CN115881971A

CN115881971A - Carbon-coated lithium manganese iron phosphate cathode material and preparation method thereof

Info

Publication number: CN115881971A
Application number: CN202310085648.2A
Authority: CN
Inventors: 李爱霞; 余海军; 谢英豪; 李长东
Original assignee: Hunan Brunp Recycling Technology Co Ltd; Guangdong Brunp Recycling Technology Co Ltd
Current assignee: Hunan Brunp Recycling Technology Co Ltd; Guangdong Brunp Recycling Technology Co Ltd
Priority date: 2023-01-17
Filing date: 2023-01-17
Publication date: 2023-03-31

Abstract

The invention discloses a carbon-coated lithium manganese iron phosphate positive electrode material and a preparation method thereof, and belongs to the technical field of lithium ion batteries. According to the preparation method of the carbon-coated lithium manganese iron phosphate anode material, the precursor of the iron metaphosphate with a porous structure is constructed in advance, and then the precursor is mixed with the manganese source, the reducing agent and other raw materials for thermal reduction, so that the reducing agent and the manganese source are uniformly dispersed on the precursor of the iron metaphosphate and sufficient reduction reaction is realized, meanwhile, a carbon coating layer can be synchronized in the thermal reduction process, and the carbon coating layer can permeate into the porous structure of the precursor of the iron metaphosphate, so that the bonding strength with the obtained lithium manganese iron phosphate anode material is high, the compaction density is high, and the conductivity of the whole product is more uniformly and effectively improved.

Description

Carbon-coated lithium manganese iron phosphate cathode material and preparation method thereof

Technical Field

The invention relates to the technical field of lithium ion batteries, in particular to a carbon-coated lithium manganese iron phosphate positive electrode material and a preparation method thereof.

Background

Compared with other batteries, the lithium ion battery has the advantages of high output voltage, long storage life and the like, and is widely developed and applied to a plurality of fields of automobiles, energy storage and the like. Currently, the positive electrode materials used in lithium ion batteries mainly include lithium cobaltate, lithium nickelate, lithium manganate, lithium nickel manganese manganate, lithium iron phosphate, lithium manganese phosphate, and the like. The lithium manganese phosphate and the lithium iron phosphate have similar crystal structures, the advantages of both the lithium manganese phosphate and the lithium iron phosphate can be combined after the two materials are compounded, and the lithium manganese phosphate and the lithium iron phosphate have the advantages of high safety, high theoretical capacity, high working voltage and high energy density.

However, the existing lithium manganese iron phosphate is generally prepared by a ferric iron source composite manganese source and a lithium source through a direct thermal reduction method, and the processes have the problems that the precursors are not uniformly mixed, so that part of raw materials are insufficiently reduced in a thermal reduction stage, and the purity of the final product is not high. Meanwhile, the lithium iron manganese phosphate anode material which is not subjected to modification coating has the defect of low conductivity, and although the conductivity of the product can be improved to a certain extent by adopting some conventional modification processes such as carbon coating and the like, the coating layers mostly have the problems of low bonding strength and low density, and the improvement degree of the performance of the product is limited.

Disclosure of Invention

Based on the defects in the prior art, the invention aims to provide a preparation method of a carbon-coated lithium manganese iron phosphate anode material, which comprises the steps of constructing a precursor of the iron metaphosphate with a porous structure in advance, mixing the precursor with a manganese source, a reducing agent and other raw materials for thermal reduction, so that the reducing agent and the manganese source can be uniformly dispersed on the precursor of the iron metaphosphate and a sufficient reduction reaction can be realized, and simultaneously a carbon coating layer can be synchronized in the thermal reduction process and can permeate into the porous structure of the precursor of the iron metaphosphate, so that the carbon-coated lithium manganese iron phosphate anode material has high bonding strength and compaction density, and the conductivity of the whole product can be more uniformly and effectively improved.

In order to achieve the purpose, the invention adopts the technical scheme that:

a preparation method of a carbon-coated lithium manganese iron phosphate cathode material comprises the following steps:

(1) Preparing an iron source into a solution, adding the solution into an aqueous solution of organic phosphoric acid, and uniformly mixing, and adjusting the pH of the obtained mixed solution to 10-12 to obtain a reaction solution A;

(2) Heating the reaction solution A to 75-85 ℃, reacting for 2-5 h, then adjusting the pH value to 2-3, aging for 1-3 h, filtering, washing and drying the obtained solid, heating to 400-800 ℃ in air atmosphere, and keeping the temperature for 4-6 h to obtain porous meta-iron phosphate B;

(3) Mixing porous ferric phosphate B with a manganese source, a lithium source, a reducing agent and a dispersing agent, carrying out ball milling treatment under a protective atmosphere, heating to 300-400 ℃, keeping the temperature for 3-5 h, heating to 400-800 ℃, and keeping the temperature for 5-15 h to obtain the carbon-coated lithium manganese iron phosphate cathode material.

According to the preparation method of the carbon-coated lithium iron manganese phosphate cathode material, the complex of iron and organic phosphoric acid is prepared under mild conditions by a chemical precipitation method, a large amount of gases such as carbon dioxide and nitrogen dioxide can be generated in the high-temperature heat preservation process of the complex, and a loose porous structure can be left on the surface of the generated iron metaphosphate after the gases escape. When porous structure's raw materials such as meta ferric phosphate B and manganese source, reductant mixes, these raw materials all can be attached to in porous meta ferric phosphate B's the pore structure, have increased the area of contact of reductant and each raw materials for iron and manganese element homoenergetic in the raw materials are effectively fully reduced, also are favorable to promoting the efficiency that lithium ion and manganese ion got into meta ferric phosphate simultaneously, make the distribution of the lithium iron manganese phosphate positive pole material composition that generates more even. On the other hand, the carbon element contained in the raw material can synchronously generate a carbon coating layer in the thermal reduction process, and the carbon coating layer can penetrate into the porous structure due to the existence of the porous iron metaphosphate B and be combined with the porous structure, so that the carbon coating layer has higher bonding strength with the lithium iron manganese phosphate anode material, and the conductivity of the carbon coating layer is improved to a higher degree and higher uniformity.

Preferably, in the step (1), the iron source is at least one of ferric chloride, ferric sulfate, ferric nitrate and ferric acetate, and the molar concentration of the iron source prepared into the solution is 0.1-0.2 mol/L.

Preferably, the organic phosphoric acid in step (1) is at least one of ethylenediamine tetramethylene diphosphonic acid, aminotrimethylene phosphonic acid, hexamethylenediamine tetramethylene phosphonic acid and diethylenetriamine pentamethylene phosphonic acid.

More preferably, the pH of the aqueous solution of the organic phosphoric acid is 1.5 to 2.5, and the molar concentration of the organic phosphoric acid is 0.1 to 0.2mol/L.

Preferably, the volume ratio of the iron source preparation solution to the organic phosphoric acid aqueous solution in the step (1) is 1: (1-1.1).

Preferably, the manganese source in the step (3) is at least one of manganese acetate, manganese oxalate, manganese nitrate, manganese sulfate and manganese phosphate; the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium acetate, lithium oxalate, lithium chloride and lithium iodide; the reducing agent is at least one of glucose, sucrose, citric acid, ascorbic acid, malic acid, vitamin C, oxalic acid, formic acid, polyvinyl alcohol, polybutadiene and ammonium sulfide; the dispersant is at least one of ethanol and water.

Preferably, a phosphorus source is further added in the step (3) and mixed with the porous iron metaphosphate B, the manganese source, the lithium source, the reducing agent and the dispersing agent, wherein the phosphorus source is at least one of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate and phosphoric acid.

According to the actual proportioning requirement, a phosphorus source with the required proportioning amount can be further introduced into the mixture to prepare the product.

More preferably, in the step (3), the molar ratio of the iron element in the porous iron metaphosphate B to the manganese element in the manganese source is (1; (total phosphorus element in porous iron metaphosphate B, manganese source, and phosphorus source): the molar ratio of the iron element in the porous iron metaphosphate B to the manganese element in the manganese source is (1-1.1): 1.

More preferably, the mass ratio of the porous iron metaphosphate B to the reducing agent in the step (3) is 1: (0.05-0.1).

Preferably, in the step (3), the ball-to-material ratio in the ball milling treatment is (5-15) to 1, the particle size of the used ball milling beads is 5-10 mm, and the ball milling time is 3-8 h; the mass ratio of the total of the porous iron metaphosphate B, the manganese source, the lithium source and the reducing agent to the dispersing agent (porous iron metaphosphate B + manganese source + lithium source + reducing agent): dispersant =1: (1.8-2.2).

Preferably, in the step (3), the heating rate during heating is 4 to 6 ℃/min.

The invention also aims to provide the carbon-coated lithium manganese iron phosphate positive electrode material prepared by the preparation method of the carbon-coated lithium manganese iron phosphate positive electrode material.

Due to the construction of the precursor with the special morphology of the porous ferric metaphosphate B, the reduction degree and the reduction uniformity of each raw material are higher in the thermal reduction process, and the purity of the product is higher than that of the lithium manganese iron phosphate anode material prepared by the common process; meanwhile, the bonding strength of the coated and modified carbon coating layer and the contained active lithium iron manganese phosphate anode material is high, the conductivity promotion degree is high, the electrochemical activity of the whole product is high, and the discharge specific capacity under the multiplying power of 0.1C can reach more than 155 mAh/g.

The invention has the beneficial effects that the invention provides the preparation method of the carbon-coated lithium manganese iron phosphate anode material, the method is characterized in that a precursor of the iron metaphosphate with a porous structure is constructed in advance, and then the precursor is mixed with a manganese source, a reducing agent and other raw materials for thermal reduction, so that the reducing agent and the manganese source can be uniformly dispersed on the precursor of the iron metaphosphate and a sufficient reduction reaction is realized, meanwhile, the carbon coating layer can be synchronized in the thermal reduction process, and can permeate into the porous structure of the precursor of the iron metaphosphate, and the carbon-coated lithium manganese iron phosphate anode material has high bonding strength and high compaction density, and the conductivity of the whole product is more uniformly and effectively improved.

Drawings

Fig. 1 is an XRD spectrum of the products obtained in example 1, comparative example 1 and comparative example 2 of effect example 1 of the present invention.

FIG. 2 is a scanning electron micrograph of porous iron metaphosphate B obtained in example 1 of the present invention.

Detailed Description

For better illustrating the objects, technical solutions and advantages of the present invention, the present invention will be further described in the following with reference to specific examples and comparative examples, which are intended to be understood in detail, but not to limit the present invention. All other embodiments obtained by a person skilled in the art without making any inventive step are within the scope of protection of the present invention. The experimental reagents and instruments involved in the practice of the present invention are, unless otherwise specified, common reagents and instruments.

Example 1

The preparation method of the carbon-coated lithium manganese iron phosphate cathode material comprises the following steps:

(1) Preparing ferric chloride into a solution with the concentration of 0.15mol/L with water, adding the solution into an ethylenediamine tetramethylene diphosphonic acid aqueous solution with the pH =2 and the molar concentration of 0.15mol/L according to the volume ratio of 1;

(2) Heating the reaction solution A to 80 ℃, stirring for reaction for 3h, then adjusting the pH value to 2 by hydrochloric acid, aging for 2h, filtering the suspension, washing the obtained solid with water, drying at 60 ℃ for 12h, heating to 750 ℃ in the air atmosphere, and keeping the temperature for 6h to obtain porous meta-iron phosphate B;

(3) Mixing porous iron metaphosphate B with manganese oxalate, lithium hydroxide, citric acid and ethanol, carrying out ball milling treatment for 8 hours under protective atmosphere, heating to 400 ℃ at the speed of 5 ℃/min, keeping the temperature for 5 hours, heating to 750 ℃ and keeping the temperature for 8 hours to obtain the carbon-coated lithium manganese iron phosphate cathode material; wherein the molar ratio of manganese element in manganese oxalate, iron element in porous iron metaphosphate B and lithium element in lithium hydroxide is 2; the addition amount of the citric acid is 8 percent of the mass of the porous iron metaphosphate B. The mass ratio of the total of the porous iron metaphosphate B, the manganese source, the lithium source and the reducing agent to the dispersing agent (porous iron metaphosphate B + manganese source + lithium source + reducing agent): dispersant =1:2; the ball-to-material ratio in the ball milling treatment is 10, and the particle size of the used ball milling beads is 5mm.

Example 2

(2) Heating the reaction solution A to 75 ℃, stirring for reaction for 5h, then adjusting the pH value to 2 by hydrochloric acid, aging for 2h, filtering the suspension, washing the obtained solid with water, drying at 60 ℃ for 12h, heating to 700 ℃ in the air atmosphere, and keeping the temperature for 6h to obtain porous meta-iron phosphate B;

(3) Mixing porous iron metaphosphate B with manganese oxalate, lithium hydroxide, citric acid and ethanol, carrying out ball milling treatment for 8 hours under a protective atmosphere, then heating to 400 ℃ at the speed of 5 ℃/min, keeping the temperature for 5 hours, heating to 700 ℃ and keeping the temperature for 12 hours to obtain the carbon-coated lithium manganese iron phosphate cathode material; wherein the molar ratio of manganese element in manganese oxalate, iron element in porous iron metaphosphate B and lithium element in lithium hydroxide is 2; the addition amount of the citric acid is 8 percent of the mass of the porous iron metaphosphate B. The mass ratio of the total of the porous iron metaphosphate B, the manganese source, the lithium source and the reducing agent to the dispersing agent (porous iron metaphosphate B + manganese source + lithium source + reducing agent): dispersant =1:2; the ball-to-material ratio in the ball milling treatment is 10, and the particle size of the used ball milling beads is 5mm.

Example 3

(1) Preparing ferric nitrate into a solution with the concentration of 0.15mol/L by using water, adding the solution into an aqueous solution of hexamethylene diamine tetramethylene phosphonic acid with the pH =2 and the molar concentration of 0.15mol/L according to the volume ratio of 1;

(2) Heating the reaction solution A to 80 ℃, stirring for reaction for 3 hours, then adjusting the pH value to 2 by using hydrochloric acid, aging for 2 hours, filtering the suspension, washing the obtained solid by using water, drying for 12 hours at 60 ℃, heating to 750 ℃ in the air atmosphere, and keeping the temperature for 6 hours to obtain porous meta-iron phosphate B;

(3) Mixing porous iron metaphosphate B with manganese nitrate, lithium carbonate, ascorbic acid and ethanol, carrying out ball milling treatment for 8 hours under a protective atmosphere, then heating to 400 ℃ at the speed of 5 ℃/min, keeping the temperature for 5 hours, heating to 650 ℃, and keeping the temperature for 15 hours to obtain the carbon-coated lithium manganese iron phosphate cathode material; wherein the molar ratio of manganese element in manganese nitrate, iron element in porous meta-iron phosphate B and lithium element in lithium carbonate is 2; the addition amount of the ascorbic acid is 8 percent of the mass of the porous meta-iron phosphate B. The mass ratio of the sum of the porous iron metaphosphate B, the manganese source, the lithium source and the reducing agent to the dispersing agent (porous iron metaphosphate B + manganese source + lithium source + reducing agent): dispersant =1:2; the ball-to-material ratio in the ball milling treatment is 10, and the particle size of the used ball milling beads is 5mm.

Example 4

The difference between the present example and example 1 is only that the molar ratio of manganese element in manganese oxalate, iron element in porous iron metaphosphate B, lithium element in lithium hydroxide, and ammonium dihydrogen phosphate added in step (3) is 0.75; the addition amount of the citric acid is 5 percent of the mass of the porous iron metaphosphate B.

Example 5

The difference between the present example and example 1 is only that the molar ratio of manganese element in manganese oxalate, iron element in porous iron metaphosphate B, lithium element in lithium hydroxide, and ammonium dihydrogen phosphate added in step (3) is 0.8; the addition amount of the citric acid is 5 percent of the mass of the porous iron metaphosphate B.

Example 6

The difference between the present example and example 1 is only that the molar ratio of manganese element in manganese oxalate, iron element in porous metaphosphate B, lithium element in lithium hydroxide, and ammonium dihydrogen phosphate added in the step (3) is 0.9; the addition amount of the citric acid is 10 percent of the mass of the porous iron metaphosphate B.

Comparative example 1

A preparation method of a carbon-coated lithium iron manganese phosphate positive electrode material comprises the following steps:

mixing iron phosphate with manganese oxalate, lithium hydroxide, citric acid and ethanol, carrying out ball milling treatment for 8 hours under a protective atmosphere, heating to 400 ℃ at a speed of 5 ℃/min, keeping the temperature for 5 hours, heating to 750 ℃, and keeping the temperature for 8 hours to obtain the carbon-coated lithium manganese iron phosphate cathode material; wherein the manganese element in the manganese oxalate, the iron element in the iron phosphate and the lithium element in the lithium hydroxide are 2; the addition amount of citric acid is 8 percent of the mass of the ferric phosphate. The mass ratio of the total of the iron phosphate, the manganese source, the lithium source and the reducing agent to the dispersing agent (iron phosphate + manganese source + lithium source + reducing agent): dispersant =1:2; the ball-material ratio during ball milling treatment is 10. The iron phosphate is non-porous iron phosphate particles with the average particle size of 1.5-2.5 mu m.

Comparative example 2

mixing 0.015mol of ferric chloride, 0.045mol of lithium hydroxide, 0.03mol of manganese oxalate and 0.2g of ascorbic acid, transferring the mixture into an aqueous solution of ethylenediamine tetramethylene diphosphonic acid with the molar concentration of 0.15mol/L, controlling the pH to be =2, grinding and uniformly mixing the mixture at room temperature, drying the obtained mixed solution at normal temperature for 12h under a vacuum condition, heating the dried mixed solution to 400 ℃ at the speed of 5 ℃/min under a protective atmosphere in a tubular furnace, preserving the heat for 5h, heating the heated mixed solution to 750 ℃ and preserving the heat for 8h to obtain the carbon-coated lithium iron manganese phosphate cathode material.

Effect example 1

In order to verify the performance of the products prepared by the preparation method of the present invention, commercially available commercial lithium metal sheets are used as the negative electrode and commercially available polypropylene separators are used as the separators for each example and comparative example, CR2032 button cells are assembled in a glove box, and then a charge and discharge test is performed at an operating voltage of 2.5 to 4.5V and a 0.1C rate, and the test results are shown in table 1.

TABLE 1

As can be seen from Table 1, the carbon-coated lithium iron manganese phosphate cathode material prepared in the embodiments of the present invention has high electrochemical activity, and the highest specific discharge capacity of the material can reach 156mAh/g at a rate of 0.1C. The XRD analysis of the products obtained in example 1, comparative example 1 and comparative example 2 is shown in fig. 1, and it can be seen that although the raw materials used in comparative example 1 and comparative example 2 are very similar to the technical solution in example 1, no precursor of the porous material is constructed, and the characteristic peaks in the XRD spectra of the finally prepared carbon-coated lithium manganese iron phosphate cathode material are inferior to those of the product in example 1 in terms of intensity or correspondence, which indicates that the reducing agent cannot sufficiently function when the product is prepared by using a common precursor raw material or a common process, and the electrochemical performance of the prepared product is certainly inferior to that of the product in example 1. The porous iron metaphosphate B prepared in example 1 is observed by a scanning electron microscope, and the result is shown in fig. 2, which shows that the porous iron metaphosphate B has a small particle size and a rich porous structure, and can sufficiently support the manganese source, the lithium source and the reducing agent and realize a uniform reduction reaction in the thermal reduction process.

The products of example 1, comparative example 1 and comparative example 2 were subjected to cycle performance testing, 50 charge and discharge cycles were performed at the same rate, and the specific discharge capacity and capacity retention rate after 50 cycles were counted, with the results shown in table 1. It can be seen that the discharge specific capacity of the product prepared by the embodiments of the invention can still reach about 150mAh/g after 50 cycles, and the capacity retention rate is higher. Compared with the products of comparative examples 1 and 2, due to the defects of material configurations, the discharge specific capacity after 50 cycles has higher attenuation degree and lower capacity retention rate.

Finally, it should be noted that the above embodiments are only used for illustrating the technical solutions of the present invention and not for limiting the protection scope of the present invention, and although the present invention is described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions can be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A preparation method of a carbon-coated lithium iron manganese phosphate positive electrode material is characterized by comprising the following steps:

(2) Heating the reaction solution A to 75-85 ℃, reacting for 2-5 h, then adjusting the pH value to 2-3, aging for 1-3 h, filtering, washing and drying the obtained solid, heating to 400-800 ℃ in the air atmosphere, and keeping the temperature for 4-6 h to obtain porous meta-iron phosphate B;

(3) Mixing porous iron metaphosphate B with a manganese source, a lithium source, a reducing agent and a dispersing agent, carrying out ball milling treatment under a protective atmosphere, then heating to 300-400 ℃, preserving heat for 3-5 h, then heating to 400-800 ℃, preserving heat for 5-15 h, and obtaining the carbon-coated lithium manganese iron phosphate cathode material.

2. The method for preparing the carbon-coated lithium iron manganese phosphate cathode material according to claim 1, wherein the iron source in the step (1) is at least one of ferric chloride, ferric sulfate, ferric nitrate and ferric acetate, and the molar concentration of the iron source after being prepared into a solution is 0.1-0.2 mol/L.

3. The method for preparing the carbon-coated lithium iron manganese phosphate cathode material according to claim 1, wherein the organic phosphoric acid in the step (1) is at least one of ethylenediamine tetramethylene diphosphonic acid, aminotrimethylene phosphonic acid, hexamethylenediamine tetramethylene phosphonic acid, and diethylenetriamine pentamethylene phosphonic acid.

4. The method for preparing the carbon-coated lithium iron manganese phosphate cathode material according to claim 3, wherein the aqueous solution of the organic phosphoric acid has a pH of 1.5 to 2.5 and a molar concentration of the organic phosphoric acid is 0.1 to 0.2mol/L.

5. The method for preparing the carbon-coated lithium iron manganese phosphate cathode material according to claim 1, wherein the volume ratio of the solution prepared from the iron source to the aqueous solution of the organic phosphoric acid in the step (1) is 1: (1-1.1).

6. The method for preparing the carbon-coated lithium iron manganese phosphate cathode material according to claim 1, wherein the manganese source in the step (3) is at least one of manganese acetate, manganese oxalate, manganese nitrate, manganese sulfate and manganese phosphate; the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium acetate, lithium oxalate, lithium chloride and lithium iodide; the reducing agent is at least one of glucose, sucrose, citric acid, ascorbic acid, malic acid, vitamin C, oxalic acid, formic acid, polyvinyl alcohol, polybutadiene and ammonium sulfide; the dispersant is at least one of ethanol and water; preferably, a phosphorus source is further added in the step (3) and mixed with the porous iron metaphosphate B, the manganese source, the lithium source, the reducing agent and the dispersing agent, wherein the phosphorus source is at least one of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate and phosphoric acid.

7. The method for preparing the carbon-coated lithium iron manganese phosphate cathode material according to claim 6, wherein the molar ratio of the iron element in the porous iron metaphosphate B to the manganese element in the manganese source in step (3) is (1; (total phosphorus element in porous iron metaphosphate B, manganese source, and phosphorus source): the molar ratio of (iron element in the porous meta-iron phosphate B + manganese element in the manganese source) was (1-1.1): 1.

8. The method for preparing the carbon-coated lithium iron manganese phosphate cathode material according to claim 7, wherein the mass ratio of the porous iron metaphosphate B to the reducing agent in the step (3) is 1: (0.05-0.1).

9. The method for preparing the carbon-coated lithium iron manganese phosphate cathode material as claimed in claim 1, wherein in the step (3), the ball-to-material ratio during ball milling is (5-15) to 1, the particle size of ball milling beads is 5-10 mm, and the ball milling time is 3-8 h; the mass ratio of the total of the porous iron metaphosphate B, the manganese source, the lithium source and the reducing agent to the dispersing agent (porous iron metaphosphate B + manganese source + lithium source + reducing agent): dispersant =1: (1.8-2.2).

10. The carbon-coated lithium iron manganese phosphate positive electrode material prepared by the method for preparing the carbon-coated lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 9.