CN110723718A - A kind of preparation method of nitrogen-doped graphene/lithium iron phosphate composite material for lithium ion battery - Google Patents

Abstract

The invention discloses a preparation method of a nitrogen-doped graphene/lithium iron phosphate composite material. The method has simple process, is suitable for industrial mass production, and the prepared composite material has excellent and stable conductivity, can be used as a positive electrode material and applied to lithium ion batteries.

Description

A kind of preparation method of nitrogen-doped graphene/lithium iron phosphate composite material for lithium ion battery

technical field

The invention belongs to the field of composite materials, in particular to a method for preparing a nitrogen-doped graphene/lithium iron phosphate composite material for lithium ion batteries.

Background technique

Lithium-ion batteries are a new generation of green high-energy batteries. Since the commercialization of lithium-ion batteries, the research on the conductivity of cathode materials has always been a hot spot in the battery field. At present, the cathode active materials used in lithium-ion batteries are mostly transition metal oxides or transition metal phosphates, most of which are semiconductors. Or insulators have poor electrical conductivity, so it is necessary to add conductive agents to improve electrical conductivity, and play a role in collecting microcurrent between active materials, active materials and current collectors to reduce the contact resistance of electrodes, and also effectively Therefore, the mobility of lithium ions in the battery material can be greatly improved, thereby improving the charge-discharge rate performance of the battery.

CN 106992301A discloses a nitrogen-doped graphene conductive agent and a preparation method thereof. The nitrogen-doped graphene is prepared by chemical vapor deposition and applied to a lithium ion battery. Although the method successfully prepares a conductive agent with high conductivity, However, there is a problem of dispersion of the conductive agent during the preparation of the slurry, and the graphene is easy to agglomerate, which increases the internal resistance of the battery, thereby reducing the performance of the battery. CN 109103442A discloses a preparation method of graphene-coated lithium iron phosphate positive electrode material. The graphene-coated lithium iron phosphate material is prepared by microwave hydrothermal method, and the problem of graphene dispersion is solved by coating. Graphene is not very conductive, so it is not ideal for improving the performance of lithium iron phosphate batteries.

SUMMARY OF THE INVENTION

In order to solve the deficiencies of the prior art, the present invention provides a preparation method of nitrogen-doped graphene/lithium iron phosphate composite material for lithium ion battery, the preparation method of the composite material includes preparing iron phosphate by a sol-gel method The precursor of the lithium and carbon source composite material is then calcined at a high temperature in an ammonia atmosphere to obtain a nitrogen-doped graphene/lithium iron phosphate composite material. It not only solves the problem of uneven dispersion of graphene, but also further improves the conductivity of the material by nitrogen modification of graphene.

For realizing the purpose of the present invention, the technical scheme of the present invention is:

A method for preparing nitrogen-doped graphene/lithium iron phosphate composite material for lithium ion batteries, using carbon source, lithium source, phosphorus source and iron source as raw materials, and preparing nitrogen-doped graphene through a sol-gel method and a high-temperature calcination method The heterographene/lithium iron phosphate composite material specifically includes the following steps:

(1) adding lithium source, phosphorus source, iron source and carbon source into deionized water to disperse, and prepare a dispersion liquid;

(2) add citric acid in the dispersion of step (1) and use ammoniacal liquor to adjust pH to 11, stir evenly;

(3) heating in a water bath at a certain temperature, stirring until gel appears;

(4) In an ammonia atmosphere, the gel obtained in step (3) is calcined at high temperature to obtain a nitrogen-doped graphene/lithium iron phosphate composite material.

Further, the molar ratio among the lithium source, phosphorus source, iron source and carbon source described in step (1) is 0.8-1.2:1-1.5:1-1.5:0.5-3.

Further, the method for preparing the dispersion described in step (1) is high-power ultrasonic dispersion, and the power is 800W-1000W.

Further, the lithium source described in step (1) is one of lithium hydroxide, lithium carbonate and lithium hydrogen phosphate.

Further, the phosphorus source described in step (1) is one of ammonium phosphate, diammonium hydrogen phosphate, ammonium monohydrogen phosphate, and phosphoric acid.

Further, the iron source described in step (1) is one of ferrous sulfate, ferric phosphate, ferric oxide, ferric chloride, and ferric acetate.

Further, the carbon source in step (1) is one of glucose, fructose and sucrose.

Further, the heating temperature of the water bath described in step (3) is 80-100 °C.

Further, the ammonia gas described in step (4) is high-purity ammonia gas.

Further, the calcination process described in step (4) is divided into two stages, the first stage is kept at 600°C for 3-5h, the second stage is kept at 750-900°C for 3-5h, and the heating rate is 3-5°C/ min.

Compared with the prior art, the present invention has the following beneficial effects:

The nitrogen-doped graphene/lithium iron phosphate composite material provided by the present invention, the preparation method of the composite material includes preparing the precursor of the lithium iron phosphate and carbon source composite material by a sol-gel method, and then calcining at a high temperature in an ammonia gas atmosphere The nitrogen-doped graphene/lithium iron phosphate composite material was obtained by the method. Through high-temperature calcination, glucose is pyrolytically reduced to nitrogen-doped graphene, the gel is dried to form lithium iron phosphate, and the nitrogen-doped graphene is recombined with lithium iron phosphate during the reduction process. The preparation process is simple, and the method can not only solve the problem of uneven dispersion of graphene in the positive electrode slurry, but also carry out nitrogen modification on the basis of graphene to further improve its conductivity, thereby improving the lithium iron phosphate battery. charge-discharge and rate performance.

Description of drawings

1 shows the capacity retention rate of the lithium-ion battery of Example 1 after being cycled 100 times at 1C;

Figure 2 is a SEM image of nitrogen-doped graphene/lithium iron phosphate composite material.

Detailed ways

In conjunction with the embodiments, the specific preferred implementation of the present invention will be further described:

Example 1

A method for preparing nitrogen-doped graphene/lithium iron phosphate composite material, comprising the following steps:

(1) According to the molar ratio of lithium source: phosphorus source: iron source = 1:1:1:1, weigh 0.01mol lithium hydroxide, 0.01mol diammonium hydrogen phosphate, 0.01mol ferric chloride and 0.01mol glucose and add to Dispersion is prepared by high-power dispersion in ionized water;

(2) Add 0.06mol citric acid during stirring, and add ammonia water to adjust pH to 11;

(3) Heating in a water bath at 80°C and stirring until gel-like to obtain the precursor of lithium iron phosphate/graphene composite material;

(4) Transfer the material obtained in step (3) into a vacuum heating furnace, pass in high-purity ammonia gas, heat up to 600 °C for 3 hours at a heating rate of 3 °C/min, and then raise the temperature to 750 °C for 3 hours. , and finally cooled to room temperature to obtain nitrogen-doped graphene/lithium iron phosphate composites.

The obtained composite material was dispersed in N-methylpyrrolidone (NMP) to obtain a dispersion. The PVDF binder is obtained by mixing the PVDF:NMP=5:95 mass ratio, and the binder and the dispersion liquid are mixed and stirred according to the ratio of 1:8 to obtain the positive electrode slurry of the lithium ion battery.

Example 2

A method for preparing nitrogen-doped graphene/lithium iron phosphate composite material, comprising the following steps:

(1) According to the molar ratio of lithium source: phosphorus source: iron source = 1:1:1:2, weigh 0.01mol lithium acetate, 0.01mol ammonium phosphate, 0.01mol ferric acetate and 0.02mol glucose into deionized water and pass through high Power dispersion to prepare dispersion;

(2) Add 0.06mol citric acid during stirring, and add ammonia water to adjust pH to 11;

(3) Heating in a water bath at 90°C and stirring until gel-like to obtain the precursor of lithium iron phosphate/graphene composite material;

(4) Transfer the material obtained in step (3) into a vacuum heating furnace, pass in high-purity ammonia gas, heat up to 600°C for 3 hours at a heating rate of 4°C/min, and then raise the temperature to 700°C for 3 hours , and finally cooled to room temperature to obtain nitrogen-doped graphene/lithium iron phosphate composites.

The obtained composite material was dispersed in N-methylpyrrolidone (NMP) to obtain a dispersion. The PVDF binder is obtained by mixing the PVDF:NMP=5:95 mass ratio, and the binder and the dispersion liquid are mixed and stirred according to the ratio of 1:8 to obtain the positive electrode slurry of the lithium ion battery.

Example 3

A method for preparing nitrogen-doped graphene/lithium iron phosphate composite material, comprising the following steps:

(1) According to the molar ratio of lithium source: phosphorus source: iron source = 1:1:1:3, weigh 0.01mol lithium acetate, 0.01mol ammonium monohydrogen phosphate, 0.01mol iron acetate and 0.03mol glucose into deionized water Preparation of dispersions by high power dispersion;

(2) Add 0.06mol citric acid during stirring, and add ammonia water to adjust pH to 11;

(3) Heating in a water bath at 100°C, stirring until gel-like, to obtain the precursor of lithium iron phosphate/graphene composite material;

(4) Transfer the material obtained in step (3) into a vacuum heating furnace, pass in high-purity ammonia gas, heat up to 600°C for 3 hours at a heating rate of 5°C/min, and then raise the temperature to 800°C for 3 hours , and finally cooled to room temperature to obtain nitrogen-doped graphene/lithium iron phosphate composites.

The obtained composite material was dispersed in N-methylpyrrolidone (NMP) to obtain a dispersion. The PVDF binder is obtained by mixing the PVDF:NMP=5:95 mass ratio, and the binder and the dispersion liquid are mixed and stirred according to the ratio of 1:8 to obtain the positive electrode slurry of the lithium ion battery.

Example 4

A method for preparing nitrogen-doped graphene/lithium iron phosphate composite material, comprising the following steps:

(1) According to the molar ratio of lithium source: phosphorus source: iron source = 1:1:1:1, weigh 0.01mol lithium acetate, 0.01mol ammonium monohydrogen phosphate, 0.01mol iron acetate and 0.01mol glucose into deionized water Preparation of dispersions by high power dispersion;

(2) Add 0.06mol citric acid during stirring, and add ammonia water to adjust pH to 11;

(3) Heating in a water bath at 80°C, stirring until gel-like, to obtain the precursor of lithium iron phosphate/graphene composite material;

(4) Transfer the material obtained in step (3) into a vacuum heating furnace, pass in high-purity ammonia gas, heat up to 600°C for 3 hours at a heating rate of 5°C/min, and then raise the temperature to 900°C for 3 hours , and finally cooled to room temperature to obtain nitrogen-doped graphene/lithium iron phosphate composites.

The obtained composite material was dispersed in N-methylpyrrolidone (NMP) to obtain a dispersion. The PVDF binder is obtained by mixing the PVDF:NMP=5:95 mass ratio, and the binder and the dispersion liquid are mixed and stirred according to the ratio of 1:8 to obtain the positive electrode slurry of the lithium ion battery.

Example 5

A method for preparing nitrogen-doped graphene/lithium iron phosphate composite material, comprising the following steps:

(1) According to the molar ratio of lithium source: phosphorus source: iron source = 1:1:1:1, weigh 0.01mol lithium acetate, 0.01mol ammonium monohydrogen phosphate, 0.01mol iron acetate and 0.01mol glucose into deionized water Preparation of dispersions by high power dispersion;

(2) Add 0.06mol citric acid during stirring, and add ammonia water to adjust pH to 11;

(3) Heating in a water bath at 90°C, stirring until gel-like, to obtain a precursor of lithium iron phosphate/graphene composite material;

(4) Transfer the material obtained in step (3) into a vacuum heating furnace, pass in high-purity ammonia gas, heat up to 600°C for 3 hours at a heating rate of 5°C/min, and then raise the temperature to 900°C for 3 hours , and finally cooled to room temperature to obtain nitrogen-doped graphene/lithium iron phosphate composites.

The obtained composite material was dispersed in N-methylpyrrolidone (NMP) to obtain a dispersion. The PVDF binder is obtained by mixing the PVDF:NMP=5:95 mass ratio, and the binder and the dispersion liquid are mixed and stirred according to the ratio of 0.5:9 to obtain the positive electrode slurry of the lithium ion battery.

The positive electrode slurry prepared in Examples 1-5 and the mesophase carbon microspheres were used as negative electrode materials, which were applied to lithium batteries and assembled into a button battery.

Table 1 The discharge capacity and coulombic efficiency of nitrogen-doped graphene/lithium iron phosphate composites of each embodiment after 100 cycles at 1 C

It can be seen from Table 1 that the nitrogen-doped graphene/lithium iron phosphate composite material prepared by the method is prepared as a positive electrode conductive paste and applied to lithium ion batteries (1-5), which has a higher capacity retention rate at high rates and Coulombic efficiency, and the specific capacity of Example 1 reaches 165mAh/g at low rate (0.5C), which is close to the theoretical specific capacity; Figure 1 is the cycle diagram of Example 1 under 1C current density for 100 cycles, prepared using this preparation method The composites maintained high capacity retention after multiple cycles.

Claims (9)

1. a preparation method of nitrogen-doped graphene/lithium iron phosphate composite material for lithium ion battery, is characterized in that comprising the following steps: (1) adding lithium source, phosphorus source, iron source and carbon source into deionized water to disperse, and prepare a dispersion liquid; (2) in the dispersion liquid of step (1), add citric acid and use ammoniacal liquor to adjust pH to 11, stir; (3) heating in a water bath at a certain temperature, stirring until gel appears; (4) In an ammonia atmosphere, the gel obtained in step (3) is calcined at high temperature to obtain a nitrogen-doped graphene/lithium iron phosphate composite material. 2 . The preparation method according to claim 1 , wherein the molar ratio between the lithium source, phosphorus source, iron source and carbon source described in step (1) is 0.8-1.2:1-1.5:1- 1.5:0:5-3. 3 . The preparation method according to claim 1 , wherein the lithium source in step (1) is one of lithium hydroxide, lithium carbonate and lithium hydrogen phosphate. 4 . 4 . The preparation method according to claim 1 , wherein the phosphorus source in step (1) is one of diammonium hydrogen phosphate, ammonium monohydrogen phosphate and phosphoric acid. 5 . 5 . The preparation method according to claim 1 , wherein the iron source in step (1) is one of ferrous sulfate, ferric phosphate, ferric oxide, ferric chloride, and ferric acetate. 6 . 6 . The preparation method according to claim 1 , wherein the carbon source in step (1) is one of glucose, fructose and sucrose. 7 . 7 . The preparation method according to claim 1 , wherein the dispersion method of the dispersion liquid in step (1) is high-power ultrasonic dispersion, and the power is 800W-1000W. 8 . 8 . The preparation method according to claim 1 , wherein the heating temperature of the water bath in step (3) is 80-100° C. 9 . 9 . The preparation method according to claim 1 , wherein the calcination process of step (4) is divided into two stages, the first stage is kept at 600°C for 3-5 hours, and the second stage is kept at 700-900°C for 3 hours. 10 . -5h, the heating rate is 3-5°C/min.

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