Composite positive electrode material for lithium ion battery and preparation method of composite positive electrode material

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a composite positive electrode material for a lithium ion battery and a preparation method of the composite positive electrode material, and the composite positive electrode material for the lithium ion battery comprises the following raw materials: a lithium manganese iron phosphate composite material, a conductive material, polyvinylidene fluoride and N-methyl-2-pyrrolidone. On one hand, active sites are increased, the transmission resistance of lithium ions is reduced, the diffusion rate and the transmission rate of the lithium ions are improved, and the conductivity, the rate capability and the specific capacity of the lithium ion battery composite positive electrode material are effectively improved; and the structure and the volume can be effectively prevented from being changed and damaged in the charging and discharging process, the erosion of electrolyte and other chemical substances and the occurrence probability of side reaction are reduced, and the cycling stability of the composite positive electrode material of the lithium ion battery is further improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a composite positive electrode material for lithium ion batteries and a preparation method thereof. Background Art

[0002] With the development of society and the advancement of science and technology, the energy crisis has become increasingly severe. Finding low-carbon, environmentally friendly and economical new energy has become the focus of attention and research. As a new type of energy, lithium-ion batteries have the advantages of no pollution, high safety, and long cycle life. They are widely used in many fields such as transportation, electronic equipment, and medical care. As one of the core parts of lithium-ion batteries, positive electrode materials are related to the performance of lithium-ion batteries. At present, common positive electrode materials for lithium-ion batteries include lithium manganese oxide, nickel cobalt manganese oxide layered materials, and lithium iron phosphate. Lithium iron phosphate positive electrode materials are environmentally friendly, have little impact on the environment, have good stability, and have a long cycle life. As the mainstream positive electrode material for lithium-ion batteries, lithium iron phosphate is the optimized material for lithium iron phosphate. Compared with lithium iron phosphate, it has higher energy density and voltage platform, but lithium iron phosphate also has shortcomings. The conductivity and ion mobility of lithium iron phosphate are low, so the cycle performance, rate performance, and capacity retention rate are poor, which seriously restricts the development and application of lithium-ion batteries. Therefore, it is very necessary to modify the positive electrode material of lithium iron phosphate.

[0003] The patent with publication number CN109650367A discloses a lithium iron manganese phosphate and a preparation method thereof. The elemental iron, manganese dioxide and phosphoric acid aqueous solution are mixed to obtain a mixture, the mixture is ball-milled to obtain hydrogen manganese iron phosphate, hydrogen manganese iron phosphate, lithium carbonate and glucose are mixed, sand-milled to a product particle size D25 of 0.2μm-1μm, and dried and calcined to obtain lithium iron manganese phosphate. The lithium iron manganese phosphate prepared by this method is used as a positive electrode material for lithium ion batteries, which improves the discharge specific capacity. The method has simple equipment and process, low environmental pressure, and is easy to carry out industrial production. However, the conductivity, cycle performance and rate performance of the prepared lithium iron manganese phosphate material have not been effectively improved. Therefore, the present invention provides a composite positive electrode material for lithium ion batteries, which has excellent rate performance, cycle stability and discharge specific capacity, and has broad application prospects. Summary of the invention

[0004] In order to solve the technical problems mentioned in the background technology, the purpose of the present invention is to provide a composite positive electrode material for lithium ion batteries and a preparation method thereof.

[0005] The purpose of the present invention can be achieved through the following technical solutions:

[0006] A composite positive electrode material for a lithium ion battery comprises the following raw materials in parts by weight: 70-85 parts of lithium iron manganese phosphate composite material, 10-15 parts of conductive material, and 10-15 parts of polyvinylidene fluoride.

[0007] Furthermore, the conductive material is any one of carbon black, carbon nanotubes or acetylene black.

[0008] Furthermore, the preparation method of the lithium manganese iron phosphate composite material comprises the following steps:

[0009] Step A: adding a phosphorus source, a manganese source, an iron source, a lithium source and a complexing agent into deionized water, stirring at a stirring rate of 100-200 r / min for 40-60 min to obtain a mixed solution;

[0010] Step B: adding the modified composite aerogel to the mixed solution, dispersing it ultrasonically at a temperature of 50-70°C and a frequency of 80-100KHz for 1-3h, transferring the system to a reaction kettle, raising the temperature to 200-220°C, stirring the reaction for 8-10h, centrifugally washing, and vacuum drying to obtain a precursor;

[0011] Step C: The precursor is placed in a crucible, argon gas is introduced for protection, and high-temperature sintering is performed in a vacuum tube furnace. After cooling to room temperature, a lithium manganese iron phosphate composite material can be obtained.

[0012] Furthermore, in step A, the phosphorus source is any one of phosphoric acid, sodium dihydrogen phosphate or diammonium hydrogen phosphate; the manganese source is any one of manganese acetate, manganese chloride or manganese nitrate; the iron source is any one of ferrous acetate, ferric nitrate, ferric chloride or ferrous nitrate; the lithium source is any one of lithium hydroxide, lithium acetate, lithium carbonate or lithium chloride; the complexing agent is any one of citric acid, tartaric acid, malic acid or succinic acid.

[0013] Furthermore, in step B, the method for preparing the modified composite aerogel comprises the following steps:

[0014] Step T1: sodium alginate is added to deionized water, stirred thoroughly, sodium periodate solution is added, stirred for reaction for 1-2 hours in the dark, ethylene glycol is added to stop the reaction, the product is separated, precipitated, and freeze-dried to obtain aldehyded sodium alginate;

[0015] Step T2: Add the aldehyde-modified sodium alginate to N,N-dimethylformamide, mix well, add L-methionine and a catalyst, raise the temperature to 45-55°C, react for 2-4 hours, and obtain modified sodium alginate;

[0016] Step T3: Add the modified sodium alginate into deionized water to prepare a solution, slowly add the solution into anhydrous ethanol, stir for 8-10 hours, let stand for 30-50 minutes, and freeze-dry to obtain a modified composite aerogel.

[0017] By adopting the above technical scheme, the hydroxyl groups in sodium alginate are oxidized into aldehyde groups using sodium periodate solution to obtain aldehyded sodium alginate. Under the action of a catalyst, the aldehyde groups in the aldehyded sodium alginate structure can react with the amino groups in the L-methionine structure to produce a Schiff base reaction to obtain modified sodium alginate. After freeze-drying, a modified composite aerogel is obtained.

[0018] Furthermore, in step T1, the concentration of the sodium periodate solution is 10-20%.

[0019] Furthermore, in step T2, the catalyst is sodium carbonate or triethylamine.

[0020] Furthermore, in step C, the high temperature sintering is carried out at a heating rate of 4-6°C / min, the temperature is raised to 600-800°C, and the time is 1-3h.

[0021] A method for preparing a composite positive electrode material for a lithium ion battery comprises the following steps:

[0022] Step 1: weigh each raw material by weight;

[0023] Step 2: Add the lithium manganese iron phosphate composite material, conductive material and polyvinylidene fluoride into N-methyl-2-pyrrolidone, place on a magnetic stirrer and stir at 300-600 r / min to mix evenly to obtain a paste-like substance, which is the composite positive electrode material.

[0024] Beneficial effects of the present invention:

[0025] The lithium iron manganese phosphate composite material is prepared by mixing a phosphorus source, a manganese source, an iron source, a lithium source and a modified composite aerogel, conducting a hydrothermal reaction and then sintering. On the one hand, the modified composite aerogel forms nitrogen-sulfur doped carbon, which changes the structure of the carbon material and increases the active sites, thereby improving the electronic conductivity. At the same time, it can reduce the lithium ion transmission resistance, increase the diffusion rate and transmission rate of lithium ions, and improve the rate performance of the composite positive electrode material of the lithium ion battery, ensuring that the battery still has stable electrochemical properties at high rates. On the other hand, using the modified composite aerogel as a carbon substrate can disperse and control the particle size of the lithium iron manganese phosphate, inhibit excessive growth, form an interconnected conductive network, greatly improve conductivity, and improve electrochemical properties. In addition, it can effectively prevent the material structure and volume from changing during the charge and discharge process, and can avoid structural destruction, so that the lithium iron manganese phosphate composite material has good stability, and can also reduce the probability of being eroded by electrolytes and other chemical substances and the occurrence of side reactions, improve the cycle stability of the composite positive electrode material of the lithium ion battery, and extend the service life of the battery.

[0026] Of course, any product implementing the present invention does not necessarily need to achieve all of the advantages described above at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for describing the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0028] Figure 1 This is the infrared spectrum of the modified composite aerogel prepared in the present invention. DETAILED DESCRIPTION

[0029] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] The preparation method of the lithium manganese iron phosphate composite material in the following embodiment is as follows:

[0031] 1. Preparation of modified composite aerogel

[0032] Step T1: 3.2 g of sodium alginate was added to deionized water, and the mixture was stirred thoroughly. 16 g of a 20% sodium periodate solution was added, and the mixture was stirred for reaction for 2 h in the dark. Then, 3 g of ethylene glycol was added to stop the reaction, and the product was separated, precipitated, and freeze-dried to obtain aldehyde-modified sodium alginate.

[0033] Step T2: Add 3 g of aldehyde-modified sodium alginate to N,N-dimethylformamide, mix well, add 2 g of L-methionine and 0.5 g of triethylamine, raise the temperature to 50°C, react for 3 h, and obtain modified sodium alginate;

[0034] Step T3: 2.8 g of modified sodium alginate was added to deionized water to prepare a 1 wt% solution, and the solution was slowly added to anhydrous ethanol. After stirring for 10 h, the solution was allowed to stand for 40 min and freeze-dried to obtain a modified composite aerogel.

[0035] The potassium bromide tablet method was used to prepare the sample, and the modified composite aerogel was tested by infrared spectroscopy. Figure 1 As shown, analysis shows that 3422cm -1 The absorption peak of hydroxyl OH appeared at 1678 cm -1 The absorption peak of C=O in the carboxyl group appeared at 1618cm -1 The absorption peak of Schiff base C=N appeared at 1175 cm -1The absorption peak of CSC appeared at 1086 cm -1 The absorption peak of COC appeared at

[0036] 2. Preparation of lithium manganese iron phosphate composite materials

[0037] Step A: 1.22 g of sodium dihydrogen phosphate, 0.77 g of manganese chloride, 0.71 g of ferrous acetate, 0.38 g of lithium carbonate and 2 g of citric acid were added into deionized water, and stirred at a stirring rate of 200 r / min for 50 min to obtain a mixed solution;

[0038] Step B: Add 3 g of modified composite aerogel to the mixed solution, perform ultrasonic dispersion at 90 kHz for 2 h at 65 °C, transfer the system to a reactor, raise the temperature to 220 °C, stir and react for 8 h, perform centrifugal washing, and vacuum dry to obtain a precursor;

[0039] Step C: The precursor is loaded into a crucible, argon gas is introduced for protection, and high-temperature sintering is performed in a vacuum tube furnace at a heating rate of 5°C / min. The temperature is raised to 700°C for 2 hours. After cooling to room temperature, a lithium manganese iron phosphate composite material can be obtained.

[0040] The lithium iron manganese phosphate composite material and the commercially available lithium iron manganese phosphate material (purchased from Shenzhen Liyou New Energy Technology Co., Ltd., with a particle size distribution of D90) were ground and sieved with agate, and then the resistivity was tested using a powder resistivity meter. The results are as follows:

[0041] Table 1 - Resistivity test results

[0042] Resistivity (Ω·cm) Lithium manganese iron phosphate composite material 0.95 Commercially available lithium manganese iron phosphate materials 2.13

[0043] It can be seen from Table 1 that the lithium iron manganese phosphate composite material prepared by the present invention has low resistivity and excellent electrical conductivity, while the commercially available lithium iron manganese phosphate material has high resistivity and poor electrical conductivity. This is because the commercially available lithium iron manganese phosphate material is unmodified, has few active sites, has large lithium ion diffusion resistance, and has low lithium ion diffusion rate and transmission rate.

[0044] Example 1

[0045] A composite positive electrode material for a lithium ion battery comprises the following raw materials in parts by weight: 70 g of lithium iron manganese phosphate composite material, 10 g of conductive carbon nanotubes, and 10 g of polyvinylidene fluoride; a preparation method of the composite positive electrode material for a lithium ion battery comprises the following steps:

[0046] Step 1: weigh each raw material by weight;

[0047] Step 2: Add the lithium manganese iron phosphate composite material, conductive carbon nanotubes and polyvinylidene fluoride into N-methyl-2-pyrrolidone, place on a magnetic stirrer and stir at 300 r / min to mix evenly to obtain a paste-like substance, which is the composite positive electrode material.

[0048] Example 2

[0049] A composite positive electrode material for a lithium ion battery comprises the following raw materials in parts by weight: 75g of lithium iron manganese phosphate composite material, 12g of conductive carbon nanotubes, and 12g of polyvinylidene fluoride; a preparation method of the composite positive electrode material for a lithium ion battery comprises the following steps:

[0050] Step 1: weigh each raw material by weight;

[0051] Step 2: Add the lithium manganese iron phosphate composite material, conductive carbon nanotubes and polyvinylidene fluoride into N-methyl-2-pyrrolidone, place on a magnetic stirrer and stir at 400 r / min to mix evenly to obtain a paste-like substance, which is the composite positive electrode material.

[0052] Example 3

[0053] A composite positive electrode material for a lithium ion battery comprises the following raw materials in parts by weight: 80g of lithium iron manganese phosphate composite material, 14g of conductive carbon nanotubes, and 13g of polyvinylidene fluoride; a preparation method of the composite positive electrode material for a lithium ion battery comprises the following steps:

[0054] Step 1: weigh each raw material by weight;

[0055] Step 2: Add the lithium manganese iron phosphate composite material, conductive carbon nanotubes and polyvinylidene fluoride into N-methyl-2-pyrrolidone, place on a magnetic stirrer and stir at 500 r / min to mix evenly to obtain a paste-like substance, which is the composite positive electrode material.

[0056] Example 4

[0057] A composite positive electrode material for a lithium ion battery comprises the following raw materials in parts by weight: 85g of lithium iron manganese phosphate composite material, 15g of conductive carbon nanotubes, and 15g of polyvinylidene fluoride; a preparation method of the composite positive electrode material for a lithium ion battery comprises the following steps:

[0058] Step 1: weigh each raw material by weight;

[0059] Step 2: Add the lithium manganese iron phosphate composite material, conductive carbon nanotubes and polyvinylidene fluoride into N-methyl-2-pyrrolidone, place on a magnetic stirrer and stir at 600 r / min to mix evenly to obtain a paste-like substance, which is the composite positive electrode material.

[0060] Comparative Example 1

[0061] A composite positive electrode material for a lithium ion battery comprises the following raw materials in parts by weight: 80 g of commercially available lithium iron manganese phosphate material, 14 g of conductive carbon nanotubes, and 13 g of polyvinylidene fluoride; a preparation method of the composite positive electrode material for a lithium ion battery comprises the following steps:

[0062] Step 1: weigh each raw material by weight;

[0063] Step 2: Add commercially available lithium manganese iron phosphate material (purchased from Shenzhen Liyou New Energy Technology Co., Ltd., with a particle size distribution of D90), conductive carbon nanotubes and polyvinylidene fluoride to N-methyl-2-pyrrolidone, place on a magnetic stirrer and stir at 500r / min to mix evenly to obtain a paste-like substance, which is the composite positive electrode material.

[0064] Performance Testing

[0065] The composite positive electrode materials prepared in Examples 1 to 4 and Comparative Example 1 were coated on an aluminum foil current collector, placed in a vacuum oven at 90°C and dried for 8 hours to form a positive electrode sheet, which was cut into 10 mm circular positive electrode sheets using a punch, a lithium sheet was used as the negative electrode, and the electrolyte was 1 mol / L Li PF6 dissolved in a solvent of EC / DMC / EMC (volume ratio of 1:1:1), the positive electrode sheet, the electrolyte, the PP separator and the metal lithium sheet were assembled to obtain a button lithium ion battery, which was tested by cyclic voltammetry, and the test results were as follows:

[0066] The cycle stability performance test was carried out under the condition of current density of 0.2A / g. The test results are shown in Table 2:

[0067] Table 2 - Discharge capacity and cycle stability test results

[0068] Discharge specific capacity (mAh / g) Capacity retention after 1000 cycles (%) Example 1 191.1 91.3 Example 2 191.9 91.8 Example 3 192.4 92.1 Example 4 191.4 91.5 Comparative Example 1 153.2 66.4

[0069] It can be seen from Table 2 that the lithium ion battery composite positive electrode materials prepared in Examples 1 to 4 of the present invention have high specific capacity and excellent cycle stability, while the lithium ion battery composite positive electrode material prepared in Comparative Example 1 has poor cycle stability. This is because the lithium iron manganese phosphate material is not modified, undergoes volume changes during the cycle, is corroded by contact with chemical substances such as the electrolyte, and may undergo side reactions, resulting in poor cycle stability.

[0070] Table 3 - Rate performance test results

[0071]

[0072] It can be seen from Table 3 that the lithium ion battery composite positive electrode materials prepared in Examples 1 to 4 of the present invention have excellent rate performance, with a maximum capacity retention of 74% from 0.5C to 3C, while the lithium ion battery composite positive electrode material prepared in Comparative Example 1, the lithium iron manganese phosphate material is unmodified, has a capacity retention of 43.6% from 0.5C to 3C, and the rate performance is poor.

[0073] The above contents are merely examples and explanations of the concept of the present invention. The technicians in this technical field may make various modifications or additions to the specific embodiments described or replace them in a similar manner. As long as they do not deviate from the concept of the invention or exceed the scope defined by the claims, they should all fall within the protection scope of the present invention.

Claims

1. A composite positive electrode material for a lithium ion battery, characterized in that: The invention comprises the following raw materials in parts by weight: 70-85 parts of lithium manganese iron phosphate composite material, 10-15 parts of conductive material and 10-15 parts of polyvinylidene fluoride.

2. A composite positive electrode material for a lithium ion battery according to claim 1, characterized in that: The conductive material is any one of carbon black, carbon nanotubes or acetylene black.

3. The composite positive electrode material for a lithium ion battery according to claim 1, characterized in that: The preparation method of the lithium iron manganese phosphate composite material comprises the following steps: Step A: adding a phosphorus source, a manganese source, an iron source, a lithium source and a complexing agent into deionized water, stirring at a stirring rate of 100-200 r / min for 40-60 min to obtain a mixed solution; Step B: adding the modified composite aerogel to the mixed solution, dispersing it ultrasonically at a temperature of 50-70°C and a frequency of 80-100KHz for 1-3h, transferring the system to a reaction kettle, raising the temperature to 200-220°C, stirring the reaction for 8-10h, centrifugally washing, and vacuum drying to obtain a precursor; Step C: The precursor is placed in a crucible, argon gas is introduced for protection, and high-temperature sintering is performed in a vacuum tube furnace. After cooling to room temperature, a lithium manganese iron phosphate composite material can be obtained.

4. The composite positive electrode material for lithium-ion batteries according to claim 3, characterized in that: In step A, the phosphorus source is any one of phosphoric acid, sodium dihydrogen phosphate or diammonium hydrogen phosphate; the manganese source is any one of manganese acetate, manganese chloride or manganese nitrate; the iron source is any one of ferrous acetate, ferric nitrate, ferric chloride or ferrous nitrate; the lithium source is any one of lithium hydroxide, lithium acetate, lithium carbonate or lithium chloride; the complexing agent is any one of citric acid, tartaric acid, malic acid or succinic acid.

5. The composite positive electrode material for lithium ion battery according to claim 3, characterized in that: In step B, the method for preparing the modified composite aerogel comprises the following steps: Step T1: sodium alginate is added to deionized water, stirred thoroughly, sodium periodate solution is added, stirred for reaction for 1-2 hours in the dark, ethylene glycol is added to stop the reaction, the product is separated, precipitated, and freeze-dried to obtain aldehyded sodium alginate; Step T2: Add the aldehyde-modified sodium alginate to N,N-dimethylformamide, mix well, add L-methionine and a catalyst, raise the temperature to 45-55°C, react for 2-4 hours, and obtain modified sodium alginate; Step T3: Add the modified sodium alginate into deionized water to prepare a solution, slowly add the solution into anhydrous ethanol, stir for 8-10 hours, let stand for 30-50 minutes, and freeze-dry to obtain a modified composite aerogel.

6. The composite positive electrode material for lithium-ion battery according to claim 5, characterized in that: In step T1, the concentration of the sodium periodate solution is 10-20%.

7. The composite positive electrode material for lithium-ion batteries according to claim 5, characterized in that: In step T2, the catalyst is sodium carbonate or triethylamine.

8. The composite positive electrode material for lithium ion battery according to claim 3, characterized in that: In step C, the high temperature sintering is carried out at a heating rate of 4-6°C / min, the temperature is raised to 600-800°C, and the time is 1-3h.

9. A method for preparing a composite positive electrode material for a lithium ion battery as claimed in claim 1, characterized in that: The following steps are involved: Step 1: weigh each raw material by weight; Step 2: Add the lithium manganese iron phosphate composite material, conductive material and polyvinylidene fluoride into N-methyl-2-pyrrolidone, place on a magnetic stirrer and stir at 300-600 r / min to mix evenly to obtain a paste-like substance, which is the composite positive electrode material.

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