Chitosan modified lithium iron phosphate and carbon composite positive electrode material and preparation method thereof

Abstract

The invention discloses a chitosan modified lithium iron phosphate and carbon composite positive electrode material and a preparation method thereof.The preparation method comprises the steps that 1, lithium hydroxide, phosphoric acid and ferrous sulfate heptahydrate are mixed to prepare a uniform and stable solution, and a lithium iron phosphate precursor is prepared through a solvothermal method; step 2, dissolving chitosan in an acetic acid aqueous solution, and magnetically stirring under a water bath condition until the chitosan is completely dissolved to prepare a chitosan solution; and 3, adding the lithium iron phosphate precursor into a chitosan solution, uniformly mixing, carrying out ball milling and freeze drying, and carrying out high-temperature carbonization treatment in an inert gas to obtain the chitosan modified lithium iron phosphate and carbon composite positive electrode material. The method is simple to operate, low in cost and uniform in product particle size distribution.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion batteries, and in particular relates to a chitosan-modified lithium iron phosphate and carbon composite positive electrode material and a preparation method thereof. Background Art

[0002] In recent years, lithium-ion batteries have become a core component of electrochemical energy storage systems due to their high energy density and excellent rate performance. The performance of their cathode materials plays a decisive role in the overall performance of the battery. Olivine-type lithium iron phosphate is considered an ideal cathode material with great potential due to its high theoretical capacity, excellent thermal stability, and environmentally friendly properties.

[0003] However, defects such as poor intrinsic electronic conductivity and low lithium ion diffusion rate of lithium iron phosphate restrict its application potential in high-rate charge and discharge scenarios (Nano Lett. 2021, 21(6): 2572). Existing preparation methods, such as those prepared by high-temperature solid-phase methods, have poor particle size distribution uniformity and are prone to agglomeration; microwave heating methods are not conducive to large-scale industrial production; and sol-gel methods are complex, have long production cycles, are expensive, and may produce harmful byproducts.

[0004] Therefore, the existing lithium iron phosphate positive electrode material has the problem of low conductivity. Summary of the Invention

[0005] To overcome the above-mentioned problems of the prior art, the present invention aims to provide a chitosan-modified lithium iron phosphate and carbon composite cathode material and its preparation method. This preparation method combines chitosan, an economical and green biomass resource, with lithium iron phosphate to form a chitosan-derived carbon skeleton / nitrogen-doped carbon-coated lithium iron phosphate composite cathode material, thereby addressing the low conductivity of existing lithium iron phosphate cathode materials. This method is simple to operate, low-cost, and produces a uniform particle size distribution.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for preparing a chitosan-modified lithium iron phosphate and carbon composite positive electrode material comprises the following steps:

[0008] Step 1: lithium hydroxide, phosphoric acid, and ferrous sulfate heptahydrate are mixed to form a uniform and stable solution, and a lithium iron phosphate precursor is prepared by a solvothermal method;

[0009] Step 2: Dissolve chitosan in an acetic acid aqueous solution and stir magnetically in a water bath until completely dissolved to prepare a chitosan solution;

[0010] Step 3: adding the lithium iron phosphate precursor to the chitosan solution and mixing them evenly, subjecting them to ball milling, freeze drying, and then carbonizing them at high temperature in an inert gas to obtain a chitosan-modified lithium iron phosphate and carbon composite positive electrode material.

[0011] In step 1, the specific preparation process of the lithium iron phosphate precursor is as follows:

[0012] a) dissolving lithium hydroxide in a mixed solvent of deionized water and ethylene glycol (30-50 mL), and slowly adding phosphoric acid dropwise to the mixed solution to form a white suspension A, which is set aside;

[0013] b) dissolving ferrous sulfate heptahydrate in a mixed solvent of deionized water and ethylene glycol (50-70 mL) to form a mixed solution B;

[0014] c) Solution B was slowly added dropwise to the white suspension A, and after vigorous stirring for 2 to 6 hours, the resulting liquid was transferred to a hydrothermal reactor and placed in an oven at 120 to 200° C. for 8 to 15 hours. After the reaction was completed, the solution was centrifuged, washed, and dried to obtain a lithium iron phosphate precursor.

[0015] In the step 1, the molar ratio of lithium hydroxide, phosphoric acid and ferrous sulfate heptahydrate is 3:(1-3):(1-3).

[0016] In step 2, the deacetylation degree of chitosan is ≥95%;

[0017] In step 2, 0.8-1 g of chitosan is dissolved in 30-50 mL of 1-10% (v / v) acetic acid aqueous solution and magnetically stirred in a water bath at 40-60° C. for 1-3 hours to obtain a chitosan solution. The purpose is to completely dissolve the chitosan.

[0018] In the step 3, the mass ratio of the lithium iron phosphate precursor to chitosan is 1:(0-0.3).

[0019] In step 3, the inert gas is nitrogen.

[0020] In step 3, the carbonization temperature is 300-800°C, the heating rate is 4-6°C / min, and the carbonization time is 3-8 hours. Under these carbonization conditions, chitosan is derived into a three-dimensional interconnected carbon-based conductive network, and the lithium iron phosphate particles are evenly anchored on the carbon skeleton.

[0021] In step 3, the specific preparation process of the chitosan-modified lithium iron phosphate / carbon composite positive electrode material is as follows:

[0022] a) mixing the lithium iron phosphate precursor and the chitosan solution and placing the mixture in a ball mill at a speed of 100 to 300 rpm for 3 to 5 hours;

[0023] b) freeze-drying the product obtained by the ball milling for 24 to 48 hours, and carbonizing it at high temperature to obtain a chitosan-modified lithium iron phosphate / carbon composite cathode material.

[0024] The present invention also provides a chitosan-modified lithium iron phosphate and carbon composite cathode material, constructed from a chitosan-derived three-dimensional interconnected carbon-based conductive network and uniformly distributed lithium iron phosphate particles. The spherical lithium iron phosphate particles are stably adsorbed on the chitosan-derived carbon framework, effectively suppressing particle agglomeration. The surface of the lithium iron phosphate particles is coated with an amorphous carbon layer approximately 2 nm thick, ensuring a stable electrode structure while improving the material's conductivity.

[0025] The present invention also provides a lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the positive electrode sheet is a positive electrode sheet prepared from the chitosan-modified lithium iron phosphate / carbon composite positive electrode material.

[0026] Beneficial effects of the present invention:

[0027] This invention uses a solvothermal method to synthesize a lithium iron phosphate precursor. This precursor is then mechanically ball-milled with a biomass carbon source, chitosan, followed by freeze-drying and high-temperature carbonization to produce a chitosan-derived carbon skeleton / nitrogen-doped carbon-coated lithium iron phosphate composite cathode material. Ball-milling-assisted carbothermal reduction yields finer fibers and smaller lithium iron phosphate particles. This method is simple, low-cost, and produces high yields with uniform particle size.

[0028] Microstructural characterization shows that the chitosan three-dimensional porous conductive carbon network not only effectively inhibits the agglomeration of lithium iron phosphate particles, but its interconnected pore structure also provides a fast channel for lithium ion transmission. At the same time, the conductive properties of the nitrogen-doped carbon layer also accelerate the charge transfer process, improving the overall conductivity of the material. Electrochemical test results show that the chitosan-modified lithium iron phosphate / carbon composite cathode material has good electrochemical performance, with a discharge capacity of 154.8 mA h g at a rate of 0.1C. -1 , and has better cycle stability and rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a scanning electron microscope (SEM) image of the chitosan-modified lithium iron phosphate and carbon composite positive electrode material obtained in Example 2 of the present invention.

[0030] Figure 2 This is a transmission electron microscope (TEM) image of the chitosan-modified lithium iron phosphate and carbon composite positive electrode material obtained in Example 2 of the present invention.

[0031] Figure 3is the X-ray diffraction (XRD) pattern of the chitosan-modified lithium iron phosphate and carbon composite positive electrode material obtained in Example 1-3 of the present application and the carbon-coated lithium iron phosphate composite material obtained in Comparative Example 1.

[0032] Figure 4 is the first charge-discharge performance curve of the chitosan-modified lithium iron phosphate and carbon composite positive electrode material obtained in Example 1-3 of the present application at a 0.1C rate, voltage window: 2.5-4.2V.

[0033] Figure 5 is the first charge-discharge performance curve of the chitosan-modified lithium iron phosphate and carbon composite positive electrode material obtained in Example 2 of the present application at different rates, voltage window: 2.5-4.2V. DETAILED DESCRIPTION

[0034] The present application will be further described in detail below with reference to the accompanying drawings.

[0035] Example 1:

[0036] The present embodiment provides a preparation method of a chitosan-modified lithium iron phosphate and carbon composite positive electrode material, which is prepared by the following steps:

[0037] Step 1: Dissolve 0.09 mol of lithium hydroxide into a mixed solvent of the same volume of deionized water (20 mL) and ethylene glycol (20 mL) and continuously stir on a magnetic stirrer until completely dissolved; then, slowly add 0.03 mol of phosphoric acid solution to the continuously stirred mixed solution to obtain a white suspension A. Then, dissolve 0.03 mol of ferrous sulfate heptahydrate in a mixed solvent of the same volume of deionized water (30 mL) and ethylene glycol (30 mL), and in order to prevent Fe 2+ from being oxidized to Fe 3+ , weigh 0.001 mol of ascorbic acid as a reducing agent and add it to obtain a mixed solution B. Then, slowly add the mixed solution B drop by drop to the white suspension A, and after stirring vigorously (speed reaches 2000 rpm) for a period of time to make the liquid mixture uniform, transfer it to a hydrothermal reactor, and then react at 180℃ in an oven for 12 h. After the reaction time ends, cool to room temperature, and then perform centrifugal washing with deionized water and ethanol alternately, and then dry in an oven to obtain a lithium iron phosphate precursor.

[0038] Step 2: Weigh 1 g of chitosan powder with a degree of deacetylation ≥95% and add it to 50 mL of 2% (v / v) acetic acid aqueous solution in portions, and then magnetically stir under water bath conditions for a period of time until the chitosan is completely dissolved to obtain a yellow transparent chitosan solution.

[0039] Step 3: Weigh 1g of the lithium iron phosphate precursor obtained in step 1 and place it in 5mL of the chitosan solution obtained in step 2. The mixture is placed in a ball mill and placed in a ball mill at a speed of 300rpm for 4h. The product obtained by ball milling is then frozen in a refrigerator for 24h and placed in a freeze dryer for freeze drying for 48h. The freeze-dried product is then placed in an alumina crucible and placed in a tube furnace. It is calcined step by step at a heating rate of 5℃ / min under an inert gas nitrogen atmosphere. First, keep the temperature at 350℃ for 3h, then continue to heat to 800℃ for 8h to finally obtain a chitosan-modified lithium iron phosphate / carbon composite positive electrode material.

[0040] Example 2:

[0041] This embodiment provides a method for preparing a chitosan-modified lithium iron phosphate and carbon composite positive electrode material, which is prepared by the following steps:

[0042] Step 1: The basic operation of preparing the lithium iron phosphate precursor in step 1 of this embodiment is basically the same as step 1 of embodiment 1, and will not be repeated here.

[0043] Step 2: The basic operation of preparing the chitosan solution in step 2 of this embodiment is basically the same as that in step 2 of embodiment 1, and will not be repeated here.

[0044] Step 3: Weigh 1g of the lithium iron phosphate precursor obtained in step 1 and place it in 10mL of the chitosan solution obtained in step 2. The mixture is placed in a ball mill and placed in a ball mill for 4 hours at a speed of 300rpm. The product obtained by ball milling is then frozen in a refrigerator for 24 hours and then placed in a freeze dryer for freeze drying for 48 hours. The freeze-dried product is then placed in an alumina crucible and placed in a tube furnace. It is calcined step by step at a heating rate of 5°C / min under an inert gas nitrogen atmosphere. First, keep the temperature at 350°C for 3 hours, then continue to heat to 800°C for 8 hours, and finally obtain a chitosan-modified lithium iron phosphate / carbon composite positive electrode material.

[0045] Example 3:

[0046] This embodiment provides a method for preparing a chitosan-modified lithium iron phosphate and carbon composite positive electrode material, which is prepared by the following steps:

[0047] Step 1: The basic operation of preparing the lithium iron phosphate precursor in step 1 of this embodiment is basically the same as step 1 of embodiment 1, and will not be repeated here.

[0048] Step 2: The basic operation of preparing the chitosan solution in step 2 of this embodiment is basically the same as that in step 2 of embodiment 1, and will not be repeated here.

[0049] Step 3: 1 g of lithium iron phosphate precursor obtained in step 1 was weighed and put into 15 mL of chitosan solution obtained in step 2. The mixture was put into a ball mill tank and ball-milled in a ball mill at a rotation speed of 300 rpm for 4 h. Then the ball-milled product was frozen in a refrigerator for 24 h and then freeze-dried in a freeze dryer for 48 h. Then the freeze-dried product was put into an alumina crucible and placed in a tube furnace for stepwise calcination at a heating rate of 5 ℃ / min under an inert gas atmosphere of nitrogen. First, it was kept at 350 ℃ for 3 h, then the temperature was continued to rise to 800 ℃ for 8 h, and finally chitosan-modified lithium iron phosphate / carbon composite positive electrode material was obtained.

[0050] Comparative Example 1:

[0051] This comparative example provides a preparation method of carbon-coated lithium iron phosphate composite material, which is prepared by the following steps:

[0052] Step 1: The basic operation of preparing lithium iron phosphate precursor in this comparative example step 1 is basically the same as that in example 1 step 1, which is not repeated here.

[0053] Step 2: The lithium iron phosphate precursor obtained in step 1 was taken and put into an alumina crucible and placed in a tube furnace for stepwise calcination at a heating rate of 5 ℃ / min under an inert gas atmosphere of nitrogen. First, it was kept at 350 ℃ for 3 h, then the temperature was continued to rise to 800 ℃ for 8 h, and finally carbon-coated lithium iron phosphate composite material was obtained.

[0054] Performance test

[0055] (1) Preparation of positive electrode: The chitosan-modified lithium iron phosphate / carbon composite positive electrode material provided by examples 1-3 and the carbon-coated lithium iron phosphate composite material provided by comparative example 1 were respectively taken as the positive electrode active material, and then weighed according to a mass ratio of 8:1:1 of the chitosan-modified lithium iron phosphate / carbon composite positive electrode material, conductive carbon black and polyvinylidene fluoride (PVDF), and then put into an agate mortar for grinding to mix the materials uniformly. Then it was added to N-methyl pyrrolidone (NMP) solvent and magnetically stirred for 12 h to obtain a uniform black slurry. Subsequently, the black viscous slurry was uniformly coated on an aluminum foil, placed in a vacuum oven at 120 ℃ for 12 h, then cooled to room temperature and taken out, and then punched into a circular electrode piece with a diameter of 10 mm by using a manual slicer for standby use.

[0056] (2) Assembly of button cell

[0057] The above prepared positive electrode piece, negative electrode piece (metal lithium piece), separator (Celgard 2500PP separator) and electrolyte (1M LiPF6 / EC+DEC+EMC (1:1:1, vol%)) were assembled into a CR2032 type button cell in an argon-filled glove box. After the cell was placed for 12 h, the electrochemical performance test was carried out.

[0058] (3) Battery performance test

[0059] The rested battery was tested using a CT3002A test system. The test was performed using a constant current charge / discharge method, with parameters such as current density, number of cycles, and voltage range (2.5-4.2V) set.

[0060] Test Example 1:

[0061] The chitosan-modified lithium iron phosphate and carbon composite cathode material obtained in Example 2 was tested by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the images measured were as follows: Figure 1 and 2 shown.

[0062] Depend on Figure 1 It can be observed that the material is composed of a three-dimensional interconnected carbon-based conductive network and embedded lithium iron phosphate particles. The lithium iron phosphate particles are evenly anchored on the chitosan-derived carbon skeleton. The carbon skeleton network has high porosity, relatively uniform structural dispersion, and the lithium iron phosphate particles are nanometer-sized.

[0063] Figure 2 It can be observed that the surface of the lithium iron phosphate particles is wrapped by an amorphous carbon layer with a thickness of about 2 nm. The thickness of the carbon layer is uniform, which is conducive to stabilizing the structure and improving electronic conductivity.

[0064] Figure 3 X-ray diffraction (XRD) test spectra of the chitosan-modified lithium iron phosphate and carbon composite positive electrode materials obtained in Examples 1-3 of the present invention and the carbon-coated lithium iron phosphate composite material obtained in Comparative Example 1 are given. The results show that the diffraction peaks of all materials correspond well to the olivine-type lithium iron phosphate standard card (JCPDS 81-1173), and the characteristic peaks are sharp and have significant intensity, indicating that the materials have high purity and crystallinity. In addition, for the chitosan-modified lithium iron phosphate and carbon composite positive electrode materials, no other impurity signals were detected. This proves the successful preparation of the chitosan-modified lithium iron phosphate and carbon composite positive electrode materials.

[0065] Figure 4 This is the first charge and discharge curve of the chitosan-modified lithium iron phosphate / carbon composite positive electrode material obtained in Examples 1-3 of the present invention at a rate of 0.1C. All electrode materials have a relatively ideal charge and discharge voltage platform (3.42V). This characteristic reflects the reversible migration process of lithium ions in the electrode material.

[0066] Figure 5The first-cycle charge-discharge curves of the chitosan-modified lithium iron phosphate / carbon composite cathode material obtained in Example 2 of the present invention at different charge and discharge rates are shown. As can be seen from the figure, the length of the platform region gradually decreases with increasing charge and discharge rate, and the charge-discharge voltage difference also increases with increasing current density. Even at a charge and discharge rate of 5C, the voltage platform of this material remains above 3.35V, which fully demonstrates the low degree of polarization of the material.

Claims

1. A method for preparing a chitosan-modified lithium iron phosphate and carbon composite positive electrode material, characterized in that: The following steps are included: Step 1: lithium hydroxide, phosphoric acid, and ferrous sulfate heptahydrate are mixed to form a uniform and stable solution, and a lithium iron phosphate precursor is prepared by a solvothermal method; Step 2: Dissolve chitosan in an acetic acid aqueous solution and stir magnetically in a water bath until completely dissolved to prepare a chitosan solution; Step 3: adding the lithium iron phosphate precursor to the chitosan solution and mixing them evenly, subjecting them to ball milling, freeze drying, and then carbonizing them at high temperature in an inert gas to obtain a chitosan-modified lithium iron phosphate and carbon composite positive electrode material.

2. The method for preparing a chitosan-modified lithium iron phosphate and carbon composite positive electrode material according to claim 1, characterized in that: In step 1, the specific preparation process of the lithium iron phosphate precursor is as follows: a) dissolving lithium hydroxide in a mixed solvent of deionized water and ethylene glycol (30-50 mL), and slowly adding phosphoric acid dropwise to the mixed solution to form a white suspension A, which is set aside; b) dissolving ferrous sulfate heptahydrate in a mixed solvent (50-70 mL) of deionized water and ethylene glycol to form a mixed solution B; c) Solution B was slowly added dropwise to the white suspension A, and after vigorous stirring for 2 to 6 hours, the resulting liquid was transferred to a hydrothermal reactor and placed in an oven at 120 to 200° C. for 8 to 15 hours. After the reaction was completed, the solution was centrifuged, washed, and dried to obtain a lithium iron phosphate precursor.

3. The method for preparing a chitosan-modified lithium iron phosphate and carbon composite positive electrode material according to claim 2, characterized in that: In the step 1, the molar ratio of lithium hydroxide, phosphoric acid and ferrous sulfate heptahydrate is 3:(1-3):(1-3).

4. The method for preparing a chitosan-modified lithium iron phosphate and carbon composite positive electrode material according to claim 3, characterized in that: In step 2, the deacetylation degree of chitosan is ≥95%; In the step 2, 0.8-1 g of chitosan is dissolved in 30-50 mL of 1-10% (v / v) acetic acid aqueous solution, and the solution is stirred magnetically in a water bath at 40-60° C. for 1-3 h to obtain a chitosan solution.

5. The method for preparing a chitosan-modified lithium iron phosphate and carbon composite positive electrode material according to claim 4, characterized in that: In the step 3, the mass ratio of the lithium iron phosphate precursor to chitosan is 1:(0-0.3); In step 3, the inert gas is nitrogen; In step 3, the carbonization temperature is 300-800° C., the heating rate is 4-6° C. / min, and the carbonization time is 3-8 hours.

6. The method for preparing a chitosan-modified lithium iron phosphate and carbon composite positive electrode material according to claim 5, characterized in that: In step 3, the specific preparation process of the chitosan-modified lithium iron phosphate / carbon composite positive electrode material is as follows: a) mixing the lithium iron phosphate precursor and the chitosan solution and placing the mixture in a ball mill at a speed of 100 to 300 rpm for 3 to 5 hours; b) freeze-drying the product obtained by the ball milling for 24 to 48 hours, and carbonizing it at high temperature to obtain a chitosan-modified lithium iron phosphate and carbon composite positive electrode material.

7. A chitosan-modified lithium iron phosphate and carbon composite positive electrode material prepared according to the method of any one of claims 1 to 6, characterized in that: A three-dimensional interconnected carbon-based conductive network derived from chitosan is synergistically constructed with evenly distributed lithium iron phosphate particles; spherical lithium iron phosphate particles are stably adsorbed on the chitosan-derived carbon skeleton, and the surface of the lithium iron phosphate particles is wrapped by an amorphous carbon layer with a thickness of about 2nm, which ensures the stability of the electrode structure while improving the conductivity of the material.

8. A lithium-ion battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the positive electrode sheet is a positive electrode sheet prepared by a chitosan-modified lithium iron phosphate and carbon composite positive electrode material prepared by the method of any one of claims 1 to 6.

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