Lithium manganese iron phosphate positive electrode material and preparation method thereof

Abstract

The invention discloses a lithium manganese iron phosphate positive electrode material and a preparation method thereof. The invention relates to a preparation method of a lithium iron manganese phosphate-based material, which is characterized by comprising the following steps: dissolving a manganese source, an iron source, a cobalt source, a phosphorus source and sodium citrate in a mixed solution of ethanol / water in a volume ratio of (2-3): 1; after uniformly mixing, transferring the mixture into a high-pressure reaction kettle for reaction; dispersing the precursor in a Tris-HCl buffer solution with the pH value of 8-9, adding dopamine hydrochloride and NH4F, uniformly mixing, and drying; in an inert atmosphere, ball-milling and mixing the product and a lithium source, and calcining in two steps: in the first stage, preserving heat at 300-350 DEG C for 1-2 hours so as to carbonize the polydopamine to form a nitrogen-doped carbon layer; in the second stage, heat preservation is conducted for 1-2 h at the temperature of 500-650 DEG C, so that the lithium iron manganese phosphate-based positive electrode material is obtained, and the lithium iron manganese phosphate-based positive electrode material has excellent capacity performance and cycle performance when applied to a lithium ion battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion batteries, in particular to a lithium ion battery positive electrode material lithium manganese iron phosphate and a preparation method thereof. BACKGROUND

[0002] Lithium ion secondary batteries, as a kind of clean, efficient and renewable secondary batteries, are widely used in electronic products, electric vehicles, power grid energy storage and other fields. Lithium ion batteries are mainly composed of four main parts: positive electrode material, negative electrode material, separator and electrolyte. Among them, the positive electrode material is the core component of lithium ion batteries, and its performance directly affects the energy density, cycle stability and safety of the battery. Lithium manganese iron phosphate (LMFP) is a derivative material of lithium iron phosphate (LiFePO4, LFP), which is considered as an important candidate for the next generation of lithium battery anodes due to its high voltage and low cost, high structural stability and many other advantages. However, lithium manganese iron phosphate faces the following problems in practical application: (1) low electronic conductivity and lithium ion conductivity of LMFP, resulting in poor high-rate performance; (2) Jahn-Teller effect of manganese ions causes lattice distortion, and Mn 2+ is dissolved during charging and discharging, leading to capacity decay and poor cycle performance; (3) electrolyte is oxidized and decomposed at high voltage (>4.0V), generating HF to corrode active materials and accelerate Mn / Fe dissolution. To solve the above problems, the prior art proposes various improvement methods, for example, the prior art CN120136084A discloses a carbon nanotube modified lithium manganese iron phosphate material, application and preparation method. The method mixes a solution containing lithium, phosphorus, iron and manganese with carbon nanotubes and heats to obtain lithium manganese iron phosphate powder, and then sintering to generate the carbon nanotube modified lithium manganese iron phosphate material in situ. The method uses the excellent conductivity of carbon nanotubes to generate lithium manganese iron phosphate cathode material in situ, and the carbon nanotubes can form a uniform conductive network, significantly reducing the internal resistance of the material and improving the charging and discharging efficiency of the material; the prior art CN120109189A discloses a double-site co-doped lithium manganese iron phosphate-based positive electrode material and a preparation method and application thereof. The lithium manganese iron phosphate-based positive electrode material includes sodium and lanthanum co-doped lithium manganese iron phosphate. In the lithium manganese iron phosphate-based positive electrode material, the molar ratio of lithium to sodium is (1-x):x, and x is in the range of 0.002-0.02. The double-site co-doped lithium manganese iron phosphate-based positive electrode material has high electronic conductivity and lithium ion diffusion rate, and has excellent rate performance and cycle stability. However, the above prior art only improves the performance of lithium manganese iron phosphate from a single perspective, so the product is difficult to meet the actual application requirements. It is of great practical significance to develop a lithium manganese phosphate material with simple preparation process, excellent electrochemical performance and low cost for industrial production of the material. SUMMARY

[0003] The application aims to provide a preparation method of a lithium ion battery positive electrode lithium manganese iron phosphate, and the method is obtained by multi-angle regulation and has a stable structure, high lithium ion conductivity and electronic conductivity.

[0004] (1) A manganese source, an iron source, a cobalt source, a phosphorus source and sodium citrate are dissolved in a mixed solution of ethanol / water in a molar ratio of 1:1:(0.05-0.1):1:(2-3), wherein the volume ratio of ethanol / water is 2-3:1; after uniform mixing, the mixture is transferred to a high-pressure reaction kettle, and reacted at 180-200 DEG C for 18-20 h to obtain a precursor;

[0005] (2) The precursor is dispersed in a Tris-HCl buffer solution with a pH of 8-9, and dopamine hydrochloride is added; then NH4F is added, and the mixture is mixed for 5-10 h and dried;

[0006] (3) The product of step (2) is mixed with a lithium source under an inert atmosphere, and then calcined in two steps: the first stage is 300-350 DEG C for 1-2 h; the second stage is 500-650 DEG C for 1-2 h, thereby obtaining a lithium manganese iron phosphate-based positive electrode material.

[0007] Further, the manganese source is one or more of manganese sulfate, manganese acetate and manganese nitrate;

[0008] Further, the iron source is one or more of iron sulfate, iron acetate, iron nitrate and iron chloride;

[0009] Further, the cobalt source is one or more of cobalt nitrate, cobalt acetate, cobalt sulfate and cobalt chloride;

[0010] Further, the molar ratio of the phosphorus source and NH4F is 1:(0.1-0.2);

[0011] Further, the lithium source is at least one of lithium carbonate and lithium hydroxide;

[0012] Further, the phosphorus source is ammonium dihydrogen phosphate or di-ammonium hydrogen phosphate;

[0013] Further, the inert atmosphere is nitrogen or argon;

[0014] The application can achieve the following technical effects:

[0015] (1) The lithium manganese iron phosphate is doped with Co, Co replaces part of the Mn / Fe sites, can stabilize the crystal framework of the lithium manganese iron phosphate, and reduce the volume change in the charging and discharging process; defects are introduced by doping, and the electronic conductivity of the lithium iron phosphate is effectively improved;

[0016] (2) The flaky lithium manganese iron phosphate (1-1.5 microns) has a high specific surface area, providing a fast channel for the diffusion of lithium ions;

[0017] (3) NH3 and HF produced by the decomposition of NH4F, HF will preferentially react with the surface LMFP to form a F-doped or surface fluoride layer, and the surface F - forms a strong bond with Mn / Fe, inhibiting the dissolution of Mn 2+ / Fe

[0018] 2+ in the charging and discharging process; NH3 can be used as a nitrogen-doped carbon source to enhance the electrical conductivity.

[0019] (4) The carbon layer formed after the carbonization of polydopamine can effectively improve the electronic conductivity of lithium manganese iron phosphate.

[0020] (5) The preparation process is simple, the cost is low, and industrialization is easy to realize. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 SEM image of the lithium manganese iron phosphate material prepared in the present application. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical scheme and advantages of the present application more clear and explicit, the technical scheme of the present application will be further illustrated by specific examples below, which only help to understand the present application and should not be regarded as a specific limitation of the present application.

[0023] Example 1

[0024] (1) Dissolve 10 mmol of manganese nitrate, 10 mmol of iron nitrate, 1 mmol of cobalt nitrate and 10 mmol of ammonium dihydrogen phosphate, 20 mmol of sodium citrate in a mixed solution of ethanol / water, wherein the volume ratio of ethanol / water is 2:1; after mixing evenly, transfer to a high-pressure reaction kettle, react at 180℃ for 18h to obtain a precursor;

[0025] (2) Disperse the precursor in Tris-HCl buffer solution with pH of 8, add 0.15g of dopamine hydrochloride, mix evenly, then add 1mmol of NH4F, mix for 5h, and dry;

[0026] (3) Under an inert atmosphere, mix the product of step (2) with 10 mmol of lithium hydroxide by ball milling, and then calcine in two steps: the first stage is 300℃ for 1h, and the second stage is 500℃ for 1h, thereby obtaining a lithium manganese iron phosphate-based positive electrode material.

[0027] Example 2

[0028] (1) 10 mmol manganese nitrate, 10 mmol iron nitrate, 1 mmol cobalt nitrate and 10 mmol ammonium dihydrogen phosphate, 25 mmol sodium citrate were dissolved in a mixture of ethanol / water, wherein the volume ratio of ethanol / water was 3:1; after mixing evenly, it was transferred to a high-pressure reaction kettle, reacted at 180°C for 18h to obtain a precursor;

[0029] (2) The precursor was dispersed in a Tris-HCl buffer solution with a pH of 8, 0.15 g of dopamine hydrochloride was added, then 1 mmol of NH4F was added, and after mixing for 5h, drying treatment was performed;

[0030] (3) The product of step (2) was mixed with 10 mmol lithium hydroxide under an inert atmosphere, and then calcined in two steps, the first stage: 300°C for 1h, the second stage: 500°C for 1h, thereby obtaining a lithium manganese iron phosphate-based positive electrode material.

[0031] Example 3

[0032] (1) 10 mmol manganese nitrate, 10 mmol iron nitrate, 1 mmol cobalt nitrate and 10 mmol ammonium dihydrogen phosphate, 25 mmol sodium citrate were dissolved in a mixture of ethanol / water, wherein the volume ratio of ethanol / water was 3:1; after mixing evenly, it was transferred to a high-pressure reaction kettle, reacted at 180°C for 18h to obtain a precursor;

[0033] (2) The precursor was dispersed in a Tris-HCl buffer solution with a pH of 9, 0.15 g of dopamine hydrochloride was added, then 1 mmol of NH4F was added, and after mixing evenly, drying treatment was performed;

[0034] (3) The product of step (2) was mixed with 10 mmol lithium hydroxide under an inert atmosphere, and then calcined in two steps, the first stage: 300°C for 1h, the second stage: 500°C for 1h, thereby obtaining a lithium manganese iron phosphate-based positive electrode material.

[0035] Comparative Example 1

[0036] (1) 10 mmol manganese nitrate, 10 mmol iron nitrate, 10 mmol ammonium dihydrogen phosphate, 20 mmol sodium citrate were dissolved in a mixture of ethanol / water, wherein the volume ratio of ethanol / water was 2:1; after mixing evenly, it was transferred to a high-pressure reaction kettle, reacted at 180°C for 18h to obtain a precursor;

[0037] (2) The precursor was dispersed in a Tris-HCl buffer solution with a pH of 8, 0.15 g of dopamine hydrochloride was added, then 1 mmol of NH4F was added, and after mixing for 5h, drying treatment was performed;

[0038] (3) Under inert atmosphere, the product of step (2) is mixed with 10 mmol lithium hydroxide by ball milling, and then calcined in two steps, the first stage: 300℃ for 1 h, the second stage: 500℃ for 1 h, thus obtaining the lithium manganese iron phosphate-based positive electrode material.

[0039] Comparative Example 2

[0040] (1) 10 mmol manganese nitrate, 10 mmol iron nitrate, 1 mmol cobalt nitrate and 10 mmol ammonium dihydrogen phosphate, 20 mmol sodium citrate are dissolved in a mixed solution of ethanol / water, wherein the volume ratio of ethanol / water is 2:1; after mixing uniformly, it is transferred to a high-pressure reaction kettle, and reacted at 180℃ for 18 h, thus obtaining a precursor;

[0041] (2) The precursor is dispersed in Tris-HCl buffer solution with pH of 8, 0.15 g dopamine hydrochloride is added, mixed for 5 h, and dried;

[0042] (3) Under inert atmosphere, the product of step (2) is mixed with 10 mmol lithium hydroxide by ball milling, and then calcined in two steps, the first stage: 300℃ for 1 h, the second stage: 500℃ for 1 h, thus obtaining the lithium manganese iron phosphate-based positive electrode material.

[0043] Comparative Example 3

[0044] (1) 10 mmol manganese nitrate, 10 mmol iron nitrate, 1 mmol cobalt nitrate and 10 mmol ammonium dihydrogen phosphate, 20 mmol sodium citrate are dissolved in a mixed solution of ethanol / water, wherein the volume ratio of ethanol / water is 2:1; after mixing uniformly, it is transferred to a high-pressure reaction kettle, and reacted at 180℃ for 18 h, thus obtaining a precursor;

[0045] (2) The precursor is dispersed in deionized water, 1 mmol of NH4F is added, mixed for 5 h, and dried;

[0046] (3) Under inert atmosphere, the product of step (2) is mixed with 10 mmol lithium hydroxide by ball milling, and then calcined in two steps, the first stage: 300℃ for 1 h, the second stage: 500℃ for 1 h, thus obtaining the lithium manganese iron phosphate-based positive electrode material.

[0047] The positive electrode materials prepared in Examples 1-3 and Comparative Examples 1-3 are mixed with superconducting carbon black and binder PVDF at a mass ratio of 8:1:1, NMP is added to make a uniform slurry, which is coated on a positive electrode current collector, dried, cut into pieces, to obtain a positive electrode sheet; a lithium sheet is used as a negative electrode sheet, a PP film is used as a separator, an electrolyte is added dropwise, to prepare a button cell, and the button cell is subjected to electrochemical performance testing, and the experimental results are as shown in the following table. As can be clearly seen from Table 1, the discharge specific capacity and cycle performance of Example 1-2 at 0.1C and 1C are significantly better than those of Comparative Examples 1-3.

[0048] Table 1

[0049]

[0050] The preferred embodiments of the present application have been described above with reference to the drawings, but the present application is not limited to the above examples, and various modifications and changes can be made by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a lithium iron phosphate-based material, characterized in that, Includes the following steps: (1) Dissolve manganese source, iron source, cobalt source, phosphorus source, and sodium citrate in a mixture of ethanol and water at a molar ratio of 1:1:(0.05-0.1):1:(2-3), wherein the volume ratio of ethanol to water is 2-3:1; after mixing evenly, transfer to a high-pressure reactor and react at 180-200℃ for 18-20h to obtain the precursor; (2) Disperse the precursor in Tris-HCl buffer at pH 8-9, add dopamine hydrochloride, mix well, then add NH4F, mix well, and dry. (3) Under an inert atmosphere, the product of step (2) is mixed with the lithium source by ball milling and then calcined in two steps: the first stage is held at 300-350℃ for 1-2 hours; the second stage is held at 500-650℃ for 1-2 hours, thereby obtaining the lithium iron manganese phosphate-based cathode material.

2. The method for preparing a lithium iron phosphate-based material according to claim 1, wherein the manganese source is one or more of manganese sulfate, manganese acetate, and manganese nitrate.

3. The method for preparing a lithium manganese iron phosphate-based material according to claim 1, wherein the iron source is one or more of ferric sulfate, ferric acetate, ferric nitrate, and ferric chloride.

4. In the method for preparing a lithium manganese iron phosphate-based material according to claim 1, the cobalt source is one or more of cobalt nitrate, cobalt acetate, cobalt sulfate, and cobalt chloride.

5. The method for preparing a lithium manganese iron phosphate-based material according to claims 1-2, wherein the molar ratio of phosphorus source to NH4F is 1:(0.1-0.2), and the phosphorus source is ammonium dihydrogen phosphate or diammonium hydrogen phosphate.

6. The method for preparing a lithium manganese iron phosphate-based material according to claims 1-5, wherein the lithium source is at least one of lithium carbonate and lithium hydroxide.

7. A lithium iron phosphate-based material, characterized in that, It is prepared using any one of claims 1-6.

8. A lithium-ion battery, characterized in that, The lithium manganese iron phosphate-based material of claim 7 is used as the positive electrode material.

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