Lithium manganese iron phosphate positive electrode material and preparation method thereof
The preparation of a three-dimensional porous sponge-like manganese-iron composite oxide precursor by solution combustion solves the problems of uniform mixing of manganese and iron and construction of porous structure in lithium manganese iron phosphate materials, improves the conductivity and cycle stability of the material, and realizes an efficient and environmentally friendly preparation process.
Patent Information
- Application Number
- CN202610541854.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-04-23
- Publication Date
- 2026-05-19
- Estimated Expiration
- 2046-04-23
AI Technical Summary
Existing methods for preparing lithium manganese iron phosphate are difficult to achieve atomic-level uniform mixing of manganese and iron, resulting in poor conductivity and poor cycle stability of the material. In addition, traditional methods have problems such as high environmental pressure and impurity phase formation.
A three-dimensional porous sponge-like manganese-iron composite oxide precursor was prepared by solution combustion. A porous framework was formed through a self-propagating reaction. The precursor was then subjected to ultrasonic vacuum infiltration, ultrafast freezing and freeze-drying techniques, followed by gentle sintering under a protective atmosphere to avoid the formation of impurity phases.
Atomic-level uniform mixing of manganese and iron elements was achieved, constructing a porous structure conducive to ion transport, which improved the rate performance and cycle stability of the material. The process is simple and environmentally friendly, making it suitable for industrial applications.
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Figure CN122059448A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of lithium-ion battery technology, specifically to a lithium manganese iron phosphate cathode material and its preparation method. Background Technology
[0002] Lithium manganese iron phosphate (LMP) is considered a promising next-generation cathode material for lithium-ion batteries due to its combination of the high safety of lithium iron phosphate and the high voltage characteristics of manganese-based materials. However, LMP has poor conductivity, and it is difficult to achieve atomically uniform mixing of iron and manganese elements in the crystal lattice. Uneven distribution leads to localized stress concentration and manganese dissolution, severely affecting the material's rate performance and cycle stability.
[0003] Existing methods for preparing lithium manganese iron phosphate mainly suffer from the following problems: High-temperature solid-phase method: Relies on mechanical mixing, making it difficult to overcome the thermodynamic barrier and achieve uniform solid solution of manganese iron at the atomic scale. Liquid-phase co-precipitation method: Although it can improve the uniformity of element mixing, the process is complex, generating large amounts of ammonia-nitrogen-containing wastewater, posing a significant environmental burden. Furthermore, the resulting precursor particles are typically dense, hindering lithium-ion diffusion and reaction during subsequent sintering. One-step liquid-phase combustion method: Direct combustion synthesis in a phosphorus-containing source system easily triggers side reactions, generating impurities such as pyrophosphate, and the product morphology is difficult to control.
[0004] Patent document CN120793878 A discloses a technical solution involving the in-situ growth of a manganese iron phosphate precursor within the cavity of a pre-prepared polyethylene glycol-sucrose hybrid aerogel porous template. Essentially, this is a template method, where the porous morphology of the precursor replicates the structure of the template. The process is relatively complex and requires additional template preparation and removal.
[0005] CN119240633 A and another published document on a one-step combustion method suggest that these technical solutions tend to involve burning or treating phosphorus-containing sources (such as phosphates) together with other reactants. However, phosphorus-containing compounds are chemically unstable at the extreme high temperatures of combustion (typically well above 1000°C) and readily undergo dehydration polymerization to generate electrochemically inert pyrophosphates (P₂O₇). 4- Impurities such as impurities can severely contaminate the final product and impair electrochemical performance.
[0006] Therefore, developing a precursor preparation technology that is simple, green, and environmentally friendly, and can ensure atomic-level uniform mixing of manganese and iron while constructing a porous microstructure that is conducive to ion transport, is key to obtaining high-performance lithium manganese iron phosphate cathode materials. Summary of the Invention
[0007] The purpose of this invention is to provide a lithium manganese iron phosphate cathode material and its preparation method, solving the technical problem in the prior art of how to provide a precursor that can both ensure uniform mixing of manganese and iron at the atomic level and construct a porous microstructure that is conducive to ion transport.
[0008] This invention discloses a method for preparing a manganese-iron composite oxide precursor. A manganese source, an iron source, and an organic fuel are dissolved in a solvent and mixed evenly to obtain a mixed solution. A self-propagating reaction is initiated by a solution combustion synthesis method. After the reaction is completed, a manganese-iron composite oxide precursor with a three-dimensional porous sponge-like microstructure is obtained.
[0009] Working principle: During the self-propagating combustion reaction, a large amount of gas is released instantaneously, thereby creating pores in situ inside the manganese-iron composite oxide precursor, forming a loose and continuous porous framework structure.
[0010] Furthermore, the three-dimensional porous sponge-like microstructure has a non-agglomerated three-dimensional continuous open-cell foam or sponge-like micromorphology, and the pore size distribution of the three-dimensional porous sponge-like microstructure is 0.2~2 micrometers, with the chemical formula Mn. x Fe 3- x O4, where 0 < x < 3.
[0011] Furthermore, the molar ratio of the manganese source to the iron source is Mn:Fe = 1:4 ~ 4:1, and more preferably 2:1.
[0012] Furthermore, the organic fuel is selected from at least one of glycine, urea, or citric acid; The anions of the manganese source and the iron source are oxidants. In the mixed solution, the stoichiometric ratio of organic fuel to oxidant is controlled between 1.2 and 2.4, preferably 1.8.
[0013] To regulate combustion temperature and gas release, thereby controlling the porosity of the precursor.
[0014] Furthermore, the oxidant is nitrate, the iron source is ferric nitrate, and the manganese source is manganese nitrate.
[0015] Furthermore, the manganese source and the iron source are uniformly mixed at the atomic scale; The self-propagating reaction is initiated at a heating temperature of 300-550°C, with a reaction time of less than 1 minute, followed by rapid cooling at a rate of 80-100°C per minute.
[0016] Rapid cooling locks in the uniform distribution of manganese and iron elements in the oxide lattice.
[0017] A manganese-iron composite oxide precursor was prepared using the method described above.
[0018] A method for preparing lithium manganese iron phosphate cathode material involves filling and compositing the manganese iron composite oxide precursor, followed by sintering to obtain the cathode material.
[0019] Furthermore, the pore filling and composite process specifically involves mixing a lithium source, a phosphorus source, and a carbon source with the manganese-iron composite oxide precursor in a liquid medium, so that the lithium source, phosphorus source, and carbon source penetrate and fill the pore network and surface of the manganese-iron composite oxide precursor framework, and then drying to obtain the composite precursor.
[0020] Furthermore, the mixing with the manganese-iron composite oxide precursor is achieved through ultrasonic vacuum-assisted infiltration, ultra-fast freezing, and freeze-drying; The lithium source is selected from at least one of lithium carbonate, lithium hydroxide, or lithium dihydrogen phosphate. The phosphorus source is selected from at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or phosphoric acid. The carbon source is selected from at least one of glucose, sucrose, citric acid, polyethylene glycol, or ascorbic acid.
[0021] Furthermore, the ultrasonic vacuum-assisted infiltration specifically involves pumping the pressure inside the vacuum impregnation equipment to a negative pressure state of -0.08 MPa to -0.1 MPa and maintaining it for 5-30 minutes to completely remove residual air from the precursor skeleton; then releasing the negative pressure and using atmospheric pressure to force the impregnation liquid into the pores.
[0022] This vacuum-to-normal-pressure process is repeated 2-5 times. Simultaneously, throughout the permeation process, ultrasonic dispersion is continuously applied to break up soft aggregates between particles, ensuring the homogeneity of the suspension.
[0023] Furthermore, the impregnation solution contains lithium, phosphorus, and a carbon source.
[0024] Furthermore, the ultra-fast freezing is spray freezing.
[0025] Furthermore, the spray freezing is atomized at a flow rate of 10-100 mL / min and then sprayed into a low-temperature medium, the temperature of which is -196℃ to -150℃.
[0026] Furthermore, the cryogenic medium is liquid nitrogen to ensure that the droplets freeze completely upon contact with the cryogenic medium (within <1 second).
[0027] Furthermore, the freeze-drying is carried out under conditions where the cold trap temperature is below -50°C (preferably -80°C to -60°C) and the system vacuum degree is better than 10 Pa.
[0028] Furthermore, the sample temperature is maintained below the freezing point (0°C) throughout the freeze-drying process, and the drying time is 12-48 hours.
[0029] Furthermore, the sintering specifically involves calcining the composite precursor under a protective atmosphere to obtain lithium manganese iron phosphate cathode material.
[0030] Furthermore, the calcination temperature is 600~850℃, and the holding time is 4~12 hours; the protective atmosphere is argon, nitrogen, or an argon-hydrogen mixture.
[0031] Furthermore, its general chemical formula is LiMn x Fe 1-x PO4 (0 < x < 1), carbon layers and active substances are uniformly coated on the surface and pores of olivine structure particles transformed from an in-situ constructed oxide framework. Compared with the prior art, the beneficial effects of the present invention are: 1. The core advantage of this invention lies in the direct and in-situ construction of a three-dimensional porous, sponge-like precursor framework, without relying on any external template. This self-generated porous structure endows the precursor with a huge specific surface area and an interconnected network of pores; 2. This invention utilizes the large amount of gas instantaneously released during solution combustion reaction as an endogenous pore-forming agent, enabling the precursor to simultaneously construct a porous framework during its own formation. This method not only has a shorter process flow and lower cost, but also produces a porous framework that is a crystallization of the reactants themselves, resulting in a purer and more stable structure. 3. This invention also discovers that by filling a pre-constructed porous framework with lithium, phosphorus, and carbon sources in subsequent processes to form a composite precursor, the mass transfer pathway of the final solid-state reaction can be greatly improved. Lithium and phosphorus species are pre-delivered to the reaction sites, achieving nanoscale contact with manganese and iron elements on the framework. This shortens the solid-state diffusion distance during subsequent sintering from the traditional micrometer level to the nanometer level, resulting in an order-of-magnitude improvement in reaction kinetics, thus laying a solid foundation for the excellent rate performance of the final cathode material. 4. This invention ingeniously decomposes the reaction process into a two-step strategy of first constructing a phosphorus-free oxide framework and then introducing a phosphorus source for gentle sintering, effectively avoiding the formation of impurity phases; 5. In this invention, a pure oxide framework is first prepared, and then lithium, phosphorus, and carbon sources are introduced into the framework through ultrasonic vacuum-assisted infiltration, ultra-fast freezing, and freeze-drying. Finally, the reaction is carried out at a sintering temperature (650-800℃) that is far below the combustion temperature. This stepwise, temperature-controlled strategy fundamentally avoids the thermodynamic conditions for the formation of pyrophosphate impurities, ensuring the high purity and uniformity of the final product. 6. This invention achieves atomic-level uniform mixing of Fe / Mn elements through the instantaneous reaction of solution combustion, effectively suppressing the Jan Taylor effect and ensuring the long-term cycle stability of the material. At the same time, the entire process is fast and efficient, does not use ammonia water, and has no ammonia nitrogen wastewater discharge, which fully meets the requirements of green chemical industry and has significant industrial application value. Attached Figure Description
[0032] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.
[0033] Figure 1 The X-ray diffraction pattern of the porous manganese-iron composite oxide precursor obtained in Example 1 of the present invention is shown.
[0034] Figure 2 This is a scanning electron microscope image of the porous manganese-iron composite oxide precursor obtained in Example 1 of the present invention.
[0035] Figure 3 This is a comparison of the X-ray diffraction patterns of lithium manganese iron phosphate obtained in Example 1 of the present invention and that obtained in Comparative Example 1.
[0036] Figure 4 The image shows a comparison of scanning electron microscope images of lithium manganese iron phosphate obtained in Example 1 and lithium manganese iron phosphate obtained in Comparative Example 1.
[0037] Figure 5 This is a comparison chart of the 10C electrochemical performance of lithium manganese iron phosphate obtained in Example 1 of the present invention and lithium manganese iron phosphate obtained in Comparative Example 1. Detailed Implementation
[0038] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.
[0039] Example 1 This embodiment discloses a lithium manganese iron phosphate cathode material and its preparation method, including the following steps: 1. Manganese nitrate and ferric nitrate are dissolved in deionized water according to stoichiometric ratio, wherein the molar ratio of Mn to Fe is 2:1.
[0040] 2. Add glycine as an organic fuel and complexing agent, controlling the stoichiometric ratio of glycine to the oxidant (nitrate) in the metal nitrate to be 1.8. Stir until completely dissolved to form a homogeneous mixed solution.
[0041] 3. The mixed solution is placed on a heating device and heated to approximately 350°C. As the water evaporates, the solution turns into a viscous gel. Subsequently, the gel instantly initiates a self-propagating combustion reaction, releasing a large amount of gas accompanied by intense exothermic reactions. The reaction is completed within one minute, with a cooling rate of 100°C per minute.
[0042] 4. Collect the fluffy powder after combustion, which is the pure-phase manganese-iron composite oxide precursor with a porous, sponge-like morphology (see attached image). Figure 1 Appendix Figure 2 ).
[0043] 5. Weigh lithium dihydrogen phosphate and glucose according to the stoichiometric ratio (Li:Mn:Fe:P = 1:0.67:0.33:1). First, completely dissolve lithium dihydrogen phosphate and glucose in deionized water to form a transparent impregnation solution. Then, add the porous FeMn2O4 precursor obtained in step 4 above to the impregnation solution and place it in a vacuum impregnation device. First, evacuate the device to a negative pressure of -0.1 MPa and maintain it for 30 minutes to remove residual air from the precursor skeleton. Then, release the negative pressure and use atmospheric pressure to force the impregnation liquid into the pores. Repeat this vacuum-restoration process 5 times. At the same time, throughout the entire permeation process, continuously apply ultrasonic dispersion to break up soft agglomerates between particles and ensure the homogeneity of the suspension.
[0044] 6. The fully permeated suspension was atomized through an atomizing nozzle at a flow rate of 100 mL / min and directly sprayed into liquid nitrogen maintained at -196℃. This ensured that the droplets were completely frozen upon contact with the cryogenic medium (within <1 second). The solid powder obtained after ultrafast freezing was rapidly transferred to a freeze dryer for sublimation drying under conditions of a cold trap temperature below -60℃ and a system vacuum degree better than 10 Pa. The sample temperature was maintained below the freezing point (0℃) throughout the drying process, and the drying time was 48 hours, yielding the composite precursor powder.
[0045] 7. Place the composite precursor powder in a tube furnace and heat it to 700°C at a rate of 5°C / min under the protection of an argon / hydrogen (95% / 5%) mixed atmosphere, and hold it at that temperature for 8 hours.
[0046] 8. Allow to cool naturally to room temperature to obtain carbon-coated lithium manganese iron phosphate cathode material.
[0047] Example 2 The only difference from Example 1 is that the heating temperature in step 3 is adjusted to 500°C. The remaining steps and parameters are the same as in Example 1, and carbon-coated lithium manganese iron phosphate cathode material is finally obtained.
[0048] Example 3 The only difference from Example 1 is that the calcination temperature in step 7 is adjusted to 800°C. The remaining steps and parameters are the same as in Example 1, and carbon-coated lithium manganese iron phosphate cathode material is finally obtained.
[0049] Example 4 The only difference from Example 1 is that the molar ratio of Mn to Fe in step 1 is adjusted to 1:1. The remaining steps and parameters are the same as in Example 1, and carbon-coated lithium manganese iron phosphate cathode material is finally obtained.
[0050] Comparative Example 1 1. Mix manganese carbonate, ferrous oxalate, lithium carbonate, ammonium dihydrogen phosphate (and glucose in stoichiometric ratio) (Li:Mn:Fe:P = 1:0.67:0.33:1) directly.
[0051] 2. Add ethanol and ball mill for 6 hours, then dry.
[0052] 3. Calcination was carried out under the same conditions as in Example 1.
[0053] 4. Preparation of cathode material (see attached) Figure 3 Appendix Figure 4 ).
[0054] Comparative Example 2 This comparative example uses commercially available lithium manganese iron phosphate material, Kejing WMP3B.
[0055] Comparative Example 3 1. Dissolve manganese nitrate, ferric nitrate, lithium dihydrogen phosphate, and glucose simultaneously in deionized water at a stoichiometric ratio (Li:Mn:Fe:P = 1:0.67:0.33:1).
[0056] 2. Add glycine in the same proportion as in Example 1, stir well, and then heat directly to 350°C to initiate a self-propagating combustion reaction.
[0057] 3. Collect the powder after combustion and calcine it at 700°C for 8 hours under an argon-hydrogen mixed atmosphere. The positive electrode material is obtained.
[0058] Comparative Example 4 The only difference between this comparative example and Example 1 is that in step 2, the stoichiometric ratio of glycine to nitrate is adjusted to 3.0.
[0059] Comparative Example 5 The only difference between this comparative example and Example 1 is that the precursor, lithium dihydrogen phosphate, and glucose were added to deionized water at the same time, stirred and soaked for 2 hours, and then directly placed in an 80°C vacuum oven to heat and evaporate the water to obtain composite precursor powder.
[0060] Comparative Example 6 The only difference between this comparative example and Example 1 is that in the freezing step of step 6, liquid nitrogen spray freezing is not used for instant freezing. Instead, the impregnated suspension is directly placed in a -30°C low-temperature freezer for conventional slow freezing, followed by freeze drying.
[0061] Performance testing The lithium manganese iron phosphate obtained in Examples 1-4 and Comparative Examples 1-2 were used as positive electrode materials for 2032 button batteries. The specific method is as follows: The prepared positive electrode material, conductive agent (Super P), and binder (PVDF) were mixed uniformly at a mass ratio of 8:1:1. A slurry was prepared using N-methylpyrrolidone (NMP) as a solvent and uniformly coated onto an aluminum foil current collector. After vacuum drying, it was punched into circular electrode sheets with a diameter of 12 mm. The areal density of the active material was approximately 2.0~3.0 mg / cm³. 2 A lithium metal sheet was used as the negative electrode, a Celgard 2400 polypropylene microporous membrane as the separator, and 1.0 M LiPF6 dissolved in a mixed solvent of EC:DMC:EMC = 1:1:1 (volume ratio) as the electrolyte. A CR2032 coin cell was assembled in an argon-filled glove box.
[0062] The lithium-ion battery was subjected to constant current charge-discharge cycle testing on a battery testing system. The test conditions were as follows: charge / discharge rates of 0.1C and 10C, voltage range of 2.0V-4.25V, and the discharge specific capacity and capacity retention after 1600 cycles were recorded. The test results are shown in the table below. Table 1
[0063] As shown in Table 1, the lithium manganese iron phosphate prepared in Examples 1-4 have high discharge specific capacity and capacity retention rate.
[0064] In Comparative Example 1, the precursors prepared without this method exhibited poor discharge specific capacity and capacity retention.
[0065] Comparative Example 2 used commercially available lithium manganese iron phosphate material, and its discharge specific capacity and capacity retention were relatively poor.
[0066] In Comparative Example 3, only iron and manganese sources were added. The combustion synthesis reaction initiated by organic fuel resulted in poor discharge specific capacity and capacity retention.
[0067] In Comparative Example 4, the stoichiometric ratio of glycine to nitrate was too high, resulting in poor discharge specific capacity and capacity retention.
[0068] In Comparative Example 5, which did not use ultrasonic vacuum-assisted permeation, instantaneous freezing, and freeze-drying steps, the discharge specific capacity and capacity retention were relatively poor.
[0069] In Comparative Example 6, which did not employ instantaneous freezing via liquid nitrogen spray, the discharge specific capacity and capacity retention were both relatively poor.
[0070] from Figures 1-5 It can be seen that Example 1 of the present invention successfully prepared a pure, highly active spinel porous precursor ( Figure 1 , Figure 2 The final product perfectly inherits this three-dimensional porous framework and exhibits extremely high crystallinity and is free of impurities. Figure 3 , Figure 4 This unique microporous structure, combined with an extremely short ion diffusion path, fundamentally solves the technical pain points of poor conductivity and high polarization in traditional lithium manganese iron phosphate materials. This results in Example 1 exhibiting a superior initial discharge specific capacity of 160.1 mAh / g at 0.1C and significantly better high-rate (10C) discharge performance and long cycle life (89.2% capacity retention after 1600 cycles). Figure 5 ).
[0071] The above are the embodiments listed in this example. However, this example is not limited to the optional embodiments described above. Those skilled in the art can arbitrarily combine the above methods to obtain other various embodiments. Anyone can derive other various forms of embodiments based on the inspiration of this example. The above specific embodiments should not be construed as limiting the scope of protection of this example. The scope of protection of this example should be determined by the claims, and the specification can be used to interpret the claims.
Claims
1. A method for preparing a manganese-iron composite oxide precursor, characterized in that: Manganese source, iron source and organic fuel are dissolved in solvent and mixed evenly to obtain a mixed solution. A self-propagating reaction is initiated by solution combustion synthesis method. After the reaction is completed, a manganese-iron composite oxide precursor with a three-dimensional porous sponge-like microstructure is obtained.
2. The method for preparing a manganese-iron composite oxide precursor according to claim 1, characterized in that: The three-dimensional porous sponge-like microstructure is a non-agglomerated, three-dimensional continuous open-cell foam or sponge-like micromorphology. The pore size distribution of the three-dimensional porous sponge-like microstructure is 0.2~2 micrometers, and its general chemical formula is Mn. x Fe 3-x O4, where 0 < x < 3.
3. The method for preparing a manganese-iron composite oxide precursor according to claim 1, characterized in that: The molar ratio of the manganese source to the iron source is Mn:Fe = 1:4~4:
1.
4. The method for preparing a manganese-iron composite oxide precursor according to claim 1, characterized in that: The organic fuel is selected from at least one of glycine, urea, or citric acid; The anions of the manganese source and the iron source are oxidants, and the stoichiometric ratio of organic fuel to oxidant in the mixed solution is controlled between 1.2 and 2.
4.
5. The method for preparing a manganese-iron composite oxide precursor according to claim 4, characterized in that: The oxidant is nitrate, the iron source is ferric nitrate, and the manganese source is manganese nitrate.
6. A manganese-iron composite oxide precursor, characterized in that: It was prepared using the method for preparing a manganese-iron composite oxide precursor according to any one of claims 1-5.
7. A method for preparing a lithium manganese iron phosphate cathode material, characterized in that: The precursor prepared by the method of preparing a manganese-iron composite oxide precursor according to any one of claims 1-5 or the manganese-iron composite oxide precursor according to claim 6 is subjected to pore filling and composite, and then sintered to obtain the final product.
8. The method for preparing a lithium manganese iron phosphate cathode material according to claim 7, characterized in that: The pore filling and composite process specifically involves mixing a lithium source, a phosphorus source, and a carbon source with the manganese-iron composite oxide precursor in a liquid medium, allowing the lithium source, phosphorus source, and carbon source to penetrate and fill the pore network and surface of the manganese-iron composite oxide precursor framework, and then drying to obtain the composite precursor.
9. The method for preparing a lithium manganese iron phosphate cathode material according to claim 8, characterized in that: The mixing with the manganese-iron composite oxide precursor was achieved through ultrasonic vacuum-assisted infiltration, ultra-fast freezing, and freeze-drying. And / or the lithium source is selected from at least one of lithium carbonate, lithium hydroxide or lithium dihydrogen phosphate; And / or the phosphorus source is selected from at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or phosphoric acid; And / or the carbon source is selected from at least one of glucose, sucrose, citric acid, polyethylene glycol or ascorbic acid; And / or the sintering specifically involves calcining the composite precursor under a protective atmosphere to obtain lithium manganese iron phosphate cathode material.
10. The method for preparing a lithium manganese iron phosphate cathode material according to claim 9, characterized in that: The ultrasonic vacuum-assisted infiltration process involves drawing the pressure inside the vacuum impregnation equipment to a negative pressure of -0.08 MPa to -0.1 MPa and maintaining it for 5-30 minutes to completely remove residual air from the precursor skeleton; then releasing the negative pressure and using atmospheric pressure to force the impregnation fluid into the pores. And / or the ultrafast freezing is spray freezing; The freeze-drying process is carried out under conditions where the cold trap temperature is below -50°C and the system vacuum degree is better than 10 Pa.