CN102544489B - Method for preparing graphene-coated olivine type lithium ferric phosphate composite material - Google Patents

Abstract

The invention relates to a method for preparing a graphene-coated olivine-type lithium iron phosphate composite material. Under the condition of magnetic stirring at room temperature, iron salt, phosphate, reducing agent and surfactant are dissolved in deionized water and dripped Add it to the graphene oxide dispersion, then dissolve the lithium salt in deionized water and add it dropwise to the above mixed solution, stir, react, centrifuge, wash, vacuum dry and anneal to obtain the product. Compared with the prior art, the present invention synthesizes the olivine structure lithium iron phosphate/graphene composite material in situ in the liquid phase, and the lithium iron phosphate in the composite material grows in situ on the surface of the graphene, not only can achieve uniformity between the two It has a good bonding force, which greatly reduces the contact resistance between lithium iron phosphate and graphene, and greatly improves the electrical conductivity of the material itself.

Description

Preparation method based on graphene-coated olivine-type lithium iron phosphate composite material

technical field

The invention relates to a preparation method of a composite material, in particular to a preparation method of a graphene-coated olivine-type lithium iron phosphate composite material.

Background technique

Lithium iron phosphate material with olivine structure has been considered as the most potential positive electrode material for lithium-ion batteries since its discovery because of its low price, non-toxic and pollution-free, and good cycle stability. There are many preparation methods for lithium iron phosphate, mainly including solid phase method, hydrothermal method, melt-gel method and co-precipitation method. However, a potential problem of lithium iron phosphate used as an electrode for lithium-ion batteries is poor conductivity, which leads to poor rate performance of the battery when it is used as an electrode material, and cannot fully exert its electrochemical performance. Surface carbon coating is a major way to improve its electrical conductivity. At present, there are two main methods of surface coating carbon, one is to mix and calcine carbonaceous substances and raw materials in a certain proportion, and the other is to add carbonaceous substances to the precursor of the product to make it react with it . The introduction of carbon materials into lithium iron phosphate was originally proposed by Ravet et al. They added organic matter to the precursor mixture of lithium iron phosphate, and obtained carbon-coated lithium iron phosphate composite materials after treatment.

Graphene has a two-dimensional lattice structure. Carbon atoms in the plane are connected by sp ² hybrid orbitals to form a hexagonal lattice structure, that is, carbon atoms are connected to three adjacent carbon atoms through strong σ bonds, and CC bonds Make graphene have good structural rigidity. The remaining p-electron orbit is perpendicular to the graphene plane and forms π bonds with the surrounding atoms. The delocalization of π electrons in the lattice makes graphene have good electrical conductivity, and the electron mobility on the plane at room temperature is 1.5× 10 ⁴ cm ² /V·s, far exceeding the conduction rate of electrons in general conductors, so it has broad potential application space in microelectronics, aerospace military industry, energy storage devices, nanoelectronic devices, and nanocomposites. The lithium iron phosphate/graphene composite material can not only alleviate the volume effect of lithium iron phosphate particles during use through graphene, but also use the electron-hole pairs of graphene as a medium for high-speed electron migration. At present, the lithium iron phosphate/graphene mixture is mainly obtained by physical doping, that is, the prepared lithium iron phosphate and graphene are directly mixed. For example, in the Chinese patent application with the application number 20091055316.7 and the application number 201010146161.3, it is formed by simple physical doping, and the Chinese patent application with the patent application number 201110083171.1 discloses a lithium iron phosphate/graphite However, this patent adopts the method of growing graphene on the lithium iron phosphate precursor by chemical vapor deposition to obtain carbon-coated materials. The method is complicated, difficult to control, and the safety performance of high-temperature hydrogen gas is poor. Reaction time Long, unfavorable for large-scale industrialized production.

Contents of the invention

The object of the present invention is to provide a novel, simple and batch-producible graphene-coated olivine-type lithium iron phosphate composite material and a preparation method thereof in order to overcome the above-mentioned defects in the prior art.

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

Select graphene oxide solution raw materials, add iron salt, phosphate, reducing agent and surfactant to graphene oxide solution, add lithium salt to obtain precursor mother liquor, obtain primary product after hydrothermal or solvothermal, wash, After filtration and drying, thermal reduction under protective atmosphere conditions obtained a graphene-coated olivine-type lithium iron phosphate composite material.

A preparation method based on graphene-coated olivine-type lithium iron phosphate composite material, comprising the following steps:

(1) Preparation of graphene oxide dispersion: at room temperature, weigh graphene oxide and add it to deionized water or an organic solvent, sonicate for 10-60 minutes, and magnetically stir for 10-30 minutes to prepare a graphene oxide solution;

(2) Weigh raw materials according to the following mass ratio: the quality of iron salt is 1 to 100 times that of graphene oxide, and the molar ratio of lithium salt, iron salt and phosphate is (1～3): (1～3): 1 , the mass of the reducing agent is 0.1% to 5% of the mass of the iron salt; the mass of the surfactant is 1 to 10 times the mass of the lithium salt;

(3) Under the condition of magnetic stirring at room temperature, iron salt, phosphate, reducing agent and surfactant are dissolved in deionized water and added dropwise in the graphene oxide dispersion liquid, then lithium salt is dissolved in deionized water and Add it dropwise to the above mixed solution, stir for 5-15 minutes, transfer to an autoclave, and heat at 120-180°C for 1-24 hours;

(4) Take out the reactants from the autoclave, centrifuge at a speed of 1000-10000r/min, then wash with ethanol and deionized water for 3-5 times, and then vacuum-dry them at 40-60°C for 3-10 hours. Annealing at 500° C. to 1000° C. for 3 to 10 hours in an atmosphere is to prepare a graphene-coated olivine-type lithium iron phosphate composite material.

In step (1), the graphene oxide solution prepared from graphene oxide and deionized water has a concentration of 1-3 mg/ml, and the graphene oxide solution prepared from graphene oxide and an organic solvent has a concentration of 1-5 mg/ml.

The organic solvent described in the step (1) is N,N-dimethylformamide, 95% ethanol or absolute ethanol.

The iron salt is ferrous sulfate, ferric sulfate, ferric chloride, ferrous chloride, ferric nitrate, ferrous nitrate, ferric acetate, ferrous acetate, ferric oxalate, ferrous oxalate, ferric citrate or ferrous sulfate One or several mixtures in any proportion of iron diammonium, the concentration of iron salt is 0.01-1mol/L.

The phosphate is selected from one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate or ammonium phosphate, and the concentration of the phosphate is 0.01-1 mol/L.

The lithium salt is selected from one or more of lithium hydroxide, lithium carbonate or lithium phosphate, and the concentration of the lithium salt is 0.01-1 mol/L.

The reducing agent is selected from one or more of ascorbic acid, hydrazine or sodium borohydride.

The surfactant is one or more of polyethylene glycol, polyvinyl alcohol or cetyltrimethylammonium bromide.

The protective atmosphere described in step (4) is nitrogen, argon, hydrogen, or a mixture of any two or three of them in any proportion, or a mixture of an inert gas and a reducing gas.

Compared with the prior art, the present invention has the advantage of in-situ synthesis of olivine-type structure lithium iron phosphate/graphene composite material in the liquid phase. In the composite material, lithium iron phosphate grows in situ on the surface of graphene, and not only the two can It achieves uniform mixing and has good bonding force, which greatly reduces the contact resistance between lithium iron phosphate and graphene, and greatly improves the electrical conductivity of the material itself.

Description of drawings

Fig. 1 is the scanning electron microscope (SEM) pattern of graphene-coated lithium iron phosphate composite material;

Fig. 2 is an X-ray diffraction (XRD) pattern of graphene-coated lithium iron phosphate composite material.

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

Example 1

Accurately weigh 0.175g of graphene oxide, measure 35ml of N,N-dimethylformamide solution, and prepare a graphene oxide organic solution with a concentration of 5mg/ml; weigh 1.4g of FeSO ₄ 7H ₂ O, 0.6g 85% OH ₃ PO ₄ solution, 3.2g polyethylene glycol and 0.12g ascorbic acid were dissolved together in 10mL deionized water, and then added to the graphene oxide organic solution to obtain mixed solution 1; 0.64g LiOH·H ₂ O Dissolved in 5mL deionized water, and added to the above mixed solution 1 to obtain the mixed solution 2; put the mixed solution 2 into the autoclave, react at 180°C for 15h, centrifuge, wash, and vacuum dry at 60°C for 4h to obtain the primary Product, the primary product was annealed under the protection of high-purity argon at 700°C for 10 hours to obtain a composite material.

Example 2

Accurately weigh 0.105g of graphene oxide, measure 35ml of deionized water, and make a graphene oxide aqueous solution with a concentration of 3mg/ml; weigh 0.7g of FeSO ₄ 7H ₂ O, 0.3g of 85% OH ₃ PO ₄ solution , 1.6g of polyethylene glycol and 0.06g of ascorbic acid were dissolved in 10mL of deionized water, and then added to the graphene oxide organic solution to obtain a mixed solution 1; 0.32g of LiOH·H ₂ O was dissolved in 5mL of deionized water, and added In the above mixed solution 1, the mixed solution 2 was obtained; the mixed solution 2 was added to the autoclave, reacted at 180°C for 10h, centrifuged, washed, and vacuum-dried at 40°C for 10h to obtain the primary product, which was heated at 1000°C The composite material was obtained by annealing for 3 h under the protection of high-purity argon.

Example 3

Accurately weigh 0.035g of graphene oxide, weigh 35ml of N,N-dimethylformamide solution, and prepare a graphene oxide organic solution with a concentration of 1mg/ml; weigh 0.35g of FeSO4 7H ₂ O, 0.15g of 85 % O-H3PO4 solution, 0.8g polyethylene glycol and 0.03g ascorbic acid were dissolved in 10mL deionized water, and then added to the graphene oxide organic solution to obtain a mixed solution 1; 0.16g LiOH·H2O was dissolved in 5mL to Ionized water, and added to the above mixed solution 1 to obtain the mixed solution 2; the mixed solution 2 was added to the autoclave, reacted at 180°C for 12h, and centrifuged, washed, and vacuum-dried at 40°C for 10h to obtain the primary product. The primary product was annealed at 500°C for 8 hours under the protection of high-purity argon to obtain a composite material.

Example 4

Accurately weigh 0.035g of graphene oxide, weigh 35ml of absolute ethanol solution, and make a graphene oxide organic solution with a concentration of 1mg/ml; weigh 0.035g of FeSO4 7H ₂ O, 0.015g of 85% O-H3PO4 solution , 0.08g polyethylene glycol and 0.003g ascorbic acid were dissolved in 10mL deionized water together, and then added to the graphene oxide organic solution to obtain a mixed solution 1; 0.016g LiOH·H2O was dissolved in 5mL deionized water, and the above From the mixed solution 1, the mixed solution 2 was obtained; the mixed solution 2 was added to the autoclave, reacted at 180°C for 8 hours, centrifuged, washed, and vacuum-dried at 40°C for 8 hours to obtain the primary product, which was purified at 700°C Annealed under the protection of argon for 5h to obtain the composite material.

Example 5

Accurately weigh 0.035g of graphene oxide, measure 35ml of N,N-dimethylformamide solution, and prepare a graphene oxide organic solution with a concentration of 1mg/ml; weigh 3.5g of FeSO ₄ 7H ₂ O, 0.6g 85% OH ₃ PO ₄ solution, 6.4g polyvinyl alcohol and 0.12g ascorbic acid were dissolved together in 10mL deionized water, and then added to the graphene oxide organic solution to obtain mixed solution 1; 0.64g LiOH·H ₂ O was dissolved Add to 5mL deionized water, and add to the above mixed solution 1 to obtain mixed solution 2; add mixed solution 2 to the autoclave, react at 180°C for 15h, centrifuge, wash, and vacuum dry at 60°C for 4h to obtain the primary product , the primary product was annealed at 700 ° C under the protection of high-purity argon for 10 h to obtain a composite material.

Example 6

Accurately weigh 0.035g of graphene oxide, measure 35ml of N,N-dimethylformamide solution, and prepare a graphene oxide organic solution with a concentration of 1mg/ml; weigh 1.4g of FeSO ₄ 7H ₂ O, 0.6 g 85% _{OHPO solution} , 0.64g cetyltrimethylammonium bromide and 0.175g sodium borohydride were dissolved together in 10mL deionized water and then added to the graphene oxide organic solution to obtain a mixed solution 1; Dissolve 0.64g LiOH·H ₂ O in 5mL deionized water, and add it to the above mixed solution 1 to obtain mixed solution 2; add mixed solution 2 to the autoclave, react at 180°C for 15h, and centrifuge , washed, and vacuum-dried at 60°C for 4h to obtain a primary product, which was annealed at 700°C for 10h under the protection of high-purity argon to obtain a composite material.

Example 7

Accurately weigh 0.035g of graphene oxide, measure 35ml of N,N-dimethylformamide solution, and prepare a graphene oxide organic solution with a concentration of 1mg/ml; weigh 1.4g of FeSO ₄ 7H ₂ O, 0.6g 85% _{OH PO} solution, 0.64g cetyltrimethylammonium bromide and 0.0014g hydrazine were dissolved in 10mL deionized water together, then added to the graphene oxide organic solution to obtain mixed solution 1; Dissolve 0.64g LiOH·H ₂ O in 5mL deionized water, and add to the above mixed solution 1 to obtain mixed solution 2; add mixed solution 2 to the autoclave, react at 180°C for 15h, centrifuge, wash, Vacuum drying at 60°C for 4 hours to obtain a primary product, which was annealed at 700°C for 10 hours under the protection of high-purity argon to obtain a composite material.

Example 8

Accurately weigh 0.035g of graphene oxide, measure 35ml of N,N-dimethylformamide solution, and prepare a graphene oxide organic solution with a concentration of 1mg/ml; weigh 1.4g of FeSO ₄ 7H ₂ O, 0.6g 85% _{OH PO} solution, 0.64g cetyltrimethylammonium bromide and 0.0014g hydrazine were dissolved in 10mL deionized water together, then added to the graphene oxide organic solution to obtain mixed solution 1; Dissolve 0.64g LiOH·H ₂ O in 5mL deionized water, and add to the above mixed solution 1 to obtain mixed solution 2; add mixed solution 2 to the autoclave, react at 120°C for 20h, centrifuge, wash, Vacuum drying at 60°C for 4 hours to obtain a primary product, which was annealed at 700°C for 10 hours under the protection of high-purity nitrogen to obtain a composite material.

Example 9

A preparation method based on graphene-coated olivine-type lithium iron phosphate composite material, comprising the following steps:

(1) Preparation of graphene oxide dispersion: at room temperature, weigh graphene oxide and add it to deionized water at a concentration of 1 mg/ml, ultrasonication for 10 minutes, and magnetic stirring for 10 minutes to prepare a graphene oxide solution;

(2) Weigh the raw materials according to the following mass ratio: the quality of ferrous sulfate is 1 time of the graphene oxide quality, the mol ratio of lithium carbonate, ferrous sulfate and ammonium dihydrogen phosphate is 1: 1: 1, and the concentration of iron salt is 0.01mol/L, the concentration of phosphate is 0.01mol/L, the concentration of lithium salt is 0.01mol/L, the quality of reducing agent ascorbic acid is 0.1% of ferrous sulfate quality; The quality of surfactant polyethylene glycol is carbonic acid 1 times the mass of lithium;

(3) Under the condition of magnetic stirring at room temperature, ferrous sulfate, ammonium dihydrogen phosphate, reducing agent ascorbic acid and surfactant polyethylene glycol are dissolved in deionized water and added dropwise in the graphene oxide dispersion, and then Lithium carbonate was dissolved in deionized water and added dropwise to the above mixed solution, stirred for 5 minutes, moved into an autoclave, and heated at 120°C for 1 hour;

(4) Take out the reactants from the autoclave, centrifuge at a speed of 1000r/min, then cross-wash with ethanol and deionized water for 3 times, then vacuum-dry at 40°C for 3h, and anneal at 500°C under a nitrogen protective atmosphere Within 3 hours, the graphene-coated olivine-type lithium iron phosphate composite material was prepared.

Example 10

A preparation method based on graphene-coated olivine-type lithium iron phosphate composite material, comprising the following steps:

(1) Preparation of graphene oxide dispersion: at room temperature, weigh graphene oxide and add it to deionized water at a concentration of 3 mg/ml, ultrasonication for 30 minutes, and magnetic stirring for 20 minutes to prepare a graphene oxide solution;

(2) raw materials are weighed according to the following mass ratio: the quality of ferric chloride is 10 times of graphene oxide quality, the mol ratio of lithium phosphate, ferric chloride and ammonium phosphate is 2: 3: 1, and the concentration of iron salt is 0.05mol/L, the concentration of phosphate is 0.08mol/L, the concentration of lithium salt is 0.1mol/L, the quality of reducing agent sodium borohydride is 2% of iron salt quality; The quality of surfactant polyvinyl alcohol is lithium 3 times the mass of salt;

(3) Under the condition of magnetic stirring at room temperature, iron trichloride, ammonium phosphate, reducing agent sodium borohydride and surfactant polyvinyl alcohol are dissolved in deionized water and added dropwise in the graphene oxide dispersion, and then Lithium phosphate was dissolved in deionized water and added dropwise to the above mixed solution, stirred for 10 minutes, moved into an autoclave, and heated at 150°C for 6 hours;

(4) Take out the reactant from the autoclave, centrifuge at a speed of 5000r/min, then cross-wash with ethanol and deionized water for 4 times, then vacuum-dry at 50°C for 5h, and place under a protective atmosphere of nitrogen and argon. 800°C and annealing for 5 hours, the graphene-coated olivine-type lithium iron phosphate composite material was prepared.

Example 11

A preparation method based on graphene-coated olivine-type lithium iron phosphate composite material, comprising the following steps:

(1) Preparation of graphene oxide dispersion: at room temperature, weigh graphene oxide and add it to N, N-dimethylformamide at a concentration of 1 mg/ml, ultrasonic for 40 minutes, and magnetically stirred for 15 minutes to prepare a graphene oxide solution ;

(2) Weigh the raw materials according to the following mass ratio: the quality of ferric nitrate is 10 times that of graphene oxide, the molar ratio of lithium hydroxide, ferric nitrate and ammonium phosphate is 1:3:1, and the concentration of iron salt is 0.1mol/ L, the concentration of phosphate is 0.5mol/L, the concentration of lithium salt is 0.2mol/L, the quality of reducing agent sodium borohydride is 2% of iron salt quality; The quality of surfactant polyvinyl alcohol is 2% of lithium salt quality 6 times;

(3) Under the condition of magnetic stirring at room temperature, iron salt, phosphate, reducing agent and surfactant are dissolved in deionized water and added dropwise in the graphene oxide dispersion liquid, then lithium salt is dissolved in deionized water and Add it dropwise to the above mixed solution, stir for 10 minutes, transfer to an autoclave, and heat at 150°C for 18 hours;

(4) Take out the reactants from the autoclave, centrifuge at a speed of 8000r/min, then cross-wash with ethanol and deionized water 4 times, then vacuum-dry at 60°C for 10h, and anneal at 800°C under a protective atmosphere for 8 Hours, the graphene-coated olivine-type lithium iron phosphate composite material was prepared.

Example 12

A preparation method based on graphene-coated olivine-type lithium iron phosphate composite material, comprising the following steps:

(1) Preparation of graphene oxide dispersion: at room temperature, weigh graphene oxide and add it to absolute ethanol at a concentration of 5 mg/ml, sonicate for 60 minutes, and magnetically stir for 30 minutes to prepare a graphene oxide solution;

(2) Weigh the raw materials according to the following mass ratio: the quality of ferrous oxalate is 100 times that of graphene oxide, the mol ratio of lithium hydroxide, ferrous oxalate and ammonium dihydrogen phosphate is 3:2:1, the concentration of iron salt is 1mol/L, the concentration of phosphate is 1mol/L, the concentration of lithium salt is 1mol/L, the quality of reducing agent ascorbic acid is 5% of iron salt quality; Surfactant hexadecyltrimethylammonium bromide The quality is 10 times that of lithium salt;

(3) Under the condition of magnetic stirring at room temperature, dissolve ferrous oxalate, ammonium dihydrogen phosphate, reducing agent and surfactant in deionized water and add it dropwise to the graphene oxide dispersion, then dissolve the lithium salt in the deionized water Ionized water was added dropwise to the above mixed solution, stirred for 15 minutes, transferred into an autoclave, and heated at 180°C for 24 hours;

(4) Take out the reactant from the autoclave, centrifuge at a speed of 10000r/min, then cross-wash with ethanol and deionized water for 5 times, then vacuum-dry at 60°C for 10h, and anneal at 1000°C for 10 hours under a protective atmosphere. Hours, the graphene-coated olivine-type lithium iron phosphate composite material was prepared.

Claims (8)

1. the preparation method based on graphene coated olivine-type composite ferric lithium phosphate material, is characterized in that, the method comprises the following steps:

(1) prepare graphene oxide dispersion liquid: under room temperature, take graphene oxide and join in deionized water or organic solvent, ultrasonic 10～60min, magnetic agitation 10～30min, is mixed with graphene oxide solution;

(2) by following quality than raw materials weighing: molysite quality is 1～100 times of graphene oxide quality, and lithium salts, molysite and phosphatic mol ratio are (1～3) ︰ (1～3) ︰ 1, and the quality of reducing agent is 0.1%～5% of molysite quality; The quality of surfactant is 1～10 times of lithium salts quality;

(3) in the situation that of room temperature magnetic agitation, molysite, phosphate, reducing agent and surfactant dissolves in deionized water and be added drop-wise in graphene oxide dispersion liquid, again lithium salts be dissolved in deionized water and be added drop-wise in graphene oxide dispersion liquid, stir 5～15min, in immigration autoclave, 120～180 ℃ are heated 1～24 hour;

(4) in autoclave, take out reactant, at rotating speed, be centrifugal under 1000～10000r/min, then use ethanol and deionized water cross washing 3～5 times, again through 40～60 ℃ of vacuumize 3～10h, under protective atmosphere at 500 ℃～1000 ℃, anneal 3～10 hours, prepare based on graphene coated olivine-type composite ferric lithium phosphate material;

Described surfactant is one or more in polyethylene glycol, polyvinyl alcohol or softex kw.

2. the preparation method based on graphene coated olivine-type composite ferric lithium phosphate material according to claim 1, it is characterized in that, the concentration of the graphene oxide solution that in step (1), graphene oxide and deionized water preparation obtain is 1～3mg/ml, and the concentration of the graphene oxide solution that graphene oxide and organic solvent preparation obtain is 1～5mg/ml.

3. the preparation method based on graphene coated olivine-type composite ferric lithium phosphate material according to claim 1, is characterized in that, the organic solvent described in step (1) is DMF, 95% ethanol or absolute ethyl alcohol.

4. the preparation method based on graphene coated olivine-type composite ferric lithium phosphate material according to claim 1, it is characterized in that, described molysite is the mixture of one or more any ratios in ferrous sulfate, ferric sulfate, ferric trichloride, frerrous chloride, ferric nitrate, ferrous nitrate, ferric acetate, ferrous acetate, ferric oxalate, ferrous oxalate, ironic citrate or ferrous sulfate two ammoniums, and the concentration of molysite is 0.01～1mol/L.

5. the preparation method based on graphene coated olivine-type composite ferric lithium phosphate material according to claim 1, it is characterized in that, described phosphate is selected from one or more in ammonium dihydrogen phosphate, diammonium hydrogen phosphate or ammonium phosphate, and phosphatic concentration is 0.01～1mol/L.

6. the preparation method based on graphene coated olivine-type composite ferric lithium phosphate material according to claim 1, is characterized in that, described lithium salts is selected from one or more in lithium carbonate or lithium phosphate, and the concentration of lithium salts is 0.01～1mol/L.

7. the preparation method based on graphene coated olivine-type composite ferric lithium phosphate material according to claim 1, is characterized in that, described reducing agent is selected from one or more in ascorbic acid, hydrazine or sodium borohydride.

8. the preparation method based on graphene coated olivine-type composite ferric lithium phosphate material according to claim 1; it is characterized in that, the protective atmosphere described in step (4) be nitrogen, argon gas, hydrogen or wherein any both or three in the mixed atmosphere of the gaseous mixture of any ratio or inert gas and reducing gas.