A hollow iron phosphate, its preparation method and uses

Abstract

This disclosure pertains to the field of battery material preparation. It provides a hollow iron phosphate, its preparation method, and its applications. The preparation method uses a specific iron source precursor, namely a spherical ferrous glycerate salt, mixed with a phosphorus source to undergo an anion exchange reaction, continuously forming hollow iron phosphate structures. This preparation method is a template-free method, requiring no additional template agent. Instead, the specific iron source precursor serves as a consumable template. The resulting hollow iron phosphate can be further used to prepare hollow lithium iron phosphate materials, which helps shorten the lithium-ion diffusion path and improve the electrochemical performance of the cathode material.

Description

Technical Field

[0001] This disclosure pertains to the field of battery material preparation, and relates to a hollow iron phosphate, its preparation method, and its uses. Background Technology

[0002] Currently, with the increasing severity of energy and environmental issues, next-generation lithium-ion batteries have been widely used in various fields. Since the Goodenough team first reported olivine-structured lithium iron phosphate (LiFePO4) in 1997, LiFePO4 has been considered one of the most promising cathode materials for lithium-ion batteries due to its high theoretical capacity (170 mAh / g), low cost, environmental compatibility, and inherent thermal safety.

[0003] However, lithium iron phosphate (LiFePO4) suffers from poor electronic conductivity and low lithium-ion diffusion rate, which significantly affects its rate performance. Iron phosphate (FePO4) is an important precursor for the synthesis of LiFePO4. Currently, to overcome the problems of LiFePO4, in addition to modification methods such as doping, coating, and particle size reduction, preparing high-quality iron phosphate with excellent performance indicators is also an important strategy for researchers to improve the electrochemical performance of LiFePO4 cathode materials.

[0004] Related research shows that hollow nanospheres possess a unique internal cavity structure, offering advantages over solid materials such as large specific surface area, low density, high stability, good surface permeability, and high porosity, thus attracting significant attention in the field of electrode materials. Hollow lithium iron phosphate (LFP) fabricated from hollow LFP materials not only shortens the transport paths of lithium ions and electrons within the material but also exposes more electrochemical active sites, thus playing a crucial role in improving the electrical performance of LFP.

[0005] Currently, most reported studies employ soft or hard template methods to prepare hollow iron phosphate. For example, CN106082157A first uses a silica core as a template to prepare iron phosphate, then introduces an alkaline solution to consume the internal silica and remove the template, thereby obtaining nanoscale, highly active hollow iron phosphate microspheres. CN103887498A uses polystyrene emulsion as a template, then introduces a complexing agent and a precipitant to prepare iron phosphate, and obtains nanoscale hollow iron phosphate microspheres after high-temperature sintering. Although these methods improve the electrochemical performance of the resulting lithium iron phosphate precursors to some extent, they usually require a cumbersome template removal process. Moreover, template removal often requires toxic reagents, strong acids and alkalis, or high-temperature calcination, which not only increases the process time and preparation cost but also causes environmental pollution.

[0006] Therefore, it is still necessary to develop a simple, economical, and environmentally friendly method for preparing hollow-structured iron phosphate, which is of great significance for realizing its industrial-scale production. Summary of the Invention

[0007] The following is an overview of the subject matter described in detail herein. This overview is not intended to limit the scope of the claims.

[0008] In view of the problems existing in the prior art, the purpose of this disclosure is to provide a hollow iron phosphate, its preparation method and uses. The preparation method uses a specific iron source precursor, namely a spherical ferrous glycerate salt, mixed with a phosphorus source to continuously form hollow iron phosphate through an anion exchange reaction. This preparation method is a template-free method, requiring no additional template agent. Instead, the specific iron source precursor serves as a consumable template. The resulting hollow iron phosphate can be further used to prepare hollow lithium iron phosphate materials, which is beneficial for shortening the lithium-ion diffusion path and improving the electrochemical performance of the cathode material.

[0009] To achieve this objective, the present disclosure adopts the following technical solution:

[0010] In a first aspect, this disclosure provides a method for preparing hollow ferric phosphate, the method comprising:

[0011] Ferrous glycerate (Fe(III)-glycerate) microspheres were mixed with a phosphorus source and reacted to obtain hollow ferric phosphate.

[0012] The following are optional technical solutions of this disclosure, but are not intended to limit the technical solutions provided by this disclosure. The technical objectives and beneficial effects of this disclosure can be better achieved through the following technical solutions.

[0013] As an optional technical solution of this disclosure, the method for preparing the ferrous glycerate microspheres includes:

[0014] Ferrous salt, polyvinylpyrrolidone (PVP), glycerol and solvent were mixed and heated to obtain ferrous glycerate microspheres.

[0015] In one embodiment, the amount of polyvinylpyrrolidone used is 15wt% to 20wt% of the mass of the ferrous salt, such as 15wt%, 16wt%, 17wt%, 18wt%, 19wt%, or 20wt%, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0016] This disclosure uses polyvinylpyrrolidone as a specific structure-directing agent, or surfactant, to regulate the morphology and consistency of the product, while also inhibiting product aggregation. Therefore, its dosage affects the size and morphology of the product.

[0017] In one embodiment, the amount of glycerol used is 0.8 to 1.2 mL per millimole of ferrous salt, such as 0.8 mL, 0.85 mL, 0.9 mL, 0.95 mL, 1 mL, 1.05 mL, 1.1 mL, 1.15 mL, or 1.2 mL, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0018] This disclosure uses glycerol to combine with ferrous salt to generate a specific metal alkoxide precursor, and the amount of glycerol used affects the particle size of the product.

[0019] In one embodiment, the temperature of the heat treatment is 160–200°C, such as 160°C, 165°C, 170°C, 175°C, 180°C, 185°C, 190°C, 195°C, or 200°C, and the time is 8–12 hours, such as 8 hours, 8.5 hours, 9 hours, 9.5 hours, 10 hours, 10.5 hours, 11 hours, 11.5 hours, or 12 hours, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0020] The temperature of heat treatment affects the crystal structure and morphology of nanomaterials. Therefore, changes in temperature and heating time can lead to changes in the particle size of nanomaterials.

[0021] In one embodiment, the ferrous salt includes at least one of ferrous nitrate, ferrous sulfate, or ferrous oxalate, such as typical but non-limiting combinations including a combination of ferrous nitrate and ferrous sulfate, a combination of ferrous sulfate and ferrous oxalate, or a combination of ferrous nitrate and ferrous oxalate.

[0022] As an optional technical solution of this disclosure, the method for preparing the ferrous glycerate microspheres further includes preparing ferrous salt as solution A, preparing polyvinylpyrrolidone as solution B, adding solution A to solution B, and then adding glycerol to mix.

[0023] In one embodiment, the solvent of solution A includes water.

[0024] In one embodiment, the concentration of ferrous salt in solution A is 0.1–0.2 mmol / mL, for example, 0.1 mmol / mL, 0.11 mmol / mL, 0.12 mmol / mL, 0.13 mmol / mL, 0.14 mmol / mL, 0.15 mmol / mL, 0.16 mmol / mL, 0.17 mmol / mL, 0.18 mmol / mL, 0.19 mmol / mL, or 0.2 mmol / mL, but is not limited to the listed values; other unlisted values ​​within the above range are also applicable.

[0025] In one embodiment, the solvent of solution B includes isopropanol.

[0026] As an optional technical solution of this disclosure, the preparation method also uses an oxidizing agent.

[0027] In one embodiment, the oxidant includes hydrogen peroxide.

[0028] In one embodiment, the amounts of the ferrous glycerate microspheres and the oxidant are controlled according to a molar ratio of iron to oxidant of 1:(1.2 to 1.5), such as 1:1.2, 1:1.25, 1:1.3, 1:1.35, 1:1.4, 1:1.45, or 1:1.5, but are not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0029] In one embodiment, the amount of ferrous glycerate microspheres and the phosphorus source is controlled according to a molar ratio of iron to phosphorus of 1:(1 to 1.02), such as 1:1, 1:1.01, 1:1.015 or 1:1.02, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0030] In one embodiment, the phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, ammonium hydrogen phosphate, or sodium hydrogen phosphate, such as typical but non-limiting combinations including combinations of phosphoric acid and ammonium dihydrogen phosphate, combinations of phosphoric acid and ammonium hydrogen phosphate, combinations of phosphoric acid and sodium hydrogen phosphate, etc.

[0031] In one embodiment, the preparation method further includes preparing ferrous glycerate microspheres into a dispersion C, preparing a phosphorus source and an oxidant into a dispersion D, adding dispersion C into dispersion D, and reacting to obtain hollow ferric phosphate.

[0032] As an optional technical solution of this disclosure, the pH conditions of the reaction are 1.8 to 2.2, such as 1.8, 1.9, 2, 2.1 or 2.2, but are not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0033] In one embodiment, the pH is adjusted using an alkaline substance.

[0034] In one embodiment, the alkaline substance is ammonia and / or sodium hydroxide.

[0035] In one embodiment, the reaction is carried out under reflux.

[0036] In one embodiment, the reaction temperature is 85–95°C, such as 85°C, 86°C, 87°C, 88°C, 89°C, 90°C, 91°C, 92°C, 93°C, 94°C, or 95°C, and the reaction time is 6–12 hours, such as 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, or 12 hours, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0037] The reaction temperature and reaction time will affect the particle size of the product and should be adjusted reasonably within the above range.

[0038] In one embodiment, after the reaction is completed, the mixture is aged for 3 to 5 hours, such as 3 hours, 3.5 hours, 4 hours, 4.5 hours, or 5 hours, but not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0039] The purpose of aging is to gradually transform amorphous ferric phosphate into crystalline ferric phosphate, thereby improving the crystallinity and purity of the material, optimizing its crystal structure, and promoting the formation of more uniform and dense particles.

[0040] In one embodiment, the preparation method further includes heat-treating the obtained hollow ferric phosphate to remove the water of crystallization.

[0041] Before the heat treatment, the hollow iron phosphate obtained by the preparation method of this disclosure is FePO4·2H2O material. After heat treatment, hollow anhydrous FePO4 material can be obtained.

[0042] In one embodiment, the heat treatment temperature is 500–750°C, such as 500°C, 520°C, 540°C, 550°C, 580°C, 600°C, 630°C, 650°C, 680°C, 700°C, 720°C, or 750°C, and the time is 4–10 hours, such as 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours, 8 hours, 8.5 hours, 9 hours, 9.5 hours, or 10 hours, but is not limited to the listed values; other unlisted values ​​within the above range are also applicable.

[0043] Generally, the higher the heat treatment temperature, the more thoroughly the water of crystallization is removed. Adjustments should be made according to actual needs.

[0044] Secondly, this disclosure provides a hollow ferric phosphate, which is obtained by the preparation method described in the first aspect.

[0045] Thirdly, this disclosure provides a method for preparing lithium iron phosphate materials, the method comprising:

[0046] The lithium source, carbon source, and hollow iron phosphate described in the second aspect are mixed and sintered to obtain lithium iron phosphate cathode material.

[0047] In the synthesis of lithium iron phosphate, apart from limiting the use of hollow iron phosphate provided in this disclosure as the iron source, this disclosure does not restrict the use of other raw materials. Lithium sources and carbon sources that can be used in the prior art are also applicable to this disclosure.

[0048] For example, the lithium source includes at least one of lithium carbonate, lithium hydroxide, or lithium acetate, such as typical but non-limiting combinations including combinations of lithium carbonate and lithium hydroxide, combinations of lithium carbonate and lithium acetate, or combinations of lithium hydroxide and lithium acetate.

[0049] For example, the carbon source includes at least one of glucose, sucrose, starch, or cellulose, such as typical but non-limiting combinations including combinations of glucose and sucrose, glucose and starch, glucose and cellulose, sucrose and starch, sucrose and cellulose, or starch and cellulose.

[0050] In one embodiment, the molar ratio of the lithium source, carbon source, and hollow iron phosphate is 1:(0.1-0.15):(1-1.1), for example, 1:0.1:1, 1:0.1:1.05, 1:0.1:1.1, 1:0.11:1, 1:0.11:1.05, 1:0.11:1.1, 1:0.12:1, 1:0.12:1.05, 1:0. The values ​​are 12:1.1, 1:0.13:1, 1:0.13:1.05, 1:0.13:1.1, 1:0.14:1, 1:0.14:1.05, 1:0.14:1.1, 1:0.15:1, 1:0.15:1.05, or 1:0.15:1.1, etc., but are not limited to the listed values. Other unlisted values ​​within the above range also apply.

[0051] In one embodiment, the mixing method includes liquid-phase ball milling.

[0052] In one embodiment, the grinding media used in the liquid phase ball mill includes ethanol.

[0053] In one embodiment, the liquid phase ball milling time is 2 to 5 hours, such as 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, or 5 hours, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0054] In one embodiment, the sintering temperature is 650–750°C, such as 650°C, 655°C, 660°C, 665°C, 670°C, 675°C, 680°C, 685°C, 690°C, 695°C, 700°C, 705°C, 710°C, 715°C, 720°C, 725°C, 730°C, 735°C, 740°C, 745°C, or 750°C, and the sintering time is 6–10 hours, such as 6 hours, 6.5 hours, 7 hours, 7.5 hours, 8 hours, 8.5 hours, 9 hours, 9.5 hours, or 10 hours, but is not limited to the listed values; other unlisted values ​​within the above range are also applicable.

[0055] For example, the method for preparing lithium iron phosphate material includes:

[0056] (1) Preparation of ferrous glycerate microspheres:

[0057] Dissolve 2–4 mmol / L of ferrous salt in 20 mL of deionized water. The ferrous salt includes at least one of ferrous nitrate, ferrous sulfate, and ferrous oxalate to obtain solution A. Dissolve 15 wt%–20 wt% of polyvinylpyrrolidone (polyvinylpyrrolidone) in 40 mL of isopropanol and stir until homogeneous to obtain solution B. Pour solution A into solution B, stir continuously, and add 8–10 mL of glycerol dropwise to the mixture. After stirring for a period of time, transfer the mixture to a 100 mL polytetrafluoroethylene-lined stainless steel autoclave and heat it in an oven at 160–200 °C for 8–12 h. After cooling to room temperature, centrifuge to collect the ferrous glycerate microsphere product, and wash and dry it.

[0058] (2) Preparation of hollow ferric phosphate:

[0059] The ferrous glycerate microsphere powder from step (1) was ultrasonically dispersed in anhydrous ethanol to obtain dispersion C; 2-4 mmol of phosphate was ultrasonically dispersed in anhydrous ethanol, the phosphate including at least one of phosphoric acid, ammonium dihydrogen phosphate, ammonium hydrogen phosphate, and sodium hydrogen phosphate, the amount of ferrous glycerate microspheres and phosphate was controlled according to a molar ratio of iron to phosphorus of 1:(1-1.02), then 10% hydrogen peroxide was added dropwise, the amount of ferrous glycerate microspheres and hydrogen peroxide was controlled according to a molar ratio of iron to hydrogen peroxide of 1:(1.2-1.5), and after stirring evenly, dispersion D was obtained; dispersion C was poured into dispersion D. The mixture was stirred at room temperature, and an alkaline substance, including ammonia and / or sodium hydroxide, was added dropwise. The pH of the solution was controlled at 1.8–2.2. The mixture was then transferred to a reflux reactor and heated to 85–95°C for 6–12 hours to carry out the reaction. After the reaction was completed, the mixture was aged for 3–5 hours. The product was then filtered, washed, and vacuum dried to obtain hollow FePO4·2H2O material. The FePO4·2H2O material was placed in a muffle furnace and heated to 500–750°C for heat treatment. The heat was held for 4–10 hours to remove the water of crystallization, thus obtaining hollow anhydrous FePO4 material, which was then ground and ready for use.

[0060] (3) Preparation of lithium iron phosphate materials:

[0061] Under a protective atmosphere, including nitrogen and / or argon, the hollow anhydrous FePO4 material from step (2) is mixed with a lithium source and a carbon source as a precursor and ball-milled for 2-5 hours in ethanol as the grinding medium. The lithium source includes at least one of lithium carbonate, lithium hydroxide, or lithium acetate, and the carbon source includes at least one of glucose, sucrose, starch, or cellulose. The molar ratio of the lithium source, carbon source, and hollow iron phosphate is controlled to be 1:(0.1-0.15):(1-1.1). After drying, the mixture is sintered at 650-750°C for 6-10 hours to obtain LiFePO4 / C material.

[0062] Fourthly, this disclosure provides a lithium iron phosphate material, which is prepared by the method described in the third aspect, and the resulting lithium iron phosphate material is a LiFePO4 / C material. The resulting lithium iron phosphate material inherits the hollow structure of hollow iron phosphate.

[0063] Fifthly, this disclosure provides a battery containing the lithium iron phosphate material described in the fourth aspect.

[0064] Compared with existing technical solutions, this disclosure has at least the following beneficial effects:

[0065] (1) The method for preparing hollow ferric phosphate described in this disclosure does not introduce an additional template agent, but instead uses a self-templating method. That is, ferrous glycerate microspheres simultaneously serve as both an iron source and a template material in the formation of the hollow nanostructure shell of ferric phosphate, allowing the template material to be directly converted into the target product shell while leaving a hollow structure. The principle of this method is based on the use of Fe... 3+ and PO4 3- The difference in diffusion rate (v(Fe) 3+ )>v(PO4 3 -)), Fe migrates outward more quickly within the template 3+ PO4 and template outward migration 3- Upon encountering each other, they react to form a FePO4 shell. The potential energy difference generated during the diffusion of the two ions inside and outside the template causes defects to form inside the template, which gradually transforms into a hollow structure. In addition, this self-participatory mechanism can effectively control the uniformity of the product particles, and eliminates the need for an auxiliary template removal process, simplifying the synthesis process, reducing production costs, and being environmentally friendly, which is conducive to industrial-scale production.

[0066] (2) In the method for preparing ferrous glycerate microspheres disclosed herein, in addition to serving as a structure directing agent for forming ferrous glycerate microspheres, some PVP can be retained in the formed ferrous glycerate microspheres by controlling the subsequent washing times of the obtained ferrous glycerate microspheres. When reacting with the phosphorus source, it can form a hollow iron phosphate coated with a nitrogen-doped carbon layer, which is beneficial to improving conductivity.

[0067] (3) The iron phosphate prepared in this disclosure is a hollow microsphere. Compared with solid iron phosphate material, the iron phosphate with this structure has a large specific surface area and high porosity. When synthesizing lithium iron phosphate, the lithium source and carbon source can penetrate into its interior more easily, shortening the transmission distance between particles and reducing polarization. Correspondingly, the specific surface area of ​​lithium iron phosphate prepared from hollow iron phosphate will also increase, the contact area between the cathode material and the electrolyte will increase, the migration path of lithium ions will be shortened and the insertion and extraction rate will be accelerated, which is beneficial to improving the rate performance of lithium iron phosphate batteries.

[0068] After reading and understanding the accompanying diagrams and detailed descriptions, the other aspects can be understood. Attached Figure Description

[0069] The accompanying drawings are used to provide a further understanding of the technical solutions in this paper and form part of the specification. They are used together with the embodiments of this application to explain the technical solutions in this paper and do not constitute a limitation on the technical solutions in this paper.

[0070] Figure 1 This is a SEM image of the ferrous glycerate microspheres from Example 1;

[0071] Figure 2This is a SEM image of the hollow anhydrous FePO4 material in Example 1;

[0072] Figure 3 This is a schematic diagram illustrating the principle of synthesizing hollow iron phosphate using the preparation method described in Example 1. Detailed Implementation

[0073] The technical solution of this disclosure will be further illustrated below through specific implementation methods.

[0074] Those skilled in the art will understand that the embodiments described are merely illustrative of this disclosure and should not be construed as specific limitations thereof.

[0075] Example 1

[0076] This embodiment provides a method for preparing hollow iron phosphate, the method comprising:

[0077] (1) Dissolve 2.5 mmol of ferrous salt FeSO4·7H2O in 20 mL of deionized water and sonicate for 10 min to obtain solution A; dissolve 18 wt% polyvinylpyrrolidone (PPP) in 40 mL of isopropanol and stir until homogeneous to obtain solution B; pour solution A into solution B, stir continuously and add 8.5 mL of glycerol dropwise to the mixture, stir for 30 min, transfer the mixture to a 100 mL polytetrafluoroethylene-lined stainless steel autoclave, and heat in an oven at 180 °C for 10 h. After cooling to room temperature, centrifuge to collect the ferrous glycerate microsphere product, and wash repeatedly with deionized water and anhydrous ethanol. Subsequently, vacuum dry the sample at 100 °C for 12 h.

[0078] (2) Disperse the ferrous glycerate microsphere powder obtained in step (1) in 40 mL of anhydrous ethanol, and sonicate for 15 min to obtain dispersion C; take 2.5 mmol of ammonium hydrogen phosphate and sonicate it in 40 mL of anhydrous ethanol, then add 1.5 mL of 10% hydrogen peroxide dropwise, and stir evenly to obtain dispersion D; pour dispersion C into dispersion D, stir at room temperature for 20 min, continuously stir and add 25% ammonia dropwise to the mixture, control the pH of the solution at 2, and so on. The mixture was then transferred to a reflux reactor and heated to 95°C for 10 hours. After reflux, it was allowed to stand for 4 hours. The precipitate was collected and washed several times with deionized water. Then, it was dried in a vacuum oven at 100°C for 12 hours to obtain hollow FePO4·2H2O. The hollow FePO4·2H2O prepared above was then placed in a muffle furnace and heated to 650°C at a heating rate of 5°C / min for 8 hours to obtain hollow anhydrous FePO4 material.

[0079] Figure 3 This is a schematic diagram illustrating the principle of synthesizing hollow iron phosphate using the preparation method described in Example 1. Figure 1 This is a SEM image of the ferrous glycerate microspheres obtained in Example 1. Figure 2 This is a SEM image of the hollow anhydrous ferric phosphate obtained in Example 1. The image shows that the synthesized ferrous glycerate is in the form of microspheres. When it is mixed with a phosphorus source and the oxidant hydrogen peroxide to form ferric phosphate, due to the Fe... 3+ The diffusion rate is greater than that of PO4. 3- The diffusion rate of Fe within the ferrous glycerate microspheres 3+ The migration is faster outward, with PO4 on the outside of the ferrous glycerate microspheres. 3- Fe migrates slowly inwards but rapidly outwards. 3+ With PO4 3- Upon encountering each other, they react to form a FePO4 shell. The potential energy difference generated by the diffusion process of the two ions will cause defects to be generated inside the ferrous glycerate microspheres and gradually transform them into a hollow structure, eventually forming hollow iron phosphate.

[0080] Example 2

[0081] This embodiment provides a method for preparing hollow iron phosphate, the method comprising:

[0082] (1) Dissolve 2 mmol of ferrous salt FeSO4·7H2O in 20 mL of deionized water and sonicate for 10 min to obtain solution A; dissolve 15 wt% polyvinylpyrrolidone (PPP) in 40 mL of isopropanol and stir until homogeneous to obtain solution B; pour solution A into solution B, stir continuously, and add 8 mL of glycerol dropwise to the mixture. After stirring for 30 min, transfer the mixture to a 100 mL polytetrafluoroethylene-lined stainless steel autoclave and heat it in an oven at 180 °C for 8 h. After cooling to room temperature, centrifuge to collect the ferrous glycerate microsphere product and wash it repeatedly with deionized water and anhydrous ethanol. Subsequently, vacuum dry the sample at 100 °C for 12 h.

[0083] (2) The ferrous glycerate microsphere powder obtained in step (1) was dispersed in 40 mL of anhydrous ethanol and sonicated for 15 min to obtain dispersion C; 2 mmol of ammonium hydrogen phosphate was sonicated and dispersed in 40 mL of anhydrous ethanol, and then 1 mL of 10% hydrogen peroxide was added dropwise. After stirring evenly, dispersion D was obtained; dispersion C was poured into dispersion D, and stirred at room temperature for 20 min. 25% ammonia was added dropwise to the mixture while stirring continuously to control the pH of the solution at 1.8. The above mixture was transferred to a reflux reactor and heated to 85°C for 8 hours. After the reflux, it was allowed to stand for 3 hours. The precipitate was collected and washed several times with deionized water. Then it was placed in a vacuum oven at 100°C for 12 hours to obtain hollow FePO4·2H2O. The hollow FePO4·2H2O prepared above was then placed in a muffle furnace and heated to 600°C at a heating rate of 5°C / min. The temperature was held for 8 hours to obtain hollow anhydrous FePO4 material.

[0084] Example 3

[0085] This embodiment provides a method for preparing hollow iron phosphate, the method comprising:

[0086] (1) Dissolve 2.5 mmol of ferrous salt FeSO4·7H2O in 20 mL of deionized water and sonicate for 10 min to obtain solution A; dissolve 18 wt% of polyvinylpyrrolidone (PPP) in 40 mL of isopropanol and stir until homogeneous to obtain solution B; pour solution A into solution B, stir continuously, and add 8 mL of glycerol dropwise to the mixture. After stirring for 30 min, transfer the mixture to a 100 mL polytetrafluoroethylene-lined stainless steel autoclave and heat it in an oven at 180 °C for 8 h. After cooling to room temperature, centrifuge to collect the ferrous glycerate microsphere product and wash it repeatedly with deionized water and anhydrous ethanol. Subsequently, vacuum dry the sample at 100 °C for 12 h.

[0087] (2) The ferrous glycerate microsphere powder obtained in step (1) was dispersed in 40 mL of anhydrous ethanol and sonicated for 15 min to obtain dispersion C; 2.5 mmol of ammonium hydrogen phosphate was sonicated and dispersed in 40 mL of anhydrous ethanol, and then 1.5 mL of 10% hydrogen peroxide was added dropwise. After stirring evenly, dispersion D was obtained; dispersion C was poured into dispersion D, and stirred at room temperature for 2 min. 25% ammonia was added dropwise while stirring continuously to control the pH of the solution at 2. The above mixture was transferred to a reflux reactor and heated to 90°C for 10 hours. After reflux, it was allowed to stand for 4 hours. The precipitate was collected and washed several times with deionized water. Then, it was dried in a vacuum oven at 100°C for 12 hours to obtain hollow FePO4·2H2O. The hollow FePO4·2H2O prepared above was then placed in a muffle furnace and heated to 650°C at a heating rate of 5°C / min for 8 hours to obtain hollow anhydrous FePO4 material.

[0088] Example 4

[0089] This embodiment provides a method for preparing hollow ferric phosphate. In step (1), the amount of polyvinylpyrrolidone used is adjusted from 18 wt% to 15 wt% of the ferrous salt mass. Except for the above, the other conditions are exactly the same as in Example 1.

[0090] Example 5

[0091] This embodiment provides a method for preparing hollow ferric phosphate. In step (1), the amount of polyvinylpyrrolidone used is adjusted from 18 wt% to 20 wt% of the ferrous salt mass. Except for the above, the other conditions are exactly the same as in Example 1.

[0092] Example 6

[0093] This embodiment provides a method for preparing hollow iron phosphate. In step (1), the heating temperature is adjusted from 180°C to 160°C. Except for the above, the other conditions are exactly the same as in Example 1.

[0094] Example 7

[0095] This embodiment provides a method for preparing hollow iron phosphate. In step (1), the heating temperature is adjusted from 180°C to 200°C. Except for the above, the other conditions are exactly the same as in Example 1.

[0096] Example 8

[0097] This embodiment provides a method for preparing hollow ferric phosphate. In step (1), the molar amount of ferrous salt is adjusted from 2.5 mmol to 3 mmol. Except for the above, the other conditions are exactly the same as in Example 1.

[0098] Example 9

[0099] This embodiment provides a method for preparing hollow ferric phosphate. In step (1), the molar amount of ferrous salt is adjusted from 2.5 mmol to 3.5 mmol. Except for the above, the other conditions are exactly the same as in Example 1.

[0100] Example 10

[0101] This embodiment provides a method for preparing hollow iron phosphate. In step (2), the pH is adjusted from 1.8 to 2. Except for the above, the other conditions are exactly the same as in Example 1.

[0102] Example 11

[0103] This embodiment provides a method for preparing hollow iron phosphate. In step (2), the pH is adjusted from 1.8 to 2.2. Except for the above, the other conditions are exactly the same as in Example 1.

[0104] Comparative Example 1

[0105] This comparative example provides a method for preparing ferric phosphate. The method does not use ferrous glycerate microspheres, but directly replaces the ferrous glycerate microspheres used in step (2) with ferrous salt in step (1) to prepare dispersion C. Except for the above, the other conditions are exactly the same as in Example 1.

[0106] Comparative Example 2

[0107] This comparative example provides a method for preparing iron phosphate, wherein PVP is not used in step (1) of the preparation method, and all other conditions are exactly the same as in Example 1.

[0108] The iron phosphate obtained in the examples and comparative examples was ground and used as a precursor for the preparation of lithium iron phosphate. Lithium carbonate, glucose, and iron phosphate were dispersed in anhydrous ethanol at a stoichiometric ratio of lithium source, carbon source, and iron phosphate of 1:0.11:1.02. The mixture was ball-milled for 3 hours at 3000 rpm until homogeneous, followed by spray drying to obtain precursor powder. The precursor powder was then heated to 400°C for 1.5 hours under a nitrogen atmosphere at a heating rate of 8°C / min, and then sintered at 700°C for 8 hours to obtain LiFePO4 / C cathode material. The LiFePO4 / C cathode material obtained above was then used to construct coin cells for lithium-ion battery electrochemical performance testing (with charge / discharge voltage controlled between 2.5 and 4.5V). The results are recorded in Table 1.

[0109] Table 1

[0110]

[0111] As can be seen from Table 1, as obtained from Examples 1-11, the lithium iron phosphate battery made from the hollow iron phosphate precursor described in this disclosure has a 0.1C discharge specific capacity of over 157.1 mAh / g, a 0.5C discharge specific capacity of over 151.4 mAh / g, and an initial charge-discharge efficiency of over 98.23%.

[0112] A comparison of Examples 1 and 2-11 shows that factors such as the amount of ferrous salt and polyvinylpyrrolidone (PVP), the synthesis temperature of the microsphere precursor, and the precipitation pH value all affect the electrical performance of lithium-ion batteries. This may be because changes in these parameters affect the morphology and size of the iron phosphate product during the synthesis of hollow iron phosphate materials.

[0113] Compared to the solid, irregular iron phosphate bulk obtained in Comparative Example 1, the iron phosphate prepared in Example 1 has a hollow nanosphere structure. This structure increases the specific surface area of ​​the lithium iron phosphate material prepared using it as a precursor, shortens the lithium-ion diffusion path, and accelerates the diffusion rate, thus improving the electrochemical performance of the lithium-ion battery. In contrast to Example 1, Comparative Example 2 did not add the surfactant PVP when preparing the ferrous glycerate precursor, which may lead to the aggregation of the precursor product, resulting in a decrease in the electrical performance of the subsequently prepared lithium iron phosphate cathode material.

Claims

1. A method for preparing hollow ferric phosphate, comprising: mixing ferrous glycerate microspheres with a phosphorus source and an oxidizing agent to obtain hollow ferric phosphate; the method for preparing the ferrous glycerate microspheres comprises: mixing a ferrous salt, polyvinylpyrrolidone, glycerol and a solvent, and performing heat treatment to obtain ferrous glycerate microspheres; the oxidizing agent comprises hydrogen peroxide.

2. The production method according to claim 1, wherein, The amount of polyvinylpyrrolidone is 15 wt% to 20 wt% of the mass of the ferrous salt.

3. The production method according to claim 1, wherein, The amount of glycerol is 0.8 mL to 1.2 mL per millimole of ferrous salt.

4. The production method according to claim 1, wherein The temperature of the heat treatment is 160 ℃ to 200 ℃, and the time is 8 h to 12 h.

5. The production method according to claim 1, wherein The method for preparing the ferrous glycerate microspheres further comprises preparing the ferrous salt into solution A, preparing polyvinylpyrrolidone into solution B, adding solution A into solution B, and then adding glycerol.

6. The production method according to claim 5, wherein The concentration of the ferrous salt in solution A is 0.1 mmol / mL to 0.2 mmol / mL.

7. The production method according to claim 1, wherein The amount of the ferrous glycerate microspheres and the oxidizing agent is controlled according to the molar ratio of iron to oxidizing agent being 1: (1.2 to 1.5).

8. The production method according to claim 1, wherein The method further comprises preparing the ferrous glycerate microspheres into dispersion liquid C, preparing the phosphorus source and the oxidizing agent into dispersion liquid D, adding dispersion liquid C into dispersion liquid D, and then performing reaction to obtain hollow ferric phosphate.

9. The production method according to claim 8, wherein The pH condition of the reaction is 1.8 to 2.

2.

10. The production method according to claim 8, wherein The temperature of the reaction is 85 ℃ to 95 ℃, and the time is 6 h to 12 h.

11. The production method according to claim 8, wherein The aging time after the reaction is 3 h to 5 h.

12. The method of producing according to claim 1, wherein, The method further comprises performing heat treatment on the obtained hollow ferric phosphate to remove crystal water.

13. The method of making according to claim 12, wherein, The temperature of the heat treatment is 500 ℃ to 750 ℃, and the time is 4 h to 10 h. 14.Hollow ferric phosphate obtained by the method of any one of claims 1 to 13. 15.A method for preparing lithium ferric phosphate material, comprising: mixing a lithium source, a carbon source and the hollow ferric phosphate of claim 14, and then sintering to obtain lithium ferric phosphate positive electrode material.

16. The method of claim 15, wherein the lithium iron phosphate material is prepared by the method of claim 15, wherein, The molar ratio of the lithium source, the carbon source and the hollow ferric phosphate is 1: (0.1 to 0.15) : (1 to 1.1).

17. The method of claim 15, wherein the lithium iron phosphate material is prepared by the method of claim 15, wherein, The temperature of the sintering is 650 ℃ to 750 ℃, and the time is 6 h to 10 h. 18.Lithium ferric phosphate material obtained by the method of any one of claims 15 to 17. 19.A battery comprising the lithium ferric phosphate material of claim 18.

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