A high-performance carbon-coated lithium manganese iron phosphate material and its preparation method

Abstract

This invention discloses a high-performance carbon-coated lithium manganese iron phosphate material and its preparation method. The preparation method includes the following steps: S1, mixing plastic particles with fatty acid manganese salt, heat-treating in a first protective gas, and then pre-treating in an oxygen atmosphere; S2, adding lithium source, iron source, and phosphorus source to the mixture obtained in step S1 and mixing, then sintering at low temperature in a second protective gas; S3, sintering the product obtained in step S2 at high temperature in a second protective gas to obtain a carbon-coated lithium manganese iron phosphate composite material. The carbon-coated lithium manganese iron phosphate composite material provided by this invention can be used to prepare lithium-ion batteries, serving as the positive electrode active material for lithium-ion batteries. This invention uses a homogeneous carbon source coating a manganese source as a phase-forming nucleus and low-temperature sintering to further stabilize the carbon coating layer, induce uniform particle growth, and obtain a lithium manganese iron phosphate material with a uniform carbon coating layer, good particle sphericity, and significantly improved cycle stability.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode technology, and in particular to a carbon-coated lithium manganese iron phosphate material and its preparation method. Background Technology

[0002] Compared to LiFePO4, lithium manganese iron phosphate (LiMn) 1-x Fe x Lithium manganese iron phosphate (LiFePO4) possesses a high potential of ~4.0 V and almost the same theoretical capacity. Under the same capacity utilization conditions, the energy density of lithium manganese iron phosphate (LFP) batteries will be about 20% higher than that of lithium iron phosphate (LFP) batteries. Therefore, internationally, LFP is considered a next-generation high-energy-density cathode material for power lithium-ion batteries. However, existing LFP materials have poorer conductivity than LFP, and manganese leaching during cycling leads to deterioration in cycle performance, among other issues, which have become bottlenecks for its further large-scale application. Therefore, there is an urgent need to develop a high-performance LFP material with excellent comprehensive performance, including electrical and cycle performance, to meet the market demand for high-energy-density lithium battery cathode materials. Summary of the Invention

[0003] The purpose of this invention is to provide a high-performance lithium manganese iron phosphate composite material. Through a multi-step reaction, a fatty acid manganese salt oxidation catalyst is first used to induce plastic degradation to provide a homogeneous carbon source. Then, the composite material with uniform carbon coating effect is obtained by sintering. This solves the technical problems of poor carbon coating uniformity and poor cycle stability of lithium manganese iron phosphate.

[0004] The method for preparing carbon-coated lithium manganese iron phosphate composite material provided by the present invention includes the following steps:

[0005] S1. The plastic granules are mixed with fatty acid manganese salt, heat-treated in a first protective gas, and then pre-treated in an oxygen atmosphere.

[0006] S2. Add lithium source, iron source and phosphorus source to the mixture obtained in step S1 and mix them. Then place the mixture under a second protective gas for low-temperature sintering.

[0007] S3. The product obtained in step S2 is placed in the second protective gas for high-temperature sintering to obtain carbon-coated lithium manganese iron phosphate composite material.

[0008] This invention uses plastic particles as a carbon source and fatty acid manganese salt as a manganese source, which first acts as an oxidation catalyst to induce plastic degradation and provide a homogeneous carbon source on the surface of the manganese source. Then, the manganese source is coated with the homogeneous carbon source as a phase-forming nucleus and sintered at low temperature to further stabilize the carbon coating layer and induce uniform particle growth, resulting in a high-performance lithium manganese iron phosphate material with uniform carbon coating.

[0009] In the method of the present invention, in step S1, the plastic particles are any one of polyethylene particles, polypropylene particles and polystyrene particles, with an average particle size ≤100μm.

[0010] In the method of the present invention, in step S1, the fatty acid manganese is any one of C10-C20 fatty acid manganese.

[0011] In the method of the present invention, in step S1, the temperature of the heat treatment is 300-500℃ and the time is 5-8h;

[0012] The first protective gas is argon or nitrogen;

[0013] The pretreatment temperature is 120-200℃, and the time is 1-3 hours;

[0014] The mass ratio of the plastic particles to the fatty acid manganese is 0.1-0.3:1.

[0015] In the method of the present invention, in step S2, the mass ratio of lithium in the lithium source, iron in the iron source, manganese in the fatty acid manganese and phosphorus in the phosphorus source is 1.02-1.14:0.3-0.6:0.7-0.4:1;

[0016] The lithium source can be any one or more of lithium hydroxide, lithium carbonate, and lithium chloride.

[0017] The iron source can be any one or more of iron oxide, iron chloride, and iron nitrate.

[0018] The phosphorus source can be any one or more of phosphoric acid, ammonium dihydrogen phosphate, and lithium dihydrogen phosphate.

[0019] In the method of the present invention, in step S2, the temperature of the low-temperature sintering is 200-300℃ and the time is 1-3h;

[0020] The second protective gas is argon or nitrogen.

[0021] In the method of the present invention, in step S3, the high-temperature sintering temperature is 600-900℃ and the time is 2-10h.

[0022] The carbon-coated lithium manganese iron phosphate composite material provided by this invention can be used to prepare lithium-ion batteries, serving as the positive electrode active material for lithium-ion batteries.

[0023] Lithium-ion batteries incorporating the carbon-coated lithium manganese iron phosphate composite material of this invention are also within the scope of protection of this invention.

[0024] The present invention has the following beneficial technical effects:

[0025] By designing a multi-step reaction: (1) using fatty acid manganese salt to provide manganese source oxidation catalyst to induce plastic degradation and provide homogeneous carbon source on the manganese source surface; (2) using homogeneous carbon source to coat manganese source as phase nucleation nucleus and sintering at low temperature to further stabilize carbon coating layer and induce uniform particle growth; (3) the obtained lithium manganese iron phosphate material has uniform carbon coating layer on surface, good particle sphericity, and significantly improved cycle stability. Detailed Implementation

[0026] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0027] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0028] Example 1

[0029] Polyethylene plastic particles (average particle size 5 μm) were uniformly mixed with C10 fatty acid manganese at a mass ratio of 0.1:1. The mixture was first heat-treated in nitrogen for 5 hours at 300℃, followed by pretreatment in oxygen for 2 hours at 150℃. The mixture was then mixed with lithium, iron, and phosphorus sources in a mass ratio of lithium, iron, manganese, and phosphorus from the phosphorus source of 1.02:0.6:0.4:1 and sintered in argon at 200℃ for 3 hours, and finally sintered at 800℃ for 8 hours.

[0030] The prepared composite material was mixed and ground with carbon black and PVDF at a mass ratio of 8:1:1, and then coated onto aluminum foil to serve as the positive electrode for lithium-ion batteries. CR2032 coin cells were assembled in an argon-filled glove box. The negative electrode was a lithium metal sheet, the separator was a polypropylene microporous membrane, and the electrolyte was a 1 mol / L LiPF6 solution with an electrolyte solvent ratio of EC:DMC:EMC = 1:1:1 (v / v / v). The measured voltage range of the coin cell was 2V-4.0V.

[0031] Example 2

[0032] Polypropylene plastic granules (average particle size 10 μm) were uniformly mixed with C15 fatty acid manganese at a mass ratio of 0.3:1. The mixture was first heat-treated in nitrogen for 5 hours at 500℃, followed by pretreatment in oxygen for 3 hours at 120℃. Subsequently, the mixture was mixed with lithium, iron, and phosphorus sources at a mass ratio of lithium, iron, manganese, and phosphorus from the phosphorus source of 1.10:0.5:0.5:1 and sintered in argon at 250℃ for 1 hour, and finally sintered at 900℃ for 6 hours.

[0033] The prepared composite material was mixed and ground with carbon black and PVDF at a mass ratio of 8:1:1, and then coated onto aluminum foil to serve as the positive electrode for lithium-ion batteries. CR2032 coin cells were assembled in an argon-filled glove box. The negative electrode was a lithium metal sheet, the separator was a polypropylene microporous membrane, and the electrolyte was a 1 mol / L LiPF6 solution with an electrolyte solvent ratio of EC:DMC:EMC = 1:1:1 (v / v / v). The measured voltage range of the coin cell was 2V-4.0V.

[0034] Example 3

[0035] Polystyrene plastic particles (average particle size 2 μm) were uniformly mixed with C16 fatty acid manganese at a mass ratio of 0.3:1. The mixture was first heat-treated in nitrogen for 5 hours at 400°C, followed by pretreatment in oxygen for 1 hour at 120°C. Subsequently, the mixture was mixed with lithium, iron, and phosphorus sources in a mass ratio of lithium, iron, manganese, and phosphorus from the phosphorus source of 1.12:0.7:0.3:1 and sintered in argon at 250°C for 1 hour, and finally sintered at 850°C for 6 hours.

[0036] The prepared composite material was mixed and ground with carbon black and PVDF at a mass ratio of 8:1:1, and then coated onto aluminum foil to serve as the positive electrode for lithium-ion batteries. CR2032 coin cells were assembled in an argon-filled glove box. The negative electrode was a lithium metal sheet, the separator was a polypropylene microporous membrane, and the electrolyte was a 1 mol / L LiPF6 solution with an electrolyte solvent ratio of EC:DMC:EMC = 1:1:1 (v / v / v). The measured voltage range of the coin cell was 2V-4.0V.

[0037] Example 4

[0038] Polyethylene plastic particles (average particle size 8 μm) were uniformly mixed with C18 fatty acid manganese at a mass ratio of 0.25:1. The mixture was first heat-treated in nitrogen for 8 hours at 400℃, followed by pretreatment in oxygen for 2 hours at 120℃. The mixture was then mixed with lithium, iron, and phosphorus sources at a mass ratio of lithium, iron, manganese, and phosphorus from the phosphorus source of 1.08:0.7:0.3:1 and sintered in argon at 200℃ for 2 hours, and finally sintered at 750℃ for 8 hours.

[0039] The prepared composite material was mixed and ground with carbon black and PVDF at a mass ratio of 8:1:1, and then coated onto aluminum foil to serve as the positive electrode for lithium-ion batteries. CR2032 coin cells were assembled in an argon-filled glove box. The negative electrode was a lithium metal sheet, the separator was a polypropylene microporous membrane, and the electrolyte was a 1 mol / L LiPF6 solution with an electrolyte solvent ratio of EC:DMC:EMC = 1:1:1 (v / v / v). The measured voltage range of the coin cell was 2V-4.0V.

[0040] Comparative Example 1

[0041] Polyethylene plastic particles (average particle size 5 μm) were uniformly mixed with C10 fatty acid manganese at a mass ratio of 0.1:1. First, the mixture was heat-treated in nitrogen for 5 hours at a temperature of 300℃. Then, the mixture was mixed with lithium, iron and phosphorus sources at a mass ratio of lithium, iron, manganese and phosphorus in the phosphorus source of 1.02:0.6:0.4:1 and sintered in argon at a temperature of 200℃ for 3 hours. Finally, the mixture was sintered at 800℃ for 8 hours.

[0042] Comparative Example 2

[0043] Polyethylene plastic particles (average particle size 5 μm) were uniformly mixed with C10 fatty acid manganese at a mass ratio of 0.1:1. Then, the mixture was mixed with lithium source, iron source and phosphorus source at a mass ratio of lithium element, iron element, manganese element and phosphorus element in phosphorus source of 1.02:0.6:0.4:1 and sintered in argon atmosphere at a sintering temperature of 200℃ for 3 hours. Finally, it was sintered at 800℃ for 8 hours.

[0044] Comparative Example 3

[0045] Polyethylene plastic granules (average particle size 100 μm) were uniformly mixed with C5 fatty acid manganese at a mass ratio of 0.1:1. The mixture was first heat-treated in nitrogen for 5 hours at 300 °C, followed by pretreatment in oxygen for 2 hours at 150 °C. Subsequently, the mixture was mixed with lithium, iron, and phosphorus sources at a mass ratio of lithium, iron, manganese, and phosphorus from the phosphorus source of 1.02:0.6:0.4:1 and sintered in argon at 200 °C for 3 hours, and finally sintered at 800 °C for 8 hours.

[0046] Comparative Example 4

[0047] Polyethylene plastic particles (average particle size 5 μm) were uniformly mixed with C10 fatty acid manganese at a mass ratio of 0.1:1. The mixture was first heat-treated in nitrogen for 5 hours at 300℃, followed by pretreatment in oxygen for 2 hours at 150℃. Subsequently, the mixture was mixed with lithium, iron, and phosphorus sources at a mass ratio of lithium, iron, manganese, and phosphorus from the phosphorus source of 1.02:0.1:0.9:1, and then sintered in argon at 800℃ for 8 hours.

[0048] The composite materials obtained in Examples 1-4 and Comparative Examples 1-4 were subjected to physicochemical and electrical property tests. The test results are shown in Table 1.

[0049] Table 1 Electrochemical properties of the composite materials in the comparative examples and embodiments

[0050]

[0051] As can be seen from Table 1, the rate performance and cycle performance of Examples 1-4 are significantly improved compared with those of Comparative Examples 1-4, indicating that the preparation method provided by the present invention has positive significance for improving carbon coating uniformity, improving rate performance and improving cycle performance.

[0052] Compared with Example 1, Comparative Example 1 omits the pretreatment step in oxygen. Comparing the effects of the two materials, it can be seen that the initial charge capacity, initial efficiency and cycle performance of Example 1 are significantly improved, indicating that the oxygen pretreatment process plays a positive role in the present invention and has the effect of promoting the oxidation of fatty acid manganese salt catalyst.

[0053] Compared with Example 1, Comparative Example 2 omits the steps of heat treatment in nitrogen and pretreatment in oxygen. Comparing the effects of the two materials, it can be seen that the electrical properties and cycle performance of Example 1 are significantly improved. Combined with the effect of Comparative Example 1, it shows that the synthesis step of first heat treatment in nitrogen and then pretreatment in oxygen is crucial to improving the surface coating and sintering effect of the material.

[0054] The comparison between Comparative Example 1 and Comparative Example 2 shows that nitrogen heat treatment can improve the uniformity of carbon coating and enhance the electrical properties of the material to a certain extent.

[0055] Compared with Example 1, Comparative Example 3 used C5 fatty acid manganese. Comparing the effects of the two materials, it can be seen that the pretreatment method and sintering regime need to be matched with fatty acid manganese salts of appropriate chain length. Fatty acid manganese salts with shorter chain lengths may be difficult to form rings and uniformly coat under the conditions designed in this invention.

[0056] Compared with Example 1, Comparative Example 4 used a mixture with lithium, iron and phosphorus sources in a mass ratio of lithium, iron, manganese and phosphorus in the phosphorus source of 1.02:0.1:0.9:1. The electrical performance and cycle performance of Comparative Example 4 were lower than those of Example 1, indicating that when fatty acid manganese salt is used as a manganese source, carbon source and catalyst, its ratio has an impact on the phase formation and carbon layer conversion uniformity of lithium manganese iron phosphate.

Claims

1. A method for preparing a carbon-coated lithium manganese iron phosphate composite material, comprising the following steps: S1. The plastic granules are mixed with fatty acid manganese salt, heat-treated in a first protective gas, and then pre-treated in an oxygen atmosphere. The plastic particles are any one of polyethylene particles, polypropylene particles and polystyrene particles, with an average particle size ≤100μm. The fatty acid manganese is any one of the C10-C20 fatty acid manganese; The heat treatment is performed at a temperature of 300-500℃ for 5-8 hours. The pretreatment temperature is 120-200℃, and the time is 1-3 hours; S2. Add lithium source, iron source and phosphorus source to the mixture obtained in step S1 and mix them. Then place the mixture under a second protective gas for low-temperature sintering. The mass ratio of lithium in the lithium source, iron in the iron source, manganese in the fatty acid manganese, and phosphorus in the phosphorus source is 1.02–1.14: 0.3–0.6: 0.7–0.4:

1. The low-temperature sintering temperature is 200-300℃, and the time is 1-3h; S3. The product obtained in step S2 is placed in the second protective gas for high-temperature sintering to obtain carbon-coated lithium manganese iron phosphate composite material.

2. The preparation method according to claim 1, characterized in that: In step S1, the mass ratio of the plastic particles to the fatty acid manganese is 0.1-0.3:

1.

3. The preparation method according to claim 1 or 2, characterized in that: In step S1, the first protective gas is argon or nitrogen.

4. The preparation method according to claim 1 or 2, characterized in that: In step S2, the second protective gas is argon or nitrogen.

5. The preparation method according to claim 1 or 2, characterized in that: In step S3, the high-temperature sintering temperature is 600-900℃ and the time is 2-10h.

6. The carbon-coated lithium manganese iron phosphate composite material prepared by the method of any one of claims 1-5.

7. The application of the carbon-coated lithium manganese iron phosphate composite material according to claim 6 in lithium-ion batteries; The carbon-coated lithium manganese iron phosphate composite material is used to prepare the positive electrode material of the lithium-ion battery.

8. A lithium-ion battery, wherein the positive electrode is prepared from the carbon-coated lithium manganese iron phosphate composite material as described in claim 6.

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