Highly conductive three-dimensional carbon nanofilament modified composite phosphate material and preparation method thereof

Abstract

The application discloses a kind of high-conductivity three-dimensional carbon nanofilament modified composite phosphate materials and preparation method thereof, the material is Na4 (Fe x Mn y )3M z (PO4)2P2O7 / C material, wherein, 0.93≤x+y≤1, and 0≤x≤1, 0≤y≤1, 0≤z≤0.2;Its preparation method includes the following steps: 1) iron manganese transition metal source, doping ion source M and reducing agent are dissolved in water;2) adding sodium source and phosphorus source;3) dropwise adding ammonia water;4) adding polar aprotic solvent, obtain the precursor suspension of submicron level crystal nucleus formation and carry out drying;4) the dried product is placed in protective atmosphere and is sequentially calcined, to obtain the material, and the sodium ion battery positive active material has excellent sodium storage performance.The preparation method of the application can significantly reduce the production energy consumption, and the reaction process has no environmental pollution, and is suitable for industrialization batch production.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of nanomaterials and electrochemistry, and relates to a high-conductivity three-dimensional carbon nanofilament modified composite phosphate material and a preparation method thereof. x Mn y )3M z (PO4)2P2O7 / C material and a preparation method thereof, which can be used as a sodium ion battery positive electrode active material. BACKGROUND

[0002] The manganese iron-based sodium pyrophosphate material has three-dimensional channels for Na + insertion / desorption in the material crystal lattice, has high sodium ion diffusion coefficient, high structural stability, high working voltage and high thermal stability, and has outstanding advantages, and is considered to be one of the most potential sodium ion battery positive electrode materials. However, the electronic conductivity of the phosphate material is low, and researchers have explored many methods to try to solve this key technical problem. At present, the commonly used technical means in the industry include surface carbon coating, nanocrystallization, heteroatom doping and the like. However, the surface carbon coating of the material by the traditional method has technical problems such as "discontinuous conductive network", "uneven carbon layer thickness", "carbon layer segregation", "uneven distribution of doped atoms", and it is difficult to greatly improve the electronic conductivity of the material. As far as we know, it is difficult to directly prepare a "high-conductivity three-dimensional carbon nanofilament" structure in the manganese iron sodium pyrophosphate material by using conventional methods. In order to realize high electronic conductivity of the material, it is necessary to add exogenous conductive carbon fibers (such as carbon nanotubes) to improve the electronic conductivity of the surface interface. However, due to the high price of carbon nanotubes and other carbon materials, it is difficult to disperse them well, which not only increases the material preparation cost, but also increases the complexity of the preparation process, and the increase of the carbon coating amount also causes the problem of a sharp decrease in the material compaction density, which seriously affects the material processing performance. "Nanocrystallization" can improve the problem of large electrochemical polarization, but this process needs to mechanically grind the micron-sized iron phosphate raw material particles through a superfine nanometer grinding device for a long time to reduce the particle size to the order of hundreds of nanometers. This process is time-consuming and energy-consuming. In summary, the traditional preparation method faces the dilemma of complex process and high cost, and manufacturing enterprises need to invest a lot of manpower and material resources to control the process, so as to obtain a material with good comprehensive performance.

[0003] CN 118495495 A discloses a sodium iron pyrophosphate positive electrode material; it is synthesized by a simple sol-gel method, and oxalic acid is used as a chelating agent to construct a sodium iron pyrophosphate (Na4Fe3(PO4)2(P2O7)) / carbon composite material with a three-dimensional conductive carbon network. However, the material prepared has a relatively loose structure, and the gap between the particles is large, which will lead to poor volume energy density; at the same time, the pore structure of the material is large and the distribution is very irregular, which will also lead to the problems of electrolyte distribution uniformity, ion transmission efficiency, electronic conduction performance and structural stability of the material. SUMMARY

[0004] The present application aims to provide a composite phosphate material (Na4(Fe x Mn y )3M z (PO4)2P2O7 / C) with high-conductivity three-dimensional carbon nanofilament modification and a preparation method thereof. The method is simple, can realize the effect of "two birds with one stone" through "one-pot", meets the requirements of green chemistry and is convenient for scale-up production, and the material used as a sodium ion battery positive electrode material can exhibit excellent electrochemical sodium storage performance. Specifically, the "two birds with one stone" method proposed in the present application can introduce high-conductivity three-dimensional carbon nanofilaments in situ through ordinary carbon sources during the reaction process, and the doping ions M introduced in the solution can be uniformly dispersed at the atomic level in the crystal lattice. This method is a technical approach that can effectively improve the overall conductivity of the material. On the one hand, the high-conductivity three-dimensional carbon nanofilaments can construct abundant transmission channels for electron and ion migration between the core and the surface of the material, which has the same effect as adding expensive high-conductivity carbon nanotubes. On the other hand, the three-dimensional carbon structure between Na4(Fe x Mn y )3M z (PO4)2P2O7 particles can play a "net binding" effect, effectively limiting the excessive growth and agglomeration of primary crystal particles during high-temperature calcination, and realizing "nanocrystallization" without high-energy mechanical grinding. When applied to sodium ion battery positive electrode materials, the three-dimensional structure "cushion net" can effectively prevent the structure from being damaged due to volume change during sodium ion insertion / extraction, effectively improving the cycle life of the electrode material. At the same time, the high-conductivity three-dimensional carbon nanofilaments can effectively optimize the ion and electron diffusion path, significantly improve the diffusion speed in the electrode, realize the full and complete development of the electrochemical performance of the active material, and make Na4(Fe x Mn y )3M zThe PO4)2P2O7 / C material is an excellent positive electrode active material, and promotes the commercialization of high-performance, long-life and low-cost sodium ion batteries.

[0005] To achieve the above-mentioned purpose, the application specifically provides the following technical solutions:

[0006] <First aspect>

[0007] The application provides a Na4(Fe x Mn y )3M z (PO4)2P2O7 / C material (the nanomaterial is a heteroatom-doped manganese-iron-based sodium pyrophosphate (Na4(Fe x Mn y )3M z (PO4)2P2O7) and carbon (C) composite), and a significant feature of the material structure is that the Na4(Fe x Mn y )3M z (PO4)2P2O7 primary particles are coated with a (uniform) nanocarbon layer, and are highly interconnected with each other through (tight) three-dimensional continuous (highly conductive) carbon nanofilaments. Furthermore, the Na4(Fe x Mn y )3M z (PO4)2P2O7 primary crystal particles are (tightly) wrapped in a conductive network constructed by abundant three-dimensional carbon nanofilaments. The material has excellent electronic and ionic conductivity, and can be used as a high-performance sodium ion battery positive electrode active material.

[0008] As an embodiment of the application, the mass ratio of Na4(Fe x Mn y )3M z (PO4)2P2O7 and C in the material is 95:5-99:1.

[0009] As an embodiment of the application, 0.93≤x+y≤1, 0≤x≤1, 0≤y≤1, and 0≤z≤0.2.

[0010] As an embodiment of the application, M is a transition metal doping ion.

[0011] As an embodiment of the application, M is Mg 2+ , Ni 2+ , Al 3+ , V 3+ , W 3+ , Ga 3+ , La 3+ , Y 3+ , Yb3+ Sn 4+ Zr 4+ Nb 5+ Mo 6+ at least one of the ions.

[0012] As an embodiment of the present application, the Na4(Fe x Mn y )3M z (PO4)2P2O7 primary crystal particles have a particle size of 0.2-0.5 microns.

[0013] As an embodiment of the present application, the material is prepared by mixing a raw sodium source compound, an iron source compound, a phosphorus source compound and a reducing agent uniformly, and then crystallizing, shaping and calcining.

[0014] <Second aspect>

[0015] The present application provides a preparation method of a high-conductivity three-dimensional carbon nanofilament modified composite phosphate material, the material being Na4(Fe x Mn y )3M z (PO4)2P2O7 / C material, wherein 0.93≤x+y≤1, 0≤x≤1, 0≤y≤1, 0≤z≤0.2, and M is a transition metal doping ion; the method comprising the following steps:

[0016] S1, stirring a transition metal source and a reducing agent in water until completely dissolved; the transition metal source including at least one of a manganese source, an iron source and an M source;

[0017] S2, adding a sodium source and a phosphorus source again, and stirring until completely dissolved to obtain a mixed solution A;

[0018] S3, adding ammonia water dropwise to the mixed solution A to control the pH of the solution to rise to a state close to a hydrogen phosphate (M z Fe 3x Mn 3y )·(HPO4)4·nH2O precipitation reaction occurs, to obtain a metastable precursor solution B of a sodium ion and a transition metal phosphate compound mixture;

[0019] S4, adding a polar aprotic solvent dropwise to the precursor solution B to form submicron crystal nuclei containing Na4[(M z Fe 3x Mn 3y )·(HPO4)4·nH2O] components in the solution to obtain a suspension C;

[0020] S5, drying the suspension C to obtain a precursor powder; calcining the precursor powder under an inert atmosphere (nitrogen, argon, etc.) to obtain a Na4(Fe x Mn y )3M z (PO4)2P2O7 / C material.

[0021] As an embodiment of the present application, the manganese source is at least one of water-soluble organic manganese salt, water-soluble inorganic manganese salt. In some embodiments, the manganese source includes at least one of citric acid chelated manganese, sodium ethylenediaminetetraacetate manganese, manganese nitrate, manganese sulfate, manganese acetate.

[0022] As an embodiment of the present application, the iron source is at least one of water-soluble organic iron salt and water-soluble inorganic iron salt. In some embodiments, the iron source includes at least one of ferric citrate, sodium ethylenediaminetetraacetate iron, ferrous citrate, ferric ammonium citrate, ferrous nitrate, ferric sulfate, ferrous sulfate, ferric nitrate, ferrous ammonium sulfate, ferric acetate.

[0023] As an embodiment of the present application, the M source is at least one of water-soluble inorganic salt, organic compound containing Mg 2+ , Ni 2+ , Al 3+ , V 3+ , W 3+ , Ga 3+ , La 3+ , Y 3 + , Yb 3+ , Sn 4+ , Zr 4+ , Nb 5+ , Mo 6+ ion.

[0024] As an embodiment of the present application, the reducing agent is an organic compound with low carbonization temperature and reducing chemical groups. In some embodiments, the reducing agent includes at least one of glucose, fructose, lactose, ascorbic acid, citric acid.

[0025] As an embodiment of the present application, the phosphorus source includes at least one of phosphoric acid, phosphorous acid, ammonium dihydrogen phosphate, sodium dihydrogen phosphate.

[0026] As an embodiment of the present application, the sodium source includes at least one of sodium carbonate, sodium bicarbonate, sodium citrate, sodium oxalate, sodium acetate, sodium tripolyphosphate, sodium tetrapolyphosphate, sodium pyrophosphate, sodium hexametaphosphate, sodium amino-tris-methylene phosphonate, sodium diethylene triamine penta-methylene phosphonate, sodium hydroxy-ethylidene diphosphonate, and sodium nitrate.

[0027] As an embodiment of the present application, the polar aprotic organic solvent is miscible with water. In some embodiments, the polar aprotic organic solvent is at least one of dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), acetone (C3H6O), isopropyl ketone, acetonitrile (C2H3N), pyridine (C5H5N), ethylene glycol, ethanol.

[0028] As an embodiment of the present application, in step S1, the molar ratio of the transition metal element to the reducing agent is optimized to achieve sufficient reduction of the transition metal element. In some embodiments, the molar ratio of the transition metal element to the reducing agent is 1:0.2-1:5.

[0029] As an embodiment of the present application, in step S2, the molar ratio of the phosphorus source to the transition metal source is optimized to achieve a dihydrogen phosphate salt (Fe x Mn y M z (H2PO4)2) solution. In some embodiments, the molar ratio of the manganese-iron transition metal source to the phosphorus source is 3:4. The molar ratio of the manganese-iron transition metal source to the sodium source is 3:4.

[0030] As an embodiment of the present application, in step S3, the ammonia water is either concentrated ammonia water (25-28%) or diluted ammonia water.

[0031] As an embodiment of the present application, in step S3, the pH of the solution is increased to 2-5. During this process, the hydroxide ions (OH-) dissociated from the ammonia water neutralize one hydrogen ion (H+) in the dihydrogen phosphate salt in the solution, making the solution state close to the monobasic phosphate salt (M z Fe 3x Mn 3y )·(HPO4)4·nH2O) precipitation reaction occurs, obtaining a metastable precursor solution B in which sodium ions and transition metal phosphate compounds are uniformly mixed.

[0032] In step S4, the solubility of the hydrated ions of inorganic salts in the aqueous solution is reduced by the miscibility of the polar aprotic solvent with water, and the precipitation reaction equilibrium is shifted to form submicron crystal nuclei containing (Na4[(M z Fe 3x Mn 3y )·(HPO4)4·nH2O]) components in the solution.

[0033] As an embodiment of the present application, in step S4, the volume ratio of the polar aprotic solvent to the precursor solution B is 1:1-10:1

[0034] As an embodiment of the present application, in step S5, the drying method is any one of hot air drying, spray drying, drum drying, vacuum drying, infrared drying or a combination thereof.

[0035] As an embodiment of the present application, in step S5, the organic solvent vapor is recycled after being recovered during the drying process.

[0036] As an embodiment of the present application, in step S5, the calcination temperature is 500-700℃; the calcination time is 2-20 hours.

[0037] <Third aspect>

[0038] The aforementioned high-conductivity three-dimensional carbon nanofilament modified composite phosphate material, or the high-conductivity three-dimensional carbon nanofilament modified composite phosphate material prepared by the aforementioned method, in the application as a positive active material of a sodium ion battery also belongs to the protection scope of the present application.

[0039] Compared with the prior art, the present application has the following beneficial effects:

[0040] (1) The Na4(Fe x Mn y )3M z (PO4)2P2O7 / C material with complex high-conductivity three-dimensional carbon nanofilament modification, which is difficult to achieve by traditional methods, can be prepared by a simple and easy solution method combined with a solid-phase sintering method.

[0041] (2) The raw materials can be completely dissolved in water, ensuring uniform mixing of various ion sources at the molecular level, and the prepared material has good consistency.

[0042] (3) The limited structure of the "solvent cage" is constructed after the organic solvent is mixed with water, and the addition of the organic solvent can rapidly reduce the solubility of the inorganic salt as a driving force, so that the sodium ions, transition metal ions and phosphate ions in the water solution can be precipitated in the nanoscale "solvent cage" limited space to form submicron crystals with stoichiometric composition, and the particle distribution is tight, the particle size is uniform, and the bulk density is high, which can effectively solve the pain points such as high energy consumption and long time caused by the "nanometerization" of raw materials through ultra-fine mechanical grinding in traditional methods.

[0043] (4) Through the "microscopic phase separation" effect after the organic solvent is mixed with water, the high-conductivity three-dimensional continuous carbon nanofilament network can be obtained by carbonization of the ordinary reducing agent carbon source in the solution after drying, without introducing expensive carbon nanotubes and other exogenous conductive carbon, and relying on its unique three-dimensional carbon nanofilament network, the overall electronic conductivity of the material can be greatly improved.

[0044] (5) When used as a positive electrode active material for sodium-ion batteries, it has ultra-high rate performance and ultra-long cycle life, and its comprehensive sodium storage performance is outstanding. This method is easy to scale up, has no pollutant emissions, and has high atomic utilization efficiency. It conforms to the characteristics of green chemistry and is conducive to market promotion.

[0045] (6) The present invention shows obvious advantages in terms of microstructural density, pore structure and carbon filament structure of the material, which are helpful to improve the performance of sodium ion batteries, including energy density, rate performance and cycle stability. Specifically, it is manifested as follows: a. Structural density: The electron microscope image of the material of the present invention ( Figure 2 ) The positive electrode material exhibited has a high structural density, with clear boundaries between particles and dense arrangement of particles, which is a beneficial result of the special effect of organic solvent and water. This dense structure helps to improve the volume energy density of the material, because the dense arrangement of particles can reduce the voids inside the material. b. Pore structure: The material prepared by the present invention shows a pore structure with high uniformity, and the pores are moderate in size and evenly distributed, which is a beneficial result of the special effect of organic solvent and water. This pore structure is conducive to the penetration of electrolyte and the transport of ions, thereby improving the rate performance and cycle stability of the battery. C. Carbon filament structure: An obvious carbon nanofilament structure similar to carbon nanotubes can be seen inside the material prepared by the present invention. These carbon filaments have two significant characteristics: A. The diameter is at the nanometer level and the length is at the submicron level. B. The number is very rich and the degree of cross-linking is very high, showing a true three-dimensional network structure feature, which can provide sufficient transmission paths for electron conduction inside the particles, significantly improving the electronic conductivity of the material, and the presence of the carbon filament network may also enhance the structural stability of the material and reduce volume expansion during the cycle. Other carbon nanowire structures that do not meet the two characteristics AB will not be as good as the present invention in terms of electronic conductivity and structural stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Other features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0047] Figure 1 The highly conductive three-dimensional carbon nanowire modified Na4(Fe 2.85 Mn 0.1 Mg 0.05 XRD spectrum of )(PO4)2P2O7 / C material;

[0048] Figure 2 The highly conductive three-dimensional carbon nanowire modified Na4(Fe 2.85 Mn 0.1 Mg 0.05SEM images of )(PO4)2P2O7 / C material; the upper left image is a 1k-fold magnification, the upper right image is a 10k-fold magnification, the lower left image is a 40k-fold magnification of the particle's internal cross-section, and the lower right image is a 160k-fold magnification of the particle's internal cross-section, illustrating the existence of a three-dimensional continuous conductive carbon nanowire network and primary particles therein;

[0049] Figure 3 The highly conductive three-dimensional carbon nanowire modified Na4(Fe 2.85 Mn 0.1 Mg 0.05 )(PO4)2P2O7 / C material charge and discharge characteristic curves at different rates; the black, red, green, blue, cyan, and pink curves represent the test results at 0.2C, 1C, 2C, 5C, 10C, and 20C rates, respectively;

[0050] Figure 4 The highly conductive three-dimensional carbon nanowire modified Na4(Fe 2.85 Mn 0.1 Mg 0.05 The charge-discharge characteristic curves of the )(PO4)2P2O7 / C material at the 1C rate at the 1st and 1000th cycles;

[0051] Figure 5 The highly conductive three-dimensional carbon nanowires modified Na4(Mn 1.8 Fe 0.96 Nb 0.04 XRD spectrum of )(PO4)2P2O7 / C material;

[0052] Figure 6 The highly conductive three-dimensional carbon nanowires modified Na4(Mn 1.8 Fe 0.96 Nb 0.04 SEM image of )(PO4)2P2O7 / C material;

[0053] Figure 7 The highly conductive three-dimensional carbon nanowires modified Na4(Mn 1.8 Fe 0.96 Nb 0.04 Charge and discharge characteristic curve of )(PO4)2P2O7 / C material at 0.1C rate;

[0054] Figure 8 The comparative example is the Na4(Mn 1.8 Fe 0.96 Nb 0.04 SEM image of )(PO4)2P2O7 / C material. DETAILED DESCRIPTION

[0055] The present invention will be described in detail below with reference to the examples. The following examples will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that those skilled in the art may make several adjustments and improvements without departing from the scope of the present invention. These all fall within the scope of protection of the present invention.

[0056] Example 1

[0057] This embodiment provides a highly conductive three-dimensional carbon nanowire modified Na4 (Fe 2.85 Mn 0.1 Mg 0.05 The preparation method of (PO4)2P2O7 / C material comprises the following steps:

[0058] 1) 187g glucose (C6H 12 06) and 5g of citric acid as a reducing agent were dissolved in 2L of deionized water. After complete dissolution, 834.03g of ferrous sulfate heptahydrate, 16.90g of manganese sulfate monohydrate, and 12.3g of magnesium sulfate heptahydrate were added and mixed for 10 minutes to obtain a clear, light green solution.

[0059] 2) Measure 273 mL of 85% phosphoric acid (H3PO4) solution and add the phosphoric acid dropwise to the light green solution obtained in step 1) to adjust the molar ratio of iron source, manganese source and phosphorus source to 3:4, and stir evenly;

[0060] 3) Weigh 328.14 g of sodium acetate and dissolve it in the solution obtained in step 2), at which point the pH of the solution is 1.8;

[0061] 4) adding aqueous ammonia diluted to 10% by mass to the solution dropwise under stirring until the pH of the solution reaches 4.1, thereby obtaining a precursor solution thermodynamically close to the boundary of the precipitation reaction;

[0062] 5) Add 6 L of polar aprotic solvent dimethyl sulfoxide (DMSO) dropwise to the precursor solution and continue stirring for 15 minutes to obtain a suspension containing structurally stable submicron-sized precursor crystals;

[0063] 5) drying and granulating the precursor microcrystal suspension through a closed-cycle spray dryer to obtain a brown precursor powder;

[0064] 6) The precursor powder was heated to 600° C. in a nitrogen atmosphere and kept warm for 10 hours to obtain the highly conductive three-dimensional carbon nanowire-modified Na4(Fe 2.85 Mn 0.1 Mg 0.05(PO4)2P2O7 / C material.

[0065] The product obtained in the present example was subjected to powder X-ray diffraction spectrum analysis (see Figure 1 ), and the results showed that the product obtained was consistent with Na4(Fe 2.85 Mn 0.1 Mg 0.05 )(PO4)2P2O7 / C standard sample PDF card (89-0579), and had high crystallinity and no impurities. Figure 2 The scanning electron microscope image of the product obtained in the present example showed that the product had good structural consistency, and the Na4(Fe 2.85 Mn 0.1 Mg 0.05 )(PO4)2P2O7 primary crystal particles were tightly packed and were highly efficiently interconnected through a three-dimensional conductive carbon nanofilament network, which could effectively enhance the electronic conductivity of the material. The material prepared was used to prepare a positive electrode sheet according to an active material, conductive carbon and polyvinylidene fluoride (PVDF) mass ratio of 8:1:1, with metallic sodium as the negative electrode, glass fiber as the separator, and an electrolyte of 1 mol / L sodium hexafluorophosphate, a solvent volume ratio of 3:2 of propylene carbonate and ethyl methyl carbonate electrolyte (NaPF6 / PC+EMC), to assemble a half-cell for charge-discharge testing. The test conditions were constant current charge-discharge testing, and the voltage interval was 2.0-4.3 V. Figure 3 The high-conductivity three-dimensional carbon nanofilament modified Na4(Fe 2.85 Mn 0.1 Mg 0.05 )(PO4)2P2O7 / C electrode material obtained in the present example was subjected to charge-discharge curve testing at different current densities. At current densities of 0.2, 1, 2, 5, 10 and 20 C, the reversible discharge specific capacity of the electrode material could reach 105, 104, 104, 103, 102 and 98.5 mAh g -1 , respectively, and at a large current density of 20 C (i.e. 3 minutes for one charge) the specific capacity could still be maintained at nearly 94% relative to the current density of 0.2 C, indicating that the material had excellent large-rate fast charge-discharge performance. As can be seen from the charge-discharge curve, the voltage polarization of the material was small and the voltage platform was well maintained, indicating that the material had excellent electronic conductivity. Figure 4 It was shown that at a current density of 1 C, the capacity retention rate was close to 100% after 1000 cycles, and the voltage platform was almost not attenuated, indicating that it had very outstanding structural stability and low resistance interface. The above results showed that, due to the unique structure of the sub-micron crystal particles coupled with the high-conductivity three-dimensional carbon nanofilament network constructed by the new preparation method, the phosphoric acid pyrophosphate sodium Na4(Fe 2.85 Mn 0.1 Mg0.05 The (PO4)2P2O7 / C material has excellent rate, capacity performance and cycle life.

[0066] Example 2

[0067] This embodiment provides a high-conductivity three-dimensional carbon nanofilament modified Na4(Mn 1.8 Fe 0.96 Nb 0.04 The preparation method of the (PO4)2P2O7 / C material comprises the following steps:

[0068] 1) Dissolve 104 g of fructose and 10 g of ascorbic acid as a reducing agent in 2 L of deionized water, and after complete dissolution, add 700.43 g of manganese ethylenediaminetetraacetate sodium, 266.93 g of ferrous sulfate heptahydrate, and 21.5216 g of niobium oxalate, mix and stir for 30 minutes to obtain a clear and transparent light greenish-yellow solution;

[0069] 2) Measure 252 mL of a 85% mass fraction phosphoric acid (H3PO4) solution, and drop the phosphoric acid into the light greenish-yellow solution obtained in step 1) drop by drop, so that the molar ratio of the transition metal source and the phosphorus source is 3:4, and stir uniformly;

[0070] 3) Weigh 38.70 g of sodium aminotri(methylphosphine) acid and dissolve it into the solution obtained in step 2), at which time the pH value of the solution is 1.9;

[0071] 4) Under stirring, add ammonia water diluted to 15% mass fraction drop by drop into the solution until the pH value of the solution rises to 4.3, to obtain a precursor solution that is thermodynamically adjacent to the precipitation reaction occurrence boundary;

[0072] 5) Add 5 L of a polar aprotic solvent N,N-dimethylformamide (DMF) dropwise to the precursor solution, continue stirring for 30 min after the dropwise addition is completed, to obtain a suspension containing nano-sized precursor microcrystals with stable structure;

[0073] 5) Dry the precursor microcrystal suspension through a closed air blowing dryer to obtain a brownish precursor powder;

[0074] 6) Heat the precursor powder to 650℃ under an argon atmosphere and keep it for 10 hours to obtain the high-conductivity three-dimensional carbon nanofilament modified Na4(Mn 1.8 Fe 0.96 Nb 0.04 )(PO4)2P2O7 / C material.

[0075] The product obtained in this embodiment is analyzed by powder X-ray diffraction spectrum (see Figure 5 ), and it can be seen that the material has good crystallinity and phase purity. Figure 6The scanning electron microscope image of the product obtained in this example shows that although the regular spherical particles are not prepared by spray drying, Na4(Mn 1.8 Fe 0.96 Nb 0.04 The P2O7 / C primary crystal particles are closely packed, and the microstructure retains the characteristics of a three-dimensional network structure with abundant pores and compactness, providing good permeability and mass transfer performance for the electrolyte and active material contact, effectively ensuring the electronic conductivity of the material, and fully exerting the electrochemical performance of the material. The charge-discharge curve of the button-type sodium ion half-cell assembled with the positive active material at 0.1C is shown in Figure 7 The discharge specific capacity is 102mAh / g, and the energy density is as high as 360Wh / kg, and the capacity retention rate is greater than 95% after 200 cycles.

[0076] Example 3

[0077] This example provides a preparation method of a high-conductivity three-dimensional carbon nanofilament modified Na4(Mn 1.8 Fe 0.96 V 0.07 )(PO4)2P2O7 / C material, comprising the following steps:

[0078] 1) Dissolve 93g of glucose and 10g of citric acid as a reducing agent in 2L of deionized water, and then add 700.43g of manganese ethylenediaminetetraacetate, 352.37g of iron ethylenediaminetetraacetate, and 8.18g of ammonium metavanadate, and mix and stir for 30 minutes to obtain a clear and transparent light yellow solution;

[0079] 2) Add 38.99g of phosphorous acid (H3PO3) to the light yellow solution obtained in step 1), and stir to dissolve;

[0080] 3) Weigh 40.90g of sodium citrate and 440.56g of hydroxyethylidene diphosphonic acid disodium and dissolve them into the solution obtained in step 2), at which time the pH value of the solution is 1.7;

[0081] 4) Under stirring, ammonia water diluted to 10% by mass is added dropwise to the solution until the pH value of the solution rises to 4.3, to obtain a precursor solution in which the precipitation reaction occurs at the boundary of the thermodynamic proximity;

[0082] 5) Add 4.5L of polar aprotic solvent N,N-dimethylformamide (DMF) dropwise to the precursor solution, and continue stirring for 30min after the addition is completed, to obtain a suspension containing nano-sized precursor microcrystals with stable structure;

[0083] 5) The precursor microcrystal suspension is dried by a closed air blowing dryer to obtain a yellow-green precursor powder;

[0084] 6) The precursor powder is heated to 650°C under an argon atmosphere for 10 hours to obtain the high-conductivity three-dimensional carbon nanofilament modified Na4(Mn 1.8 Fe 0.96 V 0.07 )(PO4)2P2O7 / C material.

[0085] The product obtained in this example is analyzed by powder X-ray diffraction pattern, and it is found that the material has good crystallinity and phase purity. The morphology of the product is analyzed by scanning electron microscopy, and it is found that the microstructure also maintains the characteristics of a three-dimensional network structure with abundant pores and compactness, and the primary crystal particles are tightly packed. A coin-type sodium ion half-battery assembled using the product as the positive active material is tested for charge and discharge at a rate of 0.1C, and the discharge specific capacity is 103.6 mAh / g, the energy density is as high as 362 Wh / kg, and the capacity retention rate is maintained at more than 99% after 100 cycles.

[0086] Comparative Example 1

[0087] This comparative example is Na4(Mn 1.8 Fe 0.96 Nb 0.04 )(PO4)2P2O7 / C nanomaterial prepared without using a polar aprotic solvent, as follows:

[0088] 1) 104 g of fructose and 10 g of ascorbic acid as a reducing agent are dissolved in 2 L of deionized water, and after complete dissolution, 700.43 g of manganese ethylenediaminetetraacetate sodium, 266.93 g of ferrous sulfate heptahydrate, and 21.5216 g of niobium oxalate are added, and the mixture is stirred for 30 minutes to obtain a clear and transparent light greenish-yellow solution;

[0089] 2) 252 mL of a 85% mass fraction phosphoric acid (H3PO4) solution is measured, and the phosphoric acid is added dropwise to the light greenish-yellow solution obtained in step 1) to make the molar ratio of transition metal source to phosphorus source 3:4, and the solution is stirred uniformly;

[0090] 3) 38.70 g of sodium aminotri(methylphosphine) acid is weighed and dissolved in the solution obtained in step 2), and at this time the pH value of the solution is 1.9;

[0091] 4) Under stirring, ammonia water diluted to 15% mass fraction is added dropwise to the solution until the pH value of the solution rises to 4.3 to obtain a precursor solution that is thermodynamically adjacent to the precipitation reaction boundary;

[0092] 5) The precursor solution is dried by a closed air blowing dryer to obtain a brownish precursor hard block;

[0093] 6) After the precursor hard block was ground into powder in a mortar, the temperature was raised to 650°C and kept for 10 hours under a nitrogen atmosphere to obtain Na4(Mn 1.8 Fe 0.96 Nb 0.04 )(PO4)2P2O7 / C material.

[0094] Figure 8 The scanning electron microscope image of the product obtained in the comparative example shows that its morphology and structure have changed significantly compared to the material prepared in Example 2. Due to the lack of the "solvent cage" effect between the aprotic polar solvent and the aqueous solution, the growth process of the primary crystal particles lacks effective restriction, resulting in a significant increase in the particles and an irregular geometric structure in the crystal morphology. On the other hand, due to the lack of the "microphase separation" effect of the mixed solvent, the reducing agent does not form a rich three-dimensional conductive carbon network around the primary crystal particles after carbonization. Some carbon exists in the form of separate segregation. This structure causes the material to lack good electronic and ionic conductivity. The button-type sodium ion half-cell assembled with it as the positive active material has a discharge capacity of only 82mAh / g at 0.1C, and the capacity retention rate after 100 cycles of energy-dense cycling is only 65%.

[0095] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various variations or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A highly conductive three-dimensional carbon nanowire-modified composite phosphate material, characterized in that: The material is Na4(Fe x Mn y )3M z (PO4)2P2O7 / C material, wherein 0.93≤x+y≤1, and 0≤x≤1, 0≤y≤1, 0≤z≤0.2; M is a transition metal doping ion; Na4(Fe x Mn y )3M z The surface of the (PO4)2P2O7 primary particles is coated with a nanocarbon layer, and the primary particles are connected to other adjacent primary particles through three-dimensional carbon nanofilaments; The composite phosphate material is prepared by a method comprising the following steps: S1. Add water to a transition metal source and a reducing agent and stir until completely dissolved; the transition metal source comprises at least one of a manganese source, an iron source, and an M source; S2, then add the sodium source and phosphorus source, stirring until completely dissolved to obtain mixed solution A; S3, add ammonia water to the mixed solution A, and adjust the pH of the solution to a state close to that of hydrogen phosphate (M z Fe 3x Mn 3y )·(HPO4)4·nH2O precipitation reaction occurs to obtain a metastable precursor solution B mixed with sodium ions and transition metal phosphate compounds; S4, add polar aprotic solvent to the precursor solution B, and form a solution containing Na4[(M z Fe 3x Mn 3y )·(HPO4)4·nH2O] component to obtain suspension C; S5, drying the suspension C to obtain a precursor powder; calcining the precursor powder under an inert atmosphere to obtain Na4(Fe x Mn y )3M z (PO4)2P2O7 / C material.

2. The composite phosphate material according to claim 1, characterized in that It includes at least one of the following technical features: A1, Na4(Fe x Mn y )3M z The mass ratio of (PO4)2P2O7 and C is 95:5-99:1; A2, M is Mg 2+ 、Ni 2+ 、Al 3+ 、V 3+ 、W 3+ 、Ga 3+ 、La 3+ 、Y 3+ 、Yb 3+ 、Sn 4+ 、Zr 4+ 、Nb 5+ 、Mo 6+ At least one of the ions; A3, the Na4 (Fe x Mn y )3M z The particle size of the primary crystal particles of (PO4)2P2O7 is 0.2-0.5 microns.

3. A method for preparing a highly conductive three-dimensional carbon nanowire-modified composite phosphate material, characterized in that: The material is Na4(Fe x Mn y )3M z (PO4)2P2O7 / C material, wherein 0.93≤x+y≤1, and 0≤x≤1, 0≤y≤1, 0≤z≤0.2, M is a transition metal doping ion; Na4(Fe x Mn y )3M z The surface of the (PO4)2P2O7 primary particles is coated with a nanocarbon layer, and the primary particles are connected to other adjacent primary particles through three-dimensional carbon nanowires. The method comprises the following steps: S1. Add water to a transition metal source and a reducing agent and stir until completely dissolved; the transition metal source comprises at least one of a manganese source, an iron source, and an M source; S2, then add the sodium source and phosphorus source, stirring until completely dissolved to obtain mixed solution A; S3, add ammonia water to the mixed solution A, and adjust the pH of the solution to a state close to that of hydrogen phosphate (M z Fe 3x Mn 3y )·(HPO4)4·nH2O precipitation reaction occurs to obtain a metastable precursor solution B mixed with sodium ions and transition metal phosphate compounds; S4, add polar aprotic solvent to the precursor solution B, and form a solution containing Na4[(M z Fe 3x Mn 3y )·(HPO4)4·nH2O] component to obtain suspension C; S5, drying the suspension C to obtain a precursor powder; calcining the precursor powder under an inert atmosphere to obtain Na4(Fe x Mn y )3M z (PO4)2P2O7 / C material.

4. The method for preparing the composite phosphate material according to claim 3, characterized in that: Include at least one of the following technical features: B1. The manganese source is at least one of a water-soluble organic manganese salt and an inorganic manganese salt; B2, the manganese source includes at least one of citric acid chelated manganese, sodium manganese EDTA, manganese nitrate, manganese sulfate, and manganese acetate; B3, the iron source is at least one of a water-soluble organic iron salt and an inorganic iron salt; B4, the iron source includes at least one of ferric citrate, sodium ferric EDTA, ferrous citrate, ammonium ferric citrate, ferrous nitrate, ferrous sulfate, ferrous sulfate, ferric nitrate, ferrous ammonium sulfate, and ferric acetate; B5, the M source contains Mg 2+ 、Ni 2+ 、Al 3+ 、V 3+ 、W 3+ 、Ga 3+ 、La 3+ 、Y 3+ 、Yb 3+ 、Sn 4+ 、Zr 4+ 、Nb 5+ 、Mo 6+ At least one of an ionic, water-soluble inorganic salt or organic compound.

5. The method for preparing the composite phosphate material according to claim 3, wherein: Include at least one of the following technical features: C1. The reducing agent is an organic compound with a low carbonization temperature and a reducing chemical group; C2. The reducing agent comprises at least one of glucose, fructose, lactose, ascorbic acid, and citric acid; C3, the phosphorus source is at least one of phosphoric acid, phosphorous acid, ammonium dihydrogen phosphate, and sodium dihydrogen phosphate; C4, the sodium source includes at least one of sodium carbonate, sodium bicarbonate, sodium citrate, sodium oxalate, sodium acetate, sodium tripolyphosphate, sodium tetrapolyphosphate, sodium pyrophosphate, sodium hexametaphosphate, sodium aminotri(methylene)phosphonate, sodium diethylenetriamine penta(methylene)phosphonate, sodium hydroxyethylidene diphosphonate and sodium nitrate; C5, the polar aprotic organic solvent is miscible with water; C6. The polar aprotic organic solvent is at least one of dimethyl sulfoxide, N,N-dimethylformamide, acetone, isopropyl ketone, acetonitrile, pyridine, ethylene glycol, and ethanol.

6. The method for preparing the composite phosphate material according to claim 3, characterized in that: In step S1 , the molar ratio of the transition metal element to the reducing agent is based on achieving sufficient reduction of the transition metal element.

7. The method for preparing the composite phosphate material according to claim 3, characterized in that: In step S2, the molar ratio of the phosphorus source to the transition metal source is such as to obtain a dihydrogen phosphate (Fe x Mn y M z (H2PO4)2) solution shall prevail.

8. The method for preparing the composite phosphate material according to claim 3, characterized in that: In step S4, the volume ratio of the polar aprotic solvent to the precursor solution B is 1:1-10:1; and / or, in step S1, the molar ratio of the transition metal element to the reducing agent is 1:0.2-1:

5.

9. The method for preparing the composite phosphate material according to claim 3, characterized in that: In step S5, the calcination temperature is 500-700° C., and the calcination time is 2-20 hours.

10. Use of the composite phosphate material according to claim 1 or 2, or the composite phosphate material prepared by the method according to any one of claims 3 to 9, as a positive electrode active material for a sodium ion battery.

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