CN101950801A - A kind of preparation method of lithium-ion battery cathode material LiFePO4/C - Google Patents

Abstract

The invention discloses a method for preparing the positive electrode material LiFePO4/C of a lithium ion battery by adopting carbon sources of the polystyrene sphere series. A carbon thermal reduction and solid phase sintering method is adopted in the preparation process. The method comprises the following concrete steps: firstly, uniformly mixing a lithium source, an iron source and a phosphorus source, and then, mixing the mixture with a carbon source; then, adding a solvent for ball milling, and drying to obtain a precursor; and pretreating and calcining the precursor in the protective atmosphere, and then, cooling the precursor in a furnace to synthesize the LiFePO4/C of which the particle size distribution is uniform and the particle diameter is 200-500nm. The invention has the advantages of wide sources of raw materials, simple preparation method, convenient control and operation, good carbon coating effect, excellent electrochemical performance and the like.

Description

A kind of preparation method of lithium-ion battery cathode material LiFePO4/C

technical field

The invention relates to a preparation method of a positive electrode material of a lithium ion battery, in particular to a preparation method of LiFePO ₄ /C, a positive electrode material of a lithium ion battery.

Background technique

Cathode materials are an important part of lithium-ion batteries, and the development of new cathode materials has become one of the keys to the development of lithium-ion batteries. Olivine structure LiFePO ₄ has the advantages of low cost, environmental friendliness, high capacity, good safety performance, stable cycle, and stable discharge voltage. However, LiFePO ₄ also has two major defects: one is low electrical conductivity, and the other is low ion diffusion coefficient, resulting in poor high-rate discharge and low-temperature performance of the material. To overcome this shortcoming, technologies such as carbon coating, particle size reduction and ion doping are often used to improve.

At present, the main synthesis methods of LiFePO ₄ are: high-temperature solid-state reaction method, liquid-phase co-precipitation method, hydrothermal method, sol-gel method, solid-phase microwave sintering method, etc. The high-temperature solid-state method is widely used in actual production, that is, lithium source, iron source, phosphorus source and carbon source are mixed, and sintered in a protective atmosphere to form LiFePO ₄ /C. Although this method is easy to synthesize LiFePO ₄ /C, it also has major defects: if the sintering temperature is low, the crystallinity of LiFePO ₄ is not high, which will affect the electrochemical performance of the material; if the reaction sintering temperature is increased, the product particles will be coarse , the particle size distribution is not uniform; and the amorphous carbon often cannot be completely coated on the surface of LiFePO ₄ particles.

Patent CN100404413C proposes a method for preparing carbon-coated lithium ferrous phosphate by using ferric source and doping metal niobium ions: mixing lithium source, phosphorus source, ferric source, niobium source and carbon source to finely grind, 500-800 Sintering at ℃ to obtain lithium iron phosphate. CN1581537 proposes a mechanical solid-phase synthesis method of LiFePO ₄ /C: after mixing metal iron powder, lithium phosphate, iron phosphate, doping element phosphate, conductive agent or conductive agent precursor in proportion and ball milling, iron phosphate is prepared by solid-state sintering lithium. Patent CN1004860004C uses organic ferric iron source and lithium dihydrogen phosphate to mix wet ball milling, the slurry is dried, pulverized, granulated, pressed into tablets, and calcined at 500-800°C to obtain lithium iron phosphate material. Patent CN100405638C adopts a high-temperature solid-phase two-step reaction method to prepare 1-2 μm lithium iron phosphate materials. CN1280185C prepares lithium ferrous phosphate by high-temperature solid-state reaction after mixing organic or polymer compound additives with lithium salt, iron salt and phosphorus salt.

Although the process of the solid-state reaction method is simple, the prepared LiFePO ₄ tends to have coarse particles and a wide range of particle size distribution, and the use of traditional carbon sources such as graphite, glucose, sucrose, etc. The indirect contact area is small, and the fluidity of the carbon source is poor during the high-temperature pyrolysis process, which cannot ensure that the conductive carbon layer is uniform and completely covers the LiFePO ₄ particles, which affects the high-current discharge and cycle performance of the material.

Contents of the invention

In view of the above problems, the object of the present invention is to propose a method of using polystyrene balls as a carbon source to prepare LiFePO ₄ /C, a positive electrode material for lithium ion batteries, so as to obtain LiFePO ₄ /C with uniform particle size distribution and a particle size of 200-500 nm. C,

Improve the conductivity of lithium-ion battery cathode materials.

The preparation method of the positive electrode material LiFePO ₄ /C for lithium ion battery of the present invention is characterized in that the material is formed by coating the surface of LiFePO ₄ with amorphous carbon accounting for 1.5-4.9% by weight of the LiFePO ₄ /C composite Powder, its preparation steps are as follows:

1) The lithium source, the iron source and the phosphorus source are uniformly mixed according to the molar ratio of Li:Fe:P element of 1.02～1.05:1～1.02:1, and then the obtained mixture is uniformly mixed with the carbon source according to the mass ratio of 10:0.3～1.9 ;

2) Using deionized water or a mixture of deionized water and ethanol in any proportion as a solvent, adding the solvent to the carbon source mixture prepared in step 1), the mass ratio of the solvent to the carbon source mixture is 1- 3:1, ball milled for 2-5 hours and then dried to obtain the precursor;

3) The precursor is pre-calcined at 300-400°C for 3-5 hours in a protective atmosphere, then calcined at 500-800°C for 4-10 hours, and then cooled with the furnace to obtain LiFePO ₄ /C.

The above-mentioned carbon source can be polystyrene spheres with a particle size of 150nm-1000nm, or a mixture of polystyrene spheres with a particle size of 150nm-1000nm and glucose, starch or petroleum coke in any proportion.

Said lithium source is lithium hydroxide or lithium carbonate. Said iron source is ferrous oxalate, ferric oxide or ferric phosphate. Said phosphorus source is ammonium dihydrogen phosphate or iron phosphate.

The drying in the above step 2) can be oven drying at 40-80°C or spray drying at 120-300°C.

The above-mentioned protective atmosphere may be nitrogen, argon or a mixture of nitrogen and argon.

Small-sized (150nm-1000nm) polystyrene spheres are monodisperse and have good fluidity. They can be fully mixed with lithium source, iron source and phosphorus source compound powder during ball milling, and evenly distributed on the surface of precursor particles after drying. During the pre-calcination process at 300-400°C, the rheology and partial cracking of the polystyrene spheres occur, so that the surface of the precursor particles is evenly coated with a mixture film of polystyrene and amorphous carbon. The coating film of the mixture hinders the agglomeration of the precursor and the growth of lithium iron phosphate particles during the calcination process at 500-800 ° C, so that the calcined lithium iron phosphate powder has the characteristics of high crystallinity, small particle size and uniform size distribution . After pyrolysis of polystyrene spheres, the carbon film coated on the surface of lithium iron phosphate particles also has a certain degree of crystallinity, which improves the conductivity of the material.

The present invention has the following advantages: (1) the source of raw materials used is wide and the price is low; (2) the preparation method is simple, the control and operation are convenient, the process is reliable, and it is suitable for large-scale production; (3) the obtained LiFePO4/C particle size distribution Uniform, small particle size, good carbon coating effect, excellent electrochemical performance.

Detailed ways

The present invention will be further described below in conjunction with specific embodiment:

Example 1:

Lithium carbonate, ferrous oxalate and ammonium dihydrogen phosphate are uniformly mixed according to the molar ratio of Li:Fe:P element 1.02:1:1, and then 334.9kg of the resulting mixture and 10kg of polystyrene balls with a particle size of 1000nm are put into the ball In the tank (zirconia balls, the ball-to-material ratio is 3:1); then add 300kg of deionized water to ball mill for 4 hours, and dry the ball-milled slurry in an oven at 60°C to make a precursor powder.

In a nitrogen protective atmosphere, first raise the temperature to 350°C and keep it for 3h, then raise the temperature to 550°C and keep it for 8h. The LiFePO ₄ /C cathode material with a particle diameter of 200-300 nm and a coating carbon mass percentage content of 1.9% is synthesized in situ by solid-state reaction.

Example 2:

Lithium carbonate, iron oxide and ammonium dihydrogen phosphate are evenly mixed according to the molar ratio of Li:Fe:P element 1.03:1.01:1, and then 233.9kg of the resulting mixture and 10kg of polystyrene balls with a particle size of 150nm are put into a spherical tank medium (zirconia balls, the ball-to-material ratio is 3:1); then 300 kg of deionized water was added for ball milling for 4 hours, and the ball-milled slurry was dried in an oven at 60° C. to make a precursor powder.

In a nitrogen protective atmosphere, first raise the temperature to 350°C and keep it for 3h, then raise the temperature to 600°C and keep it for 8h. The LiFePO ₄ /C positive electrode material with a particle size of 200-300 nm and a coating carbon mass percentage content of 1.5% is synthesized in situ by solid-state reaction.

Example 3:

Lithium hydroxide, ferrous oxalate and ammonium dihydrogen phosphate are evenly mixed according to the molar ratio of Li:Fe:P element 1.05:1.02:1, and then 342.8kg of the resulting mixture and 30kg of polystyrene balls with a particle size of 150nm are put into the In a spherical tank (zirconia balls, the ball-to-material ratio is 3:1); then add 600kg of deionized water and ethanol (volume ratio 1:1) for ball milling for 4 hours, and the ball-milled slurry is spray-dried at 200°C Make precursor powder.

In an argon protective atmosphere, first raise the temperature to 350°C and keep it for 3h, then raise the temperature to 650°C and keep it for 8h. The LiFePO ₄ /C positive electrode material with a particle size of 250-400 nm and a coating carbon mass percentage content of 3.2% is synthesized in situ by solid-state reaction.

Example 4:

Lithium hydroxide and ferric phosphate are uniformly mixed according to the molar ratio of Li:Fe:P element 1.02:1:1, and then 265.8kg of the resulting mixture is put into a spherical tank together with 60kg of carbon sources (zirconia balls, the ball-to-material ratio is 3:1), wherein the carbon source is a mixture of polystyrene spheres with a particle size of 500nm and glucose in a mass ratio of 9:1; then add 900kg of deionized water for ball milling for 4 hours, and the milled slurry is prepared by spray drying at 250°C into precursor powder.

In a nitrogen protective atmosphere, first raise the temperature to 300°C and keep it for 3h, then raise the temperature to 750°C and keep it for 6h. The LiFePO ₄ /C cathode material with a particle diameter of 250-500 nm and a coating carbon mass percentage content of 4.9% is synthesized in situ by solid-state reaction.

Using the LiFePO ₄ /C composite positive electrode material of the present invention to make a lithium ion battery positive electrode, the steps are as follows:

Mix the lithium-ion battery composite positive electrode material with the binder polyvinylidene fluoride (PVDF) and conductive carbon black in a ratio of 92:4:4, add the solvent NMP and stir into a slurry, evenly coat the surface of the aluminum foil, and then The pole pieces were dried at 60°C for 12 hours. The electrode sheet was compacted and weighed, and then dried in a vacuum oven at 90°C for 12 hours.

The powders synthesized in Example 1, Example 2, Example 3 and Example 4 are all pure phase LiFePO ₄ with high crystallinity through X-ray diffraction analysis, and the coated carbon layer has an amorphous structure. Further analysis by transmission electron microscopy shows that the thickness of the amorphous carbon coating layer on the surface of LiFePO ₄ particles is about 7-26nm, the thickness of the coating layer is uniform, and there is a certain degree of graphitization crystallization. The particle size distribution range of the powder synthesized in Example 1, Example 2, Example 3 and Example 4 is narrow by laser particle size analyzer analysis.

The prepared electrode sheet is used as the positive electrode of the lithium ion battery and the carbon negative electrode is assembled into a battery. The electrolyte is DEC+EC containing 1mol/L LiPF ₆ (volume ratio DEC:EC=7:3), and the diaphragm is made of polypropylene Celgard2300. The cell assembly process was done in a dry glove box with a relative humidity below 1%. After the assembled battery was placed for 24 hours, a constant current charge and discharge test was performed. The charge and discharge voltage range was 2.5 to 3.9V. The reversible lithium intercalation capacity of the positive electrode of the lithium ion battery was measured in an environment of 25°C. There are two measurement schemes: 1) rate performance test, 0.1C charge, rate performance test with 0.1C, 0.2C, 0.5C, 1C, 2C and 5C discharge respectively; 2) cycle performance test with 1C charge and discharge.

The cathode material for lithium ion batteries prepared by the present invention has the following advantages:

1. The production process is simple and the production efficiency is high. The preparation process of the precursors of Example 1, Example 2, Example 3 and Example 4 is simple; the two-step solid-state reaction can obtain LiFePO ₄ /C cathode material with high crystallinity and excellent performance.

2. High reversible capacity and excellent high rate performance. As shown in FIG. 1 , the LiFePO ₄ /C cathode material of the present invention has excellent rate discharge performance. The materials of Example 1, Example 2, Example 3 and Example 4 have a discharge capacity of 153.5mAh/g, 166.7mAh/g, 156.2mAh/g and 155.4mAh/g at a rate of 0.1C, respectively; at a rate of 5C The capacities of 105.9mAh/g, 102.3mAh/g, 119.7mAh/g and 118.5mAh/g can still be maintained after discharge.

3. Excellent cycle performance. The lithium-ion batteries using the materials of Example 1, Example 2, Example 3 and Example 4 of the present invention have the first discharge capacity of 143.7mAh/g, 145.8mAh/g, 143.5mAh/g and 138.5 mAh/g, the discharge capacity decay of Example 1 after 100 cycles is 114.3mAh/g, and the discharge capacity of Embodiment 2, Example 3 and Example 4 after 100 cycles reaches 144.1mAh/g, 143.3mAh/g And 137.6mAh/g, the capacity has almost no decay.

Claims (7)

1. anode material for lithium-ion batteries LiFePO ₄The preparation method of/C is characterized in that this material is by accounting for LiFePO ₄The amorphous carbon of/C compound quality percentage 1.5～4.9% is coated on LiFePO ₄The surface and the powder that forms, its preparation process is as follows:

1) with lithium source, source of iron and phosphorus source according to Li: Fe: the mol ratio 1.02～1.05: 1～1.02: 1 of P element is evenly mixed, and the gained mixture is evenly mixed by mass ratio 10: 0.3～1.9 with carbon source again;

2) mixed liquor of pressing arbitrary proportion with deionized water or deionized water and ethanol is as solvent, solvent is added in the mixture of the carbonaceous sources that step 1) makes, and the mass ratio of solvent and carbonaceous sources mixture is 1～3: 1, and is dry behind ball milling 2～5h, presoma;

3) presoma is in protective atmosphere, and 300～400 ℃ of pre-burning 3～5h cool off with stove behind 500-800 ℃ of calcining 4～10h then earlier, get LiFePO ₄/ C.

2. anode material for lithium-ion batteries LiFePO according to claim 1 ₄The preparation method of/C is characterized in that carbon source is that particle diameter is the polystyrene spheres of 150nm～1000nm, or particle diameter is the polystyrene spheres of 150nm～1000nm and glucose, starch or the petroleum coke mixture by arbitrary proportion.

3. anode material for lithium-ion batteries LiFePO according to claim 1 ₄The preparation method of/C is characterized in that said lithium source is lithium hydroxide or lithium carbonate.

4. anode material for lithium-ion batteries LiFePO according to claim 1 ₄The preparation method of/C is characterized in that said source of iron is ferrous oxalate, iron oxide or ferric phosphate.

5. anode material for lithium-ion batteries LiFePO according to claim 1 ₄The preparation method of/C is characterized in that said phosphorus source is ammonium dihydrogen phosphate or ferric phosphate.

6. anode material for lithium-ion batteries LiFePO according to claim 1 ₄The preparation method of/C is characterized in that step 2) said drying is 40-80 ℃ of oven drying or 120～300 ℃ of spray dryings.

7. anode material for lithium-ion batteries LiFePO according to claim 1 ₄The preparation method of/C is characterized in that protective atmosphere is the gaseous mixture of nitrogen, argon gas or nitrogen and argon gas.