or reducing atmosphere, and then calcined at high temperature for a period of time to obtain the product sodium pyrophosphate iron phosphate positive electrode material;

[0022] (4) mixing sodium pyrophosphate iron phosphate, conductive agent and polyvinylidene fluoride with N-methyl-2-pyrrolidone to form a uniform active slurry, then using an automatic applicator to apply the active slurry on an aluminum foil current collector, drying in a drying oven for a certain time, then rolling the electrode sheet through an electric motorized roller, and then cutting the obtained electrode sheet into a circular electrode sheet.

[0023] The phosphorus salt includes at least one or more than one mixture of iron phosphate, iron phosphate dihydrate and iron phosphate tetrahydrate;

[0024] The iron source includes at least one or more than one mixture of iron (II) oxalate dihydrate, iron powder, iron oxide, magnetite, iron pyrophosphate, iron sulfate, ferrous sulfate, iron citrate, ammonium iron citrate, iron acetylacetonate, iron (II) acetate, sodium oxalate hydrate, iron ethyl alcohol, sucrose iron and ammonium iron (II) sulfate hexahydrate;

[0025] The sodium salt includes at least one or more than one mixture of sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium acetate, sodium nitrate, sodium oxalate, sodium citrate, sodium carbonate, sodium bicarbonate, sodium pyrophosphate, sodium trihydrogen pyrophosphate, sodium dihydrogen pyrophosphate, sodium methoxide and sodium ascorbate;

[0026] The pyrophosphoric acid or pyrophosphate compound includes at least one or more than one mixture of pyrophosphoric acid, sodium pyrophosphate, iron pyrophosphate, tributylammonium pyrophosphate, sodium trihydrogen pyrophosphate, sodium dihydrogen pyrophosphate, isoprene pyrophosphate triammonium salt, sodium trihydrogen pyrophosphate monohydrate and sodium pyrophosphate decahydrate;

[0027] The dispersant includes at least one or more than one mixture of polyethylene glycol and Triton X-100;

[0028] The carbon source includes at least one or more than one mixture of sucrose, glucose, fructose, oxalic acid, ascorbic acid, starch, lactic acid, citric acid monohydrate and malic acid.

[0029] The temperature rising rate in the low-temperature pre-burning process in step (3) is 1-10 ℃ / min, the pre-burning temperature is 200-400 ℃, and the pre-burning time is 1-5 h; the temperature rising rate in the calcination process is 1-10 ℃ / min, the calcination temperature is 450-600 ℃, and the calcination time is 5-18 h; the inert protective atmosphere or reducing atmosphere includes any one or more than one mixture of high-purity argon, high-purity nitrogen, high-purity hydrogen argon, high-purity helium and high-purity carbon dioxide.

[0030] The mass ratio of sodium pyrophosphate iron phosphate, conductive agent and polyvinylidene fluoride in step (4) is 7+ x :2- x :1 (0≤ xThe conductive agent includes one of acetylene black, conductive carbon black, conductive graphite and ketjen black; the normal-pressure drying temperature is 80-150 DEG C, and the time is 1-4 h.

[0031] Another technical solution of the application is the application of the prepared in-situ polymerization coated sodium iron pyrophosphate composite material in a sodium ion battery, and the structural formula of the in-situ polymerization coated sodium iron pyrophosphate composite material is Na:Fe:P:P2O7, wherein the ratio of Na:Fe:P is 1:3:2, the ratio of Fe:P2O7 is 1:1, and the ratio of Na:P2O7 is 1:1. x : y : z :1(3≤ x ≤4.5、2≤ y ≤3、1≤ z ≤2)。

[0032] Compared with the prior art, the application has at least the following beneficial effects:

[0033] (1) The application adopts an electrochemical in-situ polymerization method to uniformly coat organic monomers on the surface of sodium iron pyrophosphate particles, effectively improves the electronic conductivity between particles by using the excellent conductivity of the conductive polymer, effectively reduces the overpotential in the charging and discharging process, increases the peak current, shortens the Na + transport path, and further improves the electrochemical performance of the material.

[0034] (2) When a specific current is applied, the organic monomer molecules are anchored on the -OH and -COOH functional groups of the sodium iron pyrophosphate electrode (from carbon) through the electrostatic force between atoms, then the oxidation current oxidizes the organic monomer molecules to form monomer radicals. These radicals initiate subsequent oxidation reactions, and the organic monomers form conductive polymers to ensure better electron and ion access to the redox active sites. Since the organic monomers are used as electrolyte additives, and they are electrochemically polymerized inside the battery to form a protective conductive coating, without any additional non-in-situ process. The conductive polymer coating layer on the surface of the particles can greatly reduce the corrosion of the electrolyte on the electrode material, the occurrence of side reactions, and improve the surface stability of the material, which is beneficial to the long cycle life of the material under strong current.

[0035] (3) The "electrochemical in-situ polymerization method" used in the application has the advantages of simple process, low cost and high preparation efficiency, and is suitable for most application scenarios. Compared with wet chemical technology, gas phase chemistry and dry process, it has obvious advantages. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 The X-ray diffraction pattern of Na4Fe3(PO4)2P2O7@1% PANI in Example 1 of the application.

[0037] Figure 2This is a scanning electron microscope image of Na4Fe3(PO4)2P2O7@1%PANI in Example 1 of the present invention.

[0038] Figure 3 This is the first charge-discharge curve of Na4Fe3(PO4)2P2O7@1%PANI in Example 1 of the present invention.

[0039] Figure 4 The constant current cycling performance diagram of Na4Fe3(PO4)2P2O7@1%PANI in Example 1 of this invention is shown.

[0040] Figure 5 This is a rate performance diagram of Na4Fe3(PO4)2P2O7@1%PANI in Example 1 of the present invention.

[0041] Figure 6 This is a scanning electron microscope image of Na3Fe2(PO4)P2O7@2%PPy in Example 2 of the present invention.

[0042] Figure 7 This is a rate performance diagram of Na3Fe2(PO4)P2O7@2%PPy in Example 2 of the present invention.

[0043] Figure 8 This is the X-ray diffraction pattern of Na4Fe3(PO4)2P2O7 in Comparative Example 2 of this invention.

[0044] Figure 9 This is a rate performance diagram of Na4Fe3(PO4)2P2O7 in Comparative Example 2 of the present invention. Detailed Implementation

[0045] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0046] To better understand the present invention, the following description, in conjunction with embodiments and related drawings, further illustrates the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0047] Example 1

[0048] This embodiment provides a method for preparing an in-situ polymerized sodium iron phosphate pyrophosphate composite material, as detailed below:

[0049] Step one: 12.0656 g of FePO4, 7.196 g of FeC2O4 2H2O and 100 mL of deionized water were mixed in a ratio of 4:3:2:1 and sand-milled for 1.5 h to form slurry A. 10.636 g of Na4P2O7, 10 g of polyethylene glycol and 3.0026 g of C6H 12 O6 were stirred with 200 mL of deionized water in a ratio of 1:1:1:1 for 60 min to form solution B;

[0050] Step two: solution B was added to slurry A and sand-milled again for 1 h to obtain precursor solution C. The precursor solution was spray-dried to obtain a powder precursor;

[0051] Step three: the obtained precursor powder was placed in an atmosphere furnace and heated to 300℃ at a rate of 3℃ / min under the protection of high-purity argon, and then heated to 550℃ at a rate of 4℃ / min and kept for 12 h, and then cooled to room temperature. Finally, the product sodium iron pyrophosphate positive electrode material was obtained;

[0052] Step four: the sodium iron pyrophosphate material, the conductive agent and the polyvinylidene fluoride (PVDF) were mixed with N-methyl-2-pyrrolidone (NMP) in a ratio of 7:2:1 by mass to form a uniform active slurry, and then the active slurry was coated on an aluminum foil current collector using an automatic coater. After drying in a drying oven at a temperature of 100℃ for 2 h, the electrode sheet was rolled by an electric roller machine, and then the obtained electrode sheet was cut into a circular electrode sheet, weighed and the corresponding value was recorded;

[0053] Step five: after the sodium iron pyrophosphate electrode sheet was dried in a vacuum drying oven at a temperature of 150℃ for 12 h, it was placed in a glove box. The sodium iron pyrophosphate electrode sheet was used as the positive electrode, sodium metal was used as the negative electrode, glass fiber was used as the separator, and the sodium metal, 150 μL of mixed electrolyte (sodium battery electrolyte was NaPF6 (1M) in a volume ratio of 1:1:1 (v / v / v) ethylene carbonate (EC) / diethyl carbonate (DEC) / ethyl methyl carbonate (EMC). The organic monomer was aniline monomer. The volume ratio of the organic monomer and the sodium battery electrolyte was 1:99), the separator, 150 μL of mixed electrolyte, the positive electrode, the gasket, the spring, and the positive electrode shell were sequentially assembled, and finally a button cell was packaged using a button sealer;

[0054] Step six: the button cell was placed on a new power test instrument, and a discharge capacity of 64.5 mAh g -1The aniline organic monomer material in the mixed electrolyte will be in-situ polymerized on the surface of the sodium pyrophosphate ferric phosphate positive electrode sheet to form a protective conductive polymer PANI coating layer after 5 cycles, and finally obtain Na4Fe3(PO4)2P2O7@1%PANI composite material.

[0055] Figure 1 The XRD pattern of the Na4Fe3(PO4)2P2O7@1%PANI material used in this embodiment; from the figure, it can be seen that the diffraction peaks of the material are fine, narrow and sharp, and the diffraction peak position and relative intensity are compared with the Na4Fe3(PO4)2P2O7 standard card, and it is found that the diffraction peak position and relative intensity of the XRD pattern of the sample are consistent with the phase composition of the Na4Fe3(PO4)2P2O7 material, and the diffraction peak position is 9.794°, 15.949°, 16.74°, 19.496°, 23.951°, 25.417°, 25.823°, 25.981°, 27.279°, 28.838°, 29.409°, 29.494°, 30.286°. It shows that the polyaniline conductive layer formed by the in-situ polymerization of aniline monomer will not affect the phase structure and element composition of the material itself.

[0056] Figure 2 The SEM image of the Na4Fe3(PO4)2P2O7@1%PANI material used in this embodiment; from the figure, it can be seen that the SEM image of the sample shows that the material has good crystallinity, and the particle size diameter is between 0.5-5 μm.

[0057] Figure 3 The first charge-discharge curve of the Na4Fe3(PO4)2P2O7@1%PANI material used in this embodiment at 0.5 C; the specific capacity of the sample during the first discharge is 101.23 mAh g -1 , the first cycle coulombic efficiency is 81.82 %, and the charge-discharge platform conforms to the related characteristics of Na4Fe3(PO4)2P2O7 material. Compared with the Na4Fe3(PO4)2P2O7 material without conductive polymer composite, the capacity and coulombic efficiency are greatly improved.

[0058] Figure 4 The constant current cycle performance graph of the Na4Fe3(PO4)2P2O7@1%PANI material used in this embodiment at a current density of 1 C; the specific capacity of the sample during the first discharge is 100.91 mAh g -1 , and the discharge specific capacity after 50 charge-discharge cycles is 87.62 mAh g -1The capacity retention rate is 86.82%. This is because the conductive polymer coating layer on the surface of the particles can greatly reduce the corrosion of the electrolyte on the electrode material, the occurrence of side reactions, and improve the surface stability of the material.

[0059] Figure 5 Figure for the rate performance of the Na4Fe3(PO4)2P2O7@1% PANI material used in this example. As can be seen from the figure, the first discharge specific capacity of the material at 0.5 C, 1 C, 2 C, 5 C and 10 C is 96.35 mAh g -1 , 87.83 mAh g -1 , 83.75 mAh g -1 , 79.09 mAh g -1 , 71.99 mAh g -1 , respectively, all with high initial discharge specific capacity at different rates.

[0060] In summary, the pyrophosphate sodium iron phosphate composite material Na4Fe3(PO4)2P2O7@1% PANI coated by in-situ polymerization shows higher discharge specific capacity, better cycle life and more stable high-rate performance, indicating that after being compounded with the conductive polymer PANI, it has higher energy density and more excellent electrochemical performance and cycle performance.

[0061] Example 2

[0062] The present example provides a preparation method of an in-situ polymerization coated sodium pyrophosphate iron phosphate composite material, which is as follows:

[0063] Step one: 18.8737 g of FePO4 2H2O, 18.3084 g of C4H6O4Fe and 200 mL of deionized water are mixed in a ratio of 3:2:1:1, and sand grinding is performed for 1 h to form slurry A. 18.7347 g of H4P2O7, 8 g of Triton X-100 and 34.23 g of C 12 H 22 O 11 are stirred and mixed with 400 mL of deionized water for 30 min to form solution B;

[0064] Step two: solution B is added to slurry A, and sand grinding is performed again for 2 h to obtain precursor solution C. The precursor solution is freeze-dried to obtain a powder-shaped precursor;

[0065] Step three: the precursor powder obtained is placed in an atmosphere furnace, and is heated to 350°C at a heating rate of 2°C / min under the protection of high-purity hydrogen argon, and is kept at 350°C for 5h, and then is heated to 500°C at a heating rate of 5°C / min, and is kept at 500°C for 15h, and is then cooled to room temperature, and finally the product sodium iron pyrophosphate positive electrode material is obtained;

[0066] Step four: the sodium iron pyrophosphate material, the conductive agent and polyvinylidene fluoride (PVDF) and N-methyl-2-pyrrolidone (NMP) are mixed in a mass ratio of 8:1:1 to form a uniform active slurry, and then the active slurry is coated on an aluminum foil current collector using an automatic coater. After drying in a drying oven at a temperature of 120°C for 1.5h, the electrode sheet is rolled by an electrically driven roll mill, and then the obtained electrode sheet is cut into a circular electrode sheet, weighed and the corresponding value is recorded;

[0067] Step five: after the sodium iron pyrophosphate electrode sheet is dried in a vacuum drying oven at a temperature of 180°C for 8h, it is placed in a glove box. The sodium iron pyrophosphate electrode sheet is used as a positive electrode, sodium metal is used as a negative electrode, glass fiber is used as a separator, and the negative electrode shell, sodium metal, 200 μL of mixed electrolyte (sodium battery electrolyte is NaPF6 (1M) in a volume ratio of 1:1:1 (v / v / v) ethylene carbonate (EC) / diethyl carbonate (DEC) / ethyl methyl carbonate (EMC). The organic monomer is a pyrrole monomer. The volume ratio of the organic monomer and the sodium battery electrolyte is 1:49), the separator, 200 μL of mixed electrolyte, the positive electrode, the gasket, the spring, and the positive electrode shell are sequentially assembled, and finally a button cell is packaged using a button sealer;

[0068] Step six: the button cell is placed on a Neware tester, a current of 2 C=258mAh g -1 is applied, and after 8 cycles, the pyrrole organic monomer material in the mixed electrolyte will be in-situ polymerized on the surface of the sodium iron pyrophosphate positive electrode sheet to form a protective conductive polymer PPy coating layer, and finally the Na3Fe2(PO4)P2O7@2%PPy composite material is obtained.

[0069] Figure 6 SEM image of the Na3Fe2(PO4)P2O7@2%PPy material used in this embodiment; as can be seen from the figure, the SEM image of the sample shows that the material has good crystallinity, and the particle size diameter is between 0.1~5 μm.

[0070] Figure 7This is a rate performance graph of the Na3Fe2(PO4)P2O7@2%PPy material used in this embodiment. As can be seen from the graph, the initial discharge specific capacity of this material at 0.2 C, 0.5 C, 1 C, 2 C, and 5 C is 101.77 mAh g⁻¹. -1 95.99mAh g -1 91.07 mAh g -1 87.47 mAh g -1 83.99 mAh g -1 It exhibits high initial discharge specific capacity at different discharge rates.

[0071] In summary, the sodium iron pyrophosphate composite material Na3Fe2(PO4)P2O7@2%PPy, after in-situ polymerization coating, exhibits more stable high-rate performance, indicating that the material, after being combined with the conductive polymer PPy, exhibits superior electrochemical performance at high current densities.

[0072] Example 3

[0073] This embodiment provides a method for preparing an in-situ polymerized sodium iron phosphate pyrophosphate composite material. The preparation method is the same as in Embodiment 2, except that in this embodiment, the volume ratio of pyrrole organic monomer to sodium electrolyte in the mixed electrolyte in step five is 1:99. The specific current applied in step six is ​​4 C = 516 mAhg. -1 Finally, a composite Na3Fe2(PO4)P2O7@1%PPy cathode material was obtained.

[0074] Example 4

[0075] This embodiment provides a method for preparing an in-situ polymerized sodium iron phosphate pyrophosphate composite material. The preparation method is the same as in Example 1, except that in this embodiment, the volume ratio of aniline organic monomer to sodium electrolyte in the mixed electrolyte in step five is 1:9. The specific current applied in step six is ​​3C = 387 mAhg. -1 Finally, a composite Na3Fe2(PO4)P2O7@10%PANI cathode material was obtained.

[0076] Comparative Example 1

[0077] Compared with Example 1, the only difference is that the electrolyte used in step five is 1 M NaPF6 (1M) in a volume ratio of 1:1:1 (v / v / v) ethylene carbonate (EC) / diethyl carbonate (DEC) / ethyl methyl carbonate (EMC), instead of a mixed electrolyte with added organic monomers. Other operations and parameters are the same as in Example 1.

[0078] The pure sodium iron pyrophosphate material prepared in the present comparative example 1 was found to have the following electrochemical performance after being tested by the half-cell test: 0.1 C reversible capacity reached 90 mAh g -1 , 1 C reversible capacity was 65 mAh g -1 , the reversible capacity attenuated to 21 mAh g -1 after 50 cycles, and the capacity retention rate was as low as 32.3 %. The lower 1 C reversible capacity of the half-cell of the present comparative example than that of the example 1 was due to the fact that no aniline organic monomer was added to the electrolyte used in step five, and the sodium iron pyrophosphate obtained was not compounded with the conductive polymer, and thus had poor conductivity and structural stability, and the capacity retention rate was lower than that of the battery prepared in example 1.

[0079] Comparative example 2

[0080] The difference between the present example and example 1 was that the current applied in step six was 5 C = 645 mAh g -1 , and the other operations and parameters were the same as those in example 1.

[0081] As shown in Figure 9 , the positive electrode material prepared in the present comparative example 2 was tested for rate performance after being assembled into a half-cell, and the first discharge specific capacity of the material at 0.5 C, 1 C, 2 C, 5 C and 10 C was 86.35 mAh g -1 , 72.83 mAh g -1 , 61.75 mAh g -1 , 45.09 mAh g -1 , and 21.99 mAh g -1 , respectively. Compared with the Na4Fe3(PO4)2P2O7@1% PANI composite material in example 1, the material in the present comparative example 2 had smaller discharge capacity at different current densities and poorer structural stability. This indicated that when the organic monomer was used at a too large current density, the coating layer of the conductive polymer formed by in-situ electrochemical polymerization would be broken, and the particles could not be effectively coated, resulting in a significant decrease in the electrochemical performance of the material.

[0082] In summary, the sodium iron pyrophosphate@conductive polymer composite positive electrode material with excellent electrochemical performance and structural stability can be obtained by the method of the present application.

[0083] The above embodiments are merely examples for clearly illustrating the made instances, but are not a limitation on the embodiments. Based on the above description, other different forms of changes or variations can also be made by those skilled in the art, and here, all the embodiments do not need to be exhausted, and the extended obvious changes or variations are still within the protection scope of the present application.

Claims

1. An in-situ polymerized coated sodium ferric pyrophosphate composite material, characterized in that, The positive electrode material has a composition of Na x Fe y (PO4) z P2O7, 3≤ x ≤4.5, 2≤ y ≤3, 1≤ z ≤2, and the in-situ polymerized coating material is an organic monomer material; the organic monomer includes at least one of an aniline monomer and a pyrrole monomer.

2. The in-situ polymerized coated sodium ferric pyrophosphate composite material according to claim 1, wherein, The XRD powder diffraction peak of the in-situ polymerization coated sodium pyrophosphate ferric phosphate mainly includes peak positions of about 9.7±0.2°, 15.9±0.2°, 16.7±0.2°, 19.4±0.2°, 23.9±0.2°, 25.4±0.2°, 25.8±0.2°, 25.9±0.2°, 27.2±0.2°, 28.8±0.2°, 29.4±0.2°, 29.4±0.2°, and 30.2±0.2°.

3. The method for preparing an in-situ polymerized coated sodium iron phosphate pyrophosphate composite material according to claim 1 or 2, characterized in that, The preparation steps are as follows: Step one: an electrode sheet of active material containing sodium pyrophosphate ferric phosphate is used as a positive electrode sheet, sodium metal is used as a negative electrode, glass fiber is used as a separator, and a mixed electrolyte formed by mixing a sodium battery electrolyte and an organic monomer is assembled into a button cell; Step two: after the button cell is subjected to a certain number of cycles by applying a current, the organic monomer in the mixed electrolyte is in-situ polymerized on the surface of the positive electrode sheet of sodium pyrophosphate ferric phosphate to form a protective conductive polymer coating layer, thereby obtaining an in-situ polymerization coated sodium pyrophosphate ferric phosphate composite material.

4. The method for preparing an in-situ polymerized coated sodium iron phosphate pyrophosphate composite material according to claim 3, characterized in that, The preparation steps of the electrode sheet of active material containing sodium pyrophosphate ferric phosphate are as follows: (1) a phosphorus salt, an iron source, and water are mixed and ground to form slurry A; a sodium salt, pyrophosphoric acid or a pyrophosphate compound, a dispersing agent, and a carbon source are stirred and mixed with water according to a certain proportion to form solution B; (2) solution B is added to slurry A, and then ground again to obtain precursor liquid C; after drying, a powder precursor is obtained; (3) the powder precursor is placed in an atmosphere furnace, and is pre-fired at low temperature for a period of time in an inert protective atmosphere or a reducing atmosphere, and then is calcined at high temperature for a period of time to obtain the product sodium pyrophosphate ferric phosphate positive electrode material; (4) sodium pyrophosphate ferric phosphate, a conductive agent, and polyvinylidene fluoride are mixed with N-methyl-2-pyrrolidone to form a uniform active slurry, and then the active slurry is coated on an aluminum foil current collector using an automatic coater; after drying in a drying oven for a certain period of time, the electrode sheet is rolled by an electric roller machine, and then the obtained electrode sheet is cut into a circular electrode sheet.

5. The preparation method of the in-situ polymerization coated sodium pyrophosphate ferric phosphate composite material according to claim 4, wherein the phosphorus salt comprises at least one of ferric phosphate, dihydrate ferric phosphate, and tetrahydrate ferric phosphate; the iron source comprises at least one of iron (II) oxalate dihydrate, iron powder, iron oxide, magnetite, pyrophosphate iron, iron sulfate, ferrous sulfate, ferric citrate, ferric ammonium citrate, acetylacetone iron, iron (II) acetate, sodium oxalate hydrate, iron ethyl alcohol, sucrose iron, and ammonium iron (II) sulfate hexahydrate; the sodium salt comprises at least one of sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium acetate, sodium nitrate, sodium oxalate, sodium citrate, sodium carbonate, sodium bicarbonate, sodium pyrophosphate, trisodium pyrophosphate monohydrate, disodium pyrophosphate dihydrate, sodium methoxide, and sodium ascorbate; ​ The pyrophosphoric acid or pyrophosphate compound includes at least one of pyrophosphoric acid, sodium pyrophosphate, iron pyrophosphate, tributylammonium pyrophosphate, trisodium monohydrogen pyrophosphate, disodium dihydrogen pyrophosphate, isoprenyl pyrophosphate triammonium salt, trisodium monohydrogen pyrophosphate monohydrate and sodium pyrophosphate decahydrate; The dispersant includes at least one of polyethylene glycol and Triton X-100; The carbon source includes at least one of sucrose, glucose, fructose, oxalic acid, ascorbic acid, starch, lactic acid, citric acid monohydrate and malic acid.

6. A process for the preparation of in-situ polymerized coated sodium ferric pyrophosphate composite material as claimed in claim 4, wherein the process is characterized by, The temperature increasing rate in the low-temperature pre-burning process in the step (3) is 1-10 ℃ / min, the pre-burning temperature is 200-400 ℃, and the pre-burning time is 1-5 h; the temperature increasing rate in the calcination process is 1-10 ℃ / min, the calcination temperature is 450-600 ℃, and the calcination time is 5-18 h; the inert protective atmosphere or reducing atmosphere includes any one of high-purity argon, high-purity nitrogen, high-purity hydrogen argon, high-purity helium and high-purity carbon dioxide.

7. A process for the preparation of in-situ polymerized coated sodium ferric pyrophosphate composite material as claimed in claim 4, wherein the process is characterized by, The mass ratio of sodium pyrophosphate, conductive agent and polyvinylidene fluoride in the step (4) is 7+ x :2- x :1 (0≤ x <2), the conductive agent includes one of acetylene black, conductive carbon black, conductive graphite and ketjen black; the normal pressure drying temperature is 80~150℃, and the time is 1~4 h.

8. A process for the preparation of in-situ polymerized coated sodium ferric pyrophosphate composite material as claimed in claim 4, wherein the process is characterized by, The sodium battery electrolyte in the step one is a mixed solution of ethylene carbonate / diethyl carbonate / methyl ethyl carbonate with a volume ratio of 1:9-999, containing 0.5-2M NaPF6; the organic monomer includes at least one of aniline monomer, pyrrole monomer and thiophene monomer.

9. A process for the preparation of in-situ polymerized coated sodium ferric pyrophosphate composite material as claimed in claim 4, wherein, The current in the step two is 0.1-4 C, and the charge-discharge cycle number is 2-10 cycles.

10. Use of an in-situ polymerized coated sodium pyrophosphate ferric phosphate composite material prepared according to the method of any one of claims 3 to 9, having a structure of the ratio Na:Fe:P:P207 of 1 < x < 4.5, 2 < y < 3, 1 < z < 2. x : y : z : x : y : z :

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