CN118651838A - A modification method of lithium manganese iron phosphate - Google Patents

Abstract

The invention provides a modification method of lithium iron manganese phosphate, which belongs to the technical field of lithium ion batteries. The modification method is specifically as follows: step 1: mixing a lithium iron manganese phosphate precursor or lithium iron manganese phosphate with a carbon source and a fluorine-boron compound; step 2: subjecting the mixture obtained in step 1 to high-temperature heat treatment to obtain modified lithium iron manganese phosphate; the outermost layer of the modified lithium iron manganese phosphate is a fluorine-metal compound, the middle layer is amorphous carbon doped with fluorine-boron, and the inner core is lithium iron manganese phosphate doped with fluorine-boron or/and metal ions; the chemical composition of the lithium iron manganese phosphate precursor or lithium iron manganese phosphate is , , the space group is Pnma, and it belongs to the olivine structure. The modified lithium manganese iron phosphate synthesized by the present invention has high capacity, energy density, good cycle stability and rate performance, and has the advantage of a simple synthesis method.

Description

A modification method of lithium manganese iron phosphate

Technical Field

The invention relates to the technical field of lithium ion batteries, and in particular to a modification method of lithium manganese iron phosphate.

Background Art

Lithium-ion battery is a clean, efficient and renewable secondary battery, and cathode material is an important component. Lithium iron phosphate is one of the most important lithium-ion cathode materials, with the advantages of excellent cycle stability, high safety and low cost. However, its further development is limited by the low energy density it provides, which is caused by the low platform voltage. Lithium manganese iron phosphate (LMFP) has a higher redox potential and is considered to be a very promising cathode material for lithium-ion batteries, but its conductivity and cycle performance are poor.

Bulk doping is an important modification method for LMFP. Foshan Defang Nanotechnology Co., Ltd. discloses a lithium-ion battery positive electrode material and a preparation method thereof (CN109192935A). Fudan University discloses a phosphorus-boron-doped lithium manganese phosphate/carbon composite material and a preparation method thereof (CN102931404A). Foshan Defang Nanotechnology Co., Ltd. discloses a phosphorus-boron-doped nano-disk-shaped lithium manganese iron phosphate positive electrode material and a preparation method and application thereof (CN116666624A). Jiangsu Beite Nanotechnology Co., Ltd. discloses a fluorine-doped lithium manganese iron phosphate positive electrode material and a preparation method thereof (CN114373912A). The common feature of the above methods is that fluorine or boron elements are doped in the phosphate olivine system to improve the conductivity and lithium ion transfer rate.

Surface coating can make the material show better electrochemical performance. Carbon coating has low cost and simple method. It can not only improve the conductivity of the material, but also prevent the oxidation of transition metals during sintering. Doping of the carbon coating layer helps to further improve the conductivity and enhance the cycle stability. Zhejiang Narada Power Co., Ltd. discloses a doped carbon coating layer modified lithium iron phosphate positive electrode material and its preparation method and application (CN118164457A). Guangxi Liugong Machinery Co., Ltd. discloses a coated lithium manganese iron phosphate material and its preparation method and application (CN118136813A). The common feature of the above methods is that doping the carbon layer with fluorine or other elements can further improve the conductivity of the carbon layer and can inhibit the formation of CEI film during the cycle process.

The two methods of coating and doping are more advanced modification methods. Foshan Defang Nanotechnology Co., Ltd. disclosed a lithium iron phosphate positive electrode material and its preparation method and lithium ion battery (CN117720113A). Hangzhou Barter New Energy Technology Co., Ltd. disclosed a lithium ion battery for portable mobile power supply and its preparation method (CN117174831A). The common feature of the above methods is to use the sol-gel method or solid phase method to simultaneously dope fluorine or fluorine boron into the carbon layer and the phosphate lattice.

The above modification methods have proven that fluorine/boron doping of carbon or bulk phase can effectively improve the electrochemical properties of materials. However, each method faces at least one of the following problems:

(1) The process is complicated and not conducive to industrial application;

(2) The doping source is expensive;

(3) The battery performance after modification still cannot meet expectations.

Summary of the invention

The object of the present invention is to provide a method for modifying lithium manganese iron phosphate. The modified lithium manganese iron phosphate synthesized by the present invention has high capacity, energy density, good cycle stability and rate performance, and has the advantage of a simple synthesis method.

In order to solve the above technical problems, the technical solution adopted by the present invention is:

A modification method for lithium manganese iron phosphate, the specific method is as follows:

Step 1: mixing a lithium iron manganese phosphate precursor or lithium iron manganese phosphate with a carbon source and a fluorine boron compound;

Step 2: subjecting the mixture obtained in step 1 to high-temperature heat treatment to obtain modified lithium manganese iron phosphate; the outermost layer of the modified lithium manganese iron phosphate is a fluorine metal compound, the middle layer is amorphous carbon doped with fluorine and boron, and the inner core is lithium manganese iron phosphate doped with fluorine, boron and/or metal ions;

Among them, the chemical composition of lithium iron manganese phosphate precursor or lithium iron manganese phosphate , , the space group is Pnma, and it belongs to the olivine structure.

Wherein, the lithium manganese iron phosphate precursor comes from at least one of a solvent thermal method or a sol-gel method.

Among them, lithium manganese iron phosphate comes from at least one of a solvent thermal method, a sol-gel method or a solid phase method.

Furthermore, the carbon source includes a mixture of one or more of glucose, polyethylene glycol, sucrose, and oleic acid.

Among them, the amount of carbon source added is lithium iron manganese phosphate precursor or lithium iron manganese phosphate. .

Among them, the fluoroboric compound includes at least one of lithium fluoroborate, sodium fluoroborate, magnesium fluoroborate, potassium fluoroborate, calcium fluoroborate, manganese fluoroborate, iron fluoroborate, cobalt fluoroborate, nickel fluoroborate, copper fluoroborate, zinc fluoroborate, stannous fluoroborate and ammonium fluoroborate.

Furthermore, in step 1, the mixing method is to add at least one dispersion medium of water and ethanol, ball mill for 30 minutes and then dry.

The high temperature heat treatment conditions are as follows: in an argon or nitrogen atmosphere, the heating temperature is Heat to 200-800 ℃, keep warm for 2-12 hours Allow to cool to room temperature.

The present invention also discloses lithium manganese iron phosphate, which is prepared by the above-mentioned lithium manganese iron phosphate modification method.

A positive electrode plate is made from the above-mentioned lithium manganese iron phosphate.

Compared with the prior art, the present invention has the following beneficial effects:

Compared with the original synthesis scheme, the precursor-based process in the present invention only adds fluorine-boron compounds, without other redundant steps. Through simple modification steps, bulk doping of lithium manganese iron phosphate, carbon coating layer doping and fluorine-metal compound coating are simultaneously achieved.

At the same time, the combined effect of multiple modification effects makes the modified lithium manganese iron phosphate have very excellent electrochemical properties, which alleviates the voltage decay and manganese dissolution during the cycle process. This modification method is flexible. By adding different fluorine-boron compounds, different types of ions can be doped and/or coated. The types of fluorine-boron compounds and the ratio of multiple fluorine-boron compounds can be specifically changed to improve the material's cycle performance, rate performance, etc., so as to achieve different modification schemes for different application scenarios.

The present invention mixes a lithium iron manganese phosphate precursor or lithium iron manganese phosphate with a carbon source and a fluorine boron compound and then performs a high-temperature heat treatment to obtain a modified lithium iron manganese phosphate. The outermost layer of the modified lithium iron manganese phosphate is a fluorine metal compound, the middle layer is a fluorine boron doped amorphous carbon, and the inner core is a lithium iron manganese phosphate doped with fluorine boron or/and metal ions. The fluorine metal compound can regulate the CEI film and hinder the damage of HF generated by the decomposition of the electrolyte to the lithium iron manganese phosphate. The fluorine boron doped amorphous carbon can play a role in conducting electricity and promoting ion transmission. The fluorine boron or/and metal ion doping can improve the structural stability of the lithium iron manganese phosphate, inhibit the Jahn-Teller effect and manganese/iron dissolution, and improve the electronic and ionic conductivity of the material. In addition, the fluorine boron compound can also improve the utilization rate of the coated carbon source and the residual carbon content of the coated carbon layer. The modified lithium iron manganese phosphate has higher voltage stability and capacity stability, and exhibits higher energy retention rate and rate performance.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

FIG1 is a schematic diagram of the structure of the material synthesized by this method.

Figure 2 is the prepared in Example 1 SEM image of the precursor.

Figure 3 is a modified SEM images of .

FIG. 4 is a diagram of the Precursors and Modifications XRD pattern of .

Figure 5 is a modified Rate charge and discharge curve.

Figure 6 is a modified SEM images of .

Figure 7 is a modified XRD pattern of .

Figure 8 is a modified Rate charge and discharge curve.

Figure 9 is a modified SEM images of .

Figure 10 is a modified XRD pattern of .

Figure 11 is a modified example 3 Rate charge and discharge curve.

Figure 12 is a modified SEM images of .

Figure 13 is a modified XRD pattern of .

Figure 14 is a modified Rate charge and discharge curve.

Figure 15 is a modified SEM images of .

Figure 16 is a modified XRD pattern of .

Figure 17 is a modified example 5 Rate charge and discharge curve.

Figure 18 is prepared in Comparative Example 1 SEM images of .

Figure 19 is prepared in Comparative Example 1 XRD pattern of .

Figure 20 is in comparative example 1 Rate charge and discharge curve.

Figure 21 is prepared in Comparative Example 2 SEM images of .

Figure 22 is prepared in Comparative Example 2 XRD pattern of .

Figure 23 is a comparative example 2 Rate charge and discharge curve.

Reference numerals:

101 outer layer, 102 inner layer, 103 middle layer.

DETAILED DESCRIPTION

In the following, only some exemplary embodiments are briefly described. As those skilled in the art will appreciate, the described embodiments may be modified in various ways without departing from the spirit or scope of the embodiments of the present invention. Therefore, the drawings and descriptions are considered to be exemplary and non-restrictive in nature. Embodiments of the present invention are described in detail below in conjunction with the accompanying drawings.

Example 1

In this example, the lithium manganese iron phosphate precursor was synthesized by the solvothermal method, and then the precursor was modified. The mixture was transferred to a reactor containing a water/ethylene glycol mixed solvent in a molar ratio of 3:0.8:0.2:1, and argon protective gas was passed through the reactor. The reactor was then sealed and heated to 180°C and kept at this temperature for 2 h. After the reaction was completed, the mixture was washed and dried to prepare Precursor.

The precursor and of glucose and The lithium fluoroborate was placed in a ball mill, ethanol was used as the dispersion medium, and the mixture was ball milled at 400 rpm for 30 min and then dried.

The mixture was placed in a corundum boat, heated to 700°C at a rate of 5°C/min in an argon atmosphere, and naturally cooled to room temperature after being kept for 2 hours. The outermost layer of the obtained product is lithium fluoride, the middle layer is fluorine-boron doped amorphous carbon, and the inner core is fluorine-boron doped lithium manganese iron phosphate.

According to Figures 2-5, the precursor is a Pnma structure, and the subsequent modification steps did not change the crystal structure of lithium manganese iron phosphate. After sintering, the particle shape did not change significantly. The modified lithium manganese iron phosphate has better rate performance and smaller polarization (higher discharge voltage).

Referring to FIG. 1 , a lithium manganese iron phosphate, whose outer layer is a fluorine-metal compound layer, the inner layer is a fluorine-boron or/and metal ion-doped lithium manganese iron phosphate layer, and the middle layer is a fluorine-boron-doped carbon layer, is used to prepare a positive electrode plate.

Example 2

In this example, a solvent thermal method is used to synthesize a lithium iron manganese phosphate precursor, and the precursor is synthesized into lithium iron manganese phosphate, and then the lithium iron manganese phosphate is modified. Compared with Example 1, the only difference is that Precursor. The glucose was placed in a ball mill, ethanol was used as the dispersion medium, and the mixture was ball milled at 400 rpm for 30 min and then dried. The mixture was placed in a corundum boat, and the temperature was raised to 700°C at a rate of 5°C/min in an argon atmosphere. After being kept warm for 2 h, it was naturally cooled to room temperature. The outermost layer of the obtained product was amorphous carbon, and the inner core was lithium manganese iron phosphate. The lithium manganese iron phosphate was mixed with The lithium fluoroborate was placed in a ball mill, ethanol was used as the dispersion medium, and the mixture was ball milled at 400 rpm for 30 min and then dried. The mixture was placed in a corundum boat, heated to 700°C at a rate of 5°C/min in an argon atmosphere, kept warm for 2 h, and then naturally cooled to room temperature. The outermost layer of the obtained product is lithium fluoride, the middle layer is fluorine-boron doped amorphous carbon, and the inner core is fluorine-boron doped lithium manganese iron phosphate.

6-8, the subsequent modification steps did not change the crystal structure of lithium iron manganese phosphate, and the particle shape did not change significantly. The morphological characteristics and electrochemical properties were slightly different from those of Example 1, proving that fluorine boron compounds also have a good modification effect on lithium iron manganese phosphate products.

Example 3

In this example, lithium manganese iron phosphate was synthesized by solid phase method and then modified. and After the glucose is evenly ground, add After ball milling at 400 rpm for 6 hours, the material was dried, and the dried powder was pre-calcined at 350 ° C for 2 hours in an argon atmosphere, and heat treated at 600 ° C for 6 hours to obtain solid-phase lithium manganese iron phosphate. The lithium fluoroborate was placed in a ball mill, ethanol was used as the dispersion medium, and the mixture was ball milled at 400 rpm for 30 min and then dried. The mixture was placed in a corundum boat, and the temperature was raised to 700°C at a rate of 5°C/min in an argon atmosphere. After being kept for 2 hours, it was naturally cooled to room temperature. The outermost layer of the obtained product is lithium fluoride, the middle layer is fluorine-boron doped amorphous carbon, and the inner core is fluorine-boron doped lithium manganese iron phosphate.

Referring to Figures 9 to 11, the modified lithium manganese iron phosphate still has a crystal structure, and the modified lithium manganese iron phosphate has good electrochemical properties. Fluoroboric compounds also have a good modification effect on lithium manganese iron phosphate synthesized by solid phase method.

Example 4

In this example, a lithium iron manganese phosphate precursor is synthesized by a solvothermal method, and then the precursor is modified. Compared with Example 1, the only difference is that lithium fluoroborate is replaced by sodium fluoroborate. The outermost layer of the obtained product is sodium fluoride, the middle layer is fluorine-boron doped amorphous carbon, and the inner core is fluorine-boron and sodium doped lithium iron manganese phosphate.

Referring to Figures 12 to 14, the subsequent modification steps did not change the crystal structure of lithium manganese iron phosphate, and the particle shape did not change significantly. The modified lithium manganese iron phosphate has good electrochemical properties. This proves that compared with lithium fluoroborate, sodium fluoroborate can also play a modification role.

Example 5

In this example, a lithium iron manganese phosphate precursor is synthesized by a solvothermal method, and then the precursor is modified. Compared with Example 1, the only difference is that lithium fluoroborate is replaced by an equimolar mixture of lithium fluoroborate and sodium fluoroborate. The outermost layer of the obtained product is lithium fluoride and sodium fluoride, the middle layer is fluorine-boron doped amorphous carbon, and the inner core is fluorine-boron and sodium doped lithium iron manganese phosphate.

Referring to Figures 15 to 17, the subsequent modification steps did not change the crystal structure of lithium iron manganese phosphate, and the particle shape did not change significantly. The modified lithium iron manganese phosphate has good electrochemical properties. This proves that the mixture of multiple fluorine-boron compounds can also play a role in modifying lithium iron manganese phosphate.

Comparative Example 1

In this example, a solvent thermal method is used to synthesize a lithium iron manganese phosphate precursor, and the precursor is synthesized into lithium iron manganese phosphate. Compared with Example 1, the only difference is that lithium fluoroborate is not added. The outer layer of the obtained product is amorphous carbon, and the inner core is lithium iron manganese phosphate.

18 to 20 , the synthesized product is a crystal structure of lithium manganese iron phosphate, and the particle shape is substantially the same as that of Examples 1, 2, 4, and 5. Compared with Examples 1, 2, 4, and 5, the electrochemical performance is poor, indicating that the modification of the fluorine boron compound can improve the electrochemical performance of lithium manganese iron phosphate based on the solvothermal method.

Comparative Example 2

In this example, lithium manganese iron phosphate was synthesized by solid phase method. and After the glucose is evenly ground, add After ball milling at 400 rpm for 6 hours, the material was dried, and the dried powder was pre-calcined at 350°C for 2 hours in an argon atmosphere, and heat treated at 600°C for 6 hours to obtain solid-phase lithium manganese iron phosphate. The outer layer of the obtained product is amorphous carbon, and the inner core is lithium manganese iron phosphate.

21-23, the synthesized product is a crystal structure of lithium manganese iron phosphate, and the particle shape is substantially the same as that of Example 3. Compared with Example 3, the electrochemical performance is poor, indicating that the modification of fluorine boron compound can improve the electrochemical performance of lithium manganese iron phosphate based on the solid phase method.

Table 1 shows the electrochemical performance of battery materials at 1C.

As can be seen from Table 1, all Examples 1-5 have improved initial coulombic efficiency and cycle stability compared to Comparative Examples 1-2, and both manganese dissolution and voltage decay are suppressed after 500 cycles, indicating that the all-in-one modification scheme used in this patent can greatly alleviate the side reactions on the surface of the material and the failure of the bulk structure of the material during the charge and discharge process. At the same time, due to the low cost and simple process of this method, it is a modification strategy for the industrialization of lithium manganese iron phosphate.

According to the SEM morphology of the final product of the embodiment or the control example in Figures 3, 6, 9, 12, 15, 18 and 21, it is explained that the modification method will not significantly change the product morphology.

According to the XRD phase analysis of the final product of the embodiment or the control example in Figures 4, 7, 10, 13, 16, 19 and 22, it is shown that the modification method will not significantly change the crystal structure of the product, and the synthesized product does not have other impurities.

According to the electrochemical performance of the final product of the embodiment or the control example in Figures 5, 8, 11, 14, 17, 20 and 23, it is shown that the electrochemical performance of the embodiment is better than that of the control example.

Although the preferred embodiments of the present invention have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. It should be pointed out that any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims (8)

1. The modification method of the lithium iron manganese phosphate is characterized by comprising the following specific steps of:

step 1: mixing a lithium iron manganese phosphate precursor or lithium iron manganese phosphate with a carbon source and a fluorine boron compound;

step 2: carrying out high-temperature heat treatment on the mixture obtained in the step 1 to obtain modified lithium manganese iron phosphate; the outermost layer of the modified lithium manganese iron phosphate is a fluorine metal compound, the middle layer is amorphous carbon doped with fluorine boron, and the inner core is boron fluoride or/and metal ion doped lithium manganese iron phosphate;

wherein the chemical components of the precursor of the lithium iron manganese phosphate or the lithium iron manganese phosphate are The space group is Pnma, which belongs to the olivine structure.

2. The method for modifying lithium iron manganese phosphate according to claim 1, wherein the method comprises the following steps: the lithium iron manganese phosphate precursor is derived from at least one of a solvothermal method or a sol-gel method.

3. The method for modifying lithium iron manganese phosphate according to claim 1, wherein the method comprises the following steps: the lithium iron manganese phosphate is derived from at least one of solvothermal method, sol-gel method or solid phase method.

4. The method for modifying lithium iron manganese phosphate according to claim 1, wherein the method comprises the following steps: the carbon source comprises one or more of glucose, polyethylene glycol, sucrose and oleic acid.

5. The method for modifying lithium iron manganese phosphate according to claim 4, wherein the method comprises the following steps: the carbon source is added in the form of a precursor of lithium iron manganese phosphate or lithium iron manganese phosphate。

6. The method for modifying lithium iron manganese phosphate according to claim 1, wherein the method comprises the following steps: the fluoroboric acid compound includes at least one of lithium fluoroborate, sodium fluoroborate, magnesium fluoroborate, potassium fluoroborate, calcium fluoroborate, manganese fluoroborate, iron fluoroborate, cobalt fluoroborate, nickel fluoroborate, copper fluoroborate, zinc fluoroborate, stannous fluoroborate and ammonium fluoroborate.

7. The method for modifying lithium iron manganese phosphate according to claim 1, wherein the method comprises the following steps: in the step 1, the mixing mode is that at least one dispersion medium of water and ethanol is added, ball milling is carried out for 30min, and then drying is carried out.

8. The method for modifying lithium iron manganese phosphate according to claim 1, wherein the method comprises the following steps: the high-temperature heat treatment condition is that the temperature rise temperature is under the argon or nitrogen atmosphereHeating to 200-800 deg.C, maintaining the temperature for 2-12 h deg.CCooled to room temperature.