Preparation method of microcrystalline cellulose loaded carbon-coated lithium iron phosphate composite positive electrode material

Abstract

The application discloses a preparation method of a microcrystalline cellulose loaded carbon coated lithium iron phosphate composite positive electrode material. Firstly, sulfuric acid and nitric acid mixed acid is used to extract microcrystalline cellulose from cotton as an attachment base and a structure regulator for carbon coated lithium iron phosphate; a mixture is prepared by mixing a lithium source and a phosphorus source according to a certain ratio, mixed with an iron source solution, and then a certain ratio of carbon source and microcrystalline cellulose is added, so that the mixture is changed from sol to gel, and then the gel is dried and finely ground to prepare a precursor; then the microcrystalline cellulose loaded carbon coated lithium iron phosphate composite positive electrode material is prepared by pre-burning and high-temperature calcining in a tube furnace under a protective atmosphere. Through the carrier and template effect of the microcrystalline cellulose, the structure and morphology of the carbon coated lithium iron phosphate are effectively regulated; the carbon coated lithium iron phosphate particles, the carbon net derived from the carbon source and the carbon rod derived from the microcrystalline cellulose constitute an efficient three-dimensional space network conductive structure, the lithium ion diffusion coefficient and the electrochemical performance are improved, and the material is suitable for being used as a positive electrode material of a power battery.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material preparation technology, specifically relating to a method for preparing a microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material. Background Technology

[0002] With the increasing depletion of fossil fuels and the worsening environmental pollution caused by their use, the search for sustainable energy sources is urgently needed. Lithium-ion batteries, with their advantages of high redox potential, high theoretical specific capacity, stable electrochemical performance, low cost, and environmental friendliness, are widely used in the new energy industry. To date, lithium-ion battery cathode materials have developed into a large family, led by lithium iron phosphate (LFP). LFP is a second-generation lithium-ion cathode material, and due to its advantages such as a high charge / discharge platform, structural stability, stable performance, mild reaction, high theoretical capacity, and long cycle life, it has become a typical representative in the current battery field. However, the original LFP is limited by its low intrinsic conductivity (~10). - 9 cm·s -1 ) and low lithium-ion diffusion coefficient (10 -13 ~10 -16 cm 2 ·s -1 The high conductivity of lithium iron phosphate (LFP) has severely hampered its application in the battery field. To overcome these shortcomings, numerous improvement measures have been developed, such as surface modification, ion doping, and particle nanostructuring. Surface modification primarily utilizes carbon coating to coat the surface of LFP particles with a carbon layer to improve conductivity; this modification method is the most widely used.

[0003] Carbon coating modification is the most common method to improve the electrochemical performance of lithium iron phosphate. The coated carbon layer has two main functions: (1) it can effectively enhance the conductivity between material ions, avoid direct contact between the material and the electrolyte, form a stable SEI film, and thus reduce electrode polarization; (2) it provides electron channels for the material, restricts the growth of material grains, and increases the specific surface area. The carbon sources used in the carbon coating process are widely available and can be divided into single carbon sources and multiple carbon sources. Single carbon sources include glucose, sucrose, citric acid, graphene, etc., and usually deposit a carbon layer on the surface of the material to form a conductive layer that effectively connects the particles. Multiple carbon sources are mainly two or more of the above carbon sources, or carbon nanotubes, carbon fibers, etc. are introduced on the basis of a single carbon source to obtain different forms of carbon structure to further improve the electrochemical performance of lithium iron phosphate. Since the planar conductive structure generated by a single carbon source has limited effect on improving the electrochemical performance of lithium iron phosphate, a three-dimensional conductive network structure constructed by multiple carbon sources has been gradually developed, which has unique advantages in improving the conductivity and lithium-ion diffusion coefficient of lithium iron phosphate.

[0004] Currently, the main methods for constructing three-dimensional conductive network structures include electrospinning, melting, and hydrothermal methods. While electrospinning can control the conductive network structure and effectively improve the electrochemical properties of lithium iron phosphate, it is overly reliant on equipment, and the expensive and precise machinery required is prohibitive for industrial production. The melting method, although simple, suffers from low uniformity in raw material mixing, requires long melting times and high temperatures, and exhibits poor product uniformity and reproducibility. The hydrothermal method produces products with high crystallinity, uniform particle size, and good electrochemical performance, but this method requires maintaining high temperatures and pressures, placing stringent demands on equipment and making it unsuitable for industrial production. Therefore, there is an urgent need to develop novel and efficient methods and technologies for constructing three-dimensional conductive network structures to further improve the performance of lithium iron phosphate cathode materials and drive technological advancements and cost reductions in lithium-ion batteries. Summary of the Invention

[0005] Given the inherent low conductivity and low lithium-ion diffusion coefficient of lithium iron phosphate, and the aforementioned drawbacks of current methods for constructing three-dimensional conductive network structures, this invention provides a method for preparing a microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material that can improve the conductivity and lithium-ion diffusion coefficient of lithium iron phosphate. Its features include: (1) introducing microcrystalline cellulose generated from cotton acid hydrolysis as an attachment substrate and structural regulator, thereby achieving structural regulation and morphology optimization of carbon-coated lithium iron phosphate; (2) lithium iron phosphate particles and their surface carbon layer, together with carbon nanotubes derived from the carbonization of microcrystalline cellulose, constitute an effective three-dimensional spatial network conductive structure, significantly improving the conductivity and lithium-ion diffusion coefficient of lithium iron phosphate, and enhancing its electrochemical performance; (3) the preparation method is simple, has a short process flow, low energy consumption, a wide range of raw material sources, easy reaction control, and minimal waste discharge, making it green and environmentally friendly. This invention opens up a new carbon source for the carbon-coated modification of lithium iron phosphate and provides a new strategy and method for constructing cathode materials with efficient three-dimensional spatial network conductive structures, as well as a new cathode material with excellent electrochemical performance for lithium-ion batteries.

[0006] The technical solution of this invention is as follows:

[0007] A method for preparing a carbon-coated lithium iron phosphate composite cathode material supported on microcrystalline cellulose for lithium-ion batteries is disclosed. Microcrystalline cellulose extracted from cotton is used as the substrate and structure regulator for the formation of carbon-coated lithium iron phosphate. The method employs a sol-gel process, with the reaction taking place in the liquid phase, allowing for maximum mixing of raw materials and effective control of product uniformity. Furthermore, the carbon-coated lithium iron phosphate is uniformly formed along the surface of the microcrystalline cellulose, enabling effective control of the structure and morphology of the carbon-coated lithium iron phosphate. The carbon-coated lithium iron phosphate particles, carbon source-derived carbon network, and microcrystalline cellulose-derived carbon rods can easily form an effective and stable three-dimensional spatial network conductive structure.

[0008] The preparation method of the above-mentioned microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material includes the following steps:

[0009] (1) Chop the cotton and add it to the reactor. Then, prepare a mixed acid with a volume ratio of nitric acid, sulfuric acid and distilled water of 1-2:1:4. Add the mixed acid to the reactor with a volume ratio of 25-35 mL to 1 g of cotton. Reflux the acid at 70-90°C for 4-5 hours. After natural sedimentation, pour out and collect the supernatant. Separate the lower slurry by filtration. Combine the filtrate and the supernatant for the preparation of the next round of acid hydrolysis mixed acid. Wash the filter cake with distilled water until neutral and then dry it to constant weight to obtain microcrystalline cellulose, denoted as MCC.

[0010] (2) According to the molar ratio of lithium source, phosphorus source and iron source of 1.00~1.05∶1∶1, first dissolve lithium source and phosphorus source in distilled water and stir until the white precipitate no longer increases to obtain mixture A with a molar concentration of 0.22~0.26mol / L; then dissolve iron source in distilled water to prepare solution B with a molar concentration of 0.60~0.65mol / L.

[0011] (3) Add solution B to mixture A while stirring, and then continue stirring until the mixture turns grayish-green and no longer changes color, to obtain mixture C;

[0012] (4) According to the mass ratio of carbon source, MCC and lithium iron phosphate theoretical yield of 0.12~0.15∶0.05~0.2∶1, add carbon source and MCC to mixture C, stir to form sol, then heat to 80~95℃ and continue stirring for 4~6h, the mixture will turn into gel.

[0013] (5) Dry and grind the gel to obtain the precursor;

[0014] (6) The precursor is pre-calcined at 300-400℃ for 2-5 hours under a protective atmosphere, then heated to 600-800℃ for 6-10 hours, cooled to room temperature, and ground to obtain the product microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material.

[0015] Furthermore, in step (1), the cotton is degreased cotton after impurities have been removed.

[0016] Further, in step (1), the nitric acid is concentrated nitric acid with a mass percentage concentration of 65% to 68%; the sulfuric acid is concentrated sulfuric acid with a mass percentage concentration of 98%.

[0017] Furthermore, in step (1), the reflux acid hydrolysis is carried out under stirring conditions, with a stirring speed of 150-250 r / min.

[0018] Furthermore, in step (2), the lithium source is a soluble lithium compound, preferably one of lithium carbonate or lithium hydroxide.

[0019] Furthermore, in step (2), the phosphorus source is a soluble phosphate or phosphoric acid, preferably ammonium dihydrogen phosphate or / and phosphoric acid.

[0020] Furthermore, in step (2), the iron source is a soluble iron salt, preferably ferrous sulfate heptahydrate extracted and converted from pyrite slag.

[0021] Furthermore, in step (4), the carbon source is sucrose, oxalic acid or citric acid, preferably citric acid and / or oxalic acid.

[0022] Furthermore, in steps (1) and (5), the drying is carried out in a vacuum drying oven at a temperature of 60–80°C.

[0023] Furthermore, in step (6), the protective atmosphere is argon or nitrogen, preferably argon.

[0024] Furthermore, in step (6), the pre-calcination and calcination heating rate is 1 to 7 °C / min.

[0025] This invention first extracts microcrystalline cellulose from cotton fibers using a mixed acid of sulfuric and nitric acids as a hydrolysant. This microcrystalline cellulose serves as the substrate and structure regulator for carbon-coated lithium iron phosphate. A lithium source and a phosphorus source are then mixed in a specific ratio to form a mixture, which is then mixed with an iron source solution. A certain proportion of carbon source and microcrystalline cellulose are added, causing the mixture to transform from a sol to a gel. After drying and further grinding, a precursor is obtained. This precursor is then placed in a tube furnace and pre-calcined and calcined at high temperature under a protective atmosphere to generate a microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material. Through the carrier, template, and structure regulation effects of microcrystalline cellulose, the carbon-coating raw material and the lithium iron phosphate precursor can be uniformly generated and distributed along the surface of the microcrystalline cellulose. When the microcrystalline cellulose is calcined at high temperature to form carbon nanotubes, the generated carbon-coated lithium iron phosphate can be well loaded and fused onto the carbon nanotubes, forming a highly efficient three-dimensional spatial network conductive structure. This improves its intrinsic conductivity and lithium-ion diffusion coefficient, thereby enhancing its electrochemical performance and application performance. The prepared microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material has good cycle performance and rate performance, and is particularly suitable as a cathode material for power batteries.

[0026] The advantages of this invention compared to the prior art are as follows:

[0027] (1) This invention uses microcrystalline cellulose extracted from cotton as the substrate and structure regulator for carbon-coated lithium iron phosphate. The carbon-coating raw material and lithium iron phosphate precursor can be uniformly generated and distributed along the surface of microcrystalline cellulose. When microcrystalline cellulose is calcined at high temperature to degenerate into carbon nanotubes, the generated carbon-coated lithium iron phosphate can be well loaded and fused onto the carbon nanotubes, achieving effective control over the structure and morphology of carbon-coated lithium iron phosphate. Furthermore, the generated carbon-coated lithium iron phosphate particles, carbon source-derived carbon network, and microcrystalline cellulose-derived carbon rods can form a highly efficient and stable three-dimensional spatial network conductive structure, which can effectively reduce electrode polarization, significantly improve the intrinsic conductivity and lithium-ion diffusion coefficient of the prepared composite cathode material, improve the electrode conductivity, promote the diffusion of lithium ions throughout the electrode, and thus improve the Li-ion conductivity. + It improves the diffusion rate and rate performance of LFP@C, and effectively reduces electron transport resistance, enabling the prepared battery to have good cycle performance and rate performance. Therefore, it is particularly suitable as a cathode material for high-power batteries.

[0028] (2) The present invention uses the sol-gel method to prepare materials, which can promote the generation and distribution of lithium iron phosphate on the surface of microcrystalline cellulose by utilizing the oxygen-containing functional groups on the surface of microcrystalline cellulose and the chemical interaction between the raw materials in the liquid environment. Microcrystalline cellulose, as an attachment substrate, can act as a carrier and template agent, so as to effectively control and optimize the structure and morphology of the generated lithium iron phosphate. The carbon-coated raw materials (such as citric acid) and the generated lithium iron phosphate precursor are uniformly distributed and grown along the microcrystalline cellulose. During high-temperature calcination, the carbon layer generated by the decomposition of the carbon-coated raw materials (such as citric acid) tightly coats the surface of the lithium iron phosphate particles and is well loaded on the microcrystalline cellulose-derived carbon nanotubes and fused with them, producing a synergistic effect. This not only improves the stability of the composite cathode material, but also significantly improves its electrochemical performance.

[0029] (3) The cotton acid hydrolysis process of the present invention is simple, mild, and easy to operate. The mixed acid after acid hydrolysis is fully recovered and recycled, and the process is environmentally friendly.

[0030] (4) This invention not only opens up a new carbon source for the carbon coating modification of lithium iron phosphate, but also provides a new strategy and method for the construction of cathode materials with efficient three-dimensional spatial network conductive structure, and provides a new cathode material with excellent electrochemical performance for lithium-ion batteries.

[0031] (5) The composite cathode material of the present invention has a simple preparation process, a short reaction time, and high efficiency; it is easy to operate, has mild reaction conditions, and low energy consumption; it generates less "three wastes" and the process is environmentally friendly; the required equipment is conventional equipment, which is easy to realize industrial production and has broad application prospects. Attached Figure Description

[0032] Figure 1This is a process flow diagram for preparing the microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material of the present invention.

[0033] Figure 2 The images show scanning electron microscope (SEM) images of samples prepared according to the present invention. In the images, (a) corresponds to the microcrystalline cellulose prepared in Example 1, (b) corresponds to the carbon-coated lithium iron phosphate (LFP-C-15%) prepared in Example 1 without MCC, and (c) and (d) correspond to the microcrystalline cellulose loaded with carbon-coated lithium iron phosphate (LFP / C / MCC-0.3g) prepared in Example 3 with MCC.

[0034] Figure 3 The X-ray diffraction patterns of the samples prepared in this invention are shown in the figure. In the figure, (a) corresponds to LFP / C-15% obtained in Comparative Example 1, (b) corresponds to LFP / C / MCC-0.1g obtained in Example 1, (c) corresponds to LFP / C / MCC-0.2g obtained in Example 2, (d) corresponds to LFP / C / MCC-0.3g obtained in Example 3, and (e) corresponds to LFP / C / MCC-0.4g obtained in Example 4.

[0035] Figure 4 The charge-discharge curves of the samples prepared for this invention are shown. The charge-discharge regime is: C / 10 rate, voltage range 2.0 to 4.2V, constant current and constant voltage charging (cutoff current is 1mA / g).

[0036] Figure 5 The cycling performance diagram of the sample prepared in this invention is shown. The charge / discharge regime is: C / 10 rate, voltage range 2.0 to 4.2V, constant current and constant voltage charging (cutoff current is 1mA / g).

[0037] Figure 6 The following is a rate performance diagram of the samples prepared in this invention. The charge / discharge regimes are: C / 20, C / 10, C / 5, C / 2, 1C, 2C, and C / 20, with a voltage range of 2.0 to 4.2V, and constant current and constant voltage charging (cutoff current is 1mA / g).

[0038] Figure 7 The CV and EIS curves of the samples prepared for this invention are shown in the figure. (a) Cyclic voltammetry curves of LFP-15% and LFP / C / MCC-0.3g, (b)-(c) CV curves of LFP-15% and LFP / C / MCC-0.3g at different scan rates, (d) Lithium-ion diffusion system of LFP-C-15% and LFP / C / MCC-0.3g, and (e) EIS test results of LFP-C-15% and LFP / C / MCC-t (t=0.1, 0.3, 0.4g). Detailed Implementation

[0039] To better understand the present invention, the present invention will be further described below with reference to the embodiments and accompanying drawings, but the scope of protection of the present invention is not limited to the scope described in the embodiments.

[0040] Example 1

[0041] (1) Take 2g of cotton, chop it and add it to a 250mL three-necked flask equipped with a mechanical stirrer, a dropping device and a reflux condenser. Prepare a mixed acid according to the volume ratio of nitric acid, sulfuric acid and distilled water of 1:1:4. Then add 60mL of the mixed acid to the reactor according to the volume mass ratio of the mixed acid to cotton of 30mL:1g. Reflux acid hydrolysis at 200r / min and 70℃ for 5h. Allow it to settle naturally, pour it out and collect the supernatant. Filter the lower slurry and collect the filtrate and combine it with the collected supernatant for the preparation of mixed acid for the next round of acid hydrolysis. Wash the filter cake with distilled water until neutral, and then place it in a vacuum drying oven and dry it at 70℃ to constant weight to obtain 1.6840g MCC.

[0042] (2) According to the molar ratio of Li, P and Fe of 1.05∶1∶1, first dissolve 0.5699g LiOH and 1.4730g NH4H2PO4 in 50mL of distilled water and stir until the white precipitate no longer increases, to obtain mixture A with a molar concentration of 0.25mol / L; then dissolve 3.5965g FeSO4·7H2O prepared from pyrite slag in 20mL of distilled water to obtain solution B with a molar concentration of 0.63mol / L.

[0043] (3) Add solution B to mixture A while stirring, and then continue stirring for a period of time until the solution turns grayish-green and no longer changes, to obtain mixture C;

[0044] (4) According to the mass ratio of carbon source, MCC and lithium iron phosphate theoretical yield of 0.15∶0.05∶1, 0.30g citric acid and 0.10g microcrystalline cellulose were added to mixture C and stirred slowly to form a sol; then the temperature was raised to 95℃ and stirred for 4h to obtain a gel.

[0045] (5) Place the gel in a vacuum drying oven and dry it at 70°C to constant weight, then grind it to obtain the precursor;

[0046] (6) The precursor was pre-calcined at 300°C for 5 hours at a heating rate of 2°C / min under an argon atmosphere, and then heated to 700°C at a heating rate of 5°C / min for 8 hours. After natural cooling to room temperature, and after grinding, 1.9024 g of microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material was obtained, which was denoted as LFP / C / MCC-0.1 g.

[0047] Example 2

[0048] (1) Take 2g of cotton, chop it and add it to a 250mL three-necked flask equipped with a mechanical stirrer, a dropping device and a reflux condenser. Add the mixed acid solution recovered in Example 1 to the required amount according to the volume ratio of nitric acid, sulfuric acid and distilled water of 1:1:4 to prepare mixed acid. Then add 70mL of mixed acid to the reactor according to the volume mass ratio of mixed acid to cotton of 35mL:1g. Reflux acid hydrolysis at 250r / min and 80℃ for 4.5h. Allow it to settle naturally, pour it out and collect the supernatant. Filter the lower slurry and collect the filtrate and combine it with the collected supernatant for the preparation of mixed acid for the next round of acid hydrolysis. Wash the filter cake with distilled water until neutral and then place it in a vacuum drying oven and dry it at 80℃ to constant weight to obtain 1.5841g MCC.

[0049] (2) According to the molar ratio of Li, P and Fe of 1.05∶1∶1, first dissolve 0.5699g LiOH and 1.4616g H3PO4 in 50mL of distilled water and stir until the white precipitate no longer increases, to obtain mixture A with a molar concentration of 0.25mol / L; then dissolve 3.5965g FeSO4·7H2O prepared from pyrite slag in 21mL of distilled water to obtain solution B with a molar concentration of 0.60mol / L.

[0050] (3) Add solution B to mixture A while stirring, and then continue stirring for a period of time until the solution turns grayish-green and no longer changes, to obtain mixture C;

[0051] (4) According to the mass ratio of carbon source, MCC and lithium iron phosphate theoretical yield of 0.12∶0.1∶1, 0.24g of citric acid and 0.20g of microcrystalline cellulose were added to mixture C and stirred slowly to form a sol; then the temperature was raised to 80℃ and stirred for 6h to obtain a gel.

[0052] (5) Place the gel in a vacuum drying oven and dry it at 70°C to constant weight, then grind it to obtain the precursor;

[0053] (6) The precursor was pre-calcined at 350°C for 3 hours at a heating rate of 5°C / min under an argon atmosphere, and then heated to 600°C at a heating rate of 1°C / min for 10 hours. After natural cooling to room temperature, it was ground to obtain 1.9271 g of microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material, denoted as LFP / C / MCC-0.2 g.

[0054] Example 3

[0055] (1) Take 2g of cotton, chop it and add it to a 250mL three-necked flask equipped with a mechanical stirrer, a dropping device and a reflux condenser. Add the mixed acid solution recovered in Example 2 to the required amount according to the volume ratio of nitric acid, sulfuric acid and distilled water of 1:1:4 to prepare mixed acid. Then add 50mL of mixed acid to the reactor according to the volume mass ratio of mixed acid to cotton of 25mL:1g. Reflux acid hydrolysis at 250r / min and 90℃ for 4.0h. Allow it to settle naturally, pour it out and collect the supernatant. Filter the lower slurry and collect the filtrate and combine it with the collected supernatant for the preparation of mixed acid for the next round of acid hydrolysis. Wash the filter cake with distilled water until neutral and then place it in a vacuum drying oven and dry it at 70℃ to constant weight to obtain 1.1842g MCC.

[0056] (2) According to the molar ratio of Li, P and Fe of 1.05∶1∶1, first dissolve 0.4949g Li2CO3 and 1.4616g H3PO4 in 57mL of distilled water and stir until the white precipitate no longer increases, to obtain mixture A with a molar concentration of 0.22mol / L; then dissolve 3.5965g FeSO4·7H2O prepared from pyrite slag in 19.4mL of distilled water to obtain solution B with a molar concentration of 0.65mol / L.

[0057] (3) Add solution B to mixture A while stirring, and then continue stirring for a period of time until the solution turns grayish-green and no longer changes, to obtain mixture C;

[0058] (4) According to the mass ratio of carbon source, MCC and lithium iron phosphate theoretical yield of 0.15∶0.15∶1, 0.30g of oxalic acid and 0.30g of microcrystalline cellulose were added to mixture C and stirred slowly to form a sol; then the temperature was raised to 85℃ and stirred for 5.5h to obtain a gel.

[0059] (5) Place the gel in a vacuum drying oven and dry it at 60°C to constant weight, then grind it to obtain the precursor;

[0060] (6) The precursor was pre-calcined at 400°C for 2 hours under a nitrogen atmosphere at a heating rate of 7°C / min, and then heated to 700°C at a heating rate of 5°C / min for 8 hours. After natural cooling to room temperature, it was ground to obtain 1.9386 g of microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material, denoted as LFP / C / MCC-0.3 g.

[0061] Example 4

[0062] (1) Take 2g of cotton, chop it and add it to a 250mL three-necked flask equipped with a mechanical stirrer, a dropping device and a reflux condenser. Add the mixed acid solution recovered in Example 3 according to the required amount of nitric acid, sulfuric acid and distilled water volume ratio of 1.5:1:4 to prepare mixed acid. Then add 70mL of mixed acid to the reactor according to the mixed acid to cotton volume mass ratio of 35mL:1g. Reflux acid hydrolysis at 150r / min and 90℃ for 4.5h. Allow it to settle naturally, pour it out and collect the supernatant. Filter the lower slurry and collect the filtrate and combine it with the collected supernatant for the preparation of mixed acid for the next round of acid hydrolysis. Wash the filter cake with distilled water until neutral and then place it in a vacuum drying oven and dry it at 70℃ to constant weight to obtain 1.0958g MCC.

[0063] (2) According to the molar ratio of Li, P and Fe of 1.05∶1∶1, first dissolve 0.5699g LiOH and 1.4730g NH4H2PO4 in 48mL of distilled water and stir until the white precipitate no longer increases, to obtain mixture A with a molar concentration of 0.26mol / L; then dissolve 3.5965g FeSO4·7H2O prepared from pyrite slag in 20mL of distilled water to obtain solution B with a molar concentration of 0.63mol / L.

[0064] (3) Add solution B to mixture A while stirring, and then continue stirring for a period of time until the solution turns grayish-green and no longer changes, to obtain mixture C;

[0065] (4) According to the mass ratio of carbon source, MCC and lithium iron phosphate theoretical yield of 0.15∶0.2∶1, 0.30g of oxalic acid and 0.40g of microcrystalline cellulose were added to mixture C and stirred slowly to form a sol; then the temperature was raised to 90℃ and stirred for 5.0h to obtain a gel.

[0066] (5) Place the gel in a vacuum drying oven and dry it at 70°C to constant weight, then grind it to obtain the precursor;

[0067] (6) The precursor was pre-calcined at 350°C for 3 hours at a heating rate of 1°C / min under an argon atmosphere, and then heated to 800°C at a heating rate of 7°C / min for 6 hours. After natural cooling to room temperature, and after grinding, 1.9544 g of microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material was obtained, which was denoted as LFP / C / MCC-0.4 g.

[0068] Example 5

[0069] (1) Take 2g of cotton, chop it and add it to a 250mL three-necked flask equipped with a mechanical stirrer, a dropping device and a reflux condenser. Add the mixed acid solution recovered in Example 4 to the required amount according to the volume ratio of nitric acid, sulfuric acid and distilled water of 2:1:4 to prepare mixed acid. Then add 60mL of mixed acid to the reactor according to the volume mass ratio of mixed acid to cotton of 30mL:1g. Reflux acid hydrolysis at 200r / min and 70℃ for 5.0h. Allow it to settle naturally, pour it out and collect the supernatant. Filter the lower slurry and collect the filtrate and combine it with the collected supernatant for the preparation of mixed acid for the next round of acid hydrolysis. Wash the filter cake with distilled water until neutral and then place it in a vacuum drying oven and dry it at 60℃ to constant weight to obtain 1.2457g MCC.

[0070] (2) According to the molar ratio of Li, P and Fe of 1.00∶1∶1, first dissolve 0.5428g LiOH and 1.4730g NH4H2PO4 in 50mL of distilled water and stir until the white precipitate no longer increases, to obtain mixture A with a molar concentration of 0.25mol / L; then dissolve 3.5965g FeSO4·7H2O prepared from pyrite slag in 20mL of distilled water to obtain solution B with a molar concentration of 0.63mol / L.

[0071] (3) Add solution B to mixture A while stirring, and then continue stirring for a period of time until the solution turns grayish-green and no longer changes, to obtain mixture C;

[0072] (4) According to the mass ratio of carbon source, MCC and lithium iron phosphate theoretical yield of 0.15∶0.2∶1, 0.30g citric acid and 0.40g microcrystalline cellulose were added to mixture C and stirred slowly to form a sol; then the temperature was raised to 90℃ and stirred for 6.0h to obtain a gel.

[0073] (5) Place the gel in a vacuum drying oven and dry it at 70°C to constant weight, then grind it to obtain the precursor;

[0074] (6) The precursor was pre-calcined at 300°C for 5 hours at a heating rate of 3°C / min under an argon atmosphere, and then heated to 600°C at a heating rate of 5°C / min for 10 hours. After natural cooling to room temperature, it was ground to obtain 1.9544 g of microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material, which was denoted as LFP / C / MCC-0.4(1) g.

[0075] Comparative Example 1

[0076] Except for the absence of microcrystalline cellulose (MCC), the preparation conditions were the same as in Example 1, as detailed below:

[0077] (1) According to the molar ratio of Li, P and Fe of 1.05∶1∶1, first dissolve 0.5699g LiOH and 1.4730g NH4H2PO4 in 50mL of distilled water and stir until the white precipitate no longer increases, to obtain mixture A with a molar concentration of 0.25mol / L; then dissolve 3.5965g FeSO4·7H2O prepared from pyrite slag in 20mL of distilled water to obtain solution B with a molar concentration of 0.63mol / L.

[0078] (2) Add solution B to mixture A while stirring, and then continue stirring for a period of time until the solution turns grayish-green and no longer changes, to obtain mixture C;

[0079] (3) According to the mass ratio of carbon source to lithium iron phosphate theoretical yield of 0.15∶1, add 0.30g of citric acid to mixture C and stir slowly to form a sol; then heat to 95℃ and continue stirring for 4h to obtain a gel;

[0080] (4) Place the gel in a vacuum drying oven and dry it at 70°C to constant weight, then grind it to obtain the precursor;

[0081] (5) The precursor was pre-calcined at 300°C for 5 hours at a heating rate of 2°C / min under an argon atmosphere, and then heated to 700°C at a heating rate of 5°C / min for 8 hours. After natural cooling to room temperature, 1.8794 g of carbon-coated lithium iron phosphate composite cathode material was obtained after grinding, which was denoted as LFP / C-15%.

[0082] Half-cells were prepared using carbon-coated lithium iron phosphate obtained in Comparative Example 1 and microcrystalline cellulose-supported carbon-coated lithium iron phosphate obtained in Examples 1-4 as cathode materials. The specific preparation methods and steps are as follows:

[0083] Using N-methylpyrrolidone (NMP) as a solvent, a mixture was prepared at a mass ratio of m(positive electrode material):m(acetylene black):m(PVDF) = 80:10:10. The mixture was ground in an agate mortar for 30 minutes. Using a 25μm preparation tool, the thoroughly mixed slurry was uniformly coated onto clean, dry, and flat aluminum foil. The foil was then vacuum-dried at 120℃ for 12 hours, followed by vacuum drying at 80℃ for 4 hours. The dried electrode sheets were cut into circular positive electrode sheets with a diameter of Φ = 12 mm using a slicing machine. The mass of each positive electrode sheet was accurately weighed on an analytical balance and recorded as m, and then numbered. The mass of a blank aluminum foil of the same size was weighed, which was 6.1 mg. Therefore, the mass of active material in each positive electrode sheet is: mactive = (m – 6.1 mg) × 80%.

[0084] After drying and weighing, the positive electrode sheet was transferred to a super-clean glove box for battery assembly. The half-cell used a lithium sheet as the negative electrode, and the separator was a Celgard 2400 polypropylene microporous membrane. 1 mol / L LiPF6 was dissolved in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) with a volume ratio of V(EC):V(DMC):V(EMC) = 1:1:1 as the electrolyte, and the cells were assembled into 2025 coin cells.

[0085] Figure 2 These are scanning electron microscope (SEM) images of the microcrystalline cellulose (a, MCC) prepared in Example 1, the carbon-coated lithium iron phosphate (b, LFP-C-15%) prepared in Comparative Example 1 without MCC, and the microcrystalline cellulose-supported carbon-coated lithium iron phosphate (cd, LFP / C / MCC-0.3g) prepared in Example 3 with MCC. Figure 2 (a) It can be seen that after acid hydrolysis, the cotton fibers decompose into short rods or granules, with only a small portion continuing to maintain a strip structure. Figure 2 (b) shows that the LFP-C-15% prepared according to the method and steps of Comparative Example 1 without the addition of MCC consists of nanosheets with a particle size of several hundred, which are stacked together and have a dense and compact structure. In contrast, the LFP / C / MCC-0.3g prepared according to the method and steps of Example 3 with the addition of MCC is shown in the scanning electron microscope image below. Figure 2 As shown in (c)-(d), microcrystalline cellulose retains its original structural morphology after high-temperature calcination. The sheet-like carbon-coated lithium iron phosphate particles and the flocculent amorphous carbon network generated by citric acid pyrolysis are loaded on MCC-derived carbon rods, forming a three-dimensional spatial network conductive structure with MCC-derived carbon rods as conductors. This indicates that the addition of MCC can significantly regulate the structure and morphology of the carbon-coated lithium iron phosphate formed on it, and effectively promote the formation of the three-dimensional spatial network conductive structure.

[0086] Figure 3 X-ray diffraction patterns of the products obtained in Example 1 (b, LFP / C / MCC-0.1g), Example 2 (c, LFP / C / MCC-0.2g), Example 3 (d, LFP / C / MCC-0.3g), Example 4 (e, LFP / C / MCC-0.4g), and Comparative Example 1 (a, LFP / C-15%) are shown. Figure 3It can be seen that all samples exhibit characteristic peaks at 2θ = 17.1°, 20.7°, 25.5°, 26.5°, 32.2°, 35.6°, 42.3°, 52.5°, and 61.9°, which is consistent with the orthorhombic LiFePO4 standard card (JCPDS No. 83-2092) with space group Pnma, corresponding to its (200), (101), (111), (121), (301), (311), (122), (222), and (113) crystal planes, respectively. Moreover, the diffraction peaks of all samples are sharp and free of impurity peaks, indicating that the LFP composite material prepared by the sol-gel method described above in this invention has a complete crystal phase structure, high crystallinity, and high purity. Figure 3 (a) shows the XRD pattern of LFP-C-15%. It can be seen that the diffraction peaks produced by the sample on the (200), (101), (111), (211), and (311) crystal planes have high intensity and sharp peak shape, indicating that the LFP-C-15% sample prepared with 15% citric acid is of high purity. Figure 3 (b)-(e) are the diffraction patterns of LFP / C / MCC-t (t = 0.1, 0.2, 0.3, 0.4 g) samples. With 15% citric acid added, the amount of microcrystalline cellulose was gradually increased. As shown in the figures, the diffraction peak intensity of the samples gradually decreased with increasing MCC content, because the amorphous carbon content gradually increased. The above results indicate that Examples 1, 2, 3, and 4 successfully prepared microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode materials.

[0087] Figure 4 The images show the initial charge-discharge curves of the cathode materials prepared in Examples 1, 2, 3, 4, and Comparative Example 1, respectively. Figure 4 It can be seen that at a 0.1C rate, the discharge specific capacity of LFP-C-15% with only citric acid added is 74.3 mAh·g. -1 With the addition of 0.3g of microcrystalline cellulose, the initial discharge specific capacity increased to 150.3mAh·g. -1 This indicates that the addition of MCC has a significant effect on improving the electrochemical performance of lithium iron phosphate.

[0088] Figure 5 The LFP / C / MCC-0.3g prepared in Example 3 was tested for its cyclic discharge performance at a 0.1C rate. After 50 cycles, the specific capacity was 147.7 mAh·g. -1 The capacity retention rate was 98.2%. Figure 6The rate performance of LFP / C / MCC-0.3g at different discharge rates is shown. The specific discharge capacities at 0.05, 0.1, 0.2, 0.5, 1, 2, and 0.05C are 157.3, 139.7, 120.8, 117.9, 108.0, 96.5, and 146.4 mAh·g, respectively. -1 When the temperature returns to 0.05C, the specific capacity recovers to 93.1% of the initial capacity.

[0089] Using Comparative Example 1 (LFP-C-15%) and Example 3 (LFP / C / MCC-0.3g) as cathode materials, at 0.1 mV·s -1 Cyclic voltammetry curves were tested at the scan rate, and AC impedance values ​​were tested using Example 1 (LFP / C / MCC-0.1g), Example 3 (LFP / C / MCC-0.3g), Example 4 (LFP / C / MCC-0.4g), and Comparative Example 1 (LFP-C-15%) as the positive electrode material. The results are as follows: Figure 7 As shown. From Figure 7 The cyclic voltammetry results in (a) show that the cathode material with added MCC has a sharper peak shape and a narrower half-peak width. The oxidation and reduction peak potentials are 3.58V and 3.28V, respectively, and the ΔV is 0.3V, which is less than the 0.39V potential difference of LFP-C-15%. This indicates that the addition of MCC can effectively reduce electrode polarization. Figure 7 (b)-(c) show the CV curves at different scan rates. The oxidation and reduction peaks during the test correspond to the extraction and insertion of lithium ions, and the phase transition between LiFePO4 and FePO4, respectively. As the scan rate increases, the electrode polarization increases, but the polarization amplitude of LFP / C / MCC-0.3g is smaller, indicating that the addition of MCC is beneficial for improving the electrode conductivity and reducing electrode polarization. Figure 7 (d) The lithium-ion diffusion coefficient D of LFP-C-15% and LFP / C / MCC-0.3g can be obtained. Li + They are 1.593×10 -9 cm·s -1 and 1.554×10 -8 cm·s -1 It is evident that the addition of MCC significantly improves the conductivity of the three-dimensional spatial network structure. Li + This indicates that the three-dimensional spatial network conductive structure formed by carbon generated from the decomposition of carbon nanotubes derived from MCC and carbon sources such as citric acid, together with lithium iron phosphate particles, is conducive to promoting the diffusion of lithium ions throughout the electrode, thereby improving the rate performance of LFP@C. Figure 7(e) shows the EIS test results for LFP-C-15% and LFP / C / MCC-t (t = 0.1, 0.3, 0.4 g). The EIS curves mainly consist of a semi-circular shape in the high-frequency region and a sloping line in the low-frequency region. The charge transfer resistances of the four samples are 721.5 Ω, 654.6 Ω, 155.4 Ω, and 293.6 Ω, respectively, indicating that the three-dimensional spatial network conductive structure composed of MCC-derived carbon rods, carbon-coated LFP particles, and carbon source-derived carbon mesh can effectively reduce electron transport resistance.

[0090] In summary, this invention uses MCC extracted from cotton as the adhesion substrate and structure modifier for carbon-coated lithium iron phosphate, enabling lithium iron phosphate particles and their surface carbon layer to form an LFP / C / MCC composite cathode material with a three-dimensional spatial network conductive structure, together with microcrystalline cellulose-derived carbon nanotubes. This effectively reduces electrode polarization, improves electrode conductivity, and promotes lithium-ion diffusion throughout the electrode, thereby enhancing the Li-P / C / MCC conductivity. + It exhibits high diffusion rate and high rate performance of LFP@C, and effectively reduces electron transport resistance, making it particularly suitable as a cathode material for high-power batteries.

[0091] The above are merely preferred embodiments of the present invention. Based on the above concept of the present invention, those skilled in the art can make various modifications and transformations. For example, within the range of proportions and process conditions given by the present invention, the proportions and process conditions can be combined and transformed. Such transformations and modifications are all within the scope of the present invention.

Claims

1. A method for preparing a microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material, characterized in that, Microcrystalline cellulose extracted from cotton is used as the attachment substrate and structure regulator for carbon-coated lithium iron phosphate. Carbon-coated lithium iron phosphate is uniformly generated along the surface of the microcrystalline cellulose, regulating its structure and morphology. This allows the carbon-coated lithium iron phosphate particles, carbon source-derived carbon network, and microcrystalline cellulose-derived carbon rods to form an effective three-dimensional conductive network structure. The specific steps include: (1) Chop the cotton and add it to the reactor. Then, prepare a mixed acid with a volume ratio of nitric acid, sulfuric acid and distilled water of 1~2:1:

4. Add the mixed acid to the reactor with a volume mass ratio of 25~35 mL:1 g of mixed acid to cotton. Reflux the acid at 70~90℃ for 4~5 h. After natural sedimentation, pour out and collect the supernatant. Separate the lower slurry by filtration. Combine the filtrate and the supernatant for the preparation of the next round of acid hydrolysis mixed acid. Wash the filter cake with distilled water until neutral and then dry it to constant weight to obtain microcrystalline cellulose, denoted as MCC. (2) According to the molar ratio of lithium source, phosphorus source and iron source of 1.00~1.05∶1∶1, first dissolve lithium source and phosphorus source in distilled water and stir until the white precipitate no longer increases to obtain mixture A with a molar concentration of 0.22~0.26 mol / L; then dissolve iron source in distilled water to prepare solution B with a molar concentration of 0.60~0.65 mol / L. (3) Add solution B to mixture A while stirring, and continue until the mixture turns grayish-green and no longer changes color, to obtain mixture C; (4) According to the mass ratio of carbon source, MCC and lithium iron phosphate theoretical yield of 0.12~0.15∶0.05~0.2∶1, add carbon source and MCC to mixture C, stir slowly to form a sol, then heat to 80~95℃ and continue stirring for 4~6 h, the mixture will turn into a gel. (5) Dry and grind the gel to obtain the precursor; (6) The precursor is pre-calcined at 300~400℃ for 2~5 h under a protective atmosphere, then heated to 600~800℃ for 6~10 h, cooled to room temperature, and ground to obtain the product microcrystalline cellulose supported carbon coated lithium iron phosphate composite cathode material.

2. The method for preparing the microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material according to claim 1, characterized in that, In step (1), the cotton is degreased cotton after impurities have been removed.

3. The method for preparing the microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material according to claim 1, characterized in that, In step (1), the nitric acid is concentrated nitric acid with a mass percentage concentration of 65% to 68%; the sulfuric acid is concentrated sulfuric acid with a mass percentage concentration of 98%.

4. The method for preparing the microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material according to claim 1, characterized in that, In step (1), the reflux acid hydrolysis is carried out under stirring conditions, with a stirring speed of 150~250 r / min.

5. The method for preparing the microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material according to claim 1, characterized in that, In step (2), the lithium source is a soluble lithium compound; the phosphorus source is a soluble phosphate or phosphoric acid; and the iron source is a soluble iron salt.

6. The method for preparing the microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material according to claim 1, characterized in that, The lithium source is lithium carbonate or lithium hydroxide; the phosphorus source is ammonium dihydrogen phosphate or / and phosphoric acid; the iron source is ferrous sulfate heptahydrate extracted and converted from pyrite slag.

7. The method for preparing the microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material according to claim 1, characterized in that, In step (4), the carbon source is at least one of sucrose, oxalic acid or citric acid.

8. The method for preparing the microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material according to claim 1, characterized in that, In step (6), the protective atmosphere is argon or nitrogen; the heating rate of the pre-calcination and calcination is 1~7℃ / min.

9. The method for preparing the microcrystalline cellulose-supported carbon-coated lithium iron phosphate composite cathode material according to claim 1, characterized in that, In steps (1) and (5), the drying is carried out in a vacuum drying oven at a temperature of 60~80℃.

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