Modified iron phosphate material, method for preparing same, and use thereof
By modifying lithium iron phosphate materials through nitrogen doping and iron phosphide coating, a three-dimensional petal-like structure is formed, which solves the problems of conductivity and diffusion rate of lithium iron phosphate materials and improves the electrochemical performance and material stability of lithium batteries.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- GUANGDONG BRUNP RECYCLING TECH CO LTD
- Filing Date
- 2024-04-07
- Publication Date
- 2026-07-10
AI Technical Summary
Existing lithium iron phosphate materials suffer from poor intrinsic conductivity and slow lithium-ion diffusion rate, resulting in incomplete capacity release, low energy density, and poor cycle performance in lithium batteries. Existing modification methods have problems such as reduced material capacity, reduced compaction density, or increased side reactions.
Modified iron phosphate material with nitrogen doping and iron phosphide coating is used to form a three-dimensional petal-like structure, which shortens the lithium-ion diffusion path and increases the contact area between the electrode material and the electrolyte. The three-dimensional petal-like structure is formed through a combination of self-assembly, water bath reaction and calcination steps. Nitrogen doping adjusts the electronic structure, and the iron phosphide coating improves conductivity and alleviates volume expansion.
It improves the electrochemical performance of lithium-ion batteries, enhances the lithiation/delithiation process, reduces side reactions between active particles and electrolyte, and improves the overall electrochemical performance of cathode materials.
Smart Images
Figure CN118183663B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of battery materials technology, specifically relating to a modified iron phosphate material, its preparation method, and its application. Background Technology
[0002] Lithium-ion batteries are currently the most widely used energy storage devices, and improving battery energy density is one of the main directions of lithium battery development at this stage. Among them, lithium iron phosphate (LiFePO4) cathode materials suffer from poor intrinsic conductivity and slow lithium-ion diffusion rate due to their own structural limitations. This results in incomplete capacity release, low energy density, and poor cycle performance in lithium batteries made from them.
[0003] Iron phosphate (FePO4) is an important precursor for the preparation of lithium iron phosphate (LiFePO4). Its morphology, structure, and performance parameters are carried over to LiFePO4 cathode materials and affect their electrical performance. To improve these issues, iron phosphate is often modified by doping, reducing particle size, and carbon coating. While these methods can improve the conductivity and rate performance of LiFePO4 materials to some extent, they also have the following drawbacks: excessive ion doping leads to a decrease in material capacity and specific energy; excessively nano-sized particles reduce the compaction density of the material, thus affecting the energy density of the lithium battery; and amorphous carbon is prone to uneven coating and may generate byproducts during the coating process, which is detrimental to improving conductivity and reduces the energy density of the battery.
[0004] Therefore, developing an iron phosphate precursor with high conductivity and good lithium-ion diffusion is of great significance for improving the overall electrochemical performance of lithium iron phosphate cathode materials. Summary of the Invention
[0005] To address the shortcomings of existing technologies, the present invention aims to provide a modified iron phosphate material, its preparation method, and its applications. The modified iron phosphate material prepared by this invention possesses a unique three-dimensional petal-like structure, which not only shortens the lithium-ion diffusion path but also increases the contact area between the electrode material and the electrolyte, promoting the lithiation / delithiation process. Furthermore, nitrogen doping and the iron phosphate coating improve the material's conductivity, and this coating also alleviates the volume expansion of the cathode material during charge and discharge, reducing side reactions between active particles and the electrolyte. Together with the three-dimensional petal-like structure, these factors enhance the overall electrochemical performance of the cathode material.
[0006] To achieve this objective, the present invention employs the following technical solution:
[0007] In a first aspect, the present invention provides a modified iron phosphate material, wherein the modified iron phosphate material has a core-shell structure, the core being nitrogen-doped iron phosphate and the outer shell being an iron phosphate coating layer;
[0008] The modified iron phosphate material has a three-dimensional petal-like structure.
[0009] The modified iron phosphate material prepared by this invention possesses a unique three-dimensional petal-like structure, which provides ample mass transfer channels for lithium ion storage and migration, shortening the lithium ion diffusion path. Simultaneously, this structure increases the contact area between the electrode material and the electrolyte, promoting the lithiation / delithiation process. Furthermore, nitrogen doping effectively modulates the electronic structure of the iron phosphate material, thereby improving its conductivity. The iron phosphate coating layer exhibits excellent conductivity, reducing the amorphous carbon content of the cathode material and thus increasing its compaction density. Moreover, this coating layer alleviates the volume expansion of the cathode material during charge and discharge, reducing side reactions between active particles and the electrolyte. Together with the three-dimensional petal-like structure, these features enhance the overall electrochemical performance of the cathode material.
[0010] As a preferred embodiment of the present invention, the modified iron phosphate material has a specific surface area of 14 m². 2 / g-17m 2 / g, for example, could be 14m 2 / g, 14.5m 2 / g, 14.8m 2 / g, 15.2m 2 / g, 15.5m 2 / g, 16.0m 2 / g, 16.4m 2 / g or 17m 2 / g.
[0011] In this invention, if the specific surface area of the modified iron phosphate material is too small, it will be detrimental to the diffusion and transfer of lithium ions, thereby affecting the electrical performance of the cathode material; if the specific surface area of the modified iron phosphate material is too large, the powder will easily agglomerate, which will cause problems such as difficulty in electrode processing and uneven coating.
[0012] Preferably, the porosity of the modified iron phosphate material is 12%-18%, for example, it can be 12%, 12.5%, 13%, 13.2%, 14.7%, 15.5%, 16.8%, 17%, 17.5% or 18%, etc.
[0013] In this invention, if the porosity of the modified iron phosphate material is too small, it will be detrimental to the wetting of the electrolyte; if the porosity of the modified iron phosphate material is too large, it will easily cause the material structure to collapse.
[0014] As a preferred embodiment of the present invention, the nitrogen doping amount in the core is 3wt%-7wt%, for example, it can be 3wt%, 3.5wt%, 3.8wt%, 4wt%, 5.8wt%, 6wt%, 6.5wt%, or 7wt%.
[0015] In this invention, if the amount of nitrogen doping is too small, the electrochemical reactivity of the lithium iron phosphate cathode material may be reduced; if the amount of nitrogen doping is too large, it will not be conducive to a significant improvement in the conductivity of the cathode material.
[0016] Preferably, the thickness of the iron phosphide coating is 18nm-21nm, for example, it can be 18nm, 18.5nm, 18.8nm, 19.5nm, 20.2nm, 20.7nm or 21nm.
[0017] In this invention, if the thickness of the iron phosphide coating layer is too thin, it will be detrimental to improving the conductivity of the cathode material and alleviating the volume stress generated during charging and discharging; if the thickness of the iron phosphide coating layer is too thick, it may hinder the migration of lithium ions, thereby affecting the performance of the battery.
[0018] In a second aspect, the present invention provides a method for preparing the modified iron phosphate material as described in the first aspect, the method comprising the following steps:
[0019] (1) 4,6-diaminopyrimidine (DE), benzene-1,3,5-trimethyltris(methylene)triphosphonic acid (BTMT) and solvent were mixed and self-assembled to obtain a hydrogen-bonded organic framework two-dimensional precursor product (BTMT-DE);
[0020] (2) The hydrogen-bonded organic framework two-dimensional precursor product and the trivalent iron source were mixed and subjected to water bath heating reaction to obtain a three-dimensional iron-based coordination polymer precursor material.
[0021] (3) The three-dimensional iron-based coordination polymer precursor material is calcined and then reduced to obtain the modified iron phosphate material.
[0022] The preparation method provided by this invention is simple, reliable, and environmentally friendly, and has good prospects for industrialization.
[0023] In this invention, the two-dimensional precursor product of the hydrogen-bonded organic framework is obtained through hydrogen bonding forces during the self-assembly process.
[0024] As a preferred technical solution of the present invention, the molar ratio of 4,6-diaminopyrimidine, phenyl-1,3,5-triyltris(methylene)triphosphonic acid in step (1) and the trivalent iron source in step (2) is (3-5):(1-3):1, wherein the selection range of 4,6-diaminopyrimidine "3-5" can be, for example, 3, 4 or 5, and the selection range of phenyl-1,3,5-triyltris(methylene)triphosphonic acid "1-3" can be, for example, 1, 1.5, 2, 2.5 or 3.
[0025] In this invention, the molar ratio of 4,6-diaminopyrimidine, phenyl-1,3,5-trimethyltris(methylene)triphosphonic acid and the trivalent iron source described in step (2) is (3-5):(1-3):1, which within this range helps the three to self-assemble into a three-dimensional iron-based coordination polymer.
[0026] Preferably, the solvent in step (1) includes alcohol and water.
[0027] The present invention does not limit the type of alcohol; for example, it may be ethanol.
[0028] Preferably, the mixing process in step (1) is accompanied by stirring.
[0029] Preferably, the mixing temperature in step (1) is room temperature, and the mixing time is 3-5 hours, for example, 3 hours, 3.5 hours, 4 hours, 4.5 hours or 5 hours.
[0030] Preferably, the trivalent iron source in step (2) includes trivalent iron salts.
[0031] Preferably, the trivalent iron salt includes ferric nitrate and / or ferric sulfate.
[0032] Preferably, before the water bath heating reaction in step (2), the pH of the mixed solution is adjusted so that the pH value of the mixed solution is 1.8-2.2, for example, it can be 1.8, 1.9, 2, 2.1 or 2.2.
[0033] In this invention, adjusting the pH of the mixture to 1.8-2.2 can provide an acidic environment for the formation of the phosphate-iron precursor polymer.
[0034] Preferably, the pH adjuster used for pH adjustment is an alkaline substance.
[0035] Preferably, the alkaline substance includes ammonia and / or sodium hydroxide.
[0036] Preferably, the temperature of the water bath heating reaction in step (2) is 85-95℃, for example, 85℃, 90℃ or 95℃, and the time of the water bath heating reaction is 4-6h, for example, 4h, 5h or 6h.
[0037] In this invention, if the temperature of the water bath heating reaction is too low, it will affect the reaction rate, prolong the reaction time, and reduce the yield of the precursor product; if the temperature of the water bath heating reaction is too high, it will cause the precursor product to decompose, thereby affecting its performance indicators.
[0038] As a preferred technical solution of the present invention, the calcination atmosphere in step (3) is an inert atmosphere, and the gas in the inert atmosphere includes nitrogen.
[0039] In this invention, the purpose of calcining in an inert atmosphere is to provide a protective atmosphere and to convert the trivalent iron-based coordination polymer into iron phosphate.
[0040] Preferably, the calcination temperature in step (3) is 500-600℃, for example, 500℃, 520℃, 540℃, 560℃, 580℃ or 600℃, and the time is 4-6h, for example, 4h, 4.5h, 5h, 5.5h or 6h.
[0041] In this invention, if the calcination temperature is too low, the iron phosphate material will have poor crystallinity and will easily contain impurities; if the calcination temperature is too high, it will affect the particle morphology and crystal structure of the iron phosphate material, resulting in its unstable phase.
[0042] Preferably, the heating rate of calcination in step (3) is 4-6℃ / min, for example, it can be 4℃ / min, 5℃ / min or 6℃ / min, etc.
[0043] Preferably, the atmosphere for the reduction reaction in step (3) is a hydrogen atmosphere.
[0044] In this invention, a reduction reaction is carried out in a hydrogen atmosphere, which can convert iron phosphate into an iron phosphide layer coating its surface, thereby improving its electrical conductivity.
[0045] Preferably, the temperature of the reduction reaction in step (3) is 850-950℃, for example, 850℃, 900℃ or 950℃, and the time is 0.5-1h, for example, 0.5h, 0.7h or 1h.
[0046] In this invention, both excessively low and excessively high temperatures in the reduction reaction will affect the formation of iron phosphate, while excessively long reaction times will lead to over-burning of iron phosphate, affecting its physicochemical properties and further impacting the electrical performance of the cathode material.
[0047] Preferably, the heating rate of the reduction reaction in step (3) is 8-12℃ / min, for example, it can be 8℃ / min, 9℃ / min, 10℃ / min, 11℃ / min or 12℃ / min, etc.
[0048] As a preferred technical solution of the present invention, the preparation method includes the following steps:
[0049] (1) Dissolve 4,6-diaminopyrimidine in an aqueous ethanol solution, stir until homogeneous, then add benzene-1,3,5-trimethyltris(methylene)triphosphonic acid, and stir at room temperature for 3-5 h to carry out self-assembly to obtain a hydrogen-bonded organic framework two-dimensional precursor product.
[0050] (2) Add ferric salt to the hydrogen-bonded organic framework two-dimensional precursor product, stir and mix evenly, then add alkaline substance dropwise to adjust the pH so that the pH of the solution is 1.8-2.2, then heat to 85-95℃ for water bath heating reaction, the reaction time is 4-6h, after which centrifugation, washing and drying are performed to obtain three-dimensional iron-based coordination polymer precursor material;
[0051] The molar ratio of 4,6-diaminopyrimidine, phenyl-1,3,5-trimethyltris(methylene)triphosphonic acid and ferric salt is (3-5):(1-3):1;
[0052] (3) The three-dimensional iron-based coordination polymer precursor material is placed in a muffle furnace and heated to 500-600°C at a heating rate of 4-6°C / min in a nitrogen atmosphere and held for 4-6 hours for calcination. After calcination, it is cooled to room temperature, the product is taken out and ground, and then placed in a muffle furnace in a hydrogen atmosphere and heated to 850-950°C at a heating rate of 8-12°C / min and held for 0.5-1 hours for reduction reaction. After the reaction is completed, it is cooled to room temperature and the product is collected to obtain the modified iron phosphate material.
[0053] Thirdly, the present invention provides a cathode material, which is obtained by sintering a modified iron phosphate material as described in the first aspect with a lithium source and a carbon source.
[0054] The positive electrode material prepared by the present invention based on modified iron phosphate material has a high compaction density, and its volume expansion during the charging and discharging process is suppressed, which effectively reduces the occurrence of side reactions between active particles and electrolyte and obtains excellent rate performance.
[0055] As a preferred embodiment of the present invention, the specific steps for sintering the modified iron phosphate material with a lithium source and a carbon source include:
[0056] The modified iron phosphate material, lithium source, and carbon source are blended and then sintered to obtain the cathode material.
[0057] In this invention, since iron phosphate is coated on the surface of iron phosphate, it has a better electrical conductivity than carbon-coated iron phosphate. It can reduce the amorphous carbon content of lithium iron phosphate cathode material to a certain extent, thereby increasing the compaction density of lithium iron phosphate material. This modified iron phosphate material can also alleviate the volume expansion of the material during charging and discharging, reduce the occurrence of side reactions between active particles and electrolyte, and thus enhance the rate performance of cathode material.
[0058] Preferably, the lithium source includes any one or a combination of at least two of lithium carbonate, lithium hydroxide, or lithium acetate.
[0059] Preferably, the carbon source includes any one or a combination of at least two of glucose, starch, or cellulose.
[0060] Preferably, the molar ratio of iron, lithium, and carbon in the modified iron phosphate material is 1:1:(0.04-0.06), for example, it can be 1:1:0.04, 1:1:0.05, or 1:1:0.06.
[0061] In this invention, the molar ratio of iron, lithium, and carbon in the modified iron phosphate material satisfies 1:1:0.04-0.06, which can obtain lithium iron phosphate material with high purity and stable crystal structure.
[0062] Preferably, the blending method includes ball milling, and the blending time is 2-5 hours, for example, 2 hours, 3 hours, 4 hours or 5 hours.
[0063] Preferably, the sintering step includes primary sintering and secondary sintering.
[0064] The present invention employs a multi-stage sintering method, which can obtain materials with a denser and more uniform structure and better crystallinity.
[0065] Preferably, the temperature of the first-stage sintering is 400-550℃, for example, 400℃, 450℃, 500℃ or 550℃, and the time is 3-5h, for example, 3h, 3.5h, 4h, 4.5h or 5h.
[0066] In this invention, primary sintering at 400-550℃ for 3-5 hours helps to improve the crystallinity of lithium iron phosphate materials.
[0067] Preferably, the temperature of the secondary sintering is 650-750℃, for example, 650℃, 670℃, 690℃, 710℃, 730℃ or 750℃, and the time is 6-10h, for example, 6h, 7h, 8h, 9h or 10h.
[0068] In this invention, secondary sintering at 650-750℃ for 6-10 hours helps to obtain lithium iron phosphate material with complete crystal transformation.
[0069] Fourthly, the present invention provides a lithium-ion battery, the lithium-ion battery comprising the positive electrode material as described in the third aspect.
[0070] The lithium-ion battery prepared based on the cathode material provided by this invention has excellent electrochemical performance.
[0071] The numerical range described in this invention includes not only the point values listed above, but also any point values within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values included in the range.
[0072] Compared with the prior art, the present invention has the following beneficial effects:
[0073] (1) The modified iron phosphate material prepared by this invention has a unique three-dimensional petal-like structure, which provides sufficient mass transfer channels for active sites for the storage and migration of lithium ions and shortens the diffusion path of lithium ions. At the same time, this structure also increases the contact area between the electrode material and the electrolyte, promoting the lithiation / delithiation process. In addition, nitrogen doping can effectively adjust the electronic structure of the iron phosphate material, thereby improving its conductivity. The iron phosphate coating layer has excellent conductivity, which can reduce the amorphous carbon content of the cathode material, thereby increasing the compaction density of the cathode material. Moreover, the coating layer can also alleviate the volume expansion of the cathode material during the charging and discharging process, reduce the occurrence of side reactions between active particles and electrolyte, and together with the three-dimensional petal-like structure, can improve the overall electrochemical performance of the cathode material.
[0074] (2) The preparation method provided by the present invention is simple, reliable and has good prospects for industrial application. Attached Figure Description
[0075] Figure 1 This is a process flow diagram of the preparation of modified iron phosphate material in Example 1 of the present invention.
[0076] Figure 2 The image shows the XRD pattern of the modified iron phosphate material prepared in Example 1 of this invention.
[0077] Figure 3 This is a scanning electron microscope image of the modified iron phosphate material prepared in Example 1 of the present invention.
[0078] Figure 4 This is a transmission electron microscope (TEM) image of the modified iron phosphate material prepared in Example 1 of the present invention.
[0079] Figure 5 The image shows the cycle stability curves of lithium-ion coin cells prepared based on the modified iron phosphate materials prepared in Example 4 and Comparative Example 1 of this invention at a current density of 0.1C. Detailed Implementation
[0080] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.
[0081] It should be noted that the room temperature below refers to 25℃.
[0082] Example 1
[0083] This embodiment provides a modified iron phosphate material, which has a core-shell structure, with the core being nitrogen-doped iron phosphate and the outer shell being an iron phosphate coating layer;
[0084] The modified iron phosphate material has a three-dimensional petal-like structure;
[0085] The modified iron phosphate material has a specific surface area of 15.8 m². 2 / g; the porosity of the modified iron phosphate material is 14.7%; the nitrogen doping amount in the core is 6wt%; the thickness of the iron phosphate coating layer is 20.6nm.
[0086] This embodiment also provides a method for preparing the above-mentioned modified iron phosphate material, the process flow diagram of which is shown below. Figure 1 As shown, the preparation method includes the following steps:
[0087] (1) Dissolve 0.004 mol of 4,6-diaminopyrimidine in 100 mL of ethanol aqueous solution, stir until uniform, then add 0.002 mol of benzene-1,3,5-trimethyltris(methylene)triphosphonic acid, and stir at room temperature for 3 h to carry out self-assembly to obtain a hydrogen-bonded organic framework two-dimensional precursor product.
[0088] (2) 0.001 mol of ferric nitrate nonahydrate was added to the hydrogen-bonded organic framework two-dimensional precursor product. After stirring and mixing for 30 min, ammonia was added dropwise to adjust the pH to 1.8. The solution was then heated to 85°C in a water bath for 4 h. After the reaction, the product was collected by centrifugation, washed with deionized water, and then dried overnight in a vacuum oven at 80°C to obtain the three-dimensional iron-based coordination polymer precursor material (Fe). 3+ -BTMT-DE);
[0089] The molar ratio of 4,6-diaminopyrimidine, phenyl-1,3,5-trimethyltris(methylene)triphosphonic acid and ferric nitrate nonahydrate is 4:2:1.
[0090] (3) The three-dimensional iron-based coordination polymer precursor material was placed in a muffle furnace and heated to 500°C at a heating rate of 5°C / min in a nitrogen atmosphere and held for 4 hours for calcination. After calcination, it was cooled to room temperature, the product was taken out and ground, and then placed in a muffle furnace in a hydrogen atmosphere and heated to 900°C at a heating rate of 10°C / min and held for 0.5 hours for reduction reaction. After the reaction was completed, it was cooled to room temperature and the product was collected to obtain three-dimensional nitrogen-doped petal-shaped iron phosphate (surface coated with iron phosphide layer), which is the modified iron phosphate material.
[0091] This embodiment also provides a cathode material, which is obtained by sintering the above-mentioned modified iron phosphate material with a lithium source and a carbon source. The specific steps include:
[0092] (a) Lithium carbonate, modified iron phosphate material and glucose were dispersed in anhydrous ethanol, ball-milled for 3 hours until uniformly mixed at a speed of 3000 rpm, and then spray-dried to obtain precursor powder.
[0093] The molar ratio of iron in the modified iron phosphate material, lithium in lithium carbonate, and glucose is 1:1:0.05.
[0094] (b) The precursor powder was heated to 400°C for 3 hours at a heating rate of 8°C / min under a nitrogen atmosphere, and then calcined at 700°C for 8 hours to obtain lithium iron phosphate cathode material.
[0095] Figure 2 The XRD pattern of the modified iron phosphate material prepared in this embodiment is shown. As can be seen from the figure, the characteristic diffraction peaks of the iron phosphate material are obvious, indicating that it has good crystallinity. At the same time, characteristic peaks of iron phosphide also appear in the spectrum.
[0096] Figure 3 The image shows a scanning electron microscope (SEM) image of the modified iron phosphate material prepared in this embodiment. As can be seen from the image, the material has a three-dimensional petal-like structure.
[0097] Figure 4 The image shows a transmission electron microscope (TEM) image of the modified iron phosphate material prepared in this embodiment. As can be seen from the image, the surface of the iron phosphate material is coated with a layer of iron phosphate.
[0098] Example 2
[0099] The difference between this embodiment and embodiment 1 is that the amount of ammonia added in step (2) is adjusted so that the pH value of the solution is 2, and then the temperature is raised to 90°C for water bath heating reaction, and the reaction time is 5 hours.
[0100] The remaining preparation methods and parameters are consistent with those in Example 1.
[0101] Example 3
[0102] The difference between this embodiment and embodiment 1 is that in step (3), the temperature is raised to 550°C in a nitrogen atmosphere at a heating rate of 5°C / min and held for 5 hours for calcination.
[0103] The remaining preparation methods and parameters are consistent with those in Example 1.
[0104] Example 4
[0105] The difference between this embodiment and embodiment 1 is that the amount of ammonia added in step (2) is adjusted so that the pH value of the solution is 2.2, and then the temperature is raised to 95°C for water bath heating reaction, and the reaction time is 6 hours.
[0106] Adjust the calcination temperature in step (3) to 600℃ and keep it warm for 6 hours.
[0107] The remaining preparation methods and parameters are consistent with those in Example 1.
[0108] Example 5
[0109] This embodiment provides a modified iron phosphate material, which has a core-shell structure, with the core being nitrogen-doped iron phosphate and the outer shell being an iron phosphate coating layer;
[0110] The modified iron phosphate material has a three-dimensional petal-like structure;
[0111] The modified iron phosphate material has a specific surface area of 15.2 m². 2 / g; the porosity of the modified iron phosphate material is 14.3%; the nitrogen doping amount in the core is 4wt%; the thickness of the iron phosphate coating layer is 18.8nm.
[0112] This embodiment also provides a method for preparing the above-mentioned modified iron phosphate material, the preparation method comprising the following steps:
[0113] (1) Dissolve 0.003 mol of 4,6-diaminopyrimidine in 100 mL of ethanol aqueous solution, stir until uniform, then add 0.003 mol of benzene-1,3,5-trimethyltris(methylene)triphosphonic acid, and stir at room temperature for 4 h to carry out self-assembly to obtain a hydrogen-bonded organic framework two-dimensional precursor product.
[0114] (2) 0.001 mol of ferric nitrate nonahydrate was added to the hydrogen-bonded organic framework two-dimensional precursor product. After stirring and mixing for 30 min, ammonia was added dropwise to adjust the pH to 2. The solution was then heated to 90 °C in a water bath for 5 h. After the reaction, the product was collected by centrifugation, washed with deionized water, and then dried overnight in a vacuum oven at 80 °C to obtain the three-dimensional iron-based coordination polymer precursor material (Fe). 3+ -BTMT-DE);
[0115] The molar ratio of 4,6-diaminopyrimidine, phenyl-1,3,5-triyltris(methylene)triphosphonic acid and ferric nitrate nonahydrate is 3:3:1.
[0116] (3) The three-dimensional iron-based coordination polymer precursor material is placed in a muffle furnace and heated to 550°C at a heating rate of 4°C / min in a nitrogen atmosphere and held for 5 hours for calcination. After calcination, it is cooled to room temperature, the product is taken out and ground, and then placed in a muffle furnace in a hydrogen atmosphere and heated to 850°C at a heating rate of 8°C / min and held for 1 hour for reduction reaction. After the reaction is completed, it is cooled to room temperature and the product is collected to obtain the modified iron phosphate material.
[0117] This embodiment also provides a cathode material, which is obtained by sintering the above-mentioned modified iron phosphate material with a lithium source and a carbon source. The specific steps include:
[0118] (a) Lithium carbonate, modified iron phosphate material and glucose were dispersed in anhydrous ethanol, ball-milled for 2 hours until uniformly mixed at a speed of 3000 rpm, and then spray-dried to obtain precursor powder.
[0119] The molar ratio of iron in the modified iron phosphate material, lithium in lithium carbonate, and glucose is 1:1:0.04.
[0120] (b) The precursor powder was heated to 500°C for 4 hours at a heating rate of 8°C / min under a nitrogen atmosphere, and then calcined at 650°C for 10 hours to obtain lithium iron phosphate cathode material.
[0121] Example 6
[0122] This embodiment provides a modified iron phosphate material, which has a core-shell structure, with the core being nitrogen-doped iron phosphate and the outer shell being an iron phosphate coating layer;
[0123] The modified iron phosphate material has a three-dimensional petal-like structure;
[0124] The modified iron phosphate material has a specific surface area of 15.5 m². 2 / g; the porosity of the modified iron phosphate material is 15%; the nitrogen doping amount in the core is 6.6wt%; the thickness of the iron phosphate coating layer is 19.5nm.
[0125] This embodiment also provides a method for preparing the above-mentioned modified iron phosphate material, the preparation method comprising the following steps:
[0126] (1) Dissolve 0.005 mol of 4,6-diaminopyrimidine in 100 mL of ethanol aqueous solution, stir until uniform, then add 0.003 mol of benzene-1,3,5-trimethyltris(methylene)triphosphonic acid, and stir at room temperature for 5 h to carry out self-assembly to obtain a hydrogen-bonded organic framework two-dimensional precursor product.
[0127] (2) 0.001 mol of ferric nitrate nonahydrate was added to the hydrogen-bonded organic framework two-dimensional precursor product. After stirring and mixing for 30 min, ammonia was added dropwise to adjust the pH to 2. The solution was then heated to 90 °C in a water bath for 5 h. After the reaction, the product was collected by centrifugation, washed with deionized water, and then dried overnight in a vacuum oven at 80 °C to obtain the three-dimensional iron-based coordination polymer precursor material (Fe). 3+ -BTMT-DE);
[0128] The molar ratio of 4,6-diaminopyrimidine, phenyl-1,3,5-triyltris(methylene)triphosphonic acid and ferric nitrate nonahydrate is 5:3:1.
[0129] (3) The three-dimensional iron-based coordination polymer precursor material is placed in a muffle furnace and heated to 600°C at a heating rate of 6°C / min in a nitrogen atmosphere and held for 4 hours for calcination. After calcination, it is cooled to room temperature, the product is taken out and ground, and then placed in a muffle furnace in a hydrogen atmosphere and heated to 950°C at a heating rate of 12°C / min and held for 0.5 hours for reduction reaction. After the reaction is completed, it is cooled to room temperature and the product is collected to obtain the modified iron phosphate material.
[0130] This embodiment also provides a cathode material, which is obtained by sintering the above-mentioned modified iron phosphate material with a lithium source and a carbon source. The specific steps include:
[0131] (a) Lithium carbonate, modified iron phosphate material and glucose were dispersed in anhydrous ethanol, ball-milled for 5 hours until uniformly mixed at a speed of 3000 rpm, and then spray-dried to obtain precursor powder.
[0132] The molar ratio of iron in the modified iron phosphate material, lithium in lithium carbonate, and glucose is 1:1:0.06.
[0133] (b) The precursor powder was heated to 550°C at a heating rate of 8°C / min under a nitrogen atmosphere and held for 5 hours, and then heated to 750°C for calcination for 6 hours to obtain lithium iron phosphate cathode material.
[0134] Example 7
[0135] The difference between this embodiment and Embodiment 1 is that the temperature of the muffle furnace under a hydrogen atmosphere is adjusted to 650°C, so that the thickness of the iron phosphate coating layer is 11.5 nm.
[0136] The remaining preparation methods and parameters are consistent with those in Example 1.
[0137] Example 8
[0138] The difference between this embodiment and Embodiment 1 is that the thickness of the iron phosphide coating layer was 21.8 nm by adjusting the holding time in a muffle furnace under a hydrogen atmosphere for 3 hours (900°C). The remaining preparation methods and parameters are consistent with those of Embodiment 1.
[0139] Example 9
[0140] The difference between this embodiment and embodiment 1 is that the water bath heating reaction temperature in step (2) is 70°C.
[0141] The remaining preparation methods and parameters are consistent with those in Example 1.
[0142] Example 10
[0143] The difference between this embodiment and embodiment 1 is that the temperature of the water bath heating reaction in step (2) is 110°C.
[0144] The remaining preparation methods and parameters are consistent with those in Example 1.
[0145] Comparative Example 1
[0146] The difference between this comparative example and Example 1 is that the iron phosphate material was prepared using a co-precipitation method, the specific steps of which include:
[0147] 0.001 mol ferric nitrate nonahydrate and 0.001 mol ammonium dihydrogen phosphate were dispersed in 50 mL of deionized water and stirred until homogeneous. Ammonia water was then added dropwise to the mixture, and the pH of the solution was controlled to be 1.8. The mixture was then heated to 85 °C and maintained for 4 h. After the reaction was completed, the mixture was allowed to stand and age for 5 h. The precipitate was collected by centrifugation and washed several times with deionized water and anhydrous ethanol. The precipitate was then dried in a vacuum oven at 100 °C for 12 h to obtain the ferric phosphate product. The product was then placed in a muffle furnace and heated to 650 °C at a heating rate of 8 °C / min and held for 8 h to remove the water of crystallization, thus obtaining the anhydrous ferric phosphate material.
[0148] The remaining preparation methods and parameters are consistent with those in Example 1.
[0149] Figure 5 The cycling stability curves of lithium-ion coin cells prepared based on the modified iron phosphate materials prepared in Example 4 and Comparative Example 1 are shown at a current density of 0.1C. As can be seen from the figure, the discharge specific capacity of the LiFePO4 / C sample prepared in Example 4 did not decrease significantly after 50 cycles at 0.1C, indicating that the material has good structural stability and reversibility.
[0150] Comparative Example 2
[0151] The difference between this comparative example and Example 1 is that no iron phosphide coating layer is provided, that is, no reduction reaction is carried out in step (3).
[0152] The remaining preparation methods and parameters are consistent with those in Example 1.
[0153] Performance testing
[0154] The lithium iron phosphate cathode materials prepared in the above embodiments and comparative examples are formulated into coin cells. The specific steps include:
[0155] The specific steps include: uniformly mixing lithium iron phosphate cathode material, conductive agent acetylene black, and adhesive polyvinylidene fluoride in N-methylpyrrolidone at a mass ratio of 92:4:4 to form a slurry, then coating it onto aluminum foil and drying it in a vacuum drying oven, then pressing it into a positive electrode sheet using a tablet press, with a negative electrode sheet being a lithium metal sheet, an electrolyte being 1 mol / L LiPF6-EC:DMC (volume ratio of 1:1), a polypropylene porous membrane as the separator, and assembling the battery in an argon glove box.
[0156] Electrochemical performance tests were conducted on the above-mentioned button cells, and the charging and discharging voltages were controlled between 2.5 and 4.5V during the tests.
[0157] Table 1
[0158]
[0159]
[0160] analyze:
[0161] As shown in the table above, the modified iron phosphate material prepared by this invention has a three-dimensional nitrogen-doped, petal-like structure with a highly conductive iron phosphide thin layer coated on its surface. The lithium iron phosphate cathode material prepared using this precursor has a large specific surface area, high conductivity, a short and fast lithium-ion diffusion path, and low resistance to mass transfer, thus resulting in superior electrochemical performance of the lithium-ion battery. In particular, the electrochemical performance of the lithium iron phosphate products prepared in the examples is significantly better than that of the comparative examples, especially Example 4.
[0162] As can be seen from Examples 1 and 7-8, if the thickness of the iron phosphide coating layer is too thin, it will not be conducive to improving the conductivity of the cathode material and alleviating the volume stress generated during charging and discharging; if the thickness of the iron phosphide coating layer is too thick, it may hinder the migration of lithium ions, thereby affecting the performance of the battery.
[0163] As can be seen from Examples 1 and 9-10, if the temperature of the water bath heating reaction is too low, it will affect the reaction rate, prolong the reaction time, and reduce the yield of the precursor product; if the temperature of the water bath heating reaction is too high, it will cause the precursor product to decompose, thereby affecting its performance indicators.
[0164] As can be seen from Example 1 and Comparative Example 1, the iron phosphate prepared in Comparative Example 1 is an irregular bulk, while the iron phosphate prepared in this invention is a three-dimensional nitrogen-doped petal-like structure with a highly conductive iron phosphate thin layer on the surface. The lithium iron phosphate cathode material prepared with this precursor has a large specific surface area, high conductivity, short lithium-ion diffusion path, fast rate and low resistance to mass transfer process. Therefore, the electrochemical performance of the lithium-ion battery is superior.
[0165] As can be seen from Example 1 and Comparative Example 2, if the iron phosphate coating layer is not provided, it will not be conducive to the significant improvement of the electrical conductivity and compaction density of the iron phosphate material, thereby affecting the electrical performance of its cathode material.
[0166] The applicant declares that the present invention is illustrated by the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.
Claims
1. A modified iron phosphate material, characterized in that, The modified iron phosphate material has a core-shell structure, with the core being nitrogen-doped iron phosphate and the outer shell being an iron phosphate coating layer; The modified iron phosphate material has a three-dimensional petal-like structure; The modified iron phosphate material is prepared by the following method, which includes the following steps: (1) 4,6-diaminopyrimidine, benzene-1,3,5-trimethyltris(methylene)triphosphonic acid and solvent were mixed and self-assembled to obtain a hydrogen-bonded organic framework two-dimensional precursor product; (2) The hydrogen-bonded organic framework two-dimensional precursor product and the trivalent iron source are mixed and subjected to water bath heating reaction to obtain a three-dimensional iron-based coordination polymer precursor material; Before the water bath heating reaction in step (2), the pH of the mixed solution is adjusted so that the pH value of the mixed solution is 1.8-2.2; The water bath heating reaction in step (2) is carried out at a temperature of 85-95℃ for 4-6 hours. (3) The three-dimensional iron-based coordination polymer precursor material is calcined and then reduced to obtain the modified iron phosphate material.
2. The modified iron phosphate material according to claim 1, characterized in that, The modified iron phosphate material has a specific surface area of 14 m². 2 / g-17m 2 / g.
3. The modified iron phosphate material according to claim 1, characterized in that, The porosity of the modified iron phosphate material is 12%-18%.
4. The modified iron phosphate material according to claim 1, characterized in that, The nitrogen doping amount in the core is 3wt%-7wt%.
5. The modified iron phosphate material according to claim 1, characterized in that, The thickness of the iron phosphate coating is 18nm-21nm.
6. A method for preparing the modified iron phosphate material according to any one of claims 1-5, characterized in that, The preparation method includes the following steps: (1) 4,6-diaminopyrimidine, benzene-1,3,5-trimethyltris(methylene)triphosphonic acid and solvent were mixed and self-assembled to obtain a hydrogen-bonded organic framework two-dimensional precursor product; (2) The hydrogen-bonded organic framework two-dimensional precursor product and the trivalent iron source are mixed and subjected to water bath heating reaction to obtain a three-dimensional iron-based coordination polymer precursor material; Before the water bath heating reaction in step (2), the pH of the mixed solution is adjusted so that the pH value of the mixed solution is 1.8-2.2; The water bath heating reaction in step (2) is carried out at a temperature of 85-95℃ for 4-6 hours. (3) The three-dimensional iron-based coordination polymer precursor material is calcined and then reduced to obtain the modified iron phosphate material.
7. The preparation method according to claim 6, characterized in that, The molar ratio of 4,6-diaminopyrimidine, phenyl-1,3,5-trimethyltris(methylene)triphosphonic acid in step (1) to the trivalent iron source in step (2) is (3-5):(1-3):
1.
8. The preparation method according to claim 6, characterized in that, The calcination atmosphere in step (3) is an inert atmosphere, and the gas in the inert atmosphere includes nitrogen.
9. The preparation method according to claim 6, characterized in that, The calcination temperature in step (3) is 500-600℃ and the time is 4-6h.
10. The preparation method according to claim 6, characterized in that, The atmosphere for the reduction reaction in step (3) is a hydrogen atmosphere.
11. The preparation method according to claim 6, characterized in that, The temperature of the reduction reaction in step (3) is 850-950℃ and the time is 0.5-1h.
12. The preparation method according to claim 6, characterized in that, The preparation method includes the following steps: (1) Dissolve 4,6-diaminopyrimidine in an aqueous ethanol solution, stir until homogeneous, then add benzene-1,3,5-trimethyltris(methylene)triphosphonic acid, and stir at room temperature for 3-5 h to carry out self-assembly to obtain a hydrogen-bonded organic framework two-dimensional precursor product. (2) Add ferric salt to the hydrogen-bonded organic framework two-dimensional precursor product, stir and mix evenly, then add alkaline substance dropwise to adjust the pH so that the pH of the solution is 1.8-2.2, then heat to 85-95℃ for water bath heating reaction, the reaction time is 4-6h, after which centrifugation, washing and drying are performed to obtain three-dimensional iron-based coordination polymer precursor material; The molar ratio of 4,6-diaminopyrimidine, phenyl-1,3,5-trimethyltris(methylene)triphosphonic acid and ferric salt is (3-5):(1-3):1; (3) The three-dimensional iron-based coordination polymer precursor material is placed in a muffle furnace and heated to 500-600°C at a heating rate of 4-6°C / min in a nitrogen atmosphere and held for 4-6 hours for calcination. After calcination, it is cooled to room temperature, the product is taken out and ground, and then placed in a muffle furnace in a hydrogen atmosphere and heated to 850-950°C at a heating rate of 8-12°C / min and held for 0.5-1 hours for reduction reaction. After the reaction is completed, it is cooled to room temperature and the product is collected to obtain the modified iron phosphate material.
13. A positive electrode material, characterized in that, The cathode material is obtained by sintering the modified iron phosphate material as described in any one of claims 1-5 with a lithium source and a carbon source.
14. The cathode material according to claim 13, characterized in that, The specific steps for sintering the modified iron phosphate material with lithium and carbon sources include: The modified iron phosphate material, lithium source, and carbon source are blended and then sintered to obtain the cathode material.
15. The cathode material according to claim 14, characterized in that, The molar ratio of iron, lithium, and carbon in the modified iron phosphate material is 1:1:(0.04-0.06).
16. The cathode material according to claim 14, characterized in that, The sintering process includes primary sintering and secondary sintering.
17. The cathode material according to claim 16, characterized in that, The temperature of the first-stage sintering is 400-550℃, and the time is 3-5 hours.
18. The cathode material according to claim 16, characterized in that, The secondary sintering temperature is 650-750℃, and the time is 6-10h.
19. A lithium-ion battery, characterized in that, The lithium-ion battery includes the positive electrode material as described in any one of claims 13-18.