Lithium iron phosphate / graphene composite cathode material and preparation method thereof

Abstract

The application discloses a lithium iron phosphate / graphene composite positive electrode material and a preparation method thereof. Based on the electrostatic adsorption principle, PAH, PSS and LiFePO4 are sequentially adsorbed on the surface of graphene oxide to obtain the final product through calcination. During the calcination process, on one hand, the non-conductive graphene oxide is reduced to high-conductive graphene, and the reduction of the graphene oxide creates many microscopic pore defects on the surface of the graphene, which is convenient for the back-and-forth shuttling of lithium ions; on the other hand, the electrostatic layer of PAH / PSS can be carbonized and used as a reducing agent for reducing Fe3 + to Fe 2+ during the calcination of lithium iron phosphate. The application can prepare the material with a particle size of 2-100 nm, greatly reduces the particle size of lithium iron phosphate, reduces the transmission path of electrons and lithium ions, and graphene can be used as a carrier of lithium iron phosphate to play a current collecting role and improve the power performance of the lithium iron phosphate positive electrode.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery materials, specifically to a lithium iron phosphate / graphene composite cathode material and its preparation method. Background Technology

[0002] Lithium-ion batteries have gained increasing attention due to their high energy density, environmental friendliness, and lack of memory effect. Since Sony released the first commercial lithium-ion battery in 1991, it has become increasingly integrated into people's lives, bringing about a technological revolution. Especially in recent years, against the backdrop of fossil fuel depletion and environmental pollution, new energy vehicles powered by lithium-ion batteries have gradually come into focus and are widely used. Commercial lithium-ion batteries generally use natural graphite, artificial graphite, silicon, etc., as negative electrodes, and lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, ternary lithium, etc., as positive electrode materials. Lithium iron phosphate and ternary lithium are the most widely used in power batteries. Ternary lithium has the advantage of high energy density, but its high cost and poor safety performance have limited its further development. Lithium iron phosphate, due to its stable structure, good safety performance, readily available raw materials, low price, and long cycle life, has gradually become the positive electrode material with the highest installed capacity. However, its inherently low electronic conductivity limits its application. There are currently two main solutions: ① Nanostructuring: reducing the particle size of the material to shorten the electron conduction path; ② Carbon coating: improving electronic conductivity through carbon coating, while the coating layer can also alleviate the volume expansion caused by the lithium insertion process. However, these solutions have not yet fully solved the problem.

[0003] Chinese patent CN 109935803 A discloses a method for preparing lithium iron phosphate cathode material. The method involves adding a graphene dispersion to a lithium iron phosphate precursor solution and sonicating at 70-90W for 1-3 hours to obtain a graphene-lithium iron phosphate precursor solution. The graphene-lithium iron phosphate precursor solution is then placed in an oil bath at 160-190℃ and reacted for 15-20 hours. Afterward, it is naturally cooled to room temperature, and the precipitate is washed multiple times with deionized water and dried at 30-50℃ for 12-15 hours to obtain the graphene-lithium iron phosphate precursor. This preparation method is a physical mixing process. There is no bond between graphene and lithium iron phosphate, and the large density difference between them easily leads to uneven mixing, poor contact between graphene and lithium iron phosphate, and high interfacial resistance. Summary of the Invention

[0004] This invention aims to solve the problem of poor electronic conductivity of lithium iron phosphate and proposes a method for preparing lithium iron phosphate / graphene composite cathode materials through electrostatic adsorption layer-by-layer self-assembly.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for preparing a lithium iron phosphate / graphene composite cathode material includes the following steps:

[0007] Graphene oxide (GO) was prepared using the Hummers method.

[0008] GO is dispersed in water by ultrasonication to form a GO aqueous solution; preferably, the concentration of the GO aqueous solution is 0.01-1 mg / mL;

[0009] Under stirring conditions, an aqueous solution of polyallyl ammonium chloride (PAH) is added dropwise to a GO aqueous solution, followed by filtration, washing, and low-temperature drying. Since the surface of GO contains abundant carboxyl, hydroxyl, and epoxy groups, it carries a negative charge, while PAH contains large molecular weight positively charged groups. GO and PAH electrostatically assemble to form a GO / PAH complex. PAH with excessively large molecular weights is difficult to dissolve, while those with excessively low molecular weights have weak electrostatic adsorption capabilities. Therefore, preferably, the molecular weight of the PAH is 50,000-80,000 Da, the solubility of the PAH aqueous solution is 0.1-1 mg / mL, and the low-temperature drying temperature is 40-60℃.

[0010] The GO / PAH complex was ultrasonically dispersed in water, and an aqueous solution of sodium polystyrene sulfonate (PSS) was added dropwise under stirring. The mixture was then filtered, washed, and dried at low temperature. Because the GO / PAH complex has a positively charged surface, while PSS contains large negatively charged molecular weight groups, GO / PAH and PSS electrostatically assemble to form a GO / PAH / PSS complex. Since excessively large molecular weight PSS is difficult to dissolve, and excessively low molecular weight PSS has weak electrostatic adsorption capacity, preferably, the molecular weight of the PSS is 50,000-80,000 Da, the solubility of the PSS aqueous solution is 0.1-1 mg / mL, and the low-temperature drying temperature is 40-60℃.

[0011] The GO / PAH / PSS complex was ultrasonically dispersed in water, and a ferric salt solution was added dropwise under stirring. Because the GO / PAH / PSS complex has a negatively charged surface, Fe... 3+ Carrying a positive charge, the two electrostatically adsorb and assemble to form GO / PAH / PSS / Fe. 3+ Complex. As phosphate solution is continuously added dropwise to this solution system, the negatively charged phosphate ions can react with GO / PAH / PSS / Fe. 3+A GO / PAH / PSS / FePO4 complex is formed through electrostatic adsorption assembly and reaction. Lithium salt is then added to this solution system, followed by grinding in a sand mill and spray drying to obtain the GO / PAH / PSS / LiFePO4 complex. Preferably, the trivalent iron salt is at least one of ferric nitrate, ferric acetate, ferric chloride, and ferric sulfate; the phosphate is at least one of diamine hydrogen phosphate and ammonium dihydrogen phosphate; and the lithium salt is at least one of lithium hydroxide, lithium carbonate, lithium acetate, and lithium oxalate. The molar ratio of the iron salt, phosphate, and lithium salt is 0.9-1.1:0.9-1.1:0.9-1.1.

[0012] The GO / PAH / PSS / LiFePO4 composite was calcined under nitrogen protection. GO was reduced to graphene, increasing its conductivity. The PAH / PSS electrostatic layer, being a polymer, was carbonized at high temperature under N2 protection. Carbon at high temperatures exhibits reducing properties and can undergo a carbothermic reaction, acting as a Fe... 3+ Reduced to Fe 2+ The reducing agent, LiFePO4, transforms from an amorphous state to a crystalline state. The composite material is then subjected to airflow pulverization and classification screening to obtain the final product. Conventional lithium iron phosphate production sintering processes require a N2+H2 mixture, with H2 as the reducing agent. This invention uses carbon as the reducing agent, unlike existing technologies. Preferably, the calcination temperature is 500-1000℃, the calcination time is 2-20 hours, and the particle size of the final product is 2-100 nm.

[0013] The beneficial effects of this invention are as follows:

[0014] The lithium iron phosphate / graphene composite cathode material prepared in this invention uses graphene oxide as a carrier. Through layer-by-layer electrostatic self-assembly modification, the graphene oxide surface acquires negative charges from macromolecular groups, resulting in strong electrostatic adsorption capacity. Lithium iron phosphate nuclei are grown on the graphene surface, and the crystal size is controlled by regulating the adsorption process. The final heat treatment reduces the non-conductive graphene oxide to highly conductive graphene. Simultaneously, the reduction of graphene oxide creates numerous micropores on the graphene surface, facilitating the shuttle movement of lithium ions. Furthermore, it carbonizes the electrostatic layer of PAH / PSS, which is then used to calcine Fe3+ during lithium iron phosphate calcination. + Reduced to Fe 2+ The reducing agent. By controlling the electrostatic adsorption time, the particle size can be controlled within 2-100nm, which greatly reduces the particle size of lithium iron phosphate particles and reduces the transport path of electrons and lithium ions. At the same time, graphene, as a carrier of lithium iron phosphate, plays a current collection role, which can improve the power performance of lithium iron phosphate cathode. Attached Figure Description

[0015] Figure 1This is a scanning electron microscope image of the lithium iron phosphate / graphene composite cathode material prepared in Example 1;

[0016] Figure 2 The discharge capacity of batteries made from the materials prepared in Example 1 and the comparative example of the present invention is compared at different discharge rates. Detailed Implementation

[0017] The present invention will be further described below with reference to embodiments, so that those skilled in the art can better understand and implement the present invention, but the embodiments are not intended to limit the present invention.

[0018] In addition, unless otherwise specified, the preparation processes in the following embodiments are all conventional methods in the prior art, and therefore will not be described in detail.

[0019] Example 1:

[0020] A method for preparing lithium iron phosphate / graphene composite cathode materials by electrostatic layer-by-layer self-assembly includes the following steps:

[0021] Graphene oxide (GO) was prepared using the Hummers method.

[0022] 50 mg of GO was dispersed in 100 mL of water by ultrasonication to form an aqueous solution of GO with a concentration of 0.5 mg / mL;

[0023] Under stirring conditions, 100 mL of a 0.5 mg / mL polyallyl ammonium chloride (PAH, Mw = 60000 Da) aqueous solution was added dropwise to the GO aqueous solution, followed by filtration, washing, and drying at 50 °C. Because the GO surface contains abundant carboxyl, hydroxyl, and epoxy groups, it carries a negative charge, while PAH contains large positively charged molecular weight groups. GO and PAH electrostatically assemble to form a GO / PAH complex.

[0024] 50 mg of the GO / PAH complex was ultrasonically dispersed in 100 mL of water. While stirring, 100 mL of a 0.5 mg / mL sodium polystyrene sulfonate (PSS, Mw = 70000 Da) aqueous solution was added dropwise. The mixture was then filtered, washed, and dried at 50 °C. Because the GO / PAH complex has a positive surface charge, while PSS contains large negatively charged molecular groups, GO / PAH and PSS electrostatically assemble to form a GO / PAH / PSS complex.

[0025] 50 mg of the GO / PAH / PSS complex was ultrasonically dispersed in water, and 100 mL of a 0.5 mol / L ferric nitrate aqueous solution was added dropwise under stirring. The adsorption time was 60 min. Because the GO / PAH / PSS complex has a negative charge on its surface, Fe... 3+Carrying a positive charge, the two electrostatically assemble to form GO / PAH / PSS / Fe 3+ Complex. 100 mL of 0.5 mol / L diamine hydrogen phosphate aqueous solution was added dropwise to this solution system, and the mixture was stirred for 60 min for adsorption. Because the phosphate group carries a negative charge, it can react with GO / PAH / PSS / Fe. 3+ A GO / PAH / PSS / FePO4 complex was formed through electrostatic assembly and reaction. 100 mL of a 0.5 mol / L lithium carbonate aqueous solution was then added to this solution, followed by grinding in a sand mill and spray drying to obtain the GO / PAH / PSS / LiFePO4 complex.

[0026] The GO / PAH / PSS / LiFePO4 composite was calcined at 800℃ for 10 h under nitrogen protection. GO was reduced to graphene, enhancing its conductivity, while the PAH / PSS electrostatic layer was carbonized and acted as Fe. 3+ Reduced to Fe 2+ The reducing agent, LiFePO4, transforms from an amorphous state to a crystalline state. The composite material is then subjected to air jet milling and classification sieving to obtain the final product with a particle size of 20 nm.

[0027] Example 2:

[0028] A method for preparing lithium iron phosphate / graphene composite cathode materials by electrostatic layer-by-layer self-assembly includes the following steps:

[0029] Graphene oxide (GO) was prepared using the Hummers method.

[0030] 40 mg of GO was dispersed in 100 mL of water by ultrasonication to form an aqueous solution of GO with a concentration of 0.4 mg / mL;

[0031] Under stirring conditions, 100 mL of a 0.4 mg / mL polyallyl ammonium chloride (PAH, Mw = 70000 Da) aqueous solution was added dropwise to the GO aqueous solution, followed by filtration, washing, and drying at 60 °C. Because the GO surface contains abundant carboxyl, hydroxyl, and epoxy groups, it carries a negative charge, while PAH contains large molecular positively charged groups. GO and PAH electrostatically assemble to form a GO / PAH complex.

[0032] 40 mg of the GO / PAH complex was ultrasonically dispersed in 100 mL of water. While stirring, 100 mL of a 0.4 mg / mL sodium polystyrene sulfonate (PSS, Mw = 60000 Da) aqueous solution was added dropwise. The mixture was then filtered, washed, and dried at 60 °C. Because the GO / PAH complex has a positive surface charge, while PSS contains large negatively charged molecular weight groups, GO / PAH and PSS electrostatically assemble to form a GO / PAH / PSS complex.

[0033] 40 mg of the GO / PAH / PSS complex was ultrasonically dispersed in water, and 100 mL of a 0.4 mol / L ferric nitrate aqueous solution was added dropwise under stirring. The adsorption time was 75 min. Due to the negative charge on the surface of the GO / PAH / PSS complex, Fe... 3+ Carrying a positive charge, the two electrostatically assemble to form GO / PAH / PSS / Fe 3+ Complex. 100 mL of 0.4 mol / L diamine hydrogen phosphate aqueous solution was added dropwise to this solution system, and the mixture was stirred for 75 min for adsorption. Because the phosphate group carries a negative charge, it can react with GO / PAH / PSS / Fe. 3+ A GO / PAH / PSS / FePO4 complex was formed through electrostatic assembly and reaction. 100 mL of a 0.4 mol / L lithium carbonate aqueous solution was then added to this solution, followed by grinding in a sand mill and spray drying to obtain the GO / PAH / PSS / LiFePO4 complex.

[0034] The GO / PAH / PSS / LiFePO4 composite was calcined at 1000℃ for 8 hours under nitrogen protection. GO was reduced to graphene, enhancing its conductivity. The PAH / PSS electrostatic layer was carbonized and acted as Fe. 3+ Reduced to Fe 2+ The reducing agent, LiFePO4, transforms from an amorphous state to a crystalline state. The composite material is then subjected to air jet milling and classification sieving to obtain the final product with a particle size of 10 nm.

[0035] Comparative Example

[0036] Compared with Example 1, the difference in the comparative example is that the process of adding polyallyl ammonium chloride aqueous solution and sodium polystyrene sulfonate aqueous solution is omitted in the preparation process. That is, the GO aqueous solution is directly mixed with the subsequent ferric nitrate aqueous solution, and then the subsequent steps are carried out to prepare lithium iron phosphate / graphene composite cathode material.

[0037] Performance characterization and testing

[0038] Figure 1 This is a scanning electron microscope (SEM) image of the lithium iron phosphate / graphene composite cathode material prepared in Example 1. Figure 1 It can be seen that lithium iron phosphate particles of about 10 nm are uniformly loaded on the graphene surface, and there are no lithium iron phosphate particles existing alone on the outside of the graphene.

[0039] Batteries were assembled using lithium iron phosphate / graphene composite cathode materials as a comparative example. Except for the cathode material, all other materials were the same. The discharge capacity of the batteries was tested at different rates. It was observed that graphene loading significantly improved the electronic and ionic conductivity of lithium iron phosphate nanoparticles, greatly enhancing the rate performance of the batteries.

[0040] Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing a lithium iron phosphate / graphene composite cathode material, characterized in that: Includes the following steps: (1) Graphene oxide and polyallyl ammonium chloride are dispersed in water, mixed evenly and stirred, and then the water in the system is removed to obtain GO / PAH composite; the molecular weight of polyallyl ammonium chloride is 50,000-80,000 Da. (2) Disperse the GO / PAH complex in water, then add sodium polystyrene sulfonate to the water, mix evenly and stir, then remove the water from the system to obtain the GO / PAH / PSS complex; the molecular weight of the sodium polystyrene sulfonate is 50,000-80,000 Da; (3) Disperse the GO / PAH / PSS complex in water, add ferric salt and mix evenly, then stir and electrostatically adsorb, then add phosphate, mix evenly and continue stirring and electrostatically adsorb, then add lithium salt to obtain a mixture; the mixture is then ground and spray dried to obtain the GO / PAH / PSS / LiFePO4 complex. (4) The GO / PAH / PSS / LiFePO4 composite was calcined under a protective atmosphere and then crushed and screened to obtain the final product.

2. The method for preparing the lithium iron phosphate / graphene composite cathode material according to claim 1, characterized in that: In step (3), the ferric salt is at least one of ferric nitrate, ferric acetate, ferric chloride, and ferric sulfate. The adsorption time for electrostatic adsorption after adding the ferric salt and mixing it evenly is 30-120 min. The adsorption time for electrostatic adsorption after adding the phosphate and mixing it evenly is 30-120 min.

3. The method for preparing the lithium iron phosphate / graphene composite cathode material according to claim 1, characterized in that: In step (3), the phosphate is at least one of diammonium hydrogen phosphate and diammonium dihydrogen phosphate.

4. The method for preparing the lithium iron phosphate / graphene composite cathode material according to claim 1, characterized in that: In step (3), the lithium salt is at least one of lithium hydroxide, lithium carbonate, lithium acetate, and lithium oxalate.

5. The method for preparing the lithium iron phosphate / graphene composite cathode material according to any one of claims 2 to 4, characterized in that: The molar ratio of the trivalent iron salt, phosphate, and lithium salt is 0.9-1.1 : 0.9-1.1 : 0.9-1.

1.

6. The method for preparing the lithium iron phosphate / graphene composite cathode material according to claim 1, characterized in that: In step (4), the calcination temperature is 500-1000℃ and the calcination time is 2-20 hours.

7. The method for preparing the lithium iron phosphate / graphene composite cathode material according to claim 1, characterized in that: In step (4), the particle size of the final product is 2-100 nm.

8. A lithium iron phosphate / graphene composite cathode material, characterized in that: It is prepared by the preparation method as described in any one of claims 1 to 7.

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