Preparation method of nitrogen-doped porous carbon composite lithium iron phosphate positive electrode material

Abstract

The invention relates to the technical field of lithium batteries, and provides a preparation method of a nitrogen-doped porous carbon composite lithium iron phosphate positive electrode material, which comprises the following steps: mixing a lithium source, anhydrous iron phosphate, titanium dioxide and a carbon source, cleaning, drying and crushing waste plastic, stirring, grinding, spray-drying and calcining to obtain a porous carbon coated precursor; and doping a nitrogen source into the porous carbon coated precursor, grinding, carrying out spray drying, and carrying out secondary calcination to obtain the nitrogen-doped porous carbon composite lithium iron phosphate positive electrode material. The nitrogen-doped porous carbon can inhibit the growth of lithium iron phosphate crystal grains, control the particle size of lithium iron phosphate and shorten the diffusion path of lithium ions; the porosity and the specific surface area are improved, and charge conduction and ion transmission are promoted; the nitrogen doping improves the conductivity of the porous carbon coating layer and improves the electron conductivity. A trace amount of Cr, Cd, Zn and other elements in the waste plastic are introduced into lithium iron phosphate through two times of calcination, so that a lithium ion diffusion channel can be expanded or the symmetry of a crystal field can be optimized, and a lithium ion migration barrier can be reduced.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material technology, and in particular to a nitrogen-doped porous carbon composite lithium iron phosphate cathode material, its preparation method, and its application. Background Technology

[0002] Among the many cathode materials for lithium-ion batteries, lithium iron phosphate (LiFePO4, or LFP) has attracted much attention due to its abundant resources, low cost, environmental friendliness, and good thermal and cycle stability.

[0003] However, LFP materials also have some inherent drawbacks that limit their widespread use in high-power applications. Firstly, LFP has a low ion diffusion coefficient (compared to one-dimensional Li₂). + Diffusion channel, 10 -14 cm 2 ·s -1 Although PO4 3- The strong covalent bond in PO stabilizes the oxygen atom, ensuring the stability of Li. + The ability to extract and insert within a stable crystal structure gives olivine-type cathode materials good safety and cycle stability; however, the strong covalent bonds in PO also make Li... + It is difficult for Li to penetrate the PO tetrahedron; it can only diffuse one-dimensionally along the b-axis, thus limiting Li's diffusion. + The diffusion rate is low. Secondly, LFP has low electronic conductivity. LFP has an olivine-type structure, which results in a long electron conduction path, slow electron migration speed, and a large band gap for electron transitions, leading to a conductivity of less than 10. -10 Sm -1 This is mainly because of Fe. 2+ The d-electron configuration of ions in the olivine structure is unfavorable for the formation of free electrons, resulting in poor overall electronic conductivity of the material. This characteristic prevents LFP materials from effectively transporting electrons when operating at high current densities, thus limiting the rate performance of the battery. In recent years, organic compounds containing pyridine ring structures have often been used as carbon sources to prepare high-performance nano-LiFePO4 / C, such as glucose, sucrose, and starch. However, the coating effect and function of different carbon sources are different, and the introduction of some carbon sources does not significantly improve the conductivity and ion diffusion of lithium iron phosphate. Summary of the Invention

[0004] Based on this, it is necessary to address the technical problem that the introduction of some carbon sources does not significantly improve the conductivity of lithium iron phosphate, nor does it significantly improve the rate performance and cycle performance of lithium iron phosphate. Therefore, a nitrogen-doped porous carbon composite lithium iron phosphate cathode material, its preparation method, and its applications are proposed.

[0005] The present invention proposes a method for preparing a nitrogen-doped porous carbon composite lithium iron phosphate cathode material, comprising the following steps: Step 1: Mix lithium source, anhydrous iron phosphate, titanium dioxide, and carbon source, and put the washed, dried, and crushed waste plastic (main component is polyethylene terephthalate, PET) into a sand mill. Add ultrapure water and zirconium oxide grinding balls for sand milling to obtain a mixed slurry with a particle size of 300~600nm. Step 2: Spray dry the above-mentioned mixed slurry to obtain a powdered mixture. Then, calcine the powdered mixture at 380℃~620℃ for 2~8 hours under inert gas protection to obtain a porous carbon-coated precursor.

[0006] Step 3: The porous carbon-coated precursor, pyrophosphoromolybdate or pyrophosphorus stannate, is mixed with melamine foam and ball-milled in ultrapure water. The resulting mixture is then spray-dried and transferred to a tube furnace for secondary calcination at 650-800℃ for 5-10 hours to obtain nitrogen-doped porous carbon-coated lithium iron phosphate cathode material.

[0007] Preferably, in step one, the lithium source is one or more of lithium hydroxide, anhydrous lithium carbonate, lithium acetate, lithium nitrate, lithium oxalate, lithium phosphate, and lithium dihydrogen phosphate.

[0008] Preferably, in step one, the mixing ratio of lithium source and anhydrous iron phosphate is: 1:1.05-1.10 based on the total molar amount of iron to the molar ratio of lithium.

[0009] Preferably, in step one, the carbon source is one or more of glucose, sucrose, citric acid, maltose, and starch, and the amount used is 15% to 25% of the total mass of the lithium source and anhydrous iron phosphate.

[0010] Preferably, in step one, the mass of the waste plastic is 1% to 10% of the total mass of the lithium source and anhydrous iron phosphate, and the amount of titanium dioxide is 0.1% to 0.5% of the mass of anhydrous iron phosphate.

[0011] Preferably, in step one, the mass of ultrapure water is 100% to 230% of the total mass of lithium source, anhydrous iron phosphate, carbon source, and titanium dioxide; the mass ratio of zirconia grinding beads to the total mass of porous carbon-coated precursor is 3 to 6:1; the grinding time is 0.5 to 10 hours; the grinding speed is 1500 to 3000 rpm; and the particle size range of the mixed slurry is D50: 300 to 600 nm.

[0012] Preferably, in step two, the spray drying inlet temperature is 180℃~240℃, the outlet temperature is set to 70℃~110℃, the spray drying spray pressure is 0.2MPa~0.3MPa, and the feed rate is 1000~2000ml / h; the calcination conditions are: after introducing nitrogen gas at room temperature for 20~60min, the temperature is raised to 380℃~620℃ at a heating rate of 2~8℃ / min, and held for 2~8h.

[0013] Preferably, in step three, the nitrogen source is melamine foam; the mass of the nitrogen source is 5% to 13% of the mass of the porous carbon-coated precursor, and the amount of pyrophosphoromolybdate or pyrophosphorostannate added is 0.5% to 3% of the mass of the porous carbon-coated precursor.

[0014] Preferably, in step three, the mass ratio of the zirconium oxide grinding beads to the total mass of the lithium source, anhydrous iron phosphate, carbon source, and titanium dioxide is 3~6:1, the ball milling time is 2~6 hours, and the sand milling speed is 300~1000 rpm.

[0015] Preferably, in step three, the secondary calcination conditions are as follows: after introducing nitrogen gas at room temperature for 20-60 minutes, the temperature is increased to 650-800℃ at a heating rate of 2-8℃ / min, and held for 5-10 hours.

[0016] The preparation method provided by this invention introduces waste plastic (mainly polyethylene terephthalate, PET) and performs pyrolysis. Since PET can decompose at high temperatures to produce volatile substances, a large number of nanoscale pores can be formed on the surface of the agglomerated particles. In the process of preparing lithium iron phosphate cathode active material, a carbon source is introduced to construct a carbon coating layer on the surface of the lithium iron phosphate cathode active material. Therefore, in the finished product, the carbon coating layer, combined with the densely interconnected nanoscale pore structure, can provide more electrons around the active material and ensure the Li... +There are more sites for insertion / extraction, and both the first carbon source involved in the above operations and the incompletely decomposed PET can be used as carbon sources for constructing the carbon coating layer. Based on this, the preparation method provided by the present invention can form a porous carbon coating layer on the surface of lithium iron phosphate. First, the porous carbon coating layer can inhibit the growth of lithium iron phosphate grains, control the particle size of lithium iron phosphate, and shorten the diffusion path of lithium ions. Second, the porous structure of the porous carbon coating layer gives the active material suitable porosity and specific surface area, promotes charge conduction and ion transport, improves the electronic conductivity and lithium ion transport dynamics of the positive electrode active material, and gives it excellent rate performance and cycle performance. At the same time, the porous structure also facilitates the contact between pyrophosphoromolybdate or pyrophosphorus stannate and the positive electrode material at the pores, so that it can be doped into the lattice of the positive electrode material during secondary calcination, thereby improving the intrinsic conductivity and cycle stability of the material. Nitrogen doping using a nitrogen source improves the conductivity of the porous carbon coating. Nitrogen atoms can form stable covalent bonds with carbon atoms, and the CN and CC bonds are of similar length. The influence of nitrogen doping on the lattice distortion of porous carbon is relatively small, and nitrogen atoms are more easily incorporated into carbon materials. Nitrogen doping also increases the carrier concentration, thereby further improving the electronic conductivity of the composite material. Finally, in addition to its main component (polyethylene terephthalate), waste plastics also contain auxiliary components such as plasticizers, fillers, lubricants, and colorants. Commonly used auxiliary components include phthalates, sebacic acid esters, chlorinated paraffins, stearic acid and its metal salts, calcium carbonate, talc and other natural minerals, titanium dioxide, zinc oxide, cadmium red, ferric oxide, chrome yellow, and zinc yellow. Through two calcinations, the metal cations in the above auxiliary components are introduced into the lithium iron phosphate material. The introduction of high-valence cations (such as Zn) further enhances the process. 2+ Cd 3+ Cr 6+ (etc.) partially replaces Li + or Fe 2+ Doping expands lithium-ion diffusion channels or optimizes crystal field symmetry, lowering the lithium-ion migration barrier. Simultaneously, introducing impurity energy levels reduces the band gap, promoting carrier transitions. Doping elements can suppress lattice distortion and phase transitions during charging and discharging, alleviate volume expansion, reduce stress accumulation, and thus improve cycle stability. Partial doping can also suppress Fe dissolution, maintaining material integrity. Attached Figure Description

[0017] Figure 1 This is a SEM image of the material obtained in Embodiment 1 of this application. Detailed Implementation

[0018] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0019] To better understand the present invention, the following description, in conjunction with embodiments and related drawings, further illustrates the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0020] Unless otherwise specified, the reagents, methods and equipment used in this invention are conventional reagents, methods and equipment in this technical field.

[0021] Unless otherwise specified, all reagents and materials used in the following examples are commercially available.

[0022] The present invention proposes a method for preparing a nitrogen-doped porous carbon composite lithium iron phosphate cathode material, comprising the following steps: Step 1: Mix lithium source, anhydrous iron phosphate, titanium dioxide, and carbon source, and put the washed, dried, and crushed waste plastic (polyethylene terephthalate, PET) into a sand mill. Add ultrapure water and zirconium oxide grinding balls for sand milling to obtain a mixed slurry with a particle size of 300~600nm. Step 2: Spray dry the above-mentioned mixed slurry to obtain a powdered mixture. Then, calcine the powdered mixture at 380℃~620℃ for 2~8 hours under inert gas protection to obtain a porous carbon-coated precursor.

[0023] Step 3: The porous carbon-coated precursor and melamine-doped foam are ball-milled in ultrapure water. Then, the resulting mixture is spray-dried and transferred to a tube furnace for secondary calcination at 650-800℃ for 5-10 hours to obtain nitrogen-doped porous carbon-coated lithium iron phosphate cathode material.

[0024] Example 1 A method for preparing a nitrogen-doped porous carbon composite lithium iron phosphate cathode material includes the following steps: (1) First, add 1135ml of deionized water to the premix tank, then weigh 500g, 130g, 45g, 4g and 20g of iron phosphate, lithium carbonate, glucose, titanium dioxide and waste plastic respectively, stir at 20℃ for 1h to obtain solid-liquid mixture, then transfer it to a sand mill for grinding, grind at 2500rpm for 1.5h until the particle size D50 of the mixed slurry reaches 400nm; (2) The mixed slurry was spray-dried. The spray drying inlet temperature was 220℃, the outlet temperature was 85℃, the spray pressure was 0.25MPa, and the feed rate was 1500ml / h to obtain a precursor powder. (3) Then place it in a box furnace filled with nitrogen and heat treat it at 600°C for 6 hours to obtain a porous carbon-coated precursor; (4) Next, weigh 400g, 40g and 50g of porous carbon-coated precursor, pyrophosphoromolybdate and melamine foam respectively and add them to 735ml of ultrapure water. After premixing evenly, transfer to a ball mill for grinding. Grind at 600rpm for 5h until a nano-slurry with D50 of 500nm is formed. (5) The nano slurry is spray-dried. The spray drying inlet temperature is 220℃, the outlet temperature is 85℃, the spray pressure is 0.25MPa, and the feed rate is 1500ml / h. (6) The material is then placed in a nitrogen-filled box furnace and nitrogen is introduced at room temperature for 30 minutes. The temperature is then increased to 750°C at a rate of 3°C / min, and heat-treated at 750°C for 8 hours. Finally, it is crushed into powder with a D50 of approximately 0.5 μm, which is the nitrogen-doped porous carbon-coated lithium iron phosphate cathode material. The SEM image of the obtained material is shown below. Figure 1 As shown.

[0025] Example 2 The method and steps are the same as in Example 1, except that pyrophosphoromolybdate is replaced with pyrophosphorus stannate, thus obtaining a nitrogen-doped porous carbon-coated lithium iron phosphate cathode material.

[0026] Comparative Example 1 A method for preparing a porous carbon-coated lithium iron phosphate cathode material includes the following steps: Steps (1)-(3), (5)-(6) are the same as in Example 1; (4) Next, weigh 400g and 50g of the porous carbon-coated precursor and melamine foam and add them to 735ml of ultrapure water. After premixing evenly, transfer to a ball mill for grinding. Grind at 600rpm for 5h until a nano slurry with D50 of 500nm is formed. The difference between Comparative Example 1 and Example 1 is that no pyrophosphomolybdate was added.

[0027] Comparative Example 2 A method for preparing a nitrogen-doped carbon composite lithium iron phosphate cathode material includes the following steps: (1) First, add 1135ml of deionized water to the premixing tank, then weigh 500g, 130g, 45g, 4g and 20g of iron phosphate, lithium carbonate, glucose, titanium dioxide and polyethylene glycol respectively, stir at 20℃ for 1h to obtain a solid-liquid mixture, then transfer it to a sand mill for grinding, grind at 2500rpm for 1.5h until the particle size D50 of the mixed slurry reaches 400nm; Steps (2)-(6) are the same as in Example 1; The difference between Comparative Example 2 and Example 1 is that no waste plastic was added.

[0028] Comparative Example 3 A method for preparing a nitrogen-doped carbon composite lithium iron phosphate cathode material includes the following steps: Steps (1)-(3), (5)-(6) are the same as in Example 1; (4) Next, weigh 400g and 40g of the porous carbon-coated precursor and pyrophosphoromolybdate, add them to 735ml of ultrapure water, premix evenly, and then transfer to a ball mill for grinding. Grind at 600rpm for 5h until a nano-slurry with D50 of 500nm is formed. The difference between Comparative Example 1 and Example 1 is that melamine was not added.

[0029] Comparative Example 4 A method for preparing a nitrogen-doped porous carbon composite lithium iron phosphate cathode material includes the following steps: (1) First, add 1135ml of deionized water to the premix tank, then weigh 500g, 130g, 45g, 4g and 20g of iron phosphate, lithium carbonate, glucose, titanium dioxide and high-purity PET respectively, stir at 20℃ for 1h to obtain solid-liquid mixture, then transfer it to a sand mill for grinding, grind at 2500rpm for 1.5h until the particle size D50 of the mixed slurry reaches 400nm; Steps (2)-(6) are the same as in Example 1; The difference between Comparative Example 4 and Example 1 is that high-purity PET was used instead of waste plastic.

[0030] Physicochemical performance testing: The composite materials prepared in each embodiment and comparative example were subjected to physical and electrochemical performance tests. The carbon content was defined as the percentage of carbon per unit mass of the material, and was detected using an infrared carbon-sulfur analyzer. The electrochemical performance of each embodiment and comparative example was experimentally studied using CR2032 button half-cells. The specific preparation process of the positive electrode is as follows: The composite materials prepared using the various embodiments and comparative examples were assembled into button cells as follows: The composite materials, acetylene black, and binder PVDF (polyvinylidene fluoride) were weighed in a mass ratio of 80:10:10, added to an agate mortar, and ground thoroughly until uniformly mixed; an appropriate amount of N-methylpyrrolidone was added dropwise to the above mixture, and the mixture was ground again until a uniform slurry of a certain viscosity was formed; the slurry was coated onto dry carbon-coated aluminum foil using a scraper, and first dried in a forced-air drying oven at 80°C for 3 hours, and then dried in a vacuum environment at 120°C for 12 hours. After the temperature dropped to room temperature, the electrode sheet was removed; the removed electrode sheet was compacted using a roller press, and then cut into circular positive electrode sheets with a diameter of 14 mm using a punching machine, ready for use.

[0031] Lithium foil was selected as the negative electrode, and the electrolyte was 1.0 mol / L LiPF6 / (EC+EMC+DMC) (where LiPF6 is lithium hexafluorophosphate, EC is ethylene carbonate, EMC is ethyl methyl carbonate, DMC is dimethyl carbonate, and the volume ratio of EC, EMC, and DMC is 1:1:1). Celgard 2400 polypropylene microporous membrane was used as the separator. All materials, including the positive electrode, negative electrode, electrolyte, and separator, were assembled into a CR2032 coin cell in a glove box filled with high-purity argon gas (H2O < 0.1 ppm, O2 < 0.1 ppm). The coin cell was then sealed using a sealing machine. Finally, the assembled CR2032 coin cell was allowed to stand at room temperature for 12 hours for activation before use.

[0032] The test results are shown in Table 1: Table 1 Electrochemical performance test results

[0033] As shown in Table 1, the modified lithium iron phosphate material prepared by this invention exhibits excellent capacity and cycle performance. Furthermore, a comparison between Example 1 and Comparative Examples 1-4 shows that the addition of waste plastics significantly improves capacity and cycle performance, and the added melamine has a synergistic effect. Porous carbon coating and nitrogen doping both enhance the rate performance of lithium iron phosphate. Based on the above experimental examples, it can be concluded that the comprehensive modification of lithium iron phosphate materials using a two-stage calcination process combined with porous carbon coating and nitrogen doping can significantly improve the rate performance of lithium iron phosphate materials.

[0034] The above embodiments are merely illustrative examples and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations; therefore, any obvious variations or modifications derived therefrom remain within the scope of protection of this invention.

Claims

1. A method for preparing a nitrogen-doped porous carbon composite lithium iron phosphate cathode material, characterized in that, Includes the following steps: Step 1: Mix lithium source, anhydrous iron phosphate, titanium dioxide, and carbon source, then add waste plastic, water, and zirconium oxide, and mill to obtain a mixed slurry with a particle size of 300~600nm. Step 2: Spray dry and calcine the above mixed slurry at high temperature to obtain a porous carbon-coated precursor; Step 3: The porous carbon-coated precursor, pyrophosphoromolybdate or pyrophosphorus stannate, and nitrogen source are placed in water for ball milling, vacuum drying, and secondary calcination to obtain nitrogen-doped porous carbon-coated lithium iron phosphate cathode material.

2. The preparation method of the nitrogen-doped porous carbon composite lithium iron phosphate cathode material as described in claim 1, characterized in that, In step one, the lithium source is one or more of lithium hydroxide, anhydrous lithium carbonate, lithium acetate, lithium nitrate, lithium oxalate, lithium phosphate, and lithium dihydrogen phosphate.

3. The method for preparing the nitrogen-doped porous carbon composite lithium iron phosphate cathode material as described in claim 1, characterized in that, In step one, the mixing ratio of lithium source and anhydrous iron phosphate is: 1:1.05-1.10 based on the total molar amount of iron to the molar ratio of lithium.

4. The method for preparing the nitrogen-doped porous carbon composite lithium iron phosphate cathode material as described in claim 1, characterized in that, In step one, the carbon source is one or more of glucose, sucrose, citric acid, maltose, and starch, and the amount used is 15% to 25% of the total mass of lithium source and anhydrous iron phosphate.

5. The method for preparing the nitrogen-doped porous carbon composite lithium iron phosphate cathode material as described in claim 1, characterized in that, In step one, the mass of the waste plastic is 1% to 10% of the total mass of the lithium source and anhydrous iron phosphate, and the amount of titanium dioxide is 0.1% to 0.5% of the mass of anhydrous iron phosphate.

6. The method for preparing the nitrogen-doped porous carbon composite lithium iron phosphate cathode material as described in claim 1, characterized in that, In step one, the grinding time is 0.5~10h, the grinding speed is 1500~3000rpm, and the particle size range of the mixed slurry is D50: 300~600nm.

7. The method for preparing the nitrogen-doped porous carbon composite lithium iron phosphate cathode material as described in claim 1, characterized in that, In step two, the spray drying inlet temperature is 180℃~240℃, the outlet temperature is set to 70℃~110℃, the spray drying pressure is 0.2MPa~0.3MPa, and the feed rate is 1000~2000ml / h; the calcination conditions are: after introducing nitrogen gas at room temperature for 20~60min, the temperature is raised to 380℃~620℃ at a heating rate of 2~8℃ / min, and held for 2~8h.

8. The method for preparing the nitrogen-doped porous carbon composite lithium iron phosphate cathode material as described in claim 1, characterized in that, In step three, the nitrogen source is melamine foam; the mass of the nitrogen source is 5% to 13% of the mass of the porous carbon-coated precursor; the sand milling time is 2 to 6 hours; the sand milling speed is 300 to 1000 rpm; the secondary calcination conditions are: after introducing nitrogen gas at room temperature for 20 to 60 minutes, the temperature is increased to 650 to 800°C at a heating rate of 2 to 8°C / min, and held for 5 to 10 hours.

9. The method for preparing the nitrogen-doped porous carbon composite lithium iron phosphate cathode material as described in claim 1, characterized in that, The amount of pyrophosphosimolybdate or pyrophosphositanate added is 0.5% to 3% of the mass of the porous carbon-coated precursor.

10. A nitrogen-doped porous carbon composite lithium iron phosphate cathode material, characterized in that, It is prepared by the method for preparing nitrogen-doped porous carbon composite lithium iron phosphate cathode material according to any one of claims 1-9.

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