Composite lithium iron phosphate electrode material applied to lithium ion battery and preparation method of composite lithium iron phosphate electrode material

Abstract

The invention relates to the technical field of electrode materials, and discloses a composite lithium iron phosphate electrode material applied to a lithium ion battery and a preparation method, the electrode material comprises the following raw materials: a porous carbon coated lithium iron phosphate precursor, a conductive agent and a binder; according to the porous carbon-coated lithium iron phosphate precursor, lithium iron phosphate is formed in pores of porous carbon in situ, a C-N coating layer is formed on the surface of the lithium iron phosphate, the electronic conductivity of the electrode material is improved, meanwhile, the formed fluorine-doped carbon coating layer promotes transmission of electrons and diffusion of lithium ions, and the performance of the electrode material is improved. According to the present invention, with the porous carbon coating, the electrochemical performance of the electrode material can be effectively improved, and in addition, the overgrowth of the lithium iron phosphate crystal grains and the agglomeration among the crystal grains are limited by the porous carbon coating, such that the lithium ion diffusion during the charge-discharge process can be easily accelerated, and the rate performance and the cycle stability of the electrode material can be improved;

Description

Technical Field

[0001] The present invention relates to the technical field of electrode materials, and particularly relates to a composite lithium iron phosphate electrode material applied to a lithium ion battery and a preparation method thereof. Background Art

[0002] Due to characteristics such as high specific energy, no memory effect, long cycle life, small self-discharge, and green safety, lithium ion batteries are widely used in fields such as electric vehicles, aerospace, and communication equipment. The positive electrode material is the core part of a lithium ion battery, and its performance directly determines the performance indicators of the lithium ion battery product. Currently, lithium manganate, lithium cobaltate, lithium nickel cobalt manganate, and lithium iron phosphate are several lithium ion battery positive electrode materials with the highest marketization degree. Among them, lithium iron phosphate is considered to be one of the most promising lithium ion battery positive electrode materials, and it has advantages such as large capacity, wide working temperature range, wide raw material sources, and safety and environmental protection.

[0003] With the continuous progress of lithium ion battery technology and the continuous expansion of application fields, higher requirements are put forward for lithium iron phosphate electrode materials. During the charge and discharge process of ordinary lithium iron phosphate, the migration path of lithium ions is limited to one-dimensional channel diffusion. Furthermore, due to its special crystal structure, lithium iron phosphate exhibits low electronic conductivity and ion diffusion coefficient at room temperature, resulting in poor rate performance and cycle stability in practical applications, which greatly limits the improvement of its electrochemical performance. Therefore, it is of great practical significance to develop a lithium iron phosphate positive electrode material with excellent high rate performance and cycle stability.

[0004] Currently, researchers mainly improve the conductivity and lithium ion diffusion rate of lithium iron phosphate electrode materials through methods such as carbon coating, metal cation doping, and particle nanosizing, so as to improve the rate performance and cycle stability of lithium iron phosphate electrode materials. Among them, carbon coating is a simple, direct, and effective method to improve the performance of lithium iron phosphate electrode materials. For example, the invention patent with the publication number CN102738462B discloses a carbon-coated lithium iron phosphate composite and a preparation method thereof. In this invention, a lithium source, an iron source, a phosphorus source, and carbon nanotubes are mixed in a solvent according to a molar ratio, and ground to form a lithium iron phosphate precursor slurry. The lithium iron phosphate precursor slurry is dried and preheated to form a lithium iron phosphate precursor. In an inert gas or vacuum state, a carburizing agent is dropped into the lithium iron phosphate precursor, and then carbon-coated lithium iron phosphate composite is formed after high-temperature roasting. This lithium iron phosphate composite has the advantages of high conductivity and good electrochemical performance. However, during the long-term high-temperature treatment process, non-in-situ carbon-coated lithium iron phosphate will agglomerate, reducing the lithium ion transport rate. Summary of the Invention

[0005] The object of the present invention is to provide a composite lithium iron phosphate electrode material for lithium-ion batteries and a preparation method thereof, and solve the following technical problems:

[0006] (1) Solve the problem of poor rate performance of the lithium iron phosphate electrode material;

[0007] (2) Solve the problem of poor cycling performance of the lithium iron phosphate electrode material.

[0008] The object of the present invention can be achieved by the following technical solutions:

[0009] The composite lithium iron phosphate electrode material for lithium-ion batteries includes the following raw materials in parts by weight: 75-85 parts of porous carbon-coated lithium iron phosphate precursor, 5-15 parts of conductive agent, and 5-15 parts of binder.

[0010] Further, the preparation method of the porous carbon-coated lithium iron phosphate precursor includes the following steps:

[0011] A1: Add sericin, perfluorooctanoic acid and ethanol solution into a reactor filled with nitrogen in sequence. After stirring evenly, add an activator and a dehydrating agent. After adding, stir at room temperature for 4-8 h, then remove nitrogen and discharge to obtain modified sericin;

[0012] A2: Mix the modified sericin and a pore-forming agent, protect with nitrogen, raise the system temperature to 500-700 °C, keep warm for 30-50 min, then cool to room temperature, wash and dry to obtain a composite porous carbon material;

[0013] A3: Mix the composite porous carbon material and deionized water, and ultrasonically disperse until a uniform suspension is formed. Place ferrous nitrate, phosphoric acid and a lithium source in deionized water, stir magnetically for 10-30 min to prepare a precursor solution. Add the precursor solution to the suspension, ultrasonically disperse for 1-3 h, then place it in a microwave reaction kettle, react at a temperature of 190-210 °C for 20-40 min, cool to room temperature, centrifuge to separate the solid material, wash and dry the solid material to obtain a porous carbon-coated lithium iron phosphate precursor.

[0014] Further, in step A1, the mass ratio of the sericin to the perfluorooctanoic acid is 1:0.1-0.3.

[0015] Further, in step A1, the activator is any one of 1-hydroxybenzotriazole, 4-dimethylaminopyridine or N-hydroxysuccinimide.

[0016] Further, in step A1, the dehydrating agent is any one of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide or N,N'-diisopropylcarbodiimide.

[0017] Further, in step A2, the pore-forming agent is a potassium hydroxide solution.

[0018] Further, in step A3, the lithium source is any one of lithium carbonate, lithium nitrate, or lithium sulfate.

[0019] Further, in step A3, the molar ratio of ferrous nitrate, phosphoric acid, and the lithium source is 1:1:3.

[0020] Further, in step A3, the parameter settings of the microwave reactor are as follows: the microwave power is 70 - 90 W, and the microwave frequency is 1400 - 1600 MHz.

[0021] Further, the conductive agent is any one of acetylene black, conductive carbon black, or conductive graphite; the binder is polyvinylidene fluoride.

[0022] A method for preparing a composite lithium iron phosphate electrode material for use in a lithium-ion battery, comprising the following steps:

[0023] Put the porous carbon-coated lithium iron phosphate precursor, conductive agent, and binder into an N-methylpyrrolidone solvent, and under the condition of a rotation speed of 500 - 700 r / min, stir at room temperature until a uniform paste-like material is formed to obtain the composite lithium iron phosphate electrode material.

[0024] Advantages of the present invention:

[0025] (1) By preparing a porous carbon-coated lithium iron phosphate precursor as the positive electrode material of a lithium-ion battery, the present invention effectively improves the rate performance and cycle stability of the electrode material. Using the carbon coating method significantly enhances the electronic conductivity of the electrode material, optimizes the kinetic performance of the electrode material. Through the steric hindrance effect, the carbon coating can, to a certain extent, prevent the secondary aggregation of particles between lithium iron phosphate grains, limit the growth of lithium iron phosphate grain size, avoid its excessive growth, thereby achieving the effect of refining the grains. Furthermore, it can effectively shorten the diffusion path of lithium ions, improve the transport rate of lithium ions, thus greatly enhancing the rate performance of the electrode material, and is beneficial to maintaining the stability of the lithium iron phosphate structure. In addition, the porous structure of carbon provides a channel for the transmission of lithium ions and electrons, while increasing the specific surface area of the material. A large specific surface area is conducive to the contact between the electrode and the electrolyte, accelerating the progress of the electrochemical reaction, thereby improving the cycle stability and rate performance of the electrode material.

[0026] (2) In the present invention, carbon-coated lithium iron phosphate is utilized. On the one hand, the carbon-nitrogen coating layer formed on the surface of lithium iron phosphate can improve the lithium storage capacity, and a highly conductive substance is coated on the surface of lithium iron phosphate, reducing the charge transfer impedance, thereby further improving the electronic conductivity of the electrode material. On the other hand, the fluorine-doped carbon coating layer promotes the electron transport and lithium ion diffusion, and can effectively improve the electrochemical performance of the electrode material.

[0027] Of course, it is not necessary for any product implementing the present invention to achieve all the above-mentioned advantages simultaneously. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for describing the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0029] Figure 1 It is the infrared spectrum test chart of the sericin and modified sericin prepared in the present invention.

[0030] Figure 2 It is the scanning electron microscope image of the composite porous carbon material prepared in the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0032] Example 1

[0033] I. Preparation of Porous Carbon-Coated Lithium Iron Phosphate Precursor

[0034] A1: 5 g of sericin, 1.2 g of perfluoroheptanoic acid and an ethanol solution with a mass dispersion of 70% are successively added into a reactor filled with nitrogen. After stirring evenly, 0.06 g of 4-dimethylaminopyridine and 0.1 g of N,N'-diisopropylcarbodiimide are added. After addition, stirring is carried out at room temperature for 4 - 8 h, then nitrogen is removed, and the product is discharged to obtain modified sericin;

[0035] The sericin and modified sericin are analyzed by infrared spectroscopy using the FTS-13 Fourier transform infrared spectrometer produced by BIO-RAD Company of the United States. The analysis results are shown in Figure 1 , and the potassium bromide tablet pressing method is used for sample preparation. The resolution during testing is 4 cm-1 , scan in the wavelength range of 4000 cm -1 ~500 cm -1 . As can be seen from Figure 1 , in the infrared spectrum of sericin, an absorption peak of N-H appears at 3420 cm -1 , an absorption peak of C=O in amide appears at 1655 cm -1 , and an absorption peak of C-N appears at 1231 cm -1 . In the infrared spectrum of modified sericin, an absorption peak of N-H appears at 3431 cm -1 , an absorption peak of C=O in amide appears at 1650 cm -1 , the absorption peak of C=O in amide is enhanced, and an absorption peak of C-F bond appears at 1230 cm -1 .

[0036] A2: Mix 5 g of modified sericin and 15 g of potassium hydroxide solution with a concentration of 3 mol / l, protect by introducing nitrogen, raise the temperature of the system to 600 °C, keep warm for 40 min, then cool to room temperature, wash and dry to obtain a composite porous carbon material;

[0037] Use the S4800-1 type emission field scanning electron microscope produced by Hitachi, Japan to analyze the morphology of the composite porous carbon material. The test results are shown in Figure 2 . It can be observed from the figure that the composite porous carbon material has a rich pore structure. This is because the use of potassium hydroxide for pore-forming treatment can form nano-pores on the modified sericin, which is beneficial for phosphorus, iron, and lithium to flow into the pores of the composite porous carbon material and in-situ form lithium iron phosphate in the pores.

[0038] A3: Mix 10 g of the composite porous carbon material and deionized water, and ultrasonicate until a uniform suspension is formed. Mix 0.4 g of ferrous nitrate, 0.3 g of phosphoric acid, and 1.6 g of lithium sulfate, place them in deionized water, and magnetically stir for 20 min to prepare a precursor solution. Add the precursor solution to the suspension, ultrasonically disperse it at an ultrasonic frequency of 90 KHz for 2 h, then place it in a microwave reaction kettle, where the microwave power is 80 W and the microwave frequency is 1500 MHz, react at a temperature of 200 °C for 30 min, cool to room temperature, then centrifuge to separate the solid material, wash and dry the solid material to obtain a porous carbon-coated lithium iron phosphate precursor.

[0039] It can be speculated that the principle of the above scheme is as follows: under the combined action of 4-dimethylaminopyridine and N,N'-diisopropylcarbodiimide, the active groups in the sericin protein structure can react with the carboxyl groups in the perfluorooctanoic acid structure, so as to graft perfluorooctanoic acid onto the sericin protein structure to obtain modified sericin protein. After pore-forming treatment with potassium hydroxide, nano-pores can be formed on the modified sericin protein to obtain a composite porous carbon material. Then, under the action of ultrasound, phosphorus, iron, and lithium can flow into the pore structure of the composite porous carbon material, and then through the microwave hydrothermal method, lithium iron phosphate is in-situ generated in the pores of the composite porous carbon material to prepare a porous carbon-coated lithium iron phosphate precursor.

[0040] II. Preparation of Composite Lithium Iron Phosphate Electrode Material

[0041] Put 75 g of the porous carbon-coated lithium iron phosphate precursor, 15 g of conductive carbon black, and 10 g of polyvinylidene fluoride into N-methylpyrrolidone solvent, and stir at room temperature to form a uniform paste-like material at a rotation speed of 500 r / min to obtain the composite lithium iron phosphate electrode material.

[0042] Example 2

[0043] Preparation of Composite Lithium Iron Phosphate Electrode Material

[0044] Put 80 g of the porous carbon-coated lithium iron phosphate precursor, 10 g of conductive carbon black, and 10 g of polyvinylidene fluoride into N-methylpyrrolidone solvent, and stir at room temperature to form a uniform paste-like material at a rotation speed of 600 r / min to obtain the composite lithium iron phosphate electrode material.

[0045] The preparation method of the porous carbon-coated lithium iron phosphate precursor is the same as that in Example 1.

[0046] Example 3

[0047] Preparation of Composite Lithium Iron Phosphate Electrode Material

[0048] Put 85 g of the porous carbon-coated lithium iron phosphate precursor, 10 g of conductive carbon black, and 5 g of polyvinylidene fluoride into N-methylpyrrolidone solvent, and stir at room temperature to form a uniform paste-like material at a rotation speed of 700 r / min to obtain the composite lithium iron phosphate electrode material.

[0049] The preparation method of the porous carbon-coated lithium iron phosphate precursor is the same as that in Example 1.

[0050] Comparative Example 1

[0051] Preparation of Composite Lithium Iron Phosphate Electrode Material

[0052] 80 g of commercially available lithium iron phosphate, 10 g of conductive carbon black, and 10 g of polyvinylidene fluoride were put into N-methylpyrrolidone solvent. Under the condition of a rotation speed of 600 r / min, it was stirred at room temperature until a uniform paste-like material was formed to obtain a composite lithium iron phosphate electrode material.

[0053] Note: The commercially available lithium iron phosphate was purchased from Hengshengyuan (Shenzhen) New Material Technology Co., Ltd.

[0054] Performance detection:

[0055] Ⅰ: The composite lithium iron phosphate electrode materials prepared in Examples 1 to 3 and Comparative Example 1 were uniformly coated on aluminum foil using an automatic coater, and then placed in an oven at 80 °C for drying to obtain a positive electrode sheet. The positive electrode sheet was punched into a circular sheet with a diameter of 12 mm by a punching machine. The assembly of the button battery was carried out in a glove box filled with high-purity argon. The battery case used was of the CR2025 type, and the electrolyte was 1 mol / L of LiPF 6 , and the assembly structure from bottom to top was: positive electrode case, the prepared electrode sheet (current collector facing down), Celgard 2400 type separator, lithium metal sheet, negative electrode case with foam nickel installed. After assembly, it was sealed using a button battery encapsulation machine. Before testing, the assembled battery was left to stand at room temperature for 4 h. A CHI660D electrochemical workstation from Shanghai Chenhua was used to conduct cyclic voltammetry tests on the battery. Among them, the scanning voltage was 2.0 - 4.2 V, the scanning rate was 0.1 mV / s, and the ambient temperature during testing was controlled at 25 °C. The specific detection results are shown in Table 1:

[0056] Table 1 - Cyclic voltammetry test

[0057]

[0058] From the test results in Table 1, it can be seen that the batteries assembled in Examples 1 to 3 had relatively high discharge specific capacities at the rates of 0.1 C, 1 C, and 5 C. After 50 cycles, the discharge specific capacity did not decrease significantly, and the capacity retention rate was greater than 90% after 50 cycles at the 5 C rate, showing excellent rate performance and cycle stability; the batteries assembled in Comparative Example 1 had lower discharge specific capacities than those in the examples at the rates of 0.1 C, 1 C, and 5 C. After 50 cycles, the discharge specific capacity decreased significantly, and the capacity retention rate was low. The rate performance and cycle stability of the battery were both poor. It is speculated that it may be because the lithium iron phosphate was not coated with porous carbon, resulting in poor rate performance and cycle stability of the battery.

[0059] Ⅱ: The porous carbon-coated lithium iron phosphate precursor in the examples and the commercially available lithium iron phosphate in Comparative Example 1 were tested for resistivity using an XF057 type resistivity tester produced by HIOKI to evaluate the conductive performance of the electrode material. The specific detection results are shown in Table 2:

[0060] Table 2 - Resistivity Test

[0061] Resistivity (Ω·m) Lithium iron phosphate precursor coated with porous carbon 2.5 Commercially available lithium iron phosphate 4.3

[0062] As can be seen from the test results in Table 2, the resistivity of the porous carbon-coated lithium iron phosphate precursor in the example is 2.5 Ω·m, which is relatively low, and the electrical conductivity performance is relatively good. Therefore, the electrode material prepared in the example has good electrical conductivity; for the commercially available lithium iron phosphate in Comparative Example 1, since the lithium iron phosphate is not carbon-coated, compared with the example, the resistivity is higher and the electrical conductivity performance is poorer. Therefore, the electrical conductivity of the electrode material prepared in Comparative Example 1 is poor.

[0063] The above content is only an example and illustration of the concept of the present invention. Those skilled in the art of the present technology can make various modifications or supplements to the specific embodiments described or use similar methods for substitution, as long as they do not deviate from the concept of the invention or exceed the scope defined by this claims, they should all fall within the protection scope of the present invention.

Claims

1. A composite lithium iron phosphate electrode material for lithium-ion batteries, characterized in that: The invention comprises the following raw materials in parts by weight: 75 to 85 parts of porous carbon-coated lithium iron phosphate precursor, 5 to 15 parts of conductive agent and 5 to 15 parts of binder.

2. The composite lithium iron phosphate electrode material for lithium ion batteries according to claim 1, characterized in that: The method for preparing the porous carbon-coated lithium iron phosphate precursor comprises the following steps: A1: Add sericin, perfluoroheptanoic acid and ethanol solution into a reactor filled with nitrogen in sequence, stir evenly, add activator and dehydrating agent, stir at room temperature for 4-8 hours, remove nitrogen, discharge, and obtain modified sericin; A2: The modified sericin and the porogen are mixed, nitrogen is passed through the system, the temperature is raised to 500-700°C, the temperature is kept for 30-50 minutes, and then the system is cooled to room temperature, washed, and dried to obtain a composite porous carbon material; A3: Mix the composite porous carbon material and deionized water, and ultrasonicate to form a uniform suspension. Place ferrous nitrate, phosphoric acid, and a lithium source in deionized water, and magnetically stir for 10 to 30 minutes to prepare a precursor solution. Add the precursor solution to the suspension, and ultrasonically disperse it for 1 to 3 hours. Then, place it in a microwave reactor, and react at a temperature of 190 to 210°C for 20 to 40 minutes. After cooling to room temperature, separate the solid material by centrifugation, wash the solid material, and dry it to obtain a porous carbon-coated lithium iron phosphate precursor.

3. The composite lithium iron phosphate electrode material for lithium ion battery according to claim 2, characterized in that: In step A1, the mass ratio of the sericin to perfluoroheptanoic acid is 1:0.1-0.

3.

4. The composite lithium iron phosphate electrode material for lithium ion battery according to claim 2, characterized in that: In step A1, the activator is any one of 1-hydroxybenzotriazole, 4-dimethylaminopyridine or N-hydroxysuccinimide.

5. The composite lithium iron phosphate electrode material for lithium ion battery according to claim 2, characterized in that: In step A1, the dehydrating agent is any one of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide or N,N'-diisopropylcarbodiimide.

6. The composite lithium iron phosphate electrode material for lithium ion battery according to claim 2, characterized in that: In step A2, the porogen is a potassium hydroxide solution.

7. The composite lithium iron phosphate electrode material for lithium ion batteries according to claim 2, characterized in that: In step A3, the lithium source is any one of lithium carbonate, lithium nitrate or lithium sulfate.

8. The composite lithium iron phosphate electrode material for lithium ion batteries according to claim 2, characterized in that: In step A3, the molar ratio of ferrous nitrate, phosphoric acid and lithium source is 1:1:

3.

9. The composite lithium iron phosphate electrode material for lithium ion batteries according to claim 2, characterized in that: In step A3, the parameters of the microwave reactor are set as follows: microwave power is 70-90 W, and microwave frequency is 1400-1600 MHz.

10. The composite lithium iron phosphate electrode material for lithium ion battery according to claim 1, characterized in that: The conductive agent is any one of acetylene black, conductive carbon black or conductive graphite; and the binder is polyvinylidene fluoride.

11. The method for preparing a composite lithium iron phosphate electrode material for lithium ion batteries according to claim 1, characterized in that: The following steps are involved: The porous carbon-coated lithium iron phosphate precursor, the conductive agent and the binder are put into an N-methylpyrrolidone solvent, and stirred at room temperature at a rotation speed of 500 to 700 r / min until a uniform paste material is formed to obtain a composite lithium iron phosphate electrode material.

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