Method for preparing graphene-based LiFePO4/C composite material

Abstract

The present invention relates to a method for preparing a graphene-based LiFePO.sub.4/C composite material, to solve the problem of poor conductivity and rate performance of lithium iron phosphate cathode material. The main features of the present invention include the steps of: 1) preparing an iron salt solution having graphene oxide dispersed therein; 2) preparing a ferric phosphate/graphene oxide precursor; 3) preparing the graphene-based LiFePO.sub.4/C composite material. The beneficial effects of the method is that the process is simple, easy to control and the resulted graphene-based LiFePO.sub.4/C composite material has high specific capacity, good recycle performance and excellent rate capability is particularly suitable to the field of the power battery application.

Claims

1. A method for preparing a graphene-based LiFePO.sub.4/C composite material, comprising: dissolving a graphene oxide and an iron salt in a deionized water, a mass ratio of the graphene oxide over an iron element in the iron salt being 0.1:1 to 0.3:1; dispersing the graphene oxide by ultrasonic to obtain an iron salt solution having the graphene oxide dispersed therein; mixing the iron salt solution with a phosphate solution to obtain a reaction mixture, in which a molar ratio of Fe:P is 1:1 to 1:1.2; adjusting a pH value of the reaction mixture to 2 to 4; controlling a temperature of the reaction mixture to allow the reaction mixture to react at 60 C. to 80 C. to obtain an emulsion; filtering the emulsion to obtain a ferric phosphate/graphene oxide precursor; blending the ferric phosphate/graphene oxide precursor with a lithium salt at a Li:Fe molar ratio of 1:1 to 1.05:1 to form a first mixture; adding a carbon source to the first mixture to form a second mixture; and sintering the second mixture under a reducing atmosphere condition at 600 C. to 700 C., to obtain the graphene-based LiFePO.sub.4/C composite material.

2. The method according to claim 1, wherein an iron ion concentration in the iron salt solution is 0.5 to 2 mol/L.

3. The method according to claim 1, wherein dispersing the graphene oxide by ultrasonic includes dispersing the graphene oxide by ultrasonic for 2 to 5 hours.

4. The method according to claim 1, wherein controlling the temperature of the reaction mixture to allow the reaction mixture to react includes allowing the reaction mixture to react for 3 to 6 hours; and sintering includes sintering for 5 to 10 hours.

5. The method according to claim 1, wherein dissolving the iron salt solution in the deionized water includes dissolving at least one of a ferric sulphate, a ferric chloride, or a ferric nitrate in the deionized water.

6. The method according to claim 1, wherein mixing the iron salt solution with the phosphate solution includes mixing the iron salt solution with a solution of at least one of a phosphoric acid, an ammonium biphosphate, an ammonium monoacid phosphate, an ammonium phosphate, a sodium biphosphate, a sodium monoacid phosphate, or a sodium phosphate.

7. The method according to claim 1, wherein adjusting the pH value of the reaction mixture includes adjusting the pH value of the reaction mixture using an alkaline solution having a concentration of 0.5 to 5 mol/L, the alkaline solution including a solution of a sodium hydroxide, a sodium carbonate, or an ammonia.

8. The method according to claim 1, wherein blending the ferric phosphate/graphene oxide precursor with the lithium salt includes blending the ferric phosphate/graphene oxide precursor with at least one of a lithium carbonate, a lithium hydroxide, or a lithium acetate.

9. The method according to claim 1, wherein adding the carbon source includes adding at least one of a glucose, a sucrose, a fructose, a lactose, a citric acid, a starch, a polyvinyl alcohol, a polypropylene, or a phenolic resin, having an amount equaling to 5 to 20% of a theoretical mass of a lithium iron phosphate.

10. The method according to claim 1, wherein sintering under the reducing atmosphere condition includes sintering in a mixed gas having a volume ratio of Ar:H.sub.2=90:10 to 95:5 or of N.sub.2:H.sub.2=90:10 to 95:5.

11. The method according to claim 1, further comprising: washing and drying the emulsion after filtering the emulsion.

12. The method according to claim 1, further comprising: performing a ball milling on the second mixture.

13. The method according to claim 1, wherein dissolving the iron salt in the deionized water includes: dissolving a ferrous salt in the deionized water, the ferrous salt including at least one of a ferrous sulfate, a ferrous chloride, or a ferrous nitrate; and oxidizing the ferrous salt using an excess H.sub.2O.sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a SEM image of graphene-based LiFePO.sub.4/C composite material obtained in Example 1;

(2) FIG. 2 is a XDR image of graphene-based LiFePO.sub.4/C composite material obtained in Example 1;

(3) FIG. 3 is the charge-discharge curve at different rates of the lithium ion battery in which the graphene-based LiFePO.sub.4/C composite material obtained in Example 2 is used as cathode material;

(4) FIG. 4 is the recycle performance curve at different rates of the lithium ion battery in which the graphene-based LiFePO.sub.4/C composite material obtained in Example 3 is used as cathode material.

EXAMPLES

Examples of the Invention

(5) In the following description, the examples are set forth for purposes of illustration rather than limitation.

Example 1

(6) Graphene oxide and ferric sulfate are dissolved in deionized water in graphene oxide:iron weight ratio of 0.1:1, to prepare a mixture solution with an iron ion concentration of 1 mol/L, and then dispersed by ultrasonic for 3 hours, to obtain an iron salt solution having graphene oxide dispersed therein;

(7) A phosphoric acid solution is prepared, with a concentration of 1 mol/L.

(8) The resulted iron salt solution having graphene oxide dispersed therein and the resulted phosphoric acid solution are added in parallel flow to a reactor equipped with a stirrer in a Fe:P molar ratio of 1:1.1. Meanwhile, the reaction liquor is adjusted to pH 2.1 using ammonia with a concentration of 1 mol/L, and then reacts under controlled temperature of 60 C. for 5 hours to give an emulsion. The resulted emulsion is filtered, washed and the residue is dried at 80 C. in an air dry oven for 24 hours, to give ferric phosphate/graphene oxide precursor;

(9) The resulted ferric phosphate/graphene oxide precursor is blended with lithium salt in Li:Fe molar ratio of 1.05:1, subjected to ball milling after adding glucose which is 20% of the theoretical mass of lithium iron phosphate, and then sintered under a reducing atmosphere condition of Ar:H.sub.2=90:10 (v/v) at 650 C. for 8 hours, to obtain graphene-based LiFePO.sub.4/C composite materials. FIGS. 1 and 2 are the SEM image and the XRD image of the material obtained in Example 1, showing that the produced particles is small and evenly dispersed, with regular morphology and clear interface between the particles, which means that its grain morphology growth is complete. In the XRD spectrum, the characteristic peaks are relatively obvious, no impurity peak are found, and the diffraction peaks are relatively sharp, showing the synthetized product crystallizes well.

Example 2

(10) Graphene oxide and ferric nitrate are dissolved in deionized water in graphene oxide:iron weight ratio of 0.2:1, to prepare a mixture solution with an iron ion concentration of 0.5 mol/L, and then dispersed by ultrasonic for 3 hours, to obtain an iron salt solution having graphene oxide dispersed therein;

(11) An ammonium biphosphate solution is prepared, with a concentration of 0.5 mol/L.

(12) The resulted iron salt solution having graphene oxide dispersed therein and the ammonium biphosphate solution are added in a Fe:P molar ratio of 1:1 in parallel flow to a reactor equipped with a stirrer. Meanwhile, the reaction liquor is adjusted to pH 2.5 using aqueous sodium hydroxide solution with a concentration of 0.5 mol/L, and then reacts under controlled temperature of 80 C. for 3 hours to give an emulsion. The resulted emulsion is filtered, washed and the residue is dried at 80 C. in an air dry oven for 24 hours, to give a ferric phosphate/graphene oxide precursor;

(13) The resulted ferric phosphate/graphene oxide precursor is blended with lithium salt in Li:Fe molar ratio of 1.02:1, subjected to ball milling after adding sucrose which is 10% of the theoretical mass of lithium iron phosphate and then sintered under a reducing atmosphere condition of Ar:H.sub.2=95:5 (v/v) at 700 C. for 5 hours, to obtain graphene-based LiFePO.sub.4/C composite materials.

(14) FIG. 3 is the charge-discharge curve at different rates of the lithium ion battery in which the graphene-based LiFePO.sub.4/C composite material obtained in Example 2 is used as cathode material. The discharge specific capacities at 0.5C, 1C, 2C and 5C are maintained above 140 mA.Math.h/g, 137 mA.Math.h/g, 130 mA.Math.h/g and 120 mA.Math.h/g, respectively, and the discharge voltage platform maintains well.

Example 3

(15) Graphene oxide and ferrous sulfate are dissolved in deionized water in graphene oxide:iron weight ratio of 0.3:1, and are oxidized by excess H.sub.2O.sub.2, to prepare a mixture solution with an iron ion concentration of 2 mol/L, and then dispersed by ultrasonic for 3 hours, to obtain an iron salt solution having graphene oxide dispersed therein;

(16) An ammonium monoacid phosphate solution is prepared, with a concentration of 2 mol/L.

(17) The resulted iron salt solution having graphene oxide dispersed therein and the ammonium monoacid phosphate solution are added in a Fe:P molar ratio of 1:1 in parallel flow to a reactor equipped with a stirrer. Meanwhile, the reaction liquor is adjusted to pH 2.5 using aqueous sodium hydroxide solution with a concentration of 5 mol/L, and then reacts under controlled temperature of 80 C. for 3 hours to give an emulsion. The resulted emulsion is filtered, washed and the residue is dried at 80 C. in an air dry oven for 24 hours, to give ferric phosphate/graphene oxide precursor;

(18) The resulted ferric phosphate/graphene oxide precursor is blended with lithium salt in Li:Fe molar ratio of 1:1, subjected to ball milling after adding starch which is 5% of the theoretical mass of lithium iron phosphate and then sintered under a reducing atmosphere condition of Ar:H.sub.2=90:10 (v/v) at 600 C. for 10 hours, to obtain graphene-based LiFePO.sub.4/C composite materials.

(19) FIG. 4 is the recycle performance curve at different rates of the lithium ion battery in which the graphene-based LiFePO.sub.4/C composite material obtained in Example 3 is used as cathode material. The product has good recycle stability at different rates, discharge specific capacity of cathode material has no significant attenuation at each ratio, and the discharge specific capacity under rate of 5C keeps at 125 mA.Math.h/g. After tests at different rates, recovery is performed at 0.2C rate and the capacity remains well, indicating that the composite material has good structural stability.