A kind of iron phosphate material and its preparation method and use

Abstract

The present application belongs to the field of cathode material preparation. The present application provides an iron phosphate material, a preparation method, and a use thereof. The preparation method uses modified nanocellulose having double bonds and carboxyl groups to form iron phosphate seed crystals. On the one hand, the modified nanocellulose is interspersed within the iron phosphate of the seed crystals, making the seed crystals dispersed, which helps reduce the difficulty of grinding, improves grinding efficiency, and facilitates obtaining small-particle iron phosphate. On the other hand, the modified nanocellulose is cross-linked and forms a three-dimensional network coating on the surface of the seed crystals, giving the seed crystals a high specific surface area and a large number of active groups to attract iron ions. Therefore, when the resulting seed crystals are used to further grow the iron phosphate material, the reaction can be greatly accelerated, thereby increasing the yield of the iron phosphate material.

Description

Technical Field

[0001] The present application relates to the field of positive electrode material preparation, such as raw materials used in the preparation of lithium-ion positive electrode materials, and in particular to an iron phosphate material and its preparation method and use. Background Art

[0002] Lithium battery positive electrode materials mainly include lithium cobalt oxide, lithium iron phosphate, lithium manganese oxide and ternary materials. Compared with other lithium battery positive electrode materials, lithium iron phosphate positive electrode material has the advantages of high safety, long cycle life and low manufacturing cost. It is one of the most promising lithium-ion battery positive electrode materials.

[0003] The main methods for preparing lithium iron phosphate cathode materials include solid-phase method, hydrothermal / solvothermal method, co-precipitation method, sol-gel method, microwave method, etc. Among them, the solid-phase method is the most widely used due to its low cost and suitability for large-scale industrial production.

[0004] The main process for synthesizing lithium iron phosphate using the solid-phase method involves mixing solid raw materials such as an iron phosphate precursor, a lithium source, and a carbon source. Mechanical grinding is then performed to achieve micrometer- or nanometer-scale uniformity. The resulting mixture is then subjected to high-temperature sintering to produce the lithium iron phosphate cathode material. For example, CN114196377A discloses a method for preparing lithium iron phosphate using the solid-phase sintering method, which requires the addition of a grinding aid and dispersant to improve the grinding of the raw materials, such as the iron phosphate.

[0005] It can be seen that as one of the important raw materials for the preparation of lithium iron phosphate, slight changes in the microstructure and chemical composition of iron phosphate will have a huge impact on the performance of lithium iron phosphate.

[0006] However, the existing preparation technology of iron phosphate often faces the following problems, resulting in product performance that needs to be improved. The preparation method of iron phosphate mainly uses phosphoric acid and ferrous salt to directly oxidize in a hydrogen peroxide atmosphere to generate iron phosphate precipitate, or uses inorganic acid to dissolve phosphate and ferrous salt first, and then adds hydrogen peroxide to oxidize to generate iron phosphate precipitate. The traditional preparation process of iron phosphate has problems such as low iron phosphate reaction rate and low crystallinity. Moreover, the obtained iron phosphate product mostly presents tightly packed secondary agglomerates. When used to prepare lithium iron phosphate, it requires higher grinding intensity, so it is more difficult to grind to nanoparticle size. The smaller the target particle size achieved by grinding, the more time and energy it consumes, and the higher the cost.

[0007] Therefore, a new scheme is proposed to control the properties of the prepared iron phosphate material so that it can have the characteristics of easy grinding, small particle size, high crystallinity and stable production, which is of great significance for the preparation of high-performance lithium iron phosphate materials. Summary of the Invention

[0008] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0009] The present application provides an iron phosphate material, a preparation method and use thereof. The preparation method uses modified nanocellulose having double bonds and carboxyl groups to form iron phosphate seeds. On the one hand, the modified nanocellulose is interspersed in the iron phosphate of the seed crystals, making the interior of the seed crystals evacuated, which is conducive to reducing the difficulty of grinding, improving grinding efficiency, and facilitating the acquisition of small-particle iron phosphate; on the other hand, the modified nanocellulose is cross-linked and forms a three-dimensional network coating on the surface of the seed crystals, so that the seed crystals have a high specific surface area and a large number of active groups to attract iron ions. Therefore, when the obtained seed crystals are used to further grow the iron phosphate material, the reaction can be greatly accelerated and the yield of the iron phosphate material can be improved.

[0010] In a first aspect, the present application provides a method for preparing an iron phosphate material, the preparation method comprising:

[0011] The modified nanocellulose having double bonds and carboxyl groups is mixed with an iron phosphate raw material to perform a formation reaction to form a first precursor;

[0012] mixing an initiator and the first precursor, performing a polymerization reaction, and coating the surface of the first precursor with the modified nanocellulose to obtain a second precursor;

[0013] The obtained second precursor is used as a seed crystal and mixed with the iron phosphate raw material again to carry out a formation reaction to obtain the iron phosphate material.

[0014] The present application prepares small-particle modified nanocellulose-coated iron phosphate as a seed crystal, and then uses the seed crystal to grow the final iron phosphate material, so that the particle size distribution of the final iron phosphate material is inherited to a certain extent from the seed crystal. The particle size distribution of the final iron phosphate material is relatively uniform and is conducive to improving the crystallinity.

[0015] Specifically, this application requires the use of modified nanocellulose with active functional groups such as carboxyl groups and double bonds. The carboxyl groups are used to attract iron and accelerate the reaction rate during the subsequent growth of the iron phosphate material. The double bonds are used to cause polymerization reactions between the modified nanocellulose and produce crosslinks.

[0016] When the modified nanocellulose is mixed with the iron phosphate raw material, the modified nanocellulose is intercalated in the iron phosphate during the process of generating the first precursor, so that the interior of the first precursor contains the modified nanocellulose, and the modified nanocellulose makes the internal structure of the first precursor sparse. This sparse structural state is not only conducive to the grinding of the subsequently formed crystal seeds and facilitates the control of the particle size of the crystal seeds, but also helps to improve the grinding properties of the final iron phosphate material, reduce the grinding difficulty of obtaining small-particle iron phosphate products, improve grinding efficiency, and thus effectively reduce costs.

[0017] After obtaining the first precursor, the present application uses an initiator to cause the modified nanocellulose contained in the first precursor (especially the modified nanocellulose exposed on the surface) to undergo a polymerization reaction with the modified nanocellulose outside the first precursor, so that the modified cellulose gradually cross-links on the surface of the first precursor and forms a three-dimensional network structure that covers the surface of the first precursor to form a second precursor; when the second precursor is ground into seed crystals to grow iron phosphate, the seed crystals with a high specific surface area and a large number of active functional groups such as carboxyl groups can easily attract iron ions, thereby accelerating the iron phosphate formation reaction and increasing the yield of iron phosphate.

[0018] The following are optional technical solutions for this application, but are not intended to limit the technical solutions provided in this application. Through the following technical solutions, the technical objectives and beneficial effects of this application can be better achieved and realized.

[0019] As an optional technical solution of the present application, the method for preparing the modified nanocellulose having double bonds and carboxyl groups includes: treating the nanocellulose with a low eutectic solvent.

[0020] In one embodiment, the nanocellulose comprises cellulose nanocrystals (CNCs) and / or cellulose nanofibers (CNFs).

[0021] In one embodiment, the deep eutectic solvent comprises a hydrogen bond acceptor, a saturated amide hydrogen bond donor, a carboxylic acid hydrogen bond donor, and an unsaturated amide hydrogen bond donor.

[0022] When nanocellulose is modified in a deep eutectic solvent, the hydroxyl groups on the surface of the nanocellulose react with the carboxylic acid components in the deep eutectic solvent, making the surface of the nanocellulose rich in carboxyl groups. At this time, unsaturated amide hydrogen bond donors such as maleimide can enhance the activity of the amide groups in the deep eutectic solvent system and the carboxyl groups on the cellulose surface, causing the amides (including saturated amide hydrogen bond donors such as urea, and unsaturated amide hydrogen bond donors such as maleimide itself) to react with the surface carboxyl groups of cellulose, grafting unsaturated double bonds onto the cellulose, resulting in the nanocellulose surface containing both carboxyl groups and double bonds. The introduction of amide groups can also increase the surface charge of nanocellulose, thereby improving its dispersibility.

[0023] The carboxylic acid hydrogen bond donor may be a saturated carboxylic acid; the saturated carboxylic acid mentioned above means that it does not contain a carbon-carbon double bond and a carbon-carbon triple bond, and the unsaturated carboxylic acid means that it contains a carbon-carbon double bond and / or a carbon-carbon triple bond.

[0024] In one embodiment, the method for preparing the modified nanocellulose having double bonds and carboxyl groups comprises: first mixing a hydrogen bond acceptor, a saturated amide hydrogen bond donor, and a carboxylic acid hydrogen bond donor to obtain a transparent solution, then adding an unsaturated amide hydrogen bond donor and nanocellulose to carry out a modification reaction to obtain modified nanocellulose having double bonds and carboxyl groups.

[0025] Because carboxyl groups need to be formed on the surface of nanocellulose when using a low eutectic solvent to modify nanocellulose, the nanocellulose is first treated with a low eutectic solvent composed of hydrogen bond acceptors, saturated amide hydrogen bond donors and carboxylic acid hydrogen bond donors, and then the unsaturated amide hydrogen bond donor component is added. This modification effect is better than directly using a low eutectic solvent containing an unsaturated amide hydrogen bond donor.

[0026] In one embodiment, the transparent solution is obtained under stirring at 70-90°C, for example, 70°C, 74°C, 78°C, 80°C, 82°C, 86°C or 90°C, and cooled to 25-40°C, for example, 25°C, 28°C, 30°C, 33°C, 35°C, 38°C or 40°C, before adding the unsaturated amide hydrogen bond donor and nanocellulose.

[0027] In one embodiment, the temperature of the modification reaction is 90-110°C, for example, 90°C, 94°C, 98°C, 100°C, 102°C, 105°C, 108°C or 110°C, and the time is 2-4h, for example, 2h, 2.4h, 2.8h, 3h, 3.2h, 3.6h or 4h, but is not limited to the listed values, and other unlisted values ​​within the above numerical range are also applicable.

[0028] As an optional technical solution of the present application, the hydrogen bond acceptor includes a quaternary ammonium salt, and the quaternary ammonium salt includes choline chloride.

[0029] In one embodiment, the saturated amide hydrogen bond donor comprises urea.

[0030] In one embodiment, the carboxylic acid hydrogen bond donor includes at least one of oxalic acid, citric acid, malonic acid, succinic acid, glutaric acid or adipic acid. For example, typical but non-limiting combinations include a combination of oxalic acid and citric acid, a combination of malonic acid and oxalic acid, a combination of citric acid and succinic acid, a combination of oxalic acid and glutaric acid, or a combination of malonic acid and adipic acid.

[0031] In one embodiment, the unsaturated amide hydrogen bond donor comprises maleimide.

[0032] In one embodiment, the molar ratio of the hydrogen bond acceptor, the saturated amide hydrogen bond donor, and the carboxylic acid hydrogen bond donor is 1:(3-5):(1-3), for example, 1:3:1, 1:3:1.5, 1:3:2, 1:3:2.5, 1:3:3, 1:3.5:1, 1:3.5:1.5, 1:3.5:2, 1:3.5:2.5, 1:3.5:3, 1:4:1, 1: 4∶1.5, 1∶4∶2, 1∶4∶2.5, 1∶4∶3, 1∶4.5∶1, 1∶4.5∶1.5, 1∶4.5∶2, 1∶4.5∶2.5, 1∶4.5∶3, 1∶5∶1, 1∶5∶1.5, 1∶5∶2, 1∶5∶2.5 or 1∶5∶3, etc., but are not limited to the listed values, other values ​​not listed within the above numerical range are also applicable.

[0033] In one embodiment, the molar ratio of the unsaturated amide hydrogen bond donor to the hydrogen bond acceptor is (0.8-1.2):1, for example, 0.8:1, 0.85:1, 0.9:1, 0.95:1, 1:1, 1.05:1, 1.1:1, 1.15:1 or 1.2:1, etc., but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0034] In one embodiment, the amount of the nanocellulose is 10% to 20% by mass of the transparent solution, for example, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19% or 20%, etc., but is not limited to the listed values. Other values ​​not listed within the above numerical range are also applicable.

[0035] As an optional technical solution of the present application, the ferric phosphate raw material includes an iron source and a phosphorus source.

[0036] In one embodiment, the iron source comprises a ferrous salt and / or a ferric salt.

[0037] In one embodiment, the iron source includes at least one of iron sulfate, iron nitrate or iron chloride. Specifically, the iron source includes at least one of ferric sulfate, ferric nitrate, ferric chloride, ferrous sulfate, ferrous nitrate or ferrous chloride.

[0038] In one embodiment, the phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or ammonium phosphate. For example, typical but non-limiting combinations include a combination of phosphoric acid and ammonium dihydrogen phosphate, a combination of ammonium dihydrogen phosphate and diammonium hydrogen phosphate, or a combination of phosphoric acid and ammonium phosphate. In one embodiment, the phosphoric acid is concentrated phosphoric acid with a concentration of 85%.

[0039] In one embodiment, the amount of the iron source and the phosphorus source is controlled according to the molar ratio of iron to phosphate being 1:(0.98-1.02), that is, the iron-phosphorus ratio is controlled to be 0.98-1.02, for example, 1:0.98, 1:0.985, 1:0.99, 1:0.995, 1:1, 1:1.01, 1:1.015, 1:1.02 or 1:1.025, etc., but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0040] As an optional technical solution of the present application, when the iron source includes a divalent iron salt, the ferric phosphate raw material also includes an oxidant.

[0041] In one embodiment, the oxidizing agent comprises hydrogen peroxide.

[0042] In one embodiment, the amount of the oxidant is 1 to 1.2 times the molar amount of the iron element in the iron source, for example, 1 time, 1.03 times, 1.05 times, 1.08 times, 1.1 times, 1.13 times, 1.15 times, 1.18 times or 1.2 times, etc., but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0043] As an optional technical solution of the present application, the generation reaction is carried out at a pH of 1.8 to 2.2, for example, 1.8, 1.85, 1.9, 1.95, 2, 2.05, 2.1, 2.15 or 2.2, etc., but is not limited to the listed values, and other unlisted values ​​within the above numerical range are also applicable.

[0044] In one embodiment, aqueous ammonia is used to adjust the pH of the formation reaction.

[0045] In one embodiment, the reaction time is 1 to 5 hours, for example, 1 hour, 1.5 hours, 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 values ​​not listed within the above numerical range are also applicable.

[0046] In one embodiment, the amount of the modified nanocellulose is 1 to 5 g per mole of iron, for example, 1 g, 1.3 g, 1.5 g, 1.8 g, 2 g, 2.3 g, 2.5 g, 2.8 g, 3 g, 3.3 g, 3.5 g, 3.8 g, 4 g, 4.2 g, 4.5 g, 4.8 g or 5 g, but is not limited to the listed values. Other values ​​not listed within the above numerical range are also applicable.

[0047] In one embodiment, the initiator comprises dicumyl peroxide and / or benzoyl peroxide.

[0048] In one embodiment, the mass ratio of the initiator to the modified nanocellulose is 1:(20-60), for example, 1:20, 1:23, 1:25, 1:28, 1:32, 1:35, 1:38, 1:43, 1:46, 1:48, 1:51, 1:54, 1:57 or 1:60, etc., but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0049] As an optional technical solution of the present application, the second precursor is ground to 1 to 3 μm and then used as the seed crystal, for example, 1 μm, 1.3 μm, 1.5 μm, 1.8 μm, 2 μm, 2.3 μm, 2.5 μm, 2.8 μm or 3 μm, etc., but is not limited to the listed values, and other unlisted values ​​within the above numerical range are also applicable.

[0050] In one embodiment, the amount of the second precursor and the ferric phosphate raw material is controlled so that the mass of the seed crystal accounts for 5% to 20% of the mass of the obtained ferric phosphate material, for example, 5%, 8%, 10%, 12%, 14%, 16%, 18% or 20%, etc., but is not limited to the listed values. Other values ​​not listed within the above numerical range are also applicable.

[0051] In one embodiment, the preparation method further comprises calcining the obtained ferric phosphate material.

[0052] The preparation method described in the present application utilizes crystal seeds, a phosphorus source, and an iron source to generate ferric phosphate in a solution system to obtain dihydrated ferric phosphate, which can be further calcined to remove the crystal water to obtain an anhydrous ferric phosphate material.

[0053] In one embodiment, the calcination temperature is 400-600°C, for example, 400°C, 430°C, 450°C, 480°C, 500°C, 530°C, 550°C, 580°C or 600°C, and the calcination time is 2-4h, for example, 2h, 2.3h, 2.5h, 2.8h, 3h, 3.2h, 3.5h, 3.8h or 4h, but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0054] As an optional technical solution of this application, the preparation method includes:

[0055] A hydrogen bond acceptor, a saturated amide hydrogen bond donor, and a carboxylic acid hydrogen bond donor are placed in a container at a molar ratio of 1:(3-5):(1-3) and mixed evenly, heated to 70-90° C. and continuously stirred until the mixture becomes a transparent solution, and then the temperature is lowered to 25-40° C. to obtain a first mixed solution;

[0056] placing an unsaturated amide hydrogen bond donor and nanocellulose into the first mixed solution, controlling the molar ratio of the unsaturated amide hydrogen bond donor to the hydrogen bond acceptor to be (0.8-1.2):1, and using the nanocellulose in an amount of 10%-20% by mass of the transparent solution to form a deep eutectic solvent, performing a modification reaction on the nanocellulose at 90-110° C. for 2-4 hours, and then filtering, washing, and drying the reaction product to obtain modified nanocellulose having carboxyl groups and double bonds on the surface;

[0057] The modified nanocellulose is uniformly dispersed in pure water, and a phosphorus source and a trivalent iron salt are then added and mixed uniformly. The amount of the modified nanocellulose is controlled to be 1 to 5 g per mole of iron, and the molar ratio of the iron element to the phosphate in the trivalent iron source and the phosphorus source is controlled to be 1: (0.98 to 1.02). The pH is then adjusted to 1.8 to 2.2, and a formation reaction is carried out for 1 to 5 hours to generate iron phosphate containing the modified nanocellulose as a first precursor, thereby obtaining a second mixed solution.

[0058] adding an initiator to the second mixed solution, controlling the mass ratio of the initiator to the modified nanocellulose to be 1:(20-60), causing the modified nanocellulose contained in the first precursor and the modified nanocellulose in the second mixed solution to undergo a polymerization reaction, obtaining cross-linked nanocellulose that is uniformly coated on the surface of the first precursor, thereby forming a second precursor;

[0059] The obtained second precursor is ground to a particle size of 1 to 3 μm and then used as a seed crystal, mixed with a phosphorus source, a divalent iron salt and an oxidant, the molar ratio of the iron element in the divalent iron source and the phosphorus source to the phosphate ion is controlled to be 1: (0.98 to 1.02), the amount of the oxidant is controlled to be 1 to 1.2 times the molar amount of the iron element in the iron source, and then ammonia water is added to adjust the pH to 1.8 to 2.2, and the formation reaction is carried out for 1 to 5 hours to obtain ferric phosphate dihydrate particles, wherein the mass of the seed crystal is controlled to account for 5% to 20%;

[0060] The obtained ferric phosphate dihydrate is calcined at 400-600° C. for 2-4 hours to remove crystal water, thereby obtaining anhydrous ferric phosphate.

[0061] In a second aspect, the present application provides an iron phosphate material, which is obtained using the preparation method described in the first aspect.

[0062] In a third aspect, the present application provides a positive electrode active material, which is prepared using the iron phosphate material described in the second aspect.

[0063] The positive electrode active material includes lithium iron phosphate or lithium manganese iron phosphate.

[0064] It should be noted that iron phosphate components have been formed in the first precursor and the second precursor, and the first precursor and / or the second precursor can also be used to directly synthesize lithium iron phosphate. However, the performance of lithium iron phosphate will be different from that of lithium iron phosphate synthesized by using the obtained second precursor as a seed crystal, mixing it with the iron phosphate raw material again, and performing a generation reaction.

[0065] Compared with the related technical solutions, this application has at least the following beneficial effects:

[0066] The preparation method described in the present application utilizes modified nanocellulose having double bonds and carboxyl groups to form iron phosphate seed crystals. On the one hand, the modified nanocellulose is intercalated in the iron phosphate of the seed crystals, making the interior of the seed crystals dispersed, which is conducive to reducing the difficulty of grinding, improving the grinding efficiency, and facilitating the acquisition of small-particle iron phosphate; on the other hand, the modified nanocellulose is cross-linked and forms a three-dimensional network coating on the surface of the seed crystals, so that the seed crystals have a high specific surface area and a large number of active groups to attract iron ions. Therefore, when the obtained seed crystals are used to further grow the iron phosphate material, the reaction can be greatly accelerated and the yield of the iron phosphate material can be increased.

[0067] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] The accompanying drawings are used to provide a further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of this application, they are used to explain the technical solution of this article and do not constitute a limitation on the technical solution of this article.

[0069] Figure 1 This is a SEM test image of the anhydrous ferric phosphate obtained in Example 1;

[0070] Figure 2 This is the XRD test pattern of the anhydrous ferric phosphate obtained in Example 1. DETAILED DESCRIPTION

[0071] The technical solution of this application is further explained below through specific implementation methods.

[0072] Those skilled in the art should understand that the embodiments are only intended to help understand the present application and should not be regarded as specific limitations of the present application.

[0073] Example 1

[0074] This embodiment provides a method for preparing iron phosphate, which comprises:

[0075] (1) Choline chloride, urea, and oxalic acid were mixed uniformly in a molar ratio of 1:4:2 in a container, heated to 80°C and stirred continuously until the mixture became a transparent solution, and then cooled to 30°C to obtain a deep eutectic solvent;

[0076] (2) placing maleimide and cellulose nanocrystals (CNCs) as nanocellulose in the above-mentioned deep eutectic solvent and reacting them at 100°C for 3 hours, then filtering, washing, and drying the reaction product to obtain modified nanocellulose containing carboxyl groups and double bonds on the surface, wherein the molar ratio of maleimide to choline chloride is 1:1, and the amount of nanocellulose added is 15% of the mass of the deep eutectic solvent;

[0077] (3) 3 g of modified nanocellulose was evenly dispersed in pure water, and then 85% concentrated phosphoric acid and ferric sulfate solution were added and mixed evenly. The pH was adjusted to 2 and the reaction was carried out for 2 h to obtain ferric phosphate containing modified nanocellulose. The iron concentration in the ferric sulfate solution was 1.5 mol / L, and the iron-phosphorus ratio Fe:P was controlled to be 1;

[0078] (4) After the reaction in step 3 is completed, 0.15 g of dicumyl peroxide is added to the reaction solution of step (3) and the reaction is continued for 3 hours. The modified nanocellulose contained in the ferric phosphate and the modified nanocellulose in the solution undergo a polymerization reaction so that the cross-linked modified nanocellulose is evenly coated on the surface of the ferric phosphate. After the reaction is completed, the product is filtered, washed, and dried to obtain the modified nanocellulose-coated ferric phosphate;

[0079] (5) Grinding the modified nanocellulose-coated ferric phosphate obtained in step 4 to a particle size of 0.5 μm as seed crystals, adding the seed crystals to a reactor, then adding ferrous sulfate, phosphoric acid, and hydrogen peroxide to mix, and adding ammonia water to adjust the pH of the mixture to 2 and reacting for 3 hours to obtain dihydrate ferric phosphate particles grown on the modified nanocellulose-coated ferric phosphate seed crystals, wherein the molar amount of hydrogen peroxide added is 1.1 times the molar amount of ferrous sulfate, the iron-phosphorus ratio Fe:P is controlled to be 1, and the mass of the seed crystals accounts for 10% of the dihydrate ferric phosphate particles;

[0080] (6) The obtained ferric phosphate dihydrate was placed in a muffle furnace and sintered at 500°C for 3 h to remove crystal water and obtain anhydrous ferric phosphate.

[0081] Example 2

[0082] This embodiment provides a method for preparing iron phosphate, which comprises:

[0083] (1) Choline chloride, urea, and citric acid were mixed uniformly in a molar ratio of 1:5:1 in a container, heated to 70°C and stirred continuously until the mixture became a transparent solution, and then the temperature was lowered to 40°C to obtain a deep eutectic solvent;

[0084] (2) placing maleimide and cellulose nanocrystals (CNCs) as nanocellulose in the above-mentioned deep eutectic solvent and reacting them at 110°C for 2 hours, then filtering, washing, and drying the reaction product to obtain modified nanocellulose containing carboxyl groups and double bonds on the surface, wherein the molar ratio of maleimide to choline chloride is 1:1, and the amount of nanocellulose added is 20% of the mass of the deep eutectic solvent;

[0085] (3) 5 g of modified nanocellulose was evenly dispersed in pure water, and then 85% concentrated phosphoric acid and ferric sulfate solution were added and mixed evenly. The pH was adjusted to 1.8 and the reaction was carried out for 1.5 h to obtain ferric phosphate containing modified nanocellulose. The iron concentration in the ferric sulfate solution was 1 mol / L, and the iron-phosphorus ratio Fe:P was controlled to be 0.98.

[0086] (4) After the reaction in step 3 is completed, 0.125 g of benzoyl oxide is added to the reaction solution of step (3) and the reaction is continued for 3 hours. The modified nanocellulose contained in the ferric phosphate and the modified nanocellulose in the solution undergo a polymerization reaction so that the cross-linked modified nanocellulose is evenly coated on the surface of the ferric phosphate. After the reaction is completed, the product is filtered, washed, and dried to obtain the modified nanocellulose-coated ferric phosphate;

[0087] (5) Grinding the modified nanocellulose-coated ferric phosphate obtained in step 4 to a particle size of 3 μm as seed crystals, adding the seed crystals to a reactor, then adding ferrous sulfate, phosphoric acid, and hydrogen peroxide to mix, and adding ammonia water to adjust the pH of the mixture to 1.8 and reacting for 1 hour to obtain dihydrate ferric phosphate particles grown on the modified nanocellulose-coated ferric phosphate seed crystals, wherein the molar amount of hydrogen peroxide added is 1 times the molar amount of ferrous sulfate, the iron-phosphorus ratio Fe:P is controlled to be 1.02, and the mass of the seed crystals accounts for 20% of the dihydrate ferric phosphate particles;

[0088] (6) The obtained ferric phosphate dihydrate was placed in a muffle furnace and sintered at 400°C for 4 h to remove crystal water and obtain anhydrous ferric phosphate.

[0089] Example 3

[0090] This embodiment provides a method for preparing iron phosphate, which comprises:

[0091] (1) Choline chloride, urea, and citric acid were mixed uniformly in a molar ratio of 1:3:3 in a container, heated to 90°C and stirred continuously until the mixture became a transparent solution, and then cooled to 25°C to obtain a deep eutectic solvent;

[0092] (2) placing maleimide and cellulose nanocrystals (CNCs) as nanocellulose in the above-mentioned deep eutectic solvent and reacting them at 110°C for 2 hours, then filtering, washing, and drying the reaction product to obtain modified nanocellulose containing carboxyl groups and double bonds on the surface, wherein the molar ratio of maleimide to choline chloride is 1:1, and the amount of nanocellulose added is 10% of the mass of the deep eutectic solvent;

[0093] (3) 5 g of modified nanocellulose was evenly dispersed in pure water, and then 85% concentrated phosphoric acid and ferric sulfate solution were added and mixed evenly. The pH was adjusted to 2.2 and the reaction was carried out for 1.5 h to obtain ferric phosphate containing modified nanocellulose. The iron concentration in the ferric sulfate solution was 2 mol / L, and the iron-phosphorus ratio Fe:P was controlled to be 0.98.

[0094] (4) After the reaction in step 3 is completed, 0.083 g of benzoyl oxide is added to the reaction solution of step (3) and the reaction is continued for 3 hours. The modified nanocellulose contained in the ferric phosphate and the modified nanocellulose in the solution undergo a polymerization reaction so that the cross-linked modified nanocellulose is evenly coated on the surface of the ferric phosphate. After the reaction is completed, the product is filtered, washed, and dried to obtain the modified nanocellulose-coated ferric phosphate;

[0095] (5) Grinding the modified nanocellulose-coated ferric phosphate obtained in step 4 to a particle size of 0.7 μm as seed crystals, adding the seed crystals to a reactor, then adding ferrous sulfate, phosphoric acid, and hydrogen peroxide to mix, and adding ammonia water to adjust the pH of the mixture to 1.8 and reacting for 1 hour to obtain dihydrate ferric phosphate particles grown on the modified nanocellulose-coated ferric phosphate seed crystals, wherein the molar amount of hydrogen peroxide added is 1.2 times the molar amount of ferrous sulfate, the iron-phosphorus ratio Fe:P is controlled to be 0.98, and the mass of the seed crystals accounts for 5% of the dihydrate ferric phosphate particles;

[0096] (6) The obtained ferric phosphate dihydrate was placed in a muffle furnace and sintered at 600°C for 2 h to remove crystal water and obtain anhydrous ferric phosphate.

[0097] Example 4

[0098] This embodiment provides a method for preparing iron phosphate. In the preparation method, the amount of modified nanocellulose is adjusted from 3 g to 1 g. Other conditions are exactly the same as those in Example 1.

[0099] Example 5

[0100] This embodiment provides a method for preparing iron phosphate. In the preparation method, the amount of modified nanocellulose is adjusted from 3 g to 0.5 g. Other conditions are exactly the same as those in Example 1.

[0101] Example 6

[0102] This embodiment provides a method for preparing iron phosphate. In the preparation method, the amount of modified nanocellulose is adjusted from 3 g to 7.5 g. Apart from this, other conditions are exactly the same as those in Example 1.

[0103] Example 7

[0104] This embodiment provides a method for preparing iron phosphate. In the preparation method, the amount of modified nanocellulose is adjusted from 3 g to 9 g. Other conditions are exactly the same as those in Example 1.

[0105] Example 8

[0106] This embodiment provides a method for preparing ferric phosphate. The preparation method adjusts the amount of seed crystals so that the mass fraction of the seed crystals in the ferric phosphate dihydrate particles is adjusted from 10% to 2%. Other conditions are exactly the same as those in Example 1.

[0107] Example 9

[0108] This embodiment provides a method for preparing ferric phosphate. The preparation method adjusts the amount of seed crystals so that the mass fraction of the seed crystals in the ferric phosphate dihydrate particles is adjusted from 10% to 5%. Other conditions are exactly the same as those in Example 1.

[0109] Example 10

[0110] This embodiment provides a method for preparing ferric phosphate. The preparation method adjusts the amount of seed crystals so that the mass fraction of the seed crystals in the ferric phosphate dihydrate particles is adjusted from 10% to 20%. Other conditions are exactly the same as those in Example 1.

[0111] Example 11

[0112] This embodiment provides a method for preparing ferric phosphate. The preparation method adjusts the amount of seed crystals so that the mass fraction of the seed crystals in the ferric phosphate dihydrate particles is adjusted from 10% to 23%. Other conditions are exactly the same as those in Example 1.

[0113] Comparative Example 1

[0114] This comparative example provides a method for preparing iron phosphate, which does not use modified nanocellulose but performs secondary growth through seed crystals. The method comprises:

[0115] 85% concentrated phosphoric acid, ferrous sulfate solution, and hydrogen peroxide are mixed evenly, and ammonia water is added to adjust the pH to 2 and react for 2 hours to obtain ferric phosphate, wherein the iron concentration in the ferrous sulfate solution is 1.5 mol / L, and the iron-phosphorus ratio Fe:P is controlled to be 1;

[0116] The iron phosphate obtained in step 1 was ground to a particle size of 0.5 μm as seed crystals, and the seed crystals were added to a reactor at a ratio of 10% of the seed crystals. Ferrous sulfate, phosphoric acid, and hydrogen peroxide were then added and mixed. Ammonia was added to adjust the pH of the mixture to 2 and the reaction was continued for 3 hours to obtain dihydrate iron phosphate particles grown on the basis of the doped and coated nanocellulose iron phosphate seed crystals. The molar amount of hydrogen peroxide added was 1.1 times the molar amount of the newly added ferrous sulfate, and the iron-phosphorus ratio was controlled to be Fe:P=1.

[0117] (3) Place the dihydrated ferric phosphate in a muffle furnace and sinter at 500°C for 3 hours to remove the crystal water to obtain anhydrous ferric phosphate.

[0118] Comparative Example 2

[0119] This comparative example provides a method for preparing iron phosphate, which does not use modified nanocellulose and does not perform secondary growth through seed crystals. The preparation method comprises:

[0120] 85% concentrated phosphoric acid, ferrous sulfate solution, and hydrogen peroxide are mixed evenly, and ammonia water is added to adjust the pH to 2 and react for 2 hours to obtain ferric phosphate dihydrate, wherein the iron concentration in the ferrous sulfate solution is 1.5 mol / L, the iron-phosphorus ratio Fe:P is controlled to be 1, and the molar amount of hydrogen peroxide added is 1.1 times the molar amount of ferrous sulfate;

[0121] The dihydrated ferric phosphate was placed in a muffle furnace and sintered at 500° C. for 3 h to remove the crystal water and obtain anhydrous ferric phosphate.

[0122] Comparative Example 3

[0123] This comparative example provides a method for preparing iron phosphate, wherein the preparation method uses unmodified nanocellulose, i.e., cellulose nanocrystals (CNC), instead of modified nanocellulose. Apart from this, other conditions are exactly the same as those in Example 1.

[0124] Performance testing:

[0125] (2) The anhydrous ferric phosphate prepared in the examples and comparative examples was subjected to an iron-phosphorus ratio test. The iron content was determined by chemical titration, and the phosphorus content was determined by the quinoline molybdate weight method. The iron-phosphorus ratio was calculated as the ratio of the amount of iron substance to the amount of phosphorus substance, Ms: Ms = w1 / w2 × 0.5545, where w1 is the iron content (mass fraction) in the ferric phosphate sample, w2 is the phosphorus content (mass fraction) in the ferric phosphate sample, and 0.5545 is the ratio of the relative molecular mass of phosphorus to the relative molecular mass of iron.

[0126] (3) For the lithium iron phosphate precursor prepared in the examples and comparative examples, iron phosphate and lithium carbonate were weighed at a molar ratio of P:Li of 1:1, and sucrose accounting for 10% of the mass fraction of the iron phosphate was mixed evenly, and then calcined at 720°C for 6 hours under a nitrogen atmosphere, and naturally cooled to room temperature to finally obtain the lithium iron phosphate positive electrode material. The positive electrode active material lithium iron phosphate, the conductive agent (Super P), and the binder polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 90:5:5, and added to the solvent N-methylpyrrolidone (NMP), stirred to form a uniform positive electrode active material slurry, and the slurry was evenly coated on the positive electrode current collector aluminum foil and dried to obtain the positive electrode sheet. Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed at a mass ratio of 1:1:1 to prepare an electrolyte with a LiPF6 concentration of 1.0 mol / L. A 2025 button-type battery was assembled using polyethylene film (PE) as the separator and a metal lithium sheet as the negative electrode to test the electrochemical performance, and charge and discharge tests were performed at room temperature between 2 and 3.75 V.

[0127] The above results are recorded in Table 1.

[0128] Table 1

[0129]

[0130]

[0131] Figure 1 and Figure 2 3 are SEM and XRD test images of the anhydrous ferric phosphate material obtained in Example 1. It can be seen from the figure that the particle size distribution of the obtained ferric phosphate is relatively uniform and the crystallinity is high.

[0132] Compared with Comparative Example 1 and Comparative Example 2, Example 1 shows that the addition of the seeds described in the present application for the preparation of iron phosphate can produce purer iron phosphate dihydrate, with higher sphericity and better particle consistency. The iron-phosphorus of the iron phosphate finally obtained is relatively high because a portion of the material is first crystallized and used as a seed to induce the remaining material to undergo crystallization. If iron phosphate is not used as a seed, more phosphoric acid is required for the crystallization of the material, which can easily produce impure iron phosphate dihydrate. Compared with Comparative Example 1, Example 1 shows that modified nanocellulose is added to the formation of primary seeds, which is easy to grind when synthesizing lithium iron phosphate, which is beneficial to the synthesis of lithium iron phosphate from iron phosphate, and the porosity of the lithium iron phosphate increases after calcination of nanocellulose, which is beneficial to improving the mobility of lithium ions. Therefore, the lithium iron phosphate synthesized using the iron phosphate as a precursor has high capacity and high rate characteristics.

[0133] The above describes in detail the optional implementation methods of the present application. However, the present application is not limited to the specific details of the above implementation methods. Within the technical concept of the present application, various simple modifications can be made to the technical solution of the present application, and these simple modifications all fall within the scope of protection of the present application.

[0134] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner unless there is any contradiction. In order to avoid unnecessary repetition, this application will not further describe various possible combinations.

[0135] In addition, the various implementation methods of the present application may be arbitrarily combined, and as long as they do not violate the concept of the present application, they should also be regarded as the contents disclosed in the present application.

Claims

1. A method for preparing an iron phosphate material, comprising: The modified nanocellulose having double bonds and carboxyl groups is mixed with an iron phosphate raw material to perform a formation reaction to form a first precursor; The method for preparing the modified nanocellulose having double bonds and carboxyl groups comprises: treating the nanocellulose with a deep eutectic solvent; the deep eutectic solvent comprises a hydrogen bond acceptor, a saturated amide hydrogen bond donor, a carboxylic acid hydrogen bond donor, and an unsaturated amide hydrogen bond donor; and the amount of the modified nanocellulose used is 1 to 5 g per mole of iron element; mixing an initiator and the first precursor, wherein the initiator includes dicumyl peroxide and / or benzoyl peroxide, and performing a polymerization reaction to coat the surface of the first precursor with modified nanocellulose to obtain a second precursor; The obtained second precursor is used as a seed crystal and mixed with the iron phosphate raw material again to carry out a formation reaction to obtain the iron phosphate material.

2. The preparation method according to claim 1, wherein The method for preparing the modified nanocellulose having double bonds and carboxyl groups comprises: first mixing a hydrogen bond acceptor, a saturated amide hydrogen bond donor and a carboxylic acid hydrogen bond donor to obtain a transparent solution, then adding an unsaturated amide hydrogen bond donor and nanocellulose to carry out a modification reaction to obtain the modified nanocellulose having double bonds and carboxyl groups.

3. The preparation method according to claim 2, wherein The transparent solution is obtained under stirring at 70-90° C. and cooled to 25-40° C. before adding the unsaturated amide hydrogen bond donor and nanocellulose.

4. The preparation method according to claim 2 or 3, wherein The modification reaction temperature is 90-110° C. and the reaction time is 2-4 hours.

5. The preparation method according to any one of claims 1 to 3, wherein The hydrogen bond acceptor comprises a quaternary ammonium salt, and the quaternary ammonium salt comprises choline chloride.

6. The preparation method according to any one of claims 1 to 3, wherein The saturated amide hydrogen bond donor includes urea.

7. The preparation method according to any one of claims 1 to 3, wherein The carboxylic acid hydrogen bond donor includes at least one of oxalic acid, citric acid, malonic acid, succinic acid, glutaric acid or adipic acid.

8. The preparation method according to any one of claims 1 to 3, wherein The unsaturated amide hydrogen bond donor includes maleimide.

9. The preparation method according to any one of claims 1 to 3, wherein The molar ratio of the hydrogen bond acceptor, the saturated amide hydrogen bond donor and the carboxylic acid hydrogen bond donor is 1:(3-5):(1-3).

10. The preparation method according to any one of claims 1 to 3, wherein The molar ratio of the unsaturated amide hydrogen bond donor to the hydrogen bond acceptor is (0.8-1.2):

1.

11. The preparation method according to claim 2 or 3, wherein The amount of the nanocellulose used is 10% to 20% of the mass of the transparent solution.

12. The preparation method according to claim 1, wherein The ferric phosphate raw material includes an iron source and a phosphorus source.

13. The preparation method according to claim 12, wherein The iron source includes a divalent iron salt and / or a trivalent iron salt.

14. The preparation method according to claim 13, wherein The iron source includes at least one of iron sulfate, iron nitrate or iron chloride.

15. The preparation method according to claim 12, wherein The phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate or ammonium phosphate.

16. The preparation method according to claim 12, wherein The amount of the iron source and the phosphorus source is controlled according to the molar ratio of iron element to phosphate radical being 1: (0.98-1.02).

17. The preparation method according to claim 13 or 14, wherein When the iron source includes a ferrous salt, the ferric phosphate raw material further includes an oxidizing agent.

18. The preparation method according to claim 17, wherein The oxidant includes hydrogen peroxide.

19. The preparation method according to claim 17, wherein The amount of the oxidant used is 1 to 1.2 times the molar amount of the iron element in the iron source.

20. The preparation method according to claim 1, wherein The reaction for forming the first precursor and the reaction for forming the ferric phosphate material are both performed at a pH of 1.8 to 2.

2.

21. The preparation method according to claim 20, wherein Ammonia was used to adjust the pH.

22. The preparation method according to claim 20, wherein The time for the formation reaction of the first precursor and the formation of the iron phosphate material is 1 to 5 hours.

23. The preparation method according to claim 1, wherein The mass ratio of the initiator to the modified nanocellulose is 1:(20-60).

24. The preparation method according to claim 1, wherein The second precursor is ground into 1-3 μm and then used as the seed crystal.

25. The preparation method according to claim 1, wherein The amount of the second precursor and the ferric phosphate raw material is controlled so that the mass of the seed crystal accounts for 5% to 20% of the mass of the obtained ferric phosphate material.

26. The preparation method according to claim 1, wherein The preparation method further comprises calcining the obtained iron phosphate material.

27. The preparation method according to claim 26, wherein The calcination temperature is 400-600° C. and the calcination time is 2-4 hours.

28. The preparation method according to claim 1, comprising: A hydrogen bond acceptor, a saturated amide hydrogen bond donor, and a carboxylic acid hydrogen bond donor are placed in a container at a molar ratio of 1:(3-5):(1-3) and mixed evenly, heated to 70-90° C. and continuously stirred until the mixture becomes a transparent solution, and then the temperature is lowered to 25-40° C. to obtain a first mixed solution; placing an unsaturated amide hydrogen bond donor and nanocellulose into the first mixed solution, controlling the molar ratio of the unsaturated amide hydrogen bond donor to the hydrogen bond acceptor to be (0.8-1.2):1, and using the nanocellulose in an amount of 10%-20% by mass of the transparent solution to form a deep eutectic solvent, performing a modification reaction on the nanocellulose at 90-110° C. for 2-4 hours, and then filtering, washing, and drying the reaction product to obtain modified nanocellulose having carboxyl groups and double bonds on the surface; The modified nanocellulose is uniformly dispersed in pure water, and a phosphorus source and a trivalent iron salt are then added and mixed uniformly. The amount of the modified nanocellulose is controlled to be 1 to 5 g per mole of iron, and the molar ratio of the iron element to the phosphate in the trivalent iron source and the phosphorus source is controlled to be 1:(0.98 to 1.02). The pH is then adjusted to 1.8 to 2.2, and a formation reaction is carried out for 1 to 5 hours to generate iron phosphate containing the modified nanocellulose as a first precursor, thereby obtaining a second mixed solution. adding an initiator to the second mixed solution, controlling the mass ratio of the initiator to the modified nanocellulose to be 1:(20-60), causing the modified nanocellulose contained in the first precursor and the modified nanocellulose in the second mixed solution to undergo a polymerization reaction, obtaining cross-linked nanocellulose that is uniformly coated on the surface of the first precursor, thereby forming a second precursor; The obtained second precursor is ground to a particle size of 1 to 3 μm and then used as a seed crystal, mixed with a phosphorus source, a divalent iron salt and an oxidant, the molar ratio of the iron element in the divalent iron source and the phosphorus source to the phosphate ion is controlled to be 1: (0.98 to 1.02), the amount of the oxidant is controlled to be 1 to 1.2 times the molar amount of the iron element in the iron source, and then ammonia water is added to adjust the pH to 1.8 to 2.2, and the formation reaction is carried out for 1 to 5 hours to obtain dihydrate ferric phosphate particles, wherein the mass of the seed crystal is controlled to account for 5% to 20%; The obtained ferric phosphate dihydrate is calcined at 400-600° C. for 2-4 hours to remove crystal water, thereby obtaining anhydrous ferric phosphate.

29. An iron phosphate material, wherein: The method is obtained by the preparation method according to any one of claims 1 to 28.

30. A positive electrode active material, wherein Prepared using the iron phosphate material according to claim 29.

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