Lithium manganese iron phosphate / graphene positive electrode material and preparation method thereof

Abstract

The invention provides a lithium manganese iron phosphate / graphene positive electrode material and a preparation method thereof. Graphene oxide strips are obtained through a liquid nitrogen shock cooling method, in the process of synthesizing lithium manganese iron phosphate through a sol-gel method, the graphene oxide strips are uniformly dispersed in a sol precursor, and in the sintering process, thermal reduction is performed to form graphene to tightly coat lithium manganese iron phosphate, so that the lithium manganese iron phosphate / graphene positive electrode material is formed. According to the preparation method provided by the invention, a rapid conductive network is economically and effectively constructed, an effective channel is provided for lithium ion diffusion, the conductivity of the material is improved, and the prepared lithium manganese iron phosphate / graphene positive electrode material has relatively good electrochemical performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a lithium iron manganese phosphate / graphene positive electrode material and a preparation method thereof. Background Art

[0002] Positive electrode materials are the core factor restricting the development of high-performance batteries, because positive electrode materials account for a considerable proportion of the cost, weight and volume of batteries. Among various positive electrode materials for electric vehicles, olivine-type lithium iron phosphate materials have the characteristics of low cost and high safety, and are considered to be one of the most competitive materials. However, as people put forward higher requirements for the cruising range of electric vehicles. The energy density of lithium iron phosphate can no longer meet the needs because its operating voltage and theoretical capacity are not satisfactory. Lithium manganese iron phosphate material has the same safety performance as lithium iron phosphate, and has a higher discharge platform, which can provide higher energy density, so it can become an upgraded substitute for lithium iron phosphate.

[0003] However, lithium iron manganese phosphate materials have the problem of low electronic and ionic conductivity, resulting in poor electrochemical performance that makes it difficult to meet the needs of industrial production and application. Based on this, how to effectively synthesize lithium iron manganese phosphate materials and explore more economical and effective carbon coating methods to improve the electrochemical properties of lithium iron manganese phosphate materials such as cycle rate performance and energy density are technical problems that technicians in this field need to solve urgently in their research. Summary of the invention

[0004] One of the purposes of the present invention is to provide a lithium iron manganese phosphate / graphene positive electrode material. Another purpose of the present invention is to provide a method for preparing the lithium iron manganese phosphate / graphene positive electrode material.

[0005] The present invention is designed to provide a lithium iron manganese phosphate / graphene positive electrode material, in which graphene oxide strips are uniformly dispersed in a sol precursor, and graphene is thermally reduced during a sintering process to form graphene that is tightly coated on the lithium iron manganese phosphate to form the lithium iron manganese phosphate / graphene positive electrode material.

[0006] Furthermore, the lithium manganese iron phosphate / graphene positive electrode material has the general formula LiFe x Mn 1-x PO4, where 0.2≤x≤0.8;

[0007] Furthermore, the lithium manganese iron phosphate / graphene positive electrode material has small and evenly distributed particles, with a particle size range of 20nm to 250nm.

[0008] Furthermore, the mass content of graphene in the lithium manganese iron phosphate / graphene positive electrode material is 0.5% to 15%.

[0009] The present invention provides a method for preparing a lithium iron manganese phosphate / graphene positive electrode material, which specifically comprises the following steps:

[0010] S1. Preparation of graphene oxide ribbons using liquid nitrogen quenching method;

[0011] S2. The chelating agent, lithium source, manganese source, iron source and phosphorus source are mixed and dissolved in deionized water to obtain a metal salt solution;

[0012] S3. heating and stirring the metal salt solution to form a sol, adding graphene oxide strips, heating and stirring the sol to form a gel, and drying the gel to obtain a xerogel;

[0013] S4. After grinding the dry gel, sintering it in an inert atmosphere, and thermally reducing the graphene oxide to obtain the lithium manganese iron phosphate / graphene positive electrode material.

[0014] Further, in S2, the chelating agent is one or more of citric acid and ascorbic acid; the lithium source is one or more of lithium hydroxide, lithium carbonate, lithium nitrate, and lithium acetate; the manganese source is one or more of manganese acetate, manganese oxalate, manganese nitrate, and manganese dihydrogen phosphate; the iron source is one or more of ferric sulfate, ferric phosphate, ferrous phosphate, ferric chloride, ferrous chloride, and ferric nitrate; and the phosphorus source is one or more of phosphoric acid, lithium dihydrogen phosphate, and ammonium dihydrogen phosphate.

[0015] Furthermore, in S3, the sol is heated and stirred at a temperature of 40 to 100°C, and the stirring time is 1 to 12 hours.

[0016] Further, in S4, the inert atmosphere is one or more of helium, argon or nitrogen, the sintering temperature is 350-800°C, the sintering time is 3-24 hours, the heating rate is 2-10°C / min, and segmented sintering can be performed during the sintering process.

[0017] Furthermore, the preparation of the graphene strips is carried out in accordance with our authorized patent (patent authorization number: CN105947973B), and the specific preparation parameter conditions are slightly adjusted: commercially purchased graphene oxide can be used or the improved Hummers method can be used to prepare graphene oxide, deionized water is used to prepare the graphene oxide dispersion, the concentration of the graphene oxide dispersion is 0.1-0.7 mg / mL, and then the graphene oxide dispersion is sprayed into liquid nitrogen at a rate of 5 mL / min, and the graphene oxide strips are obtained after freeze drying, and the graphene oxide strips are evenly dispersed in the metal salt sol, and the amount of graphene oxide strips added is 0.5% to 20% of the mass of the product lithium iron manganese phosphate / graphene positive electrode material, and thermal reduction is achieved in the subsequent sintering process to obtain the lithium iron manganese phosphate / graphene positive electrode material.

[0018] The present invention is designed to provide a lithium iron manganese phosphate / graphene positive electrode material and a preparation method thereof. The method economically and effectively constructs a fast conductive network, provides an effective channel for lithium ion diffusion, improves the material conductivity, and the prepared lithium iron manganese phosphate / graphene positive electrode material has good electrochemical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0020] Figure 1 A scanning electron microscope (SEM) image of a graphene oxide ribbon prepared in an embodiment of the present invention;

[0021] Figure 2 This is a scanning electron microscope (SEM) image of the lithium iron manganese phosphate / graphene positive electrode material prepared in an embodiment of the present invention;

[0022] Figure 3 This is the XRD diagram of the lithium manganese iron phosphate / graphene positive electrode material prepared in an embodiment of the present invention. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0024] Example 1

[0025] This embodiment provides a lithium manganese iron phosphate / graphene positive electrode material and a preparation method thereof, comprising the following steps:

[0026] S1. Using commercial graphene oxide; ultrasonically treating graphene oxide in deionized water to form a graphene oxide solution with a concentration of 0.3 mg / mL; spraying the graphene oxide dispersion into liquid nitrogen at a rate of 5 mL / min, and freeze-drying to obtain graphene oxide strips;

[0027] S2. Add a chelating agent, a lithium source, a manganese source, an iron source, and a phosphorus source in a stoichiometric ratio, and stir until all are dissolved to obtain a metal salt solution; citric acid, lithium hydroxide, manganese dihydrogen phosphate, iron phosphate, and phosphoric acid;

[0028] S3. The metal salt solution is heated and stirred to form a sol, and graphene oxide strips are added, wherein the amount of graphene oxide strips added is 1% of the mass of the product lithium manganese iron phosphate, and the sol is heated and stirred to form a gel, and the suspected gel is dried to obtain a xerogel;

[0029] S4. After grinding the above dry gel, sintering it in a nitrogen atmosphere, the sintering is divided into two stages, the first stage sintering temperature is 350 ° C, the sintering time is 6 hours, the heating rate is 5 ° C / min, the second stage sintering temperature is 700 ° C, the sintering time is 12 hours, the heating rate is 5 ° C / min, and finally cooled to room temperature, washed with deionized water, and finally dried to obtain lithium manganese iron phosphate / graphene positive electrode material LiFe 0.8 Mn 0.2 PO4 / C;

[0030] Example 2

[0031] A method for preparing a lithium manganese iron phosphate / graphene positive electrode material, which differs from Example 1 only in that: in step S3, the addition amount of graphene oxide strips accounts for 5% of the total solid addition amount.

[0032] Comparative Example 1

[0033] A method for preparing a lithium iron manganese phosphate / graphene positive electrode material, which differs from Example 1 only in that: in step S3, the amount of graphene oxide strips added accounts for 0% of the total solid addition amount

[0034] In order to verify the beneficial effects of the technical solution of the embodiment of the present application, the lithium iron manganese phosphate / graphene positive electrode material, conductive agent, and binder prepared in the above embodiments and comparative examples were mixed and stirred in N-methylpyrrolidone (NMP) at a mass ratio of 80:10:10 to prepare a mixed slurry, and the mixed slurry was coated on a carbon-coated aluminum foil and dried to obtain a positive electrode sheet, which was assembled with a separator and a negative metal lithium sheet into a 2032 button half-cell for electrochemical performance testing; wherein the surface density of the positive electrode sheet is 1.5 mg / cm 2 .

[0035] The test environment temperature was 25°C, the test voltage range was 2.5V-4.5V, and the charge and discharge tests were conducted at 0.1C, 0.2C, 0.5C, and 1C rates. The test results are shown in Table 1.

[0036]

[0037] As can be seen from Table 1, relative to Example 1, the amount of graphene oxide strips added in Example 2 is increased, so the proportion of graphene in the prepared lithium iron manganese phosphate / graphene positive electrode material is increased, the discharge specific capacity is slightly reduced, the conductivity is improved, and it has better rate performance; relative to Comparative Example 1 without adding graphene oxide strips, Examples 1 and 2 both show higher conductivity, and the three-dimensional ribbon structure of the graphene oxide strips constructs a fast conductive network, thereby obtaining excellent electrochemical performance.

[0038] Figure 1 This is an SEM image of the above-mentioned graphene oxide strip, showing that it has a three-dimensional ribbon structure. During the synthesis process, it is evenly dispersed in the sol precursor and dried. In the subsequent sintering process, its three-dimensional ribbon structure is used as a framework for thermal reduction to form graphene and tightly wrap around lithium iron manganese phosphate to form a lithium iron manganese phosphate / graphene positive electrode material.

[0039] Figure 2 This is a SEM image of the above-mentioned lithium iron manganese phosphate / graphene positive electrode material, which shows that the particles of the lithium iron manganese phosphate / graphene positive electrode material are small and evenly distributed, with a particle size range of 20nm to 250nm.

[0040] Figure 3 This is the XRD diagram of the above-mentioned lithium manganese iron phosphate / graphene positive electrode material. By comparing the intensity and position of the diffraction peak with the standard card (PDF#13-0336), it is proved that the prepared material is lithium manganese iron phosphate.

[0041] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. A lithium manganese iron phosphate / graphene positive electrode material, characterized in that: The graphene oxide strips are evenly dispersed in the sol precursor and are thermally reduced during the sintering process to form graphene tightly wrapped around the lithium manganese iron phosphate.

2. The lithium iron manganese phosphate / graphene composite material according to claim 1, wherein the general formula is LiFe x Mn 1-x PO4, wherein 0.2≤x≤0.8; the particle size of the lithium iron manganese phosphate particles is 20nm~250nm.

3. The lithium iron manganese phosphate / graphene composite material according to claim 1, characterized in that: The mass content of graphene in the lithium iron manganese phosphate / graphene composite material is 0.5% to 15%.

4. A method for preparing a lithium manganese iron phosphate / graphene positive electrode material, characterized in that: The method comprises the following steps: S1. Preparation of graphene oxide ribbons using liquid nitrogen quenching method; S2. The chelating agent, lithium source, manganese source, iron source and phosphorus source are mixed and dissolved in deionized water to obtain a metal salt solution; S3. heating and stirring the metal salt solution to form a sol, adding graphene oxide strips, heating and stirring the sol to form a gel, and drying the gel to obtain a xerogel; S4. Grind the dry gel and sinter it under an inert atmosphere to obtain the lithium manganese iron phosphate / graphene positive electrode material.

5. The method for preparing the lithium iron manganese phosphate / graphene positive electrode material according to claim 4, characterized in that: In S1, a liquid nitrogen quenching method is used to obtain graphene oxide strips, wherein the graphene oxide strips have a length range of 0.1 to 250 μm and a width range of 0.01 to 1 μm.

6. The method for preparing the lithium manganese iron phosphate / graphene positive electrode material according to claim 4, characterized in that: In S2, the chelating agent is one or more of citric acid and ascorbic acid; the lithium source is one or more of lithium hydroxide, lithium carbonate, lithium nitrate, and lithium acetate; the manganese source is one or more of manganese acetate, manganese oxalate, manganese nitrate, and manganese dihydrogen phosphate; the iron source is one or more of ferric sulfate, ferric phosphate, ferrous phosphate, ferric chloride, ferrous chloride, and ferric nitrate; and the phosphorus source is one or more of phosphoric acid, lithium dihydrogen phosphate, and ammonium dihydrogen phosphate.

7. The method for preparing the lithium iron manganese phosphate / graphene positive electrode material according to claim 4, characterized in that: In the step S3, the amount of graphene oxide strips added is 0.5% to 20% of the mass of the product lithium iron manganese phosphate / graphene positive electrode material, the sol heating and stirring temperature is 50 to 100° C., and the stirring time is 1 to 12 hours.

8. The method for preparing the lithium iron manganese phosphate / graphene positive electrode material according to claim 4, characterized in that: In S4, the inert atmosphere is one or more of helium, argon or nitrogen, the sintering temperature is 350-800°C, the sintering time is 3-24 hours, and the heating rate is 2-10°C / min.

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