Composite iron phosphate, and preparation method and application thereof

Abstract

The application provides a composite iron phosphate and a preparation method and application thereof, and the preparation method comprises the following steps: (1) mixing nanocellulose, glacial acetic acid, acetic anhydride, perchloric acid and an organic solvent to obtain acetylated nanocellulose through reaction; (2) mixing nanocellulose, TEMPO and sodium bromide with water, adding sodium hypochlorite solution to obtain oxidized nanocellulose through reaction; (3) mixing the oxidized nanocellulose and the acetylated nanocellulose with an acidic aqueous solution, then mixing with an oily solvent to obtain a water-in-oil emulsion; (4) mixing an iron source, a phosphorus source and the water-in-oil emulsion, and then obtaining the composite iron phosphate through sintering treatment after reaction. The application utilizes the different hydrophilic / hydrophobic difference of acetylated and oxidized nanocellulose to induce the generation of a spherical shell-shaped iron phosphate filled with rod-shaped structures, the nanocellulose forms carbon in situ after high-temperature calcination, and the uniform distribution of the carbon layer is realized, and the conductivity is further improved.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, and relates to a composite iron phosphate, its preparation method and application. Background Technology

[0002] Lithium iron phosphate (LiFePO4) is a type of lithium... + Battery cathode materials have a series of unique advantages such as high operating voltage, high energy density, long cycle life, and environmental friendliness. They also support stepless expansion and can be used to store large-scale electrical energy when combined with an energy storage system.

[0003] However, the crystal structure of LiFePO4 results in its low electronic conductivity, which to some extent limits the potential of Li... + The diffusion motion causes it to diffuse along the one-dimensional channel, resulting in an extremely low ion diffusion rate (10⁻⁶) for LiFePO₄ materials. -17 ~10 -14 cm 2 / s); Because a continuous FeO6 octahedral network is not formed, a good electronic conductivity network cannot be formed, resulting in the low electronic conductivity of LiFePO4 (approximately 10 at room temperature). -9 ~10 -10 (S / cm). Therefore, the structure of LiFePO4 itself determines that it is not suitable for high-rate discharge.

[0004] CN117794853A discloses a modified iron phosphate, its preparation method and application. The preparation method includes the following steps: (1) preparing NH2-MIL-53(Al) nanoparticles, mixing the nanoparticles with a solvent, adding an acid solution, heating and stirring to obtain hollow NH2-MIL-53(Al) powder; (2) preparing a dispersion of the hollow NH2-MIL-53(Al) powder, mixing the dispersion with an iron salt solution, stirring and drying, mixing the obtained powder with a phosphorus source solution, controlling the pH, and reacting; (3) sintering the aged material to obtain the modified iron phosphate.

[0005] CN117509593A discloses a lithium iron phosphate material and its preparation method. The method uses a microwave-assisted solvothermal method to construct nano-lithium iron phosphate material, and the obtained material has a hollow bowl-shaped loose structure.

[0006] The above-mentioned methods reduce the lithium-ion transport path by preparing hollow lithium iron phosphate or iron phosphate, and improve electronic conductivity through carbon coating. However, hollow lithium iron phosphate reduces the space capacity density of the cathode material, and forming a uniform carbon coating layer on the surface of lithium iron phosphate material is also quite difficult. Summary of the Invention

[0007] The purpose of this invention is to provide a composite iron phosphate, its preparation method, and its application. This invention utilizes the different hydrophilicity / hydrophobicity of acetylated and oxidized nanocellulose crystals to induce the generation of a rod-shaped, spherical iron phosphate filled with a rod structure. After high-temperature calcination, the nanocellulose crystals form carbon in situ, achieving a uniform distribution of the carbon layer and further improving the electrical conductivity.

[0008] To achieve this objective, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a method for preparing composite iron phosphate, the method comprising the following steps:

[0010] (1) Nanocellulose, glacial acetic acid, acetic anhydride, perchloric acid and organic solvent are mixed and reacted in one step to obtain acetylated nanocellulose crystals;

[0011] (2) Mix nanocellulose, tetramethylpiperidine oxide (TEMPO), sodium bromide and water, and add sodium hypochlorite solution to carry out a two-step reaction to obtain oxidized nanocellulose crystals;

[0012] (3) Mix oxidized nanocellulose crystals, acetylated nanocellulose crystals and acidic aqueous solution to obtain a mixed solution, and mix the mixed solution with an oily solvent to obtain a water-in-oil emulsion;

[0013] (4) The iron source, phosphorus source and water-in-oil emulsion are mixed to obtain a mixed emulsion, which is then subjected to a three-step reaction and sintering treatment to obtain the composite iron phosphate.

[0014] This invention does not limit the order of operations for steps (1) and (2). Step (1) can be performed first, or step (2) can be performed first. The terms one-step reaction, two-step reaction, and three-step reaction used in this invention are merely for distinction and do not refer to the number of reactions.

[0015] This invention pre-prepares acetylated and oxidized cellulose nanocrystals separately. Utilizing their different hydrophilic / hydrophobic properties, an emulsion is obtained through oil-water mixing. The acetylated cellulose nanocrystals, being amphiphilic, act as emulsion stabilizers and simultaneously utilize their ketone groups to coordinate with iron ions, thus locating the iron ions. The oxidized cellulose nanocrystals, possessing excellent hydrophilicity, are distributed in the aqueous solution, utilizing the adsorption of iron ions by their carboxyl and hydroxyl groups. Phosphate ions are then added, reacting with the located iron ions to generate iron phosphate. The acetylated cellulose nanocrystals serve as a template to form the iron phosphate shell, which, together with the oxidized cellulose nanocrystals forming iron phosphate rods within the shell, constitutes the final composite iron phosphate structure.

[0016] Preferably, the length of the nanocellulose in step (1) is 110 to 150 nm, for example: 110 nm, 120 nm, 130 nm, 140 nm or 150 nm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0017] Preferably, the diameter of the nanocellulose in step (1) is 10-20 nm, for example: 10 nm, 12 nm, 15 nm, 18 nm or 20 nm, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0018] The structural formula of the nanocellulose described in this invention is as follows:

[0019] R2 includes -COCH3 and / or -H.

[0020] Preferably, the organic solvent in step (1) includes toluene.

[0021] Preferably, the mass-to-volume ratio of nanocellulose and glacial acetic acid in step (1) is 1:(15-25) g / mL, for example: 1:15 g / mL, 1:18 g / mL, 1:20 g / mL, 1:22 g / mL or 1:25 g / mL, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0022] Preferably, the mass-to-volume ratio of nanocellulose and acetic anhydride in step (1) is 1:(5-10)g / mL, for example: 1:5g / mL, 1:6g / mL, 1:8g / mL, 1:9g / mL or 1:10g / mL, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0023] Preferably, the mass-volume ratio of nanocellulose and perchloric acid in step (1) is (8-12):1g / mL, for example: 8:1g / mL, 9:1g / mL, 10:1g / mL, 11:1g / mL or 12:1g / mL, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0024] Preferably, the mass-to-volume ratio of nanocellulose and organic solvent in step (1) is 1:(20-30) g / mL, for example: 1:20 g / mL, 1:22 g / mL, 1:25 g / mL, 1:28 g / mL or 1:30 g / mL, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0025] Preferably, the temperature of the one-step reaction in step (1) is 20 to 30°C, for example: 20°C, 22°C, 25°C, 28°C or 30°C, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0026] Preferably, the reaction time in step (1) is 2 to 6 hours, for example: 2 hours, 3 hours, 4 hours, 5 hours or 6 hours, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0027] Preferably, the one-step reaction in step (1) is terminated by adding anhydrous ethanol, and after dialysis, acetylated cellulose nanocrystals are obtained by freeze-drying.

[0028] Preferably, the mass-to-volume ratio of nanocellulose and anhydrous ethanol in step (1) is 1:(40-60) g / mL, for example: 1:40 g / mL, 1:45 g / mL, 1:50 g / mL, 1:55 g / mL or 1:60 g / mL, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0029] Preferably, the mass ratio of nanocellulose and tetramethylpiperidine oxide in step (2) is (380-410):1, and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0030] Preferably, the mass ratio of nanocellulose and sodium bromide in step (2) is (35-50):1, for example: 35:1, 38:1, 40:1, 45:1 or 50:1, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0031] Preferably, the molar concentration of the sodium hypochlorite solution in step (2) is 0.3 to 0.8 mol / L, for example: 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.7 mol / L or 0.8 mol / L, etc.

[0032] Preferably, the mass-to-volume ratio of nanocellulose and sodium hypochlorite solution in step (2) is 1:(20-30) g / mL, for example: 1:20 g / mL, 1:22 g / mL, 1:25 g / mL, 1:28 g / mL or 1:30 g / mL, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0033] Preferably, the temperature of the two-step reaction in step (2) is 20 to 30°C, for example: 20°C, 22°C, 25°C, 28°C or 30°C, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0034] Preferably, the pH of the two-step reaction in step (2) is 10 to 10.5, for example: 10, 10.1, 10.2, 10.4 or 10.5, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0035] Preferably, the two-step reaction in step (2) is terminated by adding anhydrous ethanol, and the pH is adjusted to 6.8-7.2 by adding acid (e.g., 6.8, 6.9, 7, 7.1 or 7.2, etc., not limited to the listed values, other unlisted values ​​within this range are also applicable), and after dialysis, acetylated cellulose nanocrystals are obtained by freeze drying.

[0036] Preferably, the mass-to-volume ratio of nanocellulose and anhydrous ethanol in step (2) is 1:(20-30)g / mL, for example: 1:20g / mL, 1:22g / mL, 1:25g / mL, 1:28g / mL or 1:30g / mL, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0037] Preferably, the mass ratio of oxidized cellulose nanocrystals to acetylated cellulose nanocrystals in step (3) is 1:(2-3), for example: 1:2, 1:2.2, 1:2.5, 1:2.8 or 1:3, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0038] Preferably, the pH of the acidic solution in step (3) is 1.8 to 2.2, for example: 1.8, 1.9, 2, 2.1 or 2.2, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0039] Preferably, the solute in the acidic solution in step (3) includes any one or a combination of at least two of sulfuric acid, nitric acid, or hydrochloric acid.

[0040] Preferably, the mass fraction of acetylated cellulose nanocrystals in the mixed solution in step (3) is 5% to 8%, for example: 5%, 5.5%, 6%, 7% or 8%, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0041] Preferably, the oily solvent in step (3) includes any one or a combination of at least two of the following: vegetable oil, n-hexane, liquid paraffin, petroleum ether, glycerin, n-heptane, toluene, xylene, dichloromethane, chloroform, and acrylate. Typical but non-limiting combinations include combinations of vegetable oil and n-hexane, liquid paraffin and petroleum ether, liquid paraffin and glycerin, or dichloromethane and toluene.

[0042] Preferably, the volume ratio of the mixed solution and the oily solvent in step (3) is 5:(6-8), for example: 5:6, 5:6.5, 5:7, 5:7.5 or 5:8, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0043] Preferably, the mixed solution in step (3) is stirred after being mixed with an oily solvent.

[0044] Preferably, the stirring time is 5 to 10 minutes, for example: 5 minutes, 6 minutes, 7 minutes, 8 minutes or 10 minutes, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0045] Preferably, the iron source in step (4) includes ferric chloride and / or ferric nitrate.

[0046] Preferably, the phosphorus source in step (4) includes any one or a combination of at least two of H3PO4, (NH4)2HPO4, NH4H2PO4, (NH4)3PO4, NaH2PO4, or Na2HPO4. Typical but non-limiting combinations include combinations of H3PO4 and (NH4)2HPO4, combinations of NH4H2PO4 and (NH4)3PO4, or combinations of NaH2PO4 and Na2HPO4, etc.

[0047] Preferably, the concentration of iron ions in the mixed emulsion in step (4) is 0.5 to 1 mol / L, for example: 0.5 mol / L, 0.6 mol / L, 0.8 mol / L, 0.9 mol / L or 1 mol / L, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0048] Preferably, the temperature of the three-step reaction in step (4) is 50 to 70°C, for example: 50°C, 55°C, 60°C, 65°C or 70°C, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0049] Preferably, stirring is performed during the three-step reaction process described in step (4).

[0050] Preferably, the stirring speed is 700 to 1000 rpm, for example: 700 rpm, 750 rpm, 800 rpm, 900 rpm or 1000 rpm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0051] Preferably, the reaction time of the three steps in step (4) is 5 to 8 hours, for example: 5 hours, 5.5 hours, 6 hours, 7 hours or 8 hours, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0052] Preferably, the sintering temperature in step (4) is 500 to 700°C, for example: 500°C, 550°C, 600°C, 650°C or 700°C, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0053] Preferably, the sintering time in step (4) is 6 to 8 hours, for example: 6 hours, 6.5 hours, 7 hours, 7.5 hours or 8 hours, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0054] In a second aspect, the present invention provides a composite iron phosphate, which is prepared by the method described in the first aspect.

[0055] The composite iron phosphate obtained by the method described in this invention has a hollow structure, with rod-shaped structures embedded inside. Both structures are beneficial for lithium-ion insertion / extraction. This invention uses nanocellulose crystals to induce their growth. During the preparation of the lithium iron phosphate cathode material, the nanocellulose crystals form carbon in situ after high-temperature calcination, achieving a uniform distribution of the carbon layer and further improving conductivity.

[0056] Thirdly, the present invention provides a lithium iron phosphate cathode material, which is prepared by sintering a composite iron phosphate and a lithium source as described in the second aspect.

[0057] Preferably, the lithium source includes lithium hydroxide and / or lithium carbonate.

[0058] Fourthly, the present invention provides a lithium-ion battery comprising the lithium iron phosphate cathode material as described in the third aspect.

[0059] Compared with the prior art, the present invention has the following beneficial effects:

[0060] (1) The present invention prepares a spherical composite iron phosphate material filled with a rod-shaped structure. The outer shell of the composite iron phosphate is a hollow structure, and the inside contains a rod-shaped structure, which is beneficial to shorten the lithium ion insertion and extraction distance and improve the rate performance. At the same time, the nanocellulose crystals, which serve as the shell and rod templates respectively, are carbonized at high temperature, which is beneficial to the uniform distribution of carbon and thus improves the conductivity.

[0061] (2) The tap density of lithium iron phosphate prepared by the method described in this invention can reach 1.27 g / cm³. 3 The batteries produced can achieve a 0.2C rate of over 161 mAh / g, a 1C rate of over 156 mAh / g, a 2C rate of over 148 mAh / g, a 5C rate of over 138 mAh / g, and a 10C rate of over 124 mAh / g. Attached Figure Description

[0062] Figure 1 This is a SEM image of the composite iron phosphate prepared in Example 1 of the present invention. Detailed Implementation

[0063] 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.

[0064] Example 1

[0065] This embodiment provides a composite iron phosphate, which is prepared by the following method:

[0066] (1) Nanocellulose with a length of 110-150 nm and a width of 10-20 nm was ultrasonically dispersed in toluene solution for 15 min in an ice-water bath to obtain a suspension. The mass-to-volume ratio of nanocellulose to toluene was 1 g: 25 mL. Glacial acetic acid, acetic anhydride and perchloric acid were added to the above solution in sequence. The volume-to-mass ratio of glacial acetic acid, acetic anhydride and perchloric acid to nanocellulose crystals was 20 mL: 7 mL: 10 mL: 1 g. The reaction was carried out at a reaction temperature of 25 °C and stirred continuously for 5 h. Then, anhydrous ethanol with a volume-to-mass ratio of 50 mL: 1 g to nanocellulose crystals was added to terminate the reaction. The obtained white product was dialyzed by anhydrous ethanol and deionized water, and finally freeze-dried to obtain acetylated nanocellulose crystals.

[0067] (2) Nanocellulose crystals with dimensions ranging from 110-150 nm in length and 10-20 nm in width were ultrasonically dispersed in an aqueous solution containing TEMPO and NaBr for 8 min in an ice-water bath. The mass ratio of nanocellulose crystals to TEMPO was 400:1, and the mass ratio of nanocellulose crystals to NaBr was 40:1. NaClO with a concentration of 0.5 mol / L was slowly added under continuous stirring to induce a catalytic oxidation reaction. The mass-to-volume ratio of nanocellulose crystals to NaClO was 1 g:25 mL. During the reaction, NaOH was used to maintain the pH at 10-10.5 until the pH stabilized. Anhydrous ethanol was then added to stop the reaction, with a volume-to-mass ratio of ethanol to nanocellulose crystals of 50 mL:1 g. The pH of the solution was adjusted to 7 with hydrochloric acid. Finally, the solution was dialyzed with deionized water, and then freeze-dried to obtain oxidized nanocellulose crystals.

[0068] (3) Dissolve oxidized nanocellulose crystals and acetylated nanocellulose crystals in deionized water to obtain a mixed solution. After stirring evenly, adjust the pH of the solution to 1.8-2.2 with hydrochloric acid. The mass fraction of acetylated nanocellulose crystals in the mixed solution is 7%, and the mass ratio of oxidized nanocellulose crystals to acetylated nanocellulose crystals is 1:2. Add the solution to liquid paraffin, an oil-based solvent, with a volume ratio of 7:5 between the oil-based solvent and the mixed solution. Stir at 10,000 rpm for 8 minutes to obtain a water-in-oil emulsion.

[0069] (4) Ferric chloride is added to the obtained water-in-oil emulsion and stirred until completely dissolved. Then, ammonium hydrogen phosphate is added and stirred until completely dissolved to obtain a mixed emulsion. The concentration of ferric chloride in the mixed emulsion is 1 mol / L, and the molar ratio of ammonium hydrogen phosphate to ferric chloride is 1.2:1. After reacting at 60°C and 800 rpm for 7 hours, the mixture is filtered, washed, dried, and then calcined at 700°C in a nitrogen atmosphere to obtain the composite ferric phosphate.

[0070] The SEM image of the composite ferric phosphate is shown below. Figure 1 As shown, by Figure 1 It can be seen that the composite iron phosphate prepared by this invention has a hollow outer shell and contains rod-shaped structures inside.

[0071] Example 2

[0072] This embodiment provides a composite iron phosphate, which is prepared by the following method:

[0073] (1) Nanocellulose with a length of 110-150 nm and a width of 10-20 nm was ultrasonically dispersed in toluene solution for 15 min in an ice-water bath to obtain a suspension. The mass-to-volume ratio of nanocellulose to toluene was 1 g: 20 mL. Glacial acetic acid, acetic anhydride and perchloric acid were added to the above solution in sequence. The volume-to-mass ratio of glacial acetic acid, acetic anhydride and perchloric acid to nanocellulose crystals was 15 mL: 5 mL: 8 mL: 1 g. The reaction was carried out at a reaction temperature of 20 °C and stirred continuously for 6 h. Then, anhydrous ethanol with a volume-to-mass ratio of 50 mL: 1 g to nanocellulose crystals was added to terminate the reaction. The obtained white product was dialyzed by anhydrous ethanol and deionized water, and finally freeze-dried to obtain acetylated nanocellulose crystals.

[0074] (2) Nanocellulose crystals with dimensions ranging from 110-150 nm in length and 10-20 nm in width were ultrasonically dispersed in an aqueous solution containing TEMPO and NaBr for 10 min in an ice-water bath. The mass ratio of nanocellulose crystals to TEMPO was 380:1, and the mass ratio of nanocellulose crystals to NaBr was 35:1. NaClO with a concentration of 0.3 mol / L was slowly added under continuous stirring to induce a catalytic oxidation reaction. The mass-to-volume ratio of nanocellulose crystals to NaClO was 1 g:30 mL. During the reaction, NaOH was used to maintain the pH at 10-10.5 until the pH stabilized. Anhydrous ethanol was then added to stop the reaction, with a volume-to-mass ratio of ethanol to nanocellulose crystals of 50 mL:1 g. The pH of the solution was adjusted to 7 with hydrochloric acid. Finally, the solution was dialyzed with deionized water, and then freeze-dried to obtain oxidized nanocellulose crystals.

[0075] (3) Dissolve oxidized nanocellulose crystals and acetylated nanocellulose crystals in deionized water to obtain a mixed solution. After stirring evenly, adjust the pH of the solution to 1.8-2.2 with hydrochloric acid. The mass fraction of acetylated nanocellulose crystals in the mixed solution is 5%, and the mass ratio of oxidized nanocellulose crystals to acetylated nanocellulose crystals is 1:2.5. Add the solution to liquid paraffin, an oil-based solvent, with a volume ratio of 6:5 between the oil-based solvent and the mixed solution. Stir at 15000 rpm for 5 minutes to obtain a water-in-oil emulsion.

[0076] (4) Ferric chloride is added to the obtained water-in-oil emulsion and stirred until completely dissolved. Then, ammonium hydrogen phosphate is added and stirred until completely dissolved to obtain a mixed emulsion. The concentration of ferric chloride in the mixed emulsion is 0.8 mol / L, and the molar ratio of ammonium hydrogen phosphate to ferric chloride is 1:1. After reacting at 50°C and 1000 rpm for 8 hours, the mixture is filtered, washed, dried, and then calcined at 600°C in a nitrogen atmosphere to obtain the composite ferric phosphate.

[0077] Example 3

[0078] This embodiment provides a composite iron phosphate, which is prepared by the following method:

[0079] (1) Nanocellulose with a length of 110-150 nm and a width of 10-20 nm was ultrasonically dispersed in toluene solution for 15 min in an ice-water bath to obtain a suspension. The mass-to-volume ratio of nanocellulose to toluene was 1 g: 30 mL. Glacial acetic acid, acetic anhydride and perchloric acid were added to the above solution in sequence. The volume-to-mass ratio of glacial acetic acid, acetic anhydride and perchloric acid to nanocellulose crystals was 25 mL: 10 mL: 12 mL: 1 g. The reaction was carried out at a reaction temperature of 30 °C and stirred continuously for 2 h. Then, anhydrous ethanol with a volume-to-mass ratio of 60 mL: 1 g to nanocellulose crystals was added to terminate the reaction. The obtained white product was dialyzed by anhydrous ethanol and deionized water, and finally freeze-dried to obtain acetylated nanocellulose crystals.

[0080] (2) Nanocellulose crystals with dimensions ranging from 110-150 nm in length and 10-20 nm in width were ultrasonically dispersed in an aqueous solution containing TEMPO and NaBr for 10 min in an ice-water bath. The mass ratio of nanocellulose crystals to TEMPO was 410:1, and the mass ratio of nanocellulose crystals to NaBr was 50:1. NaClO with a concentration of 0.8 mol / L was slowly added under continuous stirring to induce a catalytic oxidation reaction. The mass-to-volume ratio of nanocellulose crystals to NaClO was 1 g:20 mL. During the reaction, NaOH was used to maintain the pH at 10-10.5 until the pH stabilized. Anhydrous ethanol was then added to stop the reaction, with a volume-to-mass ratio of ethanol to nanocellulose crystals of 60 mL:1 g. The pH of the solution was adjusted to 7 with hydrochloric acid. Finally, the solution was dialyzed with deionized water, and then freeze-dried to obtain oxidized nanocellulose crystals.

[0081] (3) Dissolve oxidized nanocellulose crystals and acetylated nanocellulose crystals in deionized water to obtain a mixed solution. After stirring evenly, adjust the pH of the solution to 1.8-2.2 with hydrochloric acid. The mass fraction of acetylated nanocellulose crystals in the mixed solution is 8%, and the mass ratio of oxidized nanocellulose crystals to acetylated nanocellulose crystals is 1:3. Add the solution to liquid paraffin, an oil-based solvent, with a volume ratio of 8:5 between the oil-based solvent and the mixed solution. Stir at 8000 rpm for 10 min to obtain a water-in-oil emulsion.

[0082] (4) Ferric chloride is added to the obtained water-in-oil emulsion and stirred until completely dissolved. Then, ammonium hydrogen phosphate is added and stirred until completely dissolved to obtain a mixed emulsion. The concentration of ferric chloride in the mixed emulsion is 0.5 mol / L, and the molar ratio of ammonium hydrogen phosphate to ferric chloride is 1:1. After reacting at 70°C and 1000 rpm for 8 hours, the mixture is filtered, washed, dried, and then calcined at 500°C in a nitrogen atmosphere to obtain the composite ferric phosphate.

[0083] Example 4

[0084] The only difference between this embodiment and Example 1 is that the mass ratio of oxidized cellulose nanocrystals to acetylated cellulose nanocrystals is 1:1; all other conditions and parameters are exactly the same as in Example 1.

[0085] Example 5

[0086] The only difference between this embodiment and Example 1 is that the mass ratio of oxidized cellulose nanocrystals to acetylated cellulose nanocrystals is 1:4; all other conditions and parameters are exactly the same as in Example 1.

[0087] Example 6

[0088] The only difference between this embodiment and Example 1 is that the volume ratio of the mixed solution to the oily solvent is 1:1, while the other conditions and parameters are exactly the same as in Example 1.

[0089] Example 7

[0090] The only difference between this embodiment and Embodiment 1 is that the volume ratio of the mixed solution to the oily solvent is 5:10, while the other conditions and parameters are exactly the same as in Embodiment 1.

[0091] Example 8

[0092] The only difference between this embodiment and Example 1 is that the mass fraction of acetylated cellulose nanocrystals in the mixed solution is 4%, while the other conditions and parameters are exactly the same as in Example 1.

[0093] Example 9

[0094] The only difference between this embodiment and Example 1 is that the mass fraction of acetylated cellulose nanocrystals in the mixed solution is 10%, while the other conditions and parameters are exactly the same as in Example 1.

[0095] Comparative Example 1

[0096] The only difference between this comparative example and Example 1 is that only acetylated cellulose nanocrystals are used; all other conditions and parameters are exactly the same as in Example 1.

[0097] Comparative Example 2

[0098] The only difference between this comparative example and Example 1 is that only oxidized cellulose nanocrystals are used; all other conditions and parameters are exactly the same as in Example 1.

[0099] Performance testing:

[0100] The iron phosphate and lithium hydroxide prepared in the examples and comparative examples were mixed in a molar ratio of 1:1, and ethanol was used as a dispersant to obtain a mixed material. The mixture was then ground and stirred in a grinder for 5 hours and dried. The dried lithium iron phosphate precursor was then placed in a tube furnace in a nitrogen environment and kept at 800°C for 10 hours to obtain lithium iron phosphate cathode material.

[0101] A uniform slurry was prepared by mixing acetylene black and PVDF in a mass ratio of 75:15:10 for the positive electrode, and then evenly coated onto an aluminum foil substrate to serve as the positive electrode of the simulated battery. The negative electrode of the simulated battery used a lithium sheet, and the separator was a polypropylene porous membrane. The electrolyte was 1 mol of LiPF6 dissolved in 1 L of a mixed solvent of EC and DMC (volume ratio 1:1). The positive electrode, negative electrode, electrolyte, and separator were assembled into a battery in an argon-protected glove box.

[0102] Tap density test: Weigh the mass m1 of the dry graduated cylinder. Add a certain amount of solid sample (to about 5 ml) into the graduated cylinder. Seal the cylinder opening and vibrate it vertically until the sample volume no longer decreases. Record the sample volume V (ml). Weigh the mass m2 (g) of the graduated cylinder and sample. Use the formula ρ = (m2 - m1) / V to obtain ρ (g / cm³). 3 The tap density of the sample to be tested is obtained.

[0103] The simulated battery rate and cycle test procedure is as follows: First, charge the battery with a constant current to 4.2V, then discharge it with a higher rate current to 2.0V. The capacity discharged is the discharge capacity at that rate. After discharging, discharge it again with a constant current to 2.0V. Then, test at the next rate. The test results are shown in Table 1.

[0104] Table 1

[0105]

[0106]

[0107] As shown in Table 1, and based on Examples 1-3, the tap density of lithium iron phosphate prepared by the method of the present invention can reach 1.27 g / cm³. 3 The batteries produced can achieve a 0.2C rate of over 161 mAh / g, a 1C rate of over 156 mAh / g, a 2C rate of over 148 mAh / g, a 5C rate of over 138 mAh / g, and a 10C rate of over 124 mAh / g.

[0108] A comparison of Examples 1 and 4-5 shows that the ratio of oxidized cellulose nanocrystals to acetylated cellulose nanocrystals affects the performance of the composite iron phosphate material described in this invention. Maintaining a mass ratio of oxidized cellulose nanocrystals to acetylated cellulose nanocrystals of 1:2-3 yields composite iron phosphate with better performance. Excessive addition of acetylated cellulose nanocrystals increases emulsion stability and improves spherical particle size stability, but also increases material cost. Simultaneously, the increased cellulose nanocrystal content leads to increased carbon content in the final lithium iron phosphate material, resulting in reduced capacity and slightly better rate performance. Insufficient addition of acetylated cellulose nanocrystals slightly reduces emulsion stability, decreases droplet sphericity stability, and slightly reduces the tap density of the lithium iron phosphate material. Reduced cellulose nanocrystal content also reduces carbon source content, leading to decreased rate performance.

[0109] A comparison of Examples 1 and 6-7 shows that the ratio of the mixed solution to the oily solvent affects the performance of the composite iron phosphate prepared according to the present invention. Controlling the volume ratio of the mixed solution to the oily solvent to 5:6-8 yields composite iron phosphate with better performance. If the amount of oily solvent added is too large, the particle size of the water-in-oil emulsion increases, affecting the final particle size of lithium iron phosphate, thus reducing the tap density of the cathode material. If the amount of oily solvent added is too small, the particle size of the water-in-oil emulsion increases, and the sphericity is insufficient and unstable, affecting the final particle size and shape of lithium iron phosphate, thus reducing the tap density of the cathode material and decreasing the rate performance.

[0110] A comparison of Examples 1 and 8-9 shows that the mass fraction of acetylated cellulose nanocrystals in the mixed solution affects the performance of the composite iron phosphate material described in this invention. Controlling the mass fraction of acetylated cellulose nanocrystals in the mixed solution to 5-8% results in better performance of the composite iron phosphate. If the mass fraction of acetylated cellulose nanocrystals in the mixed solution is too high, the carbon content in the final lithium iron phosphate material increases, leading to a decrease in capacity. If the mass fraction of acetylated cellulose nanocrystals in the mixed solution is too low, the stability of the emulsion and the spherical stability of the droplets are poor, and the tap density of the lithium iron phosphate material decreases; simultaneously, the carbon source content decreases, resulting in a decline in rate performance.

[0111] Comparing Example 1 and Comparative Examples 1-2, it can be seen that the acetylated nanocellulose crystals of the present invention are amphiphilic, thus serving as a stabilizer for emulsions. Simultaneously, they can utilize the coordination of their ketone groups with iron ions to locate the iron ions. The oxidized nanocellulose crystals possess excellent hydrophilicity, thus distributing in aqueous solutions and utilizing the adsorption of iron ions by their carboxyl and hydroxyl groups. Subsequently, phosphate ions are added, reacting with the located iron ions to generate iron phosphate. The acetylated nanocellulose crystals serve as a template to form the iron phosphate shell, which, together with the oxidized nanocellulose crystals forming iron phosphate rods within the shell, constitutes the final composite iron phosphate structure. The hollow structure contains a rod-like internal structure; both structures facilitate the insertion and extraction of lithium ions.

[0112] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a composite iron phosphate, characterized by, The preparation method comprises the following steps: (1) mixing nanocellulose, glacial acetic acid, acetic anhydride, perchloric acid and an organic solvent to obtain acetylated nanocellulose crystals through one-step reaction; (2) mixing nanocellulose, tetramethylpiperidinooxy, sodium bromide and water, and adding sodium hypochlorite solution to perform two-step reaction to obtain oxidized nanocellulose crystals; (3) mixing the oxidized nanocellulose crystals and the acetylated nanocellulose crystals with an acidic aqueous solution to obtain a mixed solution, mixing the mixed solution with an oily solvent to obtain a water-in-oil emulsion; (4) mixing an iron source, a phosphorus source and the water-in-oil emulsion to obtain a mixed emulsion, and performing three-step reaction and sintering treatment to obtain the composite iron phosphate.

2. The production method according to claim 1, wherein In step (1), the length of the nanocellulose is 110-150 nm.

3. The production method according to claim 1, wherein In step (1), the diameter of the nanocellulose is 10-20 nm.

4. The production method according to claim 1, wherein In step (1), the organic solvent comprises toluene.

5. The production method according to claim 1, wherein In step (1), the mass-volume ratio of the nanocellulose to glacial acetic acid is 1:(15-25) g / mL.

6. The production method according to claim 1, wherein In step (1), the mass-volume ratio of the nanocellulose to acetic anhydride is 1:(5-10) g / mL.

7. The production method according to claim 1, wherein In step (1), the mass-volume ratio of the nanocellulose to perchloric acid is (8-12):1 g / mL.

8. The production method according to claim 1, wherein In step (1), the mass-volume ratio of the nanocellulose to the organic solvent is 1:(20-30) g / mL.

9. The production method according to claim 1, wherein In step (1), the temperature of the one-step reaction is 20-30℃.

10. The production method according to claim 1, wherein In step (1), the time of the one-step reaction is 2-6 h.

11. The production method according to claim 1, wherein In step (1), the one-step reaction is terminated by adding anhydrous ethanol, and the acetylated nanocellulose crystals are obtained through dialysis and freeze-drying.

12. The production method according to claim 1, wherein In step (1), the mass-volume ratio of the nanocellulose to anhydrous ethanol is 1:(40-60) g / mL.

13. The production method according to claim 1, wherein In step (2), the mass ratio of the nanocellulose to tetramethylpiperidinooxy is (380-410):

1.

14. The production method according to claim 1, wherein In step (2), the mass ratio of the nanocellulose to sodium bromide is (35-50):

1.

15. The production method according to claim 1, wherein In step (2), the molar concentration of the sodium hypochlorite solution is 0.3-0.8 mol / L.

16. The production method according to claim 1, wherein In step (2), the mass-volume ratio of the nanocellulose to the sodium hypochlorite solution is 1:(20-30) g / mL.

17. The production method according to claim 1, wherein In step (2), the temperature of the two-step reaction is 20-30℃.

18. The production method according to claim 17, wherein The pH of the two-step reaction is 10-10.

5.

19. The production method according to claim 1, wherein In step (2), the two-step reaction is terminated by adding anhydrous ethanol, and the pH is adjusted to 6.8-7.2 by adding acid, and the acetylated nanocellulose crystals are obtained through dialysis and freeze-drying.

20. The production method according to claim 1, wherein In step (2), the mass-volume ratio of the nanocellulose to anhydrous ethanol is 1:(20-30) g / mL.

21. The production method according to claim 1, wherein In step (3), the mass ratio of the oxidized nanocellulose crystals to the acetylated nanocellulose crystals is 1:(2-3).

22. The production method according to claim 1, wherein In step (3), the pH of the acidic solution is 1.8-2.

2.

23. The production method according to claim 1, wherein In step (3), the solute of the acidic solution comprises any one or a combination of at least two of sulfuric acid, nitric acid or hydrochloric acid.

24. The production method according to claim 1, wherein In step (3), the mass fraction of the acetylated nanocellulose crystals in the mixed solution is 5%-8%.

25. The production method according to claim 1, wherein The oil solvent in step (3) includes any one or a combination of at least two of vegetable oil, n-hexane, liquid paraffin, petroleum ether, glycerol, n-heptane, toluene, xylene, dichloromethane, trichloromethane, and acrylate.

26. The production method according to claim 1, wherein The volume ratio of the mixed solution to the oil solvent in step (3) is 5:6-8.

27. The production method according to claim 1, wherein The mixed solution and the oil solvent are stirred after mixing in step (3).

28. The production method according to claim 27, wherein The stirring time is 5-10 min.

29. The production method according to claim 1, wherein The iron source in step (4) includes ferric chloride and / or ferric nitrate.

30. The production method according to claim 1, wherein The phosphorus source in step (4) includes any one or a combination of at least two of H3PO4, (NH4)2HPO4, NH4H2PO4, (NH4)3PO4, NaH2PO4, and Na2HPO4.

31. The production method according to claim 1, wherein The concentration of iron ions in the mixed emulsion in step (4) is 0.5-1 mol / L.

32. The production method according to claim 1, wherein The temperature of the three-step reaction in step (4) is 50-70℃.

33. The production method according to claim 1, wherein The three-step reaction is stirred in step (4).

34. The method of claim 33, wherein the step of preparing is performed by a method comprising: The stirring speed is 700-1000 rpm.

35. The preparation method according to claim 1, characterized in that, The three-step reaction time in step (4) is 5-8 h.

36. The method of claim 1, wherein the compound is ###00023### 36A. The sintering temperature in step (4) is 500-700℃.

37. The preparation method according to claim 1, characterized in that, The sintering time in step (4) is 6-8 h.

38. A composite iron phosphate, characterized in that, The composite iron phosphate is prepared by the method of any one of claims 1-37.

39. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate anode material is prepared by mixing and sintering the composite iron phosphate of claim 38 with a lithium source.

40. A lithium-ion battery, characterized by, The lithium ion battery contains the lithium iron phosphate anode material of claim 39.

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